JP2017048452A - Aluminum alloy sheet for automobile body panel excellent in filiform rust resistance, coating galling curability and processability and manufacturing method therefor, automobile body panel using the same and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an aluminum alloy sheet for an automobile body panel excellent in filiform rust resistance and recycling property and a manufacturing method therefor, an automobile body panel using the same and a manufacturing method therefor.SOLUTION: An aluminum alloy sheet for an automobile body panel excellent in filiform rust resistance and recycling property, is made of an Al alloy containing Mg:0.30 to 0.80 mass% (hereafter referred to as "%"), Si:0.80 to 1.40%, Mn:0.20 to 0.65%, Zn:0.44 to 0.60%, Fe:0.25 to 0.40% and Cu:0.17 to 0.25%, has elongation of 23% at 100 days after solution heat treatment, bearing force after conducting coating galling treatment of 200 MPa or more and further point number in a Heming test based on JISH7701 of 0 to 2 point. The present invention also provides a manufacturing method for the aluminum alloy sheet, an automobile body panel using the aluminum alloy sheet and a manufacturing method therefor.SELECTED DRAWING: None

Description

  The present invention relates to an aluminum alloy plate for an automobile body panel and a method for producing the same, and an automobile body panel using the same and a method for producing the same, and in particular, it is excellent even when subjected to a buff grinding process for improving design properties. The present invention relates to an aluminum alloy plate for an automobile body panel that has yarn rust resistance and is excellent in paint bake hardenability and workability, and a manufacturing method thereof, and an automotive body panel using the same and a manufacturing method thereof.

  In recent years, there is an increasing demand for recycling resources in various fields. Since aluminum alloys produced by reducing bauxite are produced by consuming enormous electric power, recycling can be said to be essential. In general, when aluminum alloys are recycled, impurities tend to increase. Since the increase in impurities in the aluminum alloy lowers the quality of the aluminum alloy, the recycled aluminum alloy is not currently used in automobile body panels.

  In order to overcome this situation, for example, Patent Document 1 proposes a countermeasure using aluminum alloy scraps. However, the stabilization treatment after solution treatment for the purpose of improving the formability and BH properties by controlling the metal structure has not been carried out, and it is possible to secure the high material properties currently required for automobile body panels and the like. Since it is difficult, it has not been put to practical use.

  On the other hand, an automobile body panel may be buffed to remove wrinkles on the surface of the body panel before chemical conversion treatment or subsequent coating. In general, it is known that machining including buffing lowers corrosion resistance, but there are many unclear points about the factors that reduce corrosion resistance. Furthermore, the corrosive form that becomes a problem in automobile body panels is thread-like corrosion (yarn rust corrosion) that deteriorates the appearance quality, but the relationship between buffing and yarn rust corrosion is unknown. For these reasons, it has been difficult to take measures to prevent the yarn rust resistance from being reduced by buffing.

JP 11-293363 A

  The present invention has been made against the background of the above circumstances, excellent yarn rust resistance, which is difficult to reduce yarn rust resistance even after buffing, excellent paint bake hardenability, and press workability and bending workability. It is an object of the present invention to provide an aluminum alloy plate for automobile body panels having such excellent workability and a manufacturing method thereof, and an automobile body panel using the same and a manufacturing method thereof.

  In order to solve the above-mentioned problems, the present inventors focused on the machined layer portion by buffing, ascertained that precipitates exist in this machined portion after paint baking, and the precipitates around the precipitates. It has been found that the potential decreases, and as a result, the corrosion resistance decreases. Specifically, by controlling the deposits containing elements that make the matrix pitting corrosion potential noble by depositing by heating at 180 ° C., which is the coating baking temperature, for 25 minutes, it is resistant to yarn rust even when buffing is applied. The present invention has been completed by finding that the properties do not deteriorate, and also by finding that it has excellent paint baking properties and processability.

  That is, the present invention according to claim 1, Mg: 0.30-0.80 mass%, Si: 0.80-1.40 mass%, Mn: 0.20-0.65 mass%, Zn: 0.44-0 .60 mass%, Fe: 0.25 to 0.40 mass%, Cu: 0.17 to 0.25 mass%, the balance being made of an aluminum alloy consisting of Al and impurities, and coating baking after 100 days of solution treatment Yield rust resistance, paint bakeability, characterized in that the yield strength after the treatment is 200 MPa or more, the elongation is 23% or more, and further, the score is 0 to 2 in the hemming test based on JISH7701. And it was set as the aluminum alloy plate for automobile body panels excellent in workability.

  The present invention is the method of manufacturing an aluminum alloy plate for an automobile body panel according to claim 1 according to claim 2, wherein the casting step of casting the aluminum alloy, and the homogenization processing step of homogenizing the ingot , A hot rolling process for hot rolling the homogenized ingot, an intermediate annealing process for the hot rolled sheet, a cold rolling process for cold rolling the intermediate annealed sheet, and a solution of the cold rolled sheet A solution treatment step for heat treatment, and a stabilization treatment step for stabilizing the solution-treated rolled plate, wherein the temperature of the solution treatment is 480 ° C. or more, and the stabilization treatment step is the solution treatment. An automotive body panel excellent in yarn rust resistance, paint bakeability and workability, which is started within 1 hour after the treatment step and is characterized by holding the rolled sheet at 80 to 120 ° C. for 1 hour or more. For aluminum alloy sheet And the production method.

Furthermore, the present invention provides a vehicle body panel according to claim 3, wherein the aluminum body plate for automobile body panel according to claim 1 is partially or entirely painted and baked, and is formed from a surface layer of the aluminum alloy plate. A machining layer having a thickness of 0.5 to 5.0 μm is formed along the thickness direction, and the number density of the Si-based intermetallic compound and the Al—Cu-based intermetallic compound in the machining layer is two. An automobile body panel excellent in yarn rust resistance, paint bakeability and workability, characterized by being / μm ² or less.

  The present invention provides the method for manufacturing an automobile body panel according to claim 3, wherein the surface of the aluminum alloy plate is buffed with a buff to which alumina particles having a particle size of 80 to 320 are attached. Yarn rust resistance, paint bake resistance and processing characterized by comprising a polishing step, a painting step for painting a buffed aluminum alloy plate, and a baking step for baking the painted aluminum alloy plate at 180 ° C. for 25 minutes This is a method for manufacturing an automobile body panel having excellent properties.

  According to the present invention, even when buffing is performed, the yarn rust resistance is not easily lowered, and the aluminum alloy plate for an automobile body panel excellent in paint bake hardenability and workability and the manufacturing method thereof, In addition, an automobile body panel using the same and a manufacturing method thereof can be obtained.

1. 1. Aluminum alloy plate for automobile body panel 1-1. Alloy Composition The alloy composition of the aluminum alloy plate for an automobile body panel according to the present invention (hereinafter sometimes simply referred to as “aluminum alloy plate”) will be described below.
The aluminum alloy plate according to the present invention has Mg: 0.30 to 0.80 mass% (hereinafter simply referred to as “%”), Si: 0.80 to 1.40%, Mn: 0.20 to 0.65. %, Zn: 0.44 to 0.60%, Fe: 0.25 to 0.40%, Cu: 0.17 to 0.25%, and the balance is made of an aluminum alloy consisting of Al and impurities. Si-based intermetallic compound in the machining layer formed by buffing by heat treatment in the baking step after the coating step performed after buffing (hereinafter, simply referred to as “heat treatment after buffing”). And the precipitation of Al—Cu intermetallic compounds.

  When Si and Cu are dissolved in the matrix, the pitting corrosion potential of the matrix is made noble. The precipitation of Si-based intermetallic compounds mainly containing Si and Al—Cu-based intermetallic compounds containing Cu means that the amount of Si and Cu dissolved in the matrix decreases. For this reason, the pitting corrosion potential around these precipitates is reduced to cause a decrease in yarn rust resistance. Therefore, by suppressing the precipitation of the Si-based intermetallic compound and the Al—Cu-based intermetallic compound by the heat treatment after buffing, it is possible to suppress the decrease in yarn rust resistance.

Mg:
Mg forms an Mg—Si-based intermetallic compound together with Si before buffing to reduce the amount of Si solid solution in the matrix. The decrease in the amount of Si solid solution in the matrix further suppresses the precipitation of the Si-based intermetallic compound by the heat treatment after buffing, and as a result, the addition of Mg contributes to the improvement of the yarn rust resistance. On the other hand, the solid solution of Mg in the matrix lowers the pitting corrosion potential of the matrix and lowers the yarn rust resistance regardless of the presence or absence of buffing. When the Mg content is less than 0.30%, the precipitation amount of the Mg—Si-based intermetallic compound becomes insufficient. As a result, the precipitation of the Si-based intermetallic compound due to the heat treatment after buffing is not sufficiently suppressed, and the rust resistance is lowered. On the other hand, when the Mg content exceeds 0.80%, the effect of Mg in solid solution in the matrix increases, leading to a decrease in yarn rust resistance and a bending workability. The Mg content is preferably 0.40 to 0.80%.

Si:
As described above, Si precipitates as a Si-based intermetallic compound on the machined layer produced by buffing and reduces the yarn rust resistance. However, when the Si content is less than 0.80%, the material strength decreases. On the other hand, when the Si content exceeds 1.40%, precipitation of Si-based intermetallic compounds increases due to the heat treatment after buffing, and as a result, the yarn rust resistance is lowered. The Si content is preferably 0.80 to 1.20%.

Mn:
Mn forms an Al—Mn—Si intermetallic compound with Si before buffing to reduce the amount of Si solid solution in the matrix. The decrease in the amount of Si solid solution in the matrix further suppresses the precipitation of the Al—Si intermetallic compound by the heat treatment after buffing, and as a result, the addition of Mn contributes to the improvement of the yarn rust resistance. When the Mn content is less than 0.20%, the precipitation amount of the Al—Mn—Si intermetallic compound becomes insufficient. As a result, the precipitation of the Al—Si intermetallic compound due to the heat treatment after buffing is not sufficiently suppressed, and the rust resistance is lowered. On the other hand, if the Mn content exceeds 0.65%, bending workability is lowered due to the generation of a large number of intermetallic compounds. The Mn content is preferably 0.30 to 0.60%.

Zn:
Zn is dissolved in the precipitated Si-based intermetallic compound or Al-Cu-based intermetallic compound by heat treatment after buffing. The solid solution of Zn in the precipitated Si-based intermetallic compound or Al-Cu-based intermetallic compound makes the pitting corrosion potential around those precipitates noble, so the yarn resistance before and after the heat treatment after buffing Does not reduce rust. On the other hand, the solid solution of Zn in the matrix lowers the pitting corrosion potential of the matrix and lowers the yarn rust resistance regardless of the presence or absence of buffing. If the Zn content is less than 0.44%, the solid solution amount of Zn in the precipitated Si-based intermetallic compound or Al—Cu-based intermetallic compound becomes insufficient, and the yarn resistance after heat treatment after buffing Rust deterioration is caused. On the other hand, if the Zn content exceeds 0.60%, the effect of lowering the pitting potential in the matrix is increased, and the yarn rust resistance is lowered.

Fe:
Fe forms an Al—Fe—Si intermetallic compound together with Si before buffing to reduce the amount of Si solid solution in the matrix. The decrease in the amount of Si solid solution in the matrix further suppresses the precipitation of the Al—Si intermetallic compound by the heat treatment after buffing, and as a result, the addition of Fe contributes to the improvement of the yarn rust resistance. When the Fe content is less than 0.25%, the precipitation amount of the Al—Fe—Si intermetallic compound is insufficient. As a result, the precipitation of the Al—Si intermetallic compound due to the heat treatment after buffing is not sufficiently suppressed, and the rust resistance is lowered. On the other hand, when the Fe content exceeds 0.40%, bending workability is reduced due to the generation of a large number of intermetallic compounds. The Fe content is preferably 0.25 to 0.35%.

Cu:
Cu is deposited as an Al—Cu intermetallic compound on the machined layer produced by buffing by heat treatment, and reduces the rust resistance. When the Cu content is less than 0.17%, the bending workability decreases. On the other hand, when the Cu content exceeds 0.25%, the precipitation of Al-Cu intermetallic compounds is increased by the heat treatment after buffing, and as a result, the yarn rust resistance is lowered.

  In the aluminum alloy plate according to the present invention, in addition to the above elements, one or more of Ni, Bi and the like may be contained as 0.05% or less and 0.15% or less as a whole as impurities.

1-2. Mechanical properties As mechanical properties, the properties after 100 days from the production of the aluminum alloy sheet were listed. That is, the proof stress after the paint baking process showing the paint bakeability and the workability are evaluated by the hemming test based on JISH7701 which affects the elongation and bending workability affecting the press workability.

Yield strength after paint baking process:
In automobile body panels, etc., the strength is improved by precipitation hardening during paint baking after molding. In the present invention, assuming the general use of an aluminum plate material for automobile body panels, the proof stress after paint baking is defined as 200 MPa or more 100 days after solution heat treatment. If the yield strength is less than 200 MPa, the strength is insufficient and it is not suitable for an automobile body panel.

Elongation:
Furthermore, when forming from aluminum plate material to automobile body panel, it is necessary to stretch the material. Assuming general use with aluminum plate material for automobile body panel, the elongation after 100 days of solution is 23% or more. It prescribes. If this elongation is less than 23%, the moldability becomes insufficient.

Bendability Furthermore, when forming from an aluminum plate material to an automobile body panel, bendability of the material is also necessary. In the present invention, assuming a general use of an aluminum plate material for an automobile body panel, a score obtained by a hemming test based on JIS 7701 using a JIS No. 5 test piece in a 90 ° direction with respect to the rolling direction 100 days after solution treatment. Is defined as 0 to 2 points. If this score exceeds two, the bendability is insufficient. In addition, this score is so preferable that it is small, and 0 point is the most preferable.

1-3. Manufacturing method Next, the manufacturing method of the aluminum alloy plate for motor vehicle body panels which concerns on this invention is demonstrated.

  First, an ingot is produced by melting an aluminum alloy having the above alloy composition in accordance with a conventional method and casting a molten metal. The obtained ingot is subjected to homogenization treatment, hot rolling, intermediate annealing, cold rolling, solution treatment, and stabilization treatment in this order. The preferable conditions for each step for satisfying the material properties defined in the present invention will be described below.

Casting process:
In the casting process, the molten metal is cast by a normal casting method such as a DC casting method to obtain an ingot.

Homogenization process:
The main purpose of the homogenization treatment is to eliminate segregation of additive elements. The homogenization temperature is preferably 480 ° C. or higher and lower than the melting point. If the treatment temperature is less than 480 ° C., the effect of eliminating segregation cannot be obtained sufficiently. On the other hand, when the processing temperature is equal to or higher than the melting point, it is difficult to suppress the occurrence of eutectic melting. The time for the homogenization treatment depends on the amount of added elements, but is preferably 20 minutes to 24 hours within the above temperature range. If the treatment time is less than 20 minutes, it may be difficult to sufficiently eliminate segregation. On the other hand, when the processing time exceeds 24 hours, the manufacturing cost increases.

Hot rolling process:
In the subsequent hot rolling step, the starting temperature is preferably set to 450 to the melting point. If it is less than 450 degreeC, a deformation resistance will increase and production efficiency will fall. On the other hand, eutectic melting occurs when the starting temperature is equal to or higher than the melting point. Moreover, it is preferable that the completion | finish temperature of hot rolling shall be 200-400 degreeC. If the end temperature is less than 200 ° C., the deformation resistance increases and the production efficiency decreases. On the other hand, when the end temperature exceeds 400 ° C., the precipitates become coarse and it becomes difficult to form a solution in the subsequent steps.

Intermediate annealing process:
The subsequent intermediate annealing is aimed at solution and recrystallization of the additive elements. The holding temperature of the intermediate annealing is preferably 480 ° C. or higher and lower than the melting point. This step is important for dissolving Mg ₂ Si, Si-based compounds and the like in the matrix, thereby imparting bake hardenability and improving the strength after baking. Furthermore, this process, Mg 2 _Si, thereby decreasing the distribution density of the second phase particles by solid solution, such as Si-based compound, which contributes to the improvement of ductility and bendability. Furthermore, this step is important for obtaining a desired crystal structure and obtaining good formability together with the subsequent cold rolling step and solution treatment step.

If the holding temperature of the intermediate annealing is less than 480 ° C., the above effects may not be sufficiently obtained. On the other hand, if the processing temperature is higher than the melting point, eutectic melting may occur. The holding time for the intermediate annealing is preferably 5 minutes or less. When the holding time exceeds 5 minutes, productivity is lacking. Also, in order to prevent a large amount of Mg ₂ Si, Si compounds, etc. from precipitating at the grain boundaries during the cooling of the intermediate annealing, from the holding temperature to the temperature range of 150 ° C. or less at a cooling rate of 100 ° C./min or more. Cooling (quenching) is preferable. In addition, you may provide a cold rolling process as needed between this intermediate annealing process and the hot rolling which is the previous process.

Cold rolling process:
In the subsequent cold rolling process, the hot-rolled sheet is rolled by a conventional method to a desired sheet thickness. By increasing the cold rolling rate, the crystal grain size is refined, and the effect of improving the bendability and preventing rough skin is exhibited. Therefore, the cold rolling rate is preferably 25% or more. From the viewpoint of controlling the metal structure, there is no positive reason to limit the upper limit of the cold rolling rate, but when the cold rolling rate is excessively increased, the cold rolling rate is 90% or less because the productivity is reduced. Is preferable. In order to stably obtain a crystal structure having a desired Δr (indicating a different directionality of the Rankford value), it is desirable to set the cold rolling rate as described above.

Solution treatment process:
After the cold rolling is completed, the cold rolled sheet is subjected to a solution treatment. The purpose of the solution treatment is the same as in the intermediate annealing, and is in the solid solution and recrystallization of the additive elements. The final crystal structure is determined by recrystallization during the solution treatment. The solution treatment temperature is 480 ° C. or higher, preferably 490 ° C. or higher and lower than the melting point. When the solution treatment temperature is less than 480 ° C., it is advantageous for suppressing the aging of the room temperature aging, but the amount of solid solution decreases and sufficient bake hardenability cannot be obtained, and ductility and bendability are also obtained. Remarkably worse. On the other hand, in order to suppress the occurrence of eutectic melting during solution treatment, the melting point is set to less than the melting point. Moreover, it is preferable that the retention time of the solution treatment is 5 minutes or less. When the holding time exceeds 5 minutes, the productivity is lowered. Furthermore, in order to prevent a large amount of Mg ₂ Si, elemental Si, etc. from precipitating at the grain boundaries during cooling after holding the solution treatment, at a cooling rate of 100 ° C./min or higher, the holding temperature is 150 ° C. or lower. It is preferable to cool (quenify) to a temperature range.

Stabilization process:
Within 1 hour after the completion of the solution treatment step, it is necessary to perform a stabilization treatment in which the rolled plate is heated and held at a temperature of 80 to 120 ° C. for 1 hour or more. This stabilization treatment contributes to the improvement of strength during paint baking. P. The purpose is to form an atomic group called cluster II that easily migrates to the zone, and is a treatment necessary for securing the strength after paint baking. If it is more than 1 hour after the solution treatment process is completed or if it is heated and held at a temperature of less than 80 ° C., cluster I will be formed, which will compete with cluster II and prevent strength improvement during paint baking. Later strength is insufficient. On the other hand, when the heating and holding temperature exceeds 120 ° C., the cluster II grows excessively and bending workability and formability deteriorate. Furthermore, when the heating and holding time is less than 1 hour, the formation of the cluster II is insufficient and the strength after baking is insufficient. The upper limit of the heating and holding time is not particularly limited, but is preferably within 24 hours from the viewpoint of production efficiency.

2. Next, an automobile body panel manufactured using the aluminum alloy plate according to the present invention will be described.

2-1. Surface structure of aluminum alloy sheet Buffing is used to make it difficult to see wrinkles on automobile body panels (hereinafter, simply referred to as “body panels”) generated during press molding and assembly of aluminum alloy sheets. To be implemented. Buffing provides a machined layer on the surface of the aluminum alloy plate of the body panel. The machined layer accumulates strain, and this strain becomes a driving force for precipitation of Si-based intermetallic compounds and Al—Cu-based intermetallic compounds in the heat treatment after buffing. The machined layer on which the Si-based intermetallic compound or the Al—Cu-based intermetallic compound is deposited has a lower pitting corrosion potential than the matrix. Therefore, the machined layer is preferentially corroded and the yarn rust resistance is reduced.

  The machining layer is formed with a thickness of 0.5 to 5.0 μm, preferably 2.0 to 5.0 μm along the thickness direction from the surface layer of the aluminum alloy plate. If the thickness is less than 0.5 μm, it is difficult to make the wrinkles difficult to see. On the other hand, when the thickness exceeds 5 μm, the potential of the machined layer is greatly reduced, and the rust resistance is remarkably lowered.

  The thickness of the machined layer is measured by observing an arbitrary portion of the buffed portion with a transmission electron microscope. For example, a cross section along the thickness direction is observed. The thickness of the machined layer is defined as the maximum value of the measured values at a plurality of locations.

The total number density of the Si-based intermetallic compound and the Al—Cu-based intermetallic compound in the machined layer is defined as 2 pieces / μm ² or less. When this number density exceeds 2 pieces / μm ² , the pitting corrosion potential in the machined layer is remarkably reduced and the rust resistance is lowered. In addition, this number density is so preferable that it is small, and 0 piece / micrometer < ² > is the most preferable.

  The number density of the Si-based intermetallic compound and the Al—Cu-based intermetallic compound in the machining layer is measured by a transmission electron microscope having an energy dispersive X-ray spectroscopic function. For example, the number of Si-based intermetallic compounds containing 50 mass% or more of Si or Al—Cu based intermetallic compounds containing 20 mass% or more of Cu is measured. The number density here is defined as the maximum value measured at three or more locations.

2-2. Manufacturing Method Next, a manufacturing method of the body panel according to the present invention will be described.

Buffing process:
When buffing is performed on the aluminum alloy plate of the body panel, the machining layer is formed on the surface layer of the aluminum alloy plate. The buffing condition affects the thickness of the machining layer. The Si-based intermetallic compound and the Al—Cu-based intermetallic compound are precipitated in the coating baking process performed after the buffing.

  For buffing, a buff to which alumina particles having a particle size of 80 to 320 are attached is used. When the particle size of the alumina particles is less than 80, the machining layer becomes thick and the yarn rust resistance decreases. On the other hand, when the particle size of the alumina particles exceeds 320, the effect of hiding soot is reduced.

Painting process: Baking process Paint is applied to the buffed aluminum alloy sheet using a normal coating method. Furthermore, the coated aluminum alloy plate is usually made into a body panel by baking at 180 ° C. for 25 minutes.

  Next, the present invention will be described in more detail based on examples. The following examples are merely illustrative for explaining the present invention, and do not limit the technical scope of the present invention.

  In the aluminum alloy material, alloys having the compositions shown in Table 1 were melted in accordance with conventional methods, and cast into slabs by DC casting. After casting, the slab was chamfered.

In order to eliminate segregation of the additive elements, the face-cut slab was homogenized at a temperature of 540 ° C. for 10 hours, cooled to room temperature, reheated, and heated at a start temperature of 530 ° C. and an end temperature of 250 ° C. Cold rolling was performed to obtain a rolled plate having a thickness of 3 mm. The obtained hot-rolled sheet was cold-rolled to a thickness of 2 mm. Next, the cold-rolled sheet was subjected to intermediate annealing by being held at 530 ° C. for 5 seconds in a salt bath furnace, and forcibly air-cooled from the holding temperature to room temperature at a cooling rate of 300 ° C./min using a fan. Subsequently, after the sheet thickness was 1 mm by cold rolling, solution treatment was performed, and then forced air cooling was performed from the solution treatment temperature to room temperature using a fan at a cooling rate of 300 ° C./min. Next, the final plate was obtained by subjecting the forced air-cooled rolled plate to a stabilization treatment. The solution treatment conditions were a holding temperature of 530 ° C. and a holding time of 0 seconds. Here, 0 seconds means cooling immediately after reaching 530 ° C. The stabilization treatment was performed by heating and holding at the temperature shown in Table 2 for 1 hour after the solution treatment. Finally, the stabilized aluminum alloy plate was immersed in 10 mass% H ₂ SO ₄ at 80 ° C. and then dried in the air. The following evaluation was performed about the sample of the aluminum alloy plate produced in this way.

(A) Strength after painting and baking after 100 days A sample of the aluminum alloy plate was held at room temperature for 100 days. And the baking condition after painting was 180 degreeC for 1 hour, and the aluminum alloy plate sample was heat-processed. The proof stress measurement was performed by processing an aluminum alloy plate sample into a JIS No. 5 test piece and performing an Instron type tensile test at a tensile speed of 10 mm / min. The results are shown in Table 4. Those with a yield strength of 200 MPa or more were judged as acceptable (◯), and those with a yield strength of less than that were judged as unacceptable (x).

(B) Elongation after 100 days The sample of the aluminum alloy sheet was held at room temperature for 100 days. The elongation was determined by measuring the elongation when an aluminum alloy plate sample was processed into a JIS No. 5 test piece and subjected to an Instron type tensile test at a tensile rate of 10 mm / min. Those with an elongation rate of 23% or more were evaluated as acceptable (◯), and those with an elongation of less than 23% were evaluated as unacceptable (x). The results are shown in Table 4.

(C) Hemming test based on JISH7701 after 100 days The sample of the aluminum alloy plate was held at room temperature for 100 days. The aluminum alloy sheet sample was processed into a JIS No. 5 test piece in a direction of 90 ° with respect to the rolling direction, and a hemming test based on JIS 7701 was performed. The pre-strain was 8%, the punch tip radius during pre-hemming was 0.5 mm, and the thickness of the intermediate plate during main hemming was 1.0 mm. After the hemming test, 0 to 2 points on the outer peripheral surface described in JISH7701 were accepted (◯), and 3 to 4 outer peripheral surfaces were rejected (x). The results are shown in Table 4.

  Next, the surface of the aluminum alloy plate sample was buffed using a buff to which alumina particles having a particle size shown in Table 3 were attached. Specifically, a machined layer was formed from the surface layer of the aluminum alloy plate sample along the plate thickness direction by polishing the surface of the aluminum alloy plate sample for 60 seconds using a buff attached to the sander. And the following measurements were performed about this machined layer.

(D) Thickness of machined layer, and surface density of Al—Si intermetallic compound and Al—Cu intermetallic compound Each aluminum alloy plate sample buffed as described above corresponds to the baking conditions for coating. After heat treatment at 180 ° C. for 25 minutes, a test piece having a thickness of about 100 to 200 nm was produced from the surface by FIB (Focused Ion Beam). Three arbitrary positions of this test piece were observed at a magnification of 20,000 times using a transmission electron microscope (TEM). In the transmission electron image at each location, the thickness where the machining layer is thickest is the machining layer thickness at that location, and the arithmetic thickness of the 3 machining thicknesses is the machining thickness of the sample. . Furthermore, elemental analysis using an EDS (Energy Dispersive Spectroscopy) detector is performed on the precipitates observed in each of the above locations, and Si-containing intermetallic compounds containing 30% or more of Si are obtained, and Cu is 10% or more. What is contained is an Al—Cu-based intermetallic compound, and the total of both is converted per unit area (μm ² ) to obtain the surface density of the part, and the arithmetic average value of the surface density of the three parts is used as the sample density. The surface density was taken. The results are shown in Table 4.

(E) Yarn rust resistance Zinc phosphate treatment was carried out under standard treatment conditions using commercially available zinc phosphate treatment chemicals on each of the above buffed aluminum alloy sheet materials. A commercially available electrodeposition coating was applied to each aluminum alloy sheet sample after the zinc phosphate treatment, followed by a heat treatment (baking) at 180 ° C. for 25 minutes. The baked sample was further subjected to intermediate coating and top coating to prepare a body panel sample that was baked. Cuts were made with a cutter in the L direction and C direction of the painted surface of this body panel sample, and the yarn rust resistance was evaluated in accordance with ASTM D2083 method. The swelling width of less than 2 mm was accepted and 2 mm or more was rejected. The results are shown in Table 4.

  In Inventive Examples 1 to 18, the evaluation results of proof stress, elongation, bendability and yarn rust resistance were acceptable. On the other hand, in Comparative Examples 1-15, either of these evaluations was disqualified.

  Specifically, in Comparative Example 1, since the holding temperature of the stabilization treatment was too low, the yield strength was rejected, and the strength required for the automobile body panel could not be obtained.

  In Comparative Example 2, since the holding temperature of the stabilization treatment was too high, the bendability and elongation rate were rejected, and the workability required for the automobile body panel was not obtained.

  In Comparative Example 3, since the particle size of the alumina particles used for buffing was too low, the machined layer was too thick and the yarn rust resistance was rejected.

  In Comparative Example 4, since the Mg content was too high, the bendability and the elongation rate were rejected, the workability required for the automobile body panel was not obtained, and the pitting potential was too low and the yarn rust resistance was too low. It was rejected.

  In Comparative Example 5, since the Mg content was too small, the surface density of the Si-based intermetallic compound deposited during the heat treatment after buffing increased, and the yarn rust resistance was rejected.

  In Comparative Example 6, since the Si content was too high, the surface density of the Si-based intermetallic compound deposited during the heat treatment after buffing increased, and the yarn rust resistance was rejected.

  In Comparative Example 7, since the Si content was too small, the yield strength was rejected, and the strength required for the automobile body panel could not be obtained.

  In Comparative Example 8, since the Mn content was too large, the bendability and elongation rate were rejected, and the workability required for the automobile body panel was not obtained.

  In Comparative Example 9, since the Mn content was too small, the surface density of the Si-based intermetallic compound deposited during the heat treatment after buffing increased, and the yarn rust resistance was rejected.

  In Comparative Example 10, since the Zn content was too high, the pitting potential was too low, and the yarn rust resistance was rejected.

  In Comparative Example 11, since the Zn content was too small, the potential around the Al—Si-based intermetallic compound and Al—Cu-based intermetallic compound deposited during the heat treatment after buffing was too low, resulting in yarn rust resistance. Sex was rejected.

  In Comparative Example 12, since the Fe content was too large, the bendability and the elongation were rejected, and the workability (bendability) required for the automobile body panel was not obtained.

  In Comparative Example 13, since the Fe content was too small, the surface density of the Al—Si-based intermetallic compound deposited during the heat treatment after buffing increased, and the yarn rust resistance was rejected.

  In Comparative Example 14, since the Cu content was excessive, the surface density of the Al—Cu intermetallic compound deposited during the heat treatment after buffing increased, and the yarn rust resistance was rejected.

  In Comparative Example 15, since the Cu content was too small, the bendability and the elongation were rejected, and the workability required for the automobile body panel was not obtained.

  INDUSTRIAL APPLICABILITY The present invention uses an aluminum alloy plate for an automobile body panel that has excellent yarn rust resistance that hardly deteriorates even when buffed, and also has excellent paint bake hardenability and workability, and the same. Auto body panels can be provided, and the industrial applicability is excellent.

Claims (4)

  Mg: 0.30 to 0.80 mass%, Si: 0.80 to 1.40 mass%, Mn: 0.20 to 0.65 mass%, Zn: 0.44 to 0.60 mass%, Fe: 0.25 It contains 0.40 mass%, Cu: 0.17 to 0.25 mass%, and consists of an aluminum alloy composed of the balance Al and impurities. After 100 days of solution treatment, the elongation is 23% or more, and the paint baking treatment is performed. An aluminum alloy sheet for automobile body panels excellent in yarn rust resistance and recyclability, characterized by having a yield strength of 200 MPa or more after application and a score of 0 to 2 according to a hemming test based on JISH7701.   A method for producing an aluminum alloy plate for an automobile body panel according to claim 1, wherein a casting step of casting the aluminum alloy, a homogenization step of homogenizing the ingot, and a homogenized ingot A hot rolling step for hot rolling, an intermediate annealing step for the hot rolled plate, a cold rolling step for cold rolling the intermediate annealed rolled plate, and a solution treatment step for solution treating the cold rolled plate And a stabilization treatment step for stabilizing the solution-treated rolled plate, the temperature of the solution treatment is 480 ° C. or more, and the stabilization treatment step is started within 1 hour after the solution treatment step And a rolled plate is heated and held at a temperature of 80 to 120 ° C. for 1 hour or longer, and a method for producing an aluminum alloy plate for an automobile body panel excellent in yarn rust resistance and recyclability. An automobile body panel that is painted and baked using part or all of the aluminum alloy plate for an automobile body panel according to claim 1, wherein 0.5 to 0.5 to the thickness direction from the surface layer of the aluminum alloy plate A machined layer having a thickness of 5.0 μm is formed, and the number density of the Si-based intermetallic compound and the Al—Cu-based intermetallic compound in the machined layer is 2 / μm ² or less. Automotive body panel with excellent rust resistance and recyclability.   4. The method of manufacturing an automobile body panel according to claim 3, wherein a buffing step of buffing the surface of the aluminum alloy plate with a buff to which alumina particles having a particle size of 80 to 320 are attached, and buffed aluminum. A manufacturing method of an automobile body panel excellent in yarn rust resistance and recyclability, comprising: a coating step of coating an alloy plate; and a baking step of baking the coated aluminum alloy plate at 180 ° C. for 25 minutes.