LOW-SILICON AND LOW-CARBON EQUIVALENT GPA GRADE MULTI-PHASE STEEL PLATE/STEEL STRIP AND MANUFACTURING METHOD THEREFOR

Abstract

Disclosed are a low-silicon and low-carbon equivalent GPa grade multi-phase steel plate/steel strip and a manufacturing method therefor. The steel plate/steel strip comprises the following components in percentages by weight: 0.03-0.07% of C, 0.1-0.5% of Si, 1.7-2.0% of Mn, P<0.02%, S<0.01%, N<0.01%, 0.01-0.05% of Al, 0.4-0.7% of Cr, 0.001-0.005% of B, and 0.07-0.15% of Ti, and also comprises one or both of 0.15-0.4% of Mo or 0.02-0.08% of Nb, with the balance being Fe and other inevitable impurities; and at the same time, the steel plate/steel strip satisfies: the content of available B*>0.001, the content of available B*=B-[Ti-3.4N-1.2(C—Nb/7.8)]/22, CE<0.58, and CE=C+Mn/6+(Cr+Mo+V)/5+(Si+Ni+Cu)/15. The steel plate has a tensile strength of >980 MPa and a yield strength of >780 MPa, and the hole expansion rate satisfies: if an original hole is a punched hole, the hole expansion rate is >50%; and if the original hole is a reamed hole, the hole expansion rate is >60%. The steel plate is mainly used in the preparation of vehicle chassis and suspension system parts.

Claims

1. A low silicon low carbon equivalent GPa grade multiphase steel plate/steel strip, comprising the following chemical elements by weight percentage: C: 03-0.07%, Si: 0.1-0.5%, Mn: 1.7-2.0%, P≤0.02%, S≤0.01%, N≤0.01%, Al: 0.01-0.05%, Cr: 0.4-0.7%, B: 0.001-0.005%, Ti: 0.07-0.15%, and further comprising Mo: 0.15-0.4%, and/or Nb: 0.02-0.08%, and a balance of Fe and other unavoidable impurities; at the same time, it satisfies: an effective content of B*≥0.001, the effective content of B*=B-[Ti-3.4N-1.2(C—Nb/7.8)]/22;
CE<0.58, CE=C+Mn/6+(Cr+Mo+V)/5+(Si+Ni+Cu)/15.

2. The low silicon low carbon equivalent GPa grade multiphase steel plate/steel strip according to claim 1, wherein the C content is 0.045-0.06%, in weight percentage.

3. The low silicon low carbon equivalent GPa grade multiphase steel plate/steel strip according to claim 1, wherein the Si content is 0.15-0.27%, in weight percentage.

4. The low silicon low carbon equivalent GPa grade multiphase steel plate/steel strip according to claim 1, wherein the B content is 0.002-0.004%, in weight percentage.

5. The low silicon low carbon equivalent GPa grade multiphase steel plate/steel strip according to claim 1, wherein the microstructure of the steel plate/steel strip contains ferrite and lower bainite, as well as a small amount of carbide precipitation phase, other inclusion phase and/or trace martensite phase, wherein the content of ferrite is ≤20%, and the content of ferrite+lower bainite is ≥95%.

6. The low silicon low carbon equivalent GPa grade multiphase steel plate/steel strip according to claim 5, wherein the microstructure of the steel plate/steel strip further contains TiN particles, and a single particle has the longest side length of <8 μm or an area of <50 μm.sup.2.

7. The low silicon low carbon equivalent GPa grade multiphase steel plate/steel strip according to claim 5, wherein the average diameter of ferrite grains is <6 μm, or a grain size ASTM rating of ferrite grains is >11.8.

8. The low silicon low carbon equivalent GPa grade multiphase steel plate/steel strip according to claim 1, wherein the steel plate/steel strip has a tensile strength of ≥980 MPa, a yield strength of ≥780 MPa; a hole expansion ratio performance satisfies that if the original hole is a punched hole, the hole expansion ratio is >50%; if the original hole is a reamed hole, the hole expansion ratio is >60%.

9. The manufacturing method for the low silicon low-carbon equivalent GPa grade multiphase steel plate/steel strip according to claim 1, which comprises the following steps: 1) Smelting, continuous casting wherein the chemical elements according to any one of claims 1-4 is smelt and cast into a slab by continuous casting, wherein a cooling rate of the slab is ≥5 C./s during continuous casting; 2) Slab hot transferring, rolling 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 two passes of hot rolling is ≥55%, and a final rolling temperature of finish rolling is 850-950° C.; 3) Cooling after rolling, coiling wherein water cooling is performed after rolling, and the coiling temperature is 550-630° C.; 4) Pickling.

10. The manufacturing method for the low silicon low-carbon equivalent GPa grade multiphase steel plate/steel strip according to claim 9, wherein after the step 3) pickling, the method further comprises the hot dip galvanizing annealing process to obtain the finished hot-rolled hot-dip galvanized steel plate.

11. The manufacturing method for the low silicon low-carbon equivalent GPa grade multiphase steel plate/steel strip according to claim 9, wherein the thickness of the steel plate/steel strip is 0.7 to 4.0 mm.

12. The manufacturing method for the low silicon low-carbon equivalent GPa grade multiphase steel plate/steel strip according to claim 9, wherein the C content of the multiphase steel plate/steel strip is 0.045-0.06%, in weight percentage.

13. The manufacturing method for the low silicon low-carbon equivalent GPa grade multiphase steel plate/steel strip according to claim 9, wherein the Si content of the multiphase steel plate/steel strip is 0.15-0.27%, in weight percentage.

14. The manufacturing method for the low silicon low-carbon equivalent GPa grade multiphase steel plate/steel strip according to claim 9, wherein the B content of the multiphase steel plate/steel strip is 0.002-0.004%, in weight percentage.

15. The manufacturing method for the low silicon low-carbon equivalent GPa grade multiphase steel plate/steel strip according to claim 9, wherein the microstructure of the steel plate/steel strip contains ferrite and lower bainite, as well as a small amount of carbide precipitation phase, other inclusion phase and/or trace martensite phase, wherein the content of ferrite is ≤20%, and the content of ferrite+lower bainite is ≥95%.

16. The manufacturing method for the low silicon low-carbon equivalent GPa grade multiphase steel plate/steel strip according to claim 9, wherein the microstructure of the steel plate/steel strip further contains TiN particles, and a single particle has the longest side length of >8 μm or an area of >50 μm.sup.2.

17. The manufacturing method for the low silicon low-carbon equivalent GPa grade multiphase steel plate/steel strip according to claim 9, wherein the average diameter of ferrite grains is <6 μm, or a grain size ASTM rating of ferrite grains is >11.8.

18. The manufacturing method for the low silicon low-carbon equivalent GPa grade multiphase steel plate/steel strip according to claim 9, wherein the steel plate/steel strip has a tensile strength of 980 MPa, a yield strength of 780 MPa; a hole expansion ratio performance satisfies that if the original hole is a punched hole, the hole expansion ratio is >50%; if the original hole is a reamed hole, the hole expansion ratio is >60%.

Description

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0054] 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).

[0055] 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).

[0056] FIG. 3 is a photo of hot-rolled red iron scale (tiger winkle) defects on the surface of the strip steel when the Si element exceeds 0.5% (FIG. 3 shows that the Si content is 0.55%, Comparative Example L).

[0057] FIG. 4 is a photo of the surface of the strip steel when the Si element is less than 0.5% (FIG. 4 shows that the Si content is 0.25%, Example C).

[0058] FIG. 5 shows that in the microstructure of the steel plate/steel strip of the present disclosure, the content of ferritE+lower bainite is ≥95%.

DETAILED DESCRIPTION

[0059] The present disclosure will be further described with reference to the following examples and figures.

[0060] The steels with different components 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.

[0061] In Table 1, Examples A-I are the steels of the present disclosure. Comparative Examples J-M are comparative steels, wherein the content of carbon or manganese or other alloying elements exceeds the scope of the composition of the present disclosure. Comparative Examples O and P use the component and process according to published patent application. Comparative Example O is an example of CN201380022062.6, of which the alloying ingredient is different from that of the present disclosure and the carbon equivalent is higher than that of the present disclosure; Comparative Example P is an example of CN201180067938.X, of which the alloying ingredient is also different from that of the present disclosure and the carbon equivalent is higher than that of the present disclosure.

[0062] 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 processes disclosed in the corresponding patent applications. But Comparative Example O is a cold-rolled product that does not involve a hot-rolling process, and its product performance is the product performance after cold-rolling and annealing. Some parameters in Comparative Example P are not mentioned, and other parameters are partially different from those of the present disclosure. Table 3 lists the tested mechanical property value of the above-mentioned Examples and Comparative Examples.

[0063] It can be seen that when the content of C, Mn, Ti, Nb, B or B* deviates from the scope of the present disclosure, for example, when the content of Mn, Ti and Nb, or B* is relatively low, such as Comparative Examples K, L and N, it leads to a strength of the steel plate lower than the design requirements; and when the content of C or B is higher than the composition range of the present disclosure, such as Comparative Examples J and M, it leads to the production of a large amount of martensite in the structure, which deteriorates the hole expansion performance of the material, not meeting the purpose of the present disclosure.

[0064] When the Si element content is higher than the scope of the present disclosure, such as Comparative Example L, serious red iron scale (tiger winkle) defects appears on the surface of the steel plate after hot rolling and pickling, as shown in FIG. 3; and when the Si element is within the scope of the present disclosure, the surface of the steel plate is normal in color after hot rolling and pickling, such as Example C as shown in FIG. 4.

[0065] 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; when the coiling temperature is too high, such as Comparative Example D-2, a large amount of coarse carbide particles are generated in the steel plate after coiling, which deteriorates the elongation and hole expansion performance. 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 elongation and hole expansion performance of the steel plate, 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 and hole expansion performance of the material, such as Comparative Example C-2.

[0066] Based on the above, the present disclosure adopts the design with low silicon and low carbon equivalent and optimizes the ratio of each element by reasonably designing the content range of effective B element on the basis of carbon-manganese steel. By further increasing 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 GPa grade ultra-high-strength hot-rolled steel plate/steel strip with a combination of high strength, high hole expansion performance, excellent surface quality and weldability performance, which has a yield strength of not less than 780 MPa, a tensile strength of not less than 980 MPa, and a hole expansion ratio of larger than 50% (the original hole is punched) or larger than 60% (the original hole is reamed), to make up for the urgent demand of the automotive industry market for chassis and suspension materials with a combination of ultra-high strength, high hole expansion performance and low carbon equivalent.

TABLE-US-00001 TABLE 1 (unit: percentage) Steel No. C Si Mn P S N Al Cr Ti Mo Nb B CE B* Ex. A 0.059 0.30 1.88 0.012 0.002 0.004 0.02 0.53 0.081 0.32 0.05 0.0015 0.562 0.0013 Ex. B 0.040 0.45 1.98 0.013 0.004 0.003 0.01 0.66 0.113 0.18 0.02 0.0045 0.568 0.0019 Ex. C 0.069 0.25 1.8 0.015 0.003 0.005 0.02 0.44 0.141 0.37 0.02 0.0031 0.548 0.0011 Ex. D 0.054 0.36 1.75 0.014 0.001 0.005 0.04 0.58 0.090 0.22 0.06 0.0020 0.530 0.0012 Ex. E 0.048 0.15 1.85 0.010 0.001 0.004 0.01 0.69 0.104 0.28 0.04 0.0035 0.560 0.0017 Ex. F 0.033 0.42 1.94 0.008 0.005 0.002 0.03 0.42 0.075 0.25 0.07 0.0043 0.518 0.0025 Ex. G 0.037 0.33 1.73 0.011 0.001 0.007 0.02 0.47 0.121 0.39 0.07 0.0040 0.519 0.0011 Ex. H 0.065 0.18 1.77 0.009 0.006 0.006 0.04 0.50 0.131 0.35 0.03 0.0037 0.542 0.0020 Ex. I 0.051 0.20 1.83 0.016 0.007 0.006 0.01 0.62 0.097 0.16 0.03 0.0025 0.525 0.0016 Comp. Ex. J 0.075 0.23 1.75 0.012 0.002 0.005 0.03 0.44 0.092 0.19 0.05 0.0044 0.472 0.0020 Comp. Ex. K 0.062 0.47 1.56 0.015 0.001 0.005 0.04 0.42 0.084 0.35 0.05 0.002 0.497 0.0022 Comp. Ex. L 0.051 0.55 1.86 0.01 0.001 0.003 0.03 0.5 0.061 0.35 0.01 0.0022 0.568 0.0026 Comp. Ex. M 0.042 0.25 1.96 0.01 0.002 0.005 0.03 0.47 0.09 0.32 0.04 0.0071 0.543 0.0053 Comp. Ex. N 0.045 0.22 1.93 0.01 0.001 0.005 0.01 0.55 0.135 0.29 0.03 0.0039 0.549 0.0008 Comp. Ex. O 0.14 0.06 2.29 0.001 0.0012 0.001 0.292 0.54 0.029 0 0 0.0015 0.634 0.0080 Comp. Ex. P 0.16 0.86 2.05 Not Not 0.004 0.033 0.33 0.12 0 0 0.002 0.625 0.0059 disclosed disclosed

TABLE-US-00002 TABLE 2 Temperature of Finish Continuous slab entering Reheating Reduction Reduction rolling Coiling cast cooling into furnace in temperature rate of the rate of the temperature temperature Steel No. rate ° C./s hot rolling ° C. ° C. first pass % second pass % ° C. ° C. Ex. A-1 12 720 1200 55 57 910 590 Comp. Ex. A-2 10 580 1200 56 57 890 600 Ex. B-1 15 800 1220 58 58 900 610 Comp. Ex. B-2 10 780 1220 45 40 880 615 Ex. C-1 10 770 1210 57 58 870 570 Comp. Ex. C-2  1 780 1210 57 57 920 575 Ex. D-1 13 750 1230 60 59 930 580 Comp. Ex. D-2 10 750 1200 58 58 860 640 Ex. E 11 790 1240 61 55 925 620 Ex. F 17 730 1190 65 55 875 605 Ex. G 20 810 1180 57 56 850 595 Ex. H 16 780 1250 58 59 940 560 Ex. I 14 740 1215 60 60 915 585 Comp. Ex. J 15 730 1200 58 57 890 600 Comp. Ex. K 14 720 1200 55 55 910 590 Comp. Ex. L 13 760 1230 57 55 900 610 Comp. Ex. M 10 720 1210 55 57 910 600 Comp. Ex. N 15 710 1210 60 55 900 610 Comp. Ex. O Not disclosed Not disclosed Not disclosed Not disclosed Not disclosed Not disclosed Not disclosed Comp. Ex. P Not disclosed Not disclosed 1260 900  Not disclosed Not disclosed 500

TABLE-US-00003 TABLE 3 Yield Tensile Hole expansion ratio Hole expansion ratio Steel No. strength, MPa strength, MPa A50% (punched hole), % (reamed hole), % Ex. A-1 799 1011 11.3 63 76 Comp. Ex. A-2 723 942 14 79 92 Ex. B-1 827 1039 10.8 57 71 Comp. Ex. B-2 800 1015 11.1 43 55 Ex. C-1 884 1097 10.2 55 69 Comp. Ex. C-2 832 1054 7.7 37 48 Ex. D-1 785 992 12.1 66 79 Comp. Ex. D-2 811 1046 10.0 47 58 Ex. E 839 1025 10.6 61 76 Ex. F 802 1040 10.9 56 67 Ex. G 812 1012 10.6 59 70 Ex. H 891 1066 10.1 55 65 Ex. I 765 1012 11.3 60 72 Comp. Ex. J 893 1087 9.5 44 61 Comp. Ex. K 711 922 15 73 90 Comp. Ex. L 696 877 16.5 80 95 Comp. Ex. M 902 1103 8.1 39 52 Comp. Ex. N 752 966 12.3 68 84 Comp. Ex. O 748 1094 6.1(A80) Not disclosed Not disclosed Comp. Ex. P 825 1203 9.6(A80) Not disclosed Not disclosed