CN114807688B

CN114807688B - 6-series aluminum alloy plate strip with high durability for automobile body and preparation method thereof

Info

Publication number: CN114807688B
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: 2023-10-13
Anticipated expiration: 2042-04-13
Also published as: CN114807688A

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 1 kind of zirconium and titanium, and the chemical components in percentage by mass are as follows: 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 unavoidable 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 of flanging performance detection is more than 2, the average diameter of crystal grains is less than or equal to 60 mu m, and the number of intermetallic compound particles with the diameter of more than 2 mu m in terms of circles 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 an important measure for energy conservation and emission reduction at present. The steel car body has large weight, which is not beneficial to the weight reduction, energy conservation and emission reduction of the car body. The density of the aluminum alloy is about one third of that of steel, and the weight of the aluminum alloy plate can be reduced by about 30% on the premise of ensuring that the rigidity of the vehicle body meets the design requirement when the aluminum alloy plate is applied to the steel plate comprising an outer plate (a vehicle door, an engine cover and a trunk outer plate), an inner plate (a vehicle door, an engine cover and a trunk inner plate) and a structural member. In particular with Mg ₂ Si is an Al-Mg-Si alloy with an aging strengthening phase, has the characteristics of moderate strength, heat treatment strengthening and good corrosion resistance, and is supplied in a state of lower yield strength after solution treatment, so that the alloy plate has good stamping forming capability, the strength can be further increased in the baking varnish process of a stamped car body, and the capability of resisting external collision damage is improved. The characteristics make the aluminum alloy material very suitable for processing the automobile body outer plate, and the aluminum alloy material is a preferred material for replacing steel and reducing the weight of an automobile.

However, since there are problems such as lower press formability and higher manufacturing cost than steel materials, the conventional aluminum alloy sheet cannot completely replace steel materials, and both are often used as automobile parts. In order to prevent contact corrosion generated when an aluminum alloy is connected 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 to the surface of the aluminum alloy as a protective layer. For this oxide film, it is required to have both good bonding strength with the underlying aluminum sheet strip and good corrosion resistance itself. The performance of the oxide film has close relation with the internal structure and the surface quality of the aluminum alloy plate strip below the oxide film. Therefore, in order to obtain an oxide film with good performance, strict control over the internal structure of the aluminum alloy sheet strip and the surface quality is necessary. On the other hand, there are often complex shape requirements as vehicle body parts. This requires that the aluminum alloy sheet strip of its base material must have good press formability. Moreover, in order to improve the capability of the parts against external impact damage while ensuring good formability, it is also required that the aluminum alloy sheet strip has as high strength as possible.

However, there is often a complex causal relationship between the aforementioned properties required, and it is difficult to achieve the best level of both. Therefore, in order to obtain the oxide film having the excellent properties and the excellent press formability, it is necessary to strictly manage the various production conditions from the alloy composition, the working heat treatment process, and the final surface coating treatment in a comprehensive manner as high as possible. Existing techniques have not met 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 of hot rolling and cold rolling, and the heat treatment conditions such as solution hardening 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-free and phosphorus-free oxide film from the viewpoint of improving the corrosion resistance of an aluminum alloy. However, it is not designed for an aluminum alloy sheet strip for a vehicle body, and is not suitable for continuous rapid and large-scale coating treatment of the surface of a sheet strip for a vehicle body.

Disclosure of Invention

The invention aims at solving 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 above purpose, the invention adopts the following technical scheme:

an aluminum alloy sheet strip having a surface protective film, at least one surface of which has 1 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. And alsoThe aluminum alloy sheet strip is composed of 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, the balance of Al and other unavoidable impurity elements, and has a tensile strength of 180MPa or more, a yield strength of 140MPa or less, an elongation of 23% or more, a flanging grade of 2 or more, an average grain diameter of 60 μm or less, and a number of intermetallic compound particles with a round conversion diameter of 2 μm or more of 2000 pieces/mm or less ² The tensile strength of the baking varnish after the equivalent heat treatment is more than or equal to 260MPa, and the yield strength is more than or equal to 190 MPa.

Further, a surface oxide film containing zirconium and titanium: the function of the zirconium and titanium containing oxide film on the surface of the aluminum alloy sheet strip is to provide good corrosion resistance and durability. The content of zirconium and titanium in the surface oxide film is converted into metallic zirconium and metallic titanium, and must be ensured to be 1-12mg/m ² Within the range. If the content is low, the thickness of the surface oxide film is insufficient, and the corrosion resistance and durability cannot meet the requirements of the vehicle body plate strip. If the content is too high, the surface oxide film becomes brittle and is liable to crack. The corrosion resistance and durability required for the vehicle body panel are not obtained, and the manufacturing cost is increased due to the large amount of materials.

Further, si content is in the range of 0.50-1.20% and Mg content is in the range of 0.30-0.90%. When Si and Mg are added simultaneously, the microscopic existence state of the Si and the Mg in an aluminum matrix can be controlled through proper heat treatment, so that the macroscopic mechanical property of the material can be changed. After heat preservation at a high temperature above 520 ℃, si and Mg tend to be solid-dissolved in the aluminum matrix. The amount of solid solution and whether there is undissolved Si and Mg remaining are related to Si and Mg contents, and the holding temperature and time. If quenched to room temperature in a rapid cooling manner, si and Mg are present in an aluminum matrix in a supersaturated state. Then when the aluminum matrix is stored for a long time at normal temperature or heated for aging treatment, the supersaturated state of Si and Mg dissolved in the aluminum matrix can be formed by GP zone-Mg ₂ Si metastable phase-Mg ₂ The order of the Si stable phases is separated from the aluminum matrix to form intermetallic particles, which cause a change in the macroscopic mechanical properties of the material. In general, GP zones or Mg ₂ The metastable phase of Si has small size and large distribution density and can greatlyThe degree increases the strength of the material and reduces the plasticity. While Mg is ₂ The stable phase of Si has large size and small distribution density, and the amplitude of improving the strength of the material and the amplitude of reducing the plasticity are small. GP zone, mg ₂ Metastable phase of Si, mg ₂ The size, distribution density and the ratio of the stable phase Si, namely the macroscopic mechanical property of the material, can be comprehensively controlled by adjusting the content of Si and Mg and the treatment conditions of solution quenching and aging.

The above GP zone and Mg when the Si and Mg contents are less than 0.50 and 0.30, respectively ₂ Metastable phase of Si, mg ₂ The overall number of so-called strengthening phase particles, equal to Si stabilization, tends to be insufficient, so that the tensile strength and yield strength of the material are lower than the required range of the present patent, and the requirements of the vehicle body panel are not met. When the Si and Mg contents are greater than 1.20 and 0.90, respectively, some coarse Mg may be formed in the casting stage ₂ Si phase particles. If the material contains a large amount of Fe, si forms coarse AlFeSi intermetallic particles together with Fe. If a large amount of Cu is present in the material, coarse AlFeCuMg intermetallic particles may be formed during casting. In either form of coarse particles, subsequent homogenization and solution treatment make it difficult to re-dissolve them back into the aluminum matrix. Since these coarse intermetallic compound particles are different from the aluminum matrix in their physicochemical properties, these particles exposed on the surface during the surface coating treatment cannot form an oxide film like the surrounding aluminum matrix, and become defects of the oxide film, and deteriorate the quality of the oxide film on the surface of the aluminum sheet strip, resulting in deterioration of corrosion resistance and durability. In severe cases, these particles may fall off the surface, leaving a tiny hole, and the corrosion resistance is reduced. At the same time, these coarse intermetallic compound particles also become crack nucleation sites when the aluminum alloy sheet strip is hemmed, and the tendency of surface cracks to occur during hemming is increased.

Further, the Fe content is less than 0.30% range. Fe is slightly dissolved in the aluminum matrix, and has the effect of improving the strength. While the vast majority of Fe forms intermetallic particles with other alloying elements. As previously mentioned, when the Fe content is high, some coarse AlFeSi, alFeCuMg intermetallic particles may be formed during the casting stage. These coarse intermetallic compound particles are generally brittle, and when an aluminum alloy sheet strip is subjected to plastic deformation such as rolling, press forming, flanging, etc., they themselves are liable to crack, and microscopic or macroscopic cracks are induced to occur, resulting in a decrease in formability of the aluminum alloy sheet strip. Meanwhile, they also become defects of oxide films during surface coating, and deteriorate the quality of the oxide films on the surfaces of aluminum strips, resulting in reduced corrosion resistance and durability. To further reduce these negative effects, the addition amount of Fe may be controlled to 0.20% or less.

Further, the Cu content is in the range of 0.01-0.25%. Cu added in a small amount is generally solid-dissolved in aluminum, and can improve the strength of the aluminum alloy sheet strip. At the same time, cu dissolved in the matrix has an effect of being biased to grain boundaries and suppressing coarse grains during recrystallization. If the Cu content is less than 0.01%, the risk of the aforementioned positive effect becoming insufficient increases. On the other hand, when aging is performed, cu in solid solution tends to be biased toward grain boundaries, and the electrode potential of the grain boundaries increases, leading to preferential corrosion in the vicinity of the grain boundaries, and increasing the risk of lowering corrosion resistance. If the Cu content exceeds 0.25%, the aforementioned negative effects will be increased to a level that cannot be ignored. In addition, solid-solution Cu can reduce the plastic deformability of the material while improving the strength of the aluminum alloy sheet.

Further, the Mn content is in the range of 0.05 to 0.25%. In the ingot, most of Mn is solid-dissolved in the aluminum matrix, and precipitates during the homogenization treatment to form nano-scale fine Al together with Al ₆ Mn intermetallic compound particles. These fine Al ₆ Mn particles have an inhibitory effect on coarsening of recrystallized grains in hot rolling, intermediate annealing and finish annealing, and the final grain size is refined. If the content is less than 0.05%, fine Al ₆ Mn particles are too small and the inhibition is insufficient. On the other hand, a part of Mn may replace Fe during casting to form coarse Al (Fe, mn) Si, al (Fe, mn) CuMg intermetallic compound particles. As mentioned before, they induce micro-or macro-crack generation, resulting in a decrease in formability of the aluminum alloy sheet strip. At the same time, coat the surface withThey also become defects of oxide films during the film process, spoil the quality of oxide films on the surfaces of aluminum strips, and cause the reduction of corrosion resistance and durability. If the Mn content exceeds 0.25%, the aforementioned coarse Al (Fe, mn) Si, al (Fe, mn) CuMg intermetallic compound particles will be excessive, causing a non-negligible negative effect.

Further, the Zn content is in the range of 0.005-0.15%. Zn is mainly solid-dissolved in aluminum. When the surface coating film is treated by alkali washing and acid washing, the surface coating film has the effects of promoting the removal of a surface oxide layer generated during the early aluminum alloy plate strip rolling heat treatment, maintaining a fresh material surface layer and enabling the adhesion of an oxide film generated during the subsequent coating film 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 the effect of comprehensively reducing the electrode potential of the aluminum alloy plate strip substrate. If the Zn content is more than 0.15%, the amount of change in the electrode potential of the matrix will be too large, and the balance between the electrode potential of the matrix and the grain boundary and between the intermetallic compound particles will be destroyed, leading to a decrease in corrosion resistance and durability.

Further, 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%; the tensile strength of the baking varnish after the equivalent heat treatment is more than or equal to 260MPa, and the yield strength is more than or equal to 190MPa.

After the aluminum alloy plate strip for the automobile body is supplied to an automobile manufacturer, the inner cover plate and the outer cover plate of the automobile body with the appearance shape meeting the design requirement are manufactured through stamping deformation, the inner cover plate is wrapped and fixed into an integral automobile cover through flanging of the outer cover plate, and finally, the outside of the automobile cover is subjected to paint baking coating. This requires a deformation resistance during press forming, that is, a lower yield strength is better. In order to press a desired external shape having a strong three-dimensional effect, plastic deformability of the material is required, and in short, the higher the elongation is, the better. On the other hand, the roof is required to be resistant to external impact pressure after being mounted on a vehicle, and in short, the higher the tensile strength and the higher the yield strength.

In order to meet the complex requirements, the yield strength of the aluminum alloy plate strip for the automobile body is often controlled below 140MPa which meets the basic requirement of stamping forming, and after qualified stamping parts are obtained, the heat generated during the final paint baking coating is utilized to promote the material to age harden, so that the yield strength and the tensile strength are improved. The heat generated during baking varnish application is generally equivalent to a rapid heat treatment at 170-190 ℃ for 15-30 minutes. In particular, how to select the baking finish conditions, the automobile manufacturers often depend on their own needs. The baking finish treatment of this patent was simulated by rapid heat treatment at 185 ℃ for 20 minutes. This treatment is only a criterion for judging the strength increasing ability of the material during baking. In general, the tensile strength is improved to a considerably smaller extent than the yield strength at the time of baking finish treatment, so that it is required that the tensile strength of an aluminum alloy sheet strip used as a body panel should be at least satisfied to be more than 180MPa. Therefore, the tensile strength and the yield strength after the baking varnish is coated can 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 previously mentioned, elongation is an indicator of the plastic deformation ability of a material, and the higher it is, the easier it is to work with the material. When the elongation of the material exceeds 23%, the material for a vehicle body panel can substantially satisfy the requirement of stamping an aluminum alloy sheet strip into an inner and outer cover sheet of a prescribed shape. Too high an elongation generally does not have an adverse effect. However, the manufacturing costs of such high elongation materials will increase substantially.

Further, the flanging level of the flanging performance detection is more than level 2. Automotive decking is typically formed from an inner panel and an outer panel laminated together, which are stamped into a specific three-dimensional shape. The inner plate and the outer plate are connected by guiding the edge part of the outer plate with the size slightly larger than that of the inner plate, and after the inner plate is bent and turned over towards the direction of the inner plate to wrap the edge of the inner plate, the inner plate and the outer plate are pressed together by applying pressure. This process is commonly referred to as flanging. The flanging process requires that the outer periphery of the edge part of the outer plate which is bent 180 degrees cannot generate cracks, and the inner periphery of the edge part of the outer plate must be pressed against two sides of the end face of the inner plate after being bent. The flanging performance detection is a detection method for measuring whether the material has the capability of not cracking after 180-degree bending flanging processing. When the flanging performance is detected, if the flanging grade of the material can reach more than 2 grades, the requirements of flanging processing can be met.

Further, the average grain diameter is less than or equal to 60 mu m. In the case of plastic deformation such as press forming and flanging, the microstructure is often deformed in grain units. This causes the microscopic surface of the deformed material to be rugged. If the crystal grains are too large, the microscopic irregularities become macroscopic irregularities perceived by the human eye, i.e., so-called orange peel surface defects, which impair the aesthetic appearance of the vehicle body parts. If the average grain size is controlled to 60 μm or less, the occurrence of such defects can be suppressed.

Further, the number of intermetallic compound particles having a diameter of more than 2 μm in terms of circles is less than 2000/mm ² . Intermetallic particles are different from aluminum substrates in their physicochemical properties, are generally harder and more brittle than aluminum substrates, and are also more different in electrode potential. If these intermetallic compound particles are relatively coarse in size, they themselves are liable to crack when plastic deformation processing such as rolling, press forming, flanging and the like is performed on the aluminum alloy sheet strip, and microscopic or macroscopic cracks are induced to occur, resulting in deterioration in formability, flanging performance and the like of the aluminum alloy sheet strip. Meanwhile, in the case of surface coating, these coarse particles exposed on the surface cannot form an oxide film like the surrounding aluminum matrix, and the oxide film becomes a defect, and the quality of the oxide film on the surface of the aluminum sheet strip is deteriorated, resulting in a decrease in corrosion resistance and durability. In severe cases, these particles will fall off the surface, leaving a tiny hole. If the number of intermetallic compound particles having a diameter of more than 2 μm in terms of circle is reduced to 2000 pieces/mm ² It is possible to significantly reduce the occurrence of cracks during plastic deformation processing and oxide film defects during surface coating, and to control these adverse effects within a negligible range.

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

after the aluminum alloy plate and the strip are proportioned according to the chemical components, the aluminum alloy plate and the strip are smelted by a common method and 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, and the 6-series aluminum alloy plate strip for the automobile body with high durability is obtained.

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

Further, an intermediate annealing treatment: the intermediate annealing is performed by a continuous heat treatment line that rapidly increases and decreases in temperature. Heating to 450-520 ℃ at a rate of more than 10 ℃/s, preserving heat for 30-120 seconds, and then cooling to room temperature at a rate of more than 30 ℃/s. It has mainly two purposes, one is to properly soften the hardened aluminum alloy sheet strip after cold rolling by a reversion and recrystallization process so that the cold rolling can be continued to a desired thickness. The other is to promote the occurrence of new grains with different crystallographic orientations in the coarse grain groups with similar crystallographic orientations generated in the previous hot rolling and cold rolling processes through a recrystallization process generated in the intermediate annealing process, thereby inhibiting the surface defects of so-called paint brush lines on the surface of the final finished plate strip.

The annealing temperature and holding time of the intermediate annealing may be selected according to the recrystallization temperature of the aluminum alloy sheet strip, for example, 450 to 520 ℃, for 30 to 120 seconds. The annealing temperature is too low or the heat preservation time is too short, which is unfavorable for the complete completion of the recrystallization process. And in turn too high or too long, would cause unnecessary thermal energy losses.

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 as much cold rolling plastic deformation energy storage to a high temperature area above the recrystallization temperature as much as possible, promote the mass production of recrystallization nuclei, and further achieve the purpose of grain refinement. At the same time, the crystallographic orientation of such fast recrystallized grains occurring in the high temperature region is often not as dependent on the old grains before recrystallization as the slow 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 having similar crystallographic orientations generated during hot rolling and cold rolling, and in achieving the second objective of the aforementioned intermediate annealing, suppressing the occurrence of so-called paint brush lines on the surface of the final sheet strip.

The cooling to room temperature at a rate of more than 30 ℃/sec is to suppress Mg during cooling ₂ The precipitation of Si phase particles grows up and is continuously subjected to one-step rapid heat treatment. Although the solution hardening treatment of the rear surface has solid solution Mg ₂ Ability of Si phase particles, however, if Mg is used in intermediate annealing due to limitation of length of heat-retaining region of solution quenching line ₂ The Si phase particles grow excessively, and it is difficult to make them sufficiently solid-dissolved completely. A rapid drop in temperature is required to limit their growth during the intermediate anneal.

Further, continuous rapid solution quenching treatment: the continuous rapid solution quenching treatment is carried out in one continuous treatment line consisting of alkali washing, water washing, solution quenching and subsequent stretching straightening processes. Wherein the solution hardening process is continuous rapid heat treatment of raising the temperature to 520-580 ℃ at a rate of more than 10 ℃/sec, and then reducing the temperature to room temperature at a rate of more than 30 ℃/sec after heat preservation for 40-120 seconds.

The continuous rapid solution quenching treatment line is particularly an aluminum alloy plate strip continuous rapid production line with the functions of unreeling, alkali washing, water washing, rapid heating, heat preservation, rapid cooling and reeling. The alkaline washing is carried out by adopting a mode of spraying 20+/-1 wt% sodium hydroxide aqueous solution with the temperature controlled at 55-65 ℃ on the two surfaces of the aluminum alloy plate strip continuously passing through the alkaline washing tank, wherein the spraying time is 8-14 seconds, so as to clean and remove surface greasy dirt, aluminum powder and the like generated during the early 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 cleaned aluminum alloy plate strip cannot meet the requirement, and the risk of surface defects during surface coating treatment is increased. In severe cases, the color of the finished sheet strip may also change. Conversely, if the concentration and temperature of the sodium hydroxide solution are higher, the spraying time is longer, and the surface layer of the aluminum alloy plate strip is excessively dissolved, so that unnecessary material loss is caused.

The alkali-washed and water-washed aluminum alloy sheet strip is then heated to 520-580 ℃ at a rate of more than 10 ℃/sec on an air-cushion type continuous rapid annealing line, and is cooled to room temperature at a rate of more than 30 ℃/sec after heat preservation for 40-120 seconds. The heating rate of more than 10 ℃/s is to ensure that recovery occurs as little as possible in the heating process, and the cold-rolled deformed energy storage which can become the driving force of recrystallization is kept to a high temperature area above the recrystallization temperature as much as possible. Thus, a high recrystallization nucleation rate can be obtained, and the grain size of the finished product after annealing is finer. If the temperature rise rate is less than 10 ℃/sec, the risk of grain size increase after annealing of the finished product increases. 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; mg precipitated during early rolling heat treatment ₂ The effect of re-dissolving the intermetallic compound particles such as Si is selected. If the temperature is too low and the heat preservation time is too short, recrystallization is incomplete, mg ₂ The solid solution degree of Si is insufficient, and finally, one or more of the tensile strength, the yield strength and the elongation of the finished product, the tensile strength and the yield strength after paint baking can not meet the requirements of the vehicle body plate. Conversely, if the temperature is too high, the material may be locally melted, and the elongation, press formability, flanging workability, and the like may be greatly deteriorated. The heat preservation time is too long, and no obvious bad part is caused except that the crystal grains slightly become larger. But is generally difficult to exceed 120 seconds because of equipment and production efficiency limitations. Without these restrictions, it is considered that the heat-retaining time is appropriately prolonged under the premise of ensuring that the crystal grains are not more than 60. Mu.m, as defined in claim 1).

Further, the aging treatment must be started within 2 hours after the solution quenching treatment, and the aging temperature is 70-100 ℃ for 4-9 hours. One purpose of this aging is to control the yield strength of the solution quenched aluminum alloy sheet strip to be as stable as possible under 140MPa for at least half a year before reaching the client to ensure that sufficient formability is still maintained in the press forming of the cover sheet. The other is to obtain the maximum strength increase of the cover plate after stamping during the baking varnish treatment.

The Al-Mg-Si aluminum alloy sheet strip after solution hardening is aged and precipitated in the form of GP zones even at ordinary temperature, and gradually and continuously causes an increase in yield strength. If the storage time at ordinary temperature is too long, the yield strength may rise to a point where the press formability is adversely affected. In addition, the internal structure of the GP zone changes with time and temperature, which affects the strength increase during the baking finish treatment. In general, the more so-called GP zones that are precipitated in the low-temperature region below 70 ℃, the smaller the strength increase will become. The more so-called GP regions that precipitate in the higher temperature region above 70 ℃, the greater the strength increase will become. However, if GP is too much, the yield strength of the sheet and strip exceeds 140MPa, which reduces press formability. The condition that the aging treatment of the patent must be started within 2 hours after the solution hardening treatment is to limit precipitation of the GP technical area. The aging temperature of 70-100 ℃ and the time of 4-9 hours are used for controlling the precipitation amount of the GP-type region within a proper range.

Further, in order to obtain a surface oxide film of high post quality, the surface coating treatment is continuously performed in the order of unreeling, acid washing, water washing, spray coating, drying, and reeling. The pickling is to clean oxide films and the like remaining on the surface of the aluminum plate strip during the preceding continuous rapid annealing. The pickling is carried out by spraying an aqueous solution of 0.5wt% sulfuric acid and 0.3wt% ammonium bifluoride at a temperature controlled between 50 and 60 ℃ onto both surfaces of an aluminum alloy plate strip continuously passing through a pickling tank for 6 to 12 seconds. The concentration of the pickling solution can be controlled by adjusting the conductivity of the liquid to 50.+ -.3 ms/cm and the fluoride ion concentration to 100-400 ppm. If the concentration or the temperature of the sulfuric acid and the ammonium bifluoride are low and the spraying time is short, the surface cleanliness of the cleaned aluminum alloy plate strip cannot meet the requirement. Conversely, if the sulfuric acid and ammonium bifluoride are at a higher concentration or temperature, the spraying time is longer, and the surface layer of the aluminum alloy sheet strip will be dissolved away too much, resulting in unnecessary material loss. The surface cleanliness of the aluminum plate strip after pickling is more than 2 levels (the lower the numerical value is, the higher the cleanliness is). If the cleanliness is less than level 2, the quality of the surface oxide film formed during the subsequent surface coating treatment will be poor, surface defects will occur, and not only will the defects be excessive, the risk of decreasing the corrosion resistance and durability of the aluminum alloy sheet for vehicle body will be increased, but also the gloss properties of the aluminum alloy sheet itself will be affected.

Spraying to obtain a reduced content of 1-12mg/m of zirconium metal and titanium metal ² Surface oxide films in the range. Can be obtained by coating or spraying a solution containing at least one or more zirconium fluoride compound and titanium fluoride compound in suitable concentrations onto the surface of the aluminum alloy sheet strip. The zirconium fluoride compound here means K ₂ ZrF ₆ 、(NH ₄ ) ₂ ZrF ₆ Isofluorozirconates, H ₂ ZrF ₆ Isofluorozirconic acid, and ZrF ₄ Zirconium fluoride, and the like. The titanium fluoride is K ₂ TiF ₆ ，(NH ₄ )TiF ₆ Isofluorotitanates, H ₂ TiF ₆ Isofluorotitanic acid, tiF ₄ Titanium fluoride, and the like. The specific coating conditions or spray conditions may be selected according to conventional surface treatment methods. For example, spraying is performed by using a mixed aqueous solution of 2wt% titanium fluoride and 4wt% zirconium fluoride at a temperature of 40-55 ℃ onto both surfaces of an aluminum alloy sheet strip continuously passing through a coating tank for 6-12 seconds. And then drying for 5-15 seconds by an infrared drying box, and winding. The concentration of the aqueous mixture for spraying can be controlled by adjusting the conductivity of the liquid to 1500-2500. Mu.s/cm and the pH to 3.5-4.5. However, no matter how the coating is carried out, the conversion content of the zirconium and the titanium on the surface after drying is necessarily ensured to be 1-12mg/m ² Within the range. If the content is low, the thickness of the surface oxide film is insufficient, and the corrosion resistance and durability cannot meet the requirements of the vehicle body plate strip. If the content is too high, the surface oxide film Can become brittle and crack easily. The corrosion resistance and durability required for the vehicle body panel are not obtained, and the manufacturing cost is increased due to the large amount of materials.

In order to meet the width requirement of large-sized vehicle body panels, the coating surface treatment of the invention is carried out by adopting treatment equipment with the treatment width of more than 1000-2200 mm. In order to ensure good oxide film quality, the whole surface film coating treatment processes such as acid washing, water washing, film coating, drying and the like are required to be completed continuously and rapidly at one time. If the aluminum strip stays intermittently in the middle, the residual dirt on the surface of the aluminum strip is easy to solidify, and the dirt is not easy to clean in the subsequent working procedures. Meanwhile, the disposable continuous rapid processing production line can also improve the yield and reduce the cost.

The invention has the remarkable advantages that according to the requirements of good baking varnish hardening performance, flanging and bending performance, stamping formability, as few surface defects as possible, excellent corrosion resistance and the like required by the plate strip for the automobile body, particularly the outer cover plate of the automobile body, the influences of the composition components, rolling conditions, heat treatment conditions, coating film and other surface treatment conditions are comprehensively and comprehensively considered, an aluminum alloy plate strip which can simultaneously and well meet the requirements of various special performances of the material for the automobile body, particularly the outer cover plate of the automobile body, and at least one side surface of the aluminum alloy plate strip is provided with at least 1 layer of oxide film containing at least 1 kind of zirconium and titanium as a protective layer, and the continuous, rapid and large-scale manufacturing method of the aluminum alloy plate strip is provided.

Drawings

FIG. 1 is a graph showing the corrosion conditions in the CASS corrosion test.

Detailed Description

The invention is further illustrated below in connection with specific examples, but the invention is not limited to these examples only.

Method for testing various performances

1) Tensile Strength, yield Strength and elongation

The tensile strength and elongation are measured based on the national standard GB/T16865. The long axis of the tensile sample is parallel to the rolling direction, the shape is punched according to the prescribed rectangular pattern of the national standard, and the elongation punctuation distance is 50mm. The loading speed at stretching was 10mm/min.

2) Average diameter of crystal grains

The average grain size was measured according to the GB/T3246.1 national standard method. And (3) after the cut sample is inlaid, selecting the vertical 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 far separated visual fields are randomly selected in the observation surface for polarized light microscopic observation measurement. The crystal grains observed in each field of view were measured for their diameters by the intercept method, and then the average value thereof was calculated.

3) Flanging performance detection

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

4) Number of intermetallic compound particles having diameter of more than 2 μm in terms of circle

Taking an area of more than 1cm at any position of the finished plate ² Sample 1 was subjected to surface polishing in the order of mechanical polishing, electrolytic polishing and the conventional conditions of aluminum alloy plate, 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 under the conventional conditions of 15kv accelerating voltage, 500 times magnification and back reflection electron acceptor. The intermetallic particles in the 10 field recordings were then counted and size measured using conventional image count measurement software, and the circle in unit area was generalized to be large in diameterThe number of intermetallic compound particles at 2. Mu.m.

5) Detection of surface defect paint brush line

The paint brush performance is detected by referring to the method of GB/T33227-2016 national standard annex D. The test piece is 90 degrees along the rolling direction, and rectangular samples with the length of 250mm and the width of 35mm are cut from the aluminum alloy plate to be tested. The sample is stretched at a rate of 3mm/min until reaching a strain of (10+/-2)%, and then the surface of the sample except the clamping part is integrally polished by a polishing tool along the length direction of the sample until the protruding part is bright, the polishing force is gentle and uniform, the surface of the sample is prevented from being scratched, and the polishing tool is preferably made of oilstone or millstone which is coated with enough lubricating oil and has a hard plane, and the granularity is F320. The surface of the sample was visually observed, and the paint line rating of the sample was determined. The first level has no obvious white stripes along the width direction; the second level has intermittent white stripes along the width direction; a small amount of through white stripes exist in the three stages along the width direction; the four stages have a large number of through white stripes in the width direction.

6) Surface cleanliness

The method is carried out based on a nonferrous metal industry standard YST 799-2012 aluminum plate foil surface cleanliness test method. And cutting a sample with the surface area not smaller than 15cm from the aluminum alloy plate strip to be detected, dripping not more than three drops of vegetable oil on the surface to be detected of the sample, and slightly pressing the oil drops by using a plexiglass disc so that the oil uniformly spreads over the surface of the sample. And folding the white tissue paper for 3-4 layers on the surface of the sample, pressing the white tissue paper by using a cylinder with a certain weight and contact area, and then moving the cylinder to wipe in a specified area, so that the aluminum powder foreign matters and residual oil dirt on the surface of the sample are adsorbed on the white tissue paper. The gray scale of the white tissue adhered with dirt is judged by comparing the colors of the white tissue and the standard KODAK GRAY SCALE gray scale color card. The higher the grey scale value, the lower the sample surface cleanliness. If the surface cleanliness is lower than the gray level 2, marking the surface as OK if the surface cleanliness is qualified; otherwise, the failure is marked as NG.

7) Testing of surface coating film

The measurement of the surface coating film was carried out using an XRF1800 type X fluorescence spectrometer manufactured by shimadzu corporation. The characteristic spectral lines generated by the sample are excited by the X-rays irradiating the flat sample surface. The secondary X-rays emitted from each element of the excited sample have a measured wavelength characteristic, and the amount of secondary X-rays generated is proportional to the content of the corresponding element in the irradiated region of the sample. If there is metallic zirconium or metallic titanium in the irradiated region, the content of metallic zirconium or metallic titanium in the irradiated region can be measured. By measuring and comparing the content of the metal zirconium or the metal titanium in the sample before and after the film coating, the weight of the metal zirconium or the metal titanium in the unit area of the film layer on the surface of the sample can be calculated.

8) Corrosion resistance evaluation

A rectangular sample with the length and width of 150×75 mm is cut from the finished aluminum alloy plate strip after surface treatment and subjected to electrophoresis treatment. The electrophoretic paint is a Guanzhu gray electrophoretic paint, the electrophoresis time is 3min, the electrophoresis voltage is 150V, and the baking time after electrophoresis is 160 ℃ for 10min. The cross-wires were scored with a 0.5mm blade on the specimen surface to a depth of the aluminum alloy substrate below the surface electrocoat layer. Subsequently, CASS corrosion test was performed, using 5wt% NaCl+0.26g/L CuCl as the corrosive solution ₂ The spraying time is 24 hours, and the pH value of the aqueous solution is 3.1-3.3. After CASS corrosion test, the test specimen was taken out, and the corrosion condition of the edge of the scribing intersection line was observed. If the occurrence of the unilateral expansion is not seen, the corrosiveness is evaluated as OK. If one-sided expansion occurs, the corrosiveness is evaluated as NG, as shown in fig. 1.

Example 1

1) Composition control

The ingredients specified in Table 1 were mixed and melted by a conventional method, and then deaerated, deslagged and added with Al-Ti-B grain refiner by a conventional method, and a flat ingot with a width of 2100mm and a thickness of 650mm was cast by a semi-continuous casting method. After homogenization treatment at 540 ℃ for 8 hours, hot rolling is finished by rolling at 300 ℃ after hot rolling to 6 mm. After the subsequent cold rolling to 3.0mm, an intermediate annealing treatment was performed. The intermediate annealing is performed by a continuous heat treatment line that rapidly increases and decreases in temperature. Raising the temperature to 480 ℃ at a rate of more than 10 ℃ per second, keeping the temperature for 60 seconds, and then cooling at 20 ℃/s The cooling rate was cooled to room temperature. Subsequently, after continuing cold rolling to a finished product thickness of 1.0mm, alkali washing, water washing and solution quenching treatment are performed on a continuous rapid solution quenching treatment line. The alkaline washing is carried out by spraying for 10 seconds at 55 ℃ with 20wt% sodium hydroxide aqueous solution and then washing. The solution hardening is carried out by adopting an air cushion type continuous rapid annealing line, the temperature is kept for 70 seconds after the temperature is raised to 550 ℃ at the speed of 12 ℃/sec, and the temperature is cooled to the normal temperature at the speed of 25 ℃/sec. Thereafter, about 0.4% stretch straightening was performed under conventional conditions and then aging was started. The time from completion of solution hardening to the start of aging treatment is in the range of 1 hour 25 to 50 minutes for all materials. Aging treatment is carried out by preserving heat at 90 ℃ for 8 hours. Finally, the aluminum alloy plate coil is continuously subjected to surface coating treatment by adopting a surface continuous treatment line according to the sequence of acid washing, water washing, spraying and drying. The pickling was performed by continuously spraying a solution of 0.5% sulfuric acid+0.3% ammonium bifluoride in water at 55 ℃ onto both surfaces of the aluminum alloy sheet strip passing through the pickling tank for 9 seconds. The spraying was performed by continuously spraying a mixed solution of 2% titanium fluoride and 4% zirconium fluoride at 50 c onto both surfaces of the aluminum alloy sheet strip passing through the coating tank for 10 seconds. And then drying for 5-15 seconds by an infrared drying box, and winding. Various aluminum alloy sheet strips for vehicle bodies with a surface oxide film as a protective layer were obtained. The surface oxide film amount (single side) of the plate and strip materials is 5.6-6.7mg/m ² Range.

Finally, samples of these processed aluminum alloy sheet strips were taken to examine various mechanical properties, internal structural parameters and paint line grades. The mechanical properties (tensile strength, yield strength, elongation) and flanging properties, which significantly changed with the room temperature storage time, and the tensile strength and yield strength after the baking finish equivalent heat treatment were measured by sampling from aluminum alloy sheet strips stored at room temperature for 7 days and 180 days after the surface coating treatment process was completed, and the results are shown in tables 2 and 3, respectively. The average grain diameter, the number of intermetallic compound particles having a diameter of more than 2 μm in terms of circles and the paint line grade, which do not significantly change with the increase of the storage time at room temperature, were measured by sampling from an aluminum alloy sheet strip stored at room temperature for 7 days after the surface coating treatment process was completed. The corrosion resistance is generally a high possibility of deterioration of materials stored for a long period of time at room temperature, and this measurement is performed by sampling from an aluminum alloy sheet strip stored for 180 days at room temperature. The measurement results of the average grain diameter, the number of intermetallic compound particles having a diameter in terms of circles of more than 2 μm, and the corrosion resistance are also shown in table 2 or table 3. And the measurement results of the paint brush line grade, including all the materials of the examples and the comparative examples are grade 1, all meet the requirements of the aluminum alloy plate strip for the vehicle body.

As can be seen from a combination of the results of tables 1-3, the materials having the composition within the scope of the patent claims all have suitable grain size and less coarse intermetallic particles, and all have good mechanical properties, flanging properties and strength increase after a considerable heat treatment of baking finish when stored for no more than 180 days at room temperature. If a certain alloy composition is out of the scope of the patent claims, the risk of oxide film defects increasing is increased due to either higher yield strength, lower flanging performance, or more coarse intermetallic compound particles.

2) Intermediate annealing treatment condition control

Using the same composition and processing heat treatment conditions as in example 1 of Table 1, a cold rolled sheet coil of 2.0mm thickness was produced by melting casting, homogenization treatment, hot rolling and cold rolling, and then an intermediate annealing treatment was performed under the conditions of Table 4. And then cold-rolled to a finished thickness of 1.0mm in 3 further passes. The edge cracking condition after each cold rolling was evaluated. If a crack of more than 0.5mm is observed at the edge, NG is determined, and if not, OK is determined, and the results are also shown in table 4. Subsequently, as in example 1 of Table 1, continuous rapid solution hardening treatment, stretching straightening, aging treatment, and surface coating treatment were performed to obtain various aluminum alloy sheet strips for automobile bodies having a surface oxide film. Finally, the sampling performed a grade test for surface defects "paint lines". The results of the detection are also shown in Table 3.

As is clear from the results in Table 4, when 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 'paint brush line', if the temperature rising speed, the annealing temperature, the heat preservation time and the cooling speed are not in the scope of the patent requirement, the degree of the paint brush line is increased.

3) Continuous rapid solution quenching process condition control

Using the same composition and the same processing heat treatment conditions as in example 1 of Table 1, a cold rolled sheet roll having a finished thickness of 1.0mm was produced by melting casting, homogenization treatment, hot rolling, cold rolling, intermediate annealing, and secondary cold rolling. Then, alkali washing, water washing and rapid solution quenching treatment are carried out on a continuous rapid solution quenching heat treatment line. Wherein, the alkali washing was performed under the conditions shown in Table 5 and the solution hardening treatment was performed under the conditions shown in Table 6. In Table 5, the alkali concentration OK represents that the concentration of the sodium hydroxide solution is within a suitable range of 20.+ -. 1%, and NG represents that it is not within this suitable range. The cleaning state of the material surface was evaluated by sampling after alkali washing, and the evaluation results are also shown in table 5. From the results of table 5, it is found that if the alkaline washing conditions are not suitable, the surface cleanliness cannot be satisfied.

The aluminum alloy plate strip coil after the continuous and rapid solution quenching treatment is subjected to continuous stretching straightening, ageing treatment and surface continuous coating treatment under the same conditions of component control of 1), so that the aluminum alloy plate strip with the surface oxide film is obtained. The results of the properties measured from these plate and strip samples were also shown in table 6, stored at room temperature for 7 days. As can be seen from table 6, when the heating rate, solution quenching temperature, holding time or cooling rate is not within the range required by the present patent, either a certain index of mechanical properties is not satisfied, or the detection level of the flanging experiment is lowered, or the grain diameter is enlarged, so that the performance requirements of the vehicle body panel cannot be satisfied well.

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4) Aging treatment condition control

The aluminum alloy sheet strip with surface oxide film was obtained by subjecting the same composition and working heat treatment conditions as those of example 1 of Table 1 to melting casting, homogenization treatment, hot rolling, cold rolling, intermediate annealing, secondary cold rolling, continuous rapid solution quenching heat treatment, and stretching and straightening, aging treatment was performed under the conditions of Table 7, and then surface continuous coating treatment was continued under the same conditions as those of "1) composition control". The results of the properties measured from these plate and strip samples were also shown in Table 7 after storage at room temperature for 7 days. As is clear from table 7, if the conditions of the time from solution hardening to aging start, aging temperature, and holding time are not within the range of the present patent requirements, either a certain index of mechanical properties is not satisfied, the test grade of the flanging test is lowered, or the tensile strength or yield strength after the baking varnish is quite heat-treated is not satisfied.

5) Surface coating treatment condition control

Aluminum alloy sheet strips having a thickness of 1.0mm were produced by the same composition and working heat treatment conditions as in example 1 of Table 1 through the steps of melting casting, homogenization treatment, hot rolling, cold rolling, intermediate annealing, secondary cold rolling, continuous rapid solution quenching heat treatment, and stretch straightening, and aging treatment. Subsequently, surface coating treatment was continuously performed in the order of unreeling, acid washing, water washing, spray coating, drying, and reeling on a continuous surface treatment line under the conditions of table 8, to obtain aluminum alloy plate strips for vehicle body having oxide films of various properties and thicknesses. The pickling solution concentration OK in Table 8 represents that the conductivity of the liquid is within 50.+ -.3 ms/cm, and the fluoride ion concentration is adjusted within a proper range of 100-400 ppm; NG represents that the conductivity and the fluoride ion concentration are more than one outside the aforementioned suitable ranges. Likewise, spray concentration OK represents that the conductivity of the liquid is within 1500-2500 mu s/cm, and the PH value is adjusted within a proper range of 3.5-4.5; NG represents that the conductivity and PH are above the appropriate range. The surface cleanliness was also tested by sampling at the post-pickling stage, and the results are also shown in table 8.

Finally, from the aluminum alloy sheet strip treated with the above surface coating film, the amount of oxide film and corrosion resistance were evaluated, and the evaluation results are also shown in table 8. From the results of Table 8, it can be seen that if the conditions for pickling or spraying are outside the scope of the patent claims, the surface cleaning degree after pickling is insufficient, or the amount of the surface oxide film is too large or too small, which causes a decrease in corrosion resistance.

The above examples are only for clarity of illustration of the present invention, and the embodiments of the present invention are not limited thereto. Any modification, replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. Preparation method of 6-series aluminum alloy plate strip with high durability for automobile bodyThe method is characterized in that: weighing raw materials according to the proportion of each component, casting into a flat large ingot by adopting a semi-continuous casting method, homogenizing, hot rolling, cold rolling, intermediate annealing treatment, secondary cold rolling, continuous rapid solution quenching treatment, stretching straightening and aging treatment, and then carrying out surface coating treatment to obtain a 6-series aluminum alloy plate strip with high durability for an automobile body; the aluminum alloy plate strip is provided with 1 layer of oxide film containing at least 1 kind of zirconium and titanium on at least one side surface, wherein the total content of zirconium and titanium in the oxide film is calculated as zirconium metal and titanium metal: 1-12mg/m ² ；

The intermediate annealing treatment is to heat up to 450-520 ℃ at a speed of more than 10 ℃/s, heat preservation is carried out for 30-120 seconds, and then the temperature is reduced to room temperature at a speed of more than 30 ℃/s;

the continuous rapid solution quenching treatment comprises alkali washing, water washing and solution quenching, wherein the alkali washing is performed by adopting a mode of spraying a 20+/-1 wt% sodium hydroxide solution at 55-65 ℃ on two surfaces of an aluminum alloy plate strip, and the spraying time is 8-14 seconds; the solution hardening is to heat up to 520-580 ℃ at a speed of more than 10 ℃/s, heat preservation is carried out for 40-120 seconds, and then the solution hardening is cooled to room temperature at a speed of more than 30 ℃/s;

the aging treatment is carried out within 2 hours after solution quenching treatment, the aging treatment temperature is 70-100 ℃ and the time is 4-9 hours;

the surface coating treatment is that firstly, the aluminum alloy plate strip is washed by acid and water, then at least one of zirconium fluoride compound and titanium fluoride compound solution is coated or sprayed on the surface of the aluminum alloy plate strip, and then the aluminum alloy plate strip is dried;

the pickling is carried out by spraying aqueous solution of 0.5wt% sulfuric acid and 0.3wt% ammonium bifluoride on the two surfaces of the aluminum alloy plate strip at 50-60 ℃ for 6-12 seconds.

2. The method for producing a 6-series aluminum alloy sheet strip for automotive body having high durability according to claim 1, characterized in that: 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 unavoidable impurity elements.

3. The method of manufacturing according to claim 1, characterized in that: 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.

4. The method of manufacturing according to claim 1, characterized in that: the tensile strength of the 6-series aluminum alloy plate strip for the automobile body with high durability 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 of flanging performance detection is more than 2, the average grain diameter is less than or equal to 60 mu m, and the number of intermetallic compound particles with the diameter of more than 2 mu m in terms of circles is less than 2000/mm ² The tensile strength of the baking varnish after the equivalent heat treatment is more than or equal to 260MPa, and the yield strength is more than or equal to 190MPa.