STEEL PLATE AND METHOD OF PRODUCTION OF SAME

Abstract

A steel plate improved in formability and wear resistance, having a predetermined chemical composition, having a metal structure of the steel plate having a ratio of number of carbides at ferrite grain boundaries with respect to a number of carbides in ferrite grains of over 1 and having a ferrite grain size of 5 m to 50 m, and having a Vickers hardness of steel plate of 100 HV to 170 HV.

Claims

1. A steel plate comprising, by mass %, C: 0.10 to 0.40%, Si: 0.01 to 0.30%, Mn: 1.00 to 2.00%, P: 0.020% or less, S: 0.010% or less, Al: 0.001 to 0.10%, N: 0.010% or less, O: 0.020% or less, Cr: 0.50% or less, Mo: 0.10% 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, Ni: 0.10% or less, Sn: 0.050% or less, Sb: 0.050% or less, As: 0.050% or less, Mg: 0.050% or less, Ca: 0.050% or less, Y: 0.050% or less, Zr: 0.050% or less, La: 0.050% or less, Ce: 0.050% or less and a balance of Fe and unavoidable impurities, herein a metal structure of the steel plate has a ratio of a number of carbides at ferrite grain boundaries with respect to a number of carbides in ferrite grains of over 1, has a ferrite grain size of 5m to 50 m, and has an area ratio of pearlite of 6% or less; and a Vickers hardness of the steel plate is 100 HV to 170 HV.

2. The steel plate according to claim 1 containing one or both of Ti: 0.10% or less and B: 0.010% or less instead of part of the Fe.

3. A method of production of a steel plate, comprising hot rolling a steel slab of a chemical composition according to claim 1 during which completing finish hot rolling at a 750 C. to 850 C. temperature region to obtain hot rolled steel plate, coiling the hot rolled steel plate at 400 C. to 550 C., pickling the coiled up hot rolled steel plate, holding the pickled hot rolled steel plate at a 650 C. to 720 C. temperature region for 3 hours to 60 hours as first stage annealing, then holding the hot rolled steel plate at a 725 C. to 790 C. temperature region for 3 hours to 50 hours as second stage annealing, and cooling the annealed hot rolled steel plate to 650 C. by a cooling rate of 1 C./hour to 30 C./hour.

4. A method of production of a steel plate, comprising hot rolling a steel slab of a chemical composition according to claim 2 during which completing finish hot rolling at a 750 C. to 850 C. temperature region to obtain hot rolled steel plate, coiling the hot rolled steel plate at 400 C. to 550 C., pickling the coiled up hot rolled steel plate, holding the pickled hot rolled steel plate at a 650 C. to 720 C. temperature region for 3 hours to 60 hours as first stage annealing, then holding the hot rolled steel plate at a 725 C. to 790 C. temperature region for 3 hours to 50 hours as second stage annealing, and cooling the annealed hot rolled steel plate to 650 C. by a cooling rate of 1 C./hour to 30 C./hour.

Description

EXAMPLES

[0125] Next, examples will be explained, but the levels in the examples are illustrations of conditions 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.

[0126] The cold workability was evaluated by taking a JIS No. 5 tensile test piece from the plate thickness 3 mm material as annealed and conducting a tensile test. The total elongations in the direction of 0 from the rolling direction and the direction of 90 from the rolling direction were evaluated. In the case where, in both directions, they were 35% or more and the difference of the total elongations |EL| in the two directions was 4% or less, it is judged that the cold workability was excellent.

[0127] The hardenability was evaluated by grinding a plate thickness 3 mm material as annealed to a plate thickness 1.5 mm, holding it in a vacuum atmosphere at 880 C.10 minutes, hardening it by a 30 C./sec cooling rate, and judging the hardenability was excellent if the fraction of martensite was 60% or more.

Example 1

[0128] Continuously cast slabs of the chemical compositions shown in Table 1 (steel ingots) were heated at 1240 C. for 1.8 hours, then hot rolled. The finish hot rolling was completed at 890 C. After that, the plates were coiled up at 510 C. to produce plate thickness 3 mm hot rolled coils. The hot rolled coils were pickled and loaded into a box type annealing furnace. The atmosphere was controlled so as to include 95% hydrogen-5% nitrogen. The coils were heated from room temperature to 705 C. and held at 705 C. for 36 hours to make the temperature distribution inside the hot rolled coils uniform, then were heated to 760 C. and held at 760 C. for 10 hours.

[0129] After that, the steel plates 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. Note that, the structures of the samples were measured by the above-mentioned method.

TABLE-US-00001 TABLE 1 C Si Mn P S Al N O Ti Cr Mo B Nb V Cu W A 0.34 0.23 1.47 0.0044 0.004 B 0.34 0.19 1.53 0.0084 0.0064 C 0.16 0.13 1.15 0.0055 0.0052 0.062 0.071 0.033 D 0.11 0.26 1.76 0.0076 0.0027 0.01 0.019 E 0.35 0.25 1.71 0.0046 0.001 F 0.32 0.19 1.04 0.0155 0.0043 0.0082 0.086 0.085 G 0.67 0.08 1.29 0.0065 0.0038 H 0.22 0.12 1.84 0.0018 0.0091 0.046 I 0.16 0.06 1.15 0.004 0.0053 0.087 0.342 0.0091 J 0.21 0.12 1.35 0.0082 0.0078 0.02 0.003 0.037 K 0.13 0.21 1.76 0.003 0.0004 0.0148 0.005 L 0.18 0.13 1.34 0.0053 0.0064 0.0002 M 0.19 0.04 1.95 0.0087 0.0038 N 0.21 0.16 1.86 0.02 0.0098 O 0.37 0.3 1.68 0.0112 0.0044 0.0121 1.06 0.39 0.062 P 0.37 0.19 2 0.0074 0.0093 Q 0.12 1.18 1.09 0.0071 0.0074 R 0.38 0.09 1.11 0.0081 0.0016 0.0056 0.003 S 0.31 0.08 1.84 0.0104 0.0011 0.053 T 0.19 0.19 1.91 0.0086 0.0057 U 0.21 0.02 1.02 0.0113 0.28 V 0.17 0.02 1.07 0.0194 0.0073 W 0.11 0.01 1.41 0.0081 0.0034 0.014 0.043 X 0.35 0.28 1.16 0.0013 0.0005 0.09 Y 0.23 0.02 1.44 0.0006 0.0057 O 0.36 0.03 1.06 0.0132 0.002 0.093 AA 0.3 0.27 0.71 0.0106 0.001 AB 0.31 0.2 1.52 0.0153 0.0075 0.0021 0.011 AC 0.31 0.11 1.4 0.0045 0.0031 0.0182 0.003 0.052 AD 0.14 0.26 1.34 0.011 0.0009 0.61 0.006 Ta Ni Sn Sb As Mg Ca Y Zr La Ce Remarks A Com. steel B Inv. steel C 0.048 Inv. steel D Inv. steel E Inv. steel F 0.0059 0.045 Inv. steel G Inv. steel H 0.027 0.048 Inv. steel I 0.009 0.011 Com. steel J 0.002 Inv. steel K 0.041 0.043 Com. steel L 0.009 0.008 Inv. steel M Com. steel N Com. steel O 0.008 0.006 Com. steel P Inv. steel Q Inv. steel R 0.064 0.0416 Inv. steel S 0.003 0.093 0.035 Inv. steel T Inv. steel U Inv. steel V Com. steel W 0.023 0.026 0.004 Inv. steel X 0.039 0.02 Inv. steel Y Inv. steel O 0.018 0.037 Inv. steel AA Inv. steel AB 0.002 Inv. steel AC 0.033 Inv. steel AD 0.093 0.05 0.033 Inv. steel

[0130] Table 2 shows the results of measurement or evaluation of the Vickers hardness of the produced samples, the ratio of the number of carbides at the ferrite grain boundaries to the number of carbides inside the ferrite grains, the pearlite area ratio, the cold workability, and the hardenability.

TABLE-US-00002 TABLE 2 Hot rolling conditions Carbide Ferrite Pearlite Vickers No. of grain Finish hot rolling Coiling size grain size area ratio hardness boundary carbides/ temp. [ C.] temp. [ C.] [m] [m] [%] [HV] No. of grain carbides A-1 776 591 0.94 18.6 1.1 130 8.65 B-1 815 490 1.09 22.1 0.3 123 7.94 C-1 798 369 1.23 27.2 1.0 110 9.14 D-1 921 500 1.08 32.0 1.6 118 6.94 E-1 763 410 1.18 24.5 1.2 125 6.17 F-1 824 442 1.15 21.6 0.6 121 8.07 G-1 773 427 1.17 18.4 1.6 149 9.14 H-1 820 485 1.09 26.1 1.5 114 6.88 I-1 710 481 1.09 22.7 0.6 110 9.42 J-1 810 497 1.08 23.8 1.8 113 9.52 K-1 784 407 1.19 32.5 2.0 114 7.64 L-1 759 543 1.02 23.1 0.3 114 9.00 M-1 791 541 1.02 26.4 0.3 109 8.48 N-1 806 499 1.08 25.9 2.1 116 8.17 O-1 778 478 1.04 19.0 0.9 135 8.22 P-1 807 453 1.13 25.2 1.2 121 8.47 Q-1 832 542 1.02 18.9 8.3 178 5.99 R-1 758 511 1.07 20.5 1.9 119 7.94 S-1 840 391 1.20 27.0 0.7 111 6.97 T-1 756 538 1.02 25.4 1.9 119 6.18 U-1 817 510 1.07 22.6 1.5 106 7.70 V-1 788 633 0.87 19.4 1.3 110 6.35 W-1 761 446 1.14 31.9 1.4 100 9.85 X-1 831 455 1.14 20.9 0.7 129 8.74 Y-1 818 440 1.15 25.6 1.9 106 7.94 Z-1 763 456 1.14 21.9 0.6 112 8.14 AA-1 824 414 1.19 21.8 0.7 123 6.09 AB-1 843 454 1.13 23.1 1.5 122 8.94 AC-1 834 508 1.07 21.8 0.9 117 9.51 AD-1 791 460 1.13 18.0 1.8 138 7.13 Total elongation [%] Elongation Martensite 0 90 anisotropy fraction direction direction | EL.sub.0 EL.sub.90 | [%] Remarks A-1 33.8 34.3 0.5 93 Comp. steel B-1 39.0 39.7 0.7 95 Inv. steel C-1 41.5 43.0 1.5 62 Comp. steel D-1 40.0 40.7 0.7 72 Comp. steel E-1 38.7 39.6 0.9 94 Inv. steel F-1 39.4 40.5 1.1 74 Inv. steel G-1 34.3 35.6 1.3 98 Comp. steel H-1 40.7 41.6 0.9 96 Inv. steel I-1 41.4 42.0 0.6 69 Comp. steel J-1 40.8 42.3 1.5 75 Inv. steel K-1 40.7 42.1 1.4 83 Inv. steel L-1 40.6 41.1 0.5 72 Inv. steel M-1 41.6 42.9 1.3 97 Inv. steel N-1 40.3 41.6 1.3 95 Inv. steel O-1 36.9 37.5 0.6 38 Comp. steel P-1 39.4 40.8 1.4 100 Inv. steel Q-1 28.8 29.6 0.8 7 Comp. steel R-1 39.8 40.6 0.8 82 Inv. steel S-1 41.2 42.6 1.4 105 Comp. steel T-1 39.8 40.7 0.9 95 Inv. steel U-1 38.6 43.4 4.8 69 Comp. steel V-1 32.8 33.8 1.0 68 Comp. steel W-1 43.2 44.2 1.0 67 Inv. steel X-1 38.5 39.4 0.9 81 Inv. steel Y-1 42.1 42.9 0.8 81 Inv. steel Z-1 41.0 41.5 0.5 78 Inv. steel AA-1 39.0 40.3 1.3 51 Comp. steel AB-1 39.3 39.8 0.5 92 Inv. steel AC-1 40.1 41.6 1.5 87 Inv. steel AD-1 34.8 35.2 0.4 5 Comp. steel

[0131] As shown in Table 2, Invention Steels B-1, E-1, F-1, H-1, J-1, K-1, L-1, M-1, N-1, P-1, R-1, T-1, W-1, X-1, Y-1, Z-1, AB-1, and AC-1 all have a ratio of the number of carbides at the ferrite grain boundaries with respect to the number of carbides inside the ferrite grains of over 1, a Vickers hardness of 170 HV or less, and excellent cold workability and hardenability.

[0132] As opposed to this, Comparative Steel G-1 is high in amount of C and deteriorates in cold workability. Comparative Steel O-1 is high in amount of Mo and amount of Cr and is high in stability of carbides, so the carbides do not dissolve at the time of hardening, the amount of formation of austenite is small, and the hardenability is inferior.

[0133] Comparative Steels Q-1 and AD-1 are high in amounts of Si and Al and high in A3 point, so the amount of formation of austenite at the time of hardening is small and the hardenability is inferior. Comparative Example U-1 is high in amount of S, has coarse MnS formed in the steel, and is low in cold workability. Comparative Example AA-1 is low in amount of Mn and inferior in hardenability.

[0134] Comparative Example I-1 is low in finishing temperature of hot rolling and deteriorates in productivity. Comparative Example D-1 is high in finishing temperature of hot rolling and has scale flaws formed at the steel plate surface. Comparative Examples C-1 and S-1 are low in coiling temperature of hot rolling, are increased in number of bainite, martensite, and other low temperature transformed structures, become brittle resulting in frequent fracture at the time of pay out of the hot rolled coil, and deteriorates in productivity.

[0135] Comparative Examples A-1 and V -1 are high in coiling temperature of hot rolling and have hot rolled structures formed with large lamellar spacing bulky pearlite and high heat stability needle-shaped coarse carbides. The carbides remain in the steel plate even after two-stage step type annealing and the cold workability deteriorates.

Example 2

[0136] To investigate the effects of the annealing conditions, steel slabs of the chemical compositions shown in Table 1 were heated at 1240 C. for 1.8 hours, then used for hot rolling. The finish hot rolling was ended at 820 C., then the plates were cooled on the ROT by a 45 C./sec cooling rate down to 520 C. and coiled at 510 C. to produce plate thickness 3.0 mm hot rolled coils. These were annealed by two-stage step type box annealing under the annealing conditions shown in Table 3 to prepare plate thickness 3.0 mm samples.

[0137] Table 3 shows the results of measurement or evaluation of the carbide size, ferrite grain size, Vickers hardness, ratio of the number of carbides at the ferrite grain boundaries to the number of carbides in the ferrite grains, pearlite area ratio, cold workability, and hardenability of the produced samples.

TABLE-US-00003 TABLE 3 1st stage annealing 2nd stage Holding Holding Holding Holding Cooling Carbide Ferrite Pearlite Vickers temp. time temp. time rate size grain size area ratio hardness [ C.] [hr] [ C.] [hr] [ C./sec] [m] [m] [%] [HV] A-2 748 25 787 38 18 1.23 26.3 7.5 176 B-2 659 60 789 9 8 1.51 32.1 0.8 134 C-2 657 39 755 38 17 0.96 21.6 1.2 117 D-2 687 50 763 15 36 0.87 23.3 0.4 154 E-2 667 48 766 44 17 1.10 21.1 0.3 135 F-2 701 47 739 47 11 2.23 26.0 1.9 131 G-2 692 26 734 27 13 0.92 12.6 0.0 158 H-2 710 40 733 23 19 1.02 15.2 0.8 143 I-2 677 49 783 8 9 1.33 31.7 1.9 117 J-2 665 23 773 24 11 1.28 28.6 0.3 118 K-2 654 22 785 20 28 1.01 30.6 0.8 120 L-2 705 1 739 21 8 1.17 17.4 0.9 165 M-2 658 9 776 50 27 1.01 26.7 1.9 110 N-2 715 15 774 19 10 1.37 30.7 0.5 128 O-2 680 46 760 13 18 0.92 15.5 8.3 175 P-2 674 6 731 1 18 0.93 10.9 13.2 213 Q-2 673 15 786 44 13 1.16 35.4 0.5 162 R-2 680 11 769 29 12 1.17 22.3 1.9 122 S-2 692 45 749 9 9 1.30 20.7 1.9 150 T-2 618 25 773 35 5 1.77 18.6 1.6 161 U-2 705 22 779 27 18 1.07 27.4 0.2 103 V-2 669 25 787 42 26 0.98 30.7 0.3 106 W-2 677 89 772 42 28 0.93 29.5 0.7 102 X-2 684 34 710 34 5 0.74 7.4 1.1 160 Y-2 652 48 761 54 13 1.14 24.5 8.0 173 Z-2 677 38 730 7 17 0.79 9.4 1.3 149 AA-2 668 23 732 39 25 0.56 13.2 1.8 134 AB-2 669 42 811 35 24 1.12 25.7 10.2 188 AC-2 698 5 748 37 2 1.87 27.6 1.5 131 AD-2 679 59 777 21 11 1.19 24.7 1.1 134 Total elongation No. of grain [%] Elongation Martensite boundary carbides/ 0 90 anisotropy fraction No. of grain carbides direction direction | EL.sub.0 EL.sub.90 | [%] Remarks A-2 1.9 29.2 30.4 1.2 93 Comp. steel B-2 8.9 37.0 37.5 0.5 95 Inv. steel C-2 5.3 40.0 40.9 0.9 69 Inv. steel D-2 4.7 33.3 34.0 0.7 81 Comp. steel E-2 5.4 36.8 38.0 1.2 91 Inv. steel F-2 8.3 37.5 38.3 0.8 74 Inv. steel G-2 7.8 32.5 33.5 1.0 98 Comp. steel H-2 4.1 35.3 35.8 0.5 96 Inv. steel I-2 7.1 40.1 41.2 1.1 62 Inv. steel J-2 8.2 40.0 41.0 1.0 75 Inv. steel K-2 4.7 39.6 40.3 0.7 83 Inv. steel L-2 7.4 31.3 32.0 0.7 72 Comp. steel M-2 3.8 41.4 42.4 1.0 97 Inv. steel N-2 9.6 38.1 39.6 1.5 95 Inv. steel O-2 7.5 29.4 29.9 0.5 35 Comp. steel P-2 0.4 22.3 23.8 1.5 98 Comp. steel Q-2 3.1 31.8 32.3 0.5 7 Comp. steel R-2 6.0 39.2 40.2 1.0 82 Inv. steel S-2 7.8 39.4 40.5 1.1 96 Inv. steel T-2 7.7 31.9 33.0 1.1 95 Comp. steel U-2 3.3 38.1 43.8 5.7 70 Comp. steel V-2 2.7 44.2 45.0 0.8 72 Inv. steel W-2 3.2 43.0 44.2 1.2 67 Comp. steel X-2 0.5 38.1 38.5 0.4 81 Comp. steel Y-2 5.7 29.7 30.3 0.6 81 Comp. steel Z-2 9.4 36.8 37.8 1.0 78 Inv. steel AA-2 3.1 36.9 37.4 0.5 49 Comp. steel AB-2 2.5 27.0 27.5 0.5 92 Comp. steel AC-2 7.9 37.5 38.5 1.0 87 Inv. steel AD-2 5.7 37.0 37.5 0.5 4 Comp. steel

[0138] As shown in Table 3, the Invention Steels B-2, C-2, E-2, F-2, H-2, 1-2, J-2, K-2, M-2, N-2, R-2, S-2, V-2, Z-2, and AC-2 all have a ratio of the number of carbides at the ferrite grain boundaries with respect to the number of carbides inside the ferrite grains of over 1 and a Vickers hardness of 170 HV or less and are excellent in cold workability and hardenability.

[0139] As opposed to this, Comparative Steel G-1 is high in amount of C and deteriorates in cold workability. Comparative Steel O-1 is high in amount of Mo and amount of Cr and deteriorates in cold workability. Further, the carbides are high in stability, so at the time of hardening, the carbides will not dissolve, the amount of production of austenite is small, and the hardenability is inferior.

[0140] Comparative Steel Q-1 is high in amount of Si and high in hardness of ferrite, so deteriorates in workability. Further, it is high in the A3 point, so the amount of production of austenite at the time of hardening is small and the hardenability is inferior. Comparative Steel AD-1 is high in amount of Al and high in A3 point, so the amount of production of austenite at the time of hardening is small and the hardenability is inferior. Comparative Steel U-1 is high in amount of S and is formed with coarse MnS in the steel, so deteriorates in cold workability. Comparative Steel AA-1 is low in amount of Mn and inferior in hardenability.

[0141] Comparative Steel T-2 is low in holding temperature at the time of the first stage annealing of the two-stage step type box annealing, is insufficient in coarsening treatment of carbides at the Ac1 temperature or less, and is insufficient in thermal stability of the carbides, so is reduced in carbides remaining at the time of second stage annealing, cannot be suppressed in pearlite transformation in the structure after gradual cooling, and deteriorates in cold workability.

[0142] Comparative Steel A-2 has a high holding temperature at the time of the first stage annealing of the two-stage step type box annealing, is formed with austenite during the annealing, cannot be raised in stability of carbides, is decreased in carbides remaining at the time of second stage annealing, cannot be suppressed in pearlite transformation in the structure after gradual cooling, and deteriorates in cold workability.

[0143] Comparative Steel L-2 is short in holding time at the time of the first stage annealing of the two-stage step type annealing, is insufficient in the coarsening treatment of the carbides at the Ac1 temperature or less, and is insufficient in the thermal stability of the carbides, so is decreased in the carbides remaining at the time of the second stage annealing, cannot suppress pearlite transformation in the structure after gradual cooling, and deteriorates in cold workability.

[0144] Comparative Steel W-2 is long in holding time at the time of the first stage annealing of the two-stage step type annealing and deteriorates in productivity. Comparative Steel X-2 is low in holding temperature at the time of the second stage annealing at the time of two-stage step annealing, is small in amount of production of austenite, cannot be increased in number ratio of carbides at the grain boundaries, and deteriorates in cold workability.

[0145] Comparative Steel AB-2 is high in holding temperature at the time of second stage annealing in the two-stage step type box annealing and is promoted in dissolution of the carbides, so decreases the residual carbides, cannot suppress pearlite transformation in the structure after gradual cooling, and deteriorates in cold forgeability.

[0146] Comparative Steel P-2 is low in holding temperature at the time of second stage annealing of the two-stage step type box annealing, has little formation of austenite, cannot be increased in the number ratio of carbides at the ferrite grain boundaries, and deteriorates in cold workability Comparative Steel Y-2 is long in holding time at the time of second stage annealing of the two-stage step type box annealing and is promoted in dissolution of carbides, so is decreased in remaining carbides, cannot suppress pearlite transformation in the structure after gradual cooling, and deteriorates in cold forgeability.

[0147] Comparative Steel D-2 is large in cooling rate from the end of the second stage annealing of the two-stage step type box annealing down to 650 C., experiences pearlite transformation at the time of cooling, and deteriorates in cold workability.

INDUSTRIAL APPLICABILITY

[0148] As explained above, according to the present invention, it is possible to produce and provide steel plate excellent in formability and wear resistance. The steel plate of the present invention is steel plate suitable as a material for auto parts, edged tools, and other machine parts produced through stamping, bending, press-forming, and other working processes, so the present invention is high in industrial applicability.