STEEL SHEET AND METHOD FOR PRODUCING SAME

Abstract

A steel sheet of the present invention is a steel sheet having a predetermined chemical composition and containing at least ferrite, residual austenite, and/or martensite in a microstructure, and furthermore, is a steel sheet in which, in a plane parallel to a rolled surface, an average distance between centers of high Mn regions adjacent to each other is 1.00 mm or less, a density D.sub.A of the high Mn regions at a sheet width center portion and a density D.sub.B of the high Mn regions at a ¼ position from a sheet width end portion satisfy a relationship of 0.77≤D.sub.A/D.sub.B≤1.30, a ratio of an average hardness of the high Mn regions to an average hardness of the low Mn regions is 1.1 to 2.0, and a difference between an average of a top 5% and an average of a bottom 5% of Mn contents in the low Mn regions is 0.3% or more.

Claims

1. A steel sheet comprising, as a chemical composition, by mass %: C: 0.040% to 0.400%; Si: 0.01% to 2.50%; Mn: 0.10% to 4.00%; Al: 0.010% to 1.500%; P: 0.001% to 0.100%; S: 0.0005% to 0.0100%; N: 0.0005% to 0.0100%; Ti: 0% to 0.200%; Mo: 0% to 0.300%; Nb: 0% to 0.200%; Cr: 0% to 4.00%; B: 0% to 0.0050%; V: 0% to 0.300%; Ni: 0% to 4.00%; Cu: 0% to 4.00%; W: 0% to 2.00%; Ca: 0% to 0.0100%; Ce: 0% to 0.0100%; Mg: 0% to 0.0100%; Zr: 0% to 0.0100%; La: 0% to 0.0100%; REM other than Ce and La: 0% to 0.0100%; Sn: 0% to 1.000%; Sb: 0% to 0.200%; and a remainder: Fe and impurities, wherein a microstructure in a range from a ⅛ thickness position in a sheet thickness direction from a surface of the steel sheet to a ⅜ thickness position in the sheet thickness direction from the surface includes, by area fraction, ferrite: 10% to 97%, residual austenite and martensite: 3% to 90%, bainite: 0% to 87%, and pearlite: 0% to 10%, in a plane parallel to a rolling direction at a ¼ thickness position in the sheet thickness direction from the surface, when a maximum value of Mn contents in a measurement range is indicated as Mnmax, an average value of the Mn contents is indicated as Mnave, regions where the Mn content is (Mnave+Mnmax)/2 or more are indicated as high Mn regions, and the other regions are indicated as low Mn regions, an average distance between centers of the high Mn regions adjacent to each other is 1.00 mm or less, a density D.sub.A of the high Mn regions at a sheet width center portion and a density D.sub.B of the high Mn regions at a ¼ width position from a sheet width end portion satisfy Expression (1), a ratio of an average hardness of the high Mn regions to an average hardness of the low Mn regions is 1.1 to 2.0, and a difference between an average of a top 5% and an average of a bottom 5% of the Mn contents at measurement points in the low Mn regions is 0.3 mass % or more,
0.77≤D.sub.A/D.sub.B≤1.30  Expression (1).

2. The steel sheet according to claim 1, wherein a hot-dip galvanized layer is formed on the surface.

3. The steel sheet according to claim 2, wherein the hot-dip galvanized layer is a hot-dip galvannealed layer.

4. A method for producing the steel sheet according to claim 1, comprising: a casting step of producing a slab by melting a steel having the chemical composition according to claim 1, casting the melted steel to produce a slab, and cooling the slab at a temperature of 950° C. to 550° C. while applying a pressure of 10 N/cm.sup.2 or more to the slab in a thickness direction so that an average cooling rate is 100° C./h or faster; a heating step of heating the slab to a temperature range of 1100° C. to 1280° C. after cooling the slab to room temperature or before cooling the slab to room temperature so that an average heating rate in a temperature range of 650° C. to 850° C. is 50° C./min or slower; a hot rolling step of hot-rolling the slab after the heating step in a temperature range of 1050° C. or higher at a cumulative rolling reduction of 35% or more to obtain a hot-rolled steel sheet; a cooling step of cooling the hot-rolled steel sheet to 650° C. or lower, the cooling being started within three seconds after completion of the hot rolling step, so that an average cooling rate to 700° C. is 20° C./s or faster; a coiling step of coiling the hot-rolled steel sheet after the cooling step in a temperature range of 300° C. to 650° C.; a pickling step of performing pickling on the hot-rolled steel sheet after the coiling step to obtain a pickled steel sheet; a cold rolling step of performing cold rolling on the pickled steel sheet to obtain a cold-rolled steel sheet; an annealing step of heating the cold-rolled steel sheet to an annealing temperature of Ac1° C. to 1000° C. at an average heating rate of 10.0° C./s or slower and performing holding at the annealing temperature for five seconds to 600 seconds; a post-annealing cooling step of cooling the cold-rolled steel sheet after the annealing step to a retention temperature of 150° C. to 550° C. at an average cooling rate of 1° C./s to 200° C./s; a retaining step of performing retention at the retention temperature for 15 seconds to 1000 seconds; and a final cooling step of cooling the cold-rolled steel sheet after the retaining step to room temperature.

5. The method for producing the steel sheet according to claim 4, further comprising: a hot-dip galvanizing step of immersing the cold-rolled steel sheet in a molten zinc bath, between the retaining step and the final cooling step.

6. The method for producing the steel sheet according to claim 5, further comprising: an alloying step of reheating the cold-rolled steel sheet to 470° C. to 550° C. and performing holding for 60 seconds or shorter, between the hot-dip galvanizing step and the final cooling step.

7. The method for producing the steel sheet according to claim 4, further comprising: a leveling step of working the cold-rolled steel sheet using a leveler, between the cold rolling step and the annealing step.

8. The method for producing the steel sheet according to claim 5, further comprising: a leveling step of working the cold-rolled steel sheet using a leveler, between the cold rolling step and the annealing step.

9. The method for producing the steel sheet according to claim 6, further comprising: a leveling step of working the cold-rolled steel sheet using a leveler, between the cold rolling step and the annealing step.

Description

EXAMPLES

[0232] Next, examples of the present invention will be described. However, the conditions in the examples are one example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one example of conditions. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

[0233] Molten steels having the compositions shown in Table 1 were continuously cast according to a normal method to obtain cast slabs. In Table 1, the chemical compositions of Kind of steel A to T satisfy the chemical composition of the present invention.

[0234] On the other hand, the Nb content in Kind of steel aa, the C content in Kind of steel bb, the Si content in Kind of steel cc, the Mn content in Kinds of steel dd and ee, the P content in Kind of steel ff, the S content in Kind of steel gg, the Al content in Kind of steel hh, and the Ti content in Kind of steel ii did not satisfy the ranges of the chemical composition of the present invention.

TABLE-US-00001 TABLE 1 Kind of Chemical composition (mass %) (remainder consists of Fe and impurities) steel C Si Mn P S N Al Nb Ti Mo Cr B Ni V A 0.085 0.60 2.08 0.008 0.0027 0.0028 0.027 — — — — — — — B 0.191 1.19 1.82 0.009 0.0021 0.0028 0.025 — — — — — — — C 0.071 0.40 1.80 0.008 0.0010 0.0013 0.010 — — — — — — — D 0.150 1.60 1.60 0.001 0.0006 0.0019 0.010 0.050 — — — — — — E 0.084 0.60 2.06 0.008 0.0027 0.0028 0.027 0.050 0.040 — — — — — F 0.180 1.45 1.80 0.018 0.0005 0.0018 0.035 — — 0.210 — — — — G 0.220 0.47 1.80 0.005 0.0010 0.0010 0.800 — — — 1.10 — — — H 0.300 1.30 2.00 0.040 0.0008 0.0016 0.010 — — — — 0.0015 — — I 0.069 0.40 1.75 0.008 0.0010 0.0013 0.010 — — — — — — — J 0.081 0.60 2.06 0.008 0.0027 0.0028 0.027 — — — — — — — K 0.195 1.19 1.82 0.009 0.0021 0.0028 0.025 — — — — — 0.30 — L 0.074 0.40 1.80 0.008 0.0010 0.0013 0.010 — — — — — — — M 0.150 1.60 1.20 0.001 0.0006 0.0019 0.010 — — — — — 0.100 N 0.080 0.60 2.06 0.008 0.0027 0.0028 0.027 — — — — — — — O 0.175 1.46 1.90 0.018 0.0005 0.0018 0.035 — — — — — — — P 0.220 0.47 1.80 0.005 0.0010 0.0010 0.800 — — — — — — — Q 0.080 0.60 2.06 0.008 0.0027 0.0028 0.027 — — — — — — — R 0.192 1.25 1.91 0.009 0.0021 0.0028 0.025 — — — — — — — S 0.071 0.40 1.80 0.008 0.0010 0.0013 0.010 — — — — — — — T 0.151 1.60 1.60 0.001 0.0006 0.0019 0.010 — — — — — — — aa 0.071 0.40 1.80 0.008 0.0010 0.0013 0.010 0.300 — — — — — — bb 0.030 0.70 2.80 0.040 0.0005 0.0018 0.010 — — — — — — — cc 0.240 3.00 3.30 0.040 0.0009 0.0010 0.010 — — — — — — — dd 0.080 0.06 4.50 0.020 0.0008 0.0011 0.010 — — — — — — — ee 0.078 0.06 0.05 0.020 0.0009 0.0016 0.010 — — — — — — — ff 0.085 0.06 2.90 0.120 0.0009 0.0016 0.010 — — — — — — — gg 0.084 0.06 1.80 0.020 0.0120 0.0010 0.010 — — — — — — — hh 0.086 0.06 2.00 0.010 0.0005 0.0011 1.800 — — — — — — — ii 0.083 0.06 1.60 0.020 0.0009 0.0016 0.010 — 0.400 — — — — — U 0.081 0.03 1.35 0.013 0.0041 0.0045 0.452 — — 0.093 0.62 — — — V 0.123 0.09 2.42 0.009 0.0034 0.0066 1.184 — 0.019 — — 0.0032 — — W 0.057 0.13 1.64 0.012 0.0013 0.0053 0.215 0.013 — 0.251 — — — — X 0.134 0.32 0.93 0.008 0.0042 0.0041 0.097 — 0.028 0.056 0.41 0.0012 0.35 — Y 0.102 0.73 3.17 0.016 0.0028 0.0047 0.028 0.023 0.013 — — — — — Z 0.077 0.35 2.82 0.012 0.0015 0.0027 0.073 — 0.032 — 0.16 — — — Kind of Chemical composition (mass %) (remainder consists of Fe and impurities) steel W Cu Others Ac1 Note A — — — 718 Invention Steel B — — — 738 Invention Steel C — — — 715 Invention Steel D — — — 752 Invention Steel E — — — 718 Invention Steel F — — — 746 Invention Steel G — — — 736 Invention Steel H — — — 739 Invention Steel I — — — 716 Invention Steel J — — Mg: 0.002 718 Invention Steel K — 0.50 — 733 Invention Steel L — — Ca: 0.0020 715 Invention Steel M — — — 757 Invention Steel N 0.11 — — 719 Invention Steel O — — Ce: 0.0025 745 Invention Steel P — — Zr: 0.0040 717 Invention Steel Q — — La: 0.0025 718 Invention Steel R — — REM: 0.0027 739 Invention Steel S — — Sn: 0.100 715 Invention Steel T — — Sb: 0.200 752 Invention Steel aa — — — 715 Comparative Steel bb — — — 713 Comparative Steel cc — — — 775 Comparative Steel dd — — — 677 Comparative Steel ee — — — 724 Comparative Steel ff — — — 694 Comparative Steel gg — — — 705 Comparative Steel hh — — — 703 Comparative Steel ii — — — 708 Comparative Steel U — — — 720 Invention Steel V — — Y: 0.0017 700 Invention Steel W — — Sc: 0.0015 709 Invention Steel X — 0.07 — 723 Invention Steel Y — — — 710 Invention Steel Z — — — 706 Invention Steel

[0235] The cast slabs having the chemical compositions shown in Table 1 were heated, hot-rolled, cooled, coiled, pickled, subjected to cold rolling, and then subjected to a leveling treatment as necessary under the conditions shown in Tables 2-1 to 2-4. The sheet thickness after cold rolling was set to 0.35 to 1.2 mm. The steel sheets were annealed and cooled under the conditions shown in Tables 2-1 to 2-4. Depending on the conditions, hot dip galvanizing was further performed. In addition, some of the steel sheets were subjected to an alloying treatment. In the pickling, the steel sheet cooled to room temperature was immersed in 5 to 10 mass % hydrochloric acid as hydrogen chloride whose temperature was controlled to 80° C. to 90° C. for a total of 30 seconds to 100 seconds, whereby scale on the surface was removed.

[0236] In addition, the cooling temperature in the cooling step in the present example is equal to the coiling temperature in the coiling step. That is, in the cooling step, the hot-rolled steel sheet was cooled to the coiling temperature shown in Tables 2-1 to 2-4.

[0237] In Tables 2-1 to 2-4, in Kind of plating of Hot-dip galvanizing step, “GI” indicates hot-dip galvanizing, and “GA” indicates hot-dip galvannealing.

TABLE-US-00002 TABLE 2-1 Hot rolling Cooling step Heating step step Time Average Cumulative until Casting step heating rolling start of Average Average rate reduction rapid cooling cooling in in cooling rate Cold rolling step Leveling rate at Pressure temper- temper- after from Coiling Sheet step 950° C. to applied ature ature completion start of step thickness Presence 550° C. in at range of Heating range of of hot cooling Coiling Cold after or Treat- Kind cooling 950° C. to 650° C. to temper- 1050° C. or railing to temper- rolling cold absence ment of of slab 550° C. 850° C. ature higher step 700° C. ature reduction rolling of No. steel ° C./h N/cm.sup.2 ° C./min ° C. % sec ° C./s ° C. % mm leveling 1 A 158 35 35 1240 41 2 96 490 52 1.20 Present 18 C  50 35 35 1240 40 1 92 550 52 1.20 Present 37 D  60 35 35 1240 43 1 94 530 52 1.20 Present 2 A  50 35 35 1240 40 1 90 530 52 1.20 Present 3 A 120 35 35 1240 41 2 95 560 52 1.20 Present 86 E 164  0 35 1240 43 2 98 540 52 1.20 Present 87 F 157  5 35 1240 40 2 98 560 52 1.20 Present 4 A 161  0 35 1240 43 2 95 570 52 1.20 Present 5 A 158 15 35 1240 40 1 95 520 52 1.20 Present 90 G 161 35 70 1240 42 1 99 470 52 1.20 Present 91 H 161 35 60 1240 40 2 99 530 52 1.20 Present 6 A 163 35 60 1240 45 1 90 540 52 1.20 Present 7 A 157 35 45 1240 45 2 100  520 80 0.35 Present 8 A 165 35 35 1050 45 2 90 530 52 1.20 Present 10 A 159 35 35 1240 25 1 100  490 52 1.20 Present 11 A 159 35 35 1240 43 5 99 490 52 1.20 Present 12 A 155 35 35 1240 40 10  98 480 52 1.20 Present 13 A 155 35 35 1240 43 1 10 470 52 1.20 Present 14 A 164 35 35 1240 45 2 15 550 52 1.20 Present 15 A 162 35 35 1240 44 1 30 450 52 1.20 Present 16 A 157 35 35 1240 44 2 95 670 52 1.20 Present 119 A 157 35 35 1240 44 2 95 280 52 1.20 Present 17 A 156 35 35 1240 40 2 100  520 52 1.20 Present 122 C 159 35 35 1240 40 1 92 550 78 0.40 Present 19 A 165 35 35 1240 42 1 95 540 52 1.20 Present 20 A 156 35 35 1240 40 2 95 520 52 1.20 Present Post- annealing Annealing step cooling Average step Retaining heating Average step Hot-dip rate cooling rate Retention galvanizing step during from time in Presence heating annealing temperature or Alloying step to Ac1 temperature range of absence Reheating Treat- or Annealing Retention to 550° C. or 150° C. to of Kind (alloying) Retention ment higher temperature AC1 time lower 550° C. plating of temperature time No. ° C./s ° C. ° C. sec ° C./s sec treatment plating ° C. sec Note 1 4.4 740 718 110 29 394 Absent — — — Invention Steel 18 3.0 810 715 110 26 170 Absent — — — Comparative Steel 37 4.4 820 752 220 55 290 Absent — — — Comparative Steel 2 3.3 760 718 220 32 229 Absent — — — Comparative Steel 3 4.4 770 718 110 39 476 Absent — — — Invention Steel 86 4.2 820 718 160 54 276 Absent — — — Comparative Steel 87 4.1 780 746 230 53 321 Absent — — — Comparative Steel 4 3.2 740 718 190 53 378 Absent — — — Comparative Steel 5 4.7 750 718 100 24 402 Absent — — — Invention Steel 90 4.9 770 736 170 30 308 Absent — — — Comparative Steel 91 2.9 810 739 80 34 232 Absent — — — Comparative Steel 6 3.5 800 718 210 48 186 Absent — — — Comparative Steel 7 2.9 750 718 250 41 385 Absent — — — Invention Steel 8 4.3 820 718 200 41 443 Absent — — — Comparative Steel 10 4.2 760 718 160 50 411 Absent — — — Comparative Steel 11 2.9 800 718 110 26 280 Absent — — — Comparative Steel 12 2.9 750 718 240 36 434 Absent — — — Comparative Steel 13 3.3 770 718 200 37 484 Absent — — — Comparative Steel 14 3.4 820 718 190 33 450 Absent — — — Comparative Steel 15 4.4 780 718 90 33 340 Absent — — — Invention Steel 16 4.5 810 718 140 49 288 Absent — — — Comparative Steel 119 4.5 810 718 140 49 288 Absent — — — Comparative Steel 17 20.0  790 718 220 34 475 Absent — — — Comparative Steel 122 30.0  870 715 110 26 170 Absent — — — Comparative Steel 19 4.1 1100  718 250 55 223 Absent — — — Comparative Steel 20 4.5 860 718 230 31 154 Absent — — — Invention Steel

TABLE-US-00003 TABLE 2-2 Hot rolling Cooling step Heating step step Time Average Cumulative until Casting step heating rolling start of Average Average rate reduction rapid cooling cooling in in cooling rate Cold rolling step Leveling rate at Pressure temper- temper- after from Coiling Sheet step 950° C. to applied ature ature completion start of step thickness Presence 550° C. in at range of Heating range of of hot cooling Coiling Cold after or Treat- Kind cooling 950° C. to 650° C. to temper- 1050° C. or railing to temper- rolling cold absence ment of of slab 550° C. 850° C. ature higher step 700° C. ature reduction rolling of No. steel ° C./h N/cm.sup.2 ° C./min ° C. % sec ° C./s ° C. % mm leveling 21 A 155 35 35 1240 41 2 90 480 52 1.20 Present 23 A 156 35 35 1240 40 2 98 550 52 1.20 Present 24 A 159 35 35 1240 45 1 93 500 52 1.20 Present 25 A 156 35 35 1240 44 1 90 500 52 1.20 Present 94 I 160 35 35 1240 40 2 90 530 52 1.20 Present 95 J 159 35 35 1240 40 1 96 560 52 1.20 Present 26 A 155 35 35 1240 41 1 94 470 52 1.20 Present 27 B 159 35 35 1240 43 1 91 530 52 1.20 Present 28 A 157 35 35 1240 43 1 98 540 70 0.50 Present 29 A 157 35 35 1240 43 2 90 550 52 1.20 Present 30 E 156 35 35 1240 42 1 94 500 52 1.20 Present 31 A 157 35 35 1240 40 2 100 540 52 1.20 Present 32 A 162 35 35 1240 42 1 92 490 52 1.20 Present 33 A 157 35 35 1240 45 1 93 500 52 1.20 Present 123 D 164 35 35 1240 43 1 94 530 52 1.20 Present 40 F 159 35 35 1240 42 2 100 570 52 1.20 Present 47 G 160 35 35 1240 42 2 95 540 52 1.20 Present 48 H 157 35 35 1240 44 1 94 480 52 1.20 Present 62 B 161 35 35 1240 42 1 100 570 52 0.80 Present 63 C 165 35 35 1240 43 2 91 530 52 1.20 Present 64 D 157 35 35 1240 41 2 97 510 52 1.20 Present 77 I 162 35 35 1240 41 2 98 530 52 1.20 Present 79 B 160 35 35 1240 45 2 90 560 52 1.20 Present 80 B 156 35 35 1240 40 2 94 560 52 1.20 Present 81 C 163 35 35 1240 42 1 99 540 52 1.20 Present 82 C 161 35 35 1240 44 1 96 560 52 1.20 Present Post- annealing Annealing step cooling Average step Retaining heating Average step Hot-dip rate cooling rate Retention galvanizing step during from time in Presence heating annealing temperature or Alloying step to Ac1 temperature range of absence Reheating Treat- or Annealing Retention to 550° C. or 150° C. to of Kind (alloying) Retention ment higher temperature AC1 time lower 550° C. plating of temperature time No. ° C./s ° C. ° C. sec ° C./s sec treatment plating ° C. sec Note 21 5.8 820 718  1 51 289 Absent — — — Comparative Steel 23 4.0 820 718 140   0.5 236 Absent — — — Comparative Steel 24 3.7 760 718 120  3 314 Absent — — — Invention Steel 25 5.1 790 718 160 10 348 Present GI — — Invention Steel 94 3.9 760 716 130 37 359 Absent — — — Invention Steel 95 5.1 740 718 160 47 400 Absent — — — Invention Steel 26 5.7 780 718 210 48  10 Absent — — — Comparative Steel 27 5.2 820 738 230 23  20 Absent — — — Invention Steel 28 4.3 820 718 150 28 800 Absent — — — Invention Steel 29 4.7 760 718 140 32 1200  Absent — — — Comparative Steel 30 4.3 760 718 160 34 312 Present GI — — Invention Steel 31 4.0 820 718  80 35 366 Present GI — — Invention Steel 32 4.5 810 718 190 25 386 Present GA 480 28 Invention Steel 33 4.5 750 718  90 50 235 Present GA 545 13 Invention Steel 123 4.5 820 752 220 55 290 Absent — — — Invention Steel 40 3.2 770 746 190 28 374 Absent — — — Invention Steel 47 5.6 750 736 180 20 314 Absent — — — Invention Steel 48 3.6 800 739 150 29 199 Absent — — — Invention Steel 62 3.6 750 738 240 42 332 Absent — — — Invention Steel 63 5.3 770 715 190 24 210 Absent — — — Invention Steel 64 4.9 810 752 230 22 379 Absent — — — Invention Steel 77 5.9 810 716 160 50 443 Absent — — — Invention Steel 79 4.3 760 738 200 41 242 Absent — — — Invention Steel 80 4.4 800 738 200 28 488 Absent — — — Invention Steel 81 2.9 740 715 120 36 237 Absent — — — Invention Steel 82 4.0 790 715 120 36 205 Absent — — — Invention Steel

TABLE-US-00004 TABLE 2-3 Hot rolling Cooling step Heating step step Time Average Cumulative until Casting step heating rolling start of Average Average rate reduction rapid cooling cooling in in cooling rate Cold rolling step Leveling rate at Pressure temper- temper- after from Coiling Sheet step 950° C. to applied ature ature completion start of step thickness Presence 550° C. in at range of Heating range of of hot cooling Coiling Cold after or Treat- Kind cooling 950° C. to 650° C. to temper- 1050° C. or railing to temper- rolling cold absence ment of of slab 550° C. 850° C. ature higher step 700° C. ature reduction rolling of No. steel ° C./h N/cm.sup.2 ° C./min ° C. % sec ° C./s ° C. % mm leveling 83 D 161 35 35 1240 41 1 96 570 52 1.20 Present 84 D 155 35 35 1240 45 1 92 490 52 1.20 Present 85 E 159 35 35 1240 42 2 90 550 52 1.20 Present 124 E 164 35 35 1240 43 2 98 540 52 1.20 Present 125 F 157 35 35 1240 40 2 98 560 52 1.20 Present 88 F 159 35 35 1240 45 2 100 540 52 1.20 Present 89 G 165 35 35 1240 41 1 92 550 52 1.20 Present 126 G 161 35 35 1240 42 1 99 470 52 1.20 Present 127 H 161 35 35 1240 40 2 99 530 52 1.20 Present 92 H 164 35 35 1240 43 1 93 540 52 1.20 Present 93 I 156 35 35 1240 44 1 90 490 52 1.20 Present 128 I 160 35 35 1240 40 2 90 530 52 1.20 Present 129 J 159 35 35 1240 40 1 96 560 52 1.20 Present 96 K 155 35 35 1240 45 1 92 500 52 1.20 Present 97 L 160 35 35 1240 40 2 100 560 52 1.20 Present 98 M 161 35 35 1240 40 1 96 570 52 1.20 Present 99 N 158 35 35 1240 41 1 92 470 52 1.20 Present 100 O 158 35 35 1240 42 1 94 480 52 1.20 Present 101 P 160 35 35 1240 41 1 95 500 36 1.60 Present 102 Q 160 35 35 1240 44 2 91 570 50 2.00 Present 103 Q 163 35 35 1240 43 1 96 570 52 1.20 Present 104 R 160 35 35 1240 44 2 97 480 52 1.20 Present 105 R 165 35 35 1240 41 1 97 530 52 1.20 Present 106 S 165 35 35 1240 45 1 99 500 52 1.20 Present 107 S 155 35 35 1240 45 2 99 530 52 1.20 Present 108 T 160 35 35 1240 43 2 96 500 52 1.20 Present Post- annealing Annealing step cooling Average step Retaining heating Average step Hot-dip rate cooling rate Retention galvanizing step during from time in Presence heating annealing temperature or Alloying step to Ac1 temperature range of absence Reheating Treat- or Annealing Retention to 550° C. or 150° C. to of Kind (alloying) Retention ment higher temperature AC1 time lower 550° C. plating of temperature time No. ° C./s ° C. ° C. sec ° C./s sec treatment plating ° C. sec Note 83 6.0 760 752 100 31 214 Absent — — — Invention Steel 84 5.6 800 752 180 34 301 Absent — — — Invention Steel 85 3.9 820 718 170 41 476 Absent — — — Invention Steel 124 5.4 820 718 160 54 276 Absent — — — Invention Steel 125 4.1 780 746 230 53 321 Absent — — — Invention Steel 88 3.4 760 746 80 36 225 Absent — — — Invention Steel 89 5.0 760 736 190 29 185 Absent — — — Invention Steel 126 5.7 770 736 170 30 308 Absent — — — Invention Steel 127 2.9 810 739 80 34 232 Absent — — — Invention Steel 92 3.8 770 739 120 36 163 Absent — — — Invention Steel 93 4.3 750 716 170 49 162 Absent — — — Invention Steel 128 3.9 760 716 130 37 359 Absent — — — Invention Steel 129 5.1 740 718 160 47 400 Absent — — — Invention Steel 96 4.3 770 733 210 45 202 Absent — — — Invention Steel 97 4.4 740 715 90 52 407 Absent — — — Invention Steel 98 5.7 788 757 140 49 498 Absent — — — Invention Steel 99 4.1 800 719 150 48 463 Absent — — — Invention Steel 100 3.0 760 745 190 50 299 Absent — — — Invention Steel 101 3.1 790 717 90 28 367 Absent — — — Invention Steel 102 5.9 820 718 190 39 174 Absent — — — Invention Steel 103 4.3 790 718 180 22 346 Absent — — — Invention Steel 104 5.9 800 739 140 51 270 Absent — — — Invention Steel 105 3.6 750 739 190 35 198 Absent — — — Invention Steel 106 3.7 790 715 200 46 233 Absent — — — Invention Steel 107 6.0 810 715 170 40 413 Absent — — — Invention Steel 108 3.2 820 752 130 28 205 Absent — — — Invention Steel

TABLE-US-00005 TABLE 2-4 Hot rolling Cooling step Heating step step Time Average Cumulative until Casting step heating rolling start of Average Average rate reduction rapid cooling cooling in in cooling rate Cold rolling step Leveling rate at Pressure temper- temper- after from Coiling Sheet step 950° C. to applied ature ature completion start of step thickness Presence 550° C. in at range of Heating range of of hot cooling Coiling Cold after or Treat- Kind cooling 950° C. to 650° C. to temper- 1050° C. or railing to temper- rolling cold absence ment of of slab 550° C. 850° C. ature higher step 700° C. ature reduction rolling of No. steel ° C./h N/cm.sup.2 ° C./min ° C. % sec ° C./s ° C. % mm leveling 109 T 162 35 35 1240 42 2 100 480 52 1.20 Present 130 A 158 35 35 1240 41 2 96 490 52 1.20 Absent 120 B 161 35 35 1240 42 1 100 570 52 1.20 Absent 121 C 163 35 35 1240 42 1 99 540 52 1.20 Absent 110 aa 155 35 35 1240 44 2 98 500 52 1.20 Present 111 bb 156 35 35 1240 42 1 98 470 52 1.20 Present 112 cc 158 35 35 1240 43 2 92 570 52 1.20 Present 113 dd 156 35 35 1240 42 2 95 540 52 1.20 Present 114 ee 160 35 35 1240 45 1 94 550 52 1.20 Present 115 ff 165 35 35 1240 44 2 98 520 52 1.20 Present 116 gg 160 35 35 1240 41 1 97 540 52 1.20 Present 117 hh 165 35 35 1240 40 2 90 510 52 1.20 Present 118 ii 155 35 35 1240 41 1 99 490 52 1.20 Present a01 J 108 17 25 1225 70 1 68 580 75 0.50 Present a02 L 326 25 28 1240 55 1 70 535 40 1.60 Present a03 N 203 67 15 1280 80 1 69 540 60 0.80 Present a04 P 116 12 18 1280 70 2 59 515 70 1.20 Present a05 R 278 44 46 1265 75 1 73 550 80 0.40 Present a06 T 125 15 8 1185 60 2 65 620 50 1.00 Present a07 U 223 24 41 1195 60 1 68 630 75 0.40 Present a08 V 216 23 19 1195 85 1 90 470 60 0.80 Present a09 W 278 16 28 1260 76 2 58 520 70 0.45 Present a10 X 137 56 24 1185 79 2 64 565 50 0.80 Present a11 Y 158 37 27 1225 63 1 101 575 50 2.00 Present a12 Z 236 31 40 1245 58 2 71 605 75 0.50 Present Post- annealing Annealing step cooling Average step Retaining heating Average step Hot-dip rate cooling rate Retention galvanizing step during from time in Presence heating annealing temperature or Alloying step to Ac1 temperature range of absence Reheating Treat- or Annealing Retention to 550° C. or 150° C. to of Kind (alloying) Retention ment higher temperature AC1 time lower 550° C. plating of temperature time No. ° C./s ° C. ° C. sec ° C./s sec treatment plating ° C. sec Note 109 4.8 780 752 130 24 225 Absent — — — Invention Steel 130 4.8 750 718 120 30 285 Absent — — — Invention Steel 120 3.8 760 738 250 45 328 Absent — — — Invention Steel 121 3.3 750 715 130 38 230 Absent — — — Invention Steel 110 4.9 790 715 80 27 355 Absent — — — Comparative Steel 111 3.3 810 713 200 34 362 Absent — — — Comparative Steel 112 3.1 810 775 120 46 382 Absent — — — Comparative Steel 113 5.7 750 677 180 31 385 Absent — — — Comparative Steel 114 4.7 780 724 160 36 178 Absent — — — Comparative Steel 115 3.8 780 694 230 37 407 Absent — — — Comparative Steel 116 3.5 820 705 250 48 304 Absent — — — Comparative Steel 117 5.8 740 703 210 55 281 Absent — — — Comparative Steel 118 5.5 770 708 130 54 454 Absent — — — Comparative Steel a01 3.3 765 718 100 35 87 Present GA 540 20 Invention Steel a02 3.0 780 715 75 20 270 Present GI — — Invention Steel a03 1.8 790 719 120 23 63 Absent — — — Invention Steel a04 2.3 800 717 90 33 107 Present GA 535 16 Invention Steel a05 3.9 785 739 125 65 45 Absent — — — Invention Steel a06 2.5 810 752 100 25 105 Present GI — — Invention Steel a07 2.2 765 720 40 29 79 Present GA 501 47 Invention Steel a08 2.1 790 700 80 47 58 Absent — — — Invention Steel a09 4.5 785 709 150 16 49 Present GI — — Invention Steel a10 3.5 805 723 60 69 630 Absent — — — Invention Steel a11 3.7 765 710 170 25 258 Present GA 495 33 Invention Steel a12 1.6 790 706 80 29 438 Absent — — — Invention Steel

[0238] The obtained steel sheet was evaluated by observing the microstructure and measuring the mechanical properties.

[0239] In the observation of the microstructure, a sample with a sheet thickness direction cross section parallel to the rolling direction as an observed section was collected, and in a range between a ⅛ thickness and a ⅜ thickness with a ¼ thickness as the center, the area fraction of each structure was measured by the above-described method.

[0240] In addition, the average distance between the centers of high Mn regions at the ¼ thickness in a plane parallel to the rolling direction of the steel sheet and a plane perpendicular to the sheet thickness direction, the ratio (D.sub.A/D.sub.B) between the density D.sub.A of the high Mn regions at a sheet width center portion and the density D.sub.B of the high Mn regions at a ¼ width, the ratio of the average hardness of the high Mn regions to the average hardness of low Mn regions, and the difference between the average value of a top 5% and the average value of a bottom 5% of Mn contents in the low Mn regions were obtained by the above-described method.

[0241] A tensile test was conducted according to JIS Z 2241:2011, and the mechanical properties (yield stress, tensile strength, and elongation) were evaluated. The measured position of the sheet was the sheet width center portion, and the test direction was a direction perpendicular to the rolling direction. The shape of the test piece was a No. 5 test piece shown in JIS Z 2241:2011.

[0242] Shape-fixability is improved as long as the yield stress can be reduced while increasing the tensile strength. Therefore, the shape-fixability was evaluated by the tensile strength and the yield ratio (YP/TS). In a case where the tensile strength TS was 590 MPa or more and YP/TS≤0.80, excellent shape-fixability was determined.

[0243] when the ductility decreases, there is a possibility that press forming itself may not be possible. Therefore, workability was evaluated by the product of the tensile strength and elongation (TS×EL). A case of TS×EL≥14,000 MPa.Math.% was determined to have sufficient workability.

[0244] Dimensional precision after pressing, which is an object of the present invention, is improved by reducing the difference in work hardening depending on the location. Therefore, in the present invention, the dimensional precision after pressing was evaluated by the difference in the amount of work hardening depending on the location.

[0245] The difference in the amount of work hardening depending on the location was defined as follows.

[0246] First, the true stress is indicated as σ, the true strain is indicated as E, and the true stress is differentiated by the true strain to obtain dσ/dε. Then, a graph of dσ/dε and σ is drawn. When the yield stress at the true stress is indicated as σYP, dσ/dε has almost the same value when σ is 0 to σYP. Thereafter, dσ/dε decreases. The slope (the graph of dσ/dε and σ) has an inflection point in the middle and becomes gentle from the middle. The true stress at the inflection point is defined as σin. This is because the amount of work hardening at σin may vary greatly depending on the location of the steel sheet.

[0247] In addition, dσ/dε (σin) at a sheet width center portion and dσ/dε (σin) at a ¼ width portion are obtained, and the absolute value of the difference therebetween is defined as |Δdσ/dε (σin)|W and obtained. Similarly, in order to investigate the difference in the amount of work hardening in the rolling direction at the sheet width center portion, when any position of the sheet width center portion is defined as a sheet width center portion 1 and the position of the sheet width center portion at a position 500 to 1000 mm away therefrom is defined as a sheet width center portion 2, the absolute value of the difference in dσ/dε (σin) is defined as |Δdσ/dε (σin)|L. The sheet width center portion 1 is the same as the sheet width center portion used for |Δdσ/dε (σin)|W.

[0248] In the present example, the difference in the amount of work hardening depending on the location was evaluated by |Δdσ/dε (σin)|L and |Δdσ/dε (σin)|W.

[0249] In a case of |Δdσ/dε (σin)|L≤1500 and |Δdσ/dε (σin)|W≤1500, it was determined that the difference in work hardening depending on the location was small.

[0250] Tables 3-1 to 3-4 show the measurement results and evaluation results.

[0251] The chemical composition of each of the obtained steel sheets was substantially the same as the chemical composition of the corresponding molten steel.

TABLE-US-00006 TABLE 3-1 Difference Ratio of between average average Average hardness value of distance Density of high top 5% between D.sub.A of Mn and centers high Density regions average Fraction of high Mn D.sub.B of to value of of Mn regions high Average Average average bottom residual regions at sheet Mn hardness hardness hardness 5% of Mn austenite in plane width regions of high of low of low contents Treat- Ferrite and Bainite Pearlite at ¼ center at ¼ D.sub.A/ Mn Mn Mn in low Mn ment fraction martensite fraction fraction thickness portion width D.sub.B regions regions regions regions No. area % area % area % area % mm /mm.sup.2 /mm.sup.2 — HV HV — mass % 1 78 13 9 0 0.60 2.35 2.08 1.13 321 204 1.57 0.42 18 86  8 6 0 0.53 1.91 1.32 1.45 221 95 2.34 0.41 37 59  7 34  0 0.59 1.27 1.80 1.42 213 88 2.43 0.44 2 79 12 9 0 0.75 2.71 1.87 1.45 419 165 2.53 0.48 3 78 13 9 0 0.74 2.42 1.94 1.25 320 163 1.96 0.48 86 78 13 9 0 1.52 2.41 1.71 1.41 397 171 2.32 0.41 87 16 24 60  0 1.62 1.49 2.11 0.71 570 239 2.38 0.45 4 79 13 8 0 1.72 2.52 1.75 1.44 374 161 2.32 0.50 5 78 14 8 0 0.90 2.43 1.94 1.25 319 168 1.90 0.47 90 29 22 49  0 0.50 1.84 1.76 1.05 367 334 1.10 0.12 91 30 18 52  0 0.48 2.09 1.91 1.09 367 306 1.20 0.11 6 77 15 8 0 0.47 2.18 1.98 1.10 298 271 1.10 0.13 7 79 13 8 0 0.45 2.23 1.93 1.15 294 268 1.10 0.35 8 91  2 7 0 0.63 1.83 2.08 0.88 260 166 1.57 0.47 10 76 15 9 0 0.53 2.41 1.75 1.38 397 167 2.38 0.50 11 78 13 9 0 1.23 2.37 1.79 1.32 398 171 2.33 0.47 12 79 12 9 0 1.22 2.37 1.79 1.33 393 169 2.32 0.48 13 79 12 9 0 1.24 2.37 1.79 1.33 395 170 2.33 0.45 14 78 13 9 0 1.22 2.38 1.78 1.34 399 172 2.32 0.43 15 77 14 9 0 0.64 2.21 1.95 1.11 290 181 1.60 0.48 16 77 14 9 0 1.20 2.43 1.73 1.38 392 178 2.22 0.49 119 77 14 9 0 0.61 2.43 1.73 1.12 392 178 1.20 0.11 17 78 13 9 0 0.78 2.40 1.76 1.10 390 177 1.58 0.11 122 87  7 6 0 0.77 2.40 1.76 1.04 390 177 1.52 0.13 19  0  4 96  0 0.69 1.84 2.08 0.88 340 203 1.68 0.45 20 30 20 50  0 0.56 2.18 1.90 1.15 286 194 1.48 0.46 dσ/dε dσ/dε (σin) (σin) in in sheet sheet dσ/dε width width (σin) center center in ¼ Treat- Yield Tensile Yield TS × portion portion width |Δdσ/dε |Δdσ/dε ment stress strength ratio Elongation EL 1 2 portion (σin)|L (σin)|W Note No. MPa MPa — % MPa-% MPa MPa MPa MPa MPa — 1 497 841 0.59 26 21450 12504 13167 11864 663 640 Invention Steel 18 333 650 0.51 30 19500 10224 10696 8332 472 1892 Comparative Steel 37 319 632 0.51 36 23000 7353 7001 9142 352 1789 Comparative Steel 2 501 845 0.59 25 20800 12552 13128 10542 576 2010 Comparative Steel 3 503 847 0.59 25 20800 12576 11928 11373 648 1203 Invention Steel 86 507 852 0.60 21 17550 13136 12517 11445 619 1691 Comparative Steel 87 903 1214 0.74 18 21467 5230 5480 6749 250 1519 Comparative Steel 4 497 841 0.59 22 18850 12504 11871 10554 633 1950 Comparative Steel 5 499 843 0.59 22 18850 12528 11920 11330 608 1198 Invention Steel 90 631 1052 0.60 20 21467 6640 5125 6341 1515 299 Comparative Steel 91 616 1010 0.61 20 20700 6850 5321 6477 1529 373 Comparative Steel 6 508 853 0.59 21 19500 12648 10787 13309 1861 661 Comparative Steel 7 514 843 0.61 25 21450 12528 13871 11867 1343 661 Invention Steel 8 512 857 0.60 16 13980 12696 12102 13311 594 615 Comparative Steel 10 501 845 0.59 24 20150 13052 12389 11371 663 1682 Comparative Steel 11 508 853 0.61 21 20800 12148 12810 13780 662 1632 Comparative Steel 12 499 843 0.59 22 18850 13028 13689 11374 661 1654 Comparative Steel 13 503 847 0.61 25 21450 13076 13726 11435 650 1641 Comparative Steel 14 512 857 0.60 20 20800 13196 13840 11551 644 1644 Comparative Steel 15 526 849 0.62 24 20150 11600 11314 11030 286 559 Invention Steel 16 510 855 0.83 24 17550 12672 12051 14480 620 1808 Comparative Steel 119 510 854 0.83 24 20150 12674 14478 12051 1804 623 Comparative Steel 17 506 851 0.84 25 18200 12624 10808 13263 1816 639 Comparative Steel 122 333 650 0.82 30 21450 10224 8500 9753 1723 471 Comparative Steel 19 790 1120 0.71 12 13306 15836 15112 16584 724 748 Comparative Steel 20 520 865 0.60 23 19809 12791 12150 12127 641 664 Invention Steel

TABLE-US-00007 TABLE 3-2 Difference Ratio of between average average Average hardness value of distance Density of high top 5% between D.sub.A of Mn and centers high Density regions average Fraction of high Mn D.sub.B of to value of of Mn regions high Average Average average bottom residual regions at sheet Mn hardness hardness hardness 5% of Mn austenite in plane width regions of high of low of low contents Treat- Ferrite and Bainite Pearlite at ¼ center at ¼ D.sub.A/ Mn Mn Mn in low Mn ment fraction martensite fraction fraction thickness portion width D.sub.B regions regions regions regions No. area % area % area % area % mm /mm.sup.2 /mm.sup.2 — HV HV — mass % 21 93  0 7 0 0.53 2.16 1.91 1.13 228 168 1.36 0.45 23 67  8 6 19  0.51 1.81 2.09 0.87 204 154 1.33 0.47 24 75 12 7 6 0.56 1.84 2.08 0.89 235 168 1.40 0.42 25 76 15 9 0 0.57 2.35 2.07 1.14 325 220 1.48 0.46 94 89  5 6 0 0.70 1.84 1.60 1.15 200 120 1.66 0.45 95 81 12 7 0 0.57 2.31 2.06 1.12 293 224 1.31 0.48 26 74 16 10  0 0.77 2.22 1.94 1.14 390 177 2.10 0.46 27 51 15 34  0 0.72 1.71 1.93 0.88 390 177 1.98 0.45 28 68 13 19  0 0.63 1.82 2.07 0.88 257 179 1.44 0.48 29 74  2 24  0 0.52 1.97 2.25 0.88 298 216 1.38 0.45 30 79 13 8 0 0.50 2.15 1.87 1.15 277 210 1.32 0.44 31 76 15 9 0 0.57 2.15 1.90 1.13 277 195 1.42 0.46 32 79 12 9 0 0.64 1.84 2.08 0.88 259 157 1.65 0.41 33 80 11 9 0 0.55 2.16 1.91 1.14 276 206 1.34 0.43 123 60  6 34  0 0.51 1.40 1.59 0.88 190 140 1.36 0.48 40 20 24 56  0 0.58 1.73 1.95 0.88 429 270 1.59 0.48 47 20 22 58  0 0.62 2.02 1.81 1.11 394 272 1.45 0.48 48 34 17 49  0 0.52 2.07 1.84 1.13 327 244 1.34 0.43 62 47 11 42  0 0.51 1.91 1.66 1.15 266 189 1.41 0.45 63 87  7 6 0 0.63 1.87 1.64 1.14 210 131 1.61 0.47 64 60  7 33  0 0.62 1.55 1.73 0.90 221 137 1.62 0.49 77 87  6 7 0 0.62 1.53 1.76 0.87 186 119 1.57 0.48 79 45 11 44  0 0.60 1.91 1.66 1.15 266 160 1.66 0.43 80 45 13 42  0 0.59 2.05 1.82 1.12 305 189 1.62 0.47 81 87  7 6 0 0.53 1.58 1.79 0.88 191 137 1.40 0.46 82 87  7 6 0 0.57 2.03 1.80 1.13 246 165 1.49 0.48 dσ/dε dσ/dε (σin) (σin) in in sheet sheet dσ/dε width width (σin) center center in ¼ Treat- Yield Tensile Yield TS × portion portion width |Δdσ/dε |Δdσ/dε ment stress strength ratio Elongation EL 1 2 portion (σin)|L (σin)|W Note No. MPa MPa — % MPa-% MPa MPa MPa MPa MPa — 21 372 699 0.53 19 13082 10809 10274 10298 535 511 Comparative Steel 23 346 666 0.52 19 12689 10415 9924 10981 491 566 Comparative Steel 24 439 777 0.56 19 14890 11740 11196 12278 545 538 Invention Steel 25 506 851 0.60 21 17550 12624 13212 11935 588 689 Invention Steel 94 295 600 0.49 36 21450 9627 9183 9127 444 500 Invention Steel 95 438 776 0.56 27 20800 11728 12323 11155 595 573 Invention Steel 26 505 849 0.59 25 21450 13200 12569 11338 631 1862 Comparative Steel 27 470 812 0.58 30 24533 7440 7092 8633 348 1193 Invention Steel 28 643 857 0.75 26 22231 12696 12017 13298 679 602 Invention Steel 29 693 845 0.82 25 21234 12552 13133 13241 581 689 Comparative Steel 30 496 840 0.59 22 18850 12493 11814 11839 678 654 Invention Steel 31 512 857 0.60 25 21450 12696 11999 12108 696 587 Invention Steel 32 510 855 0.60 21 17690 12672 12080 13268 592 596 Invention Steel 33 499 843 0.59 22 18209 12528 11918 11928 611 601 Invention Steel 123 319 632 0.51 40 25300 8740 8286 9160 454 420 Invention Steel 40 901 1212 0.74 18 22233 5840 6152 6143 312 303 Invention Steel 47 708 1048 0.68 20 21467 6660 7023 6356 363 304 Invention Steel 48 665 1008 0.66 21 20700 6860 6519 6551 341 309 Invention Steel 62 458 798 0.57 31 24533 7910 7518 7480 392 430 Invention Steel 63 327 642 0.51 29 18850 10128 9606 9630 523 499 Invention Steel 64 318 630 0.50 34 21467 8750 9210 9158 460 408 Invention Steel 77 303 610 0.50 33 20150 9746 9292 10245 455 499 Invention Steel 79 459 800 0.57 26 20700 7900 7512 7483 388 417 Invention Steel 80 467 808 0.58 28 23000 7860 8250 7471 390 389 Invention Steel 81 322 636 0.51 28 17550 10057 9534 10540 522 483 Invention Steel 82 330 646 0.51 29 18850 10176 10710 9661 534 515 Invention Steel

TABLE-US-00008 TABLE 3-3 Difference Ratio of between average average Average hardness value of distance Density of high top 5% between D.sub.A of Mn and centers high Density regions average Fraction of high Mn D.sub.B of to value of of Mn regions high Average Average average bottom residual regions at sheet Mn hardness hardness hardness 5% of Mn austenite in plane width regions of high of low of low contents Treat- Ferrite and Bainite Pearlite at ¼ center at ¼ D.sub.A/ Mn Mn Mn in low Mn ment fraction martensite fraction fraction thickness portion width D.sub.B regions regions regions regions No. area % area % area % area % mm /mm.sup.2 /mm.sup.2 — HV HV — mass % 83 64 7 29 0 0.69 1.67 1.46 1.14 204 121 1.69 0.49 84 60 7 33 0 0.61 1.66 1.46 1.14 206 142 1.45 0.49 85 77 15 8 0 0.57 2.15 1.88 1.14 280 215 1.30 0.41 124 78 14 8 0 0.51 2.15 1.88 1.14 280 204 1.37 0.45 125 17 17 66 0 0.61 1.73 1.95 0.89 428 304 1.41 0.49 88 16 24 60 0 0.59 1.87 1.65 1.13 396 259 1.53 0.42 89 25 19 56 0 0.58 1.73 1.94 0.89 366 265 1.38 0.43 126 31 20 49 0 0.58 1.87 1.66 1.13 344 246 1.40 0.48 127 23 21 56 0 0.53 2.08 1.84 1.13 331 245 1.35 0.42 92 32 19 49 0 0.58 2.07 1.82 1.14 325 237 1.37 0.42 93 89 5 6 0 0.51 1.82 1.60 1.14 197 151 1.30 0.49 128 89 5 6 0 0.58 1.96 1.76 1.12 226 163 1.39 0.43 129 79 12 9 0 0.64 1.97 2.22 0.89 271 175 1.55 0.42 96 47 12 41 0 0.64 2.03 1.82 1.12 293 189 1.55 0.45 97 85 8 7 0 0.60 1.86 1.65 1.13 211 150 1.41 0.43 98 61 7 32 0 0.54 1.35 1.21 1.12 227 150 1.51 0.46 99 79 13 8 0 0.55 2.14 1.89 1.13 258 176 1.47 0.41 100 11 24 65 0 0.65 2.00 1.74 1.15 394 246 1.60 0.41 101 74 17 9 0 0.54 1.87 1.65 1.13 321 221 1.45 0.44 102 79 13 8 0 0.51 1.82 2.06 0.88 239 184 1.30 0.49 103 82 11 7 0 0.61 2.17 1.89 1.15 263 189 1.39 0.46 104 51 11 38 0 0.61 2.00 1.75 1.14 260 175 1.49 0.48 105 54 11 35 0 0.62 1.98 1.75 1.14 254 172 1.48 0.43 106 85 9 6 0 0.54 1.89 1.64 1.15 198 143 1.38 0.48 107 88 6 6 0 0.63 2.01 1.80 1.12 225 157 1.43 0.49 108 62 10 28 0 0.61 1.79 1.60 1.11 225 145 1.55 0.41 dσ/dε dσ/dε (σin) (σin) in in sheet sheet dσ/dε width width (σin) center center in ¼ Treat- Yield Tensile Yield TS × portion portion width |Δdσ/dε |Δdσ/dε ment stress strength ratio Elongation EL 1 2 portion (σin)|L (σin)|W Note No. MPa MPa — % MPa-% MPa MPa MPa MPa MPa — 83 310 620 0.50 33 20700 8800 8362 8365 438 435 Invention Steel 84 316 628 0.50 34 21467 8760 8289 8321 471 439 Invention Steel 85 507 852 0.60 25 21450 12636 11994 12012 642 624 Invention Steel 124 507 852 0.60 25 21450 12636 11973 12000 663 636 Invention Steel 125 903 1214 0.74 14 17589 5830 6129 6122 299 292 Invention Steel 88 898 1210 0.74 15 17684 5850 5572 5575 278 275 Invention Steel 89 711 1050 0.68 23 23767 6650 6953 6967 303 317 Invention Steel 126 713 1052 0.68 20 20700 6640 6339 6335 301 305 Invention Steel 127 667 1010 0.66 20 20700 6850 6522 6524 328 326 Invention Steel 92 658 1002 0.66 24 23767 6890 6515 6560 375 330 Invention Steel 93 294 598 0.49 32 18850 9603 9114 9122 489 481 Invention Steel 128 295 600 0.49 29 17550 9627 10132 9171 505 456 Invention Steel 129 438 776 0.56 25 19500 11728 12260 12318 532 590 Invention Steel 96 443 782 0.57 27 21467 7990 8373 7616 383 374 Invention Steel 97 338 656 0.52 28 18200 10296 9739 9828 556 468 Invention Steel 98 292 596 0.49 37 22233 8922 9459 8504 537 418 Invention Steel 99 453 793 0.57 22 17550 11931 11322 11366 609 566 Invention Steel 100 861 1180 0.73 20 23767 6000 5717 5682 284 318 Invention Steel 101 641 986 0.65 22 21450 14236 13534 13580 702 656 Invention Steel 102 452 792 0.57 26 20800 11919 11331 12475 588 555 Invention Steel 103 447 786 0.57 22 17550 11848 11313 11219 535 629 Invention Steel 104 449 788 0.57 26 20700 7960 7570 7556 390 404 Invention Steel 105 440 778 0.57 28 21467 8010 7603 7626 407 384 Invention Steel 106 292 596 0.49 32 18850 9579 9106 9076 473 503 Invention Steel 107 295 600 0.49 35 20800 9627 10074 9168 447 459 Invention Steel 108 297 602 0.49 37 22233 8890 9331 8482 441 408 Invention Steel

TABLE-US-00009 TABLE 3-4 Difference Ratio of between average average Average hardness value of distance Density of high top 5% between D.sub.A of Mn and centers high Density regions average Fraction of high Mn D.sub.B of to value of of Mn regions high Average Average average bottom residual regions at sheet Mn hardness hardness hardness 5% of Mn austenite in plane width regions of high of low of low contents Treat- Ferrite and Bainite Pearlite at ¼ center at ¼ D.sub.A/ Mn Mn Mn in low Mn ment fraction martensite fraction fraction thickness portion width D.sub.B regions regions regions regions No. area % area % area % area % mm /mm.sup.2 /mm.sup.2 — HV HV — mass % 109 63  8 29  0 0.62 1.55 1.74 0.89 211 132 1.60 0.42 130 72 20 8 0 0.65 2.34 1.82 1.28 331 215 1.54 0.43 120 42 17 41  0 0.58 2.05 1.59 1.29 376 200 1.88 0.46 121 82 13 5 0 0.59 2.01 1.59 1.27 201 151 1.33 0.48 110 88  6 6 0 0.67 1.68 1.92 0.88 279 118 2.37 0.43 111 98  0 2 0 0.60 3.16 2.79 1.13 156 110 1.42 0.46 112 51 13 36  0 0.51 3.53 3.07 1.15 368 159 2.31 0.47 113 80 12 8 0 0.50 4.80 4.20 1.14 364 155 2.35 0.47 114 95  0 5 0 0.49 0.05 0.05 1.15 153 118 1.32 0.48 115 81 11 8 0 0.55 3.09 2.71 1.14 268 192 1.43 0.48 116 81 11 8 0 0.58 1.92 1.68 1.14 272 182 1.54 0.47 117 80 12 8 0 0.56 1.88 2.12 0.89 364 153 2.38 0.50 118 79 12 9 0 0.59 1.51 1.69 0.89 364 157 2.32 0.42 a01 74 12 12  2 0.68 1.75 2.00 0.88 287 174 1.65 0.51 a02 68 13 15  4 0.43 2.85 2.65 1.08 228 149 1.53 0.48 a03 66 24 10  0 0.45 2.66 2.80 0.95 335 261 1.28 0.34 a04 70 15 13  2 0.60 2.01 1.74 1.16 272 195 1.39 0.53 a05 61 28 11  0 0.38 3.05 2.45 1.24 361 254 1.42 0.37 a06 70  8 19  3 0.73 1.36 1.50 0.91 232 135 1.72 0.58 a07 92  6 0 2 0.64 1.99 2.17 0.92 224 163 1.37 0.35 a08 83 12 5 0 0.47 2.70 2.37 1.14 253 191 1.32 0.51 a09 90  7 2 1 0.53 2.32 2.17 1.07 235 129 1.82 0.45 a10 68  4 25  3 0.40 2.96 3.02 0.98 216 175 1.23 0.32 a11 82 11 6 1 0.48 2.61 2.83 0.92 293 155 1.89 0.66 a12 87  9 4 0 0.50 2.25 2.38 0.95 260 183 1.42 0.61 dσ/dε dσ/dε (σin) (σin) in in sheet sheet dσ/dε width width (σin) center center in ¼ Treat- Yield Tensile Yield TS × portion portion width |Δdσ/dε |Δdσ/dε ment stress strength ratio Elongation EL 1 2 portion (σin)|L (σin)|W Note No. MPa MPa — % MPa-% MPa MPa MPa MPa MPa — 109 291 594 0.49 43 25300 8930 9451 9370 521 440 Invention Steel 130 508 851 0.59 25 21350 12609 12178 11288 431 1321 Invention Steel 120 468 802 0.57 29 23582 8021 7618 6693 403 1328 Invention Steel 121 333 651 0.51 26 16999 10150 9642 8835 508 1315 Invention Steel 110 292 596 0.49 35 13890 9079 9527 10586 448 1507 Comparative Steel 111 170 410 0.41 44 18200 7358 7713 6976 355 382 Comparative Steel 112 450 790 0.57 29 23000 8550 8108 6915 442 1635 Comparative Steel 113 440 778 0.57 23 18050 10652 11194 12198 542 1545 Comparative Steel 114 190 444 0.43 44 19500 7764 7363 7349 401 415 Comparative Steel 115 445 711 0.57 26 13209 11824 11269 11227 555 597 Comparative Steel 116 452 702 0.57 27 13940 11919 12505 11294 586 626 Comparative Steel 117 438 776 0.56 28 21450 11228 10620 12795 608 1566 Comparative Steel 118 443 782 0.57 27 20800 11300 10778 12841 522 1541 Comparative Steel a01 517 807 0.64 22 17518 9277 8675 9002 602 275 Invention Steel a02 408 712 0.57 24 17169 9750 9701 9080 49 670 Invention Steel a03 617 997 0.62 15 14561 10592 10259 10663 333 71 Invention Steel a04 547 867 0.63 21 18034 9991 10362 9698 371 293 Invention Steel a05 484 1145 0.42 18 20948 11944 11165 12045 779 101 Invention Steel a06 368 598 0.62 26 15316 10895 11403 10524 508 371 Invention Steel a07 261 601 0.43 25 15205 8023 7558 8245 465 222 Invention Steel a08 352 725 0.49 23 16670 10372 10008 10818 364 446 Invention Steel a09 303 623 0.49 25 15393 7733 7333 7346 400 387 Invention Steel a10 443 653 0.68 24 15943 6895 7023 6737 128 158 Invention Steel a11 439 883 0.50 20 17315 11039 10265 10240 774 799 Invention Steel a12 371 791 0.47 19 15189 10852 10138 10523 714 329 Invention Steel

[0252] In Treatment Nos. 2, 18, and 37, the average cooling rate at 950° C. to 550° C. was slow in the cooling of the slab, and Expression (1) and the ratio of the average hardness of the high Mn regions to the average hardness of the low Mn regions were outside the ranges of the present invention. As a result, |Δdσ/dε (σin)|W was outside the target range.

[0253] In Treatment Nos. 4, 86, and 87, the pressure applied at 950° C. to 550° C. was low, the average distance between the centers of the high Mn regions at the ¼ thickness, Expression (1), and the ratio of the average hardness of the high Mn regions to the average hardness of the low Mn regions were outside the ranges of the present invention. As a result, |Δdσ/dε (σin)|W was outside the target range.

[0254] In Treatment Nos. 6, 90, and 91, the heating rate in the temperature range of 650° C. to 850° C. in the heating before the hot rolling was fast, and the difference between the average value of the top 5% and the average value of the bottom 5% of the Mn contents in the low Mn regions was small. As a result, |Δdσ/dε (σin)|L was outside the target range.

[0255] In Treatment No. 8, the heating temperature during the hot rolling was low, and the fraction of residual austenite and martensite was low. As a result, TS×EL was outside the target range.

[0256] In Treatment No. 10, the rolling reduction in the temperature range of 1050° C. or higher was low, D.sub.A/D.sub.B did not satisfy Expression (1), and the ratio of the average hardness of the high Mn regions to the average hardness of the low Mn regions was high. As a result, |Δdσ/dε (σin)|W was outside the target range.

[0257] In Treatment Nos. 11 and 12, the time from the end of the hot rolling to the start of the cooling was long, D.sub.A/D.sub.B did not satisfy Expression (1), and the ratio of the average hardness of the high Mn regions to the average hardness of the low Mn regions was high. As a result, |Δdσ/dε (σin)|W was outside the target range.

[0258] In Treatment Nos. 13 and 14, the average cooling rate from the start of the cooling to 700° C. was slow, D.sub.A/D.sub.B did not satisfy Expression (1), and the ratio of the average hardness of the high Mn regions to the average hardness of the low Mn regions was high. As a result, |Δdσ/dε (σin)|W was outside the target range.

[0259] In Treatment No. 16, the coiling temperature was high, D.sub.A/D.sub.B did not satisfy Expression (1), and the ratio of the average hardness of the high Mn regions to the average hardness of the low Mn regions was high, so that |Δdσ/dε (σin)|W was outside the target range.

[0260] In Treatment No. 119, the coiling temperature was low, and the difference between the average value of the top 5% and the average value of the bottom 5% of the Mn contents in the low Mn regions was small. As a result, |Δdσ/dε (σin)|L was outside the target range.

[0261] In Treatment Nos. 17 and 122, the average heating rate during the heating to Ac1° C. or higher was fast, and the difference between the average value of the top 5% and the average value of the bottom 5% of the Mn contents in the low Mn regions was small. As a result, |Δdσ/dε (σin)|L was outside the target range.

[0262] In Treatment No. 19, the annealing temperature was high, and the ferrite fraction was outside the range of the present invention. As a result, TS×EL was outside the target range.

[0263] In Treatment No. 21, the retention time at the annealing temperature was short, and the fraction of residual austenite and martensite was low. As a result, TS×EL was outside the target range.

[0264] In Treatment No. 23, the average cooling rate from the annealing temperature to 550° C. was slow, and the pearlite fraction was higher than the range of the present invention. As a result, TS×EL was outside the target range.

[0265] In Treatment No. 26, the retention time in the temperature range of 150° C. to 550° C. was short, and the ratio of the average hardness of the high Mn regions to the average hardness of the low Mn regions was higher than the range of the present invention. As a result, |Δdσ/dε (σin)|W was outside the target range.

[0266] In Treatment No. 29, the retention time in the temperature range of 150° C. to 550° C. was long, and residual austenite and martensite were low. As a result, the yield ratio was outside the target range.

[0267] In Treatment No. 110, since Kind of steel aa was used, the Nb content was higher than the range of the present invention, and the ratio of the average hardness of the high Mn regions to the average hardness of the low Mn regions was higher than the range of the present invention. As a result, TS×EL was low and |Δdσ/dε (σin)|W was outside the target range.

[0268] In Treatment No. 111, since Kind of steel bb was used, the C content was lower than the range of the present invention, so that the ferrite fraction was high and the residual austenite fraction was low. As a result, TS was outside the target range.

[0269] In Treatment No. 112, since Kind of steel cc was used, the Si content was higher than the range of the present invention, and the ratio of the average hardness of the high Mn regions to the average hardness of the low Mn regions was higher than the range of the present invention. As a result, |Δdσ/dε (σin)|W was outside the target range.

[0270] In Treatment No. 113, since Kind of steel dd was used, the Mn content was higher than the range of the present invention, and the ratio of the average hardness of the high Mn regions to the average hardness of the low Mn regions was higher than the range of the present invention. As a result, |Δdσ/dε (σin)|W was outside the target range.

[0271] In Treatment No. 114, since Kind of steel ee was used, the Mn content was lower than the range of the present invention, and residual austenite and martensite were lower than the ranges of the present invention. As a result, TS was outside the target range.

[0272] In Treatment No. 115, since Kind of steel ff was used, the P content was higher than the range of the present invention. As a result, TS×EL was outside the target range.

[0273] In Treatment No. 116, since Kind of steel gg was used, the S content was higher than the range of the present invention. As a result, TS×EL was outside the target range.

[0274] In Treatment No. 117, since Kind of steel hh was used, the Al content was higher than the range of the present invention. As a result, the ratio of the average hardness of the high Mn regions to the average hardness of the low Mn regions was higher than the range of the present invention, and |Δdσ/dε (σin)|W was outside the target range.

[0275] Treatment No. 118 is a component of symbol ii, the Ti content was higher than the range of the present invention, and the ratio of the average hardness of the high Mn regions to the average hardness of the low Mn regions was higher than the range of the present invention. As a result, |Δdσ/dε (σin)|W was outside the target range.

[0276] Regarding the other conditions, the structure was within the range of the present invention, and the tensile strength, yield ratio, TS×EL, and |Δdσ/dε (σin)|W were within the specified ranges.