6-series high-strength aluminum alloy plate for automobile and preparation method of 6-series high-strength aluminum alloy plate

Abstract

The invention discloses a 6-series high-strength aluminum alloy plate for an automobile and a preparation method of the 6-series high-strength aluminum alloy plate, and belongs to the technical field of aluminum alloy preparation. The plate comprises the following metal element components in percentage by mass: 0.7%-1.0% of Si, 0.2%-0.3% of Fe, 0.5%-0.9% of Cu, 0.15%-0.3% of Mn, 0.6%-1.0% of Mg, 0.15%-0.25% of Cr, 0.05%-0.1% of Zn, more than or equal to 0.2% and less than or equal to 0.35% of Cr and Zn, and the balance of Al and inevitable impurities. The preparation method comprises the following steps: smelting and casting according to alloy components and proportions to obtain a cast ingot, homogenizing, carrying out hot rolling and coiling, cooling, carrying out cold rolling and coiling, carrying out solid solution treatment, carrying out pre-aging treatment, and detecting whether the mechanical property meets the requirements or not. By adding the Cr and Zn elements and controlling the adding amount of the Cr and Zn elements, the obtained aluminum alloy plate is high in strength, good in plasticity and low in anisotropy.

Description

Technical Field

[0001] The invention relates to the technical field of aluminum alloy automobile sheet materials, and in particular to a 6 series high-strength aluminum alloy sheet material for automobiles and a preparation method thereof. Background Art

[0002] At present, automobile lightweighting is the most feasible and effective measure to achieve energy conservation and emission reduction. Reducing the weight of the automobile body is one of the main goals of responding to automobile lightweighting. It can not only meet the existing functions of the automobile, such as safety, comfort, and aesthetics, but also reduce fuel consumption, reduce air pollution and automobile exhaust emissions. At the same time, in the field of new energy vehicles, automobile lightweighting can effectively improve problems such as cruising range and power. Aluminum alloy is the preferred material for automobile lightweighting due to its low density, light weight, and easy processing. Among them, the aluminum alloys used in automobile body panels mainly include 5 series aluminum alloys and 6 series aluminum alloys. 5 series aluminum alloys are non-heat-treatable alloys, and their surfaces are prone to Luders bands and orange peels during deformation. Therefore, 5 series aluminum alloys are mostly used in automobile interior panels. The 6 series aluminum alloy is a heat-treatable alloy. Due to its excellent forming properties and bake hardening properties, it is widely used in automotive exterior coverings, such as 6005A, 6014, and 6016 aluminum alloys. In addition, it is also used in automotive structural parts and battery bottom plates, such as 6451 and 6111 aluminum alloys.

[0003] In order to accelerate the process of lightweighting of automobiles, higher-strength aluminum alloy materials can be developed to reserve for further weight reduction of automobile bodies, especially in the field of new energy vehicles. The development of high-strength aluminum alloy plates can better solve the problem of insufficient driving range and improve power. For example, Chinese patent application CN114107746 A discloses a high-performance wide-width 6XXX aluminum alloy plate and strip and its manufacturing method, which adopts semi-solid processing, particle reinforcement and double-roller thin strip continuous casting process. The aluminum alloy plate produced can reach a yield strength of 160-240MPa and a tensile strength of 240-320MPa in T4 or T42 state; the yield strength after baking can reach 260-300MPa and the tensile strength is 340-380MPa; although this method improves the strength of the plate and strip at room temperature and after baking through the particle reinforcement process, it is difficult to implement on the existing production line of automobile thin plate and strip, and the cost of replacing equipment is too high; Chinese patent application CN112119176 A discloses high-strength 6XXX and 7XXX aluminum alloys and preparation methods thereof, and adopts the method of adding different alloying elements such as Zr, Sc, Sn, Zn, Ni, etc. to achieve the preparation of high-strength 6XXX aluminum alloys with a yield strength and / or tensile strength greater than 450MPa. The method makes the yield strength greater than 450MPa by adding multiple alloying elements, but the addition of rare earth elements in production is not conducive to the control of overall production costs; Chinese patent application CN110079716A discloses a high-manganese-chromium 6XXX series aluminum alloy material for automobile body and a preparation method thereof, and increases the Mn+Cr content to 40% on the basis of the existing JIS-6N01 alloy. 0.5-0.75%, the tensile strength of the alloy after peak aging, overaging and short-cycle heating is not less than 260MPa, the yield strength is controlled between 241-280MPa, and the yield strength after long-cycle heating is not less than 230MPa, which can effectively resist the strength exhaustion after heat exposure. This method mainly analyzes the change of strength after peak aging, overaging and short-cycle and long-cycle heating by increasing the addition ratio of Mn+Cr. However, the production of automobile body panels mainly adopts the under-aging state. Although the coarse-grained layer on the surface of the profile can be suppressed after adding Mn, adding a large amount of Mn will promote the growth of grains with specific orientation, which is not conducive to the regulation of anisotropy.

[0004] Although the above methods have done a lot of research on the preparation of 6 series high-strength plates, mainly focusing on particle reinforcement, spray forming, adding alloy elements and other methods to achieve strength improvement, in the actual production process of automobile plates, it is mainly based on the production equipment of existing plates, and the production difficulty and production cost also need to be considered, which is crucial to the development and subsequent application of 6 series high-strength plates for automobiles. Based on this, a 6 series high-strength aluminum alloy plate for automobiles that can be produced using existing plate production equipment and a preparation method thereof are proposed. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a 6 series high-strength aluminum alloy plate for automobiles and a preparation method thereof. By optimizing the alloy composition, an aluminum alloy plate with high strength, high plastic strain ratio and low anisotropy can be obtained.

[0006] The technical solution adopted by the present invention is:

[0007] The invention provides a 6 series high-strength aluminum alloy plate for automobiles, comprising the following metal element components in mass percentage: Si 0.7-1.0%, Fe 0.2-0.3%, Cu 0.5-0.9%, Mn 0.15-0.3%, Mg 0.6-1.0%, Cr 0.15-0.25%, Zn 0.05-0.1%, 0.2%≤Cr+Zn≤0.35%, and the balance is Al and unavoidable impurities.

[0008] Furthermore, the total content of Cr and Zn is 0.25%≤Cr+Zn≤0.30%.

[0009] The present invention also provides a method for preparing a 6 series high-strength aluminum alloy plate for automobiles, comprising the following steps:

[0010] According to the alloy composition and proportion, the ingot is obtained by melting and casting → homogenization treatment → hot rolling and coiling → cooling → cold rolling and coiling → solution treatment → pre-aging treatment → mechanical properties test to see if it meets the requirements.

[0011] Furthermore, in the homogenization process, the homogenization temperature is 530-560° C., and the insulation time is 6-12 hours.

[0012] Furthermore, in the hot rolling process, 1+5 rolling is adopted, the hot rolling coiling temperature is 300-400° C., and the coil thickness after hot rolling is 4.0-7.0 mm.

[0013] Furthermore, in the cold rolling process, the cold rolling reduction ratio is 50 to 70%.

[0014] Furthermore, in the solution treatment process, the solution treatment temperature is 520 to 560° C., and the solution treatment time is 20 to 60 seconds.

[0015] Furthermore, in the pre-aging treatment step, the pre-aging temperature is 60-100°C.

[0016] The beneficial effects of the present invention are:

[0017] The invention provides a 6 series high-strength aluminum alloy plate for automobiles and a preparation method thereof. By adding elements Cr and Zn and controlling the added content, the plastic strain ratio r value is regulated, so that the plate can reach a yield strength of more than 170MPa, a tensile strength of 300-310MPa, an elongation of ≥25%, r0°≥0.62, r45°≥0.65, and r90°≥0.7 in a T4P state; after baking and hardening (185°C*20min), the yield strength can reach 320-330MPa, the tensile strength can reach 380-390MPa, and the elongation can reach ≥17%, that is, an aluminum alloy plate with high strength, high plastic strain ratio and low anisotropy is obtained; and the Cr and Zn elements added in the invention are conventional elements, which are easy to realize in an on-site smelting and casting process, and do not need to modify the existing equipment, which is beneficial to cost control. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0019] Figure 1 This is a grain morphology image of the plate sample prepared in Example 1;

[0020] Figure 2 This is a grain morphology image of the plate sample prepared in Example 2;

[0021] Figure 3 This is a grain morphology image of the plate sample prepared in Example 3;

[0022] Figure 4 This is a grain morphology image of the plate sample prepared in Comparative Example 1;

[0023] Figure 5 This is a grain morphology image of the plate sample prepared in Comparative Example 2;

[0024] Figure 6 This is a grain morphology image of the plate sample prepared in Comparative Example 3;

[0025] Figure 7 This is the grain morphology of the plate sample prepared in Comparative Example 4. DETAILED DESCRIPTION

[0026] The present invention provides a 6 series high strength aluminum alloy plate for automobiles and a preparation method thereof. To make the purpose, technical solution and effect of the present invention clearer and more specific, the present invention is further described in detail below. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0027] A 6 series high-strength aluminum alloy plate for automobiles comprises the following metal element components in mass percentage: Si 0.7-1.0%, Fe 0.2-0.3%, Cu 0.5-0.9%, Mn 0.15-0.3%, Mg 0.6-1.0%, Cr 0.15-0.25%, Zn 0.05-0.1%, 0.2%≤Cr+Zn≤0.35%, and the balance is Al and unavoidable impurities; preferably, the total content of Cr and Zn is 0.25%≤Cr+Zn≤0.30%.

[0028] The method for preparing the above-mentioned 6 series high-strength aluminum alloy sheet for automobiles comprises the following steps:

[0029] According to the alloy composition and ratio, the ingot is obtained by smelting and casting → homogenization treatment → hot rolling and coiling → cooling → cold rolling and coiling → solution treatment → pre-aging treatment → mechanical properties test to see if it meets the requirements;

[0030] Specifically, in the homogenization process, the homogenization temperature is 530-560° C., and the holding time is 6-12 hours;

[0031] In the hot rolling process, 1+5 rolling is adopted, the hot rolling coiling temperature is 300-400°C, and the coil thickness after hot rolling is 4.0-7.0mm;

[0032] In the cold rolling process, the cold rolling processing rate is 50-70%;

[0033] In the solution treatment process, the solution treatment temperature is 520 to 560° C., and the solution treatment time is 20 to 60 seconds.

[0034] In the pre-aging treatment process, the pre-aging temperature is 60-100°C.

[0035] The present invention mainly obtains a high-strength and high-plasticity 6-series aluminum alloy sheet for automobiles by adding Cr and Zn elements and controlling the addition amount of Cr and Zn elements. The addition of Cr can play the role of dispersion strengthening, fine grain strengthening and solid solution strengthening, specifically: the Cr element will combine with the Mn element to precipitate the α-Al(FeMnCr)Si dispersed phase during homogenization treatment, and the dispersed α phase hinders the dislocation movement, thereby playing the role of dispersion strengthening; the addition of the Cr element is also easy to generate a high melting point phase during the casting solidification process, which will act as a heterogeneous nucleation point to make the sheet grain refine, playing the role of fine grain strengthening; in addition, the Cr element can be dissolved back into the matrix during high-temperature heat treatment, playing the role of solid solution strengthening. Among them, the dispersion strengthening effect contributes the most to the strength of the sheet, which can not only effectively improve the strength, but also improve the plasticity of the sheet, which is mainly because the dispersed phase is a particle that is not easy to deform and is not easy to be cut during the processing process, thereby avoiding the occurrence of stress concentration. Of course, if too much Cr is added, part of the Cr will occupy more Si, reduce the amount of Mg2Si phase formed, and have a certain effect on strength and plasticity. In addition, the improvement of strength and elongation by Zn is very significant. The Cr+Zn combination can achieve a significant improvement in strength. If Zn is added alone, although the strength can be significantly improved, the Zn element has a certain inhibitory effect on the improvement of r values ​​in three directions. Therefore, the present invention adopts the Cr+Zn combination, which can not only improve the strength and plasticity of the plate, but also improve the plastic strain ratio r value, and at the same time reduce the anisotropy of the plate.

[0036] Example 1

[0037] This embodiment provides a 6 series high-strength aluminum alloy plate for automobiles, whose element composition is Si 0.9%, Fe 0.25%, Cu 0.85%, Mn 0.22%, Mg 0.86%, Cr 0.18%, Zn 0.08%, and the rest is Al.

[0038] The above-mentioned 6 series high-strength aluminum alloy sheet for automobiles is prepared according to the following process:

[0039] According to the alloy composition and proportion, the ingot is obtained by melting and casting → homogenization treatment → hot rolling and coiling → cooling → cold rolling and coiling → solution treatment → pre-aging treatment → mechanical properties test to see if it meets the requirements.

[0040] Specifically, in the homogenization treatment process, the homogenization treatment temperature is 560°C and the holding time is 6h; then after the homogenization treatment, it is directly hot-rolled after being taken out of the furnace, using 1+5 rolling, the hot rolling coiling temperature is 390°C, and the coil thickness after hot rolling is 7.0mm; after the hot rolling is completed, it is cooled, and then cold rolled after cooling, and the cold rolling processing rate is 70%; after cold rolling, solution treatment and pre-aging treatment are carried out, the solution treatment temperature is 550°C, the solution treatment time is 35s, and after the solution treatment, it is water-cooled to room temperature, and the pre-aging temperature is 90°C.

[0041] Example 2

[0042] This embodiment provides a 6 series high-strength aluminum alloy plate for automobiles, whose element composition is Si 0.88%, Fe 0.24%, Cu 0.83%, Mn 0.22%, Mg 0.83%, Cr 0.25%, Zn 0.05%, and the rest is Al.

[0043] The above-mentioned 6 series high-strength aluminum alloy sheet for automobiles is prepared according to the following process:

[0044] According to the alloy composition and proportion, the ingot is obtained by melting and casting → homogenization treatment → hot rolling and coiling → cooling → cold rolling and coiling → solution treatment → pre-aging treatment → mechanical properties test to see if it meets the requirements.

[0045] Specifically, in the homogenization treatment process, the homogenization treatment temperature is 540°C and the holding time is 12h; then after the homogenization treatment, it is directly hot-rolled out of the furnace, using 1+5 rolling, the hot rolling coiling temperature is 360°C, and the coil thickness after hot rolling is 6.0mm; after the hot rolling is completed, it is cooled and cold-rolled after cooling, and the cold rolling processing rate is 65%; after cold rolling, solution treatment and pre-aging treatment are carried out, the solution treatment temperature is 530°C, the solution treatment time is 40s, and it is water-cooled to room temperature after solution treatment, and the pre-aging temperature is 80°C.

[0046] Example 3

[0047] This embodiment provides a 6 series high-strength aluminum alloy plate for automobiles, whose element composition is Si 0.75%, Fe 0.21%, Cu 0.76%, Mn 0.20%, Mg 0.73%, Cr 0.20%, Zn 0.10%, and the rest is Al.

[0048] The above-mentioned 6 series high-strength aluminum alloy sheet for automobiles is prepared according to the following process:

[0049] According to the alloy composition and proportion, the ingot is obtained by melting and casting → homogenization treatment → hot rolling and coiling → cooling → cold rolling and coiling → solution treatment → pre-aging treatment → mechanical properties test to see if it meets the requirements.

[0050] Specifically, in the homogenization treatment process, the homogenization treatment temperature is 550°C and the holding time is 8h; then, after the homogenization treatment, it is directly hot-rolled after being taken out of the furnace, using 1+5 rolling, the hot rolling coiling temperature is 340°C, and the coil thickness after hot rolling is 5.0mm; after the hot rolling is completed, it is cooled, and then cold rolled after cooling, and the cold rolling processing rate is 60%; after cold rolling, solution treatment and pre-aging treatment are carried out, the solution treatment temperature is 520°C, the solution treatment time is 60s, and after the solution treatment, it is water-cooled to room temperature, and the pre-aging temperature is 100°C.

[0051] Comparative Example 1

[0052] This comparative example provides a 6 series high-strength aluminum alloy plate for automobiles, whose element composition is Si 0.78%, Fe 0.20%, Cu 0.79%, Mn 0.18%, Mg 0.76%, Cr 0.15%, Zn 0.12%, and the rest is Al.

[0053] The above-mentioned 6 series high-strength aluminum alloy sheet for automobiles is prepared according to the following process:

[0054] According to the alloy composition and proportion, the ingot is obtained by melting and casting → homogenization treatment → hot rolling and coiling → cooling → cold rolling and coiling → solution treatment → pre-aging treatment → mechanical properties test to see if it meets the requirements.

[0055] Specifically, in the homogenization treatment process, the homogenization treatment temperature is 550°C and the holding time is 8h; then, after the homogenization treatment, it is directly hot-rolled after being taken out of the furnace, using 1+5 rolling, the hot rolling coiling temperature is 340°C, and the coil thickness after hot rolling is 5.0mm; after the hot rolling is completed, it is cooled, and then cold rolled after cooling, and the cold rolling processing rate is 60%; after cold rolling, solution treatment and pre-aging treatment are carried out, the solution treatment temperature is 520°C, the solution treatment time is 60s, and after the solution treatment, it is water-cooled to room temperature, and the pre-aging temperature is 100°C.

[0056] Comparative Example 2

[0057] This comparative example provides a 6 series high-strength aluminum alloy plate for automobiles, whose element composition is Si 0.89%, Fe 0.24%, Cu 0.84%, Mn 0.21%, Mg 0.85%, Cr 0.20%, and the rest is Al.

[0058] The above-mentioned 6 series high-strength aluminum alloy sheet for automobiles is prepared according to the following process:

[0059] According to the alloy composition and proportion, the ingot is obtained by melting and casting → homogenization treatment → hot rolling and coiling → cooling → cold rolling and coiling → solution treatment → pre-aging treatment → mechanical properties test to see if it meets the requirements.

[0060] Specifically, in the homogenization treatment process, the homogenization treatment temperature is 560°C and the holding time is 6h; then after the homogenization treatment, it is directly hot-rolled after being taken out of the furnace, using 1+5 rolling, the hot rolling coiling temperature is 390°C, and the coil thickness after hot rolling is 7.0mm; after the hot rolling is completed, it is cooled, and then cold rolled after cooling, and the cold rolling processing rate is 70%; after cold rolling, solution treatment and pre-aging treatment are carried out, the solution treatment temperature is 550°C, the solution treatment time is 35s, and after the solution treatment, it is water-cooled to room temperature, and the pre-aging temperature is 90°C.

[0061] Comparative Example 3

[0062] This comparative example provides a 6 series high-strength aluminum alloy plate for automobiles, whose element composition is Si 0.86%, Fe 0.21%, Cu 0.85%, Mn 0.23%, Mg 0.85%, Zn 0.18%, and the rest is Al.

[0063] The above-mentioned 6 series high-strength aluminum alloy sheet for automobiles is prepared according to the following process:

[0064] According to the alloy composition and proportion, the ingot is obtained by melting and casting → homogenization treatment → hot rolling and coiling → cooling → cold rolling and coiling → solution treatment → pre-aging treatment → mechanical properties test to see if it meets the requirements.

[0065] Specifically, in the homogenization treatment process, the homogenization treatment temperature is 540°C and the holding time is 12h; then after the homogenization treatment, it is directly hot-rolled out of the furnace, using 1+5 rolling, the hot rolling coiling temperature is 360°C, and the coil thickness after hot rolling is 6.0mm; after the hot rolling is completed, it is cooled and cold-rolled after cooling, and the cold rolling processing rate is 65%; after cold rolling, solution treatment and pre-aging treatment are carried out, the solution treatment temperature is 530°C, the solution treatment time is 40s, and it is water-cooled to room temperature after solution treatment, and the pre-aging temperature is 80°C.

[0066] Comparative Example 4

[0067] This comparative example provides a 6 series high-strength aluminum alloy plate for automobiles, whose element composition is Si 0.85%, Fe 0.22%, Cu 0.83%, Mn 0.20%, Mg 0.88%, and the rest is Al.

[0068] The above-mentioned 6 series high-strength aluminum alloy sheet for automobiles is prepared according to the following process:

[0069] According to the alloy composition and proportion, the ingot is obtained by melting and casting → homogenization treatment → hot rolling and coiling → cooling → cold rolling and coiling → solution treatment → pre-aging treatment → mechanical properties test to see if it meets the requirements.

[0070] Specifically, in the homogenization treatment process, the homogenization treatment temperature is 540°C and the holding time is 12h; then after the homogenization treatment, it is directly hot-rolled out of the furnace, using 1+5 rolling, the hot rolling coiling temperature is 360°C, and the coil thickness after hot rolling is 6.0mm; after the hot rolling is completed, it is cooled and cold-rolled after cooling, and the cold rolling processing rate is 65%; after cold rolling, solution treatment and pre-aging treatment are carried out, the solution treatment temperature is 530°C, the solution treatment time is 40s, and it is water-cooled to room temperature after solution treatment, and the pre-aging temperature is 80°C.

[0071] The above examples and comparative examples were sampled for room temperature mechanical property testing, as shown in Table 1, and metallographic testing was performed. Figure 1-7 As shown, the average grain size of the longitudinal section is calculated and recorded in Table 1.

[0072] In addition, samples were taken and mechanical properties were tested after hardening at 185°C*20min, as shown in Table 2.

[0073] Table 1 Mechanical properties at room temperature

[0074]

[0075] Table 2 Baking mechanical properties

[0076]

[0077] By analyzing the room temperature mechanical properties and baking mechanical properties of Table 1 and Table 2, the plates of Examples 1-3 of the present invention can reach a yield strength of more than 170MPa, a tensile strength of 300-310MPa, an elongation of ≥25%, r0°≥0.62, r45°≥0.65, and r90°≥0.7 in the T4P state; after baking hardening (185℃*20min), the yield strength can reach 320-330MPa, a tensile strength of 380-390MPa, and an elongation of ≥17%, that is, it can simultaneously ensure high strength and high plastic strain ratio, and reduce the anisotropy of the plate. In addition, through comparative examples 1-4, it is also reflected that when adding Cr and Zn elements, the present invention needs to control the addition amount, otherwise it affects the strength, plasticity and anisotropy, and cannot simultaneously achieve high strength and high plastic strain ratio.

[0078] It should be noted that some parts not described in the present invention can be implemented by adopting or drawing on existing technologies.

[0079] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A 6 series high strength aluminum alloy sheet for automobiles, characterized in that: The invention comprises the following metal element components in mass percentage: Si 0.7-1.0%, Fe 0.2-0.3%, Cu 0.5-0.9%, Mn 0.15-0.3%, Mg 0.6-1.0%, Cr0.15-0.25%, Zn 0.05-0.1%, 0.2%≤Cr+Zn≤0.35%, and the balance is Al and unavoidable impurities.

2. The 6 series high strength aluminum alloy sheet for automobiles according to claim 1, characterized in that: The total content of Cr and Zn is 0.25%≤Cr+Zn≤0.30%.

3. A method for preparing a 6 series high strength aluminum alloy sheet for automobiles as claimed in claim 1 or 2, characterized in that: Including the following processes: According to the alloy composition and proportion, the ingot is obtained by melting and casting → homogenization treatment → hot rolling and coiling → cooling → cold rolling and coiling → solution treatment → pre-aging treatment → mechanical properties test to see if it meets the requirements.

4. The method for preparing a 6 series high strength aluminum alloy sheet for automobile according to claim 3, characterized in that: In the homogenization process, the homogenization temperature is 530-560° C. and the insulation time is 6-12 hours.

5. The method for preparing a 6 series high strength aluminum alloy sheet for automobile according to claim 3, characterized in that: In the hot rolling process, 1+5 rolling is adopted, the hot rolling coiling temperature is 300-400°C, and the coil thickness after hot rolling is 4.0-7.0 mm.

6. The method for preparing a 6 series high strength aluminum alloy sheet for automobile according to claim 3, characterized in that: In the cold rolling process, the cold rolling reduction ratio is 50 to 70%.

7. The method for preparing a 6 series high strength aluminum alloy sheet for automobile according to claim 3, characterized in that: In the solution treatment process, the solution treatment temperature is 520 to 560° C., and the solution treatment time is 20 to 60 seconds.

8. The method for preparing a 6 series high strength aluminum alloy sheet for automobiles according to claim 3, characterized in that: In the pre-aging treatment process, the pre-aging temperature is 60-100°C.

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