High-fatigue-performance cold-rolled steel for automobile auxiliary frame and preparation method of high-fatigue-performance cold-rolled steel

Abstract

The invention discloses high-fatigue-performance cold-rolled steel for an automobile auxiliary frame and a preparation method thereof, and belongs to the technical field of automobile material preparation. The cold-rolled steel comprises the following chemical components in percentage by mass: 0.06 to 0.16 percent of C, 0.15 to 0.40 percent of Si, 1.0 to 2.5 percent of Mn, less than or equal to 0.02 percent of P, less than or equal to 0.01 percent of S, 0.02 to 0.1 percent of Al, 0.01 to 0.06 percent of Ti, 0.3 to 1.5 percent of Cr, 0.1 to 0.6 percent of Mo, 0.005 to 0.03 percent of Nb and the balance of Fe and inevitable impurity elements. The preparation method comprises the steps of smelting, continuous casting, heating, hot rolling, coiling, acid pickling, cold rolling and annealing. The cold-rolled steel has excellent mechanical properties and fatigue properties, meets the use requirements of steel for the automobile auxiliary frame, can replace currently used hot-rolled automobile structural steel, prolongs the service life of the auxiliary frame, and meanwhile, realizes light weight of an automobile body structure.

Description

Technical Field

[0001] The invention belongs to the technical field of automobile material preparation, and relates to a cold-rolled steel for an automobile subframe with high fatigue performance and a preparation method thereof. Background Art

[0002] High-end, electrified, lightweight and intelligent have become the main trends in the development of the automotive industry. With the continuous development of automotive material preparation technology, a variety of advanced high-strength steels have been developed and widely used in the manufacture of automotive body structural parts. The continuous innovation and optimization of passenger car body materials by various automobile factories have continuously improved the quality of automobiles. In contrast, the attention paid to chassis component materials is slightly insufficient. As an important structural component of the automobile, the chassis contains huge room for innovation and development in material selection and application technology. Among them, the subframe is a device connecting the suspension and the body, and has high requirements for strength and fatigue performance. At present, most passenger car subframes are made of hot-rolled pickled carbon manganese steel, cold-formed hot-rolled automotive structural steel and other materials. Such materials have a tensile strength of about 500MPa, good forming performance, simple preparation process, and certain cost advantages, but the thickness of the hot-rolled plate used is above 2.0mm, which is not conducive to the lightweighting of automobiles. Some high-end models also use aluminum alloy subframes, which have significant lightweighting effects, but the purchase price of aluminum alloy materials is expensive, and the manufacturing cost has increased significantly.

[0003] Therefore, there is an urgent need to develop cold-rolled steel for automobile subframes with high fatigue performance, which can significantly reduce the material thickness and effectively improve the lightweight level of the entire vehicle while meeting the requirements of high fatigue performance and high strength. Summary of the invention

[0004] In view of the deficiencies in the prior art, the present invention provides a high fatigue performance cold-rolled steel for an automobile subframe and a preparation method thereof.

[0005] In order to achieve the above-mentioned invention object, the present invention adopts the following technical scheme:

[0006] A cold-rolled steel for automobile subframe with high fatigue performance, the chemical composition and mass percentage of which are C: 0.06-0.16%, Si: 0.15-0.40%, Mn: 1.0-2.5%, P≤0.02%, S≤0.01%, Al: 0.02-0.1%, Ti: 0.01-0.06%, Cr: 0.3-1.5%, Mo: 0.1-0.6%, Nb: 0.005-0.03%, and the balance is Fe and unavoidable impurity elements.

[0007] Furthermore, the chemical composition and mass percentage of the cold-rolled steel are C: 0.08-0.14%, Si: 0.18-0.3%, Mn: 1.35-2.0%, P≤0.02%, S≤0.01%, Al: 0.03-0.08%, Ti: 0.02-0.04%, Cr: 0.6-1.2%, Mo: 0.2-0.5%, Nb: 0.008-0.02%, and the remainder is Fe and unavoidable impurity elements.

[0008] Furthermore, the thickness of the cold-rolled steel is 1.0-2.5 mm, and the microstructure is ferrite+bainite.

[0009] Furthermore, the cold-rolled steel has a yield strength of 280-420 MPa, a tensile strength of 440-550 MPa, an elongation after fracture A80 ≥ 23%, and a 7 Cyclic fatigue strength ≥360MPa, fatigue ratio ≥0.65.

[0010] The present invention also provides a method for preparing the above-mentioned cold-rolled steel for automobile subframe with high fatigue performance, which comprises the steps of smelting, continuous casting, heating, hot rolling, coiling, pickling, cold rolling and annealing;

[0011] The smelting process uses LF+RH refining process;

[0012] In the coiling process, the coiling temperature is controlled at 580-700°C;

[0013] The annealing process adopts a continuous annealing process, which includes heating, soaking, slow cooling, rapid cooling, over-aging and leveling steps in sequence; wherein the soaking temperature is 780-850°C, and the soaking time is 80-140s; then slowly cool to the slow cooling temperature of 680-750°C at a cooling rate of 1-5°C / s, then quickly cool to the over-aging temperature of 300-380°C at a cooling rate of 20-35°C / s, keep warm for 400-600s, and finally leveling, with an elongation range of 0.30-0.50%.

[0014] Furthermore, the smelting process specifically includes molten iron pretreatment, converter smelting, and LF+RH refining;

[0015] Hot metal pretreatment: Desulfurization is carried out by composite injection process, and S is less than or equal to 0.003wt% after pretreatment;

[0016] Converter smelting: blowing oxygen into slag, adding deoxidizer and alloy material to make the content of each element reach the content range of finished product;

[0017] LF+RH refining: LF treatment time is controlled to be not less than 20min, RH vacuum treatment time is 30-60min, N content is controlled below 0.0025%, S content is controlled below 0.002%, A, B, C, D inclusions are all controlled below level 1.5, and the total rating of various inclusions is controlled below 6.0.

[0018] Furthermore, in the continuous casting process, the molten steel obtained in the smelting process is cast using argon protection, the superheat of the molten steel in the tundish is controlled at 25-30°C, and after mixing and diverting in the tundish, it is injected into the crystallizer for cooling and solidification for continuous casting.

[0019] Furthermore, in the heating process, the continuous casting billet is heated to a temperature of 1250-1300° C., the holding time is 1-2 hours, and the air-fuel ratio is controlled to be ≤1.3.

[0020] Furthermore, in the hot rolling process, the rough rolling outlet temperature is controlled at 980-1080°C; the start rolling temperature of the finishing rolling is controlled at 900-1020°C, and the final rolling temperature is 790-860°C; after finishing rolling, the steel billet is cooled to the coiling temperature at a rate of 5-20°C / s.

[0021] Furthermore, in the pickling process, a hydrochloric acid shallow tank turbulent pickling process is adopted, and the pickling temperature is 50-100° C.; in the cold rolling process, the reduction rate is 60-70%.

[0022] The composition design principle of the present invention is as follows:

[0023] C: The main strengthening element in steel, which can play a role of solid solution strengthening and improve the strength of steel. However, excessive content will affect the welding performance of steel. In order to ensure good comprehensive performance, the C content is controlled at 0.06-0.16% after comprehensive consideration.

[0024] Si: The main deoxidizing element has a strong solid solution strengthening effect, but too high Si content will affect the surface quality and welding performance. Taking all factors into consideration, the Si content is controlled at 0.15-0.4%.

[0025] Mn: It has a certain solid solution strengthening effect and can improve the hardness and strength of steel. However, excessive Mn will cause segregation. Considering all factors, the Mn content is controlled at 1.0-2.5%.

[0026] Al: An important deoxidizer that can refine grains and form alumina inclusions, which can improve the strength and hardness of steel. However, too much Al will affect the toughness of steel. Taking all factors into consideration, the Al content is controlled at 0.02-0.1%.

[0027] Cr: It can improve the hardenability of steel and produce carbides, thereby increasing the tensile strength and fatigue strength of steel. However, too much Cr will reduce the plasticity and toughness of the steel plate. Taking all factors into consideration, the Cr content is controlled at 0.3% to 1.5%.

[0028] Ti: It forms nano-scale carbides with C, and has a significant precipitation strengthening effect. In addition, it can effectively hinder the growth of austenite grains, thereby improving the strength of the steel plate. However, when the Ti content is too high, the weldability and plasticity of the steel will be reduced. Considering all factors, the Ti content is controlled at 0.01-0.06%.

[0029] Mo: The element can be dissolved in ferrite, austenite and carbide, reducing the austenite phase area, improving the hardenability of steel, and at the same time improving the tempering stability and fatigue performance of steel. However, too much Mo will increase the cost. Considering comprehensively, the Mo content is controlled at 0.1-0.6%.

[0030] Nb: It can form stable carbides with C, refine grains, enhance the strength and hardness of steel, and improve welding performance. Since Nb is expensive, excessive amount will also lead to increased costs. After comprehensive consideration, the Nb content is controlled at 0.005-0.03%.

[0031] The technical conception of the main process parameters of the present invention is as follows: In order to improve the fatigue performance of steel, the present invention selects the LF+RH double treatment process for refining outside the steelmaking furnace, which can effectively reduce the content of non-metallic inclusions in the steel and improve the purity of the material, thereby suppressing the initiation of fatigue cracks and improving fatigue strength. The curling temperature is controlled at 580-700°C to reduce the growth momentum of ferrite grains and reduce the grain size, thereby reducing the banded structure, which is conducive to the subsequent annealing to produce uniform and fine ferrite grains. The control of the continuous annealing process is to enable the structure to fully recrystallize and form a mixed structure of ferrite and bainite. The cooperation between bainite and fine ferrite grains can not only improve the tensile strength of steel, but also hinder the formation and expansion of fatigue cracks, greatly improving the fatigue performance of steel.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention adopts a low-carbon, high-manganese chemical composition system based on high strength and toughness, and refines the structure by adding common alloying elements such as Si, Nb, Ti, Cr, and Mo to improve the fatigue performance of steel. The smelting adopts the LF+RH double refining process technology to improve the cleanliness of molten steel, reduce the content of non-metallic inclusions, and further improve the fatigue performance of steel. Annealing adopts a continuous annealing process, and the final structure of the steel plate is a fine ferrite and bainite mixed structure. The steel plate has excellent mechanical properties and fatigue properties, with a yield strength of 280~420MPa, a tensile strength of 440~550MPa, an elongation after fracture A80≥23%, 10 7 The cyclic fatigue strength is ≥360MPa, and the fatigue ratio is ≥0.65. The present invention meets all the requirements for the use of automobile subframe steel, can replace the currently used hot-rolled automobile structural steel, improve the service life of the subframe, and achieve lightweight body structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is the microstructure diagram of the cold-rolled steel in Example 1 of the present invention. DETAILED DESCRIPTION

[0034] The present invention is further described in detail below in conjunction with the embodiments and drawings.

[0035] Examples 1 to 10

[0036] The cold-rolled steel for automobile subframe with high fatigue performance in each embodiment of the present invention is produced according to the following steps:

[0037] (1) Smelting: Smelting is carried out according to the composition in Table 1, specifically including molten iron pretreatment, converter smelting, and LF+RH refining;

[0038] a) Hot metal pretreatment: Desulfurization is carried out by composite injection process, and S is less than or equal to 0.003wt% after pretreatment;

[0039] b) Converter smelting: blowing oxygen into slag, adding deoxidizer and alloy material to make the content of each element reach the content range of finished product;

[0040] c) LF+RH refining: LF treatment time is controlled to be not less than 20min, RH vacuum treatment time is 30-60min, N content is controlled below 0.0025%, S content is controlled below 0.002%, A, B, C, D inclusions are all controlled below grade 1.5, and the total rating of various inclusions is controlled below 6.0.

[0041] (2) Continuous casting: The molten steel obtained in step (1) is cast under argon protection, and the superheat of the molten steel in the tundish is controlled at 25-30°C. After mixing and diverting in the tundish, it is injected into the crystallizer for cooling and solidification for continuous casting.

[0042] (3) Heating: The continuous casting billet is heated at a temperature of 1250-1300°C for 1-2 hours, and the air-fuel ratio is controlled to be ≤1.3.

[0043] (4) Hot rolling: The outlet temperature of rough rolling is controlled at 980-1080°C; the starting temperature of finishing rolling is controlled at 900-1020°C, and the final rolling temperature is controlled at 790-860°C; after finishing rolling, the billet is cooled to the coiling temperature, and the cooling rate is controlled at 5-20°C / s.

[0044] (5) Coiling: The coiling temperature is controlled at 580-700°C.

[0045] (6) Pickling: The hydrochloric acid shallow tank turbulent pickling process is adopted, and the pickling temperature is 50-100℃.

[0046] (7) Cold rolling: The reduction rate is 60-70%, and the thickness of the cold rolled sheet is 1.0-2.5 mm.

[0047] (8) Annealing: A continuous annealing process is adopted, wherein the soaking temperature is 780-850°C, the soaking time is 80-140s, and then slowly cooled to the slow cooling temperature of 680-750°C at a cooling rate of 1-5°C / s, and then quickly cooled to the over-aging temperature of 300-380°C at a cooling rate of 20-35°C / s, and kept at this temperature for 400-600s. Finally, flattening is performed, and the flattening elongation range is 0.30-0.50%.

[0048] The chemical composition of the cold-rolled steel of Examples 1-10 is shown in Table 1; the production process parameter control is shown in Table 2-5, and the mechanical properties and fatigue properties are shown in Table 6.

[0049] Table 1. Chemical composition of cold-rolled steel in various embodiments (wt%)

[0050] Group No. C Si Mn P S Al Ti Cr Mo Nb Example 1 0.085 0.180 1.99 0.0046 0.0047 0.042 0.035 0.91 0.32 0.009 Example 2 0.120 0.223 2.00 0.0047 0.0042 0.049 0.035 0.73 0.27 0.013 Example 3 0.092 0.217 1.37 0.0048 0.0045 0.058 0.020 1.00 0.41 0.010 Example 4 0.080 0.190 1.80 0.0051 0.0048 0.033 0.022 0.79 0.45 0.018 Example 5 0.115 0.197 1.41 0.0044 0.0043 0.038 0.037 0.68 0.30 0.009 Example 6 0.060 0.150 1.00 0.0043 0.0042 0.020 0.010 0.30 0.10 0.005 Example 7 0.082 0.181 1.35 0.0046 0.0043 0.030 0.020 0.60 0.20 0.008 Example 8 0.160 0.400 2.50 0.0043 0.0043 0.100 0.060 1.20 0.60 0.020 Example 9 0.140 0.300 2.00 0.0042 0.0045 0.080 0.040 1.50 0.50 0.030 Example 10 0.112 0.239 1.63 0.0045 0.0044 0.056 0.031 0.89 0.35 0.017

[0051] Table 2. Parameters of smelting, continuous casting and heating processes in various embodiments

[0052]

[0053]

[0054] Table 3 Non-metallic inclusions in various examples

[0055]

[0056] Table 4. Rolling production control parameters of various embodiments

[0057]

[0058] Table 5: Annealing process parameters of various embodiments

[0059]

[0060] Table 6 Mechanical properties and fatigue properties of cold-rolled steel in various embodiments

[0061]

[0062] It can be seen from Table 6 that the cold-rolled yield strength produced by the present invention is 280-420 MPa, the tensile strength is 440-550 MPa, the elongation after fracture A80 is ≥ 23%, and the 10 7 Cyclic fatigue strength ≥360MPa, fatigue ratio ≥0.65.

Claims

1. A cold-rolled steel for automobile subframe with high fatigue performance, characterized in that: Its chemical composition and mass percentage are C: 0.06-0.16%, Si: 0.15-0.40%, Mn: 1.0-2.5%, P≤0.02%, S≤0.01%, Al: 0.02-0.1%, Ti: 0.01-0.06%, Cr: 0.3-1.5%, Mo: 0.1-0.6%, Nb: 0.005-0.03%, and the balance is Fe and unavoidable impurity elements.

2. The high fatigue performance cold-rolled steel for automobile subframe according to claim 1, characterized in that: Its chemical composition and mass percentage are C: 0.08-0.14%, Si: 0.18-0.3%, Mn: 1.35-2.0%, P≤0.02%, S≤0.01%, Al: 0.03-0.08%, Ti: 0.02-0.04%, Cr: 0.6-1.2%, Mo: 0.2-0.5%, Nb: 0.008-0.02%, and the balance is Fe and unavoidable impurity elements.

3. The high fatigue performance cold-rolled steel for automobile subframe according to claim 2, characterized in that: The thickness of the cold-rolled steel is 1.0-2.5 mm, and the microstructure is ferrite+bainite.

4. The high fatigue performance cold-rolled steel for automobile subframe according to claim 3, characterized in that: The cold-rolled steel has a yield strength of 280-420 MPa, a tensile strength of 440-550 MPa, an elongation after fracture A80 ≥ 23%, and a 7 Cyclic fatigue strength ≥360MPa, fatigue ratio ≥0.

65.

5. The method for preparing the cold-rolled steel for automobile subframe with high fatigue performance according to any one of claims 1 to 4, characterized in that: It includes smelting, continuous casting, heating, hot rolling, coiling, pickling, cold rolling and annealing processes; The smelting process uses LF+RH refining process; In the coiling process, the coiling temperature is controlled at 580-700°C; The annealing process adopts a continuous annealing process, which includes heating, soaking, slow cooling, rapid cooling, over-aging and leveling steps in sequence; wherein the soaking temperature is 780-850°C, and the soaking time is 80-140s; then slowly cool to the slow cooling temperature of 680-750°C at a cooling rate of 1-5°C / s, then quickly cool to the over-aging temperature of 300-380°C at a cooling rate of 20-35°C / s, keep warm for 400-600s, and finally leveling, with an elongation range of 0.30-0.50%.

6. The method for preparing cold-rolled steel for automobile subframe with high fatigue performance according to claim 5, characterized in that: The smelting process specifically includes molten iron pretreatment, converter smelting, and LF+RH refining; Hot metal pretreatment: Desulfurization is carried out by composite injection process, and S is less than or equal to 0.003wt% after pretreatment; Converter smelting: blowing oxygen into slag, adding deoxidizer and alloy material to make the content of each element reach the content range of finished product; LF+RH refining: LF treatment time is controlled to be not less than 20min, RH vacuum treatment time is 30-60min, N content is controlled below 0.0025%, S content is controlled below 0.002%, A, B, C, D inclusions are all controlled below level 1.5, and the total rating of various inclusions is controlled below 6.

0.

7. The method for preparing cold-rolled steel for automobile subframe with high fatigue performance according to claim 6, characterized in that: In the continuous casting process, the molten steel obtained in the smelting process is cast using argon protection, the superheat of the molten steel in the tundish is controlled at 25-30°C, and after mixing and diverting in the tundish, it is injected into the crystallizer for cooling and solidification for continuous casting.

8. The method for preparing cold-rolled steel for automobile subframe with high fatigue performance according to claim 7, characterized in that: In the heating process, the continuous casting billet is heated to a temperature of 1250-1300°C, the holding time is 1-2 hours, and the air-fuel ratio is controlled to be ≤1.

3.

9. The method for preparing cold-rolled steel for automobile subframe with high fatigue performance according to claim 8, characterized in that: In the hot rolling process, the rough rolling outlet temperature is controlled at 980-1080°C; the start rolling temperature of the finishing rolling is controlled at 900-1020°C, and the final rolling temperature is 790-860°C; after finishing rolling, the steel billet is cooled to the coiling temperature at a rate of 5-20°C / s.

10. The method for preparing the cold-rolled steel for automobile subframe with high fatigue performance according to any one of claims 1 to 9, characterized in that: The pickling process adopts the hydrochloric acid shallow tank turbulent pickling process, and the pickling temperature is 50-100°C; the cold rolling process has a reduction rate of 60-70%.

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