WO2021157356A1 - Production method of high-strength aluminum alloy extruded material - Google Patents

Abstract

The purpose of the present invention is to provide a high-strength aluminum alloy extruded material which has excellent corrosion resistance and formability, good hardenability, and high productivity.　This production method is characterized by: extrusion processing using cast billet of an aluminum alloy that comprises, in mass %, Zn: 6.0-7.0%, Mg: 1.5-2.0%, Cu: 0.20-1.50%, Zr: 0.10-0.25%, Ti: 0.005-0.05%, Mn: 0.15-0.35%, Sr: less than or equal to 0.25%, and [Mn + Zr + Sr]: 0.10-0.50%, the remainder comprising Al and unavoidable impurities; immediately after the extrusion processing, cooling at a cooling speed in the range 50-750°C/min until reaching less than or equal to 100°C; and thereafter, performing a one-stage or two-stage aging treatment, and a short heat treatment at a temperature higher than that of the aging treatment.

Description

Manufacturing method of high-strength aluminum alloy extruded material

The present invention relates to a method for producing an extruded material using an aluminum alloy, and relates to a method for producing an extruded aluminum alloy material having particularly high strength, excellent moldability and corrosion resistance, and high productivity.

In the fields of automobiles and various industrial machines, further weight reduction and miniaturization are required, and as one of the means for achieving this, it is considered to manufacture a structural member with a high-strength aluminum alloy member.
Al—Mg—Si based (6000 series) alloys and Al—Zn—Mg based (7000 series) alloys are known as high-strength aluminum alloys.
The 6000 series alloy _{aims at high strengthening by precipitation hardening of Mg 2} Si, but there is a technical problem that the extrudability is remarkably lowered when the amount of Mg and Si added is large.
The 7000 series alloy is a natural aging alloy, and is characterized by the fact that the addition of Zn has less influence on the extrudability than Mg and Si, but the heat treatment time is long to obtain high strength by artificial aging treatment. There are technical issues.

Patent Document 1 discloses a method for producing a high-strength aluminum alloy extruded material using the baking temperature at the time of painting, but it is a 6000 series alloy and sufficient high strength has not been obtained.

Japanese Patent Application Laid-Open No. 2002-235158

An object of the present invention is to provide a method for producing a high-strength aluminum alloy extruded material having excellent corrosion resistance and moldability, good hardenability, and high productivity.

The method for producing a high-strength aluminum alloy extruded material according to the present invention is as follows, in mass%, Zn: 6.0 to 7.0%, Mg: 1.5 to 2.0%, Cu: 0.20 to 1. .50%, Zr: 0.10 to 0.25%, Ti: 0.005 to 0.05%, Mn: 0.15 to 0.35%, Sr: 0.25% or less, and [Mn + Zr + Sr]: It is extruded using a cast billet of an aluminum alloy consisting of 0.10 to 0.50% and the balance is Al and unavoidable impurities. It is characterized in that it is cooled to the extent that it becomes, and then a one-stage or two-stage aging treatment and a heat treatment at a higher temperature than the aging treatment for a short time are performed.

In the present invention, in an Al—Zn—Mg—Cu based alloy, the recrystallization depth of the surface of the extruded material can be suppressed by air cooling immediately after the extrusion processing, and good quenching and high strength can be obtained.
The reason for setting the aluminum alloy composition will be described below.

<Zn component>
Even if a relatively large amount of Zn is added, it is easy to obtain high strength while suppressing a decrease in extrusion property, but if it is added excessively, the stress corrosion cracking resistance is reduced. It is preferably in the range of 0 to 7.0%.
<Mg component>
It is the most effective component for increasing the strength of the extruded material, but the extrudability tends to decrease, and the extruded material tends to crack during plastic working such as bending.
Therefore, the Mg component was set in the range of 1.5 to 2.0%.
If the Mg component is in this range, a decrease in extrusion property can be suppressed and high strength can be obtained.
<Cu component>
The strength of the Cu component can be increased by the solid solution effect in the metal structure, but if the amount added is large, it tends to cause deterioration of extrusion property and moldability, and general corrosion resistance is lowered.
Therefore, the Cu component was set in the range of 0.20 to 1.50%, preferably 0.20 to 0.30%.
<Mn, Cr, Zr components>
The Mn, Cr, and Zr components are all transition elements, and act to suppress recrystallization that tends to occur on the surface of the extruded material during extrusion processing, and to reduce the depth of the recrystallized layer on the surface.
However, when the amount of these components increases, the quenching sensitivity becomes sharp in the cooling (press end quenching) immediately after the extrusion process.
Among them, the Cr component has a particularly large influence, so that the Cr component is not contained, or when it is contained, it is suppressed to 0.05% or less, preferably 0.01% or less.
The Mn component is not as sensitive to quenching as the Cr component, and the range of 0.15 to 0.35% is preferable for adopting fan air cooling for press edge quenching.
In the present invention, a Zr component was added to suppress the depth of the recrystallized layer.
Since Zr has a limit of being able to be dissolved in molten aluminum, the range of Zr: 0.10 to 0.25% was set.
<Sr, Ti component>
The Sr component can suppress the coarsening of crystal grains in the billet cast structure, and as a result, has the effect of suppressing the depth of the recrystallized layer on the surface of the extruded material, which tends to occur during extrusion.
On the other hand, when the amount of the Sr component added is large, coarse crystals having Sr as a nucleus are likely to be generated.
Therefore, when Sr is added, it is preferable to suppress Sr: 0.25% or less, and the total amount of [Mn + Zr + Sr] is 0.10 to 0.50% in order to achieve both strength and suppression of the recrystallized layer. It is preferable to set it in the range of.
When Cr is contained, the total amount of [Mn + Zr + Sr + Cr] is set in the range of 0.10 to 0.50%.
The Ti component is effective for refining crystal grains when casting billets, and is added in the range of Ti: 0.005 to 0.05%.
<Other ingredients>
Examples of impurities that are easily mixed in the billet casting of an aluminum alloy include Fe and Si.
If the amount of these components is large, the strength is lowered and the bending formability is lowered. Therefore, it is preferable to suppress Fe to 0.2% or less and Si to 0.1% or less.

In the present invention, when the above-mentioned aluminum alloy billet is cast and extruded, the cooling rate is 50 to 750 ° C./min, preferably the cooling rate of 50 to 500 ° C./min, as the press end quenching immediately after the extrusion process. Air-cool the fan within the range of.
When the cooling rate exceeds 750 ° C./min, a cooling difference occurs depending on the part of the extruded material, and strain is likely to occur.
In addition, water cooling makes the cooling device large.
Cool the extruded material to near room temperature below 100 ° C.
Further, the cast billet used in the present invention is cast, then homogenized at 480 to 520 ° C., and then cooled at a cooling rate of 50 ° C./hour or more so that the average crystal grain size becomes 250 μm or less. It is preferable to set it to.

The high-strength aluminum alloy extruded material according to the present invention is also characterized in the chemical composition of the alloy, but the most characteristic is that high strength can be obtained by a short-time heat treatment.
As described above, after the fan is air-cooled immediately after extrusion, heat treatment is performed to obtain excellent stress corrosion cracking resistance (SCC resistance) and high strength.
In the past, in general, a relatively low temperature first-stage aging treatment was performed to generate primary crystals, followed by a relatively high-temperature second-stage aging treatment to grow primary crystals. In this case, the total heat treatment time is long, which causes a decrease in productivity.

Therefore, in the present invention, the total time of the heat treatment can be shortened as compared with the conventional case by performing the heat treatment at a temperature higher than the aging treatment temperature for a short time after the aging treatment of the first stage or the second stage.
For example, the temperature of the first stage is set to 90 to 180 ° C., preferably 110 to 150 ° C., and the temperature of the second stage is set to a temperature higher than that of the first stage.
In the present invention, the subsequent heat treatment temperature is set to a temperature higher than those.
For example, the heat treatment is performed at a temperature higher than that of the first stage or the second stage and in the range of 140 to 250 ° C. for a short time of 0.1 to 6 hours.
By such a heat treatment step, high strength having a tensile strength of 460 MPa or more and a proof stress of 430 MPa or more can be obtained.
In the present invention, the second-stage aging treatment may be omitted, in which case the heat treatment is performed after the first-stage aging treatment.

The extruded material of the high-strength aluminum alloy according to the present invention may be subjected to plastic working such as bending molding by press molding, bender bending or the like, and then artificial aging treatment or the like.

The cast aluminum alloy billet used in the present invention is continuously cast into a columnar billet by adjusting the chemical composition of the molten aluminum alloy to the above range, but the casting speed is set to 50 mm / min or more and the above-mentioned. By cooling at a rate of 50 ° C./hour or more after the homogenization treatment as described above, the average crystal grain size in the billet structure becomes 250 μm or less, and the depth of the surface recrystallized layer during extrusion processing can be suppressed to a small size. can.

The extruded material produced by the method for producing a high-strength aluminum alloy extruded material according to the present invention has so-called “stickiness” in which cracking is less likely to occur, and stress corrosion cracking resistance is improved.
Further, the press end quenching may be performed by an air cooling means such as fan air cooling, the extruded material is less likely to be distorted or deformed, and the heat treatment time for the subsequent artificial aging treatment or the like can be shortened, and the productivity is improved.

The composition example (Example) of the aluminum alloy used for evaluation is shown. A composition example (comparative example) of the aluminum alloy used for the evaluation is shown. The manufacturing conditions (examples) of the cast billet and the extruded material used for the evaluation are shown. The manufacturing conditions (comparative example) of the cast billet and the extruded material used for the evaluation are shown. The evaluation result (Example) of the extruded material quality is shown. The evaluation result (comparative example) of the extruded material quality is shown.

The molten aluminum alloy having the composition shown in the tables of FIGS. 1 and 2 was adjusted, and a cylindrical billet was cast.
The casting speeds are shown in the tables of FIGS. 3 and 4, and homogenization treatment was performed at the HOMO temperature (° C.).
The HOMO temperature is preferably in the range of 480 to 520 ° C.
After the homogenization treatment, the mixture was cooled at the cooling rate shown in the table.
In the table, the billet crystal grain size indicates the average crystal grain size in the structure of the cast billet.
The average particle size of the billet crystal is preferably 250 μm or less.
Billets preheated to the BLT temperature shown in the table were loaded into a container of an extruder and extruded.
Immediately after the extrusion process, the fan was air-cooled at a cooling rate (° C./min) in the table until the extruded material became at least 100 ° C. or lower.
The cooling rate is preferably in the range of 50 to 750 ° C./min.
If necessary, for example, in the case of car parts, the shape of a bumper reinforcement, a door beam, or the like is assumed, and the shape is bent into a bow shape.
For example, bending molding with a curvature of 500 to 3000 mm.
After that, heat treatment was performed under the heat treatment conditions shown in the table.
In the heat treatment conditions in the table, the first stage indicates the heat treatment temperature and time of the first stage, the second stage indicates the heat treatment temperature and time of the second stage, and the BH temperature and BH time indicate the high temperature heating conditions (heat treatment). show.

The evaluation results are shown in the tables of FIGS. 5 and 6.
The value of T5 indicates the tensile strength (MPa), 0.2% proof stress (MPa) and elongation (%) after the artificial aging treatment.
The target values according to the present invention are shown in the table.
These mechanical properties were measured with a tensile tester conforming to JIS standards by manufacturing JIS-5 pieces from extruded materials based on JIS-Z2241.
The crystal grain size of the billet and the surface recrystallization depth of the extruded material were measured by performing predetermined etching treatments after mirror finishing the cross section and then performing image processing by observation with an optical microscope.
In the table, SCC property shows the results of stress corrosion cracking resistance test.
With a stress of 80% of the proof stress applied to the test piece, the following conditions were set as one cycle, and the target was achieved when no cracks occurred in 720 cycles.
<1 cycle>
Immerse in 3.5% NaCl aqueous solution at 25 ° C. for 10 minutes, then leave at 25 ° C. at humidity 40% for 50 minutes, and then air dry.

<Discussion of evaluation results>
Examples 1 to 45 clear all quality targets.
In particular, in Examples 19, 22, 25 and 34 to 38, even if the total heat treatment time (overall time) is suppressed to 10 hours or less, the tensile strength is 460 MPa or more and the proof stress is 430 MPa or more, and the elongation and resistance are achieved. It also has excellent SCC properties.
On the other hand, in Comparative Examples 8 to 11, the addition amounts of Mn and Mg were small, and the tensile strength and proof stress were not achieved.
In Comparative Example 12, since the amount of Mg and the amount of Cu were large, the SCC resistance did not reach the target.
In Comparative Examples 1 to 4, the heat treatment time of the first stage was set long as in the conventional case, but the tensile strength was not reached the level of Examples 1 to 3.

According to the manufacturing method according to the present invention, it can be applied to structural members of various vehicles and industrial machines because it has high strength, excellent corrosion resistance, and good moldability.

Claims (3)

In the following mass%, Zn: 6.0 to 7.0%, Mg: 1.5 to 2.0%, Cu: 0.20 to 1.50%, Zr: 0.10 to 0.25%, Ti: 0.005 to 0.05%, Mn: 0.15 to 0.35%, Sr: 0.25% or less, and [Mn + Zr + Sr]: 0.10 to 0.50%, the balance is unavoidable with Al. Extruded using cast billets of aluminum alloy consisting of impurities,
Immediately after the extrusion process, the mixture is cooled at a cooling rate of 50 to 750 ° C./min to 100 ° C. or lower, followed by one-stage or two-stage aging treatment and heat treatment at a temperature higher than that of the aging treatment for a short time. A method for producing a high-strength aluminum alloy extruded material. The first aspect of claim 1, wherein the cast billet is cast and then homogenized at 480 to 520 ° C. and then cooled at a cooling rate of 50 ° C./hour or more so that the average crystal grain size is 250 μm or less. How to manufacture high-strength aluminum alloy extruded material. The method for producing an aluminum alloy extruded material according to claim 2, wherein the tensile strength is 460 MPa or more and the proof stress is 430 MPa or more.

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