Recycled material-containing 7000 series aluminum alloy and billet using the same, extrusion material and producing method

Abstract

To provide a high strength extrusion material excellent in SCC resistance, while aiming to improve a service rate of recycled materials (scrap materials), in a 7000-series aluminum alloy and a producing method thereof.SOLUTION: The 7000-series aluminum alloy contains, in terms of mass%, Zn: 6.0 to 8.0%, Mg: 1.0 to 2.0%, Cu: 0.10 to 1.0%, Mn: 0.004 to 0.40%, Zr: 0.10 to 0.25%, Ti: 0.005 to 0.05%, Cr: 0.05% or less, with the content range of Si and Fe being Si: 0.01 to 0.25%, Fe: 0.10 to 0.40%, and [Fe+Mn+Zr]: 0.23 to 1.05%, and contains 30% or more of recycled material.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a 7000 series aluminum alloy and a billet and extrusion material using the same, and in particular to a manufacturing method for an extrusion material having excellent stress corrosion cracking resistance (SCC resistance) and high strength while improving the utilization rate of recycled materials (scrap materials). [Background technology]

[0002] Al-Zn-Mg and Al-Zn-Mg-Cu aluminum alloys are known to have excellent high strength. In these 7000 series aluminum alloys, Fe and Si are alloying components classified as impurities, and their influence is large in wrought materials. If the amount of these components increases during the recycling process of scrap materials, it can cause a decrease in stress corrosion cracking resistance and a failure to obtain high strength.

[0003] Patent Document 1 discloses a technology that enables the use of scrap material by forming intermetallic particles into a circular or spherical shape with an aspect ratio of 4 or less. However, this does not provide sufficient strength or SCC resistance. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2023-17976 A Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention aims to provide a 7000 series aluminum alloy extruded material that is excellent in SCC resistance and has high strength while improving the utilization rate of recycled materials (scrap materials), and a method for producing the same. [Means for solving the problem]

[0006] The 7000 series aluminum alloy according to the present invention is characterized in that it contains, in mass%, Zn: 6.0-8.0%, Mg: 1.0-2.0%, Cu: 0.10-1.0%, Mn: 0.004-0.40%, Zr: 0.10-0.25%, Ti: 0.005-0.05%, and Cr: 0.05% or less, with Si and Fe contents in the ranges of Si: 0.01-0.25%, Fe: 0.10-0.40%, and [Fe+Mn+Zr]: 0.23-1.05%, and contains 30% or more recycled material.

[0007] Recycled materials include offcuts and chips generated during the manufacturing process of aluminum products, and scrap materials collected from the city. To use these as recycled materials during extrusion, they are melted, the components are adjusted as molten aluminum alloy, and then cast into a cylindrical billet for extrusion processing. Therefore, impurity elements contained in commercially available scrap materials and Fe and Si are easily mixed in from the furnaces and molds that come into contact with the materials during the remelting and casting processes. Wrought materials are more affected by impurities than cast materials, and in particular in 7000 series alloys, Fe is usually kept below 0.1%, and at most 0.2%. Additionally, it was also considered preferable for Si to be 0.1% or less. This limits the rate at which recycled materials can be used. The present invention is characterized in that it is possible to increase the content of recycled material by suppressing the length and ratio of crystallized particles that appear in the metal structure.

[0008] In the present invention, the billet for extrusion is a billet for extrusion cast using the 7000 series aluminum alloy according to claim 1, and it is preferable that the billet has an average length of crystallized particles in a metal structure of 100 μm or less and a crystallized particle ratio of 6% or less.

[0009] A billet having such a metal structure can be obtained, for example, with a billet diameter of 174 mm or more to 204 mm, by setting the casting speed to 45 mm / min or more and the solidification rate to a high speed of 5° C. / sec or more. Billets for extrusion are produced by continuous or semi-continuous casting, in which molten aluminum alloy is poured into a mold from above and cast downward as it cools and solidifies. In this case, by rapidly cooling the material so that the solidification rate is 5°C / sec or more, it is possible to make the average length of the needle-like crystals, etc., made of intermetallic compounds in the metal structure, 100 µm or less, and the ratio of the crystals 6% or less. As a result, even if the amount of impurities such as Fe and Si increases, the extruded material can maintain its excellent SCC resistance and high strength in the next extrusion process.

[0010] In the present invention, in the extruded material using the billet for extrusion processing as described in claim 2, it is preferable that the average length of the crystallites in the metal structure of the extruded material is 20 μm or less and the crystallite ratio is 6% or less.

[0011] Such an extruded material can be obtained by setting the cross-sectional area of ​​the extruded material relative to the billet diameter so that the extrusion ratio, which should increase the degree of processing during extrusion, is 12 or more. Effect of the Invention

[0012] As a result, even if the material contains 30% or more recycled material, die edge quenching immediately after extrusion and two-stage artificial aging treatment result in an extruded material with excellent SCC resistance and high strength, with a tensile strength of 400 MPa or more and a 0.2% yield strength of 380 MPa or more. [Brief description of the drawings]

[0013] [Figure 1] The composition of the aluminum alloy used in the evaluation is shown below. [Diagram 2] The billet casting conditions and evaluation results are shown. [Diagram 3] The extrusion conditions are shown below. [Figure 4] The evaluation results of the extruded material are shown. [Diagram 5] (a) shows the evaluation area of ​​the billet, and (b) shows an example of the metal structure. [Figure 6] An example of the metal structure of an extruded material is shown. [Figure 7] The evaluation conditions for SCC resistance are shown below. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Molten aluminum alloys having the compositions of Examples 1 to 14 and Comparative Examples 1 to 3 shown in the table of FIG. 1 were prepared, and billets having a diameter of 204 mm were cast under the conditions shown in the table of FIG. In the table of FIG. 1, the total amount of recycled material indicates the mass percentage of the recycled material contained in the molten metal used in billet casting. In-process scrap and city scrap represent the breakdown of the total amount recycled. In-process scrap refers to scraps and other materials generated during the manufacturing process of aluminum products within the company, while city scrap refers to sash scraps, cutting chips, and scraps procured from outside the company. In the present invention, the target usage rate of recycled materials is 30% or more.

[0015] Figure 2 shows the billet casting conditions. For the evaluation, a cylindrical billet with a diameter of 204 mm was manufactured and used. The hydrogen gas content in the molten metal was set to 0.2 cc / 100 g Al or less, and a water-cooled vertical continuous casting machine was used, with the targets being a casting speed of 45 mm / min or more and a solidification speed of 5°C / sec or more. The cast billet was subjected to homogenization treatment (HOMO) at 470 to 540°C for 2 to 8 hours, and then cooled to 100°C or less at a cooling rate of 50°C / hr or more. The average grain size, length and percentage of crystallized particles in the metal structure of the billet thus obtained are shown in the table in FIG. The measurement method is as follows. From the billet cast material, samples of the center, 1 / 2 radius, and surface layer as shown in FIG. 5(a) are cut out, and the billet cast cross section is mirror-polished and then etched with Keller's reagent. The metal structure was observed by optical microscopy, and the lengths of the crystallized particles inside the billet were measured from 500x images and calculated as an average value. The crystallized matter in the 500x image was measured in terms of area ratio by image analysis. The targets were an average crystallite length of 100 μm or less and a crystallite rate of 6% or less. FIG. 5(b) shows an example of a photograph of the metal structure of the billet according to the example.

[0016] Extrusion was performed under the conditions shown in FIG. 3, and the evaluation results of the extruded material obtained are shown in the table of FIG. The cross-sectional area of ​​the extrusion material used in the evaluation was 981 mm 2 and 1307 mm 2 The extrusion ratios are 33 and 25, respectively. The billet temperature (BLT temperature) was set to the temperature shown in Table 3, and the extrusion process was performed so that the extruded profile temperature immediately after extrusion after preheating was 440°C or higher. The conditions for die end quenching (air cooling) immediately after extrusion were as follows: natural cooling was performed over 0.1 min or more until the extruded material temperature (cooling start temperature) after extrusion reached 400 to 550°C, and thereafter fan air cooling was started at a cooling rate of 50 to 750°C / min (cooling rate after cooling start). Next, two-stage artificial aging treatment was performed under the conditions shown in the table of FIG.

[0017] The table in Figure 4 shows the evaluation results of the extruded material. The evaluation items and evaluation conditions are as follows: <Mechanical properties> Based on JIS-Z2241, JIS-5 tensile test pieces are prepared from the extruded material, and tensile tests are carried out using a tensile testing machine conforming to the JIS standard. <Microscopic observation of extruded material> A sample was cut out from the extruded material, and the extruded cross section was mirror-polished. The metal structure was then observed under an optical microscope, and the length of the crystallized particles inside the extrusion was measured from a 1000x image and calculated as an average value. The crystallized matter in the 1000x image was measured in terms of area ratio by image analysis. <SCC resistance> A test piece measuring t3 mm x w20 mm x L100 mm was cut out from the extruded profile, and a stress of 80% of the yield strength was applied to the test material using a three-point bending stress load as shown in Figure 7. The following conditions were counted as one cycle, and if no cracks occurred after 720 cycles, the target was achieved. If cracks occurred up to that point, the number of cycles (cyc) was recorded. [1 cycle] Immerse in a 3.5% NaCl aqueous solution at 25°C for 10 minutes, then leave at 25°C and 40% humidity for 50 minutes, and then air dry. <Charpy impact value> Based on JIS-Z2242, JIS-V notch No. 4 test pieces are prepared from the extruded shapes, and Charpy impact tests are carried out using a Charpy impact tester conforming to JIS standards. <Surface recrystallization> A sample is cut out from the extruded shape, the extruded cross section is mirror-polished, and then etched with sodium hydroxide reagent (3% NaOH). The metal structure is observed under an optical microscope, and the thickness of the recrystallized structure from the extrusion surface is measured from a 1000x image. <Surface texture> After extrusion, the surface of the extruded shape was visually inspected for the presence of foreign matter or cracks.

[0018] In Examples 1 to 14, all quality targets were cleared even though the recycled material usage rate was 30% or more. In contrast, in Comparative Example 1, the Si content exceeded 0.30% and 0.25%, and the casting speed was 40 mm / min and 45 mm / min or less, so that the average length of the crystallized particles became large and the tensile strength and yield strength did not reach the targets. In Comparative Example 2, the Cu content was 1.10%, which was slightly higher than the 1.00% in the present invention, and the strength was insufficient under the cooling conditions after extrusion and the artificial aging treatment conditions in the present invention. Comparative Example 3 was used as a reference because it did not contain any recycled materials. In the present invention, by including recycled materials, even if the amounts of Si and Fe components are high, the SCC resistance can be improved by selecting appropriate billet casting conditions and extrusion processing conditions.

Claims

1. In mass%, Zn: 6.0 to 8.0%, Mg: 1.0 to 2.0%, Cu: 0.10 to 1.0%, Mn: 0.004 to 0.40%, Zr: 0.10 to 0.25%, Ti: 0.005 to 0.05%, Cr: 0.05% or less, A 7000 series aluminum alloy, characterized in that the Si and Fe contents are in the ranges of Si: 0.01-0.25%, Fe: 0.10-0.40%, [Fe+Mn+Zr]: 0.23-1.05%, and that recycled materials are contained in an amount of 30% or more.

2. A billet for extrusion casting using the 7000 series aluminum alloy according to claim 1, The billet for extrusion is characterized in that the average length of crystallites in the metal structure is 100 μm or less and the crystallite ratio is 6% or less.

3. An extrusion material using the billet for extrusion processing according to claim 2, An aluminum alloy extrusion material, characterized in that the average length of crystallites in the metal structure of the extrusion material is 20 μm or less and the crystallite ratio is 6% or less.

4. Using the 7000 series aluminum alloy according to claim 1, A method for producing an extruded material, comprising the steps of: casting a billet with a diameter of 174 mm or more at a casting speed of 45 mm / min or more and a solidification rate during casting of 5°C / sec or more; and extruding the cast billet under conditions such that the extrusion ratio is 12 or more.

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