JP7750334B2 - Aluminum alloy castings, aluminum alloy forgings, and their manufacturing methods - Google Patents

Description

The present invention relates to an aluminum alloy casting, an aluminum alloy forging, and a method for manufacturing the same.
This application claims priority based on Japanese Patent Application No. 2022-210255, filed on December 27, 2022, the contents of which are incorporated herein by reference.

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

Furthermore, iron-based materials have traditionally been used exclusively for automobile parts, particularly suspension parts. However, in recent years, these have increasingly been replaced with aluminum or aluminum alloy materials, primarily for the purpose of reducing weight.

Since these automotive parts require excellent corrosion resistance, high strength, and excellent workability, Al-Mg-Si alloys, particularly A6061, are often used as the aluminum alloy material. To improve strength, these automotive parts are manufactured using aluminum alloy materials as the processing material through forging, a type of plastic processing.

In addition, due to the need to reduce costs, suspension parts have recently begun to be put into practical use in which cast components are forged as raw materials without extrusion, and then subjected to solution treatment and artificial aging treatment (T6 treatment). With the aim of further reducing weight, development is underway to develop high-strength alloys to replace the conventional A6061 (see, for example, Patent Documents 1 to 3).

Japanese Patent Application Publication No. 5-59477 Japanese Patent Application Publication No. 5-247574 Japanese Patent Application Publication No. 6-256880

However, the high-strength Al-Mg-Si alloys mentioned above have the problem that the processed structure recrystallizes during the forging and heat treatment processes, resulting in the formation of coarse crystal grains, making it impossible to achieve sufficient strength. Therefore, some alloys contain zirconium (Zr) to prevent recrystallization and prevent the formation of coarse recrystallized grains (see, for example, Patent Documents 1 and 2 mentioned above).

Although the addition of Zr is effective in preventing recrystallization, it has the following problems.
(1) The addition of Zr weakens the grain refining 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) The effect of refining the crystal grains of the ingot itself is weakened, making the ingot more susceptible to cracking, increasing internal defects, and reducing yields.
(3) Zr forms compounds with Al-Ti-B alloys, and these compounds accumulate at the bottom of the furnace storing the molten alloy, contaminating the furnace. In addition, these compounds also crystallize coarsely in the produced ingot, reducing its strength.

Thus, although adding Zr is effective in preventing recrystallization, it was difficult to maintain strength stability.

The present invention was made in light of this technical background, and aims to provide aluminum alloy forging materials and aluminum alloy forgings that have excellent mechanical properties at room temperature, as well as methods for manufacturing the same.

To solve the above problems, the present invention provides the following means.

Aspect 1 of the present invention is a steel sheet containing Cu in the range of 0.25% by mass or more and 0.55% by mass or less, Mg in the range of 0.60% by mass or more and 1.25% by mass or less, Si in the range of 0.90% by mass or more and 1.4% by mass or less, Mn in the range of 0.35% by mass or more and 0.60% by mass or less, Fe in the range of 0.15% by mass or more and 0.30% 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 0.30% by mass or less, Ti in the range of 0.01% by mass or more and 0.1% by mass or less, and B in the range of 0.0010% by mass or less. % or more and 0.030% or less by mass, Zr or Zr in the range of 0.0010% or more and 0.050% or less by mass, the ratio of Fe to Mn (Fe/Mn) is less than 1.4 by mass, and the balance is Al and unavoidable impurities. This aluminum alloy forging material is composed of an aluminum alloy having an alloy composition, and has a post-cast electrical conductivity of 25% IACS or more and 35% IACS or less, and a Rockwell hardness HRF of 62 or more and 82 or less.

Aspect 2 of the present invention is a steel containing Cu in the range of 0.25% by mass or more and 0.55% by mass or less, Mg in the range of 0.60% by mass or more and 1.25% by mass or less, Si in the range of 0.90% by mass or more and 1.4% by mass or less, Mn in the range of 0.35% by mass or more and 0.60% by mass or less, Fe in the range of 0.15% by mass or more and 0.30% 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 0.30% by mass or less, Ti in the range of 0.01% by mass or more and 0.1% by mass or less, and B in the range of 0.0010% by mass or more. This aluminum alloy forging material is composed of an aluminum alloy having an alloy composition containing 0.030% by mass or less of Zr, 0.0010% by mass or more and 0.050% by mass or less of Zr, a ratio of Fe to Mn (Fe/Mn) of 0.3 to 1.2 by mass, and the balance consisting of Al and unavoidable impurities, and has a post-cast electrical conductivity of 25% IACS to 35% IACS and a Rockwell hardness HRF of 62 to 82.

Aspect 3 of the present invention is a steel sheet containing Cu in the range of 0.25 mass% or more and 0.55 mass% or less, Mg in the range of 0.60 mass% or more and 1.25 mass% or less, Si in the range of 0.90 mass% or more and 1.4 mass% or less, Mn in the range of 0.35 mass% or more and 0.60 mass% or less, Fe in the range of 0.15 mass% or more and 0.30 mass% or less, Zn in the range of 0.25 mass% or less, Cr in the range of 0.050 mass% or more and 0.30 mass% or less, and Ti in the range of 0.01 mass% or more and 0.1 % by mass or less, B in the range of 0.0010% by mass or more and 0.030% by mass or less, Zr in the range of 0.0010% by mass or more and 0.050% by mass or less, a ratio of the Fe content to the Mn content (Fe/Mn) of less than 1.4 in mass ratio, and the balance being Al and unavoidable impurities, wherein the number density of precipitates containing Mn within 2.0 μm including the grain boundaries is 4 precipitates/μm2 or ^more , the ratio of high-angle grain boundaries with a crystal orientation difference of 15° or more is 27% or less, and the aluminum alloy forging has an impact value of 10 J/cm2 or ^more at room temperature.

Aspect 4 of the present invention relates to the aluminum alloy forged product of aspect 3, wherein the size of the precipitates is 0.5 μm or less.

Aspect 5 of the present invention is a steel sheet containing Cu in the range of 0.25 mass% or more and 0.55 mass% or less, Mg in the range of 0.60 mass% or more and 1.25 mass% or less, Si in the range of 0.90 mass% or more and 1.4 mass% or less, Mn in the range of 0.35 mass% or more and 0.60 mass% or less, Fe in the range of 0.15 mass% or more and 0.30 mass% or less, Zn in the range of 0.25 mass% or less, Cr in the range of 0.050 mass% or more and 0.30 mass% or less, and Ti in the range of 0.01 mass% or more and 0.1 mass% or less. % or less, B in the range of 0.0010% by mass to 0.030% by mass, Zr in the range of 0.0010% by mass to 0.050% by mass, a ratio of the Fe content to the Mn content (Fe/Mn) of 0.3 to 1.2 in mass ratio, and the balance being Al and unavoidable impurities, wherein the number density of precipitates containing Mn within 2.0 μm including the grain boundaries is 4 precipitates/ ^μm2 or more, the ratio of high-angle grain boundaries with a crystal orientation difference of 15° or more is 27% or less, and the aluminum alloy forging has an impact value of 10 J/cm2 or ^more at room temperature.

Aspect 6 of the present invention relates to the aluminum alloy forged product of aspect 5, wherein the size of the precipitates is 0.5 μm or less.

Aspect 7 of the present invention is a method for manufacturing an aluminum alloy forged product according to any one of Aspects 3 to 6, comprising the following steps: a molten metal forming step of obtaining a molten aluminum alloy; a casting step of casting the obtained molten metal to obtain a cast product; a forging step of heating the cast product at a temperature between 500°C and the melting point and plastically processing it to obtain a forged product; a solution treatment step of heating the obtained forged product from 20°C to 500°C at a heating rate of 5.0°C/min or more and holding it at 530-560°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 it in a water tank for more than 1 minute but not more than 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.

Aspect 8 of the present invention relates to the method for manufacturing an aluminum alloy forged product of aspect 4, and further includes a homogenization heat treatment step between the casting step and the forging step, in which the aluminum alloy cast product is subjected to homogenization heat treatment at a temperature in the range of 370°C to 560°C for 2 hours to 10 hours.

According to the present invention, it is possible to provide an aluminum alloy material for forging that has excellent mechanical properties at room temperature.
According to the present invention, it is possible to provide an aluminum alloy forged product having excellent mechanical properties at room temperature.
Furthermore, according to the present invention, the homogenization treatment step that has conventionally been performed after casting of the molten aluminum alloy to remove segregation can be eliminated, thereby providing a low-cost, energy-saving method for manufacturing aluminum alloy forged products.

1 is a perspective view showing an example of an aluminum alloy forged product according to an embodiment of the present invention. FIG. 2 is a plan view showing another example of an aluminum alloy forged product according to an embodiment of the present invention. FIG. 10 is a perspective view showing yet another example of an aluminum alloy forged product according to an embodiment of the present invention. 1 is a cross-sectional view showing an example of the vicinity of a mold of a horizontal continuous casting apparatus for producing an aluminum alloy forged product according to an embodiment of the present invention. FIG. 5 is an enlarged cross-sectional view of a main portion near a cooling water cavity of the horizontal continuous casting machine shown in FIG. 4 . FIG. 2 is an explanatory diagram illustrating the heat flux of a cooling wall portion of the horizontal continuous casting device. FIG. 2 is a plan view showing the position at which the central portion was sampled from the aluminum alloy forging obtained in this example for preparing a test piece for evaluating mechanical properties. FIG. 1 is a plan view showing a test piece for evaluating mechanical properties produced in this example.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
In the drawings used in the following description, characteristic portions may be enlarged for convenience in order to make the features easier to understand, and the dimensional ratios of the components may not 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 can be implemented with appropriate changes within the scope that does not change the effects.

[Aluminum alloy forging materials]
First, an aluminum alloy forging material according to one embodiment of the present invention.
The aluminum alloy forging material of this embodiment contains Cu in the range of 0.25 mass% or more and 0.55 mass% or less, Mg in the range of 0.60 mass% or more and 1.25 mass% or less, Si in the range of 0.90 mass% or more and 1.4 mass% or less, Mn in the range of 0.35 mass% or more and 0.60 mass% or less, Fe in the range of 0.15 mass% or more and 0.30 mass% or less, Zn in the range of 0.25 mass% or less, Cr in the range of 0.050 mass% or more and 0.30 mass% or less, and Ti in the range of 0.01 mass% or more and 0.02 mass% or less. 1% by mass or less, B in the range of 0.0010% by mass to 0.030% by mass, Zr in the range of 0.0010% by mass to 0.050% by mass, the ratio of the Fe content to the Mn content (Fe/Mn) is less than 1.4 by mass, and the balance is Al and unavoidable impurities, and the aluminum alloy forging material has an alloy composition in which the electrical conductivity after casting is 25% IACS to 35% IACS and the Rockwell hardness HRF is 62 to 82.

An aluminum alloy material for forging according to another embodiment of the present invention contains Cu in the range of 0.25% by mass to 0.55% by mass, Mg in the range of 0.60% by mass to 1.25% by mass, Si in the range of 0.90% by mass to 1.4% by mass, Mn in the range of 0.35% by mass to 0.60% by mass, Fe in the range of 0.15% by mass to 0.30% by mass, Zn in the range of 0.25% by mass, Cr in the range of 0.050% by mass to 0.30% by mass, and Ti in the range of 0.01% by mass to 0.1% by mass. the aluminum alloy forging material is an aluminum alloy having an alloy composition containing 0.0010% by mass or more and 0.030% by mass or less, B in the range of 0.0010% by mass or more and 0.030% by mass or less, Zr in the range of 0.0010% by mass or more and 0.050% by mass or less, a ratio of the Fe content to the Mn content Fe/Mn in mass ratio of 0.3 or more and 1.2 or less, and the balance being Al and unavoidable impurities, and the aluminum alloy forging material has an electrical conductivity after casting of 25% IACS or more and 35% IACS or less, and a Rockwell hardness HRF of 62 or more and 82 or less.
The Fe/Mn mass ratio can be set to 0.3 or more and 1.0 or less, or 0.4 or more and 0.8 or less, or 0.4 or more and 0.7 or less.

The aluminum alloy forging material in the above embodiment corresponds to a 6000 series aluminum alloy in that it contains Mg and Si.

(Electrical conductivity: 25% IACS or more and 35% IACS or less)
The electrical conductivity of the aluminum alloy forging material of the above embodiment is 25% IACS or more and 35% IACS or less. This electrical conductivity is the electrical conductivity at room temperature, which is approximately 20°C ± 15°C.
If the electrical conductivity is less than 25% IACS, the material will be too hard and its workability will be reduced, and if it exceeds 35% IACS, the material will be too soft, which will result in a deterioration in machinability (ability to separate chips), and the guaranteed strength of the final product may not be met.

(Rockwell hardness HRF: 62 or more and 82 or less)
The Rockwell hardness HRF of the aluminum alloy forging material of the above embodiment is equal to or greater than 62 and equal to or less than 82. Here, the Rockwell hardness HRF is a value measured in accordance with JIS Z2245:2016 "Rockwell hardness test - Test method."
This is because a Rockwell hardness HRF within this range provides good workability. That is, if the Rockwell hardness HRF is less than 62, the material is too soft, resulting in poor machinability (chip breakability) and the guaranteed strength of the final product may not be satisfied, while if it exceeds 82, the material is too hard, resulting in poor workability.

[Aluminum alloy forgings]
An aluminum alloy forged product according to one embodiment of the present invention will be described.
FIG. 1 is a perspective view of an aluminum alloy forging according to one embodiment of the present invention.
As shown in Figure 1, an aluminum alloy forging 1a has a long portion 2 and connecting portions 4a, 4b connected to both ends of the long portion 2 in the longitudinal direction. The long portion has a rectangular cross section. Each of the two connecting portions 4 may have a through hole. An aluminum alloy forging 1a having this shape can be used as, for example, an I-type suspension arm.

The aluminum alloy forged product of this embodiment contains Cu in the range of 0.25 mass% or more and 0.55 mass% or less, Mg in the range of 0.60 mass% or more and 1.25 mass% or less, Si in the range of 0.90 mass% or more and 1.4 mass% or less, Mn in the range of 0.35 mass% or more and 0.60 mass% or less, Fe in the range of 0.15 mass% or more and 0.30 mass% or less, Zn in the range of 0.25 mass% or less, Cr in the range of 0.050 mass% or more and 0.30 mass% or less, and Ti in the range of 0.01 mass% or less. and Zr in the range of 0.0010% by mass or more and 0.1% by mass or less, B in the range of 0.0010% by mass or more and 0.030% by mass or less, Zr in the range of 0.0010% by mass or more and 0.050% by mass or less, a ratio of the Fe content to the Mn content (Fe/Mn) of less than 1.4 in mass ratio, and the balance being Al and unavoidable impurities, wherein the number density of precipitates containing Mn within 2.0 μm including grain boundaries is 4 precipitates/ ^μm2 or more, and the ratio of high-angle grain boundaries with a crystal orientation difference of 15° or more is 27% or less.
Furthermore, the aluminum alloy forged product of this embodiment has an impact value of 10 J/cm ² or more at room temperature.

For example, the size of precipitates containing Mn within 2.0 μm, including grain boundaries, is 0.5 μm or less.

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

(Cu: 0.25% by mass or more, 0.55% 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 aluminum alloy forging 1a at room temperature can be improved.

(Mg: 0.60 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 matrix or by precipitating as Mg-Si compounds (Mg ₂ Si) such as the β″ phase, or Al-Cu-Mg-Si compounds (AlCuMgSi) such as the Q phase. Mg ₂ Si also has the effect of suppressing the formation of CuAl ₂ phase in the aluminum alloy. Suppressing the formation of CuAl ₂ phase improves the corrosion resistance of the aluminum alloy forging 1a. When the Mg content is within the above range, the corrosion resistance as well as the mechanical properties at room temperature of the aluminum alloy forging 1a can be improved.

(Si: 0.90 mass% or more, 1.4 mass% or less)
Like Mg, Si has the effect of improving the mechanical properties and corrosion resistance of the aluminum alloy forging 1a at room temperature. However, if excessive Si is added to the aluminum alloy, coarse primary 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 aluminum alloy forging 1a at room temperature while suppressing the crystallization of primary Si.

(Mn: 0.35 mass% or more, 0.60 mass% or less)
Mn has the effect of improving the tensile strength of the aluminum alloy by forming fine granular precipitates containing intermetallic compounds such as Al-Mn-Fe-Si and Al-Mn-Cr-Fe-Si in the aluminum alloy. By ensuring that the Mn content is within the above range, the mechanical properties of the aluminum alloy forging 1a at room temperature can be improved.

(Fe: 0.15% by mass or more, 0.30% by mass or less)
Fe has the effect of improving the tensile strength of the aluminum alloy by crystallizing in the aluminum alloy as fine crystallized particles including intermetallic compounds such as Al-Mn-Fe-Si, Al-Mn-Cr-Fe-Si, Al-Fe-Si, Al-Cu-Fe, and Al-Mn-Fe. By keeping the Fe content within the above range, the mechanical properties of the aluminum alloy forging 1a at room temperature can be improved.
The Fe/Mn relationship is less than 1.4. By making the Fe/Mn relationship less than 1.4, it is possible to suppress the crystallization of AlFeSi-based compounds having a size of 3.0 μm or more, and to increase the number density of AlMn-based compounds in the crystal grains.

(Cr: 0.050 mass% or more, 0.30 mass% or less)
Cr acts to improve the tensile strength of the aluminum alloy by forming fine granular crystallized products containing 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 aluminum alloy forging 1a at room temperature can be improved.

(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 aluminum alloy forged product 1a, 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.

(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 be formed and may be mixed into the aluminum alloy forged product 1a 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 more.

(Zr: 0.0010% by mass or more, 0.05% by mass or less)
When Zr is 0.05% by mass or less, it precipitates in the form of Al ₃ Zr and Al-(Ti,Zr), thereby suppressing recrystallization and contributing to improving the strength of the aluminum alloy forged product 1a through precipitation strengthening. If the Zr content exceeds 0.050% by mass, it may crystallize as coarse Zr compounds, which may lead to a decrease in the corrosion resistance of the aluminum alloy forged product 1a. For this reason, the Zr content is set to 0.050% by mass or less. In addition, in order to obtain the above-mentioned effects of suppressing recrystallization and improving the strength of the forged product through precipitation strengthening, the Zr content is preferably 0.0010% by mass or more.

(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 at grain boundaries, causing intergranular corrosion and leading to a decrease in the corrosion resistance of the aluminum alloy forged product. Therefore, it is preferable that the Zn content be 0.250% by mass or less, or that no Zn be included at all.

(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% by mass.

The longitudinal central portion 2a of the long portion 2 of the aluminum alloy forging 1a of this embodiment is the portion to which the maximum principal stress is applied when the aluminum alloy forging 1a is used, for example, as a suspension arm for a vehicle. The central portion 2a is, for example, a region that is 1% to 80% of the entire long portion 2, including the longitudinal center of the long portion 2. The aspect ratio of the long portion 2 (longitudinal length/length of the short side perpendicular to the longitudinal direction) is, for example, 2 to 100. The cross section of the central portion 2a of the long portion 2 is a cross section (hereinafter sometimes referred to as the central cross section) taken along the direction in which pressure was applied when manufacturing the aluminum alloy forging 1a by forging.

(Does not contain AIFeSi(Mn) compounds with an average particle size of 3.0 μm or more)
The alloy structure in the central cross section of the aluminum alloy forging 1a is made to contain no AIFeSi(Mn)-based compounds with an average particle size of 3.0 μm or more, as the presence of AIFeSi(Mn)-based compounds with an average particle size of 3.0 μm or more may result in a decrease in mechanical properties (tensile properties, fatigue properties, etc.).

(Impact value is 10 J/cm2 or ^more )
The central cross section of the aluminum alloy forged product 1a has mechanical properties such that the impact value at room temperature (20°C) is 10 J/ ^cm2 or more. If this impact value is less than 10 J/ ^cm2 , the durability of the part may be reduced.
In this specification, the term "impact value" refers to the Charpy impact strength measured in accordance with the "Charpy impact test method for metallic materials" specified in JIS Z2242-2005. The measurement is performed using a cylindrical test piece.

(The ratio of high-angle grain boundaries with a crystal orientation difference of 15° or more is 27% or less)
In the cross section of the central portion of the aluminum alloy forging 1a, grain boundaries with a crystal orientation difference of 15° or more (high-angle grain boundaries) are an indicator of the degree of recrystallization in the long portion 2 of the aluminum alloy forging 1a. A ratio of these high-angle grain boundaries of 27% or less indicates that recrystallization is sufficiently suppressed. Sufficient suppression of recrystallization improves the mechanical properties of the long portion 2. The ratio of high-angle grain boundaries can be obtained from an EBSD image.

The aluminum alloy forging 1a of this embodiment, configured as described above, is made of an aluminum alloy having the above-described alloy composition, and therefore is less likely to undergo recrystallization during production. Therefore, excessively coarse crystal grains are less likely to form. Furthermore, the cross section of the central portion 2a of the long portion 2 of the aluminum alloy forging 1a of this embodiment has an alloy structure in which Mn-containing precipitates with a size of 0.5 μm or less are present at a number density of 4/μm2 or ^more within 2.0 μm, including the grain boundaries, and the proportion of high-angle grain boundaries with a crystal orientation misorientation of 15° or more is 27% or less.
Therefore, the central portion 2a of the long portion 2 has high tensile properties and fatigue properties, excellent toughness, and improved impact resistance.

The aluminum alloy forged product 1a of this embodiment has high strength and durability in the central portion 2a of the long portion 2, and is lightweight, making it advantageous for use as a suspension arm for vehicles such as automobiles.

In the aluminum alloy forging 1a of this embodiment shown in Figure 1, one connecting portion 4a is cylindrical with a relatively small diameter, and one connecting portion 4b is cylindrical with a relatively large diameter, and the long portion 2 has a shape that widens in width from the end edge on the side of one connecting portion 4a toward the end edge on the side of the other connecting portion 4b. However, the shape of the aluminum alloy forging 1a is not limited to this. For example, one connecting portion 4a and the other connecting portion 4b of the aluminum alloy forging 1a may have the same shape. The width of the long portion 2 may be constant. Furthermore, the long portion 2 may have a curved shape. Three or more connecting portions 4 may be formed.

FIG. 2 is a plan view of another example of an aluminum alloy forged product according to an embodiment of the present invention.
The aluminum alloy forging 1b shown in Fig. 2 has three connecting portions 4c, 4d, and 4e. The connecting portions 4c and 4d are connected by a long portion 2, and the connecting portions 4d and 4e are connected by a short portion 5 that is relatively shorter than the long portion 2. A through hole is provided in the connecting portion 4c. This aluminum alloy forging 1b can be used, for example, as an L-shaped suspension arm.

FIG. 3 is a plan view of yet another example of an aluminum alloy forged product according to an embodiment of the present invention.
The aluminum alloy forging 1c shown in Fig. 3 has three connecting portions 4f, 4g, and 4h. The connecting portions 4f and 4g are connected to each other by the long portion 2, and the connecting portions 4f and 4h are connected to each other by the long portion 2. A through hole is provided in the connecting portion 4f. This aluminum alloy forging 1b can be used, for example, as an A-type suspension arm.

In another embodiment of the present invention, an aluminum alloy forging contains Cu in the range of 0.25% by mass to 0.55% by mass, Mg in the range of 0.60% by mass to 1.25% by mass, Si in the range of 0.90% by mass to 1.4% by mass, Mn in the range of 0.35% by mass to 0.60% by mass, Fe in the range of 0.15% by mass to 0.30% by mass, Zn in the range of 0.25% by mass, Cr in the range of 0.050% by mass to 0.30% by mass, and Ti in the range of 0.01% by mass. and Zr in the range of 0.0010% by mass or more and 0.1% by mass or less, B in the range of 0.0010% by mass or more and 0.030% by mass or less, Zr in the range of 0.0010% by mass or more and 0.050% by mass or less, a ratio of the Fe content to the Mn content (Fe/Mn) in mass ratio is 0.3 or more and 1.2 or less, and the balance is Al and unavoidable impurities, wherein the number density of precipitates containing Mn within 2.0 μm including grain boundaries is 4 precipitates/μm2 or ^more , and the ratio of high-angle grain boundaries with a crystal orientation difference of 15° or more is 27% or less.
Furthermore, the aluminum alloy forged product of this embodiment has an impact value of 10 J/cm ² or more at room temperature.
The Fe/Mn mass ratio can be set to 0.3 or more and 1.0 or less, or 0.4 or more and 0.8 or less, or 0.4 or more and 0.7 or less.

For example, the size of precipitates containing Mn within 2.0 μm, including grain boundaries, is 0.5 μm or less.

[Method for manufacturing aluminum alloy forged products]
Next, a method for manufacturing an aluminum alloy forged product according to this embodiment will be described.
The method for manufacturing an aluminum alloy forged product 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.

(Molten metal forming process)
The molten metal forming step is a step of melting raw materials to obtain a molten aluminum alloy having a composition adjusted to include Cu in a range of 0.25 mass% to 0.55 mass%, Mg in a range of 0.60 mass% to 1.25 mass%, Si in a range of 0.90 mass% to 1.4 mass%, Mn in a range of 0.35 mass% to 0.60 mass%, Fe in a range of 0.15 mass% to 0.30 mass%, Zn in a range of 0.25 mass%, and Cr in a range of 0.050 mass% to 0.30 mass%. The molten metal of a 6000 series aluminum alloy is obtained by adjusting the alloy composition to contain the following alloy elements: Ti in the range of 0.01% by mass or more and 0.1% by mass or less; B in the range of 0.0010% by mass or more and 0.030% by mass or less; Zr in the range of 0.0010% by mass or more and 0.050% by mass or less; a ratio of the Fe content to the Mn content (Fe/Mn) of less than 1.4 in terms of mass ratio, with the balance being Al and unavoidable impurities.
The Fe/Mn mass ratio may be adjusted to be 0.3 or more and 1.2 or less.

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

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

Also, the raw materials for the molten aluminum alloy may be 10% or more scrap material of 1000, 2000, 3000, 4000, 5000, 6000, or 7000 series aluminum alloys, with the remainder being virgin aluminum ingots and the above-mentioned additive elements. These may be melted to obtain a molten aluminum alloy with an adjusted composition. In this case, it is possible to obtain Al-Mg-Si aluminum alloy forgings that are less prone to recrystallization and have excellent mechanical properties at room temperature. Note that virgin aluminum ingots are aluminum with a purity of, for example, 99% or more, obtained by subjecting alumina produced from minerals to electrolysis, a process known as electrolytic refining.

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

4 and 5 show a horizontal continuous casting apparatus that can be used to manufacture the aluminum alloy cast product of this embodiment.
4 is a cross-sectional view showing an example of the vicinity of the mold 12 of the horizontal continuous casting apparatus 10. FIG. 5 is an enlarged cross-sectional view of a main portion of the horizontal continuous casting apparatus 10 near the cooling water cavity 24.

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

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

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

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

A withdrawal drive device (not shown) that withdraws the cast aluminum alloy rod B at a constant speed may be installed at the other end 12b of the mold 12. It is also preferable to install a synchronized cutting machine (not shown) that cuts the continuously withdrawn aluminum alloy rod B to any desired length.

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

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

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

The inner peripheral surface 21a of the hollow portion 21 of the mold 12 is formed at an elevation angle of 0° to 3° (more preferably 0° to 1°) relative to the mold center axis C toward the casting direction of the aluminum alloy bar B (see Figure 1). In other words, the inner peripheral surface 21a is configured with a tapered shape that opens out like a cone toward the casting direction. The angle of this taper is the elevation angle.

If the elevation angle is less than 0°, the aluminum alloy rod B may encounter resistance at the other end 12b, which is the mold outlet, when it is pulled out of the mold 12, making casting difficult. On the other hand, if the elevation angle exceeds 3°, the inner surface 21a will not make sufficient contact with the molten aluminum alloy M, reducing the heat transfer effect from the molten aluminum alloy M and its solidified shell to the mold 12, which may result in insufficient solidification. This is undesirable as it may result in casting problems such as the formation of a remelted skin on the surface of the aluminum alloy rod B or the ejection of unsolidified molten aluminum alloy M from the end of the aluminum alloy rod B.

The cross-sectional shape of the hollow portion 21 of the mold 12 (the planar shape of the hollow portion 21 of the mold 12 when viewed from the other end 21b) may be selected to match the shape of the aluminum alloy rod to be cast, such as a triangular or rectangular cross-sectional shape, a polygonal, semicircular, elliptical, or an irregular cross-sectional shape that does not have an axis or plane of symmetry, in addition to the circular shape of this embodiment.

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

In this embodiment, pressurized lubricating fluid is supplied from the lubricant supply port 22a to the inner surface 21a of the mold 12. Alternatively, the liquid lubricant may be heated to decompose into a gas, which is then supplied to the inner surface 21a of the mold 12. Alternatively, a porous material may be placed in the lubricant supply port 22a, and the lubricating fluid may be allowed to seep out onto the inner surface 21a of the mold 12 through this porous material.

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

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

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

The cooling water injection passage 25 also sprays cooling water W directly from the shower opening 25a facing the hollow portion 21 toward the aluminum alloy rod B at the other end 12b of the mold 12, thereby cooling the aluminum alloy rod B. The vertical cross-sectional shape of the cooling water injection passage 25 may be, in addition to the circular shape of this embodiment, a semicircular, pear-shaped, or horseshoe-shaped shape, for example.

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

The length from the position where the extension of the central axis of the shower opening 25a of the cooling water injection passage 25 hits the surface of the cast aluminum alloy rod B to the contact surface between the mold 12 and the refractory plate 13 is called the effective mold length L, and this effective mold length L is preferably, for example, 10 mm or more and 40 mm or less. If this effective mold length L is less than 10 mm, casting is impossible because a good coating will not be formed. If it exceeds 40 mm, the effect of forced cooling will be reduced, solidification will occur due to the mold wall, and contact resistance between the mold 12 and the molten aluminum alloy M or aluminum alloy rod B will increase, which may lead to unstable casting, such as cracks on the casting surface or tearing inside the mold, and this is undesirable.

It is preferable that the supply of cooling water W to these cooling water cavities 24 and the spraying of cooling water W from the shower openings 25a of the cooling water spray passages 25 can each be controlled by control signals from a control device (not shown).

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

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

As shown in Figure 4, the cooling wall portion 27 of the mold 12, which is the portion where the inner bottom surface 24a of the cooling water cavity 24 faces the inner surface 21a of the hollow portion 21 of the mold 12, is formed so that the heat flux value per unit area from the molten aluminum alloy M in the hollow portion 21 toward the cooling water W in the cooling water cavity 24 is in the range of 10 x ¹⁰⁵ W/ ^m2 or more and 50 x ¹⁰⁵ W/ ^m2 or less.

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

In Figure 4, the molten aluminum alloy M in the molten metal receiver 11 is supplied through a refractory plate 13 to one end 12a of a mold 12, which is held so that the mold center axis C is approximately horizontal, and is forcibly cooled at the other end 12b of the mold 12 to form an aluminum alloy rod B.

The aluminum alloy rod B is drawn out at a constant speed by a drawing drive device (not shown) installed near the other end 12b of the mold 12, so that it is continuously cast into a long aluminum alloy rod B. The drawn aluminum alloy rod B is then cut to the desired length, for example, by a synchronized cutting machine (not shown).

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

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

Lubricating vegetable oils can be used as liquid lubricants. Examples include rapeseed oil, castor oil, and salad oil. These are preferred because they have little adverse impact on the environment.

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

The casting speed, which is the speed at which the aluminum alloy rod B is withdrawn from the mold 12, is preferably 200 mm/min or more and 1500 mm/min or less (more preferably 400 mm/min or more and 1000 mm/min or less). This is because a casting speed within this range results in a uniform and fine network structure of the crystals formed during casting, which increases the resistance of the aluminum matrix to deformation at high temperatures and improves its high-temperature mechanical strength.

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

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

Furthermore, by setting the heat flux value per unit area from the molten aluminum alloy M in the hollow portion 21 to the cooling water W in the cooling water cavity 24 in the cooling wall portion 27 of the mold 12 to a range of 10 x 10 ⁵ W/m ² or more and 50 x 10 ⁵ W/m ² or less, it is possible to prevent the aluminum alloy rod B from sticking.

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

Based on the mold material, thickness, and temperature measurement data that showed good results even when the amount of lubricating oil was reduced during casting, the cooling wall portion 27 of the mold 12 is configured so that the heat flux value per unit area is 10×10 ⁵ W/m ² or more, thereby preventing seizure of the cast aluminum alloy bar B. It is also preferable that the heat flux value per unit area is 50×10 ⁵ W/m ² or less.

To ensure that the cooling wall portion 27 of the mold 12 falls within this heat flux value range, the mold 12 may be formed so that the thickness t of the cooling wall portion 27 of the mold 12 is, for example, in the range of 0.5 mm or more and 3.0 mm or less. Furthermore, the thermal conductivity of at least the cooling wall portion 27 of the mold 12 may be in the range of 100 W/m·K or more and 400 W/m·K or less.

When producing the aluminum alloy rod B of this embodiment, the horizontal continuous casting apparatus 10 described above is used to continuously supply the molten aluminum alloy M stored in the molten metal receiving portion 11 from one end side 12a of the mold 12 into the hollow portion 21. Cooling water W is also supplied to the cooling water cavity 24, and a lubricating fluid, such as lubricating oil, is supplied from the fluid supply pipe 22.

The molten aluminum alloy M supplied into the hollow portion 21 is then cooled and solidified under conditions where the heat flux value per unit area at the cooling wall portion 27 is 10× ¹⁰ W/ ^m2 or more, thereby casting the aluminum alloy rod B. Furthermore, when casting the aluminum alloy rod B, it is preferable to set the wall surface temperature of the cooling wall portion 27 of the mold 12, which is cooled by cooling water W, to 100°C or less.

The aluminum alloy rod B thus obtained is cooled and solidified under conditions where the heat flux value per unit area in the cooling wall portion 27 is 10×10 ⁵ W/m ² or more, thereby suppressing the adhesion of reaction products, such as carbides, caused by contact between the lubricating oil gas and the molten aluminum alloy M. This eliminates the need to cut and remove carbides and the like from the surface of the aluminum alloy rod B, and allows the aluminum alloy rod B to be produced with a high yield.

The casting process for obtaining a cast product from the molten aluminum alloy M is not limited to the horizontal continuous casting method described above, and known continuous casting methods such as vertical continuous casting can also be used. Vertical continuous casting methods are classified into the float method and the hot top method depending on the method by which the molten aluminum alloy M is supplied to the mold (casting die 12). Below, we will briefly explain the case where the hot top method is used.

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

In addition, cooling water is sprayed directly onto the casting as it is removed from the mold, allowing the metal to solidify all the way to the inside of the casting as it is continuously pulled out. Molds are generally made of metal components with good thermal conductivity and have a hollow structure to allow a coolant to be introduced inside.

The refrigerant used can be selected from those available industrially, but water is recommended from the standpoint of ease of use.

The mold used in this embodiment is made of a metal such as copper or aluminum, or graphite, selected from the viewpoint of heat transfer performance and durability at the contact point with the molten metal. The header is generally made of a refractory material and is installed above the mold. There are no particular restrictions on the material and size of the header, and they can be selected appropriately depending on the composition range of the alloy being cast and the dimensions of the cast product.

The average cooling rate during casting may be selected appropriately from a generally recommended range, such as 10 to 300°C/sec. The casting speed may be selected appropriately from a range generally used in horizontal continuous casting, such as 200 to 600 mm/min.

The casting method described above allows for the production of uniform metal structures even in medium to large castings. There are no particular restrictions on the diameter of the castings, and it is suitable for use with rods with diameters of 30 to 100 mm.

(Forging process)
The forging process involves cutting the cast aluminum alloy casting to a predetermined size, heating the resulting forging material to a predetermined temperature, and then applying pressure in a press to mold it into a die. In this embodiment, forging is performed without the homogenization treatment that was conventionally performed after casting to remove segregation. Therefore, the segregation removal that was previously performed in the homogenization treatment must be performed by heating the material during forging, so heating must be performed at a temperature above 500°C and below the melting point. Forging is then performed to obtain a forged product (e.g., an automobile suspension arm part, etc.). If the material heating temperature during forging is below 500°C, compounds such as AlFeSi and Mg ₂ Si in the alloy structure remain in a segregated state, increasing deformation resistance and preventing sufficient processing, and cracks will occur. Furthermore, if the temperature exceeds the melting point, defects such as eutectic melting are likely to occur.

(Solution treatment process)
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.

In this embodiment, the forged product is subjected to solution treatment by holding it at a treatment temperature of 530°C or higher and 560°C or lower for a period of 0.3 to 3 hours. The heating rate from room temperature to the above-mentioned treatment temperature is preferably 5.0°C/min or higher. If the treatment temperature is lower 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 easily, but eutectic melting and recrystallization may occur. Furthermore, if the heating rate is lower than 5.0°C/min, coarse precipitation of _Mg2Si may occur. On the other hand, if the treatment temperature is lower than 530°C, the solution treatment may not progress, making it difficult to achieve high strength by aging precipitation.

(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.

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 all surfaces of the forged product come into 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.

(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, thereby imparting an appropriate hardness.

In this embodiment, the forged product after the quenching process is heated to a temperature of 170°C or higher and 210°C or lower and held at that temperature for 0.5 hours or longer and 7 hours or shorter to perform aging treatment. If the treatment temperature is lower than 170°C or the holding time is shorter than 0.5 hours, the _Mg2Si -based precipitates that improve 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 tensile strength.

In another embodiment of the present invention, the method for producing an aluminum alloy forged product further includes, between the casting step and the forging step, a homogenization heat treatment step in which the aluminum alloy cast product is subjected to homogenization heat treatment by holding the aluminum alloy cast product at a temperature in the range of 370°C to 560°C for 2 hours to 10 hours.
The method for manufacturing an aluminum alloy forged product of this embodiment differs from the method for manufacturing an aluminum alloy forged product of the above embodiment in that it includes a homogenization heat treatment step.

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

[Examples 1 to 8 and Comparative Examples 1 to 3]
(Production of continuous cast products)
First, an aluminum alloy was prepared having the alloy composition (the balance being aluminum) shown in Table 1 below. Using the prepared aluminum alloy, a continuous cast product having a circular cross section and a diameter of 82 mm was produced.

(Manufacturing of aluminum alloy forged products)
Next, the obtained continuous cast product was subjected to a homogenization heat treatment step (only Comparative Examples 1 to 3), a forging step, a solution treatment step, a quenching treatment step, and an artificial aging treatment step in this order to obtain an aluminum alloy forged product 1a having the shape shown in Figure 1. The conditions for the homogenization heat treatment step (only Comparative Examples 1 to 3), the forging step, the solution treatment step, the quenching treatment step, and the artificial aging treatment step are shown in Table 2 below.

[evaluation]
The following evaluations were carried out on the central portion 2a in the longitudinal direction of the long portion 2 of the aluminum alloy forgings 1a of Examples 1 to 8 and Comparative Examples 1 to 3 obtained as described above. The evaluation results of the central portion 2a of the long portion 2 are shown in Table 3 below.

<Fe/Mn>
The Fe/Mn ratio was adjusted to 0.3 or more and 1.2 or less in the examples, and to more than 1.2 in the comparative examples.
"Good": 0.3 or more and 1.2 or less.
"×": Over 1.2.

<Electrical conductivity (%IACS)>
The electrical conductivity of the obtained continuous cast product was measured at room temperature.
(Judgment criteria)
"Good": 25% IACS or more and 35% IACS or less.
"X": Less than 25% IACS or more than 35% IACS.

<Rockwell hardness (HRF)>
The Rockwell hardness (HRF) of the obtained continuous cast product was measured in accordance with JIS Z2245:2016 "Rockwell hardness test - Test method".
(Judgment criteria)
"〇": Between 62 and 82.
"X": Less than 62 or more than 82.

<Mechanical property (impact property) evaluation>
The central portion 2a of the long portion 2 of the aluminum alloy forged product 1a was cut as shown in Figure 7 to obtain a rectangular column for preparing a test specimen for evaluating mechanical properties (impact properties). The obtained rectangular column was processed to prepare a cylindrical test specimen for evaluating mechanical properties as shown in Figure 8. The parallel portion diameter A of the test specimen for evaluating mechanical properties was 8.0 mm, and the gauge length G was 30.0 mm. The test specimen for evaluating mechanical properties was subjected to a Charpy impact test at room temperature (25°C) in accordance with JIS Z2242 to measure the impact value. The obtained impact value was evaluated based on the following criteria.
(Judgment criteria)
"Good": Impact value is 10 J/cm ² or more at room temperature.
"X": Impact value is less than 10 J/cm ² at room temperature.

<Refinement evaluation, recrystallization/crystal coarsening evaluation>
The central portion 2a of the long portion 2 of the aluminum alloy forging 1a was cut perpendicular to the surface of the central portion 2a to obtain a plate-like body (2 mm thick) for preparing a test piece for refinement evaluation. The obtained plate-like body was cut into a 7 mm square to obtain a 7 mm x 7 mm x 2 mm thick test piece for refinement evaluation. The surface (cross section of the central portion 2a) of the obtained test piece for refinement evaluation was measured using an SEM-EBSD (scanning electron microscope-electron backscatter diffraction) to measure the number of AlFeSi-based compounds of 3.0 μm or more and the ratio of high-angle grain boundaries with a crystal orientation difference of 15° or more. The number of AlFeSi-based compounds of 3.0 μm or more and the ratio of high-angle grain boundaries obtained were each judged based on the following criteria to evaluate the refinement of crystal grains and the recrystallization/crystal coarsening. The measurement conditions for SEM-EBSD were an acceleration voltage of 15 kV, a measurement pitch of 0.5 μm/px, an analysis area of 500×500 μm ² , and a grain boundary definition angle of 15°.
(Judgment criteria)
(Criteria for determining the number of AlFeSi-based compounds of 3.0 μm or more)
"O": If none (0).
"×": If available.
(Criteria for determining the ratio of high-angle grain boundaries)
"O": Less than 27%.
"X": Over 27%.

<Method for measuring the number density of precipitates containing Mn in aluminum alloy forgings>
In addition, for each aluminum alloy forging, a FE-SEM (field emission scanning electron microscope) was used to measure the number density of precipitates containing Mn within 2.0 μm, including grain boundaries. A sample piece for structure observation measuring approximately 10 mm x 10 mm x 10 mm was cut from the central portion 2a of the long portion 2 of the aluminum alloy forging 1a, and this sample piece was polished using a cross-section polisher. An FE-SEM (field emission scanning electron microscope) photograph of the polished sample piece was then taken, and the number density of precipitates containing Mn within 2.0 μm, including grain boundaries, was determined within a field area of 1.5815 ^mm2 in the SEM photograph, and the number density was evaluated according to the following criteria.
(Number density criteria)
"O": 4 particles/μm ² or more.
"x": 4 particles/ ^μm2 or less.

<Overall rating>
Seven evaluation results, namely, Fe/Mn ratio, electrical conductivity, Rockwell hardness, impact properties, the number of AlFeSi-based compounds of 3.0 μm or more, the number density of AlMn-based compounds, and the ratio of high-angle grain boundaries with a crystal orientation difference of 15° or more, were evaluated based on the following criteria.
(Judgment criteria)
"O": All seven ratings are "O".
"X": One or more of the seven evaluations is "X".

10...Horizontal continuous casting device 11...Molten metal receiving portion (tundish)
11a...molten metal inlet portion 11b...molten metal holding portion 11c...outlet portion 12...mold 12a...one end side 12b...other end side 13...refractory plate-shaped body (heat insulating member)
13a... Passage for pouring molten metal 21... Hollow portion 21a... Inner peripheral surface 21b... Other end side 22... Fluid supply pipe 22a... Lubricant supply port 23... Cooling device 24... Cooling water cavity 24a... Inner bottom surface 25... Cooling water injection passage 25a... Shower opening 26... Cooling water supply pipe 27... Cooling wall portion B... Aluminum alloy rod M... Molten alloy W... Cooling water 100... Aluminum alloy forging

Claims (2)

1. An aluminum alloy forging comprising an aluminum alloy having an alloy composition containing Cu in the range of 0.25 mass% to 0.55 mass%, Mg in the range of 0.60 mass% to 1.25 mass%, Si in the range of 0.90 mass% to 1.4 mass%, Mn in the range of 0.35 mass% to 0.60 mass%, Fe in the range of 0.15 mass% to 0.30 mass%, Zn in the range of 0.25 mass% or less, Cr in the range of 0.050 mass% to 0.30 mass%, Ti in the range of 0.01 mass% to 0.1 mass%, B in the range of 0.0010 mass% to 0.030 mass%, Zr in the range of 0.0010 mass% to 0.050 mass%, and the balance consisting of Al and unavoidable impurities, wherein the number density of precipitates containing Mn within 2.0 μm including grain boundaries is 4 precipitates/μm ² or more, the ratio of high-angle grain boundaries with a crystal orientation difference of 15° or more is 27% or less, and the impact value at room temperature is 10 J/ ^cm2 or more,
a molten metal forming step for obtaining a molten metal of the aluminum alloy;
a casting step of obtaining a casting by casting the obtained molten metal;
a forging step in which the casting is heated at a temperature of 500°C to the melting point and subjected to plastic working to obtain a forged product;
a solution treatment step in which the obtained forged product is heated from 20°C to 500°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;
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 not more than 40 minutes;
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 hours to 8 hours to undergo aging treatment ;
A method for manufacturing an aluminum alloy forged product , which does not include a homogenization heat treatment step in which homogenization heat treatment is performed between the casting step and the forging step . The method for producing an aluminum alloy forged product according to claim 1, wherein the precipitates have a size of 0.5 μm or less.