Manufacturing method of high-strength aluminum alloy extrusion material with excellent SCC resistance

Abstract

The purpose of the present invention is to provide a method for producing an aluminum alloy extruded material, which is effective in improving strength while ensuring excellent SCC resistance. In the method, a blank is cast using an aluminum alloy having a composition comprising, in mass%, 6.0 to 8.0% of Zn, 1.5 to 2.8% of Mg, 0.10 to 0.50% of Cu, 0.10 to 0.50% of Zr, 0.005 to 0.05% of Ti, 0.10 to 0.40% of Mn, and 0.05% or less of Cr, with the total of Mn + Cr + Zr being 0.10 to 0.50%, with the balance being Al and unavoidable impurities; the blank obtained through casting is subjected to homogenization treatment at the temperature of 470-560 DEG C for 1-14 hours and then cooled at the cooling speed of 50 DEG C / hr or above; and the blank is subjected to extrusion processing, then air cooling is immediately carried out, after the temperature of the extruded material reaches 300-480 DEG C, the extruded material is cooled to 200 DEG C or below within the cooling speed range of 150-950 DEG C / min, and then artificial aging treatment is carried out.

Description

Technical Field

[0001] The present invention relates to a method for producing an extruded material using an Al-Zn-Mg-based aluminum alloy, and in particular, to a method for producing an extruded material having high strength and excellent SCC resistance. Background Art

[0002] Al-Zn-Mg series (7000 series) aluminum alloys can be used in various fields such as vehicles and industrial machinery because they can be extruded into high-strength materials.

[0003] The quality of the extruded material of such an aluminum alloy varies greatly depending on the casting of the billet, homogenization conditions, extrusion conditions, and heat treatment conditions.

[0004] For example, Patent Document 1 describes a technology for performing recovery heat treatment using a 7000 series aluminum alloy to which a large amount of transition elements such as Mn, Cr, and Zr are added. However, when used as is, the stress corrosion cracking resistance (SCC resistance) is insufficient.

[0005] In Patent Document 2, a high-strength material is obtained by adding transition elements such as Mn, Cr, and Zr and performing rolling or drawing processing after extrusion, but the SCC resistance is still poor, and it is difficult to achieve both high strength and SCC resistance.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application Publication No. 2014-145119

[0009] Patent Document 2: Japanese Patent Application Laid-Open No. 2928445 Summary of the invention

[0010] Technical problem to be solved by the invention

[0011] An object of the present invention is to provide a method for producing an aluminum alloy extruded material which is effective for achieving high strength while ensuring excellent SCC resistance.

[0012] Technical solutions for solving technical problems

[0013] The method for producing a high-strength aluminum alloy extruded material having excellent SCC resistance according to the present invention is characterized in that:

[0014] A billet is cast using an aluminum alloy, wherein the composition of the aluminum alloy is: in terms of mass %, Zn: 6.0-8.0%, Mg: 1.5-2.8%, Cu: 0.10-0.50%, Zr: 0.10-0.50%, Ti: 0.005-0.05%, Mn: 0.10-0.40%, Cr: 0.05% or less, and the total of Mn+Cr+Zr is 0.10-0.50%, and the balance is Al and unavoidable impurities;

[0015] The cast billet is homogenized at 470-560°C for 1-14 hours and then cooled at a cooling rate of 50°C / hr or more;

[0016] The billet is extruded and immediately air-cooled. After the temperature of the extruded material reaches 300-480°C, it is cooled to below 200°C at a cooling rate of 150-950°C / min, and then artificial aging is performed.

[0017] The artificial aging treatment is as follows: the first stage is 80-130°C for 2-8 hours, the second stage is 130-160°C for 2-16 hours,

[0018] The high-strength aluminum alloy extruded material preferably has a 0.2% proof stress of 460 MPa or more and excellent SCC resistance, and the average crystal grain size of the extruded material is 250 μm or less.

[0019] When a billet is cast using an Al-Zn-Mg aluminum alloy and then subjected to extrusion processing, the temperature of the extruded material immediately after extrusion reaches a high temperature of 440 to 585° C. due to processing heat or the like.

[0020] Therefore, quenching is possible by cooling immediately after extrusion, and this is called die edge quenching.

[0021] As the die edge quenching, a method of air cooling using a fan or the like and a method of water cooling using water or the like are known.

[0022] In this case, by adding transition elements such as Mn, Cr, and Zr to the aluminum alloy, the crystal grains are refined and the hardenability is improved.

[0023] However, as disclosed in Documents 1 and 2, when the amount of Cr added increases, the quenching sensitivity becomes acute, and a sufficient quenching effect cannot be obtained unless the quenching is performed at a water-cooling level.

[0024] However, when the extruded material is water-cooled at a high temperature immediately after the extrusion process, the extruded material undergoes cooling deformation, making it difficult to ensure the shape quality of the extruded material, and there is a technical problem that the stress corrosion cracking resistance (SCC resistance) is reduced.

[0025] Therefore, the present invention is characterized in that by controlling the amount of Cr added, Mn: 0.1-0.40% and Zr: 0.10-0.25% which are relatively insensitive to quenching are added, and the total amount of Mn+Cr+Zr is controlled in the range of 0.10-0.50%, air cooling at a cooling rate of 50-150°C / min is applied until the temperature of the extruded material immediately after extrusion reaches 300-480°C from 500-585°C, and then rapid cooling at a cooling rate of 150-950°C / min is applied, thereby preventing a decrease in SCC resistance and achieving high strength.

[0026] Next, the reasons for selecting the composition of the aluminum alloy in the present invention will be described.

[0027] <Zn、Mg>

[0028] Zn and Mg components contribute to obtaining high strength.

[0029] Among them, Zn can be added in high concentration without relatively reducing extrudability, but if too much, SCC resistance decreases. Therefore, the following, in terms of mass %, preferably Zn is in the range of 6.0 to 8.0%, and Mg is in the range of 1.5 to 2.8%.

[0030] <cu>

[0031] The Cu component is effective for achieving high strength due to the solid solution effect, but when too much, extrudability is reduced and general corrosion resistance is reduced. Therefore, the Cu content is preferably in the range of 0.10 to 0.50%.

[0032] <Mn、Zr、Cr>

[0033] Mn, Zr, and Cr are all transition elements, which can suppress the refinement of crystal grains and the depth of surface recrystallization during extrusion molding.

[0034] In the present invention, while ensuring hardenability by cooling immediately after extrusion, in consideration of air cooling in the high temperature region of the extruded material and rapid cooling starting from 300 to 480°C, Mn: 0.10 to 0.40%, Zr: 0.10 to 0.25%, and Cr are reduced as much as possible, preferably at least 0.05% or less.

[0035] In addition, the total content of Mn+Cr+Zr is preferably controlled within the range of 0.10 to 0.50%.

[0036] <ti>

[0037] The Ti component is effective for refining crystal grains during billet casting.

[0038] Considering the amount of Ti dissolved, the Ti content is preferably in the range of 0.005 to 0.05%.

[0039] <Fe、Si>

[0040] In the present invention, Fe and Si components are impurities, and are preferably controlled to be Fe: 0.4% or less and Si: 0.3% or less.

[0041] Effects of the Invention

[0042] In the present invention, even if the quenching air immediately after extrusion molding is cooled to an extrusion temperature of 300 to 480°C and then rapidly cooled at a cooling rate of 150 to 950°C / min, cooling deformation and reduction in SCC resistance can be suppressed, and high strength can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 The compositions of the aluminum alloys used for evaluation are shown.

[0044] Figure 2 The production conditions of the billets and extruded materials used for evaluation are shown.

[0045] Figure 3 Evaluation results of the extruded materials are shown. DETAILED DESCRIPTION

[0046] preparation Figure 1 The molten aluminum alloy composition shown in Figure 2 Under the manufacturing conditions shown, 8-inch cylindrical billets were produced and homogenized. Figure 2 The extruded materials were produced under the manufacturing conditions shown.

[0047] The blank is produced by a continuous casting method such as a hot top casting method or a heat insulation mold casting method.

[0048] At this time, when the casting speed is set to 50 mm / min or more, the casting structure of the billet becomes a fine structure with an average crystal grain size of 250 μm or less.

[0049] The average grain size is an average value of 3×3 points at the center of the cross section of the front end, middle part, and rear end of the billet and the middle position of the peripheral part.

[0050] Homogenization of the billet (HOMO) is carried out to eliminate the microsegregation produced during billet casting.

[0051] The homogenization treatment is to maintain the temperature at 470-560°C for 1-14 hours and then cool at a cooling rate of 50°C / hr or more.

[0052] Next, the billet (BLT) is preheated to 400° C. or higher, preferably 440 to 500° C., and then filled into a container of an extruder and extruded.

[0053] Extrusion processing directly uses an extruder.

[0054] The temperature of the extruded material (extruded profile) immediately after the extrusion process may be 440°C or higher, preferably 480 to 510°C.

[0055] The temperature is 440°C or higher immediately after extrusion from the die, and the material is air-cooled at the ambient temperature.

[0056] The present invention is characterized in that air cooling is performed immediately for about 0.1 min or more immediately after extrusion, and water cooling or strong air cooling by a fan is performed at a rate of 150 to 950°C / min from the stage when the temperature of the extruded material (extruded profile) reaches 300 to 480°C until the temperature reaches below 200°C.

[0057] In this way, in the die edge quenching during extrusion molding, even when the temperature of the extruded material reaches 300~480℃, the quenching effect can be fully obtained by rapid cooling at a rate of 150~950℃ / min. Compared with the existing die edge quenching using water cooling, cooling deformation is not easy to occur in the extruded material.

[0058] Thereby, high strength can be obtained while suppressing a decrease in SCC resistance.

[0059] This point will be described later by comparing Examples with Comparative Examples.

[0060] Next, the extruded material is subjected to artificial aging treatment to obtain an extruded material having excellent SCC resistance and high strength. The first stage of the artificial aging treatment is 80-130° C.×2-8 hr, and the second stage is 130-160° C.×2-8 hr.

[0061] In the present invention, the targets are a T5 tensile strength of 480 MPa or more, a 0.2% T5 yield strength of 460 MPa or more, and a T5 elongation of 10% or more.

[0062] The SCC resistance (stress corrosion cracking resistance) is aimed at preventing cracking from occurring for 720 cycles or more under the sample conditions described later.

[0063] In addition, the depth of the recrystallized layer formed on the surface of the extruded material is targeted to be 150 μm or less on average.

[0064] Next, the evaluation method of the extruded material will be described.

[0065] (1) Mechanical properties: Based on JIS-Z2241, JIS-5 tensile test pieces were prepared along the extrusion direction of the extruded material and measured using a tensile tester in accordance with JIS standards.

[0066] (2) Regarding the depth of the recrystallized layer formed on the surface of the extruded material, a cross section of a sample cut out from the extruded material was mirror-polished and then etched with a 3% NaOH sodium hydroxide aqueous solution.

[0067] Next, the average depth of the recrystallized layer was measured from a 100-times-magnified image of the metal structure by optical microscope observation.

[0068] Regarding SCC resistance, the following conditions were defined as one cycle under a state where a stress of 80% of the proof strength was applied to the test piece, and the occurrence of cracks at 720 cycles or more was considered to have achieved the target.

[0069] <1 cycle>

[0070] The samples were immersed in a 3.5% NaCl aqueous solution at 25°C for 10 minutes, and then placed at 25°C and 40% humidity for 50 minutes, followed by natural drying.

[0071] (Evaluation results)

[0072] Figures 1 to 3 The production conditions of Examples 1 to 15 and Comparative Examples 1 to 4 and the evaluation results of the extruded materials obtained therefrom are shown in FIG.

[0073] All evaluation items of Examples 1 to 15 achieved the target, and the tensile strength was 480 MPa or more, the 0.2% proof stress was 460 MPa or more, and the SCC resistance was excellent.

[0074] On the other hand, in Comparative Example 1, the quenching start temperature of the extruded material at 150 to 950° C. / min was 500° C., exceeding the prescribed upper limit of 480° C., and therefore the SCC resistance did not achieve the target.

[0075] In Comparative Example 2, Mn was 0.70%, exceeding the upper limit of the specification of 0.40%, and the rapid cooling start temperature was high, so the SCC resistance did not achieve the target.

[0076] In addition, the strength of Comparative Example 2 is lower than that of Comparative Example 1.

[0077] In Comparative Example 3, the Cu component may be 1.60%, which is more than the upper limit of the specification of 0.50%, and even if the strength is high, the SCC resistance is significantly reduced.

[0078] In Comparative Example 4, the Mg component was 1.21%, which was lower than the lower limit of 1.5% of the specification, and therefore the strength did not achieve the target.

[0079] Industrial Applicability

[0080] When the method for producing an aluminum alloy extruded material according to the present invention is used, a high-strength extruded material having excellent SCC resistance is obtained, and thus the extruded material can be used for various structural parts in vehicles, industrial machinery, and the like.< / ti> < / cu>

Claims

1. A method for producing a high-strength aluminum alloy extrusion material having excellent SCC resistance, characterized in that: A billet is cast using an aluminum alloy, wherein the composition of the aluminum alloy is: in terms of mass %, Zn: 6.0-8.0%, Mg: 1.5-2.8%, Cu: 0.10-0.50%, Zr: 0.10-0.50%, Ti: 0.005-0.05%, Mn: 0.10-0.40%, Cr: 0.05% or less, and the total of Mn+Cr+Zr is 0.10-0.50%, and the balance is Al and unavoidable impurities, The billet obtained by casting is subjected to a homogenization treatment at 470-560° C. for 1-14 hours, and then cooled at a cooling rate of 50° C. / hr or more. The billet is extruded and then immediately air-cooled. After the temperature of the extruded material reaches 300-480° C., it is cooled to below 200° C. at a cooling rate of 150-950° C. / min, and then artificially aged.

2. The method for producing a high-strength aluminum alloy extrusion material having excellent SCC resistance according to claim 1, characterized in that: The artificial aging treatment is as follows: the first stage is 80-130°C for 2-8 hours, the second stage is 130-160°C for 2-16 hours, The high-strength aluminum alloy extrusion material has a 0.2% yield strength of 460 MPa or more and excellent SCC resistance.

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