JP7699938B2 - Aluminum alloy plate for car body parts - Google Patents

Description

The present invention relates to an aluminum alloy sheet for vehicle body parts.

In recent years, in order to protect the global environment, there is a demand for the adoption of materials that can reduce the environmental load. In the case of aluminum alloy plates, there is a high demand for recycling scraps of various used aluminum alloy parts (hereinafter referred to as aluminum scrap material) to manufacture them. When this aluminum scrap material is recycled to manufacture aluminum alloy plates, the amount of CO ₂ generated can be significantly reduced and the environmental load can be reduced compared to the case of using aluminum ingots. When such aluminum alloys recycled from aluminum scrap material are used as car body parts (e.g., automobile rainforce members), it is important to ensure press formability and paintability during part molding, structural strength as a product, corrosion resistance, etc. However, since aluminum alloy plates recycled from aluminum scrap material contain various additive elements, it has been difficult to satisfy all the required characteristics required for these structures. For example, with regard to material strength, it is necessary to keep it within a certain strength range in order to ensure press formability during part molding, and if the strength is too high or too low, the press formability is poor. On the other hand, high strength is required to ensure structural strength as a final product. The material strength of these rainforce components is improved by the temperature applied in the painting process (baking process) after press forming, which determines the strength of the final product. However, even taking into account the strength improvement caused by baking, it was difficult to achieve both press formability and the structural strength of the final product.
As such an aluminum alloy plate made from recycled aluminum scrap material, for example, the aluminum alloy plate described in Patent Document 1 is known.

For example, the aluminum alloy sheet of Patent Document 1 is formed from an aluminum alloy containing 0.4-2.0 mass% Si, 0.2-0.6 mass% Fe, 0.1-0.7 mass% Cu, 0.5-1.5 mass% Mn, 0.5-2.0 mass% Mg, 0.05-1.0 mass% Zn, with the remainder being Al and unavoidable impurities. The aluminum alloy sheet described in Patent Document 1 has a recrystallized structure throughout the entire cross section of the sheet thickness, a recrystallized grain size of 50 μm or less, and an electrical conductivity of 43.0-49.5% IACS, which improves press formability and corrosion resistance.

Patent No. 5323673

Incidentally, although Patent Document 1 describes the press formability and corrosion resistance of aluminum alloy sheets, the target product is a heat insulator for an automobile, etc., which is different from aluminum alloy sheets for vehicle body parts (e.g., aluminum alloy sheets for automobile rainforce), and therefore the required properties do not necessarily match. In addition, it is considered difficult to achieve both the above-mentioned press formability and the structural strength of the final product.

The present invention has been made in consideration of the above circumstances, and has an object to provide an aluminum alloy sheet for vehicle body parts that has formability, product strength, and corrosion resistance while reducing the environmental load by using aluminum scrap as a raw material.

As a result of intensive research, the researchers analyzed the composition of aluminum scrap generated at aluminum rolling plants, investigated in detail the effects of the alloying elements contained in large amounts in aluminum scrap, and by appropriately controlling the selection of the alloying composition range and the combination of subsequent manufacturing conditions, discovered an aluminum alloy material for car body parts that has formability, product strength and corrosion resistance.

In other words, in consideration of the above problems, the inventors have optimized the components of the aluminum alloy made from recycled aluminum scrap material, as well as the manufacturing processes such as casting, rolling and heat treatment, to control the solid solution/precipitation state of each added component, the size and dispersion state such as number density of various intermetallic compounds, and the texture, and have obtained an invention that satisfies at a high level all the required characteristics required for aluminum alloy sheets for car body parts. For example, with regard to the dispersion state of the intermetallic compounds, the proportion of coarse Mg-Si compounds is optimized to ensure the material strength required for press formability during part molding, while the heat load in the subsequent part painting process causes age hardening, resulting in high strength in the final product.

The aluminum alloy sheet for vehicle body parts of the present invention contains Fe: 0.5 mass% or less, Si: 1.0 mass% to 2.0 mass% or less, Cu: 0.2 mass% or less, Mn: 0.5 mass% to 1.5 mass% or less, Mg: 0.4 mass% to 1.2 mass% or less, Zn: 1.0 mass% or less, Zr: 0.10 mass% to 0.20 mass% or less, with the remainder being Al and unavoidable impurities, has an average crystal grain size of 100 μm or less, a 0.2% yield strength of 120 MPa to 175 MPa, an elongation of 15% or more, and a 0.2% yield strength of 180 MPa or more after applying a uniaxial strain of 2% in the rolling direction and then heat treatment at 170°C for 20 minutes.

In the present invention, the 0.2% yield strength is 120 MPa or more and 175 MPa or less, and the elongation is 15% or more, so press processing can be performed appropriately. In addition, after applying 2% uniaxial strain and then heat treating at 170°C for 20 minutes, the 0.2% yield strength can be made very high at 180 MPa or more, which can increase the strength of the final product. In addition, the Cu content is low at 0.2 mass% or less and the Zn content is low at 1.0 mass% or less, so corrosion resistance can be improved. Furthermore, the average crystal grain size is small at 100 μm or less, so a certain level of workability can be ensured.

If the average grain size exceeds 100 μm, the workability decreases. If the 0.2% yield strength is less than 120 MPa, the aluminum alloy sheet may break during press processing, and if it exceeds 175 MPa, the workability decreases. Furthermore, if the elongation is less than 15%, the workability decreases. In addition, if the 0.2% yield strength after applying 2% uniaxial strain in the rolling direction and then heat treatment at 170°C for 20 minutes is less than 180 MPa, the strength of the final product (e.g., automobile reinforcement) decreases.

Fe contributes to improving the yield strength of the aluminum alloy sheet, but if it exceeds 0.5 mass %, the proportion of intermetallic compounds increases, and the yield strength becomes too high, resulting in deterioration of formability.
Si contributes to improving the yield strength of the aluminum alloy sheet, and if it is less than 1.0 mass%, sufficient yield strength after heat treatment at 170° C. is not obtained, resulting in insufficient product strength. On the other hand, if it exceeds 2.0 mass%, the proportion of intermetallic compounds increases, and the yield strength becomes too high, resulting in poor formability.
Cu contributes to improving the yield strength and corrosion resistance of the aluminum alloy plate, and if its content exceeds 0.2 mass %, the corrosion resistance is significantly reduced.
Mn contributes to improving the yield strength of the aluminum alloy sheet, and if it is less than 0.5 mass%, sufficient yield strength after heat treatment at 170° C. is not obtained, resulting in insufficient product strength. On the other hand, if it exceeds 1.5 mass%, intermetallic compounds become coarse, and sufficient yield strength after heat treatment at 170° C. is also not obtained.
Mg contributes to improving the yield strength of the aluminum alloy sheet, and if it is less than 0.4 mass %, sufficient yield strength cannot be obtained after heat treatment at 170° C. On the other hand, if it exceeds 1.2 mass %, the yield strength becomes too high, and formability deteriorates.
Zn contributes to the corrosion resistance of the aluminum alloy sheet, and if its content exceeds 1.0 mass %, the corrosion resistance deteriorates.
Zr contributes to improving the yield strength of the aluminum alloy sheet and refining the crystal grains. If the content is less than 0.10% by mass, the 0.2% yield strength and the yield strength after heat treatment at 170°C cannot be improved, whereas if the content exceeds 0.20% by mass, the crystal grains become coarse and the formability deteriorates.

In a preferred embodiment of the aluminum alloy sheet for vehicle body parts of the present invention, it further contains one or more of Cr: 0.01% by mass to 0.10% by mass and Ti: 0.01% by mass to 0.10% by mass.
Cr contributes to improving strength, refining crystal grains, and stabilizing the structure. If the content is less than 0.01% by mass, the above effects cannot be fully obtained. If the content exceeds 0.10% by mass, the above effects become saturated and a large number of intermetallic compounds are generated, which may adversely affect formability.
Ti contributes to improving strength and refining the ingot structure. If the content is less than 0.01% by mass, the above effects cannot be sufficiently obtained. If the content exceeds 0.10% by mass, the above effects become saturated and coarse crystallized products may occur.

In a preferred embodiment of the aluminum alloy sheet for vehicle body parts of the present invention, the number of Mg-Si based second phase particles having an equivalent circle diameter of 1.0 μm or more is 5.0 × ¹⁰³ particles/ ^mm2 or less, and the number of Al-Zr based second phase particles having an equivalent circle diameter of 0.1 μm or less is 2.0 × 10 particles/ ^mm2 or more.
In the above embodiment, the proportion of coarse Mg-Si compounds is set to 5.0 x ¹⁰³ particles/ ^mm2 or less, and the proportion of Al-Zr compounds is set to 2.0 x 103 particles/ ^mm2 or more. This ensures the material strength required for press formability during part molding, while causing age hardening due to the load heat in the subsequent part painting process, thereby making it possible to obtain high strength in the final product.

In a preferred embodiment of the aluminum alloy sheet for vehicle body parts of the present invention, the sum of the area ratio of crystal grains having an orientation that differs from the cube orientation of the cross section in the rolling direction by 15° or less to all crystal grains and the area ratio of crystal grains having an orientation that differs from the Goss orientation by 15° or less to all crystal grains is 5% or more.
Here, in the case of a texture of a plate material obtained by rolling, the texture in the EBSD (Electron Backscatter Diffraction) method is expressed by the rolling surface and the rolling direction, where the rolling surface is expressed by {hkl} and the rolling direction is expressed by <uvw>. When this expression is used, the cube orientation is expressed as {001}<100> and the Goss orientation is expressed as {110}<001>.
In the above-mentioned Cube orientation {001}<100> and Goss orientation {110}<001>, the slip lines on the crystal plane (rolled surface) can be well symmetrical with respect to the bending axis at 45° and 135°. Therefore, in the above-mentioned embodiment, by increasing the sum of the area ratio of crystal grains having an orientation with an orientation difference of 15° or less from the cube orientation to all crystal grains in all crystal grains on the rolled surface and the area ratio of crystal grains having an orientation with an orientation difference of 15° or less from the Goss orientation to all crystal grains (hereinafter referred to as the orientation area ratio) to a certain level, the formation of shear bands on the outer side of bending can be suppressed, and bending workability can be significantly improved.

The method for producing an aluminum alloy sheet for vehicle body parts of the present invention comprises melting and casting an aluminum alloy containing Fe: 0.5% by mass or less, Si: 1.0% by mass to 2.0% by mass, Cu: 0.2% by mass or less, Mn: 0.5% by mass to 1.5% by mass, Mg: 0.4% by mass to 1.2% by mass, Zn: 1.0% by mass or less, Zr: 0.10% by mass to 0.20% by mass or less, with the balance being Al and unavoidable impurities, and then holding the aluminum alloy at 500°C to 600°C for 2 hours or more to homogenize the aluminum alloy. The plate is subjected to a tempering treatment, then to multiple passes of hot rolling at a rolling speed of 50 m/min or more, followed by cold rolling to form a plate material with a thickness of 0.8 mm to 2.5 mm. The plate material is then heated to 500°C to 550°C at a heating rate of 100°C/sec or more, held for 15 to 120 seconds, and then cooled to 100°C at a cooling rate of 200°C/sec or more for solution treatment, after which it is aged by storing at room temperature for 14 days or more, or aged at 50°C for 72 hours.

In the present invention, a plate material having a final thickness of 0.8 mm to 2.5 mm is formed by hot rolling and cold rolling, and this plate material is then subjected to solution treatment and industrial aging treatment or aging treatment by storing at room temperature for 14 days or more, thereby making it possible to manufacture an aluminum alloy plate for vehicle body parts that has formability, strength, and corrosion resistance.

If the homogenization temperature is less than 500° C., segregation occurring during casting remains, making it impossible to perform sufficient homogenization, whereas if the holding temperature exceeds 600° C., the ingot may melt. Furthermore, if the holding time is less than 2 hours, homogenization may not proceed sufficiently.
If the rolling speed of one pass of hot rolling is less than 50 m/min, the elongation of the material and the sum of the area ratio of crystal grains having an orientation misorientated by 15° or less from the cube orientation to all crystal grains and the area ratio of crystal grains having an orientation misorientated by 15° or less from the Goss orientation to all crystal grains (orientation area ratio) decrease, and the 0.2% yield strength after heat treatment at 170° C. decreases, resulting in a decrease in formability and product strength of the aluminum alloy sheet.
If the heating rate of the solution treatment is less than 100° C./sec, the productivity of the aluminum alloy sheet decreases, if the holding temperature is less than 500° C., the re-dissolution of the solute elements does not proceed sufficiently, and if the holding temperature exceeds 600° C., the sheet may melt and break. In addition, if the holding time is less than 15 seconds, the re-dissolution does not proceed sufficiently, and if it exceeds 120 seconds, the productivity of the aluminum alloy sheet decreases.
Furthermore, if the cooling rate in the solution treatment is less than 10° C./sec, the productivity of the aluminum alloy sheet decreases.
If the storage at room temperature after the solution treatment is less than 14 days, the age hardening will be insufficient, the strength of the aluminum alloy sheet will be insufficient, and the formability will be reduced.

According to the present invention, by using scrap aluminum as the raw material, it is possible to provide an aluminum alloy sheet for vehicle body parts that has excellent formability, strength, and corrosion resistance while reducing the environmental impact.

The following describes an embodiment in which the aluminum alloy sheet for vehicle body parts (hereinafter referred to as aluminum alloy sheet) according to the present invention is used as a reinforcement material.

[Configuration of aluminum alloy plate]
The aluminum alloy plate of the present embodiment is processed into a so-called reinforcement material that is inserted, for example, inside the hood or trunk of an automobile and is used as a reinforcement for increasing the rigidity of a structure. This aluminum alloy plate is made of an aluminum alloy made of aluminum scrap material. Specifically, the aluminum alloy that becomes the aluminum alloy plate contains 50 or 60 mass % or more of aluminum scrap material, and may be made entirely of aluminum scrap material.
Such aluminum scrap material may consist of scrap from aluminum rolling mills and various used aluminum alloy parts.

As described above, the aluminum alloy plate contains multiple elements because it is made primarily from aluminum scrap. Specifically, the aluminum alloy plate contains Fe: 0.5 mass% or less, Si: 1.0 mass% to 2.0 mass% or less, Cu: 0.2 mass% or less, Mn: 0.5 mass% to 1.5 mass% or less, Mg: 0.4 mass% to 1.2 mass% or less, Zn: 1.0 mass% or less, Zr: 0.10 mass% to 0.20 mass% or less, with the remainder being Al and unavoidable impurities.

Fe contributes to improving the yield strength of the aluminum alloy sheet, but if it exceeds 0.5 mass %, the proportion of intermetallic compounds increases, and the yield strength becomes too high, resulting in deterioration of formability.
Si contributes to improving the yield strength of the aluminum alloy sheet, and if it is less than 1.0 mass%, sufficient yield strength after heat treatment at 170° C. is not obtained, resulting in insufficient product strength. On the other hand, if it exceeds 2.0 mass%, the proportion of intermetallic compounds increases, and the yield strength becomes too high, resulting in poor formability.
Cu contributes to improving the yield strength and corrosion resistance of the aluminum alloy plate, and if its content exceeds 0.2 mass %, the corrosion resistance is significantly reduced.
Mn contributes to improving the yield strength of the aluminum alloy sheet, and if it is less than 0.5 mass%, sufficient yield strength after heat treatment at 170° C. is not obtained, resulting in insufficient product strength. On the other hand, if it exceeds 1.5 mass%, intermetallic compounds become coarse, and sufficient yield strength after heat treatment at 170° C. is also not obtained.
Mg contributes to improving the yield strength of the aluminum alloy sheet, and if it is less than 0.4 mass %, sufficient yield strength cannot be obtained after heat treatment at 170° C. On the other hand, if it exceeds 1.2 mass %, the yield strength becomes too high, and formability deteriorates.
Zn contributes to the corrosion resistance of the aluminum alloy sheet, and if its content exceeds 1.0 mass %, the corrosion resistance deteriorates.
Zr contributes to improving the yield strength of the aluminum alloy sheet and refining the crystal grains. If the content is less than 0.10% by mass, the 0.2% yield strength and the yield strength after heat treatment at 170°C cannot be improved, whereas if the content exceeds 0.20% by mass, the crystal grains become coarse and the formability deteriorates.

In addition, the aluminum alloy plate preferably further contains one or more of Cr: 0.01 mass % or more and 0.10 mass % or less, and Ti: 0.01 mass % or more and 0.1 mass % or less.
Cr contributes to improving strength, refining crystal grains, and stabilizing the structure. If the content is less than 0.01% by mass, the above effects cannot be fully obtained. If the content exceeds 0.10% by mass, the above effects become saturated and a large number of intermetallic compounds are generated, which may adversely affect formability.
Ti contributes to improving strength and refining the ingot structure. If the content is less than 0.01% by mass, the above effects cannot be sufficiently obtained. If the content exceeds 0.10% by mass, the above effects become saturated and coarse crystallized products may occur.

This aluminum alloy sheet has an average crystal grain size of 100 μm or less, a 0.2% yield strength of 120 MPa to 175 MPa, an elongation of 15% or more, and a 0.2% yield strength of 180 MPa or more after heat treatment at 170°C for 20 minutes after applying 2% uniaxial strain in the rolling direction. The application of 2% uniaxial strain is a condition assuming press forming as a reinforcement, and the heat treatment at 170°C for 20 minutes is a condition assuming painting after processing into a reinforcement. In other words, the 0.2% yield strength after heat treatment at 170°C for 20 minutes after applying 2% uniaxial strain in the rolling direction (sometimes referred to as the yield strength after 170°C heat treatment) assumes the strength of the reinforcement product.

Moreover, if the average grain size exceeds 100 μm, the workability is reduced. Furthermore, if the 0.2% proof stress is less than 120 MPa, the aluminum alloy sheet may break during press working, and if it exceeds 175 MPa, the workability is reduced. In addition, if the elongation is less than 15%, the workability is reduced. Furthermore, if the 0.2% proof stress after applying 2% uniaxial strain in the rolling direction and then performing heat treatment at 170° C. for 20 minutes is less than 180 MPa, the strength of the final product, which is an automobile part (e.g., automobile reinforcement), is reduced.
The average crystal grain size is more preferably 25 μm or less, the 0.2% proof stress is more preferably 145 MPa or more and 170 MPa or less, and the proof stress after heat treatment at 170° C. is more preferably 195 MPa or more.

The aluminum alloy plate has 5.0× ¹⁰³ Mg-Si second phase particles/ ^mm2 or less having an equivalent circle diameter of 1.0 μm or more and 2.0×103 Al-Zr second phase particles/mm2 or more having an equivalent circle diameter of 0.1 μm or less. If the number of Mg-Si second phase particles/ ^mm2 having an equivalent circle diameter of 1.0 μm or ^more exceeds 5.0× ¹⁰³ or the number of Al-Zr second phase particles having an equivalent circle diameter of 0.1 μm or less is less than 2.0× ¹⁰³ , it is difficult to ensure the material strength required for press formability during part molding, and age hardening may be difficult to occur due to the heat load in the subsequent part painting process.

Furthermore, the aluminum alloy sheet has an orientation area ratio (cube orientation area ratio) of crystal grains having an orientation difference of 15° or less with respect to all crystal grains in the cross section in the rolling direction, and an area ratio (Goss orientation area ratio) of crystal grains having an orientation difference of 15° or less with respect to all crystal grains, which is 5% or more. Although this orientation area ratio of 5% or more contributes to improving bending workability, even if the orientation area ratio value significantly exceeds 5%, the effect is saturated, so that it is sufficient that it is 5% or more. Note that if the area ratio of the above crystal grains to all crystal grains is less than 5%, it becomes difficult to suppress the formation of shear bands on the outside of the bend, making it difficult to significantly improve bending workability.

In addition, the aluminum alloy sheet preferably has a material electrical conductivity of 38% IACS or more and 43% IACS or less. The electrical conductivity is also an index showing the solid solution/precipitation state of each added element (especially Si and Mg), and affects the strength of the aluminum alloy sheet (0.2% proof stress and 0.2% proof stress after heat treatment at 170°C for 20 minutes). As the solid solution in the aluminum matrix progresses, the electrical conductivity decreases, and as precipitation as an intermetallic compound progresses, the electrical conductivity increases. Therefore, if the electrical conductivity of the material is too low or too high, the material strength and strength after aging tend to decrease, and it is desirable to keep it within the above range.
In addition, when the electrical conductivity is less than 38% IACS or exceeds 43% IACS, the solid solubility of each added element in the aluminum alloy sheet falls outside the appropriate range, so that the 0.2% proof stress and the 0.2% proof stress after heat treatment at 170° C. for 20 minutes may decrease. In addition, when the electrical conductivity exceeds 43% IACS, the intermetallic compounds tend to become coarse, so that the corrosion resistance may decrease slightly.
This electrical conductivity is more preferably 40% IACS or more and 42% IACS or less.

[Method of manufacturing aluminum alloy sheet]
The aluminum alloy plate is manufactured by the following procedure. First, an aluminum alloy having the above composition containing 50% by mass or more of aluminum scrap material is subjected to a melting and casting treatment, a homogenization treatment, a soaking treatment, a hot rolling treatment, a cold rolling treatment, a solution treatment, and an aging treatment in this order. The process will be described in detail below.

[Melting and casting process]
An aluminum alloy containing 50 mass% or more of aluminum scrap material, Fe: more than 0.5 mass% to 1.2 mass% or less, Si: 1.0 mass% to 2.0 mass% or less, Cu: 0.2 mass% or less, Mn: 0.5 mass% to 1.5 mass% or less, Mg: 0.4 mass% to 1.2 mass% or less, Zn: 1.0 mass% or less, Zr: 0.10 mass% to 0.20 mass% or less, with the balance being Al and unavoidable impurities, is melted to produce a molten aluminum alloy. The molten aluminum alloy is then cast by a semi-continuous casting method (DC casting).
The casting method is not limited to semi-continuous casting, and other conventional methods such as continuous casting may be used. Furthermore, the aluminum alloy ingot may be subjected to a surface cutting process before or after the homogenization treatment.

[Homogenization treatment]
Homogenization is performed on the ingot obtained by the semi-continuous casting method in order to remove inhomogeneous structures such as segregation. The high-temperature homogenization causes the added elements that were supersaturated in the matrix during casting to precipitate as intermetallic compounds. The size and dispersion of the precipitated intermetallic compounds are affected by the temperature and time of the homogenization, so it is necessary to select heat treatment conditions according to the type of added elements.

For example, since the aluminum alloy containing the aluminum scrap material has the above composition, the obtained ingot is subjected to homogenization treatment at a temperature of 500° C. to 600° C. for 2 hours or more. More preferably, the homogenization treatment is performed at a temperature of 535 to 595° C. for 3 to 8 hours.
If the holding temperature for the homogenization treatment is less than 500° C., segregation occurring during casting remains, making it impossible to perform sufficient homogenization, whereas if the holding temperature exceeds 600° C., the ingot may melt. Furthermore, if the holding time is less than 2 hours, homogenization may not proceed sufficiently.

[Soaking treatment]
The ingot that has been subjected to the homogenization treatment is subjected to a soaking treatment. This soaking treatment is performed at a temperature slightly lower than that of the homogenization treatment, and is maintained at a temperature of, for example, 480° C. to 550° C. for 1 hour or more. Note that the soaking treatment may be performed in combination with the soaking treatment before hot rolling.

[Hot rolling process]
The homogenized ingot (or the soaked ingot, if soaked) is subjected to hot rolling. This hot rolling is performed at a high temperature of about 500°C. Specifically, after rough hot rolling with an outlet temperature of 400°C to 460°C, the plate is passed through a single-reverse hot finishing mill three times at a rolling speed of 50 m/min or more to obtain a thickness of 2 mm to 6 mm. Specifically, the first pass of hot finishing is performed under the conditions of a rolling speed of 50 m/min to 150 m/min and a coiling temperature of 350°C to 400°C. Next, the second pass of hot finishing is performed under the conditions of a rolling speed of 50 m/min to 150 m/min and a coiling temperature of 330°C to 380°C. Finally, a third pass of hot finishing is carried out under conditions of a rolling speed of 150 m/min to 300 m/min and a coiling temperature of 230° C. to 330° C.

If the rolling speed of the hot rolling is less than 50 m/min, the elongation and cube orientation rate of the material decrease, and the 0.2% yield strength after heat treatment at 170° C. decreases, resulting in a decrease in formability and product strength of the aluminum alloy sheet.
In this embodiment, the conditions of the hot finish rolling are variously changed to control the texture of the material, and the sum (orientation area ratio) of the area ratio of crystal grains having an orientation within 15° of the cube orientation of the rolled surface in the final rolled product to all crystal grains and the area ratio of crystal grains having an orientation within 15° of the Goss orientation to all crystal grains is adjusted to a desired range.

[Cold rolling]
Next, the plate material after the hot rolling is subjected to a cold rolling process. The method of this cold rolling process is not particularly limited, but it can be performed, for example, by passing the plate material through a rolling mill. The thickness of the plate material after this cold rolling is, for example, 0.8 mm or more and 2.5 mm or less.

[Solution treatment]
The cold-rolled plate is then subjected to a solution treatment, in which the plate is heated to 500° C. to 550° C. at a heating rate of 100° C./s or more, held at this temperature for 15 to 120 seconds, and then cooled to 100° C. or less at a cooling rate of 200° C./s or more.
If the heating rate of the solution treatment is less than 100°C/sec, the productivity of the aluminum alloy sheet decreases, if the holding temperature is less than 500°C, the recrystallization does not proceed sufficiently, and if the holding temperature exceeds 600°C, the aluminum alloy sheet may melt and break. If the holding time is less than 15 seconds, the recrystallization does not proceed sufficiently, and if it exceeds 120 seconds, the productivity of the aluminum alloy sheet decreases. If the cooling rate of the solution treatment is less than 200°C/sec, the productivity of the aluminum alloy sheet decreases.

[Aging treatment]
Finally, the solution-treated sheet material is stored at room temperature for 14 days or more, or aged at 50°C for 72 hours, which is equivalent to room temperature. The proof stress and elongation of the aluminum alloy sheet after aging treatment following rolling have a large effect on the press formability of the product. The 0.2% proof stress of the aluminum alloy sheet after this aging treatment is 120 MPa or more and 175 MPa or less, and the elongation is 15% or more.

Furthermore, the 0.2% yield strength of the aluminum alloy sheets manufactured in this manner (aluminum alloy sheets after aging treatment) was measured by the following method, assuming the thermal load in the painting process after press forming of the product. Specifically, the test pieces were subjected to a 2% strain by a tensile test based on JIS Z2241, and then subjected to a heat treatment in which the test pieces were heated to 170°C at a heating rate of 10°C/sec or more, held for 20 minutes, and then cooled at a cooling rate of 10°C/sec or more, and then the yield strength was measured. The yield strength of these aluminum alloy sheets after the 170°C heat treatment is equivalent to the product strength of the product. The 0.2% yield strength of the aluminum alloy sheets after the 170°C heat treatment is 180 MPa or more.

Next, we will explain the crystal orientation. In the expression of the texture in the EBSD method, the cube orientation is expressed as {001}<100>, and the Goss orientation is expressed as {110}<001>. The cube orientation and the Goss orientation show similar characteristics in three directions: the thickness direction ND of the rolled surface, the rolling direction LD, and the direction perpendicular to the rolling direction TD. In the cube orientation {001}<100> and the Goss orientation {110}<001>, the slip lines on the crystal plane (rolled surface) can be made to have good symmetry at 45° and 135° with respect to the bending axis. For this reason, it was found that by increasing the sum (orientation area ratio) of the area ratio of crystal grains with orientations that are within 15° of the cube orientation to all crystal grains and the area ratio of crystal grains with orientations that are within 15° of the Goss orientation to all crystal grains to a certain level, the formation of shear bands on the outside of the bend can be suppressed, and bending workability can be significantly improved.

The aluminum alloy sheet produced in this manner has an average crystal grain size of 100 μm or less, a 0.2% yield strength of 120 MPa to 175 MPa, an elongation of 15% or more, and a 0.2% yield strength of 180 MPa or more after heat treatment at 170°C for 20 minutes after application of 2% uniaxial strain, i.e., an aluminum alloy sheet for vehicle body parts that has formability, strength, and corrosion resistance. In addition, since the aluminum alloy sheet can be produced from an aluminum alloy containing 50% or more by mass of aluminum scrap material, the environmental load can be reduced.

Specifically, the aluminum alloy plate of this embodiment has a 0.2% yield strength of 120 MPa or more and 175 MPa or less, and an elongation of 15% or more, so that press working can be performed appropriately. In addition, after applying uniaxial strain of 2% in the rolling direction and then heat treatment at 170°C for 20 minutes, the 0.2% yield strength can be made very high at 180 MPa or more, and the strength of the final product (e.g., automobile reinforcement) can be increased. In addition, the Cu content is low at 0.2 mass% or less and the Zn content is low at 1.0 mass% or less, so that corrosion resistance can be increased. Furthermore, the average crystal grain size is small at 100 μm or less, so that workability can be improved.

In addition, by optimizing the abundance ratio of coarse Mg-Si compounds to 5.0 × ¹⁰³ particles/ ^mm2 or less and the abundance ratio of Al-Zr compounds having an equivalent circle diameter of 0.1 μm or less to 2.0 × 10 particles/ ^mm2 or more, it is possible to ensure the material strength required for press formability during part molding, while causing age hardening by the load heat in the subsequent part painting process, thereby obtaining high strength in the final product. Furthermore, the sum of the area ratio of the crystal grains having an orientation difference of 15° or less with respect to the cube orientation of the rolled surface to all the crystal grains and the area ratio of the crystal grains having an orientation difference of 15° or less with respect to the Goss orientation to all the crystal grains (orientation area ratio) is 5% or more, and in this cube orientation (Cube orientation) {001}<100> and Goss orientation {110}<001>, the slip lines on the crystal plane (rolled surface) can be made symmetrical at 45° and 135° with respect to the bending axis, respectively, so that by increasing the orientation area ratio to a certain level, the formation of shear bands on the outside of the bend can be suppressed and bending workability can be significantly improved. In addition, since the electrical conductivity is set to 38% IACS or more and 43% IACS or less, the 0.2% proof stress of the aluminum alloy sheet and the 0.2% proof stress after heat treatment at 170°C for 20 minutes can each be within the above numerical range. Furthermore, if the electrical conductivity is 43% IACS or less, the intermetallic compounds do not become coarse, and therefore the deterioration of corrosion resistance can be suppressed.

Then, by performing press forming on the aluminum alloy plate for vehicle body parts as described above and then painting it, it is possible to provide an automobile reinforcement having excellent formability, strength and corrosion resistance.

The present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention.
For example, in the above embodiment, an example has been described in which the aluminum alloy sheet is processed into automobile reinforcements, but the aluminum alloy sheet for vehicle body parts can also be applied to vehicle frame parts other than reinforcements, vehicle body parts such as outer panels, etc. Also, the aluminum alloy sheet can be applied to chassis parts, and in the present invention, the term "vehicle body parts" includes chassis parts.

The aluminum alloys of Examples 1 to 17 and Comparative Examples 1 to 14 were produced by the methods described below, and the 0.2% proof stress and elongation of each of the obtained samples were measured, and then the formability was evaluated.
The compositions (ingredients) of the aluminum alloys used as the raw materials for Examples 1 to 17 and Comparative Examples 1 to 14 were as shown in Table 1.

These aluminum alloys were melted to generate molten aluminum alloys, which were then cast by semi-continuous casting. The ingots obtained by the semi-continuous casting method were subjected to homogenization treatment at 540°C for 8 hours, followed by soaking treatment at 510°C for 1 hour, hot rolling under the various conditions shown in Table 2, and cold rolling to form plates with a thickness of 1.5 mm. The plates were heated to 520°C at a heating rate of 100°C/s or more and held for 20 seconds or less, then cooled to 100°C or less at a cooling rate of 50°C/s, followed by aging treatment at 50°C for 72 hours to obtain the respective samples.
In addition, in the hot rolling conditions in Table 2, the rolling speed is shown in a range of not less than the lower limit value and not more than the upper limit value in each column for rolling conditions B to D (for example, not less than 20 m/min and not more than 50 m/min in the first pass of rolling condition D), and in the rolling condition A, the range is shown in a range of not less than the lower limit value and not more than the upper limit value in each column (not less than 100 m/min and not more than 150 m/min in the first pass).

(Measurement of 0.2% Yield Strength)
The 0.2% proof stress was measured by a method conforming to JIS Z 2241. Specifically, a sample was cut out from each obtained sample parallel to the rolling direction to prepare a test piece of JIS No. 5 shape, and a tensile test was performed at room temperature to measure the proof stress (MPa). The tensile speed was 5 mm/min.

(Measurement of Elongation)
The elongation was measured by a method conforming to JIS Z2241. Specifically, a sample was cut out from each obtained sample parallel to the rolling direction to prepare a test piece of JIS No. 5 shape, and a tensile test was performed at room temperature to measure the elongation. Note that the elongation referred to here is the permanent elongation after break based on JIS Z2241 expressed as a percentage of the original gauge length.

(conductivity)
The electrical conductivity was measured by a four-terminal method. A current of 500 mA was applied to the sample in a room temperature environment of 20 to 25° C., and the resistance was calculated from the voltage value, and then the electrical conductivity was calculated.

(Observation of compound particle distribution state)
The cross section parallel to the rolling direction of the produced aluminum alloy was observed. The cross section was subjected to CP processing (cross-section processing) based on the ion milling method, and the observation was performed using a field emission scanning electron microscope (FE-SEM). The circle equivalent diameter and distribution density of the compound particles (Mg-Si compound particles and Al-Zr compound particles) were calculated by image analysis based on the observed images, and the number density (pieces/mm ² ) of Mg-Si compound particles with a circle equivalent diameter of 1.0 μm or more and the number density (pieces/mm ² ) of Al-Zr compounds with a circle equivalent diameter of 0.1 μm or less are shown in Table 3.

(crystal grain size)
The method for exposing the metal structure was to polish a cross section of the aluminum alloy plate parallel to the rolling direction with emery paper, roughly buff polish and finish polish, rinse with water, dry, and further perform anodizing treatment in Barker's solution under the conditions of a bath temperature of 25° C., an applied voltage of 30 V, and an application time of 120 seconds. The treated sample was photographed using a polarized optical microscope, and the average crystal grain size was calculated by a cut-off method.

(azimuth area ratio)
The sum (orientation area ratio) of the area ratio of crystal grains having an orientation within 15° of the Cube orientation to all crystal grains and the area ratio of crystal grains having an orientation within 15° of the Goss orientation to all crystal grains was measured as the sum of the area fraction of crystal grains having an orientation within 15° of the Cube{001}<100> orientation to all crystal grains and the area fraction of crystal grains having an orientation within 15° of the Goss orientation to all crystal grains in an OIM (Orientation Imaging Microscopy) image of a crystal grain orientation distribution map measured by an EBSD (Electron Backscatter Diffraction) method. Specifically, a measurement region of 0.20 mm × 1.5 mm (plate thickness) of the plate thickness cross section in the rolling direction of the above-mentioned aluminum alloy plate was scanned with an electron beam at a pitch of 1 μm to measure the crystal orientation at each measurement point, and among the crystal grains determined from the orientation difference between the measurement points, crystal grains having an orientation with an orientation difference of 15° or less from the Cube orientation and crystal grains having an orientation with an orientation difference of 15° or less from the Goss orientation were each measured for their average area ratio (%) relative to the measured area to calculate the Cube orientation area ratio and the Goss orientation area ratio, and the sum of these was taken as the orientation area ratio.

(Evaluation of formability)
The formability was evaluated by visual inspection of the occurrence of cracks and wrinkles when bending at 180° in close contact according to JIS Z 2248. In this case, a specimen with some wrinkles but no cracks was evaluated as very good (◎), a specimen with some cracks or wrinkles but no cracks overall was evaluated as good (◯), and a specimen with noticeable cracks over the entire surface was evaluated as poor (×).

(Product strength evaluation)
After applying 2% uniaxial strain in the rolling direction to the samples, the 0.2% yield strength after heat treatment for 20 minutes at 170° C. was assumed to be the product strength. Those with a yield strength of 200 MPa or more were evaluated as very good (◎), those with a yield strength of 180 MPa or more but less than 200 MPa were evaluated as good (◯), and those with a yield strength of less than 180 MPa were evaluated as unacceptable (×).

(Evaluation of corrosion resistance)
To evaluate corrosion resistance, a salt spray test (SST) was performed for 800 hours. After the corrosion test, the samples were subjected to the measurement of corrosion weight loss after removing the corrosion products with chromium phosphate. Based on the results, samples with corrosion weight loss of less than 15.0 mg/ ^cm2 were evaluated as good (◯), and samples with corrosion weight loss of 15.0 mg/ ^cm2 or more were evaluated as poor (×).
The yield strength, elongation, electrical conductivity, number density of Mg—Si compounds having an equivalent circle diameter of 1.0 μm or more, number density of Al—Zr compounds having an equivalent circle diameter of 0.1 μm or less, crystal grain size, Cube orientation area ratio, Goss orientation area ratio, orientation area ratio, and yield strength after heat treatment at 170° C. are shown in Table 3, and the various evaluations are shown in Table 4.

As shown in Tables 3 and 4, Examples 1 to 17 contain Fe: 0.5% by mass or less, Si: 1.0% by mass to 2.0% by mass, Cu: 0.2% by mass or less, Mn: 0.5% by mass to 1.5% by mass, Mg: 0.4% by mass to 1.2% by mass, Zn: 1.0% by mass or less, Zr: 0.10% by mass to 0.20% by mass, with the remainder being Al and unavoidable impurities, have an average crystal grain size of 100 μm or less, a 0.2% yield strength of 120 MPa to 175 MPa, an elongation of 15% or more, and a 0.2% yield strength of 180 MPa or more after applying 2% uniaxial strain in the rolling direction and then heat treatment at 170°C for 20 minutes. All of these have good or very good formability and product strength, and all have good corrosion resistance.

On the other hand, in Comparative Examples 1 and 2, the Fe content was too high, resulting in high 0.2% yield strength and poor formability. In Comparative Example 3, the Si content was too low, resulting in low 0.2% yield strength after 170°C heat treatment, resulting in poor product strength. In Comparative Example 4, the Si content was too high, resulting in poor corrosion resistance, high 0.2% yield strength, and poor formability. In Comparative Example 5, the Cu content was too high, resulting in poor corrosion resistance, high 0.2% yield strength, and poor formability. In Comparative Example 6, the Mn content was too low, resulting in low 0.2% yield strength and low 0.2% yield strength after 170°C heat treatment, resulting in poor formability and product strength. In Comparative Example 7, the Mn content was too high, resulting in low 0.2% yield strength after 170°C heat treatment, resulting in poor product strength.

In addition, in Comparative Example 8, the Mg content was too low, so both the 0.2% yield strength and the 0.2% yield strength after 170°C heat treatment were low, and the formability and product strength were poor. In Comparative Example 9, the Mg content was too high, so the 0.2% yield strength was low and the formability was poor. In Comparative Example 10, the Zn content was too high, so the corrosion resistance was poor. In Comparative Example 11, the Zr content was too low, so both the 0.2% yield strength and the 0.2% yield strength after 170°C heat treatment were low, and the formability and product strength were poor. In Comparative Examples 12 to 14, the hot rolling condition was D, and the rolling speeds in the first and second passes were lower than in the other conditions A to C. For this reason, Comparative Examples 12 to 14 had low elongation. Of these, in Comparative Example 13, the crystal grain size was small due to the influence of Fe, Mn, and other elements other than Zr, but the elongation was low, making the formability unacceptable, and the 0.2% yield strength after heat treatment at 170°C was low, so the product strength was also unacceptable. In Comparative Example 14, the crystal grain size was large and the elongation was low, making the formability unacceptable, and the 0.2% yield strength after heat treatment at 170°C was low, so the product strength was also unacceptable. On the other hand, in Comparative Example 12, the crystal grains were coarsened by heat treatment at a certain temperature such as homogenization treatment due to the large content of Zr components, and the crystal grain size became large, and the elongation was also low, so the formability was unacceptable.

Claims (3)

Fe: 0.5 mass% or less, Si: 1.0 mass% or more and 2.0 mass% or less, Cu: 0.2 mass% or less, Mn: 0.5 mass% or more and 1.5 mass% or less, Mg: 0.4 mass% or more and 1.2 mass% or less, Zn: 1.0 mass% or less, Zr: 0.10 mass% or more and 0.20 mass% or less, with the balance being Al and unavoidable impurities, the average crystal grain size is 100 μm or less, the 0.2% proof stress is 120 MPa or more and 175 MPa or less, the elongation is 15% or more, and the 0.2% proof stress after applying a uniaxial strain of 2% in the rolling direction and then performing a heat treatment at 170° C. for 20 minutes is 180 MPa or more,
An aluminum alloy sheet for vehicle body parts, characterized in that Mg-Si based second phase particles having an equivalent circle diameter of 1.0 μm or more are 5.0 × 103 ^particles /mm2 ^or less, and Al-Zr based second phase particles having an equivalent circle diameter of 0.1 μm or less are 2.0 × 10 particles/mm2 ^or more . The aluminum alloy sheet for vehicle body parts according to claim 1, further comprising one or more of the following: Cr: 0.01% by mass to 0.10% by mass; Ti: 0.01% by mass to 0.10% by mass. 3. The aluminum alloy sheet for vehicle body parts according to claim 1 or 2, characterized in that the sum of an area ratio of crystal grains having an orientation with an orientation difference of 15° or less from a cube orientation in a cross section in the rolling direction to all crystal grains and an area ratio of crystal grains having an orientation with an orientation difference of 15° or less from a Goss orientation to all crystal grains is 5% or more.