Aluminum alloy sheet for vehicle body parts

Abstract

To provide an aluminum alloy sheet for car body parts 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 body parts contains 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 and Zr: 0.10 mass% or more to 0.20 mass% or less with the balance being Al and inevitable impurities. The aluminum alloy sheet has an average crystal grain size of 100 μm or less, a 0.2% proof stress of 120 MPa or more and 175 MPa or less, and an elongation of 15% 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 180 MPa or more.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to an aluminum alloy for vehicle body parts. On the board

Background Art

[0002] In recent years, in order to protect the global environment, the adoption of materials that can reduce the environmental load has been demanded. Also in the case of aluminum alloy sheets, there is a very high need to reuse scraps of various used aluminum alloy parts (hereinafter referred to as aluminum scrap materials) for manufacturing. When manufacturing an aluminum alloy sheet by reusing this aluminum scrap material, compared with the case of adopting primary aluminum ingots, the generation amount of CO2 can be significantly suppressed, and the environmental load can be reduced. When using an aluminum alloy obtained by reusing such aluminum scrap material as a vehicle body part (for example, an automotive rain force member), it is important to ensure press formability, paintability, structural strength as a product, corrosion resistance, etc. during part molding. However, since the aluminum alloy sheet obtained by reusing aluminum scrap material contains various additive elements, it has been difficult to satisfy all the required characteristics demanded for these structures. For example, regarding material strength, in order to ensure press formability during part molding, it is necessary to keep it within a certain strength range. If the strength is too high or too low, the press formability deteriorates. On the other hand, as a final product, it is necessary to have high strength in order to ensure structural strength. These rain force members have their material strength improved by the temperature applied in the painting process (baking process) after press forming and become the strength of the final product. However, even considering the strength improvement by this baking, it has been difficult to achieve both press formability and ensuring the structural strength of the final product. As such an aluminum alloy sheet obtained by reusing aluminum scrap material, for example, the aluminum alloy sheet described in Patent Document 1 is known.

[0003] ​For example, the aluminum alloy plate of Patent Document 1 is formed of an aluminum alloy containing Si: 0.4 to 2.0% by mass, Fe: 0.2 to 0.6% by mass, Cu: 0.1 to 0.7% by mass, Mn: 0.5 to 1.5% by mass, Mg: 0.5 to 2.0% by mass, Zn: 0.05 to 1.0% by mass, with the balance being Al and unavoidable impurities. The aluminum alloy plate described in this Patent Document 1 has a recrystallized structure throughout the entire cross-section of the plate thickness, and the recrystallized grain size is 50 μm or less, and the conductivity is 43.0 to 49.5% IACS, thereby improving the press formability and corrosion resistance.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] By the way, in Patent Document 1, although there is a description about the press formability and corrosion resistance of the aluminum alloy plate, since the target product is a heat insulator for automobiles etc., which is different from an aluminum alloy plate for vehicle body parts (for example, an aluminum alloy plate for automobile rain force), the necessary characteristics do not necessarily match. Also, it is considered difficult to achieve both the above-mentioned press formability and ensuring the structural strength of the final product.

[0006] The present invention has been made in view of the above circumstances, and aims to provide an aluminum alloy for vehicle body parts having formability, product strength, and corrosion resistance while suppressing the environmental load by using aluminum scrap as a raw material. The board with the aim of providing.

Means for Solving the Problems

[0007] As a result of intensive research, the present researchers analyzed the components of aluminum scrap generated in an aluminum rolling factory, carefully examined the effects of alloying elements abundantly contained in the aluminum scrap, and found an aluminum alloy material for vehicle body parts having formability, product strength, and corrosion resistance by appropriately controlling the selection of the alloy component range and the combination of subsequent manufacturing conditions.

[0008] That is, in view of the above problems, in addition to optimizing the components of the aluminum alloy using recycled aluminum scrap, the present inventors optimized manufacturing processes such as casting, rolling, and heat treatment to control the solid solution and precipitation states of each additive component, the dispersion states such as the size and number density of various intermetallic compounds, and the microstructure, thereby obtaining an invention product that satisfies all the required characteristics for an aluminum alloy sheet for vehicle body parts at a high level. For example, regarding the dispersion state of the intermetallic compound, by optimizing the proportion of the presence of coarse Mg-Si compounds, while ensuring the material strength required for press formability during part molding, high strength in the final product can be obtained by causing age hardening due to the heat load in the subsequent part painting process.

[0009] The aluminum alloy sheet for vehicle body parts of the present invention contains 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, and the balance consists of Al and inevitable impurities, has an average crystal grain size of 100 μm or less, a 0.2% proof stress of 120 MPa or more and 175 MPa or less, an elongation of 15% or more, and a 0.2% proof stress of 180 MPa or more after heat treatment at 170 °C for 20 minutes after applying a uniaxial strain of 2% in the rolling direction.

[0010] In the present invention, since the 0.2% proof stress is 120 MPa or more and 175 MPa or less, and the elongation is 15% or more, appropriate pressing can be performed. Further, after applying a uniaxial strain of 2% and then performing a heat treatment at 170°C for 20 minutes, the 0.2% proof stress can be made extremely high, at 180 MPa or more, enhancing the strength of the final product. Also, since the amount of Cu is 0.2 mass% or less and the amount of Zn is 1.0 mass% or less, the corrosion resistance can be enhanced. Furthermore, since the average crystal grain size is as small as 100 μm or less, a certain workability can be ensured.

[0011] Note that when the average crystal grain size exceeds 100 μm, the workability deteriorates. Also, if the 0.2% proof stress is less than 120 MPa, the aluminum alloy sheet may break during pressing, and if it exceeds 175 MPa, the workability deteriorates. Further, if the elongation is less than 15%, the workability deteriorates. In addition, if 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 less than 180 MPa, the strength of the final product (e.g., automotive rain reinforcement) decreases.

[0012] Fe contributes to improving the proof stress of the aluminum alloy sheet. When it exceeds 0.5 mass%, the proportion of intermetallic compounds increases, and the proof stress becomes too high, deteriorating the formability. Si contributes to improving the proof stress of the aluminum alloy sheet. If it is less than 1.0 mass%, sufficient proof stress after a 170°C heat treatment cannot be obtained, resulting in insufficient product strength. On the other hand, if it exceeds 2.0 mass%, the proportion of intermetallic compounds increases, and the proof stress becomes too high, deteriorating the formability. Cu contributes to improving the proof stress and corrosion resistance of the aluminum alloy sheet. When it exceeds 0.2 mass%, the corrosion resistance significantly decreases. Mn contributes to improving the proof stress of the aluminum alloy sheet. If it is less than 0.5 mass%, sufficient proof stress after a 170°C heat treatment cannot be obtained, resulting in insufficient product strength. On the other hand, if it exceeds 1.5 mass%, the intermetallic compounds coarsen, and again, sufficient proof stress after a 170°C heat treatment cannot be obtained. Mg contributes to improving the yield strength of the aluminum alloy sheet, and if it is less than 0.4% by mass, sufficient yield strength after heat treatment at 170 °C cannot be obtained. On the other hand, if it exceeds 1.2% by mass, the yield strength becomes too high and the formability deteriorates. Zn contributes to the corrosion resistance of the aluminum alloy sheet, and if it exceeds 1.0% by mass, the corrosion resistance deteriorates. Zr contributes to improving the yield strength of the aluminum alloy sheet and refining the crystal grains. If it 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. If it exceeds 0.20% by mass, the crystal grains rather coarsen and the formability deteriorates.

[0013] As a preferred embodiment of the aluminum alloy sheet for vehicle body parts of the present invention, it is preferable to further contain one or more of Cr: 0.01% by mass or more and 0.10% by mass or less, and Ti: 0.01% by mass or more and 0.10% by mass or less. Cr contributes to improving strength, refining crystal grains, and stabilizing the structure. If it is less than 0.01% by mass, the above effects cannot be sufficiently obtained. If it exceeds 0.10% by mass, the above effects are saturated, and in addition, a large number of intermetallic compounds are generated, which may have an adverse effect on formability. Ti contributes to improving strength and refining the ingot structure. If it is less than 0.01% by mass, the above effects cannot be sufficiently obtained. If it exceeds 0.10% by mass, the above effects are saturated, and in addition, there is a possibility of generating coarse precipitates.

[0014] As a preferred embodiment of the aluminum alloy sheet for vehicle body parts of the present invention, the Mg-Si-based second-phase particles having an equivalent circle diameter of 1.0 μm or more are 5.0×10 3 pieces / mm 2 or less, and the Al-Zr-based second-phase particles having an equivalent circle diameter of 0.1 μm or less are 2.0×10 pieces / mm 2 or more. In the above embodiment, the proportion of the existence of coarse Mg-Si compounds is 5.0×10 3 pieces / mm 2 or less, and the proportion of the existence of Al-Zr compounds is 2.0×10 pieces / mm 2By optimizing the above, while ensuring the material strength required for press formability during component molding, aging hardening can be caused by the heat load in the subsequent component painting process to obtain high strength in the final product.

[0015] As 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 with an orientation difference of 15° or less 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 with an orientation difference of 15° or less from the Goss orientation to all crystal grains is preferably 5% or more. Here, the representation of the microstructure in the EBSD (Electron BackScatter Diffraction) method is represented by the rolling plane and the rolling direction in the case of the microstructure of the sheet material by rolling. The rolling plane is represented by {hkl}, and the rolling direction is <uvw>It is expressed by this expression. When using this expression, the cube orientation (Cube orientation) is expressed as {001}<100>, and the Goss orientation is expressed as {110}<001>. In the above cube orientation {001}<100> and Goss orientation {110}<001>, the slip lines on the crystal plane (rolling plane) can have good symmetry at 45° and 135° with respect to the bending axis. For this reason, in the above aspect, the area ratio of the crystal grains having an orientation within 15° of the orientation difference from the cube orientation in all the crystal grains of the rolling plane to all the crystal grains, and the area ratio of the crystal grains having an orientation within 15° of the orientation difference from the Goss orientation to all the crystal grains (hereinafter referred to as the orientation area ratio) are increased to a certain level or more, so that the formation of shear bands on the outer side of bending is suppressed, and the bending workability can be greatly improved.

[0016] The manufacturing method of the aluminum alloy plate for vehicle body parts of the present invention contains 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, and the balance is composed of Al and inevitable impurities. After melting and casting the aluminum alloy, a homogenization treatment of holding at 500°C or more and 600°C or less for 2 hours or more is performed, and then hot rolling in a plurality of passes is performed at a rolling speed of 50 m / min or more, and then cold rolling is performed to form a plate material with a thickness of 0.8 mm or more and 2.5 mm or less. The plate material is heated to 500°C or more and 550°C or less at a heating rate of 100°C / second or more and held for 15 seconds or more and 120 seconds or less, and then solution treatment of cooling to 100°C or less at a cooling rate of 200°C / second or more is performed, and then aging treatment by storage at room temperature for 14 days or more or aging treatment at 50°C for 72 hours is performed.

[0017] In the present invention, by hot rolling and cold rolling, a plate material with a final thickness of 0.8 mm or more and 2.5 mm or less is formed, and by performing solution treatment and aging treatment by industrial or storage at room temperature for 14 days or more on this plate material, an aluminum alloy plate for vehicle body parts having formability, strength, and corrosion resistance can be manufactured.

[0018] If the temperature of the homogenization treatment is less than 500 °C, segregation occurring during casting remains, and sufficient homogenization cannot be carried out. If the holding temperature exceeds 600 °C, there is a risk that the ingot will melt. Also, if the holding time is less than 2 hours, homogenization may not proceed sufficiently. If the rolling speed in one pass of hot rolling is less than 50 m / min, the elongation of the material, the area ratio of crystal grains having an orientation with an orientation difference within 15° from the cube orientation to all crystal grains, and the sum (orientation area ratio) of the area ratio of crystal grains having an orientation with an orientation difference within 15° from the Goss orientation to all crystal grains decrease, and the 0.2% proof stress after the 170 °C heat treatment decreases. As a result, the formability and product strength of the aluminum alloy sheet decrease. 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 solute elements does not proceed sufficiently. If it exceeds 600 °C, there is a risk that the sheet will melt and break. Also, if the holding time is less than 15 seconds, re-dissolution does not proceed sufficiently. If it exceeds 120 seconds, the productivity of the aluminum alloy sheet decreases. Also, if the cooling rate of the solution treatment is less than 10 °C / second, the productivity of the aluminum alloy sheet decreases. If the room temperature storage after the solution treatment is less than 14 days, age hardening is insufficient, the strength of the aluminum alloy sheet is insufficient, and the formability decreases.

Advantages of the Invention

[0019] According to the present invention, by using an aluminum scrap material as a raw material, it is possible to provide an aluminum alloy sheet for vehicle body parts having excellent formability, strength, and corrosion resistance while suppressing the environmental load.

Embodiments for Carrying Out the Invention

[0020] Hereinafter, an embodiment in which an aluminum alloy sheet for vehicle body parts (hereinafter referred to as an aluminum alloy sheet) according to the present invention is applied to a rain force material will be described.

[0021] [Configuration of Aluminum Alloy Sheet] The aluminum alloy plate of this embodiment is inserted, for example, inside the hood and trunk of an automobile, and is processed into a so-called reinforcement material used as a reinforcement or the like for enhancing the rigidity of a structure. This aluminum alloy plate is formed from an aluminum alloy using aluminum scrap as a raw material. Specifically, the aluminum alloy that forms the aluminum alloy plate contains 50% or 60% by mass or more of aluminum scrap, and may be entirely formed of aluminum scrap. In addition, such aluminum scrap consists of scraps from aluminum rolling factories and various used aluminum alloy parts.

[0022] Also, as described above, since the aluminum alloy plate uses aluminum scrap as the main raw material, it contains a plurality of elements. Specifically, the aluminum alloy plate contains Fe: 0.5% by mass or less, Si: 1.0% by mass or more and 2.0% by mass or less, Cu: 0.2% by mass or less, Mn: 0.5% by mass or more and 1.5% by mass or less, Mg: 0.4% by mass or more and 1.2% by mass or less, Zn: 1.0% by mass or less, Zr: 0.10% by mass or more and 0.20% by mass or less, with the balance being Al and unavoidable impurities.

[0023] Fe contributes to improving the yield strength of the aluminum alloy plate. When it exceeds 0.5% by mass, the proportion of intermetallic compounds increases, and the yield strength becomes too high, resulting in deteriorated formability. Si contributes to improving the yield strength of the aluminum alloy plate. When it is less than 1.0% by mass, sufficient yield strength after heat treatment at 170°C cannot be obtained, and the product strength is insufficient. On the other hand, when it exceeds 2.0% by mass, the proportion of intermetallic compounds increases, and the yield strength becomes too high, resulting in deteriorated formability. Cu contributes to improving the yield strength and corrosion resistance of the aluminum alloy plate. When it exceeds 0.2% by mass, the corrosion resistance significantly decreases. Mn contributes to improving the yield strength of the aluminum alloy plate. When it is less than 0.5% by mass, sufficient yield strength after heat treatment at 170°C cannot be obtained, and the product strength is insufficient. On the other hand, when it exceeds 1.5% by mass, the intermetallic compounds coarsen, and sufficient yield strength after heat treatment at 170°C cannot be obtained either. Mg contributes to improving the yield strength of the aluminum alloy sheet. If it is less than 0.4% by mass, sufficient yield strength after heat treatment at 170 °C cannot be obtained. On the other hand, if it exceeds 1.2% by mass, the yield strength becomes too high and the formability deteriorates. Zn contributes to the corrosion resistance of the aluminum alloy sheet. If it exceeds 1.0% by mass, the corrosion resistance deteriorates. Zr contributes to improving the yield strength of the aluminum alloy sheet and refining the crystal grains. If it 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. If it exceeds 0.20% by mass, the crystal grains rather coarsen and the formability deteriorates.

[0024] Further, the aluminum alloy sheet preferably further contains one or more of Cr: 0.01% by mass or more and 0.10% by mass or less, and Ti: 0.01% by mass or more and 0.1% by mass or less. Cr contributes to improving the strength, refining the crystal grains, and stabilizing the structure. If it is less than 0.01% by mass, the above effects cannot be sufficiently obtained. If it exceeds 0.10% by mass, the above effects are saturated, and in addition, a large number of intermetallic compounds are generated, which may have an adverse effect on the formability. Ti contributes to improving the strength and refining the ingot structure. If it is less than 0.01% by mass, the above effects cannot be sufficiently obtained. If it exceeds 0.10% by mass, the above effects are saturated, and in addition, there is a possibility of generating coarse precipitates.

[0025] This aluminum alloy sheet has an average crystal grain size of 100 μm or less, a 0.2% yield strength of 120 MPa or more and 175 MPa or less, 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 a uniaxial strain of 2% in the rolling direction. The application of a 2% uniaxial strain is a condition assuming press forming as a rain force, and the heat treatment at 170 °C for 20 minutes is a condition assuming painting after processing into a rain force. That is, the 0.2% yield strength (sometimes referred to as the yield strength after heat treatment at 170 °C) after heat treatment at 170 °C for 20 minutes after applying a 2% uniaxial strain in the rolling direction assumes the strength of the product as a rain force.

[0026] In addition, when the average crystal grain size exceeds 100 μm, the workability deteriorates. 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 deteriorates. In addition, when the elongation is less than 15%, the workability deteriorates. Also, if the 0.2% proof stress after heat treatment at 170 °C for 20 minutes after applying a uniaxial strain of 2% in the rolling direction is less than 180 MPa, the strength of the final product, automotive parts (for example, automotive rain reinforcements), decreases. Incidentally, 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. Also, the proof stress after the 170 °C heat treatment is more preferably 195 MPa or more.

[0027] In addition, the aluminum alloy sheet has 5.0×10 3 particles / mm 2 or less of Mg-Si-based second-phase particles with an equivalent circle diameter of 1.0 μm or more, and 2.0×10 particles / mm 2 or more of Al-Zr-based second-phase particles with an equivalent circle diameter of 0.1 μm or less. Incidentally, if the number of Mg-Si-based second-phase particles with an equivalent circle diameter of 1.0 μm or more exceeds 5.0×10 3 particles / mm 2 or if the number of Al-Zr-based second-phase particles with an equivalent circle diameter of 0.1 μm or less is less than 2.0×10 particles / mm 2 it becomes difficult to ensure the material strength required for press formability during part forming, and there is a possibility that age hardening becomes difficult to occur due to the heat load in the subsequent part painting process.

[0028] Furthermore, the aluminum alloy sheet has an area ratio of crystal grains having an orientation with an orientation difference of within 15° from the cube orientation of the cross-section in the rolling direction to the total crystal grains (Cube orientation area ratio), and an area ratio of crystal grains having an orientation with an orientation difference of within 15° from the Goss orientation to the total crystal grains (Goss orientation area ratio), and the sum of these (orientation area ratio) is 5% or more. This orientation area ratio contributes to the improvement of bend formability if it is 5% or more. However, since the effect saturates even if the numerical value of the orientation area ratio greatly exceeds 5%, it suffices if it is 5% or more. Note that if the area ratio of the above crystal grains to the total crystal grains is less than 5%, it becomes difficult to suppress the formation of shear bands on the outer side of the bend, and thus it is difficult to significantly improve the bend formability.

[0029] Also, the aluminum alloy sheet preferably has a conductivity of the material of 38% IACS or more and 43% IACS or less. The conductivity also serves as an index representing the solid solution and precipitation state of each additive 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 conductivity decreases, and as the precipitation progresses as an intermetallic compound, the conductivity increases. For this reason, if the conductivity of the material is too low or too high, the material strength and the strength after aging tend to decrease, and it is desirable to be within the above range. Note that when the conductivity is less than 38% IACS and when it exceeds 43% IACS, the solid solubility of each additive element in the aluminum alloy sheet is outside the appropriate range, and thus the 0.2% proof stress and the 0.2% proof stress after heat treatment at 170 °C for 20 minutes may decrease. Further, when the conductivity exceeds 43% IACS, the intermetallic compound tends to coarsen, and thus the corrosion resistance may slightly decrease. This conductivity is more preferably 40% IACS or more and 42% IACS or less.

[0030] [Manufacturing method of aluminum alloy sheet] The aluminum alloy plate is manufactured through the following procedure. First, an aluminum alloy with the above composition containing 50% by mass or more of aluminum scrap material is subjected to melting and casting treatment, homogenization treatment, soaking treatment, hot rolling treatment, cold rolling treatment, solution treatment, and aging treatment in this order. The following is a specific description.

[0031] [Melting and Casting Treatment] An aluminum alloy containing 50% by mass or more of aluminum scrap material, with Fe: exceeding 0.5% by mass and not exceeding 1.2% by mass, Si: 1.0% by mass or more and 2.0% by mass or less, Cu: 0.2% by mass or less, Mn: 0.5% by mass or more and 1.5% by mass or less, Mg: 0.4% by mass or more and 1.2% by mass or less, Zn: 1.0% by mass or less, Zr: 0.10% by mass or more and 0.20% by mass or less, and the balance being Al and unavoidable impurities, is melted to produce an aluminum alloy melt. Then, the aluminum alloy melt is cast by the semi - continuous casting method (DC casting). Note that the casting method is not limited to the semi - continuous casting method, and other conventional methods such as the continuous casting method may also be used. Also, surface machining may be performed on the aluminum alloy ingot before and after the homogenization treatment.

[0032] [Homogenization Treatment] For the ingot obtained by the semi - continuous casting method, a homogenization treatment is carried out for the purpose of removing inhomogeneous structures such as segregation. By the high - temperature homogenization treatment, the additive elements supersaturated in the matrix during casting precipitate as intermetallic compounds. Since the size and dispersion amount of the precipitated intermetallic compounds are affected by the temperature and time of the homogenization treatment, it is necessary to select heat treatment conditions according to the type of additive element.

[0033] For example, since the aluminum alloy containing aluminum scrap material has the above composition, the homogenization treatment for the obtained ingot is carried out at a temperature of 500°C or more and 600°C or less for 2 hours or more. This homogenization treatment is more preferably maintained at a temperature of 535 - 595°C for 3 - 8 hours. In addition, if the holding temperature of the homogenization treatment is less than 500°C, segregation generated during casting will remain, and sufficient homogenization cannot be achieved. If the holding temperature exceeds 600°C, there is a risk that the ingot will melt. Also, if the holding time is less than 2 hours, homogenization may not proceed sufficiently.

[0034] [Soaking heat treatment] Soaking heat treatment is performed on the homogenized ingot. This soaking heat treatment is carried out at a temperature slightly lower than that of the homogenization treatment. For example, it is held at a temperature of 480°C or higher and 550°C or lower for 1 hour or more. Note that the soaking heat treatment may also be combined with the soaking heat treatment before hot rolling.

[0035] [Hot rolling process] Hot rolling process is performed on the homogenized ingot (if soaking heat treatment is performed, the ingot after soaking heat treatment). This hot rolling is carried out at a high temperature around 500°C. Specifically, after hot rough rolling with the outlet temperature of 400°C to 460°C, it is passed through a single reverse type hot finishing rolling mill three times at a rolling speed of 50 m / min or more to make the thickness of the sheet material 2 mm to 6 mm. Specifically, the first pass of hot finishing is executed under the conditions that the rolling speed is 50 m / min or more and 150 m / min or less, and the coiling temperature is 350°C or higher and 400°C or lower. Next, the second pass of hot finishing is executed under the conditions that the rolling speed is 50 m / min or more and 150 m / min or less, and the coiling temperature is 330°C or higher and 380°C or lower. Finally, the third pass of hot finishing is executed under the conditions that the rolling speed is 150 m / min or more and 300 m / min or less, and the coiling temperature is 230°C or higher and 330°C or lower.

[0036] In addition, if the rolling speed of hot rolling is less than 50 m / min, the elongation and cube orientation ratio of the material will decrease, and the 0.2% proof stress after the 170°C heat treatment will decrease, resulting in a decrease in the formability and product strength of the aluminum alloy sheet. In this embodiment, by varying the conditions of the hot finish rolling in various ways, the microstructure of the material is controlled, and the sum (orientation area ratio) of the area ratio of the crystal grains having an orientation with an orientation difference of within 15° from the cube orientation of the rolling surface in the final rolled product to the total crystal grains and the area ratio of the crystal grains having an orientation with an orientation difference of within 15° from the Goss orientation to the total crystal grains is adjusted to a desired range.

[0037] [Cold rolling] Next, cold rolling treatment is performed on the steel sheet after hot rolling. The method of this cold rolling treatment is not particularly limited, but for example, it can be carried out by passing the steel sheet through a rolling mill. The thickness of the steel sheet after this cold rolling is, for example, 0.8 mm or more and 2.5 mm or less.

[0038] [Solution treatment] Solution treatment is performed on the steel sheet after cold rolling treatment. In this solution treatment, the steel sheet is heated to 500°C or more and 550°C or less at a heating rate of 100°C / second or more, held for 15 seconds or more and 120 seconds or less, and then cooled to 100°C or less at a cooling rate of 200°C / second or more. 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, recrystallization does not proceed sufficiently, and if it exceeds 600°C, the aluminum alloy sheet may melt and break. Also, 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. Further, if the cooling rate of the solution treatment is less than 200°C / second, the productivity of the aluminum alloy sheet decreases.

[0039] [Aging treatment] Finally, the steel sheet subjected to the solution treatment is stored at room temperature for 14 days or more, or an aging treatment equivalent to room temperature is performed by holding it at 50°C for 72 hours. The yield strength and elongation after the aging treatment after the rolling of these aluminum alloy sheets greatly affect the press formability of the product. The 0.2% yield strength 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.

[0040] Furthermore, for the aluminum alloy sheet thus manufactured (aluminum alloy sheet after aging treatment), assuming the heat load in the painting process after press forming in the product, the 0.2% proof stress was measured by the following method. Specifically, for the test piece after applying a strain of 2% by a tensile test based on JIS Z2241, it was heated to 170°C at a heating rate of 10°C / second or more, held for 20 minutes, and then heat-treated by cooling at a cooling rate of 10°C / second or more, and then the proof stress was measured. The proof stress of these aluminum alloy sheets after the 170°C heat treatment is equivalent to the product strength of the product. The 0.2% proof stress of the aluminum alloy sheet after the 170°C heat treatment is 180 MPa or more.

[0041] Next, the crystal orientation will be described. In the representation of the texture in the EBSD method, the Cube orientation is represented as {001}<100>, and the Goss orientation is represented as {110}<001>. These Cube orientation and Goss orientation exhibit similar characteristics in the three directions of the thickness direction ND, rolling direction LD, and the direction perpendicular to the rolling direction TD of the rolling surface. In this Cube orientation {001}<100> and Goss orientation {110}<001>, the slip lines on the crystal plane (rolling surface) can have good symmetry at 45° and 135° with respect to the bending axis. Therefore, by increasing the sum (orientation area ratio) of the area ratio of the crystal grains having an orientation difference within 15° from the Cube orientation to the total crystal grains and the area ratio of the crystal grains having an orientation difference within 15° from the Goss orientation to the total crystal grains to a certain level, it was found that the formation of shear bands on the outer side of bending can be suppressed and the bend formability can be significantly improved.

[0042] The aluminum alloy sheet produced in this way has an average crystal grain size of 100 μm or less, a 0.2% proof stress of 120 MPa or more and 175 MPa or less, an elongation of 15% or more, and a 0.2% proof stress of 180 MPa or more after heat treatment at 170 °C for 20 minutes after applying a uniaxial strain of 2%. That is, it becomes an aluminum alloy sheet for vehicle body parts having formability, strength, and corrosion resistance. In addition, since an aluminum alloy sheet can be produced from an aluminum alloy containing 50% by mass or more of aluminum scrap material, the environmental load can be reduced.

[0043] Specifically, since the aluminum alloy sheet of the present embodiment has a 0.2% proof stress of 120 MPa or more and 175 MPa or less and an elongation of 15% or more, press working can be appropriately performed. In addition, the 0.2% proof stress after heat treatment at 170 °C for 20 minutes after applying a uniaxial strain of 2% in the rolling direction can be made extremely high at 180 MPa or more, and the strength of the final product (for example, an automobile rain reinforcement) can be increased. Further, since the amounts of Cu and Zn are as small as 0.2% by mass or less and 1.0% by mass or less, respectively, the corrosion resistance can be enhanced. Furthermore, since the average crystal grain size is as small as 100 μm or less, the workability can be improved.

[0044] In addition, the proportion of the presence of coarse Mg-Si compounds is 5.0×10 3 pieces / mm 2 or less, and the proportion of the presence of Al-Zr compounds having a circle equivalent diameter of 0.1 μm or less is 2.0×10 pieces / mm 2 By optimizing as described above, while ensuring the material strength required for press formability during component molding, age hardening can be caused by the heat load in the subsequent component painting process to obtain high strength in the final product. Furthermore, the sum of the area ratio of crystal grains with an orientation having an orientation difference of within 15° from the cube orientation of the rolling surface to the total crystal grains and the area ratio of crystal grains with an orientation having an orientation difference of within 15° from the Goss orientation to the total crystal grains (orientation area ratio) is 5% or more. In this cube orientation (Cube orientation) {001}<100> and Goss orientation {110}<001>, the slip lines on the crystal plane (rolling surface) can be made symmetric with respect to the bending axis at 45° and 135°. Therefore, by increasing the orientation area ratio to a certain level or higher, the formation of shear bands on the outer side of bending can be suppressed, and the bending processability can be significantly improved. In addition, since the conductivity is set to be 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 be within the above numerical ranges. Also, if the conductivity is 43% IACS or less, the intermetallic compound does not coarsen, so a decrease in corrosion resistance can be suppressed.

[0045] And after performing press forming on the aluminum alloy sheet for vehicle body parts as described above and then applying painting, an automobile rain reinforcement having excellent formability, strength, and corrosion resistance can be provided.

[0046] Note that 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 of being processed into an automobile rain reinforcement was described. However, the present invention is not limited to this, and it is also possible to apply this aluminum alloy sheet for vehicle body parts to vehicle body parts such as vehicle frame parts and outer panels other than rain reinforcements. It is also possible to apply it to chassis parts, and in the present invention, vehicle body parts include chassis parts.

Examples

[0047] The aluminum alloys of Examples 1 to 17 and Comparative Examples 1 to 14 were produced by the method shown below. After measuring the 0.2% proof stress and elongation of each obtained sample, the formability was evaluated. This will be described in detail below. The compositions (components) of the aluminum alloys used as raw materials in Examples 1 to 17 and Comparative Examples 1 to 14 were as shown in Table 1.

[0048] These aluminum alloys were melted to produce a molten aluminum alloy, and then cast by semi - continuous casting. The ingots obtained by the semi - continuous casting method were subjected to a homogenization treatment at 540°C for 8 hours, followed by a soaking heat treatment at 510°C for 1 hour. After hot rolling under various conditions shown in Table 2, cold rolling was performed to form a sheet with a thickness of 1.5 mm. This sheet was heated to 520°C at a heating rate of 100°C / second or more and held for 20 seconds or less, and then subjected to a solution treatment in which it was cooled to 100°C or less at a cooling rate of 50°C / second. After that, an aging treatment was carried out by holding at 50°C for 72 hours to obtain each sample. In addition, in the hot rolling conditions of Table 2, the rolling speed is shown as a range of not less than the lower limit value and less than the upper limit value for each column in Rolling Conditions B to D (for example, in the first pass of Rolling Condition D, it is not less than 20 m / min and less than 50 m / min), and in Rolling Condition A, it is shown as a range of not less than the lower limit value and not more than the upper limit value for each column (100 m / min or more and 150 m / min or less for the first pass).

[0049] (Measurement of 0.2% proof stress) For the 0.2% proof stress, it was measured by a method conforming to JIS Z2241. Specifically, samples were cut out from each obtained sample parallel to the rolling direction to prepare test pieces of JIS No. 5 shape, and a tensile test was carried out at room temperature to measure the proof stress (MPa). The tensile speed was 5 mm / min.

[0050] (Measurement of elongation) Regarding elongation, it was measured by a method conforming to JIS Z2241. Specifically, samples were cut out from each of the obtained samples parallel to the rolling direction to prepare test pieces of JIS No. 5 shape, and a tensile test was carried out at room temperature to measure the elongation. Here, the elongation is expressed as the percentage of the permanent elongation after fracture based on JIS Z2241 with respect to the original gauge length.

[0051] (Electrical conductivity) Regarding electrical conductivity, it was measured by the four-terminal method. A current of 500 mA was passed through the sample in a room temperature environment of 20 - 25 °C, the resistance was calculated from the voltage value, and then the electrical conductivity was calculated.

[0052] (Observation of compound particle distribution state) Regarding the produced aluminum alloy, a cross-section parallel to the rolling direction was observed. The observation was carried out using a field emission scanning electron microscope (FE-SEM) on a cross-section subjected to CP processing (cross-section processing) based on the ion milling method. Based on the observed images, the equivalent circle diameter and distribution density of compound particles (Mg-Si compound particles and Al-Zr compounds) were calculated by image analysis, and the number density (pieces / mm 2 ) of Mg-Si compound particles with an equivalent circle diameter of 1.0 μm or more, and the number density (pieces / mm 2 ) of Al-Zr compounds with an equivalent circle diameter of 0.1 μm or less were shown in Table 3.

[0053] (Crystal grain size) As a method for exposing the metal structure, a cross-section cut parallel to the rolling direction of the aluminum alloy plate was polished with emery paper, subjected to rough buff polishing and finish polishing, then washed with water and dried, and further, an anodizing treatment was applied in Barker's solution under the conditions of bath temperature: 25 °C, applied voltage: 30 V, and applied time: 120 seconds. Regarding the treated sample, it was photographed using an optical microscope with polarized light, and the average crystal grain size was calculated by the cutting method.

[0054] (Orientation area ratio) The sum of the area ratio of the crystal grains having an orientation with an orientation difference of within 15° from the Cube orientation to all crystal grains (Cube orientation area ratio) and the area ratio of the crystal grains having an orientation with an orientation difference of within 15° from the Goss orientation to all crystal grains (Goss orientation area ratio) (orientation area ratio) was measured as the sum of the area fraction of the crystal grains having an orientation with an orientation difference of within 15° from the Cube {001}<100> orientation to all crystal grains and the area fraction of the crystal grains having an orientation with an orientation difference of within 15° from the Goss orientation in the crystal grain orientation distribution map OIM (Orientation Imaging Microscopy) image measured by the EBSD (Electron BackScatter Diffraction) method. Specifically, for a measurement region of 0.20 mm × 1.5 mm (plate thickness) in the plate thickness cross-section in the rolling direction of the above-described aluminum alloy plate, an electron beam was scanned at a pitch of 1 μm to measure the crystal orientation of each measurement point. Among the crystal grains determined from the orientation difference between the measurement points, for each of the crystal grains having an orientation with an orientation difference of within 15° from the Cube orientation and the crystal grains having an orientation with an orientation difference of within 15° from the Goss orientation, the average area ratio (%) with respect to the measurement 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) Regarding the evaluation of formability, the occurrence of cracks and wrinkles when performing 180° adhesion bending based on JIS Z2248 was visually evaluated. In this case, those with wrinkles but no cracks were evaluated as very good (◎), those with some cracks and wrinkles but no overall cracks were evaluated as good (〇), and those with prominent cracks on the entire surface were evaluated as unacceptable (×).

[0056] (Evaluation of product strength) After applying a uniaxial strain of 2% in the rolling direction to the sample, the 0.2% proof stress after heat treatment at 170°C for 20 minutes was assumed to be the product strength. Those with a proof stress of 200 MPa or more were evaluated as very good (◎), those with a proof stress of 180 MPa or more and less than 200 MPa were evaluated as good (〇), and those with a proof stress of less than 180 MPa were evaluated as unacceptable (×).

[0057] (Evaluation of corrosion resistance) As an evaluation of corrosion resistance, a salt spray test (SST) was carried out for 800 hours. For the samples after this corrosion test, after removing the corrosion products with phosphoric acid chromium, the corrosion weight loss was measured. Based on this result, those with a corrosion weight loss of less than 15.0 mg / cm 2 were evaluated as good (〇), and those with 15.0 mg / cm 2 or more were evaluated as unacceptable (×). The yield strength, elongation, electrical conductivity, number density of Mg-Si compounds with an equivalent circle diameter of 1.0 μm or more, number density of Al-Zr compounds with 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 various evaluations are shown in Table 4.

[0058]

Table 1

[0059]

Table 2

[0060]

Table 3

[0061]

Table 4

[0062] As shown in Table 3 and Table 4, Examples 1 to 17 contain 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 inevitable impurities, having an average crystal grain size of 100 μm or less, a 0.2% proof stress of 120 MPa or more and 175 MPa or less, an elongation of 15% or more, and a 0.2% proof stress of 180 MPa or more after heat treatment at 170 °C for 20 minutes after applying a uniaxial strain of 2% in the rolling direction, all had good or very good formability and product strength, and all had good corrosion resistance.

[0063] On the other hand, in Comparative Examples 1 and 2, since the Fe component was too much, the 0.2% proof stress became high and the formability became impossible. In Comparative Example 3, since the Si component was too little, the 0.2% proof stress after the 170 °C heat treatment became low, so the product strength became impossible. In Comparative Example 4, since the Si component was too much, the corrosion resistance became impossible, and in addition, the 0.2% proof stress became high and the formability became impossible. In Comparative Example 5, since the Cu component was too much, the corrosion resistance became impossible, and in addition, the 0.2% proof stress became high and the formability also became impossible. In Comparative Example 6, since the Mn component was too little, the 0.2% proof stress and the 0.2% proof stress after the 170 °C heat treatment became low, so both the formability and the product strength became impossible. In Comparative Example 7, since the Mn component was too much, the 0.2% proof stress after the 170 °C heat treatment became low, so the product strength became impossible.

[0064] Moreover, in Comparative Example 8, since the Mg content was too low, both the 0.2% proof stress and the 0.2% proof stress after heat treatment at 170°C were low, resulting in unacceptable formability and product strength. In Comparative Example 9, since the Mg content was too high, the 0.2% proof stress was low and the formability became unacceptable. In Comparative Example 10, since the Zn content was too high, the corrosion resistance became unacceptable. In Comparative Example 11, since the Zr content was too low, both the 0.2% proof stress and the 0.2% proof stress after heat treatment at 170°C were low, resulting in unacceptable formability and product strength. In Comparative Examples 12 to 14, the hot rolling conditions were D, and compared with the other conditions A to C, the rolling speeds in the first and second passes were low. Therefore, in Comparative Examples 12 to 14, the elongation was low. Among these, in Comparative Example 13, although the crystal grain size was small due to the influence of Fe, Mn, etc. other than Zr, the elongation was low, resulting in unacceptable formability. In addition, the 0.2% proof stress after heat treatment at 170°C was low, so the product strength was also unacceptable. Also, in Comparative Example 14, the crystal grain size was large and the elongation was low, resulting in unacceptable formability. In addition, the 0.2% proof stress after heat treatment at 170°C was low, so the product strength was also unacceptable. On the other hand, in Comparative Example 12, since a large amount of Zr component was contained, the crystal grains were coarsened by heat treatment at a certain temperature such as homogenization treatment, and the crystal grain size became large. In addition, the elongation was low, so the formability became unacceptable.< / uvw>

Claims

1. 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, having a composition consisting of the balance being Al and unavoidable impurities, an average crystal grain size of 100 μm or less, a 0.2% proof stress of 120 MPa or more and 175 MPa or less, an elongation of 15% or more, and a 0.2% proof stress of 180 MPa or more after heat treatment at 170 °C for 20 minutes after applying a uniaxial strain of 2% in the rolling direction, an aluminum alloy sheet for vehicle body parts, characterized in that the number of Mg-Si based second phase particles having an equivalent circle diameter of 1.0 μm or more is 5.0×103 pieces / 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 pieces / mm2 or more.

2. The aluminum alloy sheet for vehicle body parts according to claim 1, further containing one or more of Cr: 0.01 mass% or more and 0.10 mass% or less, and Ti: 0.01 mass% or more and 0.10 mass% or less.

3. The aluminum alloy sheet for vehicle body parts according to claim 1 or 2, characterized in that the sum of the area ratio of crystal grains having an orientation with an orientation difference of 15° or less from the cube orientation in the cross-section in the rolling direction to all crystal grains 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 is 5% or more.

Images