Aluminum alloy plates for automotive reinforcement

Abstract

To provide an aluminum alloy sheet for car reinforcements that is made from aluminum scrap material to reduce the environmental load, while having moldability, product strength and corrosion resistance, and a method for producing the same.SOLUTION: An aluminum alloy sheet for car reinforcements contains Fe: 0.5 mass% or less, Si: 1.2 mass% or more and 1.6 mass% or less, Cu: 0.2 mass% or less, Mn: 0.8 mass% or more and 1.2 mass% or less, Mg: 0.45 mass% or more and 0.7 mass% or less, and Zn: 0.7 mass% or less with the balance being Al and inevitable impurities. The aluminum alloy sheet has an average crystal grain size of 30 μm or less, a 0.2% proof stress of 100 MPa or more and 155 MPa or less, and an elongation of 20% or more. The aluminum alloy sheet is subjected to 2% uniaxial strain in a rolling direction and then heated at 170°C for 20 minutes, resulting in the 0.2% proof stress of 170 MPa or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an aluminum alloy for automobile reinforcement. On the board Regarding. In addition, reinforcement is also called reinforce, reinforce, reinforce, or reinforcement. [Background technology]

[0002] In recent years, the adoption of materials that reduce environmental impact has become necessary to protect the global environment. There is a strong demand for aluminum alloy sheets manufactured using scrap aluminum from various used aluminum alloy parts (hereinafter referred to as aluminum scrap). Manufacturing aluminum alloy sheets from this scrap aluminum significantly reduces CO2 emissions and environmental impact compared to using aluminum bullion. When using aluminum alloys recycled from such scrap aluminum for automotive reinforcement components, ensuring press formability and paintability during part molding, as well as structural strength and corrosion resistance as a finished product, are important. However, because aluminum alloy sheets recycled from scrap aluminum contain various additive elements, it has been difficult to satisfy all of the required properties for these structures. For example, material strength must fall within a certain strength range to ensure press formability during part molding; either too high or too low strength results in poor press formability. On the other hand, high strength is required to ensure the structural strength of the final product. The strength of these reinforcement components is improved by the temperature they are exposed to during the painting process (baking process) after press molding, which contributes to the strength of the final product.However, even taking into account the strength improvement caused by this baking process, 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.

[0003] For example, the aluminum alloy sheet of Patent Document 1 is formed from an aluminum alloy containing 0.4 to 2.0 mass% Si, 0.2 to 0.6 mass% Fe, 0.1 to 0.7 mass% Cu, 0.5 to 1.5 mass% Mn, 0.5 to 2.0 mass% Mg, 0.05 to 1.0 mass% Zn, and 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 to 49.5% IACS, thereby improving press formability and corrosion resistance. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5323673 Summary of the Invention [Problem to be solved by the invention]

[0005] Incidentally, although Patent Document 1 describes the press formability and corrosion resistance of the aluminum alloy sheet, the target product is a heat insulator for an automobile, etc., which is different from the aluminum alloy sheet for automobile reinforcement of the present invention, and therefore the required properties do not necessarily match. In addition, it is thought to be difficult to achieve the above-mentioned press formability and ensure the structural strength of the final product.

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an aluminum alloy sheet for automobile reinforcement, which has formability, product strength and corrosion resistance while reducing the environmental load by using aluminum scrap as a raw material, and a manufacturing method thereof. [Means for solving the problem]

[0007] As a result of intensive research, the researchers analyzed the composition of aluminum scrap generated at aluminum rolling plants and studied in detail the effects of the alloying elements contained in large quantities in aluminum scrap. By selecting the range of alloying elements and appropriately controlling the combination of subsequent manufacturing conditions, they discovered an aluminum alloy material for automotive reinforcements that has formability, product strength, and corrosion resistance.

[0008] In other words, in consideration of the above-mentioned problems, the inventors have optimized the composition of aluminum alloys made from recycled aluminum scrap, as well as the manufacturing processes such as casting, rolling, and heat treatment, to control the solid solution and precipitation states of each added component, the dispersion state (size, number density, etc.) of various intermetallic compounds, and the texture, thereby obtaining an invention that satisfies all of the required properties for automotive rainforce to a high degree. For example, with regard to the dispersion state of intermetallic compounds, optimizing the proportion of coarse Mg-Si compounds ensures the material strength required for press formability during part molding, while the thermal load in the subsequent part painting process causes age hardening, resulting in high strength in the final product.

[0009] The aluminum alloy sheet for automotive reinforcement of the present invention contains 0.5% by mass or less Fe, 1.2 to 1.6% by mass Si, 0.2% by mass or less Cu, 0.8 to 1.2% by mass Mn, 0.45 to 0.7% by mass Mg, 0.7% by mass or less Zn, and the balance being Al and unavoidable impurities, and has an average grain size of 30 μm or less, a 0.2% yield strength of 100 to 155 MPa, an elongation of 20% or more, and a 0.2% yield strength of 170 MPa or more after applying a uniaxial strain of 2% in the rolling direction and then heat treatment at 170°C for 20 minutes.

[0010] In the present invention, the 0.2% yield strength is 100 MPa or more and 155 MPa or less, and the elongation is 20% or more, allowing for appropriate press working. Furthermore, after applying a 2% uniaxial strain and then heat treating at 170°C for 20 minutes, the 0.2% yield strength can be increased to 170 MPa or more, thereby enhancing the strength of the final product. Furthermore, the Cu content is low, at 0.2 mass% or less, and the Zn content is low, at 0.7 mass% or less, allowing for enhanced corrosion resistance. Furthermore, the average crystal grain size is small, at 30 μm or less, allowing for improved workability.

[0011] If the average grain size exceeds 30 μm, the workability decreases. If the 0.2% yield strength is less than 100 MPa, the aluminum alloy sheet may break during press working, and if it exceeds 150 MPa, the workability decreases. Furthermore, if the elongation is less than 20%, the workability decreases. In addition, if the 0.2% yield strength after applying 2% uniaxial strain in the rolling direction and then heat treating at 170°C for 20 minutes is less than 170 MPa, the strength of the final product (automotive reinforcement) decreases.

[0012] 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 a deterioration in formability. Si contributes to improving the yield strength of the aluminum alloy sheet, and if it is less than 1.2% by mass, sufficient yield strength cannot be obtained after heat treatment at 170°C, resulting in insufficient product strength. On the other hand, if it exceeds 1.6% by 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 sheet, and if it exceeds 0.2 mass %, the corrosion resistance will be significantly reduced. Mn contributes to improving the yield strength of the aluminum alloy sheet, and if it is less than 0.8% by mass, sufficient yield strength cannot be obtained after heat treatment at 170°C, resulting in insufficient product strength. On the other hand, if it exceeds 1.2% by mass, intermetallic compounds become coarse, and sufficient yield strength cannot be obtained after heat treatment at 170°C. Mg contributes to improving the yield strength of the aluminum alloy sheet, and if its content is less than 0.45% by mass, sufficient yield strength cannot be obtained after heat treatment at 170° C. On the other hand, if its content exceeds 0.7% by mass, the yield strength becomes too high, resulting in a decrease in elongation and deterioration in formability. Zn contributes to the corrosion resistance of the aluminum alloy sheet, and if the content exceeds 0.7 mass %, the corrosion resistance deteriorates.

[0013] In a preferred embodiment of the aluminum alloy sheet for automobile reinforcement of the present invention, it may further contain one or more of Cr: 0.01% by mass or more and 0.10% by mass or less, Ti: 0.01% by mass or more and 0.10% by mass or less, and Zr: 0.01% by mass or more and 0.10% by mass or less. Cr and Zr each contribute to improving strength, refining crystal grains, and stabilizing the structure. If the content is less than 0.01% by mass, the above effects are not fully achieved, whereas if the content exceeds 0.10% by mass, the above effects become saturated and many intermetallic compounds are generated, which may adversely affect formability. Ti contributes to improving strength and refining the ingot structure, and if it is less than 0.01% by mass, the above effects cannot be sufficiently obtained, while if it exceeds 0.10% by mass, the above effects become saturated and coarse crystallized particles may be formed.

[0014] In a preferred embodiment of the aluminum alloy sheet for automobile reinforcement of the present invention, the Mg-Si based second phase particles having an equivalent circle diameter of 1.0 μm or more are contained in an amount of 5.0×10 3 pieces / mm 2 It would be better if it was below. In the above embodiment, the proportion of coarse Mg-Si compounds is 5.0 × 10 3 pieces / mm 2 By optimizing the following, it is possible to ensure the material strength required for press formability during part molding, while also achieving high strength in the final product by causing age hardening due to the heat load during the subsequent part painting process.

[0015] In a preferred embodiment of the aluminum alloy sheet for automobile reinforcement of the present invention, the sum of the area ratio of crystal grains having an orientation misorientated within 15° from the cube orientation of the cross section in the rolling direction to all crystal grains and the area ratio of crystal grains having an orientation misorientated within 15° from the Goss orientation to all crystal grains is 3% or more. Here, the texture in the EBSD (Electron Backscatter Diffraction) method is expressed by the rolling plane and rolling direction in the case of the texture of a rolled sheet material. The rolling plane is expressed as {hkl} and the rolling direction is <uvw>When this expression is used, the cube orientation is {001} <100> The Goss direction is expressed as {110} <001> It is expressed as follows. Above Cube orientation {001} <100> and Goss orientation {110} <001> In this case, the slip lines on the crystal plane (rolled surface) can be made well symmetrical at 45° and 135° to the bending axis. Therefore, in the above embodiment, by increasing the sum of the area ratio of crystal grains having an orientation that is within 15° of the cubic orientation to all crystal grains in all crystal grains on the rolled surface and the area ratio of crystal grains having an orientation that is within 15° of 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 outside of the bend can be suppressed and bending workability can be significantly improved.

[0016] The method for producing an aluminum alloy sheet for automobile reinforcement of the present invention includes melting and casting an aluminum alloy containing aluminum scrap material, Fe: 0.5 mass% or less, Si: 1.2 mass% to 1.6 mass% or less, Cu: 0.2 mass% or less, Mn: 0.8 mass% to 1.2 mass% or less, Mg: 0.45 mass% to 0.7 mass% or less, Zn: 0.7 mass% or less, and the balance being Al and unavoidable impurities, and then holding the melting and casting at 500°C to 600°C for 2 hours or more. After homogenization, the plate is hot-rolled in multiple passes at a rolling speed of 50 m / min or more, and then cold-rolled 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 at this temperature for 15 to 120 seconds, and then solution-treated by cooling to 100°C at a cooling rate of 200°C / sec or more. The plate material is then aged by storing at room temperature for 14 days or more, or aged at 50°C for 72 hours.

[0017] In the present invention, a plate having a final thickness of 0.8 mm or more and 2.5 mm or less is formed by hot rolling and cold rolling, and this plate is then subjected to a solution treatment and an industrial aging treatment or an aging treatment by storing at room temperature for 14 days or more, whereby an aluminum alloy plate for automobile reinforcement having formability, strength, and corrosion resistance can be produced.

[0018] If the homogenization temperature is less than 500°C, segregation that occurs during casting remains, preventing sufficient homogenization, while if the holding temperature exceeds 600°C, the ingot may melt. Also, if the holding time is less than 2 hours, homogenization may not proceed sufficiently. If the rolling speed for 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 that is within 15° of the cubic orientation to all crystal grains and the area ratio of crystal grains having an orientation that is within 15° of the Goss orientation to all crystal grains (orientation area ratio) will decrease, and the 0.2% yield strength after heat treatment at 170°C will decrease, resulting in a decrease in the formability and product strength of the aluminum alloy sheet. If the heating rate of the solution treatment is less than 100°C / second, 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. Furthermore, 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 / second, 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. [Effects of the Invention]

[0019] According to the present invention, by using aluminum scrap as the raw material, it is possible to provide an automobile reinforcement member that has excellent formability, strength, and corrosion resistance while reducing the environmental load. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, an embodiment of an aluminum alloy sheet for automobile reinforcement (hereinafter referred to as aluminum alloy sheet) according to the present invention will be described.

[0021] [Aluminum alloy plate composition] The aluminum alloy plate of this embodiment is processed into a so-called reinforcement material that is inserted, for example, inside the hood or trunk of an automobile and used as a reinforcement to increase the rigidity of the structure. This aluminum alloy plate is formed from an aluminum alloy made from aluminum scrap. Specifically, the aluminum alloy that becomes the aluminum alloy plate may contain 50 or 60 mass % or more of aluminum scrap, and may be formed entirely from aluminum scrap. Such aluminum scrap material is made up of scrap from aluminum rolling mills and various used aluminum alloy parts.

[0022] As described above, the aluminum alloy plate is mainly made from aluminum scrap and therefore contains a plurality of elements, specifically, the aluminum alloy plate contains 0.5 mass% or less of Fe, 1.2 to 1.6 mass% of Si, 0.2 mass% or less of Cu, 0.8 to 1.2 mass% of Mn, 0.45 to 0.7 mass% of Mg, and 0.7 mass% or less of Zn, with the balance being Al and unavoidable impurities.

[0023] 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 a deterioration in formability. Si contributes to improving the yield strength of the aluminum alloy sheet, and if it is less than 1.2% by mass, the yield strength after the heat treatment at 170°C described below is insufficient, resulting in insufficient product strength. On the other hand, if it exceeds 1.6% by 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 sheet, and if it exceeds 0.2 mass %, the corrosion resistance will be significantly reduced. Mn contributes to improving the yield strength of the aluminum alloy sheet, and if it is less than 0.8% by mass, sufficient yield strength cannot be obtained after heat treatment at 170°C, resulting in insufficient product strength. On the other hand, if it exceeds 1.2% by mass, intermetallic compounds become coarse, and sufficient yield strength cannot be obtained after heat treatment at 170°C. Mg contributes to improving the yield strength of the aluminum alloy sheet, and if its content is less than 0.45% by mass, sufficient yield strength cannot be obtained after heat treatment at 170° C. On the other hand, if its content exceeds 0.7% by mass, the yield strength becomes too high, resulting in a decrease in elongation and deterioration in formability. Zn contributes to the corrosion resistance of the aluminum alloy sheet, and if the content exceeds 0.7 mass %, the corrosion resistance deteriorates.

[0024] The aluminum alloy plate preferably further contains one or more of Cr: 0.01% by mass to 0.10% by mass, Ti: 0.01% by mass to 0.1% by mass, and Zr: 0.01% by mass to 0.10% by mass. Cr and Zr each contribute to improving strength, refining crystal grains, and stabilizing the structure. If the content is less than 0.01% by mass, the above effects are not fully achieved, whereas if the content exceeds 0.10% by mass, the above effects become saturated and many intermetallic compounds are generated, which may adversely affect formability. Ti contributes to improving strength and refining the ingot structure, and if it is less than 0.01% by mass, the above effects cannot be sufficiently obtained, while if it exceeds 0.10% by mass, the above effects become saturated and coarse crystallized particles may be formed.

[0025] This aluminum alloy sheet has an average grain size of 30 μm or less, a 0.2% yield strength of 100 MPa to 155 MPa, an elongation of 20% or more, and a 0.2% yield strength of 170 MPa or more after applying a 2% uniaxial strain in the rolling direction and then heat treating at 170°C for 20 minutes. The application of 2% uniaxial strain is a condition assuming press forming for use as reinforcement, and the heat treatment at 170°C for 20 minutes is a condition assuming painting after processing into reinforcement. In other words, the 0.2% yield strength after applying a 2% uniaxial strain in the rolling direction and then heat treating at 170°C for 20 minutes (sometimes referred to as the yield strength after 170°C heat treatment) is expected to be the strength required for a reinforcement product.

[0026] Furthermore, if the average grain size exceeds 30 μm, the workability decreases. Furthermore, if the 0.2% yield strength is less than 100 MPa, the aluminum alloy sheet may break during press working, and if it exceeds 150 MPa, the workability decreases. In addition, if the elongation is less than 20%, the workability decreases. Furthermore, 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 170 MPa, the strength of the final product (automotive reinforcement) decreases. The average crystal grain size is more preferably 25 μm or less, the 0.2% yield strength is more preferably 125 MPa or more and 150 MPa or less, and the yield strength after heat treatment at 170° C. is more preferably 185 MPa or more.

[0027] In addition, the aluminum alloy plate contains 5.0 × 10 Mg-Si second phase particles with a circle equivalent diameter of 1.0 μm or more. 3 pieces / mm 2 The Mg-Si second phase particles having a circle equivalent diameter of 1.0 μm or more are 5.0 × 10 3 pieces / mm 2 If the temperature exceeds this range, it may be 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.

[0028] Furthermore, the aluminum alloy sheet has an orientation area ratio (cube orientation area ratio) of crystalline grains having an orientation misorientation within 15° from the cube orientation in the cross section in the rolling direction relative to all crystalline grains, and an orientation area ratio (Goss orientation area ratio) of crystalline grains having an orientation misorientation within 15° from the Goss orientation relative to all crystalline grains, of 3% or more. This orientation area ratio of 3% or more contributes to improving bending workability, but even if the orientation area ratio value significantly exceeds 3%, the effect saturates, so a value of 3% or more is sufficient. Note that if the area ratio of the above-mentioned crystalline grains relative to all crystalline grains is less than 3%, it becomes difficult to suppress shear band formation on the outer side of the bend, making it difficult to significantly improve bending workability.

[0029] Furthermore, the aluminum alloy sheet preferably has a material electrical conductivity of 40% IACS or more and 45% IACS or less. Electrical conductivity is also an index showing the state of solid solution or precipitation of each added element (particularly 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 solid solution into the aluminum matrix progresses, electrical conductivity decreases, and as precipitation as intermetallic compounds progresses, electrical conductivity increases. For this reason, if the electrical conductivity of the material is too low or too high, the material strength and strength after aging tend to decrease, so it is desirable to keep it within the above range. If the electrical conductivity is less than 40% IACS or exceeds 45% IACS, the solid solubility of each added element in the aluminum alloy sheet will be outside the appropriate range, which may result in a decrease in 0.2% yield strength and 0.2% yield strength after heat treatment at 170°C for 20 minutes. Furthermore, if the electrical conductivity exceeds 45% IACS, the intermetallic compounds tend to coarsen, which may result in a slight decrease in corrosion resistance. This conductivity is more preferably 42% IACS or more and 44% IACS or less.

[0030] [Method of manufacturing aluminum alloy sheets] 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 is subjected to a melting and casting treatment, a homogenizing treatment, a soaking treatment, a hot rolling treatment, a cold rolling treatment, a solution treatment, and an aging treatment in this order. The process is specifically described below.

[0031] [Melting and casting process] An aluminum alloy containing 50% by mass or more of aluminum scrap material and containing 0.5% by mass or less of Fe, 1.2% by mass to 1.6% by mass of Si, 0.2% by mass or less of Cu, 0.8% by mass to 1.2% by mass of Mn, 0.45% by mass to 0.7% by mass of Mg, and 0.7% by mass or less of Zn, with the balance being Al and unavoidable impurities, is melted to produce a molten aluminum alloy, and 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 also be used. Furthermore, the aluminum alloy ingot may be subjected to a facing process before or after the homogenization treatment.

[0032] [Homogenization] Homogenization is performed on ingots obtained by semi-continuous casting to remove inhomogeneous structures such as segregation. The high-temperature homogenization process 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 process, so it is necessary to select heat treatment conditions according to the type of added elements.

[0033] For example, since the aluminum alloy containing aluminum scrap has the above composition, the resulting ingot is subjected to homogenization treatment at a temperature of 500°C to 600°C for 2 hours or more. More preferably, this 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 that occurs during casting will remain, preventing sufficient homogenization, while 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.

[0034] [Soaking treatment] The homogenized ingot is then subjected to a soaking treatment. This soaking treatment is carried out at a temperature slightly lower than that of the homogenization treatment, for example, by holding the ingot at a temperature of 480°C to 550°C for at least one hour. This soaking treatment may also serve as the soaking treatment before hot rolling.

[0035] [Hot rolling process] The homogenized ingot (or the soaked ingot, if soaked) is then subjected to hot rolling. This hot rolling is performed at a high temperature of around 500°C. Specifically, after rough hot rolling where the outlet temperature is 400°C to 460°C, the ingot is passed through a single-reverse hot finishing mill three times at a rolling speed of 50 m / min or more to produce a sheet material with a thickness of 2 mm to 6 mm. Specifically, the first pass of hot finishing is performed at 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 at 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 the conditions of a rolling speed of 150 m / min to 300 m / min and a coiling temperature of 230°C to 330°C.

[0036] If the rolling speed of the hot rolling is less than 50 m / min, the elongation and cube orientation rate of the material will decrease, and the 0.2% yield strength after heat treatment at 170°C will also decrease, resulting in a decrease in formability and product strength of the aluminum alloy sheet. In this embodiment, the texture of the material was controlled by changing the conditions of the hot finish rolling in various ways, and the sum (orientation area ratio) of the area ratio of crystal grains having an orientation within 15° of the cubic 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 was adjusted to a desired range.

[0037] [Cold rolling] Next, the plate material after hot rolling is subjected to cold rolling. The method of this cold rolling 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.

[0038] [Solution treatment] The cold-rolled sheet material is then subjected to solution treatment, in which the sheet material is heated to 500°C to 550°C at a heating rate of 100°C / s or faster, held at this temperature for 15 to 120 seconds, and then cooled to 100°C or lower at a cooling rate of 200°C / s or faster. If the heating rate in the solution treatment is less than 100°C / second, the productivity of the aluminum alloy sheet decreases, if the holding temperature is less than 500°C, 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, recrystallization does not proceed sufficiently, and if it exceeds 120 seconds, the productivity of the aluminum alloy sheet decreases. If the cooling rate in the solution treatment is less than 200°C / second, the productivity of the aluminum alloy sheet decreases.

[0039] [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, equivalent to room temperature. The yield strength and elongation of these aluminum alloy sheets after aging treatment following rolling have a significant effect on the press formability of the product. After this aging treatment, the 0.2% yield strength of the aluminum alloy sheet is 100 MPa to 155 MPa, and the elongation is 20% or more.

[0040] Furthermore, the 0.2% yield strength of the aluminum alloy sheet (aluminum alloy sheet after aging treatment) manufactured in this manner was measured by the following method, assuming the thermal load in the painting process after press forming of the product. Specifically, a test piece was subjected to a 2% strain in a tensile test based on JIS Z 2241, and then subjected to a heat treatment in which the test piece was 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 corresponds to the product strength of the product. The 0.2% yield strength of the aluminum alloy sheet after this 170°C heat treatment is 170 MPa or more.

[0041] Next, we will explain the crystal orientation. The cube orientation is expressed as {001} <100> The Goss direction is expressed as {110} <001> The Cube orientation and Goss orientation exhibit 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). <100> and Goss orientation {110} <001> In this case, the slip lines on the crystal plane (rolled surface) can be made symmetrical at 45° and 135° to the bending axis. Therefore, by increasing the sum of the area ratio of crystal grains with orientations that are within 15° of the cubic orientation relative to all crystal grains and the area ratio of crystal grains with orientations that are within 15° of the Goss orientation relative to all crystal grains (orientation area ratio) to a certain level, it was found that the formation of shear bands on the outside of the bend can be suppressed, and bending workability can be significantly improved.

[0042] The aluminum alloy sheet produced in this manner has an average crystal grain size of 30 μm or less, a 0.2% yield strength of 100 MPa or more and 155 MPa or less, an elongation of 20% or more, and a 0.2% yield strength of 170 MPa or more after being subjected to a uniaxial strain of 2% and then heat treated at 170°C for 20 minutes, i.e., an aluminum alloy sheet for automotive reinforcement having formability, strength, and corrosion resistance. Furthermore, since the aluminum alloy sheet can be produced from an aluminum alloy containing 50% by mass or more of aluminum scrap, the environmental load can be reduced.

[0043] Specifically, the aluminum alloy sheet of this embodiment has a 0.2% yield strength of 100 MPa or more and 155 MPa or less, and an elongation of 20% or more, allowing for appropriate press working. Furthermore, after applying a uniaxial strain of 2% in the rolling direction and then heat treating at 170°C for 20 minutes, the 0.2% yield strength can be increased to 170 MPa or more, thereby enhancing the strength of the final product (automotive reinforcement). Furthermore, the Cu content is as low as 0.2 mass% or less, and the Zn content is as low as 0.7 mass% or less, allowing for enhanced corrosion resistance. Furthermore, the average crystal grain size is as small as 30 μm or less, allowing for improved processability.

[0044] In addition, the proportion of coarse Mg-Si compounds was 5.0 × 10 3 pieces / mm 2 By optimizing the following, it is possible to ensure the material strength required for press formability during part molding, while at the same time, age hardening occurs due to the heat load during the subsequent part painting process, resulting in high strength in the final product. Furthermore, the sum of the area ratio of crystal grains with orientations within 15° of the cube orientation on the rolled surface to all crystal grains and the area ratio of crystal grains with orientations within 15° of the Goss orientation to all crystal grains (orientation area ratio) is 3% or more, and this cube orientation (Cube orientation {001} <100> and Goss orientation {110} <001> In this case, the slip lines on the crystal plane (rolled surface) can be made symmetrical at 45° and 135° relative to the bending axis, and by increasing the orientation area ratio to a certain level, the formation of shear bands on the outside of the bend can be suppressed, significantly improving bending workability. In addition, since the electrical conductivity is set to 40% IACS or more and 45% 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 ranges. Furthermore, if the electrical conductivity is 45% IACS or less, coarsening of intermetallic compounds does not occur, and therefore a decrease in corrosion resistance can be suppressed.

[0045] Then, by press-forming the above-described aluminum alloy sheet for automobile reinforcement and then painting it, an automobile reinforcement having excellent formability, strength and corrosion resistance can be provided.

[0046] 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. [Example]

[0047] The aluminum alloys of Examples 1 to 17 and Comparative Examples 1 to 12 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 raw materials in Examples 1 to 17 and Comparative Examples 1 to 12 were as shown in Table 1.

[0048] These aluminum alloys were melted to produce molten aluminum alloys, which were then cast by semi-continuous casting. The ingots obtained by semi-continuous casting were subjected to homogenization treatment at 565°C for 2 hours, followed by soaking treatment at 510°C for 1 hour, hot rolling under the various conditions shown in Table 2, and then 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 / sec or more, held at this temperature for 20 seconds or less, and then rolled into sheets. 、1 After solution treatment by cooling to 00°C or below, each sample was subjected to aging treatment by holding at 50°C for 72 hours. In the hot rolling conditions in Table 2, the rolling speed for rolling conditions B to D is shown as a range between the lower limit and the upper limit for each column (for example, 20 m / min or more and less than 50 m / min for the first pass under rolling condition D), and for rolling condition A, the range is shown as a range between the lower limit and the upper limit for each column (100 m / min or more and 150 m / min for the first pass).

[0049] (0.2% proof stress measurement) The 0.2% yield strength was measured according to a method in accordance with JIS Z 2241. Specifically, a sample was cut out from each obtained specimen parallel to the rolling direction to prepare a JIS No. 5 shaped test piece, and a tensile test was carried out at room temperature to measure the yield strength (MPa). The tensile speed was 5 mm / min.

[0050] (Measurement of elongation) Elongation was measured by a method conforming to JIS Z 2241. Specifically, a sample was cut out from each obtained specimen parallel to the rolling direction to prepare a JIS No. 5 shaped test piece, and a tensile test was carried out at room temperature to measure the elongation. Note that the elongation referred to here is the permanent elongation after break based on JIS Z 2241, expressed as a percentage of the original gauge length.

[0051] (conductivity) The conductivity was measured by the 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 conductivity was calculated.

[0052] (Observation of compound particle distribution) The cross section of the manufactured aluminum alloy parallel to the rolling direction was observed. The cross section was subjected to CP processing (cross-section processing) based on the ion milling method, and the observation was carried out using a field emission scanning electron microscope (FE-SEM). Based on the observed image, the circle equivalent diameter and distribution density of the compound particles (Mg-Si compound particles) were calculated by image analysis.

[0053] (crystal grain size) The method used to expose the metal structure was to polish a cross section of the aluminum alloy plate cut parallel to the rolling direction with emery paper, then roughly buff polish and finish polish, followed by rinsing with water and drying, and then anodizing 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 light optical microscope, and the average crystal grain size was calculated by the intercept method.

[0054] (azimuth area ratio) The sum of the area ratio of crystal grains with orientations that are within 15° of the Cube orientation to all crystal grains (Cube orientation area ratio) and the area ratio of crystal grains with orientations that are within 15° of the Goss orientation to all crystal grains (Goss orientation area ratio) was calculated by adding the Cube{001} <100> The sum of the area fraction of all crystal grains of crystal grains having an orientation misorientation within 15° from the Cube orientation and the area fraction of all crystal grains of crystal grains having an orientation misorientation within 15° from the Goss orientation was measured as the orientation area fraction. Specifically, an electron beam was scanned at a pitch of 1 μm over a measurement area of ​​0.20 mm × 1.5 mm (plate thickness) of the thickness cross section in the rolling direction of the aluminum alloy plate described above, to measure the crystal orientation at each measurement point, and among the crystal grains determined from the orientation misorientation between the measurement points, measurements were made on each of the crystal grains having an orientation misorientation within 15° from the Cube orientation and the crystal grains having an orientation misorientation within 15° from the Goss orientation. of all grains in the region The average area ratio (%) relative to the area was measured 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.

[0055] (Evaluation of formability) The formability was evaluated by visual inspection of the occurrence of cracks and wrinkles when bending the sheet through 180° in close contact according to JIS Z 2248. In this case, sheets with almost no wrinkles and no cracks were evaluated as very good (◎), sheets with some wrinkles but no cracks were evaluated as good (◯), and sheets with cracks were evaluated as poor (×).

[0056] (Product strength evaluation) After applying 2% uniaxial strain to the samples in the rolling direction, the 0.2% yield strength after heat treatment at 170°C for 20 minutes was assumed to be the product strength. Yield strengths of 190 MPa or more were rated as very good (◎), those between 170 MPa and 190 MPa were rated as good (◯), and those under 170 MPa were rated as unacceptable (×).

[0057] (Corrosion resistance evaluation) To evaluate corrosion resistance, a salt spray test (SST) was carried out for 1000 hours. After this corrosion test, the corrosion products were removed from the samples using chromium phosphate, and then the corrosion weight loss was measured. Based on this result, the corrosion weight loss was 15.0 mg / cm 2 Less than 15.0 mg / cm is considered good (〇), 2 The above were evaluated as unacceptable (×). Table 3 shows the yield strength, elongation, electrical conductivity, number density of Mg-Si compounds, crystal grain size, Cube orientation area ratio, Goss orientation area ratio, orientation area ratio, and yield strength after heat treatment at 170°C, and Table 4 shows the results of various evaluations.

[0058] [Table 1]

[0059] [Table 2]

[0060] [Table 3]

[0061] [Table 4]

[0062] As shown in Tables 3 and 4, Examples 1 to 17 contained Fe: 0.5% by mass or less, Si: 1.2% by mass to 1.6% by mass, Cu: 0.2% by mass or less, Mn: 0.8% by mass to 1.2% by mass, Mg: 0.45% by mass to 0.7% by mass, Zn: 0.7% by mass or less, with the balance being Al and unavoidable impurities, had an average crystal grain size of 30 μm or less, a 0.2% yield strength of 100 MPa to 155 MPa, an elongation of 20% or more, and had a 0.2% yield strength of 170 MPa or more after applying a uniaxial strain of 2% in the rolling direction and then heat treatment at 170 ° C for 20 minutes. All of these had good or very good formability and product strength, and good corrosion resistance.

[0063] On the other hand, in Comparative Example 1, the Fe content was too high, resulting in a high 0.2% yield strength, and the formability deteriorated, resulting in an unacceptable product. In Comparative Example 2, the Si content was too low, resulting in low 0.2% yield strength and 0.2% yield strength after heat treatment at 170°C, resulting in both unacceptable formability and product strength. In Comparative Example 3, the Si content was too high, resulting in low 0.2% yield strength and elongation, resulting in unacceptable formability. In Comparative Example 4, the Cu content was too high, resulting in unacceptable corrosion resistance. In Comparative Example 5, the Mn content was too low, resulting in a low electrical conductivity and a low 0.2% yield strength after heat treatment at 170°C, resulting in unacceptable product strength. In Comparative Example 6, the Mn content was too high, resulting in a high electrical conductivity and an excessive increase in the number density of Mg-Si compounds, resulting in a low 0.2% yield strength after heat treatment at 170°C, resulting in unacceptable product strength.

[0064] In Comparative Example 7, the Mg content was too low, resulting in low 0.2% proof stress and low 0.2% proof stress after heat treatment at 170°C, and therefore both formability and product strength were unacceptable. In Comparative Example 8, the Mg content was too high, resulting in low 0.2% proof stress and elongation, and therefore formability was unacceptable. In Comparative Example 9, the Zn content was too high, resulting in poor corrosion resistance and a low orientation area ratio, and therefore formability was unacceptable. In Comparative Examples 10 and 11, the hot rolling condition was D, and the rolling speeds in the first and second passes were lower than those in the other conditions A to C. As a result, the elongation and orientation area ratio were low in Comparative Example 10, and therefore formability was unacceptable. In Comparative Example 11, the elongation and orientation area ratio were low, and the 0.2% proof stress after heat treatment at 170°C was low, and therefore both formability and product strength were unacceptable.< / uvw>

Claims

1. the steel sheet has a composition containing Fe: 0.5% by mass or less, Si: 1.2% by mass or more and 1.6% by mass or less, Cu: 0.02% by mass or more and 0.2% by mass or less, Mn: 0.8% by mass or more and 1.2% by mass or less, Mg: 0.45% by mass or more and 0.7% by mass or less, Zn: 0.05% by mass or more and 0.7% by mass or less, with the balance being Al and unavoidable impurities, an average crystal grain size of 30 μm or less, a 0.2% yield strength of 100 MPa or more and 155 MPa or less, an elongation of 20% or more, and a 0.2% yield strength of 170 MPa or more after being subjected to a uniaxial strain of 2% in the rolling direction and then a heat treatment at 170°C for 20 minutes, 5.0 × 10 Mg-Si based second phase particles having a circle equivalent diameter of 1.0 μm or more ３ pieces / mm ２ is as follows:

1. An aluminum alloy sheet for automobile reinforcement, characterized in that the sum of the area ratio of crystal grains having an orientation that is within 15° of the cubic orientation of a cross section in the rolling direction to all crystal grains and the area ratio of crystal grains having an orientation that is within 15° of the Goss orientation to all crystal grains is 3% or more.

2. 2. The aluminum alloy sheet for automobile reinforcement according to claim 1, further containing one or more of Cr: 0.01% by mass or more and 0.10% by mass or less, Ti: 0.01% by mass or more and 0.10% by mass or less, and Zr: 0.01% by mass or more and 0.10% by mass or less.

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