STEEL SHEET AND METHOD FOR MANUFACTURING SAME

Abstract

This steel sheet has a predetermined chemical composition, in which a microstructure at a ¼ depth position of a sheet thickness from a surface of the steel sheet contains, by volume fraction, ferrite: 0% to 50%, residual austenite: 6% to 30%, bainite: 5% to 60%, tempered martensite: 5% to 50%, fresh martensite: 0% to 10%, and pearlite: 0% to 5%, at the ¼ depth position of the sheet thickness from the surface, a number proportion of the residual austenite having an aspect ratio of 2.0 or more to an entire residual austenite is 50% or more, and a number density of inclusions and precipitates having a grain size of 1 μm or more is 30/mm.sup.2 or less, and at a 1/20 depth position of the sheet thickness from the surface, an average interval between Mn-concentrated portions in a direction perpendicular to a rolling direction is 300 μm or less, and a standard deviation of Mn concentrations in the residual austenite is 0.40% or less.

Claims

1. A steel sheet comprising, as a chemical composition, by mass %: C: 0.150% to 0.400%; Si: 0.01% to 2.50%; Mn: 1.50% to 3.50%; P: 0.050% or less; S: 0.0100% or less; Al: 0.001% to 1.500%; Si and Al: 0.50% to 3.00% in total; N: 0.0100% or less; O: 0.0100% or less; Ti: 0% to 0.200%; V: 0% to 1.00%; Nb: 0% to 0.100%; Cr: 0% to 2.00%; Ni: 0% to 1.00%; Cu: 0% to 1.00%; Co: 0% to 1.00%; Mo: 0% to 1.00%; W: 0% to 1.00%; B: 0% to 0.0100%; Sn: 0% to 1.00%; Sb: 0% to 1.00%; Ca: 0% to 0.0100%; Mg: 0% to 0.0100%; Ce: 0% to 0.0100%; Zr: 0% to 0.0100%; La: 0% to 0.0100%; Hf: 0% to 0.0100%; Bi: 0% to 0.0100%; REM other than Ce and La: 0% to 0.0100%; and a remainder of Fe and impurities, wherein a microstructure at a ¼ depth position of a sheet thickness from a surface of the steel sheet contains, by volume fraction, ferrite: 0% to 50%, residual austenite: 6% to 30%, bainite: 5% to 60%, tempered martensite: 5% to 50%, fresh martensite: 0% to 10%, and pearlite: 0% to 5%, at the ¼ depth position of the sheet thickness from the surface, a number proportion of the residual austenite having an aspect ratio of 2.0 or more to an entire residual austenite is 50% or more, and a number density of inclusions and precipitates having a grain size of 1 μm or more is 30/mm.sup.2 or less, and at a 1/20 depth position of the sheet thickness from the surface, an average interval between Mn-concentrated portions in a direction perpendicular to a rolling direction is 300 μm or less, and a standard deviation of Mn concentrations in the residual austenite is 0.40% or less.

2. The steel sheet according to claim 1, wherein a ratio of a Vickers hardness Hv.sub.sur at a depth position of 30 μm from the surface to a Vickers hardness [Hv] at the ¼ depth position of the sheet thickness from the surface satisfies Expression (1),
Hv.sub.sur/[Hv]≤0.80  (1).

3. The steel sheet according to claim 1, further comprising: a plating layer on the surface.

4. A method for manufacturing the steel sheet according to claim 1, the method comprising: a casting step of casting a molten steel having the chemical composition according to claim 1 into a slab having a thickness of 200 to 300 mm; a hot rolling step of performing hot rolling on the slab to obtain a hot-rolled steel sheet; a coiling step of coiling the hot-rolled steel sheet in a temperature range of 25° C. to 680° C.; a cold rolling step of performing cold rolling on the hot-rolled steel sheet after the coiling step at a rolling reduction of 20% or more to obtain a cold-rolled steel sheet; a first annealing step of performing first annealing on the cold-rolled steel sheet; a second annealing step of performing second annealing on the cold-rolled steel sheet after the first annealing step; and a soaking step of holding the cold-rolled steel sheet after the second annealing step in a temperature range of 260° C. to 450° C. for 10 to 1000 seconds, wherein, in the casting step, a solidification rate at a depth position of 10 mm from a surface of the molten steel is set to 100 to 1000° C./min, a molten steel casting amount per unit time is set to 2.0 to 6.0 tons/min, and cooling is performed at an average cooling rate set to 4° C./sec or higher between a liquidus temperature and a solidus temperature of a surface layer area at a depth position of 5 mm from the surface of the molten steel, in the hot rolling step, an average heating rate of the slab between Ac1 and Ac1+30° C. is 2 to 50° C./min, and the hot rolling is performed after the slab is heated to 1200° C. or higher for 20 minutes or longer, in the first annealing step, an average heating rate between Ac1 and Ac1+30° C. is 0.5° C./min or higher, holding is performed at a highest heating temperature of Ac3 to 950° C. for 1 second to 1000 seconds, an average cooling rate in a temperature range up to 650° C. is 1° C./sec or higher, and a cooling stop temperature is 25° C. to 450° C., and in the second annealing step, holding is performed at a highest heating temperature of Ac1+20° C. or higher and lower than Ac3 for 1 second to 1000 seconds, and then cooling to 250° C. or lower is performed.

5. A method for manufacturing the steel sheet according to claim 1, the method comprising: a casting step of casting a molten steel having the chemical composition according to claim 1 into a slab having a thickness of 200 to 300 mm; a hot rolling step of performing hot rolling on the slab to obtain a hot-rolled steel sheet; a coiling step of coiling the hot-rolled steel sheet in a temperature range of 25° C. to 450° C.; a cold rolling step of performing cold rolling on the hot-rolled steel sheet at a rolling reduction of 30% or less to obtain a cold-rolled steel sheet as necessary; a first annealing step of performing first annealing on the hot-rolled steel sheet or the cold-rolled steel sheet; and a soaking step of holding the hot-rolled steel sheet or the cold-rolled steel sheet after the first annealing step in a temperature range of 260° C. to 450° C. for 10 to 1000 seconds, wherein, in the casting step, a solidification rate at a depth position of 10 mm from a surface of the molten steel is set to 100 to 1000° C./min, a molten steel casting amount per unit time is set to 2.0 to 6.0 tons/min, and cooling is performed at an average cooling rate set to 4° C./sec or higher between a liquidus temperature and a solidus temperature of a surface layer area at a depth position of 5 mm from the surface of the molten steel, in the hot rolling step, an average heating rate of the slab between Ac1 and Ac1+30° C. is 2 to 50° C./min, the hot rolling is performed after the slab is heated to 1200° C. or higher for 20 minutes or longer, and after completing finish rolling at a temperature of 850° C. or higher, cooling to 600° C. is performed at an average cooling rate of 10° C./sec or higher, and in the first annealing step, holding is performed at a highest heating temperature of Ac1+20° C. or higher and lower than Ac3 for 1 second to 1000 seconds, and then cooling to 250° C. or lower is performed.

6. The method for manufacturing the steel sheet according to claim 4, wherein, in at least one of the first annealing step and the second annealing step, when the holding at the highest heating temperature is performed for 1 second to 1000 seconds, an atmosphere log(PH.sub.2O/PH.sub.2) in a heating furnace is −1.10≤log(PH.sub.2O/PH.sub.2)≤−0.07.

7. The method for manufacturing the steel sheet according to claim 5, wherein, in the first annealing step, when the holding at the highest heating temperature is performed for 1 second to 1000 seconds, an atmosphere log(PH.sub.2O/PH.sub.2) in a heating furnace is −1.10≤log(PH.sub.2O/PH.sub.2)≤−0.07.

8. The method for manufacturing the steel sheet according to claim 4, further comprising: a hot-dip galvanizing step of immersing the cold-rolled steel sheet in a hot-dip galvanizing bath to obtain a hot-dip galvanized steel sheet during the cooling from the highest heating temperature to 250° C. or lower in the second annealing step before the soaking step.

9. The method for manufacturing the steel sheet according to claim 5, further comprising: a hot-dip galvanizing step of immersing the hot-rolled steel sheet or the cold-rolled steel sheet in a hot-dip galvanizing bath to obtain a hot-dip galvanized steel sheet during the cooling from the highest heating temperature to 250° C. or lower in the first annealing step before the soaking step.

10. The method for manufacturing the steel sheet according to claim 4, further comprising: a hot-dip galvanizing step of immersing the cold-rolled steel sheet after the soaking step in a hot-dip galvanizing bath to obtain a hot-dip galvanized steel sheet.

11. The method for manufacturing the steel sheet according to claim 5, further comprising: a hot-dip galvanizing step of immersing the hot-rolled steel sheet or the cold-rolled steel sheet after the soaking step in a hot-dip galvanizing bath to obtain a hot-dip galvanized steel sheet.

12. The steel sheet according to claim 2, further comprising: a plating layer on the surface.

13. The method for manufacturing the steel sheet according to claim 6, further comprising: a hot-dip galvanizing step of immersing the cold-rolled steel sheet in a hot-dip galvanizing bath to obtain a hot-dip galvanized steel sheet during the cooling from the highest heating temperature to 250° C. or lower in the second annealing step before the soaking step.

14. The method for manufacturing the steel sheet according to claim 7, further comprising: a hot-dip galvanizing step of immersing the hot-rolled steel sheet or the cold-rolled steel sheet in a hot-dip galvanizing bath to obtain a hot-dip galvanized steel sheet during the cooling from the highest heating temperature to 250° C. or lower in the first annealing step before the soaking step.

15. The method for manufacturing the steel sheet according to claim 6, further comprising: a hot-dip galvanizing step of immersing the cold-rolled steel sheet after the soaking step in a hot-dip galvanizing bath to obtain a hot-dip galvanized steel sheet.

16. The method for manufacturing the steel sheet according to claim 7, further comprising: a hot-dip galvanizing step of immersing the hot-rolled steel sheet or the cold-rolled steel sheet after the soaking step in a hot-dip galvanizing bath to obtain a hot-dip galvanized steel sheet.

Description

EXAMPLES

[0245] Next, examples of the present invention will be described. Conditions in the examples are one example of conditions adopted to confirm the feasibility and effects of the present invention. 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.

[0246] The transformation temperatures Ac1 (° C.) and Ac3 (° C.) used to limit the manufacturing conditions were calculated using the following formulas: (element symbols in the formulas indicate the mass % of the corresponding elements in steel)

Ac1=723−10.7×Mn−16.9×Ni+29.1×Si+16.9×Cr

Ac3=912−230.5×C+31.6×Si−20.4×Mn−39.8×Cu−18.1×Ni−14.8×Cr+16.8×Mo

[0247] Example 1 relates to inventions according to claims 1, 2, 3, 4, 6, 8, and 10 and show examples of conditions thereof, and Example 2 relates to inventions according to claims 1, 2, 3, 5, 7, 9, and 11 and show examples of conditions thereof.

Example 1

[0248] Steels having the chemical composition shown in Table 1 were cast to produce slabs having a thickness of 200 to 300 mm. In Table 1, the unit is mass %, and the remainder other than the components shown in Table 1 is Fe and impurities. These slab were subjected to hot rolling under the conditions shown in Tables 2-1 to 2-3 to manufacture hot-rolled steel sheets. During the hot rolling, rough rolling was performed so that the total rolling reduction at 1050° C. or higher became 60% to 90%, and finish rolling was performed by a plurality of rolling stands so that a finishing temperature became 800° C. or higher. Thereafter, the hot-rolled steel sheets were pickled to remove the scale on the surface. Thereafter, under the conditions shown in Tables 2-1 to 2-6, a cold rolling step, a first annealing step, a second annealing step, and a soaking step were performed. Under a condition in which hot-dip galvanizing or further alloying was performed before the soaking step or after the soaking step, hot-dip galvanizing is described.

TABLE-US-00001 TABLE 1 Kind of steel C Si Mn P S Al N O Ti V Nb Cr A 0.172 0.53 2.55 0.011 0.0021 0.030 0.0033 0.0011 0.11 B 0.225 1.85 2.34 0.009 0.0019 0.015 0.0021 0.0012 0.12 C 0.156 0.66 3.43 0.008 0.0031 0.713 0.0031 0.0013 D 0.214 1.52 2.22 0.013 0.0045 0.026 0.0041 0.0015 0.011 E 0.285 1.67 1.52 0.021 0.0022 0.033 0.0022 0.0017 0.031 F 0.156 2.05 1.71 0.011 0.0021 0.034 0.0022 0.0031 G 0.188 0.96 2.15 0.012 0.0033 0.512 0.0021 0.0021 H 0.152 0.12 2.96 0.010 0.0052 1.432 0.0025 0.0022 I 0.213 1.52 2.31 0.009 0.0015 0.032 0.0024 0.0024 J 0.388 0.98 1.65 0.015 0.0013 0.312 0.0033 0.0011 0.22 K 0.151 2.35 1.52 0.011 0.0015 0.387 0.0035 0.0012 L 0.195 1.11 2.35 0.012 0.0021 0.055 0.0041 0.0014 M 0.222 1.35 2.65 0.008 0.0032 0.422 0.0048 0.0015 0.042 N 0.195 1.85 2.72 0.011 0.0022 0.041 0.0021 0.0018 0.022 O 0.231 1.73 2.43 0.007 0.0022 1.450 0.0032 0.0008 0.021 P 0.091 1.65 2.34 0.008 0.0025 0.043 0.0039 0.0011 Q 0.196 0.31 2.46 0.013 0.0031 0.044 0.0031 0.0016 R 0.182 1.55 0.99 0.012 0.0035 0.195 0.0028 0.0021 S 0.190 1.21 3.65 0.007 0.0033 0.045 0.0029 0.0025 T 0.421 1.65 1.82 0.012 0.0041 0.041 0.0028 0.0024 U 0.172 2.65 2.10 0.009 0.0035 0.031 0.0034 0.0018 V 0.196 1.11 2.55 0.012 0.0021 1.755 0.0025 0.0011 W 0.173 0.53 2.54 0.012 0.0021 0.030 0.0036 0.0011 0.11 X 0.179 1.55 1.45 0.012 0.0035 0.195 0.0028 0.0021 Kind of Si + steel Ni Cu Co Mo W B Sn Sb Others Al A Bi: 0.0052 0.56 B 1.87 C 0.21 0.22 1.37 D 1.55 E 0.0021 1.70 F 0.21 0.05 2.08 G 0.11 REM: 0.00031 1.47 H Mg: 0.0052 1.55 I 0.05 0.06 1.55 J 0.22 Ca: 0.0031 1.29 K 0.41 0.06 Ce: 0.0049, 2.74 Zr: 0.0055 L Hf: 0.0039 1.17 M 0.0018 1.77 N 0.09 0.0021 1.89 O 0.21 0.0022 3.18 P 1.69 Q 0.35 R 1.75 S 1.26 T 1.69 U 2.68 V 2.87 W Bi: 0.0052, 0.56 La: 0.003 X 1.75 Bold underlines indicate outside of the range of the present invention. Blanks in the table indicate that the corresponding chemical element was not intentionally added.

TABLE-US-00002 TABLE 2-1 Casting conditions Average cooling rate between liquidus temperature Hot rolling step Solidification Molten and solidus Average Cold rate at steel temperature of heating rolling position of casting surface layer rate of slab Heating step 10 mm from amount area at depth (Ac1 to Heating time at Cold Kind surface of per unit position of 5 Ac1 + temperature 1200° C. Coiling rolling of molten steel time mm 30° C.) of slab higher temperature ratio No. steel ° C./min tons/min ° C./sec ° C./min ° C. min ° C. % 1 120 2.2 20 4.3 1230 81 650 52 2 A 130 2.4 24 4.6 1215 49 540 41 3 A 105 2.2 15 5.1 1224 72 580 43 4 A 130 3.5 12 6.4 1225 69 550 42 5 A 130 4.1 16 3.3 1224 68 720 44 6 B 160 4.2 19 4.5 1224 71 590 50 7 B 180 3.3 18 4.8 1224 75 580 52 8 B 170 3.4 12 4.0 1225 78 480 45 9 B 1035  2.5 21 — — — — — 10 C 110 2.4 22 4.6 1241 98 550 50 11 C 140 2.6 11 5.4 1233 74 540 48 12 C 160 2.2  8 6.1 1235 102  580 52 13 C  90 3.1  8 5.8 1245 127  575 52 14 D 160 4.1  9 4.7 1210 45 655 58 15 D 130 4.0 12 4.6 1231 89 630 52 16 D 170 3.1 19 4.1 1211 49 590 49 17 D 120 2.8  3 3.9 1231 83 555 52 18 D 110 1.8  8 4.5 1241 91 540 51 19 D 130 6.8  7 4.6 1222 81 495 53 20 E 120 2.1  7 4.8 1225 79 480 52 21 E 150 3.3 10 5.5 1226 91 670 52 22 E 120 3.4 12 5.6 1223 44 575 54 23 E 140 3.5 14 1.8 1241 91 580 55 24 F 110 3.6 22 4.2 1249 100  560 54 25 F 110 3.1 21 5.1 1251 88 540 48 26 F 120 4.4 13 4.8 1195 105  550 49 27 F 130 4.5 15 5.4 1215 15 570 55 28 G 140 3.2 16 5.6 1234 77 565 51 29 G 130 3.5 22 4.2 1223 91 570 52 30 G 120 3.8 13 4.2 1234 90 480 52 31 G 160 3.4  8 4.3 1241 108  380 35 32 H 140 3.5 11 4.2 1232 85 370 34 Bold underlines indicate outside of the range of the present invention.

TABLE-US-00003 TABLE 2-2 Casting conditions Average cooling rate between liquidus temperature Hot rolling step Solidification Molten and solidus Average Cold rate at steel temperature of heating rolling position of casting surface layer rate of slab Heating step 10 mm from amount area at depth (Ac1 to Heating time at Cold Kind surface of per unit position of 5 Ac1 + temperature 1200° C. Coiling rolling of molten steel time mm 30° C.) of slab higher temperature ratio No. steel ° C./min tons/min ° C./sec ° C./min ° C. min ° C. % 33 I 150 3.4 9 3.7 1243 89 250 35 34 J 130 3.3 14 3.8 1244 99  85 31 35 K 120 2.5 15 4.1 1238 109 440 44 36 K 120 2.6 16 3.8 1240 99 500 47 37 L 120 2.5 9 3.2 1241 107 540 49 38 M 130 2.6 10 4.8 1226 76 550 48 39 M 140 3.8 15 3.8 1228 81 535 46 40 M 130 4.1 19 3.7 1231 85 620 52 41 N 140 4.4 22 3.4 1227 99 610 51 42 N 120 2.3 11 3.5 1223 71 615 55 43 N 130 3.3 13 3.7 1219 75 480 45 44 O 120 2.6 14 — — — — — 45 P 150 3.4 13 4.1 1244 108 650 52 46 Q 130 3.1 14 4.2 1219 67 590 57 47 R 120 2.6 21 4.6 1244 99 625 59 48 S 120 2.5 22 4.5 1234 108 630 — 49 T 120 2.9 8 5.1 1211 45 585 — 50 U 130 2.1 7 — — — — — 51 V 140 2.5 11 — — — — — 52 A 120 2.2 20 4.3 1230 81 650 52 53 E 120 2.1 7 4.8 1225 79 480 52 54 E 120 2.1 7 4.8 1225 79 480 52 55 E 120 2.1 7 4.8 1225 79 480 52 56 E 120 2.1 7 4.8 1225 79 480 52 57 A 120 2.2 20 4.3 1230 81 650 52 58 A 120 2.2 20 4.3 1230 81 650 52 59 B 170 3.4 12 4.0 1225 78 480 45 60 B 170 3.4 12 4.0 1225 78 480 45 61 N 120 2.3 11 3.5 1223 71 615 55 62 G 130 3.5 22 4.2 1223 91 570 52 63 W 120 2.3 20 4.3 1231 81 650 52 64 X 120 2.7 21 4.6 1245 99 625 59

TABLE-US-00004 TABLE 2-3 First annealing step Heating Cooling Second annealing step Average Average Heating heating cooling rate Holding Cooling rate Holding between time Cooling stop between time highest between temperature Ac1 and Highest between heating Cooling Highest Ac1 + from highest Kind Ac1 + heating Ac3 to temperature stop heating 20° C. to heating of 30° C. temperature 950° C. and 650° C. temperature temperature Ac3° C. temperature No. steel ° C./sec ° C. sec ° C./sec ° C. ° C. sec ° C. 1 A 4.4 845 121 5.5 265 785 170 240 2 A 5.2 850 145 6.7 270 765 180 235 3 A 5.4 860 215 9.3 470 755 220 210 4 A 4.8 880 95 4.7 330 765 150 225 5 A 4.4 890 130 5.8 235 760 175 220 6 B 4.8 875 134 6.1 220 780 180 225 7 B 3.8 880 145 6.0 210 755 175 230 8 B 4.4 875 155 6.7 195 795 150 235 9 B — — — — — — — — 10 C 4.4 830 152 6.6 230 750 190 195 11 C 5.1 835 134 6.3 430 770 150 275 12 C 4.6 840 89 4.4 250 835 120 195 13 C 4.2 830 205 8.3 310 780 190 190 14 D 5.1 870 139 6.4 250 805 160 210 15 D 4.9 830 148 6.7 265 810 165 215 16 D 5.1 885 144 6.6 310 820 155 220 17 D 5.2 890 156 7.1 245 780 145 180 18 D 4.9 880 164 7.2 260 790 165 185 19 D 3.8 880 172 7.0 275 780 135 170 20 E 3.7 870 190 7.5 280 780 190 165 21 E 4.9 875 218 9.1 290 785 205 220 22 E 4.8 880 105 5.1 320 790 210 265 23 E 4.4 880 192 8.0 335 790 180 230 24 F 4.8 920 182 7.8 315 805 185 215 25 F 4.9 910 161 7.1 185 810 195 215 26 F 4.7 910 167 7.2 215 800 165 220 27 F 4.9 915 130 6.0 180 790 170 235 28 G 4.8 860 197 8.3 225 795 205 240 29 G 4.4 865 199 8.2 265 780 190 230 30 G 4.4 880 210 0.8 245 785 195 220 31 G 0.4 870 130 6.0 250 810 165 225 32 H 4.9 840 150 6.7 280 790 175 195 Bold underlines indicate outside of the range of the present invention.

TABLE-US-00005 TABLE 2-4 First annealing step Second annealing step Heating Cooling Average Average Heating heating cooling rate Holding Cooling rate Holding between time Cooling stop between time highest between temperature Ac1 and Highest between heating Cooling Highest Ac1 + from highest Kind Ac1 + heating Ac3 to temperature stop heating 20° C. to heating of 30° C. temperature 950° C. and 650° C. temperature temperature Ac3° C. temperature No. steel ° C./sec ° C. sec ° C./sec ° C. ° C. sec ° C. 33 I 4.8 880 160 7.0 290 780 205 190 34 J 5.0 840 130 6.1 235 785 220 195 35 K 5.2 920 124 6.0 250 790 155 175 36 K 5.1 915 130 5.8 265 795 160 175 37 L 5.1 865 134 6.3  45 785 170 190 38 M 4.4 860  99 4.7 175 790 160 185 39 M 3.8 875 137 5.7 165 780 195 210 40 M 3.8 880 204 8.1 280 740 220 225 41 N 3.9 885 215 8.5 240 805 210 230 42 N 4.2 880 230 9.2 230 810 195 235 43 N 4.2 890 210 8.5 240 795 210 270 44 O — — — — — — — — 45 P 3.8 910 195 7.8 260 790 205 220 46 Q 3.7 840 140 5.8 240 785 195 235 47 R 3.8 905 144 6.0 250 805 170 225 48 S — — — — — — — — 49 T — — — — — — — — 50 U — — — — — — — — 51 V — — — — — — — — 52 A 4.4 847 122 5.5 265 788 173 240 53 E 3.7 873 191 7.5 280 781 191 165 54 E 3.7 874 194 7.5 280 782 193 165 55 E 3.7 873 191 7.5 280 781 191 165 56 E 3.7 873 191 7.5 280 781 191 165 57 A 4.4 854 130 5.5 265 794 179 240 58 A 4.4 856 132 5.5 265 796 181 240 59 B 4.4 875 155 6.7 195 795 152 241 60 B 4.4 875 155 6.7 195 795 152 241 61 N 4.2 880 230 9.2 230 810 194 225 62 G 4.4 865 199 8.2 266 779 190 232 63 W 4.4 844 121 5.5 264 785 170 241 64 X 3.8 904 144 6.0 249 805 170 226 Bold underlines indicate outside of the range of the present invention.

TABLE-US-00006 TABLE 2-5 Soaking step Hot-dip Holding Atmosphere in heating furnace galvanizing temperature Holding First Second Hot-dip after time after annealing annealing galvanizing Kind stopping stopping step step before soaking step of cooling cooling log(PH.sub.2O/H.sub.2) log(PH.sub.2O/H.sub.2) or after soaking step No. steel ° C. sec — — — Note 1 A 395 390 −1.70 −1.72 — Example of Present Invention 2 A 385 410 −1.69 −0.73 Hot-dip Example of Present galvanizing Invention 3 A 360 380 −1.63 −1.65 — Comparative Example 4 A 470 375 −1.79 −1.72 — Comparative Example 5 A 385 350 −0.72 −0.68 Hot- dip galvanizing Comparative Example 6 B 375 360 −1.65 −1.63 — Example of Present Invention 7 B 380 410 −0.73 −1.71 — Comparative Example 8 B 410 420 −1.68 −1.73 — Example of Present Invention 9 B — — — — — Comparative Example 10 C 390 340 −0.69 −0.73 Hot-dip Example of Present galvanizing Invention 11 C 350 330 −0.67 −0.66 Hot-dip Comparative Example galvanizing 12 C 365 310 −1.66 −0.72 Hot-dip Comparative Example galvanizing 13 C 350 380 −1.65 −1.66 — Comparative Example 14 D 375 375 −0.73 −1.68 — Example of Present Invention 15 D 380 365 −1.68 −1.66 — Comparative Example 16 D 240 310 −1.71 −0.71 Hot-dip Comparative Example galvanizing 17 D 355 350 −0.69 −0.72 Hot-dip Comparative Example galvanizing 18 D 360 340 −0.68 −0.73 — Comparative Example 19 D 390 330 −1.68 −0.74 — Comparative Example 20 E 375 350 −1.71 −1.65 — Example of Present Invention 21 E 380 365 −0.69 −0.73 Hot-dip Example of Present galvanizing Invention 22 E 370 380 −1.69 −0.73 Hot-dip Comparative Example galvanizing 23 E 405 350 −1.65 −1.66 — Comparative Example 24 F 375 250 −1.71 −1.73 — Example of Present Invention 25 F 240 210 −1.71 −1.71 — Comparative Example 26 F 380 290 −1.69 −1.70 — Comparative Example 27 F 395 380 −1.66 −1.65 — Comparative Example 28 G 400 400 −1.71 −1.66 — Example of Present Invention 29 G 405 410 −1.69 −0.73 Hot-dip Example of Present galvanizing Invention 30 G 410 380 −1.67 −0.68 Hot-dip Comparative Example galvanizing 31 G 390 380 −1.66 −1.72 — Comparative Example 32 H 395 340 −1.71 −0.73 — Example of Present Invention Bold underlines indicate outside of the range of the present invention.

TABLE-US-00007 TABLE 2-6 Soaking step Hot-dip Holding Atmosphere in heating furnace galvanizing temperature Holding First Second Hot-dip after time after annealing annealing galvanizing Kind stopping stopping step step before soaking step of cooling cooling log(PH.sub.2O/H.sub.2) log(PH.sub.2O/H.sub.2) or after soaking step No. steel ° C. sec — — — Note 33 I 380 345 −1.65 −1.66 — Example of Present Invention 34 J 375 360 −0.71 −0.68 Hot-dip Example of Present galvanizing Invention 35 K 390 330 −1.72 −0.71 Hot-dip Example of Present galvanizing Invention 36 K 300 550 −1.70 −0.70 Hot-dip Example of Present galvanizing Invention 37 L 380 320 −1.72 −0.77 Hot-dip Example of Present galvanizing Invention 38 M 395 310 −0.69 −0.71 Hot-dip Example of Present galvanizing Invention 39 M 470 290 −1.72 −1.66 — Comparative Example 40 M 385 295 −1.69 −1.72 — Comparative Example 41 N 390 390 −0.69 −0.73 — Example of Present Invention 42 N 385 395 −0.71 −0.73 Hot-dip Example of Present galvanizing Invention 43 N 395 340 −0.69 −0.73 Hot-dip Comparative Example galvanizing 44 O — — — — — Comparative Example 45 P 415 360 −1.72 −1.60 — Comparative Example 46 Q 405 210 −1.72 −0.71 Hot-dip Comparative Example galvanizing 47 R 415 175 −0.69 −0.72 Hot-dip Comparative Example galvanizing 48 S — — — — — Comparative Example 49 T — — — — — Comparative Example 50 U — — — — — Comparative Example 51 V — — — — — Comparative Example 52 A 395 390 −1.70 −1.72 — Example of Present Invention 53 E 375 350 −1.71 −1.65 — Example of Present Invention 54 E 255 350 −1.71 −1.65 — Comparative Example 55 E 375 350 −1.71 −1.65 — Example of Present Invention 56 E 375 350 −1.71 −1.65 — Example of Present Invention 57 A 395 390 −1.70 −1.72 — Example of Present Invention 58 A 455 390 −1.70 −1.72 — Comparative Example 59 B 410 420 −1.68 −1.73 — Example of Present Invention 60 B 410 420 −1.68 −1.73 — Example of Present Invention 61 N 385 395 −0.71 −0.73 Hot-dip Example of Present galvanizing Invention 62 G 405 410 −1.69 −0.73 Hot-dip Example of Present galvanizing Invention 63 W 395 391 −1.70 −1.72 — Example of Present Invention 64 X 415 176 −0.69 −0.72 Hot-dip Comparative Example galvanizing Bold underlines indicate outside of the range of the present invention.

[0249] Next, for each of the steel sheets obtained in this manner, a microstructure in a range of a ⅛ thickness to a ⅜ thickness centered on a ¼ thickness position from the surface was observed by the above-described method, and volume fractions of ferrite, residual austenite, bainite, tempered martensite, fresh martensite, and pearlite were examined.

[0250] In addition, by the above-described method, a number proportion of residual austenite having an aspect ratio of 2.0 or more in the entire residual austenite in the range of the ⅛ thickness to the ⅜ thickness centered on the ¼ thickness position from the surface were examined.

[0251] Furthermore, by the above-described method, a number density of inclusions and precipitates having a grain size of 1 μm or more in the range of the ⅛ thickness to the ⅜ thickness centered on the ¼ thickness position from the surface, an average interval between Mn-concentrated portions at a 1/20 depth position of a sheet thickness from the surface, and a standard deviation of Mn concentrations in the residual austenite were examined.

[0252] For mechanical properties, a JIS No. 5 tensile test piece was collected from a direction perpendicular to a rolling direction, a tensile test was performed according to JIS 22241 (2011), and tensile strength (TS) and total elongation (El) were measured. A gauge length was set to 50 mm, and a tension speed was set to 10 mm/min.

[0253] In addition, a hole expansion ratio (λ) was measured by performing “JFS T 1001 Method of hole expanding test” of The Japan Iron and Steel Federation Standard. A punch diameter was set to 10 mm, a punching clearance was set to 12%, and a punch shape was set to a conical punch with a 60° tip end portion.

[0254] Those having a TS of 980 MPa or more and a TS×El×λ.sup.0.5/1000 of 80 or more were determined to have good mechanical properties and preferable press formability for use as vehicle component.

[0255] Toughness after introduction of plastic strain (toughness after press forming) was evaluated by the following method. A JIS No. 5 tensile test piece was collected from the direction perpendicular to the rolling direction, 5% plastic strain (prestrain) was applied by the tensile test, and a heat treatment of 170° C.×20 minutes was performed to simulate age hardening due to a heat input during baking coating. A Charpy test piece with a 2 mm V notch was collected from a parallel portion of the tensile test piece after the application of strain and the heat treatment simulating baking coating. Thereafter, a Charpy test was performed at a test temperature of −20° C. according to JIS Z 2242 (2018). Those in which Charpy absorbed energy after the application of strain/Charpy absorbed energy before the application of plastic strain exceeded 0.7 were determined to be Very Good (VG), those of 0.5 to 0.7 were determined to be Good (G), and those of less than 0.5 were determined to be Bad (B). The evaluation of VG and G was regarded as acceptable.

[0256] In addition, in order to measure a ratio of a Vickers hardness Hv.sub.sur at a depth position of 30 μm from the surface to a Vickers hardness [Hv] at a ¼ depth position of the sheet thickness from the surface, three Vickers hardnesses were measured with a load set to 20 g at the depth position of a sample with an embedded cross section according to JIS Z 2244 (2009), and Hv.sub.sur/[Hv] was obtained from each average value.

[0257] These results are shown in Tables 3-1 to 3-4.

[0258] In the tables, a cold-rolled steel sheet that had not been galvanized is indicated as CR, a hot-dip galvanized steel sheet is indicated as GI, and a galvannealed steel sheet is indicated as GA.

TABLE-US-00008 TABLE 3-1 Microstructure (⅛ to ⅜ thickness position) Number proportion of residual austenite Kind Residual Tempered Fresh having aspect ratio of of Steel Ferrite austenite Bainite martensite martensite Pearlite 2.0 or more No. steel sheet % % % % % % % 1 A CR 21 7 43 26 3 0 72 2 A GA 26 10 37 22 5 0 65 3 A CR 23 5 42 15 15 0 35 4 A CR 27 4 17 29 17 6 45 5 A GA 25 5 27 24 12 7 48 6 B CR 18 19 25 36 2 0 75 7 B CR 55 4 11 15 12 3 25 8 B CR 33 21 28 15 3 0 68 9 B — — — — — — — — 10 C GA 25 10 28 35 2 0 62 11 C GA 22 7 37 19 13 2 57 12 C GA 12 5 22 55 5 1 55 13 C CR 24 10 30 34 2 0 65 14 D CR 26 21 28 22 2 1 71 15 D CR 25 5 31 24 12 3 34 16 D GA 32 5 45 13 9 1 65 17 D GA 28 18 29 22 1 2 62 18 D CR 25 14 36 21 3 1 67 19 D CR 22 15 40 20 2 1 65 20 E CR 19 22 13 42 4 0 68 21 E GA 17 21 16 39 5 2 65 22 E GA 16 8 43 15 15 3 62 23 E CR 19 9 40 17 13 2 72 24 F CR 26 14 30 26 3 1 69 25 F CR 25 8 37 15 12 3 72 26 F CR 24 12 26 25 11 2 65 27 F CR 33 12 24 15 13 3 66 28 G CR 30 16 17 35 2 0 72 29 G GA 31 14 13 33 6 3 76 30 G GA 41 5 17 23 12 2 48 31 G CR 32 6 29 19 11 3 68 32 H CR 28 18 23 25 4 2 65 Bold underlines indicate outside of the range of the present invention.

TABLE-US-00009 TABLE 3-2 Microstructure (⅛ to ⅜ thickness position) Number proportion of residual austenite Kind Residual Tempered Fresh having aspect ratio of of Steel Ferrite austenite Bainite martensite martensite Pearlite 2.0 or more No. steel sheet % % % % % % % 33 I CR 29 19 20 27 4 1 69 34 J GI 22 21 14 35 6 2 68 35 K GA 31 12 17 34 5 1 65 36 K GA 30 9 19 39 3 0 62 37 L GA 32 19 21 25 3 0 79 38 M GI 22 18 24 34 2 0 68 39 M CR 21 5 26 31 11 6 72 40 M CR 35 5 23 22 14 1 45 41 N CR 23 16 21 33 5 2 71 42 N GA 19 18 26 31 6 0 65 43 N GA 23 5 30 21 18 3 65 44 O — — — — — — — — 45 P CR 35 3 12 45 3 2 48 46 Q GA 23 2 34 33 2 6 45 47 R GA 62 5 21  3 2 7 44 48 S — — — — — — — — 49 T — — — — — — — — 50 U — — — — — — — — 51 V — — — — — — — — 52 A CR 21 6 46 24 3 0 72 53 E CR 19 29  6 42 4 0 68 54 E CR 19 32  3 42 4 0 68 55 E CR 19 27  8 42 4 0 50 56 E CR 19 25 10 42 4 0 68 57 A CR 16 6 60 16 2 0 72 58 A CR 15 5 62 16 2 0 72 59 B CR 37 23 34  5 1 0 68 60 B CR 35 22 32 10 1 0 68 61 N GA 18 17 24 31 10 0 65 62 G GA 31 14 12 33 6 4 76 63 W CR 21 7 44 26 2 0 72 64 X GA 57 5 26  3 2 7 44 Bold underlines indicate outside of the range of the present invention.

TABLE-US-00010 TABLE 3-3 Inclusions and Mechanical properties precipitates Toughness Number after density of Mn segregation application inclusions Average of 5% and interval prestrain precipitates between and heat having Mn- Standard Press formability treatment grain size concentrated deviation of TS* of 170° Hardness Kind of 1 μm or portions at Mn El* C. × 20 Hv.sub.sur/ of Steel more 1/20 depth concentrations TS El λ λ.sup.0.5/ minutes [Hv] No. steel sheet /mm.sup.2 μm % MPa % % 1000 — — Note 1 A CR 15 220 0.21 995 19.7 39 122 G 0.89 Example of Present Invention 2 A GA 17 200 0.19 985 21.5 25 106 VG 0.76 Example of Present Invention 3 A CR 19 210 0.22 1052 14.0 20 66 B 0.88 Comparative Example 4 A CR 12 205 0.19 1035 14.2 18 62 B 0.91 Comparative Example 5 A GA 16 220 0.43 1044 12.9 25 67 B 0.75 Comparative Example 6 B CR 12 195 0.22 1055 23.5 35 147 G 0.92 Example of Present Invention 7 B CR 13 210 0.22 975 17.2 18 71 B 0.78 Comparative Example 8 B CR 11 190 0.18 1029 24.2 21 114 G 0.91 Example of Present Invention 9 B — — — — — — — — — — Comparative Example 10 C GA 10 210 0.24 995 18.0 31 100 VG 0.76 Example of Present Invention 11 C GA 9 190 0.21 1045 15.0 23 75 B 0.77 Comparative Example 12 C GA 9 185 0.20 1210 11.0 35 79 VG 0.75 Comparative Example 13 C CR 25 315 0.45 1062 17.0 18 77 B 0.92 Comparative Example 14 D CR 12 220 0.23 1021 22.3 35 135 VG 0.76 Example of Present Invention 15 D CR 9 220 0.23 1055 13.2 19 61 B 0.86 Comparative Example 16 D GA 10 205 0.21 1065 14.5 16 62 VG 0.76 Comparative Example 17 D GA 35 225 0.23 1022 21.1 19 94 B 0.77 Comparative Example 18 D CR 32 205 0.22 998 23.2 18 98 B 0.76 Comparative Example 19 D CR 41 185 0.18 997 22.5 19 98 B 0.75 Comparative Example 20 E CR 12 205 0.21 1210 15.2 35 109 G 0.86 Example of Present Invention 21 E GA 11 210 0.23 1224 14.8 38 112 VG 0.79 Example of Present Invention 22 E GA 12 200 0.20 1294 10.2 19 58 B 0.75 Comparative Example 23 E CR 12 205 0.42 1264 10.5 21 61 B 0.82 Comparative Example 24 F CR 13 195 0.19 985 24.1 25 119 G 0.85 Example of Present Invention 25 F CR 11 200 0.21 1024 13.5 22 65 B 0.89 Comparative Example 26 F CR 13 205 0.41 1055 14.5 21 70 B 0.82 Comparative Example 27 F CR 12 210 0.42 1032 13.5 26 71 B 0.88 Comparative Example 28 G CR 9 195 0.21 998 21.6 31 120 G 0.89 Example of Present Invention 29 G GA 13 210 0.26 1003 22.5 35 134 VG 0.75 Example of Present Invention 30 G GA 10 205 0.24 1051 16.1 18 72 B 0.77 Comparative Example 31 G CR 12 210 0.43 1025 15.4 19 69 B 0.86 Comparative Example 32 H CR 13 190 0.25 1021 19.5 28 105 VG 0.75 Example of Present Invention Bold underlines indicate outside of the range of the present invention.

TABLE-US-00011 TABLE 3-4 Inclusions Mechanical properties and Toughness precipitates after Number Mn segregation application density of Average of 5% inclusions interval prestrain and between and heat precipitates Mn- Standard Press formability treatment having concentrated deviation of TS* of 170° Hardness Kind grain size portions at Mn El* C. × 20 Hv.sub.sur/ of Steel of 1 μm or 1/20 depth concentrations TS El λ λ.sup.0.5/ minutes [Hv] No. steel sheet more μm % MPa % % 1000 — — Note 33 I CR 12 170 0.18 986 24.3 35 141 G 0.88 Example of Present Invention 34 J GI 11 195 0.22 1195 16.2 32 110 VG 0.77 Example of Present Invention 35 K GA 12 205 0.18 995 17.5 42 113 VG 0.79 Example of Present Invention 36 K GA 11 195 0.21 1020 16.8 40 108 VG 0.79 Example of Present Invention 37 L GA 11 210 0.21 1006 24.2 35 144 VG 0.75 Example of Present Invention 38 M GI  9 190 0.22 1222 15.2 45 125 VG 0.77 Example of Present Invention 39 M CR 10 205 0.22 1105 12.2 25 67 B 0.89 Comparative Example 40 M CR 13 210 0.22 1075 10.2 22 51 B 0.88 Comparative Example 41 N CR 10 190 0.21 1195 14.8 45 119 VG 0.77 Example of Present Invention 42 N GA  9 195 0.22 1203 14.9 35 106 VG 0.79 Example of Present Invention 43 N GA 11 210 0.22 1272 11.1 32 79 B 0.75 Comparative Example 44 O — — — — — — — — — — Comparative Example 45 P CR 12 205 0.19 891 16.1 30 79 G 0.89 Comparative Example 46 Q GA 11 200 0.18 1085 11.9 32 73 VG 0.75 Comparative Example 47 R GA 15 195 0.16 921 21.0 35 114 VG 0.77 Comparative Example 48 S — — — — — — — — — — Comparative Example 49 T — — — — — — — — — — Comparative Example 50 U — — — — — — — — — — Comparative Example 51 V — — — — — — — — — — Comparative Example 52 A CR 15 220 0.21 999 19.7 39 123 G 0.89 Example of Present Invention 53 E CR 12 205 0.21 1212 15.4 37 114 G 0.86 Example of Present Invention 54 E CR 12 205 0.21 1211 15.2 12 64 B 0.86 Comparative Example 55 E CR 12 205 0.21 1215 15.9 35 114 G 0.86 Example of Present Invention 56 E CR 30 205 0.21 1208 16.6 35 119 G 0.86 Example of Present Invention 57 A CR 15 300 0.21 1003 13.8 47 95 G 0.89 Example of Present Invention 58 A CR 15 220 0.21 980 12.8 39 78 G 0.89 Comparative Example 59 B CR 11 190 0.35 1039 24.4 28 134 VG 0.78 Example of Present Invention 60 B CR 11 190 0.18 1037 24.5 25 127 VG 0.77 Example of Present Invention 61 N GA  9 195 0.22 1208 14.9 30 99 VG 0.79 Example of Present Invention 62 G GA 13 210 0.26 1004 22.6 37 138 VG 0.75 Example of Present Invention 63 W CR 15 220 0.21 992 19.7 38 120 G 0.89 Example of Present Invention 64 X GA 15 195 0.16 920 21.6 34 116 VG 0.77 Comparative Example

[0259] The steel sheets of the examples of the present invention were steel sheets having high strength, a good balance between strength, elongation, and hole expansibility, and little deterioration in Charpy absorbed energy after the plastic deformation (the application of prestrain) and the heat treatment of 170° C.×20 minutes simulating the baking coating.

[0260] On the other hand, in comparative examples, steel sheets could not be manufactured, or steel sheets were inferior in any of strength, a balance between strength, elongation, and hole expansibility, and Charpy absorbed energy after the plastic deformation (the application of prestrain) and the heat treatment 170° C.×20 minutes simulating the baking coating.

[0261] In Example No. 9, since the solidification rate was high at a position of 10 mm from the surface of the molten steel, and cracks occurred in the slab, steps subsequent to the hot rolling could not be performed.

[0262] In Example No. 13, since the solidification rate at a position of 10 mm from the surface of the molten steel was slow, the average interval between the Mn-concentrated portions at the 1/20 depth was wide, and as a result, the standard deviation of the Mn concentrations was high, in addition to the deterioration of the balance of mechanical properties, the deterioration of the Charpy absorbed energy also increased.

[0263] In Example No. 18, since the molten steel casting amount per unit time was small, the trapping of inclusions and precipitates in the vicinity of the surface layer of the slab during casting increased, and the Charpy absorbed energy deteriorated.

[0264] In Example No. 19, since the molten steel casting amount per unit time was large, the trapping of inclusions and precipitates in the vicinity of the surface layer of the slab due to entrainment of mold powder increased. Therefore, the Charpy absorbed energy deteriorated.

[0265] In Example No. 17, since the average cooling rate between the liquidus temperature and the solidus temperature of the surface layer area at a depth position of 5 mm of the molten steel was too low, grain growth of inclusions and precipitates progressed during the solidification process, and the Charpy absorbed energy deteriorated.

[0266] In Example No. 23, since the heating rate of the slab from Ac1 to Ac1+30° C. was too low, distribution of Mn at α/γ during an increase in temperature progressed, and the standard deviation of the Mn concentrations increased. As a result, a decrease in martensitic transformation temperature due to partial Mn concentration occurred and the volume fraction of fresh martensite increased. As a result, the balance of mechanical properties deteriorated, and the Charpy absorbed energy also deteriorated.

[0267] In Example No. 26, since the slab heating temperature was low, Mn was not sufficiently uniform, and in addition to an increase in the volume fraction of fresh martensite, the standard deviation of the Mn concentrations increased. As a result, the balance of mechanical properties deteriorated, and the Charpy absorbed energy also deteriorated.

[0268] In Example No. 27, since the heating time of the slab at 1200° C. or higher was short, Mn was not sufficiently uniform and the standard deviation of the Mn concentrations increased. As a result, the balance of mechanical properties deteriorated, and the Charpy absorbed energy also deteriorated.

[0269] In Example No. 5, since the coiling temperature in the hot rolling was high, the distribution of Mn progressed in phase transformation after the coiling, and the standard deviation of the Mn concentrations increased. As a result, the balance of mechanical properties deteriorated, and the Charpy absorbed energy also deteriorated.

[0270] In Example No. 31, since the average heating rate between Ac1 and Ac1+30° C. in the first annealing step was low, the standard deviation of the Mn concentrations increased. As a result, the balance of mechanical properties deteriorated, and the Charpy absorbed energy also deteriorated.

[0271] In Example No. 15, since the highest heating temperature in the first annealing step was low, the microstructure after the first annealing step did not become a lath-like structure over the entire surface. Therefore, the volume fraction of residual austenite was insufficient and the volume fraction of fresh martensite was increased, which deteriorated the balance of mechanical properties and also deteriorated the Charpy absorbed energy.

[0272] In Example No. 30, since the average cooling rate from the highest heating temperature to 650° C. in the first annealing step was low, ferritic transformation occurred during cooling, and the microstructure after the first annealing step did not become a lath-like structure over the entire surface. As a result, the balance of mechanical properties deteriorated, and the Charpy absorbed energy also deteriorated.

[0273] In Example No. 3, since the cooling stop temperature in the first annealing step was high, a complete lath-like structure could not be obtained during the subsequent holding, and the volume fraction of fresh martensite in the final product increased, or the number proportion of residual austenite having an aspect ratio of 2.0 or more decreased. As a result, the balance of mechanical properties deteriorated, and the Charpy absorbed energy also deteriorated.

[0274] In Example Nos. 7 and 40, since the highest heating temperature in the second annealing step was low, the amount of residual austenite from which a final product could be obtained decreased, the balance of mechanical properties deteriorated, and the Charpy absorbed energy also deteriorated.

[0275] In Example No. 12, since the highest heating temperature in the second annealing step was high, the lath-like structure formed in the first annealing step collapsed and the desired microstructure could not be obtained. As a result, the balance of mechanical properties deteriorated.

[0276] In Example Nos. 11, 22, and 43, since the cooling stop temperature in the second annealing step was high, the volume fraction of tempered martensite decreased. As a result, bainitic transformation did not sufficiently progress during subsequent holding, and the volume fraction of fresh martensite generated after final cooling increased. As a result, the balance of mechanical properties deteriorated, and the Charpy absorbed energy also deteriorated.

[0277] In Example Nos. 4 and 39, since the holding temperature after stopping cooling in the soaking step was high, cementite was generated in residual austenite exposed to a high temperature, resulting in instability. Therefore, the volume fraction of residual austenite that could be finally obtained decreased. As a result, the balance of mechanical properties deteriorated, and the Charpy absorbed energy also deteriorated.

[0278] In Example Nos. 16 and 25, since the holding temperature after stopping cooling in the soaking step was low, bainitic transformation did not sufficiently progress during holding, and the amount of fresh martensite generated after final cooling increased. As a result, the balance of mechanical properties deteriorated. In Example 25, furthermore, the Charpy absorbed energy also deteriorated.

[0279] In Example No. 54, since the holding temperature after stopping cooling in the soaking step was low, the volume fraction of bainite was insufficient. In addition, the volume fraction of residual austenite was also excessive, which deteriorated the balance of mechanical properties.

[0280] In Example No. 58, since the holding temperature after stopping cooling in the soaking step was high, bainitic transformation progressed excessively, resulting in an excessive volume fraction of bainite. Therefore, the balance of mechanical properties deteriorated.

[0281] In Example No. 44, since the total amount of Si and Al was large, cracks occurred in the slab. Therefore, steps subsequent to the hot rolling could not be performed.

[0282] In Example No. 45, since the C content was low, the Ms point did not decrease significantly, and as a result, the volume fraction of residual austenite was small. In addition, the number proportion of residual austenite having an aspect ratio of 2.0 or more was small. Therefore, the balance of mechanical properties deteriorated.

[0283] In Example No. 46, since the total amount of Si and Al was small, carbon concentration in untransformed austenite was small. As a result, the volume fraction of residual austenite was small, the volume fraction of pearlite was large, and the number proportion of residual austenite having an aspect ratio 2.0 or more was small. Therefore, the balance of mechanical properties deteriorated.

[0284] In Example Nos. 47 and 64, since the Mn content was low, the Ms point did not decrease. As a result, the volume fractions of ferrite and pearlite were large, the volume fractions of residual austenite and tempered martensite were large, and the number proportion of residual austenite having an aspect ratio 2.0 or more was small. Therefore, the tensile strength was insufficient.

[0285] In Example No. 48, since the Mn content was high, cracks occurred in the steel sheet during the cold rolling step. Therefore, subsequent steps could not be performed.

[0286] In Example No. 49, since the C content was high, cracks occurred in the steel sheet during the cold rolling step. Therefore, subsequent steps could not be performed.

[0287] In Example 50, since the Si content was high, cracks occurred in the slab. Therefore, steps subsequent to the hot rolling could not be performed.

[0288] In Example No. 51, since the Al content was high, cracks occurred in the slab. Therefore, steps subsequent to the hot rolling could not be performed.

[0289] In Example Nos. 2, 10, 14, 21, 29, 32, 34, 35, 36, 37, 38, 41, 42, 61, and 62, since the oxygen potential was high in one or both of the first annealing step or the second annealing step, internal oxidation occurred in the vicinity of the surface layer of the steel sheet, and at the same time, decarburization progressed, resulting in softening. Therefore, the ratio of the Vickers hardness Hv.sub.sur at a depth position of 30 μm from the surface to the Vickers Hardness [Hv] at a ¼ depth position of the sheet thickness from the surface decreased, and the bendability was improved, so that the Charpy absorbed energy was improved.

Example 2

[0290] Using the same slab as in Example 1, hot rolling was performed under the conditions shown in Tables 4-1 and 4-2 to manufacture hot-rolled steel sheets. Thereafter, the hot-rolled steel sheets were pickled to remove the scale on the surface. During the hot rolling, rough rolling was performed so that the total rolling reduction at 1050° C. or higher became 60% to 90%. Thereafter, under the conditions shown in Tables 4-3 and 4-4, cold rolling, a first annealing step, and a soaking step were performed. Under a condition in which hot-dip galvanizing was performed before the soaking step or after the soaking step, hot-dip galvanizing is described. In addition, some of the hot-rolled steel sheets after the pickling were subjected to cold rolling at a rolling reduction of 2% for shape correction.

TABLE-US-00012 TABLE 4-1 Casting conditions Average cooling rate between Hot rolling step liquidus Average Solidification Molten temperature and heating rate at steel solidus rate of position of casting temperature of slab Heating Average 10 mm from amount surface layer area (Ac1 to Heating time at Finish cooling Kind surface of per unit at depth position Ac1 + temperature 1200° C. rolling end rate to Coiling of molten steel time of 5 mm 30° C.) of slab or higher temperature 600° C. temperature No. steel ° C./min tons/min ° C./sec ° C./min ° C. min ° C. ° C./sec ° C. 65 A 130 2.2 20 4.5 1230 85 870 75 380 66 A 130 2.4 24 4.6 1235 91 875 85 350 67 A 105 2.2 15 4.4 1220 60 830 80 370 68 A 130 3.5 12 5.1 1215 55 910 95 380 69 A 130 4.1 16 5.5 1225 78 890 70 390 70 B 160 4.2 19 5.6 1235 95 870 65 395 71 B 180 3.3 18 4.8 1230 80 910 8: 355 72 B 170 3.4 12 6.1 1220 68 920 95 475 73 B 1035 2.5 21 — — — — — — 74 C 110 2.4 22 4.5 1215 55 940 70 355 75 C 140 2.6 11 4.6 1210 60 935 75 360 76 C 160 2.2 8 5.1 1240 120 940 60 355 77 C 90 3.1 8 5.5 1230 95 910 80 380 78 D 160 4.1 9 5.8 1235 98 890 95 360 79 D 150 3.9 10 6.3 1240 105 880 85 320 80 D 130 4.0 12 5.1 1240 105 895 100 380 81 D 170 3.1 19 4.8 1235 93 905 95 375 82 D 120 2.8 3 4.9 1220 65 910 90 380 83 D 110 1.8 8 6.2 1220 68 920 80 385 84 D 130 6.8 7 6.3 1215 68 915 85 390 85 E 120 2.1 7 5.4 1210 45 915 80 370 86 E 150 3.3 10 5.9 1205 38 920 85 375 87 E 120 3.4 12 4.8 1210 48 890 85 510 88 E 140 3.5 14 6.7 1205 17 895 65 350 89 F 110 3.6 22 6.8 1210 18 900 75 340 90 F 110 3.1 21 5.5 1230 81 905 70 330 91 F 120 4.4 13 4.6 1235 90 915 70 290 92 F 130 4.5 15 1.8 1240 108 870 75 380 93 G 140 3.2 16 5.5 1245 135 905 80 350 94 G 130 3.5 22 6.8 1250 105 950 95 360 95 G 120 3.8 13 6.4 1245 115 920 90 380 96 G 160 3.4 8 5.4 1190 45 915 90 390 97 H 140 3.5 11 4.5 1235 85 905 100 375 98 I 150 3.4 9 4.4 1230 78 910 95 380 Bold underlines indicate outside of the range of the present invention.

TABLE-US-00013 TABLE 4-2 Casting conditions Average cooling rate between Hot rolling step liquidus Average Solidification Molten temperature and heating rate at steel solidus rate of position of casting temperature of slab Heating Average 10 mm from amount surface layer area (Ac1 to Heating time at Finish cooling Kind surface of per unit at depth position Ac1 + temperature 1200° C. rolling end rate to Coiling of molten steel time of 5 mm 30° C.) of slab or higher temperature 600° C. temperature No. steel ° C./min tons/min ° C./sec ° C./min ° C. min ° C. ° C./sec ° C. 99 J 130 3.3 14 4.6 1250 130 920 100  385 100 K 120 2.5 15 4.8 1220 68 935 95 395 101 K 130 3.0 12 4.6 1230 75 940 95 360 102 L 120 2.5 9 4.9 1225 75 925 85 340 103 M 130 2.6 10 5.2 1225 71 930 95 345 104 M 140 3.8 15 5.6 1230 90 940 95 345 105 M 130 4.1 19 5.9 1255 130 840 95 350 106 N 140 4.4 22 5.8 1250 122 925 85 230 107 N 120 2.3 11 5.7 1240 105 915 85 225 108 N 130 3.3 13 6.2 1250 110 930 65 650 109 O 120 2.6 14 — — — — — — 110 P 150 3.4 13 4.5 1230 80 915 80 230 111 Q 130 3.1 14 4.6 1250 130 920 80 260 112 R 120 2.6 21 4.8 1240 105 940 95 315 113 S 120 2.5 22 4.6 1245 120 950 80 320 114 T 120 2.9 8 4.8 1230 80 920 95 350 115 U 130 2.1 7 — — — — — — 116 V 140 2.5 11 — — — — — — 117 K 120 2.5 15 4.8 1220 68 935 95 395 118 E 120 2.1 7 5.4 1210 45 915 80 370 119 E 120 2.1 7 5.4 1210 45 915 80 370 120 E 120 2.1 7 5.4 1210 45 915 80 370 121 E 120 2.1 7 5.4 1210 45 915 80 370 122 E 120 2.1 7 5.4 1210 45 915 80 370 123 E 120 2.1 7 5.4 1210 45 915 80 370 124 B 160 4.2 19 5.6 1235 95 870 65 395 125 B 160 4.2 19 5.6 1235 95 870 65 395 126 M 130 2.6 10 5.2 1225 71 930 95 345 127 M 130 2.6 10 5.2 1225 71 930 95 345 128 J 130 3.3 14 4.6 1250 130 920 100  385 129 G 130 3.5 22 6.8 1250 105 950 95 360 130 W 130 2.3 20 4.5 1231 85 870 75 380 131 R 120 2.7 21 4.8 1241 105 940 95 315 Bold underlines indicate outside of the range of the present invention.

TABLE-US-00014 TABLE 4-3 First annealing step Cooling Heating Cooling Holding stop Atmosphere Hot-dip Cold time temperature Soaking step in heating galvanizing rolling between of cooling Holding Holding furnace Hot-dip step Ac1 + from temperature time First galvanizing Cold Highest 20° C. highest after after annealing before soaking Kind rolling heating and heating stopping stopping step step or after of ratio temperature Ac3° C. temperature cooling cooling log(PH.sub.2O/H.sub.2) soaking step No. steel % ° C. sec ° C. ° C. sec — — Note 65 A 24 795 210 230 380 250 −1.77 — Example of Present Invention 66 A 24 770 225 225 385 255 −0.75 Hot-dip Example of Present galvanizing Invention 67 A 21 770 230 210 360 350 −1.66 Hot-dip Comparative Example galvanizing 68 A 22 850 210 235 365 320 −0.72 — Comparative Example 69 A 18 775 205 290 370 315 −0.73 — Comparative Example 70 B 24 790 195 205 410 290 −0.74 — Example of Present Invention 71 B 15 780 190 210 430 285 −1.71 — Example of Present Invention 72 B 21 790 180 220 425 265 −0.73 Hot-dip Comparative Example galvanizing 73 B — — — — — — — — Comparative Example 74 C 22 760 210 240 415 150 −0.73 Hot-dip Example of Present galvanizing Invention 75 C 16 785 188 210 380 180 −0.72 Hot-dip Example of Present galvanizing Invention 76 C 14 700 185 235 395 165 −0.75 Hot-dip Comparative Example galvanizing 77 C 22 790 190 210 380 315 −0.73 Hot-dip Comparative Example galvanizing 78 D 24 820 205 235 375 320 −1.71 — Example of Present Invention 79 D 40 825 201 240 385 300 −1.65 — Comparative Example 80 D 17 810 250 215 375 280 −0.74 Hot-dip Example of Present galvanizing Invention 81 D 22 810 280 195 480 215 −1.65 — Comparative Example 82 D 23 775 275 185 395 150 −1.61 — Comparative Example 83 D 25 790 265 205 410 220 −1.71 — Comparative Example 84 D 19 765 255 200 385 145 −0.73 Hot-dip Comparative Example galvanizing 85 E 18 790 260 175 375 150 −0.72 Hot-dip Example of Present galvanizing Invention 86 E 16 795 195 215 380 180 −0.71 — Example of Present Invention 87 E 24 780 185 220 390 255 −1.71 — Comparative Example 88 E 25 810 195 235 395 265 −1.65 — Comparative Example 89 F 22 815 210 240 415 260 −0.69 Hot-dip Example of Present galvanizing Invention 90 F 17 805 225 235 240 270 −0.71 Hot-dip Comparative Example galvanizing 91 F 12 810 230 220 410 280 −1.65 — Example of Present Invention 92 F 22 805 165 225 400 290 −0.75 — Comparative Example 93 G — 790 160 225 390 215 −1.72 — Example of Present Invention 94 G 24 795 195 210 395 205 −0.77 Hot-dip Example of Present galvanizing Invention 95 G 18 860 185 230 380 200 −0.76 Hot-dip Comparative Example galvanizing 96 G 22 810 210 240 375 195 −0.74 Hot-dip Comparative Example galvanizing 97 H 17 805 225 240 390 180 −1.71 — Example of Present Invention 8 I 16 780 230 225 365 170 −0.74 Hot-dip Example of Present galvanizing Invention Bold underlines indicate outside of the range of the present invention.

TABLE-US-00015 TABLE 4-4 First annealing step Cooling Heating Cooling Holding stop Atmosphere Hot-dip Cold time temperature Soaking step in heating galvanizing rolling between of cooling Holding Holding furnace Hot-dip step Ac1 + from temperature time First galvanizing Cold Highest 20° C. highest after after annealing before soaking Kind rolling heating and heating stopping stopping step step or after of ratio temperature Ac3° C. temperature cooling cooling log(PH.sub.2O/H.sub.2) soaking step No. steel % ° C. sec ° C. ° C. sec — — Note 99 J 23 785 228 235 410 260 −0.76 — Example of Present Invention 100 K 22 795 240 240 405 290 −0.72 Hot-dip Example of Present galvanizing Invention 101 K 21 800 230 250 300 550 −0.68 Hot-dip Example of Present galvanizing Invention 102 L 24 780 235 225 395 340 −1.69 — Example of Present Invention 103 M 22 810 210 215 355 355 −0.71 — Example of Present Invention 104 M 16 795 225 205 375 215 −0.73 Hot-dip Example of Present galvanizing Invention 105 M 18 800 230 200 355 220 −0.76 Hot-dip Comparative Example galvanizing 106 N 22 795 290 195 375 280 −0.76 Hot-dip Example of Present galvanizing Invention 107 N 21 810 280 180 220 250 −0.75 — Comparative Example 108 N 14 805 240 185 385 240 −0.76 — Comparative Example 109 O — — — — — — — — Comparative Example 110 P 19 805 210 200 380 250 −0.74 Hot-dip Comparative Example galvanizing 111 Q 23 790 195 205 375 280 −0.74 — Comparative Example 112 R 21 790 185 215 360 220 −0.72 Hot-dip Comparative Example galvanizing 113 S 22 — — — — — — — Comparative Example 114 T 19 — — — — — — — Comparative Example 115 U — — — — — — — — Comparative Example 116 V — — — — — — — — Comparative Example 117 K 22 795 241 241 406 291 −0.72 Hot-dip Example of Present galvanizing Invention 118 E 18 790 263 176 376 151 −0.72 Hot-dip Example of Present galvanizing Invention 119 E 18 790 267 155 243 153 −0.72 Hot-dip Comparative Example galvanizing 120 E 18 790 263 176 270 151 −0.72 Hot-dip Example of Present galvanizing Invention 121 E 18 790 263 176 245 154 −0.72 Hot-dip Comparative Example galvanizing 122 E 18 790 262 178 377 157 −0.72 Hot-dip Example of Present galvanizing Invention 123 E 18 790 261 181 376 160 −0.72 Hot-dip Example of Present galvanizing Invention 124 B 24 790 195 205 440 990 −0.74 — Example of Present Invention 125 B 24 790 195 205 463 1012  −0.74 — Comparative Example 126 M 22 810 209 215 355 356 −0.71 — Example of Present Invention 127 M 22 810 208 215 357 355 −0.71 — Example of Present Invention 128 J 23 786 228 222 410 260 −0.76 — Example of Present Invention 129 G 24 795 193 211 395 205 −0.77 Hot-dip Example of Present galvanizing Invention 130 W 24 794 210 230 380 251 −1.77 — Example of Present Invention 131 R 21 789 185 215 360 221 −0.72 Hot-dip Comparative Example galvanizing Bold underlines indicate outside of the range of the present invention.

[0291] Tables 5-1 to 5-4 show the results of examining the steel sheets manufactured under the above conditions under the same conditions as in Example 1.

TABLE-US-00016 TABLE 5-1 Microstructure (⅛ to ⅜ thickness position) Number proportion of residual austenite Kind Residual Tempered Fresh having aspect ratio of Ferrite austenite Bainite martensite martensite Pearlite of 2.0 or more No. steel Plating % % % % % % % 65 A CR 20 14 34 28 3 1 77 66 A GA 28 13 30 26 2 1 65 67 A GA 30 5 26 25 12 2 32 68 A CR  4 5 53 25 11 2 29 69 A CR 24 8 51 2 14 1 45 70 B CR 16 22 17 42 3 0 65 71 B CR 17 20 20 40 2 1 62 72 B GA 20 8 39 15 12 6 35 73 B — — — — — — — — 74 C GA 23 12 38 25 2 0 66 75 C GA 21 13 33 32 1 0 63 76 C GA 45 4 29 6 13 3 15 77 C GA 23 9 29 36 3 0 62 78 D CR 25 19 25 25 5 1 74 79 D CR 20 17 30 26 4 3 35 80 D GA 35 19 13 31 2 0 73 81 D CR 32 5 19 32 6 6 62 82 D CR 31 18 26 22 2 1 68 83 D CR 21 16 37 22 3 1 66 84 D GA 22 14 36 21 5 2 67 85 E GI 16 23 19 38 3 1 71 86 E CR 15 20 26 35 3 1 67 87 E CR 16 12 34 25 11 2 32 88 E CR 19 14 21 32 12 2 35 89 F GA 24 11 29 30 5 1 72 90 F GA 22 5 27 35 9 2 69 91 F CR 25 12 28 31 3 1 65 92 F CR 21 11 29 25 13 1 71 93 G CR 33 16 26 21 3 1 69 94 G GA 31 13 24 26 4 2 67 95 G GA 15 5 23 52 3 2 31 96 G GI 41 12 10 28 8 1 68 97 H CR 22 14 28 31 4 1 72 98 I GA 29 18 18 32 2 1 75 Bold underlines indicate outside of the range of the present invention.

TABLE-US-00017 TABLE 5-2 Microstructure (⅛ to ⅜ thickness position) Number proportion of residual austenite Kind Residual Tempered Fresh having aspect ratio of Ferrite austenite Bainite martensite martensite Pearlite of 2.0 or more No. steel Plating % % % % % % % 99 J CR 24 21 13 35 5 2 72 100 K GA 29 9 21 35 4 2 65 101 K GA 27 8 23 37 5 0 68 102 L CR 25 22 23 26 3 1 64 103 M CR 22 19 33 23 2 1 66 104 M GA 21 18 32 25 3 1 68 105 M GA 31 5 26 15 21 2 43 106 N GA 22 16 35 28 3 1 77 107 N CR 23 8 36 20 12 1 73 108 N CR 2 5 35 22 11 6 46 109 O — — — — — — — — 110 P GA 35 3 12 45 3 2 46 111 Q CR 22 3 53 22 0 0 44 112 R GA 59 5 23  3 2 8 46 113 S — — — — — — — — 114 T — — — — — — — — 115 U — — — — — — — — 116 V — — — — — — — — 117 K GA 29 6 24 35 4 2 65 118 E GI 16 29 13 38 3 1 71 119 E GI 19 32 4 41 3 1 71 120 E GI 19 29 6 40 4 2 71 121 E GI 20 29 4 40 4 2 71 122 E GI 19 22 8 45 4 2 50 123 E GI 14 21 10 49 4 2 71 124 B CR 10 16 59 12 3 0 65 125 B CR 10 14 62 11 3 0 65 126 M CR 30 27 36  5 1 1 66 127 M CR 29 24 35 10 1 1 66 128 J CR 23 19 11 35 10 2 72 129 G GA 31 13 22 26 4 4 67 130 w CR 20 14 35 28 2 1 77 131 R GA 53 5 29  3 2 8 46 Bold underlines indicate outside of the range of the present invention.

TABLE-US-00018 TABLE 5-3 Inclusions Mechanical properties and Mn segregation Toughness precipitates Average after Number interval application density of between of 5% inclusions Mn- prestrain and concentrated and heat precipitates portions at Standard Press formability treatment having 1/20 deviation of TS* of 170° Hardness Kind grain size thickness Mn El* C. × 20 Hv.sub.sur/ of of 1 μm or position concentrations TS El λ λ.sup.0.5/ minutes [Hv] No. steel Plating more μm % MPa % % 1000 — — Note 65 A CR 15 220 0.21 1015 19.2 43 128 G 0.92 Example of Present Invention 66 A GA 17 200 0.19 1002 22.1 23 106 VG 0.75 Example of Present Invention 67 A GA 19 210 0.22 1043 13.5 25 70 B 0.89 Comparative Example 68 A CR 12 205 0.19 1051 12.9 20 61 B 0.91 Comparative Example 69 A CR 16 220 0.43 1072 11.5 32 70 B 0.76 Comparative Example 70 B CR 12 195 0.22 1035 21.5 31 124 VG 0.75 Example of Present Invention 71 B CR 13 210 0.22 1025 22.1 30 123 G 0.95 Example of Present Invention 72 B GA 11 190 0.18 1051 13.5 25 71 B 0.91 Comparative Example 73 B — — — — — — — — — — Comparative Example 74 C GA 10 210 0.24 989 19.2 25 95 VG 0.78 Example of Present Invention 75 C GA 9 190 0.21 1010 18.9 35 113 VG 0.76 Example of Present Invention 76 C GA 9 185 0.20 982 19.0 18 79 B 0.86 Comparative Example 77 C GA 25 315 0.45 1010 19.0 17 79 B 0.75 Comparative Example 78 D CR 12 220 0.23 1015 22.4 31 127 G 0.89 Example of Present Invention 79 D CR 8 220 0.24 1025 19.2 18 83 G 0.85 Comparative Example 80 D GA 9 220 0.23 1020 23.4 31 133 VG 0.75 Example of Present Invention 81 D CR 10 205 0.21 1062 13.5 25 72 G 0.84 Comparative Example 82 D CR 35 225 0.23 1015 21.5 21 100 B 0.87 Comparative Example 83 D CR 32 205 0.22 1008 22.1 23 107 B 0.89 Comparative Example 84 D GA 41 185 0.18 1007 21.3 21 98 B 0.79 Comparative Example 85 E GI 12 205 0.21 1195 14.5 38 107 VG 0.79 Example of Present Invention 86 E CR 11 210 0.23 1205 14.2 41 110 VG 0.78 Example of Present Invention 87 E CR 12 200 0.20 1292 11.5 25 74 B 0.89 Comparative Example 88 E CR 12 205 0.41 1312  9.5 26 64 B 0.85 Comparative Example 89 F GA 13 195 0.19 1011 19.2 45 130 VG 0.77 Example of Present Invention 90 F GA 11 200 0.21 1044 12.1 33 73 B 0.76 Comparative Example 91 F CR 13 205 0.35 1012 19.8 26 102 G 0.88 Example of Present Invention 92 F CR 12 210 0.42 1035 13.2 25 68 B 0.76 Comparative Example 93 G CR 9 195 0.21 1003 22.3 24 110 G 0.87 Example of Present Invention 94 G GA 13 210 0.26 1011 23.1 25 117 VG 0.76 Example of Present Invention 95 G GA 10 205 0.24 1105 10.5 35 69 B 0.77 Comparative Example 96 G GI 12 210 0.41 1031 18.5 31 106 B 0.75 Comparative Example 97 H CR 13 190 0.25 1041 17.8 29 100 G 0.83 Example of Present Invention 98 I GA 12 170 0.18 1006 24.2 36 146 VG 0.76 Example of Present Invention Bold underlines indicate outside of the range of the present invention.

TABLE-US-00019 TABLE 5-4 Inclusions Mechanical properties and Mn segregation Toughness precipitates Aerage after Number interval application density of between of 5% inclusions Mn- prestrain and concentrated and heat precipitates portions at Standard Press formability treatment having 1/20 deviation of TS* of 170° Hardness Kind grain size thickness Mn El* C. × 20 Hv.sub.sur/ of of 1 μm or position concentrations TS El λ λ.sup.0.5/ minutes [Hv] No. steel Plating more μm % MPa % % 1000 — — Note 99 J CR 11 195 0.22 1215 14.8 35 106 VG 0.78 Example of Present Invention 100 K GA 12 205 0.18 993 17.9 35 105 VG 0.75 Example of Present Invention 101 K GA 13 200 0.17 1005 17.1 32 97 VG 0.72 Example of Present Invention 102 L CR 11 210 0.21 1020 24.2 31 137 G 0.85 Example of Present Invention 103 M CR 9 190 0.22 1215 16.2 38 121 VG 0.76 Example of Present Invention 104 M GA 10 205 0.22 1209 15.4 39 116 VG 0.77 Example of Present Invention 105 M GA 13 210 0.22 1276 11.5 25 73 B 0.76 Comparative Example 106 N GA 10 190 0.21 1201 14.1 43 111 VG 0.74 Example of Present Invention 107 N CR 9 195 0.22 1274 12.1 25 77 B 0.77 Comparative Example 108 N CR 11 210 0.22 1254 11.5 25 72 B 0.88 Comparative Example 109 O — — — — — — — — — — Comparative Example 110 P GA 12 205 0.19 885 14.2 35 74 VG 0.76 Comparative Example 111 Q CR 11 200 0.18 1021 11.2 31 64 G 0.88 Comparative Example 112 R GA 15 195 0.16 911 16.5 33 86 VG 0.77 Comparative Example 113 S — — — — — — — — — — Comparative Example 114 T — — — — — — — — — — Comparative Example 115 U — — — — — — — — — — Comparative Example 116 V — — — — — — — — — — Comparative Example 117 K GA 12 205 0.18 996 17.5 35 103 VG 0.75 Example of Present Invention 118 E GI 12 205 0.21 1197 15.1 36 108 VG 0.79 Example of Present Invention 119 E GI 12 205 0.21 1197 14.6 18 74 B 0.79 Comparative Example 120 E GI 12 205 0.21 1205 13.9 31 93 VG 0.79 Example of Present Invention 121 E GI 12 205 0.21 1175 12.9 27 79 B 0.79 Comparative Example 122 E GI 12 205 0.21 1206 14.3 32 98 VG 0.79 Example of Present Invention 123 E GI 30 205 0.21 1203 14.1 33 97 VG 0.79 Example of Present Invention 124 B CR 9 300 0.22 1037 15.0 35 92 VG 0.75 Example of Present Invention 125 B CR 13 195 0.26 1036 13.5 32 79 B 0.75 Comparative Example 126 M CR 9 190 0.35 1225 16.0 43 129 VG 0.76 Example of Present Invention 127 M CR 9 190 0.22 1222 15.9 45 130 VG 0.76 Example of Present Invention 128 J CR 11 195 0.22 1219 14.7 30 98 VG 0.78 Example of Present Invention 129 G GA 13 210 0.26 1012 23.2 26 120 VG 0.76 Example of Present Invention 130 W CR 15 220 0.21 1013 19.3 42 127 G 0.92 Example of Present Invention 131 R GA 15 195 0.16 910 16.2 34 86 VG 0.77 Comparative Example

[0292] The steel sheets of the examples of the present invention were steel sheets having high strength, a good balance between strength, elongation, and hole expansibility, and little deterioration in Charpy absorbed energy after plastic deformation and 170° C.×20 minutes simulating baking coating.

[0293] In Example No. 73, since the solidification rate was high at a position of 10 mm from the surface of the molten steel, and cracks occurred in the slab, steps subsequent to the hot rolling could not be performed.

[0294] In Example No. 77, since the solidification rate at a position of 10 mm from the surface of the molten steel was slow, the average interval between the Mn-concentrated portions at the 1/20 thickness position was wide, and as a result, the standard deviation of the Mn concentrations in residual austenite was high. Therefore, in addition to the deterioration of the balance of mechanical properties, the deterioration of the Charpy absorbed energy also increased.

[0295] In Example 83, since the molten steel casting amount per unit time was small, the trapping of inclusions and precipitates in the vicinity of the surface layer of the slab during casting increased. Therefore, the Charpy absorbed energy deteriorated.

[0296] In Example No. 84, since the molten steel casting amount per unit time was large, the trapping of inclusions and precipitates in the vicinity of the surface layer of the slab due to entrainment of mold powder increased. Therefore, the Charpy absorbed energy deteriorated.

[0297] In Example 82, since the average cooling rate between the liquidus temperature and the solidus temperature of the surface layer area at a depth position of 5 mm of the molten steel was too low, grain growth of inclusions and precipitates progressed during the solidification process, and the Charpy absorbed energy deteriorated.

[0298] In Example 92, since the heating rate of the slab from Ac1 to Ac1+30° C. was too low, distribution of Mn at α/γ during an increase in temperature progressed, and the standard deviation of the Mn concentrations in residual austenite increased. As a result, a decrease in martensitic transformation temperature due to partial Mn concentration occurred and the volume fraction of fresh martensite increased. As a result, the balance of mechanical properties deteriorated, and the Charpy absorbed energy also deteriorated.

[0299] In Example No. 96, since the slab heating temperature was low, Mn was not sufficiently uniform, and the standard deviation of the Mn concentrations in residual austenite increased. As a result, the Charpy absorbed energy deteriorated.

[0300] In Example No. 88, since the heating time of the slab at 1200° C. or higher was short, Mn was not sufficiently uniform and the standard deviation of the Mn concentrations in residual austenite increased. As a result, the balance of mechanical properties deteriorated, and the Charpy absorbed energy also deteriorated.

[0301] In Example 67 and 105, since the finishing temperature in the hot rolling was low, a dislocation density in austenite immediately after the hot rolling increased and ferritic transformation was promoted. Therefore, a lath-like structure could not be obtained on the entire surface in the microstructure after the hot rolling. As a result, the volume fraction of residual austenite was insufficient or the volume fraction of fresh martensite was increased, which deteriorated the balance of mechanical properties and also deteriorated the Charpy absorbed energy.

[0302] In Example Nos. 72, 87, and 108, since the coiling temperature in the hot rolling was high, ferrite was generated by phase transformation after the coiling. Therefore, a lath-like structure could not be obtained on the entire surface in the microstructure after the hot rolling. As a result, the volume fraction of fresh martensite increased, so that the balance of mechanical properties deteriorated, and the Charpy absorbed energy also deteriorated.

[0303] In Example No. 79, the number proportion of residual austenite having an aspect ratio of 2.0 or more decreased due to the high rolling reduction. As a result, the Charpy absorbed energy deteriorated.

[0304] In Example No. 76, since the highest heating temperature in the first annealing step was low, the amount of residual austenite finally obtained in the product decreased, the balance of mechanical properties deteriorated, and the Charpy absorbed energy also deteriorated.

[0305] In Example Nos. 68 and 95, since the highest heating temperature in the first annealing step was high, the lath-like structure formed in the hot rolling step collapsed, and the number proportion of residual austenite having an aspect ratio of 2.0 or more decreased. As a result, the balance of mechanical properties deteriorated, and the Charpy absorbed energy also deteriorated.

[0306] In Example No. 69, since the cooling stop temperature in the first annealing step was high, the volume fraction of tempered martensite decreased. As a result, bainitic transformation did not sufficiently progress during subsequent holding, and the volume fraction of fresh martensite generated after final cooling increased. As a result, the balance of mechanical properties deteriorated, and the Charpy absorbed energy also deteriorated.

[0307] In Example No. 81, since the holding temperature after stopping cooling in the soaking step was high, cementite was generated in residual austenite exposed to a high temperature, resulting in instability. Therefore, the volume fraction of residual austenite that could be finally obtained decreased. As a result, the balance of mechanical properties deteriorated.

[0308] In Example Nos. 90 and 107, since the holding temperature after stopping cooling in the soaking step was low, bainitic transformation did not sufficiently progress during holding, and the amount of fresh martensite generated after final cooling increased. As a result, the balance of mechanical properties deteriorated, and the Charpy absorbed energy also deteriorated.

[0309] In Example No. 119, since the holding temperature after stopping cooling in the soaking step was low, the volume fraction of bainite was insufficient. In addition, the volume fraction of residual austenite was also excessive, which deteriorated the balance of mechanical properties.

[0310] In Example No. 121, since the holding temperature after stopping cooling in the soaking step was low, the volume fraction of bainite was insufficient. Therefore, the balance of mechanical properties deteriorated.

[0311] In Example No. 125, since the holding temperature after stopping cooling in the soaking step was high and the holding time after stopping cooling was long, bainitic transformation progressed excessively, resulting in an excessive volume fraction of bainite. Therefore, the balance of mechanical properties deteriorated.

[0312] In Example No. 109, since the total amount of Si and Al was large, cracks occurred in the slab. Therefore, steps subsequent to the hot rolling could not be performed.

[0313] In Example 110, since the C content was low, the Ms point did not decrease significantly, and as a result, the volume fraction of residual austenite was small. In addition, the number proportion of residual austenite having an aspect ratio of 2.0 or more was small. Therefore, the balance of mechanical properties deteriorated.

[0314] In Example No. 111, since the total amount of Si and Al was small, carbon concentration in untransformed austenite was small. As a result, the volume fraction of residual austenite was small, and the number proportion of residual austenite having an aspect ratio 2.0 or more was small. Therefore, the balance of mechanical properties deteriorated.

[0315] In Example Nos. 112 and 131, since the Mn content was low, the Ms point did not decrease. As a result, the volume fractions of ferrite and pearlite were large, the volume fractions of residual austenite and tempered martensite were large, and the number proportion of residual austenite having an aspect ratio 2.0 or more was small. Therefore, the tensile strength was insufficient.

[0316] In Example No. 113, since the Mn content was high, cracks occurred in the steel sheet during the cold rolling step. Therefore, subsequent steps could not be performed.

[0317] In Example No. 114, since the C content was high, cracks occurred in the steel sheet during the cold rolling step. Therefore, subsequent steps could not be performed.

[0318] In Example No. 115, since the Si content was high, cracks occurred in the slab. Therefore, steps subsequent to the hot rolling could not be performed.

[0319] In Example No. 116, since the Al content was high, cracks occurred in the slab. Therefore, steps subsequent to the hot rolling could not be performed.

[0320] In Example Nos. 66, 70, 74, 75, 80, 85, 86, 89, 94, 98, 99, 100, 101, 103, 104, 106, 117, 118, 120, 122, 123, 124, 126, 127, 128, and 129, since the oxygen potential was high in the first annealing step, internal oxidation occurred in the vicinity of the surface layer of the steel sheet, and at the same time, decarburization progressed, resulting in softening. Therefore, the ratio of the Vickers hardness Hv.sub.sur at a depth position of 30 μm from the surface to the Vickers Hardness [Hv] at a ¼ depth position of the sheet thickness from the surface decreased, and the bendability was improved, so that the Charpy absorbed energy was improved.

[0321] While the preferred embodiments and examples of the present invention have been described above, these embodiments and examples are merely examples within the scope of the gist of the present invention, and additions, omissions, substitutions, and other changes of the configuration can be made without departing from the gist of the present invention. That is, the present invention is not limited by the above description, but is limited only by the appended claims, and can be appropriately changed within the scope as a matter of course.