JP2024139746A - Manufacturing method for high-strength aluminum alloy extrusion material with excellent SCC resistance and aluminum alloy used therein - Google Patents

Abstract

[Problem] The objective is to obtain a high-strength aluminum alloy extrusion material with excellent SCC resistance by using an Al-Zn-Mg alloy having a predetermined composition and obtaining the extrusion material under predetermined extrusion conditions. [Solution] The following, by mass%, is Zn: 6.0-10.0%, Mg: 1.5-3.5%, Cu: 0.20-2.50%, Zr: 0.10-0.25%, Ti: 0.005-0.05%, Mn: 0.30 or less, Cr: 0.25% or less, the total of [Mn+Cr+Zr] is 0.10-0.50%, and the balance is Al and unavoidable impurities, and the average crystal grain size after casting is 250 μm or less. The method is characterized in that an aluminum alloy is used, the extrusion is performed so that the temperature of the extruded material immediately after the extrusion is 440°C or higher, the extruded material is allowed to cool in the air for 1.0 to 3.4 minutes at a cooling rate of less than 100°C/min until the temperature of the extruded material reaches 250 to 400°C, and then the extruded material is forced to cool at a cooling rate of 100 to 2000°C/min until the temperature of the extruded material reaches 150°C or lower, followed by aging treatment. [Selected Figure] Figure 1

Description

The present invention relates to a method for producing an extruded material made of an Al-Zn-Mg aluminum alloy that has high strength and excellent SCC resistance, and to an aluminum alloy used in the method.

In the field of vehicles and the like, there is a demand for high-strength aluminum alloy extrusion materials for the purpose of reducing weight.
Among aluminum alloys, Al-Zn-Mg alloys are known to have high strength. However, there is a technical problem in that when attempting to obtain even higher strength, resistance to stress corrosion cracking (SCC resistance) decreases.

For example, in Patent Document 1, the thickness of the surface recrystallization is set to 7% or less of the wall thickness, and the average grain size of the surface recrystallization is set to 150 μm or less, thereby improving SCC resistance, but the tensile strength is only at the level of approximately 450 MPa.

Patent No. 2928445

The present invention aims to obtain a high-strength aluminum alloy extrusion material with excellent SCC resistance by using an Al-Zn-Mg alloy with a specified composition and obtaining an extrusion material under specified extrusion conditions.

The high-strength aluminum alloy extrusion material with excellent SCC resistance according to the present invention uses an aluminum alloy having, in mass%, the following: Zn: 6.0-10.0%, Mg: 1.5-3.5%, Cu: 0.20-2.50%, Zr: 0.10-0.25%, Ti: 0.005-0.05%, Mn: 0.30 or less, Cr: 0.25% or less, a total of [Mn+Cr+Zr] of 0.10-0.50%, with the balance being Al and unavoidable impurities, and having an average crystal grain size after casting of 250 μm or less.
Here, in order to obtain a cast structure having an average crystal grain size of 250 μm or less, a molten aluminum alloy having the above alloy composition is cast at a casting speed of 50 mm/min or more, and then homogenized at 480 to 520° C. for 1 to 12 hours (HOMO treatment), followed by cooling at a cooling speed of 50° C. or more.

The method for producing a high-strength aluminum alloy extrusion material having excellent SCC resistance according to the present invention is characterized in that the aluminum alloy is extruded so that the temperature of the extrusion material immediately after the extrusion processing is 440°C or higher, the extrusion material is allowed to cool in the atmosphere at a cooling rate of less than 100°C/min for 1.0 to 3.4 minutes until the temperature of the extrusion material reaches 250 to 400°C from immediately after the extrusion processing until the temperature of the extrusion material reaches 250 to 400°C, and then the extrusion material is cooled at a cooling rate of 100 to 2000°C/min until the temperature of the extrusion material reaches 150°C or lower, followed by aging treatment.
This provides a tensile strength of 500 MPa or more and a 0.2% yield strength of 470 MPa or more.
Here, natural cooling refers to extruding so that the temperature of the extruded material immediately after extrusion is 440° C. or higher, and then allowing it to cool naturally in the air at a cooling rate of less than 100° C./min over 1.0 to 3.4 minutes.
The forced cooling is performed by forced air cooling using a fan or water cooling at a cooling rate in the range of 100 to 2000°C.
Furthermore, if the extruded material is cooled to 150° C. or below, there is no effect on its strength and SCC resistance.

In addition, it is preferable that the extruded material is cooled in the air for at least one minute immediately after the extrusion process until the temperature of the extruded material reaches 300 to 400°C, and then the material is forced to cool at a cooling rate of 200 to 2000°C/min until the extrusion temperature reaches 150°C or less, and then aging treatment is performed.

The aluminum alloy composition used in the present invention was selected for the following reasons.
<Zn and Mg Components>
Zn does not reduce extrudability even at relatively high concentrations and contributes to improving strength, and the addition of Mg causes _MgZn2 to precipitate in the structure, increasing strength.
However, if the amount of Mg added is large, the extrudability decreases, and the amount of _MgZn2 precipitated becomes too large, which may decrease the toughness.
Therefore, a combination of Zn: 6.0 to 10.0% and Mg: 1.5 to 3.5% is preferable.
<Cu Component>
The addition of Cu is effective in improving strength due to the solid solution effect, but if the amount added is too large, general corrosion resistance decreases, so the Cu content is preferably in the range of 0.20 to 2.50%.
<Zr, Mn and Cr Components>
These components are all transition elements, and are effective in suppressing the depth of recrystallization formed on the surface of the extruded material during extrusion processing, and in making the crystal grains finer.
This improves the stress corrosion cracking resistance.
Of these, the Cr component is the most sensitive to hardening, and although the Mn component is not as sensitive to hardening as Cr, it has a greater effect than Zr.
Therefore, in the present invention, Zr is set to 0.10 to 0.25%, and when Mn is added, the Mn content is suppressed to 0.30% or less, and when Cr is added, the Cr content is suppressed to 0.25% or less, and the total of [Mn + Cr + Zr] is suppressed to the range of 0.10 to 0.50%.
<Ti Component>
The Ti component is effective in refining crystal grains when a billet for use in extrusion is cast, and a very small amount of B is also generally added.
Ti: A small amount of 0.005 to 0.05% is sufficient.
<Other ingredients>
In the casting process of 7000 series aluminum alloys, Fe and Si are often contained as impurities. If the amounts are large, they affect the extrudability, stress corrosion cracking resistance, etc., so it is preferable to restrict Fe to 0.2% or less and Si to 0.1% or less.

By extruding an aluminum alloy of a specified composition and controlling the cooling conditions (die edge quenching) immediately after extrusion, a high-strength extruded material can be obtained while maintaining excellent SCC resistance.

The composition of the aluminum alloy used in the evaluation is shown, with the remainder being Al and unavoidable impurities. The casting conditions and homogenization treatment (HOMO treatment) conditions for the extrusion billet are shown below. The extrusion conditions and the cooling conditions immediately after extrusion of the extruded material are shown below. The evaluation results of the extruded material are shown.

Molten aluminum alloys having the compositions shown in the table of FIG. 1 were prepared, and billets were cast and homogenized (HOMO) under the conditions shown in the table of FIG.
Next, as shown in FIG. 3, the billet (BLT) was preheated to 440° C. or higher, and extruded at a speed of 5 m/min or higher to produce a hollow extrusion or solid extrusion having a wall thickness of 2 to 3 mm, a height of 50 to 60 mm, and a width of 110 to 120 mm.
The temperature of the extruded material immediately after extrusion is shown in the table of FIG.
In all cases, the temperatures were above 440°C.

As shown in the table of FIG. 3, the cooling conditions immediately after extrusion were varied and evaluated for Examples 1 to 14 and Comparative Examples 1 to 16.
The target range for each condition is shown in the table in FIG. 3, with conditions that fall within that range indicated with an "O" and conditions that fall outside that range indicated with an "X".

The evaluation results are shown in FIG. 4. The evaluation conditions are as follows:
<Mechanical properties>
Based on JIS-Z2241, JIS-5 test pieces were prepared, and the tensile strength, σ _0.2 yield strength, and elongation were measured using a tensile tester conforming to the JIS standard.
<Billet grain size and microstructure of extruded material>
The sample surface was mirror-polished and etched with Keller's reagent.
The metal structure was observed by optical microscopy, and the image at 100 times magnification was processed to determine the average crystal grain size.
<Stress corrosion cracking resistance (SCC resistance)>
The test piece was subjected to a stress of 80% of its yield strength, and the following conditions were counted as one cycle. If no cracks occurred after 720 cycles, the target was achieved.
If cracks occurred during testing, the number of cycles at which they occurred was recorded.
[1 cycle]
The sample is immersed in a 3.5% NaCl aqueous solution at 25° C. for 10 minutes, then left to stand at 25° C. and a humidity of 40% for 50 minutes, and then naturally dried.

As can be seen from the table in Figure 4, Examples 1 to 14 all had excellent SCC resistance because the cooling conditions immediately after extrusion were controlled within the target range, whereas Comparative Examples 1 to 14 had a tensile strength of 500 MPa or more, a 0.2% yield strength of 470 MPa or more, and an elongation of 8% or more, but all did not achieve the target SCC resistance.

When examining the evaluation results in detail, the following can be said:
In the table of FIG. 3, the cooling start extrusion temperature indicates that the extrusion material immediately after extrusion was allowed to cool naturally in the air, and from the point in time when the cooling start extrusion temperature shown in the table of FIG. 3 was reached, the extrusion material was forcedly cooled at a cooling rate of 100 to 2000° C./min.
In the table of FIG. 3, the time to reach the cooling start time refers to the above-mentioned time for natural cooling in the atmosphere.

As shown in the table of FIG. 3, in Examples 1 to 10, the cooling rate of the extrusion material was 279 to 350° C./min, which indicates forced air cooling by a fan.
In contrast, in Examples 11 to 14, the extrusion material was cooled at a cooling rate of 1500° C./min, which indicates that forced cooling was performed by water cooling.
This shows that the temperature of the extruded material immediately after extrusion is 440°C or higher, and by providing a process for slowly allowing it to cool naturally in the air over a period of 1.0 to 3.4 minutes, it is possible to achieve excellent SCC resistance, regardless of whether the subsequent forced cooling is air-cooling or water-cooling.
In addition, Comparative Example 15 was allowed to slowly cool naturally in the air over 5.02 minutes, and Comparative Example 16 was allowed to cool naturally in the air over 3.45 minutes. As a result, although elongation and SCC resistance were ensured, the tensile strength and yield strength did not reach the target.

The cooling rate by natural cooling immediately after extrusion can be calculated by the formula: (temperature of extruded material after extrusion)-(extrusion temperature at which cooling starts)/(time to reach the start of cooling).
For example, the cooling rate by natural cooling in the air in Example 1 is (470-382)/1.24=about 70° C./min.
Moreover, the cooling rate in Example 12 is (500-350)/2.40=approximately 62.5°C/min.
As explained above, in Comparative Examples 15 and 16, the SCC resistance was ensured but the strength did not reach the target. This shows that, although the SCC resistance can be ensured if the natural cooling in the air immediately after extrusion is slow at less than 100° C./min, in order to ensure the strength, it is necessary to limit the cooling time in the air to between 1.0 and 3.4 minutes.
From the viewpoint of ensuring strength, the cooling start temperature is preferably in the range of 300 to 400°C.
Furthermore, it is preferable that the forced cooling by a fan or water cooling is 200° C./min or more.

In Comparative Example 1, the cooling start time was short at 0.10 minutes, and the forced cooling was fast at 2,300°C/min, so SCC resistance was not achieved.

Claims (3)

An aluminum alloy having, in mass %, Zn: 6.0 to 10.0%, Mg: 1.5 to 3.5%, Cu: 0.20 to 2.50%, Zr: 0.10 to 0.25%, Ti: 0.005 to 0.05%, Mn: 0.30 or less, Cr: 0.25% or less, a total of [Mn + Cr + Zr] of 0.10 to 0.50%, with the balance being Al and unavoidable impurities, and having an average crystal grain size after casting of 250 μm or less,
The extrusion is performed so that the temperature of the extruded material immediately after the extrusion is 440° C. or higher.
A method for producing a high-strength aluminum alloy extrusion material with excellent SCC resistance, comprising the steps of: cooling the extrusion material in the atmosphere at a cooling rate of less than 100°C/min for 1.0 to 3.4 minutes until the temperature of the extrusion material reaches 250 to 400°C immediately after extrusion; and then forcibly cooling the extrusion material at a cooling rate of 100 to 2000°C/min until the temperature of the extrusion material reaches 150°C or less, followed by aging treatment. Immediately after the extrusion process, the extruded material is cooled in the air for at least 1 minute until the temperature of the extruded material reaches 300 to 400 ° C.
The method for producing a high-strength aluminum alloy extrusion material having excellent SCC resistance according to claim 1, characterized in that the extrusion temperature is then forcedly cooled at a cooling rate of 200 to 2000 ° C./min until the extrusion temperature becomes 150 ° C. or less, and then aging treatment is performed. The method for manufacturing high-strength aluminum alloy extrusion material with excellent SCC resistance described in claim 2, characterized in that the tensile strength is 500 MPa or more and the 0.2% yield strength is 470 MPa or more.

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