STEEL SHEET AND METHOD FOR MANUFACTURING STEEL SHEET

Abstract

In a steel sheet according to the present embodiment, a Ti content and a N content satisfy Ti−3.5×N≥0.003, at a sheet thickness ¼ position, a metallographic structure includes 90% or more of martensite in terms of volume fraction, at the sheet thickness ¼ position, a number density of TiC having a circle equivalent diameter of 1 to 500 nm is 3.5×10.sup.4 particles/mm.sup.2 or more, at the sheet, thickness ¼ position, a value of a median value of a Mn concentration+3σ is 5.00% or less, and a hardness measured at the sheet thickness ¼ position is 1.30 times or more a hardness measured at a position 50 μm deep from a surface of the steel sheet.

Claims

1. A steel sheet comprising, as a chemical composition, in a unit of mass %: C: 0.20% or more and 0.45% or less; Si: 0.01% or more and 2.50% or less; Mn: 1.20% or more and 3.50% or less; P: 0.040% or less; S: 0.010% or less; Al: 0.001% or more and 0.100% or less; N: 0.0001% or more and 0.0100% or less; Ti: 0.005% or more and 0.100% or less; B: 0% or more and 0.010% or less; O: 0.006% or less; Mo: 0% or more and 0.50% or less; Nb: 0% or more and 0.20% or less; Cr: 0% or more and 0.50% or less; V: 0% or more and 0.50% or less; Cu: 0% or more and 1.00% or less; W: 0% or more and 0.100% or less; Ta: 0% or more and 0.10% or less; Ni: 0% or more and 1.00% or less; Sn: 0% or more and 0.050% or less; Co: 0% or more and 0.50% or less; Sb: 0% or more and 0.050% or less; As: 0% or more and 0.050% or less; Mg: 0% or more and 0.050% or less; Ca: 0% or more and 0.040% or less; Y: 0% or more and 0.050% or less; Zr: 0% or more and 0.050% or less; La: 0% or more and 0.050% or less; Ce: 0% or more and 0.050% or less; and a remainder consisting of Fe and impurities, wherein a Ti content and a N content satisfy the following formula 1, at a sheet thickness ¼ position, a metallographic structure includes 90% or more of martensite in terms of volume fraction, at the sheet thickness ¼ position, a number density of TiC having a circle equivalent diameter of 1 to 500 nm is 3.5×10.sup.4 particles/mm.sup.2 or more, at the sheet thickness ¼ position, a value of a median value of a Mn concentration+3σ is 5.00% or less, a hardness measured at the sheet thickness ¼ position is 1.30 times or more a hardness measured at a position 50 μm deep from a surface of the steel sheet, and a tensile strength is 1310 MPa or more,
Ti−3.5×N≥0.003  (formula 1) here, element symbols Ti and N in the formula 1 mean the Ti content and the N content of the steel sheet.

2. The steel sheet according to claim 1, comprising: hot-dip galvanizing, hot-dip galvannealing, electro plating, or aluminum plating.

3. A method for manufacturing a steel sheet comprising: hot-rolling a cast piece having the chemical composition according to claim 1 with a finish rolling end temperature set to an Ac3 point or higher to obtain a steel sheet; coiling the steel sheet at a coiling temperature set to 500° C. or lower; cold-rolling the steel sheet at a rolling reduction set to 0% to 20%; and annealing the steel sheet in a temperature range of the Ac3 point or higher with an oxygen potential in a temperature range of 700° C. or higher set to −1.2 or higher and 0 or lower, wherein, when the steel sheet is heated up to the temperature range of the Ac3 point or higher in the annealing, the steel sheet is held in a temperature range of 500° C. to 700° C. for 70 to 130 seconds, and when the steel sheet is cooled from the temperature range of the Ac3 point or higher in the annealing, the steel sheet is held in a temperature range of 700° C. to 500° C. for 4 to 25 seconds.

4. The method for manufacturing a steel sheet according to claim 3, further comprising: tempering the annealed steel sheet.

5. The method for manufacturing a steel sheet according to claim 3, further comprising: performing hot-dip galvanizing, hot-dip galvannealing, electro plating, or aluminum plating on the annealed steel sheet.

6. The method for manufacturing a steel sheet according to claim 4, further comprising: performing hot-dip galvanizing, hot-dip galvannealing, electro plating, or aluminum plating on the annealed steel sheet.

7. A steel sheet comprising, as a chemical composition, in a unit of mass %: C: 0.20% or more and 0.45% or less; Si: 0.01% or more and 2.50% or less; Mn: 1.20% or more and 3.50% or less; P: 0.040% or less; S: 0.010% or less; Al: 0.001% or more and 0.100% or less; N: 0.0001% or more and 0.0100% or less; Ti: 0.005% or more and 0.100% or less; B: 0% or more and 0.010% or less; O: 0.006% or less; Mo: 0% or more and 0.50% or less; Nb: 0% or more and 0.20% or less; Cr: 0% or more and 0.50% or less; V: 0% or more and 0.50% or less; Cu: 0% or more and 1.00% or less; W: 0% or more and 0.100% or less; Ta: 0% or more and 0.10% or less; Ni: 0% or more and 1.00% or less; Sn: 0% or more and 0.050% or less; Co: 0% or more and 0.50% or less; Sb: 0% or more and 0.050% or less; As: 0% or more and 0.050% or less; Mg: 0% or more and 0.050% or less; Ca: 0% or more and 0.040% or less; Y: 0% or more and 0.050% or less; Zr: 0% or more and 0.050% or less; La: 0% or more and 0.050% or less; Ce: 0% or more and 0.050% or less; and a remainder comprising Fe and impurities, wherein a Ti content and a N content satisfy the following formula 1, at a sheet thickness ¼ position, a metallographic structure includes 90% or more of martensite in terms of volume fraction, at the sheet thickness ¼ position, a number density of TiC having a circle equivalent diameter of 1 to 500 nm is 3.5×10.sup.4 particles/mm.sup.2 or more, at the sheet thickness ¼ position, a value of a median value of a Mn concentration+3σ is 5.00% or less, a hardness measured at the sheet thickness ¼ position is 1.30 times or more a hardness measured at a position 50 μm deep from a surface of the steel sheet, and a tensile strength is 1310 MPa or more,
Ti−3.5×N≥0.003  (formula 1) here, element symbols Ti and N in the formula 1 mean the Ti content and the N content of the steel sheet.

Description

EXAMPLES

[0152] The effect of one aspect of the present invention will be more specifically described using examples. Here, conditions in the examples are simply examples of conditions adopted to confirm the feasibility and effect of the present invention. The present invention is not limited to these examples of the conditions. The present invention is capable of adopting a variety of conditions within the scope of the gist of the present invention as long as the object of the present invention is achieved.

[0153] Various cast pieces having the chemical composition shown in Table 1 to Table 3 were hot-rolled, coiled, cold-rolled, and annealed, thereby manufacturing steel sheets. The remainders of the chemical composition of these steel sheets were iron and impurities. In Table 1 to Table 3, for the contents of elements that were intentionally not added, the cells are left blank. Finish rolling end temperatures, coiling temperatures, cold rolling reductions, heating temperatures during annealing (annealing temperatures), tempering temperatures, holding times during heating, holding times during cooling, and oxygen potentials in a temperature range of 700° C. or higher were as shown in Table 4-1 and Table 4-2. In addition, for the steel sheets for which the cold rolling reduction of 0% is shown, in Table 4-1 and Table 4-2, the cold rolling was skipped. For part of the steel sheets, tempering was performed after annealing, and the tempering conditions are shown in Table 4-1 and Table 4-2.

[0154] The volume fractions of martensite at the sheet thickness ¼ position, the number densities of TiC having a circle equivalent diameter of 1 to 500 nm at the sheet thickness ¼ position, the values of the median value of the Mn concentration+3σ at the sheet thickness ¼ position, the hardness of the steel sheets at the sheet thickness ¼ position, and the hardness at the position 50 μm deep from the surface of the steel sheet of the various steel sheets obtained by the above-described manufacturing method were measured and shown in Table 5-1 and Table 5-2. Methods for measuring these values were as described above. In addition, the proportions between the hardness measured at the sheet thickness ¼ position and the hardness measured at the position 50 μm deep from the surface of the steel sheet were calculated and also shown in Table 5-1 and Table 5-2.

[0155] Additionally, the delayed fracture resistance properties of the steel sheets were evaluated by a method to be described, below and shown in Table 6-1 and Table 6-2. For the steel sheets manufactured using the method for manufacturing a steel sheet according to the present embodiment, the delayed fracture resistance properties were evaluated according to the method described in Materia Japan (Bulletin of the Japan Institute of Metals), Vol. 44, No. 0.3 (2005) pp. 254 to 256. Specifically, steel sheet was sheared with a clearance of 10%, and then a U bending test was performed at 10R. A strain gauge was attached to the center of the obtained test piece, and stress was applied by tightening both ends of the test piece with bolts. The applied stress was calculated from the monitored strain in the strain gauge. As a load stress, a stress corresponding to 0.8 times the tensile strength (TS) was applied. This is because the residual stress that is introduced during forming is considered to correspond to the TS of the steel sheet. The obtained U-bending test piece was immersed in an HCl aqueous solution having a pH of 3 at a liquid temperature of 25° C. and held under an atmospheric pressure of 950 to 1070 hPa for 48 hours, and the presence or absence of cracking was investigated.

[0156] The pass/fail criterion for the tensile strength, which is the strength of the steel sheet, was set to 1310 MPa or more. A steel sheet that satisfied this pass/fail criterion was judged to, be a steel sheet having a high strength.

[0157] The pass/fail criterion for the balance between strength and ductility of the steel sheet was set to tensile strength (TS)×elongation (EL) of 15000 MPa % or more. A steel sheet that satisfied this pass/fail criterion was judged to be a steel sheet having an excellent strength.

[0158] As the pass/fail criterion for the delayed fracture resistance properties of the steel sheet, a case where cracks having a length of more than 3 mm were observed in the U-bending test piece was evaluated as C, a case where fine cracks having a length of less than 3 mm were observed on the end surface was evaluated as B, a case where cracks were not observed was evaluated as A, cases where the evaluation was A were regarded as pass, and cases where the evaluation was B or C were regarded as fail. A steel sheet that satisfied these pass/fail criterion was judged to be a steel sheet having excellent delayed fracture resistance properties.

[0159] The pass/fail criterion for the fatigue resistance properties of the steel sheet was set to a yield ratio of 0.65 or more. A steel sheet that satisfied this pass/fail criterion was judged to be a steel sheet having excellent fatigue resistance properties.

TABLE-US-00001 TABLE 1 No. C Si Mn P S Al N Ti A 0.291 0.38 2.0 0.0037 0.0008 0.085 0.0009 0.095 B 0.336 2.41 1.4 0.0035 0.0086 0.020 0.0001 0.049 C 0.265 1.09 3.4 0.0172 0.0080 0.008 0.0088 0.083 D 0.396 0.59 3.1 0.0024 0.0038 0.009 0.0042 0.070 E 0.232 0.15 2.2 0.0337 0.0017 0.007 0.0014 0.057 F 0.211 1.71 2.5 0.0312 0.0012 0.043 0.0006 0.056 G 0.315 0.90 2.8 0.0022 0.0008 0.012 0.0018 0.058 H 0.368 2.22 1.7 0.0023 0.0009 0.007 0.0009 0.062 I 0.408 1.43 1.5 0.0067 0.0009 0.079 0.0008 0.068 J 0.438 1.85 2.9 0.0041 0.0008 0.011 0.0003 0.053 K 0.271 1.87 1.9 0.0301 0.0008 0.068 0.0001 0.008 L 0.318 1.26 2.3 0.0125 0.0021 0.011 0.0005 0.020 M 0.281 0.90 2.3 0.0319 0.0075 0.017 0.0005 0.079 N 0.447 1.45 2.0 0.0026 0.0009 0.013 0.0002 0.052 O 0.411 1.14 1.4 0.0048 0.0085 0.007 0.0099 0.095 P 0.254 1.60 1.5 0.0018 0.0008 0.007 0.0008 0.082 Q 0.232 2.13 2.7 0.0025 0.0042 0.076 0.0018 0.058 R 0.382 0.06 2.9 0.0052 0.0019 0.042 0.0008 0.060 S 0.362 1.85 3.4 0.0155 0.0006 0.009 0.0010 0.089 T 0.217 0.34 2.5 0.0349 0.0006 0.008 0.0013 0.067 U 0.333 0.56 1.8 0.0075 0.0005 0.088 0.0001 0.050 V 0.308 2.34 3.1 0.0029 0.0009 0.013 0.0078 0.090 W 0.190 0.77 2.7 0.0299 0.0017 0.041 0.0013 0.091 X 0.459 0.71 1.4 0.0031 0.0083 0.093 0.0012 0.076 Y 0.380 1.40 1.1 0.0044 0.0009 0.006 0.0014 0.057 Z 0.358 0.31 2.3 0.0335 0.0006 0.045 0.0106 0.082 AA 0.393 0.90 2.1 0.0340 0.0007 0.050 0.0001 0.004 AB 0.326 0.45 1.5 0.0018 0.0009 0.055 0.0009 0.005 AC 0.250 1.19 3.0 0.0273 0.0009 0.006 0.0010 0.108

TABLE-US-00002 TABLE 2 No. B O Mo Nb Cr V Co Ni Cu A B C D E F G H I J K L M 0.0010 N 0.080 0.047 0.390 O 0.010 0.395 0.100 0.083 P 0.015 0.064 Q 0.037 0.060 R 0.002 S 0.019 0.040 T 0.0020 U 0.001 0.016 0.034 V W 0.159 0.299 0.782 X 0.001 0.128 0.406 Y 0.0026 0.313 0.041 Z AA AB AC

TABLE-US-00003 TABLE 3 Ti − Ac3 No. W Ta Sn Sb As Mg Ca Y Zr La Ce Note 3.5 * N point A Example 0.092 808 B Example 0.049 883 C Example 0.052 798 D Example 0.055 746 E Example 0.052 800 F Example 0.054 868 G Example 0.052 778 H Example 0.059 862 I Example 0.065 841 J Example 0.052 797 K Example 0.008 863 L Example 0.018 801 M Example 0.077 830 N Example 0.051 805 O Example 0.060 823 P Example 0.079 876 Q Example 0.052 860 R 0.002 0.005 0.020 0.004 0.004 Example 0.057 733 S 0.009 0.085 0.004 0.039 0.007 0.005 Example 0.086 816 T 0.037 0.004 0.002 Example 0.062 808 U 0.003 0.002 0.021 0.005 Example 0.050 803 V 0.012 0.004 0.005 0.006 0.019 Example 0.063 849 W Comparative 0.086 823 Example X 0.021 0.025 0.021 0.028 0.012 Comparative 0.072 848 Example Y 0.012 0.086 0.031 0.005 0.004 0.004 Comparative 0.052 840 Example Z Comparative 0.045 795 Example AA Comparative 0.004 791 Example AB Comparative 0.002 779 Example AC Comparative 0.105 835 Example

TABLE-US-00004 TABLE 4-1 Cold Cold-rolled sheet annealing Hot rolling step rolling Holding Holding Finish step time at time, at rolling end Coiling Cold Heating 500° C. 700° C. Tempering Tempering Steel temperature temperature rolling temperature to 700° to 500° Oxygen temperature time Ac3 No. kind ° C. ° C. reduction % ° C. C. [s] C. [s] potential ° C. s Note point 1 A 990 76 7 862 102 17 −1.1 234 39 Example 808 2 B 893 402 12 885 93 14 −1.05 238 45 Example 883 3 C 978 146 1 830 75 23 −0.85 203 51 Example 798 4 D 871 433 6 849 81 21 −0.95 392 27 Example 746 5 E 909 457 13 803 77 18 −0.77 315 48 Example 800 6 F 983 153 17 870 111 9 −0.62 — — Example 868 7 G 965 313 2 876 78 10 −1.02 245 36 Example 778 8 H 881 233 9 898 103 12 −1.08 353 37 Example 862 9 I 912 192 7 852 100 13 −0.72 324 21 Example 841 10 J 932 367 3 824 87 20 −0.98 347 18 Example 797 11 K 880 90 17 870 106 23 −0.92 227 13 Example 863 12 L 860 34 12 830 102 19 −1.02 218 41 Example 801 13 M 962 112 15 880 98 16 −1.07 239 20 Example 830 14 N 947 180 19 867 119 22 −1.06 293 24 Example 805 15 O 890 263 11 889 102 11 −1.1 287 31 Example 823 16 P 939 468 13 878 76 13 −1.09 184 53 Example 876 17 Q 973 35 15 872 71 10 −1.1 — — Example 860 18 R 895 402 19 834 97 20 −0.97 189 7150  Example 733 19 S 924 104 5 844 102 16 −0.7 — — Example 816 20 T 878 284 1 885 82 7 −0.77 — — Example 808 21 U 864 264 9 880 72 14 −0.85 206 3000  Example 803 22 V 947 330 17 876 99 11 −0.77 213 56 Example 849 23 A 966 422 9 831 91 17 −1.08 213 29 Example 808 24 B 912 30 13 892 103 15 −1.03 188 31 Example 883 25 C 947 284 18 800 113 11 −0.82 198 43 Example 798

TABLE-US-00005 TABLE 4-2 Cold Cold rolled sheet annealing Hot rolling step rolling Holding Holding Finish step time at time at rolling end Coiling Cold Heating 500° C. 700° C. Tempering Tempering Steel temperature temperature rolling temperature to 700° to 500° Oxygen temperature time Ac3 No. kind ° C. ° C. reduction ° C. C. [s] C. [s] potential ° C. s Note point 26 D 893 486  9 760 83  8 −1.06 403 28 Example 746 27 E 867 136  6 835 93 10 −0.82 280 33 Example 800 28 F 943  57  0 875 79  9 −0.8  — — Example 868 29 G 959 388  6 811 92 19 −1   258 47 Example 778 30 H 969 398 13 873 92  7 −0.95 352 32 Example 862 31 I 923 449 14 894 101  10 −1.06 291 16 Example 841 32 J 860 173 19 820 109  18 −0.96 342 19 Example 797 33 M 910 309  2 891 75  9 −0.47 362 12 Example 830 34 N 861 204 15 886 96 21 −0.92 245 28 Example 805 35 O 978 102 18 852 129  22 −0.95 276 29 Example 823 36 W 900 160 12 836 81 16 −1.08 291 53 Comparative 823 Example 37 X 933 234  5 864 97 11 −0.83 198 26 Comparative 848 Example 38 Y 911 357 10 892 93  9 −1.05 182 141 Comparative 840 Example 39 Z 937 257  2 865 77 15 −0.81 248 33 Comparative 795 Example 40 AA 888 372 16 827 73 12 −1.1  201 47 Comparative 791 Example 41 AB 853 162 15 825 76  9 −1.02 267 52 Comparative 779 Example 42 P 846  39  7 879 98 12 −1.07 349 3671 Comparative 876 Example 43 Q 920 516 19 870 81 11 −1.02 287 43 Comparative 860 Example 44 R 903  97 21 868 74 18 −0.92 172 6231 Comparative 733 Example 45 F 874 330 13 798 95 12 −0.65 232 29 Comparative 868 Example 46 T 995 439  9 813 85  7 −1.3  312 36 Comparative 808 Example 47 AC 937 132 17 865 101  19 −0.89 271 62 Comparative 835 Example 48 U 903 352  9 807 67  5 −1.05 223 1231 Comparative 803 Example 49 V 934 258  1 851 134  23 −0.75 286 105 Comparative 849 Example 50 A 872 482 15 811 73  2 −0.93 241 53 Comparative 808 Example 51 B 917 103  4 884 125  28 −1.02 238 72 Comparative 883 Example

TABLE-US-00006 TABLE 5-1 Vickers hardness Total of Hardness ratio martensite TiC (hardness at and Mn concentration Number Sheet sheet thickness tempered Median density thickness 1/4 position/ Steel martensite value + Median particles/ Surface 1/4 hardness of No. kind % 3σ value σ mm.sup.2 layer position surface layer) 1 A 98.2 4.39 2.35 0.68 4.8.E+05 392 527 1.34 2 B 96.2 4.68 2.46 0.74 9.0.E+04 408 540 1.32 3 C 99.6 4.89 2.58 0.77 3.7.E+04 358 492 1.38 4 D 95.3 4.81 2.53 0.76 5.2.E+04 403 562 1.39 5 E 95.9 4.82 2.54 0.76 4.0.E+04 330 429 1.30 6 F 95.6 4.22 2.21 0.67 8.2.E+05 389 510 1.31 7 G 97.1 4.86 2.55 0.77 3.9.E+04 375 534 1.42 8 H 95.3 4.39 2.32 0.69 5.8.E+05 392 539 1.38 9 I 94.2 4.40 2.33 0.69 5.4.E+05 403 546 1.35 10 J 93.4 4.74 2.52 0.74 6.0.E+04 409 540 1.32 11 K 94.8 4.29 2.10 0.73 5.7.E+06 358 506 1.41 12 L 96.7 4.40 2.45 0.65 4.4.E+06 372 536 1.44 13 M 97.9 4.10 2.15 0.65 1.9.E+06 364 528 1.45 14 N 96.4 3.80 2.09 0.57 4.2.E+07 419 552 1.32 15 O 95.2 4.29 2.28 0.67 6.7.E+05 411 554 1.35 16 P 96.1 4.87 2.56 0.77 3.8.E+04 353 461 1.30 17 Q 99.6 4.17 2.16 0.67 1.5.E+06 349 486 1.39 18 R 94.2 4.48 2.41 0.69 4.0.E+05 398 556 1.40 19 S 99,1 4.48 2.41 0.69 4.4.E+05 401 564 1.41 20 T 95.6 4.98 2.61 0.79 3.5.E+04 386 502 1.30 21 U 98.5 4.49 2.42 0.69 4.5.E+05 378 516 1.37 22 V 94.5 4.38 2.31 0.69 6.0.E+05 372 524 1.41 23 A 97.9 4.73 2.51 0.74 6.7.E+04 364 534 1.47 24 B 97.3 3.97 2.11 0.62 6.5.E+06 382 546 1.43 25 C 98.2 4.30 2.29 0.67 9.6.E+05 358 480 1.34

TABLE-US-00007 TABLE 5-2 Vickers hardness Total of Hardness ratio martensite TiC (hardness at and Mn concentration Number Sheet sheet thickness tempered Median density thickness 1/4 position/ Steel martensite value + Median particles/ Surface 1/4 hardness of No. kind % 3σ value σ mm.sup.2 layer position surface layer) 26 D 96.4 4.91 2.60 0.77 3.5.E+04 404 560 1.39 27 E 96.2 4.58 2.45 0.71 1.5.E+05 338 442 1.31 28 F 97.3 4.89 2.58 0.77 3.7.E+04 367 497 1.35 29 G 96.2 4.73 2.51 0.74 7.1.E+04 376 539 1.43 30 H 94.2 4.58 2.45 0.71 1.3.E+05 392 547 1.40 31 I 93.8 4.73 2.48 0.75 8.6.E+04 407 544 1.34 32 J 94.3 3.79 2.08 0.57 1.6.E+07 407 531 1.30 33 M 92.3 4.87 2.56 0.77 3.9.E+04 358 495 1.38 34 N 93.4 4.07 2.12 0.65 3.5.E+06 410 536 1.31 35 O 94.8 3.67 2.02 0.55 1.0.E+08 408 554 1.36 36 W 96.6 4.45 2.38 0.69 5.2.E+05 298 391 1.31 37 X 93.4 4.54 2.41 0.71 2.7.E+05 421 612 1.45 38 Y 96.3 5.09 2.63 0.82 1.6.E+05 397 562 1.42 39 Z 96.2 4.44 2.37 0.69 4.7.E+05 392 534 1.36 40 AA 95.6 4.73 2.51 0.74 3.2.E+04 390 560 1.44 41 AB 92.8 4.41 2.37 0.68 3.4.E+04 374 576 1.54 42 P 96.7 5.22 2.73 0.83 2.4.E+05 351 462 1.31 43 Q 97.2 5.14 2.62 0.84 4.0.E+04 343 477 1.39 44 R 92.6 5.18 2.69 0.83 1.4.E+04 396 554 1.40 45 F 88.2 4.44 2.37 0.69 2.8.E+05 315 410 1.30 46 T 96.1 4.86 2.55 0.77 4.0.E+04 372 428 1.15 47 AC 93.4 4.64 2.48 0.72   5.E+06 275 380 1.38 48 U 92.7 4.61 2.51 0.7 3.0.E+04 351 493 1.40 49 V 94.5 4.55 2.42 0.71 3.4.E+04 348 512 1.47 50 A 96.2 4.91 2.57 0.78 2.0.E+04 368 519 1.41 51 B 91.8 4.44 2.4 0.68 3.4.E+04 382 537 1.41

TABLE-US-00008 TABLE 6-1 Hydrogen Yield Tensile Elon- Strength × embrit- Steel stress Yield strength gation elongation tlement No. kind MPa ratio MPa % MPa % resistance 1 A 1184 0.68 1738 8.9 15468 A 2 B 1174 0.66 1783 8.8 15690 A 3 C 1198 0.74 1625 9.3 15113 A 4 D 1286 0.69 1853 8.3 15380 A 5 E 1192 0.84 1416 10.8 15293 A 6 F 1199 0.71 1683 11.1 18681 A 7 G 1179 0.67 1762 8.6 15153 A 8 H 1274 0.72 1780 8.8 15664 A 9 I 1278 0.71 1802 8.7 15677 A 10 J 1275 0.72 1783 8.5 15156 A 11 K 1203 0.72 1670 9.0 15030 A 12 L 1187 0.67 1770 8.5 15045 A 13 M 1183 0.68 1742 8.7 15155 A 14 N 1281 0.70 1820 8.3 15106 A 15 O 1283 0.70 1829 8.9 16278 A 16 P 1201 0.79 1520 10.2 15504 A 17 Q 1140 0.71 1603 9.4 15068 A 18 R 1284 0.70 1835 8.2 15047 A 19 S 1287 0.69 1862 8.5 15827 A 20 T 1199 0.72 1656 11.0 18216 A 21 U 1189 0.70 1704 8.9 15166 A 22 V 1185 0.69 1728 8.7 15034 A 23 A 1179 0.67 1762 8.7 15329 A 24 B 1175 0.65 1802 8.4 15137 A 25 C 1201 0.76 1584 9.8 15523 A

TABLE-US-00009 TABLE 6-2 Hydrogen Yield Tensile Elon- Strength × embrit- Steel stress Yield strength gation elongation tlement No. kind MPa ratio MPa % MPa % resistance 26 D 1285 0.70 1847 8.2 15145 A 27 E 1184 0.81 1460 10.3 15038 A 28 F 1180 0.72 1640 10.2 16728 A 29 G 1175 0.66 1779 8.5 15122 A 30 H 1279 0.71 1805 8.6 15523 A 31 I 1277 0.71 1794 8.4 15070 A 32 J 1269 0.72 1752 8.8 15418 A 33 M 1198 0.73 1632 9.4 15341 A 34 N 1272 0.72 1768 8.7 15382 A 35 O 1283 0.70 1830 8.3 15189 A 36 W 1200 0.93 1290 9.7 12513 A 37 X 1315 0.65 2018 6.6 13319 C 38 Y 1287 0.69 1856 8.2 15219 B 39 Z 802 0.63 1280 6.4  8192 C 40 AA 1083 0.59 1847 8.6 15884 C 41 AB 1092 0.57 1902 7.9 15026 C 42 P 1201 0.79 1523 10.8 16448 B 43 Q 1151 0.73 1574 11.3 17786 B 44 R 1074 0.59 1827 8.3 15164 C 45 F 1005 0.82 1230 13.1 16113 A 46 T 1193 0.84 1413 10.9 15402 C 47 AC 890 0.71 1254 13.5 16929 A 48 U 1069 0.64 1682 9.7 16315 C 49 V 1081 0.63 1716 8.9 15272 C 50 A 1059 0.61 1731 8.8 15233 C 51 B 1053 0.59 1772 8.6 15239 C

[0160] The examples that satisfied all of the requirements of the present invention were steel sheets having a high strength, an excellent balance between strength and ductility, excellent delayed fracture resistance properties, and excellent fatigue resistance properties. On the other hand, comparative examples that lacked one or more of the requirements of the present invention were evaluated as fail in one or more of the above-described evaluation criteria. In the tables, numerical values outside the scope of the invention or numerical values that do not meet the pass/fail criterion are underlined.

[0161] In a steel sheet 36, the C content was insufficient. In this steel sheet 36, it was not possible to ensure the tensile strength and TS×EL.

[0162] In a steel sheet 37, the C content was excessive. In this steel sheet 37, the strength became excessive, which made the yield ratio and TS×EL insufficient, and, furthermore, it was not possible to ensure the delayed fracture resistance properties.

[0163] In a steel sheet 38, Mn was insufficient. In this steel sheet 38, the value of the median value of the Mn concentration+3σ at the sheet thickness ¼ position became excessive. This is considered because ferrite appeared after hot rolling and thus strain was formed in the steel sheet nonuniformly due to the subsequent cold rolling. Therefore, in this steel sheet 38, it was not possible to ensure the delayed fracture resistance properties.

[0164] In a steel sheet 39, the N content was excessive. In this steel sheet 39, the steel sheet embrittled, and it was not possible to ensure the yield ratio, the tensile strength, and TS×EL.

[0165] In a steel sheet 40, the Ti content was insufficient, and the number density of TiC having a circle equivalent diameter of 1 to 500 nm at the sheet thickness ¼ position was insufficient. Therefore, in the steel sheet 40, it was not possible to ensure the delayed fracture resistance properties.

[0166] In a steel sheet 41, the chemical composition did not satisfy the relational formula between Ti and N. In this steel sheet 41, the number, density of TiC having a circle equivalent diameter of 1 to 500 nm at the sheet thickness ¼ position was insufficient. Therefore, in the steel sheet 41, it was not possible to ensure the delayed fracture resistance properties.

[0167] In a steel sheet 42, the value of the median value of the Mn concentration+3σ at the sheet thickness ¼ position became excessive. This is considered because the finish rolling end temperature of the steel sheet 42 was below the Ac3 point, and ferrite appeared after the end of hot rolling and thus strain was formed in the steel sheet nonuniformly due to the subsequent cold rolling. Therefore, in the steel sheet 42, it was not possible to ensure the delayed fracture resistance properties.

[0168] In a steel sheet 43, the value of the median value of the Mn concentration+3σ at the sheet thickness ¼ position became excessive. This is considered because the coiling temperature of the steel, sheet 43 was high, and ferrite appeared and thus strain was formed in the steel sheet nonuniformly due to the subsequent cold rolling. Therefore, in the steel sheet 43, it was not possible to ensure the delayed fracture resistance properties.

[0169] In a steel sheet 44, the value of the median value of the Mn concentration+3σ at the sheet thickness ¼ position became excessive, and, furthermore, the number density of TiC having a circle equivalent diameter of 1 to 500 nm at the sheet thickness ¼ position was insufficient. This is considered because the cold rolling reduction of the steel sheet 44 was too high. Therefore, in the steel sheet 44, it was not possible to ensure the yield ratio and the delayed fracture resistance properties.

[0170] In a steel sheet 45, the volume fraction of martensite at the sheet thickness ¼ position was insufficient. This is considered because the heating temperature during annealing of the steel sheet 45 was insufficient. Therefore, in the steel sheet 45, the tensile strength was insufficient.

[0171] In a steel sheet 46, the hardness measured at the position 50 μm deep from the surface of the steel sheet was excessive with respect to the hardness measured at the sheet thickness ¼ position. This is considered because the annealing atmosphere of the steel sheet 46 was inappropriate. Therefore, in the steel sheet 46, it was not possible to ensure the delayed fracture resistance properties.

[0172] In a steel sheet 47, the Ti content was excessive. Therefore, in the steel sheet 47, a large amount of TiC was precipitated, and the amount of a C solid solution decreased, and thus it was not possible to ensure the tensile strength.

[0173] In a steel sheet 48, the number density of TiC having a circle equivalent diameter of 1 to 500 nm at the sheet thickness ¼ position was insufficient. This is considered because, in the annealing of the steel sheet 48, the holding time at 500° C. to 700° C. vas insufficient at the time of heating the steel sheet up to a temperature range of the Ac3 point or higher. Therefore, in the steel sheet 48, it was not possible to ensure the yield ratio and the delayed fracture resistance properties.

[0174] In a steel sheet 49, the number density of TiC having a circle equivalent diameter of 1 to 500 nm at the sheet thickness ¼ position was insufficient. This is considered because, in the annealing of the steel sheet 49, the holding time at 500° C. to 700° C. was too long at the time of heating the steel sheet up to a temperature range of the Ac3 point or higher, Therefore, in the steel sheet 49, it was not possible to ensure the yield ratio and the delayed fracture resistance properties.

[0175] In a steel sheet 50, the number density of TiC having a circle equivalent diameter of 1 to 500 nm at the sheet thickness ¼ position was insufficient. This is considered because, in the annealing of the steel sheet 50, the holding time at 700° C. to 500° C. was insufficient at the time of cooling the steel sheet from the temperature range of the Ac3 point or higher. Therefore, in the steel sheet 50, it was not possible to ensure the yield ratio and the delayed fracture resistance properties.

[0176] In a steel sheet 51, the number density of TiC having a circle equivalent diameter of 1 to 500 nm at the sheet thickness ¼ position was insufficient. This is considered because, in the annealing of the steel sheet 51, the holding time at 700° C. to 500° C. was too long at the time of cooling the steel sheet from the temperature range of the Ac3 point or higher. Therefore, in the steel sheet 51 it was not possible to ensure the yield ratio and the delayed fracture resistance properties.