ULTRAHIGH-STRENGTH DUAL-PHASE STEEL AND MANUFACTURING METHOD THEREFOR

Abstract

Disclosed in the present disclosure is an ultrahigh-strength dual-phase steel. The matrix structure of the ultrahigh-strength dual-phase steel is ferrite and martensite, wherein the ferrite and the martensite are evenly distributed in an island shape. The ultrahigh-strength dual-phase steel contains the following chemical elements in percentage by mass: 0.12-0.2% of C, 0.5-1.0% of Si, 2.5-3.0% of Mn, 0.02-0.05% of Al, 0.02-0.05% of Nb, 0.02-0.05% of Ti, and 0.001-0.003% of B. Further disclosed in the present disclosure is a manufacturing method for the ultrahigh-strength dual-phase steel, comprising the steps of smelting and continuous casting, hot rolling, cold rolling, annealing, tempering, and leveling. The ultrahigh-strength dual-phase steel in the present disclosure has not only good mechanical properties but also excellent delayed cracking resistance and low initial hydrogen content, and can be suitable for manufacturing of vehicle safety structural parts.

Claims

1. An ultra-high-strength dual-phase steel, wherein the ultra-high-strength dual-phase steel has a matrix structure of ferrite + martensite, wherein the ferrite and the martensite are distributed evenly like islands, and wherein the ultra-high-strength dual-phase steel comprises the following chemical elements in mass percentages, in addition to Fe: C: 0.12-0.2%, Si: 0.5-1.0%, Mn: 2.5-3.0%, Al: 0.02-0.05%, Nb: 0.02-0.05%, Ti: 0.02-0.05%, B: 0.001%-0.003%.

2. The ultra-high-strength dual-phase steel according to claim 1, wherein the chemical elements have the following mass percentages: C: 0.12-0.2%, Si: 0.5-1.0%, Mn: 2.5-3.0%, Al: 0.02-0.05%, Nb: 0.02-0.05%, Ti: 0.02-0.05%, B: 0.001%-0.003%, and a balance of Fe and other unavoidable impurities.

3. The ultra-high-strength dual-phase steel 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 ≤0.01%, S≤0.002%, N≤0.004%.

4. The ultra-high-strength dual-phase steel according to claim 1, wherein the mass percentages of the chemical elements satisfy at least one of: $C:0.14-0.18\%,$ $Mn:2.5-2.8\%.$ .

5. The ultra-high-strength dual-phase steel according to claim 1, wherein the martensite has a phase proportion of >90%.

6. The ultra-high-strength dual-phase steel according to claim 1, wherein the martensite comprises coherently distributed ε carbides.

7. The ultra-high-strength dual-phase steel according to claim 1, wherein the ultra-high-strength dual-phase steel has performances that meet at least one of the following: yield strength ≥900 MPa, tensile strength ≥1300 MPa, elongation after fracture ≥5%, initial hydrogen content ≤10 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.

8. The ultra-high-strength dual-phase steel according to claim 1, wherein the ultra-high-strength dual-phase steel has performances that meet at least one of the following: yield strength ≥930 MPa, tensile strength ≥1320 MPa, elongation after fracture ≥5.5%, initial hydrogen content ≤7 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 1.2 times of the tensile strength.

9. The ultra-high-strength dual-phase steel according to claim 1, wherein the ultra-high-strength dual-phase steel has a yield ratio of 0.70-0.75.

10. A manufacturing method for the ultra-high-strength 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: heating to an annealing soaking temperature of 800-850° C. at a heating rate of 3-10° C./s, the 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 670-730° C.; (5) Tempering: tempering temperature: 260-320° C.; tempering time: 100-400 s; (6) Temper rolling; and (7) Electrogalvanizing.

11. The manufacturing method according to claim 10, wherein in 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 1220-1260° C.; then rolled with a finishing rolling temperature being controlled at 880-920° C.; then cooled at a rate of 20-70° C./s after the rolling; then coiled at a coiling temperature of 600-650° C.; and then subjected to heat preservation treatment after the 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%; and/or in step (7), double-side electrogalvanizing is performed with a plating layer weight of 10-100 g/m.sup.2 on each side.

15. The manufacturing method according to claim 10, wherein in step (2), a cast slab is controlled to be soaked at a temperature of 1220-1250° C., and a coiling temperature is 605-645° C.; in step (4), the annealing soaking temperature is 805-845° C.; in step (5), the tempering temperature is 260-310° C., and the tempering time is 100-300 s.

16. The ultra-high-strength dual-phase steel according to claim 1, wherein the mass percentages of the chemical elements satisfy at least one of: C: 0.14-0.18%, Mn: 2.5-2.8%.

17. The ultra-high-strength dual-phase steel according to claim 2, wherein the martensite has a phase proportion of >90%, and/or the martensite comprises coherently distributed ε carbides, and/or the ultra-high-strength dual-phase steel has a yield ratio of 0.70-0.75.

18. The ultra-high-strength dual-phase steel according to claim 2, wherein the ultra-high-strength dual-phase steel has performances that meet at least one of the following: yield strength ≥900 MPa, tensile strength ≥1300 MPa, elongation after fracture ≥5%, initial hydrogen content ≤10 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, the chemical elements have the following mass percentages: C: 0.12-0.2%, Si: 0.5-1.0%, Mn: 2.5-3.0%, Al: 0.02-0.05%, Nb: 0.02-0.05%, Ti: 0.02-0.05%, B: 0.001%-0.003%, and a balance of Fe and other unavoidable impurities.

20. The manufacturing method according to claim 10, wherein: the martensite has a phase proportion of >90%; and/or, the martensite comprises coherently distributed ε carbides; and/or, the ultra-high-strength dual-phase steel has a yield ratio of 0.70-0.75; and/or, the ultra-high-strength dual-phase steel has performances that meet at least one of the following: yield strength ≥900 MPa, tensile strength ≥1300 MPa, elongation after fracture ≥5%, initial hydrogen content ≤10 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.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0054] FIG. 1 shows the structure of the cold-rolled and annealed dual-phase steel of Example 1. Detailed Description

[0055] The ultra-high-strength 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

[0056] Table 1 lists the mass percentages of various chemical elements in the steel grades corresponding to the ultra-high-strength dual-phase steels in Examples 1-7 and the steels in Comparative Examples 1-14.

TABLE-US-00001 (wt%, the balance is Fe and other unavoidable impurities except for P, S and N) Steel grade C Si Mn P S Nb Ti Al N B Ex. 1 A 0.122 0.54 2.52 0.01 0.001 0.036 0.033 0.034 0.0030 0.0011 Ex. 2 B 0.147 0.72 2.64 0.008 0.0008 0.044 0.038 0.043 0.0027 0.0024 Ex. 3 C 0.131 0.63 2.98 0.009 0.002 0.022 0.045 0.028 0.0033 0.0029 Ex. 4 D 0.158 0.84 2.56 0.007 0.0007 0.043 0.037 0.022 0.0028 0.0015 Ex. 5 E 0.164 0.55 2.82 0.01 0.001 0.027 0.025 0.038 0.0032 0.0018 Ex. 6 F 0.173 0.95 2.73 0.005 0.0015 0.033 0.038 0.034 0.0029 0.0016 Ex. 7 G 0.194 0.66 2.65 0.009 0.0009 0.048 0.04 0.049 0.0026 0.0021 Comp. Ex. 1 H 0.118 0.67 2.65 0.006 0.002 0.025 0.043 0.034 0.0028 0.0013 Comp. Ex. 2 I 0.205 0.58 2.53 0.009 0.0008 0.038 0.025 0.027 0.0034 0.0025 Comp. Ex. 3 J 0.144 0.47 2.42 0.01 0.0016 0.043 0.035 0.022 0.0037 0.0016 Comp. Ex. 4 K 0.153 0.65 3.05 0.0007 0.0013 0.037 0.043 0.037 0.0028 0.0022 Comp. Ex. 5 L 0.149 0.72 2.75 0.01 0.0008 0.025 0.013 0.025 0.0022 0.0017 Comp. Ex. 6 M 0.162 0.64 2.58 0.0005 0.0012 0.01 0.035 0.039 0.0032 0.0024 Comp. Ex. 7-14 N 0.175 0.77 2.55 0.008 0.0009 0.033 0.026 0.046 0.0028 0.0018

[0057] The ultra-high-strength 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: [0058] (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; [0059] (2) Hot rolling: The cast slab was soaked at a temperature controlled at 1220-1260° C., and then rolled, wherein the finishing rolling temperature was controlled at 880-920° 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 600-650° C. After coiling, an insulation cover was used to perform heat preservation treatment; [0060] (3) Cold rolling: The cold rolling reduction rate was controlled at 45-65%; [0061] (4) Annealing: The temperature was raised to the annealing soaking temperature of 800-850° 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 670-730° C.; [0062] (5) Tempering: The tempering temperature was 260-320° C., and the tempering time was 100-400 s; [0063] (6) Temper rolling: The temper rolling reduction rate was controlled at ≤0.3%; [0064] (7) Double-side electro-galvanization: The weight of the plating layer on each side was 10-100 g/m.sup.2.

[0065] It should be noted that the chemical compositions of the ultra-high-strength 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.

[0066] Tables 2-1 and 2-2 list the specific process parameters for the ultra-high-strength dual-phase steels in Examples 1-7 and the steels in Comparative Examples 1-14.

TABLE-US-00002 No. Steel grade Step (1) Step (2) Step (3) Drawing speed in continuous casting (m/min) Soaking temperature (°C) Finishing rolling temperature (°C) Cooling rate (°C/s) Coiling temperature (°C) Cold rolling reduction rate (%) Ex. 1 A 1.0 1250 895 25 605 55 Ex. 2 B 1.2 1245 880 30 625 60 Ex. 3 C 1.5 1220 890 45 645 45 Ex. 4 D 0.9 1234 905 50 625 50 Ex. 5 E 1.1 1224 910 35 615 52 Ex. 6 F 1.3 1240 890 60 600 48 Ex. 7 G 1.0 1250 885 65 630 62 Comp. Ex. 1 H 1.2 1230 895 40 615 50 Comp. Ex. 2 I 0.9 1228 915 55 620 56 Comp. Ex. 3 J 1.4 1250 890 70 625 49 Comp. Ex. 4 K 1.3 1255 900 45 615 52 Comp. Ex. 5 L 0.6 1230 905 35 615 62 Comp. Ex. 6 M 1.0 1225 890 65 620 58 Comp. Ex. 7 N 1.2 1199 905 30 625 50 Comp. Ex. 8 N 1.1 1273 900 35 610 55 Comp. Ex. 9 N 1.3 1255 885 60 580 55 Comp. Ex. 10 N 1.4 1230 895 50 665 52 Comp. Ex. 11 N 1.1 1225 905 55 600 56 Comp. Ex. 12 N 1.2 1240 910 45 610 60 Comp. Ex. 13 N 0.9 1245 895 40 625 52 Comp. Ex. 14 N 1.5 1238 885 60 612 50

TABLE-US-00003 No. Step (4) Step (5) Step (6) Heating rate (°C/s) Annealing soaking temperature (°C) Annealing time (s) Rapid cooling rate (°C/s) Starting temperature of rapid cooling (°C) Tempering temperatur e (°C) Tempering time (s) Temper rolling reduction rate (%) Ex. 1 5 825 120 45 705 280 200 0.2 Ex. 2 8 820 75 35 670 290 300 0.1 Ex. 3 10 845 180 80 690 265 210 0.1 Ex. 4 9 805 100 55 700 305 250 0.3 Ex. 5 4 810 45 48 670 285 120 0.2 Ex. 6 3 828 55 56 680 290 300 0.3 Ex. 7 6 824 85 55 725 266 125 0.2 Comp. Ex. 1 6 832 150 48 695 286 330 0.1 Comp. Ex. 2 5 840 90 65 675 315 205 0.3 Comp. Ex. 3 8 800 105 70 705 307 190 0.1 Comp. Ex. 4 9 818 180 62 720 274 180 0.3 Comp. Ex. 5 5 835 60 58 715 264 240 0.2 Comp. Ex. 6 4 812 75 80 670 292 225 0.1 Comp. Ex. 7 8 834 120 55 720 305 325 0.2 Comp. Ex. 8 7 826 135 38 700 269 290 0.1 Comp. Ex. 9 6 819 95 80 680 296 175 0.2 Comp. Ex. 10 10 830 105 55 685 288 205 0.1 Comp. Ex. 11 3 794 145 62 695 308 380 0.1 Comp. Ex. 12 8 865 65 56 705 275 280 0.3 Comp. Ex. 13 5 807 175 48 710 340 300 0.2 Comp. Ex. 14 6 814 125 54 690 230 290 0.1

[0067] A variety of performance tests were performed on the ultra-high-strength dual-phase steels in Examples 1-7 and the steels in Comparative Examples 1-14. The test results obtained are listed in Table 3.

[0068] 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.

[0069] 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.

[0070] Table 3 lists the performance test results for the ultra-high-strength dual-phase steels in Examples 1-7 and the steels in Comparative Examples 1-14.

TABLE-US-00004 No. Yield strength (MPa) Tensile strength (MPa) Elongation after fracture (%) Initial hydrogen content (ppm) Stress level 0.6*TS Stress level 0.8*TS Stress level 1.2*TS Ex. 1 932 1329 9.7 5 O O O Ex. 2 955 1338 9.2 7 O O O Ex. 3 961 1340 8.5 3 O O O Ex. 4 987 1364 7.6 6 O O O Ex. 5 1004 1385 6.8 4 O O O Ex. 6 1046 1407 6.2 1 O O O Ex. 7 1065 1421 5.7 2 O O O Comp. Ex. 1 877 1285 11.3 3 O O O Comp. Ex. 2 1126 1469 4.3 9 O X X Comp. Ex. 3 854 1276 11.5 2 O O O Comp. Ex. 4 1134 1480 3.9 8 O X X Comp. Ex. 5 865 1291 11.8 3 O O O Comp. Ex. 6 839 1274 12.2 7 O O O Comp. Ex. 7 852 1258 12.8 4 O O O Comp. Ex. 8 1100 1446 4.7 8 O O X Comp. Ex. 9 1098 1439 4.4 2 O O X Comp. Ex. 10 870 1268 12.6 6 O O O Comp. Ex. 11 886 1275 11.3 5 O O O Comp. Ex. 12 1133 1485 3.9 8 O O X Comp. Ex. 13 865 1286 11.6 6 O O O Comp. Ex. 14 1108 1455 5.1 3 O O X Note: The results of soaking the steel plates in 1 mol/L hydrochloric acid for 300 hours under a certain internal stress level: O represents no cracking, X represents cracking.

[0071] As it can be seen from Table 3, high-strength steels having a strength of at least 1300 MPa can be manufactured according to the present disclosure. Each Example according to the present disclosure has a yield strength of ≥ 900 MPa, a tensile strength of ≥ 1300 MPa, an elongation after fracture of ≥ 5%, and an initial hydrogen content of ≤10 ppm. The ultra-high-strength dual-phase steel in each Example has an ultra-high strength and a delayed cracking performance that is significantly better than that of a comparative steel grade of the same level. No delayed cracking occurs when the steel plate is soaked in 1 mol/L hydrochloric acid for 300 hours under a pre-stress of greater than or equal to the tensile strength. The ultra-high-strength dual-phase steel in each Example has excellent performances. It is suitable for manufacture of automotive safety structural parts, and it is highly valuable and promising for popularization and application.

[0072] 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.

[0073] 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.