HIGH STRENGTH STEEL SHEET AND METHOD FOR MANUFACTURING THE SAME

Abstract

A high strength steel sheet having 1180 MPa or higher tensile strength and a method for manufacturing the same are disclosed. The high strength steel sheet has a specific chemical composition and is such that in a region at sheet thickness, the area fraction of martensite is 80% or more, the volume fraction of retained austenite is 3% or more and 15% or less, the area fraction of the total of ferrite and bainitic ferrite is 10% or less, the average grain size of prior austenite is 20 m or less, and the average of the proportions of packets having the largest area in prior austenite grains is 70% by area or less of the prior austenite grain.

Claims

1. A high strength steel sheet having a chemical composition comprising, in mass %, C: 0.030% or more and 0.500% or less, Si: 0.50% or more and 2.50% or less, Mn: 1.50% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, a balance being Fe and incidental impurities, the high strength steel sheet being such that in a region at sheet thickness, an area fraction of martensite is 80% or more, a volume fraction of retained austenite is 3% or more and 15% or less, an area fraction of a total of ferrite and bainitic ferrite is 10% or less, an average grain size of prior austenite is 20 m or less, and an average of proportions of packets having the largest area in prior austenite grains is 70% by area or less of the prior austenite grain.

2. The high strength steel sheet according to claim 1, wherein the chemical composition further comprises at least one element selected from, in mass %, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Co: 0.010% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less.

3. The high strength steel sheet according to claim 1, which has a coated layer on a surface of the steel sheet.

4. The high strength steel sheet according to claim 2, which has a coated layer on a surface of the steel sheet.

5. A method for manufacturing the high strength steel sheet according to claim 1, the method comprising: providing a cold rolled steel sheet produced by subjecting a steel having the chemical composition to hot rolling, pickling, and cold rolling; annealing the steel sheet by heating at an annealing temperature of 750 C. or above and 950 C. or below for a holding time at the annealing temperature of 10 seconds or more and 1000 seconds or less; bending and unbending the steel sheet 1 to 15 times in total with a roll having a radius of 800 mm or less during the annealing; cooling the steel sheet at an average cooling rate of 20 C./s or more in a temperature range from 700 C. to 600 C. and at an average cooling rate of 20 C./s or more in a temperature range from 499 C. to Ms; bending and unbending the steel sheet in the temperature range from 499 C. to Ms, 1 to 15 times in total with a roll having a radius of 800 mm or less; cooling the steel sheet at an average cooling rate of 150 C./s or less in a temperature range from Ms to a cooling stop temperature Ta; applying a tension to the steel sheet in the temperature range from Ms to the cooling stop temperature Ta while controlling the tension to 5 MPa or more and 100 MPa or less, the cooling stop temperature Ta being 100 C. or above and (Ms80 C.) or below where Ms is martensite start temperature ( C.) defined by formula (1); and tempering the steel sheet at a tempering temperature of Ta or above and 450 C. or below for a holding time at the tempering temperature of 10 seconds or more and 1000 seconds or less, $\begin{array}{cc}\mathrm{Ms}=519-474\left[\%C\right]-30.4\left[\%\mathrm{Mn}\right]-12.1\left[\%\mathrm{Cr}\right]-7.5\left[\%\mathrm{Mo}\right]-17.7\left[\%\mathrm{Ni}\right]& \left(1\right)\end{array}$ wherein [% C], [% Mn], [% Cr], [% Mo], and [% Ni] indicate the contents (mass %) of C, Mn, Cr, Mo, and Ni, respectively, and are zero when the element is absent.

6. A method for manufacturing the high strength steel sheet according to claim 2, the method comprising: providing a cold rolled steel sheet produced by subjecting a steel having the chemical composition to hot rolling, pickling, and cold rolling; annealing the steel sheet by heating at an annealing temperature of 750 C. or above and 950 C. or below for a holding time at the annealing temperature of 10 seconds or more and 1000 seconds or less; bending and unbending the steel sheet 1 to 15 times in total with a roll having a radius of 800 mm or less during the annealing; cooling the steel sheet at an average cooling rate of 20 C./s or more in a temperature range from 700 C. to 600 C. and at an average cooling rate of 20 C./s or more in a temperature range from 499 C. to Ms; bending and unbending the steel sheet in the temperature range from 499 C. to Ms, 1 to 15 times in total with a roll having a radius of 800 mm or less; cooling the steel sheet at an average cooling rate of 150 C./s or less in a temperature range from Ms to a cooling stop temperature Ta; applying a tension to the steel sheet in the temperature range from Ms to the cooling stop temperature Ta while controlling the tension to 5 MPa or more and 100 MPa or less, the cooling stop temperature Ta being 100 C. or above and (Ms80 C.) or below where Ms is martensite start temperature ( C.) defined by formula (1); and tempering the steel sheet at a tempering temperature of Ta or above and 450 C. or below for a holding time at the tempering temperature of 10 seconds or more and 1000 seconds or less, $\begin{array}{cc}\mathrm{Ms}=519-474\left[\%C\right]-30.4\left[\%\mathrm{Mn}\right]-12.1\left[\%\mathrm{Cr}\right]-7.5\left[\%\mathrm{Mo}\right]-17.7\left[\%\mathrm{Ni}\right]& \left(1\right)\end{array}$ wherein [% C], [% Mn], [% Cr], [% Mo], and [% Ni] indicate the contents (mass %) of C, Mn, Cr, Mo, and Ni, respectively, and are zero when the element is absent.

7. The method for manufacturing the high strength steel sheet according to claim 5, further comprising performing a coating treatment.

8. The method for manufacturing the high strength steel sheet according to claim 6, further comprising performing a coating treatment.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 is a set of views illustrating a structure of a packet having the largest area in a prior austenite grain according to aspects of the present invention, and how the proportion of the packet is calculated.

[0020] FIG. 2 is a set of views illustrating the concept of the steepness A in the width direction of a steel sheet according to aspects of the present invention, and how the steepness is calculated.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0021] Embodiments of the present invention will be described below.

[0022] First, appropriate ranges of the chemical composition of the high strength steel sheet and the reasons why the chemical composition is thus limited will be described. In the following description, % indicating the contents of constituent elements of steel means mass % unless otherwise specified.

[C: 0.030% or More and 0.500% or Less]

[0023] Carbon is one of the important basic components of steel. Particularly in accordance with aspects of the present invention, carbon is an important element that affects the amount of martensite and the total amount of ferrite and bainitic ferrite. When the C content is less than 0.030%, the amount of martensite is lowered and the total amount of ferrite and bainitic ferrite is increased, with the result that realizing 1180 MPa or higher TS is difficult. When, on the other hand, the C content is more than 0.500%, martensite becomes brittle to cause deterioration in working embrittlement resistance. Thus, the C content is limited to 0.030% or more and 0.500% or less. The lower limit of the C content is preferably 0.050% or more. The upper limit of the C content is preferably 0.400% or less. The lower limit of the C content is more preferably 0.100% or more. The upper limit of the C content is more preferably 0.350% or less.

[Si: 0.50% or More and 2.50% or Less]

[0024] Silicon is one of the important basic components of steel and is an important element that affects TS and the amount of retained austenite. When the Si content is less than 0.50%, the strength of martensite decreases to make it difficult to achieve 1180 MPa or higher TS. When, on the other hand, the Si content is more than 2.50%, the amount of retained austenite is increased excessively to cause deterioration in working embrittlement resistance. Thus, the Si content is limited to 0.50% or more and 2.50% or less. The lower limit of the Si content is preferably 0.55% or more. The upper limit of the Si content is preferably 2.00% or less. The lower limit of the Si content is more preferably 0.60% or more. The upper limit of the Si content is more preferably 1.80% or less.

[Mn: 1.50% or More and 5.00% or Less]

[0025] Manganese is one of the important basic components of steel and is an important element that affects the amount of martensite and the total amount of ferrite and bainitic ferrite. When the Mn content is less than 1.50%, the amount of martensite is lowered and the total amount of ferrite and bainitic ferrite is increased, with the result that realizing 1180 MPa or higher TS is difficult. When, on the other hand, the Mn content is more than 5.00%, martensite becomes brittle to cause deterioration in working embrittlement resistance. Thus, the Mn content is limited to 1.50% or more and 5.00% or less. The lower limit of the Mn content is preferably 2.00% or more. The upper limit of the Mn content is preferably 4.50% or less. The lower limit of the Mn content is more preferably 2.20% or more. The upper limit of the Mn content is more preferably 4.00% or less.

[P: 0.100% or Less]

[0026] Phosphorus is segregated at prior austenite grain boundaries and makes the grain boundaries brittle, thereby lowering the ultimate deformability of steel sheets and causing deterioration in working embrittlement resistance. Thus, the P content needs to be 0.100% or less. The lower limit of the P content is not particularly specified. In view of the fact that phosphorus is a solid solution strengthening element and can increase the strength of steel sheets, the lower limit is preferably 0.001% or more. For the reasons above, the P content is limited to 0.100% or less. The lower limit of the P content is preferably 0.001% or more. The upper limit of the P content is preferably 0.070% or less.

[S: 0.0200% or Less]

[0027] Sulfur forms sulfides and lowers the ultimate deformability of steel sheets to cause deterioration in working embrittlement resistance. Thus, the S content needs to be 0.0200% or less. The lower limit of the S content is not particularly specified but is preferably 0.0001% or more due to production technique limitations. For the reasons above, the S content is limited to 0.0200% or less. The lower limit of the S content is preferably 0.0001% or more. The upper limit of the S content is preferably 0.0050% or less.

[Al: 1.000% or Less]

[0028] Aluminum forms the oxide and lowers the ultimate deformability of steel sheets to cause deterioration in working embrittlement resistance. Thus, the Al content needs to be 1.000% or less. The lower limit of the Al content is not particularly specified. In view of the fact that aluminum suppresses the occurrence of carbides during continuous annealing and promotes the formation of retained austenite, the Al content is preferably 0.001% or more. For the reasons above, the Al content is limited to 1.000% or less. The lower limit of the Al content is preferably 0.001% or more. The upper limit of the Al content is preferably 0.500% or less.

[N: 0.0100% or Less]

[0029] Nitrogen forms nitrides and lowers the ultimate deformability of steel sheets to cause deterioration in working embrittlement resistance. Thus, the N content needs to be 0.0100% or less. The lower limit of the N content is not particularly specified, but the N content is preferably 0.0001% or more due to production technique limitations. For the reasons above, the N content is limited to 0.0100% or less. The lower limit of the N content is preferably 0.0001% or more. The upper limit of the N content is preferably 0.0050% or less.

[O: 0.0100% or Less]

[0030] Oxygen forms oxides and lowers the ultimate deformability of steel sheets to cause deterioration in working embrittlement resistance. Thus, the O content needs to be 0.0100% or less. The lower limit of the O content is not particularly specified but the O content is preferably 0.0001% or more due to production technique limitations. For the reasons above, the O content is limited to 0.0100% or less. The lower limit of the O content is preferably 0.0001% or more. The upper limit of the O content is preferably 0.0050% or less.

[0031] The chemical composition of the high strength steel sheet according to an embodiment of the present invention includes the components described above, and the balance is Fe and incidental impurities. Here, the incidental impurities include Zn, Pb, As, Ge, Sr, and Cs. A total of 0.100% or less of these impurities is acceptable.

[0032] In addition to the components in the proportions described above, the high strength steel sheet according to aspects of the present invention may further include at least one element selected from, in mass %, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less. These elements may be contained singly or in combination.

[0033] When the contents of Ti, Nb, and V are each 0.200% or less, coarse precipitates and inclusions will not occur in large amounts and thus will not cause lowering of the ultimate deformability of steel sheets; hence there will be no deterioration in working embrittlement resistance. Thus, the contents of Ti, Nb, and V are each preferably 0.200% or less. The lower limits of the contents of Ti, Nb, and V are not particularly specified. These elements form fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing to increase the strength of steel sheets. In view of this fact, the contents of Ti, Nb, and V are each more preferably 0.001% or more. When titanium, niobium, and vanadium are added, the contents thereof are each limited to 0.200% or less for the reasons above. The lower limits of the contents of Ti, Nb, and V, when added, are each more preferably 0.001% or more. The upper limits of the contents of Ti, Nb, and V, when added, are each more preferably 0.100% or less.

[0034] When the contents of Ta and W are each 0.10% or less, coarse precipitates and inclusions will not occur in large amounts and thus will not cause lowering of the ultimate deformability of steel sheets; hence there will be no deterioration in working embrittlement resistance. Thus, the contents of Ta and W are each preferably 0.10% or less. The lower limits of the contents of Ta and W are not particularly specified. These elements form fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing to increase the strength of steel sheets. In view of this fact, the contents of Ta and W are each more preferably 0.01% or more. When tantalum and tungsten are added, the contents thereof are each limited to 0.10% or less for the reasons above. The lower limits of the contents of Ta and W, when added, are each more preferably 0.01% or more. The upper limits of the contents of Ta and W, when added, are each more preferably 0.08% or less.

[0035] When the B content is 0.0100% or less, inner cracks that lower the ultimate deformability of steel sheets will not form during casting or hot rolling and thus there will be no deterioration in working embrittlement resistance. Thus, the B content is preferably 0.0100% or less. The lower limit of the B content is not particularly specified. The B content is more preferably 0.0003% or more in view of the fact that this element is segregated at austenite grain boundaries during annealing and enhances hardenability. When boron is added, the content thereof is limited to 0.0100% or less for the reasons above. The lower limit of the content of B, when added, is more preferably 0.0003% or more. The upper limit of the content of B, when added, is more preferably 0.0080% or less.

[0036] When the contents of Cr, Mo, and Ni are each 1.00% or less, coarse precipitates and inclusions will not occur in increased amounts and thus will not cause lowering of the ultimate deformability of steel sheets; hence there will be no deterioration in working embrittlement resistance. Thus, the contents of Cr, Mo, and Ni are each preferably 1.00% or less. The lower limits of the contents of Cr, Mo, and Ni are not particularly specified. In view of the fact that these elements enhance hardenability, the contents of Cr, Mo, and Ni are each more preferably 0.01% or more. When chromium, molybdenum, and nickel are added, the contents thereof are each limited to 1.00% or less for the reasons above. The lower limits of the contents of Cr, Mo, and Ni, when added, are each more preferably 0.01% or more. The upper limits of the contents of Cr, Mo, and Ni, when added, are each more preferably 0.80% or less.

[0037] When the Co content is 0.010% or less, coarse precipitates and inclusions will not occur in increased amounts and thus will not cause lowering of the ultimate deformability of steel sheets; hence there will be no deterioration in working embrittlement resistance. Thus, the Co content is preferably 0.010% or less. The lower limit of the Co content is not particularly specified. In view of the fact that this element enhances hardenability, the Co content is more preferably 0.001% or more. When cobalt is added, the content thereof is limited to 0.010% or less for the reasons above. The lower limit of the content of Co, when added, is more preferably 0.001% or more. The upper limit of the content of Co, when added, is more preferably 0.008% or less.

[0038] When the Cu content is 1.00% or less, coarse precipitates and inclusions will not occur in increased amounts and thus will not cause lowering of the ultimate deformability of steel sheets; hence there will be no deterioration in working embrittlement resistance. Thus, the Cu content is preferably 1.00% or less. The lower limit of the Cu content is not particularly specified. In view of the fact that this element enhances hardenability, the Cu content is preferably 0.01% or more. When copper is added, the content thereof is limited to 1.00% or less for the reasons above. The lower limit of the content of Cu, when added, is more preferably 0.01% or more. The upper limit of the content of Cu, when added, is more preferably 0.80% or less.

[0039] When the Sn content is 0.200% or less, inner cracks that lower the ultimate deformability of steel sheets will not form during casting or hot rolling and thus there will be no deterioration in working embrittlement resistance. Thus, the Sn content is preferably 0.200% or less. The lower limit of the Sn content is not particularly specified. The Sn content is more preferably 0.001% or more in view of the fact that tin enhances hardenability (in general, is an element that enhances corrosion resistance). When tin is added, the content thereof is limited to 0.200% or less for the reasons above. The lower limit of the content of Sn, when added, is more preferably 0.001% or more. The upper limit of the content of Sn, when added, is more preferably 0.100% or less.

[0040] When the Sb content is 0.200% or less, coarse precipitates and inclusions will not occur in increased amounts and thus will not cause lowering of the ultimate deformability of steel sheets; hence there will be no deterioration in working embrittlement resistance. Thus, the Sb content is preferably 0.200% or less. The lower limit of the Sb content is not particularly specified. In view of the fact that this element enables control of the thickness of surface layer softening and the strength, the Sb content is more preferably 0.001% or more. When antimony is added, the content thereof is limited to 0.200% or less for the reasons above. The lower limit of the content of Sb, when added, is more preferably 0.001% or more. The upper limit of the content of Sb, when added, is more preferably 0.100% or less.

[0041] When the contents of Ca, Mg, and REM are each 0.0100% or less, coarse precipitates and inclusions will not occur in increased amounts and thus will not cause lowering of the ultimate deformability of steel sheets; hence there will be no deterioration in working embrittlement resistance. Thus, the contents of Ca, Mg, and REM are each preferably 0.0100% or less. The lower limits of the contents of Ca, Mg, and REM are not particularly specified. In view of the fact that these elements change the shapes of nitrides and sulfides into spheroidal and enhance the ultimate deformability of steel sheets, the contents of Ca, Mg, and REM are each more preferably 0.0005% or more. When calcium, magnesium, and rare earth metal(s) are added, the contents thereof are each limited to 0.0100% or less for the reasons above. The lower limits of the contents of Ca, Mg, and REM, when added, are each more preferably 0.0005% or more. The upper limits of the contents of Ca, Mg, and REM, when added, are each more preferably 0.0050% or less.

[0042] When the contents of Zr and Te are each 0.100% or less, coarse precipitates and inclusions will not occur in increased amounts and thus will not cause lowering of the ultimate deformability of steel sheets; hence there will be no deterioration in working embrittlement resistance. Thus, the contents of Zr and Te are each preferably 0.100% or less. The lower limits of the contents of Zr and Te are not particularly specified. In view of the fact that these elements change the shapes of nitrides and sulfides into spheroidal and enhance the ultimate deformability of steel sheets, the contents of Zr and Te are each more preferably 0.001% or more. When zirconium and tellurium are added, the contents thereof are each limited to 0.100% or less for the reasons above. The lower limits of the contents of Zr and Te, when added, are each more preferably 0.001% or more. The upper limits of the contents of Zr and Te, when added, are each more preferably 0.080% or less.

[0043] When the Hf content is 0.10% or less, coarse precipitates and inclusions will not occur in increased amounts and thus will not cause lowering of the ultimate deformability of steel sheets; hence there will be no deterioration in working embrittlement resistance. Thus, the Hf content is preferably 0.10% or less. The lower limit of the Hf content is not particularly specified. In view of the fact that this element changes the shapes of nitrides and sulfides into spheroidal and enhances the ultimate deformability of steel sheets, the Hf content is more preferably 0.01% or more. When hafnium is added, the content thereof is limited to 0.10% or less for the reasons above. The lower limit of the content of Hf, when added, is more preferably 0.01% or more. The upper limit of the content of Hf, when added, is more preferably 0.08% or less.

[0044] When the Bi content is 0.200% or less, coarse precipitates and inclusions will not occur in increased amounts and thus will not cause lowering of the ultimate deformability of steel sheets; hence there will be no deterioration in working embrittlement resistance. Thus, the Bi content is preferably 0.200% or less. The lower limit of the Bi content is not particularly specified. In view of the fact that this element reduces the occurrence of segregation, the Bi content is more preferably 0.001% or more. When bismuth is added, the content thereof is limited to 0.200% or less for the reasons above. The lower limit of the content of Bi, when added, is more preferably 0.001% or more. The upper limit of the content of Bi, when added, is more preferably 0.100% or less.

[0045] When the content of any of Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi is below the preferred lower limit, the element does not impair the advantageous effects according to aspects of the present invention and is regarded as an incidental impurity.

[0046] Next, the steel microstructure of the high strength steel sheet according to aspects of the present invention will be described.

[Area Fraction of Martensite: 80% or More]

[0047] This configuration is a very important requirement that constitutes an aspect of the present invention. 1180 MPa or higher TS can be achieved when the area fraction of martensite is 80% or more. Thus, the area fraction of martensite is limited to 80% or more. The area fraction is preferably 82% or more, and more preferably 84% or more.

[Volume Fraction of Retained Austenite: 3% or More and 15% or Less]

[0048] This configuration is a very important requirement that constitutes an aspect of the present invention. When the volume fraction of retained austenite is less than 3%, it is difficult to realize excellent bendability because the anti-cracking effect of retained austenite cannot be obtained at the time of bending. When the amount of retained austenite is more than 15%, retained austenite is excessively transformed into hard martensite at the time of working and the steel sheet is lowered in ultimate deformability and will not attain excellent working embrittlement resistance. Thus, the amount of retained austenite is limited to 3% or more and 15% or less. The lower limit of the amount of retained austenite is preferably 5% or more. The upper limit of the amount of retained austenite is preferably 14% or less. The lower limit of the amount of retained austenite is more preferably 7% or more. The upper limit of the amount of retained austenite is more preferably 13% or less.

[0049] Here, retained austenite is measured as follows. The steel sheet is polished to expose a face 0.1 mm below sheet thickness and is thereafter further chemically polished to expose a face 0.1 mm below the face exposed above. The face is analyzed with an X-ray diffractometer using CoK radiation to determine the integral intensity ratios of the diffraction peaks of {200}, {220}, and {311}planes of fcc iron and {200}, {211}, and {220}planes of bcc iron. Nine integral intensity ratios thus obtained are averaged to determine retained austenite.

[Area Fraction of the Total of Ferrite and Bainitic Ferrite: 10% or Less]

[0050] This configuration is a very important requirement that constitutes an aspect of the present invention. When the total amount of ferrite and bainitic ferrite is more than 10%, it is difficult to achieve 1180 MPa or higher TS. Thus, the total amount of ferrite and bainitic ferrite is limited to 10% or less. The total amount is preferably 8% or less, and more preferably 5% or less. The lower limit of the total amount of ferrite and bainitic ferrite is not particularly limited. The total amount may be 0%.

[0051] Here, the total amount of ferrite and bainitic ferrite is measured as follows. A longitudinal cross section of the steel sheet is polished and is etched with 3 vol % Nital. A portion at sheet thickness (a location corresponding to of the sheet thickness in the depth direction from the steel sheet surface) is observed using SEM in 10 fields of view at a magnification of 2000. In the microstructure images, ferrite and bainitic ferrite are recessed structures having a flat interior and containing no inner carbides. The values thus obtained are averaged to determine the total amount of ferrite and bainitic ferrite.

[0052] The amount of martensite is measured as follows. The amount of martensite can be determined by measuring the amounts of retained austenite, ferrite, and bainitic ferrite based on the methods described above, and subtracting the total thereof from 100%. Thus, the amount of martensite in accordance with aspects of the present invention includes both quenched martensite and tempered martensite. Because the volume fraction of retained austenite is almost equal to the area fraction, the amount is subtracted as such from 100% together with the amounts of ferrite and bainitic ferrite expressed in area fraction.

[Average Grain Size of Prior Austenite: 20 m or Less]

[0053] This configuration is a very important requirement that constitutes an aspect of the present invention. Reducing the average grain size of prior austenite can suppress crack propagation and thereby enhances the working embrittlement resistance of steel sheets. In order to obtain these effects, the average grain size of prior austenite needs to be 20 m or less. The lower limit of the average grain size of prior austenite is not particularly specified but is preferably 2 m or more due to production technique limitations. For the reasons above, the average grain size of prior austenite is limited to 20 m or less. The average grain size is preferably 2 m or more. The average grain size is preferably 15 m or less. The average grain size is more preferably 3 m or more. The average grain size is more preferably 10 m or less.

[0054] Here, the average grain size of prior austenite is measured as follows. A longitudinal cross section of the steel sheet is polished and is etched with, for example, a mixed solution of picric acid and ferric chloride to expose prior austenite grain boundaries. Portions at sheet thickness (locations corresponding to of the sheet thickness in the depth direction from the steel sheet surface) are photographed with an optical microscope each in 3 to 10 fields of view at a magnification of 400. Twenty straight lines including 10 vertical lines and 10 horizontal lines are drawn at regular intervals on the image data obtained, and the grain size is determined by a linear intercept method.

[Average of the Proportions of Packets Having the Largest Area in Prior Austenite Grains: 70% by Area or Less]

[0055] This configuration is a very important requirement that constitutes an aspect of the present invention. The proportion of a packet having the largest area in a prior austenite grain affects the flatness in the width direction and the working embrittlement resistance. As illustrated in FIG. 1, a prior austenite grain contains up to four kinds of packets distinguished by crystal habit plane formed by transformation. The packet having the largest area in a prior austenite grain is the packet that occupies the largest area among such packets.

[0056] The proportion of one packet in a prior austenite grain is determined by dividing the area of the packet of interest by the area of the whole prior austenite grain.

[0057] As a result of extensive studies, the present inventors have found that strain among the packets is reduced and the flatness in the width direction is improved by lowering the proportion of a packet having the largest area in a prior austenite grain. The present inventors have also found that lowering the proportion of a packet having the largest area in a prior austenite grain leads to a fine microstructure and suppresses crack propagation, thereby enhancing the working embrittlement resistance of the steel sheet. Thus, the average of the proportions of packets having the largest area in prior austenite grains is limited to 70% or less. The average proportion is preferably 60% or less. The lower limit of the average proportion of packets having the largest area in prior austenite grains is not particularly limited. The grains contain up to four kinds of packets. When four packets are evenly distributed, the proportion of a packet having the largest area in the prior austenite grain is 25%. Thus, the lower limit of the average proportion of packets having the largest area in prior austenite grains is preferably 25% or more. However, the lower limit of the average proportion is not necessarily limited thereto.

[0058] Here, the average proportion of packets having the largest area in prior austenite grains is measured as follows. First, a test specimen for microstructure observation is sampled from the cold rolled steel sheet. Next, the sampled test specimen is polished by vibration polishing with colloidal silica to expose a cross section in the rolling direction (a longitudinal cross section) for use as observation surface. The observation surface is specular. Next, electron backscatter diffraction (EBSD) measurement is performed with respect to a portion at sheet thickness (a location corresponding to of the sheet thickness in the depth direction from the steel sheet surface) to obtain local crystal orientation data. Here, the SEM magnification is 1000, the step size is 0.2 m, the measured region is 80 m square, and the WD is 15 mm. The local orientation data obtained is analyzed with OIM Analysis 7 (OIM), and a map (a CP map) that shows close-packed plane groups (CP groups) with different colors is created using the method described in Non Patent Literature 1. In accordance with aspects of the present invention, a packet is defined as a region or regions belonging to the same CP group. From the CP map obtained, the area of the packet having the largest area is determined and is divided by the area of the whole prior austenite grain to give the proportion of the packet having the largest area in the prior austenite grain. This analysis is performed with respect to 10 or more adjacent prior austenite grains, and the results are averaged to give the average proportion of packets having the largest area in prior austenite grains.

[0059] Next, a manufacturing method according to aspects of the present invention will be described.

[0060] In accordance with aspects of the present invention, a steel material (a steel slab) may be obtained by any known steelmaking method without limitation, such as a converter or an electric arc furnace. To prevent macro-segregation, the steel slab (the slab) is preferably produced by a continuous casting method.

[0061] In accordance with aspects of the present invention, the slab heating temperature, the slab soaking holding time, and the coiling temperature in hot rolling are not particularly limited. For example, the steel slab may be hot rolled in such a manner that the slab is heated and is then rolled, that the slab is subjected to hot direct rolling after continuous casting without being heated, or that the slab is subjected to a short heat treatment after continuous casting and is then rolled. The slab heating temperature, the slab soaking holding time, the finish rolling temperature, and the coiling temperature in hot rolling are not particularly limited. The lower limit of the slab heating temperature is preferably 1100 C. or above. The upper limit of the slab heating temperature is preferably 1300 C. or below. The lower limit of the slab soaking holding time is preferably 30 minutes or more. The upper limit of the slab soaking holding time is preferably 250 minutes or less. The lower limit of the finish rolling temperature is preferably Ar.sub.3 transformation temperature or above. Furthermore, the lower limit of the coiling temperature is preferably 350 C. or above. The upper limit of the coiling temperature is preferably 650 C. or below.

[0062] The hot rolled steel sheet thus produced is pickled. Pickling can remove oxides on the steel sheet surface and is thus important to ensure good chemical convertibility and a high quality of coating in the final high strength steel sheet. Pickling may be performed at a time or several. The hot rolled sheet that has been pickled may be cold rolled directly or may be subjected to heat treatment before cold rolling.

[0063] The rolling reduction in cold rolling and the sheet thickness after rolling are not particularly limited. The lower limit of the rolling reduction is preferably 30% or more. The upper limit of the rolling reduction is preferably 80% or less. The advantageous effects according to aspects of the present invention may be obtained without any limitations on the number of rolling passes and the rolling reduction in each pass.

[0064] The cold rolled steel sheet obtained as described above is annealed. Annealing conditions are as follows.

[Annealing Temperature: 750 C. or Above and 950 C. or Below]

[0065] When the annealing temperature is below 750 C., the amount of martensite is lowered and the total amount of ferrite and bainitic ferrite is increased, with the result that realizing 1180 MPa or higher TS is difficult. When, on the other hand, the annealing temperature is above 950 C., prior austenite grains are excessively increased in size and the prior austenite grain size exceeds 20 m to give rise to a decrease in working embrittlement resistance. Thus, the annealing temperature is limited to 750 C. or above and 950 C. or below. The lower limit of the annealing temperature is preferably 800 C. or above. The upper limit of the annealing temperature is preferably 900 C. or below.

[Holding Time During Annealing at the Annealing Temperature: 10 Seconds or More and 1000 Seconds or Less]

[0066] When the holding time at the annealing temperature is less than 10 seconds, the amount of martensite is lowered and the total amount of ferrite and bainitic ferrite is increased, with the result that realizing 1180 MPa or higher TS is difficult. When, on the other hand, the holding time at the annealing temperature is more than 1000 seconds, prior austenite grains are excessively increased in size to cause a decrease in working embrittlement resistance. Thus, the holding time at the annealing temperature is limited to 10 seconds or more and 1000 seconds or less. The lower limit of the holding time at the annealing temperature is preferably 50 seconds or more. The upper limit of the holding time at the annealing temperature is preferably 500 seconds or less.

[During the Annealing, the Steel Sheet is Bent and Unbent 1 to 15 Times in Total with a Roll Having a Radius of 800 mm or Less.]

[0067] As a result of extensive studies, the present inventors have found that bending and unbending of the steel sheet during annealing affects the proportion of a packet having the largest area in a prior austenite grain. When the steel sheet being annealed is not subjected to bending and unbending with a roll having a radius of 800 mm or less, the amount of nucleation sites for martensite transformation is reduced. Consequently, the average proportion of packets having the largest area in prior austenite grains exceeds 70%, and the flatness in the width direction and also the working embrittlement resistance are deteriorated. When, on the other hand, the steel sheet being annealed is subjected to bending and unbending 16 times or more with a roll having a radius of 800 mm or less, the steel sheet is deteriorated in ultimate deformability and also in working embrittlement resistance. Thus, in the annealing, the total count of bending and unbending with a roll having a radius of 800 mm or less is limited to 1 or more and 15 or less. The radius of the roll is preferably 600 mm or less. The lower limit of the total count of bending and unbending is preferably 3 or more. The upper limit of the total count of bending and unbending is preferably 10 or less. The lower limit of the radius of the roll is not necessarily limited but is preferably 50 mm or more.

[0068] Incidentally, bending and unbending is a treatment that bends the steel sheet with a roll in one direction according to a known technique and unbends the steel sheet in the opposite direction to cancel the bend. Bending and unbending are not counted in pairs. That is, each bending is counted one and each unbending is counted one.

[Average Cooling Rate in the Temperature Range from 700 C. to 600 C.: 20 C./s or More]

[0069] As a result of extensive studies, the present inventors have found that the average cooling rate in the temperature range from 700 C. to 600 C. affects the proportion of a packet having the largest area in a prior austenite grain. When the average cooling rate in the temperature range from 700 C. to 600 C. is less than 20 C./s, the effects imparted by bending and unbending of the steel sheet during annealing are lowered and the amount of nucleation sites for martensite transformation is reduced. Consequently, the average proportion of packets having the largest area in prior austenite grains exceeds 70%, and the flatness in the width direction and also the working embrittlement resistance are deteriorated. Thus, the average cooling rate from 700 C. to 600 C. is limited to 20 C./s or more and is preferably 30 C./s or more. The upper limit is not necessarily limited but is preferably 100 C./s or less.

[Average Cooling Rate in the Temperature Range from 499 C. to Ms: 20 C./s or More]

[0070] The average cooling rate in the temperature range from 499 C. to Ms affects the total area fraction of ferrite and bainitic ferrite. When the average cooling rate in the temperature range from 499 C. to Ms is less than 20 C./s, the total amount of ferrite and bainitic ferrite is increased to make it difficult to realize 1180 MPa or higher TS. Thus, the average cooling rate in the temperature range from 499 C. to Ms is limited to 20 C./s or more. The average cooling rate is preferably 25 C./s or more. The upper limit is not necessarily limited but is preferably 100 C./s or less.

[0071] The martensite start temperature Ms ( C.) is defined by the following formula (1):

[00002]$\begin{array}{cc}\mathrm{Ms}=519-474\left[\%C\right]-30.4\left[\%\mathrm{Mn}\right]-12.1\left[\%\mathrm{Cr}\right]-7.5\left[\%\mathrm{Mo}\right]-17.7\left[\%\mathrm{Ni}\right]& \left(1\right)\end{array}$

wherein [% C], [% Mn], [% Cr], [% Mo], and [% Ni] indicate the contents (mass %) of C, Mn, Cr, Mo, and Ni, respectively, and are zero when the element is absent.
[The Steel Sheet in the Temperature Range from 499 C. to Ms is Bent and Unbent 1 to 15 Times in Total with a Roll Having a Radius of 800 mm or Less.]

[0072] As a result of extensive studies, the present inventors have found that bending and unbending of the steel sheet in the temperature range from 499 C. to Ms affects the proportion of a packet having the largest area in a prior austenite grain. When the steel sheet in the temperature ranges from 499 C. to Ms is not subjected to bending and unbending with a roll having a radius of 800 mm or less, the amount of martensite nucleation sites is reduced. Consequently, the average proportion of packets having the largest area in prior austenite grains exceeds 70%, and the flatness in the width direction and also the working embrittlement resistance are deteriorated. When, on the other hand, the steel sheet in the temperature ranges from 499 C. to Ms is subjected to bending and unbending 16 times or more with a roll having a radius of 800 mm or less, the steel sheet is deteriorated in ultimate deformability and also in working embrittlement resistance. Thus, the total count of bending and unbending in the temperature range from 499 C. to Ms with a roll having a radius of 800 mm or less is limited to 1 or more and 15 or less. The radius of the roll is preferably 600 mm or less. The lower limit of the total count of bending and unbending is preferably 3 or more. The upper limit of the total count of bending and unbending is preferably 10 or less. The lower limit of the radius of the roll is not necessarily limited but is preferably 50 mm or more.

[Average Cooling Rate in the Temperature Range from Ms to Cooling Stop Temperature Ta: 150 C./s or Less]

[0073] As a result of extensive studies, the present inventors have found that the average cooling rate in the temperature range from Ms to the cooling stop temperature Ta affects the proportion of a packet having the largest area in a prior austenite grain. When the average cooling rate in the temperature range from Ms to the cooling stop temperature Ta is more than 150 C./s, the martensite transformation rate is so fast that a packet grows fast easily. Consequently, the average proportion of packets having the largest area in prior austenite grains exceeds 70%, and the flatness in the width direction and also the working embrittlement resistance are deteriorated. Thus, the average cooling rate in the temperature range from Ms to the cooling stop temperature Ta is limited to 150 C./s or less. The average cooling rate is preferably 120 C./s or less. The lower limit is not necessarily limited but is preferably 5 C./s or more.

[Tension Applied to the Steel Sheet in the Temperature Range from Ms to the Cooling Stop Temperature Ta: 5 MPa or More and 100 MPa or Less]

[0074] As a result of extensive studies, the present inventors have found that the application of tension to the steel sheet in the temperature range from Ms to the cooling stop temperature Ta affects the proportion of a packet having the largest area in a prior austenite grain. When the tension applied to the steel sheet in the temperature range from Ms to the cooling stop temperature Ta is less than 5 MPa, the amount of martensite nucleation sites is reduced. Consequently, the average proportion of packets having the largest area in prior austenite grains exceeds 70%, and the flatness in the width direction and also the working embrittlement resistance are deteriorated. When, on the other hand, more than 100 MPa tension is applied to the steel sheet in the temperature range from Ms to the cooling stop temperature Ta, the ultimate deformability of the steel sheet is lowered and the working embrittlement resistance is deteriorated. Thus, the tension applied to the steel sheet in the temperature range from Ms to the cooling stop temperature Ta is limited to 5 MPa or more and 100 MPa or less. The lower limit of the tension applied to the steel sheet in the temperature range from Ms to the cooling stop temperature Ta is preferably 6 MPa or more. The upper limit of the tension applied to the steel sheet in the temperature range from Ms to the cooling stop temperature Ta is preferably 50 MPa or less. The tension is applied in a usual manner. As an example, the tension may be applied by controlling the roll speeds of the rolls in the furnace.

[0075] While the bending and unbending process increases the number of nucleation sites that are martensite start sites, the tension application process produces different effects by promoting martensite transformation itself.

[Cooling Stop Temperature Ta: 100 C. or Above and (Ms80 C.) or Below]

[0076] When the cooling stop temperature Ta is below 100 C., the amount of retained austenite decreases and bendability is lowered. When, on the other hand, the cooling stop temperature Ta is above (Ms80 C.), the amount of retained austenite is excessively increased and the prior austenite grain size is excessively enlarged to cause deterioration in working embrittlement resistance. Thus, the cooling stop temperature Ta is limited to 100 C. or above and (Ms80 C.) or below. The lower limit of the cooling stop temperature Ta is preferably 120 C. or above. The upper limit of the cooling stop temperature Ta is preferably (Ms100 C.) or below.

[Tempering Temperature: Ta or Above and 450 C. or Below]

[0077] After the cooling is stopped at the cooling stop temperature Ta, the steel sheet is held at the temperature or is reheated and held at a temperature of 450 C. or below to stabilize retained austenite. When the tempering temperature is below Ta, retained austenite cannot be obtained as desired and consequently bendability is lowered. When the tempering temperature is above 450 C., martensite is excessively tempered to make it difficult to achieve 1180 MPa or higher TS. Thus, the tempering temperature is limited to Ta or above and 450 C. or below. The lower limit of the tempering temperature is preferably (Ta+10 C.) or above. The upper limit of the tempering temperature is preferably 420 C. or below.

[Holding Time at the Tempering Temperature: 10 Seconds or More and 1000 Seconds or Less]

[0078] When the holding time at the tempering temperature is less than 10 seconds, austenite stabilization is insufficient and retained austenite cannot be obtained as desired. Consequently, bendability is lowered. When the holding time at the tempering temperature is more than 1000 seconds, martensite is excessively tempered to make it difficult to achieve 1180 MPa or higher TS. Thus, the holding time at the tempering temperature is limited to 10 seconds or more and 1000 seconds or less. The lower limit of the holding time at the tempering temperature is preferably 50 seconds or more. The upper limit of the holding time at the tempering temperature is preferably 800 seconds or less.

[0079] Post-temper cooling is not particularly limited and the steel sheet may be cooled to a desired temperature in an appropriate manner. Incidentally, the desired temperature is preferably about room temperature.

[0080] Furthermore, the high strength steel sheet described above may be worked under conditions where the amount of equivalent plastic strain is 0.10% or more and 5.00% or less. The working may be followed by reheating at 100 C. or above and 400 C. or below.

[0081] When the high strength steel sheet is a product that is traded, the steel sheet is usually traded after being cooled to room temperature.

[0082] The high strength steel sheet may be subjected to coating treatment during annealing or after annealing.

[0083] For example, the coating treatment during annealing may be hot-dip galvanizing treatment performed when the annealed steel sheet is being cooled or has been cooled from 700 C. to 600 C. at an average cooling rate of 20 C./s or more. The hot-dip galvanizing treatment may be followed by alloying. For example, the coating treatment after annealing may be Zn-Ni electrical alloy coating treatment or pure Zn electroplated coating treatment performed after tempering. A coated layer may be formed by electroplated coating, or hot-dip zinc-aluminum-magnesium alloy coating may be applied. While the coating treatment has been described above focusing on zinc coating, the types of coating metals, such as Zn coating and Al coating, are not particularly limited. Other conditions in the manufacturing method are not particularly limited. From the point of view of productivity, the series of treatments including annealing, hot-dip galvanizing, and alloying treatment of the coated zinc layer is preferably performed on hot-dip galvanizing line CGL (continuous galvanizing line). To control the coating weight of the coated layer, the hot-dip galvanizing treatment may be followed by wiping. Conditions for operations, such as coating, other than those conditions described above may be determined in accordance with the usual hot-dip galvanizing technique.

[0084] After the coating treatment after annealing, the steel sheet may be worked again under conditions where the amount of equivalent plastic strain is 0.10% or more and 5.00% or less. The working may be followed by reheating at 100 C. or above and 400 C. or below.

Examples

[0085] Steels having a chemical composition described in Table 1 and 2, with the balance being Fe and incidental impurities, were smelted in a converter and were continuously cast into slabs. Next, the slabs obtained were heated, hot rolled, pickled, cold rolled, and subjected to annealing treatment described in Table 3 and 4. High strength cold rolled steel sheets having a sheet thickness of 0.6 to 2.2 mm were thus obtained. During annealing, the steel sheet was subjected to bending and unbending with a roll having a radius of 300 mm. In the temperature range from 499 C. to Ms, the steel sheet was subjected to bending and unbending with a roll having a radius of 300 mm. Incidentally, some of the steel sheets were subjected to coating treatment during or after annealing.

[0086] The high strength cold rolled steel sheets obtained as described above were used as test steels. Tensile characteristics, bendability, flatness in the width direction, and working embrittlement resistance were evaluated in accordance with the following test methods.

TABLE-US-00001 TABLE 1 Chemical composition (mass %) Steels C Si Mn P S N O Al Ti B Nb Cu Others A 0.242 1.02 2.69 0.006 0.0012 0.002 0.006 0.034 INV. EX. B 0.246 1.31 2.67 0.005 0.0005 0.003 0.002 0.030 INV. EX. C 0.248 1.35 2.66 0.008 0.0013 0.007 0.002 0.055 INV. EX. D 0.249 1.40 2.84 0.006 0.0009 0.006 0.006 0.015 INV. EX. E 0.236 1.00 2.86 0.012 0.0009 0.002 0.004 0.036 INV. EX. F 0.034 1.19 12.48 0.015 0.0012 0.005 0.004 0.031 INV. EX. G 0.025 1.19 2.61 0.012 0.0009 0.004 0.002 0.036 COMP. EX. H 0.466 1.23 2.50 0.013 0.0012 0.006 0.006 0.034 INV. EX. I 0.504 1.34 2.43 0.007 0.0014 0.001 0.003 0.028 COMP. EX. J 0.268 0.74 2.81 0.006 0.0008 0.003 0.002 0.028 INV. EX. K 0.226 0.41 2.59 0.012 0.0014 0.004 0.006 0.031 COMP. EX. L 0.235 2.36 2.84 0.009 0.0012 0.004 0.002 0.034 INV. EX. M 0.240 2.58 2.87 0.009 0.0009 0.001 0.005 0.051 COMP. EX. N 0.241 1.03 1.57 0.012 0.0011 0.003 0.006 0.055 INV. EX. O 0.246 1.34 1.41 0.011 0.0014 0.002 0.007 0.018 COMP. EX. P 0.229 1.32 4.75 0.007 0.0010 0.005 0.001 0.014 INV. EX. Q 0.223 1.32 5.16 0.007 0.0008 0.002 0.004 0.031 COMP. EX. R 0.224 1.08 2.75 0.097 0.0012 0.004 0.007 0.017 INV. EX. S 0.225 1.39 2.54 0.109 0.0011 0.005 0.001 0.050 COMP. EX. T 0.230 1.05 2.43 0.010 0.0195 0.002 0.003 0.015 INV. EX. U 0.239 1.29 2.42 0.007 0.0204 0.003 0.006 0.044 COMP. EX. V 0.246 1.18 2.63 0.011 0.0011 0.005 0.003 0.924 INV. EX. W 0.265 1.15 2.74 0.006 0.0007 0.007 0.007 1.049 COMP. EX. X 0.261 1.21 2.59 0.009 0.0010 0.0090 0.005 0.033 INV. EX. Y 0.252 1.36 2.73 0.010 0.0007 0.0110 0.005 0.049 COMP. EX. Z 0.238 1.09 2.65 0.012 0.0013 0.004 0.0090 0.019 INV. EX. AA 0.263 1.21 2.70 0.009 0.0008 0.002 0.0110 0.058 COMP. EX. AB 0.245 1.11 2.44 0.007 0.0011 0.006 0.003 0.045 INV. EX. AC 0.252 1.08 2.71 0.015 0.0012 0.007 0.004 0.028 0.003 INV. EX. AD 0.238 1.25 2.71 0.013 0.0012 0.004 0.004 0.052 0.195 INV. EX. AE 0.227 1.04 2.75 0.009 0.0013 0.002 0.006 0.041 0.212 COMP. EX. AF 0.259 1.16 2.52 0.007 0.0014 0.005 0.004 0.058 0.0003 INV. EX. AG 0.249 1.29 2.65 0.011 0.0015 0.005 0.002 0.054 0.0072 INV. EX. AH 0.229 1.18 2.77 0.013 0.0014 0.001 0.005 0.027 0.0105 COMP. EX. AI 0.252 1.32 2.45 0.013 0.0005 0.003 0.005 0.052 0.003 INV. EX. AJ 0.251 1.02 2.87 0.013 0.0011 0.003 0.002 0.037 0.182 INV. EX. AK 0.232 1.30 2.53 0.013 0.0013 0.005 0.005 0.020 0.213 COMP. EX. AL 0.225 1.23 2.42 0.008 0.0005 0.005 0.003 0.045 0.02 INV. EX. AM 0.242 1.08 2.78 0.010 0.0009 0.002 0.004 0.041 0.92 INV. EX. Underlines indicate being outside the range of the present invention.

TABLE-US-00002 TABLE 2 Chemical composition (mass %) Steels C Si Mn P S N O Al Ti B Nb Cu Others AN 0.257 1.32 2.53 0.010 0.0015 0.002 0.003 0.032 1.12 COMP. EX. AO 0.251 1.07 2.82 0.006 0.0014 0.005 0.003 0.012 V: 0.024 INV. EX. AP 0.255 1.26 2.73 0.009 0.0011 0.002 0.004 0.035 Ta: 0.06 INV. EX. AQ 0.226 1.22 2.47 0.013 0.0014 0.007 0.006 0.044 W: 0.07 INV. EX. AR 0.240 1.28 2.65 0.009 0.0008 0.006 0.004 0.044 Cr: 0.17 INV. EX. AS 0.248 1.25 2.70 0.010 0.0006 0.004 0.006 0.023 Mo: 0.54 INV. EX. AT 0.229 1.09 2.80 0.011 0.0009 0.003 0.001 0.043 Co: 0.008 INV. EX. AU 0.245 1.19 2.79 0.007 0.0014 0.005 0.005 0.042 Ni: 0.75 INV. EX. AV 0.228 1.31 2.57 0.011 0.0013 0.004 0.003 0.049 Sn: 0.019 INV. EX. AW 0.268 1.15 2.67 0.006 0.0013 0.003 0.002 0.012 Sb: 0.056 INV. EX. AX 0.242 1.16 2.75 0.014 0.0010 0.002 0.003 0.012 Ca: 0.0024 INV. EX. AY 0.240 1.10 2.77 0.014 0.0006 0.003 0.005 0.011 Mg: 0.0090 INV. EX. AZ 0.234 1.18 2.90 0.008 0.0012 0.002 0.002 0.052 Zr: 0.064 INV. EX. BA 0.233 1.38 2.75 0.012 0.0007 0.005 0.005 0.032 Te: 0.075 INV. EX. BB 0.268 1.02 2.54 0.006 0.0013 0.004 0.002 0.042 Hf: 0.04 INV. EX. BC 0.230 1.28 2.78 0.011 0.0014 0.004 0.004 0.052 REM: 0.0079 INV. EX. BD 0.227 1.17 2.60 0.011 0.0008 0.004 0.005 0.055 Bi: 0.164 INV. EX. BE 0.227 1.17 2.61 0.014 0.0014 0.007 0.007 0.021 Zn: 0.037 INV. EX. BF 0.235 1.33 2.82 0.008 0.0011 0.001 0.001 0.021 Pb: 0.086 INV. EX. BG 0.239 1.26 2.82 0.012 0.0012 0.003 0.002 0.013 As: 0.035 INV. EX. BH 0.248 1.30 2.61 0.010 0.0006 0.006 0.007 0.055 Ge: 0.032 INV. EX. BI 0.221 1.18 2.86 0.012 0.0008 0.002 0.003 0.040 Sr: 0.050 INV. EX. BJ 0.240 1.14 2.45 0.010 0.0008 0.002 0.005 0.026 Cs: 0.012 INV. EX. BK 0.262 1.15 2.57 0.011 0.0009 0.006 0.005 0.011 INV. EX. BL 0.240 1.34 2.88 0.007 0.0006 0.006 0.002 0.055 INV. EX. BM 0.220 1.36 2.74 0.011 0.0011 0.004 0.003 0.043 INV. EX. BN 0.242 1.39 2.72 0.009 0.0007 0.002 0.006 0.016 INV. EX. BO 0.224 1.02 2.66 0.011 0.0006 0.007 0.004 0.052 INV. EX. Underlines indicate being outside the range of the present invention.

TABLE-US-00003 TABLE 3 Average Average Count of Count of cooling cooling bending and Holding bending and rate in rate in unbending in time at unbending temperature temperature temperature Annealing annealing during range of range of range of temp. temp. annealing 700-600 C. 499 C.-Ms 499 C.-Ms Ms (Ms-80) Nos. Steels ( C.) (s) (times) ( C./s) ( C./s) (times) ( C.) ( C.) 1 A 861 56 10 68 64 3 323 243 2 B 777 125 10 66 73 3 321 241 3 B 777 135 10 51 69 3 321 241 4 B 744 168 10 78 38 3 321 241 5 B 946 77 10 57 64 3 321 241 6 B 952 195 10 66 49 3 321 241 7 B 863 30 10 77 39 3 321 241 8 B 871 3 10 71 76 3 321 241 9 B 875 837 10 76 54 3 321 241 10 B 876 1008 10 67 60 3 321 241 11 B 875 107 1 69 58 3 321 241 12 B 870 73 0 64 80 3 321 241 13 B 880 148 15 53 55 3 321 241 14 B 877 64 15 78 34 3 321 241 15 B 880 83 3 38 53 3 321 241 16 B 860 197 3 12 53 3 321 241 17 B 870 80 3 72 70 3 321 241 18 B 873 126 3 76 36 3 321 241 19 B 866 140 3 64 22 3 321 241 20 B 869 169 3 55 16 3 321 241 21 B 861 72 3 72 71 3 321 241 22 B 866 111 3 55 34 3 321 241 23 B 862 180 3 79 49 1 321 241 24 B 868 198 3 66 64 0 321 241 25 B 863 62 3 63 48 15 321 241 26 B 873 184 3 73 72 15 321 241 27 B 874 191 3 72 72 3 321 241 28 B 869 150 3 50 43 3 321 241 29 B 879 103 3 56 62 3 321 241 30 B 878 176 3 79 78 3 321 241 31 B 869 81 3 70 61 3 321 241 32 B 879 151 3 74 67 3 321 241 33 B 879 134 3 70 56 3 321 241 34 B 878 102 10 79 70 3 321 241 35 B 872 88 10 76 62 3 321 241 36 B 878 128 10 57 62 3 321 241 37 B 868 95 10 59 30 3 321 241 38 B 860 115 10 56 63 3 321 241 39 B 866 187 10 67 53 3 321 241 40 B 868 125 10 53 64 3 321 241 41 B 876 133 10 50 64 3 321 241 42 B 865 120 10 57 37 3 321 241 43 B 865 141 10 50 64 10 321 241 44 B 861 134 3 54 64 10 321 241 45 B 877 153 3 72 47 10 321 241 46 B 874 82 3 77 73 10 321 241 47 B 868 138 3 65 34 10 321 241 48 B 878 101 3 67 54 10 321 241 49 C 866 70 3 53 33 10 321 241 50 D 877 85 3 54 41 10 315 235 51 E 870 125 3 69 62 3 320 240 52 F 862 119 3 70 68 3 427 347 53 G 869 70 3 64 38 3 428 348 54 H 872 125 3 59 51 3 222 142 55 I 865 82 3 77 53 3 206 126 56 J 872 112 3 59 57 3 307 227 Average Tension cooling applied to rate in steel sheet in Holding Cooling temperature temperature time at stop temp. range of range of Tempering tempering Sheet Ta Ms-Ta Ms-Ta temp. temp. thickness Nos. ( C.) ( C./s) (MPa) ( C.) (s) (mm) Type* 1 208 15 17 339 247 1.4 CR INV. EX. 2 190 14 12 297 299 1.4 CR INV. EX. 3 187 17 16 294 208 1.4 CR INV. EX. 4 177 18 12 278 214 1.4 CR COMP. EX. 5 189 13 13 261 190 1.4 CR INV. EX. 6 182 13 12 329 245 1.4 CR COMP. EX. 7 187 18 11 322 191 1.4 CR INV. EX. 8 185 13 8 349 116 1.4 CR COMP. EX. 9 184 15 11 280 101 1.4 CR INV. EX. 10 196 13 14 325 238 1.4 CR COMP. EX. 11 184 18 10 261 206 1.4 CR INV. EX. 12 200 13 18 303 270 1.4 CR COMP. EX. 13 189 14 16 332 267 1.4 CR INV. EX. 14 185 13 14 292 242 1.4 CR INV. EX. 15 186 19 18 259 117 1.4 CR INV. EX. 16 174 10 17 345 294 1.4 CR COMP. EX. 17 171 17 16 309 269 1.4 CR INV. EX. 18 194 16 17 347 237 1.4 CR INV. EX. 19 193 14 18 320 298 1.4 CR INV. EX. 20 182 11 15 335 212 1.4 CR COMP. EX. 21 175 95 13 305 211 1.4 CR INV. EX. 22 179 19 15 179 271 1.4 CR INV. EX. 23 203 17 14 308 136 1.4 CR INV. EX. 24 177 15 18 302 249 1.4 CR COMP. EX. 25 193 11 13 314 177 1.4 CR INV. EX. 26 173 12 16 280 109 1.4 CR INV. EX. 27 105 20 15 349 134 1.4 CR INV. EX. 28 97 19 12 341 212 1.4 CR COMP. EX. 29 240 15 17 343 221 1.4 CR INV. EX. 30 244 15 13 332 284 1.4 CR COMP. EX. 31 199 17 12 273 137 1.4 CR INV. EX. 32 186 19 17 336 153 1.4 CR INV. EX. 33 175 135 11 284 253 1.4 CR INV. EX. 34 191 154 14 337 221 1.4 CR COMP. EX. 35 209 17 6 336 166 1.4 CR INV. EX. 36 174 20 2 276 164 1.4 CR COMP. EX. 37 197 14 90 347 251 1.4 CR INV. EX. 38 200 16 92 325 176 1.4 CR INV. EX. 39 205 18 11 205 265 1.4 CR INV. EX. 40 175 19 15 175 140 1.4 CR INV. EX. 41 200 13 13 435 237 1.4 CR INV. EX. 42 185 19 8 444 173 1.4 CR INV. EX. 43 187 18 15 312 27 1.4 CR INV. EX. 44 192 13 15 269 6 1.4 CR COMP. EX. 45 194 11 14 275 936 1.4 CR INV. EX. 46 211 16 10 330 995 1.4 CR INV. EX. 47 182 11 10 295 254 1.4 CR INV. EX. 48 196 107 12 338 165 1.6 CR INV. EX. 49 206 97 16 280 138 2.0 CR INV. EX. 50 196 12 16 264 238 1.4 CR INV. EX. 51 182 13 10 310 270 1.4 CR INV. EX. 52 288 11 9 345 236 1.4 CR INV. EX. 53 296 17 9 325 135 1.4 CR COMP. EX. 54 110 12 12 257 177 1.4 CR INV. EX. 55 105 13 15 283 210 1.4 CR COMP. EX. 56 164 17 13 265 127 1.4 CR INV. EX. Underlines indicate being outside the range of the present invention. *CR: cold rolled steel sheet (no coating), GI: hot-dip galvanized steel sheet (no alloying of zinc coating), GA: galvannealed steel sheet, EG: electrogalvanized steel sheet

TABLE-US-00004 TABLE 4 Average Average Count of Count of cooling cooling bending and Holding bending and rate in rate in unbending in time at unbending temperature temperature temperature Annealing annealing during range of range of range of temp. temp annealing 700-600 C. 499 C.-Ms 499 C.-Ms Ms (Ms-80) Nos. Steels ( C.) (s) (times) ( C./s) ( C./s) (times) ( C.) ( C.) 57 K 868 64 3 66 37 3 333 253 58 L 876 162 3 53 73 3 321 241 59 M 867 79 10 67 65 3 318 238 60 N 880 118 10 66 32 3 357 277 61 O 871 87 10 64 72 3 360 280 62 P 868 99 10 56 43 3 266 186 63 Q 867 61 10 52 76 3 256 176 64 R 873 56 10 67 61 3 329 249 65 S 871 114 3 50 40 3 335 255 66 T 871 60 3 54 49 3 336 256 67 U 876 195 3 69 59 3 332 252 68 V 876 72 3 77 46 3 322 242 69 W 872 195 3 79 58 3 310 230 70 X 863 89 3 71 65 10 317 237 71 Y 868 70 3 54 61 10 317 237 72 Z 877 157 3 66 43 10 326 246 73 AA 861 186 3 54 73 10 312 232 74 AB 862 154 3 74 48 10 329 249 75 AC 872 154 3 51 58 10 317 237 76 AD 865 109 3 59 52 10 324 244 77 AE 872 92 3 77 71 10 328 248 78 AF 870 147 3 67 68 3 320 240 79 AG 872 113 3 69 53 3 320 240 80 AH 871 175 10 60 36 3 326 246 81 AI 861 183 10 72 64 3 325 245 82 AJ 861 78 10 54 70 3 313 233 83 AK 867 51 10 54 78 3 332 252 84 AL 879 195 10 65 31 3 339 259 85 AM 877 182 10 71 70 3 320 240 86 AN 870 192 10 71 47 3 320 240 87 AO 799 181 10 55 34 3 314 234 88 AP 917 153 10 78 74 3 315 235 89 AQ 870 19 10 51 40 3 337 257 90 AR 879 870 10 57 78 3 323 243 91 AS 860 187 1 52 30 3 315 235 92 AT 869 182 15 58 32 3 325 245 93 AU 868 112 3 22 50 3 305 225 94 AV 871 148 3 63 32 3 333 253 95 AW 864 181 10 71 23 3 311 231 96 AX 862 175 10 64 75 3 321 241 97 AY 874 51 10 64 35 1 321 241 98 AZ 872 56 10 55 71 15 320 240 99 BA 865 99 3 51 61 3 325 245 100 BB 864 166 3 62 40 3 315 235 101 BC 866 180 10 60 39 3 325 245 102 BD 862 159 10 53 74 3 332 252 103 BE 866 173 3 69 51 3 332 252 104 BF 877 158 3 56 33 3 322 242 105 BG 876 140 10 61 38 3 320 240 106 BH 870 190 10 68 52 3 322 242 107 BI 863 175 10 50 75 3 327 247 108 BJ 877 187 10 58 54 3 331 251 109 BK 877 196 10 69 32 12 317 237 110 BL 878 145 5 78 39 12 318 238 111 BM 867 92 5 77 57 3 331 251 112 BN 863 129 12 55 37 3 322 242 113 BO 876 181 12 51 43 3 332 252 Average Tension cooling applied to rate in steel sheet in Holding Cooling temperature temperature time at stop temp. range of range of Tempering tempering Sheet Ta Ms-Ta Ms-Ta temp. temp. thickness Nos. ( C.) ( C./s) (MPa) ( C.) (s) (mm) Type* 57 209 12 16 277 290 1.4 CR COMP. EX. 58 205 15 17 339 179 1.4 CR INV. EX. 59 199 15 10 262 269 1.4 GA COMP. EX. 60 240 16 9 301 292 1.4 GA INV. EX. 61 221 20 10 286 207 1.4 GA COMP. EX. 62 126 18 9 293 284 1.4 GA INV. EX. 63 146 13 17 271 194 1.4 GA COMP. EX. 64 202 17 11 259 162 1.4 CR INV. EX. 65 205 13 18 293 264 1.4 CR COMP. EX. 66 211 14 18 254 176 1.4 GA INV. EX. 67 193 14 14 302 286 1.4 GA COMP. EX. 68 182 10 10 305 206 1.4 GI INV. EX. 69 167 16 15 301 132 1.4 GA COMP. EX. 70 195 19 13 265 184 1.4 GA INV. EX. 71 169 19 8 307 135 1.4 GA COMP. EX. 72 176 13 13 259 193 1.4 GA INV. EX. 73 167 18 10 307 279 1.4 GI COMP. EX. 74 196 12 12 327 174 1.4 GA INV. EX. 75 194 11 14 349 185 1.4 GA INV. EX. 76 213 17 16 331 180 1.4 GA INV. EX. 77 202 13 12 318 112 1.4 GA COMP. EX. 78 174 13 11 258 152 1.4 GA INV. EX. 79 203 17 13 267 184 1.4 GI INV. EX. 80 182 17 14 287 164 1.4 GA COMP. EX. 81 183 17 12 331 234 1.4 GA INV. EX. 82 200 14 18 275 243 1.4 GA INV. EX. 83 207 16 11 277 166 1.4 GA COMP. EX. 84 189 18 10 273 218 1.4 CR INV. EX. 85 195 20 17 261 178 1.4 CR INV. EX. 86 189 13 15 346 275 1.4 GA COMP. EX. 87 183 13 16 265 158 1.4 GA INV. EX. 88 167 11 12 339 275 1.4 GA INV. EX. 89 218 18 16 285 232 1.4 GA INV. EX. 90 184 13 10 311 181 1.4 GA INV. EX. 91 202 13 14 293 298 1.4 GA INV. EX. 92 200 15 16 349 298 1.4 GA INV. EX. 93 186 11 15 277 141 1.5 CR INV. EX. 94 210 16 17 306 175 1.6 CR INV. EX. 95 194 16 10 318 172 1.2 CR INV. EX. 96 200 15 15 200 284 1.1 CR INV. EX. 97 209 19 8 335 277 1.4 CR INV. EX. 98 203 15 15 317 151 1.4 CR INV. EX. 99 109 16 17 339 162 1.4 CR INV. EX. 100 231 14 13 329 273 1.2 CR INV. EX. 101 198 17 13 291 280 1.1 CR INV. EX. 102 203 149 17 337 294 1.4 CR INV. EX. 103 190 14 5 307 193 1.5 CR INV. EX. 104 197 15 86 316 114 1.6 CR INV. EX. 105 172 13 11 172 172 1.2 CR INV. EX. 106 201 13 11 437 160 1.1 CR INV. EX. 107 197 19 17 311 21 1.5 CR INV. EX. 108 221 17 11 325 943 1.6 CR INV. EX. 109 196 11 10 302 294 1.2 EG INV. EX. 110 190 15 16 317 141 1.1 GI INV. EX. 111 202 13 9 333 138 1.4 EG INV. EX. 112 195 94 15 302 134 1.5 GI INV. EX. 113 188 103 17 267 162 1.8 GA INV. EX. Underlines indicate being outside the range of the present invention. *CR: cold rolled steel sheet (no coating), GI: hot-dip galvanized steel sheet (no alloying of zinc coating), GA: galvannealed steel sheet, EG: electrogalvanized steel sheet

(Microstructure Observation)

[0087] The amount of martensite, the amount of retained austenite, the total amount of ferrite and bainitic ferrite, and the average grain size of prior austenite were determined by the methods described hereinabove.

(Proportion of Packets Having the Largest Area in Prior Austenite Grains)

[0088] The average proportion of packets having the largest area in prior austenite grains was determined by the method described hereinabove.

(Tensile Test)

[0089] A JIS No. 5 test specimen (gauge length: 50 mm, parallel section width: 25 mm) was sampled so that the longitudinal direction of the test specimen would be perpendicular to the rolling direction. A tensile test was performed in accordance with JIS Z 2241 under conditions where the crosshead speed was 1.6710.sup.1 mm/sec. TS was thus measured. In accordance with aspects of the present invention, 1180 MPa or higher TS was determined to be acceptable.

(Bendability)

[0090] A 30 mm100 mm bendability test specimen was sampled and was tested by a V-block method in accordance with JIS Z 2248 to measure the minimum bending radius R that did not cause cracking on the ridge portion of the bend. The bending direction was the longitudinal direction of the test specimen. The minimum bending radius (R) was divided by the sheet thickness (t) to determine the value R/t. Bendability was evaluated as excellent when R/t was 6.0 or less. Here, the presence or absence of cracking was determined by analyzing the ridge portion of the bend top with a digital microscope (RH-2000 manufactured by HIROX CO., LTD.) at 40 magnification.

(Flatness in the Width Direction)

[0091] The cold rolled steel sheets obtained as described above were analyzed to measure the flatness in the width direction. The measurement is illustrated in FIG. 2. Specifically, a sheet with a length of 500 mm in the rolling direction (coil width500 mm Lsheet thickness) was cut out from the coil and was placed on a surface plate in such a manner that the warp at the ends would face upward. The height on the steel sheet was measured with a contact displacement meter by continuously moving the stylus over the width. Based on the results, the steepness as an index of the flatness of the steel sheet shape was measured as illustrated in FIG. 2. The flatness was rated as x when the steepness was more than 0.02, as when the steepness was more than 0.01 and 0.02 or less, and as when the steepness was 0.01 or less. The steel sheet was evaluated as excellent in the flatness in the width direction when the steepness was 0.02 or less.

(Working Embrittlement Resistance)

[0092] The working embrittlement resistance was evaluated by Charpy test. A Charpy test specimen was a 2 mm deep V-notched test piece that was a stack of steel sheets fastened together with bolts to eliminate any gaps between the steel sheets. The number of steel sheets that were stacked was controlled so that the thickness of the stack as the test piece would be closer to 10 mm. When, for example, the sheet thickness was 1.2 mm, eight sheets were stacked to give a 9.6 mm thick test piece. The sheets for stacking into the Charpy test specimen were sampled so that the width direction would be the longitudinal direction. As an index of the working embrittlement resistance, the ratio vE.sub.0%/vE.sub.10% of the absorbed impact energy at room temperature of the as-produced (unworked) steel sheet to that of the steel sheet after 10% rolling was measured. The working embrittlement resistance was rated as x when vE.sub.0%/vE.sub.10% was less than 0.6, as when vE.sub.0%/vE.sub.10% was 0.6 or more and less than 0.7, and as when VE.sub.0%/vE.sub.10% was 0.7 or more. The Charpy test specimen was evaluated as excellent in working embrittlement resistance when vE.sub.0%/vE.sub.10% was 0.6 or more. Conditions other than those described above conformed to JIS Z 2242: 2018.

[0093] The results are described in Tables 5 to 7. As shown in Tables 5 to 7, INVENTIVE EXAMPLES achieved 1180 MPa or higher TS and excellent bendability, flatness in the width direction, and working embrittlement resistance. In contrast, COMPARATIVE EXAMPLES were unsatisfactory in one or more of TS, bendability, flatness in the width direction, and working embrittlement resistance.

TABLE-US-00005 TABLE 5 Average proportion of Total of packets having Average Sheet ferrite and the largest grain size Limiting Working thick- Retained bainitic area in prior of prior bending Flatness embrit- ness Martensite austenite ferrite austenite grains austenite TS R in width tlement Nos. Steels (mm) (area %) (vol %) (area %) (area %) (m) (MPa) (mm) R/t direction resistance 1 A 1.4 81 11 7 56 10 1385 5.5 3.9 INV. EX. 2 B 1.4 85 9 4 53 11 1562 4.5 3.2 INV. EX. 3 B 1.4 81 9 10 47 8 1214 4.5 3.2 INV. EX. 4 B 1.4 79 8 13 59 10 1004 5.5 3.9 COMP. EX. 5 B 1.4 84 10 7 59 19 1410 5.5 3.9 INV. EX. 6 B 1.4 83 10 5 49 23 1514 4.5 3.2 X COMP. EX. 7 B 1.4 80 10 10 53 13 1200 4.5 3.2 INV. EX. 8 B 1.4 82 5 13 49 11 1102 5.5 3.9 COMP. EX. 9 B 1.4 84 9 6 59 16 1458 4.5 3.2 INV. EX. 10 B 1.4 84 10 5 53 24 1515 5.0 3.6 X COMP. EX. 11 B 1.4 86 8 5 65 14 1511 3.5 2.5 INV. EX. 12 B 1.4 87 9 3 91 10 1609 4.0 2.9 X X COMP. EX. 13 B 1.4 85 9 5 45 14 1509 4.0 2.9 INV. EX. 14 B 1.4 81 11 8 49 8 1364 3.5 2.5 INV. EX. 15 B 1.4 83 11 7 64 8 1409 4.0 2.9 INV. EX. 16 B 1.4 82 9 7 90 13 1407 2.0 1.4 X X COMP. EX. 17 B 1.4 84 10 5 53 14 1513 4.0 2.9 INV. EX. 18 B 1.4 82 9 7 57 11 1405 4.5 3.2 INV. EX. 19 B 1.4 81 8 10 54 14 1201 6.5 4.6 INV. EX. 20 B 1.4 74 9 17 49 14 1047 7.0 5.0 COMP. EX. 21 B 1.4 85 9 5 58 10 1514 5.5 3.9 INV. EX. 22 B 1.4 85 9 5 47 10 1511 1.0 0.7 INV. EX. 23 B 1.4 85 11 4 68 9 1554 7.0 5.0 INV. EX. 24 B 1.4 82 12 7 89 10 1413 6.0 4.3 X X COMP. EX. 25 B 1.4 86 10 3 60 14 1610 6.0 4.3 INV. EX. 26 B 1.4 86 10 4 50 13 1559 6.5 4.6 INV. EX. 27 B 1.4 92 3 2 46 11 1664 8.0 5.7 INV. EX. 28 B 1.4 95 2 3 55 13 1608 9.0 6.4 COMP. EX. 29 B 1.4 84 14 3 51 11 1604 7.0 5.0 INV. EX. 30 B 1.4 81 16 3 48 15 1609 6.0 4.3 X COMP. EX. 31 B 1.4 85 11 7 54 10 1415 5.0 3.6 INV. EX. 32 B 1.4 88 8 4 55 10 1562 5.0 3.6 INV. EX. 33 B 1.4 89 10 3 70 10 1607 5.0 3.6 INV. EX. 34 B 1.4 89 12 2 90 15 1655 6.0 4.3 X X COMP. EX. 35 B 1.4 84 9 8 68 10 1358 5.0 3.6 INV. EX. 36 B 1.4 86 10 3 81 9 1604 5.5 3.9 X X COMP. EX. 37 B 1.4 87 8 5 58 8 1510 6.0 4.3 INV. EX. 38 B 1.4 88 10 3 52 13 1606 6.5 4.6 INV. EX. 39 B 1.4 82 8 8 59 9 1356 5.0 3.6 INV. EX. 40 B 1.4 84 12 5 49 9 1512 3.5 2.5 INV. EX. 41 B 1.4 88 8 4 53 15 1199 6.0 4.3 INV. EX. 42 B 1.4 84 12 6 47 12 1207 5.0 3.6 INV. EX. 43 B 1.4 85 4 8 60 13 1360 8.0 5.7 INV. EX. Underlines indicate being outside the range of the present invention.

TABLE-US-00006 TABLE 6 Average proportion of Average Total of packets having grain Sheet ferrite and the largest area size of Limiting Working thick- Retained bainitic in prior prior bending Flatness embrit- ness Martensite austenite ferrite austenite grains austenite TS R in width tlement Nos. Steels (mm) (area %) (vol %) (area %) (area %) (m) (MPa) (mm) R/t direction resistance 44 B 1.4 95 2 2 59 12 1655 8.5 6.1 COMP. EX. 45 B 1.4 86 9 3 55 10 1207 6.0 4.3 INV. EX. 46 B 1.4 85 9 6 58 14 1195 6.5 4.6 INV. EX. 47 B 1.4 83 11 7 54 15 1408 5.0 3.6 INV. EX. 48 B 1.6 86 8 6 56 9 1456 6.0 3.8 INV. EX. 49 C 2.0 90 9 2 56 10 1665 8.0 4.0 INV. EX. 50 D 1.4 85 11 7 47 13 1440 6.0 4.3 INV. EX. 51 E 1.4 87 8 2 58 11 1627 6.0 4.3 INV. EX. 52 F 1.4 81 10 10 51 12 1189 6.5 4.6 INV. EX. 53 G 1.4 83 5 12 47 10 1103 5.0 3.6 COMP. EX. 54 H 1.4 85 11 5 59 10 1803 6.0 4.3 INV. EX. 55 I 1.4 83 8 6 48 14 1995 6.0 4.3 X COMP. EX. 56 J 1.4 82 10 7 47 11 1211 4.0 2.9 INV. EX. 57 K 1.4 86 9 2 58 14 1175 6.0 4.3 COMP. EX. 58 L 1.4 84 13 2 47 14 1711 5.0 3.6 INV. EX. 59 M 1.4 82 16 2 46 14 1749 3.0 2.1 X COMP. EX. 60 N 1.4 82 10 9 59 15 1188 6.5 4.6 INV. EX. 61 O 1.4 72 9 19 56 10 1011 7.0 5.0 COMP. EX. 62 P 1.4 85 10 5 46 9 1602 6.0 4.3 INV. EX. 63 Q 1.4 88 10 4 48 9 1669 5.5 3.9 X COMP. EX. 64 R 1.4 82 10 8 52 10 1284 3.0 2.1 INV. EX. 65 S 1.4 88 10 4 46 10 1491 0.5 0.4 X COMP. EX. 66 T 1.4 85 12 3 56 15 1520 4.0 2.9 INV. EX. 67 U 1.4 86 9 4 49 8 1525 3.0 2.1 X COMP. EX. 68 V 1.4 87 11 3 47 11 1639 6.5 4.6 INV. EX. 69 W 1.4 83 9 8 57 9 1466 5.5 3.9 X COMP. EX. 70 X 1.4 86 11 4 45 15 1587 5.5 3.9 INV. EX. 71 Y 1.4 83 10 8 60 14 1382 4.0 2.9 X COMP. EX. 72 Z 1.4 85 8 4 49 13 1515 4.5 3.2 INV. EX. 73 AA 1.4 85 10 7 54 9 1460 5.5 3.9 X COMP. EX. 74 AB 1.4 87 12 5 58 14 1479 5.0 3.6 INV. EX. 75 AC 1.4 86 11 5 50 10 1193 5.5 3.9 INV. EX. 76 AD 1.4 85 8 5 57 10 1488 6.0 4.3 INV. EX. 77 AE 1.4 86 10 4 52 13 1879 6.0 4.3 X COMP. EX. 78 AF 1.4 85 11 6 56 12 1197 6.0 4.3 INV. EX. 79 AG 1.4 85 12 5 49 10 1514 6.0 4.3 INV. EX. 80 AH 1.4 86 11 2 49 13 1880 4.5 3.2 X COMP. EX. 81 AI 1.4 81 12 7 55 14 1212 6.0 4.3 INV. EX. 82 AJ 1.4 89 10 2 47 10 1668 5.0 3.6 INV. EX. 83 AK 1.4 87 9 3 57 14 1892 5.0 3.6 X COMP. EX. 84 AL 1.4 80 11 8 58 12 1195 4.5 3.2 INV. EX. 85 AM 1.4 87 9 5 50 11 1492 5.5 3.9 INV. EX. 86 AN 1.4 88 11 3 51 14 1851 5.5 3.9 X COMP. EX. Underlines indicate being outside the range of the present invention.

TABLE-US-00007 TABLE 7 Average proportion of Total of packets having Average Sheet ferrite and the largest area grain size Limiting Working thick- Retained bainitic in prior of prior bending Flatness embrit- ness Martensite austenite ferrite austenite grains austenite TS R in width tlement Nos. Steels (mm) (area %) (vol %) (area %) (area %) (m) (MPa) (mm) R/t direction resistance 87 AO 1.4 81 8 9 55 9 1194 4.0 2.9 INV. EX. 88 AP 1.4 86 11 6 54 16 1493 5.5 3.9 INV. EX. 89 AQ 1.4 82 9 9 54 15 1195 5.5 3.9 INV. EX. 90 AR 1.4 87 9 4 54 17 1541 5.0 3.6 INV. EX. 91 AS 1.4 81 10 7 48 13 1416 6.0 4.3 INV. EX. 92 AT 1.4 82 11 8 49 12 1305 7.5 5.4 INV. EX. 93 AU 1.5 86 10 3 63 14 1611 6.5 4.3 INV. EX. 94 AV 1.6 83 12 7 58 10 1355 5.5 3.4 INV. EX. 95 AW 1.2 81 10 10 46 14 1202 4.0 3.3 INV. EX. 96 AX 1.1 88 11 3 53 11 1594 4.0 3.6 INV. EX. 97 AY 1.4 82 10 6 63 10 1438 5.0 3.6 INV. EX. 98 AZ 1.4 83 10 6 49 13 1426 7.5 5.4 INV. EX. 99 BA 1.4 90 4 5 48 11 1481 8.0 5.7 INV. EX. 100 BB 1.2 83 13 4 52 11 1602 4.5 3.8 INV. EX. 101 BC 1.1 85 8 7 47 14 1374 5.0 4.5 INV. EX. 102 BD 1.4 85 10 4 70 12 1484 3.0 2.1 INV. EX. 103 BE 1.5 83 9 6 69 14 1384 7.0 4.7 INV. EX. 104 BF 1.6 87 11 5 52 12 1483 8.5 5.3 INV. EX. 105 BG 1.2 85 10 4 47 11 1541 5.5 4.6 INV. EX. 106 BH 1.1 87 11 3 55 9 1217 4.0 3.6 INV. EX. 107 BI 1.5 88 3 5 46 12 1436 8.5 5.7 INV. EX. 108 BJ 1.6 81 9 8 56 11 1201 7.0 4.4 INV. EX. 109 BK 1.2 85 11 4 50 15 1590 5.5 4.6 INV. EX. 110 BL 0.6 86 8 2 59 11 1661 2.5 4.2 INV. EX. 111 BM 2.2 84 8 5 47 12 1441 8.0 3.6 INV. EX. 112 BN 1.5 86 12 4 51 10 1558 6.0 4.0 INV. EX. 113 BO 1.8 88 9 3 52 13 1530 8.5 4.7 INV. EX. Underlines indicate being outside the range of the present invention.