ULTRAHIGH HOLE EXPANSION STEEL AND METHOD FOR MANUFACTURING THEREFOR

Abstract

The present invention provides an ultrahigh hole expansion steel and a method for manufacturing therefor. The steel comprises the following components in percentage by mass: C: 0.03-0.09%; Si0.2%; Mn: 0.5-2.0%; P0.02%; S0.003%; Al: 0.2-1.2%; N0.004%; Ti: 0.05-0.20%; Mo: 0.05-0.5%; Mg0.005%; O0.003%; B0.001%; and the balance being Fe and inevitable impurities. wherein C, Mn, Mo and B in the steel satisfy the following formula: 0.252C+Mn/3+Mo+150B1.5; wherein each chemical element in the formula represent the numerical value before the percentage sign of the percentage by mass of corresponding chemical elements. The steel according to the present invention has excellent matching of strength, plasticity and hole expansion performance, and can be applied in passenger vehicle chassis parts that require high strength and thickness reduction, such as a control arm and a subframe.

Claims

1. A steel, comprising the following components in percentage by mass: C: 0.03-0.09%; Si0.2%; Mn: 0.5-2.0%; P0.02%; S0.003%; Al: 0.2-1.2%; N0.004%; Ti: 0.05-0.20%; Mo: 0.05-0.5%; Mg0.005%; O0.003%; B0.001%; and the balance being Fe and inevitable impurities, wherein C, Mn, Mo and B in the steel satisfy the following formula: $0.252C+\mathrm{Mn}/3+\mathrm{Mo}+150B1.5,$ wherein each chemical element in the formula represent the numerical value before the percentage sign of the percentage by mass of corresponding chemical elements.

2. The steel as claimed in claim 1, characterized in that, the steel further comprises one or more elements selected from Nb, V, Cu, Ni and Cr, wherein Nb0.06%, V0.10%, preferably 0.05%, Cu0.5%, preferably 0.3 wt %, Ni0.5%, preferably 0.3%, Cr0.5%, preferably 0.3% in percentage by mass.

3. The steel as claimed in claim 1, characterized in that, the components of the steel further satisfy at least one of the following: Si0.15 wt %, Mn: 1.0-1.6 wt %, S0.0015 wt %, Al: 0.5-1.0 wt %, N0.003 wt %, Ti: 0.07-0.11 wt %, Mo: 0.15-0.45 wt %, Ni0.03 wt %, B0.0005 wt %.

4. The steel as claimed in claim 1, characterized in that, the steel has a yield strength of 700 MPa, a tensile strength of 780 MPa, a transverse elongation A50 of 17%, and a hole expansion rate 80%.

5. The steel as claimed in claim 1, characterized in that, the steel has a structure containing 95 volume % or more, preferably 97 volume % or more of ferrite, and 5 volume % or less, preferably 3 volume % or less, of pearlite, wherein the ferrite contains dispersively distributed nanoscale carbides.

6. A method for manufacturing the steel as claimed in claim 1, comprising the following steps: 1) Smelting and casting; Smelting a molten steel in a converter or an electric furnace according to the composition as claimed in claim 1, then secondary refining in a vacuum furnace, and casting into a billet or an ingot; 2) Reheating the billet or the ingot; Heating temperature1200 C., holding time: 1-2 hours; 3) Hot rolling and cooling the billet or the ingot; wherein initial rolling temperature: 1050-1150 C., rough rolling of 3-5 passes is carried out under high pressure at 1050 C. or more to a cumulative deformation of 50%, obtaining an intermediate billet, thereafter, the intermediate billet is air-cooled or water-cooled to 950-1000 C., and finishing rolling of 5-7 passes is carried out to a cumulative deformation of 70%, a final rolling temperature is 850-950 C., obtaining a steel strip; wherein cooling adopts laminar flow cooling; after final rolling, water cooling the steel strip to 550-650 C. at a cooling speed of 10 C./s and coiling, after coiling, cooling to room temperature at a cooling speed of 50 C./h, obtaining a hot-rolled strip steel.

7. The method as claimed in claim 6, characterized in that, the method further comprises step 4) Pickling, wherein a pickling operating speed of the hot-rolled strip steel is 30-140m/min, a pickling temperature is 75-85 C., a straightening rate is 3%, rinsing is carried out at 35-50 C., and surface drying and oiling are carried out at 120-140 C.

8. The method as claimed in claim 6, characterized in that, the steel further comprises one or more elements selected from Nb, V, Cu, Ni and Cr, wherein Nb0.06%, V0.10%, preferably 0.05%, Cu0.5%, preferably 0.3 wt %, Ni0.5%, preferably 0.3%, Cr0.5%, preferably 0.3% in percentage by mass.

9. The method as claimed in claim 6, characterized in that, the components of the steel further satisfy at least one of the following: Si0.15 wt %, Mn: 1.0-1.6 wt %, S0.0015 wt %, Al: 0.5-1.0wt %, N0.003 wt %, Ti: 0.07-0.11 wt %, Mo: 0.15-0.45 wt %, Ni0.03 wt %, B0.0005 wt %.

10. The method as claimed in claim 6, characterized in that, the steel has a yield strength of 700 MPa, a tensile strength of 780 MPa, a transverse elongation A50 of 17%, and a hole expansion rate 80%.

11. The method as claimed in claim 6, characterized in that, the steel has a structure containing 95 volume % or more, preferably 97 volume % or more of ferrite, and 5 volume % or less, preferably 3 volume % or less, of pearlite, wherein the ferrite contains dispersively distributed nanoscale carbides.

12. The steel as claimed in claim 2, characterized in that, the steel has a yield strength of 700 MPa, a tensile strength of 780 MPa, a transverse elongation A50 of 17%, and a hole expansion rate 80%.

13. The steel as claimed in claim 3, characterized in that, the steel has a yield strength of 700 MPa, a tensile strength of 780 MPa, a transverse elongation A50 of 17%, and a hole expansion rate 80%.

14. The steel as claimed in claim 2, characterized in that, the steel has a structure containing 95 volume % or more, preferably 97 volume % or more of ferrite, and 5 volume % or less, preferably 3 volume % or less, of pearlite, wherein the ferrite contains dispersively distributed nanoscale carbides.

15. The steel as claimed in claim 3, characterized in that, the steel has a structure containing 95 volume % or more, preferably 97 volume % or more of ferrite, and 5 volume % or less, preferably 3 volume % or less, of pearlite, wherein the ferrite contains dispersively distributed nanoscale carbides.

16. The steel as claimed in claim 4, characterized in that, the steel has a structure containing 95 volume % or more, preferably 97 volume % or more of ferrite, and 5 volume % or less, preferably 3 volume % or less, of pearlite, wherein the ferrite contains dispersively distributed nanoscale carbides.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0063] FIG. 1 is a schematic diagram of the rolling and cooling process for the steel according to the present invention;

[0064] FIG. 2 is a typical metallographic photograph of the steel of Example 2 of the present invention:

[0065] FIG. 3 is a typical metallographic photograph of the steel of Example 4 of the present invention:

[0066] FIG. 4 is a typical metallographic photograph of the steel of Example 6 of the present invention.

DETAILED DESCRIPTION

[0067] The present invention is further described below with reference to the accompanying examples and drawings.

[0068] The steel compositions of Examples and Comparative Examples of the present invention are shown in Table 1, and the balance of the composition in Table 1 are Fe and inevitable impurities.

[0069] The process path for manufacturing the steel in Examples of the present invention is: [0070] 1) Smelting and casting: [0071] Smelting the composition shown in Table 1 in a converter or an electric furnace, then secondary refining in a vacuum furnace, and casting into a billet or an ingot: [0072] 2) Reheating the billet or the ingot: [0073] Heating temperature 1200 C., holding time: 1-2 hours. [0074] 3) Hot rolling and cooling the billet or the ingot: [0075] wherein initial rolling temperature: 1050-1150 C., rough rolling of 3-5 passes was carried out under high pressure at 1050 C. or more to a cumulative deformation of 50%, obtaining an intermediate billet: thereafter, the intermediate billet was air-cooled or water-cooled to 950-1000 C., and finishing rolling of 5-7 passes were carried out to an cumulative deformation of 70%, a final rolling was completed between 850-950 C., obtaining a strip steel: [0076] wherein cooling adopts laminar flow cooling, the strip steel is water-cooled to 550-650 C. at a cooling speed of 10 C./s and coiling, after coiling, cooling to room temperature at a cooling speed of 50 C./h after coiling.

[0077] The specific process is shown in FIG. 1.

[0078] Table 2 shows the manufacturing process parameters of the steel of the Examples of the present invention. Table 3 shows the performance evaluation of the steel of the Examples and the Comparative Examples of the present invention.

[0079] The steel in Comparative Examples 1-3 is selected from CN103602895A, and the steel in Comparative Example 4 is selected from CN114107792A. Table 1 provides composition differences between the Examples and the Comparative Examples. It can be seen from Table 1 that the composition designs of the Comparative Examples are all low aluminum designs, and the composition designs of Comparative Examples 1-3 also include high silicon designs, while the composition design of the present invention is silicon-free and high aluminum. The two are completely different in composition design.

[0080] As can be seen from Table 3, the steel coil obtained according to the compositions and processes of the present invention have a yield strength of 700 MPa, a tensile strength of780 MPa, a transverse elongation A50 of17%, and a hole expansion rate of 80%.

[0081] It can also be seen from Table 3 that, although Comparative Examples 1-3 are similar to the present invention in terms of yield strength, tensile strength and elongation, the hole expansion rate of Comparative Examples 1-3 are significantly lower than those of the Examples of the present invention.

[0082] The yield strength, tensile strength and elongation of the steel in Table 3 were tested in accordance with GB/T 228.1-2021 Tensile Test of Metallic Materials Part 1: Room Temperature Test Methods.

[0083] The hole expansion rate of steel was tested in accordance with GB/T 24524-2021 Experimental Methods for Hole Expansion of Thin Plates and Thin Strips of Metallic Materials.

[0084] FIGS. 2-4 respectively show typical metallographic photographs of the steels in Examples 2, 4 and 6 of the present invention.

[0085] It is clear from the figures that using the composition and process path designed by the present invention, a ferrite-dominated structure is obtained with a very small amount of pearlite. Specifically, the ferrite in the steel is 97 volume % or more, the pearlite is 3 volume % or less, and the ferrite contains dispersively distributed nanoscale carbides.

[0086] The steel of the Examples of the present invention exhibits a good matching of high strength, high plasticity and ultrahigh hole expansion rate with excellent comprehensive performance.

[0087] As can be seen from the above Examples and Comparative Examples, the 780 MPa high-strength steel of the present invention has a good matching of high strength, high plasticity and ultrahigh hole expansion rate, and is particularly suitable for manufacturing vehicle chassis structure and other parts that require high strength, thickness reduction, hole expansion and flanging forming such as the control arm, etc., and can be used for wheels and other complex parts that need flanging, which has a broad application prospect.

TABLE-US-00001 TABLE 1 unit: percentage by mass C Si Mn P S Al N Mo Ti O Mg Cu Ni Cr Nb V B Example 1 0.055 0.13 1.79 0.010 0.0026 0.92 0.0037 0.27 0.15 0.0030 0.001 0.3 0.3 0.02 Example 2 0.088 0.05 0.51 0.013 0.0020 0.39 0.0031 0.06 0.12 0.0025 0.005 0.04 0.0005 Example 3 0.041 0.07 1.46 0.020 0.0027 1.05 0.0028 0.5 0.05 0.0028 0.2 0.2 0.10 0.0008 Example 4 0.070 0.10 0.88 0.018 0.0029 0.62 0.0035 0.16 0.07 0.0024 0.004 Example 5 0.035 0.12 0.51 0.015 0.0019 0.21 0.0038 0.05 0.18 0.0029 0.1 0.1 0.02 0.06 0.0006 Example 6 0.062 0.06 1.98 0.008 0.0022 1.48 0.0025 0.42 0.11 0.0027 0.003 0.06 Example 7 0.045 0.14 0.75 0.017 0.0025 0.81 0.0040 0.36 0.20 0.0026 0.5 0.0010 Example 8 0.075 0.08 1.22 0.014 0.0024 0.53 0.0033 0.05 0.09 0.0023 0.002 0.5 0.03 0.04 Comparative 0.045 1.10 1.70 0.010 0.0009 0.057 0.0031 0.13 0.045 Example 1 Comparative 0.050 0.85 1.90 0.011 0.0020 0.024 0.0031 0.11 0.001 0.060 Example 2 Comparative 0.080 0.55 1.65 0.009 0.0010 0.051 0.0045 0.15 0.024 Example 3 Comparative 0.052 0.09 1.51 0.008 0.0008 0.035 0.0029 0.18 0.09 0.0020 Example 4

TABLE-US-00002 TABLE 2 Initial Cumulative Intermediate Cumulative Final Water- Air- Heating rolling deformation billet deformation rolling cooling cooling Coiling temperature temperature of rough temperature of finishing temperature rate time temperature Example C. C. rolling % C. rolling % C. C./s s C. 1 1310 1150 80 1000 95.0 950 50 8 560 2 1270 1130 65 990 94.5 890 25 15 610 3 1220 1060 75 970 97.6 860 80 5 580 4 1280 1110 60 995 95.8 920 38 12 550 5 1240 1090 71 980 96.8 880 65 6 630 6 1200 1050 50 950 95.2 850 10 20 600 7 1300 1140 55 990 96.8 930 46 10 650 8 1230 1070 75 975 91.4 870 20 16 620

TABLE-US-00003 TABLE 3 Steel plate Yield Tensile Transverse Hole thickness strength strength elongation expansion mm MPa MPa A50 % rate % Example 1 2.5 759 810 20 95.5 Example 2 4.8 776 849 18 101.2 Example 3 1.5 746 833 20 94.4 Example 4 4.2 761 842 19 98.9 Example 5 2.3 755 828 21 97.7 Example 6 6.0 732 815 19 103.6 Example 7 3.6 797 854 18 87.8 Example 8 5.4 738 803 20 110.3 Comparative 2.9 720 790 19 58 Example 1 Comparative 2.8 710 820 17 65 Example 2 Comparative 4.0 750 856 15 50 Example 3 Comparative 2.5 736 803 20 93 Example 4