HOT-ROLLED STEEL SHEET

Abstract

A hot-rolled steel sheet has a predetermined chemical composition, a microstructure of an inner region includes 40% to 80% of one or two of martensite and bainite in total and 20% to 60% of ferrite by area ratio, an area ratio of a remainder in microstructure is less than 5%, s/c, which is a ratio of a ferrite area ratio s of a surface layer region to a ferrite area ratio c of the inner region, is in a range of 1.15 to 2.50, (1Hvs/Hvc), which is a hardness difference ratio between a Vickers hardness Hvs of the surface layer region and a Vickers hardness Hvc of the inner region, is 0.20 or less, and a tensile strength is 980 MPa or more.

Claims

1. A hot-rolled steel sheet comprising, as a chemical composition, by mass %: C: 0.02% to 0.30%; Si: 0.10% to 2.00%; Mn: 0.5% to 3.0%; sol. Al: 0.10% to 1.00%; Ti: 0.06% to 0.20%; P: 0.1000% or less; S: 0.0100% or less; N: 0.0100% or less; Nb: 0% to 0.100%; Ca: 0% to 0.0060%; Mo: 0% to 0.50%; Cr: 0% to 1.00%; V: 0% to 0.40%; Ni: 0% to 0.40%; Cu: 0% to 0.40%; B: 0% to 0.0020%; and Sn: 0% to 0.20%, with a remainder of Fe and impurities, a microstructure of an inner region includes 40% to 80% of one or two of martensite and bainite in total and 20% to 60% of ferrite by area ratio, and an area ratio of a remainder in microstructure is less than 5%, s/c, which is a ratio of a ferrite area ratio s of a surface layer region to a ferrite area ratio c of the inner region, is in a range of 1.15 to 2.50, (1Hvs/Hvc), which is a hardness difference ratio between a Vickers hardness Hvs of the surface layer region and a Vickers hardness Hvc of the inner region, is 0.20 or less, and a tensile strength is 980 MPa or more.

2. The hot-rolled steel sheet according to claim 1, wherein the chemical composition comprises, by mass %, one or more of Nb: 0.010% to 0.100%, Ca: 0.0005% to 0.0060%, Mo: 0.02% to 0.50%, Cr: 0.02% to 1.00%, V: 0.01% to 0.40%, Ni: 0.01% to 0.40%, Cu: 0.01% to 0.40%, B: 0.0001% to 0.0020%, and Sn: 0.01% to 0.20%.

Description

EXAMPLES

[0145] Next, the effect of one aspect of the present invention will be more specifically described using examples. However, conditions in the examples are simply examples of the conditions employed to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples of the conditions. The present invention may employ various conditions to achieve the object of the present invention without departing from the scope of the present invention.

[0146] Steels having chemical compositions shown in Table 1 were melted, and slabs having a thickness of 240 to 300 mm were manufactured using continuous casting. Hot-rolled steel sheets shown in Tables 4 and 5 were obtained using the obtained slabs under manufacturing conditions shown in Tables 2 and 3.

[0147] The steel sheet was pushed up from below between stands with a roller to perform bending. A strain amount during bending was controlled by the adjustment of a bending angle using the push-up distance of the roller or the diameter of the roller. In this case, a strain amount during bending was obtained from Equation (2).

[0148] For the obtained hot-rolled steel sheets, area fractions and Vickers hardnesses of microstructures of an inner region and a surface layer region, a tensile strength, total elongation, absorbed energy at a temperature of 20 C., and a fatigue limit ratio were obtained by the above-mentioned method. Obtained measurement results are shown in Tables 4 and 5.

Method of Evaluating Properties of Hot-Rolled Steel Sheet

[0149] In a case where a tensile strength TS was 980 MPa or more, a hot-rolled steel sheet was considered to be excellent in strength and determined to be acceptable. On the other hand, in a case where the tensile strength TS was less than 980 MPa, a hot-rolled steel sheet was not considered to be excellent in strength and determined to be unacceptable.

[0150] In a case where total elongation was 10% or more, a hot-rolled steel sheet was considered to be excellent in ductility and determined to be acceptable. On the other hand, in a case where total elongation was less than 10%, a hot-rolled steel sheet was not considered to be excellent in ductility and determined to be unacceptable.

[0151] In a case where absorbed energy at a temperature of 20 C. was 80 J/cm.sup.2 or more, a hot-rolled steel sheet was considered to be excellent in toughness and determined to be acceptable. On the other hand, in a case where absorbed energy at a temperature of 20 C. was less than 80 J/cm.sup.2, a hot-rolled steel sheet was not considered to be excellent in toughness and determined to be unacceptable.

[0152] In a case where a fatigue limit ratio was 0.48 or more, a hot-rolled steel sheet was considered to be excellent in fatigue strength and determined to be acceptable. On the other hand, in a case where a fatigue limit ratio was less than 0.48, a hot-rolled steel sheet was not considered to be excellent in fatigue strength and determined to be unacceptable.

TABLE-US-00001 TABLE 1 Chemical composition (mass %) Remainder: Fe and impurities Ar.sub.3 Steel sol. ( type C Si Mn Al Ti P S N Nb Ca Mo Cr V Ni Cu B Sn C.) Note A 0.10 1.10 1.4 0.17 0.09 0.0150 0.0033 0.0036 792 Steel of pre- sent in- ven- tion B 0.06 0.22 1.1 0.18 0.10 0.0126 0.0038 0.0040 0.0011 798 Steel of pre- sent in- ven- tion C 0.07 0.33 1.1 0.20 0.06 0.0126 0.0033 0.0030 0.0008 0.35 803 Steel of pre- sent in- ven- tion D 0.14 1.43 1.5 0.29 0.13 0.0135 0.0010 0.0040 0.015 775 Steel of pre- sent in- ven- tion E 0.15 0.12 1.8 0.25 0.10 0.0135 0.0014 0.0033 0.20 704 Steel of pre- sent in- ven- tion F 0.08 0.72 1.2 0.30 0.07 0.0154 0.0030 0.0044 0.035 0.30 805 Steel of pre- sent in- ven- tion G 0.04 0.30 2.2 0.44 0.14 0.0130 0.0066 0.0030 0.0018 717 Steel of pre- sent in- ven- tion H 0.12 1.00 0.6 0.06 0.10 0.0135 0.0055 0.0036 846 Com- par- ative steel I 0.20 0.44 2.0 0.34 0.04 0.0150 0.0027 0.0036 0.0021 0.05 685 Com- par- ative steel J 0.28 1.20 2.5 0.35 0.11 0.0135 0.0033 0.0044 0.10 0.10 0.04 637 Steel of pre- sent in- ven- tion K 0.26 1.50 1.8 0.25 0.12 0.0154 0.0010 0.0030 0.10 0.20 0.0001 715 Steel of pre- sent in- ven- tion L 0.18 1.80 2.8 0.55 0.17 0.0130 0.0014 0.0033 0.02 0.10 0.15 0.05 0.10 0.01 670 Steel of pre- sent in- ven- tion M 0.18 1.76 2.2 0.22 - 0.0130 0.0016 0.0036 0.06 0.0018 712 Com- par- ative steel N 0.18 2.01 2.0 0.03 0.01 0.0130 0.0009 0.0041 733 Com- par- ative steel An underline represents that a condition is out of the range of the present invention.

TABLE-US-00002 TABLE 2 Rolling Primary Secondary reduction cooling cooling Heating Finishing of Strain Cooling Cooling Average Air- Cooling Average Coiling Sheet temper- temper- final during start stop tem- cooling cooling stop tem- cooling temper- thick- Test Steel ature ature pass bending time perature rate time perature rate ature ness No. type C. C. % Sec. C. C./sec Sec. C. C./sec C. mm Note 1 A 1200 962 24 0.008 1.3 639 222 6 100 220 100 1.6 Example of present invention 2 A 1200 1019 20 0.040 1.0 747 72 4 100 266 100 2.3 Comparative Example 3 A 1200 991 33 0.014 1.1 662 222 6 100 179 100 2.3 Example of present invention 4 A 1200 967 25 0.013 0.4 656 80 6 100 295 100 2.3 Example of present invention 5 A 1200 936 16 0.001 0.7 736 226 2 100 322 100 2.6 Comparative Example 6 B 1200 981 32 0.016 0.7 718 169 3 100 390 100 2.6 Example of present invention 7 B 1200 1017 21 0.010 1.0 710 220 6 100 154 100 1.6 Example of present invention 8 B 1200 992 29 0.016 0.7 554 103 3 100 390 100 2.9 Comparative Example 9 B 1200 943 19 0.013 1.2 634 151 2 100 357 100 2.9 Example of present invention 10 C 1200 940 28 0.006 0.4 715 185 2 100 374 100 2.9 Example of present invention 11 C 1200 948 29 0.014 1.1 720 79 5 100 93 100 2.0 Example of present invention 12 C 1200 966 26 0.004 0.1 622 20 6 100 128 100 3.2 Comparative Example 13 C 1250 978 32 0.012 0.9 664 99 2 100 91 100 3.2 Example of present invention 14 C 1250 1020 35 0.016 0.4 680 218 5 100 138 100 2.6 Example of present invention 15 D 1250 973 31 0.008 0.4 707 210 2 100 142 100 4.5 Example of present invention 16 D 1250 1006 15 0.002 0.5 780 131 4 100 434 100 3.9 Comparative Example 17 D 1250 918 39 0.011 1.1 650 204 6 100 371 100 3.9 Example of present invention 18 D 1250 920 33 0.017 1.2 745 97 5 100 234 100 2.3 Example of present invention 19 F 1200 966 37 0.004 1.1 625 109 4 100 88 100 2.3 Example of present invention 20 E 1200 1013 17 0.006 1.1 657 85 1 100 303 100 2.3 Comparative Example An underline represents that a manufacturing condition is not preferable.

TABLE-US-00003 TABLE 3 Rolling Primary Secondary reduction cooling cooling Heating Finishing of Strain Cooling Cooling Average Air- Cooling Average Sheet temper- temper- final during start stop tem- cooling cooling stop tem- cooling Coiling thick- Test Steel ature ature pass bending time perature rate time perature rate temper- ness No. type C. C. % Sec. C. C./sec Sec. C. C./sec ature mm Note 21 E 1200 920 39 0.014 1.6 654 62 2 100 389 100 2.9 Example of present invention 22 E 1200 973 23 0.010 0.8 725 50 12 100 288 100 2.9 Comparative Example 23 F 1200 956 33 0.005 0.5 748 77 3 100 262 100 2.9 Example of present invention 24 F 1200 956 22 0.018 0.8 719 63 3 530 243 500 2.6 Comparative Example 25 F 1200 964 33 0.006 1.3 651 130 2 100 193 100 2.6 Example of present invention 26 F 1200 927 31 0.006 0.6 733 153 6 100 130 100 1.6 Example of present invention 27 G 1250 1000 33 0.006 2.4 666 114 4 100 403 100 2.0 Comparative Example 28 G 1250 987 24 0.018 1.2 695 120 2 100 419 100 1.8 Example of present invention 29 G 1250 954 39 0.007 1.1 731 154 6 100 211 100 1.8 Example of present invention 30 G 1250 912 20 0.002 1.6 668 84 4 100 109 100 2.0 Example of present invention 31 G 1250 1004 7 0.004 1.5 725 205 5 100 247 100 1.6 Comparative Example 32 G 1200 1020 27 0.017 1.4 701 91 6 100 196 100 1.6 Example of present invention 33 H 1200 1020 25 0.004 0.6 740 145 6 100 295 100 3.6 Comparative Example 34 I 1230 968 33 0.008 0.5 709 57 4 100 93 100 3.6 Comparative Example 35 J 1200 964 28 0.006 0.4 715 130 2 100 374 100 2.6 Example of present invention 36 K 1200 941 32 0.006 1.3 731 185 3 100 193 100 2.6 Example of present invention 37 L 1250 954 38 0.007 1.2 651 154 6 100 211 100 1.8 Example of present invention 38 M 1250 918 39 0.011 1.1 666 114 4 100 403 100 3.9 Comparative Example 39 N 1200 900 20 0.006 1.3 695 120 5 100 419 100 2.6 Comparative Example 40 K 1250 954 39 0.024 1.5 725 205 5 100 247 100 1.8 Comparative Example 41 L 1200 941 32 0.006 1.3 651 130 2 100 193 250 2.0 Comparative Example An underline represents that a manufacturing condition is not preferable.

TABLE-US-00004 TABLE 4 Inner region Mar- tensite + Surface layer region Hardness Total Absorbed Fatigue Ferrite bainite Re- Ferrite Re- difference Tensile elon- energy limit Test Steel c c mainder s mainder s/c s/c ratio strength gation (20 C.) ratio No. type Area % Area % Area % Area % Area % MPa % J/cm.sup.2 Note 1 A 22 78 0 31 69 1.40 0.89 0.14 1018 16 158 0.50 Example of present invention 2 A 32 43 25 95 5 2.97 0.12 0.32 1148 16 65 0.59 Comparative Example 3 A 33 67 0 46 54 1.40 0.80 0.12 1003 15 115 0.59 Example of present invention 4 A 33 67 0 46 54 1.40 0.80 0.16 1209 16 217 0.54 Example of present invention 5 A 25 75 0 24 76 0.96 1.01 0.15 1116 18 83 0.41 Comparative Example 6 B 40 60 0 56 44 1.40 0.73 0.17 1162 19 171 0.55 Example of present invention 7 B 44 56 0 62 38 1.40 0.69 0.02 1034 14 93 0.58 Example of present invention 8 B 12 77 11 32 68 2.68 0.77 0.09 1187 11 99 0.35 Comparative Example 9 B 35 65 0 49 51 1.40 0.78 0.09 1100 13 127 0.55 Example of present invention 10 C 26 74 0 36 64 1.40 0.86 0.02 997 14 90 0.50 Example of present invention 11 C 36 64 0 64 36 1.78 0.56 0.15 1072 13 226 0.56 Example of present invention 12 C 15 80 5 50 50 3.33 0.54 0.11 1147 12 162 0.42 Comparative Example 13 C 28 72 0 52 48 1.86 0.67 0.09 1127 15 219 0.52 Example of present invention 14 C 28 72 0 52 48 1.86 0.67 0.05 1188 14 183 0.48 Example of present invention 15 D 53 47 0 75 25 1.42 0.53 0.06 987 13 202 0.55 Example of present invention 16 D 18 68 14 68 32 3.78 0.22 0.12 1137 12 145 0.39 Comparative Example 17 D 37 63 0 63 37 1.70 0.59 0.06 1054 17 160 0.60 Example of present invention 18 D 33 67 0 67 33 2.03 0.49 0.06 1007 16 158 0.49 Example of present invention 19 E 46 54 0 54 46 1.17 0.85 0.12 1079 11 163 0.57 Example of present invention 20 E 18 60 22 60 40 3.33 0.37 0.18 1093 11 206 0.46 Comparative Example An underline means that a condition is out of the range of the present invention or a property is not preferable.

TABLE-US-00005 TABLE 5 Inner region Mar- tensite + Surface layer region Hardness Total Absorbed Fatigue Ferrite bainite Re- Ferrite Re- difference Tensile elon- energy limit Test Steel c c mainder s mainder s/c s/c ratio strength gation (20 C.) ratio No. type Area % Area % Area % Area % Area % MPa % J/cm.sup.2 Note 21 E 47 53 0 62 38 1.32 0.72 0.09 996 15 211 0.57 Example of present invention 22 E 70 25 5 72 28 1.03 0.92 0.07 1157 15 65 0.45 Comparative Example 23 F 45 55 0 55 45 1.22 0.82 0.09 1189 15 141 0.50 Example of present invention 24 F 25 55 20 75 25 3.00 0.45 0.03 850 18 75 0.60 Comparative Example 25 F 40 60 0 60 40 1.50 0.67 0.04 1159 13 91 0.52 Example of present invention 26 F 30 70 0 70 30 2.33 0.43 0.13 1070 12 232 0.59 Example of present invention 27 G 25 75 0 28 72 1.12 0.96 0.07 1133 18 126 0.42 Comparative Example 28 G 34 66 0 66 34 1.94 0.52 0.06 1011 14 184 0.49 Example of present invention 29 G 33 67 0 67 33 2.03 0.49 0.09 1159 17 226 0.58 Example of present invention 30 G 51 49 0 73 27 1.43 0.55 0.10 1121 15 158 0.58 Example of present invention 31 G 15 61 24 16 84 1.07 0.98 0.03 1153 18 122 0.42 Comparative Example 32 G 24 76 0 52 48 2.17 0.32 0.04 1163 12 187 0.56 Example of present invention 33 H 15 72 13 20 80 1.33 0.80 0.07 1206 15 124 0.40 Comparative Example 34 I 75 17 8 76 24 1.01 0.61 0.17 1034 15 72 0.41 Comparative Example 35 T 33 67 0 60 40 1.82 0.60 0.09 1159 13 158 0.52 Example of present invention 36 K 28 72 0 65 35 2.32 0.49 0.08 1012 14 191 0.50 Example of present invention 37 L 39 61 0 67 33 1.72 0.54 0.03 1189 16 214 0.57 Example of present invention 38 M 34 66 0 66 34 1.94 0.52 0.25 997 14 90 0.42 Comparative Example 39 N 33 67 0 67 33 2.03 0.49 0.22 1072 13 226 0.46 Comparative Example 40 K 52 42 6 91 9 1.75 0.21 0.06 987 13 57 0.56 Comparative Example 41 L 45 55 0 84 16 1.87 0.29 0.18 875 13 62 0.46 Comparative Example An underline means that a condition is out of the range of the present invention or a property is not preferable.

[0153] From Tables 4 and 5, it is found that hot-rolled steel sheets according to examples of the present invention have high strength and excellent toughness, fatigue strength, and ductility.

[0154] On the other hand, it is found that hot-rolled steel sheets according to comparative examples are inferior in any one or more of strength, toughness, and fatigue strength.

INDUSTRIAL APPLICABILITY

[0155] According to the aspect of the present invention, it is possible to provide a hot-rolled steel sheet that has high strength and excellent fatigue strength, toughness, and ductility. According to this hot-rolled steel sheet, since it is possible to reduce the weight of a vehicle body of a vehicle or the like, to integrally form components, to shorten a working step, and the like, to improve fuel efficiency, and to reduce manufacturing costs, the industrial value of the hot-rolled steel sheet is high.