Manufacturing method of high-strength aluminum alloy extrusion material excellent in toughness and corrosion resistance

Abstract

To provide a production method for high-strength aluminum alloy extrusion material excellent in toughness and corrosion resistance.SOLUTION: A production method for high-strength aluminum alloy extrusion material is characterized by carrying out: a step of casting a billet at a casting speed of 50 mm / min or more; a step of applying a homogenization treatment for 1-14 hours at 460-540°C to a casted billet; a step of cooling the billet after the homogenization treatment at a cooling speed of 50°C / hr or more; and a step of performing extrusion using the billet and performing air cooling at a cooling speed of 50-500°C / min as a die end hardening of extrusion, and then carrying out an artificial aging treatment, using the aluminum alloy comprising in mass%, Zn:5.0-7.0%, Mg:0.50-1.60%, Cu:0.05-0.50%, Zr:0.10-0.25%, Ti 0.005-0.05%, Mn:0.3% or less, Cr:0.2% or less, 0.10-0.65% total of [Mn+Cr+Zr], and the balance Al with inevitable impurities.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a high-strength extruded material using a 7000 series aluminum alloy, and particularly to a method for manufacturing an aluminum alloy extruded material having high strength, excellent toughness and corrosion resistance.

Background Art

[0002] In the fields of vehicles and various industrial machines, weight reduction of structural members is desired due to requirements such as reduction of environmental load, energy saving, and weight reduction. One of the means is an extruded material made of an aluminum alloy. As high-strength aluminum alloys, 7000 series alloys of the Al-Zn-Mg system and 6000 series alloys of the Al-Mg-Si system are typical examples. Among them, the 7000 series alloy can achieve high strength without relatively reducing the extrusion processability.

[0003] For example, Patent Document 1 discloses an aluminum alloy extruded material using an aluminum alloy containing 0.1 to 0.5% in total of one or more of Zn: 5.5 to 9.0%, Mg: 1.0 to 2.0%, Cu: 0.1 to 1.0%, Fe: 0.01 to 0.40%, Si: 0.01 to 0.20%, Ti: 0.005 to 0.2%, further Zr: 0.01 to 0.25%, Cr: 0.01 to 0.25%, V: 0.01 to 0.25%, and Sc: 0.01 to 0.25%. However, since rapid cooling at 200°C / sec or more, that is, 12000°C / min or more, must be performed while the temperature of the extruded material is 400°C or higher, cooling strain is likely to occur in the shape of the extruded material, and overaging treatment is required to ensure energy absorption characteristics (toughness), which causes a decrease in productivity.

[0004] Patent Document 2 discloses a method of reheating an extruded material made of an aluminum alloy having one or more of Zn: 5.5 to 7.0%, Mg: 0.5 to 1.8%, Cu: 0.1 to 0.5%, Fe: 0.01 to 0.40%, Si: 0.01 to 0.20%, Ti: 0.005 to 0.2%, and further Zr: 0.01 to 0.25%, Cr: 0.01 to 0.25%, Mn: 0.01 to 0.25% by extrusion to 330 to 550 °C at a heating rate of 10 °C / sec or more, and then cooling at a cooling rate of 50 °C / sec or more. However, this requires a heat treatment furnace and a cooling device for reheating, and surface recrystallization progresses when the extruded material is reheated, which may result in poor corrosion resistance.

[0005] In the case of conventional 7000-series high-strength aluminum alloy extruded materials, when attempting to obtain high strength, the toughness decreases, the stress corrosion cracking resistance in members subjected to stress decreases, and furthermore, water cooling is required during die-end quenching by air cooling after extrusion, resulting in a problem of reduced productivity.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] An object of the present invention is to provide a method for manufacturing a high-strength aluminum alloy extruded material with good productivity, which has high strength, excellent toughness and corrosion resistance, by die-end quenching by air cooling after extrusion.

Means for Solving the Problems

[0008] The manufacturing method of the high-strength aluminum alloy extruded material according to the present invention is as follows, all in mass%: Zn: 5.0 to 7.0%, Mg: 0.50 to 1.60%, Cu: 0.05 to 0.50%, Zr: 0.10 to 0.25%, Ti: 0.005 to 0.05%, Mn: 0.3% or less, Cr: 0.2% or less, and the total of [Mn + Cr + Zr] is in the range of 0.10 to 0.65%, with the balance being composed of Al and inevitable impurities. Using this aluminum alloy, a step of casting a billet at a casting speed of 50 mm / min or more, a step of performing a homogenization treatment on the cast billet at 460 to 540 °C for 1 to 14 hours, a step of cooling at a cooling rate of 50 °C / hr or more after the homogenization treatment, and a step of performing extrusion processing using the billet and air-cooling at a cooling rate of 50 to 500 °C / min as die end quenching of the extrusion processing, and then performing artificial aging treatment. Here, the artificial aging treatment preferably consists of a first stage at 80 to 120 °C for 1 to 6 hours and a second stage at 130 to 180 °C for 1 to 14 hours, and is a two-stage artificial aging treatment within a total of 20 hours. The extruded material manufactured in this way has high strength with a yield strength of 260 MPa or more, a Charpy impact value of 25 J / cm 2 or more, a ductility with an axial crushing EA rate of 55% or more, a DSC (integrated value corresponding to the endothermic peak) value of 30 or less, and a recrystallization depth of the surface part of 150 μm or less, and is excellent in stress corrosion cracking resistance.

[0009] In the present invention, the reasons for selecting the component range of the aluminum alloy are as follows. <Zn and Mg components> Even at a relatively high concentration, Zn does not reduce the extrudability and contributes to the improvement of strength. By adding Mg, MgZn2 precipitates in the structure, increasing the strength. However, when the addition amount of Mg increases, the extrudability decreases, and the precipitation amount of MgZn2 may become too large, reducing the toughness. Therefore, a combination in the range of Zn: 5.0 to 7.0%, Mg: 0.5 to 1.60% is good. In addition, the precipitation amount of MgZn2 may be controlled by setting Zn to 5.5 to 7.0% and Mg to 0.8 to 1.3%. <Cu component> The addition of the Cu component is effective for improving the strength due to the solid solution effect. However, when the addition amount increases, the general corrosion resistance decreases. Therefore, the range of Cu: 0.05 to 0.50% is preferable, and preferably the range of Cu: 0.10 to 0.30%. <Zr, Mn and Cr components> These components are all transition elements, which are effective in suppressing the recrystallization depth formed on the surface of the extruded material during extrusion processing and in refining the crystal grains. As a result, the toughness is improved and the stress corrosion cracking resistance is improved. However, among these, the Cr component sharpens the hardening susceptibility the most. When the addition amount increases, sufficient strength cannot be obtained unless hardening (die-end hardening) by cooling immediately after extrusion processing is changed to rapid cooling at the water-cooling level. Although the Mn component is not as hardening-sensitive as Cr, its influence is greater than that of Zr. Therefore, in the present invention, it is preferable to cope with this by adding only Zr: 0.10 to 0.25% without adding Cr and Mn. When adding Mn, it is suppressed to 0.30% or less. When adding Cr, it is suppressed to 0.20% or less, and it is preferably suppressed to the range of 0.10 to 0.65% in total of [Mn + Cr + Zr]. <Ti component> The Ti component is effective in refining crystal grains when casting a billet for extrusion processing, and generally a very small amount of B is also added. A slight addition amount of Ti: 0.005 to 0.05% is sufficient. <Other components> In the casting process of the 7000 series aluminum alloy, etc., the Fe component and the Si component are often contained as impurities. However, when the amount increases, it affects the extrudability, stress corrosion cracking resistance, etc. Therefore, it is preferably suppressed to Fe: 0.2% or less and Si: 0.1% or less.

[0010] Next, the manufacturing conditions will be described. In the production of an extruded material of an aluminum alloy, a cylindrical billet is cast and used. The casting conditions of this billet and the homogenization treatment conditions thereof also affect the quality of the extruded material to be produced. The billet is continuously cast into a cylindrical shape using a molten aluminum alloy by hot top casting or the like. When the casting speed is 50 mm / min or more, the average grain diameter of the crystal grains of the casting structure of the billet becomes a fine structure of 250 μm or less, and the crystal grains of the extruded material are refined and the toughness is improved even in the subsequent extrusion process. Also, for the homogenization treatment conditions after casting, heat treatment is performed at 460 to 540 °C for 1 hour or more and 14 hours or less to sufficiently dissolve the precipitates, and then the subsequent cooling is preferably performed at a cooling rate of 50 °C / hr or more.

[0011] In the extrusion process, the billet is loaded into the container of the extruder and pressed by a stem or the like, and the extruded material is extruded and formed through a pressing die. In this case, before loading the billet into the container, it is preheated to 400 °C or more and 500 °C or less. The extruded material immediately after being extruded from the extrusion die is at a high temperature of 500 °C or more. Die end quenching means that by utilizing this high temperature immediately after extrusion and performing air cooling with a fan or the like at a cooling rate of 50 to 500 °C / min, quenching becomes possible. When using the aluminum alloy according to the present invention, it is not necessary to perform water cooling as in Reference Document 1, and it is possible to suppress deformation of the extruded material during cooling. The air cooling device has a simple structure compared to the water cooling device and is excellent in productivity.

[0012] The extruded material that has undergone the die end quenching process will then be subjected to artificial aging treatment, and the conditions thereof also affect the quality of the extruded material. In the present invention, two-stage artificial aging treatment is performed with the first stage at 80 to 120 °C × 1 to 6 hours and the second stage at 130 to 180 °C × 1 to 14 hours, with a total of 20 hours or less. Thereby, the target quality can be obtained without overaging or reheating treatment.

Effects of the Invention

[0013] Conventionally, when trying to increase the strength of 7000 series aluminum alloys, they are prone to cracking and it is difficult to ensure toughness. Since the processing resistance increases during extrusion, the extruded material becomes hot, and the thickness of the recrystallized layer formed on the surface of the extruded material becomes thick, resulting in the technical problem of reduced stress corrosion cracking resistance. In the present invention, by optimizing the chemical composition of the aluminum alloy and the manufacturing conditions, an extruded material with a good balance can be obtained.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

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Figure 7

Modes for Carrying Out the Invention

[0015] The molten aluminum alloys of Examples 1 to 18 included in the component range according to the present invention shown in the table of FIG. 1 and Comparative Examples 19 to 25 in which any component is outside the scope of the present invention were adjusted, and billets were cast at the casting speed shown in the table of FIG. 2, and then homogenized at the homogenization treatment temperature (HOMO temperature) and HOMO time, and cooled at the cooling rate after HOMO in the table. The average crystal grain size of the metal structure of the billets at that time is shown in the column of "Billet Crystal Grain Size" in the table of FIG. 2. Next, the billets were preheated at the temperature shown as "BLT Temperature" in the table of FIG. 2 and subjected to extrusion processing. The shape of the extruded material was a U-shaped cross-section with a wall thickness of approximately 3 mm. Immediately after extrusion, it was air-cooled with a fan at the cooling rate shown in the table of Fig. 2, and die-end quenching was performed. Next, after the first heat treatment at the heat treatment temperature and heat treatment time shown in Fig. 2, the second heat treatment was performed, and two-stage artificial aging treatment was carried out. In addition, in the table of Fig. 2, the preferred conditions and ranges in the present invention were shown together as manufacturing conditions.

[0016] The evaluation items and their evaluation results are shown in the table of Fig. 3. The target values for each evaluation item were shown in the table. The evaluation conditions are as follows. <Mechanical properties> Based on JIS-Z2241, a JIS No. 5 test piece was prepared, and using a tensile testing machine conforming to the JIS standard, the tensile strength, σ 0.2 yield strength, and elongation were measured. <Billet crystal grain size> The surface of the billet was polished to a mirror finish and etched with Keller's reagent. The metallographic structure was observed by optical microscope observation, the 100-fold image was processed, and the average crystal grain size was determined. <Surface recrystallization depth of the extruded material> The cross-section of the extruded material was polished to a mirror finish and then etched with 3% NaOH. The average thickness of the recrystallized layer was determined by image processing from the 100-fold image with an optical microscope. <Charpy impact test> Based on JIS-Z2242, a JIS-V notch No. 4 test piece was prepared, and the Charpy impact value was determined with a Charpy impact testing machine conforming to the JIS standard. <Stress corrosion cracking resistance (SCC resistance)> With a stress of 80% of the yield strength applied to the test piece, the condition where no cracks occurred after 720 cycles with the following conditions as one cycle was regarded as achieving the target. In addition, for those in which cracks occurred during the process, the number of cycles was displayed. [1 cycle] Immerse it in an aqueous solution of 3.5% NaCl at 25°C for 10 min, then leave it standing in an environment of 25°C and 40% humidity for 50 min, and then air-dry it. <DSC analysis> From the chart obtained using a differential scanning calorimeter (Thermo plus evo2 manufactured by Rigaku), the value of the area (integrated value) corresponding to the endothermic peak was taken as the DSC value. An example of the chart is shown in Fig. 4. In the present invention, as shown in the explanatory diagram of Fig. 5, the purpose of evaluating the DSC value is that when the precipitation amount of the precipitate in the crystal grains becomes too large, the toughness decreases and cracks are likely to occur. Therefore, the area (integrated value) of the endothermic peak at 100 - 200°C at which the precipitate redissolves was set to 30 or less. The endothermic peak portion is shown in Fig. 4. <Axial crushing property and its appearance> The test of the axial crushing property was carried out as follows. Using a square tube-shaped test piece having a "field" shape, a static load was applied along the axial length direction of the test piece at a compression speed of 50 mm / min (axial length dimension before the test: 150 mm → axial length dimension after the test: 60 mm). Fig. 6 shows the load-displacement curve when the test piece according to Example 4 was subjected to an axial crushing test. The horizontal axis represents the stroke of the crosshead of the compression testing machine when a load is applied in the axial length direction of the test piece. The vertical axis represents the magnitude of the load. As shown in Fig. 6, when the test piece is crushed and deformed in a bellows shape, peaks (P1 - P3) of the load value occur at intervals. Fig. 7(a) shows the state of the test piece according to Example 4 after the axial crushing test. After the axial crushing test, the test piece was successfully crushed and deformed into a bellows shape. When crushed and deformed in this bellows shape, the evaluation was ○. Fig. 7(b) shows the state of the test piece according to Comparative Example 3 after the axial crushing test. After the axial crushing test, the wall portion of the test piece was broken. When the wall portion was broken in this way, the evaluation was ×.

[0017] <Consideration of Evaluation Results> In Examples 1 to 18, all quality targets were cleared. On the other hand, in Comparative Example 19, the addition amount of Zr was small, the surface recrystallization depth was thick, and the Charpy impact value and stress corrosion cracking resistance also did not reach the target. In Comparative Example 20, although the addition amount of Mg was large and it had strength, the toughness was decreased. In Comparative Example 21, the artificial aging treatment conditions were outside the scope of the present invention, and the toughness did not reach the target. In Comparative Examples 22 and 23, since the addition amount of Zr was small, the toughness was poor. In Comparative Example 24, the range of [Mn + Cr + Zr] was out of range and the heat treatment conditions were also not within the predetermined range, so the quality target was not reached. In Comparative Example 25, the Mg content was high and the toughness was inferior.

Claims

1. All of the following are in mass %, Zn: 5.0 to 7.0%, Mg: 0.50 to 1.60%, Cu: 0.05 to 0.50%, Zr: 0.10 to 0.25%, Ti: 0.005 to 0.05%, Mn: 0.3% or less, Cr: 0.2% or less, and the total of [Mn + Cr + Zr] is in the range of 0.10 to 0.65%, and the balance is made of Al and inevitable impurities. Using an aluminum alloy, A step of casting a billet at a casting speed of 50 mm / min or more, A step of performing a homogenization treatment on the cast billet at 460 to 540°C for 1 to 14 hours, A step of cooling at a cooling rate of 50°C / hr or more after the homogenization treatment, Extrusion processing using the billet, and performing air cooling with a cooling rate of 50 to 500°C / min as die quenching of the extrusion processing, Thereafter, it consists of a first stage at 80 to 120°C for 1 to 6 hours and a second stage at 130 to 180°C for 1 to 14 hours, and by performing a two-stage artificial aging treatment within a total of 20 hours, A high-strength material with a tensile strength of 260 MPa or more, a Charpy impact value of 25 J / cm ２ or more, having toughness with an axial crushing EA rate of 55% or more, a DSC (integrated value corresponding to the endothermic peak) value of 30 or less, and a recrystallization depth of the surface part of 150 μm or less, and excellent stress corrosion cracking resistance. A method for manufacturing a high-strength aluminum alloy extruded material excellent in toughness and corrosion resistance, characterized by the above features.

2. The aluminum alloy used is an aluminum alloy in which Zn: 5.5 to 7.0%, Mg: 0.8 to 1.3%, Cu: 0.05 to 0.50%, Zr: 0.10 to 0.25%, Ti: 0.005 to 0.05%, does not contain Mn and Cr, and the balance is made of Al and inevitable impurities. A method for manufacturing a high-strength aluminum alloy extruded material excellent in toughness and corrosion resistance according to Claim 1.

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