CN115354179B - 6xxx aluminum alloy plate, and preparation method and application thereof - Google Patents

Abstract

The invention provides a 6xxx aluminum alloy plate, a preparation method and application thereof. The preparation method comprises the steps of casting and carrying out homogenization heat treatment on raw materials of a 6xxx aluminum alloy plate to obtain a homogenized cast ingot, carrying out hot rough rolling on the homogenized cast ingot for at least n times, wherein n is 7 to 20, the reduction rate of each time is sequentially increased, the reduction rate of the 1 st time is not more than 5%, the reduction rate of the 2 nd time is not less than 6%, the reduction rates of the n-2 nd, the n-1 st and the n-th times are not less than 30%, and the average reduction rate of the times is not less than 16%, and sequentially carrying out post-treatment on the obtained hot rough rolled plate to obtain the 6xxx aluminum alloy plate. According to the invention, the hot rough rolling pass reduction rate is strictly limited, the metal structure of the hot rough rolling plate is improved, the plate crystal grains are refined, the Goss texture at the edge part is eliminated, the Cube texture at the middle part is reduced, the plate structure is integrally improved, and the prepared 6xxx aluminum alloy plate is qualified in paint brushing performance and excellent in surface quality.

Description

6xxx aluminum alloy plate, and preparation method and application thereof

Technical Field

The invention relates to the technical field of aluminum alloy manufacturing, in particular to a 6xxx aluminum alloy plate, and a preparation method and application thereof.

Background

The light weight of the automobile is an important means for saving energy, reducing emission and reducing pollution. Aluminum is the most abundant green metal in the earth, and the aluminum alloy plate is considered as an ideal material for automobile light weight due to light weight, high specific strength and excellent comprehensive performance. The aluminum alloy is used for replacing steel, so that the weight reduction effect of more than 30% of parts in the automobile can be realized. The 5xxx aluminum alloys have good formability, but they tend to produce luders after stamping, which is highly aesthetically undesirable, and thus the 6xxx series aluminum alloys are widely used as automotive outer covers. The 6xxx aluminum alloy has good forming performance and can meet the stamping requirement, the inner plate is coated by the good edge covering performance, and more importantly, the 6xxx aluminum alloy has excellent surface quality and can meet the appearance requirement.

The quality of an aluminum alloy cast ingot is generally more than 5 tons under the industrial production condition, and the production process of the 6xxx series automobile plate comprises the following steps: semi-continuous casting, homogenization, hot rolling, cold rolling, intermediate annealing, final cold rolling, solid solution and pre-aging. The outer cover 6xxx sheet material is required to have excellent surface quality, and the surface of the sheet material after stamping can not have surface defects such as paint brush lines, so that a preparation method of the 6xxx aluminum alloy sheet is required to be provided, and the surface quality of a finished sheet material is improved. The hot rolling process is one of core processes for controlling the surface quality of the plate, particularly the hot rough rolling process, and the key for ensuring the excellent quality of the final finished plate is how to regulate and control the process so as to obtain an excellent metal structure. In recent years, much research has been conducted on how to improve the performance of 6xxx automotive panel paint-brush wires: CN 104532077B provides a short-flow preparation method of 6xxx series aluminum alloy automobile body plates without a painted and brushed line, the innovation of the method is mainly realized by preparing plates without the painted and brushed line under the condition without intermediate annealing, and the influence of a hot rough rolling process on the painted and brushed line is not concerned. CN 113166858A proposes a method for producing 6xxx aluminum sheet with high surface quality, mainly aiming at the design of inner cover sheet, which has lower requirement on surface quality than the outer cover sheet. CN 112461746A provides a painted line display method, which mainly aims at how to better evaluate painted lines and does not relate to a preparation process.

Disclosure of Invention

The invention mainly aims to provide a 6xxx aluminum alloy plate, a preparation method and application thereof, and aims to solve the problem of insufficient paint brushing performance of the aluminum alloy plate of an automobile outer covering part in the prior art. In the surface plate structure along the wide edge of the plate, the average grain size along the ND direction (normal direction) of the hot rough rolling plate prepared by the method is less than 24 mu m, and the Goss orientation ratio is less than 7 percent; in the plate structure at 1/4 of the plate thickness along the wide side part of the plate, the average grain size along the ND direction (normal direction) is less than 27 μm, and the Goss orientation ratio is less than 11%; in the surface plate structure along the middle position of the plate width, the average grain size along the ND direction (normal direction) is less than 72 mu m, and the Cube orientation ratio is less than 14 percent; in the plate structure at the position of 1/4 of the plate thickness along the middle of the plate width, the average grain size along the ND direction (normal direction) is less than 72 μm, and the Cube orientation ratio is less than 5%. The finished plate produced by the subsequent procedures of hot finish rolling, primary cold rolling, intermediate annealing, finished product cold rolling, continuous annealing, pre-aging treatment and the like has a paint brush line grade superior to 2 grade at each position along the TD direction (transverse direction), and meets the use requirement of the outer plate of the automobile plate covering part.

In order to achieve the above object, according to one aspect of the present invention, there is provided a method of producing a 6xxx aluminum alloy sheet, the composition of the aluminum alloy sheet includes, in mass percent, si 0.4 to 1.5wt.%; mg 0.2 to 1.0wt.%; cu ≦ 0.3wt.%; mn ≦ 0.2wt.%; cr is less than or equal to 0.2wt.%, fe is less than or equal to 0.3wt.%, the balance is Al and inevitable impurities, each inevitable impurity is less than 0.05wt.%, and the total impurities are less than 0.15wt.%; the preparation method comprises the following steps: step S1, casting a raw material of a 6xxx aluminum alloy plate to obtain an ingot; s2, carrying out homogenization heat treatment on the ingot to obtain a homogenized ingot; s3, carrying out hot rough rolling on the homogenized cast ingot to obtain a hot rough rolled plate, wherein the hot rough rolling comprises at least n passes, n is 7-20, the reduction rate of each pass is sequentially increased, the reduction rate of the 1 st pass is less than or equal to 5%, the reduction rate of the 2 nd pass is more than or equal to 6%, the reduction rates of the n-2 nd pass, the n-1 st pass and the n-th pass are more than or equal to 30%, and the average pass reduction rate is more than or equal to 16%; and S4, sequentially carrying out hot finish rolling, primary cold rolling, intermediate annealing, secondary cold rolling, continuous annealing and pre-aging treatment on the hot rough rolled plate to obtain the 6xxx aluminum alloy plate.

Furthermore, in the step S3, the rolling strain rate of each pass is sequentially increased, the rolling strain rate of the 1 st pass is 0.5 to 0.9/S, the rolling strain rate of the 2 nd pass is 0.6 to 1.0/S, the rolling strain rate of the n-2 nd pass and the rolling strain rate of the n-1 st pass are more than or equal to 10/S, and the rolling strain rate of the n th pass is more than or equal to 20/S.

Further, the thickness of the cast ingot is 450 to 700mm, the width w of the hot rough rolling plate is 0.8 to 3.0m, the thickness t is 20 to 35mm, and the total deformation in the hot rough rolling process is more than or equal to 95 percent.

Further, in step S3, the rolling direction of the hot rough rolled plate material is defined as RD direction, the transverse direction is defined as TD direction, the normal direction perpendicular to the RD direction and the TD direction is defined as ND direction, the surface perpendicular to the TD direction of the hot rough rolled plate material is defined as TD plane, the surface perpendicular to the RD direction of the hot rough rolled plate material is defined as RD plane, the surface perpendicular to the ND direction of the hot rough rolled plate material is defined as ND plane, a region within w/20 of the hot rough rolled plate material in the TD direction from the TD plane is defined as a first region, a region within t/10 of the ND direction from the ND plane is defined as a second region, and in the plate material structure of the overlapped region of the first region and the second region, as viewed from the TD plane, the average grain size in the ND direction is less than 24 μm, and the Goss orientation ratio is less than 7%; the area within t/5~t/4 along the ND direction is defined as a third area, and in the plate material structure of the overlapping area of the first area and the third area, the average grain size along the ND direction is less than 27 μm and the Goss orientation ratio is less than 11% when observed from the TD plane.

Further, in step S3, in the plate structure of the area of the hot rough rolled plate, which is within w/2 +/-w/10 of the TD plane along the TD direction, is defined as a fourth area, and in the overlapped area of the second area and the fourth area, the average grain size along the ND direction is less than 72 μm and the Cube orientation ratio is less than 14% when observed from the TD plane; in the plate material structure of the overlapping region of the third region and the fourth region, the average grain size in the ND direction is less than 72 μm and the Cube orientation ratio is less than 5% when observed from the TD plane.

Further, in step S2, the homogenization heat treatment includes the steps of: heating the cast ingot to 530 to 560 ℃ at the speed of 10 to 30 ℃/h, and keeping the temperature for 6 to 24h; then cooling to 500-520 ℃ at the speed of 10-30 ℃/h, and preserving heat for 1-10 h.

Further, in step S3, the hot rough rolling is directly discharged from a furnace for hot rolling after the homogenization heat treatment, the initial rolling temperature of the hot rough rolling is the same as the final heat preservation temperature of the homogenization heat treatment, and the final rolling temperature of the hot rough rolling is more than or equal to 400 ℃.

Further, step S2 includes, before the homogenization heat treatment, a step of sequentially sawing and milling the ingot.

According to another aspect of the present invention, there is provided a 6xxx aluminum alloy sheet, obtained by the preparation method of the present invention; preferably, the 6xxx aluminum alloy sheets have a paint brush line rating throughout the transverse direction that is better than grade 2.

According to another aspect of the present invention, there is provided a use of the above-described 6xxx aluminum alloy sheet in an automotive outer cover.

Compared with the prior art, the invention is mainly different from the prior art in that:

1. and (4) finely controlling the hot rough rolling process. According to the invention, through strictly limiting the pass reduction rate of hot rough rolling, further regulating and controlling technological parameters such as rolling speed, rolling temperature and the like, the hot rough rolling processing process of the plate is controlled, the metal structure of the hot rough rolling plate is improved, the crystal grains of the hot rough rolling plate are refined, and the prepared 6xxx aluminum alloy plate has no paint brush lines along the TD direction (transverse direction) and has excellent surface quality.

2. Goss texture and Cube texture are regulated and controlled in different areas. According to the invention, aiming at the difference of the principle of painting and brushing the wire at the edge part and the middle part of the aluminum alloy plate, the crystal grains are refined by regulating and controlling the reduction rate of the pass of hot rough rolling and further regulating and controlling the technological parameters such as rolling speed, rolling temperature and the like, the Goss texture at the edge part is eliminated, the Cube texture at the middle part is reduced, and the plate texture is integrally improved, so that the painting and brushing performance of the finished plate is improved.

Compared with the prior art, the invention has the following beneficial effects:

1. the preparation method provided by the invention aims at large-size and heavy ingots required by industrial production, and further sets a cooling step after homogenization heat treatment, so that the surface damage caused by aluminum adhesion of high-temperature aluminum ingots in the hot rolling process is avoided; the further arrangement of the cooling and heat preservation steps ensures that the temperatures of all parts of the large ingot are consistent, and avoids the phenomenon that the temperature difference between the surface and the core of the plate is large when the plate is directly discharged from the furnace for air cooling, so that the structures of all parts after hot rolling are different.

2. The invention strictly controls the deformation of hot rough rolling in the range of the capacity of industrial equipment, further controls the rolling speed and the rolling temperature, accords with the conditions of large-scale industrial production, does not increase or change the original equipment at the same time, and can be realized under the existing conditions.

3. Compared with other automobile plate production processes, the hot rough rolling process provided by the invention has the advantages that the pass reduction rate is improved, the rolling speed is further increased, the sales volume is increased, the productivity is increased, and the hot rough rolling process conforms to the concept of energy conservation, emission reduction, environmental protection and low-carbon manufacturing.

4. The hot rough rolling process strictly controls the structure of the hot rough rolled plate, thereby ensuring that the finished plate has excellent comprehensive properties such as forming property, flanging property, painting and brushing property and the like.

Drawings

The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate embodiments of the invention and, together with the description, serve to explain the invention and not to limit the invention. In the drawings:

FIG. 1 shows various directions and faces of a hot rough rolled sheet according to one embodiment of the present invention;

FIG. 2 illustrates an area of coincidence of a first area with a second area, and an area of coincidence of a first area with a third area, according to one embodiment of the present invention;

FIG. 3 illustrates an area of coincidence of a second area with a fourth area, and an area of coincidence of a third area with a fourth area, according to one embodiment of the present invention;

FIG. 4 illustrates a paint brush line standard grade schematic;

FIG. 5 shows the Goss texture distribution of the overlapping area of the first area and the second area of the hot rough rolled sheet according to example 1 of the present invention;

fig. 6 shows a Goss texture distribution of a first area and a second area of the hot rough rolled sheet of comparative example 1 at the overlapping area;

FIG. 7 shows Cube texture distribution of the overlapping area of the second area and the fourth area of the hot rough rolled plate according to example 1 of the present invention

Fig. 8 shows Cube texture distribution in the overlapping area of the second area and the fourth area of the hot rough rolled plate of comparative example 1.

Detailed Description

It should be noted that, in the present application, the embodiments and features of the embodiments may be combined with each other without conflict. The present invention will be described in detail below with reference to the accompanying drawings in conjunction with embodiments.

Interpretation of terms:

pass reduction rate: the thickness before each pass of rolling is H, the thickness after each pass of rolling is H, and the percentage (%) of the ratio of the pass reduction delta H to the thickness H of the rolled piece before the current pass of rolling is the pass reduction = delta H/H x 100.

Rolling strain rate: the thickness before each pass of rolling is H, the thickness after each pass of rolling is H, the rolling linear velocity is v, the radius of the roller is R, and the rolling strain rate =2 xv x (((H-H) x R) ^1/2 ÷(H＋h)。

Goss oriented grain: in the crystal orientation analysis by the electron back scattering diffraction method, the crystal grains have an orientation shifted by 11 ° or less from the Goss orientation {011} <100 >.

Cube-oriented grains: in the crystal orientation analysis by the electron back scattering diffraction method, crystal grains having an orientation shifted within 11 ° from the Cube orientation {001} <100 >.

Average crystal grain size in ND direction: the average grain size in the ND direction is obtained by dividing the unit length along the ND direction by the number of grains in the length, and the number of the grains is more than 500 for ensuring the accuracy.

As described in the background of the invention, there is a problem in the prior art that the aluminum alloy sheet of the outer cover of the automobile has insufficient paintability. The painted lines are seen from the surface as one line, and actually are the concave-convex fluctuation of the surface of the plate, and the essence is the surface fluctuation caused by uneven deformation of the aluminum alloy plate during preparation. The inventor unexpectedly discovers in the research process that in the process of producing the aluminum alloy plate by industrial hot rough rolling, the phenomenon that the heat dissipation of the edge part of the plate is faster than that of the middle part of the plate inevitably occurs, so that the structure difference is generated, the structure difference between the edge part and the middle part is inherited to cause different reasons for producing the paint brushing line on the finished plate, the edge part is caused by the fact that the Goss texture is too high, and the middle part is caused by the fact that the Cube texture is too high. Therefore, in order to solve the above problems, in an exemplary embodiment of the present invention, there is provided a method for producing a 6xxx aluminum alloy sheet, wherein the composition of the aluminum alloy sheet comprises, by mass, 0.4 to 1.5wt.% of Si; mg 0.2 to 1.0wt.%; cu ≤ 0.3wt.%; mn ≤ 0.2wt.%; cr is less than or equal to 0.2wt.%, fe is less than or equal to 0.3wt.%, and the balance is Al and inevitable impurities, such as Na, zn, ca and the like, wherein each inevitable impurity is less than 0.05wt.%, and the total impurities are less than 0.15wt.%.

The preparation method comprises the following steps: step S1, casting a raw material of a 6xxx aluminum alloy plate to obtain an ingot; s2, carrying out homogenization heat treatment on the ingot to obtain a homogenized ingot; s3, carrying out hot rough rolling on the homogenized cast ingot to obtain a hot rough rolled plate, wherein the hot rough rolling comprises at least n passes, n is 7-20, the reduction rate of each pass is sequentially increased, the reduction rate of the 1 st pass is less than or equal to 5%, the reduction rate of the 2 nd pass is more than or equal to 6%, the reduction rates of the n-2 nd pass, the n-1 st pass and the n nd pass are more than or equal to 30%, such as 30-45%, and the average pass reduction rate is more than or equal to 16%, such as 16-20%; and S4, sequentially carrying out hot finish rolling, primary cold rolling, intermediate annealing, secondary cold rolling, continuous annealing and pre-aging treatment on the hot rough rolled plate to obtain the 6xxx aluminum alloy plate.

Si and Mg are used as main strengthening elements of the 6xxx aluminum alloy, and when the content is too low, the strengthening effect may be insufficient, and when the content is too high, the strength may be too high, which is harmful to the safety of pedestrians. When the contents of Si and Mg are limited to the above ranges, both high surface quality and moderate strength can be obtained. The above-mentioned content of Cu can also improve the strength by solid solution strengthening and improve the formability. Mn, cr and Fe are effective elements for improving strength, refining crystal grains and stabilizing a structure, when the content of Mn, cr and Fe is too low, the effect is insufficient, when the content of Mn, cr and Fe is too high, various intermetallic compounds are generated, and adverse effects are generated on forming performance, so that the content of Mn, cr and Fe is limited to the range to improve metal structure and refine plate crystal grains, and the preparation method is more suitable for preparing 6xxx aluminum alloy, and the 6xxx aluminum alloy plate with more excellent painting linear performance is obtained.

Pure aluminum and various intermediate alloys are firstly smelted and cast according to the component proportion to obtain cast ingots, then the cast ingots are put into a heat treatment furnace for homogenization treatment, the cast ingots are taken out of the furnace after the homogenization treatment, hot rough rolling is directly carried out, the hot rough rolling processing process of the plate is controlled by strictly limiting the reduction rate of hot rough rolling passes, the metal structure of the hot rough rolled plate is improved, and the grains of the hot rough rolled plate are refined; after the hot rough rolling is finished, performing hot finish rolling, primary cold rolling and intermediate annealing on the obtained hot rough rolled plate, and performing secondary cold rolling, continuous annealing and pre-aging treatment to obtain a finished product 6xxx aluminum alloy plate, wherein the prepared aluminum alloy plate has no paint brush line along the normal direction and has excellent surface quality; wherein, the subsequent treatment of the obtained hot rough rolled plate can be realized by using the conventional process conditions in the field, and the details are not repeated.

Preferably, the hot rough rolling comprises at least n passes, wherein n is 11 to 16, the reduction ratios of the passes are sequentially increased, the reduction ratio of the 1 st pass is 3~5%, the reduction ratio of the 2 nd pass is 6~8%, the reduction ratio of the n-2 nd pass is 31 to 39%, the reduction ratio of the n-1 st pass is 31 to 41%, the reduction ratio of the n th pass is 35 to 45%, and the paint brush linear performance is better.

In a preferred embodiment, in step S3, the rolling strain rate of each pass sequentially increases, the rolling strain rate of the 1 st pass is 0.5 to 0.9/S, the rolling strain rate of the 2 nd pass is 0.6 to 1.0/S, and the rolling strain rates of the n-2 nd pass and the n-1 st pass are not less than 10/S, such as 10 to 28/S; the strain rate of the nth pass rolling is more than or equal to 20/s, such as 20 to 45/s. The reduction rate and the rolling strain rate of hot rough rolling are increased along with the increase of the pass, so that the shearing zones at the edge part are eliminated, the Goss texture is reduced, the middle crystal grains are refined, the Cube texture is reduced, the random texture ratio is increased, and the plate texture is integrally improved. Wherein the smaller reduction rate and the smaller rolling strain rate of the front section are convenient for biting into the ingot blank, and the more sufficient time is provided between passes to improve the recrystallization degree, thereby further obtaining fine grains which are uniformly distributed; the uniformity and the recrystallization degree of the grain size are further ensured by the gradually increased reduction rate and rolling strain rate of the intermediate pass; the rear section adopts a large reduction rate and a fast rolling strain rate, so that the grain size can be further uniform and refined while the rolling temperature is ensured, the organization structures of the edge part and the middle part of the plate are improved, the performance of painting a wire is improved, the rolling efficiency can be further improved, and the productivity is increased.

Preferably, the rolling strain rate of each pass is gradually increased, the rolling strain rate of the 1 st pass is 0.5 to 0.8/s, the rolling strain rate of the 2 nd pass is 0.7 to 1.0/s, the strain rate of the n-2 nd pass is 10 to 20/s, and the rolling strain rate of the n-1 st pass is 13 to 28/s; the strain rate of the nth pass rolling is 21-45/s, so that the grain size of the aluminum alloy plate can be more uniform and refined, and the rolling efficiency is higher.

As described above, the preparation method provided by the invention can aim at large-size large-weight large-size ingots required by industrial production, and in order to be more matched with the existing industrial production equipment, in a preferred embodiment, the thickness of the ingots is 450 to 700mm, the width w of the hot rough rolling plate is 0.8 to 3.0m, the thickness t is 20 to 35mm, and the total deformation of the hot rough rolling process is not less than 95%, so that the hot rolling pass reduction rate can be more finely and gradually controlled on the premise of adapting to production, the aluminum alloy structure is more uniform, the crystal grains are more refined, and the paint brushing performance of the aluminum alloy plate is further improved.

In a preferred embodiment, in step S3, as shown in fig. 1 and 2, the rolling direction of the hot rough rolled plate is defined as RD direction, the transverse direction is defined as TD direction, the normal directions perpendicular to RD direction and TD direction are defined as ND direction, the surface perpendicular to TD direction of the hot rough rolled plate is defined as TD plane, the surface perpendicular to RD direction of the hot rough rolled plate is defined as ND direction, the surface perpendicular to RD direction of the hot rough rolled plate is defined as DR plane, the surface perpendicular to ND direction of the hot rough rolled plate is defined as ND plane, the area of the hot rough rolled plate within w/20 of the TD direction from the TD plane is defined as a first area, the area within w/20 of the hot rough rolled plate along the TD direction from the TD plane is defined as DR plane, the area within t/t direction from the ND plane is defined as ND plane, the area of the hot rough rolled plate within w/20 of the TD direction from the TD plane is defined as a second area, and the grain orientation ratio of the second area of the Goss plane is smaller than the second area (i.e. the grain width of the second area observed area); the area within t/5~t/4 from the ND plane in the ND direction is defined as a third area, and in the plate material structure of the overlapping area of the first area and the third area (i.e., 1/4 of the plate thickness along the wide side portion of the plate), the average grain size in the ND direction is less than 27 μm and the Goss orientation ratio is less than 11% as viewed from the TD plane.

On the other hand, the hot rough rolling process can further refine the middle crystal grains, reduce the Cube texture and increase the random texture proportion, thereby integrally improving the plate texture and improving the wire painting performance of the finished plate. In a preferred embodiment, in step S3, as shown in fig. 1 and 3, a region of the hot rough rolled plate material within w/2 ± w/10 from the TD plane in the TD direction is defined as a fourth region, and in the plate material structure of the overlapping region of the second region and the fourth region (i.e., the surface at the middle position of the plate width), the average grain size in the ND direction is less than 72 μm and the Cube orientation ratio is less than 14% as viewed from the TD plane; in the plate material structure of the overlapping region of the third region and the fourth region (i.e., 1/4 of the plate thickness at the middle position along the plate width), the average grain size in the ND direction is less than 72 μm and the Cube orientation ratio is less than 5% when observed from the TD plane.

In order to further improve the structural uniformity of the ingot structure, the preparation method of the invention aims at large-size and heavy ingots required by industrial production, and in a preferred embodiment, in the step S2, the homogenization heat treatment comprises the following steps: heating the cast ingot to 530-560 ℃ at the speed of 10-30 ℃/h, and preserving heat for 6-24h; then cooling to 500-520 ℃ at the speed of 10-30 ℃/h, and preserving heat for 1-10 h. The homogenized cooling can avoid the aluminum adhesion phenomenon of the high-temperature aluminum ingot in the hot rolling process so as to damage the surface; the temperature of each part of the large cast ingot is further ensured to be consistent by cooling along with the furnace and heat preservation, and the phenomenon that the temperature difference between the surface and the core of the plate is large after the plate is directly discharged from the furnace and air-cooled, so that the structure of each part is different after hot rolling is avoided, the aluminum alloy structure is further improved, and the paint brushing performance is improved.

In a preferred embodiment, in the step S3, the hot rough rolling is hot rolling directly discharged from a furnace after the homogenization heat treatment, the initial rolling temperature of the hot rough rolling is 500 to 520 ℃ which is the same as the final holding temperature of the homogenization heat treatment, and the final rolling temperature of the hot rough rolling is not less than 400 ℃, and the hot rough rolling process of the invention can further reduce the Goss texture and the middle Cube texture of the edge of the aluminum alloy plate, thereby better improving the paint brushing performance of the plate.

In order to obtain a clean ingot surface and facilitate the subsequent processing, in a preferred embodiment, step S2 further comprises the steps of sequentially sawing and milling the ingot before the homogenization heat treatment.

In still another exemplary embodiment of the present invention, there is also provided a 6xxx aluminum alloy sheet, resulting from the method of the present invention; due to the adoption of the preparation method, the hot rough-rolled plate structure is strictly controlled, and the finished plate has excellent comprehensive performances such as forming performance, flanging performance, painting and brushing performance and the like. In the surface plate structure along the wide edge of the plate, the average grain size along the ND direction (normal direction) of the hot rough rolling plate prepared by the method is less than 24 mu m, and the Goss orientation ratio is less than 7 percent; in the plate structure at the 1/4 of the plate thickness along the wide side part of the plate, the average grain size along the ND direction (normal direction) is less than 27 μm, and the Goss orientation ratio is less than 11%; in the surface plate structure along the middle position of the plate width, the average grain size along the ND direction (normal direction) is less than 72 mu m, and the Cube orientation ratio is less than 14 percent; in the plate structure at the position of 1/4 of the plate thickness along the middle of the plate width, the average grain size along the ND direction (normal direction) is less than 72 μm, and the Cube orientation ratio is less than 5%. The finished plate produced by the following procedures of hot finish rolling, primary cold rolling, intermediate annealing, finished product cold rolling, continuous annealing, pre-aging treatment and the like has a paint brush line grade superior to grade 2 at each position along the TD direction (transverse direction).

In another exemplary embodiment of the present invention, there is also provided a use of the above 6xxx aluminum alloy plate in an automobile outer cover, wherein the aluminum alloy plate has excellent painting line performance, and also has suitable strength and forming performance when used in the automobile outer cover, meets the use requirement of an outer plate of the cover, and is beautiful and practical.

The present application is described in further detail below with reference to specific examples, which should not be construed as limiting the scope of the invention as claimed.

Example 1

Step S1, melting pure aluminum and various intermediate alloys according to the component ratio of No. 1 in Table 1, and casting a cast ingot with the thickness of 700mm by utilizing semi-continuous casting equipment after refining the melt;

s2, performing head cutting and surface milling on the cast ingot, then putting the cast ingot into a heat treatment furnace for homogenization heat treatment, raising the temperature to 550 ℃ at the speed of 20 ℃/h, and preserving the temperature for 10h; then cooling the mixture to 520 ℃ along with the furnace at the speed of 20 ℃/h, and preserving the heat for 5h;

step S3, carrying out hot rough rolling on the homogenized cast ingot to obtain a hot rough rolled plate, wherein the initial rolling temperature of the hot rough rolling is 520 ℃, the final rolling width w of the hot rough rolling is 2m, the thickness t is 35mm, the total deformation is 95.0%, the final rolling temperature of the hot rough rolling is 440 ℃, 16 passes of the hot rough rolling are totally carried out, the average pass reduction rate is 16.5%, and the pass distribution is shown in Table 2;

s4, after the hot rough rolling is finished, carrying out hot finish rolling, primary cold rolling and intermediate annealing on the obtained hot rough rolled plate, and then rolling to the thickness of a finished plate; and sequentially carrying out solid solution, water quenching and pre-aging treatment on the obtained cold-rolled sheet to obtain a finished product 6xxx aluminum alloy sheet.

Example 2

Example 2 differs from example 1 only in that ingots were prepared with composition # 2 in table 1, the ingot thickness was 550mm, the initial hot rough rolling temperature was 500 ℃, the final hot rough rolling thickness t was 20mm, the total deformation was 96.4%, the final hot rough rolling temperature was 400 ℃, the number of hot rough rolling was 13 in total, the average pass reduction was 21.5%, and the pass distribution was as shown in table 3.

Example 3

Example 3 differs from example 1 only in that ingots having a thickness of 576mm, a hot rough rolling start temperature of 510 ℃, a hot rough rolling final thickness t of 25mm, a total deformation of 95.7%, a hot rough rolling final temperature of 430 ℃, 16 passes of hot rough rolling, an average pass reduction of 17.2%, and pass distribution as shown in table 4 were prepared according to composition # 3 in table 1.

Example 4

Example 4 differs from example 1 only in that an ingot is prepared according to the composition No. 4 in the table 1, the thickness of the ingot is 450mm, the ingot is heated to 560 ℃ at the speed of 30 ℃/h in the step S2, and the temperature is kept for 6h; then cooling to 520 ℃ at the speed of 30 ℃/h, and preserving heat for 1h. In the step S3, the initial rolling temperature of the hot rough rolling is 520 ℃, the final rolling width w is 3.0m, the thickness t is 20mm, the total deformation is 95.6%, the final rolling temperature of the hot rough rolling is 400 ℃, 13 passes of the hot rough rolling are performed, the average pass reduction rate is 20.5%, and the pass distribution is shown in Table 5.

Example 5

Example 5 differs from example 1 only in that an ingot with a thickness of 450mm is prepared according to the composition No. 4 in Table 1, the ingot is heated to 530 ℃ at a speed of 10 ℃/h in step S2, and the temperature is kept for 24h; then cooling to 500 ℃ at the speed of 10 ℃/h, and preserving the heat for 10h. In step S3, the initial rolling temperature of the hot rough rolling is 500 ℃, the final rolling width w is 0.8m, the thickness t is 20mm, the total deformation is 95.6%, the final rolling temperature of the hot rough rolling is 400 ℃, the hot rough rolling is carried out for 11 passes, the average pass reduction rate is 23.5%, and the pass distribution is shown in Table 6.

Comparative example 1

Comparative example 1 differs from example 1 only in that the ingot thickness was 500mm, the hot rough rolling start temperature was 510 ℃, the hot rough rolling final thickness t was 28.6mm, the total deformation was 94.3%, the hot rough rolling final temperature was 384 ℃, the hot rough rolling average pass reduction was 15.0%, and the pass distribution is shown in table 7.

Comparative example 2

Comparative example 2 differs from example 1 only in that ingots were prepared with composition # 2 in table 1, the ingot thickness was 700mm, the initial hot rough rolling temperature was 505 ℃, the final hot rough rolling thickness t was 26mm, the total strain was 96.3%, the final hot rough rolling temperature was 420 ℃, the average pass reduction of hot rough rolling was 13.3%, and the pass distribution is shown in table 8.

In examples 1 to 5 and comparative examples 1 to 2, samples were tested after hot rough rolling for texture observation, and the finished plates were left standing at room temperature for 7 days before evaluation of the wire brushing performance.

The test method comprises the following steps:

and (3) observing the structure of the hot rough rolling plate: the test was carried out using EBSD, and a hot rolled sheet having a rolling direction length of 10mm and a rolling direction length of 5mm in a vertical direction and a full thickness was taken from the hot rough rolled sheet, and the sheet structure of the overlapping region of the first region and the second region (i.e., the surface along the wide side portion of the sheet), the sheet structure of the overlapping region of the first region and the third region (i.e., the sheet thickness along the wide side portion of the sheet at 1/4), the sheet structure of the overlapping region of the second region and the fourth region (i.e., the surface along the middle position of the sheet width), and the sheet structure of the overlapping region of the third region and the fourth region (i.e., the sheet thickness along the middle position of the sheet width at 1/4) were observed as described above. The sample was mechanically polished and then electropolished. The statistical texture orientation is: goss {011} <100>, cube {001} <100>, the orientation difference of the statistical texture is 11 degrees.

Paint brush linear performance test: cutting a strip-shaped sample with the length of 250mm and the width of 35mm from a finished plate, wherein the length direction of the sample is vertical to the rolling direction, the width direction of the sample is parallel to the rolling direction, pre-stretching the sample by 10 percent along the length direction, polishing the sample by using a 320-mesh oilstone, and carrying out grade evaluation on the surface appearance of the polished sample. The paint line performance rating is shown in fig. 4: stage 1: no obvious painting line is painted along the width direction; and 2, stage: intermittent painting lines exist along the width direction, and no penetrating painting lines exist; and 3, stage: there are sparse through-paint brush lines along the width direction; 4, level: dense through-the-paintbrush lines appeared across the width, with levels 1 and 2 being acceptable and levels 3 and 4 being unacceptable.

The results of the texture observation and paintbrush line performance test of the finished panels prepared in examples 1 to 5 and comparative examples 1 to 2 are shown in table 9. Example 1 the texture distribution of the Goss at the edge (first and second overlapping areas) of the hot-rolled rough plate is shown in FIG. 5, and the texture distribution of the Goss at the edge (first and second overlapping areas) of the hot-rolled rough plate of comparative example 1 is shown in FIG. 6; the Cube texture distribution in the middle (second and fourth overlapping areas) of the hot rough rolled sheet of example 1 is shown in fig. 7, and the Cube texture distribution in the middle (second and fourth overlapping areas) of the hot rough rolled sheet of comparative example 1 is shown in fig. 8.

As can be seen from the above, in comparative example 1, the total deformation of the hot rough rolling is small, the pass rolling strain rate is insufficient, the final rolling temperature is low, the grain size is large, the Goss texture at the edge part is too much, the Cube texture at the middle part is too much, and the paint brush linear performance inherited to the finished plate is finally unqualified. In the comparative example 2, the pass reduction rate is insufficient, the pass rolling strain rate is insufficient, the grain size is large, the Goss texture at the edge part is too much, the Cube texture at the middle part is too much, and finally the paint brush linear performance of the finished plate is inherited to be unqualified. In examples 1 to 5, by using the preparation method of the present invention, the hot rough rolling process of the plate is controlled by strictly limiting the reduction ratio of the hot rough rolling pass, adjusting and controlling the process parameters such as the rolling speed, the rolling temperature, and the like, so that the metal structure of the hot rough rolled plate is improved, the crystal grains of the hot rough rolled plate are refined, the Goss texture at the edge is eliminated, the Cube texture at the middle part is reduced, the plate structure is integrally improved, the prepared 6xxx aluminum alloy plate has no paint brush lines along the TD direction (transverse direction), the surface quality is excellent, the paint brush line performance of the finished plate is greatly improved, and the performance of the finished plate is qualified. Furthermore, it can be seen that the paint brush line performance of the 6xxx aluminum alloy sheets is best when the production process parameters are all within the preferred ranges of the present invention.

The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (9)

1. The preparation method of the 6xxx aluminum alloy plate is characterized in that the components of the aluminum alloy plate comprise, by mass percent, 0.4 to 1.5wt.% of Si; mg 0.2 to 1.0wt.%; cu ≤ 0.3wt.%; mn ≦ 0.2wt.%; cr is less than or equal to 0.2wt.%, fe is less than or equal to 0.3wt.%, the balance is Al and inevitable impurities, each inevitable impurity is less than 0.05wt.%, and total impurities are less than 0.15wt.%; the preparation method comprises the following steps:

step S1, casting the raw material of the 6xxx aluminum alloy plate to obtain an ingot;

s2, carrying out homogenization heat treatment on the ingot to obtain a homogenized ingot;

s3, carrying out hot rough rolling on the homogenized cast ingot to obtain a hot rough rolled plate, wherein the hot rough rolling comprises at least n passes, n is 7-20, the reduction rate of each pass is sequentially increased, the reduction rate of the 1 st pass is less than or equal to 5%, the reduction rate of the 2 nd pass is more than or equal to 6%, the reduction rates of the n-2 nd, n-1 st and n-th passes are more than or equal to 30%, and the average pass reduction rate is more than or equal to 16%;

s4, sequentially carrying out hot finish rolling, primary cold rolling, intermediate annealing, secondary cold rolling, continuous annealing and pre-aging treatment on the hot rough rolled plate to obtain a 6xxx aluminum alloy plate;

in the step S3, the rolling strain rate of each pass is gradually increased, the rolling strain rate of the 1 st pass is 0.5 to 0.9/S, the rolling strain rate of the 2 nd pass is 0.6 to 1.0/S, the rolling strain rates of the n-2 nd pass and the n-1 st pass are not less than 10/S, and the rolling strain rate of the n th pass is not less than 20/S; the initial rolling temperature of the hot rough rolling is 500-520 ℃, and the final rolling temperature is more than or equal to 400 ℃.

2. The preparation method according to claim 1, wherein the thickness of the cast ingot is 450 to 700mm, the width w of the hot rough rolled plate is 0.8 to 3.0m, the thickness t is 20 to 35mm, and the total deformation of the hot rough rolling process is not less than 95%.

3. The production method according to claim 2, wherein in the step S3, a rolling direction of the hot rough rolled plate material is defined as RD direction, a transverse direction is defined as TD direction, while a normal direction perpendicular to the RD direction and the TD direction is defined as ND direction, a surface of the hot rough rolled plate material perpendicular to the TD direction is defined as TD plane, a surface of the hot rough rolled plate material perpendicular to the RD direction is defined as RD plane, a surface of the hot rough rolled plate material perpendicular to the ND direction is defined as ND plane, a region within w/20 of the TD plane in the TD direction of the hot rough rolled plate material is defined as a first region,

defining a region t/10 away from the ND surface along the ND direction as a second region, wherein in the plate material structure of the overlapped region of the first region and the second region, the average grain size along the ND direction is less than 24 mu m and the Goss orientation ratio is less than 7 when observed from the TD surface;

and defining a region t/5~t/4 away from the ND surface along the ND direction as a third region, wherein in the plate material structure of the overlapped region of the first region and the third region, the average grain size along the ND direction is less than 27 μm and the Goss orientation ratio is less than 11% when observed from the TD surface.

4. The production method according to claim 3, wherein in the step S3, a region of the hot rough rolled plate material within w/2 ± w/10 of the TD plane in the TD direction is defined as a fourth region,

in the sheet material structure of the overlapping region of the second region and the fourth region, the average grain size in the ND direction is less than 72 [ mu ] m and the Cube orientation ratio is less than 14% when viewed from the TD plane;

in the plate material structure of the overlapping region of the third region and the fourth region, the average grain size in the ND direction is less than 72 [ mu ] m and the Cube orientation ratio is less than 5% when viewed from the TD plane.

5. The method according to claim 1, wherein in the step S2, the homogenization heat treatment comprises the steps of: heating the cast ingot to 530-560 ℃ at the speed of 10-30 ℃/h, and preserving heat for 6-24h; then cooling to 500-520 ℃ at the speed of 10-30 ℃/h, and preserving heat for 1-10 h.

6. The preparation method according to claim 1, wherein in the step S3, the hot rough rolling is direct hot rolling in a furnace after the homogenization heat treatment, the initial rolling temperature of the hot rough rolling is the same as the final holding temperature of the homogenization heat treatment, and the final rolling temperature of the hot rough rolling is not less than 400 ℃.

7. The preparation method according to claim 1, wherein the step S2 further comprises the steps of sawing and milling the ingot in sequence before the homogenization heat treatment.

8. A6 xxx aluminum alloy sheet as obtained by the method of any of claims 1-7, wherein the 6xxx aluminum alloy sheet has a paint-brushed line rating across the transverse direction superior to 2.

9. Use of the 6xxx aluminum alloy sheet of claim 8 in an automotive outer cover.

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