Aluminum alloy for extruded materials, billets using the same, and method for manufacturing extruded materials.

Abstract

The objective is to provide an aluminum alloy for extruded materials that is high in strength while also having excellent bendability and resistance to stress corrosion cracking, as well as a method for manufacturing cast billets using the same, and a method for manufacturing extruded materials. [Solution] The material contains, by mass%, Zn: 6.0~8.0%, Mg: 1.0~2.0%, Cu: 0.10~1.0%, Zr: 0.10~0.25%, Ti: 0.005~0.05%, further allowing Si: 0.01~0.25%, Fe: 0.10~0.40%, Mn: 0.004~0.4%, Cr: 0.05% or less, with the total [Fe+Mn+Cr+Zr] in the range of 0.20~1.10%, the remainder consisting of Al and unavoidable impurities, and is characterized by containing a total of 25% or more of recycled materials from the market and scraps or chips generated during the product manufacturing process as raw materials.

Description

Technical Field

[0001] The present invention relates to a 7000 - series aluminum alloy, a method for casting a billet using the same, and a method for manufacturing an extruded material.

Background Art

[0002] From the perspective of effective utilization of aluminum - based resources, it has been considered to recycle recycled materials recovered from the market after being used as aluminum products, off - cuts, chips, etc. generated in the manufacturing process of aluminum products, and reuse them as aluminum resources.

[0003] For example, extruded materials made of 7000 - series aluminum alloys are widely used as high - strength materials. However, when used for vehicle or aircraft parts, bending workability by various presses, benders, etc. is required, and stress corrosion cracking resistance, etc. is also required.

[0004] However, when recycling materials (scrap materials) recovered from the market or off - cuts and chips generated in the manufacturing process are remelted and reused as aluminum resources, the amount of elements called impurities such as Si and Fe increases, resulting in technical problems such as a decrease in quality characteristics such as bending properties.

[0005] For example, Patent Document 1 discloses a technique for adjusting the addition of crystal nuclei of intermetallic compounds that can be generated when molten aluminum solidifies, for the purpose of preventing the coarsening of intermetallic compounds accompanying an increase in the Fe content. However, this technique is applied to AC4C - series casting alloys, and carbides such as TiC are used as crystal nuclei. Therefore, the primary crystal nucleus formation process is unstable and not practical. Patent Document 2 discloses an Al - Mg - Si - series aluminum alloy with excellent bending properties. However, this technique relates to a rolled material made of a 6000 - series aluminum alloy and aims to control the ratio of α - phase precipitates to β - phase precipitates, and the control is unstable and not practical.

Prior Art Documents

[0006] [Patent Document 1] Japanese Patent Publication No. 2019-209362 [Patent Document 2] Japanese Patent Publication No. 2010-116594 [Overview of the project] [Problems that the invention aims to solve]

[0007] The present invention aims to provide an aluminum alloy for extruded materials that is high in strength while also having excellent bendability and resistance to stress corrosion cracking, a method for manufacturing cast billets using the same, and a method for manufacturing extruded materials. [Means for solving the problem]

[0008] The aluminum alloy for extruded materials according to the present invention contains, by mass%, Zn: 6.0-8.0%, Mg: 1.0-2.0%, Cu: 0.10-1.0%, Zr: 0.10-0.25%, Ti: 0.005-0.05%, further allowing Si: 0.01-0.25%, Fe: 0.10-0.40%, Mn: 0.004-0.4%, Cr: 0.05% or less, with the total [Fe+Mn+Cr+Zr] in the range of 0.20-1.10%, the remainder consisting of Al and unavoidable impurities, and is characterized by containing, in total by mass, 25% or more of recycled materials from the market and scraps or chips generated during the product manufacturing process as raw materials.

[0009] This invention is characterized by the fact that, by utilizing recycled materials, the amount of Si can be increased to 0.25%, and the amount of Fe can be increased to 0.40%, while the other components have been adjusted accordingly. Here, the recycled materials collected from the market include not only aluminum used in vehicle and aircraft parts, but also aluminum widely used as a wrought alloy. Furthermore, scraps and chips generated during the product manufacturing process may also be scraps and chips generated during the cutting and bending of extruded materials.

[0010] This invention is characterized not only by the chemical composition of the aluminum alloy but also by the manufacturing conditions of the billet. Specifically, the method is characterized by comprising the steps of: using the above-described aluminum alloy, melting it in the range of 740 to 780°C and then casting a billet at a solidification rate of 5°C / sec or higher; and homogenizing the billet in the range of 470 to 540°C and then cooling it at a cooling rate of 50°C / hr or higher. By manufacturing billets in this manner, the increased amounts of Si and Fe components can suppress the coarsening and increase in the amount of precipitates formed in the billet structure. The average length of precipitates formed in the metal structure of the billet manufactured in this way is 100 μm or less, and the area ratio of these precipitates can be kept to 6% or less.

[0011] Furthermore, the method for manufacturing an aluminum alloy extruded material according to the present invention is characterized by the following steps: using the above billet, extruding it at an extrusion ratio of 12 or more; immediately thereafter cooling the extruded material from a temperature of 400 to 550°C to 200°C or lower at a cooling rate of 50 to 750°C / min; followed by a first stage heat treatment at 80 to 130°C and a second stage heat treatment at 135 to 170°C, with a total heat treatment time of 8 to 22 hours. When extruded materials are manufactured in this manner, the average length of precipitates generated in the metal structure of the extruded material is 20 μm or less, and the area ratio of these precipitates is 6% or less. Furthermore, the PFZ width is 50 nm or more, and the number of precipitated particles deposited along that line is 10 particles / μm or less per unit length of 1 μm. This invention controls the conditions for die end quenching immediately after extrusion, setting the quenching period to a slightly longer duration than the conditions under which the intensity of artificial aging treatment peaks, thereby creating a sub-over-aging condition. [Effects of the Invention]

[0012] The extruded material obtained in the present invention is made of a 7000 series aluminum alloy. Even if it contains 25% or more, and further 30% or more of recycled materials, it can ensure high strength with a tensile strength of 400 MPa or more and a 0.2% proof stress of 380 MPa or more, while being excellent in bendability and stress corrosion cracking resistance (SCC resistance).

Brief Description of the Drawings

[0013] [Figure 1] Shows the chemical composition of the aluminum alloy used for evaluation. [Figure 2] Shows the production conditions of the billet. [Figure 3] Shows the production conditions of the extruded material. [Figure 4] Shows the evaluation results of the extruded material. [Figure 5] Shows the test method of three-point bending. [Figure 6] Shows an example of the nano-structure photograph of the extruded material.

Modes for Carrying Out the Invention

[0014] The aluminum alloy composition applied to the present invention will be described below. <zn> While high concentrations of Zn allow for increased strength without reducing extrudeability, excessive addition reduces stress corrosion cracking resistance. Therefore, the Zn content was kept within the range of 6.0-8.0%. <mg> While magnesium (Mg) is the most effective component for increasing strength, adding too much reduces extrudeability and moldability. Therefore, the Mg content was kept in the range of 1.0-2.0%. <cu> While the copper component is effective in increasing strength through its solid solution effect, adding too much reduces extrudeability, corrosion resistance, and moldability. Therefore, the copper content was kept in the range of 0.10 to 1.0%. <zr> Mn, Cr, and Zr are transition elements that can suppress the recrystallization depth on the surface of the extruded material during extrusion molding. However, if added in large amounts, the hardening sensitivity will be sharpened. Among these, the Zr component can be sufficiently hardened by fan air cooling immediately after extrusion without increasing the sensitivity compared to Mn and Cr, and the strength does not decrease. Also, if it exceeds 0.25%, it cannot dissolve in the molten aluminum. Therefore, the range of Zr was set to 0.10 - 0.25%. The Mn component can be sufficiently hardened by fan cooling without increasing susceptibility compared to Cr, and it is possible to achieve both stress corrosion resistance and strength. To achieve both strength with Mn content below 0.30% and suppression of recrystallization, a Fe+Mn+Cr+Zr content in the range of 0.20 to 1.10% is preferred. <ti> The Ti component is effective in refining the crystal grains when casting aluminum alloy extrusion billets, and the Ti content was set to a range of 0.005 to 0.05%. <Fe,Si> Fe and Si components are inevitably introduced as impurities during the preparation of molten aluminum alloy and the casting of billets due to the use of recycled materials (scrap). When the amount increases, strength, corrosion resistance, and moldability decrease. Furthermore, since Fe can suppress the recrystallization depth without increasing the susceptibility to quenching as a transition element, the Fe content was set to 0.10-0.40% and Si content to 0.01-0.25%.

[0015] Next, a billet was cast using molten aluminum alloy adjusted to the chemical composition shown in Figure 1, and its physical properties after extrusion and artificial aging treatment were evaluated, which are described below. In Figure 1, "in-process" refers to the amount of scrap and chips generated when cutting and bending extruded 7000 series aluminum alloy, while "commercial scrap" refers to recycled materials such as vehicle parts, aircraft parts, window frame scraps, chips, and scraps collected from the market. The "total" represents the proportion of in-process recycled materials plus locally sourced recycled materials added to the molten aluminum, relative to the total amount of molten aluminum. These recycled materials were added to the molten aluminum, and the remaining virgin material was used to adjust the composition. In this invention, the goal was to use recycled materials for an overall rate of 30% or more, and for a rate of 25% or more within the process.

[0016] Figure 2 shows the casting conditions for the billet. In this example, billets with diameters of 204 mm and 254 mm were cast. The goal is to maintain the melting temperature of the molten metal within the range of 740-780°C, and the specific maintenance temperatures are shown in the table in Figure 2. The specific examples shown below are presented in the table in Figure 2, and the target range will be explained. The molten metal was degassed by injecting Ar gas, then poured into the mold from the top, and cooled from the surroundings to ensure a solidification rate of 5°C / sec or higher. This allows for casting speeds of 45 mm / min or higher. The billets obtained in this way are then subjected to homogenization (HOMO) treatment. To ensure sufficient solid dissolution of the precipitates, a cooling period of 470-540°C for 2-8 hours was preferred, followed by a cooling rate of 50°C / hr or higher to near room temperature to refine the precipitates in the metal structure. When manufactured in this manner, the billet's metal structure exhibits an average grain size of 250 μm or less, an average length of precipitates of 100 μm or less, and an area ratio of these precipitates of 6% or less.

[0017] Figure 3 shows the processing and heat treatment conditions for the extruded material. Here, an extrusion ratio of 12 or higher refers to the ratio of the cross-sectional area of ​​the billet to the cross-sectional area of ​​the extruded material. A higher extrusion ratio results in a finer metal structure, which facilitates subsequent die end hardening. Furthermore, the heat treatment conditions involve a so-called two-stage artificial aging process. In the first stage, primary crystals are precipitated at a relatively low temperature of 80-130°C for 3-7 hours. Subsequently, a second stage heat treatment is performed at a relatively higher temperature of 135-170°C for 5-15 hours to grow the precipitates and ensure strength. In this process, the conditions were set to be slightly overaged in order to ensure flexibility.

[0018] The physical properties of the extruded material obtained in this way are shown in the table in Figure 4. The evaluation method is as follows: <Mechanical properties> Tensile test specimens of JIS-13B were taken from the extruded material, and tensile tests were conducted in accordance with JIS-Z2241. <3-point bend> A test piece with an extrusion material length of L1200 mm was cut out, and as shown in Fig. 5, the extrusion material was supported at two points with a pitch of 100 mm, and compressed with a compression testing machine (manufactured by Hodogaya Chemical Co., Ltd.) using a φ254 mm iron pole from the center up to a stroke of 150 mm at a compression speed of 50 mm / sec to evaluate the presence or absence of cracks on the appearance of the bent part. <Charpy test> Based on JIS-Z2242, a JIS-V notch No. 4 test piece was prepared from the extrusion material, and a Charpy impact test was carried out using a Charpy impact testing machine conforming to JIS standards. <SCC property> From the extrusion material, a test piece with a thickness t of 3 mm × width w of 20 mm × length L of 100 mm was cut out, and the SCC resistance was evaluated by three-point bending stress loading. With the stress of 80% of the yield strength loaded on the test material, the following conditions were taken as one cycle, and at 720 cycles, those without cracks on the appearance were considered to achieve the goal (720 cyc or 1440 cyc), and for those with cracks occurring before that, the cycle number (cyc) was taken as the evaluation value. [One cycle] Immersed in a 3.5% NaCl aqueous solution at 25°C for 10 min, then left standing in an environment of 25°C and 40% humidity for 50 min, and then air-dried naturally. <Metallographic structure> Samples were cut out from the center (C), the radius 1 / 2 part (R / 2), and the surface layer part (R) of the circular cross-section of the billet respectively, mirror-polished, and then etched with Keller's reagent. The metallographic structure was observed by optical microscope observation, and the length of the crystallized products inside the billet was measured from a 50× magnification image and calculated as the average value. The crystallized products in the 50× magnification image were measured by area ratio through image analysis. <Microscopic observation of extrusion material> Samples were cut out from the extrusion material, mirror-polished on the extrusion cross-section, and then the metallographic structure was observed by optical microscope observation. The length of the crystallized products inside the extrusion was measured from a 100× magnification image and calculated as the average value. The crystallized products in the 100× magnification image were measured by area ratio through image analysis. <Surface recrystallization> A sample is cut from the extruded material, the extruded cross-section is mirror-polished, and then etching is performed using sodium hydroxide reagent (3% NaOH). The metal structure was observed using an optical microscope, and the thickness of the recrystallized structure from the extruded surface was measured from 1000x magnification images. <Surface texture> The surface of the extruded material after extrusion was visually inspected to confirm that there was no foreign matter adhering to it or any cracks. <Extruded material nanostructure observation> Samples were cut from the extruded material, and the inside of the extrusion was observed using a TEM (transmission electron microscope) to measure the PFZ width (precipitate-free region) of the grain boundaries. Samples were cut from the extruded material, and the inside of the extrusion was observed using a TEM (transmission electron microscope). The number of grain boundary precipitates per unit length of grain boundary present at the grain boundaries was measured.

[0019] Examples 1 to 23 demonstrate that die end quenching immediately after extrusion is performed by air cooling, and subsequent artificial aging treatment ensures a tensile strength of 400 MPa or more, a 0.2% yield strength of 380 MPa or more, and an elongation of 10% or more. The extruded material obtained in this way has a recrystallization depth of 150 μm or less from the surface, and no surface defects such as cracks are present. Furthermore, in the metallic structure of the extruded material, the average length of the precipitates was 15 μm or less, and the area ratio of these precipitates was 6% or less. An example of a photograph of a nanostructure is shown in Figure 6. In terms of nanostructure, the PFZ width was 50 nm or more, and the number of particles deposited along that line was 10 particles / μm or less. Furthermore, the bending properties were evaluated using a three-point bending test, and the impact value was 8 J / cm using a Charpy test machine. 2 We were able to secure the above. Furthermore, no cracks occurred at SCC (stress corrosion cracking) levels above 720 cyc.

[0020] Compared to Examples 1-23, Comparative Example 1 had Si components exceeding 0.30% and 0.25%, a higher molten metal temperature, lower tensile strength and yield strength than the target values, and cracking occurred even in three-point bending. Comparative Example 2 had a Cu component of 1.05%, which was above 1.0%, so it had high strength but poor bendability. Comparative Example 3 had a Cu component of 1.10%, but its strength was slightly reduced because the nanostructure did not meet the target. Comparative Example 4 had a higher Mg content (2.50%) than 2.0%, and the casting conditions did not meet the requirements, resulting in high strength but poor bendability and SCC resistance. Comparative Example 5 had poor three-point bendability due to a short heat treatment time, Comparative Example 6 had poor bendability due to insufficient degassing treatment, Comparative Example 7 had a low extrusion ratio, Comparative Example 8 had a slow solidification rate, and Comparative Example 9 had a slow cooling rate of the extruded material; all of these had poor bendability. From these findings, it has become clear that by controlling the manufacturing conditions for aluminum alloys and billets, and the manufacturing conditions for extruded materials in this invention, it is possible to obtain extruded materials that are high in strength while also exhibiting excellent bendability and stress corrosion cracking resistance, even when using 30% or more, or up to 95%, of recycled materials in total.< / ti> < / zr> < / cu> < / mg> < / zn>

Claims

1. The following are the mass percentages of the materials: Zn: 6.0-8.0%, Mg: 1.0-2.0%, Cu: 0.10-1.0%, Zr: 0.10-0.25%, Ti: 0.005-0.05%. Furthermore, the content of Si: 0.01-0.25% and Fe: 0.10-0.40% is permitted. An aluminum alloy for extruded materials, characterized in that it contains Mn: 0.004 to 0.4%, Cr: 0.05% or less, the total of [Fe + Mn + Cr + Zr] is in the range of 0.20 to 1.10%, the remainder being Al and unavoidable impurities, and the raw materials include recycled materials from the market and scraps or chips generated during the product manufacturing process, totaling 25% or more.

2. A step of casting a billet using the aluminum alloy described in claim 1, by melting it in the range of 740 to 780°C and then solidifying it at a rate of 5°C / sec or higher, A method for manufacturing a billet for extruded materials, characterized by comprising the steps of homogenizing the billet at a temperature in the range of 470 to 540°C, and then cooling it at a cooling rate of 50°C / hr or higher.

3. Using the billet described in claim 2, extrusion is performed at an extrusion ratio of 12 or more, and immediately thereafter, the temperature of the extruded material is cooled from 400 to 550°C to 200°C or lower at a cooling rate of 50 to 750°C / min. Next, a method for manufacturing an aluminum alloy extruded material, characterized by comprising a first heat treatment at 80 to 130°C and a second heat treatment at 135 to 170°C, with a total heat treatment time of 8 to 22 hours.

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