WO2024142830A1 - Aluminum alloy forging material, aluminum alloy forged product, and method for manufacturing same - Google Patents

Abstract

Provided is an aluminum alloy forged product containing: 0.25-0.55 mass% of Cu; 0.60-1.25 mass% of Mg; 0.95-1.4 mass% of Si; 0.35-0.60 mass% of Mn; 0.15-0.30 mass% of Fe; at most 0.25 mass% of Zn; 0.050-0.30 mass% of Cr; 0.01-0.1 mass% of Ti; 0.0010-0.030 mass% of B; and 0.0010-0.050 mass% of Zr, wherein the Fe/Mn ratio is less than 1.4, the remainder consists of Al and unavoidable impurities, the number density of Mn-containing precipitates within 2.0 μm including grain boundaries is 4/μm2 or more, and when the fraction of high-angle grain boundaries having a crystal misorientation of 15° or more is 27% or less, the impact value at room temperature is 10 J/cm2 or more.

Description

Aluminum alloy forging material, aluminum alloy forging product and its manufacturing method

The present invention relates to an aluminum alloy material for forging, an aluminum alloy forging, and a method for producing 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 light weight. For example, high-tensile steel has traditionally been used for automobile suspension and bumper parts. However, in recent years, high-strength aluminum alloy materials have come to be used.

Furthermore, in the past, iron-based materials were used exclusively for automobile parts, particularly suspension parts. However, in recent years, they 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, especially A6061, are often used as the aluminum alloy material. In order to improve the strength of these automotive parts, they are manufactured by forging, a type of plastic processing, using the aluminum alloy material as the processing material.

In addition, recently, due to the need to reduce costs, suspension parts have begun to be put to practical use in which the cast components are used as they are without extrusion, and then subjected to a 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).

Japanese Patent Application Laid-Open No. 5-59477 Japanese Patent Application Laid-Open No. 5-247574 Japanese Patent Application Laid-Open No. 6-256880

However, the above-mentioned high-strength Al-Mg-Si alloys have the problem 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 sufficiently high strength. For this reason, some alloys have been designed to prevent recrystallization by adding Zr (zirconium) to prevent the generation of coarse recrystallized grains (see, for example, Patent Documents 1 and 2 above).

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, making the crystal grains of the ingot itself coarse, which leads to 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, so that the ingot is more likely to crack, internal defects increase, and the yield decreases.
(3) Zr forms compounds with Al-Ti-B based 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 ingot, reducing its strength.

Thus, although the addition of Zr was effective in preventing recrystallization, it was difficult to maintain strength stability.

The present invention was made in consideration of this technical background, and aims to provide an aluminum alloy forging material that has excellent mechanical properties at room temperature, an aluminum alloy forging product, and a manufacturing method thereof.

The present invention provides the following means to solve the above problems.

Aspect 1 of the present invention is a composition in which Cu is in the range of 0.25 mass% or more and 0.55 mass% or less, Mg is in the range of 0.60 mass% or more and 1.25 mass% or less, Si is in the range of 0.90 mass% or more and 1.4 mass% or less, Mn is in the range of 0.35 mass% or more and 0.60 mass% or less, Fe is in the range of 0.15 mass% or more and 0.30 mass% or less, Zn is in the range of 0.25 mass% or less, Cr is in the range of 0.050 mass% or more and 0.30 mass% or less, Ti is in the range of 0.01 mass% or more and 0.1 mass% or less, and B is in the range of 0.0010 mass% or less. This aluminum alloy forging material is composed of an aluminum alloy having an alloy composition containing 0.010% to 0.030% by mass, 0.0010% to 0.050% by mass of Zr, a ratio of the Fe content to the Mn content (Fe/Mn) of less than 1.4 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 2 of the present invention is a composition comprising 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, Ti in the range of 0.01 mass% or more and 0.1 mass% or less, and B in the range of 0.0010 mass% or less. The aluminum alloy forging material is composed of an aluminum alloy having an alloy composition containing 0.030% by mass or more, 0.0010% by mass or more and 0.050% by mass or less of Zr, a ratio of the Fe content to the Mn content Fe/Mn of 0.3 to 1.2 by mass, and the balance being 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 having a Cu content in the range of 0.25 mass% or more and 0.55 mass% or less, a Mg content in the range of 0.60 mass% or more and 1.25 mass% or less, a Si content in the range of 0.90 mass% or more and 1.4 mass% or less, a Mn content in the range of 0.35 mass% or more and 0.60 mass% or less, a Fe content in the range of 0.15 mass% or more and 0.30 mass% or less, a Zn content in the range of 0.25 mass% or less, a Cr content in the range of 0.050 mass% or more and 0.30 mass% or less, a Ti content in the range of 0.01 mass% or more and 0.1 % or less by mass, 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, 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 , the ratio of high-angle grain boundaries having 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.

In aspect 4 of the present invention, in the aluminum alloy forged product of aspect 3, the size of the precipitates is 0.5 μm or less.

Aspect 5 of the present invention is a steel sheet having a Cu content in the range of 0.25 mass% or more and 0.55 mass% or less, an Mg content in the range of 0.60 mass% or more and 1.25 mass% or less, an Si content in the range of 0.90 mass% or more and 1.4 mass% or less, an Mn content in the range of 0.35 mass% or more and 0.60 mass% or less, an Fe content in the range of 0.15 mass% or more and 0.30 mass% or less, an Zn content in the range of 0.25 mass% or less, an Cr content in the range of 0.050 mass% or more and 0.30 mass% or less, an Ti content in the range of 0.01 mass% or more and 0.1 mass% or less, and an Zn content in the range of 0.02 mass% or more and 0.05 mass% or less. % or less, B in the range of 0.0010 mass % or more and 0.030 mass % or less, Zr in the range of 0.0010 mass % or more and 0.050 mass % or less, a ratio of the Fe content to the Mn content Fe/Mn is 0.3 to 1.2 in mass ratio, and the balance is an alloy composition consisting of 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 , the ratio of high-angle grain boundaries having 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 is an aluminum alloy forged product according to aspect 5, in which the size of the precipitates is 0.5 μm or less.

Aspect 7 of the present invention is a method for producing an aluminum alloy forging according to any one of aspects 3 to 6, comprising the steps of: forming a molten metal of the aluminum alloy; casting the molten metal to obtain a casting; heating the casting at a temperature between 500°C and the melting point and performing plastic processing to obtain a forging; solution treatment of the forging at a temperature of 20°C to 500°C at a heating rate of 5.0°C/min or more and maintaining the temperature at 530 to 560°C for 0.3 to 3 hours; quenching the entire surface of the forging in contact with quenching water within 5 to 60 seconds after the solution treatment and quenching in a water tank for more than 1 minute and not exceeding 40 minutes; and aging treatment of the forging after the quenching treatment 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 is a method for manufacturing an aluminum alloy forged product according to aspect 4, which further includes a homogenization heat treatment step between the casting step and the forging step, in which the aluminum alloy casting product is held at a temperature range of 370°C to 560°C for 2 hours to 10 hours to perform homogenization heat treatment.

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

1 is a perspective view showing an example of an aluminum alloy forging according to an embodiment of the present invention. FIG. FIG. 2 is a plan view showing another example of an aluminum alloy forged product according to an embodiment of the present invention. FIG. 2 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 forging according to an embodiment of the present invention. 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 a heat flux in a cooling wall portion of a horizontal continuous casting apparatus. 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. 2 is a plan view showing a test piece for evaluating mechanical properties produced in this example.

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 a range that does not change the effects of the present invention.

[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, Ti in the range of 0.01 mass% or more and 0.02 mass% or less, and Zn in the range of 0.03 mass% or more and 0.04 mass% or less. 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 by mass, with the remainder 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.

An aluminum alloy forging material according to another embodiment of the present invention has 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 the range below, 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 being 0.3 to 1.2 in mass ratio, with the balance being Al and unavoidable impurities, and the aluminum alloy forging material is composed of an aluminum alloy having an alloy composition, the electrical conductivity after casting being 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, can be set to 0.4 or more and 0.8 or less, or can be set to 0.4 or more and 0.7 or less.

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

(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 about 20°C ± 15°C.
If the electrical conductivity is less than 25% IACS, the material will be too hard and its workability will decrease, whereas if it exceeds 35% IACS, the material will be too soft, resulting in poor machinability (ability to break away chips), and the guaranteed strength of the final product may not be satisfied.

(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 62 or more and 82 or less. Here, the Rockwell hardness HRF is a value measured in accordance with JIS Z2245:2016 "Rockwell hardness test - Test method".
This is because the workability is good if the Rockwell hardness HRF is within this range. That is, if the Rockwell hardness HRF is less than 62, the material is too soft, resulting in poor machinability (chip breakability) and inability to satisfy the guaranteed strength of the final product, while if it exceeds 82, the material is too hard, resulting in poor workability.

[Aluminum alloy forgings]
An aluminum alloy forging 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 Fig. 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. The aluminum alloy forging 1a having this shape can be used as, for example, an I-type suspension arm.

The aluminum alloy forging 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 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, and the balance being Al and unavoidable impurities.
Moreover, 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 forgings of this embodiment correspond to 6000 series aluminum alloy forgings in that they contain 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 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 aluminum alloy forging 1a at room temperature can be improved.

(Mg: 0.60% by mass or more, 1.25% by 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 (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. By suppressing the formation of CuAl ₂ phase, the corrosion resistance of the aluminum alloy forging 1a is improved. By keeping the Mg content within the above range, it is possible to improve the corrosion resistance as well as the mechanical properties at room temperature of the aluminum alloy forging 1a.

(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 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 aluminum alloy forging 1a at room temperature while suppressing the crystallization of primary crystal 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. When 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 mass% or more, 0.30 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 products 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 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 crystallization of AlFeSi-based compounds having a size of 3.0 μm or more, and to improve the number density of AlMn-based compounds in crystal grains.

(Cr: 0.050 mass% or more, 0.30 mass% or less)
Cr has the effect of improving the tensile strength of the aluminum alloy by forming fine granular crystallized products including intermetallic compounds such as Al-Mn-Cr-Fe-Si and Al-Fe-Cr in the aluminum alloy. By having 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 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 products may be formed, and the wrought workability may be reduced. In addition, if a large amount of coarse crystallized products 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 the aluminum alloy and improving the wrought workability. By adding B to the aluminum alloy together with the above-mentioned Ti, the effect of refining the crystal grains is improved. If the content of B is less than 0.0010% 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 products may be formed and may be mixed into the aluminum alloy forged product 1a as inclusions. In addition, if a large amount of coarse crystallized products containing B are 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.

(Zr: 0.0010 mass% or more, 0.05 mass% or less)
If Zr is 0.05% by mass or less, it precipitates in the form of Al ₃ Zr and Al-(Ti, Zr), which contributes to improving the strength of the aluminum alloy forging 1a by suppressing recrystallization and 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 forging 1a. 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 forging by precipitation strengthening, the Zr content is preferably 0.0010% by mass or more.

(Zn: 0.250 mass% or less)
Zn may 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 leading to a decrease in the corrosion resistance of the aluminum alloy forged product. For this reason, it is preferable that the Zn content is 0.250% by mass or less, or that it is not contained at all.

(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%.

The longitudinal center 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 of a vehicle. The central portion 2a is, for example, a region in the range of 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, in the range of 2 to 100. The cross section of the central portion 2a of the long portion 2 is a cross section in the direction along which pressure is applied when manufacturing the aluminum alloy forging 1a by forging (hereinafter, sometimes referred to as the central cross section).

(Does not contain AIFeSi(Mn) based 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 not to contain AIFeSi(Mn)-based compounds having an average particle size of 3.0 μm or more. If AIFeSi(Mn)-based compounds having an average particle size of 3.0 μm or more are present, there is a risk of deterioration of 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/cm ² or more. If this impact value is less than 10 J/cm ² , there is a risk that the durability of the part will decrease.
In this specification, the term "impact value" refers to the Charpy impact strength measured in accordance with the provisions of JIS Z2242-2005 "Charpy impact test method for metallic materials." The test specimen used for the measurement is a cylindrical test specimen.

(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 (high-angle grain boundaries) with a crystal orientation difference of 15° or more are indicators of the degree of progress of recrystallization in the long portion 2 of the aluminum alloy forging 1a. The ratio of these high-angle grain boundaries being 27% or less indicates that recrystallization is sufficiently suppressed. The mechanical properties of the long portion 2 are improved by sufficiently suppressing recrystallization. The ratio of high-angle grain boundaries can be obtained from an EBSD image.

The aluminum alloy forging 1a of the present embodiment configured as described above is made of the above-mentioned alloy composition, so that recrystallization is unlikely to occur when the forging is manufactured. Therefore, excessively coarse crystal grains are unlikely to be generated. In addition, the cross section of the central portion 2a of the long portion 2 of the aluminum alloy forging 1a of the present embodiment has an alloy structure in which the number density of precipitates containing Mn and having a size of 0.5 μm or less within 2.0 μm including the grain boundaries is 4 pieces/μm2 or ^more , and the ratio of high-angle grain boundaries with a crystal orientation difference 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 also lightweight, so it can be advantageously used for suspension arms of vehicles such as automobiles.

In the aluminum alloy forging 1a of this embodiment shown in FIG. 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 in which the width increases 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, but 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. Also, 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 as, for example, an A-type suspension arm.

In another embodiment of the aluminum alloy forging of the present invention, Cu is in the range of 0.25 mass% or more and 0.55 mass% or less, Mg is in the range of 0.60 mass% or more and 1.25 mass% or less, Si is in the range of 0.90 mass% or more and 1.4 mass% or less, Mn is in the range of 0.35 mass% or more and 0.60 mass% or less, Fe is in the range of 0.15 mass% or more and 0.30 mass% or less, Zn is in the range of 0.25 mass% or less, Cr is in the range of 0.050 mass% or more and 0.30 mass% or less, Ti is 0.01 mass% 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 is 0.3 to ^1.2 in mass ratio, and the balance is Al and unavoidable impurities.
Moreover, 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, can be set to 0.4 or more and 0.8 or less, or can be set to 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 of manufacturing aluminum alloy forged products]
Next, a method for producing an aluminum alloy forged product according to this embodiment will be described.
The method for producing 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 preparing a molten aluminum alloy by melting raw materials and adjusting the composition thereof. The molten aluminum alloy has a composition of 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%, Cr in the range of 0.050 mass% to 0.30 mass%, and Cr in the range of 0.050 mass% to 0.30 mass%. The molten aluminum alloy of 6000 series is obtained by adjusting the content of the alloy so as to have an alloy composition in which the content of Fe to the content of Mn is less than 1.4 in terms of mass ratio, the content of Ti being within the range of 0.01% by mass or more and 0.1% by mass or less, the content of B being within the range of 0.0010% by mass or more and 0.030% by mass or less, and the content of Zr being within the range of 0.0010% by mass or more and 0.050% by mass or less, and the ratio of the content of Fe to the content of Mn, Fe/Mn, is 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 steps using molten aluminum alloy with the above composition, 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 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.

A molten aluminum alloy can be obtained by heating and melting an aluminum alloy. Alternatively, the aluminum alloy may be formed by melting a mixture containing the elemental elements or compounds containing two or more elements that are the raw materials for the aluminum alloy, in a ratio that produces the desired aluminum alloy. For example, Ti and B may be mixed as grain refiners, such as Al-Ti-B rods, for the purpose of controlling the grain size of the aluminum alloy produced in the casting process.

Also, 10% or more of scrap material of 1000 series, 2000 series, 3000 series, 4000 series, 5000 series, 6000 series, or 7000 series aluminum alloys may be used as raw materials for the molten aluminum alloy, with the remainder being new aluminum ingots and the above-mentioned additive elements, and these may be melted to obtain a molten aluminum alloy having a composition adjusted. In this case, it is possible to obtain an Al-Mg-Si-based aluminum alloy forging that is less prone to recrystallization and has excellent mechanical properties at room temperature. Note that new aluminum ingots are 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.

(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 horizontal continuous casting method.

A horizontal continuous casting apparatus that can be used to manufacture the aluminum alloy casting of this embodiment is shown in FIGS.
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 showing a main portion of the horizontal continuous casting apparatus 10 near the cooling water cavity 24.

The horizontal continuous casting device 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 section 11 is composed of a molten metal inlet section 11a that receives the molten aluminum alloy M obtained in the above-mentioned molten metal forming process, a molten metal holding section 11b, and an outlet section 11c into the hollow section 21 of the mold 12.

The molten metal receiving portion 11 maintains the upper liquid level of the molten aluminum alloy M at a position higher than the upper surface of the hollow portion 21 of the mold 12, and in the case of multiple casting, distributes the molten aluminum alloy M stably 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 body 13. The molten aluminum alloy M supplied into the hollow portion 21 is then cooled and solidified by the cooling device 23 described below, and is drawn out from the other end side 12b of the mold 12 as an aluminum alloy rod B, which is a solidified ingot.

A pull-out drive device (not shown) that pulls out the cast aluminum alloy rod B at a constant speed may be installed on the other end 12b of the mold 12. It is also preferable to install a synchronized cutter (not shown) that cuts the continuously pulled aluminum alloy rod B to the desired length.

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

In this embodiment, the mold 12 is a hollow cylindrical member, and is 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 from 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 (center 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°) with respect to the central axis C of the mold toward the casting direction of the aluminum alloy bar B (see FIG. 1). In other words, the inner peripheral surface 21a is configured in 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°, when the aluminum alloy rod B is pulled out of the mold 12, it encounters resistance at the other end 12b, which is the mold outlet, and casting may become difficult. On the other hand, if the elevation angle exceeds 3°, the contact of the inner peripheral surface 21a with the molten aluminum alloy M may become insufficient, and the effect of removing heat from the molten aluminum alloy M and its solidified shell to the mold 12 may decrease, resulting in insufficient solidification. As a result, a remelted skin may appear on the surface of the aluminum alloy rod B, or unsolidified molten aluminum alloy M may erupt from the end of the aluminum alloy rod B, which is not preferable, as this may lead to casting problems.

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 side 21b) may be selected to match the shape of the aluminum alloy rod to be cast, such as a triangular or rectangular cross-sectional shape, a polygon, a semicircle, an ellipse, or an irregular cross-sectional shape that does not have an axis or plane of symmetry, other than the circular shape of this embodiment.

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

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

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

The cooling water cavity 24 is formed in the mold 12 outside the inner circumferential surface 21a of the hollow portion 21, in a ring shape surrounding the hollow portion 21, 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 from the surface in contact with the inner surface 21a of the mold 12, forming a solidified shell on the surface of the molten aluminum alloy M.

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

In this embodiment, the cooling water W supplied through the cooling water supply pipe 26 is first stored in the cooling water cavity 24 to cool the inner surface 21a of the hollow portion 21 of the mold 12, and then the cooling water W in the cooling water cavity 24 is sprayed toward the aluminum alloy bar B from the cooling water spray passage 25. However, it is also possible to configure the configuration so that each of these is supplied through a separate cooling water supply pipe.

The length from the position where the extension line of the central axis of the shower opening 25a of the cooling water injection passage 25 hits the surface of the cast aluminum alloy bar B to the contact surface between the mold 12 and the refractory plate-like body 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 not possible because a good film is not formed, and if it exceeds 40 mm, the effect of forced cooling is reduced, solidification by the mold wall becomes dominant, and the contact resistance between the mold 12 and the aluminum alloy molten metal M or aluminum alloy bar B increases, which may cause cracks on the casting surface or tearing inside the mold, making the casting unstable, and is not preferable.

It is preferable that the supply of cooling water W to the cooling water cavity 24 and the spraying of cooling water W from the shower opening 25a of the cooling water spray passage 25 can each be controlled by a control signal 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" in this context 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., 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 to 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 is formed so that the thickness t of the cooling wall 27 of the mold 12, i.e., the distance between the inner bottom surface 24a of the cooling water cavity 24 and the inner peripheral surface 21a of the hollow portion 21 of the mold 12, is within a range of, for example, 0.5 mm to 3.0 mm, and preferably 0.5 mm to 2.5 mm. In addition, the material for forming the mold 12 is selected so that the thermal conductivity of at least the cooling wall 27 of the mold 12 is within a range of 100 W/m·K to 400 W/m·K.

In FIG. 4, the molten aluminum alloy M in the molten metal receiving section 11 is supplied to one end 12a of the mold 12, which is held so that the central axis C of the mold is nearly horizontal, via a refractory plate 13, and is forcibly cooled at the other end 12b of the mold 12 to become an aluminum alloy rod B.

The aluminum alloy rod B is pulled out at a constant speed by a pull-out drive device (not shown) installed near the other end 12b of the mold 12, so that it is cast continuously to form a long aluminum alloy rod B. The pulled aluminum alloy rod B is then cut to the desired length, for example, by a synchronous cut-off machine (not shown).

The composition ratio of the cast aluminum alloy rod B can be confirmed, for example, by a method using a photoelectric emission spectrophotometric analyzer (example: Shimadzu PDA-5500, manufactured by 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 receiving portion 11 and the upper inner peripheral surface 21a of the mold 12 is preferably 0 mm to 250 mm (more preferably 50 mm to 170 mm). By setting it in this range, the pressure of the molten aluminum alloy M supplied to the mold 12 and the lubricating oil and the gas produced by vaporizing the lubricating oil are appropriately balanced, resulting in stable castability.

For the liquid lubricant, vegetable oils, which are lubricating oils, can be used. Examples include rapeseed oil, castor oil, and salad oil. These are preferred because they have a small negative 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 of the aluminum alloy rod B may not solidify and may 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 bar B and cause internal defects.

The casting speed, which is the speed at which the aluminum alloy rod B is pulled out of 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, at a casting speed within this range, the network structure of the crystals formed by casting becomes uniform and fine, which increases the resistance of the aluminum matrix to deformation at high temperatures and improves the high-temperature mechanical strength.

The amount of cooling water sprayed from the shower opening 25a of the cooling water spray passage 25 is preferably 10 L/min or more and 50 L/min or less (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 may 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 an internal defect. On the other hand, if the amount of cooling water is more than this range, the mold 12 may lose too much heat, causing it to solidify midway.

The average temperature of the molten aluminum alloy M flowing from the molten metal receiving portion 11 into the mold 12 is, for example, preferably 650°C or higher and 750°C or lower (more preferably 680°C or higher and 720°C or lower). If the temperature of the molten aluminum alloy M is too low, there is a risk that coarse crystals will form in the mold 12 or in front of it and will be incorporated into the aluminum alloy bar B as internal defects. On the other hand, if the temperature of the molten aluminum alloy M is too high, a large amount of hydrogen gas will be easily incorporated into the molten aluminum alloy M, which will be incorporated into the aluminum alloy bar B as porosity and may cause 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, the occurrence of seizure of the aluminum alloy rod B can be prevented.

The cooling wall 27 of the mold 12 receives heat from the molten aluminum alloy M and exchanges the heat by cooling it with the cooling water W contained in the cooling water cavity 24. Regarding the state of this heat exchange, attention is focused on the heat flux per unit area as shown in the explanatory diagram in Fig. 6. The heat flux per unit area is expressed by the following formula (1) according to Fourier's law.
Q = -k × (T1 - T2) / L (1)
Q: heat flux k: thermal conductivity (W/m·K) of the portion through which heat passes (the cooling wall portion 27 of the mold 12 in this embodiment)
T1: Low temperature side temperature of the portion through which heat passes (in this embodiment, the inner bottom surface 24a of the cooling water cavity 24)
T2: High temperature side temperature of the portion through which 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 through (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 show good results even when the amount of lubricating oil is 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 making it possible to prevent seizure of the cast aluminum alloy bar B. In addition, it is preferable that the heat flux value per unit area is 50×10 ⁵ W/m ² or less.

In order to set the cooling wall portion 27 of the mold 12 within this range of heat flux values, 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. In addition, the thermal conductivity of at least the cooling wall portion 27 of the mold 12 may be set in the range of 100 W/m·K or more and 400 W/m·K or less.

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

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 in the cooling wall portion 27 is 10× ¹⁰⁵ W/ ^m2 or more, to cast the aluminum alloy bar B. In addition, when casting the aluminum alloy bar 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 27 is 10×10 ⁵ W/m ² or more, thereby suppressing adhesion of reaction products, such as carbides, caused by contact between the lubricating oil gas and the molten aluminum alloy M. This makes it unnecessary to cut and remove carbides, etc., on 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 any known continuous casting method such as vertical continuous casting can be used. Vertical continuous casting methods are classified into the float method and the hot top method depending on the method of supplying the molten aluminum alloy M to the mold (casting mold 12), but the following 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 spout and through the header, where the flow rate is adjusted, and enters a cylindrical mold that is 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 pulled out of the mold, and the solidification of the metal progresses 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 the introduction of a coolant inside.

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

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

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

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

(Forging process)
The forging process is a process in which the aluminum alloy casting after casting is cut to a predetermined size, the obtained forging material is heated to a predetermined temperature, and then pressure is applied by a press machine to mold it into a die. In this embodiment, 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 of an automobile). If the material heating temperature during forging is less than 500°C, compounds such as AlFeSi and Mg ₂ Si 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.

(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.

In this embodiment, the forged product is subjected to solution treatment by holding it at a treatment temperature of 530°C or more and 560°C or less for 0.3 or more and 3 hours or less. 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 in solid solution. On the other hand, if the treatment temperature exceeds 560°C, the solute elements may be dissolved in solid solution more, but eutectic melting and recrystallization may occur easily. In addition, if the heating rate is less than 5.0°C/min, Mg ₂ Si 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.

(Quenching process)
The quenching process is a process in which the forged product in the solid-solution state obtained in 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 that stores water (quenching water) and quenched by submerging the forged product. The temperature of the water in the tank is preferably 20°C or higher and 60°C or lower. The forged product is preferably placed in the water tank for 5 seconds or higher and 60 seconds or lower after solution treatment so that all surfaces of the forged product are in contact with water. The submersion time of the forged product varies depending on the size of the casting, but is, for example, between more than 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 the elements that are supersaturated in solid solution, thereby imparting an appropriate hardness.

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 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.

In another embodiment of the present invention, a 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 range of 370° C. or higher and 560° C. or lower for 2 hours or higher and 10 hours or lower.
The method for producing an aluminum alloy forged product of this embodiment differs from the method for producing an aluminum alloy forged product of the above-described 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 having the alloy composition (the balance being aluminum) shown in the following Table 1 was prepared. A continuous cast product having a circular cross section and a diameter of 82 mm was produced from the prepared aluminum alloy.

(Manufacturing of aluminum alloy forgings)
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 of 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 performed on the central portion 2a in the longitudinal direction of the long portion 2 in 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.
"X": Over 1.2.

<Conductivity (%IACS)>
The conductivity 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) was measured in accordance with JIS Z2245:2016 "Rockwell hardness test - test method".
(Judgment criteria)
“O”: 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 FIG. 7 to obtain a rectangular column for preparing a test piece for evaluating mechanical properties (impact properties). The obtained rectangular column was processed to prepare a cylindrical test piece for evaluating mechanical properties as shown in FIG. 8. The parallel portion diameter A of the test piece for evaluating mechanical properties was 8.0 mm, and the gauge length G was 30.0 mm. The test piece 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/ ^cm2 or more at room temperature.
"X": The impact value at room temperature is less than 10 J/ ^cm2 .

<Evaluation of refinement, recrystallization and coarsening of crystals>
The central portion 2a of the long portion 2 of the aluminum alloy forging 1a was cut in a direction perpendicular to the surface of the central portion 2a to obtain a plate-shaped body (thickness 2 mm) for preparing a test piece for refinement evaluation. The obtained plate-shaped body was cut into a 7 mm square to obtain a test piece for refinement evaluation of 7 mm x 7 mm x 2 mm thick. 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 were measured using SEM-EBSD (scanning electron microscope-electron backscatter diffraction device) for the surface (cross section of the central portion 2a) of the obtained test piece for refinement evaluation. The number of AlFeSi-based compounds of 3.0 μm or more and the ratio of high-angle grain boundaries obtained were judged based on the following criteria, respectively, to evaluate the refinement of crystal grains and the recrystallization and coarsening of crystal grains. 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 the number of AlFeSi compounds of 3.0 μm or more)
"O": 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 device (field emission scanning electron microscope device) was used to measure the number density of precipitates containing Mn within 2.0 μm including grain boundaries. A sample piece for observing the structure having a size of about 10 mm x 10 mm x 10 mm was cut out from the central part 2a of the long part 2 of the aluminum alloy forging 1a, and this sample piece was polished using a cross section sample preparation device (Cross section polisher). Then, an FE-SEM photograph (field emission scanning electron microscope photograph) of the sample piece after this polishing was taken, and the number density of precipitates containing Mn within 2.0 μm including grain boundaries was obtained in the range of a field area of 1.5815 mm ² in this SEM photograph, and the number density was evaluated according to the following criteria.
(Number density criteria)
"O": 4 particles/ ^μm2 or more.
"X": 4 particles/ ^μm2 or less.

<Overall evaluation>
The seven evaluation results, namely, Fe/Mn ratio, electrical conductivity, Rockwell hardness, impact properties, the number of AlFeSi-based compounds having a size of 3.0 μm or more, the number density of AlMn-based compounds, and the ratio of high-angle grain boundaries having 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-like body (thermal insulation member)
Reference Signs List 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 (8)

1. 1. An aluminum alloy forging material having an alloy composition comprising 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, B in the range of 0.0010% by mass or more and 0.030% by mass or less, and 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) being less than 1.4 in terms of mass ratio, with the remainder being Al and unavoidable impurities,
   An aluminum alloy forging material having 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.
2. 1. An aluminum alloy forging material having an alloy composition comprising 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, B in the range of 0.0010% by mass or more and 0.030% by mass or less, and 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) being 0.3 to 1.2 in mass ratio, with the balance being Al and unavoidable impurities,
   An aluminum alloy forging material having 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.
3. 1. An aluminum alloy forging comprising an aluminum alloy having an alloy composition comprising 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, B in the range of 0.0010% by mass or more and 0.030% by mass or less, and Zr in the range of 0.0010% by mass or more and 0.050% by mass or less, wherein a ratio of the Fe content to the Mn content (Fe/Mn) is less than 1.4 in mass ratio, and the balance is composed of Al and unavoidable impurities,
   An aluminum alloy forging having a number density of Mn-containing precipitates of 4 particles/ ^μm2 or more within 2.0 μm including grain boundaries, a ratio of high-angle grain boundaries having a crystal orientation difference of 15° or more of 27% or less, and an impact value of 10 J/ ^cm2 or more at room temperature.
4. The aluminum alloy forged product according to claim 3, wherein the precipitates have a size of 0.5 μm or less.
5. 1. An aluminum alloy forging comprising an aluminum alloy having an alloy composition comprising 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, B in the range of 0.0010% by mass or more and 0.030% by mass or less, and 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) being 0.3 to 1.2 in mass ratio, with the remainder being Al and unavoidable impurities;
   An aluminum alloy forging having a number density of Mn-containing precipitates of 4 particles/ ^μm2 or more within 2.0 μm including grain boundaries, a ratio of high-angle grain boundaries having a crystal orientation difference of 15° or more of 27% or less, and an impact value of 10 J/ ^cm2 or more at room temperature.
6. The aluminum alloy forged product according to claim 5, wherein the size of the precipitates is 0.5 μm or less.
7. A method for producing an aluminum alloy forged product according to any one of claims 3 to 6,
   a molten metal forming step for obtaining a molten aluminum alloy;
   a casting step of obtaining a casting by casting the obtained molten metal;
   a forging process for heating the cast product at a temperature of 500° C. to the melting point and subjecting it to plastic processing to obtain a forged product;
   A solution treatment process in which the obtained forged product is heated to 20°C to 500°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 contacted 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;
   and an aging treatment step of heating the forged product that has been subjected to the quenching treatment step at a temperature of 180°C to 220°C for 0.5 hours to 8 hours to perform aging treatment.
8. 8. The method for producing an aluminum alloy forged product according to claim 7, further comprising a homogenization heat treatment step between the casting step and the forging step, in which the aluminum alloy cast product is held at a temperature range of 370°C or higher and 560°C or lower for 2 hours or longer and 10 hours or shorter to perform homogenization heat treatment.
   
   

Images