LOW-CARBON LOW-COST ULTRA-HIGH-STRENGTH MULTIPHASE STEEL PLATE/STEEL STRIP AND MANUFACTURING METHOD THEREFOR

Abstract

Disclosed are a low-cost ultra-high-strength multiphase steel plate/steel strip and its manufacturing method. Said steel plate/steel strip comprises the following components in percentage by weight: 0.03 to 0.07% of C, 0.1 to 0.5% of Si, 1.3 to 1.9% of Mn, less than or equal to 0.02% of P, less than or equal to 0.01% of S, 0.01 to 0.05% of Al, 0.2 to 0.5% of Cr, 0.07 to 0.14% of Ti, less than 0.03% of (Ni+Nb+Mo+V), and the balance being Fe and other inevitable impurities; and Mn+1.5Cr+5(Ti+Al+Cu)+10(Mo+Ni)+20(Nb+V)<3.0; Mn+2Cr+4Ti+4Nb+4V+4Mo—Si/3+2C≤3.0. The steel plate is mainly used for the manufacturing of automotive chassis and suspension system parts.

Claims

1. A ultra-high strength multiphase steel plate/steel strip, comprising the following chemical elements by weight percentage: C: 0.03-0.07%, Si: 0.1-0.5%, Mn: 1.3-1.9%, P≤0.02%, S≤0.01%, Al: ≤0.01-0.05%, Cr: 0.2-0.5%, Ti: 0.07-0.14%, (Ni+Nb+Mo+V)<0.03%, and a balance of Fe and unavoidable impurities; at the same time, it is required to satisfy:
[Mn+1.5Cr+5(Ti+Al+Cu)+10(Mo+Ni)+20(Nb+V)]≤3.0;
(Mn+2Cr+4Ti+4Nb+4V+4Mo—Si/3+2C)≤3.0.

2. The ultra-high strength multiphase steel plate/steel strip according to claim 1, wherein the C content is 0.04-0.06%, in weight percentage.

3. The ultra-high strength multiphase steel plate/steel strip according to claim 1, wherein the Si content is 0.1-0.27%, in weight percentage.

4. The ultra-high strength multiphase steel plate/steel strip according to claim 1, wherein the Mn content is 1.45-1.75%, in weight percentage.

5. The ultra-high strength multiphase steel plate/steel strip according to claim 1, wherein the Cr content is 0.35-0.50%, in weight percentage.

6. The ultra-high strength multiphase steel plate/steel strip according to claim 1, wherein in chemical elements Nb+Mo+V<0.03%, in weight percentage.

7. The ultra-high strength multiphase steel plate/steel strip according to claim 1, wherein the steel has a structure containing ferrite, lower bainite, and further containing carbide precipitation phase, inclusion phase and/or trace martensite phase, wherein the content of ferrite is ≤70 %, and the content of ferrite+lower bainite is ≥90 %.

8. The ultra-high strength multiphase steel plate/steel strip according to claim 7, wherein the microstructure of the steel plate/steel strip further comprises TiN particles, and a single particle has the longest side length of <10 μm or an area of <50 μm.sup.2.

9. The ultra-high strength multiphase steel plate/steel strip according to claim 7, wherein the average diameter of ferrite grains is <6 μm, or a grain size ASTM rating of ferrite is >11.8.

10. The ultra-high strength multiphase steel plate/steel strip according to claim 1, wherein the steel plate/steel strip has a tensile strength of ≥780 MPa, a yield strength of ≥680 MPa, and a hole expansion ratio performance index which satisfies: if the original hole is a punched hole, the hole expansion ratio is ≥85%; if the original hole is a reamed hole, the hole expansion ratio is ≥115%; and a bending performance which satisfies 180° bending is qualified at d=0.5 a.

11. The ultra-high strength multiphase steel plate/steel strip according to claim 1, wherein the steel plate/steel strip has a yield ratio of 0.9, and an elongation of ≥15%.

12. A manufacturing method for the ultra-high strength multiphase steel plate/steel strip according to claim 1 comprising the following steps: 1) Smelting, continuous casting wherein the chemical elements according to any one of claims 1-6 is smelt and cast into a casting slab by continuous casting, wherein a cooling rate of the slab is ≥5 C./s during continuous casting; 2) Hot entering, rolling, cooling after rolling and coiling of the slab wherein the slab enters the furnace at a temperature of not less than 700° C., and the slab is heated at a heating temperature of 1100-1250° C.; wherein each reduction rate for the first and second pass of hot rolling is ≥55%, and a final rolling temperature of finish rolling is 850-950° C.; and the coiling temperature is 550-630° C.; and 3) Pickling.

13. The manufacturing method for the ultra-high strength multiphase steel plate/steel strip according to claim 12, wherein after step 3) pickling, the method further comprises hot dip galvanizing annealing process to obtain the finished hot-rolled hot-dip galvanized steel plate.

14. The manufacturing method for the ultra-high strength multiphase steel plate/steel strip according to claim 12, wherein in step 1), a proportion of columnar crystals in the slab casting structure is ≤10%, or a thickness of the columnar crystal region is <40 mm.

15. The manufacturing method for the ultra-high strength multiphase steel plate/steel strip according to claim 12, wherein the thickness of the steel plate/steel strip is 0.7-4.0 mm.

16. The manufacturing method for the ultra-high strength multiphase steel plate/steel strip according to claim 12, wherein the C content of the steel plate/steel strip is 0.04-0.06%, in weight percentage, the Si content of the steel plate/steel strip is 0.1-0.27%, in weight percentage, the Mn content of the steel plate/steel strip is 1.45-1.75%, in weight percentage, or the Cr content of the steel plate/steel strip is 0.35-0.50%, in weight percentage.

17. The manufacturing method for the ultra-high strength multiphase steel plate/steel strip according to claim 12, wherein the steel has a structure containing ferrite, lower bainite, and further containing carbide precipitation phase, inclusion phase and/or trace martensite phase, wherein the content of ferrite is ≤70%, and the content of ferrite+lower bainite is ≥90%.

18. The manufacturing method for the ultra-high strength multiphase steel plate/steel strip according to claim 17, wherein the microstructure of the steel plate/steel strip further comprises TiN particles, and a single particle has the longest side length of <10 μm or an area of <50 μm.sup.2.

19. The manufacturing method for the ultra-high strength multiphase steel plate/steel strip according to claim 17, wherein the average diameter of ferrite grains is <6 μm, or a grain size ASTM rating of ferrite is >11.8.

20. The manufacturing method for the ultra-high strength multiphase steel plate/steel strip according to claim 17, wherein the steel plate/steel strip has a tensile strength of ≥780 MPa, a yield strength of ≥680 MPa, a yield ratio of ≥0.9, and an elongation of ≥15%, and a hole expansion ratio performance index which satisfies: if the original hole is a punched hole, the hole expansion ratio is ≥85%; if the original hole is a reamed hole, the hole expansion ratio is ≥115%; and a bending performance which satisfies 180° bending is qualified at d=0.5 a.

Description

DESCRIPTION OF THE DRAWINGS

[0072] FIG. 1 shows the size of TiN particles when the cooling rate of continuous casting reaches 5° C./s or more and their morphology after hot rolling at large reduction (photograph of the structure in hot rolling state).

[0073] FIG. 2 shows the size of TiN particles when the cooling rate of continuous casting is less than 5° C./s and their morphology after hot rolling at large reduction (photograph of the structure in hot rolling state).

[0074] FIG. 3 shows that the microstructure of the steel plate/steel strip of the present disclosure contains ferrite and lower bainite, wherein the content of ferrite+lower bainite is ≥90%.

DETAILED DESCRIPTION

[0075] The present disclosure will be further described with reference to the following examples.

[0076] The steels with different compositions after smelting shown in Table 1 were heated and hot rolled according to the process shown in Table 2 to obtain steel plates with a thickness of less than 4 mm The yield strength, tensile strength and elongation were measured for tensile specimens with a gauge length of 50 mm and 5 mm along the longitudinal direction, and the hole expansion ratio and 180° bending performance were measured in the middle area of the steel plate. The test data are shown in Table 2. Among them, the hole expansion ratio is measured by the hole expansion test. The specimen with a hole in the center was pressed into a concave die with a punch, so that the center hole of the specimen was enlarged until necking or perforated cracks appear at the edge of the hole. Since the preparation method of the original hole in the center of the specimen has a great influence on the test results of the hole expansion ratio, the original holes in the center of the specimens were prepared by punching and reaming, respectively and subsequent tests were performed in accordance with the hole expansion ratio test method specified in the ISO/DIS 16630 standard. The 180° bending test was performed using the method for the determination of bending properties in the GB/T232-2010 standard.

[0077] In Table 1, Examples A-I are the steels of the present disclosure. The content of carbon or manganese or other alloying elements in Comparative Examples J-N exceeds the scope of the composition of the present disclosure. M in the Table refers to the calculated value of Item [Mn+1.5Cr+5(Ti+Al+Cu)+10(Mo+Ni)+20(Nb+V)], R refers to the calculated value of Item (Mn+2Cr+4Ti+4V+4Nb+4Mo—Si/3+2C) in the composition. In addition, Comparative Example O and Comparative Example P are Examples disclosed in CN101906567A and CN101285156A, respectively. It can be seen from the comparison that the M and R values of Comparative Example O and Comparative Example P all exceed the range of the present disclosure, indicating that the alloy cost of these two comparative examples is higher than that of the case of the present disclosure, and the optimized alloy ratio according to the formula designed in the present invention is not adopted.

[0078] Table 2 shows the different manufacturing processes of various steel grades in Table 1, which are also divided into two categories of Examples and Comparative Examples, wherein the processes of Comparative Example O and Comparative Example P are the manufacturing processes disclosed in the corresponding patent applications. Table 3 shows the detected values for mechanical properties of the above-mentioned examples and comparative examples, wherein the properties of Comparative Example O and Comparative Example P are those disclosed in the corresponding patent applications. It can be seen from the table that the properties of Comparative Example O and Comparative Example P are inferior to those of the examples in the present disclosure.

[0079] It can be seen that when C, Mn, Ti and other alloying components deviate from the scope of the present invention, for example, when the content of Mn and Ti is low, such as Comparative Examples K and M, the strength of the steel plate is lower than the design requirements; and when the content of C, Ti or the R value is higher than the composition range of the present disclosure, such as Comparative examples J, L and N, the excessive content of C and Mn leads to the production of a large amount of martensite in the structure, which deteriorates the hole expansion and bending properties of the material, while if the content of Ti and the value of R is too high, the carbides in the structure are coarsened, and the hole expansion performance of the material is deteriorated, which is not in line with the purpose of the present disclosure.

[0080] When the temperature of the slab entering the furnace is too low, such as Comparative Steel A-2, the strength does not meet the design standards of the present disclosure; if the coiling temperature is too low, such as Comparative Example D-2, the precipitation of carbides in the steel is inhibited, resulting in too low strength of the steel plate. When the reduction rate of the first two passes of hot rolling is not sufficient, the banded structure of the steel plate cannot be completely eliminated, and the grains cannot be fully refined to achieve the uniformity of the structure, which leads to the deterioration of the bending performance of the steel plate for elongation, such as Comparative Example B-2. When the cooling rate of continuous casting is not sufficient, but a large reduction rate is pursued in hot rolling, the coarse TiN particles in the steel are broken and a potential crack source is formed, which greatly deteriorates the elongation, hole expansion performance and bending performance of the material, such as Comparative Example C-2.

[0081] Based on the above, the present disclosure greatly reduces alloying costs by controlling a reasonable composition range, limiting the content of alloying elements and optimizing the ratio of each element on the basis of carbon-manganese steel. By further controlling the cooling rate in continuous casting, the hot rolling reduction rate, and the coiling temperature on the basis of the conventional automobile steel production line, the present disclosure produces a low-cost ultra-high-strength hot-rolled steel plate/steel strip with high strength, high hole expansion performance and excellent bending performance, which has a yield strength of not less than 680 MPa, a tensile strength of not less than 780 MPa, and a hole expansion ratio of not less than 85% (the original hole is punched) or not less than 115% (the original hole is reamed), 180° bending d=0.5 a, to make up for the urgent demand of the automotive industry market for chassis and suspension materials with a combination of low cost, high strength and high forming performance

TABLE-US-00001 TABLE 1 (unit: percentage) Steel No. C Si Mn P S Al Cr Ti V Mo Ni Cu Nb M R Example A 0.058 0.30 1.55 0.012 0.002 0.02 0.43 0.090 0 0 0 0 0 2.75 2.79 Example B 0.040 0.48 1.79 0.013 0.004 0.01 0.38 0.110 0 0 0 0 0 2.96 2.91 Example C 0.068 0.25 1.32 0.015 0.003 0.02 0.40 0.130 0 0 0 0 0.01 2.87 2.73 Example D 0.053 0.36 1.85 0.014 0.002 0.04 0.29 0.071 0 0 0 0 0 2.84 2.70 Example E 0.046 0.14 1.49 0.010 0.001 0.01 0.49 0.077 0 0 0 0 0 2.66 2.82 Example F 0.035 0.43 1.37 0.008 0.005 0.03 0.33 0.101 0 0 0 0 0.02 2.92 2.440666667 Example G 0.062 0.21 1.72 0.007 0.006 0.01 0.25 0.140 0 0 0 0 0 2.85 2.838 Example H 0.038 0.39 1.65 0.014 0.004 0.01 0.35 0.135 0 0 0 0 0 2.90 2.836 Example I 0.049 0.17 1.42 0.013 0.001 0.05 0.46 0.120 0 0 0 0 0 2.96 2.861333333 Comparative Example J 0.072 0.23 1.75 0.012 0.002 0.03 0.40 0.085 0 0 0 0 0 2.93 2.96 Comparative Example K 0.052 0.47 1.28 0.015 0.001 0.04 0.42 0.102 0 0 0 0 0.01 2.82 2.52 Comparative Example L 0.056 0.20 1.42 0.01 0.001 0.03 0.45 0.148 0 0 0 0 0 2.99 2.96 Comparative Example M 0.042 0.29 1.59 0.01 0.002 0.03 0.47 0.067 0 0 0 0 0 2.78 2.79 Comparative Example N 0.049 0.16 1.56 0.01 0.001 0.01 0.48 0.132 0 0 0 0 0 2.99 3.09 Comparative Example O 0.085 0.18 1.45 0.015 0.002 0.042 0.03 0.162 0 0.21 0.01 0 0.051 5.735 3.315 Comparative Example P 0.07 0.1 1.85 0.02 0.01 Not disclosed 0.80 0.12 0 0 0 0 0 3.65 4.04

TABLE-US-00002 TABLE 2 Temperature of Continuous slab entering cast cooling into furnace in Reheating Reduction rate Reduction rate Finish rolling Coiling rate hot rolling temperature of the first pass of the second pass temperature temperature Steel No. ° C./s ° C. ° C. % % ° C. ° C. Example A-1 10 720 1190 55 57 900 600 Com. Example A-2 10 620 1220 55 55 890 600 Example B-1 15 800 1210 58 56 890 615 Com. Example B-2 10 780 1220 45 40 880 610 Example C-1 12 810 1200 59 59 880 630 Com. Example C-2 1 780 1225 57 57 890 600 Example D-1 17 750 1230 65 60 870 605 Com. Example D-2 10 850 1240 58 58 910 520 Example E 13 830 1225 63 61 920 560 Example F 14 790 1220 62 62 930 575 Example G 16 850 1245 64 59 945 595 Example H 9 710 1160 56 56 855 580 Example I 11 770 1170 58 58 865 585 Com. Example M 15 730 1210 58 57 900 610 Com. Example N 15 720 1210 55 55 890 580 Com. Example J 15 760 1220 57 55 900 610 Com. Example K 10 720 1230 55 57 890 610 Com. Example L 10 710 1230 56 55 900 600 Com. Example O Not disclosed Not disclosed >1150 Not disclosed Not disclosed 900 525 Com. Example P Not disclosed 1000° C. 1180 Not disclosed Not disclosed 930 640

TABLE-US-00003 TABLE 3 Yield Tensile Hole Hole strength, strength, expansion ratio expansion ratio 180° bending Steel No. MPa MPa A50% A5% (punched hole), % (reamed hole), % (d = 0.5a) Example A-1 762 843 17.3 21.2 99 123 qualified Comparative Example A-2 652 743 16.2 19.5 101 126 qualified Example B-1 737 822 18.4 23.1 105 136 qualified Comparative Example B-2 725 823 15.8 19.6 85 115 cracked Example C-1 803 830 15.2 19.1 99 123 qualified Comparative Example C-2 681 811 11.1 14.3 70 83 cracked Example D-1 818 873 15.3 19.1 92 118 qualified Comparative Example D-2 636 731 12.7 16.5 112 139 qualified Example E 712 785 20.7 24.3 103 132 qualified Example F 724 805 18.5 25.0 108 139 qualified Example G 829 899 15.1 19.2 86 117 qualified Example H 749 815 16.3 20.7 89 120 qualified Example I 772 833 16.0 20.5 91 122 qualified Comparative Example J 853 916 7.0 10.0 72 85 cracked Comparative Example K 601 727 20.2 24.5 112 139 qualified Comparative Example L 805 853 15.3 19.4 70 87 cracked Comparative Example M 636 731 24.7 29.6 112 139 qualified Comparative Example N 798 832 16.1 20.2 76 101 qualified Comparative Example O 698 803 18.0 Not disclosed 79 Not disclosed Not disclosed Comparative Example P 755-800 780-885 Not disclosed 17-23 Not disclosed Not disclosed 180° bending d = a