6016 aluminum alloy sheet for automobile outer covering and method for manufacturing the same

Abstract

The present application relates to the technical field of aluminum alloy plate preparation and heat treatment synergic control, in particular to a 6016 aluminum alloy plate suitable for automobile outer cover and a preparation method thereof, which comprises the following steps: batching, smelting, homogenizing, hot rolling, first-stage cold rolling, intermediate annealing, second-stage cold rolling, solid solution, quenching and finishing treatment; the 6016 aluminum alloy plate suitable for automobile outer cover comprises the following components and their weight percentages: Si 1.00%-1.10%, Mg 0.44%-0.52%, Cu 0.08%-0.14%, Fe 0.12%-0.25%, Mn 0.05%-0.10%, Ti 0.01%-0.03%, Zr 0.02%-0.05%, Cr≤0.05%, single impurity≤0.05%, total impurity≤0.15%, and the balance is Al; the prepared aluminum alloy plate has the advantages of stable improvement of stamping surface strip, uniform grain distribution, delivery state yield stability and baking hardening response.

Description

Technical Field

[0001] This invention relates to the field of synergistic control technology of aluminum alloy sheet preparation and heat treatment, specifically to a 6016 aluminum alloy sheet suitable for automotive exterior panels and its preparation method. Background Technology

[0002] With the increasing demands for lightweighting in automobiles and the surface quality of exterior body panels, the preparation technology of 6016 and similar 6xxx series aluminum alloy outer sheet thin plates has gradually evolved from relying mainly on composition design and conventional heat treatment to a process system that coordinates the entire process of homogenization, hot rolling, coiling, cold rolling, solution quenching, and pre-aging. In existing publicly available technologies, the front-end processes have formed relatively mature routes for melting and casting, milling, and homogenization; the hot rolling stage mainly revolves around a final rolling temperature window of 355~365℃ or a final stand exit temperature window of 320~360℃, and two typical schemes have been formed in the coiling stage: a low-temperature coiling route and a hot-rolled self-annealing route; the intermediate processes have two paths: setting intermediate annealing and segmented cold rolling, and not setting intermediate annealing, relying on hot-rolled self-annealing followed by direct cold rolling; the back-end processes have further developed to include rapid quenching after solution treatment, leveling, and single-segment pre-aging control, to balance formability, bake hardening response, and surface quality.

[0003] Overall, the focus of this technology has gradually shifted from simply pursuing strength or single grain refinement to comprehensively optimizing yield elongation bands, ribbed lines, orange peel texture, grain uniformity, and delivery stability. The industry trend is characterized by multi-process, multi-parameter coupled control. However, existing technologies still largely organize production using fixed temperature and time windows, and there is relatively little public disclosure regarding the systematic control of thermal history differences at different locations of the coil, thickness-oriented microstructure uniformity, and the synergistic relationship between finishing deformation and subsequent aging.

[0004] Existing technical solutions mainly focus on the conventional manufacturing processes of 6016 or 6xxx series automotive aluminum alloy sheets. The existing routes typically include: melting and casting, milling and homogenization, hot rolling, coiling, intermediate annealing or hot-rolled self-annealing, cold rolling, solution treatment, quenching, tension bending / tension straightening or conventional leveling, single-stage pre-aging, storage and transportation, and subsequent stamping and baking hardening processes.

[0005] Regarding the front-end processes, one type of existing technology employs conventional DC casting, milling, and homogenization treatment. For example, Chinese patent CN104561681A discloses a homogenization route of 560~580℃×8~10 h. Another type of technology uses a wider homogenization or reheating window. For example, existing technology EP1472380A2 / US20050028894A1 discloses a processing range of 400~570℃ and 6~24 h. In addition, Chinese patent CN108385043A discloses a method that skips traditional homogenization and proceeds directly to hot rolling. For hot rolling processes, the currently disclosed final rolling or exit temperatures are mainly concentrated in the range of 355~365℃ or 320~360℃. Among them, the existing technology US6652678B1 also emphasizes achieving hot coil self-annealing through two-stage homogenization and a lower starting rolling temperature. Chinese patent CN108385043A discloses a route that heats aluminum alloy to 440~490℃ and then hot rolls it to 4~6 mm.

[0006] In the mid-to-late stage processes after hot rolling, existing technologies mainly employ two approaches to coiling control: one is a low-temperature coiling route, requiring a coiling temperature below 350℃, preferably below 300℃, to ensure mechanical properties and avoid surface streaks; the other is a hot-rolled self-annealing route that utilizes recrystallization during the early cooling process when the hot roll is still above approximately 270~290℃. Meanwhile, the intermediate annealing settings also differ: Chinese patent CN104561681A uses intermediate annealing at 380~390℃ for 2.5~3 hours after hot rolling followed by segmented cold rolling; existing technologies EP1472380A2 / US20050028894A1 allow batch annealing or continuous annealing; while existing technology US6652678B1 discloses a route that does not require annealing. After cold rolling, existing processes typically roll the sheet to a finished thickness of approximately 0.9~1.2 mm.

[0007] In the final processing stage, existing technologies typically employ solution treatment followed by rapid quenching, combined with leveling and single-stage pre-aging to control the material's delivery condition. For example, Chinese patent CN104561681A discloses solution treatment at 560–570°C for 80–100 seconds, requiring the temperature to drop below 50°C within 5 minutes; Chinese patent CN108385043A discloses water quenching at 535–550°C for 10–15 minutes; existing technologies US5728241A and EP3765647A1 both consider rapid quenching to below 60°C after solution treatment as a prerequisite for subsequent stabilization or pre-aging. For leveling and pre-aging, existing technologies usually regard leveling as a plate shape adjustment process rather than a core control object, and often adopt a single-stage pre-aging system, such as 65~110℃ for 20~70 min, or 110~180℃ for 1~2 min, or short-term placement at room temperature followed by pre-aging treatment at 160~190℃ for 5~10 min.

[0008] In general, existing technologies mainly focus on organizing production around fixed temperature or time windows such as final rolling temperature, coiling temperature, whether to set intermediate annealing, solution quenching, and single-stage pre-aging, in order to balance formability, bake hardening response, and surface quality.

[0009] In summary, the core deficiency of existing technologies lies in the lack of coordinated control over the entire process of "thermal history difference - energy storage difference - heat exposure difference after leveling". Therefore, it is difficult to simultaneously and stably improve the uniformity of stamping surface stripes, grain distribution, yield stability in the delivery state, and bake hardening response.

[0010] In summary, there is an urgent need for an aluminum alloy sheet that can stabilize and improve the surface stripe of stamping, the uniformity of grain distribution, the yield stability in the delivery state, and the bake-hardening response. Summary of the Invention

[0011] The present invention aims to solve the technical problem of how to provide an aluminum alloy sheet that simultaneously improves the stability of stamped surface stripes, uniform grain distribution, yield stability in the delivery state, and bake-hardening response.

[0012] To achieve the above objectives, the first aspect of the present invention provides a method for preparing 6016 aluminum alloy sheet suitable for automotive exterior body panels, comprising the following steps: batching, melting and casting, homogenization, hot rolling, first-stage cold rolling, intermediate annealing, second-stage cold rolling, solution treatment, quenching, and finishing.

[0013] The components and their weight percentages in the 6016 aluminum alloy sheet material suitable for automotive exterior panels are as follows:

[0014] Si 1.00%~1.10%, Mg 0.44%~0.52%, Cu 0.08%~0.14%, Fe 0.12%~0.25%, Mn 0.05%~0.10%, Ti 0.01%~0.03%, Zr 0.02%~0.05%, Cr≤0.05%, other individual impurities≤0.05%, total impurities≤0.15%, balance Al.

[0015] The second aspect of the present invention provides an aluminum alloy sheet prepared by the above-mentioned method for preparing 6016 aluminum alloy sheet suitable for automotive exterior panels, wherein the aluminum alloy sheet has a stamping line rating of 1 to 2, a grain size variation coefficient of 12% to 28%, a yield strength drift of 0 to 8 MPa after 30 days of natural aging, and a bake hardening response of 80 to 110 MPa.

[0016] The beneficial effects of this invention are as follows:

[0017] (1) By setting segmented cold rolling and intermediate annealing to restore the dominant and incomplete recrystallization between hot rolling and final solution treatment, the present invention is beneficial to weaken the energy storage difference between the surface / center layer and the difference in the driving force of recrystallization in the thickness direction, thereby obtaining a narrower grain distribution and a more balanced surface / thickness texture, and improving the uniformity of the plate structure.

[0018] (2) By setting a first thermal regulation after melt quenching and combining it with pure tension or low wrap angle leveling and a second pre-aging adjusted according to leveling strain feedback, this invention is beneficial to control the coupling effect between vacancies, early atomic clusters and leveling-induced dislocations, avoids a significant increase in the yield strength of the delivery state, reduces the risk of yield drift and stamping surface stripes, and takes into account both plate shape and baking hardening response.

[0019] (3) The present invention does not set the final rolling temperature, coiling temperature, intermediate annealing and pre-aging separately, but rather coordinates the aluminum alloy processing technology around the thermal history difference, energy storage difference and heat exposure difference after leveling. Therefore, it is beneficial to improve the balance between surface stripes, grain distribution, yield stability in the delivery state and bake hardening response.

[0020] (4) In the preferred embodiment, by incorporating the first thermal regulation, leveling permanent strain and the second pre-aging into the restricted window control, it is also beneficial to reduce natural aging drift, improve the consistency of delivery status and the stability of subsequent stamping and baking processes. Detailed Implementation

[0021] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0022] In this invention, the thermal history control refers to controlling the temperature changes, time, heating rate, and cooling rate of the aluminum alloy throughout the entire process from heating, forming, cooling to subsequent heat treatment.

[0023] The energy storage difference is the difference in energy stored at different locations inside the aluminum alloy sheet.

[0024] The heat exposure difference after leveling refers to the cumulative difference in temperature and time experienced by different parts of the aluminum alloy sheet after leveling.

[0025] Although existing technologies have formed a production route for 6016 automotive outer panels mainly based on homogenization, hot rolling, coiling, cold rolling, solution quenching, and leveling / pre-aging, production is still mainly organized according to fixed temperature and time windows. The main controls are the final rolling temperature, coiling temperature, whether intermediate annealing is used, solution quenching, and single-stage pre-aging. However, there is insufficient systematic control over the sensitive thermal history difference within the coil, the thickness energy storage gradient, and the linkage between leveling strain and pre-aging.

[0026] In the hot rolling mill's final stand exit to the coiling stage, existing technologies often focus on a single final rolling temperature or coiling temperature window, typically failing to treat the "final stand exit - cooling - coiling - coil internal heat exposure difference" as a unified control object. This easily leads to localized self-annealing and localized non-self-annealing microstructure splitting at different locations within the coil, resulting in subsequent stamping surface stripe defects, grain dispersion, and surface quality fluctuations.

[0027] In the final cold rolling stage after hot rolling, existing technologies typically employ conventional approaches such as long-duration intermediate annealing, no intermediate annealing, or hot-rolled self-annealing. Intermediate treatments are mostly aimed at softening or process transitions, lacking specific control over the energy storage differences between the surface and core layers and the poor driving force for recrystallization in the thickness direction. This deficiency easily leads to insufficient uniformity in the thickness structure and texture, resulting in a wider grain distribution, which is detrimental to balancing surface quality and structural stability.

[0028] In the finishing and aging stages after solution quenching, existing technologies generally treat leveling / tension straightening as a simple plate-shaping process and employ single-stage pre-aging with a fixed window, failing to fully consider the coupling effect between leveling strain, post-quenching vacancies and early clustering states, and subsequent thermal exposure. Therefore, fluctuations in yield stability in the delivered state are prone to occur, the risk of stripes on the stamped surface increases, and it is difficult to simultaneously ensure formability and bake-hardening response.

[0029] This invention does not set each process parameter in isolation, but instead treats the sensitive thermal history from the hot rolling mill exit to the coiling stage, the thickness energy storage gradient from hot coiling to the final thickness cold rolling stage, and the microstructure before final solution treatment as continuous control objects. Through the coordinated adjustment of "end mill exit temperature control - rapid controlled cooling after exit - low temperature coiling - segmented cold rolling - recovery intermediate treatment - final thickness cold rolling - short-time solution rapid quenching", it achieves synchronous control of surface stripe risk, grain distribution uniformity, and delivery stability.

[0030] The first aspect of the present invention provides a method for preparing 6016 aluminum alloy sheet suitable for automotive exterior body panels, comprising the following steps: batching, melting and casting, homogenization, hot rolling, first-stage cold rolling, intermediate annealing, second-stage cold rolling, solution treatment, quenching, and finishing.

[0031] The components and their weight percentages in the 6016 aluminum alloy sheet material suitable for automotive exterior panels are as follows:

[0032] Si 1.00%~1.10%, Mg 0.44%~0.52%, Cu 0.08%~0.14%, Fe 0.12%~0.25%, Mn 0.05%~0.10%, Ti 0.01%~0.03%, Zr 0.02%~0.05%, Cr≤0.05%, other individual impurities≤0.05%, total impurities≤0.15%, balance Al.

[0033] In this invention, specific alloy compositions ensure the stability of subsequent hot working, solution treatment, and microstructure control windows.

[0034] According to the present invention, the melting and casting conditions include: a melting temperature of 730~750°C, and casting at a casting temperature of 705~715°C.

[0035] According to the present invention, the homogenization conditions include: after the cast ingot is sawn and milled, it undergoes a two-stage homogenization process, with the first stage being 565~575℃×4~8 h and the second stage being 460~480℃×2~6 h.

[0036] In this invention, a specific homogenization regime can improve as-cast segregation and stabilize the microstructure before subsequent hot rolling, providing a more stable prerequisite for final rolling temperature control and low-temperature coiling.

[0037] According to the present invention, the hot rolling conditions include: a starting rolling temperature of 460~480℃, a strip temperature of 430~460℃ after rough rolling before entering the finishing rolling stage, a final rolling temperature of 355~360℃, cooling to the coiling temperature at a cooling rate of 20~35℃ / s, a coiling temperature of 280~320℃, no isothermal holding after coiling, air cooling for 2~12 h before entering the first stage of cold rolling, and an absolute value ΔT of the difference between the final rolling temperature and the coiling temperature of 35~80℃.

[0038] In this invention, the final rolling temperature is different from the coiling temperature. The final rolling temperature is used to limit the exit state of the strip after the last hot rolling deformation, while the coiling temperature is used to limit the temperature state of the strip when it enters the coiler. By controlling the final rolling temperature, the online cooling rate after final rolling, the coiling temperature, and the temperature uniformity at different positions of the coil, the differences in the degree of local recovery, self-annealing, or recrystallization of the hot-rolled coil are reduced, thereby improving the microstructure uniformity of the hot-rolled coil and the subsequent cold-rolled sheet.

[0039] According to a preferred embodiment of the present invention, the winding temperature is 290~310°C.

[0040] According to the present invention, the hot rolling process further includes coil heat exposure difference control: after coiling, the head, middle, and tail sections in the coil length direction, the edge and middle sections in the coil width direction, and the outer, middle, and inner rings in the coil diameter direction are used as monitoring positions; wherein, the head section in the coil length direction is the end of the strip that first enters the coiler, the tail section is the end of the strip that last enters the coiler, the edge section in the coil width direction is the area 50~100 mm from both sides, the middle section is the area ±50 mm from the center of the strip width, and the outer, middle, and inner rings in the coil diameter direction are the outer diameter side, the middle layer, and the inner diameter side of the coiled material, respectively. The cumulative heat exposure difference at the above monitoring positions within the sensitive temperature range of 280~320℃ is calculated as the maximum difference of the temperature-time integral H=∫Tdt, the maximum difference of H is not greater than 120℃·min, and the absolute value of the temperature difference between each monitoring position at the same measurement time is 20~30℃.

[0041] According to the present invention, the conditions for the first stage of cold rolling include: after hot rolling and coiling, the first stage of cold rolling is performed without annealing, and the reduction rate of the first stage of cold rolling is 40-55%.

[0042] According to the present invention, the intermediate annealing conditions include: 360~420℃ × 60~180 s.

[0043] In this invention, the intermediate annealing is carried out in a continuous annealing furnace with a heating rate of 5~20℃ / s, and after holding at the temperature, it is cooled to below 150℃ at a rate of 10~30℃ / s. After intermediate annealing, the difference in microhardness between the surface 1 / 6 thickness region and the core 1 / 3 thickness region is no greater than 8 HV, and the difference in average grain size between the core layer and the surface layer is no greater than 8 μm.

[0044] In this invention, the first stage of cold rolling and intermediate annealing causes the material to mainly recover and controls the degree of recrystallization, keeping it in an incomplete recrystallization state.

[0045] According to the present invention, the homogenization treatment of the core layer includes: performing a two-stage homogenization treatment, the first stage being 565~575℃×4~8 h, and the second stage being 460~480℃×2~6 h.

[0046] According to the present invention, the conditions for the second stage of cold rolling include: performing the second stage of cold rolling after intermediate annealing, and the total reduction rate of the first stage of cold rolling and the second stage of cold rolling is 65~75%.

[0047] In this invention, unlike conventional processes that use long-term intermediate annealing to achieve complete softening, or rely on hot-rolled self-annealing, the intermediate annealing treatment of this invention is not aimed at complete softening. Instead, it reduces the deformation energy stored in the material after the first stage of cold rolling through short-time heat treatment, reduces the energy storage difference between the surface and the central layer, and improves the recrystallization uniformity in subsequent cold rolling and solution treatment. Its mechanism is as follows: the first stage of cold rolling introduces an appropriate amount of deformation energy into the sheet, and then the material is mainly restored through short-time intermediate annealing, accompanied by a limited degree of recrystallization, thereby weakening the energy storage gradient in the thickness direction; after the second stage of cold rolling and subsequent solution treatment, a microstructure that is more conducive to the final grain refinement and microstructure uniformity is formed.

[0048] According to the present invention, the conditions for solid solution treatment include: a solid solution temperature of 550~565℃ and a holding time of 60~120 s.

[0049] According to the present invention, the quenching conditions include: cooling to below 60°C at a cooling rate of 50~120°C / s, and holding at that temperature for 0.1~1 h.

[0050] The role of solution treatment and quenching in this invention is to ensure that the microstructure formed by the thermal history control and energy storage control of the preceding process is stably presented in the final state, and to provide a controllable starting state for subsequent formability and bake hardening response.

[0051] According to a preferred embodiment of the present invention, the quenching conditions include: cooling to below 40-50°C at a cooling rate of 80-100°C / s, and holding at that temperature for 0.3-0.8 hours.

[0052] According to the present invention, the conditions for the first heat regulation include: holding at 70~90℃ for 5~20 min within 30 min after quenching; followed by pure tensile leveling or low wrap angle bending straightening, with a permanent leveling strain of 0.2~0.8%.

[0053] According to the present invention, the conditions for the second pre-aging include: holding at 95~120℃ for 10~40 min after leveling; wherein, when the permanent strain of leveling is 0.2%~0.4%, the second pre-aging temperature is 95~105℃ and the time is 10~20 min; when the permanent strain of leveling is greater than 0.4% and not greater than 0.8%, the second pre-aging temperature is 105~120℃ and the time is 20~40 min.

[0054] This invention achieves continuous and coordinated control of the cumulative temperature-time difference at different locations within the sensitive temperature zone after hot rolling, the thickness energy storage difference during the intermediate cold rolling process, and the permanent strain after solution quenching and the subsequent pre-aging temperature-time regime. This enables the microstructure and performance fluctuations of 6016 aluminum alloy automotive outer panels to be reduced simultaneously in the length, width, and thickness directions of the coil, as well as during the delivery aging stage.

[0055] Specifically, the first stage controls the cumulative heating degree at different locations within the sensitive temperature zone to suppress initial microstructure splitting caused by the coexistence of localized self-annealing and non-self-annealing. The second stage uses an intermediate treatment primarily focused on recovery without complete recrystallization to weaken the energy storage gradient and recrystallization driving force difference from the surface to the center layer. The third stage uses solution quenching followed by rapid cooling, and then uses feedback adjustment of the subsequent pre-aging temperature and time based on the leveling permanent strain to control the coupling effect between vacancies, early atomic clusters, and leveling-induced dislocations. Thus, this invention can reduce the superimposed amplification effect of initial microstructure differences, thickness-oriented grain size distribution dispersion, and yield drift in the delivery state within the same process chain, thereby reducing the risk of strip defects and grain size distribution dispersion on the stamping surface, and improving the microstructure uniformity, delivery state stability, stamping surface quality consistency, and bake-hardening response stability of 6016 automotive outer panels.

[0056] The second aspect of the present invention provides an aluminum alloy sheet prepared by the above-mentioned method for preparing 6016 aluminum alloy sheet suitable for automotive exterior panels, wherein the aluminum alloy sheet has a stamping line rating of 1 to 2, a grain size variation coefficient of 12% to 28%, a yield strength drift of 0 to 8 MPa after 30 days of natural aging, and a bake hardening response of 80 to 110 MPa.

[0057] Test methods

[0058] Chemical composition was determined according to GB / T 7999-2015.

[0059] The surface stripes of stamping are determined according to the post-stamping grading method: the sheet metal is prepared into a part according to the typical stamping process of automotive exterior body panels. After cleaning, the surface stripes are observed under 45° oblique light. The stripes are rated from 1 to 5 according to the visibility of the grading line, where 1 is no visible stripes and 5 is severe stripes.

[0060] The uniformity of grain distribution was determined in accordance with GB / T 6394-2017. Samples were taken at the head, middle and tail of the roll length direction, the edge and middle of the roll width direction, and the surface and 1 / 2 thickness of the roll thickness direction. The grain size was statistically analyzed by the cross-section method and expressed as the grain size variation coefficient CV = standard deviation / average value × 100%.

[0061] The yield stability in the delivered state was determined according to GB / T 228.1-2021: the plate after solution treatment, quenching, finishing and aging was naturally aged at room temperature for 30 days, and the yield strength before and after natural aging was tested respectively. The difference between the two was taken as the 30-day yield strength drift.

[0062] The bake hardening response was determined according to GB / T 228.1-2021: After the delivered sheet was pre-stretched by 2%, it was kept at 185℃ for 20 min to simulate baking treatment, and the difference in yield strength before and after baking was calculated.

[0063] The technical solution of the present invention will be further described in detail below with reference to the embodiments. Obviously, the embodiments described herein are only some embodiments of the present invention and are not intended to limit the present invention. All other embodiments implemented by those skilled in the art based on the embodiments of the present invention without creative improvements are within the protection scope of the present invention.

[0064] Example 1

[0065] Raw materials and ingredients

[0066] The alloy uses 6016 as the main alloy, and its composition is as follows:

[0067] Si: 1.05%; Mg: 0.48%; Cu: 0.11%; Fe: 0.18%; Mn: 0.08%; Ti: 0.02%; Zr: 0.03%; Cr: 0.03%; other individual impurities ≤0.05%, total impurities ≤0.15%, balance is Al.

[0068] Melting and Homogenization

[0069] The melting temperature is 740℃, the casting temperature is 710℃, and DC casting is carried out after conventional refining, degassing, filtration and grain refinement.

[0070] After the ingot is milled on both sides, it undergoes a two-stage homogenization process:

[0071] The first stage is 560℃×6 h;

[0072] The second stage is 470℃×4 h.

[0073] Hot rolling and coiling

[0074] The starting rolling temperature for hot rolling is 470℃;

[0075] After rough rolling, the strip temperature is 445℃ before entering the finish rolling.

[0076] The final rolling temperature was 358℃;

[0077] The temperature was reduced to the coiling temperature at a cooling rate of 28℃ / s. The coiling temperature was 300℃, and the coiling time was 6h. The absolute value of the difference between the final rolling temperature and the coiling temperature, ΔT, was 58℃.

[0078] Simultaneously control the heat exposure difference of the head, middle and tail in the roll length direction, the edge and middle in the roll width direction, and the outer, middle and inner rings in the roll diameter direction. The temperature difference at the same measurement time at each monitoring position is 25℃, and the cumulative heat exposure difference is not greater than 60℃·min.

[0079] Cold rolling and intermediate processing

[0080] After hot rolling, the coils are not subjected to long-term batch annealing and are directly introduced into the first stage of cold rolling.

[0081] The reduction rate of the first cold rolling stage was 48%;

[0082] After the first stage of cold rolling, intermediate annealing was performed at 390℃ for 120 s; the heating rate was 15℃ / s, and after holding at that temperature, it was cooled to 120℃ at 20℃ / s. After intermediate annealing, the microhardness difference between the surface 1 / 6 thickness region and the core 1 / 3 thickness region was 3 HV, and the average grain size difference between the core and the surface was 3 μm.

[0083] The second stage of cold rolling is then carried out, with a total cold rolling reduction rate of 70%.

[0084] Solution treatment and quenching

[0085] Continuous solution treatment was performed at a solution temperature of 558℃ and a holding time of 90 s.

[0086] After solution treatment, rapid quenching is performed, with the plate temperature reduced to 45℃ at a cooling rate of 80℃ / s, and held at that temperature for 0.5 h.

[0087] The conditions for the first heat conditioning include: holding at 80℃ for 10 minutes within 20 minutes after quenching, and then performing pure tensile leveling after heat conditioning, with a permanent leveling strain of 0.5%;

[0088] The conditions for the second pre-aging process include: maintaining the temperature at 105℃ for 25 minutes after leveling.

[0089] A1 sheet of 6016 aluminum alloy was obtained.

[0090] Compared to the conventional 6016 automotive outer panel process, this embodiment does not adopt a production path based solely on a fixed final rolling temperature or a fixed coiling temperature, nor does it employ a route of long-batch annealing after hot rolling to completely soften the material. Instead, it focuses on controlling the sensitive thermal history during the hot rolling and coiling stage, as well as the thickness energy storage difference after segmented cold rolling. The main differences are: firstly, limiting the final rolling temperature and coordinating rapid controlled cooling to a specific low-temperature coiling temperature after exiting the coil; secondly, directly entering the first stage of cold rolling after hot rolling, and setting a recovery-type intermediate annealing instead of complete recrystallization annealing; and thirdly, through the combination of the above processes and short-time solution rapid quenching, the microstructure formed in the previous stage can be preserved in the final state.

[0091] Example 2

[0092] Raw materials and ingredients

[0093] The alloy uses 6016 as the main alloy, and its composition is as follows:

[0094] Si: 1.00%; Mg: 0.44%; Cu: 0.08%; Fe: 0.12%; Mn: 0.05%; Ti: 0.01%; Zr: 0.02%; Cr: 0%; other individual impurities ≤0.05%, total impurities ≤0.15%, balance is Al.

[0095] Melting and Homogenization

[0096] The melting temperature is 730℃, the casting temperature is 705℃, and DC casting is carried out after conventional refining, degassing, filtration and grain refinement.

[0097] After the ingot is milled on both sides, it undergoes a two-stage homogenization process:

[0098] The first stage is 565℃ × 4 h;

[0099] The second stage is 460℃×2 h.

[0100] Hot rolling and coiling

[0101] The starting rolling temperature for hot rolling is 460℃;

[0102] After rough rolling, the strip temperature is 430℃ before entering the finish rolling.

[0103] The final rolling temperature is 355℃;

[0104] The temperature was reduced to the coiling temperature at a cooling rate of 20℃ / s. The coiling temperature was 320℃, and the coiling time was 2 h. The absolute value of the difference between the final rolling temperature and the coiling temperature, ΔT, was 35℃.

[0105] Simultaneously control the heat exposure difference of the head, middle and tail in the roll length direction, the edge and middle in the roll width direction, and the outer, middle and inner rings in the roll diameter direction. The temperature difference at the same measurement time at each monitoring position is 20℃, and the cumulative heat exposure difference is not greater than 80℃·min.

[0106] Cold rolling and intermediate processing

[0107] After hot rolling, the coils are not subjected to long-term batch annealing and are directly introduced into the first stage of cold rolling.

[0108] The reduction rate of the first cold rolling stage is 40%;

[0109] After the first stage of cold rolling, intermediate annealing was performed at 360℃ for 60 s; the heating rate was 5℃ / s, and after holding at that temperature, it was cooled to 145℃ at 10℃ / s. After intermediate annealing, the microhardness difference between the surface 1 / 6 thickness region and the core 1 / 3 thickness region was 8 HV, and the average grain size difference between the core and the surface was 8 μm.

[0110] The second stage of cold rolling is then carried out, with a total cold rolling reduction of 65%.

[0111] Solution treatment and quenching

[0112] Continuous solution treatment was performed at a solution temperature of 550℃ and a holding time of 60 s.

[0113] After solution treatment, rapid quenching is performed to reduce the plate temperature to 40°C at a cooling rate of 50°C / s.

[0114] The conditions for the first heat conditioning include: holding at 70℃ for 5 minutes within 30 minutes after quenching, and then performing pure tensile leveling after heat conditioning, with a permanent leveling strain of 0.2%;

[0115] The conditions for the second pre-aging process include: maintaining the temperature at 95°C for 10 minutes after leveling.

[0116] A2 6016 aluminum alloy sheet was obtained.

[0117] Example 3

[0118] Raw materials and ingredients

[0119] The alloy uses 6016 as the main alloy, and its composition is as follows:

[0120] Si: 1.10%; Mg: 0.52%; Cu: 0.14%; Fe: 0.25%; Mn: 0.10%; Ti: 0.03%; Zr: 0.05%; Cr: 0.05%; other individual impurities ≤0.05%, total impurities ≤0.15%, balance is Al.

[0121] Melting and Homogenization

[0122] The melting temperature is 750℃, the casting temperature is 715℃, and DC casting is carried out after conventional refining, degassing, filtration and grain refinement.

[0123] After the ingot is milled on both sides, it undergoes a two-stage homogenization process:

[0124] The first stage is 575℃×8 h;

[0125] The second stage is 480℃×6 h.

[0126] Hot rolling and coiling

[0127] The starting rolling temperature for hot rolling is 480℃;

[0128] After rough rolling, the strip temperature is 460℃ before entering the finish rolling.

[0129] The final rolling temperature is 360℃;

[0130] The temperature is reduced to the coiling temperature at a cooling rate of 35℃ / s. The coiling temperature is 280℃. After coiling, the coil is air-cooled for 12 hours before entering the first stage of cold rolling. The absolute value of the difference between the final rolling temperature and the coiling temperature, ΔT, is 80℃.

[0131] Simultaneously control the heat exposure difference of the head, middle and tail in the roll length direction, the edge and middle in the roll width direction, and the outer, middle and inner rings in the roll diameter direction. The temperature difference at the same measurement time at each monitoring position is 30℃, and the cumulative heat exposure difference is not greater than 120℃·min.

[0132] Cold rolling and intermediate processing

[0133] After hot rolling, the coils are not subjected to long-term batch annealing and are directly introduced into the first stage of cold rolling.

[0134] The reduction rate of the first cold rolling stage is 55%;

[0135] After the first stage of cold rolling, intermediate annealing was performed at 420℃ for 180 s; the heating rate was 20℃ / s, and after holding at that temperature, it was cooled to 100℃ at 30℃ / s. After intermediate annealing, the microhardness difference between the surface 1 / 6 thickness region and the core 1 / 3 thickness region was 6 HV, and the average grain size difference between the core and the surface was 6 μm.

[0136] The second stage of cold rolling is then carried out, with a total cold rolling reduction rate of 75%.

[0137] Solution treatment and quenching

[0138] Continuous solution treatment was performed at a solution temperature of 565℃ for 120 seconds.

[0139] After solution treatment, rapid quenching is performed to reduce the plate temperature to 50°C at a cooling rate of 120°C / s.

[0140] The conditions for the first heat conditioning include: holding at 90℃ for 20 minutes within 30 minutes after quenching, and then straightening by low wrap angle bending after heat conditioning, with a permanent strain of 0.8%;

[0141] The conditions for the second pre-aging process include: maintaining the temperature at 120℃ for 40 minutes after leveling.

[0142] A3 6016 aluminum alloy sheet was obtained.

[0143] Comparative Example 1

[0144] The preparation method is the same as in Example 1, except that a two-stage homogenization process is performed:

[0145] The first stage is 550℃×3 h;

[0146] The second stage is 450℃×1 h.

[0147] Aluminum alloy sheet DA1 was obtained.

[0148] Comparative Example 2

[0149] The preparation method is the same as in Example 1, except that a two-stage homogenization process is performed:

[0150] The first stage is 585℃×10 h;

[0151] The second stage is 490℃×8 h.

[0152] Aluminum alloy sheet DA2 was obtained.

[0153] Comparative Example 3

[0154] The preparation method is the same as in Example 1, except that the final rolling temperature and the coiling temperature are the same in the hot rolling process, which is 320°C.

[0155] Aluminum alloy sheet DA3 was obtained.

[0156] Comparative Example 4

[0157] The preparation method is the same as in Example 1, except that the final rolling temperature and the coiling temperature are the same in the hot rolling process, which is 360°C.

[0158] Aluminum alloy sheet DA4 was obtained.

[0159] Comparative Example 5

[0160] The preparation method is the same as in Example 1, except that the absolute value of the difference between the final rolling temperature and the coiling temperature, ΔT, is 20°C.

[0161] Aluminum alloy sheet DA5 was obtained.

[0162] Comparative Example 6

[0163] The preparation method is the same as in Example 1, except that the absolute value of the difference between the final rolling temperature and the coiling temperature, ΔT, is 95°C.

[0164] Aluminum alloy sheet DA6 was obtained.

[0165] Comparative Example 7

[0166] The preparation method is the same as in Example 1, except that a long-term annealing process is performed before the first stage of cold rolling, with a temperature of 390°C for 3 hours.

[0167] Aluminum alloy sheet DA7 was obtained.

[0168] Comparative Example 8

[0169] The preparation method is the same as in Example 1, except that the first stage of cold rolling is followed by complete recrystallization annealing at 450°C for 2 hours.

[0170] Aluminum alloy sheet DA8 was obtained.

[0171] Comparative Example 9

[0172] The preparation method of Example 1 is the same, except that a continuous solution treatment is performed at a solution temperature of 540°C and a holding time of 50 s.

[0173] After solution treatment, rapid quenching is performed to reduce the plate temperature to 70°C at a cooling rate of 30°C / s.

[0174] DA9 aluminum alloy sheet was obtained.

[0175] Comparative Example 10

[0176] The preparation method of Example 1 is the same, except that a continuous solution treatment is performed at a solution temperature of 575°C and a holding time of 150 s.

[0177] After solution treatment, rapid quenching is performed to reduce the plate temperature to 80°C at a cooling rate of 40°C / s.

[0178] Aluminum alloy sheet DA10 was obtained.

[0179] Performance tests were performed on A1-A3 and DA1-DA10, as shown in Table 1.

[0180] Table 1

[0181]

[0182] By comparing the examples and the comparative examples, it can be seen that the aluminum alloy sheets of Examples A1-A3 have the advantages of stabilizing and improving the stripe on the stamped surface, uniform grain distribution, yield stability in the delivery state, and bake hardening response.

[0183] This invention takes the sensitive thermal history difference from the hot rolling exit to the coiling stage and the thickness energy storage difference after segmented cold rolling as the main control objects and designs them in a coordinated manner, so as to take into account surface quality, microstructure uniformity and delivery stability, rather than optimizing the final rolling temperature, coiling temperature, intermediate annealing or solution treatment separately.

[0184] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing 6016 aluminum alloy sheet suitable for automotive exterior body panels, characterized in that, The process includes the following steps: batching, melting and casting, homogenization, hot rolling, first-stage cold rolling, intermediate annealing, second-stage cold rolling, solution treatment, quenching, heat control, finishing, and pre-aging. The components and their weight percentages in the 6016 aluminum alloy sheet material suitable for automotive exterior panels are as follows: Si 1.00%~1.10%, Mg 0.44%~0.52%, Cu 0.08%~0.14%, Fe 0.12%~0.25%, Mn 0.05%~0.10%, Ti 0.01%~0.03%, Zr 0.02%~0.05%, Cr≤0.05%, other individual impurities≤0.05%, total impurities≤0.15%, balance Al; The melting and casting conditions include: melting temperature of 730~750℃, casting method, and casting temperature of 705~715℃; The homogenization conditions include: after the cast ingot is sawn and milled, it undergoes a two-stage homogenization process, with the first stage being 565~575℃×4~8 h and the second stage being 460~480℃×2~6 h. The hot rolling conditions include: the initial rolling temperature is 460~480℃, the strip temperature after rough rolling is 430~460℃ before entering the finishing rolling, the final rolling temperature is 355~360℃, the temperature is reduced to the coiling temperature at a cooling rate of 20~35℃ / s, the coiling temperature is 280~320℃, after coiling there is no isothermal holding, after air cooling for 2~12 hours, the first stage of cold rolling is started, and the absolute value of the difference between the final rolling temperature and the coiling temperature ΔT is 35~80℃; The conditions for the first stage of cold rolling include: after hot rolling and coiling, no annealing is performed, and the reduction rate of the first stage of cold rolling is 40-55%; The intermediate annealing conditions include: 360~420℃ × 60~180 s; The conditions for the second stage of cold rolling include: the second stage of cold rolling is carried out after intermediate annealing, and the total reduction rate of the first stage cold rolling and the second stage cold rolling is 65~75%; The conditions for solid solution treatment include: a solid solution temperature of 550~565℃ and a holding time of 60~120 s; The quenching conditions include: cooling to below 60°C at a cooling rate of 50~120°C / s and holding at that temperature for 0.1~1 h; The conditions for heat regulation include: holding at 70~90℃ for 5~20 minutes within 30 minutes after quenching; The pre-aging conditions include: after leveling, maintaining the temperature at 95~120℃ for 10~40 min.

2. The aluminum alloy sheet prepared by the method for preparing 6016 aluminum alloy sheet suitable for automotive exterior panels as described in claim 1, characterized in that, The aluminum alloy sheet has a stamping line rating of 1-2, a grain size variation coefficient of 12%-28%, a yield strength drift of 0-8 MPa after 30 days of natural aging, and a bake hardening response of 80-110 MPa.

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