HIGH-STRENGTH STEEL SHEET, HIGH-STRENGTH GALVANIZED STEEL SHEET, METHOD FOR MANUFACTURING HIGH-STRENGTH STEEL SHEET, AND METHOD FOR MANUFACTURING HIGH-STRENGTH GALVANIZED STEEL SHEET

Abstract

Provided are a high-strength steel sheet having a specified chemical composition, in which a Mn-segregation degree in a region within 100 m from a surface thereof in a thickness direction is 1.5 or less, in a plane parallel to the surface of the steel sheet in a region within 100 m from the surface of the steel sheet in the thickness direction, the number of oxide-based inclusion grains having a grain long diameter of 5 m or more is 1000 or less/100 mm.sup.2, a proportion of the number of oxide-based inclusion grains having a chemical composition containing alumina of 50 mass % or more, silica of 20 mass % or less, and calcia of 40 mass % or less to the total number of oxide-based inclusions having a grain long diameter of 5 m or more is 80% or more, a specified metallographic structure, and a TS of 980 MPa or more, a high-strength galvanized steel sheet, and a manufacturing method thereof.

Claims

1. A high-strength steel sheet having a chemical composition containing, by mass %, C: 0.07% to 0.30%, Si: 0.10% to 2.5%, Mn: 1.8% to 3.7%, P: 0.03% or less, S: 0.0020% or less, Sol. Al: 0.01% to 1.0%, N: 0.0006% to 0.0055%, O: 0.0008% to 0.0025%, and the balance being Fe and inevitable impurities, wherein a Mn-segregation degree in a region within 100 m from a surface of the steel sheet in a thickness direction is 1.5 or less, in a plane parallel to the surface of the steel sheet in a region within 1.00 m from the surface of the steel sheet in the thickness direction, the number of oxide-based inclusion grains having a grain long diameter of 5 m or more is 1000 or less per 100 mm.sup.2, a proportion of the number of oxide-based inclusion grains having a chemical composition containing alumina in an amount of 50 mass % or more, silica in an amount of 20 mass % or less, and calcia in an amount of 40 mass % or less to the total number of oxide-based inclusion grains having a grain long diameter of 5 m or more is 80% or more, a metallographic structure including, in terms of volume fraction, a martensite phase and a bainite phase in an amount of 25% to 100% in total, a ferrite phase in an amount of less than 75% (including 0%), and an austenite phase in an amount of less than 15% (including 0%), and a tensile strength of 980 MPa or more.

2. The high-strength steel sheet according to claim 1, wherein Si (mass %)/Mn (mass %) is 0.20 or more and 1.00 or less in the chemical composition.

3. The high-strength steel sheet according to claim 1, wherein the chemical composition further contains, at least any one group selected from the groups of: Group I: by mass %, Ca: 0.0002% to 0.0030%, Group II: by mass %, one, two, or more of Ti: 0.01% to 0.1%, Nb: 0.01% to 0.1%, V: 0.001% to 0.1%, and Zr: 0.001% to 0.1%, Group III: by mass %, one, two, or all of Cr: 0.01% to 1.0%, Mo: 0.01% to 0.20%, and B: 0.0001% to 0.0030%, Group IV: by mass %, one, two, or all of Cu: 0.01% to 0.5%, Ni: 0.01% to 0.5%, and Sn: 0.001% to 0.1%, Group V: by mass %, Sb: 0.005% to 0.05%, and Group VI: by mass %, one or both of REM and Mg in an amount of 0.0002% or more and 0.01% or less in total.

4. The high-strength steel sheet according to claim 2, wherein the chemical composition further contains, at least any one group selected from the groups of: Group I: by mass %, Ca: 0.0002% to 0.0030%, Group II: by mass %, one, two, or more of Ti: 0.01% to 0.1%; Nb: 0.01% to 0.1%, V: 0.001% to 0.1%, and Zr: 0.001% to 0.1%, Group III: by mass %, one, two, or all of Cr: 0.01% to 1.0%, Mo: 0.01% to 0.20%, and B: 0.0001% to 0.0030%, Group IV: by mass %, one, two, or all of Cu: 0.01% to 0.5%, Ni: 0.01% to 0.5%, and Sn: 0.001% to 0.1%, Group V: by mass %, Sb: 0.005% to 0.05%, and Group VI: by mass %, one or both of REM and Mg in an amount of 0.0002% or more and 0.01% or less in total.

5. A high-strength galvanized steel sheet having the high-strength steel sheet according to claim 1 and a galvanizing layer formed on the surface of the high-strength steel sheet.

6. A high-strength galvanized steel sheet having the high-strength steel sheet according to claim 2 and a galvanizing layer formed on the surface of the high-strength steel sheet.

7. A high-strength galvanized steel sheet having the high-strength steel sheet according to claim 3 and a galvanizing layer formed on the surface of the high-strength steel sheet.

8. A high-strength galvanized steel sheet having the high-strength steel sheet according to claim 4 and a galvanizing layer formed on the surface of the high-strength steel sheet.

9. A method for manufacturing the high-strength steel sheet according to claim 1, the method comprising performing refining in an RH vacuum degasser with a circulation time of 900 seconds or more, performing continuous casting on the refined molten steel under a condition that a flow rate of the molten steel at a solidification interface in a vicinity of a meniscus of a mold is 1.2 m/min or less, heating the cast steel obtained through the casting directly or after having cooled the steel to a temperature of 1220 C. or higher and 1300 C. or lower, performing a first pass of rough rolling with a rolling reduction of 10% or more, performing a first pass of finish rolling with a rolling reduction of 20% or more, completing hot rolling at a finishing delivery temperature equal to or higher than the Ar.sub.3 transformation temperature, performing coiling at a temperature range of 400 C. or higher and lower than 550 C. to obtain a hot-rolled steel sheet, pickling the hot-rolled steel sheet, performing cold rolling on the pickled steel sheet with a rolling reduction ratio of 40% or more to obtain a cold-rolled steel sheet, heating the cold-rolled steel sheet at a heating temperature of 800 C. to 880 C., cooling the heated steel sheet to a rapid-cooling start temperature of 550 C. to 750 C., in which a retention time in a temperature range of 800 C. to 880 C. through the heating and cooling is 10 seconds or more, performing cooling at an average cooling rate of 15 C./sec or more from the rapid-cooling start temperature to a rapid-cooling stop temperature of 350 C. or lower, and holding the rapidly cooled steel sheet in a temperature range of 150 C. to 450 C. for a retention time of 100 seconds to 1000 seconds.

10. A method for manufacturing the high-strength steel sheet according to claim 2, the method comprising performing refining in an RH vacuum degasser with a circulation time of 900 seconds or more, performing continuous casting on the refined molten steel under a condition that a flow rate of the molten steel at a solidification interface in a vicinity of a meniscus of a mold is 1.2 m/min or less, heating the cast steel obtained through the casting directly or after having cooled the steel to a temperature of 1220 C. or higher and 1300 C. or lower, performing a first pass of rough rolling with a rolling reduction of 10% or more, performing a first pass of finish rolling with a rolling reduction of 20% or more, completing hot rolling at a finishing delivery temperature equal to or higher than the Ar.sub.3 transformation temperature, performing coiling at a temperature range of 400 C. or higher and lower than 550 C. to obtain a hot-rolled steel sheet, pickling the hot-rolled steel sheet, performing cold rolling on the pickled steel sheet with a rolling reduction ratio of 40% or more to obtain a cold-rolled steel sheet, heating the cold-rolled steel sheet at a heating temperature of 800 C. to 880 C., cooling the heated steel sheet to a rapid-cooling start temperature of 550 C. to 750 C., in which a retention time in a temperature range of 800 C. to 880 C. through the heating and cooling is 10 seconds or more, performing cooling at an average cooling rate of 15 C./sec or more from the rapid-cooling start temperature to a rapid-cooling stop temperature of 350 C. or lower, and holding the rapidly cooled steel sheet in a temperature range of 150 C. to 450 C. for a retention time of 100 seconds to 1000 seconds.

11. A method for manufacturing the high-strength steel sheet according to claim 3, the method comprising performing refining in an RH vacuum degasser with a circulation time of 900 seconds or more, performing continuous casting on the refined molten steel under a condition that a flow rate of the molten steel at a solidification interface in a vicinity of a meniscus of a mold is 1.2 m/min or less, heating the cast steel obtained through the casting directly or after having cooled the steel to a temperature of 1220 C. or higher and 1300 C. or lower, performing a first pass of rough rolling with a rolling reduction of 10% or more, performing a first pass of finish rolling with a rolling reduction of 20% or more, completing hot rolling at a finishing delivery temperature equal to or higher than the Ara transformation temperature, performing coiling at a temperature range of 400 C. or higher and lower than 550 C. to obtain a hot-rolled steel sheet, pickling the hot-rolled steel sheet, performing cold rolling on the pickled steel sheet with a rolling reduction ratio of 40% or more to obtain a cold-rolled steel sheet, heating the cold-rolled steel sheet at a heating temperature of 800 C. to 880 C., cooling the heated steel sheet to a rapid-cooling start temperature of 550 C. to 750 C., in which a retention time in a temperature range of 800 C. to 880 C. through the heating and cooling is 10 seconds or more, performing cooling at an average cooling rate of 15 C./sec or more from the rapid-cooling start temperature to a rapid-cooling stop temperature of 350 C. or lower, and holding the rapidly cooled steel sheet in a temperature range of 150 C. to 450 C. for a retention time of 100 seconds to 1000 seconds.

12. A method for manufacturing the high-strength steel sheet according to claim 4, the method comprising performing refining in an RH vacuum degasser with a circulation time of 900 seconds or more, performing continuous casting on the refined molten steel under a condition that a flow rate of the molten steel at a solidification interface in a vicinity of a meniscus of a mold is 1.2 m/min or less, heating the cast steel obtained through the casting directly or after having cooled the steel to a temperature of 1220 C. or higher and 1300 C. or lower, performing a first pass of rough rolling with a rolling reduction of 10% or more, performing a first pass of finish rolling with a rolling reduction of 20% or more, completing hot rolling at a finishing delivery temperature equal to or higher than the Ar.sub.3 transformation temperature, performing coiling at a temperature range of 400 C. or higher and lower than 550 C. to obtain a hot-rolled steel sheet, pickling the hot-rolled steel sheet, performing cold rolling on the pickled steel sheet with a rolling reduction ratio of 40% or more to obtain a cold-rolled steel sheet, heating the cold-rolled steel sheet at a heating temperature of 800 C. to 880 C., cooling the heated steel sheet to a rapid-cooling start temperature of 550 C. to 750 C., in which a retention time in a temperature range of 800 C. to 880 C. through the heating and cooling is 10 seconds or more, performing cooling at an average cooling rate of 15 C./sec or more from the rapid-cooling start temperature to a rapid-cooling stop temperature of 350 C. or lower, and holding the rapidly cooled steel sheet in a temperature range of 150 C. to 450 C. for a retention time of 100 seconds to 1000 seconds.

13. A method for manufacturing a high-strength galvanized steel sheet, the method comprising forming a galvanizing layer on the surface of the high-strength steel sheet obtained by using the method according to claim 9.

14. A method for manufacturing a high-strength galvanized steel sheet, the method comprising forming a galvanizing layer on the surface of the high-strength steel sheet obtained by using the method according to claim 10.

15. A method for manufacturing a high-strength galvanized steel sheet, the method comprising forming a galvanizing layer on the surface of the high-strength steel sheet obtained by using the method according to claim 11.

16. A method for manufacturing a high-strength galvanized steel sheet, the method comprising forming a galvanizing layer on the surface of the high-strength steel sheet obtained by using the method according to claim 12.

Description

EXAMPLES

[0125] By using steels having the chemical compositions given in Table 1, steel ingots were manufactured through melting and casting processes under the conditions given in Table 2. The obtained steel ingots (slabs having a thickness of 250 mm) were subjected to hot rolling under the conditions given in Table 2 to obtain hot-rolled steel sheets having a thickness of 2.6 mm. Subsequently, cold rolling was performed in order to obtain a thickness of 1.4 mm. Furthermore, a heat treatment simulating continuous annealing was performed.

[0126] This heat treatment simulating continuous annealing was performed under the conditions given in Table 2 (the cooling rate to the rapid-cooling stop temperature was 10 C./s). Subsequently, a tempering treatment was performed by reheating the steel sheets or by holding the steel sheets at the rapid-cooling stop temperature under the conditions given in Table 2, cooling was performed thereafter, and skin pass rolling was then performed with an elongation ratio of 0.2%.

TABLE-US-00001 TABLE 1 mass % Steel No. C Si Mn P S Sol. Al N O Cr V Sb Mo Cu 1 0.081 0.69 2.63 0.019 0.0011 0.037 0.0035 0.0011 0 0 0.012 0 0 2 0.094 0.54 2.69 0.018 0.0010 0.045 0.0038 0.0012 0 0 0.011 0 0 3 0.095 0.56 2.65 0.022 0.0016 0.058 0.0049 0.0009 0 0.06 0.015 0 0 4 0.083 0.63 2.52 0.024 0.0015 0.040 0.0041 0.0013 0.09 0 0 0.18 0 5 0.089 0.60 2.66 0.021 0.0017 0.022 0.0048 0.0012 0 0 0.014 0 0.07 6 0.125 0.53 2.52 0.014 0.0018 0.056 0.0035 0.0014 0 0 0.013 0 0 7 0.132 0.06 2.62 0.009 0.0014 0.033 0.0044 0.0015 0 0 0.006 0 0 8 0.200 0.65 2.45 0.012 0.0008 0.41 0.0041 0.0021 0 0 0 0 0 9 0.310 0.65 2.43 0.015 0.0006 0.031 0.0038 0.0016 0 0 0.014 0 0 10 0.132 0.72 2.26 0.016 0.0008 0.054 0.0027 0.0042 0 0 0.013 0 0 11 0.141 0.83 2.03 0.013 0.0010 0.020 0.0039 0.0016 0.35 0 0.012 0 0 12 0.195 0.74 2.51 0.014 0.0011 0.037 0.0034 0.0011 0 0 0.009 0 0 13 0.080 2.65 3.75 0.007 0.0021 0.028 0.0033 0.0008 0 0 0.011 0 0 14 0.051 1.60 1.42 0.019 0.0041 0.036 0.0044 0.0009 0 0 0.009 0 0 15 0.106 0.65 2.45 0.015 0.0010 0.035 0.0038 0.0015 0 0 0 0 0 16 0.125 0.25 2.82 0.014 0.0008 0.038 0.0041 0.0012 0 0 0 0 0 17 0.102 0.81 3.80 0.008 0.0005 0.035 0.0031 0.0015 0 0 0 0 0 18 0.103 0.64 2.64 0.007 0.0032 0.039 0.0035 0.0016 0 0 0 0 0 19 0.101 0.65 2.65 0.006 0.0005 0.045 0.0039 0.0022 0 0 0 0 0 Steel No. Ni Sn Ti Nb Zr B Ca REM, Mg Si/Mn Note 1 0 0 0.019 0.041 0 0.0015 0.0004 0 0.26 Example 2 0 0 0.018 0.044 0 0.0012 0.0005 0 0.20 Example 3 0 0 0.015 0.046 0 0.0028 0.0011 0 0.21 Example 4 0 0 0.016 0.032 0.005 0.0008 0.0013 0 0.25 Example 5 0.06 0 0.025 0.035 0 0.0011 0.0012 0.0005 0.23 Example 6 0 0.005 0.015 0.034 0 0.0011 0.0004 0 0.21 Example 7 0 0 0.016 0.035 0 0.0017 0.0003 0 0.02 Comparative Example 8 0 0 0 0 0 0 0.0005 0 0.27 Example 9 0 0 0 0.031 0 0.0012 0.0045 0 0.27 Comparative Example 10 0 0 0.015 0.024 0 0.0015 0.0041 0 0.32 Comparative Example 11 0 0 0.016 0.022 0 0.0012 0.0003 0 0.41 Example 12 0 0 0.021 0.023 0 0.0013 0.0021 0 0.29 Example 13 0 0 0.034 0.053 0 0.0012 0.0005 0 0.71 Comparative Example 14 0 0 0.020 0.032 0 0.0014 0.0004 0 1.13 Comparative Example 15 0 0 0 0 0 0 <0.0002 0 0.27 Example 16 0 0 0 0 0 0 <0.0002 0 0.09 Example 17 0 0 0 0 0 0 0.0003 0 0.21 Comparative Example 18 0 0 0 0 0 0 0.0004 0 0.24 Comparative Example 19 0 0 0 0 0 0 0.0039 0 0.25 Comparative Example *Underlined portions indicate values out of the range of the present invention.

TABLE-US-00002 TABLE 2 Hot Rolling Condition Quenching & Tempering First First Treatment Molten Rolling Rolling Cold Steel Reduction Reduction Finishing Rolling Steel Circulation Flow Heating Heating of Rough of Finish Delivery Coiling Reduction Soaking Soaking Sheet Steel Time Rate Temperature Time Rolling Rolling Temperature Temperature Ratio Temperature Time No. No. (sec) (m/min) ( C.) (min.) (%) (%) ( C.) ( C.) (%) ( C.) (s) 1A 1 1000 1.0 1250 180 15 25 880 520 50 830 100 1B 1 600 1.0 1250 180 14 24 880 520 50 830 100 1C 1 1000 1.7 1250 180 16 23 880 520 50 830 100 1D 1 1000 1.0 1250 180 8 15 880 520 50 830 100 2A 2 1000 1.0 1250 180 15 26 880 520 50 815 100 2B 2 1150 1.0 1250 180 14 23 880 520 50 815 100 2C 2 1000 1.0 1150 180 15 24 880 520 50 815 100 3A 3 1000 1.0 1250 180 14 23 880 520 50 830 100 4A 4 1000 1.0 1250 180 14 25 880 520 50 840 100 5A 5 1000 1.0 1250 180 13 24 880 520 50 815 100 6A 6 1000 1.0 1250 180 15 26 880 520 50 860 100 7A 7 1000 1.0 1250 180 16 5 880 520 50 850 100 8A 8 750 1.5 1250 180 16 24 880 520 50 810 300 8B 8 1000 1.0 1250 180 16 25 880 520 50 810 300 9A 9 1000 1.0 1250 180 15 26 880 520 50 830 300 10A 10 1000 1.0 1250 180 17 25 880 520 50 860 300 11A 11 1000 1.0 1250 180 15 25 880 520 50 860 300 12A 12 1000 1.0 1250 180 13 25 880 520 50 860 300 12B 12 1000 1.0 1250 180 16 23 880 520 50 860 250 13A 13 1000 1.0 1180 10 14 27 880 520 50 830 300 14A 14 1000 1.5 1250 180 15 23 880 520 50 830 300 15A 15 1000 1.0 1250 180 15 25 860 500 50 830 100 16A 16 1000 1.0 1250 180 15 25 860 500 50 830 100 17A 17 1000 1.0 1250 180 15 25 860 500 50 830 100 18A 18 1000 1.0 1250 180 15 26 860 500 50 830 100 19A 19 1000 1.0 1250 180 14 24 860 500 50 830 100 Quenching & Tempering Treatment Average Cooling Rate to Rapid- Rapid- Rapid- Steel cooling Start cooling Stop cooling Stop Holding Holding Sheet Temperature Temperature Temperature Temperature Time No. ( C.) ( C./s) ( C.) ( C.) (s) Note 1A 710 18 340 300 470 Example 1B 710 18 340 300 470 Comparative Example 1C 710 18 340 300 470 Comparative Example 1D 710 18 340 300 470 Comparative Example 2A 700 24 260 260 490 Example 2B 520 14 260 260 490 Comparative Example 2C 650 20 300 260 490 Comparative Example 3A 640 26 290 260 540 Example 4A 700 17 340 350 730 Example 5A 650 19 330 300 610 Example 6A 670 15 340 380 490 Example 7A 710 5 250 250 490 Comparative Example 8A 700 650 25 350 600 Comparative Example 8B 700 650 25 350 720 Example 9A 700 650 25 300 600 Comparative Example 10A 730 700 25 200 600 Comparative Example 11A 730 700 25 250 500 Example 12A 730 700 25 300 500 Example 12B 730 850 25 200 800 Example 13A 700 650 25 250 720 Comparative Example 14A 700 650 25 300 650 Comparative Example 15A 700 19 340 300 440 Example 16A 700 19 340 300 440 Example 17A 700 700 25 300 800 Comparative Example 18A 700 700 25 300 800 Comparative Example 19A 700 18 340 340 450 Comparative Example *Underlined portions indicate values out of the range of the present invention.

[0127] The steel sheets obtained as described above were subjected to investigations and evaluations regarding the Mn-segregation degree, oxide-based inclusions, metallographic structure (phase fraction (volume fraction)), tensile properties, and bending workability as described below.

[0128] Evaluation of Mn-Segregation Degree

[0129] Mn concentration distribution was determined in a region of 150 mm.sup.2 located within 100 m from the surface in the thickness direction by using an EPMA (Electron Probe Micro Analyzer). At this time, since the determined value of the Mn-segregation degree (the maximum Mn concentration in a region within 100 m from the surface/the average Mn concentration in a region within 100 m from the surface) depends on the probe diameter of the EPMA, the segregation, of Mn was evaluated by using a probe having a diameter of 2 m. Here, since there is an increase in the apparent maximum Mn-segregation degree in the case where inclusions such as MnS exist, an evaluation was conducted with the value for inclusions being excluded in the case where inclusions were detected.

[0130] Evaluation of Oxide-Based Inclusions in Steel Sheet

[0131] The number of inclusion grains having a grain long diameter of 5 m or more was investigated in a region of 10 mm10 mm in planes parallel to the surface of the steel sheet at a depth of 50 m and at a depth of 100 m from the surface of the steel sheet in the thickness direction (since the results obtained at a depth of 50 m and at a depth of 100 m were the same (because of homogeneity), only one of the results is given in the Table). Here, it is needless to say that the plane parallel to the surface of the steel sheet was a plane including the rolling direction (a plane which includes the rolling direction and which is parallel to the surface of the steel sheet). In addition, regarding all the inclusion grains having a grain long diameter of 5 m or more, the chemical composition thereof was quantitatively analyzed by performing SEM-EDX analysis to obtain the number of inclusion grains having a chemical composition containing alumina in an amount of 50 mass % or more, silica in an amount of 20 mass % or less, and calcia in an amount of 40 mass % or less (the number of grains having the appropriate chemical composition). In addition, the proportion of the number of grains having the appropriate chemical composition obtained as described above to the total number of inclusion grains having a grain long diameter of 5 m or more ((number of grains having the appropriate chemical composition)/(total number of inclusion grains having a grain long diameter of 5 m or more)) was calculated and defined as the proportion of grains having the appropriate chemical composition.

[0132] Metallographic Structure (Phase Fraction)

[0133] A plane located at of the thickness in a cross section in the rolling direction was observed by using a scanning electron microscope (SEM). By performing observation 5 times (in 5 observation fields of view), by performing image analysis on cross-sectional microstructure photographs taken at a magnification of 2000 times to determine the occupation areas of each phase in a region of 50 m square, and by calculating the average occupation area, the average occupation area was defined as the volume fraction of the phase. Here, the occupation area of the phases other than a ferrite phase and a pearlite phase was regarded as that of a martensite phase, a bainite phase, and a retained austenite phase. Subsequently, the amount of a retained austenite phase was determined by using an X-ray diffraction method with the K ray of Mo. That is, by using a test piece prepared so that the plane located at about of the thickness of the steel sheet was observed, and by calculating the volume fraction of a retained austenite phase from the peak intensities of the (211)-plane and (220)-plane of an austenite phase and the (200)-plane and (220)-plane of a ferrite phase, the volume fraction was defined as the volume fraction of a retained austenite phase. Subsequently, the difference calculated by subtracting the volume fraction of a retained austenite phase from the volume fraction corresponding to the occupation area which was regarded as that of a martensite phase, a bainite phase, and a retained austenite phase as described above was defined as the volume fraction of a martensite phase and a bainite phase.

[0134] Tensile Properties

[0135] By performing a tensile test in accordance with JIS Z 2241 on a JIS No. 5 test piece (JIS Z 2201) which had been taken so that the longitudinal direction thereof was a direction at a right angle to the rolling direction of the steel sheet, yield strength (YS), tensile strength (TS), and total elongation (El), which is the index of ductility, were determined. In addition, in the case of the example of, the present invention, a tensile strength of 980 MPa or more was achieved.

[0136] Bending Workability

[0137] By determining the limit bending radius (R (mm)) by performing a V block bend test (tip angle of the pressing tool: 90, tip radius R: increased at intervals of 0.5 mm from 0.5 mm) in accordance with JIS Z 2248 on a JIS No. 3 test piece which had been taken from a position located at of the width of the steel sheet so that the longitudinal direction thereof was the width direction of the coil, R/t was calculated by dividing the limit bending radius by the thickness (t (mm)) and used as an index. In addition, to evaluate variation in bendability in the width direction, the bending test was performed with the radius being equal to the limit bending radius R, which was used to calculate R/t described above, 5 times each at 7 positions located at through of the width. A case where the incidence ratio of cracking was 6% or less was judged as good. In the evaluation of bendability, by performing observation with a loupe at a magnification of 10 times, a case where a crack having a length of 0.2 mm or more was observed was judged as a case where cracking occurred.

[0138] The evaluation results are given in Table 3. As the results indicate, it is clarified that the examples of the present invention had a tensile strength. TS of 980 MPa or more, a limit bending radius R/t of 1.5 or less in the case of 980 MPa grade, 2.5 or less in the case of 1180 MPa grade, and 3.0 or less in the case of 1320 MPa grade or more, that is, excellent mechanical properties and bending workability. On the other hand, the comparative examples were poor in terms of at least one of such properties. In addition, the examples of the present invention had good stretch flange formability.

TABLE-US-00003 TABLE 3 Microstructure Observation Result of Oxide-based Inclusion Volume Segregation Number of Grains Proportion of Grains Volume Volume Volume Fraction Fraction Steel Mn Having Appropriate Having Appropriate Fraction of Fraction of of Bainite and of Sheet Segregation Piece/ Chemical Chemical Ferrite Austenite Martensite Pearlite No. Degree cm.sup.2 Composition Composition (%) (%) (%) (%) 1A 1.2 710 670 0.94 39 0 61 0 1B 1.3 1340 920 0.69 40 0 60 0 1C 1.2 1510 1010 0.67 38 0 62 0 1D 2.3 750 680 0.91 39 0 61 0 2A 1.3 720 640 0.89 37 0 63 0 2B 1.3 750 660 0.88 65 11 24 0 2C 1.7 770 640 0.83 43 0 57 0 3A 1.3 630 580 0.92 42 0 58 0 4A 1.2 680 570 0.84 45 0 55 0 5A 1.1 720 630 0.88 37 0 63 0 6A 1.4 760 640 0.84 7 5 88 0 7A 2.0 680 580 0.85 77 0 8 15 8A 1.2 1450 980 0.68 42 0 58 0 8B 1.3 820 690 0.84 9 0 91 0 9A 1.2 680 560 0.82 0 0 100 0 10A 1.3 400 350 0.88 5 0 95 0 11A 1.3 470 410 0.87 37 0 63 0 12A 1.4 650 590 0.91 12 0 88 0 12B 1.3 600 520 0.87 0 0 100 0 13A 1.7 630 550 0.87 35 0 65 0 14A 1.2 1560 950 0.61 81 0 19 0 15A 1.4 500 425 0.85 41 0 59 0 16A 1.3 420 380 0.90 11 0 89 0 17A 1.6 630 540 0.86 31 0 69 0 18A 1.3 650 535 0.82 41 0 59 0 19A 1.3 750 560 0.75 40 3 57 0 Property Steel Yield Tensile Variation in Sheet Strength Strength Ductility Bendability No. (MPa) (MPa) YR (%) R/t (%) Note 1A 785 1026 0.765 16.8 1.1 3 Example 1B 781 1024 0.763 16.4 1.8 14 Comparative Example 1C 779 1032 0.755 16.3 1.8 11 Comparative Example 1D 783 1022 0.766 16.7 1.4 14 Comparative Example 2A 865 1147 0.754 14.1 0.7 0 Example 2B 623 824 0.756 22.1 0.4 0 Comparative Example 2C 812 1045 0.777 15.0 1.8 11 Comparative Example 3A 842 1132 0.744 12.0 1.1 3 Example 4A 759 1095 0.693 12.4 0.7 0 Example 5A 761 1096 0.694 12.6 0.7 3 Example 6A 892 1253 0.712 11.5 1.8 3 Example 7A 441 642 0.687 28.7 <0.4 0 Comparative Example 8A 872 1158 0.753 13.1 3.2 11 Comparative Example 8B 901 1182 0.762 11.2 2.1 0 Example 9A 1342 1750 0.767 6.4 4.3 3 Comparative Example 10A 1035 1330 0.778 7.5 3.6 6 Comparative Example 11A 724 1018 0.711 16.6 1.1 0 Example 12A 967 1268 0.763 10.2 1.9 3 Example 12B 1146 1390 0.824 8.7 2.7 5 Example 13A 768 981 0.783 15.4 2.1 11 Comparative Example 14A 574 724 0.793 31.2 0.4 14 Comparative Example 15A 732 1051 0.696 15.8 1.1 3 Example 16A 904 1242 0.728 11.3 1.8 0 Example 17A 869 1231 0.706 12.4 3.3 5 Comparative Example 18A 702 1035 0.678 15.1 2.2 15 Comparative Example 19A 691 981 0.704 14.9 1.9 11 Comparative Example *Underlined portions indicate values out of the range of the present invention.