ELECTRO-GALVANIZED SUPER-STRENGTH DUAL-PHASE STEEL RESISTANT TO DELAYED CRACKING, AND MANUFACTURING METHOD THEREFOR

Abstract

Disclosed is an electro-galvanized super-strength dual-phase steel resistant to delayed cracking. A matrix structure thereof is ferrite+tempered martensite and the steel contains the following chemical elements in the following mass percentages: C:0.07-0.1%, Si: 0.05-0.3%, Mn: 2.0-2.6%, Cr: 0.2-0.6%, Mo: 0.1-0.25%, Al: 0.02-0.05%, Nb: 0.02-0.04%, and V: 0.06-0.2%. Also disclosed is a method for manufacturing the electro-galvanized super-strength dual-phase steel resistant to delayed cracking, the method comprising the steps of: smelting and continuous casting, hot rolling, cold rolling, annealing, tempering, leveling and electroplating. The electro-galvanized super-strength dual-phase steel resistant to delayed cracking according to the present invention not only has better mechanical properties, but also has excellent delayed cracking resistance and low initial hydrogen content.

Claims

1. An electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking, wherein the electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking has a matrix structure of ferrite+tempered martensite, and comprises the following chemical elements in mass percentages, in addition to Fe: C: 0.07-0.1%, Si: 0.05-0.3%, Mn: 2.0-2.6%, Cr: 0.2-0.6%, Mo: 0.1-0.25%, Al: 0.02-0.05%, Nb: 0.02-0.04%, V: 0.06-0.2%.

2. The electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking according to claim 1, wherein the chemical elements have the following mass percentages: C: 0.07-0.1%, Si: 0.05-0.3%, Mn: 2.0-2.6%, Cr: 0.2-0.6%, Mo: 0.1-0.25%, Al: 0.02-0.05%, Nb: 0.02-0.04%, V: 0.06-0.2%, and a balance of Fe and other unavoidable impurities.

3. The electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking according to claim 1 or 2, wherein it further comprises 0.0015-0.003% of element B.

4. The electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking according to claim 2, wherein the unavoidable impurities include elements P, S and N, and contents thereof are controlled to be at least one of the following: P≤0.012%, S≤0.003%, N≤0.005%.

5. The electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking according to claim 1, wherein a phase proportion of the tempered martensite is >50%.

6. The electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking according to claim 1, wherein dispersive fine carbide particles are precipitated in the matrix structure, wherein the carbide particles include MoC, VC, Nb(C, N), and wherein the carbide particles are all distributed in the matrix structure in a coherent form.

7. The electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking according to claim 6, wherein the carbide particles have a size of 60 nm.

8. The electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking according to claim 1, wherein the tempered martensite further comprises coherently distributed ε carbide.

9. The electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking according to claim 1, wherein its properties meet at least one of the following: yield strength ≥550 MPa, tensile strength ≥980 MPa, elongation after fracture ≥12%, initial hydrogen content ≤3 ppm; no delayed cracking when soaked in 1 mol/L hydrochloric acid for 300 hours under a pre-stress of greater than or equal to the tensile strength.

10. A manufacturing method for the electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking according to claim 1, wherein the method comprises the following steps: (1) Smelting and continuous casting; (2) Hot rolling; (3) Cold rolling; (4) Annealing: heating to an annealing soaking temperature of 780-820° C. at a heating rate of 3-10° C./s, an annealing time being 40-200 s; and then rapidly cooling at a rate of 30-80° C./s, a starting temperature of the rapid cooling being 650-730° C.; (5) Tempering: tempering temperature: 200-280° C.; tempering time: 100-400 s; (6) Temper rolling; (7) Electroplating.

11. The manufacturing method according to claim 10, wherein in the step (1), a drawing speed in the continuous casting is controlled at 0.9-1.5 m/min during the continuous casting process.

12. The manufacturing method according to claim 10, wherein in step (2), a cast slab is controlled to be soaked at a temperature of 1200-1260° C.; then rolled with a finishing rolling temperature being controlled at 840-900° C.; then cooled at a rate of 20-70° C./s after rolling; then coiled at a coiling temperature of 580-630° C.; and then subjected to heat preservation treatment after coiling.

13. The manufacturing method according to claim 10, wherein in step (3), a cold rolling reduction rate is controlled at 45-65%.

14. The manufacturing method according to claim 10, wherein in step (6), a temper rolling reduction rate is controlled at ≤0.3%.

15. The manufacturing method according to claim 10, wherein in step (2), a cast slab is controlled to be soaked at a temperature of 1210-1245° C.; in step (4), heating at a heating rate of 3-10° C./s is performed to achieve an annealing soaking temperature of 790-810° C., the annealing time being 40-160 s, and then rapid cooling is performed at a rate of 35-80° C./s, a starting temperature of the rapid cooling being 650-730° C.; in step (5), the tempering temperature is 210-270° C., and the tempering time is 120-300 s.

16. The electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking according to claim 2, wherein it further comprises 0.0015-0.003% of element B.

17. The electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking according to claim 2, wherein a phase proportion of the tempered martensite is >50%; and/or dispersive fine carbide particles are precipitated in the matrix structure, wherein the carbide particles include MoC, VC, Nb(C, N), and wherein the carbide particles are all distributed in the matrix structure in a coherent form; and/or the tempered martensite further comprises coherently distributed ε carbide.

18. The electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking according to claim 2, wherein its properties meet at least one of the following: yield strength ≥550 MPa, tensile strength ≥980 MPa, elongation after fracture ≥12%, initial hydrogen content ≤3 ppm; no delayed cracking when soaked in 1 mol/L hydrochloric acid for 300 hours under a pre-stress of greater than or equal to the tensile strength.

19. The manufacturing method according to claim 10, wherein the chemical elements have the following mass percentages: C: 0.07-0.1%, Si: 0.05-0.3%, Mn: 2.0-2.6%, Cr: 0.2-0.6%, Mo: 0.1-0.25%, Al: 0.02-0.05%, Nb: 0.02-0.04%, V: 0.06-0.2%, optional B: 0.0015-0.003%, and a balance of Fe and other unavoidable impurities.

20. The manufacturing method according to claim 10, wherein a phase proportion of the tempered martensite is >50%; and/or dispersive fine carbide particles are precipitated in the matrix structure, wherein the carbide particles include MoC, VC, Nb(C, N), and wherein the carbide particles are all distributed in the matrix structure in a coherent form; and/or the tempered martensite further comprises coherently distributed c carbide.

Description

DETAILED DESCRIPTION

[0059] The electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking and the method for manufacturing the same according to the present 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 present disclosure.

Examples 1-6 and Comparative Examples 1-14

[0060] Table 1 lists the mass percentages of various chemical elements in the steel grades corresponding to the electro-galvanized ultra-high-strength dual-phase steels resistant to delayed cracking in Examples 1-6 and the steels in Comparative Examples 1-14.

TABLE-US-00001 TABLE 1 (wt %, the balance is Fe and other unavoidable impurities except for P, S and N) Steel grade C Si Mn P S Nb Cr Mo Al N V B Ex. 1 A 0.07  0.05 2.07 0.011 0.001 0.022 0.34 0.21 0.021 0.0035 0.11 0.0015 Ex. 2 B 0.073 0.08 2.14 0.008 0.0008 0.024 0.38 0.23 0.033 0.0044 0.15 0.0020 Ex. 3 C 0.078 0.13 2.48 0.009 0.003 0.032 0.45 0.25 0.028 0.0037 0.09 0.0018 Ex. 4 D 0.088 0.24 2.02 0.012 0.002 0.039 0.36 0.12 0.049 0.0028 0.17 — Ex. 5 E 0.096 0.17 2.28 0.01 0.001 0.027 0.22 0.18 0.038 0.0032 0.14 0.0024 Ex. 6 F 0.1  0.28 2.33 0.005 0.0005 0.034 0.57 0.17 0.037 0.0047 0.08 0.0029 Comp. Ex. 1 G 0.044 0.27 2.25 0.011 0.002 0.024 0.43 0.24 0.034 0.0028 0.12 0.0016 Comp. Ex. 2 H 0.123 0.09 2.19 0.009 0.0008 0.038 0.45 0.22 0.027 0.0044 0.16 0.0024 Comp. Ex. 3 I 0.085 0.17 1.92 0.012 0.001 0.033 0.35 0.12 0.032 0.0037 0.15 0.0019 Comp. Ex. 4 J 0.092 0.25 2.65 0.01 0.002 0.037 0.43 0.23 0.023 0.0028 0.13 0.0026 Comp. Ex. 5 K 0.075 0.14 2.08 0.011 0.002 0.025 0.18 0.02 0.025 0.0042 0.11 0.0015 Comp. Ex. 6 L 0.072 0.18 2.15 0.011 0.002 0.01  0.56 0.13 0.029 0.0042 0.02 0.0022 Comp. Ex. 7-14 M 0.084 0.25 2.28 0.012 0.002 0.033 0.26 0.16 0.026 0.0028 0.17 0.0017

[0061] The electro-galvanized ultra-high-strength dual-phase steels resistant to delayed cracking in Examples 1-6 according to the present disclosure and the steels in Comparative Examples 1-14 were all prepared by the following steps:

[0062] (1) Smelting and continuous casting: The drawing speed in the continuous casting was controlled to be 0.9-1.5 m/min during the continuous casting process, and the continuous casting was carried out in a secondary cooling mode with a large amount of water;

[0063] (2) Hot rolling: The cast slab was soaked at a temperature controlled at 1200-1260° C., and then rolled, wherein the finishing rolling temperature was controlled at 840-900° C. After rolling, the steel was cooled at a rate of 20-70° C./s. Then, the steel was coiled at a coiling temperature of 580-630° C. After coiling, an insulation cover was used to hold the temperature for 1-5 hours;

[0064] (3) Cold rolling: The cold rolling reduction rate was controlled at 45-65%.

[0065] (4) Annealing: The temperature was raised to the annealing soaking temperature of 780-820° C. at a heating rate of 3-10° C./s, wherein the annealing time was 40-200 s. Then, rapid cooling was performed at a rate of 30-80° C./s, wherein the starting temperature of the rapid cooling was 650-730° C.;

[0066] (5) Tempering: The tempering temperature was 200-280° C., and the tempering time was 100-400 s;

[0067] (6) Temper rolling: The temper rolling reduction rate was controlled at ≤0.3%;

[0068] (7) Double-side electro-galvanization: The weight of the plating layer on each side was 10-100 g/m.sup.2.

[0069] It should be noted that the chemical compositions of the electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking in Examples 1-6 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 M 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.

[0070] Tables 2-1 and 2-2 list the specific process parameters for the electro-galvanized ultra-high-strength dual-phase steels resistant to delayed cracking in Examples 1-6 and the steels in Comparative Examples 1-14.

TABLE-US-00002 TABLE 2-1 Step (1) Step (3) Drawing Step (2) Cold speed in Finishing rolling continuous Soaking rolling Cooling Coiling reduction Steel casting temperature temperature rate temperature rate No. grade (m/min) (° C.) (° C.) (° C./s) (° C.) (%) Ex. 1 A 0.9 1230 885 35 585 50 Ex. 2 B 1.1 1240 860 30 595 60 Ex. 3 C 1.5 1220 890 65 605 65 Ex. 4 D 1.3 1215 875 40 625 55 Ex. 5 E 1.1 1224 880 35 615 48 Ex. 6 F 1.5 1230 890 60 600 58 Comp. Ex. 1 G 1.4 1235 895 60 595 50 Comp. Ex. 2 H 1.2 1200 875 65 620 64 Comp. Ex. 3 I 1.5 1210 855 70 625 49 Comp. Ex. 4 J 0.9 1255 845 55 590 52 Comp. Ex. 5 K 1.4 1250 880 45 615 62 Comp. Ex. 6 L 1.0 1225 870 65 620 55 Comp. Ex. 7 M 0.9 1185 905 30 590 62 Comp. Ex. 8 M 1.1 1265 900 35 610 50 Comp. Ex. 9 M 0.9 1245 855 60 550 55 Comp. Ex. 10 M 1.5 1220 865 40 660 65 Comp. Ex. 11 M 1.0 1225 895 55 600 56 Comp. Ex. 12 M 1.5 1230 875 45 610 61 Comp. Ex. 13 M 1.3 1245 855 60 625 52 Comp. Ex. 14 M 1.2 1230 890 45 595 60

TABLE-US-00003 TABLE 2-2 Step (4) Step (6) Starting Temper Annealing Rapid temperature Step (5) rolling Heating soaking Annealing cooling of rapid Tempering Tempering reduction rate temperature time rate cooling temperature time rate No. (° C./s) (° C.) (s) (° C./s) (° C.) (° C.) (s) (%) Ex. 1 5 795 60 55 710 260 100 0.1 Ex. 2 8 790 80 35 680 240 300 0.1 Ex. 3 7 785 120 80 650 210 250 0.3 Ex. 4 4 794 160 45 730 270 200 0.1 Ex. 5 3 810 40 45 670 230 120 0.2 Ex. 6 10 806 160 72 660 265 300 0.1 Comp. Ex. 1 8 796 40 45 650 240 340 0.1 Comp. Ex. 2 4 785 80 50 670 200 400 0.3 Comp. Ex. 3 3 800 120 60 705 245 260 0.1 Comp. Ex. 4 10 795 160 55 650 235 170 0.3 Comp. Ex. 5 8 805 120 38 725 240 330 0.2 Comp. Ex. 6 8 786 160 80 670 225 280 0.1 Comp. Ex. 7 9 788 40 45 720 250 200 0.2 Comp. Ex. 8 6 785 80 70 700 235 160 0.1 Comp. Ex. 9 3 802 120 44 680 240 300 0.2 Comp. Ex. 10 4 814 160 50 660 240 240 0.1 Comp. Ex. 11 8 765 40 62 695 200 190 0.1 Comp. Ex. 12 7 845 80 55 708 250 300 0.3 Comp. Ex. 13 9 795 100 48 710 300 210 0.2 Comp. Ex. 14 5 805 95 56 690 160 180 0.1

[0071] A variety of performance tests were performed on the electro-galvanized ultra-high-strength dual-phase steels resistant to delayed cracking in Examples 1-6 and the steels in Comparative Examples 1-14. The test results obtained are listed in Table 3.

[0072] Table 3 lists the performance test results for the electro-galvanized ultra-high-strength dual-phase steels resistant to delayed cracking in Examples 1-6 and the steels in Comparative Examples 1-14. 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 after fracture were obtained finally.

[0073] Method for measurement of hydrogen content: The sample was heated to a certain temperature, and a hydrogen analyzer was used to measure the concentration of hydrogen released along with the change (rise) of the temperature, thereby judging the initial hydrogen content in the steel.

TABLE-US-00004 TABLE 3 Elongation Initial Yield Tensile after Stress Stress Stress Stress hydrogen strength strength fracture level level level level content No. (MPa) (MPa) (%) 0.6*TS 0.8*TS 1.0*TS 1.2*TS (ppm) Ex. 1 575 988 16.4 ◯ ◯ ◯ ◯ 0.7 Ex. 2 602 1001 15.8 ◯ ◯ ◯ ◯ 1.0 Ex. 3 628 1015 15.1 ◯ ◯ ◯ ◯ 0.6 Ex. 4 655 1028 13.9 ◯ ◯ ◯ ◯ 2.0 Ex. 5 684 1032 13.1 ◯ ◯ ◯ ◯ 2.0 Ex. 6 703 1044 12.6 ◯ ◯ ◯ ◯ 1.5 Comp. Ex. 1 477 865 20.6 ◯ ◯ ◯ ◯ 0.6 Comp. Ex. 2 745 1088 9.8 ◯ ◯ X X 1.9 Comp. Ex. 3 465 884 21.2 ◯ ◯ ◯ ◯ 0.5 Comp. Ex. 4 753 1091 10.2 ◯ ◯ X X 1.8 Comp. Ex. 5 625 1008 13.9 ◯ ◯ X X 1.5 Comp. Ex. 6 616 996 14.2 ◯ ◯ X X 1.0 Comp. Ex. 7 594 965 16.6 ◯ ◯ ◯ ◯ 0.9 Comp. Ex. 8 749 1079 10.8 ◯ ◯ ◯ X 1.8 Comp. Ex. 9 762 1085 10.4 ◯ ◯ ◯ X 2.0 Comp. Ex. 10 599 975 16.7 ◯ ◯ ◯ ◯ 0.9 Comp. Ex. 11 518 924 19.6 ◯ ◯ ◯ ◯ 0.6 Comp. Ex. 12 805 1127 7.8 ◯ ◯ ◯ X 3.5 Comp. Ex. 13 623 969 16.5 ◯ ◯ ◯ ◯ 0.8 Comp. Ex. 14 715 1065 12.8 ◯ ◯ ◯ X 1.7 Note: The results of soaking the steel plates in 1 mol/L hydrochloric acid for 300 hours under a certain internal stress level: ◯ represents no cracking, X represents cracking.

[0074] As it can be seen from Table 3, each Example according to the present disclosure had a yield strength of ≥550 MPa, a tensile strength of ≥980 MPa, an elongation after fracture of ≥12%, and an initial hydrogen content of ≤3 ppm. The electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking in each Example had an ultra-high strength and a delayed cracking performance that was significantly better than that of a comparative steel grade of the same level. No delayed cracking occurred when the steel plate was soaked in 1 mol/L hydrochloric acid for 300 hours under a pre-stress of greater than or equal to the tensile strength. The excellent performances of the electro-galvanized ultra-high-strength dual-phase steel resistant to delayed cracking according to the present disclosure can meet the industrial requirements, suitable for manufacture of automotive safety structural parts. It is highly valuable and promising for popularization and application.

[0075] 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.

[0076] 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.