WO2023233713A1 - Manufacturing method for high-strength aluminum alloy extruded material having excellent scc resistance - Google Patents

Abstract

The purpose of the present invention is to provide a manufacturing method for an aluminum alloy extruded material that is effective in achieving higher strength while maintaining excellent SCC resistance. The present invention is characterized in that a billet is casted using an aluminum alloy comprising, in terms of mass%, 6.0 to 8.0% Zn, 1.5 to 2.8% Mg, 0.10 to 0.50% Cu, 0.10 to 0.50% Zr, 0.005 to 0.05% Ti, 0.10 to 0.40% Mn, and 0.05% or less Cr, with a total of Mn+Cr+Zr being 0.10 to 0.50% and the remainder consisting of Al and unavoidable impurities, the billet obtained by the casting is homogenized at 470 to 560°C for 1 to 14 hours, and then cooled at a cooling rate of 50°C/hr or higher, the billet is used for extrusion processing, air-cooled immediately after the extrusion until the temperature of the extruded material reaches 300 to 480°C, further cooled to 200°C or less at a cooling rate in a range of 150 to 950°C/min, and then subjected to an artificial aging treatment.

Description

Method for manufacturing high-strength aluminum alloy extrusions with excellent SCC resistance

The present invention relates to a method for producing an extruded material using an Al-Zn-Mg-based aluminum alloy, and particularly to a method for producing an extruded material that has high strength and excellent SCC resistance.

BACKGROUND ART Al-Zn-Mg (7000 series) aluminum alloys are used in various fields such as vehicles and industrial machinery because they produce high-strength extruded materials.
The quality of the extruded material of this type of aluminum alloy varies greatly depending on the billet casting, homogenization treatment conditions, extrusion conditions, heat treatment conditions, etc.
For example, Patent Document 1 describes a technique in which a 7000-series aluminum alloy to which a relatively large amount of transition elements such as Mn, Cr, and Zr are added is subjected to restoring heat treatment. (SCC resistance) is insufficient.
In Patent Document 2, a high-strength material is obtained by adding transition elements such as Mn, Cr, and Zr and rolling or drawing after extrusion, but the SCC resistance is still poor, and it is difficult to achieve both high strength and SCC resistance. It is difficult to achieve this goal.

Japanese Patent Application Publication No. 2014-145119 Japanese Patent No. 2928445

An object of the present invention is to provide a method for manufacturing an aluminum alloy extrusion material that is effective in achieving high strength while ensuring excellent SCC resistance.

The method for producing a high-strength aluminum alloy extruded material with excellent SCC resistance according to the present invention is as follows in mass %: Zn: 6.0 to 8.0%, Mg: 1.5 to 2.8%, Cu: 0.10 to 0.50%, Zr: 0.10 to 0.50%, Ti: 0.005 to 0.05%, Mn: 0.10 to 0.40%, Cr: 0.05% or less A billet is cast using an aluminum alloy in which the total of Mn+Cr+Zr is 0.10 to 0.50%, and the balance is Al and unavoidable impurities, and the billet obtained by the casting is heated at 470 to 560°C for 1 to 30 minutes. After 14 hours of homogenization, the billet is cooled at a cooling rate of 50° C./hr or higher, extruded using the billet, immediately air cooled, and when the temperature of the extruded material reaches 300 to 480° C., the cooling rate is further increased to 150° C./hr. It is characterized by being cooled down to 200°C or less at a rate of 950°C/min, and then subjected to artificial aging treatment.
Here, the artificial aging treatment is performed at 80 to 130°C for 2 to 8 hours in the first stage, and 2 to 16 hours at 130 to 160°C in the second stage, and has a 0.2% yield strength of 460 MPa or more, and has excellent SCC resistance. The average crystal grain size of the extruded material is preferably 250 μm or less.

When a billet is cast using an Al-Zn-Mg-based aluminum alloy and extruded using this billet, the temperature of the extruded material immediately after extrusion is as high as 440 to 585° C. due to processing heat and the like.
Therefore, quenching is possible by cooling immediately after extrusion, and this is called die end quenching.

For this die end hardening, two methods are known: air cooling using a fan or the like, and water cooling using water or the like.
In this case, by adding transition elements such as Mn, Cr, and Zr to the aluminum alloy, grain size is refined and hardenability is improved.
However, as disclosed in References 1 and 2, when the amount of Cr added increases, the quenching sensitivity becomes sharper, and a sufficient quenching effect cannot be obtained unless the material is rapidly cooled to the level of water cooling.
However, when extruded materials are cooled in a high-temperature state immediately after extrusion processing, cooling distortion occurs in the extruded materials, making it difficult to ensure the shape quality of the extruded materials and reducing stress corrosion cracking resistance (SCC resistance). There were declining technical challenges.

Therefore, the present invention suppresses the amount of Cr added, adds Mn: 0.1 to 0.40% and Zr: 0.10 to 0.25%, which are relatively less sensitive to quenching, and increases the total amount of Mn + Cr + Zr. By controlling the temperature within the range of 0.10 to 0.50%, air cooling is performed at a cooling rate of 50 to 150 °C/min until the extruded material temperature reaches 300 to 480 °C from 500 to 585 °C immediately after extrusion, It is characterized by the fact that it is then rapidly cooled at a cooling rate of 150 to 950°C/min, thereby achieving high strength while suppressing a decrease in SCC resistance.

The reason for selecting the composition of the aluminum alloy in the present invention will be explained below.
<Zn, Mg>
Zn and Mg components contribute to obtaining high strength.
Among them, Zn can be added at a high concentration without comparably reducing extrudability, but if it is too large, the SCC resistance will decrease, so below, Zn is in the range of 6.0 to 8.0% by mass. Preferably, Mg is in the range of 1.5 to 2.8%.
<Cu>
The Cu component is effective in achieving high strength due to its solid solution effect, but if it is too large, the extrudability and general corrosion resistance will decrease, so the Cu content is preferably in the range of 0.10 to 0.50%.
<Mn, Zr, Cr>
Mn, Zr, and Cr are all transition elements, and can suppress the refinement of crystal grains and the depth of surface recrystallization during extrusion molding.
While ensuring hardenability by cooling immediately after extrusion, considering air cooling in the high temperature range of the extruded material and rapid cooling from 300 to 480°C in the present invention, Mn: 0.10 to 0.40%, Zr: The content of Cr is preferably 0.10 to 0.25%, preferably at least 0.05%.
Further, it is preferable to control the total amount of Mn+Cr+Zr within a range of 0.10 to 0.50%.
<Ti>
The Ti component is effective in refining crystal grains during billet casting.
Considering the amount of dissolved Ti, Ti is preferably in the range of 0.005 to 0.05%.
<Fe, Si>
Fe and Si components become impurities in the present invention, and it is preferable to suppress Fe to 0.4% or less and Si to 0.3% or less.

In the present invention, even if the quenching immediately after extrusion is air-cooled until the extrusion temperature reaches 300 to 480°C, and then rapid cooling is performed at a cooling rate of 150 to 950°C/min, cooling distortion can be suppressed and SCC resistance can be prevented from decreasing. Therefore, it is possible to achieve both high strength and high strength.

The composition of the aluminum alloy used in the evaluation is shown. The manufacturing conditions of the billet and extruded material used in the evaluation are shown below. The evaluation results of extruded materials are shown.

A molten metal having an aluminum alloy composition as shown in FIG. 1 was prepared, an 8-inch cylindrical billet was manufactured under the manufacturing conditions shown in FIG. 2, and after homogenization treatment, an extruded material was manufactured under the manufacturing conditions shown in FIG.
Billets are cast using continuous casting methods such as hot top casting and insulated mold casting.
At this time, when the casting speed is set to 50 mm/min or more, the billet's cast structure becomes a fine structure with an average crystal grain size of 250 μm or less.
The average grain size indicates the average value of 3×3 points at the center of the cross section of the billet's tip, middle, and rear end, and an intermediate position between the periphery and the tip.
Billet homogenization treatment (HOMO) is performed to eliminate micro-segregation that occurs during billet casting.
In the homogenization treatment, the temperature is maintained at 470 to 560°C for 1 to 14 hours, and then the mixture is cooled at a cooling rate of 50°C/hr or more.

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.
A direct extruder was used for extrusion processing.
The temperature of the extruded material (extruded shape material) immediately after extrusion processing is 440°C or higher, preferably 480 to 510°C.
Immediately after being extruded from the die, it is at a high temperature of 440° C. or higher, but it is air cooled at ambient temperature.
In the present invention, immediately after extrusion, the extruded material (extruded shape) is air cooled for at least about 0.1 min, and from the stage when the temperature of the extruded material (extruded shape) reaches 300 to 480°C, the temperature is kept at a rate of 150 to 950°C/min until the temperature reaches 200°C or less. It is characterized by water cooling or strong fan air cooling.
In this way, in die end quenching during extrusion, a sufficient quenching effect can be obtained even if the extruded material is rapidly cooled at a rate of 150 to 950°C/min once the temperature of the extruded material reaches 300 to 480°C. Cooling strain is less likely to occur in the extruded material than die end quenching.
This makes it possible to suppress deterioration in SCC resistance while maintaining high strength.
This point will be discussed later by comparing Examples and Comparative Examples.

Next, the extruded material is subjected to artificial aging treatment at 80-130°C x 2-8 hr in the first stage and 2-8 hr at 130-160°C in the second stage, resulting in excellent SCC resistance and high strength extrusion. wood is obtained.
In the present invention, the targets were T5 tensile strength of 480 MPa or more, 0.2% T5 yield strength of 460 MPa or more, and T5 elongation of 10% or more.
The goal of SCC resistance (stress corrosion cracking resistance) was that no cracking occurred for 720 cycles or more under the sample conditions described below.
Furthermore, the depth of the recrystallized layer formed on the surface of the extruded material was targeted to be 150 μm or less on average.

The evaluation method for extruded materials will be explained below.
(1) Mechanical properties were measured by preparing a JIS-5 tensile test piece along the extrusion direction of the extruded material based on JIS-Z2241 and using a tensile tester compliant with JIS standards.
(2) The depth of the recrystallized layer formed on the surface of the extruded material was determined by mirror-polishing the cross section of the sample cut out from the extruded material and etching it with a 3% NaOH aqueous sodium hydroxide solution.
Next, the average depth of the recrystallized layer was measured from a 100x image of the metal structure by optical microscopic observation.
For SCC resistance, the following conditions were set as one cycle with a stress of 80% of the proof stress being applied to the test piece, and the target was achieved if cracking occurred for 720 cycles or more.
<1 cycle>
It is immersed in a 3.5% NaCl aqueous solution at 25°C for 10 minutes, then left at 25°C and 40% humidity for 50 minutes, and then air-dried.

(Evaluation results)
Figures 1 to 3 show the manufacturing conditions of Examples 1 to 15 and Comparative Examples 1 to 4, and the evaluation results of the extruded materials obtained thereby.
Examples 1 to 15 achieved the targets in all evaluation items, had high strength of 480 MPa or more in tensile strength and 460 MPa or more in 0.2% yield strength, and had excellent SCC resistance.
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, which exceeded the upper limit of the specification of 480°C, so the SCC resistance did not reach the target.
In Comparative Example 2, Mn was 0.70%, which exceeded the upper limit of the standard 0.40%, and the quenching start temperature was high, so the SCC resistance did not reach the target.
Moreover, the strength of Comparative Example 2 was lower than that of Comparative Example 1.
In Comparative Example 3, the Cu component was 1.60%, which was higher than the upper limit of 0.50% of the specification, and even though it had high strength, the SCC resistance was significantly reduced.
In Comparative Example 4, the Mg component was 1.21%, which was lower than the lower limit of the specification of 1.5%, so the strength did not reach the target.

By using the method for producing an aluminum alloy extruded material according to the present invention, a high-strength extruded material with excellent SCC resistance can be obtained, so it can be used for various structural members in vehicles, industrial machinery, etc.

Claims (2)

The following mass % is Zn: 6.0 to 8.0%, Mg: 1.5 to 2.8%, Cu: 0.10 to 0.50%, Zr: 0.10 to 0.50%, Ti: 0.005 to 0.05%, Mn: 0.10 to 0.40%, Cr: 0.05% or less, the total of Mn + Cr + Zr is 0.10 to 0.50%, and the balance is unavoidable as Al. A billet is cast using an aluminum alloy consisting of impurities,
The billet obtained by the casting is homogenized at 470 to 560°C for 1 to 14 hours, and then cooled at a cooling rate of 50°C/hr or more,
The billet is extruded and immediately cooled in the air, and when the temperature of the extruded material reaches 300 to 480°C, it is further cooled to 200°C or less at a cooling rate of 150 to 950°C/min, and then artificially processed. A method for producing a high-strength aluminum alloy extruded material with excellent SCC resistance, which comprises aging treatment. The artificial aging treatment is performed at 80 to 130 °C for 2 to 8 hours in the first stage, and 130 to 160 °C for 2 to 16 hours in the second stage,
The method for producing a high-strength aluminum alloy extruded material with excellent SCC resistance according to claim 1, characterized in that it has a 0.2% yield strength of 460 MPa or more and is excellent in SCC resistance.

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