HOT-ROLLED STEEL SHEET

Abstract

This hot-rolled steel sheet has a desired chemical composition, a microstructure contains, in area %, ferrite: 10 to 30%, bainite: 40 to 85%, retained austenite: 5 to 30%, fresh martensite: 5% or less, and pearlite: 5% or less, the ferrite has an average particle size of 5.00 m or less, a difference between an average nanoindentation hardness of the ferrite and an average nanoindentation hardness of the bainite is 1,000 MPa or less, and the tensile strength is 980 MPa or more.

Claims

1. A hot-rolled steel sheet having a chemical composition containing, in mass %, C: 0.100 to 0.350%, Si: 0.01 to 3.00%, Mn: 1.00 to 4.00%, sol. Al: 0.001 to 2.000%, Si+sol. Al: 1.00% or more, Ti: 0.010 to 0.380%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, Nb: 0 to 0.100%, V: 0 to 0.500%, Cu: 0 to 2.00%, Cr: 0 to 2.00%, Mo: 0 to 1.00%, Ni: 0 to 2.00%, B: 0 to 0.0100%, Ca: 0 to 0.0200%, Mg: 0 to 0.0200%, REM: 0 to 0.1000%, Bi: 0 to 0.020%, one or more of Zr, Co, Zn and W: 0 to 1.00% in total, and Sn: 0 to 0.050%, in which Tief represented by the following Formula (a) is 0.010 to 0.300%, and the remainder consists of Fe and impurities, and a microstructure comprising, in area %, ferrite: 10 to 30%, bainite: 40 to 85%, retained austenite: 5 to 30%, fresh martensite: 5% or less, and pearlite: 5% or less, wherein the ferrite has an average particle size of 5.00 m or less, wherein a difference between an average nanoindentation hardness of the ferrite and an average nanoindentation hardness of the bainite is 1,000 MPa or less, and wherein the tensile strength is 980 MPa or more:
Tief=Ti-48/14N-48/32S(a) where each element symbol in Formula (a) indicates their content (mass %).

2. The hot-rolled steel sheet according to claim 1, wherein the chemical composition contains, in mass % one or more of Nb: 0.005 to 0.100%, V: 0.005 to 0.500%, Cu: 0.01 to 2.00%, Cr: 0.01 to 2.00%, Mo: 0.01 to 1.00%, Ni: 0.02 to 2.00%, B: 0.0001 to 0.0100%, Ca: 0.0005 to 0.0200%, Mg: 0.0005 to 0.0200%, REM: 0.0005 to 0.1000%, and Bi: 0.0005 to 0.020%.

3. A hot-rolled steel sheet having a chemical composition containing, in mass %, C: 0.100 to 0.350%, Si: 0.01 to 3.00%, Mn: 1.00 to 4.00%, sol. Al: 0.001 to 2.000%, Si+sol. Al: 1.00% or more, Ti: 0.010 to 0.380%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, Nb: 0 to 0.100%, V: 0 to 0.500%, Cu: 0 to 2.00%, Cr: 0 to 2.00%, Mo: 0 to 1.00%, Ni: 0 to 2.00%, B: 0 to 0.0100%, Ca: 0 to 0.0200%, Mg: 0 to 0.0200%, REM: 0 to 0.1000%, Bi: 0 to 0.020%, one or more of Zr, Co, Zn and W: 0 to 1.00% in total, and Sn: 0 to 0.050%, in which Tief represented by the following Formula (a) is 0.010 to 0.300%, and the remainder comprising Fe and impurities, and a microstructure comprising, in area %, ferrite: 10 to 30%, bainite: 40 to 85%, retained austenite: 5 to 30%, fresh martensite: 5% or less, and pearlite: 5% or less, wherein the ferrite has an average particle size of 5.00 m or less, wherein a difference between an average nanoindentation hardness of the ferrite and an average nanoindentation hardness of the bainite is 1,000 MPa or less, and wherein the tensile strength is 980 MPa or more:
Tief=Ti48/14N48/32S(a) where each element symbol in Formula (a) indicates their content (mass %).

Description

EXAMPLES

[0174] Next, effects of one aspect of the present invention will be described in more detail with reference to examples, but conditions in the examples are one condition example used for confirming the feasibility and effects of the present invention, and the present invention is not limited to this one condition example. In the present invention, various conditions can be used without departing from the gist of the present invention and as long as the object of the present invention can be achieved.

[0175] Steels having chemical compositions shown in Tables 1 and 2 were melted, and slabs with a thickness of 240 to 300 mm were produced by continuous casting. Using the obtained slabs, hot-rolled steel sheets were obtained under production conditions shown in Tables 3 and 4.

[0176] Here, before hot rolling, the sample was heated to the slab heating temperature shown in Table 3 and held for 6,000 seconds or more. In Table 4, in Production No. 10, after primary cooling, air cooling was performed in a temperature range of 530 C. or lower for an air cooling time shown in Table 4, and in Production No. 11, after primary cooling, air cooling was performed in a temperature range of higher than 700 C. and 723 C. or lower for an air cooling time shown in Table 4. In addition, in all examples, tertiary cooling was performed to a temperature range of 150 C. or lower.

[0177] For the obtained hot-rolled steel sheets, the area proportion of each structure, the average particle size of ferrite, the difference between the average nanoindentation hardness of ferrite and the average nanoindentation hardness of bainite, the tensile strength TS, the uniform elongation uEl, the hole expansion rate and the maximum bending angle were measured by the above methods. Here, a total elongation El (elongation at break according to JIS Z 2241: 2011) was obtained by a tensile test in which the tensile strength TS and the uniform elongation uEl were measured.

[0178] The obtained measurement results are shown in Table 5. Here, in Production No. 15, a 40 area % tempered martensite (a structure that could not be determined as any structure by the above structure observation method) was generated in addition to the structure shown in Table 5.

Evaluation Criteria

[0179] If the tensile strength TS was 980 MPa or more, it was determined satisfactory because the sample had excellent strength. On the other hand, if the tensile strength TS was less than 980 MPa, it was determined unsatisfactory because the sample did not have excellent strength.

[0180] If the product (TSuEl) of the tensile strength TS and the uniform elongation uEl was 8,260 MPa.Math.% or more, it was determined satisfactory because the sample had excellent ductility. On the other hand, if the TSuEl was less than 8,260 MPa.Math.%, it was determined unsatisfactory because the sample did not have excellent ductility.

[0181] If the hole expansion rate was 45% or more, it was determined satisfactory because the sample had excellent hole expansibility. On the other hand, if the hole expansion rate was less than 45%, it was determined unsatisfactory because the sample did not have excellent hole expansibility.

[0182] If the maximum bending angle was 60 or more, it was determined satisfactory because the sample had excellent bendability. On the other hand, if the maximum bending angle was less than 60, it was determined unsatisfactory because the sample did not have excellent bendability.

TABLE-US-00001 TABLE 1 Mass %, remainder being Fe and impurities Steel No. C Si Mn sol. Al Si + sol. Al Ti P S N A 0.152 0.90 2.70 0.620 1.52 0.120 0.021 0.0019 0.0034 B 0.210 2.25 2.61 0.033 2.28 0.061 0.020 0.0011 0.0026 C 0.345 1.26 1.85 0.750 2.01 0.051 0.023 0.0020 0.0023 D 0.165 0.85 2.07 0.260 1.11 0.095 0.019 0.0027 0.0031 E 0.256 1.24 2.49 1.310 2.55 0.090 0.022 0.0011 0.0016 F 0.264 1.56 1.42 0.650 2.21 0.065 0.021 0.0017 0.0038 G 0.124 1.85 3.67 0.023 1.87 0.113 0.021 0.0033 0.0028 H 0.194 2.16 2.45 0.033 2.19 0.075 0.023 0.0025 0.0019 I 0.185 2.20 2.08 0.019 2.22 0.121 0.021 0.0027 0.0022 J 0.167 2.43 3.21 0.018 2.45 0.086 0.018 0.0030 0.0028 K 0.168 1.95 2.04 0.038 1.99 0.072 0.025 0.0025 0.0031 L 0.185 1.61 2.91 0.040 1.65 0.096 0.023 0.0031 0.0028 M 0.240 2.23 1.92 0.015 2.25 0.054 0.016 0.0035 0.0038 N 0.154 2.05 2.66 0.023 2.07 0.062 0.017 0.0024 0.0015 O 0.096 2.13 2.45 0.022 2.15 0.058 0.025 0.0029 0.0036 P 0.381 2.37 2.90 0.035 2.41 0.134 0.015 0.0018 0.0019 Q 0.154 0.51 2.63 0.450 0.96 0.065 0.015 0.0036 0.0015 R 0.251 1.77 0.86 0.032 1.80 0.074 0.024 0.0011 0.0039 S 0.175 2.01 4.24 0.029 2.04 0.053 0.024 0.0029 0.0026 T 0.216 1.79 2.45 0.029 1.82 0.009 0.024 0.0029 0.0026 U 0.270 1.24 2.46 0.380 1.62 0.052 0.024 0.0056 0.0120 V 0.101 2.06 1.97 0.040 2.10 0.016 0.010 0.0030 0.0030 W 0.110 0.65 2.82 1.240 1.89 0.376 0.012 0.0049 0.0204 X 0.121 1.65 3.50 0.460 2.11 0.312 0.023 0.0018 0.0036 Mass %, remainder being Fe and impurities Steel No. O Nb V Cu Cr Mo Ni B Note A 0.0038 Steel of the present invention B 0.0031 Steel of the present invention C 0.0033 Steel of the present invention D 0.0027 Steel of the present invention E 0.0016 Steel of the present invention F 0.0025 Steel of the present invention G 0.0046 Steel of the present invention H 0.0022 0.042 Steel of the present invention I 0.0051 0.034 Steel of the present invention J 0.0042 0.04 Steel of the present invention K 0.0054 0.42 Steel of the present invention L 0.0030 0.14 Steel of the present invention M 0.0047 0.19 Steel of the present invention N 0.0032 0.0025 Steel of the present invention O 0.0037 Comparative steel P 0.0043 Comparative steel Q 0.0015 Comparative steel R 0.0034 Comparative steel S 0.0039 Comparative steel T 0.0039 Comparative steel U 0.0058 Comparative steel V 0.0031 Comparative steel W 0.0010 Steel of the present invention X 0.0025 Steel of the present invention The underline indicates that it is outside the scope of the present invention

TABLE-US-00002 TABLE 2 Steel Mass %, remainder being Fe and impurities No. Ca Mg REM Bi Zr Co Zn W Sn Tief T0 T1 T2 Note A 0.0021 0.0014 0.105 1286 884 430 Steel of the present invention B 0.050 1235 871 405 Steel of the present invention C 0.0017 0.040 1281 869 366 Steel of the present invention D 0.003 0.080 1264 877 444 Steel of the present invention E 0.083 1322 879 387 Steel of the present invention F 0.049 1277 870 419 Steel of the present invention G 0.098 1248 883 411 Steel of the present invention H 0.08 0.065 1253 888 418 Steel of the present invention I 0.03 0.109 1318 888 435 Steel of the present invention J 0.05 0.072 1251 877 406 Steel of the present invention K 0.058 1227 876 437 Steel of the present invention L 0.018 0.082 1282 893 404 Steel of the present invention M 0.036 1236 868 411 Steel of the present invention N 0.14 0.053 1194 870 430 Steel of the present invention O 0.041 1126 867 465 Comparative steel P 0.125 1459 895 315 Comparative steel Q 0.054 1201 870 431 Comparative steel R 0.059 1289 871 444 Comparative steel S 0.040 1191 871 368 Comparative steel T 0.004 1009 858 408 Comparative steel U 0.002 1248 870 382 Comparative steel V 0.001 990 856 478 Comparative steel W 0.299 1421 948 446 Steel of the present invention X 0.297 1404 932 418 Steel of the present invention The underline indicates that it is outside the scope of the present invention

TABLE-US-00003 TABLE 3 Time from Cumulative rolling Cumulative rolling Final rolling Time from completion of reduction rate in a reduction rate reduction rate completion of Slab heating rough rolling until temperature range during finish during finish finish rolling until Production Steel temperature finish rolling of T1 to T1 + 30 C. rolling rolling start of cooling No. No. C. s % % % s 1 A 1300 66 32 94 19 0.9 2 B 1250 60 32 93 17 0.7 3 B 1200 70 31 93 15 0.8 4 B 1250 165 32 93 15 0.8 5 B 1250 109 20 93 20 0.8 6 B 1250 95 32 85 20 0.9 7 B 1250 65 32 93 12 0.7 8 B 1250 65 54 93 16 1.3 9 B 1250 66 54 93 18 0.9 10 B 1250 43 32 94 18 0.7 11 B 1250 76 32 94 20 0.9 12 B 1250 59 32 93 18 0.5 13 B 1250 99 44 93 15 0.7 14 B 1250 36 32 93 15 0.8 15 B 1250 60 38 93 15 0.8 16 B 1250 75 32 93 15 0.9 17 B 1250 110 32 93 15 0.8 18 C 1290 95 41 90 15 0.7 19 D 1290 68 39 90 18 0.7 20 E 1350 77 41 91 16 0.8 21 F 1280 41 51 91 16 0.8 22 G 1260 63 51 90 20 1.0 23 H 1260 85 36 94 19 0.9 24 I 1320 95 51 94 15 0.8 25 J 1260 80 36 90 20 0.7 26 K 1250 89 40 92 19 1.0 27 L 1290 57 38 91 15 0.7 28 M 1250 81 39 90 15 0.6 29 N 1230 55 42 90 17 1.0 30 O 1230 47 41 93 16 0.9 31 P 1300 67 54 90 17 0.8 32 Q 1250 66 54 90 20 0.9 33 R 1300 76 39 90 20 0.8 34 S 1230 101 54 93 20 0.8 35 T 1230 64 40 92 20 0.7 36 U 1250 40 43 90 15 1.0 37 V 1200 122 30 94 28 0.4 38 W 1420 120 75 94 15 0.5 39 X 1410 94 72 93 16 0.7 40 B 1250 135 32 94 20 0.5 The underline indicates that conditions are not preferable

TABLE-US-00004 TABLE 4 Air cooling Average Primary time in a Average Average cooling rate cooling temperature cooling rate cooling rate of primary stop range of 600 to of secondary Coiling of tertiary Production Steel cooling temperature 700 C. cooling temperature cooling No. No. C./s C. s C./s C. C./h Note 1 A 41 681 2.9 44 436 27 Example of the present invention 2 B 49 627 2.8 44 423 20 Example of the present invention 3 B 51 618 2.7 53 408 27 Comparative Example 4 B 49 624 1.1 51 410 27 Comparative Example 5 B 50 623 2.8 45 412 27 Comparative Example 6 B 46 641 2.9 40 423 27 Comparative Example 7 B 41 665 2.5 49 409 27 Comparative Example 8 B 50 605 1.5 52 414 27 Comparative Example 9 B 13 680 1.1 58 426 25 Comparative Example 10 B 46 530 2.3 49 429 25 Comparative Example 11 B 30 723 2.2 56 412 25 Comparative Example 12 B 56 639 0.0 53 414 25 Comparative Example 13 B 48 632 3.4 43 423 25 Comparative Example 14 B 44 650 2.2 37 434 25 Comparative Example 15 B 49 628 2.3 58 352 25 Comparative Example 16 B 24 642 3.0 42 410 50 Comparative Example 17 B 65 680 2.5 47 415 10 Comparative Example 18 C 48 629 2.6 51 390 15 Example of the present invention 19 D 47 644 2.7 43 470 15 Example of the present invention 20 E 54 610 2.4 55 411 20 Example of the present invention 21 F 39 675 2.5 56 448 20 Example of the present invention 22 G 45 658 2.9 58 442 20 Example of the present invention 23 H 42 678 1.7 52 447 20 Example of the present invention 24 I 39 692 2.2 41 438 15 Example of the present invention 25 J 45 652 2.3 51 433 25 Example of the present invention 26 K 26 632 2.0 40 440 25 Example of the present invention 27 L 41 688 2.6 44 428 25 Example of the present invention 28 M 52 608 1.2 43 428 25 Example of the present invention 29 N 51 616 2.3 43 438 25 Example of the present invention 30 O 42 657 1.5 53 471 27 Comparative Example 31 P 48 653 2.8 58 328 27 Comparative Example 32 Q 44 651 2.1 56 460 27 Comparative Example 33 R 51 614 3.0 44 470 25 Comparative Example 34 S 44 649 1.0 54 374 20 Comparative Example 35 T 33 692 2.4 54 415 20 Comparative Example 36 U 41 666 1.7 46 393 20 Comparative Example 37 V 15 686 7.0 38 370 60 Comparative Example 38 W 42 684 2.8 52 448 25 Example of the present invention 39 X 100 605 3.0 67 430 35 Example of the present invention 40 B 68 620 7.1 65 406 25 Comparative Example The underline indicates that conditions are not preferable

TABLE-US-00005 TABLE 5 Average Difference in particle average hardness Retained Fresh size of between ferrite Sheet Production Steel Ferrite Bainite austenite martensite Pearlite ferrite and bainite thickness No. No. area % area % area % area % area % m MPa mm Note 1 A 25 64 9 2 0 1.40 846 3.6 Example of the present invention 2 B 12 68 20 0 0 1.52 967 2.1 Example of the present invention 3 B 11 73 16 0 0 1.43 1240 2.6 Comparative Example 4 B 11 74 15 0 0 1.40 1146 2.9 Comparative Example 5 B 9 75 15 1 0 2.42 925 2.6 Comparative Example 6 B 9 74 15 2 0 4.10 924 2.6 Comparative Example 7 B 7 77 13 0 3 4.80 18 2.9 Comparative Example 8 B 12 76 12 0 0 5.20 879 2.6 Comparative Example 9 B 32 51 12 5 0 5.43 1125 2.6 Comparative Example 10 B 28 58 10 0 4 2.84 1071 2.9 Comparative Example 11 B 25 63 12 0 0 1.85 1035 2.6 Comparative Example 12 B 0 84 12 4 0 2.9 Comparative Example 13 B 35 52 10 0 3 2.85 976 2.9 Comparative Example 14 B 12 79 4 0 5 2.10 984 2.6 Comparative Example 15 B 12 38 10 0 0 1.85 974 2.1 Comparative Example 16 B 28 57 4 11 0 3.24 954 2.1 Comparative Example 17 B 22 76 2 0 0 2.12 846 2.1 Comparative Example 18 C 10 67 23 0 0 1.62 913 2.1 Example of the present invention 19 D 18 76 6 0 0 2.14 972 4.2 Example of the present invention 20 E 28 61 8 0 3 3.47 924 2.6 Example of the present invention 21 F 29 56 15 0 0 3.20 897 1.8 Example of the present invention 22 G 12 71 12 5 0 1.87 976 2.1 Example of the present invention 23 H 28 56 16 0 0 1.24 865 2.1 Example of the present invention 24 1 17 70 13 0 0 1.65 954 2.9 Example of the present invention 25 J 12 69 14 5 0 1.23 992 2.3 Example of the present invention 26 K 15 75 10 0 0 3.10 894 2.9 Example of the present invention 27 L 10 73 13 4 0 1.46 886 2.9 Example of the present invention 28 M 12 75 13 0 0 1.79 987 2.9 Example of the present invention 29 N 10 78 12 0 0 1.23 894 4.0 Example of the present invention 30 O 42 58 0 0 0 4.30 891 2.9 Comparative Example 31 P 0 49 8 43 0 2.9 Comparative Example 32 Q 10 87 3 0 0 1.62 874 4.0 Comparative Example 33 R 48 47 5 0 0 4.82 924 2.9 Comparative Example 34 S 0 72 4 24 0 2.3 Comparative Example 35 T 23 65 12 0 0 4.95 1232 2.9 Comparative Example 36 U 10 77 13 0 0 1.26 1165 2.6 Comparative Example 37 V 43 52 4 1 0 1.80 1242 2.1 Comparative Example 38 W 15 71 14 0 0 1.54 764 2.6 Example of the present invention 39 X 11 69 16 4 0 2.36 824 2.6 Example of the present invention 40 B 37 46 15 0 2 1.56 1152 2.9 Comparative Example The underline indicates that it is outside the scope of the present invention or property values are not preferable

TABLE-US-00006 TABLE 6 Tensile Total Uniform Hole Maximum strength elongation elongation expansion bending Production Steel TS El uEl TS uEl rate angle No. No. MPa % % MPa .Math. % % Note 1 A 1044 27.0 10.0 10440 48 62 Example of the present invention 2 B 1216 20.0 12.0 14592 56 75 Example of the present invention 3 B 1086 13.6 8.1 8797 24 48 Comparative Example 4 B 1179 14.8 8.1 9550 32 48 Comparative Example 5 B 1254 15.0 6.6 8276 42 52 Comparative Example 6 B 1178 17.0 7.0 8246 43 61 Comparative Example 7 B 1211 16.2 6.8 8235 32 61 Comparative Example 8 B 1054 22.0 9.8 10329 35 51 Comparative Example 9 B 976 21.0 11.0 10736 35 57 Comparative Example 10 B 987 18.4 10.0 9870 38 56 Comparative Example 11 B 992 20.1 10.0 9920 35 52 Comparative Example 12 B 1201 15.0 6.8 8167 58 58 Comparative Example 13 B 972 23.0 15.0 14580 44 58 Comparative Example 14 B 1257 12.4 6.5 8171 52 61 Comparative Example 15 B 1262 12.1 5.9 7446 48 64 Comparative Example 16 B 1351 12.0 6.0 8106 38 47 Comparative Example 17 B 992 17.0 8.0 7936 57 61 Comparative Example 18 C 1287 21.0 10.1 12999 50 65 Example of the present invention 19 D 984 21.0 11.0 10824 48 74 Example of the present invention 20 E 1221 13.4 11.3 13797 52 75 Example of the present invention 21 F 1236 15.6 12.0 14832 49 72 Example of the present invention 22 G 1182 14.2 12.0 14184 56 76 Example of the present invention 23 H 1213 17.0 9.2 11160 55 76 Example of the present invention 24 I 1257 16.0 7.1 8925 47 67 Example of the present invention 25 J 1294 16.2 7.2 9317 47 64 Example of the present invention 26 K 1192 19.0 9.1 10847 54 69 Example of the present invention 27 L 1242 17.4 8.6 10681 46 62 Example of the present invention 28 M 1275 14.2 6.8 8670 48 60 Example of the present invention 29 N 1274 15.4 6.8 8663 49 64 Example of the present invention 30 O 804 26.2 15.0 12060 25 79 Comparative Example 31 P 1542 9.0 6.0 9252 15 41 Comparative Example 32 Q 976 15.0 7.0 6832 68 69 Comparative Example 33 R 792 23.0 12.0 9504 62 72 Comparative Example 34 S 1524 11.0 5.0 7620 25 43 Comparative Example 35 T 1023 23.0 11.0 11253 36 51 Comparative Example 36 U 1232 16.0 8.1 9979 42 51 Comparative Example 37 V 832 29.0 18.0 14976 32 48 Comparative Example 38 W 1175 16.0 9.2 10810 54 64 Example of the present invention 39 X 1215 14.2 7.2 8748 52 63 Example of the present invention 40 B 956 22.4 15.0 14340 38 56 Comparative Example The underline indicates that it is outside the scope of the present invention or property values are not preferable

[0183] As can be understood from Table 6, in examples of the present invention, hot-rolled steel sheets having excellent strength, ductility, hole expansibility and bendability were obtained.

[0184] On the other hand, in comparative examples in which the chemical composition and/or the microstructure were not within the ranges defined by the present invention, any one or more of the above properties were poor. Here, in Production No. 15, since an amount of bainite was insufficient and tempered martensite was generated, the ductility deteriorated. In addition, in Production No. 16, the amount of fresh martensite was large, the difference in hardness between overall structures was large, and thus the hole expansibility and bendability deteriorated.

INDUSTRIAL APPLICABILITY

[0185] According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet having excellent strength, ductility, hole expansibility and bendability.