Aluminum alloy forging material, aluminum alloy forged product, and method for manufacturing same

Abstract

This aluminum alloy forged product is composed of 0.25 mass % or more and 0.55 mass % or less of Cu, 0.60 mass % or more and 1.25 mass % or less of Mg, 0.90 mass % or more and 1.4 mass % or less of Si, 0.35 mass % or more and 0.60 mass % or less of Mn, 0.15 mass % or more and 0.30 mass % or less of Fe, 0.25 mass % or less of Zn, 0.050 mass % or more and 0.30 mass % or less of Cr, 0.01 mass % or more and 0.1 mass % or less of Ti, 0.0010 mass % or more and 0.030 mass % or less of B, 0.0010 mass % or more and 0.050 mass % or less of Zr with a remainder being made up of Al and unavoidable impurities.

Description

TECHNICAL FIELD

[0001] The present invention relates to an aluminum alloy forging material, an aluminum alloy forged product, and a method for manufacturing the same.

[0002] Priority is claimed on Japanese Patent Application No. 2022-210255, filed Dec. 27, 2022, the content of which is incorporated herein by reference.BACKGROUND ART

[0003] In recent years, the applications of aluminum alloys as structural members for a variety of products have been expanding utilizing their light weights. For example, for suspension systems or bumper parts of cars, high tensile strength steel has been thus far used. Incidentally, in recent years, high-purity aluminum alloy materials have come into use.

[0004] In addition, for car parts, particularly, for example, suspension parts, only iron-based materials have been used. Incidentally, in recent years, iron-based materials have been often replaced by aluminum materials or aluminum alloy materials for the main purpose of weight reduction.

[0005] For these car parts, since excellent corrosion resistance, high strength, and excellent processability are required, Al—Mg—Si-based alloys, particularly, A6061, are frequently used as aluminum alloy materials. Such car parts are manufactured by performing forging, which is a single type of plastic working, on aluminum alloy materials as materials for working.

[0006] In addition, recently, since there has been a need for cost reduction, suspension parts obtained by forging a cast member as it is as a material without performing extrusion and then performing a treatment in which a solution treatment and an artificial aging treatment are performed (T6 treatment) have been put into practical use, and development of a high strength alloy that replaces conventional A6061 has been underway for the purpose of additional weight reduction (for example, refer to Patent Documents 1 to 3).CITATION LISTPatent DocumentPatent Document 1: Japanese Unexamined Patent Application, First Publication No. H5-59477

[0008] Patent Document 2: Japanese Unexamined Patent Application, First Publication No. H5-247574

[0009] Patent Document 3: Japanese Unexamined Patent Application, First Publication No. H6-256880SUMMARY OF INVENTIONTechnical Problem

[0010] However, the above-described Al—Mg—Si-based high strength alloys have had a problem in that the worked structure recrystallizes during forging and a heat treatment step, and coarse grains are generated, which makes it impossible to obtain a sufficiently high strength. Therefore, there are alloys to which Zr (zirconium) is added to prevent recrystallization for the purpose of preventing the generation of coarse recrystallized grains (for example, refer to Patent Documents 1 and 2).

[0011] However, addition of Zr is effective for recrystallization prevention but has had the following problems.

[0012] (1) Addition of Zr weakens the grain refining effect of an Al—Ti—B-based alloy, coarsens crystal grains in an ingot itself, and decreases the strengths of worked products (forged products) after plastic working.

[0013] (2) Since the grain refining effect of the ingot itself weakens, the ingot is likely to crack, the number of internal defects increases, and the yield deteriorates.

[0014] (3) Zr forms compounds with the Al—Ti—B-based alloy, the compounds are deposited on the bottom of a furnace where a molten alloy is retained and contaminate the furnace, and in the manufactured ingot as well, these compounds coarsely crystallize in the ingot and decrease the strength.

[0015] As described above, addition of Zr is effective for recrystallization prevention, but it has been difficult to maintain the stability of the strength.

[0016] The present invention has been made in view of such a technical background, and an object of the present invention is to provide an aluminum alloy forging material and an aluminum alloy forged product that have excellent mechanical properties at normal temperature, and a method for manufacturing the same.Solution to Problem

[0017] In order to solve the above-described problems, the present invention provides the following means.

[0018] An aspect 1 of the present invention is an aluminum alloy forging material composed of an aluminum alloy having an alloy composition containing 0.25 mass % or more and 0.55 mass % or less of Cu, 0.60 mass % or more and 1.25 mass % or less of Mg, 0.90 mass % or more and 1.4 mass % or less of Si, 0.35 mass % or more and 0.60 mass % or less of Mn, 0.15 mass % or more and 0.30 mass % or less of Fe, 0.25 mass % or less of Zn, 0.050 mass % or more and 0.30 mass % or less of Cr, 0.01 mass % or more and 0.1 mass % or less of Ti, 0.0010 mass % or more and 0.030 mass % or less of B, 0.0010 mass % or more and 0.050 mass % or less of Zr with a remainder being made up of Al and unavoidable impurities, a ratio Fe / Mn of the amount of Fe to the amount of Mn in terms of the mass ratio being less than 1.4, in which the conductivity after casting is 25% IACS or higher and 35% IACS or lower, and Rockwell hardness HRF is 62 or higher and 82 or lower.

[0019] An aspect 2 of the present invention is an aluminum alloy forging material composed of an aluminum alloy having an alloy composition containing 0.25 mass % or more and 0.55 mass % or less of Cu, 0.60 mass % or more and 1.25 mass % or less of Mg, 0.90 mass % or more and 1.4 mass % or less of Si, 0.35 mass % or more and 0.60 mass % or less of Mn, 0.15 mass % or more and 0.30 mass % or less of Fe, 0.25 mass % or less of Zn, 0.050 mass % or more and 0.30 mass % or less of Cr, 0.01 mass % or more and 0.1 mass % or less of Ti, 0.0010 mass % or more and 0.030 mass % or less of B, 0.0010 mass % or more and 0.050 mass % or less of Zr with a remainder being made up of Al and unavoidable impurities, a ratio Fe / Mn of the amount of Fe to the amount of Mn in terms of the mass ratio being 0.3 or more and 1.2 or less, in which the conductivity after casting is 25% IACS or higher and 35% IACS or lower, and Rockwell hardness HRF is 62 or higher and 82 or lower.

[0020] An aspect 3 of the present invention is an aluminum alloy forged product composed of an aluminum alloy having an alloy composition containing 0.25 mass % or more and 0.55 mass % or less of Cu, 0.60 mass % or more and 1.25 mass % or less of Mg, 0.90 mass % or more and 1.4 mass % or less of Si, 0.35 mass % or more and 0.60 mass % or less of Mn, 0.15 mass % or more and 0.30 mass % or less of Fe, 0.25 mass % or less of Zn, 0.050 mass % or more and 0.30 mass % or less of Cr, 0.01 mass % or more and 0.1 mass % or less of Ti, 0.0010 mass % or more and 0.030 mass % or less of B, 0.0010 mass % or more and 0.050 mass % or less of Zr with a remainder being made up of Al and unavoidable impurities, a ratio Fe / Mn of the amount of Fe to the amount of Mn in terms of the mass ratio being less than 1.4, in which the number density of precipitates containing Mn within 2.0 μm including a grain boundary is 4 precipitates / μm2 or more, the rate of high angle grain boundaries having a crystal orientation difference of 15° or more is 27% or less, and the impact value at normal temperature is 10 J / cm2 or more.

[0021] An aspect 4 of the present invention is the aluminum alloy forged product of the aspect 3, in which the precipitate has a size of 0.5 μm or less.

[0022] An aspect 5 of the present invention is an aluminum alloy forged product composed of an aluminum alloy having an alloy composition containing 0.25 mass % or more and 0.55 mass % or less of Cu, 0.60 mass % or more and 1.25 mass % or less of Mg, 0.90 mass % or more and 1.4 mass % or less of Si, 0.35 mass % or more and 0.60 mass % or less of Mn, 0.15 mass % or more and 0.30 mass % or less of Fe, 0.25 mass % or less of Zn, 0.050 mass % or more and 0.30 mass % or less of Cr, 0.01 mass % or more and 0.1 mass % or less of Ti, 0.0010 mass % or more and 0.030 mass % or less of B, 0.0010 mass % or more and 0.050 mass % or less of Zr with a remainder being made up of Al and unavoidable impurities, a ratio Fe / Mn of the amount of Fe to the amount of Mn in terms of the mass ratio being 0.3 or more and 1.2 or less, in which the number density of precipitates containing Mn within 2.0 μm including a grain boundary is 4 precipitates / μm2 or more, the rate of high angle grain boundaries having a crystal orientation difference of 15° or more is 27% or less, and the impact value at normal temperature is 10 J / cm2 or more.

[0023] An aspect 6 of the present invention is the aluminum alloy forged product of the aspect 5, in which the precipitate has a size of 0.5 μm or less.

[0024] An aspect 7 of the present invention is a method for manufacturing the aluminum alloy forged product of any one of the aspect 3 to the aspect 6, the method having a molten metal formation step of obtaining a molten metal of the aluminum alloy, a casting step of casting the obtained molten metal to obtain a cast product, a forging step of material-heating the cast product at a temperature of 500° C. to a melting point or lower and performing plastic working to obtain a forged product, a solution treatment step of performing a solution treatment by heating the obtained forged product up to 20° C. to 500° C. at a heating rate of 5.0° C. / min and holding the forged product at 530° C. to 560° C. for 0.3 to three hours, a hardening step of bringing an entire surface of the forged product into contact with hardening water within five to 60 seconds after the solution treatment and hardening the forged product in a water tank for longer than one minute and 40 minutes or shorter, and an aging treatment step of heating a forged product that has undergone the hardening treatment step at a temperature of 180° C. to 220° C. for 0.5 hours to eight hours to perform an aging treatment.

[0025] An aspect 8 of the present invention is the method for manufacturing the aluminum alloy forged product of the aspect 4, the method further having, between the casting step and the forging step, a homogenizing heat treatment step of holding the aluminum alloy cast product in a temperature range of 370° C. or higher and 560° C. or lower for two hours or longer and 10 hours or shorter to perform a homogenizing heat treatment.Advantageous Effects of Invention

[0026] According to the present invention, it is possible to provide an aluminum alloy forging material having excellent mechanical properties at normal temperature.

[0027] According to the present invention, it is possible to provide an aluminum alloy forged product having excellent mechanical properties at normal temperature.

[0028] In addition, according to the present invention, it is possible to provide a low-cost and energy-saving method for manufacturing an aluminum alloy forged product since a homogenizing treatment step, which has been thus far performed to remove segregation after a molten aluminum alloy is cast, is reduced.BRIEF DESCRIPTION OF DRAWINGS

[0029] FIG. 1 A perspective view showing one example of an aluminum alloy forged product according to one embodiment of the present invention.

[0030] FIG. 2 A plan view showing another example of the aluminum alloy forged product according to one embodiment of the present invention.

[0031] FIG. 3 A perspective view showing still another example of the aluminum alloy forged product according to one embodiment of the present invention.

[0032] FIG. 4 A cross-sectional view showing one example of the vicinity of a mold of a horizontal continuous casting device for manufacturing the aluminum alloy forged product according to one embodiment of the present invention.

[0033] FIG. 5 An enlarged cross-sectional view of a main part in the vicinity of a cooling water cavity of the horizontal continuous casting device shown in FIG. 4.

[0034] FIG. 6 An explanatory view showing a heat flux in a cooling wall part of the horizontal continuous casting device.

[0035] FIG. 7 A plan view showing the collection position of the central part collected for production of a test piece for mechanical property evaluation from an aluminum alloy forged product obtained in the present example.

[0036] FIG. 8 A plan view showing the test piece for mechanical property evaluation produced in the present example.DESCRIPTION OF EMBODIMENTS

[0037] Hereinafter, an embodiment of the present invention will be described in detail with reference to drawings.

[0038] The drawings to be used in the following description sometimes show characteristic parts in an enlarged manner for convenience to facilitate the understanding of properties, and the dimensional ratio or the like of each configuration element is not always the same as those in actual cases. In addition, materials, dimensions, or the like to be exemplified in the following description are simply examples, and the present invention is not necessarily limited thereto and can be appropriately modified and carried out to an extent that the effect of the present invention is not changed.Aluminum Alloy Forging Material

[0039] First, an aluminum alloy forging material according to one embodiment of the present invention will be described.

[0040] The aluminum alloy forging material of the present embodiment is an aluminum alloy forging material having an alloy composition containing 0.25 mass % or more and 0.55 mass % or less of Cu, 0.60 mass % or more and 1.25 mass % or less of Mg, 0.90 mass % or more and 1.4 mass % or less of Si, 0.35 mass % or more and 0.60 mass % or less of Mn, 0.15 mass % or more and 0.30 mass % or less of Fe, 0.25 mass % or less of Zn, 0.050 mass % or more and 0.30 mass % or less of Cr, 0.01 mass % or more and 0.1 mass % or less of Ti, 0.0010 mass % or more and 0.030 mass % or less of B, 0.0010 mass % or more and 0.050 mass % or less of Zr with a remainder being made up of Al and unavoidable impurities, the ratio Fe / Mn of the amount of Fe to the amount of Mn in terms of the mass ratio being less than 1.4, in which the conductivity after casting is 25% IACS or higher and 35% IACS or lower, and the Rockwell hardness HRF is 62 or higher and 82 or lower.

[0041] The aluminum alloy forging material of the present embodiment is an aluminum alloy forging material having an alloy composition containing 0.25 mass % or more and 0.55 mass % or less of Cu, 0.60 mass % or more and 1.25 mass % or less of Mg, 0.90 mass % or more and 1.4 mass % or less of Si, 0.35 mass % or more and 0.60 mass % or less of Mn, 0.15 mass % or more and 0.30 mass % or less of Fe, 0.25 mass % or less of Zn, 0.050 mass % or more and 0.30 mass % or less of Cr, 0.01 mass % or more and 0.1 mass % or less of Ti, 0.0010 mass % or more and 0.030 mass % or less of B, 0.0010 mass % or more and 0.050 mass % or less of Zr with a remainder being made up of Al and unavoidable impurities, a ratio Fe / Mn of the amount of Fe to the amount of Mn in terms of the mass ratio being 0.3 or more and 1.2 or less, in which the conductivity after casting is 25% IACS or higher and 35% IACS or lower, and Rockwell hardness HRF is 62 or higher and 82 or lower.

[0042] Fe / Mn can be set to 0.3 or more and 1.0 or less in terms of the mass ratio, also can be set to 0.4 or more and 0.8 or less, and also can be set to 0.4 or more and 0.7 or less.

[0043] The aluminum alloy forging material of the present embodiment contains Mg and Si and thus corresponds to a 6000 series aluminum alloy.Conductivity: 25% IACS or Higher and 35% IACS or Lower

[0044] The conductivity of the aluminum alloy forging material of the present embodiment is 25% IACS or higher and 35% IACS or lower. This conductivity refers to conductivity at room temperature, and the room temperature refers to approximately 20° C.±15° C.

[0045] In a case where the conductivity is lower than 25% IACS, the aluminum alloy forging material is too hard, and the processability deteriorates, and in a case where the conductivity exceeds 35% IACS, the aluminum alloy forging material is soft, and there is thus a case where the machinability (chip breakability) deteriorates and the guaranteed strength of a final product cannot be satisfied.Rockwell Hardness HRF: 62 or Higher and 82 or Lower

[0046] The Rockwell hardness HRF of the aluminum alloy forging material of the present embodiment is 62 or higher and 82 or lower. Here, the Rockwell hardness HRF is a value measured in accordance with JIS Z 2245:2016 “Rockwell hardness test-Test method.”

[0047] This is because the processability is favorable when the Rockwell hardness HRF is within this range. That is, in a case where the Rockwell hardness HRF is lower than 62, the aluminum alloy forging material is soft, and there is thus a case where the machinability (chip breakability) deteriorates and the guaranteed strength of a final product cannot be satisfied, and in a case where the Rockwell hardness HRF is higher than 82, the aluminum alloy forging material is too hard, and the processability deteriorates.Aluminum Alloy Forged Product

[0048] An aluminum alloy forged product according to one embodiment of the present invention will be described.

[0049] FIG. 1 is a perspective view of the aluminum alloy forged product according to one embodiment of the present invention.

[0050] As shown in FIG. 1, an aluminum alloy forged product 1a has a long part 2 and connecting parts 4a and 4b connected to both ends of the long part 2 in the longitudinal direction, respectively. The long part has a square cross section. A through hole may be provided in each of these two connecting parts 4. The aluminum alloy forged product 1a having this shape can be used as, for example, an I-type suspension arm.

[0051] The aluminum alloy forged product of the present embodiment is an aluminum alloy forged product composed of an aluminum alloy having an alloy composition containing 0.25 mass % or more and 0.55 mass % or less of Cu, 0.60 mass % or more and 1.25 mass % or less of Mg, 0.90 mass % or more and 1.4 mass % or less of Si, 0.35 mass % or more and 0.60 mass % or less of Mn, 0.15 mass % or more and 0.30 mass % or less of Fe, 0.25 mass % or less of Zn, 0.050 mass % or more and 0.30 mass % or less of Cr, 0.01 mass % or more and 0.1 mass % or less of Ti, 0.0010 mass % or more and 0.030 mass % or less of B, 0.0010 mass % or more and 0.050 mass % or less of Zr with a remainder being made up of Al and unavoidable impurities, a ratio Fe / Mn of the amount of Fe to the amount of Mn in terms of the mass ratio being less than 1.4, in which the number density of precipitates containing Mn within 2.0 μm including a grain boundary is 4 precipitates / μm2 or more, and the rate of high angle grain boundaries having a crystal orientation difference of 15° or more is 27% or less.

[0052] In addition, in the aluminum alloy forged product of the present embodiment, the impact value at normal temperature is set to 10 J / cm2 or more.

[0053] The size of the precipitate containing Mn that is present within 2.0 μm including a grain boundary is, for example, 0.5 μm or less.

[0054] The aluminum alloy forged product of the present embodiment contains Mg and Si and thus corresponds to a forged product of a 6000 series aluminum alloy.Cu: 0.25 Mass % or More and 0.55 Mass % or Less

[0055] Cu has a function of finely dispersing a Mg—Si-based compound in the aluminum alloy and a function of being precipitated as an Al—Cu—Mg—Si-based compound including a Q phase, thereby improving the tensile strength of the aluminum alloy. When the amount of Cu is within the above-described range, it is possible to improve the mechanical properties of the aluminum alloy forged product 1a at normal temperature.Mg: 0.60 Mass % or More and 1.25 Mass % or Less

[0056] Mg has a function of improving the tensile strength of the aluminum alloy. Mg forms a solid solution in the aluminum mother phase or is precipitated as an Al—Cu—Mg—Si-based compound (Mg2Si), such as a β″ phase, or a Q phase, thereby contributing to the strengthening of the aluminum alloy. In addition, Mg2Si has a function of curbing the generation of a CuAl2 phase in the aluminum alloy. When the generation of a CuAl2 phase is curbed, the corrosion resistance of the aluminum alloy forged product 1a improves. When the amount of Mg is within the above-described range, it is possible to improve the corrosion resistance together with the mechanical properties of the aluminum alloy forged product 1a at normal temperature.Si: 0.90 Mass % or More and 1.4 Mass % or Less

[0057] Similar to Mg, Si has a function of improving the corrosion resistance together with the mechanical properties of the aluminum alloy forged product 1a at normal temperature. However, when Si is excessively added to the aluminum alloy, coarse primary crystal Si grains crystallize, which creates a risk of a decrease in the tensile strength of the aluminum alloy. When the amount of Si is within the above-described range, it is possible to improve the corrosion resistance together with the mechanical properties of the aluminum alloy forged product 1a at normal temperature while curbing the crystallization of primary crystal Si grains.Mn: 0.35 Mass % or More and 0.60 Mass % or Less

[0058] Mn has a function of forming fine granular crystals containing an intermetallic compound, such as Al—Mn—Fe—Si or Al—Mn—Cr—Fe—Si, in the aluminum alloy, thereby improving the tensile strength of the aluminum alloy. When the amount of Mn is within the above-described range, it is possible to improve the mechanical properties of the aluminum alloy forged product 1a at normal temperature.Fe: 0.15 Mass % or More and 0.30 Mass % or Less

[0059] Fe has a function of crystallizing as fine crystals containing an intermetallic compound, such as Al—Mn—Fe—Si, Al—Mn—Cr—Fe—Si, Al—Fe—Si, Al—Cu—Fe, or Al—Mn—Fe, in the aluminum alloy, thereby improving the tensile strength of the aluminum alloy. When the amount of Fe is within the above-described range, it is possible to improve the mechanical properties of the aluminum alloy forged product 1a at normal temperature.

[0060] The relationship of Fe / Mn is less than 1.4. When the relationship of Fe / Mn is less than 1.4, it is possible to curb the crystallization of a 3.0 μm or larger AlFeSi-based compound and to improve the number density of an AlMn-based compound in crystal grains.Cr: 0.050 Mass % or More and 0.30 Mass % or Less

[0061] Cr has a function of forming fine granular crystals containing an intermetallic compound, such as Al—Mn—Cr—Fe—Si or Al—Fe—Cr, in the aluminum alloy, thereby improving the tensile strength of the aluminum alloy. When the amount of Cr is within the above-described range, it is possible to improve the mechanical properties of the aluminum alloy forged product 1a at normal temperature.Ti: 0.01 Mass % or More and 0.1 Mass % or Less

[0062] Ti has a function of refining the crystal grains of the aluminum alloy and improving the stretching processability. In a case where the Ti content is less than 0.01 mass %, there is a risk that the crystal grain refining effect cannot be sufficiently obtained. On the other hand, when the Ti content exceeds 0.1 mass %, there is a risk that coarse crystals may be formed and the stretching processability may deteriorate. In addition, when a large amount of a coarse crystal containing Ti is mixed into the aluminum alloy forged product 1a, there is a case where the toughness deteriorates. Therefore, the amount of Ti is set to 0.012 mass % or more and 0.035 mass % or less. The amount of Ti is preferably 0.015 mass % or more and 0.050 mass % or less.B: 0.001 Mass % or More and 0.030 Mass % or Less

[0063] B has a function of refining the crystal grains of the aluminum alloy and improving the stretching processability. When B is added to the aluminum alloy together with Ti, the crystal grain refining effect improves. When the amount of B is less than 0.0010 mass %, there is a risk that the crystal grain refining effect cannot be sufficiently obtained. On the other hand, when the amount of B exceeds 0.030 mass %, there is a risk that coarse crystals may be formed and mixed into the aluminum alloy forged product 1a as an inclusion. In addition, when a large amount of a coarse crystal containing B is mixed into a final product of the aluminum alloy, there is a case where the toughness deteriorates. Therefore, the amount of B is set to 0.0010 mass % or more and 0.030 mass %. The amount of B is preferably 0.0050 mass % or more and 0.025 mass %.Zr: 0.0010 Mass % or More and 0.050 Mass % or Less

[0064] When the content is 0.05 mass % or less, Zr is precipitated in a form of Al3Zr and Al—(Ti, Zr) and thereby contributes to improving the strength of the aluminum alloy forged product 1a by a recrystallization curbing effect or precipitation strengthening. When the amount of Zr exceeds 0.050 mass %, Zr crystallizes as a coarse Zr compound, which creates a risk of the deterioration of the corrosion resistance of the aluminum alloy forged product 1a. Therefore, the amount of Zr is set to 0.050 mass % or less. In addition, in order to obtain an effect of improving the strength of the forged product by the recrystallization curbing effect or precipitation strengthening, the amount of Zr is preferably 0.0010 mass % or more.Zn: 0.250 Mass % or Less

[0065] The amount of Zr needs to be 0.250 mass % or less. When the amount of Zn exceeds 0.250 mass %, MgZn2 is generated and precipitated in grain boundaries from the Al mother phase, intergranular corrosion thus occurs, and the corrosion resistance of the aluminum alloy forged product deteriorates. Therefore, the amount of Zn is preferably 0.250 mass % or less or 0 mass %.Unavoidable Impurities

[0066] The unavoidable impurities are impurities that are unavoidably mixed into the aluminum alloy from a raw material or a manufacturing step. Examples of the unavoidable impurities include Ni, Sn, Be, and the like. The amount of these unavoidable impurities is preferably not more than 0.1 mass %.

[0067] The central part 2a in the longitudinal direction of the long part 2 of the aluminum alloy forged product 1a of the present embodiment is a part where maximum principal stress is applied in a case where the aluminum alloy forged product 1a is used as, for example, a suspension arm of a vehicle. The central part 2a is, for example, a region that includes the center in the longitudinal direction of the long part 2 and is within a range of 1% or more and 80% or less of the entire long part 2. The aspect ratio (the length in the longitudinal direction / the length of a short side in a direction perpendicular to the longitudinal direction) of the long part 2 is, for example, within a range of two or more and 100 or less. The cross section of the central part 2a of the long part 2 is a cross section in a direction along the direction in which pressure is applied at the time of manufacturing the aluminum alloy forged product 1a by forging (hereinafter referred to as central part cross section in some cases).AlFeSi (Mn)-Based Compound Having Average Particle Diameter of 3.0 μm or Larger being not Contained

[0068] An AlFeSi (Mn)-based compound having an average particle diameter of 3.0 μm or larger needs not to be contained in the alloy structure on the central part cross section of the aluminum alloy forged product 1a. When an AlFeSi (Mn)-based compound having an average particle diameter of 3.0 μm or larger is present, there is a risk that the mechanical properties (tensile properties / fatigue properties and the like) may deteriorate.Impact Value being 10 J / Cm2 or More

[0069] The central part cross section of the aluminum alloy forged product 1a has a mechanical property of the impact value at normal temperature (20° C.) being 10 J / cm2 or more. When this impact value is less than 10 J / cm2, there is a risk that the durability of a part may deteriorate.

[0070] In the present specification, “impact value” means the Charpy impact strength obtained by measurement in accordance with the regulations of JIS Z 2242-2005 “Method for Charpy pendulum impact test of metallic materials.” The impact value is measured using a cylindrical test piece as a test piece.Rate of High Angle Grain Boundaries Having Crystal Orientation Difference of 15° or More Being 27% or Less

[0071] In the central part cross section of the aluminum alloy forged product 1a, a crystal grain boundary (high angle grain boundary) having a crystal orientation difference of 15° or more serves as an index of the degree of progress of recrystallization in the long part 2 of the aluminum alloy forged product 1a. The rate of these high angle grain boundaries being 27% or less indicates that recrystallization has been sufficiently curbed. When recrystallization has been sufficiently curbed, the mechanical properties of the long part 2 improve. The rate of the high angle grain boundaries can be obtained from an EBSD image.

[0072] In the aluminum alloy forged product 1a of the present embodiment configured as described above, since the aluminum alloy, which is the material of the aluminum alloy forged product, has the above-described alloy composition, recrystallization is less likely to occur at the time of manufacturing the forged product. Therefore, coarse crystal grains are less likely to be generated excessively. In addition, the cross section of the central part 2a of the long part 2 of the aluminum alloy forged product 1a of the present embodiment has an alloy structure in which the number density of precipitates containing Mn having a size of 0.5 μm or less within 2.0 μm including a grain boundary is 4 precipitates / μm2 or more, and the rate of the high angle grain boundaries having a crystal orientation difference of 15° or more is 27% or less.

[0073] Therefore, the central part 2a of the long part 2 has high tensile properties or fatigue properties, excellent toughness, and improved impact resistance.

[0074] In the aluminum alloy forged product 1a of the present embodiment, the central part 2a of the long part 2 has high strength or durability and is lightweight and can be thus advantageously used for suspension arms for vehicles, such as cars.

[0075] In the aluminum alloy forged product 1a of the present embodiment shown in FIG. 1, one connecting part 4a has a cylindrical shape with a relatively small diameter, the other connecting part 4b has a cylindrical shape with a relatively large diameter, the long part 2 has a shape that increases in width from the edge of one connecting part 4a toward the edge of the other connecting part 4b, but the shape of the aluminum alloy forged product 1a is not limited thereto. For example, one connecting part 4a and the other connecting part 4b of the aluminum alloy forged product 1a may have the same shape. The width of the long part 2 may be constant. In addition, the long part 2 may have a curved shape. Three or more connecting parts 4 may be formed.

[0076] FIG. 2 is a plan view of another example of the aluminum alloy forged product according to one embodiment of the present invention.

[0077] An aluminum alloy forged product 1b shown in FIG. 2 has three connecting parts 4c, 4d, and 4e. The connecting part 4c and the connecting part 4d are connected to each other with the long part 2, and the connecting part 4d and the connecting part 4e are connected to each other with a short part 5 having a relatively short length than the long part 2. A through hole is provided in the connecting part 4c. This aluminum alloy forged product 1b can be used as, for example, an L-type suspension arm.

[0078] FIG. 3 is a plan view of still another example of the aluminum alloy forged product according to one embodiment of the present invention.

[0079] An aluminum alloy forged product 1c shown in FIG. 3 has three connecting parts 4f, 4g, and 4h. The connecting part 4f and the connecting part 4g, and the connecting part 4f and the connecting part 4h are each connected to each other with the long part 2. A through hole is provided in the connecting part 4f. This aluminum alloy forged product 1b can be used as, for example, an A-type suspension arm.

[0080] An aluminum alloy forged product of another embodiment of the present invention is an aluminum alloy forged product composed of an aluminum alloy having an alloy composition containing 0.25 mass % or more and 0.55 mass % or less of Cu, 0.60 mass % or more and 1.25 mass % or less of Mg, 0.90 mass % or more and 1.4 mass % or less of Si, 0.35 mass % or more and 0.60 mass % or less of Mn, 0.15 mass % or more and 0.30 mass % or less of Fe, 0.25 mass % or less of Zn, 0.050 mass % or more and 0.30 mass % or less of Cr, 0.01 mass % or more and 0.1 mass % or less of Ti, 0.0010 mass % or more and 0.030 mass % or less of B, 0.0010 mass % or more and 0.050 mass % or less of Zr with a remainder being made up of Al and unavoidable impurities, a ratio Fe / Mn of the amount of Fe to the amount of Mn in terms of the mass ratio being 0.3 or more and 1.2 or less, in which the number density of precipitates containing Mn within 2.0 μm including a grain boundary is 4 precipitates / μm2 or more, and the rate of high angle grain boundaries having a crystal orientation difference of 15° or more is 27% or less.

[0081] In addition, in the aluminum alloy forged product of the present embodiment, the impact value at normal temperature is set to 10 J / cm2 or more.

[0082] Fe / Mn can be set to 0.3 or more and 1.0 or less in terms of the mass ratio, also can be set to 0.4 or more and 0.8 or less, and also can be set to 0.4 or more and 0.7 or less.

[0083] The size of the precipitate containing Mn that is present within 2.0 μm including a grain boundary is, for example, 0.5 μm or less.Method for Manufacturing Aluminum Alloy Forged Product

[0084] Next, a method for manufacturing an aluminum alloy forged product of the present embodiment will be described.

[0085] The method for manufacturing an aluminum alloy forged product of the present embodiment includes, for example, a molten metal formation step, a casting step, a forging step, a solution treatment step, a hardening treatment step, and an aging treatment step.Molten Metal Formation Step

[0086] The molten metal formation step is a step of dissolving a raw material to obtain a molten aluminum alloy having a prepared composition. The composition of the molten aluminum alloy is adjusted to an alloy composition containing 0.25 mass % or more and 0.55 mass % or less of Cu, 0.60 mass % or more and 1.25 mass % or less of Mg, 0.90 mass % or more and 1.4 mass % or less of Si, 0.35 mass % or more and 0.60 mass % or less of Mn, 0.15 mass % or more and 0.30 mass % or less of Fe, 0.25 mass % or less of Zn, 0.050 mass % or more and 0.30 mass % or less of Cr, 0.01 mass % or more and 0.1 mass % or less of Ti, 0.0010 mass % or more and 0.030 mass % or less of B, 0.0010 mass % or more and 0.050 mass % or less of Zr with a remainder being made up of Al and unavoidable impurities, the ratio Fe / Mn of the amount of Fe to the amount of Mn in terms of the mass ratio being less than 1.4 to obtain a molten metal of a 6000 series aluminum alloy.

[0087] Fe / Mn in terms of the mass ratio may be adjusted to 0.3 or more and 1.2 or less.

[0088] When the subsequent steps are performed using the molten aluminum alloy having the above-described composition, recrystallization is less likely to occur, and an Al—Mg—Si-based aluminum alloy forged product having excellent mechanical properties at normal temperature can be obtained. A new aluminum lump refers to aluminum having a concentration of 99% or higher that is obtained by performing an electrolysis that is referred to as electrolytic refining on alumina manufactured from a mineral.

[0089] The molten aluminum alloy can be obtained by heating and melting the aluminum alloy. In addition, the molten aluminum alloy may also be formed by melting a single element or a mixture containing two or more elements in proportions in which the target aluminum alloy is generated, the element serving as a raw material of the aluminum alloy. For example, Ti or B may be mixed therewith as a crystal grain refining material, such as an Al—Ti—B rod, for the purpose of controlling crystal grain sizes in an aluminum alloy that is to be generated in the casting step.

[0090] In addition, a material containing 10% or more of a scrap material of a 1000 series, 2000 series, 3000 series, 4000 series, 5000 series, 6000 series, or 7000 series aluminum alloy with a remainder being made up of a new aluminum lump and the above-described additive element may be used as a raw material of the molten aluminum alloy, and the molten aluminum alloy having a prepared composition may be obtained by dissolving the material. In this case, recrystallization is less likely to occur, and an Al—Mg—Si-based aluminum alloy forged product having excellent mechanical properties at normal temperature can be obtained. The new aluminum lump is, for example, aluminum having a purity of 99% or higher that is obtained by performing an electrolysis that is called electrolytic refining on alumina manufactured from a mineral.Casting Step

[0091] In the casting step, the molten aluminum alloy (liquid phase) is cooled and coagulated into solid (solid phase) to obtain an aluminum alloy cast product. In the casting step, for example, a horizontal continuous casting method can be used.

[0092] Here, a horizontal continuous casting device that can be used in the manufacturing of the aluminum alloy cast product of the present embodiment is shown in FIG. 4 and FIG. 5.

[0093] FIG. 4 is a cross-sectional view showing one example of the vicinity of a mold 12 of a horizontal continuous casting device 10. FIG. 5 is an enlarged cross-sectional view of a main part in the vicinity of a cooling water cavity 24 of the horizontal continuous casting device 10.

[0094] The horizontal continuous casting device 10 shown in FIG. 4 and FIG. 5 has a molten metal receiving part (tundish) 11, the hollow cylindrical mold 12, and a refractory plate (insulation member) 13 that is disposed between one end side 12a of this mold 12 and the molten metal receiving part 11.

[0095] The molten metal receiving part 11 is composed of a molten metal inflow part 11a that receives a molten aluminum alloy M obtained in the molten metal formation step, a molten metal holding part 11b, and an outflow part 11c toward a hollow part 21 of the mold 12.

[0096] The molten metal receiving part 11 maintains the level of the upper liquid surface of the molten aluminum alloy M at a position higher than the upper surface of the hollow part 21 of the mold 12 and stably distributes the molten aluminum alloy M to each mold 12 in the case of multiple casting.

[0097] The molten aluminum alloy M held in the molten alloy holding part 11b in the molten alloy receiving part 11 is poured into the hollow part 21 of the mold 12 from a pouring path 13a provided in the refractory plate 13. In addition, the molten aluminum alloy M supplied into the hollow part 21 is cooled and coagulated with a cooling device 23, which will be described below, and drawn out from the other end side 12b of the mold 12 as an aluminum alloy rod B, which is a coagulated ingot.

[0098] On the other end side 12b of the mold 12, a drawer drive device (not shown) that draws out the cast aluminum alloy ingot B at a constant rate needs to be installed. In addition, a synchronous cutting device (not shown) that cuts the aluminum alloy rod B that has been continuously drawn out into an arbitrary length is also preferably installed.

[0099] The refractory plate 13 is a member that blocks heat transfer between the molten metal receiving part 11 and the mold 12 and may be composed of, for example, a material such as calcium silicate, alumina, silica, an alumina and silica mixture, silicon nitride, silicon carbide, or graphite. Such a refractory plate 13 can also be composed of a plurality of layers of different configuration materials.

[0100] The mold 12 is a hollow cylindrical member in the present embodiment and is formed of, for example, one material selected from aluminum, copper, or alloys thereof or a material obtained by combining two or more thereof. As the material of such a mold 12, an optimal combination needs to be selected from the viewpoint of thermal conductivity, heat resistance, and mechanical strength.

[0101] The hollow part 21 of the mold 12 is formed to have a circular cross section to make the aluminum alloy rod B be cast into a cylindrical rod shape, and the mold 12 is held such that the mold central axis (central axis) C that passes through this hollow part 21 is almost along the horizontal direction.

[0102] An inner circumferential surface 21a of the hollow part 21 of the mold 12 is formed at an elevation angle of 0° to 3° (more preferably 0° to 1°) with respect to the mold central axis C in the casting direction (refer to FIG. 1) of the aluminum alloy rod B. That is, the inner circumferential surface 21a is configured in a tapered shape that is conically open in the casting direction. In addition, the angle formed by the taper is an elevation angle.

[0103] When the elevation angle is less than 0°, the aluminum alloy rod B meets resistance on the other end side 12b, which is a casting outlet, at the time of being drawn out of the mold 12, and there is thus a risk that casting may become difficult. On the other hand, when the elevation angle exceeds 3°, the contact of the inner circumferential surface 21a with the molten aluminum alloy M becomes insufficient, and an effect of removing the heat of the molten aluminum alloy M or a coagulated shell generated due to the cooling and coagulation of the molten aluminum alloy into the mold 12 weakens, which creates a risk of insufficient coagulation. As a result, there is a risk that a casting trouble, such as the generation of a remelted skin on the surface of the aluminum alloy rod B or the spouting of the uncoagulated molten aluminum alloy M from the end part of the aluminum alloy rod B, may be generated, which is not preferable.

[0104] The cross-sectional shape of the hollow part 21 of the mold 12 (the planar shape of the hollow part 21 of the mold 12 when seen from the other end side 21b) may be selected according to the shape of the aluminum alloy rod to be cast, for example, a triangular or rectangular cross-sectional shape, a polygonal cross-sectional shape, a semi-circular cross-sectional shape, an elliptical cross-sectional shape, an irregular cross-sectional shape having no axes or planes of symmetry, or the like, aside from the circular shape in the present embodiment.

[0105] On one end side 12a of the mold 12, a fluid supply pipe 22 that supplies a lubricating fluid into the hollow part 21 of the mold 12 is disposed. As the lubricating fluid that is supplied from the fluid supply pipe 22, any one or more lubricating fluids selected from gas lubricants and liquid lubricants can be used. In the case of supplying both a gas lubricant and a liquid lubricant, a fluid supply pipe for each is preferably provided separately. The lubricating fluid that is pressurized and supplied from the fluid supply pipe 22 is supplied into the hollow part 21 of the mold 12 through an annular lubricant supply port 22a.

[0106] In the present embodiment, the sent lubricating fluid is supplied from the annular lubricant supply port 22a to the inner circumferential surface 21a of the mold 12. The liquid lubricant may be heated, turned into a decomposed gas, and supplied to the inner circumferential surface 21a of the mold 12. In addition, a porous material may be disposed in the annular lubricant supply port 22a, and the lubricating fluid may be exuded onto the inner circumferential surface 21a of the mold 12 through this porous material.

[0107] In the mold 12, the cooling device 23 that is cooling means for cooling and coagulating the molten aluminum alloy M is formed. The cooling device 23 of the present embodiment has the cooling water cavity 24 that stores cooling water W for cooling the inner circumferential surface 21a of the hollow part 21 of the mold 12 and a cooling water injection path 25 that makes this cooling water cavity 24 and the hollow part 21 of the mold 12 communicate with each other.

[0108] The cooling water cavity 24 is annularly formed on the outside of the inner circumferential surface 21a of the hollow part 21 in the mold 12 to surround the hollow part 21, and the cooling water W is supplied through a cooling water supply pipe 26.

[0109] The inner circumferential surface 21a of the mold 12 is cooled by the cooling water W that is stored in the cooling water cavity 24, whereby the heat of the molten aluminum alloy M that fills the hollow part 21 of the mold 12 is taken away from the surface that is in contact with the inner circumferential surface 21a of the mold 12, and a coagulated shell is formed on the surface of the molten aluminum alloy M.

[0110] In addition, the cooling water injection path 25 directly applies the cooling water W toward the aluminum alloy rod B on the other end side 12b of the mold 12 from a shower opening 25a that faces the hollow part 21 to cool the aluminum alloy rod B. The vertical cross-sectional view of such a cooling water injection path 25 may also be, for example, a semicircle, a pear shape, or a horseshoe shape aside from the circular shape in the present embodiment.

[0111] In the present embodiment, the cooling water W that is supplied through the cooling water supply pipe 26 is first stored in the cooling water cavity 24 to cool the inner circumferential surface 21a of the hollow part 21 of the mold 12, and the cooling water W in the cooling water cavity 24 is further injected toward the aluminum alloy rod B from the cooling water injection path 25, but these cooling waters W can be each supplied with a different cooling water supply pipe.

[0112] The length from a position where an extended line of the central axis of the shower opening 25a of the cooling water injection path 25 meets the surface of the cast aluminum alloy rod B to the contact surface between the mold 12 and the refractory plate 13 is referred to as an effective mold length L, and this effective mold length L is preferably, for example, 10 mm or longer and 40 mm or shorter. When this effective mold length L is shorter than 10 mm, a favorable film is not formed or the like, which makes casting impossible, and when the effective mold length is longer than 40 mm, a forcible cooling effect becomes weak, coagulation by the mold wall becomes dominant, the contact resistance between the mold 12 and the molten aluminum alloy M or the aluminum alloy rod B becomes large, and there is thus a risk that casting may become so unstable that cracks are generated on the cast surface or the molten aluminum alloy M or the aluminum alloy rod B may be shredded in the mold, which is not preferable.

[0113] Regarding the supply of the cooling water W to the cooling water cavity 24 or the injection of the cooling water W from the shower opening 25a of the cooling water injection path 25, it is preferable that each operation can be controlled with a control signal from a control unit (not shown).

[0114] The cooling water cavity 24 is formed so that an inner bottom surface 24a close to the hollow part 21 of the mold 12 becomes a surface parallel to the inner circumferential surface 21a of the hollow part 21 of the mold 12.

[0115] Being parallel mentioned herein includes a case where the inner circumferential surface 21a of the hollow part 21 of the mold 12 is formed at an elevation angle of 0° to 3° with respect to the inner bottom surface 24a of the cooling water cavity 24, that is, a case where the inner bottom surface 24a is inclined at more than 0° up to 3° with respect to the inner circumferential surface 21a.

[0116] As shown in FIG. 4, a cooling wall part 27 of the mold 12 that is a part where the inner bottom surface 24a of such a cooling water cavity 24 and the inner circumferential surface 21a of the hollow part 21 of the mold 12 face each other is formed so that the value of a heat flux per unit area from the molten aluminum alloy M in the hollow part 21 toward the cooling water W in the cooling water cavity 24 falls into a range of 10×105 W / m2 or more and 50×105 W / m2 or less.

[0117] The mold 12 needs to be formed so that a thickness t of such a cooling wall part 27 of the mold 12, that is, the interval between the inner bottom surface 24a of the cooling water cavity 24 and the inner circumferential surface 21a of the hollow part 21 of the mold 12 is in a range of, for example, 0.5 mm or more and 3.0 mm or less and preferably 0.5 mm or more and 2.5 mm or less. In addition, a material that forms the mold 12 needs to be selected so that the thermal conductivity of at least the cooling wall part 27 of the mold 12 is in a range of 100 W / m·K or higher and 400 W / m·K or lower.

[0118] In FIG. 4, the molten aluminum alloy M in the molten metal receiving part 11 is supplied from one end side 12a of the mold 12 held such that the mold central axis C becomes almost horizontal through the refractory plate 13, forcibly cooled on the other end side 12b of the mold 12, and turns into the aluminum alloy rod B.

[0119] The aluminum alloy rod B is drawn out at a constant rate with the drawer drive device (not shown) installed near the other end side 12b of the mold 12 and is thus continuously cast, whereby a long aluminum alloy rod B is formed. The drawn aluminum alloy rod B is cut into a desired length with the synchronous cutting device (not shown).

[0120] The composition ratio of the cast aluminum alloy rod B can be confirmed by, for example, a method in which a photoelectric photometry-type optical emission spectrometer (device example: PDA-5500 manufactured by Shimadzu Corporation) as described in “JIS H 1305” is used.

[0121] The difference between the height of the liquid surface level of the molten aluminum alloy M retained in the molten metal receiving part 11 and the height of the upper inner circumferential surface 21a of the hollow part 21 is preferably set to 0 mm to 250 mm (more preferably 50 mm to 170 mm). When the difference is set within such a range, the pressure of the molten aluminum alloy M that is supplied into the mold 12 is suitably balanced with a lubricating oil and a gas generated by the vaporization of the lubricating oil, and the castability thus stabilizes.

[0122] As the liquid lubricant, a plant oil that is a lubricating oil can be used. Examples thereof include rapeseed oil, castor oil, and salad oil. These adversely affect the environment only to a small extent and are thus preferable.

[0123] The amount of the lubricating oil supplied is preferably 0.05 mL / minute to 5 mL / minute (more preferably 0.1 mL / minute or more and 1 mL / minute or less). When the amount supplied is too small, there is a risk that the molten aluminum alloy M of the aluminum alloy rod B may not coagulate due to the lack of lubrication and may leak from the mold 12.

[0124] When the amount supplied is too large, there is a risk that a surplus of the lubricating oil may be mixed into the aluminum alloy rod B and may turn into an internal defect.

[0125] The casting rate, which is the rate of drawing the aluminum alloy rod B out of the mold 12, is preferably 200 mm / minute or faster and 1500 mm / minute or slower (more preferably 400 mm / minute or faster and 1000 mm / minute or slower). This is because when the casting rate is within this range, the network structure of crystals that are formed by casting becomes uniform and fine, the resistance to deformation of the aluminum material at high temperatures increases, and the high-temperature mechanical strength improves.

[0126] The amount of the cooling water that is injected from the shower opening 25a of the cooling water injection path 25 is preferably 10 L / minute or more and 50 L / minute or less (more preferably 25 L / minute or more and 40 L / minute or less) per mold. When the amount of the cooling water is smaller than the above, there is a risk that the molten aluminum alloy M may not coagulate and leak from the mold 12. In addition, there is a risk that the surface of the cast aluminum alloy rod B may remelts to form a nonuniform structure and may remain as an internal defect. On the other hand, in a case where the amount of the cooling water is larger than this range, there is a risk that the mold 12 may removes heat too much and the molten aluminum alloy M may be coagulated in the middle.

[0127] The average temperature of the molten aluminum alloy M that flows into the mold 12 from the molten metal receiving part 11 is preferably, for example, 650° C. or higher and 750° or lower (more preferably 680° C. or higher and 720° or lower). When the temperature of the molten aluminum alloy M is too low, there is a risk that a coarse crystal may be formed and incorporated as an internal defect into the mold 12 and the front thereof. On the other hand, when the temperature of the molten aluminum alloy M is too high, there is a risk that a large amount of hydrogen gas may be incorporated into the molten aluminum alloy M, be incorporated as a porosity into the aluminum alloy rod B, and turn into a cavity.

[0128] In addition, when the value of the heat flux per unit area from the molten aluminum alloy M in the hollow part 21 toward the cooling water W in the cooling water cavity 24 is made to be in a range of 10×105 W / m2 or more and 50×105 W / m2 or less in the cooling wall part 27 of the mold 12, it is possible to prevent the occurrence of seizure of the aluminum alloy rod B.

[0129] The cooling wall part 27 of the mold 12 receives heat due to the removal of heat from the molten aluminum alloy M, and heat exchanged is performed by cooling this heat with the cooling water W that is stored in the cooling water cavity 24, and regarding this heat exchange state, attention was paid to the heat flux per unit area as in the explanatory view shown in FIG. 6. The heat flux per unit area is represented by the following equation (1) according to Fourier's Law.Q=-k×(T⁢1-T⁢2) / L(1)Q: Heat flux

[0131] k: Thermal conductivity at a place where heat passes through (the cooling wall part 27 of the mold 12 in the present embodiment)

[0132] T1: Low-temperature side temperature at a place where heat passes through (the inner bottom surface 24a of the cooling water cavity 24 in the present embodiment)

[0133] T2: High-temperature side temperature at a place where heat passes through (the inner circumferential surface 21a of the hollow part 21 of the mold 12 in the present embodiment)

[0134] L: Section length (mm) of a place where heat passes through (the thickness t of the cooling wall part 27 of the mold 12 in the present embodiment)

[0135] When the cooling wall part 27 of the mold 12 is configured so that the value of the heat flux per unit area reaches 10×105 W / m2 or more based on the material and thickness of the mold and temperature measurement data that enable favorable results to be obtained even when the amount of the lubricating oil is reduced during casting, it is possible to prevent the seizure of the cast aluminum alloy rod B. In addition, the value of the heat flux per unit area is preferably set to 50×105 W / m2 or less.

[0136] In order to make the value of the heat flux be in such a range in the cooling wall part 27 of the mold 12, the mold 12 needs to be formed so that the thickness t of the cooling wall part 27 of the mold 12 falls into, for example, a range of 0.5 mm or more and 3.0 mm or less. In addition, the thermal conductivity of at least the cooling wall part 27 of the mold 12 needs to be in a range of 100 W / m·K or higher and 400 W / m·K or lower.

[0137] At the time of manufacturing the aluminum alloy rod B of the present embodiment, the molten aluminum alloy M retained in the molten metal receiving part 11 is continuously supplied into the hollow part 21 from one end side 12a of the mold 12 using the above-described horizontal continuous casting device 10. In addition, the cooling water W is supplied to the cooling water cavity 24, and a lubricating flux, for example, a lubricating oil, is supplied from the fluid supply pipe 22.

[0138] In addition, the molten aluminum alloy M supplied into the hollow part 21 is cooled and coagulated under a condition that the value of the heat flux per unit area in the cooling wall part 27 is 10×105 W / m2 or more to cast the aluminum alloy rod B. In addition, during the casting of the aluminum alloy rod B, the wall surface temperature of the cooling wall part 27 of the mold 12 that is cooled with the cooling water W is preferably made to be 100° C. or lower.

[0139] The aluminum alloy rod B thus obtained is cooled and coagulated under a condition that the value of the heat flux per unit area in the cooling wall part 27 is 10×105 W / m2 or more, whereby the fixation of a reaction product due to the contact between the gas of the lubricating oil and the molten aluminum alloy M, for example, a carbide, is curbed. Therefore, there is no need to cut and remove a carbide or the like on the surface of the aluminum alloy rod B, and it is possible to manufacture the aluminum alloy rod B with a high yield.

[0140] The casting step of obtaining a cast product from the molten aluminum alloy M is not limited to the above-described horizontal continuous casting method, and a well-known continuous casting method, such as a vertical continuous casting method, can be used. The vertical continuous casting method is classified into a float method or a hot top method depending on the method for supplying the molten aluminum alloy M to a mold (mold 12), and the case of using the hot top method will be simply described below.

[0141] A casting machine that is used in the hot top method include a mold, a molten metal receptor (header), or the like. A molten metal supplied to a molten metal receiving part passes through a tap outlet and the header, thereby flows at an adjusted flow rate, flows into a tubular mold installed almost horizontally, and is forcibly cooled in the mold, whereby a coagulated shell is formed on the surface of the molten metal.

[0142] Furthermore, cooling water is directly poured to a cast product drawn out from the mold, and the cast product is continuously drawn out while the coagulation of metal progresses up to the inside of the cast product. Ordinarily, a metal member having favorable thermal conductivity is used as the mold, and the mold has a hollow structure to introduce a refrigerant thereinto.

[0143] The refrigerant being used may be appropriately selected from industrially available refrigerants, and water is recommended from the viewpoint of easy use.

[0144] The mold that is used in the present embodiment is appropriately selected from metals, such as copper or aluminum, or graphite from the viewpoint of heat conduction performance and durability in a contact part with the molten metal. The header is ordinarily made of a refractory and installed in the upper side of the mold. The material or size of the header may be selected as appropriate depending on the component range of an alloy to be cast or the dimension of a cast product and is not particularly limited.

[0145] The average cooling rate during casting may be appropriately selected from an ordinarily recommended range, for example, 10 to 300° C. / second. The casting rate may be appropriately selected from an ordinary range in the horizontal continuous casting and may be appropriately selected from, for example, a range of 200 to 600 mm / minute.

[0146] The casting method described above makes it possible to obtain a uniform metal structure even in medium to large size cast products. The diameter of a cast product to which the casting method can be applied is not particularly limited, and the casting method can be suitably used for materials having a diameter of 30 to 100 mm.Forging Step

[0147] The forging step is a step of heating a material for forging obtained by cutting the aluminum alloy cast product after the casting into a predetermined size at a predetermined temperature and then performing mold forming by applying pressure thereto with a press machine. In the present embodiment, forging is performed without performing a homogenizing treatment that has been conventionally performed for segregation removal after casting. Therefore, there is a need to perform segregation removal, which has been performed by the homogenizing treatment, by heating the material during forging, and there is thus a need to heat the material at a temperature of 500° C. or higher and the melting point or lower. After that, forging is performed to obtain a forged product (for example, a car suspension part or the like). When the material heating temperature during forging is lower than 500° C., a compound, such as an AlFeSi-based compound or a Mg2Si-based compound, remains in a segregated state in the alloy structure, deformation resistance becomes high, sufficient working becomes impossible, and cracks are generated. In addition, the material heating temperature during forging is higher than the melting point, a defect, such as eutectic melting, is likely to occur.Solution Treatment Step

[0148] The solution treatment step is a step of heating the forged product obtained in the forging step to form a solid solution, thereby alleviating distortion introduced in the forging step and forming a solid solution of a solute element.

[0149] In the present embodiment, the solution treatment is performed by holding the forged product at a treatment temperature of 530° C. or higher and 560° C. or lower for 0.3 to three hours. The heating rate from room temperature up to the above-described treatment temperature is preferably 5.0° C. / min or faster. When the treatment temperature is lower than 530° C., there is a risk that the formation of the solid solution of the solute element may become insufficient. On the other hand, when the treatment temperature is higher than 560° C., the formation of the solid solution of the solute element is further promoted, but there is a risk that eutectic melting or recrystallization is less likely to occur. In addition, in a case where the heating rate is slower than 5.0° C. / min or slower, there is a risk that Mg2Si may be coarsely precipitated. On the other hand, in a case where the treatment temperature is lower than 530° C., there is a risk that the formation of the solid solution may not progress and the strength is less likely to be increased by age precipitation.Hardening Treatment Step

[0150] The hardening treatment step is a step of rapidly cooling the forged product in a solid solution state obtained by the solution treatment step to form a supersaturated solid solution.

[0151] In the present embodiment, the forged product is injected into a water tank where water (hardening water) is retained to submerge the forged product, thereby performing a hardening treatment. The water temperature in the water tank is preferably 20° C. or higher and 60° C. or lower. The forged product is preferably injected into the water tank such that the entire surface of the forged product comes into contact with water within five seconds or longer and 60 seconds or shorter after the solution treatment. The submersion time of the forged product also varies with the size of the forged product but is, for example, longer than one minute and 30 minutes or shorter.Aging Treatment Step

[0152] The aging treatment step is a step of heating and holding the forged product at a relatively low temperature to precipitate an element that has formed the supersaturated solid solution and impart appropriate hardness.

[0153] In the present embodiment, an aging treatment is performed by heating the forged product after the hardening treatment step at a temperature of 170° C. or higher and 210° C. or lower and holding the forged product at that temperature for 0.5 hours or longer and seven hours or shorter. When the treatment temperature is lower than 170° C. or the holding time is shorter than 0.5 hours, there is a risk that a Mg2Si-based precipitate that improves the tensile strength may not sufficiently grow. On the other hand, when treatment temperature is higher than 190° C. or the holding time is longer than seven hours, there is a risk that the Mg2Si-based precipitate may become too coarse and the tensile strength may not be sufficiently improved.

[0154] A method for manufacturing an aluminum alloy forged product of another embodiment of the present invention further has, between the casting step and the forging step, a homogenizing heat treatment step of holding the aluminum alloy cast product in a temperature range of 370° C. or higher and 560° C. or lower for two hours or longer and 10 hours or shorter to perform a homogenizing heat treatment.

[0155] The method for manufacturing an aluminum alloy forged product of this embodiment is different from the method for manufacturing an aluminum alloy forged product of the above-described embodiment in terms of the homogenizing heat treatment step.EXAMPLES

[0156] Next, specific examples of the present invention will be described, but the present invention is not particularly limited to these specific examples.Examples 1 to 8 and Comparative Examples 1 to 3Production of Continuous Cast Products

[0157] First, aluminum alloys having an alloy composition (the remainder was aluminum) shown in Table 1 below were prepared. Continuous cast products having a circular cross section with a diameter of 82 mm were produced using the prepared aluminum alloys.TABLE 1Element content (mass %)SiFeCuMnMgCrTiBZnZrExample 11.1180.2660.3230.4000.8290.0930.0140.0030.0040.001Example 21.1040.2500.3200.4200.8980.1100.0150.0030.0090.001Example 31.0500.2500.3080.4580.8930.0980.0260.0030.0090.001Example 41.1500.2500.3020.4400.9100.1020.0150.0030.0090.001Example 51.1380.2500.3230.4300.8290.0930.0140.0030.0040.001Example 61.1640.2500.3200.4900.8980.1100.0150.0030.0090.001Example 71.1500.2500.3080.4580.8930.0980.0260.0030.0090.001Example 81.1500.2500.3020.4400.9100.1020.0150.0030.0090.001Example 91.3500.2800.4900.5401.1800.1800.0180.0030.0050.001Example 101.0700.2300.4400.4801.0300.1500.0200.0030.0040.001Example 110.9600.2200.3100.4400.6800.1000.0180.0030.0040.001Example 121.2500.2900.4000.4901.1000.1500.0200.0030.0040.001Comparative Example 11.7000.6300.3050.4400.9110.0920.0140.0030.0040.001Comparative Example 21.1480.7500.3200.4400.8200.0900.0150.0032.2000.001Comparative Example 31.151.0000.3120.440.9100.0940.0150.0030.0040.001Manufacture of Aluminum Alloy Forged Products

[0158] Next, a homogenizing heat treatment step (only in Comparative Examples 1 to 3), a forging step, a solution treatment step, a hardening treatment step, and an artificial aging treatment step were performed in this order on the obtained continuous cast products to obtain aluminum alloy forged products 1a having a shape shown in FIG. 1. The conditions of the homogenizing heat treatment step (only in Comparative Examples 1 to 3), the forging step, the solution treatment step, the hardening treatment step, and the artificial aging treatment step are shown in Table 2 below.TABLE 2Homogenizing heat treatmentSolution treatmentHardening treatmentHeatingForgingHeatingTimeWaterArtificial aging treatmentrateTemper-HoldingTemper-rateTemper-Holdingtaken untiltemper-Submer-Temper-Holding[° C. / aturetimeature[° C. / aturetimesubmergingaturesionaturetimemin][° C.][min][° C.]min][° C.][min][s][° C.]time[° C.][min]Example 1—5201054518015601.5180300Example 25201054518015601.5180300Example 35201054518015601.5180300Example 45001054560156025180300Example 55001054560156025180300Example 65001054560156025180300Example 7500105456015601.519590Example 8500105456015601.519590Example 9510554018010405180300Example 10505105501205402190110Example 11500754512010305195110Example 12500555018010405180300Comparative1.54704205201054518015601.5180300Example 1Comparative1.54704205201054518015601.5180300Example 2Comparative1.54704205201054518015601.5180300Example 3Evaluations

[0159] Regarding central parts 2a in the longitudinal direction of long parts 2 in the aluminum alloy forged products 1a of Examples 1 to 8 and Comparative Examples 1 to 3 obtained as described above, the following evaluations were performed. The evaluation results of the central parts 2a of the long parts 2 are shown in Table 3 below.Fe / Mn's

[0160] Fe / Mn's were adjusted to 0.3 or more and 1.2 or less in the examples and to more than 1.2 in the comparative examples.

[0161] “O” . . . Fe / Mn is 0.3 or more 1.2 or less.

[0162] “X” . . . Fe / Mn is more than 1.2.Conductivity (% IACS)

[0163] The conductivity was measured at room temperature.Determination Criteria

[0164] “O” . . . The conductivity is 25% IACS or higher and 35% IACS or lower.

[0165] “X” . . . The conductivity is lower than 25% IACS or higher than 35% IACS.Rockwell Hardness HRF

[0166] The Rockwell hardness (HRF) was measured in accordance with JIS Z 2245:2016 “Rockwell hardness test-Test method.”Determination Criteria

[0167] “O” . . . The Rockwell hardness HRF is 62 or higher and 82 or lower.

[0168] “X” . . . The Rockwell hardness HRF is lower than 62 or higher than 82.Mechanical Properties (Impact Properties) Evaluation

[0169] The central part 2a of the long part 2 of the aluminum alloy forged product 1a was cut as shown in FIG. 7, and a prismatic body for the production of a test piece for mechanical properties (impact properties) evaluation was collected. The obtained prismatic body was worked to produce a cylindrical test piece for mechanical properties evaluation shown in FIG. 8. In the test piece for mechanical properties evaluation, a parallel section diameter A was set to 8.0 mm, and a gauge length G was set to 30.0 mm. On the test piece for mechanical properties evaluation, a Charpy impact test was performed at normal temperature (25° C.) in accordance with JIS Z 2242, thereby measuring the impact value. The obtained impact value was evaluated based on the following determination criteria.Determination Criteria

[0170] “O” . . . The impact value at normal temperature is 10 J / cm2 or more.

[0171] “X” . . . The impact value at normal temperature is less than 10 J / cm2.Refinement Evaluation and Recrystallization / Crystal Coarsening Evaluation

[0172] The central part 2a of the long part 2 of the aluminum alloy forged product 1a was cut in a direction perpendicular to the surface of the central part 2a to collect a plate-like body (thickness: 2 mm) for the production of a test piece for refinement evaluation. The obtained plate-like body was cut into a 7 mm×7 mm square to produce a 7 mm×7 mm×2 mm-thick test piece for refinement evaluation. On the surface (cross section of the central part 2a) of the collected test piece for refinement evaluation, the number of 3.0 μm or larger AlFeSi-based compounds and the rate of high angle grain boundaries having a crystal orientation difference of 15° or more were measured using SEM-EBSD (scanning electron microscope-electron backscatter diffraction device). The obtained number of 3.0 μm or larger AlFeSi-based compounds and rate of high angle grain boundaries were each determined based on the following criteria to evaluate the refinement and recrystallization / crystal coarsening of crystal particles. As the measurement conditions of SEM-EBSD, the acceleration voltage was set to 15 kV, the measurement pitch was set to 0.5 μm / px, the analysis region was set to 500×500 μm2, and the grain boundary definition angle was set to 15°.Determination CriteriaDetermination Criteria of Number of 3.0 μm or Larger AlFeSi-Based Compounds

[0173] “O” . . . There is no 3.0 μm or larger AlFeSi-based compound.

[0174] “X” . . . There is a 3.0 μm or larger AlFeSi-based compound.Determination Criteria of Rate of High Angle Grain Boundaries

[0175] “O” . . . The rate is 27% or less.

[0176] “X” . . . The rate is more than 27%.Method for Measuring Number Density of Mn-Containing Precipitates in Aluminum Alloy Forged Product

[0177] In addition, from each aluminum alloy forged product, the number density of precipitates containing Mn within 2.0 μm including a grain boundary was measured. A sample piece for structure observation having a size of approximately 10 mm×10 mm width×10 mm thickness was cut out from the central part 2a of the long part 2 of the aluminum alloy forged product 1a, and this sample piece was polished using a cross section polisher. In addition, a FE-SEM photograph (field emission scanning electron microscope photograph) of this polished sample piece was captured, the number density of precipitates containing Mn within 2.0 μm including a grain boundary was obtained in a range of the area of field of view of 1.5815 mm2 in this SEM photograph, and the number density was evaluated with the following determination criteria.Determination Criteria of Number Density

[0178] “O” . . . The number density is 4 precipitates / μm2 or more.

[0179] “X” . . . The number density is 4 precipitates / μm2 or less.Comprehensive Evaluation

[0180] The evaluation results of the seven items of the Fe / Mn ratio, the conductivity, the Rockwell hardness, the impact properties, the number of 3.0 μm or larger AlFeSi-based compounds, the number density of an AlMn-based compound, and the rate of high angle grain boundaries having a crystal orientation difference of 15° or more were evaluated based on the following determination criteria.Determination Criteria

[0181] “O” . . . All of the seven items are evaluated as “O.”

[0182] “X” . . . One or more of the seven items are evaluated as “X.”TABLE 3CompoundsNumber ofPhysical property valueMechanical propertiesContents3.0 μm orEvalu-Evalu-Evalu-Evalu-largerConduc-ation_25Hard-ation_62Impactation_10ation_0.3AlMnFeSitivityto 35nessto 82valueor moreFe / Mnto 1.2compoundsExample 129.24◯68.9◯13◯0.67◯0Example 229.31◯68.2◯13◯0.60◯0Example 330.2◯66.9◯11◯0.55◯0Example 431.7◯65.2◯13◯0.57◯0Example 528.8◯68.3◯14◯0.58◯0Example 629.5◯68.2◯15◯0.51◯0Example 729.4◯68.9◯12◯0.55◯0Example 830.1◯66.2◯11◯0.57◯0Example 929.12◯69.4◯14◯0.52◯0Example 1029.33◯68.3◯13◯0.48◯0Example 1129.25◯68.9◯14◯0.50◯0Example 1229.17◯69.1◯14◯0.59◯0Comparative47.17X37.0X9X1.43X12Example 1Comparative46.2X38.1X6X1.70X48Example 2Comparative46.7X38.4X5X2.27X65Example 3Recrystallization / CoarseningRate ofCompoundscrystalEvalu-Precipitates evaluationgrainEvalu-Compre-ation_0Evaluation_4boundariesationhensivecom-Numberprecipitates / μm2of 15° or(27% orevalu-poundsdensityor moremoreless)ationExample 1◯9.01◯26.9◯◯Example 2◯9.32◯26.8◯◯Example 3◯7.21◯26.2◯◯Example 4◯6.89◯25.5◯◯Example 5◯7.48◯24.7◯◯Example 6◯6.91◯26.3◯◯Example 7◯9.22◯26.8◯◯Example 8◯7.74◯25.7◯◯Example 9◯12.2◯25.3◯◯Example 10◯10.5◯25.8◯◯Example 11◯11.8◯24.2◯◯Example 12◯12.1◯25.1◯◯ComparativeX1.2◯33.8XXExample 1ComparativeX2.1◯42.2XXExample 2ComparativeX1.8◯55.1XXExample 3REFERENCE SIGNS LIST10 Horizontal continuous casting device11 Molten metal receiving part (tundish)

[0185] 11a Molten metal inflow part

[0186] 11b Molten metal holding part

[0187] 11c Outflow part

[0188] 12 Mold

[0189] 12a One end side

[0190] 12b Other end side

[0191] 13 Refractory plate (insulation member)

[0192] 13a Pouring path

[0193] 21 Hollow part

[0194] 21a Inner circumferential surface

[0195] 21b Other end side

[0196] 22 Fluid supply pipe

[0197] 22a Lubricant supply port

[0198] 23 Cooling device

[0199] 24 Cooling water cavity

[0200] 24a Inner bottom surface

[0201] 25 Cooling water injection path

[0202] 25a Shower opening

[0203] 26 Cooling water supply pipe

[0204] 27 Cooling wall part

[0205] B Aluminum alloy rod

[0206] M Molten aluminum alloy

[0207] W Cooling water

[0208] 100 Aluminum alloy forged product

Claims

1. An aluminum alloy forging material being composed of an aluminum alloy having an alloy composition containing 0.25 mass % or more and 0.55 mass % or less of Cu, 0.60 mass % or more and 1.25 mass % or less of Mg, 0.90 mass % or more and 1.4 mass % or less of Si, 0.35 mass % or more and 0.60 mass % or less of Mn, 0.15 mass % or more and 0.30 mass % or less of Fe, 0.25 mass % or less of Zn, 0.050 mass % or more and 0.30 mass % or less of Cr, 0.01 mass % or more and 0.1 mass % or less of Ti, 0.0010 mass % or more and 0.030 mass % or less of B, 0.0010 mass % or more and 0.050 mass % or less of Zr with a remainder being made up of Al and unavoidable impurities, a ratio Fe / Mn of an amount of Fe to an amount of Mn in terms of a mass ratio being less than 1.4,wherein conductivity after casting is 25% IACS or higher and 35% IACS or lower, and Rockwell hardness HRF is 62 or higher and 82 or lower.

2. An aluminum alloy forging material being composed of an aluminum alloy having an alloy composition containing 0.25 mass % or more and 0.55 mass % or less of Cu, 0.60 mass % or more and 1.25 mass % or less of Mg, 0.90 mass % or more and 1.4 mass % or less of Si, 0.35 mass % or more and 0.60 mass % or less of Mn, 0.15 mass % or more and 0.30 mass % or less of Fe, 0.25 mass % or less of Zn, 0.050 mass % or more and 0.30 mass % or less of Cr, 0.01 mass % or more and 0.1 mass % or less of Ti, 0.0010 mass % or more and 0.030 mass % or less of B, 0.0010 mass % or more and 0.050 mass % or less of Zr with a remainder being made up of Al and unavoidable impurities, a ratio Fe / Mn of an amount of Fe to an amount of Mn in terms of a mass ratio being 0.3 or more and 1.2 or less,wherein conductivity after casting is 25% IACS or higher and 35% IACS or lower, and Rockwell hardness HRF is 62 or higher and 82 or lower.

3. An aluminum alloy forged product being composed of an aluminum alloy having an alloy composition containing 0.25 mass % or more and 0.55 mass % or less of Cu, 0.60 mass % or more and 1.25 mass % or less of Mg, 0.90 mass % or more and 1.4 mass % or less of Si, 0.35 mass % or more and 0.60 mass % or less of Mn, 0.15 mass % or more and 0.30 mass % or less of Fe, 0.25 mass % or less of Zn, 0.050 mass % or more and 0.30 mass % or less of Cr, 0.01 mass % or more and 0.1 mass % or less of Ti, 0.0010 mass % or more and 0.030 mass % or less of B, 0.0010 mass % or more and 0.050 mass % or less of Zr with a remainder being made up of Al and unavoidable impurities, a ratio Fe / Mn of an amount of Fe to an amount of Mn in terms of a mass ratio being less than 1.4,wherein a number density of precipitates containing Mn within 2.0 μm including a grain boundary is 4 precipitates / μm2 or more, the rate of high angle grain boundaries having a crystal orientation difference of 15° or more is 27% or less, and an impact value at normal temperature is 10 J / cm2 or more.

4. The aluminum alloy forged product according to claim 3,wherein the precipitate has a size of 0.5 μm or less.

5. An aluminum alloy forged product being composed of an aluminum alloy having an alloy composition containing 0.25 mass % or more and 0.55 mass % or less of Cu, 0.60 mass % or more and 1.25 mass % or less of Mg, 0.90 mass % or more and 1.4 mass % or less of Si, 0.35 mass % or more and 0.60 mass % or less of Mn, 0.15 mass % or more and 0.30 mass % or less of Fe, 0.25 mass % or less of Zn, 0.050 mass % or more and 0.30 mass % or less of Cr, 0.01 mass % or more and 0.1 mass % or less of Ti, 0.0010 mass % or more and 0.030 mass % or less of B, 0.0010 mass % or more and 0.050 mass % or less of Zr with a remainder being made up of Al and unavoidable impurities, a ratio Fe / Mn of an amount of Fe to an amount of Mn in terms of a mass ratio being 0.3 or more and 1.2 or less,wherein a number density of precipitates containing Mn within 2.0 μm including a grain boundary is 4 precipitates / μm2 or more, the rate of high angle grain boundaries having a crystal orientation difference of 15° or more is 27% or less, and an impact value at normal temperature is 10 J / cm2 or more.

6. The aluminum alloy forged product according to claim 5,wherein the precipitate has a size of 0.5 μm or less.

7. A method for manufacturing an aluminum alloy forged product, the aluminum alloy forged product being the aluminum alloy forged product according to claim 3, the method comprising:a molten metal formation step of obtaining a molten metal of the aluminum alloy;a casting step of casting the obtained molten metal to obtain a cast product;a forging step of material-heating the cast product at a temperature of 500° C. to a melting point or lower and performing plastic working to obtain a forged product;a solution treatment step of performing a solution treatment by heating the obtained forged product up to 20° C. to 500° C. at a heating rate of 5.0° C. / min and holding the forged product at 530° C. to 560° C. for 0.3 to three hours;a hardening step of bringing an entire surface of the forged product into contact with hardening water within five to 60 seconds after the solution treatment and hardening the forged product in a water tank for longer than one minute and 40 minutes or shorter; andan aging treatment step of heating a forged product that has undergone the hardening treatment step at a temperature of 180° C. to 220° C. for 0.5 hours to eight hours to perform an aging treatment.

8. The method for manufacturing an aluminum alloy forged product according to claim 7, the method further comprising, between the casting step and the forging step:a homogenizing heat treatment step of holding the aluminum alloy cast product in a temperature range of 370° C. or higher and 560° C. or lower for two hours or longer and 10 hours or shorter to perform a homogenizing heat treatment.

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