STEEL SHEET, MEMBER, AND METHODS FOR PRODUCING SAME

Abstract

A base steel sheet is disclosed having a specified chemical composition and a steel microstructure at a quarter thickness position containing specified ranges of ferrite, fresh martensite, retained austenite, bainite, tempered bainite, and tempered martensite, the value obtained by dividing the total area fraction of isolated island-like fresh martensite and isolated island-like retained austenite by the sum of the area fraction of fresh martensite and the volume fraction of retained austenite in a ferrite grain is 0.65 or more, the isolated island-like fresh martensite and the isolated island-like retained austenite in the ferrite grain has an average grain size of 2.0 m or less, and the amount of diffusible hydrogen in the base steel sheet is 0.50 ppm by mass or less.

Claims

1-11. (canceled)

12. A steel sheet comprising a base steel sheet, wherein the base steel sheet has a chemical composition containing, on a mass percent basis, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 0.75% or less, Mn: 2.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, and N: 0.0100% or less, with the remainder being Fe and incidental impurities, and has a steel microstructure, as a microstructure at a quarter thickness position of the base steel sheet, in which an area fraction of ferrite: 20.0% or more and 80.0% or less, an area fraction of fresh martensite: 15.0% or less, an area fraction of retained austenite: 3.0% or less, a value obtained by dividing a total area fraction of island-like fresh martensite and island-like retained austenite in a ferrite grain by a sum of an area fraction of fresh martensite and an area fraction of retained austenite in the entire steel sheet: 0.65 or more, an area fraction of bainite and tempered bainite: 10.0% or less, an area fraction of tempered martensite: 10.0% or more and 70.0% or less, and the island-like fresh martensite and the island-like retained austenite in the ferrite grain has an average grain size of 2.0 m or less, and an amount of diffusible hydrogen in the base steel sheet is 0.50 ppm by mass or less, and the steel sheet has a tensile strength of 780 MPa or more.

13. The steel sheet according to claim 12, wherein the chemical composition further contains, on a mass percent basis, at least one element selected from Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less.

14. The steel sheet according to claim 12, comprising one or two or more selected from the following (1) to (3): (1) comprising a galvanized layer as an outermost surface layer on one or both surfaces of the steel sheet, (2) when a region of 200 m or less from a surface of the base steel sheet in the thickness direction is defined as a surface layer, the base steel sheet has, in the surface layer, a surface soft layer with a Vickers hardness of 85% or less with respect to a Vickers hardness at a quarter thickness position, and when nanohardness is measured at 300 points or more in a 50 m50 m region on a sheet surface at a quarter depth position in the thickness direction and at a half depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet, a ratio of a number of measurements with a nanohardness of 7.0 GPa or more on the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet to a total number of measurements at the quarter depth position in the thickness direction of the surface soft layer is 0.10 or less, the nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet has a standard deviation of 1.8 GPa or less, and the nanohardness of the sheet surface at the half depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet has a standard deviation of 2.2 GPa or less, and (3) comprising a metal coated layer formed on the base steel sheet on one or both surfaces of the steel sheet.

15. The steel sheet according to claim 13, comprising one or two or more selected from the following (1) to (3): (1) comprising a galvanized layer as an outermost surface layer on one or both surfaces of the steel sheet, (2) when a region of 200 m or less from a surface of the base steel sheet in the thickness direction is defined as a surface layer, the base steel sheet has, in the surface layer, a surface soft layer with a Vickers hardness of 85% or less with respect to a Vickers hardness at a quarter thickness position, and when nanohardness is measured at 300 points or more in a 50 m50 m region on a sheet surface at a quarter depth position in the thickness direction and at a half depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet, a ratio of a number of measurements with a nanohardness of 7.0 GPa or more on the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet to a total number of measurements at the quarter depth position in the thickness direction of the surface soft layer is 0.10 or less, the nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet has a standard deviation of 1.8 GPa or less, and the nanohardness of the sheet surface at the half depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet has a standard deviation of 2.2 GPa or less, and (3) comprising a metal coated layer formed on the base steel sheet on one or both surfaces of the steel sheet.

16. A member comprising the steel sheet according to claim 12.

17. A member comprising the steel sheet according to claim 13.

18. A member comprising the steel sheet according to claim 14.

19. A member comprising the steel sheet according to claim 15.

20. A method for producing a steel sheet, comprising: a hot rolling step of hot-rolling a steel slab with the chemical composition according to claim 12 under a condition of a finish rolling temperature of 820 C. or more to produce a hot-rolled steel sheet; a heating step of heating the steel sheet after the hot rolling step in a temperature range of 350 C. or more and 600 C. or less at an average heating rate of 7 C./s or more; an annealing step of annealing under conditions of an annealing temperature: 750 C. or more and 900 C. or less and an annealing time: 20 seconds or more; after the annealing step, a first cooling step of cooling under conditions of an average cooling rate of 7 C./s or more from (the annealing temperature 30 C.) to 650 C. and an average cooling rate of 14 C./s or less from 650 C. to 500 C.; after the first cooling step, a second cooling step of applying a tension of 2.0 kgf/mm.sup.2 or more to the steel sheet in a temperature range of 300 C. or more and 450 C. or less, then subjecting the steel sheet to five or more passes, each pass involving contact with a roll with a diameter of 500 mm or more and 1500 mm or less for a quarter circumference of the roll, and then cooling the steel sheet to a cooling stop temperature of 250 C. or less; a reheating step of reheating the steel sheet to a temperature range of the cooling stop temperature or more and 440 C. or less and holding the steel sheet for 20 seconds or more after the second cooling step; and optionally a cold rolling step of cold-rolling the steel sheet after the hot rolling step and before the heating step at a rolling reduction of 20% or more and 80% or less to produce a cold-rolled steel sheet.

21. A method for producing a steel sheet, comprising: a hot rolling step of hot-rolling a steel slab with the chemical composition according to claim 13 under a condition of a finish rolling temperature of 820 C. or more to produce a hot-rolled steel sheet; a heating step of heating the steel sheet after the hot rolling step in a temperature range of 350 C. or more and 600 C. or less at an average heating rate of 7 C./s or more; an annealing step of annealing under conditions of an annealing temperature: 750 C. or more and 900 C. or less and an annealing time: 20 seconds or more; after the annealing step, a first cooling step of cooling under conditions of an average cooling rate of 7 C./s or more from (the annealing temperature 30 C.) to 650 C. and an average cooling rate of 14 C./s or less from 650 C. to 500 C.; after the first cooling step, a second cooling step of applying a tension of 2.0 kgf/mm.sup.2 or more to the steel sheet in a temperature range of 300 C. or more and 450 C. or less, then subjecting the steel sheet to five or more passes, each pass involving contact with a roll with a diameter of 500 mm or more and 1500 mm or less for a quarter circumference of the roll, and then cooling the steel sheet to a cooling stop temperature of 250 C. or less; a reheating step of reheating the steel sheet to a temperature range of the cooling stop temperature or more and 440 C. or less and holding the steel sheet for 20 seconds or more after the second cooling step; and optionally a cold rolling step of cold-rolling the steel sheet after the hot rolling step and before the heating step at a rolling reduction of 20% or more and 80% or less to produce a cold-rolled steel sheet.

22. The method for producing a steel sheet according to claim 20, comprising the following one or two selected from the following (1) to (2): (1) comprising a galvanizing step of performing a galvanizing treatment on the steel sheet to form a galvanized layer on the steel sheet after the first cooling step and before the second cooling step, and (2) comprising a metal coating step of performing metal coating on one or both surfaces of the steel sheet to form a metal coated layer after the hot rolling step and before the annealing step.

23. The method for producing a steel sheet according to claim 21, comprising the following one or two selected from the following (1) to (2): (1) comprising a galvanizing step of performing a galvanizing treatment on the steel sheet to form a galvanized layer on the steel sheet after the first cooling step and before the second cooling step, and (2) comprising a metal coating step of performing metal coating on one or both surfaces of the steel sheet to form a metal coated layer after the hot rolling step and before the annealing step.

24. The method for producing a steel sheet according to claim 20, wherein the annealing in the annealing step is performed in an atmosphere with a dew point of 30 C. or more.

25. The method for producing a steel sheet according to claim 21, wherein the annealing in the annealing step is performed in an atmosphere with a dew point of 30 C. or more.

26. The method for producing a steel sheet according to claim 22, wherein the annealing in the annealing step is performed in an atmosphere with a dew point of 30 C. or more.

27. The method for producing a steel sheet according to claim 23, wherein the annealing in the annealing step is performed in an atmosphere with a dew point of 30 C. or more.

28. A method for producing a member, comprising a step of subjecting the steel sheet according to claim 12 to at least one of forming and joining to produce a member.

29. A method for producing a member, comprising a step of subjecting the steel sheet according to claim 13 to at least one of forming and joining to produce a member.

30. A method for producing a member, comprising a step of subjecting the steel sheet according to claim 14 to at least one of forming and joining to produce a member.

31. A method for producing a member, comprising a step of subjecting the steel sheet according to claim 15 to at least one of forming and joining to produce a member.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0109] FIG. 1 is an example of a SEM image of aspects of the present invention (Inventive Example No. 13 in Examples).

[0110] FIG. 2(a) is an explanatory view of U-bending (primary bending) in a U-bending+tight bending test in Examples. FIG. 2(b) is an explanatory view of tight bending (secondary bending) in a U-bending+tight bending test in Examples.

[0111] FIG. 3(a) is an explanatory view of V-bending (primary bending) in a V-bending+orthogonal VDA bending test in Examples. FIG. 3(b) is an explanatory view of orthogonal VDA bending (secondary bending) in a V-bending+orthogonal VDA bending test in Examples.

[0112] FIG. 4(a) is a front view of a test member composed of a hat-shaped member and a steel sheet spot-welded together for an axial compression test in Examples. FIG. 4(b) is a perspective view of the test member illustrated in FIG. 4(a). FIG. 4(c) is a schematic explanatory view of an axial compression test in Examples.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0113] Aspects of the present invention are described on the basis of the following embodiments.

[1. Steel Sheet]

[0114] A steel sheet according to aspects of the present invention is a steel sheet including a base steel sheet, wherein the base steel sheet has a chemical composition containing, on a mass percent basis, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 0.75% or less, Mn: 2.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, and N: 0.0100% or less, with the remainder being Fe and incidental impurities, and has a steel microstructure, as a microstructure at a quarter thickness position of the base steel sheet, in which the area fraction of ferrite: 20.0% or more and 80.0% or less, an area fraction of fresh martensite: 15.0% or less, an area fraction of retained austenite: 3.0% or less, a value obtained by dividing a total area fraction of island-like fresh martensite and island-like retained austenite in a ferrite grain by a sum of an area fraction of fresh martensite and an area fraction of retained austenite: 0.65 or more, an area fraction of bainite and tempered bainite: 10.0% or less, an area fraction of tempered martensite: 10.0% or more and 70.0% or less, and the island-like fresh martensite and the island-like retained austenite in the ferrite grain has an average grain size of 2.0 m or less, and an amount of diffusible hydrogen in the base steel sheet is 0.50 ppm by mass or less, and the steel sheet has a tensile strength of 780 MPa or more.

[0115] The steel sheet may have a galvanized layer as an outermost surface layer on one or both surfaces of the steel sheet. A steel sheet with a galvanized layer may be a galvanized steel sheet.

Chemical Composition

[0116] First, the chemical composition of a base steel sheet of a steel sheet according to an embodiment of the present invention is described. The unit in the chemical composition is % by mass and is hereinafter expressed simply in % unless otherwise specified.

C: 0.030% or More and 0.250% or Less

[0117] C is an element effective in forming an appropriate amount of tempered martensite, bainite, tempered bainite, or the like to ensure a TS of 780 MPa or more and high YS and YR. A C content of less than 0.030% results in an increase in the area fraction of ferrite and makes it difficult to achieve a TS of 780 MPa or more. This also reduces YS and YR.

[0118] On the other hand, a C content of more than 0.250% results in an increase in the area fraction of fresh martensite, excessively high TS, and lower El. This also results in an increase in the area fraction of fresh martensite, lower bendability in a V-bending test, and undesired R/t (press formability). This also results in an increase in the area fraction of retained austenite, the formation of hard fresh martensite by deformation-induced transformation of retained austenite when a steel sheet is punched in a hole expansion test, is subjected to U-bending in a U-bending+tight bending test, or is subjected to V-bending in a V-bending+orthogonal VDA test, results in void formation and crack growth in a subsequent test, and results in undesired (press formability), ST (fracture resistance characteristics in case of a collision), and SFmax (fracture resistance characteristics in case of a collision). Thus, the C content is 0.030% or more and 0.250% or less. The C content is preferably 0.050% or more. The C content is preferably 0.130% or less.

Si: 0.01% or More and 0.75% or Less

[0119] Si promotes ferrite transformation during annealing and in a cooling process after annealing. Thus, Si is an element that affects the area fraction of ferrite. A Si content of less than 0.01% results in a decrease in the area fraction of ferrite and lower ductility.

[0120] On the other hand, a Si content of more than 0.75% results in an increase in the volume fraction of retained austenite, the formation of hard fresh martensite by deformation-induced transformation of retained austenite when a steel sheet is punched in a hole expansion test, is subjected to U-bending in a U-bending+tight bending test, or is subjected to V-bending in a V-bending+orthogonal VDA test, results in void formation and crack growth in a subsequent test, and results in undesired , ST, and SFmax. Thus, the Si content is 0.01% or more and 0.75% or less. The Si content is preferably 0.10% or more. The Si content is preferably 0.70% or less.

Mn: 2.00% or More and Less than 3.50%

[0121] Mn is an element that adjusts the area fraction of tempered martensite, bainite, tempered bainite, or the like. A Mn content of less than 2.00% results in an increase in the area fraction of ferrite and makes it difficult to achieve a TS of 780 MPa or more. This also reduces YS and YR.

[0122] On the other hand, a Mn content of 3.50% or more results in a decrease in martensite start temperature Ms (hereinafter also referred to simply as an Ms temperature or Ms) and a decrease in martensite formed in a first cooling step. This results in an increase in fresh martensite formed in the second cooling step, insufficient tempering of the fresh martensite in a subsequent reheating step, an increase in the area fraction of the fresh martensite, lower bendability in a V-bending test, and undesired R/t. Thus, the Mn content is 2.00% or more and less than 3.50%. The Mn content is preferably 2.20% or more. The Mn content is preferably 3.00% or less.

P: 0.001% or More and 0.100% or Less

[0123] P is an element that has a solid-solution strengthening effect and increases TS and YS of a steel sheet. To produce such effects, the P content is 0.001% or more.

[0124] On the other hand, a P content of more than 0.100% results in segregation of P at a prior-austenite grain boundary and embrittlement of the grain boundary. This results in void formation and crack growth along the prior-austenite grain boundary and undesired R/t in a V-bending test. This also results in void formation and crack growth along the prior-austenite grain boundary when a steel sheet is punched in a hole expansion test, is subjected to U-bending in a U-bending+tight bending test, or is subjected to V-bending in a V-bending+orthogonal VDA test, and undesired , ST, and SFmax. Thus, the P content is 0.001% or more and 0.100% or less. The P content is preferably 0.030% or less.

S: 0.0200% or Less

[0125] S is present as a sulfide in steel. In particular, a S content of more than 0.0200% results in void formation and crack growth from the sulfide as a starting point in a V-bending test and undesired R/t. This also results in void formation and crack growth from the sulfide as a starting point when a steel sheet is punched in a hole expansion test, is subjected to U-bending in a U-bending+tight bending test, or is subjected to V-bending in a V-bending+orthogonal VDA test, and undesired , ST, and SFmax. Thus, the S content is 0.0200% or less. The S content is preferably 0.0080% or less.

[0126] The S content may have any lower limit but is preferably 0.0001% or more due to constraints on production technology.

Al: 0.010% or More and 2.000% or Less

[0127] Al promotes ferrite transformation during annealing and in a cooling process after annealing. Thus, Al is an element that affects the area fraction of ferrite. An Al content of less than 0.010% results in a decrease in the area fraction of ferrite and lower ductility.

[0128] On the other hand, an Al content of more than 2.000% results in an excessive increase in the area fraction of ferrite and makes it difficult to achieve a TS of 780 MPa or more. This also reduces YS and YR. Thus, the Al content is 0.010% or more and 2.000% or less. The Al content is preferably 0.015% or more. The Al content is preferably 1.000% or less.

N: 0.0100% or Less

[0129] N is present as a nitride in steel. In particular, a N content of more than 0.0100% results in void formation and crack growth from the nitride as a starting point in a V-bending test and undesired R/t. This also results in void formation and crack growth from the nitride as a starting point when a steel sheet is punched in a hole expansion test, is subjected to U-bending in a U-bending+tight bending test, or is subjected to V-bending in a V-bending+orthogonal VDA test, and undesired , ST, and SFmax. Thus, the N content is 0.0100% or less. The N content is preferably 0.0050% or less.

[0130] The N content may have any lower limit but is preferably 0.0005% or more due to constraints on production technology.

[0131] A base chemical composition of a base steel sheet of a steel sheet according to an embodiment of the present invention has been described above. A base steel sheet of a steel sheet according to an embodiment of the present invention has a chemical composition that contains the base components and the remainder other than the base components including Fe (iron) and incidental impurities. A base steel sheet of a steel sheet according to an embodiment of the present invention preferably has a chemical composition that contains the base components and the remainder composed of Fe and incidental impurities.

[0132] A base steel sheet of a steel sheet according to an embodiment of the present invention may contain, in addition to the base components, at least one selected from the following optional components. As long as the following optional components are contained in an amount equal to or less than their respective upper limits described below, the advantages of aspects of the present invention can be achieved. Thus, there is no particular lower limit. Any of the following optional elements contained in amounts below the following appropriate lower limits is considered to be an incidental impurity.

[0133] At least one selected from Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less

Nb: 0.200% or Less

[0134] Nb forms fine carbide, nitride, or carbonitride during hot rolling or annealing and thereby increases TS, YS, and YR. To produce such effects, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more.

[0135] On the other hand, a Nb content of more than 0.200% may result in a large number of coarse precipitates or inclusions. In such a case, a coarse precipitate or inclusion may act as a starting point of a void and a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, or a V-bending+orthogonal VDA bending test, and desired , R/t, ST, and SFmax may not be achieved. Thus, when Nb is contained, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.

Ti: 0.200% or Less

[0136] Like Nb, Ti forms fine carbide, nitride, or carbonitride during hot rolling or annealing and thereby increases TS, YS, and YR. To produce such effects, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more.

[0137] On the other hand, a Ti content of more than 0.200% may result in a large number of coarse precipitates or inclusions. In such a case, a coarse precipitate or inclusion may act as a starting point of a void and a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, or a V-bending+orthogonal VDA bending test, and desired , R/t, ST, and SFmax may not be achieved. Thus, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.

V: 0.200% or Less

[0138] Like Nb or Ti, V forms fine carbide, nitride, or carbonitride during hot rolling or annealing and thereby increases TS and YS. To produce such effects, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. The V content is even more preferably 0.010% or more, even further more preferably 0.030% or more.

[0139] On the other hand, a V content of more than 0.200% may result in a large number of coarse precipitates or inclusions. In such a case, a coarse precipitate or inclusion may act as a starting point of a void and a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, or a V-bending+orthogonal VDA bending test, and desired , R/t, ST, and SFmax may not be achieved. Thus, when V is contained, the V content is preferably 0.200% or less. The V content is more preferably 0.060% or less.

B: 0.0100% or Less

[0140] B is an element that segregates at an austenite grain boundary and enhances hardenability. B is also an element that controls the formation and grain growth of ferrite during cooling after annealing. To produce such effects, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more.

[0141] The B content is even more preferably 0.0005% or more, even further more preferably 0.0007% or more.

[0142] On the other hand, a B content of more than 0.0100% may result in a crack in a steel sheet during hot rolling. The internal crack may act as a starting point of a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, or a V-bending+orthogonal VDA bending test, and desired , R/t, ST, and SFmax may not be achieved. Thus, when B is contained, the B content is preferably 0.0100% or less. The B content is more preferably 0.0050% or less.

Cr: 1.000% or Less

[0143] Cr is an element that enhances hardenability, and the addition of Cr forms an appropriate amount of tempered martensite and increases TS, YS, and YR. To produce such effects, the Cr content is preferably 0.0005% or more. The Cr content is more preferably 0.010% or more.

[0144] Cr is even more preferably 0.030% or more, even further more preferably 0.050% or more.

[0145] On the other hand, a Cr content of more than 1.000% may result in an increase in the area fraction of fresh martensite, lower flangeability, lower bendability in a V-bending test, and undesired \ and R/t. Thus, when Cr is contained, the Cr content is preferably 1.000% or less. The Cr content is more preferably 0.800% or less, even more preferably 0.700% or less.

Ni: 1.000% or Less

[0146] Ni is an element that enhances hardenability, and the addition of Ni forms a large amount of tempered martensite and increases TS, YS, and YR. To produce such effects, the Ni content is preferably 0.005% or more. The Ni content is more preferably 0.020% or more. The Ni content is even more preferably 0.040% or more, even further more preferably 0.060% or more.

[0147] On the other hand, a Ni content of more than 1.000% may result in an increase in the area fraction of fresh martensite, lower flangeability, lower bendability in a V-bending test, and undesired and R/t. Thus, when Ni is contained, the Ni content is preferably 1.000% or less. The Ni content is more preferably 0.800% or less.

[0148] The Ni content is even more preferably 0.600% or less, even further more preferably 0.400% or less.

Mo: 1.000% or Less

[0149] Mo is an element that enhances hardenability, and the addition of Mo forms a large amount of tempered martensite and increases TS, YS, and YR. To produce such effects, the Mo content is preferably 0.010% or more. The Mo content is more preferably 0.030% or more.

[0150] On the other hand, a Mo content of more than 1.000% may result in an increase in the area fraction of fresh martensite, lower flangeability, lower bendability in a V-bending test, and undesired and R/t. Thus, when Mo is contained, the Mo content is preferably 1.000% or less. The Mo content is more preferably 0.500% or less, even more preferably 0.450% or less, even further more preferably 0.400% or less. The Mo content is even more preferably 0.350% or less, even further more preferably 0.300% or less.

Sb: 0.200% or Less

[0151] Sb is an element effective in suppressing the diffusion of C near the surface of a steel sheet during annealing and controlling the formation of a soft layer near the surface of the steel sheet. An excessive increase of a soft layer near the surface of a steel sheet may make it difficult to achieve a TS of 780 MPa or more. This may also reduce YS. Thus, the Sb content is preferably 0.002% or more. The Sb content is more preferably 0.005% or more.

[0152] On the other hand, an Sb content of more than 0.200% may result in no soft layer near the surface of a steel sheet and lower , R/t, ST, and SFmax. Thus, when Sb is contained, the Sb content is preferably 0.200% or less. The Sb content is more preferably 0.020% or less.

Sn: 0.200% or Less

[0153] Like Sb, Sn is an element effective in suppressing the diffusion of C near the surface of a steel sheet during annealing and controlling the formation of a soft layer near the surface of the steel sheet. An excessive increase of a soft layer near the surface of a steel sheet may make it difficult to achieve a TS of 780 MPa or more. This may also reduce YS. Thus, the Sn content is preferably 0.002% or more. The Sn content is more preferably 0.005% or more.

[0154] On the other hand, a Sn content of more than 0.200% may result in no soft layer near the surface of a steel sheet and lower , R/t, ST, and SFmax. Thus, when Sn is contained, the Sn content is preferably 0.200% or less. The Sn content is more preferably 0.020% or less.

Cu: 1.000% or Less

[0155] Cu is an element that enhances hardenability, and the addition of Cu forms a large amount of tempered martensite and increases TS, YS, and YR. To produce such effects, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.008% or more, even more preferably 0.010% or more. The Cu content is even more preferably 0.020% or more.

[0156] On the other hand, a Cu content of more than 1.000% may result in an excessive increase in the area fraction of fresh martensite. Furthermore, a large number of coarse precipitates and inclusions may be formed. In such a case, excessively formed fresh martensite or a coarse precipitate or inclusion may act as a starting point of a void and a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, or a V-bending+orthogonal VDA bending test, and desired , R/t, ST, and SFmax may not be achieved. Thus, when Cu is contained, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.200% or less.

Ta: 0.100% or Less

[0157] Like Ti, Nb, and V, Ta forms fine carbide, nitride, or carbonitride during hot rolling or annealing and increases TS, YS, and YR. Furthermore, Ta partially dissolves in Nb carbide or Nb carbonitride and forms a complex precipitate, such as (Nb, Ta) (C, N). This suppresses coarsening of a precipitate and stabilizes precipitation strengthening. This further improves TS and YS. To produce such effects, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.002% or more, even more preferably 0.004% or more.

[0158] On the other hand, a Ta content of more than 0.100% may result in a large number of coarse precipitates or inclusions. In such a case, an excessively coarse precipitate or inclusion may act as a starting point of a void and a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, or a V-bending+orthogonal VDA bending test, and desired , R/t, ST, and SFmax may not be achieved. Thus, when Ta is contained, the Ta content is preferably 0.100% or less.

[0159] The Ta content is more preferably 0.090% or less, even more preferably 0.080% or less.

W: 0.500% or Less

[0160] W is an element that enhances hardenability, and the addition of W forms a large amount of tempered martensite and increases TS, YS, and YR. To produce such effects, the W content is preferably 0.001% or more. The W content is more preferably 0.030% or more.

[0161] On the other hand, a W content of more than 0.500% may result in an increase in the area fraction of fresh martensite, lower flangeability, lower bendability in a V-bending test, and undesired and R/t. Thus, when W is contained, the W content is preferably 0.500% or less. The W content is more preferably 0.450% or less, even more preferably 0.400% or less. The W content is even further more preferably 0.300% or less.

Mg: 0.0200% or less

[0162] Mg is an element effective in spheroidizing the shape of an inclusion of sulfide, oxide, or the like and improving the flangeability and bendability of a steel sheet. To produce such effects, the Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0005% or more, even more preferably 0.0010% or more.

[0163] On the other hand, a Mg content of more than 0.0200% may result in a large number of coarse precipitates or inclusions. In such a case, an excessively coarse precipitate or inclusion may act as a starting point of a void and a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, or a V-bending+orthogonal VDA bending test, and desired , R/t, ST, and SFmax may not be achieved. Thus, when Mg is contained, the Mg content is preferably 0.0200% or less. The Mg content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

Zn: 0.0200% or Less

[0164] Zn is an element effective in spheroidizing the shape of an inclusion and improving the flangeability and bendability of a steel sheet. To produce such effects, the Zn content is preferably 0.0010% or more. The Zn content is more preferably 0.0020% or more, even more preferably 0.0030% or more.

[0165] On the other hand, a Zn content of more than 0.0200% may result in a large number of coarse precipitates or inclusions. In such a case, an excessively coarse precipitate or inclusion may act as a starting point of a void and a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, or a V-bending+orthogonal VDA bending test, and desired , R/t, ST, and SFmax may not be achieved. Thus, when Zn is contained, the Zn content is preferably 0.0200% or less. The Zn content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

Co: 0.0200% or Less

[0166] Like Zn, Co is an element effective in spheroidizing the shape of an inclusion and improving the flangeability and bendability of a steel sheet. To produce such effects, the Co content is preferably 0.0010% or more. The Co content is more preferably 0.0020% or more, even more preferably 0.0030% or more.

[0167] On the other hand, a Co content of more than 0.0200% may result in a large number of coarse precipitates or inclusions. In such a case, an excessively coarse precipitate or inclusion may act as a starting point of a void and a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, or a V-bending+orthogonal VDA bending test, and desired , R/t, ST, and SFmax may not be achieved. Thus, when Co is contained, the Co content is preferably 0.0200% or less. The Co content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

Zr: 0.1000% or Less

[0168] Like Zn and Co, Zr is an element effective in spheroidizing the shape of an inclusion and improving the flangeability and bendability of a steel sheet. To produce such effects, the Zr content is preferably 0.0010% or more. On the other hand, when the Zr content is more than 0.1000%, an excessively coarse precipitate or inclusion may act as a starting point of a void and a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, and a V-bending+orthogonal VDA bending test, and desired , R/t, ST, and SFmax may not be achieved. Thus, when Zr is contained, the Zr content is preferably 0.1000% or less.

[0169] The Zr content is more preferably 0.0300% or less, even more preferably 0.0100% or less.

Ca: 0.0200% or Less

[0170] Ca is present as an inclusion in steel. A Ca content of more than 0.0200% may result in a large number of coarse inclusions. In such a case, an excessively coarse precipitate or inclusion may act as a starting point of a void and a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, or a V-bending+orthogonal VDA bending test, and desired , R/t, ST, and SFmax may not be achieved. Thus, when Ca is contained, the Ca content is preferably 0.0200% or less. The Ca content is preferably 0.0020% or less. The Ca content is more preferably 0.0019% or less, even more preferably 0.0018% or less. The Ca content may have any lower limit but is preferably 0.0005% or more. Due to constraints on production technology, the Ca content is more preferably 0.0010% or more.

[0171] Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less

[0172] Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are elements effective in improving the flangeability and bendability of a steel sheet. To produce such effects, each of the Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM contents is preferably 0.0001% or more.

[0173] On the other hand, a Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, or REM content of more than 0.0200% or an As content of more than 0.0500% may result in a large number of coarse precipitates or inclusions. In such a case, a coarse precipitate or inclusion may act as a starting point of a void and a crack in a hole expansion test, a V-bending test, a U-bending+tight bending test, or a V-bending+orthogonal VDA bending test, and desired , R/t, ST, and SFmax may not be achieved. Thus, when at least one of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM is contained, each of the Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM contents is preferably 0.0200% or less, and the As content is preferably 0.0500% or less.

[0174] The Se content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Se content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0175] The Te content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Te content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0176] The Ge content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Ge content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0177] The As content is more preferably 0.0010% or more, even more preferably 0.0015% or more. The As content is more preferably 0.0400% or less, even more preferably 0.0300% or less.

[0178] The Sr content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Sr content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0179] The Cs content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Cs content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0180] The Hf content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Hf content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0181] The Pb content is more preferably 0.0005% or more, even more preferably 0.0008% or more. The Pb content is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0182] The Bi content is more preferably 0.0005% or more, even more preferably 0.0008% or more. Bi is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0183] REM is more preferably 0.0005% or more, even more preferably 0.0008% or more. REM is more preferably 0.0180% or less, even more preferably 0.0150% or less.

[0184] The term REM, as used herein, refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The term REM concentration, as used herein, refers to the total content of one or two or more elements selected from the REM.

[0185] REM is preferably, but not limited to, Sc, Y, Ce, or La.

Steel Microstructure

[0186] Next, the steel microstructure of a base steel sheet of a steel sheet according to an embodiment of the present invention is described.

Area Fraction of Ferrite: 20.0% or More and 80.0% or Less

[0187] Soft ferrite is a phase that improves ductility. It is also a phase necessary to form isolated island-like fresh martensite or isolated island-like retained austenite in a grain and to suppress the connection of voids and crack growth. To achieve both desired ductility and good , R/t, ST, and SFmax, the area fraction of ferrite is 20.0% or more. On the other hand, an excessive increase in the area fraction of ferrite makes it difficult to achieve a TS of 780 MPa or more. This also reduces YS and YR. Thus, the area fraction of ferrite is 80.0% or less. The area fraction of ferrite is preferably 30.0% or more.

Area Fraction of Fresh Martensite: 15.0% or Less (Including 0.0%)

[0188] In accordance with aspects of the present invention, fresh martensite with an excessively increased area fraction acts as a starting point of void formation in a hole expanding process in a hole expansion test or in a bending process in a V-bending test, and desired A and R/t cannot be achieved. Thus, the area fraction of fresh martensite is 15.0% or less. The area fraction of fresh martensite is preferably 10.0% or less. The area fraction of fresh martensite may have any lower limit and may be 0.0%. The term fresh martensite refers to as-quenched (untempered) martensite. The fresh martensite includes (isolated) island-like fresh martensite in ferrite grains described later.

Area Fraction of Retained Austenite: 3.0% or Less (Including 0.0%)

[0189] In accordance with aspects of the present invention, an excessive increase in the area fraction of retained austenite results in the formation of hard fresh martensite by deformation-induced transformation of retained austenite when a steel sheet is punched in a hole expansion test, is subjected to U-bending in a U-bending+tight bending test, or is subjected to V-bending in a V-bending+orthogonal VDA test, results in void formation and crack growth in a subsequent test, and results in undesired , ST, and SFmax. Thus, the area fraction of retained austenite is 3.0% or less. The area fraction of retained austenite is preferably 2.5% or less, more preferably 2.0% or less. The lower limit of the area fraction of retained austenite is preferably, but not limited to, 0.1% or more, more preferably 0.2% or more.

[0190] The retained austenite includes (isolated) island-like retained austenite in ferrite grains described later.

[0191] In a second cooling step in a production method described later, the desired area fraction of tempered martensite can be ensured by applying a tension of 2.0 kgf/mm.sup.2 or more to a steel sheet in the temperature range of 300 C. or more and 450 C. or less, then subjecting the steel sheet to five or more passes, each pass involving contact with a roll with a diameter of 500 mm or more and 1500 mm or less for a quarter circumference of the roll, to cause deformation-induced transformation of non-transformed austenite into fresh martensite, tempering the fresh martensite in a subsequent reheating step, and finally controlling the area fraction of fresh martensite to 15.0% or less and the volume fraction of retained austenite to 3.0% or less.

[0192] Value obtained by dividing total area fraction of island-like fresh martensite and island-like retained austenite in ferrite grain by sum of area fraction of fresh martensite and area fraction of retained austenite in entire steel sheet: 0.65 or more

[0193] In accordance with aspects of the present invention, as shown in FIG. 1, isolated island-like fresh martensite (M) and isolated island-like retained austenite (RA) in a ferrite (F) grain are finer than tempered martensite (TM) and hard second phase (fresh martensite (M)+retained austenite (RA)) present at a ferrite grain boundary, is a microstructure that may act as a void formation position but is less likely to be involved in the connection of voids or crack growth, and is a microstructure that is necessary to ensure a TS of 780 MPa or more and achieve desired , R/t, ST, and SFmax. Thus, the value ((M+RA)/(M+RA)) obtained by dividing the total area fraction of isolated island-like fresh martensite and isolated island-like retained austenite in a ferrite grain by the sum of the area fraction of fresh martensite and the area fraction of retained austenite is 0.65 or more.

[0194] In accordance with aspects of the present invention, the value obtained by dividing the total area fraction of isolated island-like fresh martensite and isolated island-like retained austenite in a ferrite grain by the sum of the area fraction of fresh martensite and the area fraction of retained austenite can be a value obtained by dividing the total area fraction of island-like fresh martensite and island-like retained austenite in the ferrite grain by the sum of the area fraction of fresh martensite and the area fraction of retained austenite in the entire steel sheet. Thus, in accordance with aspects of the present invention, the value obtained by dividing the total area fraction of island-like fresh martensite and island-like retained austenite in a ferrite grain by the sum of the area fraction of fresh martensite and the area fraction of retained austenite in the entire steel sheet is 0.65 or more.

[0195] Furthermore, the value obtained by dividing the total area fraction of island-like fresh martensite and island-like retained austenite in a ferrite grain by the sum of the area fraction of fresh martensite and the area fraction of retained austenite in the entire steel sheet is preferably 0.70 or more.

[0196] The upper limit of the value obtained by dividing the total area fraction of island-like fresh martensite and island-like retained austenite in a ferrite grain by the sum of the area fraction of fresh martensite and the volume fraction of retained austenite in the entire steel sheet is preferably, but not limited to, 0.94 or less, more preferably 0.92 or less.

Area Fraction of Bainite and Tempered Bainite: 10.0% or Less (Including 0.0%)

[0197] An excessive increase in the area fraction of bainite formed in the first cooling step or tempered bainite formed by tempering the bainite formed in the reheating step results in tempered martensite with an undesired area fraction and makes it difficult to achieve a TS of 780 MPa or more. Thus, the area fraction of bainite and tempered bainite (B+BT) is 10.0% or less. The area fraction of bainite and tempered bainite is preferably 8.0% or less. The area fraction of bainite and tempered bainite may be 0.0%.

Area Fraction of Tempered Martensite: 10.0% or More and 70.0% or Less

[0198] A hard second phase (fresh martensite+retained austenite) present at a ferrite grain boundary is a microstructure that promotes void formation and crack growth during press forming and in case of a collision. On the other hand, tempered martensite is a microstructure mostly present at a ferrite grain boundary. The microstructure is formed by applying a tension of 2.0 kgf/mm.sup.2 or more to a steel sheet in the temperature range of 300 C. or more and 450 C. or less in a second cooling step in a production method described later, then subjecting the steel sheet to five or more passes, each pass involving contact with a roll with a diameter of 500 mm or more and 1500 mm or less for a quarter circumference of the roll, to cause deformation-induced transformation of non-transformed austenite into fresh martensite, and tempering the fresh martensite in a subsequent reheating step. The tempered martensite is a microstructure necessary to achieve desired , R/t, ST, and SFmax. Thus, the area fraction of tempered martensite is 10.0% or more. The area fraction of tempered martensite is preferably 20.0% or more.

[0199] On the other hand, an excessive increase in the area fraction of tempered martensite results in ferrite with an undesired area fraction, and desired ductility cannot be ensured. Thus, the area fraction of tempered martensite is 70.0% or less. The area fraction of tempered martensite is preferably 60.0% or less.

Average grain size of island-like fresh martensite and island-like retained austenite in ferrite grain: 2.0 m or less

[0200] In accordance with aspects of the present invention, when island-like fresh martensite and island-like retained austenite in a ferrite grain have a small average grain size, it is possible to ensure a TS of 780 MPa or more, further suppress void formation, and achieve better , R/t, ST, and SFmax. Thus, the average grain size of island-like fresh martensite and island-like retained austenite (M+RA) in a ferrite grain is 2.0 m or less. The average grain size of island-like fresh martensite and island-like retained austenite in a ferrite grain is preferably 1.0 m or less.

[0201] Although the lower limit is not particularly limited, the average grain size of island-like fresh martensite and island-like retained austenite in a ferrite grain is preferably 0.1 m or more, more preferably 0.2 m or more.

[0202] The area fraction of the remaining microstructure other than the ferrite, fresh martensite, retained austenite, bainite, tempered bainite, and tempered martensite is preferably 10.0% or less. The area fraction of the remaining microstructure is more preferably 5.0% or less. The area fraction of the remaining microstructure may be 0.0%.

[0203] The remaining microstructure is, for example, but not limited to, pearlite, or carbide such as cementite, or unrecrystallized ferrite. The type of the remaining microstructure can be determined, for example, by scanning electron microscope (SEM) observation.

[0204] The area fractions of ferrite, bainite, tempered bainite, tempered martensite, and the hard second phase (fresh martensite+retained austenite) are measured at a quarter thickness position of a base steel sheet as described below.

[0205] A sample is cut out to form a thickness cross section (L cross section) parallel to the rolling direction of a steel sheet as an observation surface. The observation surface of the sample is then polished with a diamond paste and is then subjected to final polishing with alumina. The observation surface of the sample is then etched with 3% by volume nital to expose the microstructure. The steel sheet is then observed at a quarter thickness position using a SEM at a magnification of 3000 times in five visual fields. From a microstructure image thus taken, the area fraction is calculated by dividing the area of each constituent microstructure (ferrite, bainite, tempered bainite, tempered martensite, and the hard second phase (fresh martensite+retained austenite)) by the measurement area in five visual fields using Adobe Photoshop available from Adobe Systems, and the area fractions are averaged to determine the area fraction of each microstructure.

[0206] Ferrite: a massive black region. Almost no carbide is contained. The area fraction of ferrite does not include isolated island-like fresh martensite and isolated island-like retained austenite in a ferrite grain.

[0207] Bainite and tempered bainite: a black to dark gray region of a massive form, an indefinite form, or the like. A relatively small number of carbide particles are contained.

[0208] Tempered martensite: a gray region of an indefinite form. A relatively large number of carbide particles are contained.

[0209] Hard second phase (retained austenite+fresh martensite): a white to light gray region of an indefinite form. No carbide is contained.

[0210] Carbide: a dotted or linear white region. It is contained in bainite, tempered bainite, and tempered martensite.

[0211] Remaining microstructure: the pearlite, cementite, and the like of known forms.

[0212] From the SEM image used for the microstructure fraction measurement, the total area of isolated island-like fresh martensite and isolated island-like retained austenite in a ferrite grain is divided by the number of isolated island-like fresh martensite grains and isolated island-like retained austenite grains in the ferrite grain to obtain an average area, and the average area is divided by the circumference ratio n, and the square root thereof is multiplied by 2 to obtain an equivalent circular diameter as the average grain size.

[0213] For one isolated island-like fresh martensite or isolated island-like retained austenite grain in a ferrite grain, in a SEM image, an island-like region with the outer periphery surrounded by ferrite and integrally formed without interruption is regarded as one to be measured.

[0214] The area fraction of retained austenite is measured as described below.

[0215] A base steel sheet is mechanically ground to a quarter thickness position in the thickness direction (depth direction) and is then chemically polished with oxalic acid to form an observation surface. The observation surface is then observed by X-ray diffractometry. A Moka radiation source is used for incident X-rays to determine the ratio of the diffraction intensity of each of (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensity of each of (200), (211), and (220) planes of bcc iron. The volume fraction of retained austenite is calculated from the ratio of the diffraction intensity of each plane. On the assumption that retained austenite is three-dimensionally homogeneous, the volume fraction of retained austenite is defined as the area fraction of the retained austenite.

[0216] The area fraction of fresh martensite is determined by subtracting the area fraction of retained austenite from the area fraction of the hard second phase determined as described above.

[00006]$\left[\mathrm{Area}\mathrm{fraction}\mathrm{of}\mathrm{fresh}\mathrm{martensite}\left(\%\right)\right]=\left[\mathrm{area}\mathrm{fraction}\left(\%\right)\mathrm{of}\mathrm{hard}\mathrm{second}\mathrm{phase}\right]-\left[\mathrm{area}\mathrm{fraction}\left(\%\right)\mathrm{of}\mathrm{retained}\mathrm{austenite}\right]$

[0217] The area fraction of the remaining microstructure is determined by subtracting the area fraction of ferrite, the area fraction of bainite and tempered bainite, the area fraction of tempered martensite, and the area fraction of the hard second phase, which are determined as described above, from 100.0%.

[00007]$\left[\mathrm{Area}\mathrm{fraction}\mathrm{of}\mathrm{remaining}\mathrm{microstructure}\left(\%\right)\right]=100.-\left[\mathrm{area}\mathrm{fraction}\mathrm{of}\mathrm{ferrite}\left(\%\right)\right]-\left[\mathrm{area}\mathrm{fraction}\mathrm{of}\mathrm{bainite}\mathrm{and}\mathrm{tempered}\mathrm{bainite}\left(\%\right)\right]-\left[\mathrm{area}\mathrm{fraction}\mathrm{of}\mathrm{tempered}\mathrm{martensite}\left(\%\right)\right]-\left[\mathrm{area}\mathrm{fraction}\mathrm{of}\mathrm{hard}\mathrm{second}\mathrm{phase}\left(\%\right)\right]$

Amount of Diffusible Hydrogen Contained in Base Steel Sheet (in Steel): 0.50 ppm by Mass or Less

[0218] When the amount of diffusible hydrogen in a steel sheet is more than 0.50 ppm by mass, desired , R/t, ST, and SFmax cannot be achieved.

[0219] The amount of diffusible hydrogen in a steel sheet is preferably 0.25 ppm by mass or less. The amount of diffusible hydrogen in a steel sheet may have any lower limit and is preferably 0.01 ppm by mass or more due to constraints on production technology.

[0220] A base steel sheet in which the amount of diffusible hydrogen is measured may be, in addition to a high-strength steel sheet before coating treatment, a base steel sheet of a high-strength galvanized steel sheet after galvanizing treatment and before processing. It may also be a base steel sheet of a steel sheet subjected to processing, such as punching or stretch flange forming, after galvanizing treatment, or a base portion of a product produced by welding a steel sheet after processing.

[0221] The amount of diffusible hydrogen in a steel sheet is measured by the following method. A test specimen with a length of 30 mm and a width of 5 mm is taken and, when a galvanized layer is formed on the steel sheet, the hot-dip galvanized layer or hot-dip galvannealed layer is removed with an alkali. The amount of hydrogen released from the test specimen is then measured by a temperature-programmed desorption analysis method. More specifically, the test specimen is continuously heated from room temperature (5 C. to 55 C.) to 300 C. at a heating rate of 200 C./h and is then cooled to room temperature. The cumulative amount of hydrogen released from the test specimen from room temperature to 210 C. is measured as the amount of diffusible hydrogen in the steel sheet. The amount of diffusible hydrogen is preferably measured after the completion of the production of the steel sheet. The amount of hydrogen is more preferably measured within one week after the completion of the production of the steel sheet. The room temperature should be within the range of annual temperature variations at the location in consideration of global production. Typically, it preferably ranges from 10 C. to 50 C.

Surface Soft Layer

[0222] A base steel sheet of a steel sheet according to an embodiment of the present invention preferably has a surface soft layer on the surface of the base steel sheet. The surface soft layer contributes to the suppression of the development of flex cracking during press forming and in case of a collision of an automobile body and therefore further improves bending fracture resistance characteristics. The term surface soft layer means a decarburized layer and refers to a surface layer region with a Vickers hardness of 85% or less with respect to the Vickers hardness of a cross section at a quarter thickness position.

[0223] The surface soft layer is formed in a region of 200 m or less from the surface of the base steel sheet in the thickness direction. The region where the surface soft layer is formed is preferably 150 m or less, more preferably 120 m or less, from the surface of the base steel sheet in the thickness direction. The lower limit of the thickness of the surface soft layer is preferably, but not limited to, 8 m or more, more preferably more than 17 m. The surface soft layer is preferably 30 m or more, more preferably 40 m or more.

[0224] The quarter thickness position of the base steel sheet where the Vickers hardness is measured is a non-surface-soft layer (a layer that does not satisfy the condition of the hardness of the surface soft layer defined in accordance with aspects of the present invention).

[0225] The Vickers hardness is measured at a load of 10 gf in accordance with JIS Z 2244-1 (2020).

Nanohardness of Surface Soft Layer

[0226] When the nanohardness is measured at 300 points or more in a 50 m50 m region on a sheet surface at each of a quarter depth position in the thickness direction and a half depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet, the ratio of the number of measurements in which the nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet is 7.0 GPa or more is 0.10 or less with respect to the total number of measurements at the quarter depth position in the thickness direction of the surface soft layer.

[0227] In accordance with aspects of the present invention, to achieve high bendability during press forming and good bending fracture characteristics in case of a collision, when the nanohardness is measured at 300 points or more in a 50 m50 m region on a sheet surface at each of a quarter depth position in the thickness direction and a half depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet, the ratio of the number of measurements in which the nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet is 7.0 GPa or more is preferably 0.10 or less with respect to the total number of measurements at the quarter depth position in the thickness direction of the surface soft layer. When the ratio of the nanohardness of 7.0 GPa or more is 0.10 or less, it means a low ratio of a hard microstructure (martensite or the like), an inclusion, or the like, and this can further suppress the formation and connection of voids and crack growth in the hard microstructure (martensite and the like), inclusion, or the like during press forming and in case of a collision, thus resulting in good R/t and SFmax.

[0228] The nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the steel sheet has a standard deviation of 1.8 GPa or less, and the nanohardness of the sheet surface at the half depth position in the thickness direction of the surface soft layer from the surface of the steel sheet has a standard deviation of 2.2 GPa or less.

[0229] In accordance with aspects of the present invention, to achieve high bendability during press forming and good bending fracture characteristics in case of a collision, the nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the steel sheet preferably has a standard deviation of 1.8 GPa or less, and the nanohardness of the sheet surface at the half depth position in the thickness direction of the surface soft layer from the surface of the steel sheet preferably has a standard deviation of 2.2 GPa or less. When the nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the steel sheet has a standard deviation of 1.8 GPa or less, and the nanohardness of the sheet surface at the half depth position in the thickness direction of the surface soft layer from the surface of the steel sheet has a standard deviation of 2.2 GPa or less, this means a small difference in microstructure hardness in a micro region and can further suppress the formation and connection of voids and crack growth during press forming and in case of a collision, thus resulting in good R/t and SFmax.

[0230] The nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet preferably has a standard deviation of 1.7 GPa or less. The nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet more preferably has a standard deviation of 1.3 GPa or less. The standard deviation of the nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet may have any lower limit and may be 0.5 GPa or more.

[0231] The nanohardness of the sheet surface at the half depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet more preferably has a standard deviation of 2.1 GPa or less. The nanohardness of the sheet surface at the half depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet more preferably has a standard deviation of 1.7 GPa or less. The standard deviation of the nanohardness of the sheet surface at the half depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet may have any lower limit and may be 0.6 GPa or more.

[0232] The phrase nanohardness of a sheet surface at a quarter depth position and at a half depth position in the thickness direction refers to a hardness measured by the following method.

[0233] When a coated layer is formed, after the coated layer is peeled off, mechanical polishing is performed to the quarter depth position-5 m in the thickness direction of the surface soft layer from the surface of the base steel sheet, buffing with diamond and alumina is performed to the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet, and colloidal silica polishing is further performed. The coated layer to be peeled off is a galvanized layer when the galvanized layer is formed, is a metal coated layer when the metal coated layer is formed, or is a galvanized layer and a metal coated layer when the galvanized layer and the metal coated layer are formed.

[0234] The nanohardness is measured with Hysitron tribo-950 and a Berkovich diamond indenter under the conditions of a load of 500 N, a measurement area of 50 m50 m, and a dot-to-dot distance of 2 m.

[0235] Furthermore, mechanical polishing is performed to the half depth position in the thickness direction of the surface soft layer, buffing with diamond and alumina is performed, and colloidal silica polishing is further performed. The nanohardness is measured with Hysitron tribo-950 and a Berkovich diamond indenter under the conditions of a load of 500 N, a measurement area of 50 m50 m, and a dot-to-dot distance of 2 m.

[0236] The nanohardness is measured at 300 points or more at the quarter depth position in the thickness direction, and the nanohardness is measured at 300 points or more at the half depth position in the thickness direction.

[0237] For example, when the surface soft layer has a thickness of 100 m, the quarter position is a position of 25 m from the surface of the surface soft layer, and the half position is a position of 50 m from the surface of the surface soft layer. The nanohardness is measured at 300 points or more at the position of 25 m, and the nanohardness is also measured at 300 points or more at the position of 50 m.

Metal Coated Layer (First Coated Layer)

[0238] A steel sheet according to an embodiment of the present invention preferably has a metal coated layer (first coated layer, precoated layer) on one or both surfaces of a base steel sheet (the metal coated layer (first coated layer) excludes a hot-dip galvanized layer and a galvanized layer of a hot-dip galvannealed layer). The metal coated layer is preferably a metal electroplated layer, and the metal electroplated layer is described below as an example.

[0239] When the metal electroplated layer is formed on the surface of a steel sheet, the metal electroplated layer as the outermost surface layer contributes to the suppression of the occurrence of flex cracking during press forming and in case of a collision of an automobile body and therefore further improves the bending fracture resistance characteristics.

[0240] In accordance with aspects of the present invention, the dew point can be more than 20 C. to further increase the thickness of the soft layer and significantly improve axial compression characteristics. In this regard, in accordance with aspects of the present invention, due to a metal coated layer, even when the dew point is 20 C. or less and the soft layer has a small thickness, axial compression characteristics equivalent to those in the case where the soft layer has a large thickness can be achieved.

[0241] The metal species of the metal electroplated layer may be any of Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Os, Ir, Pt, Au, Hg, Ti, Pb, and Bi and is preferably Fe. Although an Fe-based electroplated layer is described below as an example, the following conditions for Fe can also be applied to other metal species.

[0242] The coating weight of the Fe-based electroplated layer is more than 0 g/m.sup.2, preferably 2.0 g/m.sup.2 or more. The upper limit of the coating weight per side of the Fe-based electroplated layer is not particularly limited, and from the perspective of cost, the coating weight per side of the Fe-based electroplated layer is preferably 60 g/m.sup.2 or less. The coating weight of the Fe-based electroplated layer is preferably 50 g/m.sup.2 or less, more preferably 40 g/m.sup.2 or less, even more preferably 30 g/m.sup.2 or less.

[0243] The coating weight of the Fe-based electroplated layer is measured as described below. A sample with a size of 1015 mm is taken from the Fe-based electroplated steel sheet and is embedded in a resin to prepare a cross-section embedded sample. Three arbitrary places on the cross section are observed with a scanning electron microscope (SEM) at an acceleration voltage of 15 kV and at a magnification of 2,000 to 10,000 times depending on the thickness of the Fe-based coated layer. The average thickness of the three visual fields is multiplied by the specific gravity of iron to convert it into the coating weight per side of the Fe-based electroplated layer.

[0244] The Fe-based electroplated layer may be, in addition to pure Fe, an alloy coated layer, such as an FeB alloy, an FeC alloy, an FeP alloy, an FeN alloy, an FeO alloy, an FeNi alloy, an FeMn alloy, an FeMo alloy, or an FeW alloy. The Fe-based electroplated layer may have any chemical composition and preferably has a chemical composition containing 10% by mass or less in total of one or two or more elements selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, with the remainder being Fe and incidental impurities. When the total amount of elements other than Fe is 10% by mass or less, this can prevent a decrease in electrolysis efficiency and can form an Fe-based electroplated layer at low cost. For an FeC alloy, the C content is preferably 0.08% by mass or less.

[0245] Next, mechanical characteristics of a steel sheet according to an embodiment of the present invention are described.

Tensile Strength (TS): 780 MPa or More

[0246] A steel sheet according to an embodiment of the present invention has a tensile strength of 780 MPa or more.

[0247] The reference values of the yield stress (YS), the yield ratio (YR), the total elongation (El), the limiting hole expansion ratio (), the critical spacer thickness (ST) in a U-bending+tight bending test and the stroke at the maximum load (SFmax) in a V-bending+orthogonal VDA bending test, and the presence or absence of fracture (appearance crack) in the axial compression test of a steel sheet according to an embodiment of the present invention are as described above.

[0248] The tensile strength (TS), the yield stress (YS), the yield ratio (YR), and the total elongation (El) are measured in the tensile test according to JIS Z 2241 (2011) described later in Examples. The limiting hole expansion ratio () is measured in the hole expansion test according to JIS Z 2256 (2020) described later in Examples. The critical spacer thickness (ST) is measured in a U-bending+tight bending test described later in Examples. The stroke at the maximum load (SFmax) in the V-bending+orthogonal VDA bending test is measured in a V-bending+orthogonal VDA bending test described later in Examples. The presence or absence of fracture (appearance crack) in the axial compression test is measured in an axial compression test described later in Examples.

Galvanized Layer (Second Coated Layer)

[0249] A steel sheet according to an embodiment of the present invention may have a galvanized layer formed on a base steel sheet (on the surface of the base steel sheet or on the surface of a metal coated layer when the metal coated layer is formed) as the outermost surface layer, and the galvanized layer may be provided on only one surface or both surfaces of the base steel sheet. A steel sheet with a galvanized layer may be a galvanized steel sheet.

[0250] Thus, a steel sheet according to aspects of the present invention may have a base steel sheet and a second coated layer (a galvanized layer, an aluminum coated layer, or the like) formed on the base steel sheet or may have a base steel sheet and a metal coated layer (a first coated layer (excluding a second coated layer of a galvanized layer)) and a second coated layer (a galvanized layer, an aluminum coated layer, or the like) sequentially formed on the base steel sheet.

[0251] The term galvanized layer, as used herein, refers to a coated layer containing Zn as a main component (Zn content: 50.0% or more), for example, a hot-dip galvanized layer or a hot-dip galvannealed layer.

[0252] The hot-dip galvanized layer is preferably composed of, for example, Zn, 20.0% by mass or less of Fe, and 0.001% by mass or more and 1.0% by mass or less of Al. The hot-dip galvanized layer may optionally contain one or two or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0% by mass or more and 3.5% by mass or less. The hot-dip galvanized layer more preferably has an Fe content of less than 7.0% by mass. The remainder other than these elements is incidental impurities.

[0253] A hot-dip galvannealed layer is preferably composed of, for example, 20% by mass or less of Fe and 0.001% by mass or more and 1.0% by mass or less of Al. The hot-dip galvannealed layer may optionally contain one or two or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0% by mass or more and 3.5% by mass or less. The hot-dip galvannealed layer more preferably has an Fe content of 7.0% by mass or more, even more preferably 8.0% by mass or more. The hot-dip galvannealed layer more preferably has an Fe content of 15.0% by mass or less, even more preferably 12.0% by mass or less. The remainder other than these elements is incidental impurities.

[0254] Furthermore, the coating weight per side of the galvanized layer is preferably, but not limited to, 20 g/m.sup.2 or more. The coating weight per side of the galvanized layer is preferably 80 g/m.sup.2 or less.

[0255] The coating weight of the galvanized layer is measured as described below. A treatment liquid is prepared by adding 0.6 g of a corrosion inhibitor for Fe (IBIT 700BK (registered trademark) manufactured by Asahi Chemical Co., Ltd.) to 1 L of 10% by mass aqueous hydrochloric acid. A steel sheet as a sample is immersed in the treatment liquid to dissolve a galvanized layer. The mass loss of the sample due to the dissolution is measured and is divided by the surface area of a base steel sheet (the surface area of a coated portion) to calculate the coating weight (g/m.sup.2).

[0256] The thickness of a steel sheet according to an embodiment of the present invention is preferably, but not limited to, 0.5 mm or more.

[0257] The thickness is more preferably more than 0.8 mm. The thickness is even more preferably 0.9 mm or more. The thickness is more preferably 1.0 mm or more. The thickness is even more preferably 1.2 mm or more.

[0258] The steel sheet preferably has a thickness of 3.5 mm or less. The thickness is more preferably 2.3 mm or less.

[0259] The width of a steel sheet according to aspects of the present invention is preferably, but not limited to, 500 mm or more, more preferably 750 mm or more. The steel sheet preferably has a width of 1600 mm or less, more preferably 1450 mm or less.

[2. Method for Producing Steel Sheet]

[0260] Next, a method for producing a steel sheet according to an embodiment of the present invention is described.

[0261] A method for producing a steel sheet according to aspects of the present invention includes a hot rolling step of hot-rolling a steel slab with the chemical composition described above under a condition of a finish rolling temperature of 820 C. or more to produce a hot-rolled steel sheet; a heating step of heating the steel sheet after the hot rolling step in the temperature range of 350 C. or more and 600 C. or less at an average heating rate of 7 C./s or more; an annealing step of annealing under conditions of an annealing temperature: 750 C. or more and 900 C. or less and an annealing time: 20 seconds or more; after the annealing step, a first cooling step of cooling under conditions of an average cooling rate of 7 C./s or more from (the annealing temperature 30 C.) to 650 C. and an average cooling rate of 14 C./s or less from 650 C. to 500 C.; after the first cooling step, a second cooling step of applying a tension of 2.0 kgf/mm.sup.2 or more to the steel sheet in the temperature range of 300 C. or more and 450 C. or less, then subjecting the steel sheet to five or more passes, each pass involving contact with a roll with a diameter of 500 mm or more and 1500 mm or less for a quarter circumference of the roll, and then cooling the steel sheet to a cooling stop temperature of 250 C. or less; a reheating step of reheating the steel sheet to the temperature range of the cooling stop temperature or more and 440 C. or less and holding the steel sheet for 20 seconds or more after the second cooling step; and optionally a cold rolling step of cold-rolling the steel sheet after the hot rolling step and before the heating step at a rolling reduction of 20% or more and 80% or less to produce a cold-rolled steel sheet.

[0262] In accordance with aspects of the present invention, a steel material (steel slab) can be melted by any method, for example, by a known melting method using a converter, an electric arc furnace, or the like. To prevent macrosegregation, a steel slab (slab) is preferably produced by continuous casting but may also be produced by ingot casting, thin slab casting, or the like. After a steel slab is produced, the steel slab may be temporarily cooled to room temperature and then reheated by a known method. Alternatively, without being cooled to room temperature, a steel slab may be subjected without problems to an energy-saving process, such as hot charge rolling or hot direct rolling, in which a hot slab is charged directly into a furnace or is immediately rolled after slight heat retention.

(Hot Rolling Step)

[0263] For heating a slab, the slab heating temperature is preferably 1100 C. or more from the perspective of melting carbide and reducing rolling force. The slab heating temperature is preferably 1300 C. or less to prevent an increase in scale loss.

[0264] The slab heating temperature is the surface temperature of the slab. A slab is formed into a sheet bar by rough rolling under typical conditions. At a low heating temperature, from the perspective of avoiding a trouble during hot rolling, the sheet bar is preferably heated with a bar heater or the like before finish rolling.

Finish Rolling Temperature: 820 C. Or More

[0265] Finish rolling reduces the ductility, flangeability, and bendability of the final material as a result of an increase in rolling load or an increase in rolling reduction in an unrecrystallized state of austenite and development of an abnormal microstructure elongated in the rolling direction. Thus, the finish rolling temperature is 820 C. or more. The finish rolling temperature is preferably 830 C. or more, more preferably 850 C. or more. The finish rolling temperature is preferably 1080 C. or less, more preferably 1050 C. or less.

[0266] The coiling temperature after hot rolling is not particularly limited, but it is necessary to consider the case where the ductility, flangeability, and bendability of the final material degrade. Thus, the coiling temperature after hot rolling is preferably 300 C. or more. The coiling temperature after hot rolling is preferably 700 C. or less.

[0267] Rough-rolled sheets may be joined together during hot rolling to continuously perform finish rolling. A rough-rolled sheet may be temporarily coiled. Furthermore, to reduce the rolling force during hot rolling, the finish rolling may be partly or entirely rolling with lubrication. The rolling with lubrication is also effective in making the shape and the material quality of a steel sheet uniform. The friction coefficient in the rolling with lubrication is preferably 0.10 or more. The friction coefficient in the rolling with lubrication is preferably 0.25 or less.

(Pickling Step)

[0268] A hot-rolled steel sheet thus produced may be pickled. Pickling can remove an oxide from the surface of a steel sheet and can therefore be performed to ensure high chemical convertibility and quality of coating of a high-strength steel sheet of the final product. Pickling may be performed once or may be divided into a plurality of times.

(Cold Rolling Step)

[0269] A pickled sheet after hot rolling or a hot-rolled steel sheet thus produced is cold-rolled as required. For cold rolling, after hot rolling, a pickled sheet may be directly cold-rolled or may be cold-rolled after heat treatment. Optionally, a cold-rolled steel sheet after the cold rolling may be pickled.

[0270] The cold rolling is, for example, multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.

Rolling Reduction of Optional Cold Rolling: 20% or More and 80% or Less

[0271] For cold rolling, the rolling reduction (cumulative rolling reduction ratio) in the cold rolling is preferably, but not limited to, 20% or more and 80% or less. A rolling reduction of less than 20% in the cold rolling tends to result in coarsening or a lack of uniformity of the steel microstructure in the annealing step and may result in the final product with lower TS or bendability. Thus, the rolling reduction in the cold rolling is preferably 20% or more. On the other hand, a rolling reduction of more than 80% in the cold rolling tends to result in a steel sheet with a poor shape and may result in an uneven galvanizing coating weight. Thus, the rolling reduction in the cold rolling is preferably 80% or less.

(Metal Coating (Metal Electroplating, First Coating) Step)

[0272] An embodiment of the present invention may include a first coating step of performing metal coating on one or both surfaces of a steel sheet after the hot rolling step (after a cold rolling step when cold rolling is performed) and before a heating step to form a metal coated layer (first coated layer).

[0273] For example, a metal electroplating treatment may be performed on the surface of the hot-rolled steel sheet or the cold-rolled steel sheet thus formed to produce a metal electroplated steel sheet before annealing in which a metal electroplated layer before annealing is formed on at least one surface thereof. The term metal coating, as used herein, excludes galvanizing (second coating).

[0274] Although the metal electroplating treatment method is not particularly limited, as described above, the metal coated layer formed on the base steel sheet is preferably a metal electroplated layer, and the metal electroplating treatment is therefore preferably performed.

[0275] For example, a sulfuric acid bath, a hydrochloric acid bath, a mixture of both, or the like can be used as an Fe-based electroplating bath. The coating weight of the metal electroplated layer before annealing can be adjusted by the energization time or the like. The phrase metal electroplated steel sheet before annealing means that the metal electroplated layer is not subjected to an annealing step, and does not exclude a hot-rolled steel sheet, a pickled sheet after hot rolling, or a cold-rolled steel sheet each annealed in advance before a metal electroplating treatment.

[0276] The metal species of the electroplated layer may be any of Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Os, Ir, Pt, Au, Hg, Ti, Pb, and Bi and is preferably Fe. Although Fe-based electroplating is described below as an example, the following conditions for the Fe-based electroplating can also be applied to another metal electroplating.

[0277] The Fe ion content of an Fe-based electroplating bath before the start of energization is preferably 0.5 mol/L or more in terms of Fe.sup.2+. When the Fe ion content of an Fe-based electroplating bath is 0.5 mol/L or more in terms of Fe.sup.2+, a sufficient Fe coating weight can be obtained. To obtain a sufficient Fe coating weight, the Fe ion content of the Fe-based electroplating bath before the start of energization is preferably 2.0 mol/L or less.

[0278] The Fe-based electroplating bath may contain an Fe ion and at least one element selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co. The total content of these elements in the Fe-based electroplating bath is preferably such that the total content of these elements in an Fe-based electroplated layer before annealing is 10% by mass or less. A metal element may be contained as a metal ion, and a non-metal element can be contained as part of boric acid, phosphoric acid, nitric acid, an organic acid, or the like. An iron sulfate coating solution may contain a conductive aid, such as sodium sulfate or potassium sulfate, a chelating agent, or a pH buffer.

[0279] Other conditions of the Fe-based electroplating bath are also not particularly limited. The temperature of an Fe-based electroplating solution is preferably 30 C. or more and 85 C. or less in view of constant temperature retention ability. The pH of the Fe-based electroplating bath is also not particularly limited, is preferably 1.0 or more from the perspective of preventing a decrease in current efficiency due to hydrogen generation, and is preferably 3.0 or less in consideration of the electrical conductivity of the Fe-based electroplating bath. The electric current density is preferably 10 A/dm.sup.2 or more from the perspective of productivity and is preferably 150 A/dm.sup.2 or less from the perspective of facilitating the control of the coating weight of an Fe-based electroplated layer. The line speed is preferably 5 mpm or more from the perspective of productivity and is preferably 150 mpm or less from the perspective of stably controlling the coating weight.

[0280] A degreasing treatment and water washing for cleaning the surface of a steel sheet and also a pickling treatment and water washing for activating the surface of a steel sheet can be performed as a treatment before Fe-based electroplating treatment. These pretreatments are followed by an Fe-based electroplating treatment. The degreasing treatment and water washing may be performed by any method, for example, by a usual method. In the pickling treatment, various acids, such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof can be used. Among them, sulfuric acid, hydrochloric acid, or a mixture thereof is preferred. The acid concentration is not particularly limited and preferably ranges from 1% to 20% by mass in consideration of the capability of removing an oxide film, prevention of a rough surface (surface defect) due to overpickling, and the like. A pickling treatment liquid may contain an antifoaming agent, a pickling accelerator, a pickling inhibitor, or the like.

(Heating Step)

[0281] An embodiment of the present invention includes a heating step of heating a steel sheet in the temperature range of 350 C. or more and 600 C. or less at an average heating rate of 7 C./s or more after the hot rolling step (after a cold rolling step when cold rolling is performed, after a metal coating step when metal coating is performed to form a metal coated layer (first coated layer), or after a metal coating step when cold rolling and metal coating are performed).

Average Heating Rate in Temperature Range of 350 C. Or More and 600 C. Or Less: 7 C./s or More

[0282] In accordance with aspects of the present invention, , R/t, ST, and SFmax can be improved by increasing the average heating rate in the temperature range of 350 C. or more and 600 C. or less to increase the ratio of an isolated fine island-like hard second phase (martensite+retained austenite) in a ferrite grain. Thus, the average heating rate in the temperature range of 350 C. or more and 600 C. or less is 7 C./s or more, preferably 9 C./s or more.

[0283] Although the upper limit is not particularly limited, the average heating rate in the temperature range of 350 C. or more and 600 C. or less is preferably 100 C./s or less, more preferably 90 C./s or less.

[0284] The average heating rate ( C./s) is calculated by (final heating temperature ( C.)-initial heating temperature ( C.))/heating time (s).

(Annealing Step)

[0285] An embodiment of the present invention includes an annealing step of annealing under conditions of an annealing temperature: 750 C. or more and 900 C. or less and an annealing time: 20 seconds or more after the heating step.

Annealing Temperature: 750 C. Or More and 900 C. Or Less

[0286] An annealing temperature of less than 750 C. results in an insufficient proportion of austenite formed during heating in a two-phase region of ferrite and austenite. This results in an excessive increase in the area fraction of ferrite after annealing and undesired TS, YS, and YR.

[0287] On the other hand, at an annealing temperature of more than 900 C., the area fraction of ferrite cannot be 20.0% or more, and the ductility decreases.

[0288] Thus, the annealing temperature is 750 C. or more and 900 C. or less. The annealing temperature is preferably 880 C. or less. The annealing temperature is the highest temperature reached in the annealing step.

Annealing Time: 20 Seconds or More

[0289] An annealing time of less than 20 seconds results in an insufficient proportion of austenite formed during heating in a two-phase region of ferrite and austenite. This results in an excessive increase in the area fraction of ferrite after annealing, and TS, YS, and YR cannot be obtained. Thus, the annealing time is 20 seconds or more. The annealing time is preferably 30 seconds or more, more preferably 50 seconds or more.

[0290] The annealing time may have any upper limit and is preferably 900 seconds or less, more preferably 800 seconds or less. The annealing time is even more preferably 300 seconds or less, even further more preferably 220 seconds or less.

[0291] The term annealing time refers to the holding time in the temperature range of (annealing temperature 40 C.) or more and the annealing temperature or less. Thus, the annealing time includes, in addition to the holding time at the annealing temperature, the residence time in the temperature range of (annealing temperature 40 C.) or more and the annealing temperature or less in heating and cooling before and after reaching the annealing temperature.

[0292] The number of annealing processes may be two or more but is preferably one from the perspective of energy efficiency.

Dew-Point Temperature of Atmosphere in Annealing Step (Annealing Atmosphere): 30 C. Or More

[0293] In an embodiment of the present invention, the dew point of the atmosphere in the annealing step (annealing atmosphere) is preferably 30 C. or more. Annealing at a dew point of 30 C. or more in the annealing atmosphere in the annealing step can promote a decarburization reaction and more deeply form a surface soft layer. The dew point in the annealing atmosphere in the annealing step is more preferably 25 C. or more, even more preferably more than 20 C., even further more preferably 15 C. or more, most preferably 5 C. or more.

[0294] The dew point of the annealing atmosphere in the annealing step may have any upper limit and is preferably 30 C. or less in order to suitably prevent oxidation of the surface of an Fe-based electroplated layer and to improve the coating adhesion when a galvanized layer is provided. The dew point in the annealing atmosphere in the annealing step is preferably 25 C. or less, more preferably 20 C. or less.

(First Cooling Step)

[0295] Aspects of the present invention include, after the annealing step, a first cooling step of cooling under conditions of an average cooling rate of 7 C./s or more from (the annealing temperature 30 C.) to 650 C. and an average cooling rate of 14 C./s or less from 650 C. to 500 C.

Average Cooling Rate from (Annealing Temperature 30 C.) to 650 C.: 7 C./s or More

[0296] In accordance with aspects of the present invention, rapid cooling in a high-temperature region of 650 C. or more results in fine austenite left in a ferrite grain boundary and finally an increase in the ratio of an isolated fine island-like hard second phase (martensite+retained austenite) in a ferrite grain. Thus, the average cooling rate from (annealing temperature 30 C.) to 650 C. is 7 C./s or more. The average cooling rate from (annealing temperature 30 C.) to 650 C. is preferably 9 C./s or more.

[0297] The average cooling rate from (annealing temperature 30 C.) to 650 C. is preferably 80 C./s or less, more preferably 60 C./s or less. The average cooling rate is even more preferably 30 C./s or less, even further more preferably 18 C./s or less.

[0298] The average cooling rate ( C./s) is calculated by (annealing temperature ( C.)30 ( C.)650 ( C.))/cooling time (s).

Average Cooling Rate from 650 C. To 500 C.: 14 C./s or Less

[0299] In accordance with aspects of the present invention, slow cooling in an intermediate-temperature region of 650 C. or less causes fine austenite at a ferrite grain boundary, after coalescence of adjacent ferrite with similar orientation resulting in one ferrite grain, to form isolated fine island-like austenite left in the ferrite grain, and finally an increase in the ratio of an isolated fine island-like hard second phase (martensite+retained austenite) in the ferrite grain. Thus, the average cooling rate from 650 C. to 500 C. (first cooling stop temperature) is 14 C./s or less, preferably 12 C./s or less. The average cooling rate from 650 C. to 500 C. is preferably 1 C./s or more, more preferably 2 C./s or more.

[0300] The average cooling rate ( C./s) is calculated by (650 ( C.)500 ( C.))/cooling time (s).

(Galvanizing Step (Second Coating Step))

[0301] In accordance with aspects of the present invention, after the first cooling step, the steel sheet may be subjected to a galvanizing treatment. A galvanized steel sheet can be produced by the galvanizing treatment.

[0302] The galvanizing treatment is, for example, a hot-dip galvanizing treatment or a galvannealing treatment.

[0303] In the hot-dip galvanizing treatment, preferably, the steel sheet is immersed in a galvanizing bath at 440 C. or more and 500 C. or less, and the coating weight is then adjusted by gas wiping or the like. The hot-dip galvanizing bath is not particularly limited as long as the galvanized layer has the composition described above. For example, the galvanizing bath preferably has a composition with an Al content of 0.10% by mass or more, the remainder being Zn and incidental impurities. The Al content is preferably 0.23% by mass or less.

[0304] In the galvannealing treatment, after the hot-dip galvanizing treatment performed in the manner described above, a hot-dip galvanized steel sheet is preferably heated to an alloying temperature of 450 C. or more to perform an alloying treatment. The alloying temperature is preferably 600 C. or less.

[0305] An alloying temperature of less than 450 C. may result in a low ZnFe alloying speed and make alloying difficult. On the other hand, an alloying temperature of more than 600 C. results in transformation of non-transformed austenite into pearlite and makes it difficult to achieve a TS of 780 MPa or more. The alloying temperature is more preferably 500 C. or more, even more preferably 510 C. or more. The alloying temperature is more preferably 570 C. or less.

[0306] The coating weight of each of the hot-dip galvanized steel sheet (GI) and the hot-dip galvannealed steel sheet (GA) is preferably 20 g/m.sup.2 or more per side. The coating weight per side of the galvanized layer is preferably 80 g/m.sup.2 or less. The coating weight can be adjusted by gas wiping or the like.

(Second Cooling Step)

[0307] Aspects of the present invention include, after the first cooling step (after a galvanizing step when the galvanizing step is performed), a second cooling step of applying a tension of 2.0 kgf/mm.sup.2 or more to a steel sheet in the temperature range of 300 C. or more and 450 C. or less, subjecting the steel sheet to five or more passes, each pass involving contact with a roll with a diameter of 500 mm or more and 1500 mm or less for a quarter circumference of the roll, and then cooling the steel sheet to a cooling stop temperature (second cooling stop temperature) of 250 C. or less.

Tension Applied in the Temperature Range of 300 C. Or More and 450 C. Or Less: 2.0 Kgf/Mm.SUP.2 .or More

[0308] In accordance with aspects of the present invention, as described above, applying a tension of 2.0 kgf/mm.sup.2 or more to a steel sheet once or more can transform most of austenite into martensite by deformation-induced transformation (stress-strain-induced transformation), and subsequent tempering in the reheating step can reduce the area fraction of fresh martensite in the final microstructure and ensure an appropriate amount of tempered martensite. This can also reduce the amount of austenite immediately after the second cooling step and reduce the volume fraction of retained austenite in the final microstructure. Consequently, desired , R/t, ST, and SFmax can be achieved.

[0309] The tension is calculated by dividing the total load (kgf) of a load cell on the left and right of the roll by the cross-sectional area of the steel sheet (=sheet thickness (mm)sheet width (mm)) (mm.sup.2). The load cells should be arranged parallel to the direction of the tension.

[0310] The load cells are preferably disposed at a position of 200 mm from both ends of the roll. The length of the roll to be used is preferably 1500 mm or more. The length of the roll to be used is preferably 2500 mm or less.

[0311] The tension is preferably 2.2 kgf/mm.sup.2 or more, more preferably 2.4 kgf/mm.sup.2 or more. The tension is preferably 15.0 kgf/mm.sup.2 or less, more preferably 10.0 kgf/mm.sup.2 or less. The tension is even more preferably 7.0 kgf/mm.sup.2 or less, even further more preferably 4.0 kgf/mm.sup.2 or less.

[0312] The number of passes to which a steel sheet is subjected, each pass involving contact with a roll with a diameter of 500 mm or more and 1500 mm or less for a quarter circumference of the roll: five or more passes

[0313] In accordance with aspects of the present invention, subjecting a steel sheet to five or more passes, each pass involving contact with a roll with a diameter of 500 mm or more and 1500 mm or less for a quarter circumference of the roll, can transform most of austenite into martensite by deformation-induced transformation (stress-strain-induced transformation), and subsequent tempering in the reheating step can reduce the area fraction of fresh martensite in the final microstructure and ensure an appropriate amount of tempered martensite. This can also reduce the amount of austenite immediately after the second cooling step and reduce the volume fraction of retained austenite in the final microstructure. Consequently, desired , R/t, ST, and SFmax can be achieved.

[0314] The number of passes is preferably six or more passes, more preferably seven or more passes.

[0315] Although the upper limit is not particularly limited, the number of passes is preferably ten or less passes, more preferably nine or less passes.

Cooling Stop Temperature: 250 C. Or Less

[0316] The cooling conditions in the second cooling step are not particularly limited and may be based on a usual method. The cooling method is, for example, gas jet cooling, mist cooling, roll cooling, water cooling, natural cooling, or the like. Setting the cooling stop temperature (second cooling stop temperature) to 250 C. or less can transform an appropriate amount of austenite into martensite, and subsequent tempering in the reheating step can reduce the area fraction of fresh martensite in the final microstructure and ensure an appropriate amount of tempered martensite. This can also reduce the amount of austenite immediately after the second cooling step and reduce the volume fraction of retained austenite in the final microstructure. Consequently, desired , R/t, ST, and SFmax can be achieved. From the perspective of preventing surface oxidation, cooling is preferably performed to 200 C. or less. The lower limit is preferably, but not limited to, room temperature (5 C. or more and 55 C. or less). The average cooling rate is preferably, for example, 1 C./s or more. The average cooling rate is preferably 50 C./s or less. The average cooling rate ( C./s) is calculated by (cooling start temperature ( C.)cooling stop temperature ( C.))/cooling time (s).

(Reheating Step)

[0317] After the second cooling step, as a reheating step, the steel sheet is reheated to the temperature range of the cooling stop temperature (second cooling stop temperature) or more and 440 C. or less and is held for 20 seconds or more.

[0318] Reheating temperature: the temperature range of the cooling stop temperature (second cooling stop temperature) or more and 440 C. or less

Reheating Holding Time: 20 Seconds or More

[0319] In accordance with aspects of the present invention, reheating to the cooling stop temperature (second cooling stop temperature) or more and holding for 20 seconds or more release diffusible hydrogen from steel. These can also reduce the area fraction of fresh martensite in the final microstructure and ensure an appropriate amount of tempered martensite. This can also reduce the amount of austenite immediately after the second cooling step and reduce the volume fraction of retained austenite in the final microstructure. Consequently, desired , R/t, ST, and SFmax can be achieved.

[0320] On the other hand, at a reheating temperature of more than 440 C., when a galvanizing treatment is performed, a zinc coating is partially melted and adheres to a roll, and a uniformly galvanized hot-dip galvanized steel sheet cannot be produced. When the reheating holding time is less than 20 seconds, a desired amount of diffusible hydrogen in steel is not released.

[0321] Thus, in accordance with aspects of the present invention, reheating is performed to the temperature range of the second cooling stop temperature or more and 440 C. or less, and holding is performed for 20 seconds or more.

[0322] The reheating temperature is preferably 40 C. or more, more preferably 160 C. or more.

[0323] The reheating temperature is preferably 420 C. or less, more preferably 320 C. or less.

[0324] The reheating holding time is preferably 25 seconds or more, more preferably 30 seconds or more.

[0325] The reheating holding time is preferably 300 seconds or less, more preferably 200 seconds or less.

[0326] The steel sheet thus produced may be further subjected to temper rolling. A rolling reduction of more than 2.00% in the temper rolling may result in an increase in yield stress and a decrease in dimensional accuracy when the steel sheet is formed into a member. Thus, the rolling reduction in the temper rolling is preferably 2.00% or less. The lower limit of the rolling reduction in the temper rolling is preferably, but not limited to, 0.05% or more from the perspective of productivity. The temper rolling may be performed with an apparatus coupled to an annealing apparatus for each step (on-line) or with an apparatus separated from the annealing apparatus for each step (off-line). The number of temper rolling processes may be one or two or more. The rolling may be performed with a leveler or the like, provided that the elongation can be equivalent to that in the temper rolling.

[0327] Other conditions of the production method are not particularly limited and, from the perspective of productivity, a series of these treatments, such as annealing, hot-dip galvanizing, and an alloying treatment of a zinc coating, are preferably performed in a continuous galvanizing line (CGL), which is a hot-dip galvanizing line. After the hot-dip galvanizing, the coating weight can be adjusted by wiping. Conditions for coating and the like other than these conditions may be based on a usual method for hot-dip galvanizing.

[3. Member]

[0328] Next, a member according to an embodiment of the present invention is described.

[0329] A member according to an embodiment of the present invention is a member produced by using the steel sheet described above (as a material). For example, the steel sheet as a material is subjected to at least one of forming and joining to produce a member.

[0330] The steel sheet has a TS of 780 MPa or more, high YS and YR, high press formability (ductility, flangeability, and bendability), and fracture resistance characteristics (bending fracture characteristics and axial compression characteristics) in case of a collision. Thus, a member according to an embodiment of the present invention has high strength and enhanced crashworthiness. Thus, a member according to an embodiment of the present invention is suitable for an impact energy absorbing member used in the automotive field.

[4. Method for Producing Member]

[0331] Next, a method for producing a member according to an embodiment of the present invention is described.

[0332] A method for producing a member according to an embodiment of the present invention includes a step of subjecting the steel sheet (for example, a steel sheet produced by the method for producing a steel sheet) to at least one of forming and joining to produce a member.

[0333] The forming method is, for example, but not limited to, a typical processing method, such as press working. A joining method is also, for example, but not limited to, typical welding, such as spot welding, laser welding, or arc welding, riveting, caulking, or the like. The forming conditions and the joining conditions are not particularly limited and may be based on a usual method.

EXAMPLES

[0334] A steel material with the chemical composition (the remainder being Fe and incidental impurities) listed in Table 1 was produced by steelmaking in a converter and was formed into a steel slab in a continuous casting method. In Table 1, - indicates the content at the level of incidental impurities.

[0335] The steel slab was heated to 1200 C. and was then subjected to rough rolling and hot rolling to produce a hot-rolled steel sheet. Hot-rolled steel sheets No. 1 to No. 56, No. 60 to No. 83, No. 92 to No. 106, and No. 112 to No. 117 thus produced were pickled and cold-rolled to produce cold-rolled steel sheets with thicknesses shown in Tables 3, 5, and 7. Hot-rolled steel sheets No. 57 to No. 59, No. 84 to No. 91, and No. 107 to No. 111 were pickled to produce hot-rolled steel sheets (pickled) with thicknesses shown in Tables 3, 5, and 7.

[0336] The cold-rolled steel sheets or hot-rolled steel sheets (pickled) were subjected to the treatments in the heating step, the annealing step, the first cooling step, the galvanizing step, the second cooling step, and the reheating step under the conditions shown in Table 2 to produce steel sheets (galvanized steel sheets).

[0337] Treatments in the heating step, the first coating step (metal coating step), the annealing step, the first cooling step, the second coating step (galvanizing step), the second cooling step, and the reheating step were performed under the conditions shown in Table 4 to produce steel sheets (galvanized steel sheets).

[0338] Treatments in the heating step, the first coating step (metal coating step), the annealing step, the first cooling step, the second cooling step, and the reheating step were performed under the conditions shown in Table 6 to produce steel sheets.

[0339] In the galvanizing step shown in Tables 2 and 4, the hot-dip galvanizing treatment or the galvannealing treatment was performed to produce a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or a hot-dip galvannealed steel sheet (hereinafter also referred to as GA). In Tables 2 and 4, the type in the coating step is also denoted by GI and GA. In the GI steel sheets in Tables 2 and 4, no alloying treatment was performed, and the alloying temperature is indicated by -. In Table 6, no galvanizing treatment was performed, and the results are indicated as CR (cold-rolled steel sheet (without coating)) or HR (hot-rolled steel sheet (without coating)).

[0340] The galvanizing bath temperature was 470 C. in the production of GI and GA.

[0341] The galvanizing coating weight ranged from 45 to 72 g/m.sup.2 per side to produce GI and was 45 g/m.sup.2 per side to produce GA.

[0342] The composition of the galvanized layer of the final hot-dip galvanized steel sheet in GI contained Fe: 0.1% to 1.0% by mass and Al: 0.2% to 0.33% by mass, the remainder being Zn and incidental impurities. GA contained Fe: 8.0% to 12.0% by mass and Al: 0.1% to 0.23% by mass, the remainder being Zn and incidental impurities.

[0343] In both cases, the galvanized layer was formed on both surfaces of the base steel sheet.

[0344] In the steel sheet thus produced, the steel microstructure of the base steel sheet was identified in the manner described above. Tables 3, 5, and 7 show the measurement results. In Tables 3, 5, and 7, F denotes ferrite, M denotes martensite, RA denotes retained austenite, M and RA denote isolated island-like fresh martensite and isolated island-like retained austenite, B and BT denote bainite and tempered bainite, TM denotes tempered martensite, P denotes pearlite, denotes carbide, and F denotes unrecrystallized ferrite.

[0345] Measurement is performed on the surface soft layer as described below. After smoothing a thickness cross section (L cross section) parallel to the rolling direction of the steel sheet by wet grinding, measurement was performed in accordance with JIS Z 2244-1 (2020) using a Vickers hardness tester at a load of 10 gf from a 1-m position to a 100-m position in the thickness direction from the surface of the steel sheet at intervals of 1 m. Measurement was then performed at intervals of 20 m to the central portion in the thickness direction. A region with hardness corresponding to 85% or less of the hardness at the quarter thickness position is defined as a soft layer (surface soft layer), and the thickness of the region in the thickness direction is defined as the thickness of the soft layer.

[0346] In Tables 1 to 7, the underlined portions indicate values outside the appropriate range of the present invention.

[0347] A tensile test, a hole expansion test, a V-bending test, a U-bending+tight bending test, a V-bending+orthogonal VDA bending test, and an axial compression test were performed in the manner described below. The tensile strength (TS), the yield stress (YS), the yield ratio (YR), the total elongation (El), the limiting hole expansion ratio (), R/t in the V-bending test, the critical spacer thickness (ST) in the U-bending+tight bending bending test, the stroke at the maximum load (SFmax) measured in the V-bending+orthogonal VDA bending test, and the presence or absence of fracture (appearance crack) in the axial compression test were evaluated in accordance with the following criteria.

TS

[0348] Good (pass): 780 MPa or more [0349] Poor (fail): less than 780 MPa

YS

[0350] Good (pass): [0351] (A) For 780 MPaTS<980 MPa, 500 MPaYS [0352] (B) For 980 MPaTS, 600 MPaYS [0353] Poor (fail): [0354] (A) For 780 MPaTS<980 MPa, 500 MPa>YS [0355] (B) For 980 MPaTS, 600 MPa>YS

YR

[0356] Good (pass): [0357] (A) For 780 MPaTS<980 MPa, 0.64YR [0358] (B) For 980 MPaTS, 0.61YR [0359] Poor (fail): [0360] (A) For 780 MPaTS<980 MPa, 0.64>YR [0361] (B) For 980 MPaTS, 0.61>YR

El

[0362] Good (pass): [0363] (A) For 780 MPaTS<980 MPa, 19.0%El [0364] (B) For 980 MPaTS, 15.0%El [0365] Poor (fail): [0366] (A) For 780 MPaTS<980 MPa, 19.0%>El [0367] (B) For 980 MPaTS, 15.0%>El

A

[0368] Good (pass): 30% or more [0369] Poor (fail): less than 30%

R/t

[0370] Good (pass); [0371] (A) For 780 MPaTS<980 MPa, 2.0R/t [0372] (B) For 980 MPaTS, 2.5R/t [0373] Poor (fail): [0374] (A) For 780 MPaTS<980 MPa, 2.0<R/t [0375] (B) For 980 MPaTS, 2.5<R/t

ST

[0376] Good (pass): [0377] (A) For 780 MPaTS<980 MPa, 2.5 mmST [0378] (B) For 980 MPaTS, 4.0 mmST [0379] Poor (fail): [0380] (A) For 780 MPaTS<980 MPa, 2.5 mm<ST [0381] (B) For 980 MPaTS, 4.0 mm<ST

SFmax

[0382] Good (pass) [0383] (A) For 780 MPaTS<980 MPa, 28.0 mmSFmax [0384] (B) For 980 MPaTS, 26.5 mmSFmax [0385] Poor (fail): [0386] (A) For 780 MPaTS<980 MPa, 28.0 mm >SFmax [0387] (B) For 980 MPaTS, 26.5 mm >SFmax

Presence or Absence of Axial Compression Fracture (Appearance Crack)

[0388] Excellent (pass): No appearance crack was observed in the sample after the axial compression test. [0389] Good (pass): No more than one appearance crack was observed in the sample after the axial compression test. [0390] Poor (fail): Two or more appearance cracks were observed in the sample after the axial compression test.

(1) Tensile Test

[0391] The tensile test was performed in accordance with JIS Z 2241 (2011). A JIS No. 5 test specimen was taken from the steel sheet such that the longitudinal direction was perpendicular to the rolling direction of the base steel sheet. TS, YS, YR, and El of the test specimen were measured at a crosshead speed of 10 mm/min in the tensile test. Tables 3, 5, and 7 show the results.

(2) Hole Expansion Test

[0392] The hole expansion test was performed in accordance with JIS Z 2256 (2020). A 100 mm100 mm test specimen was taken from the steel sheet by shearing. A hole with a diameter of 10 mm was punched in the test specimen with a clearance of 12.5%. Using a die with an inner diameter of 75 mm, a blank holding force of 9 ton (88.26 kN) was then applied to the periphery of the hole, a conical punch with a vertex angle of 60 degrees was pushed into the hole, and the hole diameter of the test specimen at the crack initiation limit (in crack initiation) was measured. The limiting hole expansion ratio A (%) was determined using the following formula. A is a measure for evaluating stretch flangeability. Tables 3, 5, and 7 show the results.

[00008]$\left(\%\right)=\left\{\left(D_f-D_0\right)/D_0\right\}100$ [0393] D.sub.f: diameter (mm) of hole of test specimen in crack initiation [0394] D.sub.0: hole diameter (mm) of initial test specimen

(3) V-Bending Test

[0395] The V (90-degree) bending test was performed in accordance with JIS Z 2248 (2014).

[0396] A 100 mm35 mm test specimen was taken from the steel sheet by shearing and end grinding. The sides of 100 mm are parallel to the width (C) direction. [0397] Bending radius R: change in 0.5 mm pitch [0398] Test method: die support, punch pressing [0399] Forming load: 10 ton [0400] Test speed: 30 mm/min [0401] Holding time: 5 s [0402] Bending direction: direction (C) perpendicular to rolling direction

[0403] The evaluation was performed three times, and R/t was calculated by dividing the minimum bending radius (critical bending radius) R with no crack by the sheet thickness t. A cleavage with a length of 200 m or more was determined as a crack using a stereomicroscope manufactured by Leica at a magnification of 25 times. At a TS of 780 MPa or more and less than 980 MPa, 2.0>R/t was determined to be good, and at a TS of 980 MPa or more, 2.5>R/t was determined to be good.

(4) U-Bending+Tight Bending Test

[0404] The U-bending+tight bending test was performed as described below.

[0405] A 60 mm30 mm test specimen was taken from the steel sheet by shearing and end grinding. The sides of 60 mm are parallel to the width (C) direction. U-bending (primary bending) was performed at a radius of curvature/thickness ratio of 4.2 in the width (C) direction with respect to an axis extending in the rolling (L) direction to prepare a test specimen. In the U-bending (primary bending), as illustrated in FIG. 2(a), a punch B1 was pressed against a steel sheet on rolls A1 to prepare a test specimen T1. Next, as illustrated in FIG. 2(b), tight bending (secondary bending) was performed in which the test specimen T1 on a lower die A2 was crushed with an upper die B2. In FIG. 2(a), D1 denotes the width (C) direction, and D2 denotes the rolling (L) direction. A spacer S described later is inserted in the test specimen.

[0406] The conditions for U-bending in the U-bending+tight bending test are as follows: [0407] Test method: roll support, punch pressing [0408] Punch tip R: 5.0 mm [0409] Clearance between roll and punch: sheet thickness+0.1 mm [0410] Stroke speed: 10 mm/min [0411] Bending direction: direction (C) perpendicular to rolling direction

[0412] The conditions for tight bending in the U-bending+tight bending test are as follows: [0413] Spacer thickness: change in 0.5 mm pitch [0414] Test method: die support, punch pressing [0415] Forming load: 10 ton [0416] Test speed: 10 mm/min [0417] Holding time: 5 s [0418] Bending direction: direction (C) perpendicular to rolling direction

[0419] The U-bending+tight bending test was performed three times, and the critical spacer thickness (ST) without cracking in any of the three tests was determined. A cleavage with a length of 200 m or more was determined as a crack using a stereomicroscope manufactured by Leica at a magnification of 25 times. ST is a measure for evaluating fracture resistance characteristics (fracture resistance characteristics of a vertical wall portion in the axial compression test) in case of a collision. Tables 3, 5, and 7 show the results.

(5) V-Bending+Orthogonal VDA Bending Test

[0420] The V-bending+orthogonal VDA bending test is performed as described below.

[0421] A 60 mm65 mm test specimen was taken from the steel sheet by shearing and end grinding. The sides of 60 mm are parallel to the rolling (L) direction. 90-degree bending (primary bending) was performed at a radius of curvature/thickness ratio of 4.2 in the rolling (L) direction with respect to an axis extending in the width (C) direction to prepare a test specimen. In the 90-degree bending (primary bending), as illustrated in FIG. 3(a), a punch B3 was pressed against a steel sheet on a die A3 with a V-groove to prepare a test specimen T1. Next, as illustrated in FIG. 3(b), the test specimen T1 on support rolls A4 was subjected to orthogonal bending (secondary bending) by pressing a punch B4 against the test specimen T1 in the direction perpendicular to the rolling direction. In FIGS. 3(a) and 3(b), D1 denotes the width (C) direction, and D2 denotes the rolling (L) direction.

[0422] The V-bending conditions in the V-bending+orthogonal VDA bending test are as follows: [0423] Test method: die support, punch pressing [0424] Forming load: 10 ton [0425] Test speed: 30 mm/min [0426] Holding time: 5 s [0427] Bending direction: rolling (L) direction

[0428] The VDA bending conditions in the V-bending+orthogonal VDA bending test are as follows: [0429] Test method: roll support, punch pressing [0430] Roll diameter: 030 mm [0431] Punch tip R: 0.4 mm [0432] Distance between rolls: (sheet thickness2)+0.5 mm [0433] Stroke speed: 20 mm/min [0434] Test specimen size: 60 mm60 mm [0435] Bending direction: direction (C) perpendicular to rolling direction

[0436] The stroke at the maximum load was determined in a stroke-load curve of the VDA bending. The average value of the stroke at the maximum load when the V-bending+orthogonal VDA bending test was performed three times was defined as SFmax (mm). SFmax is a measure for evaluating fracture resistance characteristics (fracture resistance characteristics of a bending ridge line portion in the axial compression test) in case of a collision. Tables 3, 5, and 7 show the results.

(6) Axial Compression Test

[0437] A 160 mm200 mm test specimen was taken from the steel sheet by shearing. The sides of 160 mm are parallel to the rolling (L) direction. A hat-shaped member 10 with a depth of 40 mm illustrated in FIGS. 4(a) and 4(b) was produced by forming (bending) with a die having a punch corner radius of 5.0 mm and a die corner radius of 5.0 mm. The steel sheet used as the material of the hat-shaped member was separately cut into a size of 80 mm200 mm. Next, the cut-out steel sheet 20 and the hat-shaped member 10 were spot-welded together to produce a test member 30 as illustrated in FIGS. 4(a) and 4(b). FIG. 4(a) is a front view of the test member 30 produced by spot-welding the hat-shaped member 10 and the steel sheet 20. FIG. 4(b) is a perspective view of the test member 30. As illustrated in FIG. 4(b), spot welds 40 were positioned such that the distance between an end portion of the steel sheet and a weld was 10 mm and the distance between the welds was 45 mm. Next, as illustrated in FIG. 4(c), the test member 30 was joined to a base plate 50 by TIG welding to prepare an axial compression test sample. Next, the axial compression test sample was collided with an impactor 60 at a constant collision speed of 10 mm/min to compress the axial compression test sample by 70 mm. As illustrated in FIG. 4(c), the compression direction D3 was a direction parallel to the longitudinal direction of the test member 30. Tables 3, 5, and 7 show the results.

[0438] The U-bending+tight bending test, the V-bending+orthogonal VDA bending test, and the axial compression test of a steel sheet with a thickness of more than 1.2 mm were all performed on a steel sheet with a thickness of 1.2 mm in consideration of the influence of the sheet thickness. A steel sheet with a thickness of more than 1.2 mm was ground on one side to have a thickness of 1.2 mm.

[0439] Since grinding may affect the bendability of the surface of a steel sheet, the ground surface in the U-bending+tight bending bending test was the inside of the bend (valley side), and the ground surface in the V-bending+orthogonal VDA bending test was the outside of the bend (mountain side) in the V-bending test and was the inside of the bend (valley side) in the subsequent VDA bending test. On the other hand, in the U-bending+tight bending test, the V-bending+orthogonal VDA bending test, and the axial compression test of a steel sheet with a thickness of 1.2 mm or less, the sheet thickness has a small influence, and the test was performed without the grinding treatment.

<Nanohardness Measurement>

[0440] To achieve high bendability during press forming and good bending fracture characteristics in case of a collision, when the nanohardness is measured at 300 points or more in a 50 m50 m region on the sheet surface at each of a quarter depth position in the thickness direction and a half depth position in the thickness direction of the surface soft layer from a base surface layer, the ratio of the number of measurements in which the nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet is 7.0 GPa or more is more preferably 0.10 or less with respect to the total number of measurements at the quarter depth position in the thickness direction. When the ratio of the nanohardness of 7.0 GPa or more is 0.10 or less, it means a low ratio of a hard microstructure (martensite or the like), an inclusion, or the like, and this could further suppress the formation and connection of voids and crack growth in the hard microstructure (martensite and the like), inclusion, or the like during press forming and in case of a collision, thus resulting in good R/t and SFmax.

[0441] In the present example, when coating was performed, peeling the coating was followed by mechanical polishing to the quarter depth position 5 m in the thickness direction of the surface soft layer from the surface of the base steel sheet, by buffing with diamond and alumina to the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet, and then by colloidal silica polishing. The nanohardness was measured at 512 points in total with Hysitron tribo-950 and a Berkovich diamond indenter under the conditions of [0442] Load: 500 N, [0443] Measurement area: 50 m50 m, and [0444] Dot-to-dot distance: 2 m.

[0445] Mechanical polishing, buffing with diamond and alumina, and colloidal silica polishing were then performed to the half depth position in the thickness direction of the surface soft layer. The nanohardness was measured at 512 points in total with Hysitron tribo-950 and a Berkovich diamond indenter under the conditions of [0446] Load: 500 N, [0447] Measurement area: 50 m50 m, and [0448] Dot-to-dot distance: 2 m.

TABLE-US-00001 TABLE 1-1 Steel Chemical composition (mass %) grade C Si Mn P S Al N Others Note A 0.115 0.65 2.85 0.009 0.0008 0.038 0.0031 Conforming steel B 0.120 0.55 2.80 0.010 0.0009 0.580 0.0038 Conforming steel C 0.085 0.15 2.45 0.012 0.0012 0.035 0.0032 Conforming steel D 0.350 0.50 2.50 0.015 0.0013 0.035 0.0038 Comparative steel E 0.010 0.45 2.75 0.009 0.0012 0.029 0.0048 Comparative steel F 0.110 1.25 2.80 0.011 0.0018 0.030 0.0035 Comparative steel G 0.100 0.50 3.85 0.016 0.0009 0.025 0.0034 Comparative steel H 0.110 0.55 1.50 0.014 0.0018 0.028 0.0035 Comparative steel I 0.100 0.70 2.70 0.009 0.0010 0.042 0.0028 Nb: 0.035 Conforming steel J 0.105 0.65 2.75 0.010 0.0009 0.028 0.0022 Ti: 0.035 Conforming steel K 0.095 0.65 2.60 0.008 0.0009 0.030 0.0034 Ti: 0.025, B: 0.0025 Conforming steel L 0.090 0.55 2.65 0.012 0.0015 0.042 0.0037 Nb: 0.015, Ti: 0.025, B: 0.0015 Conforming steel M 0.070 0.15 2.25 0.014 0.0033 0.025 0.0028 Nb: 0.015, Ti: 0.025, B: 0.0020, Cr: 0.550 Conforming steel N 0.090 0.55 2.75 0.013 0.0056 0.022 0.0026 Nb: 0.015, Ti: 0.025, B: 0.0020 Conforming steel O 0.135 0.55 2.70 0.010 0.0008 0.035 0.0025 Nb: 0.020, Ti: 0.015, B: 0.0015 Conforming steel P 0.045 0.60 2.65 0.015 0.0013 0.037 0.0046 Nb: 0.015, Ti: 0.020, B: 0.0020 Conforming steel Q 0.090 0.72 2.60 0.018 0.0028 0.032 0.0037 Nb: 0.015, Ti: 0.020, B: 0.0015 Conforming steel R 0.085 0.08 2.70 0.010 0.0041 0.055 0.0030 Nb: 0.020, Ti: 0.020, B: 0.0020 Conforming steel S 0.090 0.55 3.15 0.001 0.0015 0.038 0.0026 Nb: 0.015, Ti: 0.025, B: 0.0025 Conforming steel T 0.085 0.60 2.10 0.011 0.0022 0.041 0.0026 Nb: 0.025, Ti: 0.015, B: 0.0020 Conforming steel U 0.085 0.65 2.65 0.012 0.0018 0.030 0.0031 V: 0.045 Conforming steel V 0.090 0.55 2.70 0.008 0.0016 0.042 0.0036 Cu: 0.180 Conforming steel W 0.095 0.50 2.55 0.007 0.0010 0.033 0.0025 Cr: 0.090 Conforming steel X 0.075 0.45 2.65 0.008 0.0008 0.055 0.0026 Ni: 0.120 Conforming steel Y 0.065 0.55 2.85 0.007 0.0020 0.045 0.0030 Mo: 0.200 Conforming steel Z 0.120 0.60 2.90 0.006 0.0007 0.035 0.0030 Sb: 0.009 Conforming steel The remainder other than these is Fe and incidental impurities.

TABLE-US-00002 TABLE 1-2 Steel Chemical composition (mass %) grade C Si Mn P S Al N Others Note AA 0.085 0.45 2.70 0.009 0.0008 0.040 0.0036 Sn: 0.018 Conforming steel AB 0.115 0.50 2.25 0.007 0.0010 0.033 0.0025 Nb: 0.035, Ta: 0.008 Conforming steel AC 0.080 0.55 2.70 0.008 0.0008 0.055 0.0026 Ta: 0.007 Conforming steel AD 0.075 0.45 2.35 0.012 0.0028 0.045 0.0022 W: 0.040 Conforming steel AE 0.065 0.50 2.65 0.004 0.0022 0.031 0.0052 Mg: 0.0055 Conforming steel AF 0.085 0.60 2.85 0.008 0.0010 0.035 0.0034 Zn: 0.0070 Conforming steel AG 0.070 0.55 2.55 0.010 0.0012 0.039 0.0030 Co: 0.0090 Conforming steel AH 0.080 0.50 2.70 0.013 0.0015 0.029 0.0025 Zr: 0.0030 Conforming steel AI 0.100 0.55 2.45 0.026 0.0019 0.052 0.0048 Ca: 0.0017 Conforming steel AJ 0.085 0.45 2.65 0.013 0.0005 0.031 0.0042 Se: 0.0080 Conforming steel AK 0.090 0.50 2.80 0.012 0.0024 0.031 0.0062 Te: 0.0150 Conforming steel AL 0.080 0.55 2.55 0.030 0.0008 0.036 0.0032 Ge: 0.0050 Conforming steel AM 0.100 0.40 2.35 0.010 0.0036 0.033 0.0076 As: 0.0230 Conforming steel AN 0.090 0.60 2.25 0.045 0.0019 0.033 0.0045 Sr: 0.0060 Conforming steel AO 0.085 0.55 2.65 0.008 0.0023 0.035 0.0032 Cs: 0.0110 Conforming steel AP |0.125 0.25 2.50 0.010 0.0032 0.032 0.0033 Hf: 0.0080 Conforming steel AQ 0.070 0.35 2.75 0.023 0.0025 0.042 0.0035 Pb: 0.0110 Conforming steel AR 0.080 0.50 2.70 0.011 0.0014 0.045 0.0038 Bi: 0.0040 Conforming steel AS 0.095 0.45 2.60 0.045 0.0019 0.033 0.0045 REM: 0.0025 Conforming steel AT 0.065 0.12 2.21 0.012 0.0038 0.029 0.0035 Nb: 0.190, Ti: 0.190, V: 0.180, B: 0.0085, Conforming steel Cr: 0.950, Ni: 0.960, Mo: 0.950, Sb: 0.190, Sn: 0.180, Cu: 0.900, Ta: 0.095, W: 0.450, Mg: 0.0170, Zn: 0.0180, Co: 0.0180, Zr: 0.0930, Ca: 0.0180, Se: 0.0190, Te: 0.0185, Ge: 0.0190, As: 0.0400, Sr: 0.0180, Cs: 0.0185, Hf: 0.0185, Pb: 0.0190, Bi: 0.0190, REM: 0.0190 The remainder other than these is Fe and incidental impurities.

TABLE-US-00003 TABLE 2-1 Hot rolling Cold step rolling Heating First cooling step Finish step step Average Average rolling Rolling Average Annealing step cooling cooling temper- reduc- heating Annealing Annealing rate (1) rate (2) Steel ature tion rate (*1) temperature time (*2) (*3) No. grade ( C.) (%) ( C./s) ( C.) (s) ( C./s) ( C./s) 1 A 890 51.7 11 810 120 13 8 2 B 870 64.7 10 820 150 12 7 3 C 900 50.0 14 810 200 14 9 4 D 860 53.8 13 820 100 15 7 5 E 870 50.0 9 810 90 10 5 6 F 900 50.0 15 800 150 13 6 7 G 920 51.7 14 800 60 12 7 8 H 860 53.8 16 810 150 14 8 9 I 890 41.7 12 830 200 13 9 10 J 920 37.9 13 820 50 12 8 11 K 900 52.9 14 800 250 14 7 12 L 870 56.3 10 790 150 15 8 13 M 880 53.8 15 810 120 10 5 14 N 890 57.1 13 800 200 13 7 15 N 700 56.3 12 810 100 12 6 16 N 870 58.6 3 790 150 14 7 17 N 890 46.2 13 700 150 13 5 18 N 920 57.1 14 800 5 15 8 19 N 900 44.0 14 820 140 2 6 20 N 880 53.8 12 810 100 14 20 21 N 870 35.7 13 810 120 16 6 22 N 890 56.5 16 820 200 13 9 23 N 890 56.5 17 790 120 14 9 24 N 900 53.8 13 800 120 13 6 25 N 890 57.1 11 820 300 13 5 26 O 870 50.0 12 810 150 11 8 27 P 890 58.6 14 800 120 13 7 28 Q 860 44.0 13 810 100 12 9 29 R 890 48.4 16 860 150 14 7 Second cooling step Galvanizing Second Reheating step step Num- cooling Re- Alloying ber stop heating temper- of temper- temper- Holding ature Tension passes ature ature time No. Type ( C.) (kgf/mm.sup.2) () ( C.) ( C.) (s) Note 1 GA 530 2.8 9 50 200 60 Inventive example 2 GA 520 2.4 8 30 210 80 Inventive example 3 GA 530 2.6 7 50 200 100 Inventive example 4 GA 510 3.0 10 50 200 60 Comparative example 5 GA 520 2.2 7 40 180 50 Comparative example 6 GA 530 2.4 6 50 240 80 Comparative example 7 GI 2.2 8 30 220 100 Comparative example 8 GA 500 2.8 7 50 200 60 Comparative example 9 GA 520 2.6 10 80 300 200 Inventive example 10 GA 510 2.9 9 70 250 100 Inventive example 11 GI 2.6 7 30 200 80 Inventive example 12 GA 540 3.0 6 50 250 100 Inventive example 13 GA 520 2.5 8 40 200 60 Inventive example 14 GA 530 2.6 7 30 200 60 Inventive example 15 GA 520 2.9 9 50 250 100 Comparative example 16 GA 540 3.2 8 30 200 80 Comparative example 17 GI 2.6 7 40 200 60 Comparative example 18 GA 530 3.0 6 50 200 40 Comparative example 19 GA 510 2.6 8 60 250 100 Comparative example 20 GI 2.9 9 50 250 150 Comparative example 21 GA 520 0.8 9 30 170 40 Comparative example 22 GA 530 2.1 3 40 180 30 Comparative example 23 GA 530 3.4 6 350 210 100 Comparative example 24 GA 530 2.5 7 30 500 60 Comparative example 25 GA 530 2.9 7 50 200 5 Comparative example 26 GA 520 2.8 10 30 180 80 Inventive example 27 GA 540 2.4 9 40 230 100 Inventive example 28 GI 2.6 6 50 190 60 Inventive example 29 GA 540 2.5 8 70 240 80 Inventive example (*1) Average heating rate: average heating rate between 350 C. and 600 C. (*2) Average cooling rate (1): average cooling rate between (annealing temperature 30 C.) and 650 C. (*3) Average cooling rate (2): average cooling rate between 650 C. and 500 C.

TABLE-US-00004 TABLE 2-2 Hot rolling Cold step rolling First cooling step Finish step Heating Average Average rolling Rolling step Annealing step cooling cooling temper- reduc- Average Annealing Annealing rate (1) rate (2) Steel ature tion heating temperature time (*2) (*3) No. grade ( C.) (%) rate (*1) ( C./s) ( C.) (s) ( C./s) ( C./s) 30 S 870 56.3 12 820 80 13 5 31 T 880 53.8 13 810 100 13 8 32 U 900 42.9 11 800 60 12 7 33 V 890 62.5 12 810 120 14 11 34 W 860 37.9 14 820 300 12 4 35 X 900 50.0 13 810 180 11 7 36 Y 850 23.1 10 800 400 12 4 37 Z 870 42.9 14 810 200 13 7 38 AA 900 41.7 12 860 200 12 6 39 AB 840 50.0 11 850 70 14 8 40 AC 890 43.8 17 800 100 13 4 41 AD 880 61.5 13 820 150 12 7 42 AB 830 50.0 11 880 40 14 6 43 AF 950 62.5 14 800 90 12 12 44 AG 870 51.7 13 820 200 14 8 45 AH 930 38.5 13 810 150 13 6 46 Al 910 71.4 20 820 180 17 12 47 AJ 890 56.3 12 760 120 14 10 48 AK 900 53.8 12 810 90 11 7 49 AL 880 42.9 14 820 150 13 6 50 AM 920 62.5 13 890 100 14 10 51 AN 870 44.8 11 800 200 12 8 52 AO 860 53.8 14 810 250 13 7 53 AP 890 56.3 12 870 100 14 6 54 AQ 900 53.8 13 810 90 15 8 55 AR 890 60.0 11 800 150 14 8 56 AS 860 46.2 14 840 100 13 7 57 N 850 11 790 200 11 5 58 N 870 10 790 220 10 5 59 N 900 9 770 250 9 4 Second cooling step Galvanizing Second Reheating step step Num- cooling Re- Alloying ber stop heating temper- of temper- temper- Holding ature Tension passes ature ature time No. Type ( C.) (kgf/mm.sup.2) () ( C.) ( C.) (s) Note 30 GA 500 2.9 12 60 190 40 Inventive example 31 GA 500 3.2 9 220 420 60 Inventive example 32 GA 520 2.9 6 40 180 120 Inventive example 33 GA 530 2.8 8 50 240 100 Inventive example 34 GA 510 2.4 9 30 180 50 Inventive example 35 GA 520 2.6 6 150 350 60 Inventive example 36 GA 490 3.4 7 50 200 90 Inventive example 37 GA 520 3.8 11 30 250 100 Inventive example 38 GI 2.8 9 60 200 100 Inventive example 39 GA 540 3.6 7 40 220 900 Inventive example 40 GA 510 3.2 9 30 230 600 Inventive example 41 GA 520 3.1 8 50 200 180 Inventive example 42 GA 580 2.6 9 80 250 80 Inventive example 43 GA 520 3.0 6 70 200 70 Inventive example 44 GI 2.2 8 40 220 100 Inventive example 45 GA 510 2.4 7 50 240 80 Inventive example 46 GA 540 2.6 8 30 190 60 Inventive example 47 GA 520 3.0 10 100 200 180 Inventive example 48 GA 530 3.8 10 60 250 60 Inventive example 49 GA 500 3.0 7 50 200 50 Inventive example 50 GA 540 2.6 8 40 240 150 Inventive example 51 GA 520 2.2 7 50 220 60 Inventive example 52 GA 500 2.4 9 30 250 180 Inventive example 53 GA 550 2.6 7 50 200 100 Inventive example 54 GA 520 3.8 9 40 190 30 Inventive example 55 GA 530 2.6 8 40 240 50 Inventive example 56 GI 3.0 7 30 200 100 Inventive example 57 GA 510 2.6 8 50 200 60 Inventive example 58 GA 530 3.2 10 40 250 60 Inventive example 59 GA 520 3.6 9 30 200 60 Inventive example (*1) Average heating rate: average heating rate between 350 C. and 600 C. (*2) Average cooling rate (1): average cooling rate between (annealing temperature 30 C.) and 650 C. (*3) Average cooling rate (2): average cooling rate between 650 C. and 500 C.

TABLE-US-00005 TABLE 3-1 Steel microstructure Area fraction of each phase(*1) Average Amount (M + RA)/ Microstructure grain size of diffusible Sheet (M + RA) of the of M and hydrogen Steel thickness F M RA (*2) B + BT TM remainder RA (*3) in steel No. grade (mm) (%) (%) (%) () (%) (%) (*1) (m) (ppm by mass) 1 A 1.4 55.4 3.2 1.4 0.82 1.8 37.2 0.32 0.10 2 B 1.2 54.3 3.8 1.0 0.81 1.9 38.1 0.31 0.08 3 C 1.6 68.9 4.5 2.1 0.91 1.0 22.4 0.23 0.06 4 D 1.2 28.6 23.8 9.5 0.67 0.6 36.3 1.83 0.09 5 E 1.4 84.2 6.3 0.2 0.71 0.4 7.9 0.09 0.15 6 F 1.6 42.6 8.9 10.8 0.70 2.4 34.2 , P 0.34 0.06 7 G 1.4 47.8 25.8 2.8 0.71 0.4 22.8 0.59 0.09 8 H 1.2 81.9 4.8 0.5 0.89 11.2 1.4 0.05 0.10 9 I 1.4 54.2 3.5 1.8 0.81 1.6 38.6 0.33 0.12 10 J 1.8 55.1 3.1 2.0 0.87 2.1 37.4 0.27 0.15 11 K 1.6 52.9 4.0 1.6 0.89 1.4 38.9 0.25 0.09 12 L 1.4 54.2 4.2 1.9 0.85 1.9 37.5 0.29 0.07 13 M 1.2 68.2 4.3 2.2 0.93 0.8 23.4 0.20 0.09 14 N 1.2 54.8 2.8 1.8 0.85 1.3 38.8 0.28 0.08 15 N 1.4 32.4 16.8 4.8 0.54 4.3 16.7 , F 1.65 0.07 16 N 1.2 51.8 3.4 1.6 0.39 1.6 36.2 , F 2.11 0.08 17 N 1.4 81.2 2.1 1.3 0.71 0.4 8.5 , F 0.05 0.09 18 N 1.2 82.4 2.0 1.1 0.72 0.2 7.1 , F 0.03 0.12 19 N 1.4 56.7 8.6 2.0 0.38 1.1 30.4 1.82 0.07 20 N 1.2 55.8 8.8 1.8 0.35 1.4 30.9 2.18 0.06 21 N 1.8 65.2 17.2 4.5 0.74 3.2 9.2 0.78 0.05 22 N 1.0 66.2 16.3 4.7 0.73 2.7 9.1 0.85 0.09 23 N 1.0 66.2 17.1 4.3 0.77 2.4 9.3 0.82 0.13 24 N 1.2 64.8 17.2 4.8 0.72 3.1 9.7 0.69 0.65 25 N 1.2 65.1 16.5 4.9 0.75 2.9 9.5 0.72 0.68 26 O 1.6 49.1 12.1 2.3 0.81 2.0 32.8 0.82 0.24 27 P 1.2 72.9 6.3 0.2 0.73 0.4 18.8 0.03 0.08 28 Q 1.4 52.6 8.9 2.8 0.79 2.7 31.2 , P 0.22 0.28 29 R 1.6 29.4 6.6 1.0 0.81 2.2 59.5 0.28 0.07 U-bending + V-bending + Axial tight VDA compression bending bending characteristics YS TS YR El ST SFmax (appearance No. (MPa) (MPa) () (%) (%) R/t (mm) (mm) crack) Type Note 1 695 1055 0.66 18.0 42 1.07 3.5 27.4 Excellent GA Inventive example 2 674 1033 0.65 17.1 40 1.25 3.0 28.4 Excellent GA Inventive example 3 568 835 0.68 22.4 48 0.63 2.0 29.2 Excellent GA Inventive example 4 1021 1382 0.74 10.8 12 4.17 8.5 24.6 Poor GA Comparative example 5 393 698 0.56 25.4 62 1.07 1.0 30.1 Good GA Comparative example 6 725 1065 0.68 16.5 18 1.88 5.5 25.9 Poor GA Comparative example 7 924 1492 0.62 10.4 31 3.21 9.0 24.9 Poor GI Comparative example 8 378 658 0.57 26.3 61 0.42 1.0 30.3 Good GA Comparative example 9 695 1062 0.65 17.7 38 1.43 3.5 27.3 Excellent GA Inventive example 10 695 1028 0.68 17.2 43 0.83 2.5 28.5 Excellent GA Inventive example 11 695 1059 0.66 18.2 45 0.63 3.0 27.9 Excellent GI Inventive example 12 695 1044 0.67 17.9 45 1.07 2.5 28.6 Excellent GA Inventive example 13 573 815 0.70 23.4 48 0.42 1.5 29.7 Excellent GA Inventive example 14 700 1070 0.65 18.2 44 0.83 2.5 28.7 Excellent GA Inventive example 15 1021 1124 0.91 15.2 21 3.21 7.5 25.6 Poor GA Comparative example 16 635 1020 0.62 17.2 22 2.92 6.5 24.6 Poor GA Comparative example 17 385 667 0.58 24.2 54 0.71 1.0 30.3 Good GI Comparative example 18 412 693 0.59 23.8 52 0.83 1.5 30.5 Good GA Comparative example 19 712 1058 0.67 17.8 28 2.86 4.5 26.1 Poor GA Comparative example 20 724 1074 0.67 17.4 25 2.92 5.0 25.8 Poor GI Comparative example 21 615 991 0.62 17.9 27 2.78 4.5 26.2 Poor GA Comparative example 22 620 989 0.63 17.5 26 3.00 5.0 25.6 Poor GA Comparative example 23 613 992 0.62 17.3 28 3.00 4.5 26.1 Poor GA Comparative example 24 629 1021 0.62 16.7 15 4.58 6.5 24.6 Poor GA Comparative example 25 625 1007 0.62 16.9 13 4.58 7.0 24.9 Poor GA Comparative example 26 720 1124 0.64 16.1 36 2.19 3.5 26.7 Good GA Inventive example 27 512 791 0.65 24.9 52 0.83 2.0 28.5 Excellent GA Inventive example 28 667 1049 0.64 17.4 33 2.14 3.5 26.8 Good Inventive example 29 745 1124 0.66 15.9 35 1.88 3.0 27.1 Excellent GA Inventive example (*1) F: ferrite, M: fresh martensite, RA: retained austenite, M: island-like fresh martensite, RA: island-like retained austenite, B: bainite, BT: tempered bainite, TM: tempered martensite, P: pearlite, : carbide, F: unrecrystallized ferrite (*2) (M + RA)/(M + RA): the value obtained by dividing the total area fraction of island-like fresh martensite and island-like retained austenite in a ferrite grain by the sum of the area fraction of fresh martensite and the area fraction of retained austenite in the entire steel sheet (*3) The average grain size of M and RA: the average grain size of island-like fresh martensite and island-like retained austenite in a ferrite grain

TABLE-US-00006 TABLE 3-2 Steel microstructure Area fraction of each phase(*1) Average Amount (M + RA)/ Microstructure grain size of diffusible Sheet (M + RA) of the of M and hydrogen Steel thickness F M RA (*2) B + BT TM remainder RA (*3) in steel No. grade (mm) (%) (%) (%) () %) (%) (*1) (m) (ppm by mass) 30 S 1.4 49.1 13.7 2.0 0.72 1.2 33.3 0.81 0.25 31 T 1.2 73.2 5.2 1.2 0.75 3.2 12.2 0.12 0.03 32 U 1.6 60.2 2.5 0.8 0.85 1.0 34.6 0.29 0.21 33 V 1.2 50.1 5.2 1.0 0.82 0.8 42.4 0.45 0.08 34 W 1.8 52.2 4.0 1.2 0.81 1.4 40.4 0.38 0.38 35 X 1.6 54.8 2.9 2.0 0.82 1.9 37.5 0.21 0.04 36 Y 2.0 54.2 3.5 1.7 0.86 1.2 38.6 0.53 0.12 37 Z 1.6 54.8 3.8 2.1 0.83 2.1 35.8 0.38 0.09 38 AA 1.4 52.4 5.8 2.2 0.90 1.5 37.4 0.31 0.11 39 AB 1.6 56.1 2.9 1.0 0.87 0.9 38.6 0.27 0.09 40 AC 1.8 56.9 4.3 1.5 0.89 1.2 35.7 0.29 0.10 41 AD 1.0 52.2 3.9 1.4 0.84 0.8 40.6 0.39 0.08 42 AB 1.4 54.2 2.7 1.0 0.91 1.4 39.2 0.20 0.06 43 AF 1.2 54.9 3.7 1.8 0.94 1.2 37.9 0.28 0.09 44 AG 1.4 53.5 2.9 1.2 0.78 1.6 38.6 0.31 0.15 45 AH 1.6 52.1 4.8 2.0 0.85 1.5 37.2 0.47 0.09 46 AI 0.8 51.9 3.4 1.6 0.89 0.7 38.5 0.55 0.24 47 AJ 1.4 64.2 9.1 1.8 0.82 0.4 23.5 0.19 0.10 48 AK 1.2 51.2 2.6 1.0 0.93 1.5 42.6 0.20 0.11 49 AL 1.6 55.8 3.9 1.5 0.88 0.6 37.4 0.28 0.14 50 AM 1.2 35.2 2.8 1.0 0.83 6.2 53.7 0.37 0.07 51 AN 1.6 55.5 3.1 1.2 0.87 0.9 35.9 0.17 0.08 52 AO 1.2 52.9 2.7 1.8 0.82 1.3 38.1 0.29 0.07 53 AF 1.4 43.2 5.2 2.0 0.85 1.4 47.4 0.19 0.10 54 AQ 1.2 54.2 3.5 1.6 0.90 0.7 38.2 0.20 0.32 55 AR 1.6 55.1 3.1 1.2 0.92 1.0 37.1 0.32 0.09 56 AS 1.4 56.9 2.8 1.5 0.85 0.6 36.6 0.26 0.10 57 N 2.6 54.2 3.7 1.9 0.80 1.0 37.2 0.42 0.11 58 N 2.9 51.2 3.9 2.0 0.78 1.2 38.9 0.48 0.10 59 N 3.2 52.8 4.2 2.2 0.75 1.4 38.9 0.56 0.11 U-bending + V-bending + Axial tight VDA compression bending bending characteristics YS TS YR El ST SFmax (appearance No. (MPa) (MPa) () (%) (%) R/t (mm) (mm) crack) Type Note 30 712 1089 0.65 17.4 37 2.50 3.5 26.7 Good GA Inventive example 31 523 789 0.66 24.9 48 0.83 2.0 28.7 Excellent GA Inventive example 32 668 998 0.67 18.9 39 0.94 3.0 27.2 Excellent GA Inventive example 33 675 1028 0.66 17.8 43 1.25 2.5 28.2 Excellent GA Inventive example 34 694 1053 0.66 17.2 38 0.83 3.0 27.1 Excellent GA Inventive example 35 702 1061 0.66 17.7 39 0.94 3.5 27.3 Excellent GA Inventive example 36 687 1008 0.68 17.5 50 0.75 2.5 28.5 Excellent GA Inventive example 37 688 1059 0.65 18.2 42 0.94 3.0 27.2 Excellent GA Inventive example 38 667 1033 0.65 17.4 40 0.71 3.5 28.2 Excellent GI Inventive example 39 695 1022 0.68 17.8 45 0.94 2.5 27.1 Excellent GA Inventive example 40 702 1034 0.68 17.3 40 0.83 3.0 27.9 Excellent GA Inventive example 41 654 1062 0.62 18.1 41 1.00 3.5 27.3 Excellent GA Inventive example 42 700 1055 0.66 17.2 42 1.07 3.0 28.0 Excellent GA Inventive example 43 639 999 0.64 18.7 52 1.25 3.0 27.8 Excellent GA Inventive example 44 677 1012 0.67 17.7 42 1.07 3.5 27.3 Excellent GI Inventive example 45 695 1029 0.68 17.5 43 0.94 2.5 28.3 Excellent GA Inventive example 46 691 1051 0.66 17.6 38 1.25 3.0 27.9 Excellent GA Inventive example 47 685 982 0.70 18.9 52 1.07 3.5 27.1 Excellent GA Inventive example 48 698 1027 0.68 17.2 48 1.25 2.5 28.2 Excellent GA Inventive example 49 710 1033 0.69 18.2 41 0.94 3.0 27.2 Excellent GA Inventive example 50 712 1098 0.65 16.8 36 1.25 3.5 27.3 Excellent GA Inventive example 51 695 1025 0.68 18.2 43 0.94 3.0 28.4 Excellent GA Inventive example 52 690 1047 0.66 17.3 41 1.25 3.0 27.6 Excellent GA Inventive example 53 720 1091 0.66 16.4 35 1.07 3.5 27.3 Excellent GA Inventive example 54 685 1029 0.67 17.2 41 0.83 2.5 28.1 Excellent GA Inventive example 55 666 1045 0.64 17.6 42 0.94 3.0 27.9 Excellent GA Inventive example 56 684 1023 0.67 17.7 44 1.07 3.5 27.2 Excellent GI Inventive example 57 684 1025 0.67 17.5 41 0.58 2.5 27.3 Excellent GA Inventive example 58 697 1012 0.69 17.8 43 0.52 3.0 27.5 Excellent GA Inventive example 59 696 1005 0.69 18.2 45 0.47 3.0 27.8 Excellent GA Inventive example (*1) F: ferrite, M: fresh martensite, RA: retained austenite, M: island-like fresh martensite, RA: island-like retained austenite, B: bainite, BT: tempered bainite, TM: tempered martensite, P: pearlite, : carbide, F: unrecrystallized ferrite (*2) (M + RA)/(M + RA): the value obtained by dividing the total area fraction of island-like fresh martensite and island-like retained austenite in a ferrite grain by the sum of the area fraction of fresh martensite and the area fraction of retained austenite in the entire steel sheet (*3) The average grain size of M and RA: the average grain size of island-like fresh martensite and island-like retained austenite in a ferrite grain

TABLE-US-00007 TABLE 4 Hot Cold First coating step Heating rolling step rolling (Metal Coating step Finish step step) Average Annealing step rolling Rolling Presence or heating Annealing Annealing Dew Steel temperature reduction absence rate (*1) temperature time point No grade ( C.) (%) (Coating type) ( C./s) ( C.) (s) ( C.) 60 A 888 51.7 Absent 12 833 98 28 61 A 892 51.7 Absent 18 827 124 9 62 A 899 51.7 Present (Fe) 11 820 196 28 63 A 915 51.7 Present (Fe) 11 826 139 9 64 A 888 51.7 Present (Ni) 13 837 136 9 65 A 863 51.7 Absent 15 806 138 9 66 A 867 51.7 Present (Fe) 16 808 127 28 67 A 915 51.7 Present (Fe) 10 818 132 9 68 M 860 53.8 Absent 16 810 154 25 69 M 925 53.8 Absent 17 827 200 5 70 M 903 53.8 Present (Fe) 18 800 177 25 71 M 915 53.8 Present (Fe) 14 804 151 5 72 M 876 53.8 Present (Ni) 17 834 194 5 73 M 916 53.8 Absent 12 813 165 5 74 M 892 53.8 Present (Fe) 10 800 92 25 75 M 864 53.8 Present (Fe) 14 817 128 5 76 N 912 57.1 Absent 18 800 140 20 77 N 876 57.1 Absent 18 806 190 10 78 N 922 57.1 Present (Fe) 17 807 142 20 79 N 921 57.1 Present (Fe) 13 839 179 10 80 N 899 57.1 Present (Ni) 10 840 159 10 81 N 900 57.1 Absent 16 826 109 10 82 N 874 57.1 Present (Fe) 14 839 127 20 83 N 888 57.1 Present (Fe) 13 832 198 10 84 N 907 Absent 9 798 167 20 85 N 895 Absent 10 805 213 10 86 N 886 Present (Fe) 9 810 173 20 87 N 920 Present (Fe) 13 805 246 10 88 N 873 Present (Ni) 9 798 239 10 89 N 878 Absent 11 813 216 10 90 N 890 Present (Fe) 11 794 200 20 91 N 904 Present (Fe) 10 808 204 10 First cooling step Second coating step Second cooling step Average Average (Galvanizing step) Second cooling cooling Alloying Number cooling Reheating step rate rate temper- of stop Reheating Holding (1) (*2) (2) (*3) ature Tension passes temperature temperature time No ( C./s) ( C./s) Type ( C.) (kgf/mm.sup.2) () ( C.) ( C.) (s) Note 60 17 4 GA 530 2.5 6 40 208 59 Inventive example 61 16 5 GA 500 3.8 9 53 202 58 Inventive example 62 11 10 GA 540 3.5 6 34 201 28 Inventive example 63 18 4 GA 490 3.3 7 51 200 29 Inventive example 64 11 5 GA 500 2.6 8 41 199 63 Inventive example 65 18 9 GI 3.3 6 34 206 59 Inventive example 66 14 5 GI 2.9 6 39 204 46 Inventive example 67 12 8 GI 2.7 6 38 193 26 Inventive example 68 12 5 GA 520 2.5 8 35 208 52 Inventive example 69 10 10 GA 500 3.5 6 30 192 43 Inventive example 70 15 7 GA 530 2.9 10 46 201 54 Inventive example 71 18 7 GA 500 3.3 10 38 198 63 Inventive example 72 12 9 GA 490 4.0 6 33 197 64 Inventive example 73 12 5 GI 2.3 7 54 197 58 Inventive example 74 18 10 GI 3.1 9 53 199 67 Inventive example 75 10 5 GI 3.0 7 50 201 60 Inventive example 76 18 9 GA 530 3.2 10 55 198 51 Inventive example 77 19 10 GA 510 2.0 7 55 201 62 Inventive example 78 13 9 GA 540 3.9 8 35 210 65 Inventive example 79 18 6 GA 500 3.0 8 32 205 47 Inventive example 80 17 8 GA 510 4.0 9 37 199 63 Inventive example 81 19 10 GI 2.0 6 48 194 36 Inventive example 82 15 9 GI 2.5 7 46 193 58 Inventive example 83 14 10 GI 2.8 7 39 190 44 Inventive example 84 12 7 GA 520 3.0 10 55 206 63 Inventive example 85 9 5 GA 490 3.0 7 49 209 44 Inventive example 86 10 7 GA 530 3.2 9 40 206 35 Inventive example 87 11 4 GA 500 3.8 10 43 190 43 Inventive example 88 12 3 GA 510 3.4 6 43 192 26 Inventive example 89 11 6 GI 3.9 9 42 192 42 Inventive example 90 10 5 GI 3.9 7 35 201 35 Inventive example 91 12 6 GI 3.2 9 50 207 67 Inventive example (*1) Average heating rate: average heating rate between 350 C. and 600 C. (*2) Average cooling rate (1): average cooling rate between (annealing temperature 30 C.) and 650 C. (*3) Average cooling rate (2): average cooling rate between 650 C. and 500 C.

TABLE-US-00008 TABLE 5-1 Steel microstructure Average grain Area fraction of each phase(*1) Microstructure size of Sheet (M + RA)/ of the M and Steel thickness F M RA (M + RA) B + BT TM remainder RA (*3) No. grade (mm) %) (%) (%) (*2) () (%) (%) (*1) (m) 60 A 1.4 64.7 2.5 2.4 0.85 1.3 28.1 0.38 61 A 1.4 56.7 3.3 1.9 0.81 1.2 35.7 0.43 62 A 1.4 58.3 3.9 2.2 0.77 3.1 32.3 0.37 63 A 1.4 56.4 5.0 1.5 0.86 0.1 33.3 0.39 64 A 1.4 52.9 4.6 0.4 0.86 0.7 28.7 0.48 65 A 1.4 64.0 3.5 1.3 0.86 3.0 28.1 0.46 66 A 1.4 58.1 3.4 0.6 0.88 2.5 34.6 0.36 67 A 1.4 60.7 4.1 1.6 0.76 1.9 29.5 0.35 68 M 1.2 72.1 3.2 1.3 0.79 0.4 22.0 0.24 69 M 1.2 71.9 4.8 1.0 0.80 1.5 20.6 0.24 70 M 1.2 71.4 3.4 1.2 0.93 2.9 20.8 0.23 71 M 1.2 70.5 2.6 2.2 0.81 2.2 19.8 0.19 72 M 1.2 71.0 3.3 0.9 0.94 3.0 21.0 0.21 73 M 1.2 66.8 2.8 0.8 0.88 2.7 24.9 0.18 74 M 1.2 71.6 3.7 1.7 0.78 2.4 20.3 0.26 75 M 1.2 73.5 4.9 1.0 0.85 1.9 18.4 0.22 76 N 1.2 57.4 3.2 2.0 0.85 1.7 30.6 0.37 77 N 1.2 64.3 2.9 1.8 0.80 1.4 29.3 0.28 78 N 1.2 52.6 3.5 0.8 0.76 2.6 36.9 0.34 79 N 1.2 59.3 2.5 1.0 0.78 3.2 29.4 0.50 80 N 1.2 62.2 4.8 2.2 0.77 1.4 29.0 0.38 81 N 1.2 53.7 2.5 1.3 0.81 3.2 38.0 0.30 82 N 1.2 60.1 3.7 2.2 0.76 3.4 30.4 0.28 83 N 1.2 59.0 4.7 1.4 0.79 0.3 33.6 0.34 84 N 3.2 59.2 3.0 0.8 0.87 0.1 34.2 0.30 85 N 3.2 59.9 3.5 1.2 0.83 0.6 31.5 0.45 86 N 3.2 56.4 4.2 2.1 0.84 1.2 36.0 0.43 87 N 3.2 54.9 2.5 1.6 0.81 0.1 39.7 0.48 88 N 3.2 58.8 3.3 1.5 0.94 1.8 33.9 0.38 89 N 3.2 53.6 3.2 2.4 0.91 3.4 35.3 0.39 90 N 3.2 61.9 3.9 1.1 0.82 3.1 29.8 0.49 91 N 3.2 58.5 2.8 0.6 0.92 2.5 35.0 0.49 Nanohardness of sheet surface Surface layer Standard Standard Amount of Metal deviation of deviation of diffusible coating Ratio of Hn at quarter Hn at half hydrogen Soft layer weight Hn of 7.0 position position in steel thickness (g/m.sup.2) GPa or more (GPa) (GPa) No. (ppm by mass) (m) (*4) (*5) (*6) (*7) Type Note 60 0.08 11 0.17 2.0 2.4 GA Inventive example 61 0.14 38 0.05 1.4 1.6 GA Inventive example 62 0.16 14 9.0 0.19 1.6 2.0 GA Inventive example 63 0.22 48 9.0 0.02 0.6 0.7 GA Inventive example 64 0.12 47 9.0 0.03 0.7 0.9 GA Inventive example 65 0.05 37 0.06 1.5 1.5 GI Inventive example 66 0.12 15 9.0 0.18 1.6 1.9 GI Inventive example 67 0.19 50 9.0 0.01 0.5 0.7 GI Inventive example 68 0.19 9 0.16 2.0 2.4 GA Inventive example 69 0.07 36 0.05 1.3 1.4 GA Inventive example 70 0.13 12 14.0 0.18 1.7 1.9 GA Inventive example 71 0.08 44 14.0 0.03 0.4 0.6 GA Inventive example 72 0.10 43 14.0 0.03 0.7 1.0 GA Inventive example 73 0.21 35 0.04 1.2 1.3 GI Inventive example 74 0.11 15 14.0 0.17 1.6 1.8 GI Inventive example 75 0.08 51 14.0 0.02 0.3 0.5 GI Inventive example 76 0.22 8 0.19 1.9 2.3 GA Inventive example 77 0.18 35 0.06 1.5 1.6 GA Inventive example 78 0.10 11 12.0 0.20 1.7 2.0 GA Inventive example 79 0.17 46 12.0 0.01 0.7 0.8 GA Inventive example 80 0.16 44 12.0 0.02 0.8 1.0 GA Inventive example 81 0.14 36 0.05 1.4 1.5 GI Inventive example 82 0.12 13 12.0 0.19 1.6 1.9 GI Inventive example 83 0.17 49 12.0 0.01 0.6 0.7 GI Inventive example 84 0.22 8 0.18 1.9 2.4 GA Inventive example 85 0.09 37 0.08 1.4 1.6 GA Inventive example 86 0.07 15 12.0 0.20 1.8 1.9 GA Inventive example 87 0.17 49 12.0 0.03 0.7 0.8 GA Inventive example 88 0.06 50 12.0 0.04 0.7 0.9 GA Inventive example 89 0.09 35 0.07 1.4 1.5 GI Inventive example 90 0.20 17 12.0 0.18 1.5 1.8 GI Inventive example 91 0.08 48 12.0 0.01 0.6 0.7 GI Inventive example (*1) F: ferrite, M: fresh martensite, RA: retained austenite, M: island-like fresh martensite, RA: island-like retained austenite, B: bainite, BT: tempered bainite, TM: tempered martensite, : carbide (*2) (M + RA)/(M + RA): the value obtained by dividing the total area fraction of island-like fresh martensite and island-like retained austenite in a ferrite grain by the sum of the area fraction of fresh martensite and the area fraction of retained austenite in the entire steel sheet (*3) The average grain size of M and RA: the average grain size of island-like fresh martensite and island-like retained austenite in a ferrite grain (*4) Metal coating weight (g/m.sup.2): first coating weight (g/m.sup.2) (*5) The ratio of the number of measurements with a nanohardness of 7.0 GPa or more to the total number of measurements of nanohardness at a quarter depth position in the thickness direction of a surface soft layer from the surface of a base steel sheet (*6) The standard deviation (GPa) of the nanohardness of a sheet surface at a quarter position in the thickness direction of a surface soft layer from the surface of a base steel sheet (*7) The standard deviation (Gpa) of the nanohardness of a sheet surface at a half position in the thickness direction of a surface soft layer from the surface of a base steel sheet

TABLE-US-00009 TABLE 5-2 Axial compression U-bending + VDA bending characteristics YS (MPa) YR El tight bending V-bending + SFmax (appearance No. (MPa) TS () (%) (%) R/t ST (mm) (mm) crack) Type Note 60 659 1026 0.64 17.6 30 1.43 3.5 27.4 Good GA Inventive example 61 688 1056 0.65 19.0 39 0.71 2.5 28.3 Excellent GA Inventive example 62 654 1029 0.64 17.3 32 1.07 3.0 27.9 Excellent GA Inventive example 63 689 1001 0.69 17.9 37 0.00 1.5 29.0 Excellent GA Inventive example 64 661 1013 0.65 17.7 33 0.00 2.0 28.8 Excellent GA Inventive example 65 643 1011 0.64 18.7 38 0.71 2.5 28.1 Excellent GI Inventive example 66 664 1018 0.65 17.3 42 1.07 3.0 27.8 Excellent GI Inventive example 67 706 1047 0.67 17.1 35 0.00 1.5 28.9 Excellent GI Inventive example 68 550 794 0.69 22.9 37 0.83 2.0 29.0 Good GA Inventive example 69 590 792 0.74 21.4 43 0.42 1.0 29.8 Excellent GA Inventive example 70 549 839 0.65 20.3 43 0.42 1.5 29.5 Excellent GA Inventive example 71 598 853 0.70 21.3 37 0.00 0.0 30.3 Excellent GA Inventive example 72 565 842 0.67 20.9 42 0.00 0.0 30.1 Excellent GA Inventive example 73 565 849 0.67 21.4 43 0.42 1.0 29.7 Excellent GI Inventive example 74 560 854 0.66 20.7 34 0.42 1.5 29.5 Excellent GI Inventive example 75 585 794 0.74 20.3 34 0.00 0.0 30.1 Excellent GI Inventive example 76 720 1049 0.69 17.5 38 1.67 3.0 27.6 Good GA Inventive example 77 707 1067 0.66 18.7 40 0.83 2.0 28.4 Excellent GA Inventive example 78 686 1002 0.68 19.0 42 1.25 2.5 28.0 Excellent GA Inventive example 79 705 1031 0.68 17.4 45 0.00 1.0 29.2 Excellent GA Inventive example 80 681 1056 0.64 18.8 32 0.00 1.0 29.0 Excellent GA Inventive example 81 713 1033 0.69 18.5 36 0.83 2.0 28.3 Excellent GI Inventive example 82 702 1028 0.68 17.6 43 1.25 2.5 28.0 Excellent GI Inventive example 83 681 1038 0.66 18.6 40 0.00 1.0 29.3 Excellent GI Inventive example 84 660 1049 0.63 17.7| 39 1.41 4.0 26.7 Good GA Inventive example 85 702 1023 0.69 18.8 43 0.94 3.5 27.5 Excellent GA Inventive example 86 640 1024 0.63 17.3 36 1.09 4.0 27.1 Excellent GA Inventive example 87 701 1045 0.67 17.3 36 0.31 2.0 28.2 Excellent GA Inventive example 88 681 1062 0.64 18.0 30 0.31 2.0 28.1 Excellent GA Inventive example 89 710 1047 0.68 17.4 36 0.94 3.5 27.4 Excellent GI Inventive example 90 700 1005 0.70 18.8 42 1.09 4.0 27.1 Excellent GI Inventive example 91 665 1023 0.65 17.5 31 0.31 2.0 28.1 Excellent GI Inventive example

TABLE-US-00010 TABLE 6 First coating Hot Cold step Heating rolling step rolling (Metal step First cooling step Finish step Coating step) Average Annealing step Average Average rolling Rolling Presence heating Annealing Annealing Dew cooling cooling Steel temperature reduction or absence rate (*1) temperature time point rate (1) (*2) rate (2) (*3) No. grade ( C.) (%) (Coating type) ( C./s) ( C.) (s) ( C.) ( C./s) ( C./s) 92 A 876 53.8 Absent 12 785 90 18 13 6 93 A 882 53.8 Absent 13 788 84 10 12 5 94 A 872 53.8 Present (Fe) 15 784 92 18 13 6 95 A 888 53.8 Present (Fe) 12 786 88 10 12 6 96 A 870 53.8 Present (Ni) 14 788 80 10 12 5 97 M 876 53.8 Absent 13 806 103 10 11 7 98 M 890 53.8 Absent 13 808 109 15 10 8 99 M 892 53.8 Present (Fe) 15 810 100 10 10 7 100 M 890 53.8 Present (Fe) 12 807 110 15 11 8 101 M 885 53.8 Present (Ni) 11 805 107 15 10 7 102 N 879 53.8 Absent 12 800 124 20 12 6 103 N 884 53.8 Absent 13 799 120 5 13 5 104 N 890 53.8 Present (Fe) 12 802 128 20 12 5 105 N 882 53.8 Present (Fe) 14 801 130 5 12 6 106 N 884 53.8 Present (Ni) 15 798 122 5 13 5 107 N 892 Absent 9 802 202 15 9 5 108 N 883 Absent 10 808 216 10 10 6 109 N 886 Present (Fe) 9 810 208 15 9 5 110 N 894 Present (Fe) 9 803 222 10 9 5 111 N 889 Present (Ni) 9 799 228 10 10 6 112 AI 893 65.4 Absent 15 815 160 5 17 11 113 AT 854 47.8 Absent 11 800 82 12 10 7 114 AT 862 47.8 Absent 12 796 80 8 11 7 115 AT 848 47.8 Present (Fe) 11 805 85 12 10 8 116 AT 860 47.8 Present (Fe) 11 803 87 8 10 7 117 AT 855 47.8 Present (Ni) 12 799 81 8 10 8 Second coating step Second cooling step Reheating step (Galvanizing step) Number cooling Reheating Alloying of stop temper- Holding temperature Tension passes temperature ature time No. Type ( C.) (kgf/mm.sup.2) () ( C.) ( C.) (s) Note 92 CR 2.8 6 30 182 48 Inventive example 93 CR 3.5 6 32 185 45 Inventive example 94 CR 3.4 9 34 189 50 Inventive example 95 CR 3.3 8 33 181 46 Inventive example 96 CR 2.9 7 35 183 47 Inventive example 97 CR 2.8 10 37 202 55 Inventive example 98 CR 3.7 6 38 204 56 Inventive example 99 CR 2.9 8 39 203 53 Inventive example 100 CR 3.8 6 38 200 57 Inventive example 101 CF 4.1 7 36 205 58 Inventive example 102 CR 3.2 9 40 210 64 Inventive example 103 CR 2.8 9 41 211 67 Inventive example 104 CR 3.8 9 39 209 63 Inventive example 105 CR 3.6 9 42 210 68 Inventive example 106 CR 4.0 10 40 212 62 Inventive example 107 HR 3.2 7 32 195 48 Inventive example 108 HR 3.4 7 31 190 44 Inventive example 109 HR 3.1 8 29 189 41 Inventive example 110 HR 3.5 10 34 187 43 Inventive example 111 HR 3.3 10 32 192 49 Inventive example 112 CR 2.9 9 30 180 50 Inventive example 113 CR 3.3 8 35 183 55 Inventive example 114 CR 3.1 7 32 189 53 Inventive example 115 CR 2.9 9 36 185 52 Inventive example 116 CR 3.2 8 34 180 56 Inventive example 117 CR 3.4 9 30 182 51 Inventive example (*1) Average heating rate: average heating rate between 350 C. and 600 C. (*2) Average cooling rate (1): average cooling rate between (annealing temperature 30 C.) and 650 C. (*3) Average cooling rate (2): average cooling rate between 650 C. and 500 C.

TABLE-US-00011 TABLE 7-1 Steel microstructure Area fraction of each phase(*1) Microstructure Average grain Sheet (M + RA)/ of the size of M and Steel thickness F M RA (M + RA) B + BT TM remainder RA (*3) No. grade (mm) (%) (%) (%) (*2) () (%) (%) (*1) (m) 92 A 1.2 63.2 2.9 2.8 0.84 1.5 28.3 0.35 93 A 1.2 56.3 3.1 1.8 0.82 1.3 35.5 0.41 94 A 1.2 57.7 3.2 2.3 0.76 3.3 32.5 0.36 95 A 1.2 57.3 4.5 1.9 0.84 0.7 34.8 0.33 96 A 1.2 56.9 4.3 0.8 0.83 0.9 35.7 0.45 97 M 1.2 72.0 3.5 1.1 0.77 0.7 22.2 0.23 98 M 1.2 71.5 4.3 1.0 0.79 1.5 20.6 0.24 99 M 1.2 71.4 3.4 1.5 0.90 2.6 20.5 0.28 100 M 1.2 71.8 2.6 2.3 0.83 2.1 19.6 0.18 101 M 1.2 71.2 3.1 0.9 0.94 3.2 21.0 0.24 102 N 1.2 60.4 3.3 2.2 0.85 1.7 30.8 0.31 103 N 1.2 63.1 2.9 1.9 0.80 1.5 29.2 0.28 104 N 1.2 54.6 3.4 0.8 0.75 2.5 36.9 0.32 105 N 1.2 59.8 3.6 1.0 0.78 3.3 29.8 0.47 106 N 1.2 61.2 4.5 2.1 0.79 1.2 29.2 0.39 107 N 3.2 58.8 3.5 0.9 0.89 0.6 34.4 0.31 108 N 3.2 59.3 3.8 1.9 0.86 0.8 33.1 0.43 109 N 3.2 56.1 4.1 2.0 0.84 1.2 35.5 0.41 110 N 3.2 54.7 2.4 1.8 0.87 0.3 39.5 0.45 111 N 3.2 58.2 3.5 1.7 0.92 1.6 33.8 0.39 112 AI 0.9 52.5 3.1 1.8 0.88 0.9 39.9 0.52 113 AT 1.2 57.6 4.9 2.2 0.82 1.9 32.5 0.33 114 AT 1.2 60.1 3.8 2.1 0.83 1.4 31.3 0.29 115 AT 1.2 58.3 3.9 0.8 0.76 2.3 32.9 0.32 116 AT 1.2 58.8 4.2 1.0 0.79 2.7 31.8 0.43 117 AT 1.2 60.2 4.7 2.1 0.80 2.2 29.2 0.37 Nanohardness of sheet surface Surface layer Standard Standard Amount of Metal Ratio of deviation of deviation of diffusible coating Hn of Hn at quarter Hn at half hydrogen Soft layer weight 7.0 GPa position position in steel thickness (g/m.sup.2) or more (GPa) (GPa) No. (ppm by mass) (m) (*4) (*5) (*6) (*7) Type Note 92 0.09 13 0.15 1.9 2.3 CR Inventive example 93 0.13 36 0.06 1.4 1.7 CR Inventive example 94 0.12 15 10.0 0.17 1.6 1.9 CR Inventive example 95 0.17 45 10.0 0.02 0.6 0.7 CR Inventive example 96 0.13 47 10.0 0.03 0.6 0.8 CR Inventive example 97 0.17 8 0.16 1.9 2.4 CR Inventive example 98 0.08 37 0.06 1.3 1.5 CR Inventive example 99 0.12 15 14.0 0.17 1.7 2.0 CR Inventive example 100 0.08 47 14.0 0.04 0.4 0.7 CR Inventive example 101 0.11 48 14.0 0.03 0.6 0.9 CR Inventive example 102 0.20 7 0.18 1.9 2.3 CR Inventive example 103 0.13 33 0.07 1.5 1.7 CR Inventive example 104 0.10 12 12.0 0.17 1.6 2.0 CR Inventive example 105 0.12 45 12.0 0.02 0.6 0.8 CR Inventive example 106 0.15 44 12.0 0.02 0.7 0.9 CR Inventive example 107 0.22 9 0.19 2.0 2.4 HR Inventive example 108 0.08 39 0.08 1.4 1.7 HR Inventive example 109 0.09 16 12.0 0.18 1.8 1.9 HR Inventive example 110 0.13 50 12.0 0.04 0.7 0.9 HR Inventive example 111 0.07 52 12.0 0.03 0.6 0.9 HR Inventive example 112 0.18 49 0.07 1.2 1.5 CR Inventive example 113 0.12 6 0.17 2.0 2.3 CR Inventive example 114 0.10 28 0.07 1.6 1.8 CR Inventive example 115 0.09 10 10.0 0.14 1.6 2.0 CR Inventive example 116 0.11 40 10.0 0.04 0.8 1.0 CR Inventive example 117 0.12 41 10.0 0.03 0.7 0.9 CR Inventive example (*1) F: ferrite, M: fresh martensite, RA: retained austenite, M: island-like fresh martensite, RA: island-like retained austenite, B: bainite, BT: tempered bainite, TM: tempered martensite, : carbide (*2) (M + RA)/(M + RA): the value obtained by dividing the total area fraction of island-like fresh martensite and island-like retained austenite in a ferrite grain by the sum of the area fraction of fresh martensite and the area fraction of retained austenite in the entire steel sheet (*3) The average grain size of M and RA: the average grain size of island-like fresh martensite and island-like retained austenite in a ferrite grain (*4) Metal coating weight (g/m.sup.2): first coating weight (g/m.sup.2) (*5) The ratio of the number of measurements with a nanohardness of 7.0 GPa or more to the total number of measurements of nanohardness at a quarter depth position in the thickness direction of a surface soft layer from the surface of a base steel sheet (*6) The standard deviation (GPa) of the nanohardness of a sheet surface at a quarter position in the thickness direction of a surface soft layer from the surface of a base steel sheet (*7) The standard deviation (Gpa) of the nanohardness of a sheet surface at a half position in the thickness direction of a surface soft layer from the surface of a base steel sheet

TABLE-US-00012 TABLE 7-2 Axial compression U-bending + VDA bending characteristics YS (MPa) YR El tight bending V-bending + SFmax (appearance No. (MPa) TS () (%) (%) R/t ST (mm) (mm) crack) Type Note 92 654 1038 0.63 17.2 32 1.67 3.5 27.1 Good CR Inventive example 93 692 1061 0.65 18.1 37 0.83 2.5 28.4 Excellent CR Inventive example 94 646 1028 0.63 17.4 33 1.25 3.0 28.1 Excellent CR Inventive example 95 682 1003 0.68 17.5 36 0.00 1.5 29.2 Excellent CR Inventive example 96 666 1015 0.66 17.8 32 0.00 1.5 28.9 Excellent CR Inventive example 97 552 796 0.69 22.8 38 0.83 2.0 29.0 Good CR Inventive example 98 578 793 0.73 21.3 41 0.42 1.0 29.9 Excellent CR Inventive example 99 565 834 0.68 21.1 42 0.42 1.5 29.6 Excellent CR Inventive example 100 595 833 0.71 21.4 39 0.00 0.0 30.5 Excellent CR Inventive example 101 563 841 0.67 20.9 42 0.00 0.0 30.3 Excellent CR Inventive example 102 712 1040 0.68 17.6 35 1.67 3.0 27.7 Good CR Inventive example 103 708 1062 0.67 16.9 43 0.83 2.0 28.3 Excellent CR Inventive example 104 691 1012 0.68 17.0 44 1.25 2.5 27.9 Excellent CR Inventive example 105 702 1021 0.69 17.4 46 0.00 1.0 29.1 Excellent CR Inventive example 106 689 1053 0.65 16.8 33 0.00 1.0 29.0 Excellent CR Inventive example 107 660 1038 0.64 19.2 42 1.41 4.0 26.8 Good HR Inventive example 108 672 1028 0.65 18.9 44 0.94 3.5 27.6 Excellent HR Inventive example 109 682 1024 0.67 19.4 38 1.09 4.0 27.2 Excellent HR Inventive example 110 700 1034 0.68 19.2 39 0.31 2.0 28.3 Excellent HR Inventive example 111 684 1039 0.66 19.1 36 0.31 2.0 28.2 Excellent HR Inventive example 112 694 1046 0.66 19.2 37 0.56 2.5 27.8 Excellent CR Inventive example 113 682 1033 0.66 16.5 34 1.67 3.0 27.5 Good CR Inventive example 114 674 1018 0.66 16.4 41 0.83 2.0 28.1 Excellent CR Inventive example 115 679 1028 0.66 16.8 38 1.25 2.5 27.7 Excellent CR Inventive example 116 683 1010 0.68 17.0 45 0.00 1.0 29.3 Excellent CR Inventive example 117 680 1009 0.67 16.8 37 0.00 1.0 29.1 Excellent CR Inventive example

[0449] As shown in Tables 3, 5, and 7, all the inventive examples passed all the tensile strength (TS), the yield stress (YS), the yield ratio (YR), the total elongation (El), the limiting hole expansion ratio (), R/t in the V-bending test, the critical spacer thickness (ST) in the U-bending+tight bending bending test, and the stroke at the maximum load (SFmax) measured in the V-bending+orthogonal VDA bending test, and had no fracture (appearance crack) in the axial compression test.

[0450] In contrast, the comparative examples were not satisfactory in at least one of the tensile strength (TS), the yield stress (YS), the yield ratio (YR), the total elongation (El), the limiting hole expansion ratio (), R/t in the V-bending test, the critical spacer thickness (ST) in the U-bending+tight bending bending test, the stroke at the maximum load (SFmax) measured in the V-bending+orthogonal VDA bending test, and the presence or absence of fracture (appearance crack) in the axial compression test. In Tables 5 and 7, at a dew point of 30 C. or more and 5 C. or less, although the surface layer has a soft layer thickness of 17 m or less and the fracture (appearance crack) in the axial compression test is rated as Good, even when the surface layer has a soft layer thickness of 17 m or less, in the presence of the metal coated layer, the fracture (appearance crack) in the axial compression test was rated as Excellent.

[0451] It was also found that the members produced by forming or joining the steel sheets of the inventive examples had good characteristics of aspects of the present invention in all of the tensile strength (TS), the yield stress (YS), the yield ratio (YR), the total elongation (El), the limiting hole expansion ratio (), R/t in the V-bending test, the critical spacer thickness (ST) in the U-bending+tight bending bending test, and the stroke at the maximum load (SFmax) measured in the V-bending+orthogonal VDA bending test, had no fracture (appearance crack) in the axial compression test, and had good characteristics of aspects of the present invention.

REFERENCE SIGNS LIST

[0452] 10 hat-shaped member [0453] 20 steel sheet [0454] 30 test member [0455] 40 spot weld [0456] 50 base plate [0457] 60 impactor [0458] A1 die [0459] A2 support roll [0460] A3 die [0461] A4 support roll [0462] B1 punch [0463] B2 punch [0464] B3 punch [0465] B4 punch [0466] D1 width (C) direction [0467] D2 rolling (L) direction [0468] D3 compression direction [0469] S spacer [0470] T1 test specimen [0471] F ferrite [0472] M martensite [0473] RA retained austenite [0474] M isolated island-like fresh martensite [0475] RA isolated island-like retained austenite [0476] B bainite [0477] BT tempered bainite [0478] TM tempered martensite

INDUSTRIAL APPLICABILITY

[0479] Aspects of the present invention enable the production of a steel sheet and a member with a TS of 780 MPa or more, high YS and YR, high press formability (ductility, flangeability, and bendability), and fracture resistance characteristics (bending fracture characteristics and axial compression characteristics) in case of a collision. A steel sheet and a member produced by a method according to aspects of the present invention can improve, for example, fuel efficiency due to the weight reduction of automobile bodies when used in automobile structural members and have significantly high industrial utility value.