780 MPA-CLASS COLD-ROLLED AND ANNEALED DUAL-PHASE STEEL AND MANUFACTURING METHOD THEREFOR

Abstract

Disclosed is a cold-rolled and annealed dual-phase steel having a tensile strength of greater than 780 MPa. A matrix structure thereof is fine and uniform martensite+ferrite. The cold-rolled and annealed dual-phase steel contains the following chemical elements in the following mass percentages: C: 0.1%-0.13%, Si: 0.4%-0.8%, Mn: 1.65%-1.9%, Al: 0.01%-0.05%, Nb: 0.01-0.03%, and Ti: 0.01-0.03%. Furthermore, the cold-rolled annealed dual-phase steel does not contain the elements Cr or Mo. In addition, also disclosed is a method for manufacturing the cold-rolled and annealed dual-phase steel, comprising smelting and continuous casting, hot rolling, cold rolling, annealing, tempering and flattening. The cold-rolled and annealed dual-phase steel of the present invention is not only economical, but also has the characteristics of high strength, excellent elongation and cold bending properties.

Claims

1. A cold-rolled and annealed dual-phase steel having a tensile strength of >780 MPa, wherein the cold-rolled and annealed dual-phase steel has a matrix structure of fine and uniform martensite+ferrite, wherein the cold-rolled and annealed dual-phase steel comprises the following chemical elements in mass percentages, in addition to Fe: C: 0.1%-0.13%, Si: 0.4%-0.8%, Mn: 1.65%-1.9%, Al: 0.01%-0.05%, Nb: 0.01-0.03%, Ti: 0.01-0.03%; wherein the cold-rolled and annealed dual-phase steel is free of Cr and Mo elements.

2. The cold-rolled and annealed dual-phase steel according to claim 1, wherein the chemical elements have the following mass percentages: C: 0.1%-0.13%, Si: 0.4%-0.8%, Mn: 1.65%-1.9%, Al: 0.01%-0.05%, Nb: 0.01-0.03%, Ti: 0.01-0.03%, and a balance of Fe and other unavoidable impurities.

3. The cold-rolled and annealed dual-phase steel according to claim 1, wherein the chemical elements have mass percentage contents satisfying at least one of the following: C: 0.11-0.125%, Si: 0.5%-0.7%, Mn: 1.7%-1.8%, Al: 0.015%-0.045%, Nb: 0.015-0.025%, Ti: 0.015-0.025%.

4. The cold-rolled and annealed dual-phase steel according to claim 2, wherein the unavoidable impurities include P, S and N elements, and contents thereof are controlled to be at least one of the following: P≤0.015%, S≤0.003%, N≤0.005%.

5. The cold-rolled and annealed dual-phase steel according to claim 1, wherein mass percentage contents of Nb and Ti further satisfy: Nb %+Ti %×3≥0.047%.

6. The cold-rolled and annealed dual-phase steel according to claim 1, wherein the martensite has a phase proportion of >55%.

7. The cold-rolled and annealed dual-phase steel according to claim 1, wherein the martensite has a grain diameter of not greater than 5 microns, and ferrite has a grain diameter of not greater than 5 microns.

8. The cold-rolled and annealed dual-phase steel according to claim 1, wherein its performances satisfy at least one of the following: yield strength>420 MPa; tensile strength>780 MPa; elongation at break with A.sub.50 gauge length 18%; 90 degree cold bending parameter R/t≤1, where R represents bending radius in mm, t represents plate thickness in mm.

9. The cold-rolled and annealed dual-phase steel according to claim 5, wherein the mass percentage contents of Nb and Ti further satisfy: 0.047%≤Nb %+Ti %×3≤0.10%.

10. A manufacturing method for the cold-rolled and annealed dual-phase steel according to claim 1, wherein the method comprises steps of: (1) Smelting and continuous casting; (2) Hot rolling; (3) Cold rolling; (4) Annealing: annealing soaking temperature: 770-820° C.; annealing time: 40-200 s; cooling at a rate of 3-5° C./s to a starting temperature of rapid cooling; rapid cooling at a rate of 30-80° C./s, wherein the starting temperature of rapid cooling is 650-730° C., and the rapid cooling is ended at a temperature of 200-270° C.; (5) Tempering; and (6) Temper rolling.

11. The manufacturing method according to claim 10, wherein in step (2), a slab is first heated to 1160-1220° C.; held for 0.6 hours or longer; hot rolled at a temperature of 850-900° C.; rapidly cooled at a rate of 30-80° C./s after rolling; coiled with the coiling temperature being controlled at 500-600° C.; and air cooled after the coiling.

12. The manufacturing method according to claim 10, wherein in step (3), a cold rolling reduction rate is controlled at 50-70%.

13. The manufacturing method according to claim 10, wherein in step (5), a tempering temperature is controlled at 200-270° C., and a tempering time is 100-400 s.

14. The manufacturing method according to claim 10, wherein in step (6), a temper rolling reduction rate is controlled at 50.3%.

15. The manufacturing method according to claim 10, wherein in step (4), the annealing soaking temperature is 790-810° C.

16. The cold-rolled and annealed dual-phase steel according to claim 2, wherein the chemical elements have mass percentage contents satisfying at least one of the following: C: 0.11-0.125%, Si: 0.5%-0.7%, Mn: 1.7%-1.8%, Al: 0.015%-0.045%, Nb: 0.015-0.025%, and Ti: 0.015-0.025%.

17. The cold-rolled and annealed dual-phase steel according to claim 2, wherein mass percentage contents of Nb and Ti further satisfy: Nb %+Ti %×3≥0.047%.

18. The cold-rolled and annealed dual-phase steel according to claim 2, wherein: the martensite has a phase proportion of >55%; and/or, the martensite has a grain diameter of not greater than 5 microns, and ferrite has a grain diameter of not greater than 5 microns; and/or its performances satisfy at least one of the following: yield strength≥420 MPa; tensile strength>780 MPa; elongation at break with A.sub.50 gauge length 18%; 90 degree cold bending parameter R/t≤1, where R represents bending radius in mm, t represents plate thickness in mm.

19. The manufacturing method according to claim 10, wherein the chemical elements of the cold-rolled and annealed dual-phase steel have the following mass percentages: C: 0.1%-0.13%, Si: 0.4%-0.8%, Mn: 1.65%-1.9%, Al: 0.01%-0.05%, Nb: 0.01-0.03%, Ti: 0.01-0.03%, and a balance of Fe and other unavoidable impurities.

20. The manufacturing method according to claim 19, wherein the chemical elements of the cold-rolled and annealed dual-phase steel have mass percentage contents satisfying at least one of the following: C: 0.11-0.125%, Si: 0.5%-0.7%, Mn: 1.7%-1.8%, Al: 0.015%-0.045%, Nb: 0.015-0.025%, and Ti: 0.015-0.025%.

Description

DESCRIPTION OF THE DRAWING

[0060] FIG. 1 shows the structure of the cold-rolled and annealed dual-phase steel of Example 1.

DETAILED DESCRIPTION

[0061] The economical 780 MPa grade cold-rolled and annealed dual-phase steel and the method for manufacturing the same according to the disclosure will be further explained and illustrated with reference to the specific Examples. Nonetheless, the explanation and illustration are not intended to unduly limit the technical solution of the disclosure.

Examples 1-7 and Comparative Examples 1-14

[0062] Table 1 lists the mass percentages of various chemical elements in the steel grades corresponding to the cold-rolled and annealed dual-phase steels in Examples 1-7 and the steels in Comparative Examples 1-14.

TABLE-US-00001 TABLE 1 (wt %, the balance is Fe and unavoidable impurities other than P, S and N) Chemical elements Steel Nb % + grade C Si Mn Al P S N Nb Ti Ti % × 3 Ex. 1 A 0.103 0.45 1.74 0.012 0.014 0.0015 0.0035 0.012 0.021 0.075 Ex. 2 B 0.106 0.52 1.68 0.032 0.013 0.0016 0.0033 0.017 0.015 0.062 Ex. 3 C 0.110 0.55 1.85 0.022 0.009 0.0018 0.0045 0.015 0.026 0.093 Ex. 4 D 0.115 0.43 1.82 0.028 0.011 0.0020 0.0042 0.024 0.024 0.096 Ex. 5 E 0.118 0.61 1.69 0.018 0.012 0.0022 0.0038 0.029 0.023 0.098 Ex. 6 F 0.123 0.66 1.76 0.025 0.011 0.0024 0.0045 0.021 0.021 0.084 Ex. 7 G 0.129 0.72 1.80 0.045 0.009 0.0012 0.0027  0.0195 0.014 0.0615 Comp. H 0.091 0.63 1.77 0.044 0.011 0.0022 0.0037 0.015 0.018 0.069 Ex. 1 Comp. I 0.138 0.64 1.81 0.047 0.009 0.0022 0.0033 0.018 0.022 0.084 Ex. 2 Comp. J 0.121 0.58 1.62 0.033 0.012 0.0015 0.0028 0.026 0.021 0.089 Ex. 3 Comp. K 0.124 0.63 1.99 0.038 0.013 0.0015 0.0035 0.026 0.019 0.083 Ex. 4 Comp. L 0.118 0.62 1.69 0.035 0.011 0.0017 0.0034 0.005 0.024 0.077 Ex. 5 Comp. M 0.117 0.57 1.72 0.026 0.008 0.0012 0.0029 0.020 0.004 0.032 Ex. 6 Comp. N 0.109 0.72 1.75 0.024 0.010 0.0014 0.0043 0.023 0.027 0.104 Ex. 7-14

[0063] The cold-rolled and annealed dual-phase steels in Examples 1-7 according to the present disclosure and the steels in Comparative Examples 1-14 were all prepared by the following steps:

[0064] (1) Smelting and continuous casting: the required alloy components were obtained, and the contents of S and P were minimized;

[0065] (2) Hot rolling: a slab was first heated to 1160-1220° C. which was held for 0.6 hours or more; then hot-rolling at a temperature of 850-900° C. was conducted; after the rolling, rapid cooling was conducted at a rate of 30-80° C./s; the coiling temperature was controlled at 500-600° C.; air cooling was conducted after coiling;

[0066] (3) Cold rolling: the cold rolling reduction rate was controlled at 50-70%;

[0067] (4) Annealing: the annealing soaking temperature was controlled at 770-820° C., alternatively and preferably at 790-810° C.; the annealing time was controlled at 40-200 s; the temperature was decreased to a starting temperature of rapid cooling by cooling at a rate of 3-5° C./s; rapid cooling was conducted at a rate of 30-80° C./s, wherein the starting temperature of the rapid cooling was 650-730° C., and the rapid cooling was ended at a temperature of 200-270° C.;

[0068] (5) Tempering: the tempering temperature was controlled at 200-270° C., and the tempering time was 100-400 s;

[0069] (6) Temper rolling: the temper rolling reduction rate was controlled at ≤0.3%.

[0070] It should be noted that the chemical compositions of the cold-rolled and annealed dual-phase steel in Examples 1-7 and the related process parameters all met the control requirements of the design specification according to the present disclosure. The chemical compositions of the steels in Comparative Examples 1-6 all included parameters that failed to meet the requirements of the design according to the present disclosure. Although the chemical composition of steel grade N in Comparative Examples 7-14 met the requirements of the design according to the present disclosure, the related process parameters all included parameters that failed to meet the requirements of the design according to the present disclosure.

[0071] Tables 2-1 and 2-2 list the specific process parameters for the cold-rolled and annealed dual-phase steels in Examples 1-7 and the steels in Comparative Examples 1-14.

TABLE-US-00002 TABLE 2-1 Step (2) Step (3) Finishing Cold rolling Heating Holding hot rolling Cooling Coiling reduction Steel temperature time temperature rate temperature rate No. grade (° C.) (h) (° C.) (° C./s) (° C.) (%) Ex. 1 A 1210 0.75 855 32 585 52 Ex. 2 B 1205 0.65 870 30 525 67 Ex. 3 C 1195 1.2 890 47 545 58 Ex. 4 D 1187 1.5 886 54 575 50 Ex. 5 E 1169 0.8 864 68 590 55 Ex. 6 F 1211 1.1 895 74 588 62 Ex. 7 G 1191 0.9 885 44 600 48 Comp. H 1187 0.7 875 50 548 50 Ex. 1 Comp. I 1175 1.1 850 62 566 56 Ex. 2 Comp. J 1200 0.95 890 68 535 55 Ex. 3 Comp. K 1213 1.3 900 76 505 52 Ex. 4 Comp. L 1178 1.4 895 46 586 67 Ex. 5 Comp. M 1194 0.8 890 38 533 48 Ex. 6 Comp. N 1153 1.6 866 55 564 62 Ex. 7 Comp. N 1237 1.5 885 60 592 50 Ex. 8 Comp. N 1195 1.4 886 70 480 55 Ex. 9 Comp. N 1186 0.9 895 75 622 58 Ex. 10 Comp. N 1209 1.1 858 49 578 56 Ex. 11 Comp. N 1193 1.3 864 55 555 60 Ex. 12 Comp. N 1169 1.5 877 62 511 52 Ex. 13 Comp. N 1178 0.9 855 48 548 54 Ex. 14

TABLE-US-00003 TABLE 2-2 Step (4) Step (6) Ending Temper Annealing Starting Rapid temperature Step (5) rolling soaking Annealing Cooling temperature of cooling of rapid Tempering Tempering reduction temperature time rate rapid cooling rate cooling temperature time rate No. (° C.) (s) (° C./s) (° C.) (° C./s) (° C.) (° C.) (s) (%) Ex. 1 815 150 5 705 55 200 200 220 0.3 Ex. 2 785 90 4 715 44 250 250 300 0.2 Ex. 3 796 105 5 670 65 265 265 210 0.3 Ex. 4 784 180 3 720 70 270 270 250 0.2 Ex. 5 810 60 4 680 66 235 235 175 0.1 Ex. 6 808 175 5 725 58 240 240 205 0.2 Ex. 7 777 85 3 695 57 266 266 380 0.1 Comp. 790 150 3 675 48 216 216 330 0.1 Ex. 1 Comp. 796 90 3 670 36 208 208 205 0.3 Ex. 2 Comp. 800 120 4 705 48 262 262 190 0.1 Ex. 3 Comp. 811 180 5 720 62 225 225 210 0.3 Ex. 4 Comp. 789 60 5 725 58 239 239 250 0.2 Ex. 5 Comp. 812 75 5 700 80 252 252 120 0.1 Ex. 6 Comp. 786 140 4 655 70 227 227 300 0.3 Ex. 7 Comp. 793 135 4 700 62 264 264 125 0.3 Ex. 8 Comp. 811 95 3 680 48 240 240 175 0.2 Ex. 9 Comp. 806 165 5 685 80 215 215 205 0.1 Ex. 10 Comp. 758 125 4 675 35 235 235 380 0.1 Ex. 11 Comp. 835 65 4 705 56 242 242 280 0.3 Ex. 12 Comp. 810 75 3 710 48 178 178 300 0.2 Ex. 13 Comp. 795 120 3 695 72 292 292 280 0.1 Ex. 14

[0072] It should be noted that, as shown in Table 2-2, the ending temperature of rapid cooling and the tempering temperature in each Example and in each Comparative Example are the same. The reason is that, in the actual process operation, the tempering operation was performed right after the rapid cooling operation was ended.

[0073] A variety of performance tests were performed on the cold-rolled and annealed dual-phase steels in Examples 1-7 and the steels in Comparative Examples 1-14. The test results obtained are listed in Table 3. As to the performance test method, GB/T 13239-2006 Metallic Materials—Tensile Testing at Low Temperature was referred to. A standard sample was prepared, and subjected to static stretching on a tensile testing machine to obtain a corresponding stress-strain curve. After data processing, the parameters of yield strength, tensile strength and elongation at break were obtained finally.

[0074] Table 3 lists the performance test results for the cold-rolled and annealed dual-phase steels in Examples 1-7 and the steels in Comparative Examples 1-14.

TABLE-US-00004 TABLE 3 90° Plate Yield Tensile Elongation bending thickness strength strength at break A.sub.50 radius R t No. (MPa) (MPa) (%) (mm) (mm) R/t Ex. 1 454 800 22.3 1.0 1.1 0.91 Ex. 2 435 812 21.5 1.0 1.1 0.91 Ex. 3 474 856 19.5 1.0 1.1 0.91 Ex. 4 449 832 20.5 1.0 1.2 0.83 Ex. 5 458 827 20.8 1.0 1.2 0.83 Ex. 6 476 872 19.7 1.0 1.2 0.83 Ex. 7 489 884 18.4 1.0 1.1 0.91 Comp. 386 768 25.2 1.0 1.2 0.83 Ex. 1 Comp. 525 934 14.6 1.5 1.0 1.50 Ex. 2 Comp. 393 777 24.3 1.0 1.0 1.00 Ex. 3 Comp. 518 941 15.1 1.5 1.1 1.36 Ex. 4 Comp. 404 835 19.6 1.0 1.0 1.00 Ex. 5 Comp. 408 828 20.1 1.0 1.1 0.91 Ex. 6 Comp. 383 765 24.7 1.0 1.1 0.91 Ex. 7 Comp. 525 936 16.6 1.5 1.3 1.15 Ex. 8 Comp. 543 952 15.8 1.5 1.0 1.50 Ex. 9 Comp. 394 774 24.5 1.0 1.0 1.00 Ex. 10 Comp. 390 772 24.5 1.0 1.0 1.00 Ex. 11 Comp. 537 947 15.5 1.5 1.2 1.25 Ex. 12 Comp. 534 942 15.3 1.5 1.1 1.36 Ex. 13 Comp. 385 774 24.5 1.0 1.0 1.00 Ex. 14

[0075] As it can be seen from Table 3, Examples 1-7 meeting the control requirements of the design specification according to the present disclosure have excellent performances, including yield strength≥420 MPa; tensile strength≥780 MPa; elongation at break with A50 gauge length ≥18%; a 90-degree cold bending parameter R/t≤11 (R represents bending radius in mm, t represents plate thickness in mm). The various performances of the cold-rolled and annealed dual-phase steels of the various Examples are quite excellent. With no addition of precious alloy elements such as Mo and Cr, the steels achieve a tensile strength of greater than 780 MPa, and exhibit good elongation and superior cold bending performance.

[0076] It's to be noted that the prior art portions in the protection scope of the present disclosure are not limited to the examples set forth in the present application file. All the prior art contents not contradictory to the technical solution of the present disclosure, including but not limited to prior patent literature, prior publications, prior public uses and the like, may all be incorporated into the protection scope of the present disclosure. In addition, the ways in which the various technical features of the present disclosure are combined are not limited to the ways recited in the claims of the present disclosure or the ways described in the specific examples. All the technical features recited in the present disclosure may be combined or integrated freely in any manner, unless contradictions are resulted.

[0077] It should also be noted that the Examples set forth above are only specific examples according to the present disclosure. Obviously, the present disclosure is not limited to the above Examples. Similar variations or modifications made thereto can be directly derived or easily contemplated from the present disclosure by those skilled in the art. They all fall in the protection scope of the present disclosure.