Suspension member and method for manufacturing suspension member

Abstract

A suspension member made of an aluminum alloy having an alloy composition comprising CU: 0.3-0.5 mass%, Mg: 0.65-1.05 mass%, Si: 0.9-1.25 mass%, Mn: 0.4-0.6 mass%, Fe: 0.15-0.30 mass%, Cr: 0.09-0.25 mass%, Ti: 0.01-0.05 mass%, B: 0.0010-0.0050 mass%, and the balance being Al and unavoidable impurities, wherein, in a plane perpendicular to a surface of a bush portion formed along a parting line and parallel to the cylindrical axis of the bush portion, the inclination of crystal grain boundaries is 45° or less.

Description

Suspension member and method of manufacturing the suspension member

[0001] This application claims priority from Japanese Patent Application No. 2023-222058, filed on December 28, 2023, the contents of which are incorporated herein by reference.

[0002] In recent years, aluminum alloys have been increasingly used as structural components for various products, taking advantage of their light weight. For example, high-tensile steel has traditionally been used for automobile suspension and bumper parts, but in recent years high-strength aluminum alloy materials have been used. While iron-based materials were previously used exclusively for automobile parts, such as suspension parts, they are increasingly being replaced by aluminum or aluminum alloy materials, primarily for the purpose of reducing weight.

[0003] These automobile parts require excellent corrosion resistance, high strength, and excellent workability, and therefore Al-Mg-Si alloys, particularly A6061, are widely used as the aluminum alloy material. To improve the strength of these automobile parts, they are manufactured by forging, a type of plastic processing, using the aluminum alloy material as the processing material.

[0004] Furthermore, due to the need to reduce costs recently, suspension parts have begun to be put into practical use in which cast members are used as raw materials without extrusion, and then subjected to a solution treatment and artificial aging treatment (T6 treatment). With the aim of further weight reduction, development of high-strength alloys to replace the conventional A6061 is underway (see Patent Documents 1 to 3).

[0005] JP-A-5-059477 JP-A-5-247574 JP-A-6-256880

[0006] Recent CO ２With the need to reduce the weight of automobiles in order to reduce emissions, demand for aluminum is on the rise. However, as a replacement for steel, aluminum will need to be even stronger. To achieve higher strength, methods such as increasing the amount of Si, Mg, and Cu added are often used, but there are concerns that increasing the amount of added elements will result in a decrease in corrosion resistance. Among corrosion resistance factors, stress corrosion cracking (SCC) is important for materials used in automobile suspensions.

[0007] Stress corrosion cracking is a phenomenon in which corrosion occurs in a suspension part when a certain amount of stress is continuously applied to that part, causing cracks to form from the corroded part. The part of the suspension where stress corrosion cracking is most important is the part where the suspension bush is pressed in. It is necessary to increase the strength of the component while ensuring stress corrosion cracking resistance in this part.

[0008] Factors that affect stress corrosion cracking resistance include added elements, compound state, and metal crystalline state. Controlling the crystalline structure is important to ensure good stress corrosion cracking resistance while maintaining high strength. Even with the same amount of added elements, if the crystalline structure is coarse, stress corrosion cracking will occur preferentially at the grain boundaries, leading to component failure.

[0009] Forging conditions have a significant effect on controlling the crystal structure. For example, when forging is performed at a low material heating temperature before forging, the accumulation of strain in areas with large strain during processing causes recrystallization during the subsequent heat treatment process.

[0010] Automobile suspension components are generally formed by flash-removal forging due to their complex shapes.

[0011] When this burr removal forging is performed, the amount of strain is large near the parting line corresponding to the burr, making it prone to recrystallization. If this recrystallization occurs in the bushing, and if the area of ​​recrystallization is large, it can significantly impair stress corrosion cracking resistance when a ball joint or the like is pressed into the bushing, leading to damage to the component. The parting line is a line that is exposed when the burr is removed from the forged product by grinding. The parting line is slightly raised from the side of the forged product. Grinding (trimming) of the forged product is usually performed at room temperature.

[0012] However, the more complex the shape of the suspension, the more difficult it becomes to suppress recrystallization during the forging process.

[0013] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a suspension member and a method for manufacturing a suspension member that are excellent in stress corrosion cracking resistance even when recrystallization occurs near the parting line.

[0014] In order to solve the above problems, the present invention provides the following means.

[0015] [1] A suspension member according to one aspect of the present invention is made of an aluminum alloy having an alloy composition comprising: Cu in a range of 0.3% by mass to 0.5% by mass, Mg in a range of 0.65% by mass to 1.05% by mass, Si in a range of 0.9% by mass to 1.25% by mass, Mn in a range of 0.4% by mass to 0.6% by mass, Fe in a range of 0.15% by mass to 0.30% by mass, Cr in a range of 0.09% by mass to 0.25% by mass, Ti in a range of 0.01% by mass to 0.05% by mass, B in a range of 0.0010% by mass to 0.0050% by mass, and the balance being Al and unavoidable impurities; The inclination of the grain boundary is 45° or less with respect to a plane perpendicular to the plane formed by the parting line of the bushing portion and parallel to the cylindrical axis of the bushing portion.

[0016] [2] The suspension member of [1] above may have a tensile strength of 380 MPa or more.

[0017] [3] A method for manufacturing a suspension member according to one aspect of the present invention includes: a molten metal forming step of forming a molten metal of an aluminum alloy having an alloy composition containing Cu in a range of 0.3% by mass to 0.5% by mass, Mg in a range of 0.65% by mass to 1.05% by mass, Si in a range of 0.9% by mass to 1.25% by mass, Mn in a range of 0.4% by mass to 0.6% by mass, Fe in a range of 0.15% by mass to 0.30% by mass, Cr in a range of 0.09% by mass to 0.25% by mass, Ti in a range of 0.01% by mass to 0.05% by mass, B in a range of 0.0010% by mass to 0.0050% by mass, the balance being Al and unavoidable impurities; and a casting step of obtaining a casting by casting the molten metal. The method includes a forging process in which the cast product is heated and forged to obtain a forged product; a solution treatment process in which the forged product is maintained at a temperature of 500°C or higher; an aging process in which the forged product that has undergone the quenching process is heat-treated; and a hot trimming process in which the forged product that has undergone the aging process is hot-trimmed at a material temperature exceeding 100°C and not exceeding 250°C.

[0018] [4] In the method for manufacturing a suspension member according to [3] above, in the hot trimming step, the forged product that has undergone the aging treatment step may be hot trimmed at a material temperature of 125°C or higher and 250°C or lower.

[0019] [5] In the method for manufacturing a suspension member according to [3] or [4] above, in the hot trimming step, the forged product that has undergone the aging treatment step may be hot trimmed at a material temperature of 150°C or higher and 250°C or lower.

[0020] [6] In the method for manufacturing a suspension member according to any one of [3] to [5] above, the casting step may be performed by continuous casting, the cooling rate of the molten metal in the casting step may be 10°C / sec or more, and the average crystal grain size of the casting may be 80 μm or less.

[0021] [7] The manufacturing method of the suspension member according to any one of [3] to [6] above may further include a quenching step of quenching the forged product after the solution treatment step and before the hot trimming step, wherein the forging step may involve forging at a material temperature of 450°C or higher and 520°C or lower, the solution treatment step may involve holding the forged product at a temperature of 550°C or lower, the quenching step may involve water quenching the forged product with water at a temperature of 60°C or lower, and the aging treatment step may involve heating the forged product that has undergone the quenching step at a temperature of 175°C or higher and 190°C or lower for 4 hours or longer.

[0022] According to the present invention, it is possible to provide a suspension member and a method for manufacturing a suspension member that are excellent in stress corrosion cracking resistance even when recrystallization occurs near the parting line.

[0023] 4 is a plan view showing an example of the configuration of a suspension member according to one embodiment of the present invention. FIG. 5 is a schematic enlarged plan view of the vicinity of a bushing portion of the suspension member of FIG. 1. FIG. 6 is an enlarged plan view of the vicinity of the bushing portion of the suspension member of FIG. 1. FIG. 7 is a cross-sectional view showing an example of the configuration of the vicinity of a mold of a horizontal continuous casting device that can be used in the casting step of a manufacturing method of a suspension arm according to one embodiment of the present invention. FIG. 8 is an enlarged cross-sectional view of a main part near the cooling water cavity of the horizontal continuous casting device of FIG. 4. FIG. 9 is an explanatory view illustrating the heat flux of the cooling wall portion of the horizontal continuous casting device. FIG. 10 is a view for explaining a manufacturing method of a suspension member according to one embodiment of the present invention, and is a plan view showing an example of the configuration of a forged product before a hot trimming step is performed. FIG. 11 is a plan view schematically showing the configuration when an SCC test is performed in Example 1. FIG. 12 is a structural image obtained by analyzing an electron microscope image of the vicinity of a vertex 51 of the bushing portion of the suspension member of Example 1 by electron backdiffraction-scatter diffraction spectroscopy.

[0024] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that the drawings used in the following description may show characteristic portions enlarged for convenience in order to make the features easier to understand, and the dimensional ratios of each component may not necessarily be the same as those in reality. Furthermore, the materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not necessarily limited thereto, and can be implemented with appropriate changes within the scope of the present invention.

[0025] [Suspension Member] First, a suspension member according to one embodiment of the present invention will be described. The suspension member according to one embodiment of the present invention is a member for a suspension arm made of an aluminum alloy forged product.

[0026] The aluminum alloy forging of this embodiment is made of an aluminum alloy having an alloy composition containing Cu in the range of 0.3% by mass to 0.5% by mass, Mg in the range of 0.65% by mass to 1.05% by mass, Si in the range of 0.9% by mass to 1.25% by mass, Mn in the range of 0.4% by mass to 0.6% by mass, Fe in the range of 0.15% by mass to 0.30% by mass, Cr in the range of 0.09% by mass to 0.25% by mass, Ti in the range of 0.01% by mass to 0.05% by mass, B in the range of 0.0010% by mass to 0.0050% by mass, and the balance being Al and unavoidable impurities.

[0027] The aluminum alloy forgings of this embodiment correspond to 6000 series aluminum alloy forgings in that they contain Mg and Si.

[0028] In a suspension member according to one embodiment of the present invention, the inclination of the grain boundary is 45° or less with respect to a plane perpendicular to the plane formed by the parting line of the bushing portion and parallel to the cylindrical axis of the bushing portion. As will be described in detail later, the bushing portion corresponds to the structure indicated by the reference numeral 50 in the drawings, and the parting line corresponds to the structure indicated by the reference numeral PL.

[0029] (Cu: 0.3% by mass or more, 0.5% by mass or less) Cu has the effect of finely dispersing Mg-Si compounds in the aluminum alloy and improving the tensile strength of the aluminum alloy by precipitating as Al-Cu-Mg-Si compounds such as the Q phase. The Cu content is in the range of 0.30% by mass or more and 0.50% by mass or less, and may be in the range of 0.35% by mass or more and 0.45% by mass or less, or 0.42% by mass or less. By having the Cu content in the above range, the mechanical properties of the aluminum alloy forging at room temperature can be improved. Furthermore, when Cu exceeds 0.5%, Mg and Mg are easily separated from each other at the grain boundaries. ２ Since the amount of Cu coexisting with Si increases, the potential difference between the matrix and the compound at the grain boundary increases, impairing stress corrosion cracking resistance, so it is preferable that the amount is within the above range.

[0030] (Mg: 0.65 mass % or more, 1.05 mass % or less) Mg has the effect of improving the tensile strength of the aluminum alloy. Mg dissolves in the aluminum matrix or forms a Mg-Si compound (Mg ２ The magnesium content is in the range of 0.65% by mass to 1.05% by mass, and may be in the range of 0.75% by mass to 1.00% by mass, or more than 0.85% by mass to 0.95% by mass. By having the magnesium content in the above range, it is possible to improve the corrosion resistance as well as the mechanical properties at room temperature of the aluminum alloy forging.

[0031] (Si: 0.9% by mass or more, 1.25% by mass or less) Like Mg, Si has the effect of improving the corrosion resistance as well as the mechanical properties of aluminum alloy forgings at room temperature. However, if excessive Si is added to an aluminum alloy, coarse primary crystal Si grains may crystallize, potentially reducing the tensile strength of the aluminum alloy. The Si content is within the range of 0.90% by mass or more and 1.25% by mass or less, and may also be within the range of 0.95% by mass or more and 1.20% by mass or less, or 1.00% by mass or more and 1.18% by mass or less. By having the Si content within the above range, it is possible to improve the corrosion resistance as well as the mechanical properties of aluminum alloy forgings at room temperature while suppressing the crystallization of primary crystal Si.

[0032] (Mn: 0.4% by mass or more, 0.6% by mass or less) Mn forms fine granular precipitates containing intermetallic compounds such as Al-Mn-Fe-Si and Al-Mn-Cr-Fe-Si in the aluminum alloy, thereby improving the tensile strength of the aluminum alloy. The Mn content is within the range of 0.40% by mass or more and 0.60% by mass or less, and may also be within the range of 0.45% by mass or more and 0.55% by mass or less, or 0.47% by mass or more and 0.53% by mass or less. By having the Mn content within the above range, the mechanical properties of the aluminum alloy forging at room temperature can be improved.

[0033] (Fe: 0.15% by mass or more, 0.3% by mass or less) Fe crystallizes in the aluminum alloy as fine crystals containing intermetallic compounds such as Al-Mn-Fe-Si, Al-Mn-Cr-Fe-Si, Al-Fe-Si, Al-Cu-Fe, and Al-Mn-Fe, thereby improving the tensile strength of the aluminum alloy. The Fe content is within a range of 0.15% by mass or more and 0.30% by mass or less, and may be within a range of 0.20% by mass or more and 0.27% by mass or less, or 0.22% by mass or more. By having the Fe content within the above range, the mechanical properties of the aluminum alloy forging at room temperature can be improved.

[0034] (Cr: 0.09% by mass or more, 0.25% by mass or less) Cr forms fine granular precipitates containing intermetallic compounds such as Al-Mn-Cr-Fe-Si and Al-Fe-Cr in the aluminum alloy, thereby improving the tensile strength of the aluminum alloy. The Cr content is within the range of 0.09% by mass or more and 0.25% by mass or less, and may also be within the range of 0.10% by mass or more and 0.20% by mass or less, or 0.12% by mass or more and 0.18% by mass or less. By having the Cr content within the above range, the mechanical properties of the aluminum alloy forging at room temperature can be improved.

[0035] (Ti: 0.01% by mass or more, 0.05% by mass or less) Ti has the effect of refining the crystal grains of an aluminum alloy and improving its wrought workability. If the Ti content is less than 0.01% by mass, the effect of refining the crystal grains may not be sufficiently obtained. On the other hand, if the Ti content exceeds 0.05% by mass, coarse crystals may be formed, which may reduce the wrought workability. Furthermore, if a large amount of coarse crystals containing Ti are mixed into an aluminum alloy forging, the toughness may be reduced. Therefore, the Ti content is set to 0.01% by mass or more, and 0.05% by mass or less. The Ti content is preferably 0.015% by mass or more, and 0.030% by mass or less.

[0036] (B: 0.0010% by mass or more, 0.0050% by mass or less) B has the effect of refining the crystal grains of an aluminum alloy and improving its wrought workability. Adding B to an aluminum alloy together with the above-mentioned Ti improves the effect of refining the crystal grains. If the B content is less than 0.0010% by mass, the effect of refining the crystal grains may not be sufficient. On the other hand, if the B content exceeds 0.0050% by mass, coarse crystals may be formed and may be mixed into the aluminum alloy forging as inclusions. Furthermore, if a large amount of coarse crystals containing B are mixed into the final aluminum alloy product, the toughness may be reduced. Therefore, the B content is set to 0.0010 to 0.0050% by mass. The B content is preferably 0.0015 to 0.0045% by mass.

[0037] (Inevitable impurities) Inevitable impurities are impurities that are inevitably mixed into the aluminum alloy from the raw materials or manufacturing process of the aluminum alloy forged product. Examples of inevitable impurities include Zn, Zr, Ni, Sn, and Be. The content of these inevitable impurities preferably does not exceed 0.1% by mass.

[0038] Fig. 1 is a plan view showing an example of the configuration of a suspension member according to one embodiment of the present invention, Fig. 2 is a schematic enlarged view of the vicinity of a bushing of the suspension member of Fig. 1, and Fig. 3 is an enlarged plan view of the vicinity of the bushing of the suspension member of Fig. 1.

[0039] The suspension member 100 shown in Figures 1 to 3 includes, for example, a wheel side coupling portion 4, a first vehicle body side coupling portion 5, a second vehicle body side coupling portion 6, a first arm portion 1, a second arm portion 2, and an arm coupling portion 3. The first arm portion 1 connects the wheel side coupling portion 4 and the first vehicle body side coupling portion 5. The second arm portion 2 connects the wheel side coupling portion 4 and the second vehicle body side coupling portion 6. The arm coupling portion 3 connects the first vehicle body side coupling portion 5 and the second vehicle body side coupling portion 6. The first vehicle body side coupling portion 5 and the second vehicle body side coupling portion 6 are members also referred to as bushing portions. The suspension member 100 according to this embodiment includes at least one bushing portion 5, 6. The bushing portions 5, 6 may have the same shape.

[0040] 2 and 3 are diagrams illustrating features of the bushing portion in the suspension member 100 of this embodiment. In FIGS. 2 and 3, an enlarged view of the bushing portion 5 is shown as an example of the bushing portion. The bushing portion 5 is used as a product, for example, in a state in which a region (removed region) indicated by a two-dot chain line and reference character H in FIGS. 2 and 3 has been removed so as to penetrate the bushing portion 5 in the x direction. That is, the region (removed region) indicated by a two-dot chain line and reference character H in FIGS. 2 and 3 has been removed to form an opening. Because the opening continues in the depth direction (x direction) of the page in FIG. 2, it actually has a cylindrical shape. Another component, such as a ball joint, is press-fitted into the opening in the x direction. In this embodiment, forged components having cylindrical openings in the bushing portions 5 and 6 and forged components having no openings in the bushing portions 5 and 6 are collectively referred to as suspension members.

[0041] In the suspension member 100 according to this embodiment, a parting line PL is formed along the outer shape of the suspension when viewed in a plan view from the z direction. The parting line PL is formed by removing burrs 70 from the forged product in a hot trimming process during the forging step in the manufacturing process, which will be described in detail later. The parting line PL can be confirmed by observing the side surface of the suspension member 100.

[0042] As shown in Figure 2, the parting line PL in the bushing portion 5 is molded to form, for example, the xy plane. The bushing portions 5, 6 have vertices at their ends in the y direction. In Figure 2, the vertices formed in the bushing portion 5 are indicated by the reference numeral 51. The vertices of the bushing portions 5, 6 are located on the center of the bushing portions 5, 6 of the suspension member 100 in the z direction, and are at the position where the distance from the wheel-side connecting portion 4 in the y direction is greatest.

[0043] In the suspension member 100 of this embodiment, the grain boundary at the apex portion 51 has an inclination of 45° or less with respect to a plane perpendicular to the plane formed by the parting line PL of the bushing portions 5, 6 (the xy plane in FIGS. 1 to 3 ) and parallel to the cylindrical axes of the bushing portions 5, 6. The inclination of the grain boundary is preferably 35° or less, and more preferably 25° or less. The inclination of the grain boundary exceeds 0°, for example, 5° or more or 10° or more.

[0044] The more complex the shape of the suspension member produced by forging, the greater the amount of strain during the forging process, which will be described in detail later, resulting in recrystallization during the subsequent solution treatment process. Because the amount of strain is particularly large near the parting line, it is difficult to completely eliminate this recrystallization. On the other hand, areas where recrystallization occurs are problematic because they are more likely to develop stress corrosion cracking when stress is applied. A ball joint is press-fitted into the bushing portions 5 and 6 of the automotive suspension member 100, and the apex 51 of the bushing portions 5 and 6 is the location most subjected to stress. The direction of stress is the direction in which the ball joint is press-fitted and the opposite direction. In other words, the direction of stress is the axial direction of the opening formed in the bushing portions 5 and 6, which is the ±x direction in Figures 1 to 3 .

[0045] If the long sides of the crystals in the recrystallized portions are perpendicular to the direction of stress applied to the bushing portions 5 and 6, the stress corrosion cracking resistance is significantly inferior. In contrast, in the region near the apex portion 51, extending from the outermost surface to a predetermined depth inward in the y direction (i.e., from E0 in the -y direction in the figure to a depth of 300 μm), the orientation of the long sides of the crystal grains is nearly parallel to the stress direction, resulting in improved resistance to stress corrosion cracking. As described above, the orientation of the crystal grains is perpendicular to the plane of the parting line PL of the bushing portions 5 and 6 (the xy plane in FIGS. 1 to 3 ) at the outermost point (apex portion 51) in the y direction of the parting line PL where stress is concentrated, and the inclination of the grain boundaries with respect to a plane parallel to the cylindrical axis (x-axis) of the bushing portions 5 and 6 is 45° or less.

[0046] The suspension member 100 in this embodiment has a tensile strength in accordance with JIS Z2241:2011 of 380 MPa or more, preferably 385 MPa or more, more preferably 390 MPa or more, and even more preferably 392 MPa or more.

[0047] FIG. 1 shows an example in which the suspension arm is an A-type arm having a first arm portion 1, a second arm portion 2, and an arm connecting portion 3, but the present invention is not limited to the above example and can also be applied to suspension arms of other shapes that have a bushing portion and are formed by forging.

[0048] [Manufacturing Method of Suspension Member] Next, a manufacturing method of a suspension member according to one embodiment of the present invention will be described. The manufacturing method of a suspension member according to one embodiment of the present invention includes a molten metal forming step of obtaining a molten alloy having the same composition as the aluminum alloy forged product described above, a forging step of forging a cast product obtained by casting the molten alloy while heating the cast product to obtain a forged product, a solution treatment step of heating the forged product at a temperature of 500°C or higher, an aging treatment step of heat treating the forged product that has undergone the solution treatment step after the solution treatment, and a hot trimming step of hot trimming the forged product that has undergone the aging treatment step at a material temperature of more than 100°C and not more than 250°C.

[0049] A manufacturing method of a suspension member according to one embodiment of the present invention includes, for example, a molten metal forming step, a casting step, a homogenization heat treatment step, a forging step, a solution treatment step, a hardening treatment step, an aging treatment step, and a hot trimming step.

[0050] (Molten Metal Formation Step) The molten metal formation step is a step of melting raw materials to obtain a molten aluminum alloy having a composition adjusted to have an alloy composition containing Cu in the range of 0.30 mass % to 0.50 mass %, Mg in the range of 0.65 mass % to 1.05 mass %, Si in the range of 0.90 mass % to 1.25 mass %, Mn in the range of 0.40 mass % to 0.60 mass %, Fe in the range of 0.15 mass % to 0.30 mass %, Cr in the range of 0.09 mass % to 0.25 mass %, Ti in the range of 0.01 mass % to 0.05 mass %, B in the range of 0.0010 mass % to 0.0050 mass %, and the balance being Al and unavoidable impurities.

[0051] By carrying out the subsequent steps using a molten aluminum alloy of the above composition, it is possible to provide a suspension member and a method for manufacturing a suspension member that are excellent in resistance to stress corrosion cracking even when recrystallization occurs near the parting line. Note that virgin aluminum ingot is aluminum with a concentration of 99% or more obtained by subjecting alumina produced from minerals to electrolysis, a process known as electrolytic refining.

[0052] A molten aluminum alloy can be obtained by heating and melting an aluminum alloy. Alternatively, the aluminum alloy may be formed by melting a mixture containing the elements or compounds containing two or more elements that are raw materials for the aluminum alloy, in proportions that produce the desired aluminum alloy. For example, to control the grain size of the aluminum alloy produced in the casting process, Ti or B may be mixed as a grain refiner, such as an Al-Ti-B rod.

[0053] Alternatively, the molten aluminum alloy may be prepared by melting 10% or more of scrap aluminum alloys of 1000, 2000, 3000, 4000, 5000, 6000, or 7000 series, with the remainder being new aluminum ingots and the above-mentioned additive elements, to obtain a molten aluminum alloy having a composition adjusted. In this case, an Al-Mg-Si suspension arm that is less susceptible to recrystallization and has excellent mechanical properties at room temperature can be obtained. Note that new aluminum ingots are aluminum with a purity of, for example, 99% or higher, obtained by subjecting alumina produced from minerals to electrolysis, a process known as electrolytic refining.

[0054] (Casting Process) In the casting process, a molten aluminum alloy (liquid phase) is cooled and solidified into a solid (solid phase) to obtain an aluminum alloy cast product. The casting process is preferably carried out by continuous casting. For example, a vertical continuous casting method or a horizontal continuous casting method can be used for the casting process. Below, a method for manufacturing a suspension arm according to one embodiment of the present invention will be described, taking as an example a case where the casting process is carried out by a horizontal continuous casting method.

[0055] Horizontal continuous casting apparatuses that can be used to produce the aluminum alloy cast product of this embodiment are shown in Figures 4 and 5. Figure 4 is a cross-sectional view showing an example of the vicinity of the mold 12 of the horizontal continuous casting apparatus 10. Figure 5 is an enlarged cross-sectional view of a main portion of the horizontal continuous casting apparatus 10 near the cooling water cavity 24.

[0056] The horizontal continuous casting apparatus 10 shown in Figures 4 and 5 has a molten metal receiving portion (tundish) 11, a hollow cylindrical mold 12, and a refractory plate-like body (heat insulating member) 13 arranged between one end side 12a of the mold 12 and the molten metal receiving portion 11.

[0057] The molten metal receiving portion 11 is composed of a molten metal inlet portion 11 a that receives the molten aluminum alloy M obtained in the molten metal forming process described above, a molten metal holding portion 11 b, and an outlet portion 11 c that flows into the hollow portion 21 of the mold 12.

[0058] The molten metal receiving portion 11 maintains the upper liquid level of the molten aluminum alloy M at a position higher than the upper surface of the hollow portion 21 of the mold 12, and in the case of multiple casting, stably distributes the molten aluminum alloy M to each mold 12.

[0059] The molten aluminum alloy M held in the molten metal holding portion 11b in the molten metal receiving portion 11 is poured into the hollow portion 21 of the mold 12 through a pouring passage 13a provided in the refractory plate 13. The molten aluminum alloy M supplied into the hollow portion 21 is then cooled and solidified by a cooling device 23 (described later) and drawn out from the other end 12b of the mold 12 as an aluminum alloy rod B, which is a solidified ingot.

[0060] A drawing drive device (not shown) that draws out the cast aluminum alloy rod B at a constant speed may be installed at the other end 12b of the mold 12. It is also preferable that a synchronized cutting machine (not shown) that cuts the continuously drawn aluminum alloy rod B to a desired length be installed.

[0061] The refractory plate 13 is a member that blocks heat transfer between the molten metal receiver 11 and the mold 12, and may be made of materials such as calcium silicate, alumina, silica, a mixture of alumina and silica, silicon nitride, silicon carbide, graphite, etc. Such a refractory plate 13 may also be made up of multiple layers made of different materials.

[0062] In this embodiment, the mold 12 is a hollow cylindrical member made of, for example, one or a combination of two or more materials selected from aluminum, copper, or alloys thereof. The materials for the mold 12 may be selected from an optimum combination in terms of thermal conductivity, heat resistance, and mechanical strength.

[0063] The hollow portion 21 of the mold 12 is formed to have a circular cross section in order to cast the aluminum alloy rod B into a cylindrical rod shape, and the mold 12 is held so that the mold central axis (central axis) C passing through the center of this hollow portion 21 is aligned approximately horizontally.

[0064] The inner peripheral surface 21a of the hollow portion 21 of the mold 12 is formed at an elevation angle of 0° to 3° (more preferably 0° to 1°) with respect to the mold central axis C toward the casting direction of the aluminum alloy rod B (see FIG. 4). That is, the inner peripheral surface 21a is configured in a tapered shape that opens like a cone toward the casting direction. The angle of this taper is the elevation angle.

[0065] If the elevation angle is less than 0°, the aluminum alloy rod B encounters resistance at the other end 12b, which is the mold outlet, when it is drawn out of the mold 12, which may make casting difficult. On the other hand, if the elevation angle exceeds 3°, the inner peripheral surface 21a will not be in sufficient contact with the molten aluminum alloy M, which may reduce the heat transfer effect from the molten aluminum alloy M and the solidified shell formed by cooling and solidifying it to the mold 12, which may result in insufficient solidification. As a result, this is not preferable because it may lead to casting problems such as the formation of a remelted skin on the surface of the aluminum alloy rod B or the ejection of unsolidified molten aluminum alloy M from the end of the aluminum alloy rod B.

[0066] The cross-sectional shape of the hollow portion 21 of the mold 12 (the planar shape when the hollow portion 21 of the mold 12 is viewed from the other end side 21b) may be selected to match the shape of the aluminum alloy rod to be cast, such as a triangular or rectangular cross-sectional shape, a polygonal, semicircular, elliptical, or an irregular cross-sectional shape having no axis or plane of symmetry, in addition to the circular shape of this embodiment.

[0067] A fluid supply pipe 22 is disposed at one end 12a of the mold 12 to supply a lubricating fluid into the hollow portion 21 of the mold 12. The lubricating fluid supplied from the fluid supply pipe 22 can be one or more lubricating fluids selected from a gas lubricant and a liquid lubricant. When supplying both a gas lubricant and a liquid lubricant, it is preferable to provide separate fluid supply pipes for each. The lubricating fluid supplied under pressure from the fluid supply pipe 22 is supplied into the hollow portion 21 of the mold 12 through an annular lubricant supply port 22a.

[0068] In this embodiment, the lubricating fluid is supplied under pressure from the lubricant supply port 22a to the inner circumferential surface 21a of the mold 12. Note that the liquid lubricant may be heated to decompose into a gas and then supplied to the inner circumferential surface 21a of the mold 12. Alternatively, a porous material may be disposed in the lubricant supply port 22a, and the lubricating fluid may be allowed to seep out onto the inner circumferential surface 21a of the mold 12 through the porous material.

[0069] A cooling device 23, which is a cooling means for cooling and solidifying the molten aluminum alloy M, is formed inside the mold 12. The cooling device 23 of this embodiment has a cooling water cavity 24 that accommodates cooling water W for cooling the inner circumferential surface 21 a of the hollow portion 21 of the mold 12, and a cooling water injection passage 25 that connects the cooling water cavity 24 with the hollow portion 21 of the mold 12.

[0070] The cooling water cavity 24 is formed in a ring shape surrounding the hollow portion 21 outside the inner circumferential surface 21 a of the hollow portion 21 inside the mold 12 , and cooling water W is supplied to the cavity 24 via a cooling water supply pipe 26 .

[0071] The inner surface 21a of the mold 12 is cooled by the cooling water W contained in the cooling water cavity 24, which removes heat from the molten aluminum alloy M filling the hollow portion 21 of the mold 12 from the surface in contact with the inner surface 21a of the mold 12, forming a solidified shell on the surface of the molten aluminum alloy M.

[0072] The cooling water jetting passages 25 spray cooling water W directly from shower openings 25a facing the hollow portion 21 toward the aluminum alloy rods B at the other end 12b of the mold 12 to cool the aluminum alloy rods B. The vertical cross-sectional shape of the cooling water jetting passages 25 may be, for example, semicircular, pear-shaped, or horseshoe-shaped, in addition to the circular shape of this embodiment.

[0073] In this embodiment, the cooling water W supplied through the cooling water supply pipe 26 is first stored in the cooling water cavity 24 to cool the inner peripheral surface 21 a of the hollow portion 21 of the mold 12, and then the cooling water W in the cooling water cavity 24 is sprayed toward the aluminum alloy rod B through the cooling water spray passage 25. However, it is also possible to configure these to be supplied by separate cooling water supply pipes.

[0074] The length from the position where the extension of the central axis of the shower opening 25a of the cooling water injection passage 25 hits the surface of the cast aluminum alloy rod B to the contact surface between the mold 12 and the refractory plate 13 is called the effective mold length L, and this effective mold length L is preferably, for example, 10 mm or more and 40 mm or less. If this effective mold length L is less than 10 mm, a good coating cannot be formed, making casting impossible. If it exceeds 40 mm, the effect of forced cooling is reduced, solidification by the mold wall becomes dominant, and the contact resistance between the mold 12 and the molten aluminum alloy M or the aluminum alloy rod B increases, which may cause cracks on the casting surface or tearing inside the mold, making casting unstable.

[0075] It is preferable that the supply of the cooling water W to the cooling water cavity 24 and the spray of the cooling water W from the shower opening 25a of the cooling water spray passage 25 can be controlled by control signals from a control device (not shown).

[0076] The cooling water cavity 24 is formed so that an inner bottom surface 24 a near the hollow portion 21 of the mold 12 is parallel to the inner peripheral surface 21 a of the hollow portion 21 of the mold 12 .

[0077] Note that "parallel" here also includes the case where the inner peripheral surface 21a of the hollow portion 21 of the mold 12 is formed at an elevation angle of 0° to 3° relative to the inner bottom surface 24a of the cooling water cavity 24, i.e., the case where the inner bottom surface 24a is inclined from 0° to 3° relative to the inner peripheral surface 21a.

[0078] As shown in FIG. 5 , the cooling wall portion 27 of the mold 12, which is the portion where the inner bottom surface 24 a of the cooling water cavity 24 faces the inner peripheral surface 21 a of the hollow portion 21 of the mold 12, has a heat flux value per unit area of ​​10×10 200 m / s from the molten aluminum alloy M in the hollow portion 21 to the cooling water W in the cooling water cavity 24. ５ W / m ２ That's it, 50 x 10 ５ W / m ２ It is formed so as to fall within the following range.

[0079] The mold 12 may be formed so that the thickness t of the cooling wall portion 27 of the mold 12, i.e., the distance between the inner bottom surface 24a of the cooling water cavity 24 and the inner peripheral surface 21a of the hollow portion 21 of the mold 12, is within a range of, for example, 0.5 mm to 3.0 mm, preferably 0.5 mm to 2.5 mm. The material for forming the mold 12 may be selected so that the thermal conductivity of at least the cooling wall portion 27 of the mold 12 is within a range of 100 W / m·K to 400 W / m·K.

[0080] In FIG. 5 , molten aluminum alloy M in a molten metal receiving portion 11 is supplied through a refractory plate 13 to one end 12 a of a mold 12, which is held so that the central axis C of the mold is substantially horizontal, and is forcibly cooled at the other end 12 b of the mold 12 to form an aluminum alloy rod B.

[0081] The aluminum alloy rod B is drawn out at a constant speed by a drawing drive device (not shown) installed near the other end 12b of the mold 12, and is continuously cast to form a long aluminum alloy rod B. The drawn aluminum alloy rod B is then cut to a desired length by, for example, a synchronous cutting machine (not shown).

[0082] The composition ratio of the cast aluminum alloy rod B can be confirmed, for example, by a method using a photoelectric emission spectrophotometer (e.g., PDA-5500 manufactured by Shimadzu Corporation, Japan) as described in "JIS H 1305".

[0083] The difference in height between the liquid level of the molten aluminum alloy M stored in the molten metal receiver 11 and the upper inner circumferential surface 21a of the mold 12 is preferably 0 mm to 250 mm (more preferably 50 mm to 170 mm). By setting the difference in height within this range, the pressure of the molten aluminum alloy M supplied into the mold 12 and the lubricating oil and the gas produced by vaporizing the lubricating oil are suitably balanced, resulting in stable castability.

[0084] The liquid lubricant may be a vegetable oil, such as rapeseed oil, castor oil, or salad oil, which are preferred because they have little adverse effect on the environment.

[0085] The lubricating oil supply rate is preferably 0.05 mL / min to 5 mL / min (more preferably 0.1 mL / min to 1 mL / min). If the supply rate is too low, the molten aluminum alloy M in the aluminum alloy rod B may not solidify due to insufficient lubrication and may leak from the mold 12. If the supply rate is too high, the excess may get mixed into the aluminum alloy rod B and cause internal defects.

[0086] The casting speed, which is the speed at which the aluminum alloy rod B is withdrawn from the mold 12, is preferably 200 mm / min or more and 1500 mm / min or less (more preferably 400 mm / min or more and 1000 mm / min or less). This is because, if the casting speed is within this range, the network structure of the crystallized matter formed by casting becomes uniform and fine, which increases the resistance of the aluminum matrix to deformation at high temperatures and improves the high-temperature mechanical strength.

[0087] The amount of cooling water sprayed from the shower openings 25a of the cooling water spray passages 25 is preferably 10 L / min to 50 L / min (more preferably 25 L / min to 40 L / min) per mold. If the amount of cooling water is less than this range, the molten aluminum alloy M may not solidify and leak from the mold 12. Furthermore, the surface of the cast aluminum alloy rod B may remelt, forming an inhomogeneous structure that may remain as internal defects. On the other hand, if the amount of cooling water is greater than this range, the mold 12 may lose too much heat, causing it to solidify prematurely.

[0088] The average temperature of the molten aluminum alloy M flowing from the molten metal receiver 11 into the mold 12 is preferably, for example, 650°C or higher and 750°C or lower (more preferably 680°C or higher and 720°C or lower). If the temperature of the molten aluminum alloy M is too low, coarse crystallized products may be formed in the mold 12 or before that, and may be incorporated as internal defects inside the aluminum alloy rod B. On the other hand, if the temperature of the molten aluminum alloy M is too high, a large amount of hydrogen gas may be easily incorporated into the molten aluminum alloy M, which may be incorporated as porosity in the aluminum alloy rod B and cause internal cavities.

[0089] In the cooling wall portion 27 of the mold 12, the heat flux value per unit area from the molten aluminum alloy M in the hollow portion 21 to the cooling water W in the cooling water cavity 24 is 10×10 ５ W / m ２ Above 50 x 10 ５ W / m ２ By keeping the temperature within the following range, it is possible to prevent the aluminum alloy rod B from seizing.

[0090] The cooling wall 27 of the mold 12 receives heat from the molten aluminum alloy M and performs heat exchange by cooling this heat with the cooling water W contained in the cooling water cavity 24. Regarding the state of this heat exchange, attention was focused on the heat flux per unit area, as shown in the explanatory diagram in Figure 6. The heat flux per unit area is expressed by the following equation (1) using Fourier's law. Q = -k × (T1 - T2) / L (1) Q: heat flux k: thermal conductivity (W / m·K) of the location through which heat passes (in this embodiment, the cooling wall 27 of the mold 12) T1: low-temperature side temperature of the location through which heat passes (in this embodiment, the inner bottom surface 24 a of the cooling water cavity 24) T2: high-temperature side temperature of the location through which heat passes (in this embodiment, the inner circumferential surface 21 a of the hollow portion 21 of the mold 12) L: section length (mm) of the location through which heat passes (in this embodiment, the thickness t of the cooling wall 27 of the mold 12)

[0091] Good results were obtained even when the amount of lubricating oil was reduced during casting. Based on the mold material, thickness, and temperature measurement data, the heat flux value per unit area was 10 × 10 ５ W / m ２ By configuring the cooling wall portion 27 of the mold 12 so that the heat flux value per unit area is 50×10 or more, it is possible to prevent the cast aluminum alloy rod B from seizing. ５ W / m ２ It is preferable to do the following:

[0092] To set the heat flux value of the cooling wall portion 27 of the mold 12 within this range, the mold 12 may be formed so that the thickness t of the cooling wall portion 27 of the mold 12 is, for example, in the range of 0.5 mm to 3.0 mm. Also, the thermal conductivity of at least the cooling wall portion 27 of the mold 12 may be set in the range of 100 W / m·K to 400 W / m·K.

[0093] When producing the aluminum alloy rod B of this embodiment, the horizontal continuous casting apparatus 10 described above is used to continuously supply the molten aluminum alloy M stored in the molten metal receptacle 11 from one end 12a of the mold 12 into the hollow portion 21. In addition, cooling water W is supplied to the cooling water cavity 24, and a lubricating fluid, for example, lubricating oil, is supplied from the fluid supply pipe 22.

[0094] The molten aluminum alloy M supplied into the hollow portion 21 is cooled to a temperature at which the heat flux value per unit area of ​​the cooling wall portion 27 is 10×10 ５ W / m ２ The aluminum alloy rod B is cooled and solidified under the above conditions to be cast. During casting of the aluminum alloy rod B, it is preferable to set the wall surface temperature of the cooling wall portion 27 of the mold 12, which is cooled by the cooling water W, to 100° C. or less.

[0095] The aluminum alloy rod B thus obtained has a heat flux value per unit area in the cooling wall portion 27 of 10×10 ５ W / m ２ Cooling and solidification under the above conditions suppresses adhesion of reaction products, such as carbides, that occur due to contact between the lubricating oil gas and the molten aluminum alloy M. This eliminates the need to remove carbides and the like from the surface of the aluminum alloy rod B by cutting, and allows the aluminum alloy rod B to be produced with a high yield.

[0096] The casting process for obtaining a cast product from the molten aluminum alloy M is not limited to the above-mentioned horizontal continuous casting method, and known continuous casting methods such as vertical continuous casting can be used. Vertical continuous casting methods are classified into the float method and the hot top method depending on the method of supplying the molten aluminum alloy M to the mold (casting die 12), and the following will briefly explain the case where the hot top method is used.

[0097] The casting equipment used in the hot top method is equipped with a mold, a molten metal receiving vessel (header), etc. The molten metal supplied to the molten metal receiving vessel passes through a tapping port and then through the header, where the flow rate is adjusted, and enters a cylindrical mold placed almost horizontally, where it is forcibly cooled and a solidified shell is formed on the outer surface of the molten metal.

[0098] Furthermore, cooling water is sprayed directly onto the casting as it is pulled out of the mold, allowing the solidification of the metal to progress throughout the casting as it is continuously pulled out. Molds are generally made of metal materials with good thermal conductivity and have a hollow structure to allow the introduction of a coolant inside.

[0099] The refrigerant to be used may be selected from among those that are industrially available, but water is recommended from the viewpoint of ease of use.

[0100] The mold used in this embodiment is appropriately selected from metals such as copper and aluminum, or graphite, from the viewpoint of heat transfer performance and durability at the contact point with the molten metal. The header is generally made of a refractory material and is installed above the mold. The material and size of the header may be appropriately selected depending on the composition range of the alloy to be cast and the dimensions of the cast product, and are not particularly limited.

[0101] The average cooling rate during casting can be selected from the range of, for example, 10° C. / sec to 300° C. / sec, and the casting speed can be selected from the range of, for example, 200 to 600 mm / min.

[0102] (Homogenization Heat Treatment Step) Subsequently, a homogenization heat treatment step may be appropriately performed. The homogenization heat treatment step is a step in which the aluminum alloy cast product obtained by the casting step is subjected to heat treatment to homogenize microsegregations caused by solidification, precipitate supersaturated solid solution elements, and transform metastable phases into equilibrium phases.

[0103] In the homogenization heat treatment step, the aluminum alloy casting obtained in the casting step is subjected to homogenization heat treatment by holding it at a temperature of 370°C or higher and 560°C or lower for 4 to 10 hours. By performing homogenization heat treatment within this temperature range, the aluminum alloy casting is homogenized and solute atoms are sufficiently introduced. Therefore, the strength is further improved by the subsequent aging treatment. The homogenization heat treatment step may be omitted.

[0104] (Forging process) The forging process is a process in which an aluminum alloy casting is molded to a predetermined size to obtain a forging material, the obtained forging material is heated to a predetermined temperature, and then pressure is applied in a press to mold it into a die.

[0105] In this embodiment, a forged product (automobile suspension arm member) is obtained by forging a forging material at a heating temperature of 450°C or higher and 560°C or lower. In this case, the forging start temperature of the forging material is preferably 450°C or higher and 560°C or lower. If the start temperature is lower than 450°C, the deformation resistance may become high, making sufficient processing impossible. On the other hand, if the start temperature exceeds 560°C, defects such as forging cracks and eutectic melting may easily occur. Furthermore, it is more preferable that the material temperature of the forging material be within the range of 480°C or higher and 520°C or lower.

[0106] The forging material preferably has an in-plane average grain size of 100 μm or less, more preferably 80 μm or less, and for example, 30 μm or more.

[0107] (Solution Treatment Step) The solution treatment step is a step in which the forged product obtained in the forging step is heated at a temperature of 500°C or higher to cause a solution, thereby relieving the strain introduced into the cast product and causing the solute elements to dissolve.

[0108] In this embodiment, the forged product is solution-treated by, for example, holding it at a temperature of 530°C or higher and 560°C or lower for 0.3 to 5 hours, preferably at a temperature of 530°C or higher and 550°C or lower for 1 to 4 hours. The solution treatment time may be 3 hours or shorter. The heating rate from room temperature to the above-mentioned treatment temperature is preferably 5.0°C / min or higher. If the treatment temperature is insufficient, the solute elements may not be dissolved sufficiently. On the other hand, if the treatment temperature is too high, the solute elements may be dissolved more easily, but eutectic melting or recrystallization may occur. Furthermore, if the heating rate is less than 5.0°C / min, the Mg ２ On the other hand, if the treatment temperature is less than 530°C, the solution treatment does not progress and it may be difficult to achieve high strength by aging precipitation.

[0109] (Quenching Step) The quenching step is a step in which the forged product in the solid solution state obtained by the solution treatment step is rapidly cooled to form a supersaturated solid solution.

[0110] In this embodiment, the water quenching process is performed by placing the forged product in a water tank containing water (quenching water) and submerging the forged product. The water temperature in the water tank is preferably 20°C or higher and 60°C or lower. The forged product is preferably placed in the water tank after solution treatment for 5 seconds or higher and 60 seconds or lower so that the entire surface of the forged product comes into contact with water. The submersion time of the forged product varies depending on the size of the casting, but is, for example, between 5 minutes and 40 minutes.

[0111] (Aging Treatment Step) The aging treatment step is a step in which the forged product is heated and held at a relatively low temperature to precipitate supersaturated solid-solution elements, thereby imparting an appropriate hardness.

[0112] In this embodiment, the forged product after the quenching treatment step is heated to a temperature of 170°C or higher and 220°C or lower, and is then held at that temperature for 0.5 hours or longer, 7.0 hours or shorter, and preferably 4 hours or longer, to perform aging treatment. If the heating temperature is lower than 180°C or the holding time is shorter than 0.5 hours, the Mg content, which improves the tensile strength, will be reduced. ２ On the other hand, if the treatment temperature exceeds 220°C, the Si-based precipitates may not grow sufficiently. ２ There is a risk that the Si-based precipitates will become too coarse and the tensile strength will not be improved sufficiently.

[0113] (Hot Trimming Process) In the hot trimming process, the forged product that has been subjected to the aging treatment process is hot trimmed at a material temperature exceeding 100°C and not exceeding 250°C.

[0114] FIG. 7 is a diagram illustrating a manufacturing method of a suspension member according to one embodiment of the present invention, and is a plan view showing an example of the configuration of a forged product before a hot trimming process is performed. As shown in FIG. 7 , a forged product 100X before the hot trimming process has a burr (excess material) 70 formed on its outer periphery. The burr 70 is composed of forging material that did not enter the grooves or holes of the die used to form the suspension member 100 during the forging process. The burr 70 is formed on the outer peripheries of the first arm portion 1, the second arm portion 2, the arm connecting portion 3, the wheel-side connecting portion 4, the first vehicle-body-side connecting portion 5, and the second vehicle-body-side connecting portion 6 of the suspension member 100. In other words, when the forged product 100X is viewed from above in the z direction, the burr 70 is formed so as to surround these components in the x and y directions.

[0115] In the hot trimming process, the burrs 70 are trimmed (trimmed) while the forged product 100X is heated. In the hot trimming process, the burrs are removed from the forged product 100X to form the suspension member 100. When the burrs 70 are removed from the suspension member 100, a parting PL corresponding to the root portion of the burr 70 is exposed.

[0116] If the material temperature in the hot trimming process is too low, the forging burrs are removed by shearing, and it may be impossible to tilt the crystals in the parting line. On the other hand, if the material temperature in the hot trimming process is too high, the material softens, making it difficult to trim the burrs 70 properly, and they may not be removed. Therefore, the material temperature in the hot trimming process is in the range of more than 100°C and not more than 250°C, and the temperature is preferably 125°C or higher, more preferably 150°C or higher, and even more preferably 175°C or higher.

[0117] According to the method for manufacturing a suspension member of this embodiment, it is possible to manufacture the suspension member of the above embodiment. According to the above embodiment, it is possible to provide a suspension member that has excellent stress corrosion cracking resistance even when recrystallization is present near the parting line, and a suspension member. Furthermore, the suspension member of this embodiment can achieve both excellent stress corrosion cracking resistance and strength.

[0118] The upper and / or lower limits of the numerical ranges described herein can be arbitrarily combined to define a preferred range. For example, the upper and lower limits of the numerical ranges can be arbitrarily combined to define a preferred range, the upper limits of the numerical ranges can be arbitrarily combined to define a preferred range, and the lower limits of the numerical ranges can be arbitrarily combined to define a preferred range.

[0119] Furthermore, the composition of the molten metal in the embodiment of the method for manufacturing a suspension member will be the alloy composition of the manufactured suspension member.

[0120] Throughout this disclosure, singular terms should be understood to include the plural concept unless otherwise specified. Thus, singular articles (e.g., "a," "an," "the," etc. in English) should be understood to include the plural concept unless otherwise specified.

[0121] The effects of the present invention will be made clearer by the following examples. Note that the present invention is not limited to the following examples and can be practiced with appropriate modifications within the scope of the present invention.

[0122] Example 1 First, a molten aluminum alloy having the alloy composition shown in Table 1 as reference numeral 1 was prepared.

[0123] The prepared molten aluminum alloy was used as a raw material and a continuously cast rod having a diameter of 82 mm and a circular cross section was produced using the horizontal continuous casting apparatus described above.

[0124] The continuous cast rod was molded to a size that fit the die used in the forging process to produce a forging material, which was then heated to 500°C and forged to produce a forged product. The average grain size (average line segment length per grain) of the forging material was measured according to JIS G0551:2020, Appendix A, A.2 (intercept method) and was found to be 65 μm.

[0125] Next, the obtained forged product was subjected to a solution treatment under the following conditions: heating rate: 10°C / min, temperature: 530°C, holding time: 3 hours

[0126] Next, an aging treatment step was carried out under the following conditions: temperature: 180°C, holding time: 4 hours

[0127] Next, hot trimming was performed by heating the forged product so that the temperature (material temperature) reached 200° C. By performing the hot trimming process, burrs were trimmed from the outer periphery of the forged product.

[0128] In this manner, the suspension member of Example 1 having an A-shape as shown in FIGS. 1 to 3 was produced.

[0129] (Stress corrosion cracking test) A C-ring was cut out from the bush portion 5 of the prepared suspension member along the dashed line indicated with the symbol CR in Fig. 2. A SCC test was carried out on the C-ring by a method in accordance with JIS H 8711:2000, applying a load of 90% of the 0.2% yield strength to the C-ring.

[0130] The SCC test was conducted under the following conditions: Test solution: dichromic acid solution Boiling test Test time: 80 hours Test specimen: C-ring test specimen Applied stress: A load of 80% of the tensile strength of 380 MPa was applied Under the above conditions, the C-ring was visually inspected for the occurrence of stress corrosion cracking after the test time (80 hours) had elapsed. Note that the test solution means that the C-ring was immersed in a boiled dichromic acid solution and the test was conducted.

[0131] Stress was applied to the C-ring by fastening a bolt. When applying stress, a strain gauge was attached to the apex of the C-ring, and the bolt was tightened until a predetermined stress was reached. After applying stress, masking was applied to the vicinity of the bolt to prevent contact with the solution. Figure 8 is a plan view showing a schematic configuration for carrying out an SCC test in Example 1. As shown in Figure 8, the C-ring C ５０ A bolt 71 and a nut 72 were fastened to the top of the rod, and a strain gauge 73 was attached to the top.

[0132] (Tensile Strength Test) A test piece conforming to the international standard ASTM-R5 was taken from the bushing, and the tensile strength of the test piece was measured by a method conforming to JIS H 8711:2000.

[0133] (Measurement of Grain Boundary Inclination) A portion near the apex of the bushing was cut out and polished, and then subjected to electron backscattered diffraction (EBSD) analysis.

[0134] Observation equipment: JEOL FE-SEM JSM-7900 EBSD (electron backscattering) measurement Acceleration voltage: 20 kV Sample tilt: 70° from the plane perpendicular to the electron beam incident direction

[0135] Figure 9 is a structural image obtained by analyzing an electron microscope image of a cross section (a cross section along the center line indicated by the dashed-dotted line in Figure 3) at the midpoint in the x direction of the bushing portion in the suspension member of Example 1, near the apex 51 of the bushing portion, using electron backscatter diffraction / diffraction spectroscopy. Numerous grain boundaries, indicated by solid black lines, can be seen in Figure 9. Figure 9 is an image of a surface perpendicular to the parting line of the bushing portion. In Figure 9, the surface S0 located outermost in the -y direction of the bushing portion is indicated by a dashed-dotted line, and the surface S300 located 300 μm deep in the +y direction from the surface S0 is indicated by a two-dot chain line.

[0136] In Figure 9, symbol E0 denotes the point on the grain boundary closest to the point at the apex located in the -y direction. Symbol E300 in Figure 9 denotes a point on the grain boundary that is on plane S300 at a depth of 300 µm from plane S0 and passes through point E0. The angle θ of the line passing through points E0 and E300 on the grain boundary relative to plane S0 was 30°. The angle θ is a plane perpendicular to the plane formed by the parting line of the bushing portion, and is the inclination with respect to a plane parallel to the cylindrical axis of the bushing portion.

[0137] [Examples 2 and 3] Suspension members were produced in the same manner as in Example 1, except that the temperature of the forged product (material temperature) in the hot trimming process was changed. Analysis was also performed in the same manner as in Example 1. In Example 2, the temperature in the hot trimming process was set to 150°C. In Example 3, the temperature in the hot trimming process was set to 250°C.

[0138] [Comparative Examples 1 to 3] Suspension members were produced in the same manner as in Example 1, except that the temperature of the forged product (material temperature) in the hot trimming process was changed. Analysis was also performed in the same manner as in Example 1. In Comparative Example 1, the temperature in the hot trimming process was 100°C. In Comparative Example 2, the temperature in the hot trimming process was 50°C. In Comparative Example 3, the temperature in the hot trimming process was 30°C.

[0139] [Comparative Examples 4 to 6] First, a molten aluminum alloy having the alloy composition shown as reference numeral 2 in Table 1 was prepared. Other conditions were the same as in Example 1, and a suspension member of Comparative Example 4 was produced, and the same analysis as in Example 1 was carried out.

[0140] Suspension members of Comparative Examples 5 and 6 were produced by the same method as Comparative Example 4, except that the temperature of the forged product (material temperature) in the hot trimming process was changed. Analysis was also performed by the same method as in Example 1. In Comparative Example 5, the temperature in the hot trimming process was 150°C. In Comparative Example 6, the temperature in the hot trimming process was 250°C.

[0141] [Comparative Examples 7 and 8] First, a molten aluminum alloy having the alloy composition shown as reference numeral 3 in Table 1 was prepared. A suspension member of Comparative Example 7 was produced under the same conditions as in Example 1, except that the temperature of the forged product in the hot trimming process was set to 100°C, and the same analysis as in Example 1 was carried out.

[0142] A suspension member of Comparative Example 8 was produced by the same method as Comparative Example 7, except that the temperature of the forged product (material temperature) in the hot trimming process was changed. Analysis was also performed by the same method as in Example 1. In Comparative Example 8, the temperature in the hot trimming process was set to 200°C.

[0143] The compositions of the molten alloys of Examples 1 to 3 and Comparative Examples 1 to 8, the temperatures of the forged products during the hot trimming process, the inclination of the grain boundaries at the apex with respect to a plane perpendicular to the plane consisting of the parting line of the bushing and parallel to the cylindrical axis of the bushing, the occurrence of stress corrosion cracking in the SCC test, the measurement results of the tensile strength test, and the evaluation taking into account the SCC test and the tensile strength test are summarized in Table 2. In the evaluation column of Table 2, forged products with good properties are marked with "A" and forged products with poor properties are marked with "B."

[0144]

[0145]

[0146] As shown in Table 2, in Examples 1 to 3 in which the hot trimming process was performed at a temperature of 150°C or higher and 250°C or lower, no stress corrosion cracking occurred, unlike in Comparative Examples 1 to 3 in which the conditions other than the temperature during the hot trimming process were the same. These results confirm that by performing hot trimming within the above temperature range on forged products satisfying the above alloy composition, it is possible to prevent burrs from being removed by shearing from the forged product, suppress the occurrence of stress corrosion cracking, and achieve high tensile strength.

[0147] In Comparative Examples 4 and 5, which used alloy composition 2 with a low Cu content, the strength of the C-ring itself was low, and even when the grain boundary angle was 45° or less, stress corrosion cracking occurred because the ring could not withstand the stress concentration. Comparative Examples 4 to 6, which had similar alloy compositions, including Comparative Example 6, were unable to achieve both tensile strength and suppression of stress corrosion cracking.

[0148] In Comparative Examples 7 and 8, which had high Cu and Fe contents, Mg ２ The amount of Cu coexisting with Si increases, which increases the potential difference between the matrix and the compound at the grain boundaries, impairing stress corrosion cracking resistance. Furthermore, the crystallization of fine crystals containing intermetallic compounds causes stress corrosion cracking, and the tensile strength is not excellent. In contrast, the forged products of Examples 1 to 3 were confirmed to be able to suppress the occurrence of stress corrosion cracking and to have excellent tensile strength.

[0149] 1: first arm portion, 2: second arm portion, 3: arm connecting portion, 4: wheel side connecting portion, 5: first vehicle body side connecting portion (bush portion), 6: second vehicle body side connecting portion (bush portion), 70: burr, 100: suspension member, 100X: forged product, PL: parting line, θ: inclination of grain boundary with respect to plane S0

Claims

1. An aluminum alloy having an alloy composition containing Cu in the range of 0.3% by mass or more and 0.5% by mass or less, Mg in the range of 0.65% by mass or more and 1.05% by mass or less, Si in the range of 0.9% by mass or more and 1.25% by mass or less, Mn in the range of 0.4% by mass or more and 0.6% by mass or less, Fe in the range of 0.15% by mass or more and 0.30% by mass or less, Cr in the range of 0.09% by mass or more and 0.25% by mass or less, Ti in the range of 0.01% by mass or more and 0.05% by mass or less, B in the range of 0.0010% by mass or more and 0.0050% by mass or less, and the balance being Al and inevitable impurities, and a suspension member which is a plane perpendicular to the plane formed by the parting line of the bush portion and has a grain boundary inclination of 45° or less with respect to the plane parallel to the cylindrical axis of the bush portion.

2. The suspension member according to claim 1, having a tensile strength of 380 MPa or more.

3. A molten metal forming step of forming a molten metal made of an aluminum alloy having an alloy composition containing Cu in the range of 0.3% by mass or more and 0.5% by mass or less, Mg in the range of 0.65% by mass or more and 1.05% by mass or less, Si in the range of 0.9% by mass or more and 1.25% by mass or less, Mn in the range of 0.4% by mass or more and 0.6% by mass or less, Fe in the range of 0.15% by mass or more and 0.30% by mass or less, Cr in the range of 0.09% by mass or more and 0.25% by mass or less, Ti in the range of 0.01% by mass or more and 0.05% by mass or less, B in the range of 0.0010% by mass or more and 0.0050% by mass or less, and the balance being Al and inevitable impurities; a casting step of obtaining a casting by casting the molten metal; a forging step of heating and forging the casting to obtain a forged product; a solution treatment step of holding the forged product at a temperature of 500°C or more; an aging treatment step of heat-treating the forged product that has undergone the solution treatment step; and a hot trimming step of hot trimming the forged product that has undergone the aging treatment step at a material temperature exceeding 100°C and 250°C or less. A method for manufacturing a suspension member.

4. The method for manufacturing a suspension member according to claim 3, wherein in the hot trimming step, the forged product that has undergone the aging treatment step is hot trimmed at a material temperature of 125°C or higher and 250°C or lower.

5. The method for manufacturing a suspension member according to claim 3, wherein in the hot trimming step, the forged product that has undergone the aging treatment step is hot trimmed at a material temperature of 150°C or higher and 250°C or lower.

6. The method for manufacturing a suspension member according to claim 3, wherein the casting step is performed by continuous casting, the cooling rate of the molten metal in the casting step is 10°C / second or higher, and the average crystal grain size of the cast product is 80 μm or less.

7. Further comprising a quenching step of quenching the forged product after the solution treatment step and before the hot trimming step; forging while heating at a material temperature of 450°C or higher and 520°C or lower in the forging step; holding the forged product at a temperature of 550°C or lower in the solution treatment step; quenching the forged product in water with a water temperature of 60°C or lower in the quenching step; and heating the forged product that has undergone the quenching step at a temperature of 175°C or higher and 190°C or lower for 4 hours or more in the aging treatment step. The method for manufacturing a suspension member according to any one of claims 3 to 6.

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