A method for manufacturing an aluminum alloy sheet for an automobile exterior panel

Abstract

This invention discloses a method for preparing aluminum alloy sheet for automotive body panels. By improving the hot rolling process, it innovatively eliminates the intermediate annealing process that was essential in the original production of aluminum alloy sheet for body panels, shortens the production cycle of 6xxx aluminum alloy for body panels, significantly improves production efficiency, reduces energy consumption and carbon emissions, and the overall performance of the prepared sheet can meet the stringent performance requirements of the body panels.

Description

Technical Field

[0001] This invention relates to an aluminum alloy and its preparation method, specifically to a method for preparing 6xxx series aluminum alloy sheets for automotive body panels. Background Technology

[0002] Gasoline vehicle emissions are a significant source of global air pollution. Lightweighting of automobiles is a crucial means of energy conservation, emission reduction, and pollution control. Aluminum alloy sheets are considered ideal materials for automotive lightweighting due to their light weight, high specific strength, and excellent overall performance. Currently, 5xxx and 6xxx aluminum alloys are mainly used to manufacture automotive sheet metal. 5xxx aluminum alloys have defects such as Lüders bands, thus limiting their application. 6xxx alloys have excellent surface quality and are more widely used in automotive body panels. Although the surface quality of 6xxx aluminum alloys is superior to 5xxx, issues such as paint brush lines still affect surface quality.

[0003] Paint lines are mainly caused by improper processing techniques, resulting in Cube and Goss textures appearing as stripes along the rolling direction of the sheet metal. During stretching, stamping, and other deformations, uneven deformation occurs throughout the sheet metal, thus forming stripes along the rolling direction and affecting the appearance. In recent years, some universities and research institutes in China have conducted a series of studies on improving 6xxx aluminum alloy automotive sheets:

[0004] Patent CN 104532077A proposes a short-process preparation method for 6XXX series aluminum alloy body panels without paint brush lines. It adopts a short-process preparation method without intermediate annealing, but it lacks attention to hot rough rolling process and coil uniformity.

[0005] Patent CN 107109547A discloses a highly formable automotive aluminum sheet with reduced or eliminated surface streaks and its preparation method. In terms of process, the final hot rolling temperature needs to be below 300℃ to suppress the precipitation of coarse Mg2Si; the intermediate annealing temperature is a box annealing at 350-450℃, which can be carried out after hot rolling or a single cold rolling process to improve texture and eliminate brush lines. Furthermore, this process precipitates a large amount of fine, soluble Mg2Si, which is beneficial for obtaining excellent overall properties after solution treatment. This method employs an intermediate annealing process, which differs significantly from the intermediate annealing-free process used in this invention.

[0006] Patent CN 101918602A proposes an aluminum alloy sheet, particularly an aluminum alloy sheet with excellent ridging mark resistance, but its main focus is on analyzing the paint brush lines on the sheet through microstructure analysis.

[0007] Based on the traditional preparation process of 6xxx series aluminum alloy sheets, this invention provides a method for preparing 6xxx series aluminum alloy sheets for automotive body panels by improving the hot rolling process. This method omits the intermediate annealing process, resulting in a shorter process flow, lower cost, and lower carbon emissions. The prepared sheets have excellent paint brush line performance and are more conducive to industrial production. Summary of the Invention

[0008] The purpose of this invention is to provide a method for preparing 6xxx series aluminum alloy sheets for automotive body panels.

[0009] To achieve the above objectives, the present invention adopts the following technical solution:

[0010] A method for preparing an aluminum alloy sheet for an automotive body panel includes the following steps:

[0011] (1) Preparation of aluminum alloy ingots;

[0012] (2) Homogenize the aluminum alloy ingots;

[0013] (3) Hot rough rolling is performed on the homogenized ingot;

[0014] (4) The billet after hot rough rolling is hot finished rolling to produce hot rolled coils;

[0015] (5) The hot-rolled coil is cold-rolled to produce a cold-rolled coil;

[0016] (6) Solution treatment is performed on the cold-rolled coil;

[0017] (7) Pre-aging treatment of the solution-treated roll material;

[0018] (8) Unwind, straighten and slice the pre-aged roll material to obtain the finished product.

[0019] Furthermore, based on a total mass percentage of 100%, the mass percentage of each component in the aluminum alloy ingot obtained in step (1) is as follows: Si 0.50%~1.50%, Fe≤0.40%, Cu≤0.30%, Mn≤0.2%, Mg 0.3%~1.1%, Cr≤0.1%, Ti 0.01-0.10%, with the balance being Al.

[0020] Furthermore, the homogenization process described in step (2) involves holding the temperature at 540-580℃ for 4-20 h, then cooling it to 490-540℃ and holding it at that temperature for 1-10 h.

[0021] Furthermore, in step (3), the total deformation rate of hot rough rolling is not less than 90%, the total number of passes is 8-18, the average reduction per pass is not less than 30mm, the final rolling thickness is 20-40mm, and the final rolling temperature is not less than 400℃; along the width of the plate, the temperature difference between the middle and the edge is not higher than 15℃.

[0022] Furthermore, in step (4), the final rolling thickness of the hot finishing roll is 2.5-6.0 mm, the rolling speed of the final pass is 2-8 m / s, the final rolling temperature is 300-380℃, and after 10 minutes of winding, the temperature difference at each part of the roll is not higher than 10℃; the cooling rate of the hot rolled roll to room temperature is not higher than 15℃ / h.

[0023] Furthermore, the thickness of the cold-rolled sheet obtained in step (5) is 0.5-2.0 mm, and the cold rolling rate is 60-80%.

[0024] Furthermore, the solution treatment described in step (6) is carried out in a continuous annealing furnace with an air cushion, where the temperature is raised to 520-580℃ at a rate of 5-50℃ / s and held for 0.5-5min.

[0025] Furthermore, the pre-aging treatment described in step (7) involves holding the temperature at 70-100℃ for 4-12 hours.

[0026] Furthermore, the finished product obtained in step (8) is a T4P state sheet with a 7-day yield strength of less than 110 MPa and a flange factor r / t. min ≤0.5, the paint brush lines on the edges and center are better than level two.

[0027] Furthermore, after the obtained product is further pre-stretched by 2% and baked at 185℃ for 20 minutes, a T8X state plate with a yield strength greater than 200MPa can be obtained.

[0028] Compared with existing technologies, the technological advancements of this invention are mainly reflected in:

[0029] (1) This invention improves the hot rolling process and innovatively eliminates the intermediate annealing process that was essential for the production of aluminum alloy sheets for the outer cover, shortens the production cycle of 6xxx aluminum alloy for the outer cover, greatly improves production efficiency, reduces energy consumption, and further reduces carbon emissions.

[0030] (2) This invention refines the grain size by controlling the total reduction and the reduction per pass in hot rough rolling, and at the same time changes the trend of continuous distribution of texture along the rolling direction, making it uniformly distributed; at the same time, by controlling the hot finish rolling temperature, rolling speed, cooling speed and thickness, the recrystallization of the hot finish rolling structure of the plate is realized, further breaking the trend of continuous distribution of texture, thereby realizing the preparation of high-quality aluminum alloy plates that meet the paint brush line performance requirements of the outer cover plate without intermediate annealing process.

[0031] (3) The present invention strictly controls the temperature uniformity of hot roughing and hot finishing rolling to ensure uniformity throughout the entire roll, especially improving the problem of differences in paint brush lines at the edges and middle along the width of the plate.

[0032] (4) This invention does not require additional processes or production equipment and can be applied to existing 6xxx board production equipment. It does not require additional conditions or elements, has strong applicability, and can meet the needs of industrial production.

[0033] (5) The aluminum alloy sheet prepared by the method of the present invention has the characteristics of low yield strength, high room temperature stability, good flanging performance and good surface quality when it is in the T4P state. It can meet the requirements of automobile companies for stamping, connecting and painting. After further baking, it can reach the T8X state, and its yield strength is rapidly improved to meet the strength requirements during service. Detailed Implementation

[0034] Terminology Explanation:

[0035] Weight percentage: The percentage of a certain alloy component by mass (weight) out of the total mass.

[0036] Pass reduction: The thickness before each rolling pass is H, and the thickness after rolling is h. The pass reduction Δh = Hh.

[0037] Yield strength: The yield limit of an aluminum alloy when it undergoes yielding. It is defined as the stress value at which 0.2% residual deformation occurs. The stress-strain curve is obtained through a uniaxial tensile test, and the yield strength data is derived from the curve.

[0038] T4P yield strength: The yield strength of the finished sheet material obtained after melting and casting, homogenization, hot rolling, cold rolling, solution treatment, pre-aging, and slicing. After the finished sheet material is left to stand at room temperature for 7 days, its mechanical properties are tested by uniaxial tensile testing to obtain the yield strength.

[0039] T8X yield strength: After the finished sheet is left at room temperature for 7 days, it is pre-stretched by 2% using a uniaxial tensile tester, and then kept in an oil bath oven at 185℃ for 20 minutes (simulating the baking process in the actual production process of the OEM). The yield strength under the baking state is tested by uniaxial tensile test.

[0040] Flanging performance: A strip sample with a length of 250mm and a width of 30mm is cut from the finished sheet material. The sample is pre-stretched by 10% along its length, and then a 50mm long rectangular sample is cut. A 180° bending test is performed using an indenter to test the flanging factor r / t, where r is the indenter radius and t is the sheet thickness (during the test, the distance between the support rollers must be 2r + 2t + 0.1mm). Metallographic photography and grading are performed on the outer surface after bending: Grade 1: smooth surface, no microcracks and continuous necking; Grade 2: slightly rough surface, no microcracks and continuous necking; Grade 3: surface with microcracks or continuous necking; Grade 4: surface with obvious cracks. Grades 1 and 2 are acceptable, while grades 3 and 4 are unacceptable. The final flanging factor is the smallest flanging factor that satisfies the requirement of a surface quality of Grade 2 or better after flanging.

[0041] Paint brush line performance: A strip sample with a length of 250mm and a width of 35mm was cut from the finished sheet material. The length direction of the sample was perpendicular to the rolling direction, and the width direction was parallel to the rolling direction. After pre-stretching the sample by 10% along the length direction, it was polished with a 320-grit oilstone. The surface morphology of the polished sample was evaluated as follows: Grade 1: No obvious paint brush lines along the width direction; Grade 2: Discontinuous paint brush lines exist along the width direction, with no through paint brush lines; Grade 3: Sparse through paint brush lines exist along the width direction; Grade 4: Dense through paint brush lines appear along the width direction. Grades 1 and 2 are acceptable, while grades 3 and 4 are unacceptable.

[0042] The present invention will be further supplemented and explained below with reference to specific implementation schemes. Obviously, the described embodiments are only some embodiments of the present invention, and not all of them.

[0043] First, pure aluminum and various intermediate alloys were melted according to the proportions in Table 1. After refining, the melt was cast into ingots using a semi-continuous casting machine. The ingots were then trimmed and milled before being placed in a heat treatment furnace for homogenization according to the parameters in Table 2. After homogenization, the ingots were directly hot-rolled. After hot rolling, the resulting hot-rolled sheet was cold-rolled. The resulting cold-rolled sheet was then solution-treated, pre-aged, and trimmed to obtain the finished sheet. The resulting sheet was left at room temperature for 7 days to test its mechanical properties, flanging performance, and paint brush line performance. The finished sheet was then subjected to a 2% pre-stretching treatment, followed by holding in an oil bath furnace at 185℃ for 20 minutes, water-quenched to room temperature, and then its mechanical properties were tested. The test results are shown in Table 3.

[0044] Table 1 Composition ratio of aluminum alloy sheet

[0045]

[0046] Table 2 Process parameters of aluminum alloy for outer panel of body panel (I)

[0047]

[0048] Table 2 Process parameters for aluminum alloy used in the outer panel of the body panel (II)

[0049] ,

[0050]

[0051] Table 3 Performance test results of aluminum alloy for outer panels of body panels

[0052]

[0053] As shown in Tables 2 and 3, the mechanical properties of the finished plates obtained in Examples 1-6 are qualified. However, in Comparative Example 1, the total deformation rate of hot roughing was insufficient, resulting in unsatisfactory grain size and texture, leading to poor flanging and brush line performance. In Comparative Example 2, the inner ring temperature of hot finishing rolling was too low, and the temperature of the inner and outer rings was uneven, resulting in poor brush line performance at the edges. In Comparative Example 3, the final rolling temperature of hot roughing was too low, resulting in unsatisfactory brush line performance. In Comparative Example 4, the initial rolling temperature was too low, leading to a large amount of Mg2Si precipitation, resulting in insufficient strength of the final finished plate. In Comparative Example 5, the final rolling speed of hot finishing rolling was too low, resulting in poor brush line performance. In Comparative Example 6, the cooling rate was too high, and the self-annealing of the hot-rolled coil was not effectively utilized, resulting in poor flanging and brush line performance. In Comparative Example 7, the single-pass... Insufficient reduction in the second rolling resulted in unsatisfactory grain size and texture, leading to poor flanging and brushing performance. Comparative Example 8 suffered from poor flanging and brushing performance due to excessively low cold rolling ratio and excessively large grain size. Comparative Example 9 had an excessively high cold rolling ratio, and the brushing performance still failed to meet requirements. Comparative Example 10 had a low edge temperature in the hot rough rolling and a large temperature difference between the edge and the center, resulting in poor brushing performance at the edge. Comparative Example 11 also had poor brushing performance at the edge due to only a low edge temperature in the hot rough rolling. Comparative Example 12 had a low inner ring temperature in the hot finish rolling and uneven temperatures between the inner and outer rings, resulting in poor brushing performance. Comparative Example 13 also had poor brushing performance due to only a low inner ring temperature in the hot finish rolling.

[0054] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made within the scope of the claims of the present invention should be included in the scope of the present invention.

Claims

1. A method for preparing aluminum alloy sheet for automotive body panels, characterized in that, Includes the following steps: (1) Preparation of aluminum alloy ingots; (2) Homogenize the aluminum alloy ingots; (3) Hot rough rolling is performed on the homogenized ingot; (4) The billet after hot rough rolling is hot finished rolling to produce hot rolled coils; (5) The hot-rolled coil is cold-rolled to produce a cold-rolled coil; (6) Solution treatment is performed on the cold-rolled coil; (7) Pre-aging treatment of the solution-treated roll material; (8) Unwind, straighten, and slice the pre-aged roll material to obtain the finished product; The total deformation rate of hot rough rolling in step (3) shall not be less than 90%, the total number of passes shall be 8-18, the average reduction per pass shall not be less than 30 mm, the final rolling thickness shall be 20-40 mm, and the final rolling temperature shall not be less than 400℃; along the width of the plate, the temperature difference between the middle and the edge shall not be higher than 15℃. The final rolling thickness of the hot finishing roll in step (4) is 2.5-6.0 mm, the rolling speed of the final pass is 2-8 m / s, the final rolling temperature is 300-380℃, and the temperature difference of the whole roll is not higher than 10℃ after 10 minutes of winding; the cooling speed of the hot rolled roll to room temperature is not higher than 15℃ / h.

2. The method for preparing aluminum alloy sheet for automotive body panels according to claim 1, characterized in that, Based on a total mass percentage of 100%, the mass percentage of each component in the aluminum alloy ingot obtained in step (1) is as follows: Si 0.50%~1.50%, Fe≤0.40%, Cu≤0.30%, Mn≤0.2%, Mg 0.3%~1.1%, Cr≤0.1%, Ti 0.01-0.10%, with the balance being Al.

3. The method for preparing aluminum alloy sheet for automotive body panels according to claim 1, characterized in that, The homogenization process described in step (2) involves holding the temperature at 540-580℃ for 4-20 h, then cooling it to 490-540℃ and holding it at that temperature for 1-10 h.

4. The method for preparing aluminum alloy sheet for automotive body panels according to claim 1, characterized in that, The thickness of the cold-rolled coil obtained in step (5) is 0.5-2.0 mm, and the cold rolling rate is 60-80%.

5. The method for preparing aluminum alloy sheet for automotive body panels according to claim 1, characterized in that, The solution treatment in step (6) is carried out in an air-cushion continuous annealing furnace, where the temperature is raised to 520-580℃ at a rate of 5-50℃ / s and held for 0.5-5min.

6. The method for preparing aluminum alloy sheet for automotive body panels according to claim 1, characterized in that, The pre-aging treatment mentioned in step (7) involves holding the temperature at 70-100℃ for 4-12 hours.

7. The method for preparing aluminum alloy sheet for automotive body panels according to claim 1, characterized in that, The finished product obtained in step (8) is a T4P state sheet with a 7-day yield strength of less than 110 MPa and a flange factor r / t. min ≤0.5, the paint brush lines on the edges and center are better than level two.

8. The method for preparing aluminum alloy sheet for automotive body panels according to claim 1, characterized in that, The resulting product was further pre-stretched by 2% and baked at 185℃ for 20 minutes to obtain a T8X state sheet with a yield strength greater than 200MPa.

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