HOT-ROLLED STEEL SHEET

Abstract

This hot-rolled steel sheet has a predetermined chemical composition. The metallographic structure at a sheet thickness ¼ depth from a surface and at a center position in a sheet width direction in a sheet width cross section parallel to a rolling direction contains, by area %, 77.0% to 97.0% of bainite and tempered martensite in total, 0% to 5.0% of ferrite, 0% to 5.0% of pearlite, 3.0% or more of residual austenite, and 0% to 10.0% of martensite. The average grain size of the metallographic structure excluding the residual austenite is 7.0 μm or less. The C concentration in the residual austenite is 0.5 mass % or more. The number density of iron-based carbides having a diameter of 20 nm or more is 1.0×10.sup.6 carbides/mm.sup.2 or more.

Claims

1. A hot-rolled steel sheet comprising, as a chemical composition, by mass %: C: 0.100% to 0.250%; Si: 0.05% to 3.00%; Mn: 1.00% to 4.00%; Nb: 0.005% to 0.050%; sol. Al: 0.001% to 2.000%; P: 0.100% or less; S: 0.0300% or less; N: 0.1000% or less; O: 0.0100% or less; Ti: 0% to 0.300%; V: 0% to 0.500%; Cu: 0% to 2.00%; Cr: 0% to 2.00%; Mo: 0% to 1.000%; 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 two or more of Zr, Co, Zn, and W: 0% to 1.00% in total; Sn: 0% to 0.050%; and a remainder including Fe and impurities, wherein a metallographic structure at a sheet thickness ¼ depth from a surface and at a center position in a sheet width direction in a sheet width cross section parallel to a rolling direction contains, by area %, 77.0% to 97.0% of bainite and tempered martensite in total, 0% to 5.0% of ferrite, 0% to 5.0% of pearlite, 3.0% or more of residual austenite, and 0% to 10.0% of martensite, an average grain size of the metallographic structure excluding the residual austenite is 7.0 μm or less, a C concentration in the residual austenite is 0.5 mass % or more, and a number density of iron-based carbides having a diameter of 20 nm or more is 1.0×10.sup.6 carbides/mm.sup.2 or more.

2. The hot-rolled steel sheet according to claim 1, wherein in a case where area fractions of residual austenite in metallographic structures, in the sheet width cross section parallel to the rolling direction, at the sheet thickness ¼ depth from the surface and at the center position in the sheet width direction, at the sheet thickness ¼ depth from the surface and at a position 300 mm from the center position in the sheet width direction to one end side in the sheet width direction, at the sheet thickness ¼ depth from the surface and at a position 600 mm from the center position in the sheet width direction to the one end side in the sheet width direction, at the sheet thickness ¼ depth from the surface and at a position 300 mm from the center position in the sheet width direction to the other end side in the sheet width direction, and at the sheet thickness ¼ depth from the surface and at a position 600 mm from the center position in the sheet width direction to the other end side in the sheet width direction are respectively denoted by γ, γ.sub.D1, γ.sub.D2, γ.sub.W1, and γ.sub.W2 in terms of area %, γ/γ.sub.D1, γ/γ.sub.D2, γ/γ.sub.W1, and γ/γ.sub.W2 are each 0.8 or more and less than 1.2, and in a case where C concentrations in the residual austenite in the metallographic structures, at the sheet thickness ¼ depth from the surface and at the center position in the sheet width direction, at the sheet thickness ¼ depth from the surface and at the position 300 mm from the center position in the sheet width direction to the one end side in the sheet width direction, at the sheet thickness ¼ depth from the surface and at the position 600 mm from the center position in the sheet width direction to the one end side in the sheet width direction, at the sheet thickness ¼ depth from the surface and at the position 300 mm from the center position in the sheet width direction to the other end side in the sheet width direction, and at the sheet thickness ¼ depth from the surface and at the position 600 mm from the center position in the sheet width direction to the other end side in the sheet width direction are respectively denoted by Cγ.sub.C, Cγ.sub.D1, Cγ.sub.D2, Cγ.sub.W1, and Cγ.sub.W2 in terms of mass %, Cγ.sub.C/Cγ.sub.D1, Cγ.sub.C/Cγ.sub.D2, Cγ.sub.C/Cγ.sub.W1, and Cγ.sub.C/Cγ.sub.W2 are each 0.8 or more and less than 1.2.

3. The hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet includes, as the chemical composition, by mass %, one or two or more selected from the group consisting of Ti: 0.005% to 0.300%, V: 0.005% to 0.500%, Cu: 0.01% to 2.00%, Cr: 0.01% to 2.00%, Mo: 0.010% to 1.000%, 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%.

4. The hot-rolled steel sheet according to claim 2, wherein the hot-rolled steel sheet includes, as the chemical composition, by mass %, one or two or more selected from the group consisting of Ti: 0.005% to 0.300%, V: 0.005% to 0.500%, Cu: 0.01% to 2.00%, Cr: 0.01% to 2.00%, Mo: 0.010% to 1.000%, 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%.

Description

EXAMPLES

[0186] Next, the effects of one aspect of the present invention will be described more specifically by way of examples, but the conditions in the examples are condition examples adopted for confirming the feasibility and effects of the present invention. The present invention is not limited to these condition examples. In the present invention, various conditions can be adopted as long as the object of the present invention can be achieved without departing from the gist of the present invention.

[0187] Steels having chemical compositions shown in Steel Nos. A to Z in Tables 1 and 2 were melted and continuously cast to manufacture slabs having a thickness of 240 to 300 mm. The obtained slabs were used to obtain hot-rolled steel sheets under the manufacturing conditions shown in Tables 3 to 6.

[0188] In addition, Manufacturing No. 35 was cold-rolled at the cold rolling reduction shown in Table 6 after coiling and annealed at the annealing holding temperature and the annealing holding time shown in Table 6. Thereafter, the steel sheet was cooled to the cooling stop temperature at the primary cooling rate shown in Table 6 and then held for the hold time after cooling shown in Table 6. In Table 5, regarding Manufacturing No. 35, the incubation time after hot rolling and coiling are performed and before annealing in Table 6 is performed is shown.

[0189] In addition, cooling of Manufacturing Nos. 36 and 37 was once stopped at the incubation temperature shown in Table 4 in cooling after hot rolling, and the steel sheets were retained only for the incubation time shown in Table 4 at the incubation temperature and then cooled again.

TABLE-US-00001 TABLE 1 Steel Mass % Remainder including Fe and impurities No. C Si Mn Nb sol. Al P S N O Ti V Cu Cr Mo Ni B Remarks A 0.128 2.09 2.12 0.015 0.025 0.017 0.0023 0.0008 0.0045 Invention Steel B 0.191 2.17 1.90 0.020 0.027 0.012 0.0009 0.0021 0.0040 Invention Steel C 0.248 2.07 2.14 0.017 0.022 0.017 0.0020 0.0030 0.0019 Invention Steel D 0.217 0.36 2.59 0.017 1.503 0.025 0.0016 0.0026 0.0025 Invention Steel E 0.190 2.80 2.08 0.019 0.027 0.025 0.0009 0.0021 0.0012 Invention Steel F 0.204 2.01 1.11 0.012 0.033 0.017 0.0030 0.0010 0.0016 Invention Steel G 0.214 2.18 3.40 0.011 0.022 0.018 0.0038 0.0033 0.0044 Invention Steel H 0.192 1.91 1.89 0.043 0.032 0.017 0.0013 0.0029 0.0013 Invention Steel I 0.181 1.97 2.04 0.010 0.019 0.016 0.0044 0.0012 0.0014 0.038 Invention Steel J 0.209 1.89 2.11 0.010 0.029 0.026 0.0014 0.0041 0.0043 0.040 Invention Steel K 0.212 2.06 1.94 0.018 0.018 0.016 0.0009 0.0013 0.0019 0.03 Invention Steel L 0.219 1.88 2.14 0.015 0.018 0.021 0.0009 0.0048 0.0025 0.14 Invention Steel M 0.201 1.92 1.99 0.013 0.033 0.016 0.0039 0.0020 0.0006 0.180 Invention Steel N 0.208 1.91 1.90 0.020 0.030 0.014 0.0016 0.0011 0.0056 0.19 Invention Steel O 0.204 2.20 2.06 0.013 0.024 0.024 0.0023 0.0012 0.0027 0.0021 Invention Steel P 0.088 1.97 2.04 0.011 0.030 0.019 0.0047 0.0035 0.0031 Comparative Steel Q 0.299 2.02 2.16 0.010 0.025 0.013 0.0009 0.0017 0.0023 Comparative Steel R 0.200 0.02 2.11 0.009 0.029 0.011 0.0013 0.0049 0.0022 Comparative Steel S 0.187 2.02 0.85 0.014 0.023 0.018 0.0033 0.0009 0.0049 Comparative Steel T 0.208 2.11 4.12 0.017 0.032 0.022 0.0023 0.0035 0.0030 Comparative Steel U 0.208 1.90 2.19 0.000 0.020 0.013 0.0034 0.0031 0.0014 Comparative Steel V 0.169 2.19 2.11 0.058 0.021 0.024 0.0011 0.0036 0.0027 Comparative Steel W 0.109 0.94 2.03 0.025 0.030 0.009 0.0028 0.0015 0.0022 0.0013 Invention Steel X 0.150 1.50 1.50 0.030 0.031 0.021 0.0048 0.0030 0.0035 Invention Steel Y 0.180 1.95 2.07 0.000 0.042 0.015 0.0090 0.0032 0.0035 Comparative Steel Z 0.180 1.99 2.00 0.005 0.051 0.015 0.0010 0.0030 0.0027 Invention Steel

TABLE-US-00002 TABLE 2 Steel Mass % Remainder including Fe and impurities No. Ca Mg REM Bi Zr Co Zn W Sn T1 T2 T3 T4 Remarks A 0.0024 0.0017 1090 731 545 460 Invention Steel B 1177 723 547 438 Invention Steel C 0.0017 1191 680 510 403 Invention Steel D 0.003 1172 990 478 403 Invention Steel E 1170 727 532 432 Invention Steel F 1120 769 615 458 Invention Steel G 1115 611 406 377 Invention Steel H 0.08 1289 718 548 438 Invention Steel I 1083 711 538 438 Invention Steel J 0.03 1100 695 524 422 Invention Steel K 0.07 1177 706 538 426 Invention Steel L 1157 683 508 414 Invention Steel M 0.018 1128 708 522 426 Invention Steel N 1189 704 536 426 Invention Steel O 0.14 1130 707 530 426 Invention Steel P 1013 750 563 482 Comparative Steel Q 1145 658 495 378 Comparative Steel R 1082 653 526 427 Comparative Steel S 1128 792 643 474 Comparative Steel T 1167 565 343 356 Comparative Steel U 703 688 517 420 Comparative Steel V 1317 717 534 441 Comparative Steel W 1133 717 558 472 Invention Steel X 1200 751 595 470 Invention Steel Y 695 714 535 437 Comparative Steel Z 1005 722 541 440 Invention Steel

TABLE-US-00003 TABLE 3 Sheet Time Average cooling Slab thickness Hot rolling until Average rate until heating reduction completion cooling cooling incubation Incubation cooling stop Manufacturing Steel temperature at 850° C. to temperature start rate temperature time after incubation No. No. T1 (° C.) 1100° C T2 (° C.) (sec) (° C./s) (° C.) (s) (° C/s) 1 A 1090 1219 91 731 881 1.1 64 — — — 2 B 1177 1216 92 723 890 0.7 84 — — — 3 B 1177 1207 87 723 905 0.9 88 — — — 4 B 1177 1227 90 723 719 1.1 117 — — — 5 B 1177 1237 91 723 905 1.8 110 — — — 6 B 1177 1226 91 723 912 1.0 43 — — — 7 B 1177 1233 93 723 898 0.6 109 — — — 8 B 1177 1237 90 723 906 1.1 121 — — — 9 B 1177 1237 90 723 917 1.0 114 — — — 10 B 1177 1206 93 723 900 0.9 73 — — — 11 B 1177 1198 93 723 919 1.1 87 — — — 12 B 1177 1226 92 723 906 0.8 64 — — — 13 B 1177 1220 93 723 902 1.0 91 — — — 14 C 1191 1215 92 680 899 1.1 92 — — — 15 D 1172 1199 90 990 1005 0.6 100 — — — 16 E 1170 1229 91 727 897 0.9 108 — — — 17 F 1120 1201 92 769 911 0.6 79 — — — 18 G 1115 1228 91 611 884 0.7 77 — — — 19 H 1289 1294 91 718 900 0.8 76 — — — 20 I 1083 1207 91 711 917 0.8 94 — — — Average cooling Cooling rate from cooling stop stop temperature to Coiling Manufacturing temperature coiling temperature temperature No. T3 (° C.) (° C./s) T4 + 50 (° C.) T4 − 100 Remarks 1 545 426 29 510 382 360 Invention Example 2 547 400 31 488 367 338 Invention Example 3 547 400 16 488 381 338 Comparative Example 4 547 407 15 488 351 338 Comparative Example 5 547 392 25 488 384 338 Comparative Example 6 547 405 22 488 379 338 Comparative Example 7 547 560 21 488 376 338 Comparative Example 8 547 411 6 488 361 338 Comparative Example 9 547 528 18 488 492 338 Comparative Example 10 547 320 24 488 286 338 Comparative Example 11 547 519 16 488 470 338 Comparative Example 12 547 460 18 488 439 338 Comparative Example 13 547 401 21 488 377 338 Comparative Example 14 510 491 27 453 448 303 Invention Example 15 478 417 15 453 371 303 Invention Example 16 532 427 25 482 370 332 Invention Example 17 615 510 22 508 482 358 Invention Example 18 406 395 27 427 373 277 Invention Example 19 548 398 28 488 375 338 Invention Example 20 538 463 24 488 419 338 Invention Example

TABLE-US-00004 TABLE 4 Sheet Time Average cooling Slab thickness Hot rolling until Average rate until heating reduction completion cooling cooling incubation Incubation cooling stop Manufacturing Steel temperature at 850° C. to temperature start rate temperature time after incubation No. No. T1 (° C.) 1100° C T2 (° C.) (sec) (° C./s) (° C.) (s) (° C/s) 21 J 1100 1212 92 695 904 1.1 112 — — — 22 K 1177 1226 91 706 900 1.0 80 — — — 23 L 1157 1201 90 683 882 0.9 111 — — — 24 M 1128 1198 91 708 903 0.9 74 — — — 25 N 1189 1229 93 704 890 0.9 108 — — — 26 O 1130 1211 92 707 902 0.6 119 — — — 27 P 1013 1230 93 750 895 1.0 118 — — — 28 Q 1145 1219 91 658 880 0.8 93 — — — 29 R 1082 1203 92 653 899 0.9 109 — — — 30 S 1128 1201 93 792 887 1.0 93 — — — 31 T 1167 1225 93 565 904 0.9 100 — — — 32 U 703 1226 90 688 910 0.8 90 — — — 33 V 1317 1320 90 717 909 0.8 116 — — — 34 W 1133 1240 91 717 950 0.8 78 — — — 35 X 1200 1234 92 751 938 1.5 80 — — — 36 Y 695 1217 93 714 904 0.1 290 780 2 230 37 Z 1005 1220 90 722 892 0.1 250 840 1 200 Average cooling Cooling rate from cooling stop stop temperature to Coiling Manufacturing temperature coiling temperature temperature No. T3 (° C.) (° C./s) T4 + 50 (° C.) T4 − 100 Remarks 21 524 396 30 472 368 322 Invention Example 22 538 397 30 476 365 326 Invention Example 23 508 400 26 464 365 314 Invention Example 24 522 408 19 476 382 326 Invention Example 25 536 422 27 476 363 326 Invention Example 26 530 409 30 476 364 326 Invention Example 27 563 396 30 532 382 382 Comparative Example 28 495 424 27 428 370 278 Comparative Example 29 526 424 24 477 365 327 Comparative Example 30 643 396 17 524 380 374 Comparative Example 31 343 330 30 406 318 256 Comparative Example 32 517 391 31 470 364 320 Comparative Example 33 534 419 22 491 376 341 Comparative Example 34 558 432 30 522 390 372 Comparative Example 35 595 348 31 520 332 370 Comparative Example 36 535 430 80 487 400 337 Comparative Example 37 541 452 65 490 425 340 Comparative Example

TABLE-US-00005 TABLE 5 Endmost portion of hot-rolled steel Center portion of hot-rolled steel sheet in sheet width direction sheet in sheet width direction Incubation Incubation Incubation Incubation Incubation Incubation time at time at time at time at time at time at 450° C. or 400° C. or 350° C. or 450° C. or 400° C. or 350° C. or Manufacturing higher higher higher higher higher higher No. (s) (s) (s) (s) (s) (s) Remarks 1 0 0 1600 0 0 15600 Invention Example 2 0 0 1500 0 0 9200 Invention Example 3 0 0 1800 0 0 15500 Comparative Example 4 0 0 1100 0 0 2400 Comparative Example 5 0 0 1600 0 0 15300 Comparative Example 6 0 0 1500 0 0 13700 Comparative Example 7 0 0 1800 0 0 13700 Comparative Example 8 0 0 1400 0 0 14200 Comparative Example 9 800 900 7400 1700 7600 27100 Comparative Example 10 0 0 0 0 0 0 Comparative Example 11 500 900 1600 2300 7900 14900 Comparative Example 12 0 700 1200 0 15200 28500 Comparative Example 13 0 0 24700 0 0 34000 Comparative Example 14 0 500 5300 0 6000 21800 Invention Example 15 0 0 1300 0 0 9800 Invention Example 16 0 0 1800 0 0 10300 Invention Example 17 900 4500 9700 2000 8000 29500 Invention Example 18 0 0 1300 0 0 12000 Invention Example 19 0 0 1800 0 0 13500 Invention Example 20 0 600 5600 0 6400 20600 Invention Example 21 0 0 1200 0 0 11000 Invention Example 22 0 0 1600 0 0 9200 Invention Example 23 0 0 1100 0 0 7200 Invention Example 24 0 0 1700 0 0 16100 Invention Example 25 0 0 1500 0 0 7300 Invention Example 26 0 0 1300 0 0 7000 Invention Example 27 0 0 2400 0 0 16500 Comparative Example 28 0 0 1700 0 0 9500 Comparative Example 29 0 0 1500 0 0 7100 Comparative Example 30 0 0 1500 0 0 12900 Comparative Example 31 0 0 1000 0 0 13900 Comparative Example 32 0 0 1300 0 0 6700 Comparative Example 33 0 0 1900 0 0 13500 Comparative Example 34 0 0 100 0 0 900 Comparative Example 35 0 0 0 0 0 0 Comparative Example 36 0 0 3900 0 0 13000 Comparative Example 37 0 100 900 0 6500 15000 Comparative Example

TABLE-US-00006 TABLE 6 Annealing Annealing Primary Hold time Cold rolling holding holding cooling Cooling stop after Manufacturing reduction temperature time rate temperature cooling No. (%) (° C.) (s) (° C./s) (° C.) (s) 1 — — — — — — 2 — — — — — — 3 — — — — — — 4 — — — — — — 5 — — — — — — 6 — — — — — — 7 — — — — — — 8 — — — — — — 9 — — — — — — 10 — — — — — — 11 — — — — — — 12 — — — — — — 13 — — — — — — 14 — — — — — — 15 — — — — — — 16 — — — — — — 17 — — — — — — 18 — — — — — — 19 — — — — — — 20 — — — — — — 21 — — — — — — 22 — — — — — — 23 — — — — — — 24 — — — — — — 25 — — — — — — 26 — — — — — — 27 — — — — — — 28 — — — — — — 29 — — — — — — 30 — — — — — — 31 — — — — — — 32 — — — — — — 33 — — — — — — 34 — — — — — — 35 10 800 40 32 400 30 36 — — — — — — 37 — — — — — —

[0190] The metallographic structures of the hot-rolled steel sheets of Manufacturing Nos. 1 to 37 obtained were observed by the above-mentioned method and the area fraction, the average grain size, and the number density of iron-based carbides of each phase were obtained. In addition, the hot-rolled steel sheets of Manufacturing Nos. 1 to 37 were subjected to X-ray diffraction by the above-mentioned method and the C concentration in the residual austenite was obtained. The obtained measurement results are shown in Tables 7 to 9.

[0191] γ, γ.sub.D1, γ.sub.D2, γ.sub.W1, and γ.sub.W2 in Table 8 refer to the area fractions of the residual austenite in the metallographic structures at the sheet thickness ¼ depth from the surface and at the center position in the sheet width direction, at the sheet thickness ¼ depth from the surface and at a position 300 mm from the center position in the sheet width direction to one end side in the sheet width direction, at the sheet thickness ¼ depth from the surface and at a position 600 mm from the center position in the sheet width direction to the one end side in the sheet width direction, at the sheet thickness ¼ depth from the surface and at a position 300 mm from the center position in the sheet width direction to the other end side in the sheet width direction, and at the sheet thickness ¼ depth from the surface and at a position 600 mm from the center position in the sheet width direction to the other end side in the sheet width cross section parallel to the rolling direction.

[0192] Cγ.sub.C, Cγ.sub.D1, Cγ.sub.D2, Cγ.sub.W1, and Cγ.sub.W2 in Table 9 refer to the C concentrations by mass % in the residual austenite in the metallographic structures at the sheet thickness ¼ depth from the surface and at the center position in the sheet width direction, at the sheet thickness ¼ depth from the surface and at the position 300 mm from the center position in the sheet width direction to one end side in the sheet width direction, at the sheet thickness ¼ depth from the surface and at the position 600 mm from the center position in the sheet width direction to the one end side in the sheet width direction, at the sheet thickness ¼ depth from the surface and at the position 300 mm from the center position in the sheet width direction to the other end side in the sheet width direction, and at the sheet thickness ¼ depth from the surface and at the position 600 mm from the center position in the sheet width direction to the other end side in the sheet width cross section parallel to the rolling direction.

TABLE-US-00007 TABLE 7 Metallographic structure at center portion in sheet width direction C concen- Average number tration in density of Tempered Residual residual Average iron-based Sheet martensite + austenite austenite grain carbides thick- Manufacturing bainite Ferrite Pearlite (γ) Martensite (C.sub.γC) size (10.sup.6 ness No. (area %) (area %) (area %) (area %) (area %) (mass %) (μm) carbides/mm.sup.2) (mm) Remarks 1 84.0 3.0 4.0 6.0 3.0 1.5 4.8 9.4 1.3 Invention Example 2 86.0 0.0 0.0 14.0 0.0 1.5 4.5 8.4 1.3 Invention Example 3 91.0 0.0 0.0 9.0 0.0 1.1 7.9 7.1 1.8 Comparative Example 4 79.0 8.0 0.0 13.0 0.0 1.1 5.3 7.7 1.9 Comparative Example 5 85.0 0.0 0.0 15.0 0.0 1.2 7.3 9.2 3.6 Comparative Example 6 78.0 7.0 0.0 15.0 0.0 1.1 4.3 7.8 3.4 Comparative Example 7 85.0 0.0 9.0 6.0 0.0 1.2 4.7 7.5 1.5 Comparative Example 8 87.0 0.0 6.0 7.0 0.0 1.1 4.8 8.7 2.7 Comparative Example 9 91.0 0.0 0.0 9.0 0.0 0.3 5.2 20.6 2.3 Comparative Example 10 98.0 0.0 0.0 2.0 0.0 1.2 4.9 7.1 3.1 Comparative Example 11 99.0 0.0 0.0 1.0 0.0 0.4 4.4 7.1 2.2 Comparative Example 12 98.0 0.0 0.0 2.0 0.0 0.4 4.7 5.5 3.4 Comparative Example 13 95.0 0.0 0.0 5.0 0.0 0.2 4.9 8.1 1.9 Comparative Example 14 78.0 0.0 4.0 11.0 7.0 1.4 5.5 8.4 2.3 Invention Example 15 96.0 0.0 0.0 4.0 0.0 1.4 4.8 8.9 3.4 Invention Example 16 88.0 0.0 0.0 12.0 0.0 1.5 4.5 9.2 3.0 Invention Example 17 86.0 4.0 2.0 8.0 0.0 1.3 4.4 6.3 3.8 Invention Example 18 85.0 0.0 0.0 6.0 9.0 1.1 5.5 8.5 3.6 Invention Example 19 91.0 0.0 0.0 9.0 0.0 1.1 5.5 7.0 3.3 Invention Example 20 78.0 0.0 0.0 17.0 5.0 1.5 4.8 7.5 1.7 Invention Example 21 89.0 0.0 0.0 11.0 0.0 1.5 5.6 7.2 3.0 Invention Example 22 91.0 0.0 0.0 9.0 0.0 1.1 4.9 7.3 2.3 Invention Example 23 91.0 0.0 0.0 9.0 0.0 1.4 5.6 9.1 4.0 Invention Example 24 86.0 0.0 0.0 14.0 0.0 1.1 4.9 7.2 3.9 Invention Example 25 91.0 0.0 0.0 9.0 0.0 1.2 4.4 10.0 3.1 Invention Example 26 85.0 0.0 0.0 15.0 0.0 1.3 5.0 7.2 3.7 Invention Example 27 70.0 28.0 0.0 2.0 0.0 1.4 4.7 9.7 2.6 Comparative Example 28 61.0 0.0 38.0 1.0 0.0 1.4 5.1 8.1 2.5 Comparative Example 29 100.0 0.0 0.0 0.0 0.0 0.0 4.6 7.4 1.9 Comparative Example 30 75.0 14.0 0.0 11.0 0.0 1.4 5.5 6.5 3.7 Comparative Example 31 86.0 0.0 0.0 2.0 12.0 1.2 5.3 9.2 2.1 Comparative Example 32 91.0 0.0 0.0 9.0 0.0 1.1 4.8 7.5 2.6 Comparative Example 33 85.0 0.0 0.0 15.0 0.0 1.4 4.9 8.0 1.6 Comparative Example 34 100.0 0.0 0.0 0.0 0.0 — 7.3 6.0 2.9 Comparative Example 35 88.0 0.0 0.0 12.0 0.0 1.2 4.8 0.5 1.3 Comparative Example 36 80.0 4.0 0.0 16.0 0.0 1.2 5.0 7.0 2.9 Comparative Example 37 83.0 6.0 0.0 11.0 0.0 1.3 5.4 9.3 2.0 Comparative Example

TABLE-US-00008 TABLE 8 Structure change in sheet width direction Manufacturing γ γ.sub.D1 γ.sub.D2 γ.sub.W1 γ.sub.W2 γ/γ.sub.D1 γ/γ.sub.D2 γ/γ.sub.W1 γ/γ.sub.W2 No. (area %) (area %) (area %) (area %) (area %) (—) (—) (—) (—) Remarks 1 6.0 5.5 6.4 6.1 6.2 1.1 0.9 1.0 1.0 Invention Example 2 14.0 13.4 12.7 13.7 13.7 1.0 1.1 1.0 1.0 Invention Example 3 9.0 8.3 8.3 9.7 9.5 1.1 1.1 0.9 0.9 Comparative Example 4 13.0 13.0 13.9 12.5 12.5 1.0 0.9 1.0 1.0 Comparative Example 5 15.0 14.9 16.4 15.9 14.3 1.0 0.9 0.9 1.0 Comparative Example 6 15.0 16.4 13.7 14.9 16.2 0.9 1.1 1.0 0.9 Comparative Example 7 6.0 5.8 6.0 5.6 6.0 1.0 1.0 1.1 1.0 Comparative Example 8 7.0 7.4 6.4 7.6 6.9 0.9 1.1 0.9 1.0 Comparative Example 9 9.0 9.6 9.2 8.8 8.4 0.9 1-0 1.0 1.1 Comparative Example 10 2.0 2.1 2.0 1.8 1.9 1.0 1.0 1.1 1.1 Comparative Example 11 1.0 5.3 10.3 6.1 9.3 0.2 0.1 0.2 0.1 Comparative Example 12 2.0 7.8 10.0 8.2 10.4 0.3 0.2 0.2 0.2 Comparative Example 13 5.0 6.5 9.7 6.4 10.0 0.8 0.5 0.8 0.5 Comparative Example 14 11.0 11.8 11.2 9.9 10.1 0.9 1.0 1.1 1.1 Invention Example 15 4.0 3.8 4.3 3.6 3.6 1.1 0.9 1.4 1.1 Invention Example 16 12.0 11.8 12.0 11.2 11.9 1.0 1.0 1.1 1.0 Invention Example 17 8.0 8.2 7.4 8.3 7.3 1.0 1.1 1.0 1.1 Invention Example 18 6.0 6.2 5.9 6.5 6.0 1.0 1.0 0.9 1.0 Invention Example 19 9.0 9.5 8.6 8.7 9.4 0.9 1.0 1.0 1.0 Invention Example 20 17.0 15.3 18.2 18.2 17.3 1.1 0.9 0.9 1.0 Invention Example 21 11.0 10.2 10.8 11.2 9.9 1.1 1.0 1.0 1.1 Invention Example 22 9.0 8.4 9.7 8.5 9.6 1.1 0.9 1.1 0.9 Invention Example 23 9.0 8.3 8.1 8.4 8.6 1.1 1.1 1.1 1.0 Invention Example 24 14.0 14.6 15.4 13.0 13.7 1.0 0.9 1.1 1.0 Invention Example 25 9.0 9.2 9.5 8.6 9.4 1.0 0.9 1.0 1.0 Invention Example 26 15.0 14.0 15.0 15.2 14.9 1.1 1.0 1.0 1.0 Invention Example 27 2.0 2.2 1.9 2.1 2.2 0.9 1.1 1.0 0.9 Comparative Example 28 1.0 1.1 0.9 1.1 1.0 0.9 1.1 0.9 1.0 Comparative Example 29 0.0 0.0 0.0 0.0 0.0 — — — — Comparative Example 30 11.0 10.2 10.0 11.0 10.1 1.1 1.1 1.0 1.1 Comparative Example 31 2.0 2.0 1.8 2.0 2.0 1.0 1.1 1.0 1.0 Comparative Example 32 9.0 7.0 5.6 6.8 6.2 1.3 1.6 1.3 1.5 Comparative Example 33 15.0 15.2 15.2 14.6 15.2 1.0 1.0 1.0 1.0 Comparative Example 34 0.0 0.0 0.0 0.0 0.0 — — — — Comparative Example 35 12.0 12.8 11.0 10.8 12.5 0.9 1.1 1.1 1.0 Comparative Example 36 14.0 10.0 10.8 9.2 10.0 1.4 1.3 1.5 1.4 Comparative Example 37 11.0 6.8 1.5 7.0 3.4 1.6 7.3 1.6 3.2 Comparative Example

TABLE-US-00009 TABLE 9 Structure change in sheet width direction Manufacturing C.sub.γC C.sub.γD1 C.sub.γD2 C.sub.γW1 C.sub.γW2 C.sub.γC/C.sub.γD1 C.sub.γC/C.sub.γD2 C.sub.γC/C.sub.γW1 C.sub.γC/C.sub.γW2 No. (mass %) (mass %) (mass %) (mass %) (mass %) (—) (—) (—) (—) Remarks 1 1.5 1.6 1.5 1.5 1.5 0.9 1.0 1.0 1.0 Invention Example 2 1.5 1.5 1.4 1.5 1.6 1.0 1.1 1.0 0.9 Invention Example 3 1.1 1.1 1.2 1.0 1.1 1.0 0.9 1.1 1.0 Comparative Example 4 1.1 1.2 1.1 1.2 1.0 0.9 1.0 0.9 1.1 Comparative Example 5 1.2 1.2 1.2 1.1 1.1 1.0 1.0 1.1 1.1 Comparative Example 6 1.1 1.2 1.2 1.2 1.1 0.9 0.9 0.9 1.0 Comparative Example 7 1.2 1.1 1.2 1.3 1.2 1.1 1.0 0.9 1.0 Comparative Example 8 1.1 1.2 1.1 1.0 1.1 0.9 1.0 1.1 1.0 Comparative Example 9 0.3 0.3 0.3 0.3 0.3 1.0 1.0 1.0 1.0 Comparative Example 10 1.2 1.2 1.2 1.3 1.2 1.0 1.0 0.9 1.0 Comparative Example 11 0.4 1.0 1.0 1.0 1.0 0.4 0.4 0.4 0.4 Comparative Example 12 0.4 0.7 0.9 0.8 1.0 0.6 0.4 0.5 0.4 Comparative Example 13 0.2 1.0 1.0 1.1 0.8 0.2 0.2 0.2 0.3 Comparative Example 14 1.4 1.3 1.3 1.4 1.5 1.1 1.1 1.0 0.9 Invention Example 15 1.4 1.5 1.3 1.3 1.5 0.9 1.1 1.1 0.9 Invention Example 16 1.5 1.4 1.7 1.5 1.4 1.1 0.9 1.0 1.1 Invention Example 17 1.3 1.3 1.4 1.4 1.2 1.0 0.9 0.9 1.1 Invention Example 18 1.1 1.0 1.1 1.0 1.1 1.1 1.0 1.1 1.0 Invention Example 19 1.1 1.2 1.0 1.1 1.2 0.9 1.1 1.0 0.9 Invention Example 20 1.5 1.6 1.4 1.6 1.6 0.9 1.1 0.9 0.9 Invention Example 21 1.5 1.5 1.4 1.6 1.5 1.0 1.1 0.9 1.0 Invention Example 22 1.1 1.0 1.0 1.0 1.1 1.1 1.1 1.1 1.0 Invention Example 23 1.4 1.5 1.5 1.4 1.3 0.9 0.9 1.0 1.1 Invention Example 24 1.1 1.0 1.2 1.2 1.0 1.1 0.9 0.9 1.1 Invention Example 25 1.2 1.1 1.1 1.3 1.2 1.1 1.1 0.9 1.0 Invention Example 26 1.3 1.2 1.3 1.3 1.4 1.1 1.0 1.0 0.9 Invention Example 27 1.4 1.3 1.4 1.4 1.5 1.1 1.0 1.0 0.9 Comparative Example 28 1.4 1.4 1.5 1.5 1.4 1.0 0.9 0.9 1.0 Comparative Example 29 0.0 0.0 0.0 0.0 0.0 — — — — Comparative Example 30 1.4 1.3 1.3 1.5 1.4 1.1 1.1 0.9 1.0 Comparative Example 31 1.2 1.2 1.3 1.2 1.2 1.0 0.9 1.0 1.0 Comparative Example 32 1.1 1.1 1.2 1.2 1.1 1.0 0.9 0.9 1.0 Comparative Example 33 1.4 1.5 1.4 1.4 1.5 0.9 1.0 1.0 0.9 Comparative Example 34 0.0 0.0 0.0 0.0 0.0 — — — — Comparative Example 35 1.2 1.3 1.1 1.2 1.3 0.9 1.1 1.0 0.9 Comparative Example 36 1.2 1.4 1.2 1.2 1.2 0.9 1.0 1.0 1.0 Comparative Example 37 1.2 1.3 1.1 1.3 1.2 0.9 1.1 0.9 1.0 Comparative Example

[0193] Evaluation Method of Properties of Hot-Rolled Steel Sheet

[0194] (1) Tensile Strength Properties

[0195] Among the mechanical properties of the hot-rolled steel sheet, the tensile strength properties (tensile strength, total elongation) were evaluated according to JIS Z 2241: 2011. A test piece was a No. 5 test piece of JIS Z 2241: 2011. As the tensile test piece collection position, the center position in the sheet width direction, a position 300 mm from the center position in the sheet width direction to one end side in the sheet width direction (position A in Table 10), a position 600 mm from the center position in the sheet width direction to the one end side in the sheet width direction (position B in Table 10), a position 300 mm from the center position in the sheet width direction to the other end side in the sheet width direction (position C in Table 10), and a position 600 mm from the center position in the sheet width direction to the other end side in the sheet width direction (position D in Table 10) were set, and the direction vertical to the rolling direction was defined as the longitudinal direction.

[0196] In a case where (tensile strength TS (MPa))×(total elongation EL (%))≥19000 was satisfied at the center position in the sheet width direction, the steel sheet was determined to be as acceptable as a hot-rolled steel sheet having excellent strength and ductility. In addition, in a case where, when (tensile strength TS (MPa))×(total elongation EL (%)) at each of the center position in the sheet width direction, the position 300 mm from the center position in the sheet width direction to one end side in the sheet width direction (position A in Table 10), the position 600 mm from the center position in the sheet width direction to the one end side in the sheet width direction (position B in Table 10), the position 300 mm from the center position in the sheet width direction to the other end side in the sheet width direction (position C in Table 10), and the position 600 mm from the center position in the sheet width direction to the other end side in the sheet width direction (position D in Table 10) was (TS×EL).sub.C, (IS×EL).sub.D1, (TS×EL).sub.D2, (TS×EL).sub.W1, and (TS×EL).sub.W2, 0.80≤{(TS×EL).sub.C/(TS×EL).sub.D1}≤1.20, 0.80≤{(TS×EL).sub.C/(TS×EL).sub.D2}≤1.20, 0.80≤{(TS×EL).sub.C/(TS×EL).sub.W1}≤1.20, and 0.80≤{(TS×EL).sub.C/(TS×EL).sub.W2}≤1.20 were satisfied, the steel sheet was determined to be acceptable as a hot-rolled steel sheet having a small variation in the sheet width direction.

[0197] (2) Hole Expansion Rate

[0198] The hole expansion rate of the hot-rolled steel sheet was evaluated by the hole expanding test method according to the Japan Iron and Steel Federation standard JFS T 1001-1996. The test piece was collected from the same position as the tensile test piece collection position, and a punched hole was provided with a cylindrical punch. In a case where (tensile strength TS (MPa))×(hole expansion rate λ (%))≥50000 was satisfied, the steel sheet was determined to be acceptable as a hot-rolled steel sheet having excellent strength and stretch flangeability.

[0199] (3) Low Temperature Toughness

[0200] The low temperature toughness of the hot-rolled steel sheet was measured by the Charpy test. The Charpy test was carried out according to JIS Z 2242: 2005, and the fracture appearance transition temperature was measured. Since the hot-rolled steel sheets manufactured in the examples had a sheet thickness of less than 10.0 mm, the front and back sides of a hot-rolled steel sheet having a sheet thickness of 2.5 mm or more were ground to 2.5 mm, and the front and back sides of a hot-rolled steel sheet having a sheet thickness of less than 2.5 mm were ground to 1.25 mm. Then, the Charpy test was performed. In a case where ductile-brittle transition temperature (vTrs) was −50° C. or lower, the steel sheet was determined to be acceptable as a hot-rolled steel sheet having excellent low temperature toughness.

[0201] The obtained measurement results are shown in Tables 10 and 11.

TABLE-US-00010 TABLE 10 Properties (TS × Total EL).sub.C at Tensile elon- Hole sheet width (TS × (TS × (TS × (TS × strength gation expansion TS × center EL).sub.D1 at EL).sub.D2 at EL).sub.W1 at EL).sub.W2 at Manufacturing TS EL rate λ λ vTrs position position A position B position C position D No. (MPa) (%) (%) (%) (° C.) (MPa .Math. %) (MPa .Math. %) (MPa .Math. %) (MPa .Math. %) (MPa .Math. %) Remarks 1 1077 18.3 59 63543 −60 19709 19512 21483 20300 19906 Invention Example 2 1299 16.2 60 77940 −80 21044 20623 22517 21044 21044 Invention Example 3 1350 16.3 52 70200 −30 22005 20685 21565 20465 22665 Comparative Example 4 1130 18.4 41 46330 −90 20792 23911 22871 21000 21208 Comparative Example 5 1296 16.8 57 73872 −40 21773 22426 22861 21773 20466 Comparative Example 6 1125 18.6 38 42750 −90 20925 23436 20925 23227 22181 Comparative Example 7 1288 14.8 35 45080 −70 19062 16966 18872 18681 18681 Comparative Example 8 1314 15.0 36 47304 −60 19710 21878 19513 20301 18922 Comparative Example 9 1280 12.5 63 80640 −60 16000 18400 17440 15840 13600 Comparative Example 10 1285 12.7 55 70675 −80 16320 17462 15340 15667 14851 Comparative Example 11 1326 12.1 53 70278 −60 16045 21660 22462 22462 21821 Comparative Example 12 1285 12.7 59 75815 −70 16320 21052 22521 22358 24642 Comparative Example 13 1281 12.0 55 70455 −70 15372 21060 21828 20752 22289 Comparative Example 14 1170 16.7 50 58500 −90 19539 19930 19148 20125 20125 Invention Example 15 1317 15.0 60 79020 −60 19755 20545 20940 19360 19755 Invention Example 16 1288 15.2 55 70840 −80 19578 19186 22123 19969 20361 Invention Example 17 992 20.1 69 68448 −60 19939 19341 19540 22133 19142 Invention Example 18 1346 14.5 49 65954 −90 19517 19127 19517 20298 19712 Invention Example 19 1324 16.3 46 60904 −80 21581 24818 20286 21581 23524 Invention Example 20 1173 18.4 62 72726 −90 21583 20936 23094 24821 22447 Invention Example 21 1283 16.6 59 75697 −90 21298 20446 20233 21937 20446 Invention Example 22 1345 15.3 60 80700 −60 20579 22636 20990 21607 21813 Invention Example 23 1301 16.8 58 75458 −90 21857 20983 20108 21857 21201 Invention Example 24 1317 15.9 62 81654 −60 20940 21359 24710 21359 20312 Invention Example 25 1361 15.7 56 76216 −80 21368 20513 20513 21368 22009 Invention Example 26 1335 15.5 53 70755 −80 20693 20072 20279 21520 21106 Invention Example 27 890 20.0 17 15130 −70 17800 18868 16554 18512 18690 Comparative Example 28 1303 12.5 36 46908 −70 16288 17265 15799 18731 15473 Comparative Example 29 1291 9.3 73 94243 −70 12006 12847 11886 13087 11886 Comparative Example 30 1328 15.9 34 45152 −90 21115 21749 20271 21749 20059 Comparative Example 31 1324 12.4 32 42368 −80 16418 16253 15925 16746 16582 Comparative Example 32 1370 15.8 56 76720 −60 21646 16018 14286 17100 14286 Comparative Example 33 1338 16.3 36 48168 −70 21809 24208 23118 20065 22027 Comparative Example 34 1165 11.8 68 79220 −80 13747 14086 13669 13860 14681 Comparative Example 35 1328 17.0 57 75696 −30 22576 23642 22493 22648 21008 Comparative Example 36 1026 21.5 45 46170 −60 22059 16879 17058 16442 14400 Comparative Example 37 1063 20.8 47 49961 −60 22110 19836 14055 19066 14447 Comparative Example

TABLE-US-00011 TABLE 11 Properties (TS × EL).sub.C/ (TS × EL).sub.C/ (TS × EL).sub.C/ (TS × EL).sub.C/ Manufacturing (TS × EL).sub.D1 (TS × EL).sub.D2 (TS × EL).sub.W1 (TS × EL).sub.W2 No. (—) (—) (—) (—) Remarks 1 1.01 0.92 0.97 0.99 Invention Example 2 1.02 0.93 1.00 1.00 Invention Example 3 1.06 1.02 1.08 0.97 Comparative Example 4 0.87 0.91 0.99 0.98 Comparative Example 5 0.97 0.95 1.00 1.06 Comparative Example 6 0.89 1.00 0.90 0.94 Comparative Example 7 1.12 1.01 1.02 1.02 Comparative Example 8 0.90 1.01 0.97 1.04 Comparative Example 9 0.87 0.92 1.01 1.18 Comparative Example 10 0.93 1.06 1.04 1.10 Comparative Example 11 0.74 0.71 0.71 0.74 Comparative Example 12 0.78 0.72 0.73 0.66 Comparative Example 13 0.73 0.70 0.74 0.69 Comparative Example 14 0.98 1.02 0.97 0.97 Invention Example 15 0.96 0.94 1.02 1.00 Invention Example 16 1.02 0.88 0.98 0.96 Invention Example 17 1.03 1.02 0.90 1.04 Invention Example 18 1.02 1.00 0.96 0.99 Invention Example 19 0.87 1.06 1.00 0.92 Invention Example 20 1.03 0.93 0.87 0.96 Invention Example 21 1.04 1.05 0.97 1.04 Invention Example 22 0.91 0.98 0.95 0.94 Invention Example 23 1.04 1.09 1.00 1.03 Invention Example 24 0.98 0.85 0.98 1.03 Invention Example 25 1.04 1.04 1.00 0.97 Invention Example 26 1.03 1.02 0.96 0.98 Invention Example 27 0.94 1.08 0.96 0.95 Comparative Example 28 0.94 1.03 0.87 1.05 Comparative Example 29 0.93 1.01 0.92 1.01 Comparative Example 30 0.97 1.04 0.97 1.05 Comparative Example 31 1.01 1.03 0.98 0.99 Comparative Example 32 1.35 1.52 1.27 1.52 Comparative Example 33 0.90 0.94 1.09 0.99 Comparative Example 34 0.98 1.01 0.99 0.94 Comparative Example 35 0.95 1.00 1.00 1.07 Comparative Example 36 1.31 1.29 1.34 1.53 Comparative Example 37 1.11 1.57 1.16 1.53 Comparative Example

[0202] As can be seen from Tables 10 and 11, in Manufacturing Nos. 1, 2, and 14 to 26 as Invention Examples, a hot-rolled steel sheet having excellent strength, ductility, stretch flangeability, and low temperature toughness and having a material property variation in the sheet width direction could be obtained.

[0203] On the other hand, in Manufacturing Nos. 3 to 13 and 27 to 37 in which the chemical composition, the metallographic structure, the C concentration in the residual austenite, the average grain size of the metallographic structure excluding the residual austenite, and/or the number density of iron-based carbides having a diameter of 20 nm or more were not within the ranges defined in the present invention, one or more of the properties (tensile strength TS, total elongation EL, hole expansion rate γ, low temperature toughness (vTrs), and material property variation in the sheet width direction) were deteriorated.

INDUSTRIAL APPLICABILITY

[0204] According to the present invention, it is possible to provide a hot-rolled steel sheet having excellent strength, ductility, stretch flangeability, and low temperature toughness. In addition, according to a preferable aspect of the present invention, it is possible to provide a hot-rolled steel sheet having the above-mentioned various properties and having a material property variation in the sheet width direction.

[0205] The hot-rolled steel sheet according to the present invention is suitable as an industrial material used for vehicle members, mechanical structural members, and building members.