STEEL SHEET, MEMBER, AND METHOD FOR PRODUCING THEM

Abstract

A steel sheet with a tensile strength (TS) of 780 MPa or more and less than 1180 MPa, a member, and a method for producing them. In a region of the steel sheet within 4.9 μm in the thickness direction, a region with a Si concentration not more than one-third of the Si concentration in the chemical composition of the steel sheet and with a Mn concentration not more than one-third of the Mn concentration in the chemical composition of the steel sheet has a thickness of 1.0 μm or more. The lowest Si concentration L.sub.Si and the lowest Mn concentration L.sub.Mn in the region within 4.9 μm from the surface of the steel sheet and a Si concentration T.sub.Si and a Mn concentration T.sub.Mn at a quarter thickness position of the steel sheet satisfy the following formula (1):

L.sub.Si+L.sub.Mn≤(T.sub.Si+T.sub.Mn)/4  (1).

Claims

1. A steel sheet having a chemical composition comprising, by mass %; Si: 0.20% to 2.00%; Mn: 1.00% or more and less than 2.70%; C: 0.120% to 0.400%; P: 0.001% to 0.100%; S: 0.0200% or less; Al: 0.010% to 2.000%; N: 0.0100% or less: optionally at least one selected from the group consisting of: Sb: 0.200% or less, and Sn: 0.200% or less: optionally at least one selected from the group consisting of: Ti: 0.200% or less, Nb: 0.200% or less, V: 0.100% or less, B: 0.0100% or less, Cu: 1.000% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 0.500% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.020% or less, Co: 0.020% or less, Zr: 0.020% or less, Ca: 0.0200% or less, Ce: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% 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 (except Ce): 0.0200% or less: optionally an equivalent carbon content Ceq in a range of 0.490% or more and less than 0.697%; the remainder being Fe and incidental impurities, wherein: the steel sheet has a microstructure including a ferrite area fraction in a range of 15% to 70%, a bainitic ferrite area fraction in a range of 3% to 25%, a tempered martensite area fraction in a range of 1% to 15%, and a retained austenite volume fraction in a range of 5% to 30%, in a region within 4.9 μm in a thickness direction from a surface of the steel sheet, a region with a Si concentration not more than one-third of the Si concentration in the chemical composition of the steel sheet and with a Mn concentration not more than one-third of the Mn concentration in the chemical composition of the steel sheet has a thickness of 1.0 Wm or more, the lowest Si concentration L.sub.Si and the lowest Mn concentration L.sub.Mn in the region within 4.9 μm in the thickness direction from the surface of the steel sheet and a Si concentration T.sub.Si and a Mn concentration T.sub.Mn at a quarter thickness position of the steel sheet satisfy the following formula (1):
L.sub.Si+L.sub.Mn(T.sub.Si+T.sub.Mn)/4  (1), the steel sheet has a tensile strength of 780 MPa or more and less than 1180 MPa, and optionally, an amount of diffusible hydrogen in the steel sheet is 0.50 ppm or less by mass.

2-4. (canceled)

5. The steel sheet according to claim 1, wherein: the steel sheet comprises a soft layer with a thickness in a range of 1.0 to 50.0 μm in the thickness direction, the soft layer being a region with hardness corresponding to 65% or less of the hardness at a quarter thickness position from the surface of the steel sheet, optionally crystal grains containing an oxide of Si and/or Mn in the region within 4.9 μm in the thickness direction from the surface of the steel sheet have an average grain size in a range of 1 to 15 μm, and optionally the Mn concentration L.sub.Mn and the Mn concentration T.sub.Mn satisfy the following formula (2):
L.sub.Mn≤T.sub.Mn/3  (2).

6-7. (canceled)

8. The steel sheet according to claim 1, wherein the steel sheet comprises a hot-dip galvanized layer or a hot-dip galvannealed layer on a surface of the steel sheet.

9-10. (canceled)

11. A member produced by performing at least one of forming and welding on the steel sheet according to claim 1.

12. A method for producing the sheet the steel sheet according to claim 1, the method comprising: a hot-rolling step of hot-rolling a steel slab with the chemical composition followed by coiling at a coiling temperature in a range of 450° C. to 750° C.; a cold-rolling step of holding the steel sheet after the hot-rolling step in a temperature range of 400° C. or more for 3600 seconds or more, pickling the steel sheet, and cold-rolling the steel sheet at a rolling reduction of 30% or more; a first annealing step of holding the steel sheet after the cold-rolling step in a first annealing temperature range of 820° C. or more for 20 seconds or more; a second annealing step of holding the steel sheet after the first annealing step in an atmosphere with a dew-point temperature of −35° C. or more in a second annealing temperature range of 740° C. to 900° C. for 20 seconds or more, cooling the steel sheet, at an average cooling rate of 8° C./s or more from the second annealing temperature range to 550° C., to a cooling stop temperature in a range of 150° C. to 300° C., and bending and unbending the steel sheet 3 to 15 times in total using a roller having a radius in a range of 100 to 1000 mm during the cooling from 740° C. to the cooling stop temperature; a reheating step of reheating the steel sheet after the second annealing step to a reheating temperature range of (the cooling stop temperature+50° C.) to 500° C. and holding the steel sheet in the reheating temperature range for 10 seconds or more; optionally a plating step of performing hot-dip galvanizing on the steel sheet after the reheating step or performing the hot-dip galvanizing followed by reheating to a temperature in a range of 450° C. to 600° C. and performing alloying treatment; and optionally a dehydrogenation step of holding the steel sheet at a temperature in a range of 50° C. to 300° C. for 0.5 to 72.0 hours after the reheating step.

13. (canceled)

14. A method for producing steel sheet according to claim 1, the method comprising: a hot-rolling step of hot-rolling a steel slab with the chemical composition followed by coiling at a coiling temperature in a range of 450° C. to 750° C.; a cold-rolling step of holding the steel sheet after the hot-rolling step in a temperature range of 400° C. or more for 3600 seconds or more, pickling the steel sheet, and cold-rolling the steel sheet at a rolling reduction of 30% or more; a first annealing step of holding the steel sheet after the cold-rolling step in a first annealing temperature range of 820° C. or more for 20 seconds or more; a second annealing step of holding the steel sheet after the first annealing step in an atmosphere with a dew-point temperature of −35° C. or more in a second annealing temperature range of 740° C. to 900° C. for 20 seconds or more, cooling the steel sheet, at an average cooling rate of 8° C./s or more from the second annealing temperature range to 550° C., to a first cooling stop temperature in a range of 350° C. to 500° C., and bending and unbending the steel sheet 3 to 15 times in total using a roller having a radius in a range of 100 to 1000 mm during the cooling from 740° C. to the first cooling stop temperature; a plating step of performing hot-dip galvanizing on the steel sheet after the second annealing step or performing the hot-dip galvanizing followed by reheating to a temperature in a range of 450° C. to 600° C. and performing alloying treatment; a reheating step of cooling the steel sheet after the plating step to a second cooling stop temperature in a range of 50° C. to 350° C., reheating the steel sheet to a reheating temperature exceeding the second cooling stop temperature and in a range of 300° C. to 500° C., and holding the reheating temperature for 10 seconds or more; and optionally a dehydrogenation step of holding the steel sheet at a temperature in a range of 50° C. to 300° C. for 0.5 to 72.0 hours after the reheating step.

15-16. (canceled)

17. A method for producing a member, the method comprising performing at least one of forming and welding on the steel sheet produced by the method for producing the steel sheet according claim 12.

18. A steel sheet having a chemical composition comprising, by mass %; Si: 0.20% to 2.00%; Mn: 1.00% or more and less than 2.70%; C: 0.120% to 0.400%; P: 0.001% to 0.100%; S: 0.0200% or less; Al: 0.010% to 2.000%; N: 0.0100% or less; optionally at least one selected from the group consisting of: Sb: 0.200% or less, and Sn: 0.200% or less: optionally at least one selected from the group consisting of: Ti: 0.200% or less, Nb: 0.200% or less, V: 0.100% or less, B: 0.0100% or less, Cu: 1.000% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 0.500% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.020% or less, Co: 0.020% or less, Zr: 0.020% or less, Ca: 0.0200% or less, Ce: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% 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 (except Ce): 0.0200% or less: optionally an equivalent carbon content Ceq in a range of 0.490% or more and less than 0.697%; the remainder being Fe and incidental impurities, wherein: the steel sheet has a steel microstructure including a ferrite area fraction in a range of 15% to 70%, a bainitic ferrite area fraction in a range of 3% to 25%, a tempered martensite area fraction in a range of 1% to 15%, and a retained austenite volume fraction in a range of 5% to 30%, in a region within 15.0 μm in a thickness direction from a surface of the steel sheet, a region with a Si concentration not more than one-third of the Si concentration in the chemical composition of the steel sheet and with a Mn concentration not more than one-third of the Mn concentration in the chemical composition of the steel sheet has a thickness of 1.0 μm or more, the lowest Si concentration L.sub.Si and the lowest Mn concentration L.sub.Mn in the region within 15.0 μm in the thickness direction from the surface of the steel sheet and a Si concentration T.sub.Si and a Mn concentration T.sub.Mn at a quarter thickness position of the steel sheet satisfy the following formula (1);
L.sub.Si+L.sub.Mn≤(T.sub.Si+T.sub.Mn)/4  (1), and the steel sheet has a tensile strength of 780 MPa or more and less than 1180 MPa, and optionally, an amount of diffusible hydrogen in the steel sheet is 0.50 ppm or less by mass.

19-21. (canceled)

22. The steel sheet according to claim 18, wherein: the steel sheet comprises a soft layer with a thickness in a range of 1.0 to 50.0 μm in the thickness direction, the soft layer being a region with hardness corresponding to 65% or less of the hardness at a quarter thickness position from the surface of the steel sheet, optionally crystal grains containing an oxide of Si and/or Mn in the region within 4.9 μm in the thickness direction from the surface of the steel sheet have an average grain size in a range of 1 to 15 Mm, and optionally the Mn concentration L.sub.Mn and the Mn concentration T.sub.Mn satisfy the following formula (2):
L.sub.Mn≤T.sub.Mn/3  (2).

23-24. (canceled)

25. The steel sheet according to claim 18, wherein the steel sheet comprises a hot-dip galvanized layer or a hot-dip galvannealed layer on a surface of the steel sheet.

26-27. (canceled)

28. A member produced by performing at least one of forming and welding on the steel sheet according to claim 18.

29. A method for producing the steel sheet according to claim 18, the method comprising: a hot-rolling step of hot-rolling a steel slab with the chemical composition followed by coiling at a coiling temperature in a range of 450° C. to 750° C.; a cold-rolling step of holding the steel sheet after the hot-rolling step in a temperature range of 400° C. or more for 3600 seconds or more, pickling the steel sheet, and cold-rolling the steel sheet at a rolling reduction of 30% or more; a first annealing step of holding the steel sheet after the cold-rolling step in a first annealing temperature range of 820° C. or more for 20 seconds or more; a second annealing step of holding the steel sheet after the first annealing step in an atmosphere with a dew-point temperature of −35° C. to 20° C. in a second annealing temperature range of 740° C. to 900° C. for 20 seconds or more, cooling the steel sheet, at an average cooling rate of 8° C./s or more from the second annealing temperature range to 550° C., to a cooling stop temperature in a range of 150° C. to 300° C., and bending and unbending the steel sheet 3 to 15 times in total using a roller having a radius in a range of 100 mm to 1000 mm during the cooling from 740° C. to the cooling stop temperature; a reheating step of reheating the steel sheet after the second annealing step to a reheating temperature range of (the cooling stop temperature+50° C.) to 500° C. and holding the steel sheet in the reheating temperature range for 10 seconds or more; optionally a plating step of performing hot-dip galvanizing on the steel sheet after the reheating step or performing the hot-dip galvanizing followed by reheating to a temperature in a range of 450° C. to 600° C. and performing alloying treatment; and optionally a dehydrogenation step of holding the steel sheet at a temperature in a range of 50° C. to 300° C. for 0.5 to 72.0 hours after the reheating step.

30. (canceled)

31. A method for producing the steel sheet according to claim 18, the method comprising: a hot-rolling step of hot-rolling a steel slab with the chemical composition followed by coiling at a coiling temperature in a range of 450° C. to 750° C.; a cold-rolling step of holding the steel sheet after the hot-rolling step in a temperature range of 400° C. or more for 3600 seconds or more, pickling the steel sheet, and cold-rolling the steel sheet at a rolling reduction of 30% or more; a first annealing step of holding the steel sheet after the cold-rolling step in a first annealing temperature range of 820° C. or more for 20 seconds or more; a second annealing step of holding the steel sheet after the first annealing step in an atmosphere with a dew-point temperature of −35° C. to 20° C. in a second annealing temperature range of 740° C. to 900° C. for 20 seconds or more, cooling the steel sheet, at an average cooling rate of 8° C./s or more from the second annealing temperature range to 550° C., to a first cooling stop temperature in a range of 350° C. to 500° C., and bending and unbending the steel sheet 3 to 15 times in total using a roller having a radius in a range of 100 to 1000 mm during the cooling from 740° C. to the first cooling stop temperature; a plating step of performing hot-dip galvanizing on the steel sheet after the second annealing step or performing the hot-dip galvanizing followed by reheating to a temperature range of 450° C. to 600° C. and performing alloying treatment; a reheating step of cooling the steel sheet after the plating step to a second cooling stop temperature in a range of 50° C. to 350° C., reheating the steel sheet to a reheating temperature exceeding the second cooling stop temperature and in a range of 300° C. to 500° C., and holding the reheating temperature for 10 seconds or more; and optionally a dehydrogenation step of holding the steel sheet at a temperature in a range of 50° C. to 300° C. for 0.5 to 72.0 hours after the reheating step.

32-33. (canceled)

34. A method for producing a member, the method comprising performing at least one of forming and welding on the steel sheet produced by the method for producing the steel sheet according to claim 29.

35. The steel sheet according to claim 5, wherein the steel sheet comprises a hot-dip galvanized layer or a hot-dip galvannealed layer on a surface of the steel sheet.

36. A member produced by performing at least one of forming and welding on the steel sheet according to claim 5.

37. A member produced by performing at least one of forming and welding on the steel sheet according to claim 8.

38. A member produced by performing at least one of forming and welding on the steel sheet according to claim 35.

39. A method for producing a member, the method comprising performing at least one of forming and welding on the steel sheet produced by the method for producing the steel sheet according to claim 14.

40. The steel sheet according to claim 22, wherein the steel sheet comprises a hot-dip galvanized layer or a hot-dip galvannealed layer on a surface of the steel sheet.

41. A member produced by performing at least one of forming and welding on the steel sheet according to claim 22.

42. A member produced by performing at least one of forming and welding on the steel sheet according to claim 25.

43. A member produced by performing at least one of forming and welding on the steel sheet according to claim 40.

44. A method for producing a member, the method comprising performing at least one of forming and welding on the steel sheet produced by the method for producing the steel sheet according to claim 31.

Description

EXAMPLES

Example 1

[0313] The disclosed embodiments are more specifically described with reference to examples. The scope of the disclosed embodiments is not limited to the following examples.

[0314] A steel material with the chemical composition listed in Table 1 and with the remainder composed of Fe and incidental impurities was obtained by steelmaking in a converter and was formed into a steel slab by a continuous casting method. The steel slab was heated to 1250° C. and was subjected to rough rolling. The steel was then subjected to finish rolling at a finish rolling temperature of 900° C. and was coiled at different coiling temperatures listed in Table 2 as a hot-rolled steel sheet. Under the conditions shown in Table 2, a cold-rolling step, a first annealing step, and a second annealing step were then performed to produce a cold-rolled steel sheet (CR).

[0315] As described below, a steel sheet was then produced through the production process according to a first embodiment or a second embodiment.

[0316] In the first embodiment, the second annealing step was followed by reheating treatment under the conditions shown in Table 2. Some of the steel sheets were then subjected to plating treatment under the conditions shown in Table 2. Some of the steel sheets were then subjected to dehydrogenation under the conditions shown in Table 2 to produce steel sheets.

[0317] In the second embodiment, the second annealing step was followed by plating treatment under the conditions shown in Table 2. Reheating treatment was then performed under the conditions shown in Table 2 to produce a steel sheet.

[0318] In a working example of the first embodiment, the cooling stop temperature after annealing in the second annealing step ranges from 150° C. to 300° C., as shown in Table 2. In a working example of the second embodiment, the cooling stop temperature after annealing in the second annealing step ranges from 350° C. to 500° C.

[0319] In the plating step, a cold-rolled steel sheet was subjected to plating treatment to produce a hot-dip galvanized steel sheet (GI) or a hot-dip galvannealed steel sheet (GA). To produce GI, the hot-dip galvanizing bath was a zinc bath containing Al: 0.20% by mass and the remainder composed of Zn and incidental impurities. To produce GA, a zinc bath containing Al: 0.141 by mass and the remainder composed of Zn and incidental impurities was used. The bath temperature was 470° C. for both GI and GA production. The coating weight ranged from approximately 45 to 72 g/m2 per side (plating on both sides) to produce GI and was approximately 45 g/m2 per side (plating on both sides) to produce GA. Alloying treatment to produce GA was performed at the temperatures shown in Table 2. The composition of the coated layer of GI contained Fe: 0.1% to 1.0% by mass, Al: 0.21 to 1.0% by mass, and the remainder composed of Zn and incidental impurities. The composition of the coated layer of GA contained Fe: 7% to 15% by mass, Al: 0.1% to 1.0% by mass, and the remainder composed of Zn and incidental impurities.

TABLE-US-00001 TABLE 1 Steel Chemical composition (mass %) type C Si Mn P S Al N Sb Sn Others Ceq(%) A 0.132 1.43 2.09 0.009 0.0010 0.034 0.0032 — — — 0.540 B 0.124 0.84 2.21 0.010 0.0009 0.030 0.0027 — — — 0.527 C 0.129 1.38 2.12 0.014 0.0012 0.029 0.0029 0.0090 — — 0.540 D 0.162 1.24 2.34 0.012 0.0013 0.029 0.0031 — 0.0080 — 0.604 E 0.205 1.45 2.25 0.007 0.0012 0.042 0.0034 — — — 0.640 F 0.189 0.89 2.52 0.009 0.0014 0.038 0.0033 — — — 0.646 G 0.212 1.35 2.18 0.006 0.0018 0.045 0.0030 0.0080 — — 0.632 H 0.225 1.40 2.20 0.010 0.0015 0.042 0.0025 — 0.0070 — 0.650 I 0.195 1.62 2.31 0.008 0.0017 0.027 0.0029 0.0050 0.0040 — 0.648 J 0.182 1.12 1.89 0.012 0.0007 0.755 0.0040 — — — 0.544 M 0.187 0.05 2.29 0.006 0.0013 0.042 0.0045 — — — 0.571 N 0.146 2.80 2.43 0.012 0.0015 0.036 0.0034 — — — 0.668 O 0.230 1.49 0.75 0.018 0.0023 0.028 0.0028 — — — 0.417 Q 0.194 1.02 2.19 0.009 0.0009 0.032 0.0034 — — Ti: 0.032 0.602 S 0.232 1.05 2.08 0.015 0.0018 0.048 0.0036 — — Nb: 0.028 0.622 T 0.278 1.56 1.72 0.012 0.0037 0.022 0.0028 0.0120 — V: 0.035 0.633 U 0.225 1.15 2.34 0.021 0.0028 0.040 0.0032 — — Ti: 0.021, B: 0.0018 0.663 V 0.341 0.78 1.75 0.013 0.0012 0.045 0.0038 — 0.0080 Cu: 0.250 0.665 W 0.182 1.32 2.09 0.015 0.0019 0.035 0.0044 0.0090 — Cr: 0.325 0.667 X 0.130 1.93 2.61 0.012 0.0036 0.024 0.0022 — — Ni: 0.505 0.661 Y 0.235 1.18 2.02 0.020 0.0024 0.031 0.0019 — — Mo: 0.250 0.696 Z 0.385 0.49 1.42 0.016 0.0022 0.042 0.0022 0.0080 — W: 0.020 0.642 AA 0.196 1.48 2.13 0.005 0.0028 0.033 0.0052 — — Ta: 0.008 0.613 AB 0.158 1.78 2.34 0.012 0.0058 0.025 0.0026 — — Pb: 0.0060 0.622 AC 0.252 0.95 2.11 0.014 0.0022 0.032 0.0025 — 0.0070 Bi: 0.0020 0.643 AD 0.168 1.38 2.43 0.019 0.0015 0.034 0.0046 — — Se: 0.0100 0.631 AE 0.189 1.59 2.02 0.025 0.0012 0.050 0.0048 0.0250 — Te: 0.0150 0.592 AF 0.215 0.87 1.87 0.015 0.0019 0.034 0.0041 — 0.0040 Ge: 0.0130 0.563 AG 0.182 1.21 2.34 0.011 0.0030 0.022 0.0026 — — As: 0.0120 0.622 AH 0.210 1.34 2.25 0.038 0.0034 0.035 0.0062 0.0090 0.0060 Sr: 0.0080 0.641 AI 0.175 1.84 2.15 0.017 0.0025 0.031 0.0034 — 0.0260 Cs: 0.0070 0.610 AJ 0.195 1.65 1.95 0.011 0.0041 0.052 0.0032 — — Zn: 0.005 0.589 AK 0.245 1.79 1.84 0.042 0.0012 0.037 0.0044 0.0050 0.0030 Co: 0.008 0.626 AL 0.192 1.52 1.98 0.012 0.0021 0.031 0.0030 — — Ca: 0.0020 0.585 AM 0.256 1.74 1.65 0.019 0.0008 0.051 0.0028 — 0.0090 Ce: 0.0012 0.604 AN 0.159 1.32 2.22 0.031 0.0005 0.036 0.0041 0.0030 — Mg: 0.0040 0.584 AO 0.195 1.25 2.30 0.010 0.0037 0.031 0.0034 0.0080 — Zr: 0.0030 0.630 AP 0.189 1.15 2.12 0.008 0.0019 0.018 0.0022 — — Hf: 0.0050 0.590 AQ 0.245 1.64 1.98 0.009 0.0022 0.032 0.0039 0.0110 0.0020 REM: 0.0030 0.643 AS 0.149 0.91 2.08 0.010 0.0010 0.030 0.0028 — — — 0.534 AT 0.194 0.84 2.65 0.012 0.0008 0.032 0.0031 — — — 0.671 AU 0.203 0.87 2.58 0.009 0.0012 0.028 0.0035 0.0070 — — 0.669 AV 0.168 1.62 2.32 0.009 0.0009 0.032 0.0034 — — Ti: 0.022, Nb: 0.025 0.622 AW 0.198 1.12 2.01 0.008 0.0010 0.030 0.0030 — — Cr: 0.102, Ca: 0.0028 0.580 AX 0.207 0.88 2.59 0.009 0.0008 0.028 0.0038 0.0060 — Nb: 0.030, Zr: 0.0025 0.675 “—” indicates the content of the inevitable impurity level.

TABLE-US-00002 TABLE 2-1 Second annealing step Hot-rolling Cold-rolling step Average step Holding First annealing step cooling Cooling Coiling time at 400° Rolling Annealing Holding Annealing Holding Dew-point rate up stop Steel temperature C. or more reduction temperature time temperature time temperature to 550° C. temperature No. type (° C.) (seconds) (%) (° C.) (seconds) (° C.) (seconds) (° C.) (° C./s) (° C.) 1 A 500 7400 62.5 840 200 800 180 −5 12 200 2 B 480 6800 60.0 850 300 780 250 −3 10 480 3 C 520 8000 61.1 830 150 790 150 −5 11 400 4 D 540 8200 66.7 860 250 810 300 −4 14 200 5 E 600 6600 54.7 850 200 800 150 −3 12 150 6 E 350 8000 61.1 830 300 800 230 −11 10 190 7 E 500 600 61.1 900 350 780 180 −10 10 180 9 E 530 8000 66.7 700 200 790 270 −11 9 220 10 E 550 8800 61.1 840 5 800 160 −4 11 460 11 E 500 7000 60.0 870 150 650 210 −7 15 390 12 E 490 6800 55.6 840 150 950 200 −8 14 190 13 E 530 7200 66.7 830 200 820 3 −5 12 220 14 E 540 7600 55.6 840 250 800 150 −50 10 430 15 E 500 7800 55.6 890 200 810 200 −9 3 400 16 E 600 6800 64.7 850 200 820 250 −6 13 50 18 E 480 6000 64.7 870 180 790 150 −10 15 380 19 E 520 5800 61.1 840 150 780 300 −7 10 720 20 E 560 6200 64.7 870 200 800 180 −9 13 190 22 E 500 7200 53.8 870 200 810 180 −4 11 470 24 E 500 7400 57.9 840 200 800 140 −3 15 450 25 E 500 6200 47.1 860 300 780 190 −6 18 400 26 F 520 7600 55.6 890 200 800 160 −2 12 200 27 G 500 7200 56.3 860 220 790 180 −6 15 380 28 H 460 5900 60.0 840 150 810 160 −9 13 420 29 I 490 6600 58.8 880 180 780 220 −4 10 200 30 J 520 7400 58.8 900 250 800 210 −7 11 190 33 M 530 7800 58.8 850 180 820 210 −4 18 480 34 N 520 7200 46.2 870 210 800 270 −6 12 420 35 O 530 6800 62.5 880 270 810 160 −8 10 450 37 Q 650 10000 61.1 890 200 800 120 −20 13 200 38 S 580 9000 58.8 850 150 790 200 −2 14 210 39 T 500 7500 56.3 860 170 810 220 −24 12 230 40 U 480 6000 57.1 880 200 780 250 −18 25 200 41 V 630 8800 50.0 900 220 820 80 −6 10 230 Second annealing step Bending and Reheating step Reheating step unbending (first embodiment) Plating step Dehydrogenation step (second embodiment) Roller Reheating Holding Alloying Processing Holding Cooling stop Reheating Holding radius Count temperature time temperature temperature time temperature temperature time No. (mm) (times) (° C.) (seconds) Type* (° C.) (° C.) (hours) (° C.) (° C.) (seconds) 1 450 8 410 60 GA 490 100 10 2 500 9 GI 200 400 60 3 550 7 GA 500 180 420 100 4 450 10 420 300 CR 5 550 9 380 100 GA 500 80 30 6 600 7 400 100 GA 500 100 8 7 400 9 390 80 GA 520 9 550 5 400 120 GA 530 80 36 10 650 7 GI 220 350 60 11 450 5 GA 510 190 400 120 12 350 9 410 100 GA 520 100 24 13 400 10 420 600 GA 510 90 30 14 700 11 GA 510 200 400 200 15 650 8 GA 490 230 420 100 16 500 9 380 100 GA 500 80 30 18 450 1 GA 510 170 380 80 19 500 9 550 60 GA 500 100 10 20 650 10 410 3 GA 500 120 7 22 450 11 GA 500 30 400 100 24 550 8 GA 500 200 600 80 25 600 7 GA 490 200 400 4 26 400 10 400 60 GA 500 100 12 27 700 8 GA 520 210 420 60 28 500 9 GI 220 380 90 29 450 9 390 350 CR 30 400 12 400 100 GA 490 33 450 7 GA 510 200 380 120 34 650 8 GA 520 190 400 150 35 700 9 GI 200 350 90 37 450 5 380 80 GI 120 9 38 600 8 410 60 GA 530 100 10 39 550 12 430 700 CR 40 700 14 380 60 GA 480 41 550 7 410 100 GA 490 100 10 *CR: cold-rolled steel sheet (without plating), GI: hot-dip galvanized steel sheet, GA: hot-dip galvannealed steel sheet

TABLE-US-00003 TABLE 2-2 Second annealing step Hot-rolling Cold-rolling step Average step Holding First annealing step cooling Cooling Coiling time at 400° Rolling Annealing Holding Annealing Holding Dew-point rate up stop Steel temperature C. or more reduction temperature time temperature time temperature to 550° C. temperature No. type (° C.) (seconds) (%) (° C.) (seconds) (° C.) (seconds) (° C.) (° C./s) (° C.) 42 W 520 7400 46.2 910 350 800 200 −6 9 180 43 X 620 8000 64.7 950 150 810 230 −9 17 390 44 Y 550 7800 62.5 880 120 820 360 −14 15 490 45 Z 590 8200 64.7 850 600 800 270 −19 10 270 46 AA 710 10400 50.0 840 180 800 250 −4 11 190 47 AB 540 7200 53.8 860 250 790 180 −8 14 450 48 AC 600 8000 52.9 840 150 810 200 −9 42 400 49 AC 620 8300 47.1 850 300 780 170 −15 15 210 50 AE 550 7700 55.5 830 120 810 270 −13 13 190 51 AF 520 6900 56.3 860 200 800 250 −32 10 430 52 AG 590 8200 76.5 880 220 780 180 −5 11 400 53 AH 470 6000 71.4 890 250 790 200 −8 10 220 54 AI 650 8300 50.0 900 190 810 140 −9 38 200 55 AJ 600 7500 56.3 850 90 800 200 −13 14 480 56 AK 520 7400 52.6 860 580 780 450 −7 12 490 57 AL 500 7000 37.5 880 220 770 200 −4 15 210 58 AM 550 7600 47.1 890 150 750 180 −3 14 220 59 AN 500 5900 55.6 860 180 800 200 −9 30 400 60 AO 600 8100 56.3 860 200 890 210 −19 11 170 61 AP 570 6700 50.0 880 300 830 380 −26 12 210 62 AQ 540 7000 56.3 850 170 800 160 −8 11 390 64 AS 560 6800 61.1 860 150 810 220 −16 14 210 65 AS 530 7800 60.0 840 200 790 160 −14 13 490 66 AT 580 8200 61.1 850 180 770 200 −20 11 200 67 AT 560 8000 62.5 870 250 780 180 −12 13 500 68 AT 500 7400 60.0 850 220 780 150 −16 12 200 69 AT 570 7600 62.5 860 200 770 200 −15 15 480 70 AU 580 7800 61.1 850 150 780 250 −13 12 180 71 AU 530 7600 50.0 860 200 780 150 −19 13 500 72 AU 550 8000 62.5 850 250 770 200 −17 11 220 73 AU 500 7600 61.1 840 200 780 230 −15 12 480 74 AV 560 7500 62.5 840 200 810 200 −5 12 400 75 AW 500 5000 53.8 860 220 800 250 −14 12 400 76 AX 570 8000 58.8 850 180 790 220 −6 12 400 Second annealing step Bending and Reheating step Reheating step unbending (first embodiment) Plating step Dehydrogenation step (second embodiment) Roller Reheating Holding Alloying Processing Holding Cooling stop Reheating Holding radius Count temperature time temperature temperature time temperature temperature time No. (mm) (times) (° C.) (seconds) Type* (° C.) (° C.) (hours) (° C.) (° C.) (seconds) 42 450 9 400 200 GA 500 43 400 6 GA 520 90 380 200 44 500 5 GA 500 180 420 180 45 600 10 400 50 GA 530 100 10 46 450 8 370 150 GA 490 47 400 9 GA 510 180 400 60 48 500 7 GA 500 200 410 120 49 450 10 380 60 GA 490 70 50 50 500 9 360 120 GA 550 51 550 6 GI 220 380 90 52 450 12 GA 520 200 400 30 53 550 8 470 90 GA 490 90 15 54 400 9 400 600 CR 55 300 6 GA 500 170 350 400 56 400 7 GA 500 210 400 100 57 450 15 380 100 GA 490 58 750 7 410 80 GA 520 110 10 59 500 8 GI 320 470 200 60 550 9 290 90 GA 490 100 12 61 450 10 350 200 GA 520 250 1 62 400 8 GA 500 200 420 100 64 500 6 420 100 GA 490 80 20 65 450 8 GA 180 380 50 66 450 7 410 60 GA 490 70 30 67 400 9 GA 200 360 100 68 500 6 420 40 GI 500 120 5 69 450 7 GI 190 400 40 70 450 6 390 50 GA 490 80 20 71 500 8 GA 210 380 80 72 450 6 400 60 GI 500 100 10 73 500 7 GI 200 400 60 74 500 7 GA 530 190 400 60 75 450 8 GA 530 200 380 100 76 400 9 GA 500 220 400 80 *CR: cold-rolled steel sheet (without plating), GI: hot-dip galvanized steel sheet, GA: hot-dip galvannealed steel sheet

[0320] For the steel sheets and the coated steel sheets used as test steels, tensile properties, stretch-flangeability (hole expandability), LME resistance, and fatigue properties were evaluated in accordance with the following test methods. The ferrite area fraction, the tempered martensite area fraction, the bainitic ferrite area fraction, and the retained austenite volume fraction of each steel sheet were measured by the following methods. The Si concentration and Mn concentration were measured by the following methods in a region within 4.9 μm in the thickness direction from the surface of the steel sheet and at a quarter thickness position of the steel sheet. The thickness of a soft layer present in the thickness direction from the surface of the steel sheet, the average grain size of crystal grains containing an oxide of Si and/or Mn in the region within 4.9 μm in the thickness direction from the surface of the steel sheet, and the amount of diffusible hydrogen in the steel sheet were also measured by the methods described above. Table 3 shows the results.

[0321] The ferrite, bainitic ferrite, and tempered martensite area fractions are measured by the following method. The area fractions were measured at a quarter thickness position. A sample was cut such that a cross section of a steel sheet in the thickness direction parallel to the rolling direction became an observation surface. The observation surface was then mirror-polished with a diamond paste, was finally polished with colloidal silica, and was etched with 3% by volume nital to expose a microstructure. Three fields in a 17 μm×23 μm field range were observed with a scanning electron microscope (SEM) at an accelerating voltage of 15 kV and at a magnification of 5000 times. In a microstructure image thus taken, the area fraction was calculated by dividing the area of each constituent microstructure (ferrite, bainitic ferrite, tempered martensite) by the measurement area in three fields using Adobe Photoshop available from Adobe Systems, and the area fractions were averaged to determine the area fraction of each microstructure.

[0322] The retained austenite volume fraction is measured by the following method. A steel sheet was mechanically ground in the thickness direction (depth direction) to a quarter thickness and was then chemically polished with oxalic acid to form an observation surface. The observation surface was observed by X-ray diffractometry. A Mo Kα radiation source was used for incident X-rays. The ratio of the diffraction intensities of (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensities of (200), (211), and (220) planes of bcc iron was determined as the retained austenite volume fraction.

[0323] The other steel sheet microstructure (remaining microstructure) may be determined by SEM observation, for example.

[0324] The Si concentration T.sub.Si and the Mn concentration T.sub.Mn at a quarter thickness position of the steel sheet were determined with a field emission-electron probe micro analyzer (FE-EPMA) from the average of 10 points of point analysis at an electron beam diameter of 1 μm at a quarter thickness position of the steel sheet. For the Si concentration in the region within 4.9 μm in the thickness direction from the surface of the steel sheet, the concentration distribution of the Si concentration in the range of 0 to 4.9 μm in the thickness direction from the surface of the steel sheet was determined by line analysis with a field emission-electron probe micro analyzer at an electron beam diameter of 0.1 μm from the surface of the steel sheet in the thickness direction. The lowest Si concentration in the concentration distribution is defined as the concentration L.sub.Si. Also for the Mn concentration in the region within 4.9 μm in the thickness direction from the surface of the steel sheet, the concentration distribution of the Mn concentration in the range of 0 to 4.9 μm in the thickness direction from the surface of the steel sheet was determined by line analysis with a field emission-electron probe micro analyzer at an electron beam diameter of 0.1 μm from the surface of the steel sheet in the thickness direction. The lowest Mn concentration in the concentration distribution is defined as the concentration L.sub.Mn. The Si concentration, the Mn concentration, L.sub.Si, T.sub.Si, L.sub.Mn, and T.sub.Mn are expressed in % by mass. In the measurement of the Si concentration and the Mn concentration with a field emission-electron probe micro analyzer in the disclosed embodiments, 10 positions without particulate matter were measured and averaged as the Si concentration and the Mn concentration.

[0325] SEM observation and energy dispersive X-ray analysis (EDX) on a cross section (L cross section) of a steel sheet were performed to determine the type of oxide in the region within 4.9 μm in the thickness direction from the surface of the steel sheet and to measure the average grain size of crystal grains containing an oxide of Si and/or Mn. The average grain size of crystal grains is the average length of grain sizes measured by microtomy in a cross section (L cross section) of a steel sheet in a direction parallel to the surface of the steel sheet.

[0326] Measurement is performed on the soft layer as described below. After smoothing a thickness cross section (L cross section: a cross section parallel to the rolling direction and perpendicular to the surface of the steel sheet) parallel to the rolling direction of the steel sheet by wet grinding, measurement was performed with a Vickers hardness tester at a load of 10 gf from a 1-μm position to a 100-μm position in the thickness direction from the surface of the steel sheet at intervals of 1 μm. Measurement was then performed at intervals of 20 μm to the central portion in the thickness direction. A region with hardness corresponding to 65% or less of the hardness at a quarter thickness position is defined as a soft layer, and the thickness of the region in the thickness direction is defined as the thickness of the soft layer.

[0327] The amount of diffusible hydrogen in a steel sheet is measured by the following method. For a cold-rolled steel sheet, a test specimen 30 mm in length and 5 mm in width was taken. For a steel sheet with a hot-dip galvanized layer or a hot-dip galvannealed layer on its surface, a test specimen 30 mm in length and 5 mm in width was taken, and the hot-dip galvanized layer or hot-dip galvannealed layer was removed with alkali. The amount of hydrogen released from the test specimen was then measured by a temperature-programmed desorption analysis method. More specifically, a test specimen is continuously heated from room temperature 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.

[0328] <Tensile Properties>

[0329] The tensile test was performed in accordance with JIS Z 2241. 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 steel sheet. The YS, TS, and total elongation (El) of the test specimen were measured at a crosshead speed of 10 mm/min in the tensile test. In the disclosed embodiments, TS of 780 MPa or more and YS and El satisfying the following conditions were judged to be acceptable.

[0330] If 780 MPa≤TS<980 MPa, then 420 MPa≤YS, and 22%≤El

[0331] If 980 MPa≤TS, then 560 MPa≤YS, and 19%≤El

[0332] <Stretch-Flangeability (Hole Expandability)>

[0333] The stretch-flangeability (hole expandability) was evaluated in a hole expanding test. The hole expanding test was performed in accordance with JIS Z 2256. A 100 mm×100 mm sample was taken by shearing from the steel sheet. A hole with a diameter of 10 mm was punched in the sample with a clearance of 12.5%. While the periphery of the hole was held using a die with an inner diameter of 75 mm at a blank holding force of 9 ton (88.26 kN), the hole diameter at the crack initiation limit was measured by pushing a conical punch with a vertex angle of 60 degrees into the hole. The critical hole expansion ratio λ (%) was calculated using the following formula, and the hole expandability was evaluated from the critical hole expansion ratio.

λ(%)={(D.sub.f−D.sub.0)/D.sub.0}×100

[0334] In this formula, D.sub.f denotes the hole diameter (mm) at the time of cracking, and D.sub.0 denotes the initial hole diameter (mm). In the disclosed embodiments, when TS and λ satisfied the following conditions, the stretch-flangeability was judged to be good.

[0335] If 780 MPa≤TS<980 MPa, then 30%≤λ

[0336] If 980 MPa≤TS, then 20%≤λ

[0337] <LME Resistance>

[0338] The LME resistance was determined by a resistance welding cracking test. A test specimen of a steel sheet cut to 30 mm×100 mm in a longitudinal direction perpendicular to the rolling direction and another test specimen made of a 980 MPa grade hot-dip galvanized steel sheet were subjected to resistance welding (spot welding) to produce a member. A set of the two steel sheets tilted 5 degrees was subjected to resistance spot welding in a servomotor pressurization type single-phase alternating current (50 Hz) resistance welding machine attached to a welding gun. The welding conditions included a welding pressure of 3.8 kN and a holding time of 0.2 seconds. The welding current ranged from 5.7 to 6.2 kA, the weld time was 21 cycles, and the holding time was 5 cycles. A test specimen was cut in half from the welded member, and a cross section was observed with an optical microscope. A test specimen with no crack of 0.02 mm or more was judged to be very good LME cracking (⊙), a test specimen with a crack of 0.02 mm or more and less than 0.1 mm was judged to be good LME cracking (0), and a test specimen with a crack of 0.1 mm or more was judged to be poor LME cracking (X).

[0339] <Fatigue Properties>

[0340] Fatigue properties were evaluated in terms of fatigue limit strength and endurance ratio in an alternating plane bending fatigue test according to JIS Z 2275 (1978). A No. 1 test specimen with a bend radius R of 30.4 mm in a stress loading portion and with a minimum width of 20 mm was used as a test specimen for the fatigue test. In the alternating plane bending fatigue test, a load was applied to a cantilever at a frequency of 20 Hz and at a stress ratio of −1, and stress with a number of cycles of more than 107 was defined as fatigue limit strength. A value obtained by dividing the fatigue limit strength by the tensile strength (TS) was defined as the endurance ratio.

[0341] In the disclosed embodiments, when a crack of 0.02 mm or more was not observed in the evaluation of LME resistance described later, a steel sheet satisfying 300 S fatigue limit strength and 0.30 S endurance ratio was judged to have good fatigue properties. When a crack occurred but was 0.02 mm or more and less than 0.1 mm in the evaluation of LME resistance described later, a steel sheet with TS, fatigue limit strength, and an endurance ratio satisfying the following was judged to have good fatigue properties.

[0342] If 780 MPa≤TS<980 MPa, then 330 MPa≤fatigue limit strength and 0.40≤endurance ratio

[0343] If 980 MPa≤TS, then 400 MPa≤fatigue limit strength and 0.40≤endurance ratio

[0344] Table 3 shows the results.

TABLE-US-00004 TABLE 3-1 F BF TM RA Thickness area area area volume Remaining of soft Steal Thickness fraction fraction fraction fraction micro- *1 L.sub.Si L.sub.Mn T.sub.Si T.sub.Mn layer No. Type (mm) (%) (%) (%) (%) structure (μm) (mass %) (mass %) (mass %) (mass %) *2 (μm) 1 A 1.2 61 15 5 11 M, θ 3.3 0.29 0.32 1.43 2.09 5.8 42.0 2 B 1.6 66 18 2 10 M, θ 4.2 0.17 0.40 0.84 2.21 5.4 38.0 3 C 1.4 60 17 3 13 M, θ, P 3.0 0.31 0.36 1.38 2.12 5.2 32.0 4 D 1.2 58 17 3 15 M, θ 3.9 0.22 0.43 1.24 2.34 5.5 10.0 5 E 1.2 45 18 7 16 M, θ 3.1 0.31 0.39 1.45 2.25 5.3 12.0 6 E 1.4 46 15 10 12 M, θ, P 0.7 0.38 0.52 1.45 2.25 4.1 11.0 7 E 1.4 49 18 7 11 M, θ 0.8 0.37 0.50 1.45 2.25 4.3 9.0 9 E 1.2 71 6 2 6 M, θ, P 2.3 0.35 0.54 1.45 2.25 4.2 22.0 10 E 1.4 73 4 2 7 M, θ 2.9 0.37 0.51 1.45 2.25 4.2 29.0 11 E 1.2 74 5 0 3 M, θ 3.0 0.34 0.52 1.45 2.25 4.3 40.0 12 E 1.6 13 24 42 2 M, θ, P 2.5 0.37 0.47 1.45 2.25 4.4 32.0 13 E 1.2 13 22 44 3 M, θ 3.1 0.36 0.52 1.45 2.25 4.2 28.0 14 E 1.6 44 17 8 14 M, θ, P 0.5 0.58 0.79 1.45 2.25 2.7 2.0 15 E 1.6 72 8 7 6 M, θ 2.8 0.38 0.53 1.45 2.25 4.1 23.0 16 E 1.2 46 20 29 2 M, θ, P 2.5 0.31 0.52 1.45 2.25 4.5 32.0 18 E 1.2 46 16 9 9 M, θ 0.3 0.62 0.83 1.45 2.25 2.6 1.0 19 E 1.4 45 10 40 3 M, θ, P 2.6 0.33 0.52 1.45 2.25 4.4 25.0 20 E 1.2 44 8 7 4 M, θ, P 2.4 0.38 0.52 1.45 2.25 4.1 21.0 22 E 1.2 43 20 32 2 M, θ, P 3.3 0.30 0.50 1.45 2.25 4.6 43.0 24 E 1.6 42 13 38 2 M, θ, P 3.4 0.31 0.48 1.45 2.25 4.7 46.0 25 E 1.8 41 9 8 3 M, θ, P 2.8 0.38 0.47 1.45 2.25 4.4 26.0 26 F 1.6 49 19 6 15 M, θ 3.1 0.24 0.42 0.89 2.52 5.2 41.0 27 G 1.4 43 20 7 17 M, θ, P 2.8 0.30 0.41 1.35 2.18 5.0 8.0 28 H 1.2 40 22 10 16 M, θ 2.5 0.28 0.45 1.40 2.20 4.9 10.0 29 I 1.4 43 18 6 18 M, θ 3.2 0.30 0.48 1.62 2.31 5.0 9.0 30 J 1.4 50 16 9 16 M, θ 2.7 0.22 0.33 1.12 1.89 5.5 32.0 33 M 1.4 51 19 9 2 M, θ, P 0.7 0.02 0.35 0.05 2.29 6.3 18.0 34 N 1.4 55 19 5 16 M, θ 3.1 0.57 0.42 2.80 2.43 5.3 35.0 35 O 1.2 74 7 2 4 M, θ, P 3.5 0.30 0.23 1.49 0.75 4.2 32.0 37 Q 1.4 48 15 12 15 M, θ 3.5 0.25 0.31 1.02 2.19 5.7 32.0 38 S 1.4 52 12 8 16 M, θ 3.2 0.22 0.33 1.05 2.08 5.7 28.0 39 T 1.4 49 17 14 14 M, θ 3.1 0.30 0.28 1.56 1.72 5.7 10.0 40 U 1.2 47 19 12 13 M, θ 2.8 0.22 0.33 1.15 2.34 6.3 43.0 41 V 1.4 43 13 13 17 M, θ, P 2.5 0.12 0.26 0.78 1.75 6.7 8.0 Average grain size of crystal Amount of grains containing diffusible Fatigue oxide of hydrogen limit Si and/or Mn (ppm by TS YS El λ strength Endurance LME No. (μm) T.sub.Mn/L.sub.Mn mass) (MPa) (MPa) (%) (%) (MPa) ratio resistance Type* Notes 1 9.2 6.53 0.05 845 482 28.4 36 360 0.43 ◯ GA Working example 2 10.1 5.53 0.09 798 467 30.2 40 345 0.43 ◯ GI Working example 3 7.4 5.89 0.04 820 475 29.3 38 360 0.44 ◯ GA Working example 4 7.9 5.44 0.03 825 452 30.7 34 375 0.45 ◯ CR Working example 5 8.1 5.77 0.06 1020 695 24.4 29 435 0.43 ◯ GA Working example 6 5.2 4.33 0.07 1108 728 19.8 26 435 0.39 X GA Comparative example 7 5.8 4.50 0.37 1089 715 20.2 21 435 0.40 X GA Comparative example 9 8.7 4.17 0.04 985 500 19.5 17 435 0.44 ◯ GA Comparative example 10 6.8 4.41 0.20 995 530 19.3 16 435 0.44 ◯ GI Comparative example 11 10.8 4.33 0.09 745 390 22.5 42 435 0.58 ◯ GA Comparative example 12 7.9 4.79 0.26 1054 645 15.5 38 435 0.41 ◯ GA Comparative example 13 7.2 4.33 0.42 1087 669 13.9 13 445 0.41 ◯ GA Comparative example 14 4.2 2.85 0.24 1015 655 21.3 24 420 0.41 X GA Comparative example 15 5.1 4.25 0.11 1002 540 19.3 43 400 0.40 ◯ GA Comparative example 16 3.9 4.33 0.08 1054 793 12.8 46 430 0.41 ◯ GA Comparative example 18 3.7 2.71 0.10 1045 694 21.4 24 430 0.41 X GA Comparative example 19 5.4 4.33 0.05 755 685 81.2 62 330 0.44 ◯ GA Comparative example 20 6.3 4.33 0.07 1095 535 17.4 16 445 0.41 ◯ GA Comparative example 22 3.3 4.50 0.06 1063 787 12.3 43 430 0.40 ◯ GA Comparative example 24 5.2 4.69 0.05 760 658 17.9 60 330 0.43 ◯ GA Comparative example 25 6.7 4.79 0.07 1076 520 17.0 10 440 0.41 ◯ GA Comparative example 26 6.5 6.00 0.04 1035 705 23.9 28 430 0.42 ◯ GA Working example 27 3.2 5.32 0.07 1005 674 23.2 30 450 0.45 ◯ GA Working example 28 5.4 4.89 0.05 1065 665 24.4 31 460 0.43 ◯ GI Working example 29 4.4 4.81 0.02 995 508 25.2 30 440 0.44 ◯ CR Working example 30 6.3 5.73 0.34 1008 638 25.6 33 435 0.43 ◯ GA Working example 33 5.7 6.54 0.02 1085 674 12.6 43 440 0.41 ◯ GA Comparative example 34 6.9 5.79 0.04 1034 735 28.9 13 425 0.41 X GA Comparative example 35 7.5 3.26 0.21 695 400 27.7 54 290 0.42 ◯ GI Comparative example 37 5.4 7.06 0.05 1065 702 23.8 28 440 0.41 ◯ GI Working example 38 5.8 6.30 0.07 1012 634 23.5 27 430 0.42 ◯ GA Working example 39 7.4 6.14 0.02 1085 685 24.1 31 470 0.43 ◯ CR Working example 40 10.9 7.09 0.29 996 602 26.2 29 410 0.41 ◯ GA Working example 41 5.4 6.73 0.04 1027 634 25.1 33 445 0.43 ◯ GA Working example *1: the thickness of a region with a Si concentration not more than one-third of the Si concentration in a chemical composition of a steel sheet and with a Mn concentration not more than one-third of the Mn concentration in the chemical composition of the steel sheet in a region within 4.9 μm in a thickness direction from a surface of the steal sheet *2 (T.sub.Si + T.sub.Mn)/(L.sub.Si + L.sub.Mn) F: ferrite, BF: bainitic ferrite TM: tempered martensite (excluding M and RA), RA: retained austenite, M: fresh martensite P: pearlite, θ: carbides, such as cementite, *CR: cold-rolled steel sheet, GI: hot-dip galvanized steel sheet, GA: hot-dip galvannealed steel sheet

TABLE-US-00005 TABLE 3-2 F BF TM RA Thickness area area area volume Remaining of soft Steal Thickness fraction fraction fraction fraction micro- *1 L.sub.Si L.sub.Mn T.sub.Si T.sub.Mn layer No. Type (mm) (%) (%) (%) (%) structure (μm) (mass %) (mass %) (mass %) (mass %) *2 (μm) 42 W 1.4 48 15 8 15 M, θ 4.2 0.28 0.35 1.32 2.09 5.4 9.0 43 X 1.2 45 16 9 18 M, θ, P 4.5 0.42 0.38 1.93 2.61 5.7 40.0 44 Y 1.2 48 10 13 14 M, θ 3.9 0.31 0.43 1.18 2.02 4.3 35.0 45 Z 1.2 49 16 12 15 M, θ 3.1 0.11 0.27 0.49 1.42 5.0 12.0 46 AA 1.4 45 18 9 13 M, θ, P 2.5 0.28 0.47 1.48 2.13 4.8 29.0 47 AB 1.2 43 14 11 18 M, θ, P 4.0 0.30 0.41 1.78 2.34 5.8 37.0 48 AC 1.6 46 18 10 15 M, θ 2.7 0.22 0.41 0.95 2.11 4.9 19.0 49 AD 1.8 50 17 10 12 M, θ, P 3.6 0.22 0.33 1.38 2.43 6.9 45.0 50 AE 1.6 48 13 9 19 M, θ 3.2 0.27 0.41 1.59 2.02 5.3 9.0 51 AF 1.4 45 15 8 18 M, θ 2.7 0.20 0.30 0.87 1.87 5.5 11.0 52 AG 0.8 42 14 14 17 M, θ, P 3.5 0.28 0.42 1.21 2.34 5.1 52.0 53 AH 1.2 47 16 11 15 M, θ 2.5 0.33 0.47 1.34 2.25 4.5 6.0 54 AI 1.6 42 18 12 27 M, θ 2.7 0.31 0.50 1.84 2.15 4.9 12.0 55 AJ 1.4 46 19 10 16 M, θ, P 3.1 0.24 0.45 1.65 1.95 5.2 45.0 56 AK 1.8 48 15 11 17 M, θ, P 2.9 0.35 0.38 1.79 1.84 5.0 7.0 57 AL 2.0 49 18 12 14 M, θ 4.5 0.32 0.35 1.52 1.98 5.2 33.0 58 AM 1.8 44 17 9 16 M, θ 3.0 0.34 0.28 1.74 1.65 5.5 13.0 59 AN 1.6 49 20 11 15 M, θ 3.8 0.29 0.33 1.32 2.22 5.7 8.0 60 AO 1.4 27 24 14 20 M, θ, P 3.9 0.25 0.35 1.25 2.30 5.9 11.0 61 AP 1.4 45 18 5 19 M, θ, P 3.1 0.20 0.28 1.15 2.12 6.8 30.0 62 AO 1.4 48 14 11 18 M, θ 2.8 0.26 0.32 1.64 1.98 6.2 10.0 64 AS 1.4 65 19 3 12 M, θ, P 1.9 0.24 0.46 0.91 2.08 4.3 18.2 65 AS 1.6 62 17 2 13 M, θ 2.1 0.23 0.43 0.91 2.08 4.5 17.6 66 AT 1.4 48 16 5 11 M, θ, P 1.8 0.16 0.51 0.84 2.65 5.2 13.7 67 AT 1.2 49 18 4 16 M, θ 2.2 0.14 0.58 0.84 2.65 4.8 14.5 68 AT 1.6 50 17 3 12 M, θ 2.1 0.19 0.60 0.84 2.65 4.4 14.9 69 AT 1.2 48 19 4 16 M, θ 1.3 0.22 0.54 0.84 2.65 4.6 13.6 70 AU 1.4 45 20 4 12 M, θ, P 2.1 0.18 0.58 0.87 2.58 4.5 5.2 71 AU 1.6 52 16 5 17 M, θ 2.6 0.13 0.49 0.87 2.58 5.6 6.8 72 AU 1.2 49 18 3 12 M, θ 1.9 0.15 0.54 0.87 2.58 5.0 5.7 73 AU 1.4 50 15 4 15 M, θ 2.0 0.18 0.62 0.87 2.58 4.3 6.1 74 AV 1.2 47 16 8 15 M, θ 2.9 0.30 0.38 1.62 2.32 5.8 38.5 75 AW 1.2 45 17 10 12 M, θ 3.2 0.21 0.31 1.12 2.01 6.0 32.0 76 AX 1.4 48 12 9 11 M, θ 3.3 0.11 0.44 0.88 2.59 6.3 34.2 Average grain size of crystal Amount of grains containing diffusible Fatigue oxide of hydrogen limit Si and/or Mn (ppm by TS YS El λ strength Endurance LME No. (μm) T.sub.Mn/L.sub.Mn mass) (MPa) (MPa) (%) (%) (MPa) ratio resistance Type* Notes 42 7.9 5.97 0.32 1035 689 23.9 35 450 0.43 ◯ GA Working example 43 8.1 6.87 0.08 995 605 25.6 32 440 0.44 ◯ GA Working example 44 3.7 4.70 0.06 1103 722 23.4 28 450 0.41 ◯ GA Working example 45 6.4 5.26 0.03 998 612 27.6 36 440 0.44 ◯ GA Working example 46 6.9 4.53 0.32 1045 628 25.4 33 430 0.41 ◯ GA Working example 47 3.6 5.71 0.08 1078 659 24.6 29 440 0.41 ◯ GA Working example 48 6.8 5.15 0.03 1044 711 23.9 38 450 0.43 ◯ GA Working example 49 3.2 7.36 0.06 995 674 26.2 32 460 0.46 ◯ GA Working example 50 5.4 4.93 0.39 1016 665 25.4 28 440 0.43 ◯ GA Working example 51 8.4 6.23 0.04 1038 601 24.2 37 445 0.43 ◯ GI Working example 52 3.7 5.57 0.07 1002 634 27.6 27 410 0.41 ◯ GA Working example 53 5.1 4.79 0.09 1032 622 23.9 39 450 0.44 ◯ GA Working example 54 6.3 4.30 0.03 1056 674 28.2 33 460 0.44 ◯ CR Working example 55 8.2 4.33 0.06 1079 662 22.4 36 440 0.41 ◯ GA Working example 56 3.6 4.84 0.08 990 595 28.2 30 435 0.44 ◯ GA Working example 57 6.5 5.66 0.32 1106 712 21.9 28 455 0.41 ◯ GA Working example 58 3.2 5.89 0.09 1020 614 26.2 37 450 0.44 ◯ GA Working example 59 5.4 6.73 0.16 1045 635 25.4 35 460 0.44 ◯ GI Working example 60 4.0 6.57 0.03 995 602 27.2 34 440 0.44 ◯ GA Working example 61 6.3 7.57 0.04 1064 666 24.6 28 435 0.41 ◯ GA Working example 62 3.4 6.19 0.07 1078 648 25.2 32 460 0.43 ◯ GA Working example 64 7.8 4.52 0.03 803 465 28.6 38 340 0.42 ◯ GA Working example 65 8.2 4.84 0.02 845 498 29.5 41 355 0.42 ◯ GA Working example 66 6.4 5.20 0.05 1002 620 23.2 32 440 0.44 ◯ GA Working example 67 5.8 4.57 0.04 1054 695 23.9 35 445 0.42 ◯ GA Working example 68 4.9 4.42 0.02 995 630 22.8 34 435 0.44 ◯ GI Working example 69 5.6 4.91 0.01 1040 700 23.5 38 450 0.43 ◯ GI Working example 70 5.5 4.45 0.03 992 605 23.7 33 440 0.44 ◯ GA Working example 71 6.3 5.27 0.01 1036 667 24.1 35 435 0.42 ◯ GA Working example 72 5.7 4.78 0.02 1008 625 22.6 30 445 0.44 ◯ GI Working example 73 5.2 4.16 0.04 1052 689 23.3 33 450 0.43 ◯ GI Working example 74 3.4 6.11 0.10 1022 690 24.6 32 400 0.39 ◯ GA Working example 75 3.1 6.48 0.09 1035 674 23.8 30 385 0.37 ◯ GA Working example 76 3.7 5.89 0.12 1009 652 23.6 29 365 0.36 ◯ GA Working example *1: the thickness of a region with a Si concentration not more than one-third of the Si concentration in a chemical composition of a steel sheet and with a Mn concentration not more than one-third of the Mn concentration in the chemical composition of the steel sheet in a region within 4.9 μm in a thickness direction from a surface of the steel sheet *2 (T.sub.Si + T.sub.Mn)/(L.sub.Si + L.sub.Mn) F: ferrite, BF: bainitic ferrite TM: tempered martensite (excluding M and RA), RA: retained austenite, M: fresh martensite P: pearlite, θ: carbides, such as cementite, *CR: cold-rolled steel sheet, GI: hot-dip galvanized steel sheet, GA: hot-dip galvannealed steel sheet

[0345] Table 3 show that the steel sheets according to the working examples have a tensile strength (TS) of 780 MPa or more and less than 1180 MPa, a high yield stress (YS), high ductility, high stretch-flangeability (hole expandability), good fatigue properties, and high LME resistance. By contrast, the steel sheets according to the comparative examples were inferior to the working examples in at least one of these.

Example 2

[0346] A galvanized steel sheet subjected to galvanizing treatment under the production conditions No. 1 (working example) shown in Table 2 of Example 1 was pressed to produce a member of a working example. Furthermore, a galvanized steel sheet subjected to a galvanizing treatment under the production conditions No. 1 (working example) in Table 2 of Example 1 and a galvanized steel sheet subjected to a galvanizing treatment under the production conditions No. 3 (working example) in Table 2 of Example 1 were joined by spot welding to produce a member of a working example. These members according to the working examples have high LME cracking resistance rated as “0” and also have high ratings in the fatigue test of the test specimens taken from the members. The members according to the working examples have a tensile strength (TS) of 780 MPa or more and less than 1180 MPa, a high yield stress (YS), high ductility, and high stretch-flangeability (hole expandability). Thus, these members are suitably used for automotive parts and the like.

[0347] A steel sheet produced under the production conditions No. 4 (working example) in Table 2 of Example 1 was pressed to produce a member of a working example. Furthermore, a steel sheet produced under the production conditions No. 4 (working example) in Table 2 of Example 1 and a steel sheet produced under the production conditions No. 29 (working example) in Table 2 of Example 1 were joined by spot welding to produce a member of a working example. These members according to the working examples have high LME cracking resistance rated as “0” and also have high ratings in the fatigue test of the test specimens taken from the members. The members according to the working examples have a tensile strength (TS) of 780 MPa or more and less than 1180 MPa, a high yield stress (YS), high ductility, and high stretch-flangeability (hole expandability). Thus, these members are suitably used for automotive parts and the like.

Example 3

[0348] A steel material with the chemical composition of the steel G, T, W, AU, AX, or AC listed in Table 1 and with the remainder composed of Fe and incidental impurities was obtained by steelmaking in a converter and was formed into a steel slab by a continuous casting method. The steel slab was heated to 1250° C. and was subjected to rough rolling. The steel was then subjected to finish rolling at a finish rolling temperature of 900° C. and was coiled at a coiling temperature listed in Table 4 as a hot-rolled steel sheet. Under the conditions shown in Table 4, a cold-rolling step, a first annealing step, and a second annealing step were then performed to produce a cold-rolled steel sheet (CR).

[0349] As described below, a steel sheet was then produced through the production process according to a first embodiment or a second embodiment.

[0350] In the first embodiment, the second annealing step was followed by reheating treatment under the conditions shown in Table 4. Some of the steel sheets were then subjected to plating treatment under the conditions shown in Table 4. Some of the steel sheets were then subjected to dehydrogenation under the conditions shown in Table 4 to produce steel sheets.

[0351] In the second embodiment, the second annealing step was followed by plating treatment under the conditions shown in Table 4. Reheating treatment was then performed under the conditions shown in Table 4 to produce a steel sheet.

[0352] In a working example of the first embodiment, the cooling stop temperature after annealing in the second annealing step ranges from 150° C. to 300° C., as shown in Table 4. In a working example of the second embodiment, the cooling stop temperature after annealing in the second annealing step ranges from 350° C. to 500° C.

[0353] In the plating step, a cold-rolled steel sheet was subjected to plating treatment to produce a hot-dip galvannealed steel sheet (GA). The hot-dip galvanizing bath was a zinc bath containing Al: 0.14% by mass and the remainder composed of Zn and incidental impurities. The bath temperature was 470° C. The coating weight was approximately 45 g/m.sup.2 per side (plating on both sides). Alloying treatment was performed at the temperatures shown in Table 2. The composition of the coated layer of GA contained Fe: 7% to 15% by mass, Al: 0.1% to 1.0% by mass, and the remainder composed of Zn and incidental impurities.

TABLE-US-00006 TABLE 4 Second annealing step Hot-rolling Cold-rolling step Average step Holding First annealing step cooling Cooling Coiling time at 400° Rolling Annealing Holding Annealing Holding Dew-point rate up stop Steel temperature C. or more reduction temperature time temperature time temperature to 550° C. temperature No type (° C.) (seconds) (%) (° C.) (seconds) (° C.) (seconds) (° C.) (° C./s) (° C.) 77 G 620 5000 66.7 850 300 810 350 7 11 400 78 T 560 7400 55.6 850 300 810 350 12 11 200 79 W 500 8000 55.6 850 300 819 350 18 11 400 80 AU 620 5000 64.7 850 300 790 350 7 11 200 81 AX 560 7400 64.7 850 300 790 350 12 11 400 82 AC 500 8000 61.1 850 300 790 350 18 11 400 Second annealing step Blending and Reheating step Reheating step unbending (first embodiment) Plating step Dehydrogenation step (second embodiment) Roller Reheating Holding Alloying Processing Holding Cooling stop Reheating Holding radius Count temperature time temperature temperature time temperature temperature time No (mm) (times) (° C.) (seconds) Type* (° C.) (° C.) (hours) (° C.) (° C.) (seconds) 77 500 8 GA 520 200 400 50 78 500 8 400 40 GA 500 80 20 79 500 8 GA 490 200 400 50 80 500 8 400 40 GA 510 80 20 81 500 8 GA 490 200 400 50 82 500 8 GA 480 200 400 50 *CR: cold-rolled steel sheet (without plating), GI: hot-dip galvanized steel sheet, GA: hot-dip galvannealed steel sheet

[0354] For the steel sheets and the coated steel sheets used as test steels, tensile properties, stretch-flangeability (hole expandability), LME resistance, and fatigue properties were evaluated in accordance with the following test methods. The ferrite area fraction, the tempered martensite area fraction, the bainitic ferrite area fraction, and the retained austenite volume fraction of each steel sheet were measured by the following methods. The Si concentration and Mn concentration were measured by the following methods in a region within 15.0 μm in the thickness direction from the surface of the steel sheet and at a quarter thickness position of the steel sheet. The thickness of a soft layer present in the thickness direction from the surface of the steel sheet, the average grain size of crystal grains containing an oxide of Si and/or Mn in the region within 15.0 μm in the thickness direction from the surface of the steel sheet, and the amount of diffusible hydrogen in the steel sheet were also measured by the methods described above. Table 5 shows the results.

[0355] The ferrite, bainitic ferrite, and tempered martensite area fractions are measured by the following method. The area fractions were measured at a quarter thickness position. A sample was cut such that a cross section of a steel sheet in the thickness direction parallel to the rolling direction became an observation surface. The observation surface was then mirror-polished with a diamond paste, was finally polished with colloidal silica, and was etched with 3% by volume nital to expose a microstructure. Three fields in a 17 μm×23 μm field range were observed with a scanning electron microscope (SEM) at an accelerating voltage of 15 kV and at a magnification of 5000 times. In a microstructure image thus taken, the area fraction was calculated by dividing the area of each constituent microstructure (ferrite, bainitic ferrite, tempered martensite) by the measurement area in three fields using Adobe Photoshop available from Adobe Systems, and the area fractions were averaged to determine the area fraction of each microstructure.

[0356] The retained austenite volume fraction is measured by the following method. A steel sheet was mechanically ground in the thickness direction (depth direction) to a quarter thickness and was then chemically polished with oxalic acid to form an observation surface. The observation surface was observed by X-ray diffractometry. A Mo Kα radiation source was used for incident X-rays. The ratio of the diffraction intensities of (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensities of (200), (211), and (220) planes of bcc iron was determined as the retained austenite volume fraction.

[0357] The other steel sheet microstructure (remaining microstructure) was determined by SEM observation.

[0358] The Si concentration T.sub.Si and the Mn concentration T.sub.Mn at a quarter thickness position of the steel sheet were determined with a field emission-electron probe micro analyzer (FE-EPMA) from the average of 10 points of point analysis at an electron beam diameter of 1 μm at a quarter thickness position of the steel sheet. For the Si concentration in the region within 15.0 μm in the thickness direction from the surface of the steel sheet, the concentration distribution of the Si concentration in the range of 0 to 15.0 μm in the thickness direction from the surface of the steel sheet was determined by line analysis with a field emission-electron probe micro analyzer at an electron beam diameter of 0.1 μm from the surface of the steel sheet in the thickness direction. The lowest Si concentration in the concentration distribution is defined as the concentration L.sub.Si. Also for the Mn concentration in the region within 15.0 μm in the thickness direction from the surface of the steel sheet, the concentration distribution of the Mn concentration in the range of 0 to 15.0 μm in the thickness direction from the surface of the steel sheet was determined by line analysis with a field emission-electron probe micro analyzer at an electron beam diameter of 0.1 μm from the surface of the steel sheet in the thickness direction. The lowest Mn concentration in the concentration distribution is defined as the concentration L.sub.Mn. The Si concentration, the Mn concentration, L.sub.Si, T.sub.Si, L.sub.Mn, and T.sub.Mn are expressed in % by mass. In the measurement of the Si concentration and the Mn concentration with a field emission-electron probe micro analyzer in the disclosed embodiments, 10 positions without particulate matter are measured and averaged as the Si concentration and the Mn concentration.

[0359] SEM observation and energy dispersive X-ray analysis (EDX) on a cross section (L cross section) of a steel sheet were performed to determine the type of oxide in the region within 15.0 μm in the thickness direction from the surface of the steel sheet and to measure the average grain size of crystal grains containing an oxide of Si and/or Mn. The average grain size of crystal grains is the average length of grain sizes measured by microtomy in a cross section (L cross section) of a steel sheet in a direction parallel to the surface of the steel sheet.

[0360] Measurement is performed on the soft layer as described below. After smoothing a thickness cross section (L cross section: a cross section parallel to the rolling direction and perpendicular to the surface of the steel sheet) parallel to the rolling direction of the steel sheet by wet grinding, measurement was performed with a Vickers hardness tester at a load of 10 gf from a 1-μm position to a 100-μm 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 μm to the central portion in the thickness direction. A region with hardness corresponding to 65% or less of the hardness at a quarter thickness position is defined as a soft layer, and the thickness of the region in the thickness direction is defined as the thickness of the soft layer.

[0361] The amount of diffusible hydrogen in a steel sheet is measured by the following method. For a cold-rolled steel sheet, a test specimen 30 mm in length and 5 mm in width was taken. For a steel sheet with a hot-dip galvanized layer or a hot-dip galvannealed layer on its surface, a test specimen 30 mm in length and 5 mm in width was taken, and the hot-dip galvanized layer or hot-dip galvannealed layer was removed with alkali. The amount of hydrogen released from the test specimen was then measured by a temperature-programmed desorption analysis method. More specifically, a test specimen is continuously heated from room temperature 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.

[0362] <Tensile Properties>

[0363] The tensile test was performed in accordance with JIS Z 2241. 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 steel sheet. The YS, TS, and total elongation (El) of the test specimen were measured at a crosshead speed of 10 mm/min in the tensile test. In the disclosed embodiments, TS of 780 MPa or more and YS and El satisfying the following conditions were judged to be acceptable.

[0364] If 780 MPa≤TS<980 MPa, then 420 MPa≤YS, and 22%≤El

[0365] If 980 MPa≤TS, then 560 MPa≤YS, and 19%≤El

[0366] <Stretch-Flangeability (Hole Expandability)>

[0367] The stretch-flangeability (hole expandability) was evaluated in a hole expanding test. The hole expanding test was performed in accordance with JIS Z 2256. A 100 mm×100 mm sample was taken by shearing from the steel sheet. A hole with a diameter of 10 mm was punched in the sample with a clearance of 12.5%. While the periphery of the hole was held using a die with an inner diameter of 75 mm at a blank holding force of 9 ton (88.26 kN), the hole diameter at the crack initiation limit was measured by pushing a conical punch with a vertex angle of 60 degrees into the hole. The critical hole expansion ratio λ (%) was calculated using the following formula, and the hole expandability was evaluated from the critical hole expansion ratio.

λ(%)={(D.sub.f−D.sub.0)/D.sub.0}×100

[0368] In this formula, D.sub.f denotes the hole diameter (mm) at the time of cracking, and Do denotes the initial hole diameter (mm). In the disclosed embodiments, when TS and A satisfied the following conditions, the stretch-flangeability was judged to be good.

[0369] If 780 MPa≤TS<980 MPa, then 30%≤λ

[0370] If 980 MPa≤TS, then 20%≤λ

[0371] <LME Resistance>

[0372] The LME resistance was determined by a resistance welding cracking test. A test specimen of a steel sheet cut to 30 mm×100 mm in a longitudinal direction perpendicular to the rolling direction and another test specimen made of a 980 MPa grade hot-dip galvanized steel sheet were subjected to resistance welding (spot welding) to produce a member. A set of the two steel sheets tilted 5 degrees was subjected to resistance spot welding in a servomotor pressurization type single-phase alternating current (50 Hz) resistance welding machine attached to a welding gun. The welding conditions included a welding pressure of 3.8 kN and a holding time of 0.2 seconds. The welding current ranged from 5.7 to 6.2 kA, the weld time was 21 cycles, and the holding time was 5 cycles. A test specimen was cut in half from the welded member, and a cross section was observed with an optical microscope. A test specimen with no crack of 0.02 mm or more was judged to be very good LME cracking (0), a test specimen with a crack of 0.02 mm or more and less than 0.1 mm was judged to be good LME cracking (0), and a test specimen with a crack of 0.1 mm or more was judged to be poor LME cracking (X).

[0373] <Fatigue Properties>

[0374] Fatigue properties were evaluated in terms of fatigue limit strength and endurance ratio in an alternating plane bending fatigue test according to JIS Z 2275 (1978). A No. 1 test specimen with a bend radius R of 40 mm in a stress loading portion and with a minimum width of 20 mm was used as a test specimen for the fatigue test. In the alternating plane bending fatigue test, a load was applied to a cantilever at a frequency of 20 Hz and at a stress ratio of −1, and stress with a number of cycles of more than 107 was defined as fatigue limit strength. A value obtained by dividing the fatigue limit strength by the tensile strength (TS) was defined as the endurance ratio.

[0375] In the disclosed embodiments, when a crack of 0.02 mm or more was not observed in the evaluation of LME resistance described later, a steel sheet satisfying 300 fatigue limit strength and 0.30 endurance ratio was judged to have good fatigue properties.

[0376] When a crack occurred but was 0.02 mm or more and less than 0.1 mm in the evaluation of LME resistance described later, a steel sheet with TS, fatigue limit strength, and an endurance ratio satisfying the following was judged to have good fatigue properties.

[0377] If 780 MPa≤TS<980 MPa, then 330 MPa≤fatigue limit strength and 0.40≤endurance ratio

[0378] If 980 MPa≤TS, then 400 MPa≤fatigue limit strength and 0.40≤endurance ratio

[0379] Table 5 shows the results.

TABLE-US-00007 TABLE 5 F BF TM RA Thickness area area area volume Remaining of soft Steal Thickness fraction fraction fraction fraction micro- *1 L.sub.Si L.sub.Mn T.sub.Si T.sub.Mn layer No Type (mm) (%) (%) (%) (%) structure (μm) (mass %) (mass %) (mass %) (mass %) *2 (μm) 77 G 1.2 50 18 10 16 M, θ 5.9 0.31 0.47 1.45 2.25 4.7 30.3 78 T 1.6 51 17 11 15 M, θ 8.8 0.30 0.40 1.45 2.25 5.3 40.4 79 W 1.6 49 19 10 14 M, θ 12.2 0.33 0.42 1.45 2.25 4.9 47.4 80 AU 1.2 46 18 12 16 M, θ 6.3 0.14 0.53 0.84 2.65 5.2 32.2 81 AX 1.2 45 19 13 15 M, θ 9.2 0.18 0.52 0.84 2.65 5.0 41.5 82 AC 1.4 47 17 13 14 M, θ 12.5 0.12 0.58 0.84 2.65 5.0 48.3 Average grain size of crystal Amount of grains containing diffusible Fatigue oxide of hydrogen limit Si and/or Mn (ppm by TS YS El λ strength Endurance LME No (μm) T.sub.Mn/L.sub.Mn mass) (MPa) (MPa) (%) (%) (MPa) ratio resistance Type* Notes 77 4.2 4.79 0.07 1045 684 24.4 32 350 0.33 ⊙ GA Working example 78 5.8 5.63 0.08 1028 661 24.1 37 340 0.33 ⊙ GA Working example 79 6.3 5.36 0.09 997 635 24.2 43 330 0.33 ⊙ GA Working example 80 4.4 5.00 0.10 1039 681 23.6 30 355 0.34 ⊙ GA Working example 81 5.3 5.10 0.09 1024 658 23.8 39 340 0.33 ⊙ GA Working example 82 6.0 4.57 0.07 993 637 24.1 44 335 0.34 ⊙ GA Working example *1: the thickness of a region with a Si concentration not more than one-third of the Si concentration in a chemical composition of a steel sheet and with a Mn concentration not more than one-third of the Mn concentration in the chemical composition of the steel sheet in a region within 15.0 μm in a thickness direction from a surface of the steel sheet *2: (T.sub.Si + T.sub.Mn)/(L.sub.Si + L.sub.Mn) F: ferrite, BF: bainitic ferrite TM: tempered martensite (excluding M and RA), RA: retained austenite, M: fresh martensite, P: pearlite, θ: carbides, such as cementite, *CR: cold-rolled steel sheet, GI: hot-dip galvanized steel sheet, GA: hot-dip galvannealed steel sheet

[0380] Table 5 show that the steel sheets according to the working examples have a tensile strength (TS) of 780 MPa or more and less than 1180 MPa, a high yield stress (YS), high ductility, high stretch-flangeability (hole expandability), good fatigue properties, and high LME resistance. By contrast, the steel sheets according to the comparative examples were inferior to the working examples in at least one of these.

Example 4

[0381] A galvanized steel sheet subjected to galvanizing treatment under the production conditions No. 77 (working example) shown in Table 4 of Example 3 was pressed to produce a member of a working example. Furthermore, a galvanized steel sheet subjected to a galvanizing treatment under the production conditions No. 77 (working example) in Table 4 of Example 3 and a galvanized steel sheet subjected to a galvanizing treatment under the production conditions No. 80 (working example) in Table 4 of Example 3 were joined by spot welding to produce a member of a working example. These members according to the working examples have high LME cracking resistance rated as “⊙” and also have high ratings in the fatigue test of the test specimens taken from the members. The members according to the working examples have a tensile strength (TS) of 780 MPa or more and less than 1180 MPa, a high yield stress (YS), high ductility, and high stretch-flangeability (hole expandability). Thus, these members are suitably used for automotive parts and the like.