Suspension member and method for manufacturing suspension member

Abstract

To provide a suspension member having excellent stress corrosion cracking resistance even when recrystallization occurs in the vicinity of a parting line, and to provide a method for manufacturing the suspension member.SOLUTION: Provided is a suspension member made of an aluminum alloy, the 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.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present invention relates to a suspension member and a method for manufacturing the suspension member.

Background Art

[0002] In recent years, aluminum alloys have been increasingly used as structural members for various products by taking advantage of their light weight. For example, automotive undercarriage and bumper parts have hitherto been made of high-tensile steel, but in recent years, high-strength aluminum alloy materials have come to be used. Automotive parts, such as suspension parts, have hitherto been made exclusively of iron-based materials, but they are increasingly being replaced with aluminum materials or aluminum alloy materials mainly for the purpose of weight reduction.

[0003] In these automotive parts, excellent corrosion resistance, high strength, and excellent workability are required, and thus Al-Mg-Si-based alloys, particularly A6061, are frequently used as aluminum alloy materials. And such automotive parts are manufactured by performing forging, which is one of the plastic working processes, on the aluminum alloy material as a processing material in order to improve the strength.

[0004] Also, recently, since it is necessary to reduce costs, suspension parts obtained by forging a casting member as a material without extrusion and then performing solution treatment and artificial aging treatment (T6 treatment) have begun to be put into practical use, and for the purpose of further weight reduction, the development of high-strength alloys to replace conventional A6061 is underway (see Patent Documents 1 to 3).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] From the perspective of reducing CO2 emissions in recent years, while the weight reduction of automobiles is required, the demand for aluminum is on an increasing trend. However, for substitution from steel materials, further strengthening is necessary. To increase the strength, methods such as increasing the addition amounts of Si, Mg, and Cu are often adopted. However, there is a concern that the corrosion resistance may decrease with the increase in the addition amounts. Among the corrosion resistances, for the materials used for automobile suspensions, "stress corrosion cracking (SCC)" is important.

[0007] Stress corrosion cracking refers to a phenomenon in which corrosion occurs at a site where a certain stress continues to be applied in the suspension part, and cracks occur starting from the corroded part. The most important site for stress corrosion cracking in the suspension is the part where the suspension bush is press-fitted. It is necessary to increase the strength of the member while ensuring the stress corrosion cracking resistance at this site.

[0008] Typical factors affecting stress corrosion cracking resistance include additive elements, compound states, and metal crystal states. To ensure good stress corrosion cracking resistance while maintaining high strength, control of the crystal structure is important. Even with the same amount of additive elements, if the crystal structure is coarse, stress corrosion cracking preferentially occurs at the grain boundaries, leading to damage of the member.

[0009] The conditions during forging have a great influence on the control of the crystal structure. For example, when forging is performed at a low material heating temperature before forging, recrystallization occurs in the heat treatment process subsequent to that at the sites where the amount of strain during processing is large due to the accumulation of strain.

[0010] In automobile suspension members, due to the complex shape, flashless forging and then forming are common.

[0011] When performing this flash removal forging, recrystallization is likely to occur in the vicinity of the parting line corresponding to the vicinity of the flash portion because the amount of strain increases. When this recrystallization occurs in the bush portion, if the range of recrystallization is large, it significantly impairs the stress corrosion cracking resistance in the state where a ball joint or the like is press-fitted into the bush, leading to damage of the member. Incidentally, the parting line is a line exposed by removing the flash from the forged product by grinding. The parting line slightly bulges from the side portion of the forged product. Grinding (trimming) of the forged product is usually performed at room temperature.

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

[0013] The present invention has been made in view of the above circumstances, and an object thereof 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 in the vicinity of the parting line.

Means for Solving the Problems

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

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

[0016] [2] In the suspension member of [1] above, the tensile strength may be 380 MPa or more.

[0017] [3] The manufacturing method of the suspension member according to one aspect of the present invention is, containing Cu in the range of 0.3 mass% or more and 0.5 mass% or less, containing Mg in the range of 0.65 mass% or more and 1.05 mass% or less, containing Si in the range of 0.9 mass% or more and 1.25 mass% or less, containing Mn in the range of 0.4 mass% or more and 0.6 mass% or less, containing Fe in the range of 0.15 mass% or more and 0.30 mass% or less, containing Cr in the range of 0.09 mass% or more and 0.25 mass% or less, containing Ti in the range of 0.01 mass% or more and 0.05 mass% or less, containing B in the range of 0.0010 mass% or more and 0.0050 mass% or less, a molten metal forming step of forming a molten metal made of an aluminum alloy having an alloy composition consisting of 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 higher, an aging treatment step of heat-treating the forged product that has undergone the quenching step, and a hot trimming step of hot-trimming the forged product that has undergone the aging treatment step at a temperature where the material temperature exceeds 100°C and is 250°C or lower.

[0018] [4] The manufacturing method of the suspension member in [3] above may perform hot trimming on the forged product that has undergone the aging treatment process at a material temperature of 125°C or higher and 250°C or lower in the hot trimming process.

[0019] [5] The manufacturing method of the suspension member in [3] or [4] above may perform hot trimming on the forged product that has undergone the aging treatment process at a material temperature of 150°C or higher and 250°C or lower in the hot trimming process.

[0020] [6] The manufacturing method of the suspension member in [3] to [5] above may perform the casting process by continuous casting, the cooling rate of the molten metal in the casting process may be 10°C / second or higher, and the average crystal grain size of the casting may be 80μm or less.

[0021] [7] The manufacturing method of the suspension member in [3] to [6] above further has a quenching process of quenching the forged product after the solution treatment process and before the hot trimming process. The forging may be performed at a material temperature of 450°C or higher and 520°C or lower in the forging process. The forged product may be held at a temperature of 550°C or lower in the solution treatment process. The forged product may be quenched with water at a water temperature of 60°C or lower in the quenching process. The forged product that has undergone the quenching process may be heated at a temperature of 175°C or higher and 190°C or lower for 4 hours or more in the aging treatment process.

Advantages of the Invention

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

Brief Description of the Drawings

[0023]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

[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 the characteristic portions enlarged for the sake of easy understanding of the characteristics, and the dimensional ratios of the respective components are not necessarily the same as the actual ones. In addition, the materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not necessarily limited thereto, and can be appropriately changed and implemented without changing the gist thereof.

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

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

[0027] The aluminum alloy forged product of the present embodiment corresponds to a forged product of a 6000 series aluminum alloy in that it contains Mg and Si.

[0028] A suspension member according to an embodiment of the present invention is a plane perpendicular to the plane formed by the parting line of the bush portion and parallel to the cylindrical axis of the bush portion, and the inclination of the grain boundary is 45° or less. Details will be described later, but the bush portion has a configuration corresponding to reference numeral 50 in the drawing, and the parting line has a configuration corresponding to 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-based compounds in the aluminum alloy and the effect of improving the tensile strength of the aluminum alloy by precipitating as Al-Cu-Mg-Si-based compounds including 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 the Cu content being within the above range, the mechanical properties of the aluminum alloy forged product at room temperature can be improved. Also, when Cu exceeds 0.5%, the amount of Cu coexisting with Mg2Si at the grain boundary increases, so the potential difference between the matrix phase and the compound at the grain boundary increases, impairing stress corrosion cracking resistance. Therefore, it is preferably within the above range.

[0030] (Mg: 0.65 mass% or more and 1.05 mass% or less) Mg has the effect of improving the tensile strength of the aluminum alloy. By dissolving Mg in the aluminum matrix phase or precipitating as Mg-Si based compounds (Mg2Si) such as β” phase, or Al-Cu-Mg-Si based compounds including Q phase, it contributes to the strengthening of the aluminum alloy. The Mg content is in the range of 0.65 mass% or more and 1.05 mass% or less, and may also be in the range of 0.75 mass% or more and 1.00 mass% or less, or more than 0.85 mass% and 0.95 mass% or less. By the Mg content being within the above range, the corrosion resistance can be improved together with the mechanical properties at room temperature of the aluminum alloy forgings.

[0031] (Si: 0.9 mass% or more and 1.25 mass% or less) Si, like Mg, has the effect of improving the corrosion resistance together with the mechanical properties at room temperature of the aluminum alloy forgings. However, if Si is added excessively to the aluminum alloy, there is a risk that the tensile strength of the aluminum alloy will decrease due to the crystallization of coarse primary Si grains. The Si content is in the range of 0.90 mass% or more and 1.25 mass% or less, and may also be in the range of 0.95 mass% or more and 1.20 mass% or less, or 1.00 mass% or more and 1.18 mass% or less. By the Si content being within the above range, it is possible to improve the corrosion resistance together with the mechanical properties at room temperature of the aluminum alloy forgings while suppressing the crystallization of primary Si.

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

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

[0034] (Cr: 0.09% by mass or more and 0.25% by mass or less) Cr has the effect of improving the tensile strength of the aluminum alloy by forming fine granular precipitates containing intermetallic compounds such as Al-Mn-Cr-Fe-Si and Al-Fe-Cr in the aluminum alloy. The Cr content is in the range of 0.09% by mass or more and 0.25% by mass or less, and may also be in 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 the Cr content being within the above range, the mechanical properties of the aluminum alloy forgings at room temperature can be improved.

[0035] (Ti: 0.01% by mass or more and 0.05% by mass or less) Ti has the effect of refining the crystal grains of the aluminum alloy and improving the stretch formability. When 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, when the Ti content exceeds 0.05% by mass, coarse precipitates may be formed, and the stretch formability may decrease. In addition, when a large amount of coarse precipitates containing Ti is mixed into the aluminum alloy forgings, the toughness may decrease. Therefore, the Ti content should be 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 and 0.0050% by mass or less) B has the effect of refining the crystal grains of the aluminum alloy and improving the stretch formability. By adding B to the aluminum alloy together with the above-mentioned Ti, the effect of refining the crystal grains is improved. If the content of B is less than 0.001% by mass, the effect of refining the crystal grains may not be sufficiently obtained. On the other hand, when the content of B exceeds 0.0050% by mass, coarse precipitates may be formed and may be mixed into the aluminum alloy forgings as inclusions. In addition, when a large amount of coarse precipitates containing B is mixed into the final product of the aluminum alloy, the toughness may decrease. Therefore, the content of B should be 0.001 to 0.0050% by mass. The content of B is preferably 0.0015 to 0.045% by mass.

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

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

[0039] The suspension member 100 shown in FIGS. 1 to 3 includes, for example, a wheel-side connecting portion 4, a first vehicle-body-side connecting portion 5, a second vehicle-body-side connecting portion, a first arm portion 1 connecting the wheel-side connecting portion 4 and the first vehicle-body-side connecting portion 5, a second arm portion 2 connecting the wheel-side connecting portion 4 and the second vehicle-body-side connecting portion 6, and an arm connecting portion 3 connecting the first vehicle-body-side connecting portion 5 and the second vehicle-body-side connecting portion 6. The first vehicle-body-side connecting portion 5 and the second vehicle-body-side connecting portion 6 are members also referred to as bush portions. The suspension member 100 according to the present embodiment includes at least one bush portion 5, 6. The bush portions 5, 6 can have the same shape.

[0040] In FIGS. 2 and 3, for the purpose of explaining the characteristics of the bush portion in the suspension member 100 of the present embodiment, as an example, the bush portion 5 is shown enlarged. When the bush portion 5 is used as a product, for example, the region indicated by the two-dot chain line together with the symbol H in FIGS. 2 and 3 is removed so as to penetrate in the x direction. That is, in the cylindrical region indicated by the two-dot chain line as the symbol H in FIGS. 2 and 3, the aluminum alloy is removed to form an opening. In the opening, for example, other members such as ball joints are press-fitted in the x direction. In the present embodiment, the forged members in which cylindrical openings are formed in the bush portions 5, 6 and the forged members in which no openings are formed in the bush portions 5, 6 are collectively referred to as the suspension member.

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

[0042] As shown in FIG. 2, in the bush portion 5, the parting line PL is formed, for example, so as to form an xy plane. Further, the bush portions 5 and 6 have vertex portions at their ends in the y direction. In FIG. 2, the vertex portion formed in the bush portion 5 is denoted by reference numeral 51. The vertex portions of the bush portions 5 and 6 are members at positions on the x-direction center of the bush portions 5 and 6 of the suspension member 100 and having the largest distance in the y direction from the wheel connecting portion 4.

[0043] In the suspension member 100 of the present embodiment, at the vertex portion 51, the inclination of the crystal grain boundary is 45° or less with respect to a plane perpendicular to the plane formed by the parting line PL of the bush portions 5 and 6 (the xy plane in FIGS. 1 to 3) and parallel to the cylindrical axis of the bush portions 5 and 6. The inclination of the crystal grain boundary is preferably 35° or less, and more preferably 25° or less.

[0044] As the shape of the suspension member manufactured by forging becomes more complex, the amount of strain during the forging process (details of which will be described later) increases, and recrystallization occurs in the subsequent solution treatment process. In particular, the amount of strain becomes extremely large in the vicinity of the parting line, so it is difficult to completely eliminate this recrystallization. On the other hand, the sites where recrystallization has occurred are problematic because they preferentially cause stress corrosion cracking when stress is applied. In the bush portions 5 and 6 of the automobile suspension member 100, a ball joint is press-fitted, and the vertex portion 51 becomes the portion where the most stress is applied. The direction in which the stress is applied is the direction in which the ball joint is press-fitted and the opposite direction. That is, the direction in which the stress is applied is the axial direction of the opening formed in the bush portions 5 and 6, and is the ±x direction in FIGS. 1 to 3.

[0045] When the direction of the long side of the crystal in the recrystallized region is perpendicular to the direction in which stress is applied to the bush portions 5 and 6, the stress corrosion cracking resistance is significantly inferior. On the other hand, in the vicinity of the apex portion 51, when the long side direction of the crystal grains in the region from the outermost surface to a predetermined depth inward in the y direction, that is, in the region from the symbol E0 in the figure to -y direction and up to a depth of 300 μm, is oriented close to parallel to the stress direction, resistance to stress corrosion cracking is obtained. As described above, the orientation of the crystal grains is such that at the outermost peripheral point (apex portion 51) in the y direction of the parting line PL where stress concentration occurs, the inclination of the grain boundary is 45° or less with respect to the plane perpendicular to the plane formed by the parting line PL of the bush portions 5 and 6 (xy plane in FIGS. 1 to 3) and parallel to the cylindrical axis (x axis) of the bush portions 5 and 6.

[0046] The suspension member 100 in the present embodiment has a tensile strength of 380 MPa or more in accordance with JIS Z2241:2011, 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 including a first arm portion 1, a second arm portion 2, and an arm connecting portion 3. However, the present invention is not limited to the above example and is also applicable to other shaped suspension arms including bush portions and formed by forging.

[0048] [Manufacturing method of suspension member] Next, a manufacturing method of a suspension member according to an embodiment of the present invention will be described. The manufacturing method of a suspension member according to an embodiment of the present invention includes a molten metal forming step of obtaining a molten metal having the same composition as the above aluminum alloy forged product, a forging step of forging a cast product obtained by casting and processing the molten metal while heating 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 temperature where the material temperature exceeds 100°C and is 250°C or lower.

[0049] The manufacturing method of a suspension member according to an 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 quenching treatment step, an aging treatment step, and a hot trimming step.

[0050] (Molten metal forming step) The molten metal forming step is a step of obtaining an aluminum alloy molten metal by melting raw materials and adjusting the composition. The composition of the aluminum alloy molten metal is adjusted so that Cu is in the range of 0.3 mass% or more and 0.5 mass% or less, Mg is in the range of 0.65 mass% or more and 1.05 mass% or less, Si is in the range of 0.9 mass% or more and 1.25 mass% or less, Mn is in the range of 0.4 mass% or more and 0.6 mass% or less, Fe is in the range of 0.15 mass% or more and 0.30 mass% or less, Cr is in the range of 0.09 mass% or more and 0.25 mass% or less, Ti is in the range of 0.01 mass% or more and 0.05 mass% or less, B is in the range of 0.0010 mass% or more and 0.0050 mass% or less, and the balance is Al and inevitable impurities.

[0051] By performing the subsequent steps using the aluminum alloy molten metal having the above composition, it is possible to provide a suspension member and a manufacturing method of a suspension member that are excellent in stress corrosion cracking resistance even when recrystallization appears near the parting line. Note that the new aluminum ingot is aluminum having a concentration of 99% or more obtained by performing electrolysis called electrolytic refining on alumina produced from minerals.

[0052] The aluminum alloy molten metal can be obtained by heating and melting an aluminum alloy. Further, a mixture containing a simple substance of an element that is a raw material of the aluminum alloy or a compound containing two or more elements in a ratio for producing the target aluminum alloy may be melted and formed. For example, for the purpose of controlling the crystal grain size of the aluminum alloy produced in the casting step, Ti or B may be mixed as a grain refinement material such as an Al-Ti-B rod.

[0053] In addition, as raw materials for the molten aluminum alloy, scrap materials of aluminum alloys of the 1000 series, 2000 series, 3000 series, 4000 series, 5000 series, 6000 series, and 7000 series may be used in an amount of 10% or more, and the balance may be new aluminum ingots and the above-mentioned additive elements. These may be melted to obtain a molten aluminum alloy having a prepared composition. In this case, it is possible to obtain an Al-Mg-Si-based suspension arm that is less likely to generate recrystallization and has excellent mechanical properties at room temperature. Note that the new aluminum ingot is aluminum, for example, with a purity of 99% or more, obtained by performing electrolysis, called electrolytic refining, on alumina produced from minerals.

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

[0055] A horizontal continuous casting apparatus that can be used for manufacturing the cast aluminum alloy product of the present embodiment is shown in FIGS. 4 and 5. FIG. 4 is a cross-sectional view showing an example near the mold 12 of the horizontal continuous casting apparatus 10. FIG. 5 is an enlarged cross-sectional view of a main part near the cooling water cavity 24 of the horizontal continuous casting apparatus 10.

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

[0057] The molten metal receiving part 11 is composed of a molten metal inflow part 11a that receives the molten aluminum alloy M obtained in the above-mentioned molten metal forming process, a molten metal holding part 11b, and an outflow part 11c to the hollow part 21 of the mold 12.

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

[0059] The aluminum alloy molten metal 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 from the pouring passage 13a provided in the refractory plate-like body 13. Then, the aluminum alloy molten metal M supplied into the hollow portion 21 is cooled and solidified by a cooling device 23 described later, and is pulled out from the other end side 12b of the mold 12 as an aluminum alloy rod B which is a solidified ingot.

[0060] It is sufficient that a drawing drive device (not shown) for drawing out the cast aluminum alloy rod B at a constant speed is installed at the other end side 12b of the mold 12. It is also preferable that a synchronous cutting machine (not shown) for cutting the continuously drawn aluminum alloy rod B into an arbitrary length is installed.

[0061] The refractory plate-like body 13 is a member that blocks heat transfer between the molten metal receiving portion 11 and the mold 12, and may be composed of materials such as calcium silicate, alumina, silica, a mixture of alumina and silica, silicon nitride, silicon carbide, graphite, etc. Such a refractory plate-like body 13 can also be composed of a plurality of layers having different constituent materials.

[0062] The mold 12 is a hollow cylindrical member in this embodiment, and is formed of a material selected from one or a combination of two or more of, for example, aluminum, copper, or their alloys. For such a material of the mold 12, an optimal combination may be selected from the viewpoints 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 make the aluminum alloy rod B to be cast into a cylindrical rod shape, and the mold 12 is held such that the mold central axis (central axis) C passing through the center of the hollow portion 21 is substantially along the horizontal direction.

[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 in 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 in a conical shape toward the casting direction. And the angle formed by the taper is the elevation angle.

[0065] When the elevation angle is less than 0°, when the aluminum alloy rod B is pulled out from the mold 12, casting may become difficult because resistance is received at the other end side 12b which is the mold outlet. On the other hand, when the elevation angle exceeds 3°, the contact of the inner peripheral surface 21a with the molten aluminum alloy M becomes insufficient, and the heat extraction effect from the molten aluminum alloy M and the solidified shell formed by its cooling and solidification to the mold 12 decreases, which may cause insufficient solidification. As a result, remelting marks may occur on the surface of the aluminum alloy rod B, or casting troubles such as the ejection of the non-solidified molten aluminum alloy M from the end of the aluminum alloy rod B may occur, which is not preferable.

[0066] In addition, 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 according to the shape of the aluminum alloy rod to be cast, such as a shape having a triangular, rectangular cross-sectional shape, polygon, semi-circle, ellipse or an irregular cross-sectional shape without a symmetry axis or a symmetry plane, in addition to the circular shape of the present embodiment.

[0067] A fluid supply pipe 22 for supplying a lubricating fluid into the hollow portion 21 of the mold 12 is disposed at one end side 12a of the mold 12. As the lubricating fluid supplied from the fluid supply pipe 22, any one or two or more kinds of lubricating fluids selected from gas lubricants and liquid lubricants can be used. When both a gas lubricant and a liquid lubricant are supplied, it is preferable to provide separate fluid supply pipes respectively. The lubricating fluid pressurized and supplied from the fluid supply pipe 22 is supplied into the hollow portion 21 of the mold 12 through the annular lubricant supply port 22a.

[0068] In this embodiment, the pumped lubricating fluid is supplied from the lubricant supply port 22a to the inner peripheral surface 21a of the mold 12. Note that the liquid lubricant may be heated to become a decomposed gas and supplied to the inner peripheral surface 21a of the mold 12. Further, a porous material may be disposed at the lubricant supply port 22a, and the lubricating fluid may be exuded to the inner peripheral surface 21a of the mold 12 through this porous material.

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

[0070] The cooling water cavity 24 is formed in an annular shape so as to surround the hollow portion 21 outside the inner peripheral surface 21a of the hollow portion 21 inside the mold 12, and the cooling water W is supplied through a cooling water supply pipe 26.

[0071] The inner peripheral surface 21a of the mold 12 is cooled by the cooling water W accommodated in the cooling water cavity 24, so that the heat of the molten aluminum alloy M filled in the hollow portion 21 of the mold 12 is taken from the surface in contact with the inner peripheral surface 21a of the mold 12, and a solidified shell is formed on the surface of the molten aluminum alloy M.

[0072] Further, the cooling water injection passage 25 directly applies the cooling water W from the shower opening 25a facing the hollow portion 21 toward the aluminum alloy rod B at the other end side 12b of the mold 12 to cool the aluminum alloy rod B. The longitudinal sectional shape of such a cooling water injection passage 25 may be, for example, a semicircle, an oval shape, or a horseshoe shape 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 accommodated in the cooling water cavity 24 to cool the inner peripheral surface 21a of the hollow portion 21 of the mold 12, and then the cooling water W in the cooling water cavity 24 is jetted from the cooling water jet passage 25 toward the aluminum alloy rod B. However, these may be supplied by separate cooling water supply pipes for each system.

[0074] The length from the position where the extension line of the central axis of the shower opening 25a of the cooling water jet passage 25 hits the surface of the cast aluminum alloy rod B to the contact surface between the mold 12 and the refractory plate-like body 13 is referred to as the effective mold length L. 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, casting becomes impossible because a good film cannot be formed. If it exceeds 40 mm, the effect of forced cooling becomes low, 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 in the casting skin or breakage inside the mold, making the casting unstable, so it is not preferable.

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

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

[0077] Here, the parallel means that when the inner peripheral surface 21a of the hollow portion 21 of the mold 12 is formed at an elevation angle of 0° to 3° with respect to the inner bottom surface 24a of the cooling water cavity 24, that is, when the inner bottom surface 24a is inclined with respect to the inner peripheral surface 21a by more than 0° and up to 3°.

[0078] As shown in FIG. 5, the cooling wall portion 27 of the mold 12, which is the portion where the inner bottom surface 24a of such a cooling water cavity 24 and the inner peripheral surface 21a of the hollow portion 21 of the mold 12 face each other, has a 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 of 10×10 5 W / m 2 or more and 50×10 5 W / m 2 or less within the following range.

[0079] The thickness t of such a cooling wall portion 27 of the mold 12, that is, 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, may be, for example, within the range of 0.5 mm or more and 3.0 mm or less, preferably 0.5 mm or more and 2.5 mm or less. Further, the forming material of 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 the range of 100 W / m·K or more and 400 W / m·K or less.

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

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

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

[0083] The height difference between the liquid level of the molten aluminum alloy M stored in the molten metal receiving portion 11 and the upper inner peripheral surface 21a of the mold 12 is preferably 0 mm to 250 mm (more preferably 50 mm to 170 mm). By setting it within such a range, the pressure of the molten aluminum alloy M supplied into the mold 12, the lubricating oil, and the gas formed by vaporization of the lubricating oil are preferably balanced, so that the castability is stabilized.

[0084] As the liquid lubricant, vegetable oil which is a lubricating oil can be used. For example, rapeseed oil, castor oil, and salad oil can be mentioned. These are preferable because they have little adverse impact on the environment.

[0085] The lubricating oil supply amount is preferably 0.05 mL / min to 5 mL / min (more preferably 0.1 mL / min or more and 1 mL / min or less). If the supply amount is too small, the molten aluminum alloy M of the aluminum alloy rod B may leak from the mold 12 without solidifying due to insufficient lubrication. If the supply amount is too large, the excess may mix 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 pulled out 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 products formed by casting becomes uniform and fine, the resistance to deformation of the aluminum base material at high temperatures increases, and the high-temperature mechanical strength is improved.

[0087] The amount of cooling water sprayed from the shower opening 25a of the cooling water spray passage 25 is preferably 10 L / min or more and 50 L / min or less per mold (more preferably 25 L / min or more and 40 L / min or less). If the amount of cooling water is less than this, the molten aluminum alloy M may leak from the mold 12 without solidifying. Also, the surface of the cast aluminum alloy rod B may be remelted to form a non-uniform structure and remain as an internal defect. On the other hand, if the amount of cooling water is more than this range, the heat extraction of the mold 12 may be too large and solidification may occur halfway.

[0088] The average temperature of the molten aluminum alloy M flowing from inside the molten metal receiving part 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, there is a risk of forming coarse crystallized substances in the mold 12 and in front of it and being incorporated as internal defects inside the aluminum alloy bar B. On the other hand, if the temperature of the molten aluminum alloy M is too high, a large amount of hydrogen gas is likely to be incorporated into the molten aluminum alloy M, and it may be incorporated as porosity in the aluminum alloy bar B, resulting in internal cavities.

[0089] And in the cooling wall part 27 of the mold 12, the heat flux value per unit area from the molten aluminum alloy M in the hollow part 21 towards the cooling water W in the cooling water cavity 24 is 10×10 5 W / m 2 or more and 50×10 5 W / m 2 or less. By setting it within this range, seizure of the aluminum alloy bar B can be prevented.

[0090] The cooling wall part 27 of the mold 12 receives heat by extracting heat from the molten aluminum alloy M, and heat exchange is performed by cooling this heat with the cooling water W contained in the cooling water cavity 24. Regarding the state of this heat exchange, as shown in the explanatory diagram of FIG. 6, attention is paid to the heat flux per unit area. The heat flux per unit area is expressed by the following formula (1) according to Fourier's law. Q = -k×(T1 - T2) / L ··· (1) Q: Heat flux k: Thermal conductivity (W / m·K) of the location where heat passes (in this embodiment, the cooling wall part 27 of the mold 12) T1: Low-temperature side temperature of the location where heat passes (in this embodiment, the inner bottom surface 24a of the cooling water cavity 24) T2: High-temperature side temperature of the location where heat passes (in this embodiment, the inner peripheral surface 21a of the hollow part 21 of the mold 12) L: Section length (mm) of the location where heat passes (in this embodiment, the thickness t of the cooling wall part 27 of the mold 12)

[0091] Based on the cast material quality, thickness, and temperature measurement data where good results were obtained even when reducing the lubricant oil amount during casting, the heat flux value per unit area is 10×10 5 W / m 2 or more. By configuring the cooling wall portion 27 of the mold 12 in this way, sticking of the cast aluminum alloy bar B can be prevented. Also, it is preferable that the heat flux value per unit area is 50×10 5 W / m 2 or less.

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

[0093] When manufacturing the aluminum alloy bar B of the present embodiment, using the above-described horizontal continuous casting apparatus 10, the aluminum alloy molten metal M stored in the molten metal receiving portion 11 is continuously supplied into the hollow portion 21 from one end side 12a of the mold 12. Also, 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] Then, the aluminum alloy molten metal M supplied into the hollow portion 21 is cooled and solidified under the condition that the heat flux value per unit area in the cooling wall portion 27 is 10×10 5 W / m 2 or more to cast the aluminum alloy bar B. Also, when casting the aluminum alloy bar B, it is preferable that the wall surface temperature of the cooling wall portion 27 of the mold 12 cooled by the cooling water W is 100°C or less.

[0095] The thus obtained aluminum alloy bar B has a heat flux value per unit area in the cooling wall portion 27 of 10×10 5 W / m 2By cooling and solidifying under the above conditions, the adhesion of reaction products, such as carbides, due to the contact between the lubricating oil gas and the molten aluminum alloy M is suppressed. As a result, it is not necessary to cut and remove carbides and the like on the surface of the aluminum alloy rod B, and the aluminum alloy rod B can be manufactured with a high yield.

[0096] The casting process for obtaining a casting from the molten aluminum alloy M is not limited to the above-described horizontal continuous casting method, and known continuous casting methods such as the vertical continuous casting method can be used. The vertical continuous casting method is classified into a float method and a hot top method depending on the supply method of the molten aluminum alloy M to the mold (casting mold 12). Hereinafter, the case of using the hot top method will be briefly described.

[0097] The casting apparatus used for the hot top method includes a mold, a molten metal receiving vessel (header), and the like. The molten metal supplied to the molten metal receiving portion passes through the outlet port and the header to adjust the flow rate, and enters a cylindrical mold installed substantially horizontally, where it is forcibly cooled to form a solidified shell on the outer surface of the molten metal.

[0098] Furthermore, cooling water is directly radiated onto the casting drawn out from the mold, and the casting is continuously drawn out while the solidification of the metal proceeds to the inside of the casting. Generally, a metal member having good thermal conductivity is used for the mold, and it has a hollow structure for introducing a refrigerant therein.

[0099] The refrigerant to be used may be appropriately selected from 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 viewpoints of heat transfer performance and durability at the contact portion 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 component range of the alloy to be cast and the dimensions of the casting, and there are no particular restrictions.

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

[0102] (Homogenization heat treatment process) Next, a homogenization heat treatment process may be appropriately performed. The homogenization heat treatment process is a process of performing heat treatment on an aluminum alloy casting obtained by a casting process to homogenize microsegregation caused by solidification, precipitate supersaturated solid solution elements, and change metastable phases to equilibrium phases.

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

[0104] (Forging process) The forging process is a process of shaping an aluminum alloy casting into a predetermined size to obtain a forging material, heating the obtained forging material to a predetermined temperature, and then applying pressure with a press to perform die forming.

[0105] In this embodiment, for the forging material, forging is performed at a heating temperature of 450°C or more and 560°C or less to obtain a forged product (suspension arm member of an automobile). At this time, the forging start temperature of the forging material is preferably 450°C or more and 560°C or less. If the start temperature is less than 450°C, the deformation resistance may increase and sufficient processing may not be possible. On the other hand, if it exceeds 560°C, there is a risk of defects such as forging cracks and eutectic melting. Also, the material temperature of the forging material is more preferably within the range of 480°C or more and 520°C or less.

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

[0107] (Solution treatment step) The solution treatment step is a step of relaxing the strain introduced into the forged product by heating the forged product obtained in the forging step at a temperature of 500°C or higher to cause solutionization and performing solid solution of solute elements.

[0108] In this embodiment, solution treatment is performed by holding the forged product at a treatment temperature of, for example, 530°C or higher and 560°C or lower for 0.3 hours or more and 5 hours or less, preferably at a treatment temperature of 530°C or higher and 550°C or lower for 1 hour or more and 4 hours or less. The time of the solution treatment may be 3 hours or less. The heating rate from room temperature to the above-described treatment temperature is preferably 5.0°C / min or more. If the treatment temperature is not sufficient, there is a possibility that the solid solution of solute elements becomes insufficient. On the other hand, if the treatment temperature is too high, although the solid solution of solute elements is more promoted, there is a possibility that eutectic melting and recrystallization are likely to occur. Also, when the heating rate is less than 5.0°C / min, there is a possibility that Mg2Si precipitates coarsely. On the other hand, when the treatment temperature is less than 530°C, solutionization does not proceed and it is difficult to achieve high strength by age precipitation. Therefore, as described above, it is preferably performed at 530°C or higher.

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

[0110] In this embodiment, a forging is put into a water tank (quenching water) in which water is stored, and quenching treatment is performed by submerging the forging. The water temperature in the water tank is preferably 20°C or higher and 60°C or lower. The forging is preferably put into the water tank such that all surfaces of the forging come into contact with water within 5 seconds or more and 60 seconds or less after solution treatment. The immersion time of the forging varies depending on the size of the casting, but is, for example, more than 5 minutes and within 40 minutes.

[0111] (Aging treatment process) The aging treatment process is a process of heating and holding the forging at a relatively low temperature to precipitate elements supersaturated in solid solution and impart appropriate hardness.

[0112] In this embodiment, aging treatment is performed by heating the forging after the quenching treatment process to a temperature of 170°C or higher and 220°C or lower and holding it at that temperature for 0.5 hours or more and 7.0 hours or less, preferably 4 hours or more. If the heating temperature is less than 180°C or the holding time is less than 0.5 hours, Mg2Si-based precipitates that improve the tensile strength may not grow sufficiently. On the other hand, if the treatment temperature exceeds 220°C, the Mg2Si-based precipitates may become too coarse and the tensile strength may not be improved sufficiently.

[0113] (Hot trimming process) In the hot trimming process (hot trimming process), the forging that has undergone the aging treatment process is hot trimmed at a temperature where the material temperature exceeds 100°C and is 250°C or lower.

[0114] FIG. 7 is a diagram for explaining a method of manufacturing a suspension member according to an aspect 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 burr (excess metal) 70 is formed on the outer periphery of a forged product 100X before the hot trimming process is performed. The burr 70 is composed of a forging material that did not enter the groove portion or hole portion of the mold for forming the suspension member 100 during the forging process. The burr 70 is formed on the outer periphery of the first arm portion 1, the second arm portion 2, the arm connecting portion 3, the wheel side connecting portion 4 of the suspension member 100, and the first vehicle body side connecting portion 5 and the second vehicle body side connecting portion 6. That is, when the forged product 100X is viewed in plan from the z direction, the burr 70 is formed so as to surround these members in the x direction and the y direction.

[0115] In the hot trimming process, the burr 70 is trimmed (trimmed) while heating the forged product 100X. In the hot trimming process, the suspension member 100 is molded by removing the burr from the forged product 100X. In the suspension member 100, when the burr 70 is removed, 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 burr may be removed by shearing, and the crystals in the parting line portion may not be inclined. On the other hand, if the material temperature in the hot trimming process is too high, the burr 70 may not be trimmed well due to softening of the material and may not be removed. Therefore, the material temperature in the hot trimming process exceeds 100°C and is in the range of 250°C or lower, 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 manufacturing method of the suspension member according to the present embodiment, the suspension member according to the above embodiment can be manufactured. According to the above embodiment, even when recrystallization occurs near the parting line, it is possible to provide a suspension member and a suspension member that are excellent in stress corrosion cracking resistance. Further, the suspension member according to the present embodiment can achieve both excellent stress corrosion cracking resistance and strength.

Example

[0118] Hereinafter, the effects of the present invention will be made clearer by examples. Note that the present invention is not limited to the following examples, and can be appropriately modified and implemented without changing the gist thereof.

[0119] [Example 1] First, a molten aluminum alloy having the alloy composition shown as reference numeral 1 in Table 1 was prepared.

[0120] Using the prepared molten aluminum alloy as a raw material, a continuous casting rod having a circular cross-section with a diameter of 82 mm was produced using the horizontal continuous casting apparatus as described above.

[0121] The continuous casting rod was sized to fit the mold in the forging process to obtain a forging material. The obtained forging material was heated to 500°C and forged to obtain a forged product. The average crystal grain size (average line segment length per crystal) of the forging material was measured to be 65 μm by JIS G0551:2020 Appendix A A.2 (cutting method).

[0122] Next, the obtained forged product was solution-treated under the following conditions. Heating rate: 10°C / min, temperature: 530°C, holding time: 3 hours

[0123] Next, an aging treatment process was performed under the following conditions. Temperature: 180°C, holding time: 4 hours

[0124] Next, the forged product was heated so that the temperature (material temperature) became 200 °C, and hot trimming was performed. By performing the hot trimming process, burrs were trimmed from the outer periphery of the forged product.

[0125] In this way, the suspension member of Example 1 having the A-type shape as shown in FIGS. 1 to 3 was manufactured.

[0126] (Stress Corrosion Cracking Test) A C-ring was cut out from the bush portion 5 of the manufactured suspension member along the dashed-dotted line indicated by the symbol CR in FIG. 2. For the C-ring, a load of 90% of the 0.2% proof stress was applied to the C-ring, and the SCC test was carried out by a method conforming to JIS H 8711:2000.

[0127] The implementation environment of the SCC test is shown below. Test solution: Boiling test of dichromic acid solution Test time: 80 hours Test piece: C-ring test piece Magnitude of applied stress: A load of 80% of the tensile strength of 380 MPa was applied. Under the above conditions, whether or not stress corrosion cracking occurred in the C-ring after the test time (80 hours) elapsed was observed by visual inspection of the appearance. Note that regarding the test solution, it means that the test was carried out by immersing the C-ring in the boiled dichromic acid solution.

[0128] The stress application to the C-ring was performed by bolt fastening. 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 the stress, masking was performed in the vicinity of the bolt to block contact with the solution. FIG. 8 is a plan view schematically showing the configuration when the SCC test is performed in Example 1. As shown in FIG. 8, for the C-ring C 50 the bolt 71 and the nut 72 were fastened, and the strain gauge 73 was attached to the apex portion.

[0129] (Tensile Strength Test) A test piece of international standard ASTM-R5 was sampled from the bush portion. The tensile strength of the test piece was measured by a method compliant with JIS H 8711:2000.

[0130] (Measurement of the inclination of grain boundaries) The vicinity of the apex of the bush part was cut out, polished, and then the electron backscattered diffraction (EBSD) method was performed.

[0131] Observation equipment: FE-SEM JSM-7900 EBSD (backscattered electron) measurement manufactured by JEOL Accelerating voltage: 20 kV Sample tilt: 70° from the plane perpendicular to the electron beam incident direction

[0132] Figure 9 is a tissue image obtained by analyzing, using the electron backscattered diffraction method, an electron microscope image of a cross-section (a cross-section along the center line indicated by the dashed line in Fig. 3) at the midpoint in the x-direction of the bush part, in the vicinity of the apex 51 of the suspension member of Example 1. In Fig. 9, a large number of grain boundaries indicated by solid black lines are confirmed. Fig. 9 is an image of a plane perpendicular to the parting line of the bush part. In Fig. 9, the plane S0 located most outward in the -y direction in the bush part is indicated by a dashed line, and the plane S300 at a depth of 300 μm in the +y direction from the plane S0 is indicated by a double dashed line.

[0133] In Fig. 9, the symbol E0 is a point on the grain boundary closest to the location most in the -y direction at the apex. Also, in Fig. 9, the symbol E300 is a point on the grain boundary passing through the point E0 on the plane S300 at a position 300 μm deep from the plane S0. The line passing through the points E0 and E300 of the grain boundary passing through the points E0 and E300 had an angle θ of 30° with respect to the plane S0. The angle θ is the inclination with respect to the plane perpendicular to the plane formed by the parting line of the bush part and parallel to the cylindrical axis of the bush part.

[0134] [Example 2, Example 3] A suspension member was 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. Also, analysis was performed in the same manner as in Example 1. In Example 2, the temperature of the hot trimming process was set to 150°C. In Example 3, the temperature of the hot trimming process was set to 250°C.

[0135] [Comparative Examples 1 - 3] A suspension member was 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. Also, analysis was performed in the same manner as in Example 1. In Comparative Example 1, the temperature of the hot trimming process was set to 100°C. In Comparative Example 2, the temperature of the hot trimming process was set to 50°C. In Comparative Example 3, the temperature of the hot trimming process was set to 30°C.

[0136] [Comparative Examples 4 - 6] First, a molten aluminum alloy having the alloy composition shown as symbol 2 in Table 1 was prepared. Under other conditions the same as in Example 1, a suspension member of Comparative Example 4 was produced and analyzed in the same manner as in Example 1.

[0137] Suspension members of Comparative Example 5 and Comparative Example 6 were produced in the same manner as in Comparative Example 4, except that the temperature of the forged product (material temperature) in the hot trimming process was changed. Also, analysis was performed in the same manner as in Example 1. In Comparative Example 5, the temperature of the hot trimming process was set to 150°C. In Comparative Example 6, the temperature of the hot trimming process was set to 250°C.

[0138] [Comparative Examples 7, 8] First, a molten aluminum alloy having the alloy composition shown as symbol 3 in Table 1 was prepared. A suspension member of Comparative Example 7 was produced under other conditions the same as in Example 1, except that the temperature of the forged product in the hot trimming process was set to 100°C, and analysis was performed in the same manner as in Example 1.

[0139] A suspension member of Comparative Example 8 was manufactured in the same manner as in Comparative Example 7, except that the temperature of the forged product (material temperature) in the hot trimming process was changed. Also, analysis was performed in the same manner as in Example 1. In Comparative Example 8, the temperature of the hot trimming process was set to 200°C.

[0140] The composition of the molten alloy, the temperature of the forged product during the hot trimming process, the inclination of the grain boundaries with respect to the plane perpendicular to the parting line of the bush part and parallel to the cylindrical axis of the bush part at the apex, the presence or absence of stress corrosion cracking in the SCC test, the measurement results of the tensile strength test, and the determination considering the SCC test and the tensile strength test for Examples 1 to 3 and Comparative Examples 1 to 8 are summarized in Table 2.

[0141]

Table 1

[0142]

Table 2

[0143] As shown in Table 2, in Examples 1 to 3 where the hot trimming process was performed at a temperature of the forged product of 150°C or higher and 250°C or lower, stress corrosion cracking that occurred in Comparative Examples 1 to 3 with the same conditions except for the temperature during the hot trimming process did not occur. From this result, it was confirmed that for a forged product satisfying the above alloy composition, by performing hot trimming within the above temperature range, it is possible to suppress the removal of burrs from the forged product by shearing, suppress the occurrence of stress corrosion cracking, and achieve a high tensile strength.

[0144] In Comparative Examples 4 and 5 of Alloy Composition 2 with a low Cu content, due to the low strength of the C-ring itself, even when the angle of the grain boundary was 45° or less, stress corrosion cracking occurred because it could not withstand stress concentration. In Comparative Examples 4 to 6 having the same alloy composition, including Comparative Example 6, it was not possible to achieve both high tensile strength and suppression of stress corrosion cracking.

[0145] In Comparative Examples 7 and 8 with high Cu and Fe contents, the amount of Cu coexisting with Mg2Si at the grain boundaries increases, resulting in a large potential difference between the matrix and the compound at the grain boundaries, impairing stress corrosion cracking resistance. Also, by precipitating as fine precipitates containing intermetallic compounds, stress corrosion cracking occurred, and it could not be said that the tensile strength was excellent.

Explanation of Signs

[0146] 1: First arm part, 2: Second arm part, 3: Arm connecting part, 4: Wheel side connecting part, 5: First vehicle body side connecting part (bush part), 6: Second vehicle body side connecting part (bush part), 70: Burr, 100: Suspension member, 100X: Forged product, PL: Partition line, θ: Inclination of grain boundary with respect to plane S0

Claims

1. containing Cu in the range of 0.3 mass% or more and 0.5 mass% or less, containing Mg in the range of 0.65 mass% or more and 1.05 mass% or less, containing Si in the range of 0.9 mass% or more and 1.25 mass% or less, containing Mn in the range of 0.4 mass% or more and 0.6 mass% or less, containing Fe in the range of 0.15 mass% or more and 0.30 mass% or less, containing Cr in the range of 0.09 mass% or more and 0.25 mass% or less, containing Ti in the range of 0.01 mass% or more and 0.05 mass% or less, containing B in the range of 0.0010 mass% or more and 0.0050 mass% or less, composed of an aluminum alloy having an alloy composition in which the balance consists of Al and unavoidable impurities, a suspension member which is a plane perpendicular to the plane formed by the parting line of the bush portion and in which the inclination of the grain boundary is 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. containing Cu in the range of 0.3 mass% or more and 0.5 mass% or less, containing Mg in the range of 0.65 mass% or more and 1.05 mass% or less, containing Si in the range of 0.9 mass% or more and 1.25 mass% or less, containing Mn in the range of 0.4 mass% or more and 0.6 mass% or less, containing Fe in the range of 0.15 mass% or more and 0.30 mass% or less, containing Cr in the range of 0.09 mass% or more and 0.25 mass% or less, containing Ti in the range of 0.01 mass% or more and 0.05 mass% or less, containing B in the range of 0.0010 mass% or more and 0.0050 mass% or less, a molten metal forming step of forming a molten metal composed of an aluminum alloy having an alloy composition in which the balance consists of Al and unavoidable impurities; a casting step of obtaining a cast product by casting the molten metal; a forging step of heating and forging the cast product 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; 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 more and 250 °C or less.

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

7. After the solution treatment step and before the hot trimming step, the method further includes a quenching step of quenching the forged product, in the forging step, forging is performed while heating at a material temperature of 450°C or higher and 520°C or lower, in the solution treatment step, the forged product is held at a temperature of 550°C or lower, in the quenching step, the forged product is quenched in water with a water temperature of 60°C or lower, in the aging treatment step, the forged product that has undergone the quenching step is heated at a temperature of 175°C or higher and 190°C or lower for 4 hours or more. The manufacturing method of the suspension member according to any one of claims 3 to 6.

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