Forged aluminum alloy material

Abstract

To provide 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%.SOLUTION: The present invention relates to a 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, and 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.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to an aluminum alloy forging material used as an automotive underbody part or the like.

Background Art

[0002] For aluminum alloy forging materials used as automotive underbody parts such as upper arms and lower arms, high strength, high ductility, high toughness, high corrosion resistance, fracture toughness, fatigue characteristics, etc. are required, and various materials have been developed conventionally.

[0003] For example, Patent Document 1 discloses an aluminum alloy forging material containing 0.5 to 1.25% by mass of Mg, 0.4 to 1.4% by mass of Si, 0.01 to 0.7% by mass of Cu, 0.05 to 0.4% by mass of Fe, 0.001 to 1.0% by mass of Mn, 0.01 to 0.35% by mass of Cr, 0.005 to 0.1% by mass of Ti respectively, and regulated such that Zr is less than 0.15% by mass, with the balance being Al and inevitable impurities. An automotive underbody part composed of this aluminum alloy forging material is described, in which in the cross-sectional structure in the width direction at the maximum stress generation site, the precipitate density observed in the structure of the cross-sectional site where the maximum stress occurs is 1.5% or less in terms of average area ratio, and the interval between each grain boundary precipitate observed in the structure of the cross-sectional site including the parting line generated during forging is 0.7 μm or more in terms of average interval.

[0004] According to the invention described in this Patent Document 1, although it is described that even for a forging material automotive underbody part with a lightweight shape, high strength, high toughness, and high corrosion resistance can be achieved, the Fe content is limited to 0.40% by mass or less.

[0005] On the one hand, Patent Document 2 states that the mixing ratio of recycled ingots of scrap can be increased and new ingots of low purity can be used. It contains Si: 0.4% by mass or more and 1.5% by mass or less, Fe: more than 0.4% by mass and 1.0% by mass or less, Cu: 0.40% by mass or less, Mg: 0.8% by mass or more and 1.3% by mass or less, Ti: 0.01% by mass or more and 0.1% by mass or less, and is regulated to contain Zn: 0.05% by mass or less. Further, it contains at least one selected from the group consisting of Mn: 0.01% by mass or more and 1.0% by mass or less, Cr: 0.1% by mass or more and 0.4% by mass or less, and Zr: 0.05% by mass or more and 0.2% by mass or less. The hydrogen content is regulated to 0.25 ml / 100 g Al or less, the balance consisting of inevitable impurities and Al, with an average crystal grain size of 50 μm or less, a crystallized area ratio of 3% or less, and an average crystallized size of 8 μm or less. An aluminum alloy forging material is described.

[0006] According to the invention described in this Patent Document 2, although it is described that an aluminum alloy forging material having the same fracture toughness and fatigue characteristics as an aluminum alloy forging material with an Fe content of 0.4% by mass or less can be manufactured, since the Cu content is limited to 0.40% by mass or less, it is slightly inferior in strength. Also, currently, from the perspective of resource depletion, recycling of various things is progressing, and recycling of metals that are consumed in large quantities has also been carried out for a long time.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] A high-strength, high-ductility, and highly corrosion-resistant aluminum alloy forging material is usually produced from virgin aluminum produced by refining bauxite. However, refining bauxite requires a huge amount of electricity, which is supplied by thermal power generation. Therefore, when manufacturing a high-strength, high-ductility, and highly corrosion-resistant aluminum alloy forging material using virgin aluminum, there was a problem of a large amount of CO2 being emitted.

[0009] However, if the amount of virgin aluminum used is reduced and the amount of recycled aluminum used increases, impurities such as Fe contained in the molten aluminum increase, and intermetallic compounds crystallize during casting, which is known to have an adverse effect on the strength, ductility, corrosion resistance, etc. of the aluminum alloy forging material manufactured using these raw materials. Therefore, there was a problem that virgin aluminum produced by emitting a large amount of CO2 had to be used in order to limit the amount of Fe.

[0010] The present invention has been made in view of the above problems, and an object thereof is to provide an aluminum alloy forging material having the same strength, ductility, fatigue characteristics, and corrosion resistance as an aluminum alloy forging material having an Fe content of 0.4 mass% or less even when the Fe content exceeds 0.4 mass%.

Means for Solving the Problems

[0011] As a result of intensive studies on the chemical composition of the aluminum alloy forging material in order to solve the above problems, the present inventors have found that Fe, which affects tensile properties (strength, ductility (elongation)), fatigue characteristics, corrosion resistance, and microstructure, Ti, which is usually added for refining the casting structure, Cu, which affects strength and corrosion resistance, etc. By setting the contents of various additive elements within a predetermined range, it was found that an aluminum alloy forging material excellent in strength, ductility, fatigue characteristics, and corrosion resistance can be obtained despite containing more than 0.4 mass% of Fe, and the present invention was created.

[0012] That is, the present invention relates to the following. 〔1〕 Si: 0.7 mass % or more and 1.5 mass % or less, Fe: More than 0.4 mass % and 0.67 mass % or less, Cu: More than 0.4 mass % and 0.8 mass % or less, Mg: 0.85 mass % or more and 1.3 mass % or less, Ti: 0.005 mass % or more and 0.07 mass % or less, Zn: Contained at 0.25 mass % or less, and further, Mn: 0.1 mass % or more and 0.95 mass % or less, Cr: More than 0.1 mass % and 0.4 mass % or less, and at least one selected from the group of Zr: 0.05 mass % or more and 0.3 mass % or less is included, The balance consists of inevitable impurities and Al, An aluminum alloy forging material having a crystallized area ratio of 3.2% or less and an average crystallized size of 8 μm or less. [2] In the thickness center 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. The aluminum alloy forging material according to [1]. [3] The 0.2% proof stress is 345 MPa or more and the elongation is 12.5% or more. The aluminum alloy forging material according to [1] or [2]. [Advantages of the Invention]

[0013] According to the present invention, an aluminum alloy forging material excellent in strength, ductility, fatigue characteristics, and corrosion resistance can be provided, even though it contains Fe exceeding 0.4 mass %. [Brief Description of the Drawings]

[0014]

Figure 1

[0015] [Aluminum Alloy Forging Material] The aluminum alloy forging material according to the present invention is 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: contained at 0.25% by mass or less, and further, 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 at least one selected from the group of Zr: 0.05% by mass or more and 0.3% by mass or less is included, the balance being unavoidable impurities and Al, an aluminum alloy forging material having a precipitate area ratio of 3.2% or less and an average precipitate size of 8 μm or less. Hereinafter, each constituent requirement of the aluminum alloy forging material according to the present invention will be described in detail.

[0016] (Si: 0.7% by mass or more and 1.5% by mass or less) Si is an essential element that contributes to increasing the strength (yield strength). If the content of Si is too small, 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 content of Si is too large, the corrosion resistance is reduced. Furthermore, workability is also inhibited, such as a decrease in elongation. Therefore, the content of Si is 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, more preferably 0.9% by mass or more and 1.1% by mass or less.

[0017] (Fe: more than 0.4% by mass and 0.67% by mass or less) Fe, together with Mn and Cr, generates dispersed particles (dispersed phase) to prevent grain boundary movement after recrystallization, prevent coarsening of crystal grains, and has the effect of refining crystal grains. Here, in the case of a conventional aluminum alloy forging material with an Fe content of 0.4 mass% or less, when heat-treated at a high temperature, solid solution of dispersed particles also progresses, making it easier for crystal grains to coarsen due to recrystallization. When the Fe content exceeds 0.4 mass% as in the present invention, since the density of dispersed particles increases, recrystallization can be suppressed even when heated at a high temperature. Also, by performing the forging process, the Fe-based crystallized products can be refined and made spherical. Further, due to the refinement of crystal grains, the progress of fatigue cracks can be suppressed, and the fatigue characteristics can be improved.

[0018] If the Fe content is too low, these effects are not obtained. On the other hand, if the Fe content is too high, crystallized products such as Al-Fe-Si-based intermetallic compounds coarsen. The coarsened crystallized products deteriorate elongation, fatigue characteristics, corrosion resistance, etc. The inventors investigated the relationship between the Fe content and elongation and created a regression formula (Figure 1). According to that, it was clarified that sufficient elongation can be obtained if it is 0.67 mass% or less. Details will be described in the section of the examples. Therefore, the Fe content is more than 0.4 mass% and 0.67 mass% or less. Preferably, it is more than 0.40 mass% and 0.65 mass% or less, and more preferably more than 0.40 mass% and 0.55 mass% or less.

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

[0020] (Mg: 0.85 mass% or more and 1.3 mass% or less) Mg is an essential element that contributes to the increase in high strength (endurance). 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, the corrosion resistance also decreases. On the other hand, if the Mg content is too high, the strength (endurance) becomes too high, which inhibits the forging property. Also, it reduces the corrosion resistance and the like. Therefore, the Mg content is 0.85 mass% or more and 1.3 mass% or less, preferably 1.0 mass% or more and 1.2 mass% or less.

[0021] (Ti: 0.005 mass% or more and 0.07 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. Also, the crystal grains coarsen and the strength decreases. On the other hand, if the Ti content exceeds 0.07 mass%, the crystal grains of the forged material after heat treatment are likely to coarsen, reducing the fatigue characteristics. Coarse precipitates become the starting point of fracture, reducing the elongation. Therefore, the Ti content is 0.005 mass% or more and 0.07 mass% or less, preferably 0.01 mass% or more and 0.06 mass% or less.

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

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

[0024] Depending on the manufacturing conditions, these dispersed particles formed by Mn and Cr have the effect of hindering grain boundary migration after recrystallization. Therefore, coarsening of crystal grains can be prevented.

[0025] If the contents of Mn and Cr are too low, these effects cannot be expected, the crystal grains coarsen, and the strength decreases. On the other hand, excessive inclusion of these elements causes a decrease in elongation. For this reason, preferably, at least one of Mn and Cr is included, the content of Mn is 0.1% by mass or more and 0.95% by mass or less, preferably 0.2% by mass or more and 0.6% by mass or less, and the content of Cr exceeds 0.1% by mass and is 0.4% by mass or less, preferably exceeds 0.1% by mass and is 0.3% by mass or less, more preferably exceeds 0.1% by mass and is 0.2% by mass or less.

[0026] Zr generates dispersed particles (dispersed phase) in the same manner as Mn and Cr. In the case of Zr, depending on casting conditions such as when Ti is included, it rather inhibits the refinement of crystal grains in the ingot and becomes a factor for coarsening the crystal grains. It also becomes a factor for deteriorating fatigue characteristics. Therefore, it is desirable to add Zr within a range that does not coarsen the crystal grains during casting. Specifically, the content of Zr 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 such ranges, the aluminum alloy forging material according to the present invention can obtain the strength required for, for example, automotive underbody parts that require high strength. By containing Ti within such ranges, the casting structure can be refined. By containing Mn, Cr, and Zr within such ranges, recrystallization during solution treatment can be suppressed to obtain fine crystals. Therefore, high strength can be ensured. And in the present invention, even when Fe is contained in such a large amount, by performing forging, the reduction, refinement, and rounding of Fe-containing precipitates are achieved, and by refining the crystal grains, elongation, fatigue characteristics, and corrosion resistance are ensured.

[0028] (Balance: inevitable impurities and Al) The remainder consists of inevitable impurities and Al. Examples of the inevitable impurities include C, Ni, Na, Ca, V, and Hf. These are likely to be mixed in as impurities and inhibit the properties of automotive underbody parts, so it is preferable not to contain them, but they are acceptable if each is 0.05% by mass or less and the total is 0.10% by mass or less.

[0029] Also, B is an impurity, but like Ti, it also 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 and reduces the above-mentioned workability. Therefore, the content of B is preferably 300 ppm or less.

[0030] <Precipitates> The aluminum alloy forging material of the present invention has a precipitate area ratio of 3.2% or less and an average precipitate size of 8 μm or less.

[0031] (Precipitate area ratio: 3.2% or less) The precipitate area ratio is determined by the addition amounts of the additive elements and their solid solution amounts, etc. By setting the precipitate area ratio to 3.2% or less, it is possible to suppress a decrease in elongation due to an increase in the crack propagation path. Also, it is possible to suppress a decrease in corrosion resistance due to the surrounding matrix of the precipitates being easily corroded. The precipitate area ratio is more preferably 3.1% or less, and even more preferably 3.0% or less.

[0032] The precipitate area ratio can be calculated by photographing a backscattered electron image of a cross-section perpendicular to the metal flow of the forging material with an SEM at the center of the wall thickness and performing image analysis. A specific calculation method is shown in the section of the examples.

[0033] (Average precipitate size: 8 μm or less) The average crystal grain size is determined by the addition amount of the additive element, the solidification rate, etc. By setting the average crystal grain size to 8 μm or less, it is possible to suppress the crystal grains from becoming the starting points of cracks in the tensile test and the decrease in elongation. The average crystal grain size is more preferably 7 μm or less, and even more preferably 6 μm or less.

[0034] The average crystal grain size can be obtained by photographing the backscattered electron image of the cross section perpendicular to the metal flow of the forged material with SEM at the center of the wall thickness, converting it into a circle of the same area with analysis software, and calculating the average size.

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

[0036] The average crystal grain diameter can be calculated by the section method on the short axis. That is, after etching the center of the wall thickness of the cross section perpendicular to the metal flow of the forged material, photographing it with an optical microscope, drawing a straight line in the direction perpendicular to the long axis of the crystal grains, measuring the number of crystal grains on the straight line, and calculating by dividing the distance of the straight line by the measured number of crystal grains.

[0037] (Length of large-angle grain boundaries: 4.15 mm or more) In the aluminum alloy forged material according to the embodiment of the present invention, at the center of the wall thickness of the cross section perpendicular to the metal flow of the aluminum alloy forged material, the length of the 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 preferably 4.15 mm or more. The length of the large-angle grain boundaries affects the mechanical properties. From the viewpoints of tensile properties and fatigue properties, the length of the large-angle grain boundaries is more preferably 4.2 mm or more, and even more preferably 4.3 mm or more.

[0038] (Length of small-angle grain boundaries: 2.0 mm or more) The small grain boundary length affects the mechanical properties. From the viewpoints of tensile properties and fatigue properties, the small grain boundary length 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 in which an SEM is equipped with a backscattered electron diffraction image [EBSD: Electron Back Scattering (Scattered) Diffraction Pattern] system. The large-angle grain boundary length and the small-angle grain boundary length are observed by the SEM-EBSD method at a step of 0.5 μm in a range of 150 μm × 150 μm at the center of the wall thickness of a cross section perpendicular to the metal flow of the forged material for crystal orientation analysis. For the data, a boundary with an orientation difference of 15° or more between adjacent crystal grains is defined as a large-angle grain boundary, and a boundary of 5° to 15° is defined as a small-angle grain boundary, and it can be automatically calculated by analyzing using analysis software (TSL OIM Analysis 7 × 64). Note that data with a CI value (Confidence Index) of 0.1 or less is excluded from the analysis.

[0040] <Tensile properties> (Yield strength: 345 MPa or more) The 0.2% yield strength of the aluminum alloy forged material of the present invention is preferably 345 MPa or more. If it is less than 345 MPa, the strength required for a forged material for automotive underbody 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 forged material of the present invention is preferably more than 12.5%. If it is 12.5% or less, the formability may be insufficient as a forged material for automotive underbody parts. The elongation is preferably 13% or more, more preferably 13.5% or more.

[0042] (Tensile strength: 370 MPa or more) The tensile strength of the aluminum alloy forging material of the present invention is preferably 370 MPa or more from the viewpoint of the strength as a forging material for automotive underbody parts. The tensile strength is 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 forging material can be adjusted by setting the chemical composition as described above and by using the manufacturing method described below.

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

[0045] <Fatigue properties> (Number of fracture repetitions at a repeated stress of 170 MPa in a fatigue test using a rotating bending fatigue test piece without a notch: 1×10 6 cycles or more) The number of fracture repetitions at a repeated stress of 170 MPa in a fatigue test using a rotating bending fatigue test piece without a notch of the aluminum alloy forging material of the present invention is preferably 1×10 6 cycles or more from the viewpoint of the fatigue life as a forging material for automotive underbody parts. The number of fracture repetitions at a repeated stress of 170 MPa in a fatigue test using a rotating bending fatigue test piece without a notch is more preferably 2×10 6 cycles or more, and even more preferably 3×10 6 cycles or more.

[0046] (Number of fracture repetitions at a repeated stress of 70 MPa in a fatigue test using a rotating bending fatigue test piece with a notch: 1×10 6 cycles or more) In the fatigue test using a rotating bending fatigue test piece with a notch of the aluminum alloy forging material of the present invention, the number of fracture repetitions at a repeated stress of 70 MPa is preferably 1 × 10 6 cycles or more from the viewpoint of the fatigue life as a forging material for automotive underbody parts. The number of fracture repetitions at a repeated stress of 70 MPa in the fatigue test using a rotating bending fatigue test piece with a notch is more preferably 2 × 10 6 cycles or more, and even more preferably 3 × 10 6 cycles or more.

[0047] The fatigue characteristics of the aluminum alloy forging material can be measured by a rotating bending fatigue test of metallic materials according to JIS Z 2274. Specifically, for a rotating bending fatigue test piece without a notch, a rotating bending fatigue test piece (No. 2 test piece) according to JIS Z 2274 is cut out from an arbitrary location so as to be parallel to the metal flow of the aluminum alloy forging material, and a rotating bending fatigue test is performed at a repeated stress of 170 MPa. Similarly, for a rotating bending fatigue test piece with a notch, an annular semi-circular grooved test piece (where ρ, the radius of the groove corner, is 0.31 mm) according to JIS Z 2274 is cut out from an arbitrary location so as to be parallel to the metal flow of the aluminum alloy forging material, and a rotating bending fatigue test is performed at a repeated stress of 70 MPa. The N value for fatigue characteristic measurement is set to 2, and the minimum value is calculated. In this way, the fatigue characteristics can be calculated.

[0048] The fatigue characteristics of the aluminum alloy forging material can be adjusted by setting the chemical composition as described above and by using the manufacturing method described later.

[0049] The aluminum alloy forging material according to the present invention described above can have the same strength, elongation, fatigue characteristics, and corrosion resistance as those of an aluminum alloy forging material with an Fe content of 0.4 mass% or less. That is, the aluminum alloy forging material according to the present invention can contain Fe, which has an adverse effect on strength, elongation, etc., in an amount exceeding 0.4% by mass. Therefore, when manufacturing the aluminum alloy forging material, the amount of aluminum ingot used can be reduced, and the CO2 emission can be significantly reduced.

[0050] [Manufacturing Method of Aluminum Alloy Forging Material] Next, the manufacturing method of the aluminum alloy forging material according to the present invention will be described. The manufacturing method of the aluminum alloy forging material of the present invention is not particularly limited, but the aluminum alloy forging material described above can be preferably manufactured by a manufacturing method including a casting process, a homogenization heat treatment process, a heating process, a forging process, a solution treatment process, a quenching process, and an artificial aging treatment process in this order. In addition, the manufacturing method of the aluminum alloy forging material may further include any process that does not inhibit the desired effects in the present invention. Examples of such processes include an extrusion process performed between the homogenization heat treatment process and the heating process, a forging roll process performed between the heating process and the forging process, and the like.

[0051] (Casting Process) The casting process is a process of casting an ingot of an aluminum alloy having the above-described composition. Since the composition has already been described in detail, the description will be omitted. The casting process is preferably performed at a heating temperature of 710 to 810°C and a cooling rate to the liquidus temperature of 7°C / sec or more. When the heating temperature in the casting process is 710°C or higher, the melting time can be shortened, and the work can be efficiently performed. Further, when the heating temperature in the casting process is 810°C or lower, the generation of dross, which is an oxide, is suppressed, and metal loss is reduced, so that an ingot can be efficiently obtained. When the cooling rate to the liquidus temperature is less than 7°C / sec, the crystallized products become coarser, and the crystallized product area ratio cannot be made 3.2% or less. In addition, 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 casting methods such as continuous casting method, semi - continuous casting method, hot top casting method, etc., among which continuous casting method is preferably used.

[0053] (Homogenization heat treatment process) Next, the homogenization heat treatment process is a process of subjecting the ingot cast in the casting process to homogenization heat treatment at 420 - 560 °C for 2.5 - 8 hours. When the heating temperature in the homogenization heat treatment process is 420 °C or higher and the heating time is 2.5 hours or longer, the precipitates can be sufficiently dissolved and the area ratio of the precipitates becomes small, so that the elongation of the product can be ensured. On the other hand, when the heating temperature in the homogenization heat treatment process is 560 °C or lower and the heating time is 8 hours or shorter, the coarsening of the dispersed particles is suppressed, so that they can be dispersed uniformly, finely and densely. That is, the effect of refining the crystal grains is easily obtained and the average crystal grain size becomes small. In addition, the heating temperature in the homogenization heat treatment process is preferably 500 - 540 °C, and the heating time is preferably 4 - 8 hours.

[0054] (Heating process) Next, the heating process is a process of heating the ingot homogenized in the homogenization heat treatment process at 400 - 545 °C for 0.5 hour or longer. As described above, in the present invention, Fe, together with Mn and Cr, has the effect of generating dispersed particles (dispersed phase) and preventing the grain boundary movement after recrystallization. Therefore, even if a sufficient heating process is carried out by adding a large amount of Fe, the number and density of the dispersed particles can be made the same as those of the conventional material, and the coarsening of the crystal grains can be prevented, so that the crystal grains can be maintained in a fine state. Therefore, the tensile properties can be maintained at the same level as those of the conventional material. This effect can be realized by sufficiently heating the ingot in this heating process before performing the forging process, dissolving and reducing the Fe - based precipitates, and further refining them.

[0055] If the heating temperature in the heating process is 440°C or higher and the heating time is 0.5 hours or longer, even in the case of an aluminum alloy forging material containing a large amount of Fe as in the present invention, the solid solution of Fe-based precipitates proceeds, and the elongation can be maintained at the same level as that of the conventional material. On the other hand, when the heating temperature is 545°C or lower, the occurrence of eutectic melting due to heat generation during processing is suppressed, voids are less likely to occur, and the mechanical properties are less likely to deteriorate. Further, coarsening and low density of dispersed particles due to heat treatment are less likely to occur, and the grain refinement effect is easily obtained. The heating temperature in the heating process is preferably 440 to 545°C.

[0056] (Forging process) The next forging process is a process of forging the ingot heated in the heating process at a forging finish temperature of 350°C or higher and a reduction ratio of 50 to 95% to obtain a forging material of a predetermined shape. The reduction ratio is defined as (1 - (L1 / L0)) × 100, where L0 is the height of the material before forging and L1 is the height after forging. When the forging finish temperature in the forging process is 350°C or higher, residual strain does not increase, so recrystallization is less likely to occur and the grain size is less likely to coarsen. Further, when the reduction ratio in the forging process is 50% or higher, casting defects can be crimped, and furthermore, the grain size and precipitates can be made sufficiently small. When the reduction ratio is 95% or lower, the working rate does not become too high, and coarsening of the grain size due to recrystallization is less likely to occur. Note that the forging finish temperature is preferably as high as possible within a range not exceeding the heating temperature. The forging finish temperature is preferably 370°C or higher, and the reduction ratio is preferably 70 to 90%.

[0057] Forging under such conditions can be performed, for example, by a mechanical press or a hydraulic press.

[0058] (Solution treatment process) The next solution treatment process is a process of subjecting the forging material obtained in the forging process to a solution treatment at 480 to 580°C for more than 0 hours and within 24 hours. By this solution treatment, solid solution of additive elements for developing strength during the subsequent artificial aging treatment process can be promoted, and elongation due to refinement of precipitates can be increased. When the heating temperature in the solution treatment step is 480 °C or higher and the heating time exceeds 0 hours, the solution is sufficient, so good elongation and strength (tensile strength and 0.2% proof stress) can be obtained. On the other hand, when the heating temperature in the solution treatment step 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. Note that the heating temperature in the solution treatment step is preferably 540 - 560 °C, and the heating time is preferably 2.5 - 8.0 hours.

[0059] (Quenching process) The next quenching process is a process of quenching the forged material solution-treated in the solution treatment step at 75 °C or lower. By performing quenching, the strength can be improved. When the quenching temperature in the quenching process is 75 °C or lower, sufficient quenching is achieved, and the strength can be sufficiently improved in the subsequent artificial aging treatment step. Note that the lower limit of the quenching temperature may be about the normal temperature of the water for quenching, that is, 20 ± 15 °C (5 - 35 °C) defined in JIS Z 8703.

[0060] (Artificial aging treatment process) The next artificial aging treatment process is a process of performing artificial aging treatment on the forged material quenched in the quenching process at 160 - 250 °C for 0.5 - 20 hours. Note that the treatment from the solution treatment step to this artificial aging treatment step is what is called so-called artificial aging hardening treatment. By such artificial aging treatment, for example, the strength required for automotive underbody parts can be obtained. When the heating temperature in the artificial aging treatment step is 160 °C or higher and the heating time is 0.5 hours or longer, sufficient strength, fatigue characteristics, and corrosion resistance can be obtained. On the other hand, when the heating temperature in the artificial aging treatment step is 250 °C or higher and the heating time is 20 hours or longer, an excessive over-aged state occurs, and sufficient strength and elongation cannot be obtained. Note that 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 method for manufacturing an aluminum alloy forged material described above, although the Fe content exceeds 0.4% by mass, an aluminum alloy forged material having the same strength, elongation, fatigue characteristics, and corrosion resistance as an aluminum alloy forged material with an Fe content of 0.4% by mass or less can be manufactured.

Examples

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

[0063] (Production of Forged Material) The aluminum alloy forged materials according to Nos. 1 to 6 were manufactured based on the following conditions using an aluminum alloy having the chemical composition shown in Table 1. First, the molten aluminum alloy was gravity cast into a mold 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 φ55 mm × length 100 mm, and homogenization treatment, heating, hot die forging using a hydraulic press, solution treatment, quenching, and artificial aging treatment were performed under the respective conditions shown in Table 2 below to produce aluminum alloy forged materials of each number.

[0064]

Table 1

[0065]

Table 2

[0066]

Table 3

[0067] The tensile properties, fatigue properties, average crystal grain size (μm), large-angle grain boundary length (mm), small-angle grain boundary length (mm), precipitate area ratio (%), average precipitate size (μm), corrosion rate (mm / y) as general corrosion resistance, and stress corrosion cracking resistance (SCC resistance) of the forged materials No. 1 to 6 were evaluated. As for the tensile properties, the tensile strength (MPa), 0.2% proof stress (MPa), and elongation (%) were measured. For the fatigue properties, the number of fracture repetitions (cycles) at a repeated stress of 170 MPa in a fatigue test using a rotating bending fatigue test piece without a notch and the number of fracture repetitions (cycles) at a repeated stress of 70 MPa in a fatigue test using a rotating bending fatigue test piece with a notch were measured. These evaluations were conducted as follows.

[0068] <Tensile properties> The tensile properties were measured using a tensile testing machine in accordance with JIS Z 2241, by preparing a tensile test piece (No. 4 test piece) in accordance with JIS Z 2201, cut out from an arbitrary location so as to be parallel to the metal flow of the aluminum alloy forged material. Regarding the tensile properties, for the 0.2% proof stress, a value of 345 MPa or more was considered qualified, and less than 345 MPa was considered unqualified. For the elongation, more than 12.5% was considered qualified, and 12.5% or less was considered unqualified.

[0069] <Fatigue properties> For the fatigue properties, for the rotating bending fatigue test piece without a notch, a rotating bending fatigue test piece (No. 2 test piece) in accordance with JIS Z 2274 was prepared by cutting out from an arbitrary location so as to be parallel to the metal flow of the aluminum alloy forged material, and evaluated at a repeated stress of 170 MPa. For the rotating bending fatigue test piece with a notch, an annular semi-circular grooved test piece (with ρ of the groove corner R being 0.31 mm) in accordance with JIS Z 2274 was prepared by cutting out from an arbitrary location so as to be parallel to the metal flow of the aluminum alloy forged material, and evaluated at a repeated stress of 70 MPa. For both the number of fracture repetitions at a repeated stress of 170 MPa in the fatigue test using a rotating bending fatigue test piece without a notch and the number of fracture repetitions at a repeated stress of 70 MPa in the fatigue test using a rotating bending fatigue test piece with a notch, 1×106 More than one cycle was passed.

[0070] Note that Fig. 1 is a diagram plotting the relationship between the Fe content and elongation in aluminum alloy forgings No. 1 to 6. From this, a regression equation was created. It can be seen that when the Fe content is 0.67 mass% or less, the elongation exceeds 12.5%, and sufficient elongation can be obtained.

[0071] (Average crystal grain size) The average crystal grain size (μm) was calculated by etching the center of the thickness of a cross-section perpendicular to the metal flow of the aluminum alloy forging, photographing it at 400 times magnification with an optical microscope, drawing a straight line in the direction perpendicular to the major axis of the crystal grains, measuring the number of crystal grains on the straight line, and dividing the distance of the straight line by the measured number of crystal grains. When the average crystal grain size exceeds 50 μm, the tensile properties and fatigue properties deteriorate. Therefore, an average crystal grain size of 50 μm or less was considered passed, and those exceeding 50 μm were considered failed.

[0072] (Large-angle grain boundary length) The SEM-EBSD (EBSP) method is a crystal orientation analysis method in which a backscattered electron diffraction image [EBSD: Electron Back Scattering (Scattered) Diffraction Pattern] system is installed in an SEM. The large-angle grain boundary length was automatically calculated by performing crystal orientation analysis by the SEM-EBSD method in a range of 150 μm × 150 μm at the center of the thickness of a cross-section perpendicular to the metal flow of the forging material with an observation step of 0.5 μm, defining the boundary with an orientation difference of 15° or more between adjacent crystal grains as a large-angle grain boundary for the data, and analyzing it using analysis software (TSL Solutions OIM Analysis ver.7). Note that data with a CI value (Confidence Index) of 0.1 or less was excluded from the analysis.

[0073] The length of the large-angle grain boundary is measured in the range of 150 μm × 150 μm by the SEM-EBSD method in the metal structure at the center of the thickness of the aluminum alloy forging. If the length of the large-angle grain boundary with an inclination angle of 15° or more is less than 4.15 mm, the fatigue characteristics deteriorate, and the number of fracture repetitions at a repeated stress of 170 MPa in the fatigue test using a rotating bending fatigue test piece without a notch and the number of fracture repetitions at a repeated stress of 70 MPa in the fatigue test using a rotating bending fatigue test piece with a notch cannot ensure 1×10 6 cycles or more. Therefore, for the aluminum alloy forging, the length of the large-angle grain boundary measured in the range of 150 μm × 150 μm by the SEM-EBSD method at the center of the thickness of the cross-section perpendicular to the metal flow, with an inclination angle of 15° or more, a length of 4.15 mm or more is considered qualified.

[0074] (Small-angle grain boundary length) The SEM-EBSD (EBSP) method is a crystal orientation analysis method in which an SEM is equipped with a backscattered electron diffraction image [EBSD: Electron Back Scattering (Scattered) Diffraction Pattern] system. The small-angle grain boundary length is observed by the SEM-EBSD method for crystal orientation analysis in the range of 150 μm × 150 μm at the center of the thickness of the cross-section perpendicular to the metal flow of the forging material with an observation step of 0.5 μm. The boundary with an orientation difference of 5° - 15° between adjacent crystal grains in the data is defined as the small-angle grain boundary and automatically calculated by using analysis software (TSL OIM Analysis 7 ×64). Note that data with a CI value (Confidence Index) of 0.1 or less was excluded from the analysis.

[0075] The length of the small-angle grain boundary is measured in the range of 150 μm × 150 μm by the SEM-EBSD method in the metal structure at the center of the thickness of the aluminum alloy forging. If the length of the small-angle grain boundary with an inclination angle of 5° - 15° is less than 2.0 mm, the fatigue characteristics deteriorate, and the number of fracture repetitions at a repeated stress of 170 MPa in the fatigue test using a rotating bending fatigue test piece without a notch and the number of fracture repetitions at a repeated stress of 70 MPa in the fatigue test using a rotating bending fatigue test piece with a notch cannot ensure 1×106 It becomes impossible to ensure more than one cycle. Therefore, the small-angle grain boundary length is measured in the range of 150 μm × 150 μm by the SEM-EBSD method at the center of the wall thickness of the cross-section perpendicular to the metal flow of the aluminum alloy forging material, and the length of the small-angle grain boundary with an inclination angle of 5° to 15° being 2.0 mm or more is considered qualified.

[0076] <Precipitate> (Precipitate area ratio) The precipitate area ratio (%) is obtained by taking a backscattered electron image at 400 times magnification with a SEM (JEOL, JSM-IT700HR) at the center of the wall thickness of the cross-section perpendicular to the metal flow of the forging material, calculating the area of the white contrast by image analysis (WinROOF2018, Ver. 4.7.0), and dividing it by the total area for which the image analysis was performed. When the precipitate area ratio exceeds 3.2%, the elongation decreases and it becomes impossible to ensure more than 12.5%. Therefore, the precipitate area ratio of 3.2% or less is considered qualified, and those exceeding 3.2% are considered unqualified.

[0077] (Average precipitate size) The average precipitate size (μm) is obtained by taking a backscattered electron image at 400 times magnification with a SEM (JEOL, JSM-IT700HR) at the center of the wall thickness of the cross-section perpendicular to the metal flow of the forging material, and calculating the average size by converting it to a circle of the same area using analysis software. When the average precipitate size exceeds 8 μm, the elongation decreases and it becomes impossible to ensure more than 12.5%. Therefore, the average precipitate size of 8 μm or less is considered qualified, and those exceeding 8 μm are considered unqualified.

[0078] <General corrosion resistance> (Corrosion rate) The corrosion rate (mm / y) is obtained by preparing a flat plate from the aluminum alloy forging material and performing it according to the provisions of the alternate immersion method of ASTM G47. Here, the corrosion rate (mm / y) represents the amount of corrosion in mm in one year (365 days). The test conditions are to repeat immersion in and withdrawal from salt water for 30 days, measure the weight before and after the test, and calculate it using the following formula (1). Here, M is the corrosion loss (g / 30 days), and ρ is the density of 2.7 (g / cm 3)、S represents the area (cm 2 ) of the evaluation part.

[0079]

Number

[0080] When the corrosion rate exceeds 0.12 mm / y, the function as a forged aluminum alloy material used for automotive underbody parts cannot be satisfied. Therefore, a corrosion rate of 0.12 mm / y or less is considered qualified, and those exceeding 0.12 mm / y are considered unqualified.

[0081] <Stress corrosion cracking resistance> The evaluation of stress corrosion cracking resistance (SCC resistance: Stress Corrosion Cracking) was carried out by the alternate immersion method according to the provisions of JIS H 8711, by preparing C-ring-shaped test pieces from the forged aluminum alloy material. The SCC test was carried out under the conditions of a load stress of 180 MPa (tensile) and a period of 30 days, assuming use as automotive underbody parts. After the test, it was observed whether stress corrosion cracking occurred in the test pieces. For stress corrosion cracking resistance, those without stress corrosion cracking in the test pieces visually were considered qualified, and those with stress corrosion cracking were considered unqualified.

[0082] The tensile properties of the forged materials No. 1 to 6, the number of fracture repetitions (cycles) at a repeated stress of 170 MPa in the fatigue test using a rotating bending fatigue test piece without a notch, the number of fracture repetitions (cycles) at a repeated stress of 70 MPa in the fatigue test using a rotating bending fatigue test piece with a notch, the average crystal grain size (μm), the length of large-angle grain boundaries (mm), the length of small-angle grain boundaries (mm), the area ratio of precipitates (%), the average precipitate size (μm), the corrosion rate (mm / y) as general corrosion resistance, and the SCC resistance are shown in Table 1.

[0083] As shown in Table 1, since the forged aluminum alloy material No. 4 satisfied all the requirements of the present invention, good evaluation results could be obtained. On the one hand, the forged materials of No.1 and 2 have a Fe content below the lower limit. For the forged material of No.3, since the Ti content exceeded the upper limit, the large-angle grain boundary length became small. As a result, the number of fracture repetitions at a repeated stress of 170 MPa in the fatigue test using a rotating bending fatigue test piece without a notch and the number of fracture repetitions at a repeated stress of 70 MPa in the fatigue test using a rotating bending fatigue test piece with a notch failed. For the forged materials of No.5 and 6, since the Fe content exceeded the upper limit, the Al-Fe-Si-based precipitates coarsened and the precipitate area ratio increased. As a result, the elongation and corrosion rate failed.

Claims

1. 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: contained at 0.25% by mass or less, and further, 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 at least one selected from the group of Zr: 0.05% by mass or more and 0.3% by mass or less is included, the balance consisting of inevitable impurities and Al, an aluminum alloy forging material having a crystallized area ratio of 3.2% or less and an average crystallized size of 8 μm or less.

2. 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, the aluminum alloy forging material according to Claim 1.

3. The 0.2% proof stress is 345 MPa or more and the elongation is more than 12.5%, the aluminum alloy forging material according to Claim 1 or 2.

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