Aluminum alloy forgings and methods for manufacturing the same

Abstract

To provide an aluminum alloy forging having excellent mechanical properties at normal temperature.SOLUTION: An aluminum alloy forging of the present invention, in component ranges of Cu: 0.3 mass% to 1.0 mass%, Mg: 0.8 mass% to 1.8 mass%, Si: 0.9 mass% to 1.9 mass%, Mn: 0.3 mass% to 1.2 mass%, Fe: 0.2 mass% to 0.3 mass%, Zn: 0.26 mass% to 1.0 mass%, Cr: 0.05 mass% to 0.3 mass%, Ti: 0.012 mass% to 0.035 mass%, B: 0.001 mass% to 0.03 mass%, and Zr: 0.05 mass% or less, has the relationship Fe / Mn of less than 1.2, and the remainder of aluminum alloy forging made of Al and inevitable impurities. In an area where the maximum principal stress is applied, the average crystal grain size is 20 to 40 μm, a ratio of large-angle grain boundaries having a crystal orientation difference 15° or more is 27% or less, and 0.2% proof stress is 380 MPa or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to aluminum alloy forgings and methods for manufacturing them. [Background technology]

[0002] In recent years, aluminum alloys have seen increasing use as structural components in various products due to their lightweight properties. For example, high-tensile steel has traditionally been used in automotive undercarriages and bumper parts. However, in recent years, high-strength aluminum alloys have come into use.

[0003] Furthermore, in the automotive parts, particularly suspension components, iron-based materials were traditionally used exclusively. However, in recent years, with the primary objective of weight reduction, these materials have increasingly been replaced with aluminum or aluminum alloy materials.

[0004] Because these automotive parts require excellent corrosion resistance, high strength, and superior workability, Al-Mg-Si alloys, particularly A6061, are frequently used as aluminum alloy materials. Furthermore, to improve the strength of such automotive parts, aluminum alloy materials are manufactured using forging, a type of plastic deformation process, as the processing material.

[0005] Furthermore, in recent years, due to the need to reduce costs, suspension parts obtained by forging cast components directly without extrusion, followed by solution treatment and artificial aging treatment (T6 treatment), have begun to be put into practical use. To further reduce weight, development of high-strength alloys to replace the conventional A6061 is underway (see, for example, Patent Documents 1-3). [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 5-59477 [Patent Document 2] Japanese Patent Application Publication No. 5-247574 [Patent Document 3] Japanese Patent Application Publication No. 6-256880 [Overview of the project] [Problems that the invention aims to solve]

[0007] In recent years, with the need to reduce CO2 emissions, there has been a growing demand for lighter automobiles, and consequently, the demand for aluminum is on the rise. However, as a substitute for steel, even higher strength is required. One method for increasing strength is to suppress the formation of a recrystallized structure during plastic deformation and solution treatment processes, thereby refining the grain size.

[0008] However, the aforementioned Al-Mg-Si high-strength alloys had a problem in that the processed structure recrystallized during the forging and heat treatment processes, resulting in the generation of coarse grains, which prevented them from achieving sufficient high strength. Therefore, some alloys have Zr (zirconium) added to prevent the formation of coarse recrystallized grains (see, for example, the above-mentioned Patent Documents 1 and 2).

[0009] However, while adding Zr is effective in preventing recrystallization, it had the following problems. (1) The addition of Zr weakens the grain refinement effect of Al-Ti-B alloys, resulting in coarser grains in the ingot itself and a decrease in the strength of the processed product (forged product) after plastic deformation. (2) The grain refinement effect of the ingot itself is weakened, making it easier for ingot cracks to occur, increasing internal defects and worsening the yield. (3) Zr forms compounds with Al-Ti-B alloys, and these compounds accumulate at the bottom of the furnace where the molten alloy is stored, contaminating the furnace. In addition, these compounds crystallize coarsely within the manufactured ingots, reducing their strength.

[0010] Thus, while the addition of Zr was effective in preventing recrystallization, it was difficult to maintain strength stability.

[0011] This invention has been made in view of the above technical background, and aims to provide an aluminum alloy forged product with excellent mechanical properties at room temperature and a method for manufacturing the same. [Means for solving the problem]

[0012] To solve the above problems, the present invention provides the following means.

[0013] The aluminum alloy forged product according to the first aspect of the present invention comprises: Cu: 0.3 mass% to 1.0 mass%, Mg: 0.8 mass% to 1.8 mass%, Si: 0.9 mass% to 1.9 mass%, Mn: 0.3 mass% to 1.2 mass%, Fe: 0.2 mass% to 0.3 mass%, Zn: An aluminum alloy forging is made of an aluminum alloy having an alloy composition in the component range of 0.26 mass% to 1.0 mass%, Cr: 0.05 mass% to 0.3 mass%, Ti: 0.012 mass% to 0.035 mass%, B: 0.001 mass% to 0.03 mass%, and Zr: 0.001 mass% or more and 0.05 mass% or less, where the Fe / Mn relationship is less than 1.2 and the remainder is Al and unavoidable impurities, wherein in the part of the aluminum alloy forging subjected to the maximum principal stress, the average grain size is 20 to 40 μm, the ratio of large-angle grain boundaries with a crystal orientation difference of 15° or more is 27% or less, and the 0.2% yield strength is 380 MPa or more.

[0014] The method for manufacturing an aluminum alloy forged product according to the second aspect of the present invention is the method for manufacturing an aluminum alloy forged product according to the above aspect, which includes an alloy melt forming step of forming an alloy melt having the same composition as the aluminum alloy forged product, a casting step of cooling and solidifying the aluminum alloy melt obtained in the alloy melt forming step to form an aluminum alloy ingot, a homogenization heat treatment step of performing a homogenization heat treatment on the aluminum alloy ingot by holding it at a temperature of 370 °C or higher and 560 °C or lower for 4 hours or longer and 10 hours or shorter, a forging step of performing forging on the aluminum alloy casting after the homogenization heat treatment step at a heating temperature of 450 °C or higher and 560 °C or lower, a solution treatment step of performing a solution treatment on the forged product obtained in the forging step by holding it at a treatment temperature of 530 °C or higher and 560 °C or lower for 0.3 hours or longer and 3 hours or shorter, a quenching treatment step of quenching all surfaces of the forged product within 60 seconds after the solution treatment step and bringing it into contact with water, and performing quenching in a water tank for 1 minute or longer and 30 minutes or shorter, and an aging treatment step of performing an aging treatment on the forged product after the quenching treatment step at a heating temperature of 170 °C or higher and 210 °C or lower for 0.5 hours or longer and 7 hours or shorter.

Advantages of the Invention

[0015] According to the present invention, an aluminum alloy forged product excellent in mechanical properties at normal temperature can be provided.

Brief Description of the Drawings

[0016] [Figure 1] It is a cross-sectional view showing an example near the mold of a horizontal continuous casting apparatus for manufacturing an aluminum alloy casting according to an embodiment of the present invention. [Figure 2] It is an enlarged cross-sectional view of a main part near the cooling water cavity of the horizontal continuous casting apparatus shown in FIG. 1. [Figure 3] It is an explanatory view for explaining the heat flux of the cooling wall portion of the horizontal continuous casting apparatus. [Figure 4] It is a perspective view of an aluminum alloy forged product produced in an example.

Embodiments for Carrying Out the Invention

[0017] Embodiments of the present invention will be described in detail below with reference to the drawings. Please note that the drawings used in the following description may be enlarged for convenience to make the features easier to understand, and the dimensional ratios of each component may not be the same as in reality. Also, the materials, dimensions, etc. exemplified in the following description are examples only, and the present invention is not necessarily limited to them, and can be implemented with appropriate modifications without changing its effect.

[0018] [Aluminum alloy forgings] First, let me describe an aluminum alloy forged product according to one embodiment of the present invention. The aluminum alloy forged product of this embodiment has the following composition: Cu: 0.3 mass%~1.0 mass%, Mg: 0.8 mass%~1.8 mass%, Si: 0.9 mass%~1.9 mass%, Mn: 0.3 mass%~1.2 mass%, Fe: 0.2 mass%~0.3 mass%, Zn: An aluminum alloy forging is made from an aluminum alloy having an alloy composition in the component range of 0.26 mass% to 1.0 mass%, Cr: 0.05 mass% to 0.3 mass%, Ti: 0.012 mass% to 0.035 mass%, B: 0.001 mass% to 0.03 mass%, and Zr: 0.001 mass% or more and 0.05 mass% or less, where the Fe / Mn relationship is less than 1.2, and the remainder is Al and unavoidable impurities, wherein in the part of the aluminum alloy forging subjected to the maximum principal stress, the average grain size is 20 to 40 μm, the ratio of large-angle grain boundaries with a crystal orientation difference of 15° or more is 27% or less, and the 0.2% yield strength is 380 MPa or more.

[0019] The aluminum alloy forged product of this embodiment corresponds to a 6000 series aluminum alloy forged product in that it contains Mg and Si.

[0020] (Cu: 0.3 mass% or more, 1.0 mass% or less) Cu has the effect of finely dispersing Mg-Si compounds in aluminum alloys and improving the tensile strength of aluminum alloys by precipitating as Al-Cu-Mg-Si compounds, including the Q phase. By keeping the Cu content within the above range, the mechanical properties of aluminum alloy forgings at room temperature can be improved.

[0021] (Mg: 0.8 mass% or more, 1.8 mass% or less) Magnesium (Mg) has the effect of improving the tensile strength of aluminum alloys. Mg contributes to strengthening aluminum alloys by solid dissolving into the aluminum matrix, or by precipitating as Mg-Si compounds such as the β'' phase (Mg2Si), or Al-Cu-Mg-Si compounds such as the Q phase (AlCuMgSi). Furthermore, Mg2Si has the effect of suppressing the formation of the CuAl2 phase in aluminum alloys. By suppressing the formation of the CuAl2 phase, the corrosion resistance of aluminum alloy forgings is improved. By keeping the Mg content within the above range, both the mechanical properties at room temperature and the corrosion resistance of aluminum alloy forgings can be improved.

[0022] (Si: 0.9 mass% or more, 1.9 mass% or less) Like magnesium (Mg), silicon (Si) improves the mechanical properties and corrosion resistance of aluminum alloy forgings at room temperature. However, excessive addition of Si to aluminum alloys can lead to the crystallization of coarse primary Si grains, potentially reducing the tensile strength of the aluminum alloy. By keeping the Si content within the above range, it is possible to suppress the crystallization of primary Si while improving the mechanical properties and corrosion resistance of aluminum alloy forgings at room temperature.

[0023] (Mn: 0.3 mass% or more, 1.2 mass% or less) Mn improves the tensile strength of aluminum alloys by forming fine granular precipitates containing intermetallic compounds such as Al-Mn-Fe-Si and Al-Mn-Cr-Fe-Si in the aluminum alloy. By keeping the Mn content within the above range, the mechanical properties of aluminum alloy forgings at room temperature can be improved.

[0024] (Fe: 0.2 mass% or more, 0.3 mass% or less) Fe crystallizes in aluminum alloys as fine precipitates containing intermetallic compounds such as Al-Mn-Fe-Si, Al-Mn-Cr-Fe-Si, Al-Fe-Si, Al-Cu-Fe, and Al-Mn-Fe, thereby improving the tensile strength of the aluminum alloy. By having an Fe content within the above range, the room-temperature mechanical properties of aluminum alloy forgings can be improved. Furthermore, the Fe / Mn ratio is less than 1.2. This less-than-1.2 ratio suppresses the crystallization of AlFeSi compounds larger than 1.5 μm, thereby improving the mechanical properties.

[0025] (Cr: 0.05 mass% or more, 0.3 mass% or less) Cr improves the tensile strength of aluminum alloys by forming fine granular precipitates containing intermetallic compounds such as Al-Mn-Cr-Fe-Si and Al-Fe-Cr within the aluminum alloy. By keeping the Cr content within the above range, the mechanical properties of aluminum alloy forgings at room temperature can be improved.

[0026] (Ti: 0.012 mass% or more, 0.035 mass% or less) Ti has the effect of refining the crystal grains of aluminum alloys and improving their wroughtability. If the Ti content is less than 0.012 mass%, the effect of refining the crystal grains may not be sufficiently obtained. On the other hand, if the Ti content exceeds 0.035 mass%, coarse precipitates may form, which may reduce wroughtability. In addition, if a large amount of coarse precipitates containing Ti are mixed into aluminum alloy forgings, the toughness may decrease. Therefore, the Ti content should be between 0.012 mass% and 0.035 mass%. Preferably, the Ti content is between 0.015 mass% and 0.030 mass%.

[0027] (B: 0.001 mass% or more, 0.03 mass% or less) Substance B has the effect of refining the crystal grains of the aluminum alloy and improving its wroughtability. Adding B along with Ti to the aluminum alloy improves the grain refinement effect. If the B content is less than 0.001 mass%, the grain refinement effect may not be sufficiently obtained. On the other hand, if the B content exceeds 0.03 mass%, coarse precipitates may form and be mixed into the aluminum alloy forging as inclusions. Furthermore, if a large amount of coarse precipitates containing B are mixed into the final aluminum alloy product, the toughness may decrease. Therefore, the B content should be 0.001 mass% or more and 0.03 mass%. Preferably, the B content is 0.005 mass% or more and 0.025 mass%.

[0028] (Zn: 0.26 mass% or more, 1.0 mass% or less) If the Zn content is 1.0 mass% or less, it is dissolved in the Al matrix and contributes to improving the strength of aluminum alloy forgings as solid solution strengthening. If the Zn content exceeds 1.0 mass%, MgZn2 is formed and precipitates from the Al matrix to the grain boundaries, causing intergranular corrosion and leading to a decrease in the corrosion resistance of aluminum alloy forgings. In addition, the solid solution of Zn has a recrystallization suppression effect, but if it is less than 0.26 mass%, the recrystallization suppression effect is insufficient, while if it is added in excess of 1.0 mass%, MgZn2 is formed and the amount of solid solution decreases, making recrystallization more likely. Furthermore, when Mg combines with Zn to form MgZn2 and Mg is taken away by Zn, the amount of Mg2Si decreases, so the grain refinement effect of Mg2Si is reduced. For this reason, it is preferable to keep the Zn content at 1.0 mass% or less.

[0029] (Zr: 0.001 mass% or more, 0.05 mass% or less) If the Zr content is 0.05 mass% or less, it precipitates in the form of Al3Zr and Al-(Ti,Zr), contributing to improved strength of aluminum alloy forgings through recrystallization inhibition and precipitation strengthening. However, if the Zr content exceeds 0.05 mass%, it may crystallize as coarse Zr compounds, potentially leading to a decrease in the corrosion resistance of aluminum alloy forgings. Therefore, the Zr content should be 0.05 mass% or less. Furthermore, to obtain the above-mentioned effects of recrystallization inhibition and precipitation strengthening that improve the strength of the forging, a Zr content of 0.001 mass% or more is preferable.

[0030] (Inevitable impurities) Unavoidable impurities are impurities that inevitably become mixed into the aluminum alloy from the raw materials or manufacturing process of the aluminum alloy forging. Examples of unavoidable impurities include Ni, Sn, and Be. It is preferable that the content of these unavoidable impurities does not exceed 0.1% by mass.

[0031] In this embodiment, the aluminum alloy forged product has a cross-sectional structure in the area where the maximum principal stress is applied, with an average grain size of 20 μm or more and 40 μm or less, a ratio of large-angle grain boundaries with a crystal orientation difference of 15° or more of 27% or less, and a 0.2% yield strength of 380 MPa or more.

[0032] Here, "average grain size" refers to the average of the diameters (equivalent circle diameters) of each crystal grain when viewed in electron microscope or EBSD images, assuming each grain is a circle with an equivalent area. This average can also be calculated automatically by analysis software. The "average grain size" should be the average of 380 or more crystal grains.

[0033] When the average grain size exceeds 40 μm, satisfactory tensile and fatigue properties cannot be obtained according to the Hall-Petch law. On the other hand, when the average grain size is less than 20 μm, toughness deteriorates and impact resistance decreases. Therefore, it is necessary to control the average grain size within the range of 20 μm to 40 μm. Furthermore, it is preferable that the standard deviation of the grain size is 30 or less. This is because suppressing variations in grain size increases impact resistance. It is more preferable that the standard deviation of the grain size is 25 or less, and even more preferable that it is 20 or less.

[0034] Grain boundaries with a crystal orientation difference of 15% or more (large-angle grain boundaries) serve as an indicator of the degree of recrystallization. A ratio of 27% or less indicates that recrystallization is sufficiently suppressed, leading to excellent mechanical properties. The ratio of grain boundaries with a crystal orientation difference of 15% or more can be obtained from EBSD images.

[0035] The 0.2% yield strength of 380 MPa or more of the aluminum alloy forged product of this embodiment is high strength comparable to that of iron-based metal materials.

[0036] [Manufacturing method for aluminum alloy forgings] Next, the manufacturing method for the aluminum alloy forged product according to this embodiment will be described. The manufacturing method for the aluminum alloy forged product of this embodiment can be used to produce the above-mentioned aluminum alloy forged product by, for example, going through a molten metal formation step, a casting step, a homogenization heat treatment step, a forging step, a solution treatment step, a quenching step, and an aging treatment step.

[0037] (Molten metal formation process) The molten metal forming process is a process of melting raw materials to obtain molten aluminum alloy with a prepared composition. The composition of the molten aluminum alloy is the same as that of the aluminum alloy forged product. Specifically, the composition is as follows: Cu: 0.3 mass% to 1.0 mass%, Mg: 0.8 mass% to 1.8 mass%, Si: 0.9 mass% to 1.9 mass%, Mn: 0.3 mass% to 1.2 mass%, Fe: 0.2 mass% to 0.3 mass%, Zn: 0.26 mass% to 1.0 mass%, Cr: 0.05 mass% to 0.3 mass%, Ti: 0.012 mass% to 0.035 mass%, B: 0.001 mass% to 0.03 mass%, and Zr: 0.001 mass% or more and 0.05 mass% or less. The Fe / Mn ratio is less than 1.2, and the remainder consists of Al and unavoidable impurities, thereby obtaining molten 6000 series aluminum alloy. A molten aluminum alloy may be obtained by using 10% or more of scrap material from 1000 series, 2000 series, 3000 series, 4000 series, 5000 series, 6000 series, and 7000 series aluminum alloys as raw materials, with the remainder being new aluminum ingots and the above-mentioned additive elements, and then melting these to adjust the composition. By using the molten aluminum alloy of the above composition to carry out the subsequent processes, it is possible to obtain Al-Mg-Si aluminum alloy forgings that are less prone to recrystallization and have excellent mechanical properties at room temperature. Note that "new aluminum ingot" refers to aluminum with a concentration of 99% or higher obtained by electrolysis, known as electrolytic refining, on alumina produced from minerals.

[0038] Molten aluminum alloy can be obtained by heating and melting an aluminum alloy. Alternatively, it may be formed by melting a mixture containing the elements that are the raw materials for the aluminum alloy, or a compound containing two or more elements, in a proportion that produces the desired aluminum alloy. For example, to control the grain size of the aluminum alloy produced in the casting process, Ti or B may be mixed in as grain refiners such as Al-Ti-B rods.

[0039] (Casting process) In the casting process, molten aluminum alloy (liquid phase) is cooled and solidified into a solid (solid phase) to obtain an aluminum alloy casting. For example, a horizontal continuous casting method can be used for the casting process.

[0040] Figures 1 and 2 show a horizontal continuous casting apparatus that can be used to manufacture aluminum alloy castings according to this embodiment. Figure 1 is a cross-sectional view showing an example of the area around the mold 12 of the horizontal continuous casting apparatus 10. Figure 2 is an enlarged cross-sectional view of the main part around the cooling water cavity 24 of the horizontal continuous casting apparatus 10.

[0041] The horizontal continuous casting apparatus 10 shown in Figures 1 and 2 includes a molten metal receiving section (tundish) 11, a hollow cylindrical mold 12, and a refractory plate-like body (insulating member) 13 positioned between one end 12a of the mold 12 and the molten metal receiving section 11.

[0042] The molten metal receiving section 11 consists of a molten metal inlet section 11a for receiving the molten aluminum alloy M obtained in the molten metal forming process described above, a molten metal holding section 11b, and an outlet section 11c for outflow to the hollow section 21 of the mold 12.

[0043] The molten metal receiving section 11 maintains the upper liquid level of the molten aluminum alloy M at a position higher than the upper surface of the hollow section 21 of the mold 12, and in the case of multi-casting, it stably distributes the molten aluminum alloy M to each mold 12.

[0044] The molten aluminum alloy M held in the molten metal holding section 11b within the molten metal receiving section 11 is poured into the hollow section 21 of the mold 12 through the pouring passage 13a provided in the refractory plate-shaped body 13. The molten aluminum alloy M supplied into the hollow section 21 is then cooled and solidified by a cooling device 23 (described later), and is pulled out from the other end 12b of the mold 12 as a solidified ingot, an aluminum alloy rod B.

[0045] The other end 12b of the mold 12 may be equipped with a draw-out drive device (not shown) for drawing out the cast aluminum alloy rod B at a constant speed. It is also preferable to have a synchronized cutting machine (not shown) for cutting the continuously drawn aluminum alloy rod B to any desired length.

[0046] The refractory plate-like body 13 is a member that blocks heat transfer between the molten metal receiving section 11 and the mold 12, and may be composed of materials such as calcium silicate, alumina, silica, a mixture of alumina and silica, silicon nitride, silicon carbide, or graphite. Such a refractory plate-like body 13 can also be composed of multiple layers made of different materials.

[0047] In this embodiment, the mold 12 is a hollow cylindrical member and is formed from one or more materials selected from, for example, aluminum, copper, or alloys thereof. The materials for such a mold 12 should be selected in an optimal combination in terms of thermal conductivity, heat resistance, and mechanical strength.

[0048] The hollow portion 21 of the mold 12 is formed with a circular cross-section in order to cast the aluminum alloy rod B into a cylindrical shape, and the mold 12 is held such that the mold central axis (central axis) C, which passes through the center of this hollow portion 21, is aligned in a nearly horizontal direction.

[0049] The inner circumferential surface 21a of the hollow portion 21 of the mold 12 is formed at an elevation angle of 0° to 3° (more preferably 0° to 1°) with respect to the mold's central axis C, in the casting direction of the aluminum alloy rod B (see Figure 1). In other words, the inner circumferential surface 21a is configured as a tapered shape that opens in a cone shape toward the casting direction. The angle formed by this taper is the elevation angle.

[0050] If the elevation angle is less than 0°, casting may become difficult because the aluminum alloy rod B encounters resistance at the other end 12b, which is the mold exit, when it is pulled out of the mold 12. On the other hand, if the elevation angle exceeds 3°, the contact between the inner circumferential surface 21a and the molten aluminum alloy M becomes insufficient, and the heat dissipation effect from the molten aluminum alloy M and the solidified shell formed therefrom to the mold 12 decreases, which may result in insufficient solidification. As a result, this may lead to casting problems such as the formation of a remelted surface on the surface of the aluminum alloy rod B, or the ejection of unsolidified molten aluminum alloy M from the end of the aluminum alloy rod B, which is undesirable.

[0051] Furthermore, the cross-sectional shape of the hollow portion 21 of the mold 12 (the planar shape when the hollow portion 21 of the mold 12 is viewed from the other end 21b) can be selected according to the shape of the aluminum alloy rod to be cast, in addition to the circular shape of this embodiment. For example, it can be a triangular, rectangular, polygonal, semicircular, elliptical, or an irregularly shaped cross-section without an axis of symmetry or plane of symmetry.

[0052] A fluid supply pipe 22 is located at one end 12a of the mold 12 to supply lubricating fluid into the hollow portion 21 of the mold 12. The lubricating fluid supplied from the fluid supply pipe 22 can be one or more types of lubricating fluid selected from gaseous lubricants and liquid lubricants. When supplying both gaseous and liquid lubricants, it is preferable to provide separate fluid supply pipes for each. The pressurized lubricating fluid supplied from the fluid supply pipe 22 is supplied into the hollow portion 21 of the mold 12 through an annular lubricant supply port 22a.

[0053] In this embodiment, the pressurized lubricating fluid is supplied to the inner circumferential surface 21a of the mold 12 from the lubricant supply port 22a. Alternatively, the liquid lubricant may be heated and decomposed into a gas before being supplied to the inner circumferential surface 21a of the mold 12. Alternatively, a porous material may be placed at the lubricant supply port 22a, allowing the lubricating fluid to seep out onto the inner circumferential surface 21a of the mold 12 through this porous material.

[0054] A cooling device 23, which is a cooling means for cooling and solidifying the molten aluminum alloy M, is formed inside the mold 12. The cooling device 23 in this embodiment has a cooling water cavity 24 that contains cooling water W for cooling the inner circumferential surface 21a of the hollow portion 21 of the mold 12, and a cooling water injection passage 25 that connects the cooling water cavity 24 and the hollow portion 21 of the mold 12.

[0055] The cooling water cavity 24 is formed in an annular shape inside the mold 12, outside the inner circumferential surface 21a of the hollow portion 21, and cooling water W is supplied via the cooling water supply pipe 26.

[0056] The mold 12 is cooled by the cooling water W contained in the cooling water cavity 24, which removes heat from the molten aluminum alloy M filling the hollow portion 21 of the mold 12 from the surface in contact with the inner surface 21a of the mold 12, causing a solidified shell to form on the surface of the molten aluminum alloy M.

[0057] Furthermore, the cooling water injection passage 25 cools the aluminum alloy rod B by directly applying cooling water W to the aluminum alloy rod B at the other end 12b of the mold 12 from the shower opening 25a facing the hollow portion 21. The longitudinal cross-sectional shape of the cooling water injection passage 25 may be other than the circular shape of this embodiment, for example, a semicircle, a pear shape, or a horseshoe shape.

[0058] In this embodiment, the cooling water W supplied via the cooling water supply pipe 26 is first contained in the cooling water cavity 24 to cool the inner circumferential surface 21a of the hollow portion 21 of the mold 12, and then the cooling water W from the cooling water cavity 24 is injected from the cooling water injection passage 25 toward the aluminum alloy rod B. However, it is also possible to configure the system so that these are supplied by separate cooling water supply pipes.

[0059] The effective mold length L is defined as the distance from the point where the extension of the central axis of the shower opening 25a of the cooling water injection passage 25 strikes the surface of the cast aluminum alloy rod B to the contact surface between the mold 12 and the refractory plate-like body 13. This effective mold length L is preferably, for example, 10 mm or more and 40 mm or less. If the effective mold length L is less than 10 mm, casting becomes impossible due to the inability to form a good coating, and if it exceeds 40 mm, the effect of forced cooling decreases, solidification by the mold wall becomes dominant, and the contact resistance between the mold 12 and the molten aluminum alloy M or aluminum alloy rod B increases, which may cause cracks in the casting surface or breakage inside the mold, making casting unstable and therefore undesirable.

[0060] Preferably, the supply of cooling water W to these cooling water cavities 24 and the injection of cooling water W from the shower openings 25a of the cooling water injection passages 25 can be controlled by control signals from a control device (not shown).

[0061] The cooling water cavity 24 is formed such that the inner bottom surface 24a of the mold 12 near the hollow portion 21 is parallel to the inner circumferential surface 21a of the hollow portion 21 of the mold 12.

[0062] In this context, "parallel" includes cases where the inner circumferential surface 21a of the hollow portion 21 of the mold 12 is formed at an elevation angle of 0° to 3° with respect to the inner bottom surface 24a of the cooling water cavity 24, that is, cases where the inner bottom surface 24a is inclined to the inner circumferential surface 21a at an angle greater than 0° and up to 3°.

[0063] As shown in Figure 2, the cooling wall portion 27 of the mold 12, which is the portion where the inner bottom surface 24a of the cooling water cavity 24 and the inner circumferential surface 21a of the hollow portion 21 of the mold 12 face each other, has a heat flux value per unit area of ​​10 × 10⁻¹⁰ 5 W / m 2 The above 50 x 10 5 W / m 2 It is formed to be within the following range.

[0064] The mold 12 should be formed such that the thickness t of the cooling wall portion 27 of the mold 12, that is, the distance between the inner bottom surface 24a of the cooling water cavity 24 and the inner circumferential surface 21a of the hollow portion 21 of the mold 12, is within the range of, for example, 0.5 mm to 3.0 mm, preferably 0.5 mm to 2.5 mm. Furthermore, the forming material of the mold 12 should be selected such that the thermal conductivity of at least the cooling wall portion 27 of the mold 12 is within the range of 100 W / m·K to 400 W / m·K.

[0065] In Figure 2, the molten aluminum alloy M in the molten metal receiving section 11 is supplied from one end 12a of the mold 12, which is held so that the mold central axis C is approximately horizontal, via a refractory plate-like body 13, and is forcibly cooled at the other end 12b of the mold 12 to form an aluminum alloy rod B.

[0066] The aluminum alloy rod B is drawn out at a constant speed by a drawing drive device (not shown) installed near the other end 12b of the mold 12, so that it is continuously cast and a long aluminum alloy rod B is formed. The drawn-out aluminum alloy rod B is then cut to a desired length by, for example, a synchronized cutting machine (not shown).

[0067] The composition ratio of the cast aluminum alloy rod B can be confirmed, for example, using a photoelectric emission spectrometer (example: PDA-5500 manufactured by Shimadzu Corporation of Japan) as described in "JIS H 1305".

[0068] The difference in height between the liquid level of the molten aluminum alloy M stored in the molten metal receiving section 11 and the upper inner circumferential surface 21a of the mold 12 is preferably 0 mm to 250 mm (more preferably 50 mm to 170 mm). By keeping it within this range, the pressure of the molten aluminum alloy M supplied into the mold 12 and the lubricating oil and the gas produced by the vaporization of the lubricating oil are suitably balanced, resulting in stable castability.

[0069] Liquid lubricants can use vegetable oils as lubricants. Examples include rapeseed oil, castor oil, and salad oil. These are preferable because they have little adverse impact on the environment.

[0070] The lubricating oil supply rate is preferably 0.05 mL / min to 5 mL / min (more preferably 0.1 mL / min or more and 1 mL / min or less). If the supply rate is insufficient, the molten aluminum alloy M of the aluminum alloy rod B may not solidify due to insufficient lubrication and may leak from the mold 12. If the supply is excessive, the excess may be mixed into aluminum alloy rod B, potentially causing internal defects.

[0071] The casting speed, which is the speed at which the aluminum alloy rod B is drawn out of the mold 12, is preferably 200 mm / min or more and 1500 mm / min or less (more preferably 400 mm / min or more and 1000 mm / min or less). This is because, within this range of casting speeds, the network structure of the precipitates formed during casting becomes uniform and fine, increasing the resistance to deformation of the aluminum material at high temperatures and improving the high-temperature mechanical strength.

[0072] The amount of cooling water sprayed from the shower opening 25a of the cooling water injection passage 25 is preferably 10 L / min or more and 50 L / min or less per mold (more preferably 25 L / min or more and 40 L / min or less). If the amount of cooling water is less than this, the molten aluminum alloy M may not solidify and may leak from the mold 12. In addition, the surface of the cast aluminum alloy rod B may remelt, forming an uneven structure that may remain as an internal defect. On the other hand, if the amount of cooling water is more than this range, the heat dissipation from the mold 12 may be too great, causing it to solidify prematurely.

[0073] The average temperature of the molten aluminum alloy M flowing from the molten metal receiving section 11 into the mold 12 is preferably, for example, 650°C or higher and 750°C or lower (more preferably 680°C or higher and 720°C or lower). If the temperature of the molten aluminum alloy M is too low, coarse crystals may form in and in front of the mold 12 and be incorporated into the aluminum alloy rod B as internal defects. On the other hand, if the temperature of the molten aluminum alloy M is too high, a large amount of hydrogen gas is easily incorporated into the molten aluminum alloy M, which may be incorporated into the aluminum alloy rod B as porosity and form internal cavities.

[0074] Furthermore, in the cooling wall portion 27 of the mold 12, the heat flux per unit area from the molten aluminum alloy M in the hollow portion 21 to the cooling water W in the cooling water cavity 24 is 10 × 10 5 W / m 2 50 x 10 5 W / m 2 By keeping the following range, seizing of aluminum alloy rod B can be prevented.

[0075] The cooling wall portion 27 of the mold 12 receives heat from the molten aluminum alloy M, and this heat is cooled by the cooling water W contained in the cooling water cavity 24, thus performing heat exchange. Regarding the state of this heat exchange, we focused on the heat flux per unit area, as shown in the explanatory diagram in Figure 3. The heat flux per unit area is expressed by the following equation (1) according to Fourier's law. Q = -k × (T1 - T2) / L ···(1) Q: Heat flux k: Thermal conductivity (W / m·K) of the location where heat passes through (cooling wall portion 27 of the mold 12 in this embodiment) T1: Low-temperature side temperature of the location where heat passes through (inner bottom surface 24a of the cooling water cavity 24 in this embodiment) T2: High-temperature side temperature of the location where heat passes through (inner peripheral surface 21a of the hollow portion 21 of the mold 12 in this embodiment) L: Section length (mm) of the location where heat passes through (thickness t of the cooling wall portion 27 of the mold 12 in this embodiment)

[0076] Based on the quality, thickness, and temperature measurement data of the cast material for which good results were obtained even when the lubricant amount was reduced during casting, the heat flux value per unit area is 10×10 5 W / m 2 By configuring the cooling wall portion 27 of the mold 12 such that it is above this value, seizure of the cast aluminum alloy bar B can be prevented. Also, it is preferable that the heat flux value per unit area be 50×10 5 W / m 2 or less.

[0077] To make the cooling wall portion 27 of the mold 12 within such a heat flux value range, the mold 12 may be formed such that the thickness t of the cooling wall portion 27 of the mold 12 is, for example, in the range of 0.5 mm or more and 3.0 mm or less. Also, the thermal conductivity of at least the cooling wall portion 27 of the mold 12 may be in the range of 100 W / m·K or more and 400 W / m·K or less.

[0078] When manufacturing the aluminum alloy bar B of this embodiment, using the horizontal continuous casting apparatus 10 described above, the aluminum alloy molten metal M stored in the molten metal receiving portion 11 is continuously supplied into the hollow portion 21 from one end side 12a of the mold 12. Also, cooling water W is supplied to the cooling water cavity 24, and a lubricating fluid, for example, lubricating oil, is supplied from the fluid supply pipe 22.

[0079] Then, the aluminum alloy molten metal M supplied into the hollow portion 21 has a heat flux value per unit area at the cooling wall portion 27 of 10×10 5 W / m 2The aluminum alloy rod B is cast by cooling and solidifying under the above conditions. Furthermore, when casting the aluminum alloy rod B, it is preferable to keep the wall temperature of the cooling wall portion 27 of the mold 12, which is cooled by the cooling water W, below 100°C.

[0080] The resulting aluminum alloy rod B has a heat flux value of 10 × 10 per unit area in the cooling wall portion 27. 5 W / m 2 By cooling and solidifying under the above conditions, the adhesion of reaction products, such as carbides, resulting from contact between the lubricating oil gas and the molten aluminum alloy M is suppressed. As a result, there is no need to cut off carbides and other materials from the surface of the aluminum alloy rod B, and the aluminum alloy rod B can be manufactured in high yield.

[0081] The casting process for obtaining a casting from molten aluminum alloy M is not limited to the horizontal continuous casting method described above; known continuous casting methods such as vertical continuous casting can be used. Vertical continuous casting is classified into the float method and the hot-top method depending on the method of supplying the molten aluminum alloy M to the mold (casting mold 12), but the case using the hot-top method will be briefly explained below.

[0082] The casting apparatus used in the hot top method includes a mold, a molten metal receiving container (header), etc. The molten metal supplied to the receiving section passes through a spout and then through the header, where its flow rate is adjusted. It then enters a cylindrical mold that is installed almost horizontally, where it is forcibly cooled and a solidified shell is formed on the outer surface of the molten metal.

[0083] Furthermore, cooling water is directly sprayed onto the casting as it is removed from the mold, allowing the metal to solidify inside the casting as it is continuously removed. Generally, molds are made of metal materials with good thermal conductivity and have a hollow structure to introduce a coolant into the interior.

[0084] The refrigerant used can be selected from industrially available options, but water is recommended for ease of use.

[0085] The mold used in this embodiment is appropriately selected from metals such as copper or aluminum, or graphite, from the viewpoint of heat transfer performance and durability at the contact area with the molten metal. The header is generally made of refractory material and is installed on the upper side of the mold. The material and size of the header can be appropriately selected depending on the composition range of the alloy to be cast and the dimensions of the casting, and are not particularly restricted.

[0086] The average cooling rate during casting should be appropriately selected from a generally recommended range, such as 10 to 300°C / second. The casting speed should be appropriately selected from a range common in horizontal continuous casting, such as 200 to 600 mm / min.

[0087] The casting method described above allows for the acquisition of a uniform metallic structure even in medium to large castings. There are no particular restrictions on the diameter of the castings to be used, and it is suitably applied to rods with a diameter of 30 to 100 mm.

[0088] (Homogenization heat treatment process) The homogenization heat treatment process involves applying a homogenization heat treatment to the aluminum alloy casting obtained in the casting process. This process homogenizes the microsegregation caused by solidification, precipitates supersaturated solid solution elements, and transforms the metastable phase into an equilibrium phase.

[0089] In this embodiment, the casting obtained in the casting process is subjected to a homogenization heat treatment, in which it is held at a temperature of 370°C or higher and 560°C or lower for 4 to 10 hours. By performing the homogenization heat treatment within this temperature range, the ingot is sufficiently homogenized and the solute atoms are sufficiently dissolved, so that sufficient strength required by the subsequent aging treatment can be obtained.

[0090] However, if the material heating temperature during forging and the solution treatment temperature are high (for example, 530°C or higher), the homogenization process can be omitted because the microsegregation can be homogenized in the forging and solution treatment processes.

[0091] (Forging process) The forging process involves shaping an aluminum alloy casting, either after casting or after a homogenization heat treatment, to a predetermined size to obtain a material for forging. This material is then heated to a predetermined temperature and subsequently molded using a press.

[0092] In this embodiment, a forging process is performed on a forging material at a heating temperature of 450°C or higher and 560°C or lower to obtain a forged product (for example, an automobile suspension arm part). At this time, the starting temperature for forging of the forging material is set to 450°C or higher and 560°C or lower. This is because if the starting temperature is below 450°C, the deformation resistance becomes high and sufficient processing cannot be performed, and if it exceeds 560°C, defects such as forging cracks and eutectic melting are likely to occur.

[0093] (Solution treatment process) The solution treatment process involves heating the forged product obtained in the forging process to induce a solution state, thereby easing the strain introduced during the forging process and enabling the solid solution of solute elements.

[0094] In this embodiment, the forged product is subjected to solution treatment by holding it at a treatment temperature of 530°C or higher and 560°C or lower for 0.3 to 3 hours or less. The heating rate from room temperature to the above-mentioned treatment temperature is preferably 5.0°C / min or higher. If the treatment temperature is below 530°C, the solid solution of the solute element may be insufficient. On the other hand, if it exceeds 560°C, the solid solution of the solute element is further promoted, but eutectic melting and recrystallization may occur more easily. Also, if the heating rate is less than 5.0°C / min, coarse precipitation of Mg2Si may occur. On the other hand, if the treatment temperature is below 530°C, solution treatment may not progress, making it difficult to achieve high strength through age precipitation.

[0095] (Heat treatment process) The quenching process involves rapidly cooling the forged product, which is in a solid solution state obtained through the solution treatment process, to form a supersaturated solid solution.

[0096] In this embodiment, the forging is quenched by immersing it in a water tank containing water (quenching water). The water temperature in the tank is preferably between 20°C and 60°C. It is preferable that the forging is placed in the water tank within 5 seconds to 60 seconds after the solution treatment so that all surfaces of the forging come into contact with the water. The immersion time of the forging varies depending on the size of the casting, but is, for example, between 1 minute and 30 minutes.

[0097] (Statute of Limitations Process) The aging process involves heating and holding the forged product at a relatively low temperature to precipitate supersaturated dissolved elements, thereby imparting appropriate hardness.

[0098] In this embodiment, the forged product after the quenching process is subjected to aging treatment by heating it to a temperature of 170°C or higher and 210°C or lower, and holding it at that temperature for 0.5 hours or higher and 7 hours or lower. If the treatment temperature is below 170°C or the holding time is less than 0.5 hours, there is a risk that the Mg2Si-based precipitates that improve tensile strength will not grow sufficiently. On the other hand, if the treatment temperature exceeds 190°C or the holding time exceeds 7 hours, there is a risk that the Mg2Si-based precipitates will become too coarse, and it will not be possible to sufficiently improve the tensile strength. [Examples]

[0099] Next, specific embodiments of the present invention will be described, but the present invention is not particularly limited to those embodiments.

[0100] (Examples 1-9 and Comparative Examples 1-2) First, an aluminum alloy with the alloy composition shown in Table 1 below (the remainder being aluminum) was prepared. Using the prepared aluminum alloy, a continuous casting with a diameter of 49 mm and a circular cross-section was manufactured. Examples 4-6 and Examples 7-8 have the same composition as Examples 1-3, but the processing conditions for the continuous casting differ.

[0101] [Table 1]

[0102] Next, the obtained continuous castings were subjected to a homogenization heat treatment process, a forging process, a solution treatment process, a quenching process, and an artificial aging process in that order to obtain an aluminum alloy forged product 100 with the shape shown in Figure 4. The conditions for the homogenization heat treatment process, forging process, solution treatment process, quenching process, and artificial aging process are shown in Table 2 below.

[0103] [Table 2]

[0104] "evaluation" The aluminum alloy forged products obtained in Examples 1-9 and Comparative Examples 1-2 were evaluated based on the evaluation method described below. The results are shown in Table 3 below.

[0105] [Table 3]

[0106] <Method for evaluating tensile strength at room temperature> Tensile test specimens with a gauge length of 25.4 mm and a parallel section diameter of 6.4 mm were taken from the evaluation area of ​​the obtained aluminum alloy forging 100. The tensile strength of these specimens was measured by performing a room temperature (25°C) tensile test and evaluated based on the following criteria. (Judgment criteria) "〇" ... The 0.2% yield strength at room temperature is 380 MPa or higher. "×" ... The 0.2% yield strength at room temperature is less than 380 MPa.

[0107] <Method for measuring grain size, standard deviation of grain size, and ratio of large-angle grain boundaries with a crystal orientation difference of 15° or more in aluminum alloy forged products> For each aluminum alloy forged product in Examples 1-9 and Comparative Examples 1-2, the average grain size, standard deviation of grain size, and the ratio of large-angle grain boundaries with a crystal orientation difference of 15° or more were measured using a SEM-EBSD apparatus and evaluated based on the following criteria. A plate-like material measuring 7 mm × 7 mm × 2 mm thick was taken from the evaluation area of ​​aluminum alloy forged product 100 and used as the SEM-EBSD measurement sample. The measurement conditions were: acceleration voltage of 15 kV, measurement pitch of 0.5 μm / px, and analysis area of ​​500 × 500 μm. 2 The experiment was conducted with a grain boundary definition angle of 15°. The results are shown in Table 3 above. (Judgment criteria (average grain size)) "〇" ... is between 20 μm and 40 μm. "×" ... Less than 20 μm, or greater than 40 μm. (Criteria for judgment: standard deviation of crystal grain size) "〇" ... 30 or less. "×" ... It's over 30. (Judgment criteria (ratio of large-angle grain boundaries with a crystal orientation difference of 15° or more)) "〇" ... 27% or less. "×" ... This is over 27%.

[0108] <Overall Rating> The four evaluation results—0.2% yield strength at room temperature, average grain size, standard deviation of grain size, and ratio of large-angle grain boundaries of 15° or greater—were evaluated based on the following criteria. (Judgment criteria) "〇"...All four evaluations are "〇". "×" ... One or more of the four evaluations are marked "×".

[0109] By manufacturing aluminum alloy forgings containing Zr and Zn, with the content of each element set within a predetermined range, and by carrying out each process—molten metal formation, casting, homogenization, forging, solution treatment, quenching, and aging—with processing conditions within a predetermined range, aluminum alloy forgings were obtained that were sufficiently refined with suppressed variations, crystal coarsening due to recrystallization was suppressed, and aluminum alloy forgings possessed excellent mechanical properties at room temperature. [Explanation of Symbols]

[0110] 10…Horizontal continuous casting apparatus 11…Molten metal receiving section (tundish) 11a... Molten metal inlet 11b... Molten metal holding section 11c...Outflow part 12…Mold 12a...One end side 12b…Other end side 13…Refractory plate-like material (insulating material) 13a…Pouring passage 21...Hollow part 21a...Inner peripheral surface 21b...Other end side 22...Fluid supply pipe 22a…Lubricant supply port 23…Cooling device 24... Cooling water cavity 24a…Inner bottom surface 25…Cooling water injection passage 25a... Shower opening 26…Cooling water supply pipe 27...Cooling wall section B... Aluminum alloy rod M...Molten alloy W... Cooling water 100…Aluminum alloy forgings

Claims

1. Cu: 0.3% by mass to 1.0% by mass, Mg: 0.8% by mass to 1.8% by mass, Si: 0.9% by mass to 1.9% by mass, Mn: 0.3% by mass to 1.2% by mass, Fe: 0.2% by mass to 0.3% by mass, Zn: 0.26% by mass to 1.0% by mass, Cr: 0.05% by mass to 0.3% by mass, Ti: 0.012% by mass to 0.035% by mass, B: 0.001% by mass to 0.03% by mass, Zr: 0.001% by mass or more, 0.05% by mass or less An aluminum alloy forging is made from an aluminum alloy having an alloy composition in which the Fe / Mn relationship is less than 1.2 within the component range, and the remainder consists of Al and unavoidable impurities. An aluminum alloy forged product wherein, in the portion of the aluminum alloy forged product subjected to the maximum principal stress, the average grain size is 20 to 40 μm, the proportion of large-angle grain boundaries with a crystal orientation difference of 15° or more is 27% or less, and the 0.2% yield strength is 380 MPa or more.

2. A method for manufacturing an aluminum alloy forging according to claim 1, A process of forming an alloy molten metal having the same composition as the aforementioned aluminum alloy forged product, A casting process in which the molten aluminum alloy obtained in the molten alloy forming process is cooled and solidified to form an aluminum alloy ingot, The aluminum alloy ingot is subjected to a homogenization heat treatment process, which involves holding it at a temperature of 370°C or higher and 560°C or lower for 4 hours or more and 10 hours or less. A forging process is performed on the aluminum alloy casting after the homogenization heat treatment process, with a heating temperature of 450°C or higher and 560°C or lower. A solution treatment step is performed on the forged product obtained in the forging step, by holding it at a treatment temperature of 530°C or higher and 560°C or lower for 0.3 hours or more and 3 hours or less. A quenching process is performed in which, within 5 seconds to 60 seconds after the solution treatment process, all surfaces of the forged product are brought into contact with quenching water, and quenched in a water tank for 1 minute to 30 minutes. A method for manufacturing an aluminum alloy forging, comprising an aging treatment step in which the forged product after the quenching treatment step is subjected to aging treatment at a heating temperature of 170°C or higher and 210°C or lower for a period of 0.5 hours or higher and 7 hours or lower.

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