Suspension arm and manufacturing method therefor

Abstract

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

Description

[Technical field]

[0001] The present invention relates to a suspension arm and a manufacturing method thereof. [Background technology]

[0002] In recent years, aluminum alloys have been increasingly used as structural components for various products, taking advantage of their light weight. For example, high-tensile steel has traditionally been used for automobile suspension and bumper parts, but in recent years high-strength aluminum alloy materials have come to be used.

[0003] In addition, while iron-based materials have been used exclusively for automobile parts, particularly suspension parts, in recent years, aluminum materials or aluminum alloy materials have been increasingly used instead, mainly 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, especially A6061, are widely used as aluminum alloy materials. 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 materials as processing materials.

[0005] Recently, due to the need to reduce costs, suspension parts have begun to be put into practical use in which the cast parts are used as they are without extrusion, and then subjected to solution treatment and artificial aging treatment (T6 treatment). In order to further reduce weight, development of high-strength alloys to replace the conventional A6061 is underway (see, for example, Patent Documents 1 to 3). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 5-59477 [Patent Document 2] Japanese Patent Application Publication No. 5-247574 [Patent Document 3] Japanese Patent Application Publication No. 6-256880 Summary of the Invention [Problem to be solved by the invention]

[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 a sufficiently high strength. Therefore, some alloys contain zirconium (Zr) to prevent recrystallization and prevent the generation of coarse recrystallized grains (see, for example, the above-mentioned Patent Documents 1 and 2).

[0008] Although the addition of Zr is effective in preventing recrystallization, it has the following problems. (1) The addition of Zr weakens the effect of refining the crystal grains of the Al-Ti-B alloy, coarsening the crystal grains of the ingot itself, leading to a decrease in the strength of the processed product (forged product) after plastic processing. (2) The effect of refining the crystal grains of the ingot itself is weakened, so that the ingot is more likely to crack, the number of internal defects increases, and the yield decreases. (3) Zr forms compounds with Al-Ti-B alloys. These compounds accumulate at the bottom of the furnace in which the molten alloy is stored, contaminating the furnace. In addition, these compounds also crystallize out as coarse particles in the produced ingots, reducing their strength.

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

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

[0011] The present invention has been made in consideration of the above technical background, and has an object to provide a suspension arm in which the variation in tensile strength from one part to another is reduced, and a manufacturing method thereof. [Means for solving the problem]

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

[0013] A first aspect of the present invention is a composition comprising: 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; the suspension arm being a forged product having an alloy composition containing at least 0.1% by mass of C, at least 0.001% by mass of B, at least 0.03% by mass of Zr, and the balance consisting of Al and unavoidable impurities, and having a 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 of 0.84 or more and 1 or less.

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

[0015] Aspect 3 of the present invention is a composition comprising 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, Ti in the range of 0.01 mass% or more and 0.1 mass% or less, the aluminum alloy has an alloy composition containing at least one of B and Zr in the range of 0.001% by mass or more and 0.03% by mass or less, at least one of Zr in the range of 0.0010% by mass or more and 0.050% by mass or less, with the balance being Al and unavoidable impurities, and is composed of a pair of bifurcated arms, an arm connecting part disposed between the pair of arms, a wheel side connecting part located at the starting point of the bifurcated part of the pair of arms, and a vehicle body side connecting part provided at one end of each of the pair of arms, The suspension arm is a forged product in which the ratio of tensile strength of the thinnest part in 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 tensile strength of the thickest part is 350 MPa or more.

[0017] A fifth aspect of the present invention is a composition comprising: Cu in the range of 0.25% by mass to 0.37% by mass; Mg in the range of 0.95% by mass to 1.25% by mass; Si in the range of 0.6% by mass to 0.75% by mass; Mn in the range of 0.05% by mass to 0.12% by mass; Fe in the range of 0.15% by mass to 0.35% by mass; Zn in the range of 0.25% by mass or less; Cr in the range of 0.050% by mass to 0.26% by mass or less; the suspension arm being a forged product having a ratio of 0.84 to 1.0 in which the ratio of the tensile strength of a portion with the maximum ratio of high-angle grain boundaries having a crystal orientation difference of 15° or more is between 0.84 and 1.0 inclusive, 0.001% by mass or more and 0.03% by mass or less, 0.0010% by mass or more and 0.050% by mass or less of Zr, and the balance being Al and unavoidable impurities.

[0018] A sixth aspect of the present invention is a suspension arm according to the fifth aspect, wherein the tensile strength of a portion having a minimum ratio of high-angle grain boundaries with a crystal orientation difference 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, in which 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 the steps of: forming a molten metal of an aluminum alloy; casting the molten metal to obtain a cast product by casting the molten metal; heating the cast product at a temperature of 500°C to the melting point or lower without performing a homogenization treatment step, and subjecting the cast product to plastic processing to obtain a forged product; a solution treatment step in which the obtained forged product is heated to 20°C to 530°C at a heating rate of 5.0°C / min or more and held 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 and within 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 be aged.

[0021] A ninth aspect of the present invention relates to the manufacturing method of a suspension arm according to the eighth aspect, in which 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 hour or higher to 3 hours or lower, and the aging treatment step is a step in which the forged product that has undergone the hardening treatment step is heated at a temperature of 185°C or higher and 205°C or lower for 0.5 hours to 3 hours or lower for aging treatment. Effect of the Invention

[0022] According to the present invention, it is possible to provide a suspension arm in which the variation in tensile strength depending on the location is reduced. [Brief description of the drawings]

[0023] [Figure 1] 1 is a schematic plan view illustrating an example of a suspension arm according to an embodiment of the present invention. FIG. [Diagram 2] This is an equivalent strain image including the entire first arm section, generated by computer simulation. [Diagram 3] This is an equivalent strain image including the entire second arm section, generated by computer simulation. [Figure 4] FIG. 1 is a side cross-sectional view showing an example of a vertical continuous casting apparatus that can manufacture a suspension arm according to an embodiment of the present invention. [Diagram 5] FIG. 5 is a bottom view for explaining a cooling method in the vertical continuous casting apparatus shown in FIG. [Figure 6] FIG. 2 is a schematic perspective view illustrating a peeling process of an aluminum bar material that enables the manufacture of a suspension arm according to an embodiment of the present invention. [Figure 7] FIG. 4 is a schematic side view illustrating a peeling step. [Figure 8] FIG. 2 is a schematic cross-sectional view for explaining a surface profile of a peeled bar for forging. [Figure 9] FIG. 9 is a partially enlarged schematic cross-sectional view of FIG. 8. [Figure 10] 2 is a schematic plan view showing positions T1 and T4 of the suspension arm 100 shown in FIG. 1. FIG. [Figure 11] FIG. 2 is a schematic plan view showing a test piece for evaluating mechanical properties produced in this example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In addition, the drawings used in the following description may show characteristic parts in an enlarged scale for the sake of convenience in order to make the characteristics easier to understand, and the dimensional ratios of each component may not necessarily be the same as in reality. Furthermore, the materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not necessarily limited to them, and may be appropriately modified and implemented within the scope that does not change the effects of the present invention.

[0025] [Suspension arm] FIG. 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 arranged between the arm sections 10, 20, a wheel side connecting section 40 located at the starting point of the bifurcations 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 portion 40 and the vehicle body side connecting portions 50, 60 each have a hole for connecting to a wheel and a hole for connecting to the vehicle body. A suspension arm 100 shown in FIG. 1 is a member known as an A-arm or an A-type arm.

[0026] 1, the x-axis, y-axis, and z-axis are three orthogonal axes that form a Cartesian coordinate system. The paper surface 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, 60.

[0027] 1, of a pair of arm portions 10, 20, the first arm portion 10 is longer than the second arm portion 20. There is no particular limit to the length of the pair of arm portions 10, 20. In the suspension arm 100 shown in FIG. 1, the arm connector 30 is disposed near the vehicle body side connectors 50, 60 of the arm sections 10, 20, but there are no particular limitations on this position.

[0028] The suspension arm of the present invention is not particularly limited as long as it is made up of 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 part of the pair of arm sections, and two vehicle body 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, the arm connecting section, the wheel side connecting section, and the vehicle body side connecting section may be publicly known.

[0029] The suspension arm of this embodiment contains 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 within the range of 0.001% or more and 0.03% or less by mass, Zr within the range of 0.0010% or more and 0.050% or less by mass, and the balance being Al and unavoidable impurities, wherein the ratio of the tensile strength of the portion where the equivalent strain (mm / mm) is maximum to the tensile strength of the portion where the equivalent strain (mm / mm) is minimum is 0.84 or more and 1 or less.

[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 mass% or more, 0.37 mass% or less) Cu has the effect of finely dispersing Mg-Si compounds in the aluminum alloy and the effect of improving the tensile strength of the aluminum alloy by precipitating as Al-Cu-Mg-Si compounds including 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 contributes to strengthening the aluminum alloy by dissolving in the aluminum parent phase or precipitating as Mg-Si compounds (Mg2Si) such as the β″ phase, or Al-Cu-Mg-Si compounds (AlCuMgSi) such as the Q phase. Mg2Si also has the effect of suppressing the formation of CuAl2 phase in the aluminum alloy. Suppressing the formation of the CuAl2 phase improves the corrosion resistance of the suspension arm 100. By keeping the Mg 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.

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

[0034] (Mn: 0.05 mass% or more, 0.12 mass% or less) Mn has the effect of improving the tensile strength of the aluminum alloy by forming fine granular precipitates including intermetallic compounds such as Al-Mn-Fe-Si and Al-Mn-Cr-Fe-Si in the aluminum alloy. By ensuring that 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 mass% or more, 0.35 mass% or less) Fe acts to improve the tensile strength of the aluminum alloy by crystallizing in the aluminum alloy as fine crystals including intermetallic compounds such as Al-Mn-Fe-Si, Al-Mn-Cr-Fe-Si, Al-Fe-Si, Al-Cu-Fe, Al-Mn-Fe, etc. By keeping the Fe content within the above range, the mechanical properties of the suspension arm 100 at room temperature can be improved.

[0036] (Cr: 0.050 mass% or more, 0.26 mass% or less) Cr has the effect of improving the tensile strength of the aluminum alloy by forming fine granular precipitates including intermetallic compounds such as Al-Mn-Cr-Fe-Si and Al-Fe-Cr in the aluminum alloy. By keeping the Cr content within the above range, the mechanical properties of the suspension arm 100 at room temperature can be improved.

[0037] (Ti: 0.01 mass% or more, 0.1 mass% or less) Ti has the effect of refining the crystal grains of the aluminum alloy and improving the wrought workability. If the Ti content is less than 0.01% by mass, the effect of refining the crystal grains may not be sufficiently obtained. On the other hand, if the Ti content exceeds 0.1% by mass, coarse crystallized particles may be formed, and the wrought workability may be reduced. In addition, if a large amount of coarse crystallized particles 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 mass% or more, 0.03 mass% or less) B has the effect of refining the crystal grains of the aluminum alloy and improving the wrought workability. By adding B to the aluminum alloy together with Ti described above, the effect of refining the crystal grains is improved. If the content of B is less than 0.0010% by mass, the effect of refining the crystal grains may not be sufficiently obtained. On the other hand, if the content of B exceeds 0.030% by mass, coarse crystallized matter may be formed and may be mixed into the suspension arm 100 as inclusions. In addition, if a large amount of coarse crystallized matter containing B is mixed into the final product of the aluminum alloy, the toughness may decrease. Therefore, the content of B is set to 0.0010% by mass or more and 0.030% by mass. The content of B is preferably 0.0050% by mass or more and 0.025% by mass.

[0039] (Zr: 0.0010 mass% or more, 0.05 mass% or less) When Zr is 0.05% by mass or less, it precipitates in the form of Al3Zr and Al-(Ti,Zr), thereby contributing to improving the strength of the suspension arm 100 through recrystallization suppression effect and precipitation strengthening. When the Zr content exceeds 0.050% by mass, it crystallizes as coarse Zr compounds, 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% by mass or less. In order to obtain the above-mentioned recrystallization suppression effect and the effect of improving the strength of the forged product through precipitation strengthening, the Zr content is preferably 0.0010% by mass or more.

[0040] (Zn: 0.250% by mass or less) The Zn content should be 0.250% by mass or less. If the Zn content exceeds 0.250% by mass, MgZn2 is generated and precipitates from the Al matrix to the grain boundaries, causing intergranular corrosion and reducing the corrosion resistance of the suspension arm. For this reason, it is preferable that the Zn content is 0.250% by mass or less, or that it is completely absent.

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

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

[0043] Figures 2 and 3 are simulation images of equivalent strain obtained by performing a computer simulation of the suspension arm 100 shown in Figure 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, Figure 2 is an equivalent strain image including the entire first arm portion 10, and Figure 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 maximum equivalent strain is the part adjacent to the wheel side connecting part 40 of the second arm section 20, indicated by the symbol Max in Figure 3, and the part with the minimum equivalent strain is the part adjacent to the vehicle body side connecting part 50 of the first arm section 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 the equivalent strain is maximum to the tensile strength of the portion where the equivalent strain is minimum is 0.84 or more and 1 or less. Here, the "portion where the equivalent strain is maximum" refers to a range of 25.4 mm in length including the portion where the equivalent strain is maximum in the simulation, and the "portion where the equivalent strain is minimum" refers to a range of 25.4 mm in length including the portion where the equivalent strain is minimum in the simulation.

[0046] In the forging process, hot plastic processing can cause changes in the structure, such as recrystallization. This can cause strength variations depending on the location even within the same forged product. The present invention was completed by forging the material using a 6000 series aluminum alloy continuous cast rod without homogenization treatment, which makes it possible to keep the tensile strength ratio at the location with high and low equivalent strain within the range of 0.84 to 1.

[0047] It is believed that when a 6000 series aluminum alloy continuous cast rod is forged without homogenization, the precipitates become finer than those that have been homogenized. This makes the precipitation strengthening of the compounds more effective, and the strength is less likely to decrease even in areas with high equivalent strain where recrystallization would occur. As a result, it is believed that it is possible to obtain products with relatively small variations in strength, even in 6000 series aluminum alloy forgings in which the equivalent strain varies greatly depending on the area.

[0048] In addition, based on the results of the simulation, it was found that the "part with the maximum equivalent strain" and the "part with the minimum equivalent strain" are the thinnest and thickest parts of the first and second arm sections, respectively. In addition, when comparing the "part with maximum equivalent strain" and "part with minimum equivalent strain" between the long first arm section and the shorter second arm section, it was found that the "part with maximum equivalent strain" was larger in the shorter second arm section, while the "part with minimum equivalent strain" was smaller in the long first arm section.

[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 adjacent to the wheel side connecting portion 40, and their thickest portions adjacent to the vehicle body side connecting portions 50, 60.

[0050] The suspension arm of this embodiment is a forged product in which the ratio of tensile strength of the thickest part to 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 difference of 15° or more (high-angle grain boundaries) are indicators of the degree of progress of recrystallization in each portion of the suspension arm 100. The ratio of high-angle grain boundaries can be obtained from an EBSD image. The "part with the maximum equivalent strain" and the "part with the minimum equivalent strain" correspond to the part with the largest proportion of high-angle grain boundaries with a crystal orientation misorientation of 15° or more and the part with the smallest 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 difference of 15° or more to the tensile strength of the portion with the smallest proportion of high-angle grain boundaries with a crystal orientation difference of 15° or more is 0.84 or more and 1 or less.

[0053] The suspension arm 100 of this embodiment configured as described above is made of an aluminum alloy having the above alloy composition, so recrystallization is unlikely to occur during the production of the forged product, and 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] [Suspension arm manufacturing method] Next, a method for manufacturing the suspension arm of this embodiment will be described. The manufacturing method of 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 hardening treatment step, and an aging treatment step.

[0056] (Molten metal formation process) The molten metal forming step is a step of obtaining a molten aluminum alloy by melting raw materials and adjusting the composition. The molten aluminum alloy has a composition of 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% or less, 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%, The alloy composition is adjusted so that it contains Zr in the range of 0.0010 mass % or more and 0.050 mass % or less, with the balance being Al and unavoidable impurities, to obtain a molten 6000-series aluminum alloy.

[0057] By carrying out the subsequent steps using molten aluminum alloy with the above composition, it is possible to obtain an Al-Mg-Si suspension arm that is less prone 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] The molten aluminum alloy can be obtained by heating and melting the aluminum alloy. Alternatively, the aluminum alloy may be formed by melting a mixture containing the elements or compounds containing two or more elements as raw materials of the aluminum alloy in a ratio that produces the target aluminum alloy. For example, Ti or B may be mixed as a grain refiner such as an Al-Ti-B rod in order to control the grain size of the aluminum alloy produced in the casting process.

[0059] Alternatively, the raw material of the molten aluminum alloy may be 10% or more of scrap material of 1000 series, 2000 series, 3000 series, 4000 series, 5000 series, 6000 series, or 7000 series aluminum alloy, with the remainder being new aluminum ingots and the above-mentioned additive elements, and the molten aluminum alloy may be obtained by melting these to adjust the composition. In this case, an Al-Mg-Si suspension arm that is less prone to recrystallization and has excellent mechanical properties at room temperature can be obtained. Note that the new aluminum ingot is aluminum with a purity of, for example, 99% or more, which is obtained by subjecting alumina produced from minerals to electrolysis, a process known as electrolytic refining.

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

[0061] A vertical continuous casting apparatus that can be used to produce aluminum alloy castings is shown in FIG. 4 and FIG. 4 is a side cross-sectional view that shows a schematic diagram of 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 continuous 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 disposed 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 into the mold 2 as primary cooling water. At the lower 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 cooled primarily by contact with the mold 2 to become a semi-solidified ingot W2. This semi-solidified ingot W2 has a solidified film formed on its outer periphery. The ingot W2 in this state passes continuously downward through the inside of the mold 2, and cooling water M is sprayed from the nozzle 1 of the mold 2 onto the ingot W2 immediately after it passes through the mold 2, and the cooling water M comes into direct contact with the outer periphery of the ingot W2 to cool it. In this way, the ingot W2 is pulled downward and is secondary-cooled to solidify most of it, producing a round bar-shaped continuous cast material (billet).

[0065] Next, a method for cooling the ingot W2 in this casting apparatus will be described in detail. Fig. 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 along line VV in Fig. 4. Fig. 5 corresponds to a view of this 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 provided on the lower inner peripheral surface of the mold 2, and are formed at predetermined intervals along the circumferential direction. Each nozzle 1 is formed so that the spray direction D of the cooling water M sprayed therefrom is inclined in one direction from the axis (central axis) X of the ingot W2 so as not to intersect with the axis X. Specifically, as shown in Figure 5, in a state where the casting apparatus is viewed from below along the axis X of the ingot W2 (the state of Figure 5), when a virtual axis line connecting the center of the nozzle 1 and the axis X of the ingot W2 is set as the projection reference axis B, the spray direction D of the cooling water M sprayed from each nozzle 1 is arranged so as to be 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 ejection direction D of the cooling water M is inclined relative to the projection reference axis B is defined as the ejection angle θ, the ejection angles θ of the cooling water M ejected from each ejection port 1 are all set to the same angle.

[0068] In this casting apparatus, each ejection direction D starts from the ejection nozzle 1, and the tip side (ejection direction side) is inclined counterclockwise with respect to the projection reference axis B in the plane of the paper in Fig. 5, 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 direction) may also be defined as one direction. In other words, each ejection direction D may start from the ejection nozzle 1 in the plane of the paper in Fig. 5, and the tip side may be inclined clockwise with respect to the projection reference axis B.

[0069] As described above, in this casting apparatus, in a bottom view, the ejection direction D of the cooling water M ejected from the ejection port 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), so that the cooling water M is not sprayed perpendicularly to the outer peripheral surface of the ingot W2 but obliquely to the outer peripheral surface of the ingot W2. Therefore, the cooling water M ejected from each ejection port 1 flows down so as to wind in a spiral along the outer peripheral surface of the ingot W2. In other words, by projecting the cooling water M obliquely to 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 on the ingot W2 are increased, so that the cooling water M flows down so as to wind in a spiral without interruption around the entire circumference of the ingot W2, and the entire circumference of the ingot W2 is evenly cooled by the secondary cooling water M without bias. Therefore, the ingot W2 is cooled evenly throughout, and partial insufficient cooling does not occur, and sufficient cooling performance can be obtained. Furthermore, the cooling water M flows down the outer circumferential surface of the ingot W2 in a spiral shape, so that the contact length (flow path) of the cooling water M with the ingot W2 is longer, and the contact time between the cooling water M and the ingot W2 is also longer, which further improves the cooling performance. As a result, it is possible to reliably prevent the occurrence of partial high temperature abnormalities in the ingot W2, and it is possible to eliminate partial high temperature abnormalities that cause material segregation and stress concentration, prevent defects such as ingot cracking, significantly reduce the occurrence of quality defects, and produce a continuously cast material as a high-quality ingot W2.

[0070] In this casting apparatus, if some of the nozzles 1 are clogged due to some reason, such as deterioration of the mold or water supply pipes, the entire circumference of the ingot W2 can be cooled evenly. In other words, since 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 outer peripheral surface of the ingot W2 is wider than the conventional projection at a right angle 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 without interruption onto the entire circumference of the ingot W2, and as described above, the projected cooling water M flows down so as to spirally wrap around the outer peripheral surface of the ingot W2, so that the entire circumference of the ingot W2 can be cooled evenly. Even if part of the nozzle 1 is blocked in this manner, cooling water M is sprayed to compensate for the blocked portion, and the entire circumference of the ingot W2 can 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, according to the hot top casting machine of this casting device, the ejection direction 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 make the cooling water M flow down so as to wrap 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] As the forging material used in the next forging step, for example, a peeled bar obtained by cutting and removing the outer circumferential surface of a continuously cast bar obtained by casting is used. Then, a lubricant (lubricating oil) is applied to the surface of the forging material formed by this peeled bar, or a lubricant is applied to the inner circumferential surface of the die of a forging device, and the forging process is performed to obtain a forged product such as the above-mentioned automobile part. However, if the surface condition of the peeled bar used as the forging material is not stable, a uniform lubricating film is not formed on the entire outer circumferential surface of the forging material as the peeled bar, and the lubricating film is formed unevenly, and the effect of the lubricating film cannot be fully obtained. Therefore, it is preferable to perform forging peeling, which can obtain a forged product with high dimensional accuracy and can improve productivity and extend the life of the die when manufacturing the forged product.

[0074] As such forging peeling, the following method can be used.

[0075] 6 and 7 are schematic diagrams for explaining the peeling process. As shown in both figures, a cutting machine 5 is provided on the conveying line for the aluminum bar W0. The cutting machine 5 has four cutting tools 51 at equal intervals (at 90° intervals) in the circumferential direction along the outer circumferential surface of the aluminum bar W0 conveyed along the conveying line. In this embodiment, the cutting tools 51 at each location are so-called combination tools having a pair of bit groups consisting of four overlapping rough machining bits and four overlapping finish machining bits.

[0076] Although not shown, workpiece transport means such as transport rollers, support rollers, and carriages for transporting the aluminum bar W0 along the transport line are provided upstream and downstream of the cutting machine 5 with respect to the transport line.

[0077] The cutting edge material of the rough cutting tool and the finish cutting tool constituting the combination tool is not particularly limited, but super hard alloys, diamonds, etc. can be preferably used, and among them, diamond tools with diamond cutting edges are particularly preferably used as finish cutting tools. In the present invention, the number of cutting tools of the cutting tool 51 is not limited, and cutting tools other than the combination tool may be used.

[0078] In this method, the cutting tool 51 is rotated around the conveying line while the aluminum bar W0 is conveyed along the conveying line, so that the entire outer peripheral surface of the aluminum bar W0 is cut and removed by the cutting tool 51.

[0079] Here, the aluminum bar W0 is the continuous cast bar W before peeling. 11 and peeled bar material after peeling (peeled bar material for forging) W 12 and peeling bar W 12 This is a general term for the forging material obtained by cutting.

[0080] In addition, during the peeling process, by appropriately adjusting cutting conditions such as the type of cutting tool, the cutting tool tip shape, the rotation speed of the cutting tool, and the feed speed of the aluminum bar W0, a peeled bar W0 having a surface profile with a unique configuration as described below can be obtained. 12 This is what is obtained.

[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 axial center is formed with a number of mountain-shaped portions 23 connected together in a generally sawtooth or thunderbolt shape in cross section. 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 designated 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 designated 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 with respect to the axis X and the inclination angle (the other base angle) θ2 of the second inclined surface 22 with respect 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°. That is, when the inclination angles θ1 and θ2 are within the above-mentioned preferred ranges, the effect of the lubricating coating, which will be described later, can be reliably obtained. Conversely, when the inclination angles θ1 and θ2 are outside the above-mentioned preferred ranges, the effect 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 effect of the lubricating coating, which will be described later, can be reliably obtained. Conversely, when the slope lengths L1 and L2 are outside the above-mentioned preferred ranges, the effect of the lubricating coating may not be fully obtained.

[0086] The peeling bar W with this unique surface profile 12 Needless to say, the peeled bar W is also cut appropriately to form the forging material. 12It has a similar surface profile.

[0087] (Forging process) The forging process is a process in which the aluminum alloy casting after casting is cut to a predetermined size, the forging material obtained by peeling using the above method is heated to a predetermined temperature, and then pressure is applied with a press machine to mold it into a die. In the present invention, the forging process is performed without performing the homogenization process that was conventionally performed after casting to remove segregation. Therefore, since it is necessary to perform the segregation removal performed by the homogenization process by heating the material during forging, it is necessary to perform the heating at a temperature of 500°C or higher and below the melting point. Then, the forging process is performed to obtain a forged product (for example, a suspension arm part for an automobile). If the material heating temperature during forging is less than 500°C, compounds such as AlFeSi and Mg2Si in the alloy structure remain in a segregated state, the deformation resistance increases, making it impossible to perform sufficient processing, and cracks occur. Furthermore, if the temperature exceeds the melting point, defects such as eutectic melting are likely to occur. It is preferable that the forging step is carried out at a temperature of 500°C or more and 550°C or less.

[0088] (Solution treatment process) The solution treatment process is a process in which the forged product obtained in the forging process is heated to bring about a solution, thereby relieving the distortion introduced in the forging process and causing the solute elements to dissolve in solid solution.

[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 held at 530 to 560°C for 0.3 to 3 hours or less to perform 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 dissolved sufficiently. On the other hand, if the treatment temperature exceeds 560°C, the solute elements may be dissolved more, but eutectic melting and recrystallization may occur easily. In addition, if the heating rate is less than 5.0°C / min, Mg2Si may precipitate coarsely. On the other hand, if the treatment temperature is less than 530°C, the solution treatment may not proceed, making it difficult to achieve high strength by aging precipitation. The solution treatment step is preferably carried out by holding the solution treatment at 535° C. or higher and 555° C. or lower for 1 hour or longer to 3 hours or shorter.

[0090] (Hardening 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 exceeding 40 minutes.

[0091] In this embodiment, the forged product is put into a water tank that stores 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 put into the water tank after the 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 cast product, but is, for example, between more than 1 minute and 30 minutes.

[0092] (Aging treatment process) The aging treatment process is a process 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 process is heated at a temperature of 180°C to 220°C for 0.5 to 8 hours to perform the aging treatment. In the aging treatment process, it is preferable to perform the aging treatment by heating the forged product that has undergone the quenching treatment process 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 process is heated to a temperature of 170°C or more and 210°C or less, and is then held at that temperature for 0.5 hours or more and 7 hours or less to perform aging treatment. If the treatment temperature is less than 170°C or the holding time is less than 0.5 hours, the Mg2Si-based precipitates that improve the tensile strength may not grow sufficiently. On the other hand, if the treatment temperature exceeds 190°C or the holding time exceeds 7 hours, the Mg2Si-based precipitates may become too coarse to sufficiently improve the tensile strength. EXAMPLES

[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] (Manufacturing of continuous casting products) First, an aluminum alloy having the alloy composition (the balance being aluminum) shown in Table 1 below was prepared. Using the prepared aluminum alloy as a raw material, a continuous cast product having a circular cross section and a diameter of 84 mm was 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 having the same composition and produced under the same production conditions.

[0096] [Table 1]

[0097] (Suspension arm manufacturing) Next, the obtained continuous cast product was subjected to a homogenization heat treatment step (only Comparative Examples 1 to 5), a forging process 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 of the homogenization heat treatment step (only Comparative Examples 1 to 5), the forging process step, the solution treatment step, the quenching treatment step, and the artificial aging treatment step are shown in Table 2 below.

[0098] [Table 2]

[0099] [evaluation] The tensile strength (MPa) was evaluated for positions T1 and T4 (areas surrounded by dotted lines in FIG. 10) in 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 positions T1 and T4 for Examples 1 to 5, Examples 6 and 7, and Comparative Examples 1 to 5, and the ratio of the tensile strength (MPa) at position T4 to the tensile strength at position T1 using the average values. Furthermore, the equivalent strain of the entire suspension arm 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 where the equivalent strain is the smallest, the thickest portion of the first arm, and the portion where the proportion of high-angle grain boundaries with a crystal orientation difference of 15° or more is the smallest. The portion indicated by the T4 position corresponds to the portion where the equivalent strain is maximum, the thinnest part of the second arm portion, and the portion where the proportion of high-angle grain boundaries with a crystal orientation difference of 15° or more is maximum.

[0100] <Mechanical property (tensile property) evaluation> A rectangular prism for preparing a test specimen for evaluating mechanical properties was taken from each of positions T1 and T4 of the suspension arm 100. The obtained rectangular prism was processed to prepare a cylindrical test specimen for evaluating mechanical properties as shown in FIG. 11. The parallel part diameter A of the test specimen 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 specimen 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] [Table 3]

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

[0104] Furthermore, 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). [Explanation of symbols]

[0105] 10 First arm section 20 Second arm section 30 Arm connection part 40 Wheel side connection part 50, 60 Vehicle side connection 100 Suspension arm

Claims

1. Cu is in the range of 0.25 mass% or more and 0.37 mass% or less; Mg is in the range of 0.95 mass% or more and 1.25 mass% or less; Si is in the range of 0.6 mass% or more and 0.75 mass% or less; Mn is in the range of 0.05 mass% or more and 0.12 mass% or less; Fe is in the range of 0.15 mass% or more and 0.35 mass% or less; Zn is within the range of 0.25 mass% or less, Cr is in the range of 0.050 mass% or more and 0.26 mass% or less; Ti is in the range of 0.01 mass% or more and 0.1 mass% or less, B is in the range of 0.001 mass % or more and 0.03 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. 2. The suspension arm according to claim 1, wherein the portion where the equivalent strain is minimum has a tensile strength of 350 MPa or more.

3. Cu is in the range of 0.25 mass% or more and 0.37 mass% or less; Mg is in the range of 0.95 mass% or more and 1.25 mass% or less; Si is in the range of 0.6 mass% or more and 0.75 mass% or less; Mn is in the range of 0.05 mass% or more and 0.12 mass% or less; Fe is in the range of 0.15 mass% or more and 0.35 mass% or less; Zn is within the range of 0.25 mass% or less, Cr is in the range of 0.050 mass% or more and 0.26 mass% or less; Ti is in the range of 0.01 mass% or more and 0.1 mass% or less, B is in the range of 0.001 mass % or more and 0.03 mass % or less; The aluminum alloy has an alloy composition containing Zr in a range of 0.0010 mass% or more and 0.050 mass% or less, with the balance being Al and unavoidable impurities; The vehicle body is configured with 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 portion of the pair of arms, and a vehicle body side connecting portion provided at one end of each of the pair of arms, A suspension arm, wherein the pair of arms are forged products, and the ratio of tensile strength of the thinnest part to the tensile strength of the thickest part is 0.84 or more and 1 or less.

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

5. Cu is in the range of 0.25 mass% or more and 0.37 mass% or less; Mg is in the range of 0.95 mass% or more and 1.25 mass% or less; Si is in the range of 0.6 mass% or more and 0.75 mass% or less; Mn is in the range of 0.05 mass% or more and 0.12 mass% or less; Fe is in the range of 0.15 mass% or more and 0.35 mass% or less; Zn is within the range of 0.25 mass% or less, Cr is in the range of 0.050 mass% or more and 0.26 mass% or less; Ti is in the range of 0.01 mass% or more and 0.1 mass% or less, B is in the range of 0.001 mass % or more and 0.03 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. 6. A suspension arm according to claim 5, wherein a portion having a minimum ratio of high-angle grain boundaries with a crystal orientation difference of 15° or more has a tensile strength of 350 MPa or more.

7. 7. A suspension arm according to claim 1, wherein an 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 the steps of: a molten metal forming step for obtaining a molten aluminum alloy; a casting step of obtaining a casting product by casting the obtained molten metal; a forging process for obtaining a forged product by heating the cast product at a temperature of 500° C. to the melting point without performing a homogenization process and subjecting the cast product to plastic processing; A solution treatment process in which the obtained 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°C to 560°C for 0.3 to 3 hours; a quenching step in which all surfaces of the forged product are brought into contact with quenching water within 5 to 60 seconds after the solution treatment, and the forged product is quenched in a water tank for more than 1 minute and less than 40 minutes; Forged products that have undergone a hardening process are subjected to a temperature of 180℃ to 220℃ for 0.5 to 8 hours. and an aging treatment step of performing aging treatment by heating.

9. The forging step is carried out at a temperature of 500° C. or more and 550° C. or less, In the solution treatment step, a solution treatment is performed at 535°C or higher and 555°C or lower for 1 hour to 3 hours, 9. The method for manufacturing a suspension arm according to claim 8, wherein in the aging treatment step, the forged product that has been subjected to the quenching treatment step is heated at a temperature of 185° C. to 205° C. for 0.5 hours to 3 hours for aging treatment.

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