CN114807688A

CN114807688A - 6-series aluminum alloy plate strip with high durability for automobile body and preparation method thereof

Info

Publication number: CN114807688A
Application number: CN202210385108.1A
Authority: CN
Inventors: 丁力群; 黄瑞银; 冉继龙; 周青华; 吴建新; 罗富鑫
Original assignee: Zhonglv Southeast Material Institute Fujian Technology Co ltd; Chinalco Ruimin Co Ltd
Current assignee: Zhonglv Southeast Material Institute Fujian Technology Co ltd; Chinalco Ruimin Co Ltd
Priority date: 2022-04-13
Filing date: 2022-04-13
Publication date: 2022-07-29
Anticipated expiration: 2042-04-13
Also published as: CN114807688B

Abstract

The invention discloses a 6-series aluminum alloy plate strip with high durability for an automobile body and a preparation method thereof, wherein at least one side surface of the aluminum alloy plate strip is provided with 1 layer of oxide film containing at least more than 1 of zirconium and titanium, and the aluminum alloy plate strip comprises the following chemical components in percentage by mass: 0.50-1.15wt% of Si, less than 0.30 wt% of Fe, 0.01-0.25 wt% of Cu, 0.30-0.90 wt% of CuMg, 0.05-0.25 percent of Mn, 0.005-0.15 percent of Zn, and the balance of Al and other inevitable impurity elements; the tensile strength is more than or equal to 180MPa, the yield strength is less than or equal to 140MPa, the elongation is more than or equal to 23%, the flanging grade detected by the flanging performance is more than 2 grade, the average diameter of crystal grains is less than or equal to 60 mu m, and the number of intermetallic compound particles with the circle conversion diameter of more than 2 mu m is less than 2000/mm ² 。

Description

6-series aluminum alloy plate strip with high durability for automobile body and preparation method thereof

Technical Field

The invention belongs to the technical field of aluminum alloy plate strips for automobile bodies, and particularly relates to a 6-series aluminum alloy plate strip with high durability for automobile bodies and a preparation method thereof.

Background

The light weight of the automobile is a key measure for energy conservation and emission reduction at present. The steel car body has heavy weight, which is not beneficial to reducing weight of the car body, saving energy and reducing emission. The density of the aluminum alloy is about one third of that of steel, and the aluminum alloy plate can reduce weight by about 30% if being applied to outer plates (a vehicle door, an engine hood and a trunk outer plate), inner plates (a vehicle door, an engine hood and a trunk inner plate) and structural parts on the premise of ensuring that the rigidity of a vehicle body meets the design requirement. In particular Mg ₂ The Al-Mg-Si alloy with Si as an aging strengthening phase has the characteristics of moderate strength, heat treatment strengthening and good corrosion resistance, and the alloy is supplied in a state of lower yield strength after solution treatment, so that the alloy plate has better stamping forming capability, the strength can be further increased in the baking finish process of a vehicle body after stamping forming, and the capability of resisting external collision damage is improved. These characteristics make the aluminum alloy material very suitable for processing automobile body outer plates, and replace steel to lighten automobileThe preferred material for vehicle weight.

However, since the aluminum alloy sheet has problems of lower press formability and higher manufacturing cost than steel materials, the conventional aluminum alloy sheet cannot completely replace steel materials, and both of them are often used as automobile parts. In order to prevent contact corrosion when joining an aluminum alloy to a steel material and to enhance the corrosion resistance of the aluminum alloy itself, it is necessary to attach a high-quality oxide film having good corrosion resistance and insulation properties as a protective layer to the surface of the aluminum alloy. It is required for this oxide film to have both good bonding strength with the underlying aluminum sheet strip and good corrosion resistance itself. The performance of the oxide film is closely related to the internal structure and the surface quality of the aluminum alloy plate strip below the oxide film. Therefore, in order to obtain a good-performance oxide film, the internal structure and the surface quality of the aluminum alloy plate strip must be strictly controlled. On the other hand, vehicle body parts often have complex shape requirements. This requires that the aluminum alloy sheet strip of the substrate thereof must have good press formability. Further, in order to improve the resistance of the parts against external impact damage while ensuring good formability, the aluminum alloy sheet strip is required to have as high strength as possible.

However, there are often complex causal relationships between the aforementioned required properties, and it is difficult to achieve the best level of both. Therefore, in order to obtain the above-mentioned oxide film having good performance, good press formability, and as high strength as possible, it is necessary to strictly and comprehensively manage various production conditions from the alloy composition, the work heat treatment process, and the final surface coating treatment. Existing technologies have not been able to meet this requirement. For example, patent document 1) CN109897995B provides a method for producing an aluminum alloy sheet for a vehicle body by strictly controlling the composition and homogenization temperature, the total reduction amount of hot rolling and cold rolling, and the heat treatment conditions such as solution quenching and aging. However, no oxide film is added to the surface of the aluminum alloy, and the corrosion resistance and durability of the aluminum alloy sheet are not considered. Patent document 2) CN110117785A provides a method for producing a chromium-and phosphorus-free oxide film from the viewpoint of improving the corrosion resistance of an aluminum alloy. However, it is not designed for aluminum alloy sheet strips for vehicle bodies and is not suitable for continuous rapid mass coating treatment of the surfaces of sheet strips for vehicle bodies.

Disclosure of Invention

The invention aims to solve the technical problems at present and provides a 6-series aluminum alloy plate strip with high durability for an automobile body and a preparation method thereof.

In order to achieve the purpose, the invention adopts the following technical scheme:

an aluminum alloy sheet strip having a surface protective film, at least one side of which has 1 layer of an oxide film containing at least 1 or more of zirconium and titanium, wherein the total content of zirconium and titanium in the oxide film is 1 to 12mg/m in terms of metallic zirconium and metallic titanium ² Within the range. The aluminum alloy sheet strip is composed of 0.50-1.15wt% of Si, less than 0.30 wt% of Fe, 0.01-0.25 wt% of Cu, 0.30-0.90 wt% of Mg, 0.05-0.25 wt% of Mn, 0.005-0.15 wt% of Zn, and the balance of Al and other inevitable impurity elements, and has a tensile strength of not less than 180MPa, a yield strength of not more than 140MPa, an elongation of not less than 23%, a flanging grade of not less than 2 grade in flanging performance test, an average grain diameter of not more than 60 μm, and a number of intermetallic compound particles having a circle-converted diameter of more than 2 μm of less than 2000/mm ² The baking varnish has the characteristics that the tensile strength is more than or equal to 260MPa and the yield strength is more than or equal to 190MPa after heat treatment.

Further, a surface oxide film containing zirconium and titanium: the oxide film containing zirconium and titanium on the surface of the aluminum alloy sheet strip functions to provide good corrosion resistance and durability. The content of zirconium and titanium in the surface oxide film is ensured to be 1-12mg/m after being converted into metal zirconium and metal titanium ² Within the range. If the content is too low, the thickness of the surface oxide film is insufficient, and the corrosion resistance and durability cannot meet the requirements of the body panel strip. If the content is too high, the surface oxide film becomes brittle and easily cracks. The corrosion resistance and durability required for the vehicle body panel cannot be obtained, and the manufacturing cost is increased due to the increase of the used materials.

Further, the Si content is in the range of 0.50 to 1.20%, and the Mg content is in the range of 0.30 to 0.90%. When Si and Mg are added simultaneously, appropriate heat can be appliedTreated to control their microscopic state of presence in the aluminum matrix to alter the macroscopic mechanical properties of the material. After heat preservation at a high temperature of 520 ℃ or higher, Si and Mg tend to be solid-dissolved in the aluminum matrix. The amount of solid solution and whether there is any residual undissolved Si and Mg are related to the contents of Si and Mg, the temperature and the time of heat preservation. If the aluminum alloy is quenched to normal temperature by means of rapid temperature reduction, Si and Mg exist in a supersaturated state in the aluminum matrix. When the aluminum alloy is stored for a long time at normal temperature or heated for aging treatment, the supersaturated Si and Mg dissolved in aluminum matrix in a solid solution state can form GP zone-Mg ₂ Metastable phase of Si-Mg ₂ The order of the stable phases of Si evolves from the aluminum matrix, forming intermetallic particles, causing changes in the macroscopic mechanical properties of the material. In general, GP zones or Mg ₂ The metastable Si phase has small size and high distribution density, and can greatly improve the strength of the material and reduce the plasticity. And Mg ₂ The Si stable phase has large size and small distribution density, and the range of improving the strength of the material and the range of reducing the plasticity are less. GP zone, Mg ₂ Metastable phase of Si, Mg ₂ The size, distribution density and occupation ratio of the Si stable phase are controlled, namely the control of the macroscopic mechanical property of the material can be comprehensively controlled by adjusting the contents of Si and Mg and the treatment conditions of solution quenching and aging.

When the Si and Mg contents are less than 0.50 and 0.30, respectively, the GP zone and Mg are contained ₂ Metastable phase of Si, Mg ₂ The overall number of so-called reinforcing phase particles, such as Si-stable phase, tends to be insufficient, so that the tensile strength and yield strength of the material fall short of the range required by this patent, failing to meet the requirements of the body panel. When the Si and Mg contents are more than 1.20 and 0.90, respectively, some coarse Mg may be formed at the casting stage ₂ Si phase particles. If the material contains a large amount of Fe, Si forms coarse AlFeSi intermetallic compound particles together with Fe. If Cu is present in a large amount in the material, coarse AlFeCuMg intermetallic compound particles may be formed during casting. In either form, the subsequent homogenization and solution treatments make it difficult to re-dissolve the coarse particles back into the aluminum matrix. These coarse intermetallic particles are physically and chemically bonded to the aluminum matrixOn the other hand, in the surface coating treatment, these particles exposed on the surface do not form an oxide film as in the case of the surrounding aluminum matrix, and become defects of the oxide film, deteriorating the quality of the oxide film on the surface of the aluminum plate strip, and causing a decrease in corrosion resistance and durability. In severe cases, these particles fall off the surface, leaving a small void and reducing corrosion resistance. Meanwhile, these coarse intermetallic compound particles also become crack nucleation sites when the aluminum alloy sheet strip is subjected to burring, and the tendency of surface cracks occurring during burring becomes large.

Further, the Fe content is less than 0.30%. Fe is dissolved in the aluminum matrix in a small amount, and has an effect of improving the strength. While a substantial portion of Fe forms intermetallic particles with other alloying elements. As mentioned before, when the Fe content is high, some coarse AlFeSi, AlFeCuMg intermetallic particles may be formed at the casting stage. These coarse intermetallic compound particles are generally brittle, and when an aluminum alloy sheet strip is subjected to plastic deformation processing such as rolling, press forming, flanging, etc., they are themselves likely to crack, and micro-or macro-cracks are induced, resulting in a decrease in formability of the aluminum alloy sheet strip. Meanwhile, they also become defects of oxide films when coating the surfaces, spoiling the quality of the oxide films on the surfaces of the aluminum sheet strips, resulting in a decrease in corrosion resistance and durability. To further reduce these negative effects, the amount of Fe added may be controlled to 0.20% or less.

Further, the Cu content is in the range of 0.01 to 0.25%. The small amount of Cu added is generally solid-dissolved in aluminum and improves the strength of the aluminum alloy sheet strip. Meanwhile, Cu dissolved in the matrix is localized in the grain boundary, and thus the effect of suppressing the coarsening of crystal grains at the time of recrystallization is obtained. If the Cu content is less than 0.01%, the risk of the aforementioned insufficient positive effects increases. On the other hand, in the aging treatment, the dissolved Cu tends to segregate to grain boundaries, thereby increasing the electrode potential of the grain boundaries, inducing preferential corrosion in the region near the grain boundaries, and increasing the risk of lowering the corrosion resistance. If the Cu content exceeds 0.25%, the above-mentioned adverse effects are increased to an insignificant level. In addition, the solid-dissolved Cu can improve the strength of the aluminum alloy plate strip and reduce the plastic deformation capacity of the material.

Further, the Mn content is in the range of 0.05-0.25%. In the ingot, most of Mn is dissolved in the aluminum matrix in a solid state and precipitates during the homogenization treatment, and forms nanoscale, fine Al together with Al ₆ Mn intermetallic compound particles. These fine Al ₆ Mn particles have an inhibiting effect on the coarsening of recrystallized grains in hot rolling, intermediate annealing and finished product annealing, so that the final grain size is refined. If the content is less than 0.05%, Al becomes fine ₆ Too few Mn particles are present, and the inhibitory effect is insufficient. On the other hand, a part of Mn may replace Fe during casting to form coarse intermetallic compound particles of Al (Fe, Mn) Si and Al (Fe, Mn) CuMg. As mentioned above, they induce micro-and macro-cracks to occur, resulting in a decrease in formability of the aluminum alloy sheet strip. Meanwhile, they also become defects of oxide films when coating the surfaces, spoiling the quality of the oxide films on the surfaces of the aluminum sheet strips, resulting in a decrease in corrosion resistance and durability. If the Mn content exceeds 0.25%, the aforementioned coarse Al (Fe, Mn) Si and Al (Fe, Mn) CuMg intermetallic compound particles will be excessive, causing non-negligible adverse effects.

Further, the Zn content is in the range of 0.005-0.15%. Zn is mainly solid-dissolved in aluminum. When the surface coating is treated by alkali washing and acid washing, the cleaning agent has the effects of promoting the removal of a surface oxidation layer generated in the early rolling heat treatment of the aluminum alloy plate strip, maintaining a fresh material surface layer and enabling the adhesion of an oxidation film generated in the subsequent coating to be better. If the Zn content is less than 0.005%, the aforementioned positive effects will be insufficient. On the other hand, the solid-dissolved Zn also has an effect of lowering the electrode potential of the aluminum alloy sheet strip base overall. If the Zn content is more than 0.15%, the amount of change in the electrode potential of the matrix becomes too large, and the balance between the matrix and the electrode potential among the grain boundary and the intermetallic compound particles is lost, resulting in a decrease in corrosion resistance and durability.

Furthermore, the tensile strength is more than or equal to 180MPa, the yield strength is less than or equal to 140MPa, and the elongation is more than or equal to 23 percent; the tensile strength of the baking varnish after heat treatment is more than or equal to 260MPa, and the yield strength is more than or equal to 190 MPa.

After the aluminum alloy plate strip for the automobile body is supplied to an automobile manufacturer, an inner cover plate and an outer cover plate of the automobile body, which meet the design requirements and have the appearance shapes, are manufactured by stamping deformation, then the inner cover plate is wrapped and fixed into an integral automobile cover by flanging the outer cover plate, and finally the outer surface of the automobile cover is painted by baking varnish. This requires that the deformation resistance, i.e., the yield strength, is as low as possible during press forming. In addition, in order to produce a desired design shape having a strong three-dimensional feeling, the plastic deformability of the material is required, and simply, the higher the elongation, the better the elongation. On the other hand, when the cover is attached to the automobile, the cover is required to be resistant to the external impact, i.e., the higher the tensile strength and yield strength, the better.

In order to meet the complex requirements, the yield strength of the aluminum alloy plate strip for the vehicle body is controlled below 140MPa which meets the basic requirement of stamping forming, and after a qualified stamping part is obtained, the heat generated during the final baking finish coating is utilized to promote the material to generate age hardening, so that the yield strength and the tensile strength are improved. The heat generated during the paint baking coating is generally equivalent to the rapid heat treatment at 190 ℃ for 15-30 minutes at 170-. The automobile manufacturers are usually determined according to their own needs, specifically how to select the baking finish conditions. The baking finish treatment of this patent was simulated with a rapid heat treatment at 185 ℃ for 20 minutes. This treatment is only one criterion for judging the ability of the material to increase in strength when it is painted. Generally, the improvement of tensile strength in the paint baking treatment is much smaller than that of yield strength, so that it is required that the tensile strength of the aluminum alloy sheet strip for use as a vehicle body panel should at least satisfy more than 180 MPa. Therefore, the tensile strength and the yield strength after the baking finish coating respectively reach the tensile strength more than or equal to 260MPa and the yield strength more than or equal to 190MPa required by the vehicle cover.

As described above, elongation is an index indicating the plastic deformability of a material, and the higher the elongation is, the easier the material is to be formed. When the elongation of the material exceeds 23%, the material for the vehicle body panel can substantially satisfy the requirement of punching the aluminum alloy sheet strip into the inner and outer covering sheets of the predetermined shape. Too high an elongation generally does not have an adverse effect. However, the manufacturing cost of such a high elongation material will be greatly increased.

Further, the flanging grade detected by the flanging performance is more than grade 2. The automotive deck is generally formed by laminating inner and outer panels that are punched out to have a specific three-dimensional shape. The connection of the inner plate and the outer plate is formed by bending and overturning the inner plate to wrap the edge of the inner plate in the direction of the inner plate and then applying pressure for pressing. This process is commonly referred to as flanging. The flanging process requires that no crack is generated on the outer periphery of the edge part of the outer plate bent by 180 degrees, and the inner periphery of the edge part of the outer plate is required to be pressed against two sides of the end surface of the inner plate after being bent. The flanging performance detection is a detection method for judging whether the material has the non-cracking capability after being bent and flanged for 180 degrees. If the flanging grade of the material can reach more than 2 grade during flanging performance detection, the requirement of flanging processing can be met.

Furthermore, the average diameter of the crystal grains is less than or equal to 60 mu m. In plastic deformation such as press forming and burring, the microstructure is often deformed in units of crystal grains. This results in a material with a deformed microscopic surface that is uneven. If the crystal grains are too coarse, the microscopic unevenness becomes macroscopic unevenness which can be perceived by human eyes, that is, so-called orange peel surface defects, and the aesthetic appearance of the vehicle body parts is impaired. If the average grain size can be controlled to 60 μm or less, the occurrence of such defects can be suppressed.

Further, the number of intermetallic compound particles having a circle-reduced diameter of more than 2 μm is less than 2000 particles/mm ² . The physical and chemical properties of the intermetallic particles are different from those of the aluminum matrix, and they are generally higher in hardness and more brittle than the aluminum matrix, and the electrode potential is also greatly different. When these intermetallic compounds have relatively large particle sizes, they are liable to crack during plastic deformation processing such as rolling, press forming, flanging and the like of the aluminum alloy sheet strip, and cause micro-or macro-cracks, resulting in deterioration of formability, flanging performance and the like of the aluminum alloy sheet strip. Meanwhile, in the surface coating, these coarse particles exposed on the surface do not form an oxide film like the surrounding aluminum matrix, and become a defect of the oxide film, destroying the surface oxygen of the aluminum stripThe quality of the film is degraded, resulting in a decrease in corrosion resistance and durability. In severe cases, these particles fall off the surface, leaving behind a tiny hole. If the number of intermetallic compound particles having a circle-reduced diameter of more than 2 μm is reduced to 2000 particles/mm ² Therefore, the occurrence of cracks during plastic deformation processing and the occurrence of oxide film defects during surface coating can be greatly reduced, and the adverse effects thereof can be suppressed to a negligible level.

The 6-series aluminum alloy plate strip with high durability for the automobile body and the preparation method thereof comprise the following steps:

after the aluminum alloy plate strip is proportioned according to the chemical components, the aluminum alloy plate strip is smelted by a general method and then cast into a flat large ingot by a semi-continuous casting method. After homogenization, hot rolling, cold rolling, intermediate annealing treatment, secondary cold rolling, continuous rapid solution quenching treatment, stretching straightening and aging treatment, surface coating treatment is carried out to obtain the 6-series aluminum alloy plate strip for the automobile body with high durability.

Further, other processing and heat treatment such as homogenization, hot rolling, cold rolling, secondary cold rolling, and the like: these other working and heat treatment steps may be carried out under conventional conditions. For example, the homogenization treatment can be carried out at the heating temperature of 500-580 ℃ and the heat preservation time of 2-20 hours, after discharging, the ingot blank is firstly rolled into an intermediate blank with the thickness of 20-30mm by a hot roughing mill, and then continuously rolled into a hot rolled strip with the thickness of 4.0-8.0mm by a finishing mill, and the finishing temperature of the finishing mill is controlled within the range of 250-360 ℃; then cold rolling is carried out, the total processing rate in the rolling process is controlled to be 30-80%, and a cold-rolled strip is prepared; then, according to the condition of 'intermediate annealing treatment', carrying out continuous heat treatment of rapid temperature rise and rapid temperature reduction to carry out intermediate annealing; then cold rolling is carried out again, and the total reduction ratio in the rolling process is controlled between 30 and 80 percent, so as to prepare the cold-rolled strip with the finished thickness.

Further, intermediate annealing treatment: the intermediate annealing is carried out by a continuous heat treatment line with rapid temperature rise and rapid temperature fall. Heating to 450-520 ℃ at the speed of more than 10 ℃/s, and cooling to room temperature at the speed of more than 30 ℃/s after heat preservation for 30-120 seconds. It has two main purposes, one is to soften the aluminium alloy sheet strip, which has become hardened after cold rolling, properly by a recovery and recrystallization process so that the cold rolling can be continued to the desired thickness. And the other is a recrystallization process generated in the process of intermediate annealing, so that new grains with different crystallographic orientations appear in coarse grain groups with similar crystallographic orientations generated in the previous hot rolling and cold rolling processes, and the surface defects of the finished plate strips, namely the painted lines, are inhibited from being generated on the surfaces of the finished plate strips.

The annealing temperature and the holding time of the intermediate annealing can be selected according to the recrystallization temperature of the aluminum alloy plate strip, for example, 450-520 ℃ and 30-120 seconds of holding. The annealing temperature is too low or the heat preservation time is too short, which is not favorable for fully completing the recrystallization process. Too high or too long in turn will result in unnecessary loss of heat energy.

The temperature rise at a rate of more than 10 ℃/s is to inhibit the recovery process during the temperature rise as much as possible, bring the cold rolling plastic deformation energy storage as much as possible to a high-temperature region above the recrystallization temperature, promote the generation of a large amount of recrystallization crystal nuclei and further achieve the purpose of grain refinement. Meanwhile, the crystallographic orientation of the rapidly recrystallized grains occurring in the high-temperature region is often not dependent on the old crystal grains before recrystallization unlike the slowly recrystallized grains occurring in the low-temperature region. Their crystallographic orientation tends to be more disordered, which is more advantageous in dispersing the orientation of coarse grain groups with similar crystallographic orientation, generated during hot rolling and cold rolling, better fulfilling the second aim of the aforesaid intermediate annealing and inhibiting the appearance of so-called lacquered lines on the surface of the strip of the final product.

The reduction to room temperature at a rate of greater than 30 ℃/sec is to suppress Mg during cooling ₂ The precipitation and growth of Si phase particles and continuous one-time rapid heat treatment. Although the latter solution quenching treatment has solid-solution Mg ₂ The ability of Si-phase particles, but due to the length limitation of the soaking zone of the solution quenching line, Mg in the case of intermediate annealing ₂ The Si phase particles grow too large, and it is difficult to sufficiently complete the solid solution. Therefore, the intermediate annealing needs rapid temperature reduction to limit the growth of the intermediate annealing.

Further, continuous rapid solution quenching treatment: the continuous rapid solution quenching treatment is implemented at one time by a continuous treatment line consisting of alkali washing, water washing, solution quenching and subsequent stretching and straightening processes. Wherein the solution quenching process is a continuous rapid heat treatment which is carried out by raising the temperature to 520-580 ℃ at a speed of more than 10 ℃/s, keeping the temperature for 40-120 s and then lowering the temperature to room temperature at a speed of more than 30 ℃/s.

The continuous and rapid solution quenching treatment line is particularly a continuous and rapid production line of aluminum alloy plates and strips with the functions of unreeling, alkali washing, water washing, rapid heating, heat preservation, rapid cooling and reeling. The alkali washing is carried out by spraying 20 +/-1 wt% sodium hydroxide aqueous solution with the temperature controlled between 55 and 65 ℃ onto the two surfaces of the aluminum alloy plate strip continuously passing through the alkali washing tank, and the spraying time is 8 to 14 seconds, so as to clean and remove surface oil stains, aluminum powder and the like generated during the previous hot rolling and cold rolling. If the concentration and the temperature of the sodium hydroxide solution are low and the spraying time is short, the surface cleanliness of the aluminum alloy plate strip after cleaning cannot meet the requirement, and the risk of surface defects during surface coating treatment is increased. In severe cases, the color of the finished board strip can also change. On the contrary, if the concentration and temperature of the sodium hydroxide solution are too high and the spraying time is too long, the surface layer of the aluminum alloy sheet strip will be excessively dissolved, resulting in unnecessary material loss.

And (3) heating the aluminum alloy plate strip subjected to alkali washing and water washing to 520-580 ℃ on an air cushion type continuous rapid annealing line at the speed of more than 10 ℃/second, and cooling to room temperature at the speed of more than 30 ℃/second after heat preservation for 40-120 seconds. The temperature rise rate of more than 10 ℃/sec is to ensure that recovery occurs as little as possible during the temperature rise process, and to retain the cold rolling deformation energy which can be the driving force for recrystallization to a high temperature region above the recrystallization temperature as much as possible. Thus, high recrystallization nucleation rate can be obtained, and the grain size of the finished product after annealing is smaller. If the temperature rise rate is less than 10 ℃/sec, the risk of the grain size of the finished product after annealing becoming large is increased. The heat preservation temperature is 520-580 ℃, the heat preservation time is 40-120 seconds, the recrystallization can be fully completed, and obvious grain growth does not occur as much as possible; and Mg precipitated at the early rolling heat treatment ₂ The intermetallic compound particles of Si and the like are selected so as to have an effect of sufficiently dissolving in solid solution again. If the temperature is too low to keep warmToo short a period, incomplete recrystallization, Mg ₂ The insufficient solid solution degree of Si finally results in that one or more of the tensile strength, yield strength and elongation of the finished product and the tensile strength and yield strength after paint baking can not meet the requirements of the automobile body panel. Conversely, if the temperature is too high, the material may be locally melted, and the elongation, press formability, burring workability, and the like may be significantly deteriorated. The heat preservation time is too long, and no obvious adverse part exists except that crystal grains are slightly enlarged. But it is generally difficult to exceed 120 seconds due to equipment and production efficiency limitations. Without these limitations, it is considered to appropriately extend the holding time while ensuring that the crystal grains are not larger than 60 μm in claim 1).

Furthermore, the aging treatment must be started within 2 hours after the solution quenching treatment, the aging temperature is 70-100 ℃, and the time is 4-9 hours. The purpose of the aging is to control the yield strength of the aluminum alloy plate strip after the solution quenching treatment to be below 140MPa as stably as possible within at least half a year before reaching a client so as to ensure that the sufficient formability is still ensured in the cover plate stamping forming. The other is to maximize the strength increase of the cover plate after stamping and forming during the baking finish treatment.

The Al — Mg — Si-based aluminum alloy sheet strip after the solution quenching treatment is subjected to aging precipitation as GP zone even at normal temperature, and gradually and continuously increases the yield strength. If the storage time at normal temperature is too long, the yield strength may rise to the point where the formability of the stamping is adversely affected. In addition, the internal structure of GP zone changes with the variation of effective temperature, which affects the strength increase during the baking finish treatment. Generally, the more so-called GP regions precipitated in a low temperature region of less than 70 ℃, the smaller the strength increase. The more so-called GP zones are precipitated in the higher temperature region exceeding 70 ℃, the greater the strength increase will become. However, if GP separates out too much in the secondary region, the yield strength of the plate and strip material is required to exceed 140MPa, and the stamping formability is reduced. The condition that the aging treatment required in this patent must be started within 2 hours after the solution quenching treatment is to limit the precipitation of the GP zone. The aging temperature is 70-100 ℃ and the aging time is 4-9 hours, so that the precipitation amount of the GP second region is controlled in a proper range.

Further, in order to obtain a surface oxide film of high after-quality, the surface coating treatment is continuously performed in the order of unwinding, pickling, washing, spraying, drying, and winding. The pickling is performed to clean an oxide film or the like remaining on the surface of the aluminum strip during the continuous rapid annealing before the pickling. The pickling is carried out by spraying 0.5wt% sulfuric acid and 0.3wt% ammonium bifluoride aqueous solution controlled at 50-60 ℃ onto both surfaces of the aluminum alloy sheet strip continuously passing through the pickling tank for 6-12 seconds. The concentration of the acid washing solution can be controlled by adjusting the conductivity of the solution to 50 + -3 ms/cm and the fluorine ion concentration to 100-400 ppm. If the concentration or the temperature of the sulfuric acid and the ammonium bifluoride is low and the spraying time is short, the surface cleanliness of the aluminum alloy plate strip after cleaning cannot meet the requirement. On the contrary, if the concentration or temperature of the sulfuric acid and ammonium bifluoride is too high and the spraying time is too long, the surface layer of the aluminum alloy sheet strip will be excessively dissolved, resulting in unnecessary material loss. The surface cleanliness of the aluminum plate strip after acid washing is required to be more than 2 grades (the lower the value is, the higher the cleanliness is). If the cleanliness is less than 2 grade, the quality of the surface oxide film formed in the subsequent surface coating treatment is deteriorated, surface defects are generated, and not only are there many defects and the risk of lowering the corrosion resistance and durability of the aluminum alloy sheet for a vehicle body increased, but also the gloss performance of the aluminum alloy sheet itself is affected.

Spraying to obtain metal zirconium and metal titanium with conversion content of 1-12mg/m ² Surface oxide film within the range. Can be obtained by applying or spraying a solution containing at least one of zirconium fluoride compound and titanium fluoride compound in an appropriate concentration to the surface of the aluminum alloy sheet strip. The zirconium fluoride compound herein means K ₂ ZrF ₆ 、(NH ₄ ) ₂ ZrF ₆ Isofluorozirconate, H ₂ ZrF ₆ Isofluorozirconic acid, and ZrF ₄ Zirconium fluoride, and the like. Titanium fluoride means K ₂ TiF ₆ ，(NH ₄ )TiF ₆ Isofluorotitanate, H ₂ TiF ₆ Isofluorotitanic acid, TiF ₄ Titanium fluoride, and the like. The specific coating conditions or spraying conditions may be selected according to conventional surface treatment methods. For example, spraying of a mixed aqueous solution of 2wt% titanium fluoride +4wt% zirconium fluoride at a temperature of 40 to 55 ℃ for 6 to 12 seconds is performed to both surfaces of the aluminum alloy sheet strip continuously passing through the coating bath. And then drying the paper by an infrared drying oven for 5 to 15 seconds and then rolling the paper. The concentration of the mixed aqueous solution for spraying can be controlled by adjusting the conductivity of the liquid to 1500-. However, in any coating method, the content of the surface zirconium metal and titanium metal after drying must be kept at 1-12mg/m ² Within the range. If the content is too low, the thickness of the surface oxide film becomes insufficient, and the corrosion resistance and durability cannot satisfy the requirements of the body panel strip. If the content is too high, the surface oxide film becomes brittle and easily cracks. The corrosion resistance and durability required for the vehicle body panel cannot be obtained, and the manufacturing cost is increased due to the increase of the used materials.

In order to meet the width requirement of the large-scale automobile body plate, the coating surface treatment of the invention needs to be carried out by adopting treatment equipment with the treatment width of more than 1000-2200 mm. In order to ensure good quality of the oxide film, the whole surface film coating treatment process such as acid washing, water washing, film coating, drying and the like needs to be continuously and rapidly completed at one time. If the aluminum strip is intermittently left on the way, the residual dirt on the surface of the aluminum strip is easily solidified, and the dirt is not easily cleaned in the subsequent process. Meanwhile, the production line is continuously and quickly processed at one time, so that the yield can be improved, and the cost can be reduced.

The invention has the remarkable advantages that the invention designs and develops the aluminum alloy plate strip which can simultaneously and well meet various special performance requirements of the material for the automobile body, particularly the outer cover plate of the automobile body and has 1 layer of the oxide film containing more than 1 of zirconium and titanium as a protective layer on at least one side surface, and provides a continuous and rapid mass manufacturing method of the aluminum alloy plate strip according to various performance requirements of the plate strip for the automobile body, particularly good baking varnish hardening performance, flanging bending performance, stamping formability, as few as possible surface defects, excellent corrosion resistance and the like, and comprehensively considers the influence of surface treatment conditions such as composition, rolling conditions, heat treatment conditions, coating and the like.

Drawings

FIG. 1 is a graph of corrosion in the CASS corrosion test.

Detailed Description

The present invention will be further described with reference to the following examples, but the present invention is not limited to these examples.

Method for testing various performances

1) Tensile strength, yield strength and elongation

The tensile strength and the elongation are measured by taking GB/T16865 national standard as a reference. The long axis of the tensile sample is parallel to the rolling direction, the shape is made by punching according to the rectangle style specified by the national standard, and the elongation punctuation distance is 50 mm. The loading rate during stretching was 10 mm/min.

2) Average diameter of crystal grains

The average grain size was measured according to the GB/T3246.1 national standard method. And after the cut sample is embedded, selecting the longitudinal section of the aluminum alloy plate as an observation surface. After the observation surface is processed according to the sequence of mechanical polishing, electrolytic polishing, etching and anodic oxidation film making, 3 fields far apart are randomly selected from the observation surface for polarization microscopic observation and measurement. For the crystal grains observed in each field, their diameters were measured by the intercept method, and then the average value thereof was calculated.

3) Flanging performance detection

The detection of the flanging performance is carried out according to the method of GB/T33227 and 2016 national standard appendix C. The long axis direction of the sample is perpendicular to the rolling direction, and a rectangular sample with the length and width of 250mm 32mm is cut from the aluminum alloy plate to be measured. After 10% tensile deformation is carried out along the long axis direction of the sample, the proper bending pressure head radius and the gasket thickness are selected according to the actual thickness of the sample and the requirements of national standard appendix C. And then bending the sample by 180 degrees by adopting a three-point bending method, and judging the grade of the flanging performance according to the specification of national annex C by observing the crack or wrinkle state of the outer side surface of the bent part. Grade 1 is surface crack or wrinkle free; grade 2 is no surface cracks but some wrinkles; the surface of the grade 3 has microcracks; grade 4 surface had significant cracking.

4) The number of intermetallic compound particles having a circle-reduced diameter of more than 2 μm

Taking an area larger than 1cm at any position of the finished plate ² Sample 1 was subjected to surface polishing in the order of mechanical polishing and electrolytic polishing and under the conventional conditions for aluminum alloy sheets, and then placed in a scanning electron microscope for observation of intermetallic compound particles. During observation, 10 fields of view are randomly selected for photographic recording by adopting 15kv accelerating voltage, 500 times magnification and the conventional conditions of a back reflection electron receptor. The intermetallic particles in 10 field records were then counted and size measured using conventional image counting measurement software, and the number of intermetallic particles per unit area with a circle-reduced diameter of greater than 2 μm was summarized.

5) Detection of surface defect paint brush line

The detection of the performance of the paint brush is carried out according to the method of GB/T33227 and 2016 national standard appendix D. The test piece is formed into 90 degrees along the rolling direction, and a rectangular sample with the length and the width of 250mm and 35mm is cut from the aluminum alloy plate to be tested. The sample is stretched at a rate of 3mm/min until the strain reaches (10 +/-2)% and then the surface of the sample except the clamping part is integrally polished along the length direction of the sample by using a polishing tool until the protruding part is bright, the polishing force is soft and uniform, the surface of the sample is prevented from being scratched greatly, and the polishing tool is preferably oilstone or millstone which is coated with sufficient lubricating oil and has a hard plane, and the granularity is F320. And (4) visually observing the surface of the sample, and evaluating the paint brush line grade of the sample. The first stage has no obvious white stripes along the width direction; the second level has discontinuous white stripes along the width direction; a small amount of through white stripes exist in the third level along the width direction; four levels have a large number of white stripes running through in the width direction.

6) Degree of surface cleanliness

The method is carried out by taking the test method of the cleanness of the surface of the aluminum plate with the foil in the YST 799 and 2012 of the nonferrous metal industry as a reference. Cutting a sample with the surface area not less than 15 x 15cm from an aluminum alloy plate strip to be detected, dripping no more than three drops of vegetable oil on the surface of the sample to be detected, and slightly pressing the oil drops by using a plexiglass disc to uniformly spread the oil on the surface of the sample. And folding 3-4 layers of white tissue paper on the surface of the sample, pressing the white tissue paper by using a cylinder with certain weight and contact area, and then moving the cylinder to wipe in a specified area to enable dirt such as aluminum powder foreign matters, residual oil and the like on the surface of the sample to be adsorbed on the white tissue paper. The grey scale of a white tissue paper stained with dirt was determined by comparing the colour of the white tissue paper with the grey scale colour chart of standard KODAK GRAY SCALE. The higher the grey scale value, the lower the sample surface cleanliness. If the surface cleanliness is lower than the gray level 2, the surface is qualified and is marked as OK; otherwise, failing to record as NG.

7) Testing of surface coating films

The measurement of the surface coating film was carried out by using an XRF1800 type X-ray fluorescence spectrometer manufactured by Shimadzu corporation, Japan. And irradiating the flat surface of the sample by X-rays to excite the characteristic spectral line generated by the sample. The secondary X-rays emitted by each element of the excited sample have measured wavelength characteristics, and the amount of secondary X-rays generated is proportional to the content of the corresponding element in the irradiated area of the sample. If the irradiation region contains zirconium metal or titanium metal, the content of zirconium metal or titanium metal in the irradiation region can be measured. By measuring and comparing the contents of zirconium metal or titanium metal in the sample before and after coating, the weight of zirconium metal or titanium metal per unit area in the surface film layer of the sample can be calculated.

8) Evaluation of Corrosion resistance

Rectangular samples having a length and width of 150X 75 mm were cut from the surface-treated finished aluminum alloy sheet strip and subjected to electrophoresis. The electrophoretic paint is a Guanxi gray electrophoretic paint, the electrophoresis time is 3min, the electrophoresis voltage is 150V, and the baking time after electrophoresis is 160 ℃ for 10 min. The cross lines were scribed on the surface of the coupon with a 0.5mm blade to a depth below the surface electrophoretic paint layer. Then, a CASS corrosion test is carried out, and 5wt% NaCl and 0.26g/L CuCl are adopted as a corrosion solution ₂ The pH of the aqueous solution of (1) to (3.3) is adjusted to 24 hours. After the CASS corrosion test, the sample is taken out, and the corrosion condition of the edge of the scribing cross line is observed. If no unilateral extension is observed, corrosion will occurThe evaluation of the properties was OK. If the unilateral extension occurred, the corrosivity was evaluated as NG, as shown in FIG. 1.

Example 1

1) Composition control

The components are proportioned according to the table 1, smelted by a conventional method, degassed and deslagged by the conventional method, added with an Al-Ti-B grain refiner and cast into a flat large ingot with the width of 2100mm and the thickness of 650mm by a semi-continuous casting method. Homogenizing at 540 deg.C for 8 hr, hot rolling to 6 mm, and rolling at 300 deg.C. Then, after cold rolling to 3.0mm, intermediate annealing treatment was performed. The intermediate annealing is carried out by a continuous heat treatment line with rapid temperature rise and rapid temperature fall. The temperature is increased to 480 ℃ at the rate of more than 10 ℃ per second, the holding time is 60 seconds, and then the temperature is cooled to the room temperature at the cooling rate of 20 ℃/s. And then, continuously cold rolling to the thickness of a finished product of 1.0mm, and then carrying out alkali washing, water washing and solution quenching treatment on a continuous rapid solution quenching treatment line. The alkali wash was carried out by spraying 20wt% aqueous sodium hydroxide solution at 55 ℃ for 10 seconds and then washing with water. The solution quenching is carried out by adopting an air cushion type continuous rapid annealing line, the temperature is raised to 550 ℃ at the speed of 12 ℃/second, then the temperature is preserved for 70 seconds, and the solution is cooled to the normal temperature at the speed of 25 ℃/second. Thereafter, the aging treatment was started after the stretching straightening was performed by about 0.4% under the usual conditions. All materials were within 1 hour, 25-50 minutes, from the completion of solution quenching to the start of aging. The aging treatment is carried out by keeping the temperature at 90 ℃ for 8 hours. And finally, continuously performing surface coating treatment on the aluminum alloy plate strip coil by adopting a surface continuous treatment line according to the sequence of acid washing, water washing, spraying and drying. The pickling is carried out by continuously spraying 0.5% sulfuric acid and 0.3% ammonium bifluoride aqueous solution at 55 ℃ onto both surfaces of the aluminum alloy sheet strip passing through the pickling tank, and the spraying time is 9 seconds. The spraying was carried out in such a manner that a mixed solution of 2% titanium fluoride +4% zirconium fluoride at a temperature of 50 c was continuously sprayed to both surfaces of the aluminum alloy sheet strip passing through the coating bath for 10 seconds. And then drying the paper by an infrared drying oven for 5 to 15 seconds and then rolling the paper. Various aluminum alloy sheet strips for vehicle bodies having a surface oxide film as a protective layer were obtained. The surface oxidation film amount (single side) of the plate and strip materials is determined to be 5.6-6.7mg/m ² And (3) a range.

Finally, various mechanical properties, internal organization structure parameters and paint brush line grades are detected by sampling the processed aluminum alloy plates and strips. The mechanical property indexes (tensile strength, yield strength, elongation) and the flanging property indexes which are obviously changed along with the extension of the room temperature storage time, and the tensile strength and the yield strength after the baking finish equivalent heat treatment are measured by sampling aluminum alloy plate strips which are stored for 7 days and 180 days at the room temperature after the surface coating treatment process is finished, and the results are respectively shown in tables 2 and 3. On the other hand, the measurement of the average grain diameter, the number of intermetallic compound particles having a diameter of 2 μm or more in terms of circle-equivalent diameter and the grade of a paint line, which did not significantly change with the increase of the room temperature storage time, was conducted by sampling from the aluminum alloy sheet strip stored at room temperature for 7 days after the completion of the surface coating treatment step. Whereas corrosion resistance is generally a high possibility of deterioration of the material with a long storage time at room temperature, this measurement is taken by sampling the aluminum alloy sheet strip stored at room temperature for 180 days. The measurement results of the average diameter of crystal grains, the number of intermetallic compound particles having a circle-equivalent diameter of more than 2 μm, and the corrosion resistance are also shown in table 2 or table 3. The results of the paint brush line grade measurements, including all the materials of the examples and comparative examples, were grade 1, and were in accordance with the requirements of the aluminum alloy sheet and strip for automobile bodies.

From a comprehensive comparison of the results in tables 1 to 3, it can be seen that the materials whose compositions are within the scope of the patent claims all have suitable grain sizes and relatively few coarse intermetallic particles and, at room temperature storage times of not more than 180 days, all have good mechanical properties, flanging properties and increased strength after baking finish equivalent heat treatment. If a certain alloy composition is outside the scope of the claims of this patent, the risk of increased oxide film defects is increased due to either a higher yield strength, a lower burring performance, or more coarse intermetallic compound particles.

2) Intermediate annealing process condition control

A cold-rolled sheet strip coil having a thickness of 2.0mm was produced by melting casting, homogenization treatment, hot rolling and cold rolling under the same composition and processing heat treatment conditions as in example 1 of Table 1, and then subjected to intermediate annealing under the conditions shown in Table 4. Then further cold rolling to 1.0mm thickness in 3 passes. The edge cracking condition after each cold rolling was evaluated. If a crack larger than 0.5mm was observed at the edge, it was judged as NG, and if not, it was judged as OK, and the results are also shown in Table 4. Subsequently, various aluminum alloy sheet strips for vehicle bodies having surface oxide films were obtained by performing continuous rapid solution quenching treatment, stretch straightening, aging treatment and surface coating treatment under the same conditions as in example 1 of Table 1. Finally, the samples were taken for a grade test of surface defects "paint lines". The results of the measurements are also shown in Table 3.

From the results in table 4, it is understood that if the intermediate annealing temperature is too low and the holding time is too short, the softening is insufficient and the edge cracking phenomenon occurs in the subsequent cold rolling. The annealing temperature is too high, and although no obvious adverse effect is caused, unnecessary energy loss is caused. For the surface defect of the 'painted line', if the temperature rise rate, the annealing temperature, the heat preservation time and the temperature reduction rate are not in the range required by the patent, the degree of the painted line is increased.

3) Continuous and rapid solution quenching treatment condition control

Using the same composition and processing heat treatment conditions as in example 1 of Table 1, a cold rolled sheet strip coil of 1.0mm finished thickness was produced by melt casting, homogenization treatment, hot rolling, cold rolling, intermediate annealing, and secondary cold rolling. And then, carrying out alkali washing, water washing and rapid solution quenching treatment on a continuous rapid solution quenching heat treatment line. The alkali washing was performed under the conditions shown in Table 5 and the solution quenching treatment was performed under the conditions shown in Table 6. In Table 5, the alkali lye concentration OK means that the sodium hydroxide solution concentration is within the proper range of 20. + -. 1%, and NG means that it is not within the proper range. And the sample was taken after the alkali cleaning to evaluate the cleaning condition of the material surface, and the evaluation results are also shown in table 5. From the results in table 5, it is understood that if the conditions for alkali cleaning are not appropriate, the surface cleanliness cannot be satisfied.

The aluminum alloy plate strip coil after the continuous and rapid solution quenching treatment is subjected to the stretching straightening, the aging treatment and the surface continuous coating treatment continuously under the same conditions as the conditions of the component control in the step 1) to obtain the aluminum alloy plate strip with the surface oxidation film. The sheets and tapes were stored at room temperature for 7 days and the results of the properties measured on samples of these sheets and tapes are also shown in Table 6. It can be seen from table 6 that when the temperature rise rate, the solution quenching temperature, the holding time or the cooling rate are out of the range of the patent requirements, or a certain index of the mechanical properties does not meet the requirements, or the detection level of the flanging experiment is reduced, or the grain diameter is too large to meet the performance requirements of the automobile body panel well.

4) Aging treatment condition control

The aluminum alloy sheet strip with the surface oxide film was obtained by carrying out the aging treatment under the conditions shown in Table 7 after the same composition and processing heat treatment conditions as those of example 1 shown in Table 1, melting casting, homogenization treatment, hot rolling, cold rolling, intermediate annealing, secondary cold rolling, continuous rapid solution quenching heat treatment, and stretch straightening, and then carrying out the continuous surface coating treatment under the same conditions as those of "1) composition control". The sheets and tapes were stored at room temperature for 7 days and the results of the properties measured on samples of these sheets and tapes are also shown in Table 7. As can be seen from Table 7, if the time from the solution quenching to the start of aging, the aging temperature and the holding time are not within the ranges required by the present patent, or a certain index of the mechanical properties does not meet the requirements, or the detection grade of the flanging test is reduced, or the tensile strength or yield strength after the baking finish is subjected to the equivalent heat treatment does not meet the requirements.

5) Surface coating treatment condition control

Using the same composition and processing heat treatment conditions as in example 1 of Table 1, an aluminum alloy sheet strip having a thickness of 1.0mm was produced by melt casting, homogenization treatment, hot rolling, cold rolling, intermediate annealing, secondary cold rolling, continuous rapid solution quenching heat treatment, stretch straightening, and aging treatment. Subsequently, surface coating treatment was continuously performed on a continuous surface treatment line in the order of unwinding, pickling, washing, spraying, drying, and winding under the conditions shown in table 8, to obtain aluminum alloy sheet strips for vehicle bodies having oxide films of various properties and thicknesses. The acid pickling solution concentration OK in Table 8 represents that the conductivity of the solution is within 50 +/-3 ms/cm, and the fluoride ion concentration is adjusted to be within a proper range of 100-400 ppm; NG represents more than one of conductivity and fluoride ion concentration outside the aforementioned suitable ranges. Similarly, the concentration OK of the spraying liquid represents that the conductivity of the liquid is within 1500-; NG represents that more than one of the conductivity and pH value is out of the above suitable range. Samples were also taken at the acid wash stage to test for surface cleanliness, and the results are also shown in table 8.

Finally, samples were taken from the surface-coated aluminum alloy sheet and strip, and the amount of oxide film and corrosion resistance were evaluated, and the evaluation results are also shown in Table 8. As can be seen from the results in table 8, if the conditions during pickling or spraying are out of the range claimed in the present patent, the corrosion resistance is lowered either because the surface cleanliness after pickling is insufficient or the amount of surface oxide film is too large or too small.

The above examples are merely for clearly illustrating the present invention, and the embodiments of the present invention are not limited thereto. Any modification, replacement, or improvement made without departing from the spirit and principle of the present invention shall fall within the protection scope of the present invention.

Claims

1. A6-series aluminum alloy sheet strip for automobile bodies having high durability, characterized in that: the aluminum alloy plate strip has 1 layer of oxide film containing more than 1 of zirconium and titanium on at least one side surface, wherein the total content of zirconium and titanium in the oxide film is calculated according to metal zirconium and metal titanium: 1-12mg/m ² (ii) a The aluminum alloy plate strip comprises the following chemical components in percentage by mass: 0.50-1.15wt% of Si, less than 0.30% of Fe, 0.01-0.25% of Cu, 0.30-0.90% of Mg, 0.05-0.25% of Mn, 0.005-0.15% of Zn, and the balance of Al and other inevitable impurity elements.

2. The 6-series aluminum alloy sheet strip for automobile bodies having high durability according to claim 1, characterized in that: the 6-series aluminum alloy plate strip for the automobile body with high durability has the tensile strength of more than or equal to 180MPa, the yield strength of less than or equal to 140MPa, the elongation of more than or equal to 23 percent, the flanging grade detected by the flanging performance is more than 2 grade, the average diameter of crystal grains is less than or equal to 60 mu m, and the number of intermetallic compound particles with the circle-converted diameter of more than 2 mu m is less than 2000/mm ² The tensile strength of the baking varnish after equivalent heat treatment is more than or equal to 260MPa, and the yield strength is more than or equal to 190 MPa.

3. A method for producing a 6-series aluminum alloy sheet strip for automobile bodies having high durability according to any one of claims 1 to 2, characterized by: weighing raw materials according to the proportion of each component, casting into a flat large ingot by adopting a semi-continuous casting method, and performing surface coating treatment after homogenization, hot rolling, cold rolling, intermediate annealing treatment, secondary cold rolling, continuous rapid solution quenching treatment, stretching straightening and aging treatment to obtain the 6-series aluminum alloy plate strip for the automobile body with high durability.

4. The production method according to claim 3, characterized in that: the intermediate annealing treatment is carried out by raising the temperature to 450-520 ℃ at the speed of more than 10 ℃/s, and reducing the temperature to room temperature at the speed of more than 30 ℃/s after heat preservation for 30-120 s.

5. The production method according to claim 3, characterized in that: the continuous rapid annealing heat treatment comprises the working procedures of alkali washing, water washing and solid solution quenching, wherein the alkali washing is carried out in a mode of spraying 20 +/-1 wt% of sodium hydroxide solution with the temperature of 55-65 ℃ on the two surfaces of the aluminum alloy plate strip, and the spraying time is 8-14 seconds; the solution quenching is carried out by raising the temperature to 520-580 ℃ at the speed of more than 10 ℃/s, and then reducing the temperature to room temperature at the speed of more than 30 ℃/s after keeping the temperature for 40-120 s.

6. The production method according to claim 3, characterized in that: the aging treatment is carried out within 2 hours after the solution quenching treatment, the temperature of the aging treatment is 70-100 ℃, and the time is 4-9 hours.

7. The production method according to claim 3, characterized in that: the surface coating treatment is to firstly carry out acid washing and water washing on the aluminum alloy plate strip, then coat or spray at least more than one zirconium fluoride compound and titanium fluoride compound solution on the surface of the aluminum alloy plate strip, and then dry the aluminum alloy plate strip.

8. The method of claim 8, wherein: the pickling is carried out by spraying aqueous solution of 0.5wt% sulfuric acid and 0.3wt% ammonium bifluoride at 50-60 ℃ on two surfaces of the aluminum alloy plate strip, and the spraying time is 6-12 seconds.

9. The method of claim 8, wherein: the zirconium fluoride compound comprises one or more of fluorozirconate, fluorozirconic acid and zirconium fluoride; the titanium fluoride compound comprises one or more of fluotitanate, fluotitanic acid and titanium fluoride.