Forged aluminum alloy material

Abstract

The present invention provides a forged aluminum alloy material which is excellent in terms of strength, ductility, fatigue characteristics, and corrosion resistance in spite of containing Fe in excess of 0.4 mass%. The present invention relates to the forged aluminum alloy material which contains Si: 0.7 mass% to 1.5 mass% inclusive, Fe: more than 0.4 mass% but not more than 0.67 mass%, Cu: more than 0.4 mass% but not more than 0.8 mass%, Mg: 0.85 mass% to 1.3 mass% inclusive, Ti: 0.005 mass% to 0.07 mass% inclusive, and Zn: 0.25 mass% or less, which additionally contains at least one selected from the group consisting of Mn: 0.1 mass% to 0.95 mass% inclusive, Cr: more than 0.1 mass% but not more than 0.4 mass%, and Zr: 0.05 mass% to 0.3 mass% inclusive, with the balance being made up of unavoidable impurities and Al, and which has a crystallized product area ratio of 3.2% or less and an average crystallized product size of 8 μm or less.

Description

Aluminum alloy forgings

[0001] The present invention relates to an aluminum alloy forging used for automobile suspension parts and the like.

[0002] Aluminum alloy forgings used for automobile suspension parts such as upper arms and lower arms are required to have high strength, high ductility, high toughness, high corrosion resistance, fracture toughness, fatigue properties, etc., and various types have been developed to meet these requirements.

[0003] For example, Patent Document 1 describes an automobile suspension part made of an aluminum alloy forging containing 0.5 to 1.25 mass% Mg, 0.4 to 1.4 mass% Si, 0.01 to 0.7 mass% Cu, 0.05 to 0.4 mass% Fe, 0.001 to 1.0 mass% Mn, 0.01 to 0.35 mass% Cr, 0.005 to 0.1 mass% Ti, with Zr limited to less than 0.15 mass%, with the balance being Al and unavoidable impurities, wherein, in a widthwise cross-sectional structure at a maximum stress generation portion, the density of crystallized precipitates observed in the structure of the cross-sectional portion where the maximum stress occurs is 1.5% or less in average area ratio, and the spacing between grain boundary precipitates observed in the structure of the cross-sectional portion including a parting line generated during forging is 0.7 μm or more in average spacing.

[0004] According to the invention described in Patent Document 1, it is described that even forged automobile suspension parts having a lightweight shape can be made to have high strength, high toughness, and high corrosion resistance, but the amount of Fe is limited to 0.40 mass % or less.

[0005] On the other hand, Patent Document 2 states that it is possible to increase the blending ratio of recycled metal from commercial scraps and to use new metal with low purity, and specifies the following as the content: Si: 0.4 mass% or more and 1.5 mass% or less, Fe: more than 0.4 mass% and 1.0 mass% or less, Cu: 0.40 mass% or less, Mg: 0.8 mass% or more and 1.3 mass% or less, Ti: 0.01 mass% or more and 0.1 mass% or less, Zn: 0.05 mass% or less, and Mn: 0.01 mass% or more and 1.0 mass% or less. The aluminum alloy forging material is characterized in that it contains at least one selected from the group consisting of 0.0 mass% or less of Al, 0.1 mass% or more and 0.4 mass% or less of Cr, and 0.05 mass% or more and 0.2 mass% or less of Zr, the amount of hydrogen is regulated to 0.25 ml / 100 g Al or less, the remainder being unavoidable impurities and Al, the average crystal grain size is 50 μm or less, the area ratio of crystallized particles is 3% or less, and the average crystallized particle size is 8 μm or less.

[0006] According to the invention described in Patent Document 2, it is stated that an aluminum alloy forging can be produced that has fracture toughness and fatigue properties equivalent to those of an aluminum alloy forging containing 0.4 mass% or less of Fe, but the Cu content is limited to 0.40 mass% or less, so the strength is somewhat inferior. Furthermore, in light of resource depletion, recycling of various things is currently progressing, and recycling of metals that are consumed in large quantities has also been carried out for some time.

[0007] Japanese Patent Publication No. 2008-163445 Japanese Patent Publication No. 2011-214093

[0008] High-strength, high-ductility, and high-corrosion-resistant aluminum alloy forgings are usually produced from virgin aluminum metal produced by refining bauxite. However, the refining of bauxite requires a huge amount of electricity, which is supplied by thermal power plants. Therefore, when producing high-strength, high-ductility, and high-corrosion-resistant aluminum alloy forgings using virgin aluminum metal, a large amount of CO is generated. ２ There was a problem of waste being discharged.

[0009] However, it is known that when the amount of virgin aluminum metal used is reduced and the amount of recycled metal used is increased, the amount of impurities such as Fe contained in the molten aluminum increases, causing intermetallic compounds to crystallize during casting, which adversely affects the strength, ductility, corrosion resistance, etc. of aluminum alloy forgings manufactured using these raw materials. ２ However, there was a problem in that the new aluminum ingots had to be produced by discharging the waste.

[0010] The present invention has been made in view of the above-mentioned problems, and aims to provide an aluminum alloy forging having an Fe content of 0.4 mass % or less, even when the Fe content exceeds 0.4 mass %, which has the same strength, ductility, fatigue properties and corrosion resistance as aluminum alloy forgings having an Fe content of 0.4 mass % or less.

[0011] In order to solve the above problems, the present inventors conducted extensive research into the chemical composition of aluminum alloy forgings. As a result, they discovered that by setting the contents of various additive elements, such as Fe, which affects tensile properties (strength, ductility (elongation)), fatigue properties, corrosion resistance, and microstructure, Ti, which is usually added to refine the cast structure, and Cu, which affects strength and corrosion resistance, within predetermined ranges, it is possible to obtain aluminum alloy forgings that are excellent in strength, ductility, fatigue properties, and corrosion resistance, even though they contain more than 0.4 mass % Fe, and thus created the present invention.

[0012] That is, the present invention relates to the following: [1] An aluminum alloy forging containing Si: 0.7% by mass to 1.5% by mass, Fe: more than 0.4% by mass to 0.67% by mass, Cu: more than 0.4% by mass to 0.8% by mass, Mg: 0.85% by mass to 1.3% by mass, Ti: 0.005% by mass to 0.07% by mass, Zn: 0.25% by mass or less, and further containing at least one element selected from the group consisting of Mn: 0.1% by mass to 0.95% by mass, Cr: more than 0.1% by mass to 0.4% by mass, and Zr: 0.05% by mass to 0.3% by mass, with the balance consisting of unavoidable impurities and Al, and having a crystallized particle area ratio of 3.2% or less and an average crystallized particle size of 8 μm or less. [2] The aluminum alloy forging according to [1], wherein the length of a high-angle grain boundary having a tilt angle of 15° or more is 4.15 mm or more, as measured in an area of ​​150 μm × 150 μm by SEM-EBSD at the center of the wall thickness of a cross section of the aluminum alloy forging perpendicular to the metal flow direction. [3] The aluminum alloy forging according to [1] or [2], wherein the 0.2% proof stress is 345 MPa or more and the elongation exceeds 12.5%.

[0013] According to the present invention, it is possible to provide an aluminum alloy forging that is excellent in strength, ductility, fatigue properties, and corrosion resistance despite containing more than 0.4 mass % Fe.

[0014] FIG. 1 is a plot of the relationship between the Fe content and elongation in the aluminum alloy forgings of Examples and Comparative Examples.

[0015] [Aluminum alloy forging] The aluminum alloy forging according to the present invention contains: Si: 0.7% by mass to 1.5% by mass; Fe: more than 0.4% by mass to 0.67% by mass; Cu: more than 0.4% by mass to 0.8% by mass; Mg: 0.85% by mass to 1.3% by mass; Ti: 0.005% by mass to 0.07% by mass; Zn: 0.25% by mass or less; and at least one element selected from the group consisting of Mn: 0.1% by mass to 0.95% by mass; Cr: more than 0.1% by mass to 0.4% by mass; and Zr: 0.05% by mass to 0.3% by mass; the balance consisting of unavoidable impurities and Al; and a crystallized particle area ratio of 3.2% or less and an average crystallized particle size of 8 μm or less. Each constituent element of the aluminum alloy forging according to the present invention will be described below.

[0016] (Si: 0.7% by mass or more and 1.5% by mass or less) Si is an essential element that contributes to high strength (yield strength). If the Si content is too low, the crystal grains become coarse, and sufficient strength (tensile strength and 0.2% yield strength) cannot be obtained by artificial aging treatment. On the other hand, if the Si content is too high, corrosion resistance is reduced. Furthermore, workability is also impaired, such as elongation being reduced. Therefore, the Si content is set to 0.7% by mass or more and 1.5% by mass or less, preferably 0.8% by mass or more and 1.3% by mass or less, and more preferably 0.9% by mass or more and 1.1% by mass or less.

[0017] (Fe: more than 0.4% by mass and not more than 0.67% by mass) Fe, together with Mn and Cr, generates dispersed particles (dispersed phases) that hinder grain boundary migration after recrystallization, preventing grain coarsening and refining the grains. In the case of conventional aluminum alloy forgings containing 0.4% by mass or less of Fe, high-temperature heat treatment promotes solid solution of dispersed particles, making the grains more likely to coarsen due to recrystallization. When Fe is contained in an amount exceeding 0.4% by mass as in the present invention, the density of dispersed particles increases, preventing recrystallization even when heated at high temperatures. Furthermore, the forging process can refine and round the Fe-based crystallized particles. Furthermore, grain refinement can suppress fatigue crack propagation and improve fatigue properties.

[0018] If the Fe content is too low, these effects are not achieved. On the other hand, if the Fe content is too high, crystallized particles such as Al-Fe-Si intermetallic compounds become coarse. The coarse crystallized particles deteriorate elongation, fatigue properties, corrosion resistance, and the like. The inventors investigated the relationship between Fe content and elongation and created a regression equation (Figure 1). This revealed that sufficient elongation can be obtained if the Fe content is 0.67 mass% or less. Details will be explained in the Examples section. Therefore, the Fe content is set to more than 0.4 mass% and not more than 0.67 mass%. Preferably, it is more than 0.40 mass% and not more than 0.65 mass%, and more preferably more than 0.40 mass% and not more than 0.55 mass%.

[0019] (Cu: more than 0.4% by mass and not more than 0.8% by mass) Cu not only contributes to improving strength through solid solution strengthening, but also has the effect of significantly accelerating age hardening of the final product during aging treatment. If the Cu content is too low, these effects are lost. On the other hand, if the Cu content is too high, the susceptibility of the metal structure of the aluminum alloy forging to stress corrosion cracking and intergranular corrosion is significantly increased, reducing the corrosion resistance and durability of the aluminum alloy forging. Therefore, the Cu content is set to more than 0.4% by mass and not more than 0.8% by mass, more preferably more than 0.4% by mass and not more than 0.5% by mass.

[0020] (Mg: 0.85% by mass or more and 1.3% by mass or less) Mg is an essential element that contributes to high strength (yield strength). If the Mg content is too low, the amount of age hardening during artificial aging treatment decreases. Also, grain coarsening is likely to occur. Furthermore, corrosion resistance also decreases. On the other hand, if the Mg content is too high, the strength (yield strength) becomes too high, which inhibits forgeability. It also reduces corrosion resistance and the like. Therefore, the Mg content is set to 0.85% by mass or more and 1.3% by mass or less, preferably 1.0% by mass or more and 1.2% by mass or less.

[0021] (Ti: 0.005% by mass or more and 0.07% by mass or less) Ti has the effect of refining the crystal grains of the ingot. If the Ti content is too low, this effect is not exerted. In addition, the crystal grains become coarse and the strength decreases. On the other hand, if the Ti content exceeds 0.07% by mass, the crystal grains of the forged material after heat treatment tend to become coarse, which deteriorates the fatigue properties. The coarse crystallized particles become the starting point of fracture and reduce the elongation. Therefore, the Ti content is set to 0.005% by mass or more and 0.07% by mass or less, preferably 0.01% by mass or more and 0.06% by mass or less.

[0022] (Zn: 0.25% by mass or less) Zn, which is easily mixed in as an impurity, is preferably 0% by mass, and if the content exceeds 0.25% by mass, the strength, elongation, and corrosion resistance decrease. Therefore, the Zn content is set to 0.25% by mass or less, preferably 0.05% by mass or less.

[0023] (Containing at least one selected from the group consisting of Mn: 0.1% by mass or more and 0.95% by mass or less, Cr: more than 0.1% by mass and 0.4% by mass or less, and Zr: 0.05% by mass or more and 0.3% by mass or less) Of these, Mn and Cr generate dispersed particles (dispersed phases) made of Al-Mn and Al-Cr intermetallic compounds in which Fe, Mn, Cr, Si, Al, etc. are selectively bonded according to their contents during homogenization heat treatment and subsequent hot forging.

[0024] Although it depends on the manufacturing conditions, the dispersed particles of Mn and Cr have the effect of preventing grain boundary migration after recrystallization, thereby preventing the crystal grains from becoming coarse.

[0025] If the Mn and Cr contents are too low, these effects cannot be expected, and the crystal grains become coarse, resulting in a decrease in strength. On the other hand, excessive inclusion of these elements causes a decrease in elongation. Therefore, it is preferable to contain at least one of Mn and Cr, and the Mn content is 0.1 mass% to 0.95 mass%, preferably 0.2 mass% to 0.6 mass%, and the Cr content is more than 0.1 mass% to 0.4 mass%, preferably more than 0.1 mass% to 0.3 mass%, and more preferably more than 0.1 mass% to 0.2 mass%.

[0026] Zr generates dispersed particles (dispersed phases) similarly to Mn and Cr. In the case of Zr, depending on the casting conditions, for example, when Ti is included, Zr can actually hinder the refinement of the crystal grains of the ingot and cause the crystal grains to become coarse. It can also cause a decrease in fatigue properties. Therefore, it is desirable to add Zr within a range that does not cause the crystal grains to become coarse during casting. Specifically, the Zr content is 0.05% by mass or more and 0.3% by mass or less, preferably 0.05% by mass or more and 0.1% by mass or less.

[0027] By containing Si, Cu, and Mg within the above ranges, the aluminum alloy forging material according to the present invention can obtain the strength required for, for example, automobile suspension parts that require high strength. By containing Ti within the above ranges, the cast structure can be refined. Furthermore, by containing Mn, Cr, and Zr within the above ranges, recrystallization during solution treatment can be suppressed, resulting in fine crystals. Therefore, high strength can be ensured. Furthermore, in the present invention, even when the iron content is so high, forging is performed to reduce, refine, and round the iron-containing crystallites, and to refine the crystal grains, thereby ensuring elongation, fatigue properties, and corrosion resistance.

[0028] (Balance: Inevitable Impurities and Al) The balance consists of inevitable impurities and Al. Examples of inevitable impurities include C, Ni, Na, Ca, V, and Hf. These are easily mixed in as impurities and impair the properties of automobile suspension parts, so it is preferable not to include them, but it is acceptable if they are contained in an amount of 0.05% by mass or less each and 0.10% by mass or less in total.

[0029] B is also an impurity, but like Ti, it has the effect of refining the crystal grains of the ingot and improving the workability during extrusion and forging. However, if it is contained in an amount exceeding 300 ppm, it also forms coarse crystal precipitates, which reduces the workability. Therefore, it is preferable that the B content be 300 ppm or less.

[0030] <Crystallized particles> In the aluminum alloy forged material of the present invention, the area ratio of crystallized particles is 3.2% or less, and the average crystallized particle size is 8 μm or less.

[0031] (Crystallized matter area ratio: 3.2% or less) The crystallized matter area ratio is determined by the amount of added elements and their solid solution amounts, etc. By setting the crystallized matter area ratio to 3.2% or less, it is possible to suppress a decrease in elongation due to an increase in the number of crack propagation paths. It is also possible to suppress a decrease in corrosion resistance due to the parent phase around the crystallized matter becoming more susceptible to corrosion. The crystallized matter area ratio is more preferably 3.1% or less, and even more preferably 3.0% or less.

[0032] The crystallized matter area ratio can be calculated by taking a backscattered electron image of a cross section perpendicular to the metal flow direction of the forged material at the center of the wall thickness using an SEM and analyzing the image. A specific calculation method will be described in the Examples section.

[0033] (Average crystallized particle size: 8 μm or less) The average crystallized particle size is determined by the amount of added element, the solidification rate, etc. By setting the average crystallized particle size to 8 μm or less, it is possible to prevent the crystallized particles from becoming the starting point of cracks in a tensile test and reducing elongation. The average crystallized particle size is more preferably 7 μm or less, and even more preferably 6 μm or less.

[0034] The average size of crystallized particles can be determined by taking a backscattered electron image of a cross section perpendicular to the metal flow of the forged material at the center of the wall thickness using an SEM, and converting it into a circle of the same area using analysis software to calculate the average size.

[0035] (Average crystal grain size: 50 μm or less) The average crystal grain size affects mechanical properties. From the viewpoint of tensile properties and fatigue properties, the average crystal grain size is preferably 45 μm or less, and more preferably 40 μm or less.

[0036] The average grain size can be calculated by the minor axis intercept method, which involves etching the central part of the wall thickness of a cross section of a forged material perpendicular to the metal flow direction, photographing it with an optical microscope, drawing a line perpendicular to the major axis of the grains, counting the number of grains on that line, and dividing the distance of the line by the number of grains measured.

[0037] (Length of high-angle grain boundaries: 4.15 mm or more) In the aluminum alloy forgings according to the embodiments of the present invention, the length of high-angle grain boundaries with tilt angles of 15° or more, measured in an area of ​​150 μm × 150 μm by SEM-EBSD at the center of the wall thickness of a cross section perpendicular to the metal flow direction of the aluminum alloy forging, is preferably 4.15 mm or more. The length of high-angle grain boundaries affects mechanical properties. From the viewpoints of tensile properties and fatigue properties, the length of high-angle grain boundaries is more preferably 4.2 mm or more, and even more preferably 4.3 mm or more.

[0038] (Length of low-angle grain boundary: 2.0 mm or more) The length of low-angle grain boundary affects mechanical properties. From the viewpoint of tensile properties and fatigue properties, the length of low-angle grain boundary is preferably 2.0 mm or more, more preferably 2.2 mm or more, and even more preferably 2.5 mm or more.

[0039] The SEM-EBSD (EBSP) method is a crystal orientation analysis method using an SEM equipped with an electron backscattering (scattered) diffraction pattern (EBSD) system. The high-angle grain boundary length and the low-angle grain boundary length can be automatically calculated by performing crystal orientation analysis using the SEM-EBSD method at the center of the wall thickness of a cross section perpendicular to the metal flow direction of the forged material over a 150 μm × 150 μm area with an observation step of 0.5 μm. The data is then analyzed using analysis software (TSL OIM Analysis 7 × 64) to define boundaries where the orientation difference between adjacent crystal grains is 15° or more as high-angle grain boundaries and boundaries where the difference is 5° to 15° as low-angle grain boundaries. The analysis is performed excluding data with a CI (Confidence Index) value of 0.1 or less.

[0040] <Tensile properties> (0.2% yield strength: 345 MPa or more) The 0.2% yield strength of the aluminum alloy forging of the present invention is preferably 345 MPa or more. If it is less than 345 MPa, the strength required for a forging material for automobile suspension parts may not be ensured. The 0.2% yield strength is more preferably 355 MPa or more, and even more preferably 360 MPa or more.

[0041] (Elongation: More than 12.5%) The elongation (%) of the aluminum alloy forging of the present invention is preferably more than 12.5%. If it is 12.5% ​​or less, the formability as a forging material for automobile suspension parts may be insufficient. The elongation is preferably 13% or more, more preferably 13.5% or more.

[0042] (Tensile strength: 370 MPa or more) From the viewpoint of strength as a forged material for automobile suspension parts, the tensile strength of the aluminum alloy forging of the present invention is preferably 370 MPa or more, more preferably 380 MPa or more, and even more preferably 385 MPa or more.

[0043] The tensile properties and fatigue properties of the aluminum alloy forgings can be adjusted by using the above-mentioned chemical composition and the manufacturing method described below.

[0044] The elongation, 0.2% proof stress, and tensile strength of aluminum alloy forgings can be measured by a metallic material tensile test in accordance with JIS Z 2241 (revised in 2011). Specifically, a tensile test specimen (No. 4 test specimen) conforming to JIS Z 2201 is prepared by cutting out from the center of a cross section at any location so that the test specimen axis is parallel to the metal flow direction of the forging, and a tensile test is performed at room temperature (25°C). The number of samples measured for mechanical properties is set to 2, and each value is calculated as an average. In this way, the elongation, 0.2% proof stress, and tensile strength can be calculated.

[0045] <Fatigue properties> (Number of repetitions to fracture at a repeated stress of 170 MPa in a fatigue test using an unnotched rotating bending fatigue test piece: 1 × 10 ６ In a fatigue test using an unnotched rotating bending fatigue test piece, the number of repetitions to fracture of the aluminum alloy forging of the present invention at a repeated stress of 170 MPa is 1×10 or more from the viewpoint of fatigue life as a forging for automobile suspension parts. ６ The number of cycles to fracture at a repeated stress of 170 MPa in a fatigue test using an unnotched rotating bending fatigue test piece is preferably 2×10 ６ cycles or more, more preferably 3 x 10 ６ It's more than a cycle.

[0046] (Number of repetitions to fracture at a repeated stress of 70 MPa in a fatigue test using a notched rotating bending fatigue test piece: 1 × 10 ６ In a fatigue test using a notched rotating bending fatigue test piece of the aluminum alloy forging of the present invention, the number of repetitions to fracture at a repeated stress of 70 MPa is set to 1×10 or more from the viewpoint of fatigue life as a forging for automobile suspension parts. ６ The number of cycles to fracture under a repeated stress of 70 MPa in a fatigue test using a notched rotating bending fatigue test piece is preferably 2×10 ６ cycles or more, more preferably 3 x 10 ６ It's more than a cycle.

[0047] The fatigue properties of aluminum alloy forgings can be measured by a rotating bending fatigue test for metallic materials in accordance with JIS Z 2274. Specifically, for unnotched rotating bending fatigue test specimens, rotating bending fatigue test specimens (No. 2 test specimens) in accordance with JIS Z 2274 are prepared by cutting from any location parallel to the metal flow direction of the aluminum alloy forging, and a rotating bending fatigue test is performed at a cyclic stress of 170 MPa. For notched rotating bending fatigue test specimens, similarly, annular semicircular grooved test specimens (groove corner radius ρ of 0.31 mm) in accordance with JIS Z 2274 are prepared by cutting from any location parallel to the metal flow direction of the aluminum alloy forging, and a rotating bending fatigue test is performed at a cyclic stress of 70 MPa. The number of samples in the fatigue property measurement is set to 2, and the minimum value is calculated. In this manner, fatigue properties can be calculated.

[0048] The fatigue properties of the aluminum alloy forgings can be adjusted by using the above-mentioned chemical composition and the manufacturing method described below.

[0049] The aluminum alloy forgings according to the present invention described above can have the same strength, elongation, fatigue properties, and corrosion resistance as aluminum alloy forgings containing 0.4% by mass or less of Fe. In other words, the aluminum alloy forgings according to the present invention can contain Fe, which has a negative effect on strength, elongation, etc., in an amount exceeding 0.4% by mass. Therefore, when producing aluminum alloy forgings, the amount of virgin aluminum used can be reduced, and CO ２ Emissions can be significantly reduced.

[0050] [Method for manufacturing aluminum alloy forgings] Next, a method for manufacturing an aluminum alloy forging according to the present invention will be described. The method for manufacturing an aluminum alloy forging according to the present invention is not particularly limited, but the above-described aluminum alloy forgings can be suitably manufactured by a manufacturing method including, in this order, a casting step, a homogenization heat treatment step, a heating step, a forging step, a solution treatment step, a quenching step, and an artificial aging treatment step. It is acceptable for the manufacturing method for the aluminum alloy forgings to further include other steps that do not impair the desired effects of the present invention. Examples of such steps include an extrusion step performed between the homogenization heat treatment step and the heating step, and a forging roll step performed between the heating step and the forging step.

[0051] (Casting Process) The casting process is a process for casting an aluminum alloy ingot having the above-described composition. The composition has already been described in detail, so further explanation is omitted. The casting process is preferably carried out at a heating temperature of 710 to 810°C and a cooling rate to the liquidus temperature of 7°C / sec or more. A heating temperature of 710°C or more in the casting process shortens the melting time, allowing for efficient operation. Furthermore, a heating temperature of 810°C or less in the casting process suppresses the generation of dross, which is an oxide, and reduces metal loss, allowing for efficient production of an ingot. A cooling rate to the liquidus temperature of less than 7°C / sec results in coarsening of the crystallized particles, making it impossible to achieve a crystallized particle area ratio of 3.2% or less. The heating temperature is preferably 710 to 750°C, and the cooling rate to the liquidus temperature is preferably 10°C / sec or more.

[0052] Casting can be carried out by melting and casting methods such as continuous casting, semi-continuous casting, and hot-top casting, but among these, continuous casting is preferred.

[0053] (Homogenization Heat Treatment Step) The next step is homogenization heat treatment of the ingot cast in the casting step at 420 to 560°C for 2.5 to 8 hours. By using a heating temperature of 420°C or higher and a heating time of 2.5 hours or longer in the homogenization heat treatment step, the crystallized particles are sufficiently dissolved, reducing the area ratio of the crystallized particles and ensuring product elongation. On the other hand, by using a heating temperature of 560°C or lower and a heating time of 8 hours or shorter in the homogenization heat treatment step, coarsening of dispersed particles is suppressed, allowing them to be dispersed uniformly, finely, and densely. In other words, the effect of refining crystal grains is easily achieved, resulting in a smaller average crystal grain size. The heating temperature in the homogenization heat treatment step is preferably 500 to 540°C, and the heating time is preferably 4 to 8 hours.

[0054] (Heating Step) The next heating step involves heating the ingot that has been homogenized in the homogenization heat treatment step at 400 to 545°C for 0.5 hours or more. As described above, Fe, together with Mn and Cr, has the effect of generating dispersed particles (dispersed phases) and preventing grain boundary migration after recrystallization, as in the present invention. Therefore, by adding a large amount of Fe, even if a sufficient heating step is performed, the number and density of dispersed particles can be maintained at the same level as in conventional materials, preventing grain coarsening and allowing the grains to remain fine. Therefore, the tensile properties can be maintained at the same level as in conventional materials. This effect can be achieved by sufficiently heating the ingot in this heating step before the forging step, reducing the Fe-based crystallized particles through solid solution, and further refining them.

[0055] If the heating temperature in the heating step is 440°C or higher and the heating time is 0.5 hours or longer, even in the case of aluminum alloy forgings containing a large amount of Fe as in the present invention, the solid solution of Fe-based crystallized substances progresses, and elongation can be maintained at the same level as conventional materials. On the other hand, if the heating temperature is 545°C or lower, the occurrence of eutectic melting due to heat generated during processing is suppressed, making it difficult for voids to occur and mechanical properties to deteriorate. In addition, coarsening and low density of dispersed particles due to heat treatment are unlikely to occur, making it easy to achieve a crystal grain refinement effect. The heating temperature in the heating step is preferably 440 to 545°C.

[0056] (Forging Process) The next forging process is a process in which the ingot heated in the heating process is forged at a forging end temperature of 350°C or higher and a rolling reduction of 50 to 95% to obtain a forged material of a predetermined shape. The rolling reduction is defined as (1-(L1 / L0)) x 100, where L0 is the height of the material before forging and L1 is the height after forging. If the forging end temperature in the forging process is 350°C or higher, residual strain is not large, so recrystallization is unlikely to occur and grains are unlikely to coarsen. Furthermore, if the rolling reduction in the forging process is 50% or higher, casting defects can be compressed and grains and precipitates can be sufficiently reduced in size. If the rolling reduction is 95% or less, the processing rate is not too high and grain coarsening due to recrystallization is unlikely to occur. Note that the forging end temperature is preferably as high as possible within a range that does not exceed the heating temperature. The forging end temperature is preferably 370°C or higher, and the rolling reduction is preferably 70 to 90%.

[0057] Forging under these conditions can be carried out using, for example, a mechanical press or a hydraulic press.

[0058] (Solution Treatment Process) The next step is to subject the forged material obtained in the forging process to solution treatment at 480 to 580°C for more than 0 hours but not more than 24 hours. This solution treatment can promote the solid solution of added elements to enhance strength during the artificial aging process described below, and can also enhance elongation by refining the crystallized particles. When the heating temperature in the solution treatment process is 480°C or higher and the heating time is longer than 0 hours, sufficient solution treatment is achieved, resulting in good elongation and strength (tensile strength and 0.2% proof stress). On the other hand, when the heating temperature in the solution treatment process is 580°C or lower and the heating time is 24 hours or shorter, the crystal grains are less likely to coarsen, the average crystal grain size is less likely to increase, and good strength (tensile strength and 0.2% proof stress) can be obtained. The heating temperature in the solution treatment process is preferably 540 to 560°C, and the heating time is preferably 2.5 to 8.0 hours.

[0059] (Quenching process) The next quenching process is a process in which the forged material that has been solution treated in the solution treatment process is quenched at 75°C or less. Quenching can improve strength. If the quenching temperature in the quenching process is 75°C or less, sufficient quenching is achieved, and the strength can be sufficiently improved in the artificial aging process described below. The lower limit of the quenching temperature may be approximately the room temperature of the water used for quenching, that is, 20±15°C (5 to 35°C) as specified in JIS Z 8703.

[0060] (Artificial Aging Treatment Step) The next step, the artificial aging treatment step, involves artificially aging the forged material quenched in the quenching treatment step at 160 to 250°C for 0.5 to 20 hours. The processes from the solution treatment step to this artificial aging treatment step are known as artificial aging hardening treatment. This artificial aging treatment can achieve the strength required for, for example, automotive suspension parts. A heating temperature of 160°C or higher and a heating time of 0.5 hours or longer in the artificial aging treatment step can provide sufficient strength, fatigue properties, and corrosion resistance. On the other hand, a heating temperature of 250°C or higher and a heating time of 20 hours or longer in the artificial aging treatment step results in excessive overaging, making it impossible to achieve sufficient strength and elongation. The heating temperature in the artificial aging treatment step is preferably 170 to 250°C, and the heating time is preferably 3 to 12 hours.

[0061] According to the manufacturing method of the aluminum alloy forgings described above, it is possible to manufacture aluminum alloy forgings having an Fe content exceeding 0.4 mass % but having strength, elongation, fatigue properties and corrosion resistance equivalent to those of aluminum alloy forgings having an Fe content of 0.4 mass % or less.

[0062] The present invention will be specifically described below with reference to examples of the present invention, but the technical scope of the present invention is not limited thereto.

[0063] (Production of Forged Materials) The aluminum alloy forgings according to Nos. 1 to 6 were produced under the following conditions using aluminum alloys having the chemical compositions shown in Table 1. First, molten aluminum alloy was gravity cast into a die to produce an ingot. The casting conditions were a heating temperature of 720°C and a cooling rate to the liquidus temperature of 10 to 15°C / sec. Each ingot was faced to a diameter of 55 mm and a length of 100 mm, and then subjected to homogenization treatment, heating, hot die forging using a hydraulic press, solution treatment, quenching, and artificial aging under the conditions shown in Table 2 below, to produce the aluminum alloy forgings of the respective numbers.

[0064]

[0065]

[0066]

[0067] The tensile properties, fatigue properties, average grain size (μm), high-angle grain boundary length (mm), low-angle grain boundary length (mm), crystallized particle area ratio (%), average crystallized particle size (μm), and general corrosion resistance (corrosion rate (mm / y) and stress corrosion cracking resistance (SCC resistance)) of the forged materials Nos. 1 to 6 were evaluated. Tensile properties were measured in terms of tensile strength (MPa), 0.2% proof stress (MPa), and elongation (%). Fatigue properties were measured by measuring the number of cycles to fracture at a cyclic stress of 170 MPa in fatigue tests using unnotched rotating bending fatigue test pieces, and the number of cycles to fracture at a cyclic stress of 70 MPa in fatigue tests using notched rotating bending fatigue test pieces. These evaluations were performed as follows.

[0068] <Tensile Properties> Tensile test pieces (No. 4 test pieces) conforming to JIS Z 2201 were prepared by cutting out from any location of the aluminum alloy forging so as to be parallel to the metal flow direction, and the tensile properties were measured using a tensile tester conforming to JIS Z 2241. Regarding the tensile properties, a 0.2% proof stress of 345 MPa or more was considered pass, and a value less than 345 MPa was considered fail, and an elongation of more than 12.5% ​​was considered pass, and a value less than 12.5% ​​was considered fail.

[0069] <Fatigue Properties> For the unnotched rotating bending fatigue test specimens, rotating bending fatigue test specimens (No. 2 test specimens) conforming to JIS Z 2274 were prepared by cutting out from any location parallel to the metal flow direction of the aluminum alloy forging, and the fatigue properties were evaluated at a cyclic stress of 170 MPa. For the notched rotating bending fatigue test specimens, annular semicircular grooved test specimens conforming to JIS Z 2274 (groove corner R, ρ, was 0.31 mm) were prepared by cutting out from any location parallel to the metal flow direction of the aluminum alloy forging, and the fatigue properties were evaluated at a cyclic stress of 70 MPa. The number of repetitions to fracture at a cyclic stress of 170 MPa in the fatigue test using the unnotched rotating bending fatigue test specimen and the number of repetitions to fracture at a cyclic stress of 70 MPa in the fatigue test using the notched rotating bending fatigue test specimen were both 1×10 ６ cycles or more was considered a pass.

[0070] Fig. 1 is a plot of the relationship between the Fe content and elongation for the aluminum alloy forgings Nos. 1 to 6. A regression equation was created from this. It can be seen that if the Fe content is 0.67 mass% or less, the elongation exceeds 12.5%, and sufficient elongation can be obtained.

[0071] (Average grain size) The average grain size (μm) was calculated by etching the center of the wall thickness of a cross section perpendicular to the metal flow direction of an aluminum alloy forging, photographing it with an optical microscope at 400x magnification, drawing a line perpendicular to the long axis of the grains, counting the number of grains on the line, and dividing the distance of the line by the number of grains measured. If the average grain size exceeds 50 μm, the tensile properties and fatigue properties will deteriorate. Therefore, an average grain size of 50 μm or less was considered acceptable, and one exceeding 50 μm was considered unacceptable.

[0072] (High-angle grain boundary length) The SEM-EBSD (EBSP) method is a crystal orientation analysis method using an SEM equipped with a backscattered electron diffraction pattern (EBSD) system. The high-angle grain boundary length was measured by SEM-EBSD crystal orientation analysis at the center of the wall thickness of a cross section perpendicular to the metal flow of the forged material in an area of ​​150 μm x 150 μm with an observation step of 0.5 μm. The boundary where the orientation difference between adjacent crystal grains is 15 ° or more was defined as a high-angle grain boundary, and the analysis was automatically calculated using analysis software (TSL Solutions OIM Analysis ver. 7). Note that the analysis was performed excluding data with a CI value (Confidence Index) of 0.1 or less.

[0073] The length of high-angle grain boundaries is measured in an area of ​​150 μm × 150 μm by the SEM-EBSD method in the metal structure of the center of the thickness of aluminum alloy forgings. When the length of high-angle grain boundaries with an inclination angle of 15° or more is less than 4.15 mm, fatigue properties deteriorate, and the number of repetitions to fracture at a repeated stress of 170 MPa in fatigue tests using unnotched rotating bending fatigue test pieces or at a repeated stress of 70 MPa in fatigue tests using notched rotating bending fatigue test pieces is 1 × 10 ６Therefore, the length of high-angle grain boundaries, measured by SEM-EBSD in an area of ​​150 μm × 150 μm at the center of the wall thickness of a cross section perpendicular to the metal flow direction of the aluminum alloy forging, was determined to be acceptable if the length of the high-angle grain boundaries with an inclination angle of 15° or more was 4.15 mm or more.

[0074] (Low-angle grain boundary length) The SEM-EBSD (EBSP) method is a crystal orientation analysis method using an SEM equipped with an electron backscattering (scattered) diffraction pattern (EBSD) system. The low-angle grain boundary length was measured by SEM-EBSD crystal orientation analysis at the center of the wall thickness of a cross section perpendicular to the metal flow of the forged material in an area of ​​150 μm × 150 μm with an observation step of 0.5 μm. The boundary where the orientation difference between adjacent crystal grains is 5 ° to 15 ° was defined as a low-angle grain boundary, and the data was automatically calculated by analyzing it using analysis software (TSL OIM Analysis 7 × 64). Note that the analysis was performed excluding data with a CI value (Confidence Index) of 0.1 or less.

[0075] The length of low-angle grain boundaries is measured in an area of ​​150 μm × 150 μm by SEM-EBSD in the metal structure at the center of the thickness of aluminum alloy forgings. When the length of low-angle grain boundaries with an inclination angle of 5° to 15° is less than 2.0 mm, fatigue properties deteriorate, and the number of repetitions to fracture at a repeated stress of 170 MPa in a fatigue test using an unnotched rotating bending fatigue test piece or at a repeated stress of 70 MPa in a fatigue test using a notched rotating bending fatigue test piece is 1 × 10 ６ Therefore, the length of low-angle grain boundaries, measured by SEM-EBSD in an area of ​​150 μm × 150 μm at the center of the wall thickness of a cross section perpendicular to the metal flow direction of the aluminum alloy forging, was determined to be acceptable if the length of the low-angle grain boundaries with an inclination angle of 5° to 15° was 2.0 mm or more.

[0076] <Crystallized Matter> (Crystallized Matter Area Ratio) The crystallized matter area ratio (%) was calculated by taking a 400x backscattered electron image using an SEM (JEOL, JSM-IT700HR) at the center of the wall thickness of a cross section perpendicular to the metal flow of the forged material, calculating the area of ​​white contrast using image analysis (WinROOF2018, Ver. 4.7.0), and dividing by the total area where image analysis was performed. If the crystallized matter area ratio exceeds 3.2%, the elongation decreases and it becomes impossible to ensure elongation of more than 12.5%. Therefore, a crystallized matter area ratio of 3.2% or less was considered pass, and one exceeding 3.2% was considered fail.

[0077] (Average Crystal Size) The average crystal size (μm) was determined by taking a 400x backscattered electron image using a SEM (JEOL, JSM-IT700HR) at the center of the wall thickness of a cross section perpendicular to the metal flow of the forged material, converting it into a circle of the same area using analysis software, and calculating the average size. If the average crystal size exceeds 8 μm, the elongation decreases and it becomes impossible to ensure elongation of more than 12.5%. Therefore, an average crystal size of 8 μm or less was considered acceptable, and one exceeding 8 μm was considered unacceptable.

[0078] <General corrosion resistance> (Corrosion rate) The corrosion rate (mm / y) was measured by preparing a flat plate from an aluminum alloy forging and testing it according to the alternate immersion method of ASTM G47. Here, the corrosion rate (mm / y) represents the amount of corrosion (mm) in one year (365 days). The test conditions were repeated immersion in salt water and removal for 30 days, and the weights before and after the test were measured and calculated using the following formula (1). Here, M is the corrosion weight loss (g / 30 days), and ρ is the density 2.7 (g / cm ３ ), S is the area of ​​the evaluation section (cm ２ ) is shown.

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[0080] If the corrosion rate exceeds 0.12 mm / y, the aluminum alloy forging material will not be able to function satisfactorily as a material used for automobile suspension parts. Therefore, a corrosion rate of 0.12 mm / y or less was deemed acceptable, and a corrosion rate exceeding 0.12 mm / y was deemed unacceptable.

[0081] <Stress corrosion cracking resistance> Evaluation of stress corrosion cracking resistance (SCC resistance) was performed in accordance with JIS H 8711 by preparing C-ring-shaped test pieces from aluminum alloy forgings and performing an alternating immersion method. The SCC test was performed under conditions of a load stress of 180 MPa (tensile) and a period of 30 days, assuming use as an automobile suspension part. After the test, the test pieces were observed for the occurrence of stress corrosion cracking. For stress corrosion cracking resistance, test pieces that were visually observed to have no stress corrosion cracking were rated as passed, and test pieces that had stress corrosion cracking were rated as failed.

[0082] Table 1 shows the tensile properties of the forged materials Nos. 1 to 6, the number of cycles to fracture at a repeated stress of 170 MPa in a fatigue test using an unnotched rotating bending fatigue test piece, the number of cycles to fracture at a repeated stress of 70 MPa in a fatigue test using a notched rotating bending fatigue test piece, the average grain size (μm), the high-angle grain boundary length (mm), the low-angle grain boundary length (mm), the crystallized particle area ratio (%), the average crystallized particle size (μm), the corrosion rate (mm / y) as general corrosion resistance, and the SCC resistance.

[0083] As shown in Table 1, the aluminum alloy forged material No. 4 satisfied all the requirements of the present invention and thus achieved favorable evaluation results. On the other hand, the forged materials Nos. 1 and 2 had Fe contents below the lower limit. The forged material No. 3 had a Ti content exceeding the upper limit, resulting in a small high-angle grain boundary length. As a result, the number of cycles to fracture at a repeated stress of 170 MPa in a fatigue test using an unnotched rotating bending fatigue test piece and the number of cycles to fracture at a repeated stress of 70 MPa in a fatigue test using a notched rotating bending fatigue test piece were unacceptable. The forged materials Nos. 5 and 6 had Fe contents exceeding the upper limit, resulting in coarsening of the Al-Fe-Si crystals and a large crystal area ratio. As a result, the elongation and corrosion rate were unacceptable.

[0084] According to the present invention, it is possible to provide an aluminum alloy forging that is excellent in strength, ductility, fatigue properties, and corrosion resistance despite containing more than 0.4 mass % Fe.

[0085] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. This application is based on Japanese patent applications filed on January 15, 2024 (Patent Application No. 2024-003898) and October 3, 2024 (Patent Application No. 2024-174169), the contents of which are incorporated herein by reference.

Claims

1. Containing Si: 0.7% by mass or more and 1.5% by mass or less, Fe: more than 0.4% by mass and 0.67% by mass or less, Cu: more than 0.4% by mass and 0.8% by mass or less, Mg: 0.85% by mass or more and 1.3% by mass or less, Ti: 0.005% by mass or more and 0.07% by mass or less, Zn: 0.25% by mass or less, and further containing at least one selected from the group consisting of Mn: 0.1% by mass or more and 0.95% by mass or less, Cr: more than 0.1% by mass and 0.4% by mass or less, and Zr: 0.05% by mass or more and 0.3% by mass or less, with the balance consisting of inevitable impurities and Al, and having a crystallized area ratio of 3.2% or less and an average crystallized size of 8 μm or less, an aluminum alloy forging material.

2. The aluminum alloy forging material according to claim 1, wherein in the center of the thickness of a cross-section perpendicular to the metal flow of the aluminum alloy forging material, the length of large-angle grain boundaries with an inclination angle of 15° or more measured in a range of 150 μm × 150 μm by the SEM-EBSD method is 4.15 mm or more.

3. The aluminum alloy forging material according to claim 1 or 2, having a 0.2% proof stress of 345 MPa or more and an elongation of more than 12.5%.

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