JP7533746B2 - Aluminum alloy forging material, aluminum alloy forging product and manufacturing method thereof - Google Patents

Description

The present invention relates to aluminum alloy forging materials, aluminum alloy forgings, and methods for producing the same.

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.

In addition, iron-based materials have been used exclusively for automobile parts, especially long parts such as suspension arms. However, in recent years, they have often been replaced with aluminum or aluminum alloy materials, mainly for the purpose of reducing weight.

Since these automobile 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 automobile 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 into practical use by forging cast components as they are without extrusion, and then performing 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 Publication No. 5-59477 Japanese Patent Application Publication No. 5-247574 Japanese Patent Application Publication No. 6-256880

In recent years, the demand for aluminum has been increasing as automobiles have become lighter in order to reduce _CO2 emissions. However, aluminum needs to be stronger in order to replace steel. One method for increasing the strength of aluminum is to suppress the formation of recrystallized structures during the plastic processing and solution treatment processes and to refine the grain size.

However, the above-mentioned high-strength Al-Mg-Si alloys have a problem in that the processed structure recrystallizes during the forging and heat treatment processes, resulting in the generation of coarse crystal grains, making it impossible to obtain sufficiently high strength. For this reason, some alloys have been modified 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 casting 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 casting itself is weakened, so that the casting 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 castings, reducing their strength.

Thus, although the addition of Zr is effective in preventing recrystallization, it is difficult to maintain the stability of strength.
The present invention has been made in consideration of the above technical background, and aims to provide an aluminum alloy forging material having excellent mechanical properties at room temperature, an aluminum alloy forging product, and a manufacturing method thereof.

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

(1) Cu in the range of 0.25% by mass or more and 0.55% by mass or less, Mg in the range of 0.85% by mass or more and 1.25% by mass or less, Si in the range of 1.02% by mass or more and 1.4% by mass or less, Mn in the range of 0.55% by mass or more and 1.0% by mass or less, Fe in the range of 0.32% by mass or more and 0.65% 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 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 0.3 to 1.2 by mass, and the balance is Al and unavoidable impurities, and the aluminum alloy forging material has an alloy composition in which the melting start temperature of the aluminum alloy forging material is 560°C to 590°C.

(2) Cu in the range of 0.25% by mass or more and 1.0% by mass or less, Mg in the range of 0.80% by mass or more and 1.8% by mass or less, Si in the range of 0.90% by mass or more and 1.9% by mass or less, Mn in the range of 0.30% by mass or more and 1.2% by mass or less, Fe in the range of 0.20% by mass or more and 0.65% 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 less, % or more and 0.030% or less by mass, Zr in the range of 0.0010% or more and 0.050% or less 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 an alloy composition consisting of Al and unavoidable impurities, and the aluminum alloy forging material has a melting start temperature of 560°C or more and 590°C or less.

(3) Cu in the range of 0.25 mass% or more and 0.55 mass% or less, Mg in the range of 0.85 mass% or more and 1.25 mass% or less, Si in the range of 1.02 mass% or more and 1.4 mass% or less, Mn in the range of 0.61 mass% or more and 1.0 mass% or less, Fe in the range of 0.32 mass% or more and 0.65 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, B in the range of 0.0010 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 the Fe content to the Mn content, Fe/Mn, is 0.3 or more and 1.2 or less by mass, and the balance is an alloy composition consisting of Al and unavoidable impurities, and the aluminum alloy forging material has a melting start temperature of 560°C or more and 590°C or less.

(4) An aluminum alloy forging having a long portion, comprising: Cu in the range of 0.25% by mass to 0.55% by mass; Mg in the range of 0.85% by mass to 1.25% by mass; Si in the range of 1.02% by mass to 1.4% by mass; Mn in the range of 0.55% by mass to 1.0% by mass; Fe in the range of 0.32% by mass to 0.65% by mass; Zn in the range of 0.25% by mass or less; Cr in the range of 0.050% by mass to 0.30% by mass; Ti in the range of 0.01% by mass to 0.1% by mass; an aluminum alloy forging comprising: a Fe content in the range of 10% 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 terms of mass ratio, of 0.3 to 1.2, with the balance being made of Al and unavoidable impurities; a cross section of a central portion in the longitudinal direction of the long portion has an average crystal grain size of an alloy structure in the range of 5 μm to 60 μm and does not contain an AIFeSi(Mn)-based compound having an average crystal grain size of 3.0 μm or more; and a fatigue limit at ¹⁰⁷ cycles to fracture in terms of smooth fatigue properties at room temperature is 150 MPa or more.

(5) An aluminum alloy forging having a long portion, comprising Cu in the range of 0.25 mass% or more and 1.0 mass% or less, Mg in the range of 0.80 mass% or more and 1.8 mass% or less, Si in the range of 0.90 mass% or more and 1.9 mass% or less, Mn in the range of 0.30 mass% or more and 1.2 mass% or less, Fe in the range of 0.20 mass% or more and 0.65 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. % or more, and Zr 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 less than 1.4 in terms of mass ratio, with the remainder having an alloy composition consisting of Al and unavoidable impurities, a cross section of a central portion in the longitudinal direction of the long portion has an average crystal grain size of an alloy structure in the range of 5 μm or more and 60 μm or less and does not contain an AIFeSi(Mn)-based compound having an average crystal grain size of 3.0 μm or more, and a fatigue limit at ¹⁰⁷ cycles to fracture in terms of smooth fatigue properties at room temperature is 150 MPa or more.

(6) An aluminum alloy forging having a long portion, comprising: Cu in the range of 0.25% by mass or more and 0.55% by mass or less; Mg in the range of 0.85% by mass or more and 1.25% by mass or less; Si in the range of 1.02% by mass or more and 1.4% by mass or less; Mn in the range of 0.61% by mass or more and 1.0% by mass or less; Fe in the range of 0.32% by mass or more and 0.65% 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; an aluminum alloy forging comprising: a Fe content in the range of 10% 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 terms of mass ratio, of 0.3 to 1.2, with the balance being made of Al and unavoidable impurities; a cross section of a central portion in the longitudinal direction of the long portion has an average crystal grain size of an alloy structure in the range of 5 μm to 60 μm and does not contain an AIFeSi(Mn)-based compound having an average crystal grain size of 3.0 μm or more; and a fatigue limit at ¹⁰⁷ cycles to fracture in terms of smooth fatigue properties at room temperature is 150 MPa or more.

(7) An aluminum alloy forged product according to any one of (4) to (6) above, which is used for a suspension arm.

(8) A method for manufacturing an aluminum alloy forged product according to any one of (4) to (7) above, comprising: a molten alloy forming process for forming an aluminum alloy molten having the same alloy composition as the aluminum alloy forged product; a casting process for cooling and solidifying the aluminum alloy molten obtained in the molten alloy forming process to form an aluminum alloy cast product; a forging process for forging the aluminum alloy cast product obtained in the casting process at a heating temperature of 450°C or higher and 560°C or lower; and a forging process for the forged product obtained in the forging process. A method for manufacturing an aluminum alloy forged product, comprising: a solution treatment process in which a solution treatment is performed at a treatment temperature of 530°C to 560°C and held for 0.3 hours to 3 hours; a quenching process in which the entire surface of the forged product is brought into contact with quenching water within a range of 5 seconds to 60 seconds after the completion of the solution treatment process, and quenched in a water tank for 1 minute to 30 minutes; and an aging process in which the forged product after the quenching process is aged at a heating temperature of 170°C to 210°C for 0.5 hours to 7 hours.

(9) The method for producing an aluminum alloy forged product described in (8) above further includes a homogenization heat treatment step between the casting step and the forging step, in which the aluminum alloy casting product is subjected to homogenization heat treatment by holding the aluminum alloy casting 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 present invention makes it possible to provide aluminum alloy forging materials, aluminum alloy forgings, and methods for manufacturing the same that have excellent mechanical properties at room temperature.

1 is a perspective view showing an example of an aluminum alloy forging according to an embodiment of the present invention. FIG. FIG. 4 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 forged product 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 the 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.

The following describes in detail an embodiment of the present invention with reference to the drawings. Note that 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 features easier to understand, and the dimensional ratios of each component may not necessarily be the same as in reality. In addition, 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 modified as appropriate within the scope that does not change the effects of the present invention.

[Aluminum alloy materials]
First, the aluminum alloy material of the present invention will be described.

An aluminum alloy material according to one 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 0.85% by mass to 1.25% by mass, Si in the range of 1.02% by mass to 1.4% by mass, Mn in the range of 0.55% by mass to 1.0% by mass, Fe in the range of 0.32% by mass to 0.65% by mass, Zn in the range of 0.25% by mass or less, Cr in the range of 0.050% by mass to 0.30% by mass, and Ti in the range of 0.01% by mass or less. It is an aluminum alloy forging material that contains 0.1% by mass or less of Zr, 0.0010% by mass or more and 0.030% by mass or less of B, 0.0010% by mass or more and 0.050% by mass or less of Zr, the ratio of the Fe content to the Mn content, Fe/Mn, is 0.3 to 1.2 by mass, and the balance is made of Al and unavoidable impurities, and the melting start temperature of this aluminum alloy forging material is 560°C to 590°C.

In another embodiment of the present invention, the aluminum alloy material contains Cu in the range of 0.25% by mass to 1.0% by mass, Mg in the range of 0.80% by mass to 1.8% by mass, Si in the range of 0.90% by mass to 1.9% by mass, Mn in the range of 0.30% by mass to 1.2% by mass, Fe in the range of 0.20% by mass to 0.65% by mass, Zn in the range of 0.25% by mass or less, 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. , 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 Fe content to Mn content Fe/Mn is less than 1.4 in mass ratio, the balance is an alloy composition consisting of Al and unavoidable impurities, and the aluminum alloy forging material has a melting start temperature of 560°C to 590°C.

In yet another embodiment of the present invention, the aluminum alloy material contains Cu in the range of 0.25% by mass to 0.55% by mass, Mg in the range of 0.85% by mass to 1.25% by mass, Si in the range of 1.02% by mass to 1.4% by mass, Mn in the range of 0.61% by mass to 1.0% by mass, Fe in the range of 0.32% by mass to 0.65% by mass, Zn in the range of 0.25% by mass or less, 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. , 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 0.3 to 1.2 in mass ratio, and the balance is an alloy composition consisting of Al and unavoidable impurities, and the aluminum alloy forging material has a melting start temperature of 560°C to 590°C.

[Aluminum alloy forgings]
Next, 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 forged product 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 in the longitudinal direction of the long portion 2. The long portion has a rectangular cross section. The aluminum alloy forging 1a having this shape can be used as, for example, an I-type suspension arm.

The aluminum alloy forged product 1a has an aluminum alloy composition containing Cu in the range of 0.25 mass% or more and 0.55 mass% or less, Mg in the range of 0.85 mass% or more and 1.25 mass% or less, Si in the range of 1.02 mass% or more and 1.4 mass% or less, Mn in the range of 0.55 mass% or more and 1.0 mass% or less, Fe in the range of 0.32 mass% or more and 0.65 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, B in the range of 0.0010 mass% or more and 0.030 mass% or less, and 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) being 0.3 to 1.2 in mass ratio, with the balance being Al and unavoidable impurities. In addition, the cross section of the central portion 2a in the longitudinal direction of the long portion 2 of the aluminum alloy forged product 1a has an average crystal grain size of the alloy structure in the range of 5 μm to 60 μm, does not contain AIFeSi(Mn)-based compounds having an average grain size of 3.0 μm or more, and has a fatigue limit of 150 MPa or more at ¹⁰⁷ cycles to fracture in terms of smooth fatigue properties at room temperature.

The aluminum alloy that is the material of the aluminum alloy forged product 1a in this embodiment corresponds to a 6000 series aluminum alloy in that it contains Mg and Si.

(Cu: 0.25% by mass or more and 0.55% by mass or less)
Cu acts to finely disperse Mg-Si compounds in the aluminum alloy and to improve the tensile strength of the aluminum alloy by precipitating as Al-Cu-Mg-Si compounds including the Q phase. When the Cu content is within the above range, the mechanical properties of the aluminum alloy forged product 1a at room temperature can be improved.

(Mg: 0.85 mass% or more and 1.25 mass% or less)
Mg has the effect of improving the tensile strength of aluminum alloys. Mg dissolves in the aluminum matrix, or forms Mg-Si compounds (Mg ₂ Si) such as the β″ phase, or the Q phase. Mg ₂ Si contributes to strengthening the aluminum alloy by precipitating as an Al-Cu-Mg-Si compound (AlCuMgSi) that is a compound that inhibits the formation of the CuAl ₂ phase in the aluminum alloy. By suppressing the formation of the _CuAl2 phase, the corrosion resistance of the aluminum alloy forged product 1a is improved. By having the Mg content within the above range, the mechanical properties of the aluminum alloy forged product 1a at room temperature are improved. It is possible to improve both the properties and corrosion resistance.

(Si: 1.02 mass% or more and 1.4 mass% or less)
Like Mg, Si has the effect of improving the mechanical properties and corrosion resistance of the aluminum alloy forged product 1a at room temperature. However, if excessive Si is added to the aluminum alloy, coarse primary crystal Si grains will crystallize. By setting the Si content within the above range, the crystallization of primary Si crystals is suppressed, and the mechanical properties of the aluminum alloy forged product 1a at room temperature are improved. It is possible to improve the mechanical properties as well as the corrosion resistance.

(Mn: 0.55% by mass or more and 1.0% by mass or less)
Mn forms fine granular precipitates, including intermetallic compounds such as Al-Mn-Fe-Si and Al-Mn-Cr-Fe-Si, in aluminum alloys, and thereby increases the tensile strength of the aluminum alloy. When the Mn content is within the above range, the mechanical properties of the aluminum alloy forged product 1a at room temperature can be improved.

(Fe: 0.32 mass% or more and 0.65 mass% or less)
Fe is a fine particle that contains intermetallic compounds such as Al-Mn-Fe-Si, Al-Mn-Cr-Fe-Si, Al-Fe-Si, Al-Cu-Fe, and Al-Mn-Fe in aluminum alloys. The Fe content in the above range improves the mechanical properties of the aluminum alloy forging 1a at room temperature. can be improved.
The Fe/Mn relationship is 0.3 or more and 1.2 or less. By keeping the Fe/Mn relationship at 1.2 or less, it is possible to suppress the crystallization of AlFeSi-based compounds having a size of 3.0 μm or more. This makes it possible to improve the mechanical properties.

(Cr: 0.050 mass% or more, 0.30 mass% or less)
Cr improves the tensile strength of aluminum alloys by forming fine granular precipitates containing intermetallic compounds such as Al-Mn-Cr-Fe-Si and Al-Fe-Cr in the aluminum alloy. When the Cr content is within the above range, the mechanical properties of the aluminum alloy forged product 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 the wrought workability. If the Ti content is less than 0.01 mass%, the effect of refining the crystal grains may not be sufficiently obtained. On the other hand, if the Ti content exceeds 0.1 mass %, coarse crystallized products are formed, and the wrought workability may be deteriorated. If a large amount of such crystallized substances are mixed in, the toughness may decrease. Therefore, the Ti content is set to 0.012 mass % or more and 0.035 mass % or less. The Ti content is preferably 0. The content of the Cr-based alloy is 0.015 mass % or more and 0.050 mass % or less.

(B: 0.0010% by mass or more, 0.030% by mass or less)
B has the effect of refining the crystal grains of an aluminum alloy and improving the wrought workability. By adding B to an aluminum alloy together with the above-mentioned Ti, the effect of refining the crystal grains is improved. 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 are formed. There is a risk that B may be mixed as an inclusion into the aluminum alloy forged product 1a. Furthermore, if a large amount of coarse crystals containing B are mixed into the final aluminum alloy product, the toughness may decrease. The content of B is set to 0.0010% by mass or more and 0.030% by mass or less. The content of B is preferably 0.0050% by mass or more and 0.025% by mass or less.

(Zr: 0.0010 mass% or more, 0.05 mass% or less)
When the content of Zr is 0.05 mass % or less, it precipitates in the form of Al ₃ Zr and Al-(Ti, Zr), and thus has a recrystallization suppressing effect and improves the strength of the aluminum alloy forged product 1a by precipitation strengthening. If the Zr content exceeds 0.050 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. The Zr content is set to 0.050 mass% or less. In order to obtain the above-mentioned recrystallization suppression effect and the effect of improving the strength of the forged product by precipitation strengthening, the Zr content is set to 0.0010 mass% or more. It is preferable that there is.

(Zn: 0.250% by mass or less)
The Zn content may be 0.250 mass% or less. If the Zn content exceeds 0.250 mass%, _MgZn2 is generated and precipitates at the grain boundaries from the Al matrix, causing intergranular corrosion and aluminum. This leads to a decrease in the corrosion resistance of the alloy forged product. Therefore, it is preferable that the Zn content is 0.250 mass % or less, or that no Zn is 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 within a 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, within a range of 2 to 100. The cross section of the central portion 2a of the long portion 2 is a cross section along the direction in which pressure is applied when manufacturing the aluminum alloy forging 1a by forging (hereinafter, sometimes referred to as the central cross section).

Here, the crystal grain size was defined as the diameter of a circle (equivalent circle diameter) having an area equal to that of each crystal grain. The area of the crystal grains was calculated from images of the crystal grains obtained using a scanning electron microscope (SEM) or electron backscatter diffraction (EBSD). The average crystal grain size was defined as the average value of the equivalent circle diameters of 420 or more crystal grains. The average crystal grain size may be calculated, for example, using commercially available image analysis software.

(Average grain size: 5 μm or more, 60 μm or less)
The average grain size of the alloy structure in the cross section of the central portion of the aluminum alloy forging 1a is an index of the degree of grain refinement of the long portion 2 of the aluminum alloy forging 1a. In the structure of the cross section of the central portion 2a in the direction, if the average crystal grain size of the crystal grains exceeds 60 μm, the tensile properties and fatigue properties of the long portion 2 may be deteriorated due to the Hall-Petch law. If the crystal grain size is less than 5 μm, the toughness of the long portion 2 may deteriorate, and the impact resistance may decrease. The diameter is in the range of 5 μm to 60 μm.

In the cross-sectional structure of the longitudinal center portion 2a of the long portion 2 of the aluminum alloy forged product 1a, the standard deviation of the crystal grain size is not particularly limited, but is preferably 30 or less. This is because if the variation in the crystal grain size is suppressed so that the standard deviation of the crystal grain size is 30 or less, the impact resistance of the long portion 2 of the aluminum alloy forged product 1a is increased. It is more preferable that the standard deviation of the crystal grain size is 25 or less, and even more preferable that it is 20 or less.

(Does not contain AIFeSi(Mn) based compounds with an average particle size of 2.0 μm or more)
The alloy structure in the central cross section of the aluminum alloy forged product 1a should not contain AIFeSi(Mn)-based compounds with an average particle size of 2.0 μm or more, preferably 3.0 μm or more. If AIFeSi(Mn)-based compounds with an average particle size of 1.5 μm or more are present, there is a risk of a decrease in mechanical properties (tensile properties/smooth fatigue properties, etc.). Even if the AIFeSi(Mn)-based compounds have an average particle size of less than 2.0 μm, it is preferable that the cross-sectional area ratio of the AIFeSi(Mn)-based compounds in a cross-sectional structure with a field area of 1.5815 ^mm2 is 12% or less.

(Fatigue limit in repeated cycles is 150 MPa or more)
The central cross section of the aluminum alloy forged product 1a has mechanical properties such that the fatigue limit at ¹⁰⁷ cycles to fracture is 150 MPa or more in the smooth fatigue properties at room temperature (20°C). If this fatigue limit is less than 150 MPa, there is a risk of the durability of the part being reduced.

(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. A ratio of high-angle grain boundaries of 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, which has the above-mentioned configuration, is made of the aluminum alloy having 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 having an average crystal grain size in the range of 10 μm to 40 μm and not including an AIFeSi(Mn)-based compound having an average grain size of 3.0 μm or more. Therefore, the central portion 2a of the long portion 2 has high tensile properties and smooth fatigue properties, excellent toughness, and improved impact resistance. Furthermore, the aluminum alloy forging 1a of the present embodiment has a fatigue limit of 150 MPa or more at 10 ⁷ cycles to fracture of the central portion 2a of the long portion 2 of the aluminum alloy forging 1a, and has durability comparable to that of iron-based metal materials.

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, so it can be advantageously used as a suspension arm for vehicles such as automobiles.

In the aluminum alloy forged product 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 is shaped so that its width increases from the end edge of one connecting portion 4a toward the end edge of the other connecting portion 4b, but the shape of the aluminum alloy forged product 1a is not limited to this. For example, one connecting portion 4a and the other connecting portion 4b of the aluminum alloy forged product 1a may have the same shape. The width of the long portion 2 may be constant. The long portion 2 may also be curved. 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 portion 4c and the connecting portion 4d are connected by a long portion 2, and the connecting portion 4d and the connecting portion 4e are connected by a short portion 5 that is relatively shorter than the long portion 2. 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, and the connecting portions 4f and 4h are each connected by a long portion 2. This aluminum alloy forging 1b can be used as, for example, an A-type suspension arm.

[Method of 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 producing an aluminum alloy forged product of this embodiment includes, for example, a molten metal forming step, a casting step, a homogenizing heat treatment step, a forging step, a solution treatment step, a quenching treatment step, and an aging treatment step. Among these steps, the solution treatment step may be performed as necessary and is not an essential step.

(Molten metal forming process)
The molten metal forming step is a step in which raw materials are melted to obtain a molten aluminum alloy having a composition adjusted. The composition of the molten aluminum alloy is the same as that of the aluminum alloy forging. That is, a molten 6000 series aluminum alloy is obtained by adjusting the alloy composition so that it contains Cu in the range of 0.25 mass% or more and 0.55 mass% or less, Mg in the range of 0.85 mass% or more and 1.25 mass% or less, Si in the range of 1.02 mass% or more and 1.4 mass% or less, Mn in the range of 0.55 mass% or more and 1.0 mass% or less, Fe in the range of 0.32 mass% or more and 0.65 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, B in the range of 0.0010 mass% or more and 0.030 mass% or less, and Zr in the range of 0.0010 mass% or more and 0.050 mass% or less, and the ratio of the Fe content to the Mn content, Fe/Mn, is 0.3 to 1.2 in mass ratio, with the balance being Al and unavoidable impurities.
By carrying out the subsequent steps using the molten aluminum alloy of the above composition, it is possible to obtain an Al-Mg-Si aluminum alloy forging that is less susceptible to recrystallization and has excellent mechanical properties at room temperature. Note that virgin aluminum ingot is aluminum with a concentration of 99% or more that is 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 in order to control the grain size of the aluminum alloy produced in the casting process.

Also, the raw material for the molten aluminum alloy may be 10% or more of scrap material of 1000 series, 2000 series, 3000 series, 4000 series, 5000 series, 6000 series, or 7000 series aluminum alloys, with the remainder being new aluminum ingots and the above-mentioned additive elements, and the molten aluminum alloy may be obtained by melting these and adjusting the composition. 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, 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 section 11 maintains the upper liquid level of the molten aluminum alloy M at a position higher than the upper surface of the hollow section 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 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.

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

The refractory plate 13 is a member that blocks heat transfer between the molten metal receptacle 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 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 the 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. 4). 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 the 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 contact of the inner peripheral surface 21a with the molten aluminum alloy M may be insufficient, and the effect of removing heat from the molten aluminum alloy M and its solidified shell to the mold 12 may decrease, causing 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 because it 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, in addition to the circular shape of this embodiment.

A fluid supply pipe 22 is disposed at 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 may 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.

Inside the mold 12, a cooling device 23 is formed 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, in addition to 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, these can also be configured to be supplied by separate cooling water supply pipes.

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 these cooling water cavities 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. 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 from 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 Corporation, PDA-5500) 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 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 little adverse effect 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 may leak from the mold 12 due to insufficient lubrication. If the supply rate is too high, the surplus may get mixed into the aluminum alloy rod 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 a non-uniform 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 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, 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.

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 bar 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, 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 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 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 refrigerant inside.

The refrigerant to be used can be selected from those that are commercially available, but water is recommended from the viewpoint 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. The material and size of the header can be appropriately selected depending on the composition range of the alloy to be cast and the dimensions of the cast product, and are not particularly limited.

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.

(Homogenization heat treatment process)
The homogenization heat treatment step is a step in which homogenization heat treatment is performed on the aluminum alloy casting obtained in the casting step to homogenize microsegregation caused by solidification, precipitate supersaturated solid solution elements, and change metastable phases to equilibrium phases.
Incidentally, such a homogenization heat treatment step may be carried out as necessary, and the forging step may be directly carried out after the casting step.

In this embodiment, the casting obtained in the casting process is subjected to homogenization heat treatment, in which the casting is held at a temperature of 370°C or higher and 560°C or lower for 2 to 10 hours. By carrying out homogenization heat treatment in this temperature range, the casting is homogenized sufficiently and solute atoms are sufficiently dissolved, so that the sufficient strength required for the subsequent aging treatment can be obtained.

(Forging process)
The forging process is a process in which an aluminum alloy casting after casting or after the homogenization heat treatment process is shaped into a predetermined size to obtain a forging material, the obtained forging material is heated to a predetermined temperature, and then pressure is applied in a press to mold it into a die.

In this embodiment, the forging material is forged at a heating temperature of 450°C or more and 560°C or less to obtain a forged product (such as an automobile suspension arm part). At this time, the starting temperature for forging the forging material is 450°C or more and 560°C or less. If the starting temperature is less than 450°C, the deformation resistance becomes too high and sufficient processing becomes impossible, and if it exceeds 560°C, defects such as forging cracks and 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 after the solution treatment for 5 seconds or higher and 60 seconds or lower 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 forged product, 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 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 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.

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

[Examples 1 to 23 and Comparative Examples 1 to 3]
(Production of continuous cast products)
First, an aluminum alloy composition having the alloy composition shown in Table 1 below (the balance being aluminum) was prepared. Continuously cast products having a circular cross section and a diameter of 49 mm were produced using the prepared aluminum alloy composition . The continuous cast products of Examples 1 and 5, the continuous cast products of Examples 2 and 6, the continuous cast products of Examples 3 and 7, and the continuous cast products of Examples 4 and 8 each have the same alloy composition, but differ in the conditions of the treatment process when producing an aluminum alloy forged product. Moreover, in Examples 9 to 23, the homogenization heat treatment process was not performed.

(Manufacturing of aluminum alloy forgings)
Next, the obtained continuous cast product was subjected to a homogenization heat treatment step (none in Examples 9 to 23), a forging process step, a solution treatment process, a quenching process, and an artificial aging process 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, the forging process step, the solution treatment process, the quenching process, and the artificial aging process are shown in Table 2 below.

[evaluation]
The aluminum alloy forging materials and aluminum alloy forgings 1a of Examples 1 to 23 and Comparative Examples 1 to 3 obtained as described above were evaluated for the central portion 2a in the longitudinal direction of the long portion 2. The evaluation results of the central portion 2a of the long portion 2 are shown in Table 3 below.

<Evaluation of melting start temperature>
The melting start temperature of aluminum alloy forging materials was measured, and an evaluation was made as to whether the melting start temperature was within the range of 560°C to 590°C.
(Judgment criteria)
"Good": The melting start temperature is within the range of 560°C to 590°C.
"X": The melting start temperature is outside the range of 560°C to 590°C.

<Mechanical properties (smooth fatigue properties) 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 (smooth fatigue 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 fatigue life of the test piece for evaluating mechanical properties was measured by performing a rotating bending fatigue test at room temperature (25° C.). The obtained fatigue life results were evaluated based on the following criteria.
(Judgment criteria)
"Good": The fatigue limit at ¹⁰⁷ cycles to fracture is 150 MPa or more.
"X": The fatigue limit at ¹⁰⁷ cycles to fracture is less than 150 MPa.

<Component: Fe/Mn ratio>
The aluminum alloy forging material was evaluated to see whether its Fe/Mn ratio was within the range of 0.3 to 1.2.
(Judgment criteria)
"Good": The Fe/Mn ratio is within the range of 0.3 to 1.2.
"X": The Fe/Mn ratio is outside the range of 0.3 or more and 1.2 or less.

<Forged product: Number of AIMnFeSi compounds of 3.0 μm or more>
The number of AIMnFeSi compound particles having a size of 3.0 μm or more in the aluminum alloy forging was counted.
(Judgment criteria)
"Good": No AIMnFeSi compound particles of 3.0 μm or more were present (0 particles).
"X": AIMnFeSi compound particles of 3.0 μm or more are present (one or more).

<Evaluation of refinement, recrystallization and coarsening of crystals>
The central portion 2a of the long portion 2 of the aluminum alloy forged product 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 surface (cross section of the central portion 2a) of the obtained test piece for refinement evaluation was measured using a SEM-EBSD (scanning electron microscope-electron backscatter diffraction device) to measure the average grain size, the standard deviation of the grain size, and the ratio of high-angle grain boundaries with a crystal orientation difference of 15° or more. The obtained average grain size, the standard deviation of the grain size, and the ratio of high-angle grain boundaries were each judged based on the following criteria to evaluate the refinement of the crystal grains and the recrystallization and coarsening of the 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°.

(Average grain size criteria)
"O": 5 μm or more and 40 μm or less.
"X": Less than 5 μm or more than 40 μm.
(Criteria for determining standard deviation of crystal grain size)
"O": 30 or less.
"X": Over 30.
(Criteria for determining the ratio of high angle grain boundaries)
"O": Less than 27%.
"X": Over 27%.

<Overall evaluation>
The evaluation results of the melting start temperature, Fe/Mn ratio, fatigue properties at room temperature, the number of AIFeSi(Mn)-based compounds having an average crystal grain size of 1.5 μm or more, refinement evaluation, and recrystallization/crystal coarsening evaluation were evaluated based on the following criteria.
(Judgment criteria)
"O": All evaluation items are "O".
"X": At least one of the evaluation items is "X".

As shown in Table 3, by setting the content of each element within a prescribed range and carrying out each of the molten metal forming process, casting process, homogenization heat treatment process, forging process, solution treatment process, quenching process and aging treatment process under prescribed conditions within the prescribed ranges, it has been confirmed that it is possible to produce an aluminum alloy forged product in which sufficiently fine crystal grains can be formed in the center of the long section in the longitudinal direction with suppressed grain size variation, crystal coarsening due to recrystallization is suppressed, and an aluminum alloy forged product having excellent mechanical properties at room temperature can be obtained.

DESCRIPTION OF THE REFERENCE NUMERALS 1a, 1b, 1c...Aluminum alloy forging 2...Long portion 2a...Central portion 4, 4a, 4b, 4c, 4d, 4e, 4f, 4g, 4h...Connecting portion 5...Short portion 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 (insulating 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 aluminum alloy W: Cooling water

Claims (9)

An aluminum alloy forging having a long portion,
an alloy composition comprising Cu in the range of 0.25 mass% or more and 0.55 mass% or less, Mg in the range of 0.85 mass% or more and 1.25 mass% or less, Si in the range of 1.02 mass% or more and 1.4 mass% or less, Mn in the range of 0.55 mass% or more and 1.0 mass% or less, Fe in the range of 0.32 mass% or more and 0.65 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, B in the range of 0.0010 mass% or more and 0.030 mass% or less, and 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) being 0.3 to 1.2 in mass ratio, with the balance being Al and unavoidable impurities;
An aluminum alloy forged product, wherein a cross section of a central portion in a longitudinal direction of the long portion has an average crystal grain size of an alloy structure in a range of 5 μm to 60 μm and does not contain an AlFeSi (Mn)-based compound having an average crystal grain size of 3.0 μm or more, and wherein the fatigue limit at ¹⁰⁷ cycles to fracture is 150 MPa or more in smooth fatigue properties at room temperature. An aluminum alloy forging having a long portion,
an alloy composition comprising Cu in the range of 0.25 mass% or more and 1.0 mass% or less, Mg in the range of 0.80 mass% or more and 1.8 mass% or less, Si in the range of 0.90 mass% or more and 1.9 mass% or less, Mn in the range of 0.30 mass% or more and 1.2 mass% or less, Fe in the range of 0.20 mass% or more and 0.65 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, B in the range of 0.0010 mass% or more and 0.030 mass% or less, and Zr in the range of 0.0010 mass% or more and 0.050 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 forged product, wherein a cross section of a central portion in a longitudinal direction of the long portion has an average crystal grain size of an alloy structure in a range of 5 μm to 60 μm and does not contain an AlFeSi (Mn)-based compound having an average crystal grain size of 3.0 μm or more, and wherein the fatigue limit at ¹⁰⁷ cycles to fracture is 150 MPa or more in smooth fatigue properties at room temperature. An aluminum alloy forging having a long portion,
an alloy composition comprising Cu in the range of 0.25 mass% or more and 0.55 mass% or less, Mg in the range of 0.85 mass% or more and 1.25 mass% or less, Si in the range of 1.02 mass% or more and 1.4 mass% or less, Mn in the range of 0.61 mass% or more and 1.0 mass% or less, Fe in the range of 0.32 mass% or more and 0.65 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, B in the range of 0.0010 mass% or more and 0.030 mass% or less, and 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) being 0.3 to 1.2 in mass ratio, with the balance being Al and unavoidable impurities;
An aluminum alloy forged product, wherein a cross section of a central portion in a longitudinal direction of the long portion has an average crystal grain size of an alloy structure in a range of 5 μm to 60 μm and does not contain an AlFeSi (Mn)-based compound having an average crystal grain size of 3.0 μm or more, and wherein the fatigue limit at ¹⁰⁷ cycles to fracture is 150 MPa or more in smooth fatigue properties at room temperature. 4. The aluminum alloy forged product according to claim 1 , which is used for a suspension arm. A method for producing an aluminum alloy forged product according to any one of claims 1 to 3 ,
a molten alloy forming step of forming a molten aluminum alloy having the same alloy composition as the aluminum alloy forged product;
a casting step of cooling and solidifying the molten aluminum alloy obtained in the molten alloy forming step to form an aluminum alloy casting;
a forging step of forging the aluminum alloy casting having undergone the casting step at a heating temperature of 450° C. or more and 560° C. or less;
a solution treatment step in which the forged product obtained in the forging step is subjected to solution treatment at a treatment temperature of 530° C. or higher and 560° C. or lower for 0.3 hours or higher and 3 hours or lower;
a quenching process in which the entire surface of the forged product is brought into contact with quenching water within a range of 5 seconds to 60 seconds after the completion of the solution treatment process, and quenched in a water tank for 1 minute to 30 minutes;
and an aging treatment step of subjecting the forged product after the quenching treatment step to an aging treatment at a heating temperature of 170°C or higher and 210°C or lower for a period of 0.5 hours or higher and 7 hours or lower. 6. The method for producing an aluminum alloy forged product according to claim 5, 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. Cu in the range of 0.25 mass% or more and 0.55 mass% or less, Mg in the range of 0.85 mass% or more and 1.25 mass% or less, Si in the range of 1.02 mass% or more and 1.4 mass% or less, Mn in the range of 0.55 mass% or more and 1.0 mass% or less, Fe in the range of 0.32 mass% or more and 0.65 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 2. An aluminum alloy forging material for use in the aluminum alloy forging product according to claim 1, comprising an alloy composition comprising: 0.1% by mass or less of Fe, 0.0010% by mass or more and 0.030% by mass or less of B, 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, in terms of mass ratio, of 0.3 to 1.2 , with the balance being Al and unavoidable impurities;
The aluminum alloy forging material has a melting start temperature of 560°C or higher and 590°C or lower. Cu in the range of 0.25 mass% or more and 1.0 mass% or less, Mg in the range of 0.80 mass% or more and 1.8 mass% or less, Si in the range of 0.90 mass% or more and 1.9 mass% or less, Mn in the range of 0.30 mass% or more and 1.2 mass% or less, Fe in the range of 0.20 mass% or more and 0.65 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, 3. An aluminum alloy forging material for use in an aluminum alloy forging having a long portion according to claim 2, comprising: 0.010% by mass or less of B; 0.0010% by mass or more and 0.030% by mass or less of Zr; a ratio of Fe to Mn (Fe/Mn) of less than 1.4 in terms of mass ratio, with the balance being Al and unavoidable impurities;
The aluminum alloy forging material has a melting start temperature of 560°C or higher and 590°C or lower. Cu in the range of 0.25 mass% or more and 0.55 mass% or less, Mg in the range of 0.85 mass% or more and 1.25 mass% or less, Si in the range of 1.02 mass% or more and 1.4 mass% or less, Mn in the range of 0.61 mass% or more and 1.0 mass% or less, Fe in the range of 0.32 mass% or more and 0.65 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 5. An aluminum alloy forging material for use in an aluminum alloy forging having a long portion according to claim 3, comprising: 0.0010% by mass or less of Fe; 0.0010% by mass or more of B; and 0.030% by mass or less of Zr. The ratio of the Fe content to the Mn content , Fe /Mn, is 0.3 or more and 1.2 or less in mass ratio, with the balance being Al and unavoidable impurities.
The aluminum alloy forging material has a melting start temperature of 560°C or higher and 590°C or lower.

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