Suspension arm and manufacturing method therefor

Abstract

A suspension arm (100) is made of an aluminum alloy having an alloy composition containing 0.25-0.37 mass% Cu, 0.95-1.25 mass% Mg, 0.6-0.75 mass% Si, 0.05-0.12 mass% Mn, 0.15-0.35 mass% Fe, 0.25 mass% or less Zn, 0.050-0.26 mass% Cr, 0.01-0.1 mass% Ti, 0.001-0.03 mass% B, and 0.0010-0.050 mass% Zr, with the remainder being Al and unavoidable impurities, and is a forged product in which the ratio of the tensile strength of a portion where the equivalent strain is maximum to the tensile strength of a portion where the equivalent strain is minimum is 0.84-1.

Description

Suspension arm and manufacturing method thereof

[0001] This application claims priority to Japanese Patent Application No. 2023-147762, filed on September 12, 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, while high-tensile steel has traditionally been used for automobile suspension and bumper parts, high-strength aluminum alloys have recently come into use.

[0003] Furthermore, while iron-based materials have been used exclusively for automobile parts, particularly suspension parts, in recent years, aluminum or aluminum alloy materials have increasingly been used instead, primarily for the purpose of reducing weight.

[0004] 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. In order to improve the strength of these automobile parts, they are manufactured by forging, which is a type of plastic processing, using the aluminum alloy material as the processing material.

[0005] Recently, due to the need to reduce costs, 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, for example, Patent Documents 1 to 3).

[0006] JP-A-5-59477 JP-A-5-247574 JP-A-6-256880

[0007] However, the above-mentioned high-strength Al-Mg-Si alloys have a problem in that the processed structure recrystallizes during the forging and heat treatment processes, resulting in the generation of coarse crystal grains, making it impossible to obtain sufficient strength. Therefore, some alloys have been modified to prevent the generation of coarse recrystallized grains by adding Zr (zirconium) (see, for example, Patent Documents 1 and 2).

[0008] However, although adding Zr is effective in preventing recrystallization, it has the following problems: (1) The addition of Zr weakens the grain refinement effect of the Al-Ti-B alloy, coarsening the grains of the ingot itself, resulting in a decrease in the strength of the processed product (forged product) after plastic working. (2) Because the grain refinement effect of the ingot itself is weakened, the ingot is more likely to crack, increasing internal defects and reducing yield. (3) Zr forms compounds with the Al-Ti-B alloy, which accumulate at the bottom of the furnace storing the molten alloy and contaminate the furnace. In addition, these compounds also crystallize coarsely in the produced ingot, reducing its strength.

[0009] Thus, although the addition of Zr is effective in preventing recrystallization, it is difficult to maintain stability in strength.

[0010] In addition, hot plastic processing during the forging process can cause changes in the structural state, such as recrystallization, which can result in variations in tensile strength depending on the part, even within the same forged product.

[0011] The present invention has been made in view of the above technical background, and has an object to provide a suspension arm in which variations in tensile strength between locations are reduced, and a method for manufacturing the same.

[0012] In order to solve the above problems, the present invention provides the following means.

[0013] Aspect 1 of the present invention is a steel sheet containing Cu in the range of 0.25 mass% or more and 0.37 mass% or less, Mg in the range of 0.95 mass% or more and 1.25 mass% or less, Si in the range of 0.6 mass% or more and 0.75 mass% or less, Mn in the range of 0.05 mass% or more and 0.12 mass% or less, Fe in the range of 0.15 mass% or more and 0.35 mass% or less, Zn in the range of 0.25 mass% or less, Cr in the range of 0.050 mass% or more and 0.26 mass% or less, and Ti in the range of 0.01 mass% or more. and 0.1 mass% or less of Zr, 0.001 mass% or more of B, and 0.0010 mass% or more of Zr, with the remainder being Al and unavoidable impurities, and the ratio of the tensile strength of the portion (T4) where the equivalent strain is maximum to the tensile strength of the portion (T1) where the equivalent strain is minimum is 0.84 or more and 1 or less.

[0014] In a second aspect of the present invention, the suspension arm of the first aspect has a tensile strength of 350 MPa or more at a portion where the equivalent strain is minimum.

[0015] Aspect 3 of the present invention is a steel sheet containing Cu in the range of 0.25 mass% or more and 0.37 mass% or less, Mg in the range of 0.95 mass% or more and 1.25 mass% or less, Si in the range of 0.6 mass% or more and 0.75 mass% or less, Mn in the range of 0.05 mass% or more and 0.12 mass% or less, Fe in the range of 0.15 mass% or more and 0.35 mass% or less, Zn in the range of 0.25 mass% or less, Cr in the range of 0.050 mass% or more and 0.26 mass% or less, and Ti in the range of 0.01 mass% or more and 0.1 mass% or less. the suspension arm is made of an aluminum alloy having an alloy composition containing 0.001 mass % or more and 0.03 mass % or less, 0.0010 mass % or more and 0.050 mass % or less of Zr in the range below, B in the range of 0.001 mass % or more and 0.03 mass % or less, Zr in the range of 0.0010 mass % or more and 0.050 mass % or less, and the balance being Al and unavoidable impurities, and is composed of a pair of bifurcated arm portions, an arm connecting portion disposed between the pair of arms, a wheel side connecting portion located at the starting point of the bifurcated portions of the pair of arms, and a vehicle body side connecting portion provided at one end of each of the pair of arms, wherein the ratio of the tensile strength of the thinnest part of the pair of arms to the tensile strength of the thickest part is 0.84 or more and 1 or less.

[0016] A fourth aspect of the present invention is the suspension arm of the third aspect, wherein the thickest portion has a tensile strength of 350 MPa or more.

[0017] Aspect 5 of the present invention is a steel sheet containing Cu in the range of 0.25 mass% or more and 0.37 mass% or less, Mg in the range of 0.95 mass% or more and 1.25 mass% or less, Si in the range of 0.6 mass% or more and 0.75 mass% or less, Mn in the range of 0.05 mass% or more and 0.12 mass% or less, Fe in the range of 0.15 mass% or more and 0.35 mass% or less, Zn in the range of 0.25 mass% or less, Cr in the range of 0.050 mass% or more and 0.26 mass% or less, and Ti in the range of 0.01 mass% or less. % or more and 0.1% or less by mass, B in the range of 0.001% or more and 0.03% or less by mass, Zr in the range of 0.0010% or more and 0.050% or less by mass, and the balance being Al and unavoidable impurities, and the suspension arm is a forged product in which the ratio of the tensile strength of a portion with the largest proportion of high-angle grain boundaries with a crystal orientation misorientation of 15° or more to the tensile strength of a portion with the smallest proportion is 0.84 or more and 1 or less.

[0018] In a sixth aspect of the present invention, in the suspension arm of the fifth aspect, the tensile strength of the portion having the smallest proportion of high-angle grain boundaries with a crystal orientation misorientation of 15° or more is 350 MPa or more.

[0019] A seventh aspect of the present invention is the suspension arm of any one of the first to sixth aspects, wherein the equivalent strain due to forging is between 1.0 and 5.5.

[0020] Aspect 8 of the present invention is a method for manufacturing a suspension arm according to any one of Aspects 1 to 6, comprising: a molten metal forming step of obtaining a molten aluminum alloy; a casting step of obtaining a cast product by casting the obtained molten metal; a forging step of obtaining a forged product by heating the cast product at a temperature of 500 ° C. to the melting point or lower without performing a homogenization treatment step and performing plastic processing; and a solution treatment step of heating the obtained forged product at a temperature of 20 ° C. to 530 ° C. at a heating rate of 5.0 ° C. / min or more and holding at 530 to 560 ° C. for 0.3 to 3 hours; a quenching step in which all surfaces of the forged product are contacted with quenching water within 5 to 60 seconds after the solution treatment and quenched in a water tank for more than 1 minute but not more than 40 minutes; and an aging treatment step in which the forged product that has undergone the quenching treatment step is heated at a temperature of 180 ° C. to 220 ° C. for 0.5 to 8 hours to perform aging treatment. This is a method for manufacturing an aluminum alloy forged product.

[0021] A ninth aspect of the present invention is the method for manufacturing a suspension arm according to the eighth aspect, wherein the forging step is carried out at a temperature of 500°C or higher and 550°C or lower, the solution treatment step is a solution treatment in which the temperature is maintained at 535°C or higher and 555°C or lower for 1 hour to 3 hours, and the aging treatment step is an aging treatment in which the forged product that has been subjected to the quenching treatment step is heated at a temperature of 185°C or higher and 205°C or lower for 0.5 hour to 3 hours.

[0022] According to the present invention, it is possible to provide a suspension arm in which variations in tensile strength depending on the location are reduced.

[0023] 1. FIG. 1 is a schematic plan view showing an example of a suspension arm according to an embodiment of the present invention. FIG. 2 is an equivalent strain image including the entire first arm portion obtained by computer simulation. FIG. 3 is an equivalent strain image including the entire second arm portion obtained by computer simulation. FIG. 4 is a cross-sectional side view showing an example of a vertical continuous casting apparatus that can manufacture the suspension arm according to an embodiment of the present invention. FIG. 5 is a bottom view for explaining a cooling method in the vertical continuous casting apparatus shown in FIG. 4. FIG. 6 is a schematic perspective view for explaining a peeling process of an aluminum bar that can manufacture the suspension arm according to an embodiment of the present invention. FIG. 7 is a schematic side view for explaining the peeling process. FIG. 8 is a schematic cross-sectional view for explaining the surface profile of a peeled bar for forging. FIG. 9 is a partially enlarged schematic cross-sectional view of FIG. 10. FIG. 11 is a schematic plan view showing positions T1 and T4 in the suspension arm 100 shown in FIG. 1. FIG. 12 is a schematic plan view showing a test piece for evaluating mechanical properties prepared in this example.

[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. Appropriate changes can be made within the scope that does not change the effects of the present invention.

[0025] [Suspension Arm] Figure 1 is a schematic plan view of a suspension arm according to one embodiment of the present invention. The suspension arm 100 shown in Figure 1 is composed of a pair of bifurcated arm sections 10, 20, an arm connecting section 30 located between the arm sections 10, 20, a wheel-side connecting section 40 located at the starting point of the bifurcated section of the pair of arm sections 10, 20, and vehicle-body-side connecting sections 50, 60 provided at one end of each of the pair of arm sections 10, 20. The wheel-side connecting section 40 and the vehicle-body-side connecting sections 50, 60 each have a hole for connecting to a wheel and a hole for connecting to the vehicle body, respectively. The suspension arm 100 shown in Figure 1 is a component known as an A-arm or A-arm.

[0026] 1, the x-axis, y-axis, and z-axis are three orthogonal axes that form a Cartesian coordinate system. The plane of the paper is the xy plane, which is a plane that passes through the wheel side coupling part 40 and the vehicle body side coupling parts 50 and 60.

[0027] In the suspension arm 100 shown in Figure 1, of the pair of arm portions 10, 20, the first arm portion 10 is longer than the second arm portion 20. There is no particular restriction on the length of the pair of arm portions 10, 20. Furthermore, in the suspension arm 100 shown in Figure 1, the arm connecting portion 30 is disposed in a portion close to the vehicle body side connecting portions 50, 60 of the arm portions 10, 20, but there is no particular restriction on this position.

[0028] The suspension arm of the present invention is not particularly limited as long as it comprises six components: a pair of bifurcated arm sections, an arm connecting section disposed between the pair of arm sections, one wheel-side connecting section located at the starting point of the bifurcated section of the pair of arm sections, and two vehicle-side connecting sections provided at one end of each of the pair of arm sections. For example, the shapes and sizes of the arm sections, arm connecting section, wheel-side connecting section, and vehicle-side connecting section may be known.

[0029] The suspension arm of this embodiment contains Cu in the range of 0.25 mass % to 0.37 mass %, Mg in the range of 0.95 mass % to 1.25 mass %, Si in the range of 0.6 mass % to 0.75 mass %, Mn in the range of 0.05 mass % to 0.12 mass %, Fe in the range of 0.15 mass % to 0.35 mass %, Zn in the range of 0.25 mass %, Cr in the range of 0.050 mass % to 0.26 mass %, and Ti in the range of 0.01 mass % to 0.12 mass %. % or more and 0.1% or less by mass, B or Zr or Zirconia ...

[0030] The suspension arm of this embodiment corresponds to a forged product of a 6000 series aluminum alloy in that it contains Mg and Si.

[0031] (Cu: 0.25% by mass or more, 0.37% 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. By keeping the Cu content within the above range, the mechanical properties of the suspension arm 100 at room temperature can be improved.

[0032] (Mg: 0.95 mass % or more, 1.25 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 Si) or Q phase and other Al-Cu-Mg-Si compounds (AlCuMgSi), which contribute to strengthening the aluminum alloy. ２ Si is the CuAl in the aluminum alloy. ２ It has the effect of suppressing the formation of CuAl phase. ２ Suppression of the formation of the phase improves the corrosion resistance of the suspension arm 100. When the Mg content is within the above range, the corrosion resistance as well as the mechanical properties of the suspension arm 100 at room temperature can be improved.

[0033] (Si: 0.6% by mass or more and 0.75% by mass or less) Like Mg, Si has the effect of improving the corrosion resistance as well as the mechanical properties of the suspension arm 100 at room temperature. However, if excessive Si is added to an aluminum alloy, coarse primary Si grains may crystallize, potentially reducing the tensile strength of the aluminum alloy. By keeping the Si content within the above range, it is possible to improve the corrosion resistance as well as the mechanical properties of the suspension arm 100 at room temperature while suppressing the crystallization of primary Si.

[0034] (Mn: 0.05% by mass or more, 0.12% 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. When the Mn content is within the above range, the mechanical properties of the suspension arm 100 at room temperature can be improved.

[0035] (Fe: 0.15% by mass or more, 0.35% 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. When the Fe content is within the above range, the mechanical properties of the suspension arm 100 at room temperature can be improved.

[0036] (Cr: 0.050% by mass or more, 0.26% by mass or less) Cr forms fine granular crystals 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. When the Cr content is within the above range, the mechanical properties of the suspension arm 100 at room temperature can be improved.

[0037] (Ti: 0.01% by mass or more, 0.1% 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 sufficient. On the other hand, if the Ti content exceeds 0.1% 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 the suspension arm 100, the toughness may be reduced. Therefore, the Ti content is set to 0.012% by mass or more, and 0.035% by mass or less. The Ti content is preferably 0.015% by mass or more, and 0.050% by mass or less.

[0038] (B: 0.001% by mass or more, 0.03% 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 0.030% by mass, coarse crystals may form and be mixed into the suspension arm 100 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% by mass or more, and 0.030% by mass or less. The B content is preferably 0.0050% by mass or more, and 0.025% by mass or less.

[0039] (Zr: 0.0010 mass % or more, 0.05 mass % or less) When Zr is 0.05 mass % or less, Al ３ Precipitation in the form of Zr and Al-(Ti,Zr) contributes to improving the strength of the suspension arm 100 through recrystallization suppression effects and precipitation hardening. If the Zr content exceeds 0.050 mass%, coarse Zr compounds will crystallize, which may lead to a decrease in the corrosion resistance of the suspension arm 100. For this reason, the Zr content is set to 0.050 mass% or less. Furthermore, in order to obtain the above-mentioned recrystallization suppression effects and the effects of improving the strength of the forged product through precipitation hardening, the Zr content is preferably 0.0010 mass% or more.

[0040] (Zn: 0.250 mass% or less) Zn may be 0.250 mass% or less. If the Zn content exceeds 0.250 mass%, MgZn ２ This precipitates from the Al matrix at grain boundaries, causing intergranular corrosion and resulting in a decrease in the corrosion resistance of the suspension arm. For this reason, it is preferable that the Zn content be 0.250 mass% or less, or that no Zn be included at all.

[0041] (Inevitable Impurities) Inevitable impurities are impurities that are inevitably mixed into the aluminum alloy from raw materials or the manufacturing process. Examples of inevitable impurities include Ni, Sn, and Be. The content of these inevitable impurities preferably does not exceed 0.1 mass%.

[0042] The suspension arm of this embodiment is a forged product in which the ratio of the tensile strength of the portion with the maximum equivalent strain to the tensile strength of the portion with the minimum equivalent strain is 0.84 or more and 1 or less. Given the shapes of the forging material to be used and the forged product (forged member) to be manufactured, the equivalent strain at each location can be determined by performing a computer simulation based on the finite element method (FEM) of the forming process from the forging material shape to the forged product shape. Software used in this simulation can be, for example, the forging analysis software "DEFORM."

[0043] 2 and 3 are simulation images of equivalent strain obtained by performing a computer simulation of the suspension arm 100 shown in Fig. 1 using the forging analysis software "DEFORM," and are equivalent strain images viewed from direction A (positive direction of the x-axis) and direction B (negative direction of the x-axis), respectively. That is, Fig. 2 is an equivalent strain image including the entire first arm portion 10, and Fig. 3 is an equivalent strain image including the entire second arm portion 20.

[0044] As a result of the simulation, it was found that for the suspension arm 100 shown in Figure 1, the part with the greatest equivalent strain is the part close to the wheel side connecting part 40 of the second arm part 20, indicated by the symbol Max in Figure 3, and the part with the least equivalent strain is the part close to the vehicle body side connecting part 50 of the first arm part 10, indicated by the symbol Min in Figure 2.

[0045] The suspension arm of the present invention is a forged product in which the ratio of the tensile strength of the portion where equivalent strain is maximum to the tensile strength of the portion where equivalent strain is minimum is 0.84 or more and 1 or less. Here, the "portion where equivalent strain is maximum" refers to a range of 25.4 mm in length that includes the point where equivalent strain is maximum in the simulation, and the "portion where equivalent strain is minimum" refers to a range of 25.4 mm in length that includes the point where equivalent strain is minimum in the simulation.

[0046] Hot plastic working in the forging process can cause changes in the microstructure, such as recrystallization, which can result in variations in strength depending on the location, even within the same forged product.The present invention was completed by forging a 6000 series aluminum alloy continuously cast rod without homogenizing it, which makes it possible to keep the tensile strength ratio between the high and low equivalent strain locations within the range of 0.84 to 1.

[0047] It is believed that when a continuously cast aluminum alloy rod having a 6000 series composition is forged without homogenization, the precipitates become finer than those that have been homogenized. This results in stronger precipitation strengthening of the compounds, making it difficult for strength to decrease even in areas with high equivalent strain where recrystallization would occur. As a result, it is believed that even forged products made of 6000 series aluminum alloys, which have significantly different equivalent strains depending on the area, can be obtained with relatively small variations in strength.

[0048] Furthermore, based on the results of the simulation, it was found that the "portion where equivalent strain is greatest" and the "portion where equivalent strain is least" are the thinnest and thickest portions of each of the first and second arms. Furthermore, when comparing the "portion where equivalent strain is greatest" and the "portion where equivalent strain is least" between the long first arm and the shorter second arm, it was found that the "portion where equivalent strain is greatest" is larger in the shorter second arm, and the "portion where equivalent strain is least" is smaller in the long first arm.

[0049] In the suspension arm 100 shown in Figure 1, both the first arm portion 10 and the second arm portion 20 have their thinnest portions in the vicinity of the wheel side connecting portion 40 and their thickest portions in the vicinity of the vehicle body side connecting portions 50, 60.

[0050] The suspension arm of this embodiment is a forged product in which the ratio of the tensile strength of the thickest part to the tensile strength of the thinnest part in the first arm portion and the second arm portion is 0.84 or more and 1 or less.

[0051] Grain boundaries with a crystal orientation misorientation of 15° or more (high-angle grain boundaries) are indicators of the degree of recrystallization in each portion of the suspension arm 100. The proportion of high-angle grain boundaries can be obtained from an EBSD image. The "portion with the maximum equivalent strain" and the "portion with the minimum equivalent strain" correspond to the portion with the highest proportion of high-angle grain boundaries with a crystal orientation misorientation of 15° or more and the portion with the lowest proportion of high-angle grain boundaries with a crystal orientation misorientation of 15° or more, respectively.

[0052] The suspension arm of this embodiment is a forged product in which the ratio of the tensile strength of the portion with the largest proportion of high-angle grain boundaries with a crystal orientation misorientation of 15° or more to the tensile strength of the portion with the smallest proportion of high-angle grain boundaries with a crystal orientation misorientation of 15° or more is 0.84 or more and 1 or less.

[0053] The suspension arm 100 of this embodiment, which has the above-described configuration, is made of an aluminum alloy having the above-described alloy composition, so that recrystallization is unlikely to occur during the manufacturing of the forged product, and therefore excessively coarse crystal grains are unlikely to be generated.

[0054] The suspension arm 100 of this embodiment has small variations in strength and is therefore suitable as a suspension arm for vehicles such as automobiles.

[0055] [Method for Manufacturing Suspension Arm] Next, a method for manufacturing the suspension arm of this embodiment will be described. The method for manufacturing the suspension arm of this embodiment includes, for example, a molten metal forming step, a casting step, a forging step, a solution treatment step, a quenching treatment step, and an aging treatment step.

[0056] (Molten Metal Forming Step) The molten metal forming step is a step of melting raw materials to obtain a molten aluminum alloy having a composition adjusted. The composition of the molten aluminum alloy is adjusted to an alloy composition containing Cu in the range of 0.25 mass % to 0.37 mass %, Mg in the range of 0.95 mass % to 1.25 mass %, Si in the range of 0.6 mass % to 0.75 mass %, Mn in the range of 0.05 mass % to 0.12 mass %, Fe in the range of 0.15 mass % to 0.35 mass %, Zn in the range of 0.25 mass %, Cr in the range of 0.050 mass % to 0.26 mass %, Ti in the range of 0.01 mass % to 0.1 mass %, B in the range of 0.001 mass % to 0.03 mass %, Zr in the range of 0.0010 mass % to 0.050 mass %, and the balance consisting of Al and unavoidable impurities, to obtain a molten 6000 series aluminum alloy.

[0057] By carrying out the subsequent steps using a molten aluminum alloy of the above composition, it is possible to obtain an Al-Mg-Si suspension arm that is resistant to recrystallization and has excellent mechanical properties at room temperature. Note that virgin aluminum is aluminum with a concentration of 99% or more, obtained by subjecting alumina produced from minerals to electrolysis, a process known as electrolytic refining.

[0058] 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.

[0059] 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.

[0060] (Casting Step) In the casting step, a molten aluminum alloy (liquid phase) is cooled and solidified into a solid (solid phase) to obtain an aluminum alloy cast product. The casting step can be performed by, for example, a vertical continuous casting method.

[0061] Vertical continuous casting machines that can be used to produce aluminum alloy castings are shown in Figures 4 and 5. Figure 4 is a side cross-sectional view that schematically shows a hot-top casting machine, which is a vertical continuous casting machine. This vertical continuous casting machine has sufficient cooling performance and can produce high-quality continuously cast material.

[0062] The vertical continuous casting apparatus 1000 shown in FIG. 4 includes a mold 2 for solidifying molten aluminum W1 to cast an ingot W2, and a molten metal receiving tank 3 provided above the mold 2 and for pouring the molten metal W1 into the mold 2.

[0063] The mold 2 is cooled by cooling water M supplied therein as primary cooling water. At the bottom end of the mold 2, a plurality of ejection ports 1 are provided at predetermined intervals in the circumferential direction, for ejecting the cooling water M in the mold 2 as secondary cooling water, as will be described in detail later.

[0064] In this casting apparatus, molten aluminum W1 is supplied to a molten metal receiving tank 3 and poured into a cooled mold 2. The poured molten metal W1 is primarily cooled by contact with the mold 2, becoming a semi-solidified ingot W2. This semi-solidified ingot W2 has a solidified film formed on its outer periphery. The ingot W2 then passes continuously downward through the inside of the mold 2. Immediately after passing through the mold 2, cooling water M is sprayed onto the ingot W2 from a nozzle 1 of the mold 2, and the cooling water M comes into direct contact with the outer periphery of the ingot W2, cooling the ingot W2. As the ingot W2 is withdrawn downward, it is secondary cooled and most of it solidifies, producing a round bar-shaped continuously cast material (billet).

[0065] Next, the cooling method of the ingot W2 in this casting apparatus will be described in detail. Figure 5 is a schematic bottom view for explaining the cooling method of this casting apparatus, and is a cross-sectional view corresponding to the cross section taken along line V-V in Figure 4. Note that Figure 5 corresponds to a view of the casting apparatus as seen from below along the axis of the ingot W2.

[0066] As shown in Figures 4 and 5, a plurality of nozzles 1 are formed on the lower inner peripheral surface of the mold 2 at predetermined intervals along the circumferential direction. Each nozzle 1 is formed so that the direction D of the cooling water M ejected therefrom is inclined in one direction from the axis (central axis) X of the ingot W2 so that the direction D does not intersect with the axis X. Specifically, as shown in Figure 5, when the casting apparatus is viewed from below along the axis X of the ingot W2 (the state shown in Figure 5), and a virtual axis connecting the center of the nozzle 1 and the axis X of the ingot W2 is defined as the projection reference axis B, the direction D of the cooling water M ejected from each nozzle 1 is inclined in one direction with respect to the projection reference axis B.

[0067] In addition, in this casting device, when the angle at which the spray direction D of the cooling water M is inclined with respect to the projection reference axis B is defined as the spray angle θ, the spray angles θ of the cooling water M sprayed from each spray nozzle 1 are all set to the same angle.

[0068] In this casting apparatus, as viewed from the plane of the drawing in Fig. 5, each jetting direction D originates from the jetting nozzle 1, and the tip end (the jetting direction side) is inclined counterclockwise with respect to the projection reference axis B, and this counterclockwise direction is defined as one direction, but this is not limited to this, and in the present invention, the opposite direction (clockwise) may also be defined as one direction. In other words, as viewed from the plane of the drawing in Fig. 5, each jetting direction D may originate from the jetting nozzle 1, and the tip end may be inclined clockwise with respect to the projection reference axis B.

[0069] As described above, in this casting apparatus, when viewed from below, the ejection direction D of the cooling water M ejected from the ejection nozzles 1 of the mold 2 is inclined in one direction so as not to intersect with the axis X of the ingot W2 (mold 2). Therefore, the cooling water M is sprayed obliquely onto the outer peripheral surface of the ingot W2, rather than perpendicularly to the outer peripheral surface of the ingot W2. As a result, the cooling water M ejected from each ejection nozzle 1 flows down along the outer peripheral surface of the ingot W2, wrapping around in a spiral. In other words, by projecting the cooling water M obliquely onto the outer peripheral surface of the ingot W2, the wettability (adhesion) between the cooling water M and the ingot W2 is improved, and at the same time, the surface tension and Coanda effect of the cooling water M with respect to the ingot W2 are increased. As a result, the cooling water M flows down around the entire circumference of the ingot W2 in a spiral without interruption, and the entire circumference of the ingot W2 is evenly cooled by the secondary cooling water M. Therefore, the entire ingot W2 is cooled evenly, preventing partial insufficient cooling and achieving sufficient cooling performance. Furthermore, since the cooling water M flows down the outer peripheral surface of the ingot W2 in a spiral pattern, the contact length (flow path) of the cooling water M with the ingot W2 is increased, and the contact time between the cooling water M and the ingot W2 is also increased accordingly, further improving cooling performance. As a result, partial high-temperature abnormalities in the ingot W2 can be reliably prevented, partial high-temperature abnormalities that cause material segregation and stress concentration can be eliminated, defects such as ingot cracking can be prevented, the occurrence of quality defects can be significantly reduced, and a high-quality ingot W2 can be continuously cast.

[0070] In this casting apparatus, even if some of the nozzles 1 become clogged due to some reason, such as deterioration of the mold or water supply piping, the entire circumference of the ingot W2 can be cooled evenly. That is, because the cooling water M is projected obliquely onto the outer peripheral surface of the ingot W2, the projection range of the cooling water M ejected from each nozzle 1 onto the ingot W2 is wider than in the conventional case where the cooling water M is projected perpendicularly to the outer peripheral surface of the ingot W2. Therefore, even if the cooling water M is not ejected from some of the nozzles 1, the cooling water M2 can be projected continuously around the entire circumference of the ingot W2. Furthermore, as described above, the projected cooling water M flows down so as to spirally wrap around the outer peripheral surface of the ingot W2, thereby enabling the entire circumference of the ingot W2 to be cooled evenly. Even if a portion of the nozzle 1 is blocked in this way, cooling water M is sprayed to fill the blocked area, allowing the entire circumference of the ingot W2 to be cooled evenly, ensuring sufficient cooling performance, reliably reducing the occurrence of quality defects, and reliably producing high-quality continuous cast material.

[0071] As described above, in the hot top casting machine of this casting device, the direction of ejection D of the cooling water M ejected from the nozzle 1 of the mold 2 is inclined, so that the entire circumference of the ingot W2 can be cooled evenly, sufficient cooling performance can be obtained, and high-quality continuous cast material can be produced.

[0072] In this casting apparatus, as described above, in order to flow the cooling water M down so that it wraps around the outer peripheral surface of the ingot W2 and cool the entire circumference evenly, it is preferable to set the spray angle θ of the cooling water M to 3° to 45°, and more preferably to 5° to 20°.

[0073] The forging material used in the subsequent forging step is, for example, a peeled bar obtained by cutting and removing the outer surface of a continuously cast bar obtained by casting using the peeling process described above. The forging material formed from this peeled bar is then forged to produce forged products such as the above-mentioned automotive parts, with a lubricant (lubricating oil) applied to its surface or to the inner peripheral surface of the forging die. However, if the surface condition of the peeled bar used as the forging material is unstable, a uniform lubricating coating will not be formed over the entire outer peripheral surface of the peeled bar, resulting in uneven formation of the lubricating coating and insufficient benefits. Therefore, it is preferable to perform forging peeling, which can produce forged products with high dimensional accuracy, improve productivity, and extend the life of the die when manufacturing forged products.

[0074] The following method can be used for such forging peeling.

[0075] 6 and 7 are schematic diagrams for explaining the peeling process. As shown in both figures, the aluminum bar W ０ The conveying line is provided with a cutting machine 5. The cutting machine 5 cuts aluminum bars W conveyed along the conveying line. ０ Four cutting tools 51 are provided at equal intervals (at 90° intervals) in the circumferential direction along the outer peripheral surface of the workpiece. In this embodiment, the cutting tools 51 at each location are so-called combination tools having a pair of tool groups consisting of a four-piece roughing tool and a four-piece finishing tool.

[0076] Although not shown in the figure, aluminum bars W are provided on the upstream and downstream sides of the conveying line of the cutting machine 5. ０ A workpiece transport means such as transport rollers, support rollers, and a carriage is provided to transport the workpiece along the transport line.

[0077] The cutting edges of the roughing and finishing cutting bits that make up the combination cutting bit are not particularly limited, but cemented carbide or diamond can be suitably used, and a diamond cutting bit with a diamond cutting edge is particularly suitable for the finishing cutting bit. In addition, the present invention is not limited to the number of cutting bits in the cutting tool 51, and cutting tools other than a combination cutting bit may also be used.

[0078] In this method, the cutting tool 51 is rotated around the conveying line while the aluminum bar W ０ By conveying the aluminum bar W along the conveying line, ０ The entire outer peripheral surface of the is cut and removed by the cutting tool 51.

[0079] Here, the aluminum bar W ０ The continuous cast bar W before peeling １１ and peeled bar material after peeling (peeled bar material for forging) W １２ and peeling rod W １２ This is a general term for forging materials obtained by cutting.

[0080] In addition, during peeling processing, the type of cutting tool, the cutting edge shape of the cutting tool, the rotation speed of the cutting tool, the aluminum bar material W ０ By appropriately adjusting the cutting conditions such as the feed rate, a peeled bar material W having a surface profile with a specific configuration can be obtained as described below. １２ This is what you get.

[0081] Figure 8 shows the peeling bar W １２ 8 is a schematic diagram showing the surface profile of the peeling bar W, which corresponds to an enlarged cross section of the area surrounded by the dashed line in FIG. 7. FIG. 9 is a schematic diagram showing the area around the mountain-shaped portion in FIG. 8 in a further enlarged manner. As shown in both figures, １２ The outer peripheral surface of the bearing is formed in a cross section in a generally sawtooth or thunderbolt shape with a number of mountain-shaped portions 23 connected together along the axial direction. The lowest point between adjacent mountain-shaped portions 23 is defined as a valley bottom 31, and the highest point of each mountain-shaped portion 23 is defined as a peak 32.

[0082] In this method, of the two slopes (diagonal lines) that form the outline of each mountain-shaped portion 23, the slope on one side in the axial direction, i.e., the slope that slopes upward from the left side to the right side in Figure 8, is referred to as the first slope 21, and the slope on the other side in the axial direction, i.e., the slope that slopes downward from the left side to the right side in Figure 8, is referred to as the second slope 22.

[0083] In this method, the peeling bar W １２ The inclination angle (one base angle) θ1 of the first inclined surface 21 relative to the axis X and the inclination angle (the other base angle) θ2 of the second inclined surface 22 relative to the axis X are formed to be different in magnitude.

[0084] In this method, the inclination angle θ1 of the first inclined surface 21 is preferably 1° to 30°, and the inclination angle θ2 of the second inclined surface 22 is preferably 50° to 100°. In other words, when the inclination angles θ1 and θ2 are within the above-mentioned preferred ranges, the effects of the lubricating coating, described below, can be reliably obtained. Conversely, when the inclination angles θ1 and θ2 are outside the above-mentioned preferred ranges, the effects of the lubricating coating may not be fully obtained.

[0085] 9, the linear distance L1 from the valley bottom 31 to the peak 32 of the first slope 21 (the length of the first slope 21) is preferably 1 mm to 20 mm, and the linear distance L2 from the peak 32 to the valley bottom 31 of the second slope 22 (the length of the second slope 22) is preferably 0.1 μm to 50 μm. In other words, when the slope lengths L1 and L2 are within the above-mentioned preferred ranges, the effects of the lubricating coating, described below, can be reliably obtained. Conversely, when the slope lengths L1 and L2 are outside the above-mentioned preferred ranges, the effects of the lubricating coating may not be fully obtained.

[0086] The peeling bar W having such a unique surface profile １２ The forging material is cut appropriately. １２ It has a similar surface profile.

[0087] (Forging Process) The forging process involves cutting the cast aluminum alloy casting to a predetermined size, peeling the resulting forging material using the above-described method, heating it to a predetermined temperature, and then applying pressure in a press to mold it into a die. In the present invention, forging is performed without the homogenization treatment that has conventionally been performed after casting to remove segregation. Therefore, the segregation removal that was previously performed by the homogenization treatment must be performed by heating the material during forging, and the heating must be performed at a temperature of 500°C or higher but below the melting point. Thereafter, forging is performed to obtain a forged product (e.g., an automobile suspension arm part, etc.). If the material heating temperature during forging is less than 500°C, the AlFeSi-based and Mg-based alloys in the alloy structure will deteriorate. ２ The Si-based compounds and other compounds remain in a segregated state, which increases the deformation resistance, making it impossible to perform sufficient processing, and causes cracks. Furthermore, if the temperature exceeds the melting point, defects such as eutectic fusion are likely to occur. It is preferable to perform the forging process at a temperature of 500°C or higher and 550°C or lower.

[0088] (Solution Treatment Step) The solution treatment step is a step in which the forged product obtained in the forging step is heated to bring about a solution, thereby alleviating the strain introduced in the forging step and causing the solute elements to dissolve.

[0089] In this embodiment, the forged product is heated from 20°C to 530°C at a heating rate of 5.0°C / min or more, and then held at 530 to 560°C for 0.3 to 3 hours to undergo solution treatment. The heating rate from room temperature to the above-mentioned treatment temperature is preferably 5.0°C / min or more. If the treatment temperature is less than 530°C, the solute elements may not be sufficiently dissolved. On the other hand, if the treatment temperature exceeds 560°C, the solute elements are more likely to be dissolved, but eutectic melting or recrystallization may occur. Furthermore, if the heating rate is less than 5.0°C / min, the Mg ２ If the treatment temperature is lower than 530°C, the solution treatment may not progress, making it difficult to achieve high strength through aging precipitation. The solution treatment step is preferably carried out at a temperature of 535°C or higher and 555°C or lower for 1 hour to 3 hours.

[0090] (Quenching process) The quenching process is a process in which the forged product in the solid solution state obtained by the solution treatment process is rapidly cooled to form a supersaturated solid solution. After the solution treatment, the entire surface of the forged product is brought into contact with quenching water within 5 to 60 seconds, and the product is quenched in a water tank for more than 1 minute but not more than 40 minutes.

[0091] In this embodiment, the forged product is placed in a water tank containing water (quenching water) and quenched by 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 is in contact with water. The submersion time of the forged product varies depending on the size of the casting, but is, for example, between 1 minute and 30 minutes.

[0092] (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 and impart appropriate hardness. The forged product that has undergone the quenching treatment step is subjected to aging treatment by heating at a temperature of 180°C to 220°C for 0.5 to 8 hours. In the aging treatment step, it is preferable to perform aging treatment by heating the forged product that has undergone the quenching treatment step at a temperature of 185°C to 205°C for 0.5 to 3 hours.

[0093] In this embodiment, the forged product after the quenching treatment step is heated to a temperature of 170°C or higher and 210°C or lower, and is then held at that temperature for 0.5 hours or longer and 7 hours or shorter to undergo aging treatment. If the treatment temperature is lower than 170°C or the holding time is shorter than 0.5 hours, the Mg content, which improves the tensile strength, is reduced. ２ On the other hand, if the treatment temperature exceeds 190°C or the holding time exceeds 7 hours, Mg ２ There is a risk that the Si-based precipitates will become too coarse and the tensile strength will not be improved sufficiently.

[0094] Next, specific examples of the present invention will be described, but the present invention is not particularly limited to these examples.

[0095] [Examples 1 to 7 and Comparative Examples 1 to 5] (Production of Continuously Cast Products) First, aluminum alloys were prepared having the alloy compositions shown in Table 1 below (the remainder being aluminum). Using the prepared aluminum alloys as raw materials, continuous cast products with a circular cross section and a diameter of 84 mm were produced using the vertical continuous casting apparatus and cooling method described above. Examples 1 to 5 and Comparative Examples 1 to 5 are suspension arm test pieces with the same composition and produced under the same production conditions.

[0096]

[0097] (Manufacturing of Suspension Arm) Next, the obtained continuous cast product was subjected to a homogenization heat treatment step (only Comparative Examples 1 to 5), a forging step (including the above-mentioned peeling treatment), a solution treatment step, a quenching treatment step, and an artificial aging treatment step in this order to obtain a suspension arm 100 having the shape shown in Fig. 11. The conditions for the homogenization heat treatment step (only Comparative Examples 1 to 5), the forging step, the solution treatment step, the quenching treatment step, and the artificial aging treatment step are shown in Table 2 below.

[0098]

[0099] [Evaluation] The tensile strength (MPa) was evaluated at the T1 and T4 positions (see the dotted-line area in FIG. 10 ) of the suspension arms 100 of Examples 1 to 7 and Comparative Examples 1 to 5 obtained as described above. Table 3 shows the average values ​​of the tensile strength (MPa) at the T1 and T4 positions for Examples 1 to 5, Examples 6 and 7, and Comparative Examples 1 to 5, and the ratio of the tensile strength (MPa) at the T4 position to the tensile strength at the T1 position, using the average values. Furthermore, the equivalent strain of the entire suspension arms 100 of Examples 1 to 7 and Comparative Examples 1 to 5 was obtained using the forging analysis software "DEFORM." The portion indicated by the T1 position corresponds to the portion with the smallest equivalent strain, the thickest portion of the first arm, and the portion with the smallest proportion of high-angle grain boundaries with a crystal orientation misorientation of 15° or more. The portion indicated by the T4 position corresponds to the portion with the largest equivalent strain, the thinnest portion of the second arm, and the portion with the largest proportion of high-angle grain boundaries with a crystal orientation misorientation of 15° or more.

[0100] <Evaluation of Mechanical Properties (Tensile Properties)> Prismatic bodies for preparing test specimens for evaluating mechanical properties were taken from positions T1 and T4 of the suspension arm 100. The obtained prismatic bodies were processed to prepare cylindrical test specimens for evaluating mechanical properties as shown in FIG. 11. The parallel portion diameter A of the test specimens for evaluating mechanical properties was 6.4 mm, and the gauge length G was 25.4 mm. A tensile test was performed on the test specimens for evaluating mechanical properties at room temperature (25°C).

[0101] For each of Examples 1 to 7 and Comparative Examples 1 to 5, the equivalent strain (mm / mm) obtained by simulation was 1.2 to 4.9 (mm / mm).

[0102]

[0103] As shown in Table 3, the ratio of the tensile strength at the T4 position to the tensile strength at the T1 position was less than 0.84 in Comparative Examples 1 to 5. In contrast, the ratio of the tensile strength at the T4 position to the tensile strength at the T1 position was 0.84 or more (more specifically, 0.86 or more) in Examples 1 to 7, and the variation in tensile strength was reduced compared to Comparative Examples 1 to 5.

[0104] In addition, in Examples 6 and 7, the tensile strength at the T1 position was lower than in Examples 1 to 5, but the variation in tensile strength was further reduced than in Examples 1 to 5 (specifically, 0.93 or more).

[0105] 10 First arm portion 20 Second arm portion 30 Arm connecting portion 40 Wheel side connecting portion 50, 60 Vehicle body side connecting portion 100 Suspension arm

Claims

1. Cu in the range of 0.25% by mass or more and 0.37% by mass or less, Mg in the range of 0.95% by mass or more and 1.25% by mass or less, Si in the range of 0.6% by mass or more and 0.75% by mass or less, Mn in the range of 0.05% by mass or more and 0.12% by mass or less, Fe in the range of 0.15% by mass or more and 0.35% by mass or less, Zn in the range of 0.25% by mass or less, Cr in the range of 0.050% by mass or more and 0.26% by mass or less, Ti in the range of 0.01% by mass or more and 0.1% by mass or less, B in the range of 0.001% by mass or more and 0.03% by mass or less, A suspension arm, which is a forged product made of an aluminum alloy having an alloy composition containing Zr in the range of 0.0010 mass% or more and 0.050 mass% or less, with the balance being Al and unavoidable impurities, and in which the ratio of the tensile strength of a portion (T4) where the equivalent strain is maximum to the tensile strength of a portion (T1) where the equivalent strain is minimum is 0.84 or more and 1 or less.

2. A suspension arm as claimed in claim 1, in which the tensile strength of the portion where the equivalent strain is minimum is 350 MPa or more.

3. An aluminum alloy having an alloy composition containing Cu in the range of 0.25% by mass or more and 0.37% by mass or less, Mg in the range of 0.95% by mass or more and 1.25% by mass or less, Si in the range of 0.6% by mass or more and 0.75% by mass or less, Mn in the range of 0.05% by mass or more and 0.12% by mass or less, Fe in the range of 0.15% by mass or more and 0.35% by mass or less, Zn in the range of 0.25% by mass or less, Cr in the range of 0.050% by mass or more and 0.26% by mass or less, Ti in the range of 0.01% by mass or more and 0.1% by mass or less, B in the range of 0.001% by mass or more and 0.03% by mass or less, Zr in the range of 0.0010% by mass or more and 0.050% by mass or less, with the remainder being Al and unavoidable impurities; A suspension arm comprising a pair of bifurcated arm portions, an arm connecting portion disposed between the pair of arms, a wheel side connecting portion located at the starting point of the bifurcated portions of the pair of arms, and a vehicle body side connecting portion provided at one end of each of the pair of arms, wherein the ratio of tensile strength of the thinnest part of the pair of arms to the tensile strength of the thickest part is 0.84 or more and 1 or less.

4. The suspension arm according to claim 3, wherein the tensile strength of the thickest portion is 350 MPa or more.

5. Cu in the range of 0.25% by mass or more and 0.37% by mass or less, Mg in the range of 0.95% by mass or more and 1.25% by mass or less, Si in the range of 0.6% by mass or more and 0.75% by mass or less, Mn in the range of 0.05% by mass or more and 0.12% by mass or less, Fe in the range of 0.15% by mass or more and 0.35% by mass or less, Zn in the range of 0.25% by mass or less, Cr in the range of 0.050% by mass or more and 0.26% by mass or less, Ti in the range of 0.01% by mass or more and 0.1% by mass or less, B in the range of 0.001% by mass or more and 0.03% by mass or less, 1. A suspension arm comprising an aluminum alloy having an alloy composition containing Zr in the range of 0.0010 mass% or more and 0.050 mass% or less, with the balance being Al and unavoidable impurities, and a forged product in which the ratio of tensile strength of a portion having the maximum ratio of high-angle grain boundaries with a crystal orientation difference of 15° or more to the minimum ratio is 0.84 or more and 1 or less.

6. A suspension arm as claimed in claim 5, in which the tensile strength of a portion where the ratio of high-angle grain boundaries with a crystal orientation difference of 15° or more is the smallest is 350 MPa or more.

7. A suspension arm as claimed in any one of claims 1 to 6, wherein the equivalent strain due to forging is between 1.0 and 5.

5.

8. A method for manufacturing a suspension arm according to any one of claims 1 to 6, comprising: a molten metal forming step of obtaining a molten aluminum alloy; a casting step of casting the obtained molten metal to obtain a casting; a forging step of heating the said casting at a temperature of 500°C to the melting point or lower and plastically processing the same to obtain a forged product without carrying out a homogenization treatment step; a solution treatment step of heating the obtained forged product at a temperature increase rate of 5.0°C / min or more from 20°C to 530°C and holding the product at 530 to 560°C for 0.3 to 3 hours; a quenching step of contacting the entire surface of the forged product with quenching water within 5 to 60 seconds after the solution treatment, and quenching the product in a water tank for more than 1 minute but not exceeding 40 minutes; and an aging treatment step of heating the forged product that has undergone the quenching treatment step at a temperature of 180°C to 220°C for 0.5 to 8 hours to perform aging treatment.

9. A method for manufacturing a suspension arm as set forth in claim 8, wherein the forging step is carried out at a temperature of 500°C or higher and 550°C or lower, the solution treatment step is a solution treatment step in which the temperature is held at 535°C or higher and 555°C or lower for 1 to 3 hours, and the aging treatment step is a step in which the forged product that has been subjected to the quenching treatment step is heated at a temperature of 185°C or higher and 205°C or lower for 0.5 to 3 hours for aging treatment.

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