Steel sheet and method for production thereof

Abstract

The present invention provides steel sheet excellent in cold formability and ductility after heat treatment and a method for production thereof. The steel sheet of the present invention is steel sheet which has a chemical composition containing, by mass %, C: 0.10 to 0.40%, Si: 0.30 to 1.00%, Mn: 0.30 to 1.00%, Al: 0.001 to 0.10%, P: 0.0001 to 0.02%, and S: 0.0001 to 0.01% and having a balance of Fe and impurities, which steel sheet characterized in that a ratio (B/A) of the number of carbides at the ferrite grain boundaries (B) to the number of carbides inside the ferrite grains (A) is over 1, a ferrite grain size is 5 m to 50 m, an average grain size of carbides is 0.4 m to 2.0 m, a pearlite area ratio is 6% or less, and a Vicker's hardness is 120 HV to 170 HV.

Claims

1. A steel sheet comprising, by mass %: C: 0.10 to 0.40%, Si: 0.30 to 1.00%, Mn: 0.30 to 1,00%, Al: 0.001 to 0.10%, P: 0,02% or less, and S: 0.01% or less and having a balance of Fe and impurities, wherein a ratio (B/A) of a number of carbides at ferrite grain boundaries (B) with respect to a number of carbides inside ferrite grains (A) is over 1, wherein a ferrite grain size is 5 m to 50 m, wherein an average grain size of carbides is 0.4 m to 2.0 m, wherein a pearlite area ratio is 6% or less, and wherein a Vicker's hardness is 120 HV to 170 HV.

2. The steel sheet according to claim 1, wherein said steel sheet further comprises, by mass %, one or more of: Ti: 0.10% or less, Cr: 0.50% or less, Mo: 0.50% or less, B: 0.01% or less, Nb: 0.10% or less, V: 0.10% or less, Cu: 0.10% or less, W: 0.10% or less, Ta: 0.10% or less, N i: 0.10% or less, Sn: 0.05% or less, Sb: 0.05% or less, As: 0.05% or less, Mg: 0.05% or less, Ca: 0.05% or less, Y: 0.05% or less, Zr: 0.05% or less, La: 0.05% or less, Ce: 0.05% or less, N: 0.01% or less and O: 0.02% or less.

3. A method for producing the steel sheet according to claim 1, the method for producing the steel sheet comprising: (i) hot rolling a steel slab of a chemical composition according to claim 1; finishing the hot rolling in a temperature range of 800 C. to 900 C.; and coiling the hot rolled steel sheet at 400 C. to 550 C., (ii) paying out the hot rolled steel sheet; pickling the hot rolled steel sheet; then holding the hot rolled steel sheet in a temperature range of 650 C. to 720 C. for 3 hours to 60 hours as first stage annealing and further holding the hot rolled steel sheet in a temperature range of 725 C. to 790 C. for 3 hours to 50 hours as second stage annealing, and (iii) cooling the hot rolled steel sheet after annealing, at cooling rate of 1 C./hour to 30 C./hour down to 650 C.; and then cooling the hot rolled steel sheet down to room temperature.

4. The method for producing the steel sheet according to claim 3, wherein that the temperature of the steel slab used for the hot rolling is 1000 to 1250 C.

5. A method for producing the steel sheet according to claim 4, the method for producing the steel sheet comprising: (i) hot rolling a steel slab of a chemical composition according to claim 4; finishing the hot rolling in a temperature range of 800 C. to 900 C.; and coiling the hot-rolled steel sheet at 400 C. to 550 C., (ii) paying out the hot-rolled steel sheet; pickling the hot-rolled steel sheet; then holding the hot-rolled steel sheet in a temperature range of 650 C. to 720 C. for 3 hours to 60 hours as first stage annealing and further holding the hot-rolled steel sheet in a temperature range of 725 C. to 790 C. for 3 hours to 50 hours as second stage annealing, and (iii) cooling the hot-rolled steel sheet after annealing, at a cooling rate of 1 C./hour to 30 C./hour down to 650 C.; and then cooling the hot-rolled steel sheet down to room temperature.

6. The method for producing the steel sheet according to claim 4, wherein the temperature of the steel slab used for the hot rolling is 1000 to 1250 C.

Description

EXAMPLES

(1) Next, examples of the embodiments is explained, but the conditions in the examples are illustrations employed for confirming the workability and effects of the present invention. The present invention is not limited to these illustrations of conditions. The present invention can employ various conditions so long as not departing from the gist of the present invention and as achieving the object of the present invention.

Example 1

(2) To investigate the effects of the chemical composition, continuously cast slabs (steel slabs) of the chemical compositions shown in Table 1-1 and Table 1-2 (chemical compositions of steel sheets of the present invention) and Table 2-1 and Table 2-2 (chemical compositions of comparative steel sheets) were processed under the following conditions from the hot rolling process to two-stage annealing process to prepare samples for evaluation of characteristics shown in Table 3 (Invention Steels A-1 to Z-1 and Comparative Steels AA-1 to AZ-1). Further, Steel Slabs A to Z in Table 1-1 and Table 1-2 all have chemical compositions of the steel sheet of the present invention. On the other hand, the chemical compositions of the Steel Slabs AA to AZ of Table 2-1 and Table 2-2 were all outside the scope of the chemical composition of steel sheet of the present invention.

(3) TABLE-US-00001 TABLE 1-1 Steel slab C Si Mn P S Al N O A 0.16 0.43 0.86 0.0013 0.0004 0.057 0.0036 B 0.32 0.7 0.34 0.0069 0.0025 0.03 0.0020 C 0.19 0.44 0.6 0.0023 0.0026 0.069 0.0036 D 0.24 0.56 0.35 0.0051 0.007 0.059 0.0019 E 0.27 0.56 0.36 0.0030 0.0005 0.024 0.0049 F 0.19 0.73 0.79 0.0032 0.0045 0.043 0.0008 G 0.35 0.79 0.59 0.0017 0.0037 0.088 0.0041 H 0.21 0.58 0.45 0.0014 0.0067 0.093 0.0005 I 0.18 0.75 0.48 0.0019 0.0044 0.085 0.0041 J 0.17 0.69 0.82 0.0039 0.0021 0.044 0.0017 K 0.17 0.39 0.89 0.0070 0.0012 0.088 0.0006 L 0.33 0.53 0.75 0.0086 0.0012 0.095 0.0039 M 0.21 0.52 0.81 0.0023 0.002 0.011 0.0036 N 0.32 0.71 0.72 0.0029 0.0058 0.043 0.0013 0.0096 O 0.32 0.61 0.31 0.0091 0.0055 0.023 0.0045 P 0.27 0.64 0.79 0.0021 0.0018 0.044 0.0009 0.0038 Q 0.19 0.6 0.37 0.0021 0.006 0.054 0.0002 R 0.2 0.72 0.48 0.0001 0.0055 0.077 0.0033 S 0.18 0.71 0.66 0.0077 0.0048 0.025 0.0028 T 0.22 0.37 0.94 0.0058 0.0019 0.073 0.0029 U 0.2 0.7 0.44 0.0050 0.0055 0.076 0.0003 0.0097 V 0.34 0.42 0.88 0.0049 0.002 0.023 0.0011 W 0.21 0.75 0.92 0.0010 0.0044 0.025 0.0017 X 0.17 0.7 0.41 0.0065 0.0068 0.056 0.0019 Y 0.3 0.56 0.78 0.0092 0.0027 0.047 0.0027 0.003 Z 0.23 0.64 0.37 0.0061 0.0061 0.048 0.0010 Units of content of the components of Table 1-1 are mass %.

(4) TABLE-US-00002 TABLE 1-2 Steel slab Ti Cr Mo B Nb V Cu W Ta Ni Sn Sb As Mg Ca Y Zr La Ce A B C D E F G H I J 0.104 0.011 0.015 0.028 0.006 K 0.03 0.009 0.016 L 0.04 0.035 0.05 0.045 M 0.007 0.0029 0.021 N 0.211 0.042 0.075 0.041 0.039 O 0.052 0.0016 0.017 0.015 P 0.081 0.0355 Q 0.031 0.048 0.02 R 0.0019 0.0226 S 0.145 0.036 0.021 0.023 T 0.111 0.035 0.0024 0.028 U 0.067 0.042 0.019 V 0.032 0.0022 0.038 W 0.183 0.002 0.042 0.016 X 0.079 0.008 0.025 0.021 Y 0.044 0.002 Z 0.249 0.004 0.031 Units of content of the components of Table 1-2 are mass %.

(5) TABLE-US-00003 TABLE 2-1 Steel slab C Si Mn P S Al N O AA 0.18 1.5 0.51 0.0080 0.0013 0.059 0.0027 AB 0.8 0.59 0.79 0.0024 0.0015 0.023 0.0002 AC 0.32 0.61 0.74 0.0097 0.0061 0.8 0.0009 AD 0.36 0.5 2.2 0.0045 0.0004 0.032 0.0002 AE 0.32 0.15 0.37 0.0007 0.0066 0.064 0.0031 AF 0.16 0.61 0.81 0.0220 0.0029 0.082 0.0033 AG 0.23 0.6 0.72 0.0014 0.012 0.09 0.0022 AH 0.06 0.78 0.64 0.0017 0.0008 0.038 0.0044 AI 0.23 0.65 0.83 0.0029 0.0047 0.045 0.012 AJ 0.16 0.35 0.3 0.0019 0.0044 0.02 0.0005 AK 0.35 0.69 0.72 0.0029 0.0065 0.098 0.0038 AL 0.29 0.76 0.81 0.0020 0.0014 0.031 0.0029 0.0002 AM 0.3 0.51 0.84 0.0001 0.0024 0.014 0.0015 AN 0.18 0.65 0.57 0.0081 0.0029 0.032 0.0028 AO 0.33 0.57 0.31 0.0086 0.0044 0.017 0.0035 0.0062 AP 0.17 0.79 0.88 0.0033 0.0041 0.029 0.0017 AQ 0.31 0.42 0.53 0.0089 0.0055 0.081 0.0033 AR 0.29 0.45 0.82 0.0002 0.0048 0.068 0.0008 AS 0.29 0.67 0.77 0.0028 0.0066 0.054 0.0039 0.0045 AT 0.27 0.49 0.69 0.0002 0.0066 0.093 0.0016 0.02 AU 0.31 0.62 0.32 0.0047 0.0012 0.064 0.0011 AV 0.28 0.46 0.49 0.0064 0.0042 0.09 0.0029 AW 0.22 0.58 0.75 0.0095 0.0016 0.012 0.0050 AX 0.18 0.64 0.77 0.0033 0.006 0.058 0.0007 AY 0.32 0.65 0.69 0.0034 0.0057 0.066 0.0035 AZ 0.26 0.65 0.32 0.0044 0.0069 0.023 0.0003 Units of content of the components of Table 2-1 are mass %.

(6) TABLE-US-00004 TABLE 2-2 Steel slab Ti Cr Mo B Nb V Cu W Ta Ni Sn Sb As Mg Ca Y Zr La Ce AA AB AC AD AE AF AG AH AI AJ 1.22 0.041 0.01 0.08 AK 0.0013 0.018 0.076 AL 0.341 1.12 0.027 0.067 0.014 0.026 AM 0.045 0.06 AN 0.11 0.015 0.11 0.008 0.028 AO 0.022 0.004 0.064 0.014 AP 0.11 AQ 0.0016 0.02 0.072 AR 0.043 0.6 0.048 AS 0.012 0.086 0.034 0.013 AT 0.026 0.055 AU 0.035 0.2 0.087 AV 0.031 0.197 0.056 AW 0.134 0.0034 0.088 0.056 0.01 AX 0.4 AY 0.27 0.021 0.076 0.0004 0.063 AZ 0.123 0.11 0.008 0.014 Units of content of the components of Table 2-2 are mass %.

(7) That is, steel slabs of the chemical compositions shown in Tables 1 and 2 were heated at 1240 C. for 1.8 hours, then hot rolled. The finish rolling was completed at a finish temperature of 820 C. After that, the steel sheets were cooled on the ROT by a 45 C./sec cooling rate and were coiled up at the coiling temperature of 510 C. to produce hot rolled steel sheet coils. Next, the hot rolled steel sheet coils were taken out and pickled, then the pickled hot rolled steel sheet coils were loaded into a box type annealing furnace for first stage annealing. The annealing atmosphere was controlled so as to include 95% hydrogen and 5% nitrogen while the coils were heated from room temperature to 705 C. and held there for 36 hours to make the temperature distribution inside the hot rolled steel sheet coils uniform. After that, for second stage annealing, the coils were heated to 760 C., held there for 10 hours, then were cooled down to 650 C. by a 10 C./hour cooling rate, then were furnace cooled down to room temperature to prepare samples for evaluation of characteristics.

(8) The samples were examined for structure and were measured for ferrite grain size and number of carbides by the above-mentioned methods. Next, the samples were loaded into an atmosphere annealing furnace, held at 950 C. for 20 minutes, and, after holding, oil cooled at 50 C. After that, they were tempered so that the hardness became 400 HV. The ductility after heat treatment was found by examining the surfaces of the samples after annealing, preparing sheet thickness 2 mm JIS No. 5 test pieces, and conducting tensile tests at room temperature. The tensile tests were performed with a gauge length of 50 mm and test speeds of 3 mm/min. The result of 10% or more was considered good.

(9) Table 3 shows the ferrite grain size (m), Vicker's hardness (HV), ratio of the number of carbides at the ferrite grain boundaries to the number of carbides inside the ferrite grains (number of grain boundary carbides/number of grain carbides), and ductility after heat treatment (%).

(10) TABLE-US-00005 TABLE 3 Carbide No. of carbides at Ductility Ferrite average Pearlite Vicker's grain boundaries/No. after heat Steel grain grain size area ratio hardness of carbides inside treatment Sample slab size [m] [m] [%] [HV] grains [%] Remarks A-1 A 13.0 1.2 2.6 126 3.5 12.2 Inv. steel B-1 B 24.3 1.0 0.5 144 4.3 10.3 Inv. steel C-1 C 16.6 1.1 0.5 126 4.5 11.7 Inv. steel D-1 D 23.9 1.0 0.3 131 3.7 11.2 Inv. steel E-1 E 23.4 1.0 0.3 132 4.9 11.4 Inv. steel F-1 F 13.8 1.1 2.7 146 4.8 12.4 Inv. steel G-1 G 16.8 1.0 0.5 158 3.0 10.5 Inv. steel H-1 H 20.2 1.0 1.0 134 5.3 11.6 Inv. steel I-1 I 19.3 1.0 2.8 143 5.5 12.7 Inv. steel J-1 J 13.5 1.2 1.8 143 4.8 12.8 Inv. steel K-1 K 12.7 1.2 2.8 126 4.9 12.4 Inv. steel L-1 L 14.3 1.1 0.7 143 4.2 11.0 Inv. steel M-1 M 13.6 1.2 1.2 133 4.7 11.5 Inv. steel N-1 N 14.7 1.1 0.8 152 3.8 10.8 Inv. steel O-1 O 25.9 1.0 1.0 138 6.3 10.5 Inv. steel P-1 P 13.8 1.1 1.9 145 6.2 11.7 Inv. steel Q-1 Q 23.0 1.0 0.6 130 5.5 12.3 Inv. steel R-1 R 19.3 1.0 0.1 142 3.5 12.3 Inv. steel S-1 S 15.5 1.1 1.0 142 5.0 12.4 Inv. steel T-1 T 12.3 1.2 1.0 127 3.1 11.5 Inv. steel U-1 U 20.5 1.0 1.7 140 3.4 12.0 Inv. steel V-1 V 12.8 1.2 1.4 135 5.0 10.4 Inv. steel W-1 W 12.4 1.1 1.8 150 4.3 12.0 Inv. steel X-1 X 21.5 1.0 1.6 136 3.2 12.4 Inv. steel Y-1 Y 13.9 1.1 2.8 142 3.6 10.8 Inv. steel Z-1 Z 22.7 1.0 1.5 136 5.1 11.8 Inv. steel AA-1 AA 18.5 0.9 4.0 189 4.7 11.9 Comp. steel AB-1 AB 13.8 1.1 9.2 178 10.6 5.6 Comp. steel AC-1 AC 14.4 0.9 0.6 166 2.8 Comp. steel AD-1 AD 6.9 1.4 6.0 176 4.7 8.4 Comp. steel AE-1 AE 23.0 1.1 4.0 112 4.1 8.5 Comp. steel AF-1 AF 13.6 1.2 0.1 138 3.7 8.5 Comp. steel AG-1 AG 14.7 1.1 1.9 141 4.4 9.5 Comp. steel AH-1 AH 15.9 1.1 1.3 138 3.9 Comp. steel AI-1 AI 13.4 1.2 1.4 144 5.3 8.2 Comp. steel AJ-1 AJ 24.9 0.8 4.0 112 4.4 9.2 Comp. steel AK-1 AK 14.7 1.1 2.6 154 4.8 6.5 Comp. steel AL-1 AL 13.2 1.1 5.0 156 4.5 7.4 Comp. steel AM-1 AM 13.3 1.2 1.9 138 3.9 6.9 Comp. steel AN-1 AN 17.2 1.1 1.8 145 5.3 9.0 Comp. steel AO-1 AO 25.9 1.0 1.3 136 2.9 6.9 Comp. steel AP-1 AP 12.8 1.2 2.0 149 3.8 9.0 Comp. steel AQ-1 AQ 18.1 1.1 0.8 132 3.4 7.0 Comp. steel AR-1 AR 13.5 1.2 1.6 136 3.9 7.1 Comp. steel AS-1 AS 14.1 1.1 1.5 149 2.4 7.3 Comp. steel AT-1 AT 15.1 1.1 1.0 136 3.0 8.0 Comp. steel AU-1 AU 25.3 1.0 1.6 139 2.8 7.4 Comp. steel AV-1 AV 19.0 1.1 0.2 131 3.4 7.4 Comp. steel AW-1 AW 14.2 1.1 2.3 137 2.7 8.3 Comp. steel AX-1 AX 14.1 1.1 2.1 140 3.4 9.0 Comp. steel AY-1 AY 15.1 1.1 1.0 149 4.4 7.0 Comp. steel AZ-1 AZ 25.2 1.0 1.5 136 3.5 7.9 Comp. steel

(11) As shown in Table 3, in the steel sheets of the present invention (A-1 to Z-1), in each case, the Vicker's hardness was 170 HV or less and the ratio of the number of carbides at the ferrite grain boundaries to the number of carbides inside the ferrite grains (number of grain boundary carbides/number of grain carbides) was over 1. Hardness is an indicator of cold formability, so it is understood the steel sheets of the present invention (A-1 to Z-1) were excellent in cold formability.

(12) As opposed to this, in Comparative Steel sheet AA-1, the amount of Si was large, in Comparative Steel sheet AB-1, the amount of C was large, and in Comparative Steel sheet AD-1, the amount of Mn was large. In each case, the Vicker's hardness was over 170 HV.

(13) In the Comparative Steel sheet AH-1, the amount of C was small and the A.sub.3 point was high, so hardening was impossible. In Comparative Steel sheet AE-1, the amount of Si was small and the Vicker's hardness was less than 120 HV. Not only that, the ductility after heat treatment fell. In each of the other comparative steel sheets, the chemical composition was outside the scope of the chemical composition of the steel sheets of the present invention, so the ductility after heat treatment fell.

Example 2

(14) To investigate the effects of the conditions of finish rolling in hot rolling and the coiling process and two-stage annealing process of steel sheet, Test Use Steel sheets A-2 to Z-2 were prepared in the following way. That is, first, Steel Slabs A to Z of the chemical compositions shown in Table 1-1 and Table 1-2 were heated at 1240 C. for 1.8 hours, then hot rolled. The finish rolling of the hot rolling was completed under the conditions shown in Table 4, then the steel sheets were cooled on the ROT by a 45 C./sec cooling rate and were coiled up at the coiling temperature shown in Table 4 to produce sheet thickness 3.0 mm hot rolled steel sheet coils.

(15) Each of the hot rolled steel sheet coils was pickled, then annealed under the annealing conditions shown in Table 4 by two-stage step type box annealing. From the annealed hot rolled steel sheet, a sample of a sheet thickness of 3.0 mm for evaluation of the characteristics was taken and measured for ferrite grain size (m), Vicker's hardness (HV), ratio of the number of carbides at the ferrite grain boundaries to the number of carbides inside the ferrite grains (number of grain boundary carbides/number of grain carbides), and ductility after heat treatment (%). The results are shown in Table 5.

(16) TABLE-US-00006 TABLE 4 Hot rolling Annealing conditions conditions Cooling Finish hot 1st stage 2nd stage speed rolling Coiling Holding Holding Holding Holding down to Steel temp. temp. temp. time temp. time 650 C. Sample slab [ C.] [ C.] [ C.] [hr] [ C.] [hr] ( C./hr) Remarks A-2 A 820 510 700 25 760 8 10 Inv. steel B-2 B 750 510 700 25 760 6 10 Comp. steel C-2 C 880 510 710 25 760 8 5 Inv. steel D-2 D 880 650 700 25 760 8 10 Comp. steel E-2 E 880 510 600 25 760 8 10 Comp. steel F-2 F 880 510 700 25 760 8 10 Inv. steel G-2 G 880 510 730 25 760 8 10 Comp. steel H-2 H 880 510 700 1 760 8 10 Comp. steel I-2 I 880 510 700 25 760 8 10 Inv. steel J-2 J 880 510 700 25 720 8 10 Comp. steel K-2 K 880 510 700 25 760 1 10 Comp. steel L-2 L 880 510 700 25 760 8 10 Inv. steel M-2 M 880 510 700 25 760 8 100 Comp. steel N-2 N 750 510 700 25 760 8 10 Comp. steel O-2 O 880 510 700 25 760 8 10 Inv. steel P-2 P 880 510 730 25 760 8 10 Comp. steel Q-2 Q 880 510 700 1 760 8 10 Comp. steel R-2 R 880 510 700 25 760 1 10 Comp. steel S-2 S 880 510 700 25 760 8 10 Inv. steel T-2 T 880 510 700 25 760 8 100 Comp. steel U-2 U 880 650 700 25 760 8 10 Comp. steel V-2 V 880 510 700 1 760 8 10 Comp. steel W-2 W 880 510 700 25 800 8 10 Comp. steel X-2 X 750 510 700 25 760 8 10 Comp. steel Y-2 Y 880 510 730 25 760 8 10 Comp. steel Z-2 Z 880 510 700 25 760 8 10 Inv. steel

(17) As shown in Table 5, in the steel sheets of the present invention, in all cases, the Vickers hardness was 170 HV or less and the ratio of the number of carbides at the ferrite grain boundaries to the number of carbides in the ferrite grains was over 1. Hardness is an indicator of cold formability, so it is understood the steel sheets of the present invention all were excellent in cold formability. Furthermore, the steel sheets of the present invention all had 10% or more ductility after heat treatment, so it is understood they were excellent in ductility after heat treatment.

(18) As opposed to this, in the comparative steel sheets, the manufacturing conditions are outside the scope of the manufacturing conditions of the method for production of the present invention, so the Vicker's hardness rises. Further, in some of the comparative steel sheets, the number of carbides at grain boundaries/number of carbides in grains also fell.

(19) TABLE-US-00007 TABLE 5 Carbide average No. of carbides at Ductility grain Pearlite Vicker's grain boundaries/No. after beat Steel size area ratio hardness of carbides inside treatment Sample slab [m] [%] [HV] grains [%] Remarks A-2 A 1.04 1.2 146.4 4.23 13.0 Inv. steel B-2 B 0.63 10.3 188.0 3.23 11.2 Comp. steel C-2 C 1.21 0.9 140.2 3.67 13.1 Inv. steel D-2 D 0.63 9.2 177.6 3.22 13.3 Comp. steel E-2 E 0.45 8.1 174.8 4.19 12.6 Comp. steel F-2 F 1.95 0.4 164.0 7.31 13.5 Inv. steel G-2 G 0.60 12.3 196.1 2.20 11.2 Comp. steel H-2 H 0.43 7.2 178.0 3.69 13.0 Comp. steel I-2 I 0.85 2.3 155.5 4.77 13.3 Inv. steel J-2 J 0.35 2.1 204.5 0.88 12.6 Comp. steel K-2 K 0.60 5.6 190.2 2.82 13.6 Comp. steel L-2 L 0.96 1.9 159.6 4.87 11.2 Inv. steel M-2 M 0.56 13.5 182.4 3.09 13.1 Comp. steel N-2 N 0.95 6.2 198.0 1.45 11.5 Comp. steel O-2 O 0.80 0.5 146.0 4.27 11.2 Inv. steel P-2 P 1.11 10.2 187.3 5.10 12.3 Comp. steel Q-2 Q 0.82 7.8 173.8 4.52 13.4 Comp. steel R-2 R 0.89 7.2 196.8 2.89 12.8 Comp. steel S-2 S 0.91 2.0 158.3 2.27 13.8 Inv. steel T-2 T 0.59 15.3 178.4 1.74 13.3 Comp. steel U-2 U 0.64 8.4 188.6 2.75 13.0 Comp. steel V-2 V 1.01 6.5 188.0 3.10 10.8 Comp. steel W-2 W 1.30 16.2 178.0 3.11 12.9 Comp. steel X-2 X 0.84 6.3 178.6 2.68 13.5 Comp. steel Y-2 Y 0.58 8.2 202.6 0.85 12.5 Comp. steel Z-2 Z 0.78 1.5 146.6 3.50 12.5 Inv. steel

INDUSTRIAL APPLICABILITY

(20) As explained above, according to the present invention, it is possible to provide steel sheet excellent in cold formability and ductility after heat treatment and a method for production thereof. Accordingly, the present invention has a high applicability in manufacture of steel sheet and industries utilizing it.