HIGH-YIELD-RATIO COLD-ROLLED DUAL-PHASE STEEL AND MANUFACTURING METHOD THERFOR

Abstract

Disclosed is a high-yield-ratio cold-rolled dual-phase steel, having the following chemical elements in percentage by mass: 0.05%-0.08% of C, 0.9%-1.2% of Mn, 0.1%-0.6% of Si, 0.030%4060% of Nb, 0.030%-0.060% of Ti, 0.015%-0.045% of Al, and the balance being Fe and other inevitable impurities. A manufacturing method for the high-yield-ratio cold-rolled dual-phase steel, comprising: (1) smelting and casting; (2) hot rolling, wherein a casting blank is controlled and soaked at a temperature of 1200° C.-1250° C.; rolled with the finish rolling temperature being 840° C.-930° C.; cooled at a speed of 20° C./s-70° C./s, and then wound at the winding temperature being 570° C.-630° C.; (3) cold rolling; (4) annealing at the soaking temperature being 750° C.-790° C. for 40 s-200 s, cooling at a speed of 30° C./s-80° C./s, the start temperature of cooling is 650° C. to 730° C., the aging temperature is 200° C. to 260° C., and the overaging time is 100 s to 400 s; and (5) leveling.

Claims

1. A cold-rolled dual-phase steel having a high yield ratio, comprising the following chemical elements in mass percentages: C: 0.05-0.08%, Mn: 0.9-1.2%, Si: 0.1-0.6%, Nb: 0.030-0.060%, Ti: 0.030-0.060%, Al: 0.015-0.045%, and a balance of Fe and other unavoidable impurities.

2. The cold-rolled dual-phase steel having a high yield ratio according to claim 1, wherein the steel has a microstructure which is a complex phase structure of martensite+ferrite+[NbxTiy(C,N)z] carbonitride.

3. The cold-rolled dual-phase steel having a high yield ratio according to claim 2, wherein the martensite has a phase proportion of 20-30%, and the martensite is in the shape of long strips-islands.

4. The cold-rolled dual-phase steel having a high yield ratio according to claim 2, wherein the [NbxTiy(C,N)z] carbonitride has an irregular spherical shape and is uniformly distributed in ferrite grains, and the [NbxTiy(C,N)z] carbonitride has a phase proportion of 5-10%.

5. The cold-rolled dual-phase steel having a high yield ratio according to claim 4, wherein the [NbxTiy(C,N)z] carbonitride has a size of less than 2 μm.

6. The cold-rolled dual-phase steel having a high yield ratio according to claim 1, wherein among the other unavoidable impurities, mass percentages of the P, S and N elements meet at least one of the following: P≤0.015%; S≤0.005%; N≤0.005%.

7. The cold-rolled dual-phase steel having a high yield ratio according to claim 1, wherein the steel has a yield ratio of greater than 0.8.

8. The cold-rolled dual-phase steel having a high yield ratio according to claim 7, wherein the steel has a yield strength of 550-660 MPa, a tensile strength of 660 MPa, and an elongation at break of 15%.

9. A manufacturing method for the cold-rolled dual-phase steel having a high yield ratio according to claim 1, wherein the method comprises the following steps: (1) Smelting and casting; (2) controlling a cast blank for soaking at a temperature of 1200-1250° C.; rolling with a finish rolling temperature being controlled at 840-930° C.; cooling at a rate of 20-70° C./s after the rolling; then coiling with a coiling temperature being controlled at 570-630° C.; (3) Cold rolling; (4) annealing at an annealing soaking temperature of 750-790° C. for an annealing time of 40-200 s; and then cooling at a rate of 30-80° C./s, wherein the cooling begins at a temperature of 650-730° C., an aging temperature is 200-260° C., and an over-aging time is 100-400 s; and (5) Temper rolling.

10. The manufacturing method according to claim 9, wherein in Step (3), a cold rolling reduction rate is controlled to be 50-70%; and/or in Step (5), a temper rolling reduction rate is controlled to be 0.3-1.0%.

11. The manufacturing method according to claim 9, wherein in Step (2), a soaking time is 5-6 hours; the steel is cooled to 570-630 ° C. after the rolling; and then the coiling is performed.

12. The cold-rolled dual-phase steel having a high yield ratio according to claim 2, wherein the steel has a yield ratio of greater than 0.8.

13. The cold-rolled dual-phase steel having a high yield ratio according to claim 3, wherein the steel has a yield ratio of greater than 0.8.

14. The cold-rolled dual-phase steel having a high yield ratio according to claim 4, wherein the steel has a yield ratio of greater than 0.8.

15. The cold-rolled dual-phase steel having a high yield ratio according to claim 6, wherein the steel has a yield ratio of greater than 0.8.

16. The manufacturing method according to claim 9, wherein the steel has a microstructure which is a complex phase structure of martensite+ferrite+[NbxTiy(C,N)z] carbonitride.

17. The manufacturing method according to claim 9, wherein the martensite has a phase proportion of 20-30%, and the martensite is in the shape of long strips-islands, and/or wherein the [NbxTiy(C,N)z] carbonitride has an irregular spherical shape and is uniformly distributed in ferrite grains, and the [NbxTiy(C,N)z] carbonitride has a phase proportion of 5-10%.

18. The manufacturing method according to claim 17, wherein the [NbxTiy(C,N)z] carbonitride has a size of less than 2 μm.

19. The manufacturing method according to claim 17, wherein among the other unavoidable impurities of the cold-rolled dual-phase steel, mass percentages of the P, S and N elements meet at least one of the following: P≤0.015%; S≤0.005%; N≤0.005%.

20. The manufacturing method according to claim 9, wherein the steel has a yield ratio of greater than 0.8, and or the steel has a yield strength of 550-660 MPa, a tensile strength of 660 MPa, and an elongation at break of ≥15%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] FIG. 1 is a microstructure diagram of a cold-rolled dual-phase steel having a high yield ratio in Example 2.

DETAILED DESCRIPTION OF THE INVENTION

[0048] The cold-rolled dual-phase steel having a high yield ratio according to the present disclosure and the method for manufacturing the same will be further explained and illustrated with reference to the accompanying drawing of the specification and 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 1-15

[0049] Table 1-1 and Table 1-2 list the mass percentages (wt %) of the chemical elements in the high-yield-ratio cold-rolled dual-phase steels of Examples 1-6 and Comparative Examples 1-15.

TABLE-US-00001 TABLE 1-1 (wt %, the balance is Fe and other unavoidable impurities except for P, S and N) No. C Si Mn P S Nb Ti Ex. 1 0.052 0.33 1.05 0.014 0.003 0.058 0.044 Ex. 2 0.055 0.18 0.99 0.011 0.004 0.048 0.038 Ex. 3 0.061 0.35 1.17 0.009 0.003 0.042 0.045 Ex. 4 0.066 0.24 1.01 0.012 0.002 0.039 0.036 Ex. 5 0.074 0.18 0.92 0.01 0.001 0.045 0.033 Ex. 6 0.078 0.35 0.98 0.013 0.005 0.034 0.047 Comp. Ex. 1 0.044 0.29 1.08 0.011 0.002 0.044 0.043 Comp. Ex. 2 0.092 0.36 1.12 0.009 0.004 0.038 0.045 Comp. Ex. 3 0.065 0.27 0.78 0.012 0.004 0.043 0.035 Comp. Ex. 4 0.056 0.25 1.26 0.01 0.002 0.037 0.043 Comp. Ex. 5 0.075 0.38 1.08 0.011 0.005 0.025 0.056 Comp. Ex. 6 0.058 0.29 1.19 0.008 0.002 0.065 0.045 Comp. Ex. 7 0.066 0.47 1.11 0.013 0.004 0.038 0.023 Comp. Ex. 8 0.062 0.52 0.96 0.012 0.003 0.055 0.068 Comp. Ex. 9 0.073 0.29 1.08 0.011 0.002 0.044 0.043 Comp. Ex. 10 0.068 0.33 1.06 0.009 0.001 0.041 0.039 Comp. Ex. 11 0.071 0.46 0.95 0.012 0.004 0.036 0.033 Comp. Ex. 12 0.062 0.32 1.05 0.013 0.002 0.032 0.037 Comp. Ex. 13 0.068 0.29 0.99 0.009 0.003 0.039 0.041 Comp. Ex. 14 0.072 0.40 1.12 0.001 0.001 0.048 0.051 Comp. Ex. 15 0.077 0.38 1.15 0.014 0.003 0.043 0.046

TABLE-US-00002 TABLE 1-2 (wt %, the balance is Fe and other unavoidable impurities except for P, S and N) phase phase phase proportion of average size of proportion of proportion of [NbxTiy(C,N)z] [NbxTiy(C,N)z] No. Al N C+(Mn + Si)/6 ferrite (%) martensite (%) carbonitride (%) carbonitride (μm) Ex. 1 0.021 0.0035 0.282 69.2 24.3 6.5 1.2 Ex. 2 0.033 0.0044 0.250 69.9 22.7 7.4 0.8 Ex. 3 0.028 0.0037 0.314 64.6 28.2 7.2 0.7 Ex. 4 0.042 0.0028 0.274 69.6 21.6 8.8 1.0 Ex. 5 0.038 0.0032 0.257 66.5 24.5 9.0 1.5 Ex. 6 0.017 0.0047 0.300 68.2 26.2 5.6 0.6 Comp. Ex. 1 0.034 0.0028 0.272 77.6 15.6 6.8 0.8 Comp. Ex. 2 0.027 0.0044 0.339 54 38.5 7.5 1.1 Comp. Ex. 3 0.032 0.0037 0.240 79.2 12.5 8.3 0.9 Comp. Ex. 4 0.023 0.0028 0.308 51 41.2 7.8 1.6 Comp. Ex. 5 0.035 0.0042 0.318 70 25.6 4.4 2.8 Comp. Ex. 6 0.042 0.0036 0.305 67.9 18.9 13.2 0.4 Comp. Ex. 7 0.028 0.0042 0.329 74.7 21.5 3.8 3.0 Comp. Ex. 8 0.026 0.0036 0.309 62.8 22.9 14.3 0.5 Comp. Ex. 9 0.034 0.0028 0.301 76.7 16.7 6.6 0.6 Comp. Ex. 10 0.022 0.0029 0.300 55.5 37.4 7.1 3.1 Comp. Ex. 11 0.043 0.0033 0.306 69.4 25.8 4.8 1.0 Comp. Ex. 12 0.041 0.0044 0.290 70.6 24.9 4.5 1.2 Comp. Ex. 13 0.038 0.0022 0.281 74.9 16.8 8.3 0.9 Comp. Ex. 14 0.029 0.0035 0.325 55.6 36.8 7.6 1.5 Comp. Ex. 15 0.030 0.0047 0.332 69.2 24.3 6.5 1.4

[0050] The method for manufacturing the high-yield-ratio cold-rolled dual-phase steels of Examples 1-6 and Comparative Examples 1-15 is as follows (the specific process parameters are listed in Table 2-1 and Table 2-2):

[0051] (1) Smelting and casting: Smelting and casting were carried out with the chemical elements listed in Table 1-1 and Table 1-2.

[0052] (2) Hot rolling: A cast blank was controlled for soaking at a temperature of 1200-1250° C. for 5-6 hours, and then rolled, wherein the finish rolling temperature was controlled at 840-930° C. After the rolling, the steel was cooled at a rate of 20-70° C./s to 570-630° C. Then, the steel was coiled, wherein the coiling temperature was controlled at 570-630° C.

[0053] (3) Cold rolling: The cold rolling reduction rate was controlled at 50-70%.

[0054] (4) Annealing: The annealing soaking temperature was 750-790° C.; and the annealing time was 40-200 s. Then, the steel was cooled at a rate of 30-80° C./s, wherein the cooling began at a temperature of 650-730° C. The aging temperature was 200-260° C., and the over-aging time was 100-400 s.

[0055] (5) Temper rolling: The temper rolling reduction rate was 0.3-1.0%.

TABLE-US-00003 TABLE 2-1 Specific process parameters for the method for manufacturing the high-yield-ratio cold-rolled dual-phase steels of Examples 1-6 and Comparative Examples 1-15 Step (2) Step (3) Soaking Finish Rolling Cooling Coiling Cold Rolling Temperature Temperature Rate Temperature Reduction Rate No. (° C.) (° C.) (° C./s) (° C.) (%) Ex. 1 1240 925 40 585 62 Ex. 2 1230 860 30 590 70 Ex. 3 1250 900 60 615 65 Ex. 4 1215 905 55 625 55 Ex. 5 1220 855 50 580 58 Ex. 6 1230 925 30 570 65 Comp. Ex. 1 1230 890 60 595 50 Comp. Ex. 2 1220 875 65 620 64 Comp. Ex. 3 1200 915 70 580 68 Comp. Ex. 4 1240 845 35 590 56 Comp. Ex. 5 1250 880 30 570 55 Comp. Ex. 6 1200 910 65 620 60 Comp. Ex. 7 1245 860 30 595 62 Comp. Ex. 8 1225 935 45 605 54 Comp. Ex. 9 1190 905 40 590 62 Comp. Ex. 10 1265 900 35 575 50 Comp. Ex. 11 1245 855 60 550 55 Comp. Ex. 12 1220 865 65 640 65 Comp. Ex. 13 1225 895 55 600 68 Comp. Ex. 14 1230 875 45 610 70 Comp. Ex. 15 1240 925 65 585 52

TABLE-US-00004 TABLE 2-2 Specific process parameters for the method for manufacturing the high-yield-ratio cold-rolled dual-phase steels of Examples 1-6 and Comparative Examples 1-15 Step (4) Step (5) Annealing Initial Temper Soaking Cooling Cooling Aging Rolling Temperature Annealing Rate Temperature Temperature Over-aging Reduction No. (° C.) Time (s) (° C./s) (° C.) (° C.) Time (s) Rate (%) Ex. 1 765 40 75 700 230 100 0.8 Ex. 2 780 80 60 660 240 200 0.6 Ex. 3 785 120 55 650 200 400 0.9 Ex. 4 774 160 70 730 250 200 1.0 Ex. 5 782 40 45 670 230 100 0.5 Ex. 6 758 80 70 660 240 300 0.9 Comp. Ex. 1 778 120 45 650 240 300 0.6 Comp. Ex. 2 785 160 50 670 200 400 0.7 Comp. Ex. 3 755 40 60 660 250 200 0.8 Comp. Ex. 4 790 80 55 650 230 100 0.9 Comp. Ex. 5 775 120 35 730 240 300 1.0 Comp. Ex. 6 768 160 80 670 200 400 0.5 Comp. Ex. 7 786 80 65 670 260 300 0.8 Comp. Ex. 8 766 100 30 660 220 200 0.6 Comp. Ex. 9 775 40 45 720 250 200 0.7 Comp. Ex. 10 785 80 70 700 230 100 0.8 Comp. Ex. 11 768 120 45 680 240 300 0.5 Comp. Ex. 12 755 160 50 650 240 200 0.9 Comp. Ex. 13 745 40 45 695 200 100 1.0 Comp. Ex. 14 805 80 55 705 250 300 0.8 Comp. Ex. 15 774 160 60 730 230 300 1.2

[0056] The high-yield-ratio cold-rolled dual-phase steels of Examples 1-6 and Comparative Examples 1-15 were tested for their properties. The test results are listed in Table 3.

TABLE-US-00005 TABLE 3 Yield Tensile Elongation Strength Strength At Break Yield No. (MPa) (MPa) (%) Ratio Ex. 1 580 690 19.5 0.84 Ex. 2 575 686 18.4 0.84 Ex. 3 604 720 18.2 0.84 Ex. 4 652 764 15.6 0.85 Ex. 5 643 751 15.3 0.86 Ex. 6 628 708 17.5 0.89 Comp. Ex. 1 525 650 21.2 0.81 Comp. Ex. 2 693 790 14.3 0.88 Comp. Ex. 3 508 632 22.6 0.80 Comp. Ex. 4 685 814 13.8 0.84 Comp. Ex. 5 564 754 16.1 0.75 Comp. Ex. 6 632 724 17.6 0.87 Comp. Ex. 7 555 708 18.8 0.78 Comp. Ex. 8 602 697 19.3 0.86 Comp. Ex. 9 532 646 21.8 0.82 Comp. Ex. 10 683 796 14.1 0.86 Comp. Ex. 11 564 734 17.9 0.77 Comp. Ex. 12 568 727 17.6 0.78 Comp. Ex. 13 565 638 21.7 0.89 Comp. Ex. 14 684 785 14.6 0.87 Comp. Ex. 15 699 774 15.2 0.90

[0057] As can be seen from Table 3, the high-yield-ratio cold-rolled dual-phase steels of Examples 1-6 and Comparative Examples 1-15 have a tensile strength of ≥660 MPa, an elongation at break of ≥15%, and a yield ratio of greater than 0.8. Thus, it can be seen that the cold-rolled dual-phase steel having a high yield ratio according to the present disclosure has the advantages of high strength, low carbon equivalent and high yield ratio.

[0058] FIG. 1 is a microstructure diagram of a cold-rolled dual-phase steel having a high yield ratio in Example 2.

[0059] As can be seen from FIG. 1, the microstructure of the high-yield-ratio cold-rolled dual-phase steel of Example 2 is a complex phase structure of martensite+ferrite+[NbxTiy(C,N)z] carbonitride, wherein the martensite has a phase proportion of 20-30%, and has a function of phase transformation strengthening. The martensite structure is in the shape of long strips-islands (it is island-shaped when observed under a low-magnification metallographic microscope; it is lath or long strip-shaped when observing the fine structure of the martensite). Meanwhile, the [NbxTiy(C,N)z] carbonitride has an irregular spherical shape and is uniformly distributed in the ferrite grains. The carbonitride has a size of less than 2 μm, and has a function of dispersion precipitation strengthening in the structure.

[0060] 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.

[0061] 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.

[0062] It should also be noted that the above-listed Examples are only specific embodiments of the present disclosure. Obviously, the present disclosure is not limited to the above Examples, and similar changes or modifications can be directly derived from or easily associated with the disclosure of the present disclosure by those skilled in the art, and should fall within the protection scope of the present disclosure.