JP2020139228A - Method for producing aluminum alloy extrusion material - Google Patents

Abstract

To provide a method for producing a high-strength aluminum alloy extrusion material that has a good hardenability as well as that has an excellent corrosion resistance and formability.SOLUTION: The method for producing aluminum alloy extrusion material comprises: extrusion processing using a cast billet of an aluminum alloy composed of, hereinafter in mass%, Zn: 6.0 to 8.0%, Mg: 1.50 to 3.50%, Cu: 0.20 to 1.50%, Zr: 0.10 to 0.25%, Ti: 0.005 to 0.05%, Mn: 0.3% or less, Sr: 0.25% or less, and [Mn + Zr + Sr]: 0.10 to 0.50%, the balance being Al and unavoidable impurities; cooling to 100°C or less at a cooling speed in a range of 50 to 750°C/min immediately after the extrusion processing; then subjecting to heating treatment in the range of 110 to 270°C; and after the heating treatment subjecting to plastic processing within a predetermined time.SELECTED DRAWING: Figure 1

Description

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

Further weight reduction and miniaturization are required in the fields of automobiles and various industrial machines, and as one of the means for achieving this, it is considered to manufacture structural members with high-strength aluminum alloy members.
As high-strength aluminum alloys, Al-Mg-Si-based (6000-based) alloys and Al-Zn-Mg-based (7000-based) alloys are known.
The 6000 series alloy aims at high strengthening by precipitation hardening of Mg ₂ Si, but there is a technical problem that the extrusion property is remarkably lowered when the addition amount of Mg and Si is large.
The 7000 series alloy is a natural aging alloy, and is characterized in that the addition of Zn has a smaller effect on extrusion properties than Mg and Si, but there is a technical problem that stress corrosion cracking resistance tends to decrease.
Further, when the strength is increased, there is a problem that cracks are likely to occur during processing such as bending molding.

For example, in Patent Document 1, a 7000 series aluminum alloy extruded profile manufactured by press quenching is heated at a heating rate of 0.4 ° C./sec or more and held in a temperature range of 200 to 550 ° C. for more than 0 second. Then, a manufacturing method is disclosed in which a restoration process of cooling at a cooling rate of 0.5 ° C./sec or more is performed, and then a crushing process and an aging process are performed.
However, the 7000 series aluminum alloy disclosed in the same publication has a large amount of Mn, Cr, and Zr transition elements added, and does not necessarily have sufficient stress corrosion cracking resistance.
It is also inferior in extrudability.

Japanese Unexamined Patent Publication No. 2014-145119

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

The method for producing an aluminum alloy extruded material according to the present invention is as follows, in terms of mass%, Zn: 6.0 to 8.0%, Mg: 1.50 to 3.50%, Cu: 0.25 to 1.50. %, Zr: 0.10 to 0.25%, Ti: 0.005 to 0.05%, Mn: 0.3% or less, Sr: 0.25% or less, and [Mn + Zr + Sr]: 0.10 to 0 Extrude using a cast billet of aluminum alloy consisting of .50% and Al and unavoidable impurities, and immediately after the extrusion, cool to 100 ° C or less at a cooling rate of 50 to 750 ° C / min. After that, heat treatment is performed in the range of 110 to 270 ° C., and after the heat treatment, plastic processing is performed within a predetermined time.

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. Therefore, the following are all Zn: 6. It is preferably in the range of 0 to 8.0%.
<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 1.50 to 3.50%.
In order to secure a tensile strength of 580 MPa or more after T5 treatment in the artificial aging treatment described later, the range of Mg: 2.40 to 3.50% is preferable, and a tensile strength of 600 MPa level after T5 treatment is further secured. The range of Mg: 3.0 to 3.50% is good.
Further, when the tensile strength after the artificial aging treatment (T5) is at a level of 480 to 580 MPa with an emphasis on moldability, Mg: 1.50 to 3.0%, preferably 1.50 to 2.80%. It may be in the range of.
<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 0.25 to 1.50%.
Preferably, the Cu component may be in the range of 0.25 to 1.00%, more preferably 0.25 to 0.60%.
<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.
For example, in Patent Document 1 above, Cr: 0.01 to 0.3% is contained, but Cr has a particularly strong quenching sensitivity, and if the cooling immediately after extrusion is at the level of cooling by a fan or the like, then It becomes difficult to obtain sufficiently high strength by the artificial aging treatment of.
Therefore, the Cr component is preferably 0.01% or less.
The Mn component is not as sensitive to quenching as the Cr component, but even when it is added, the addition amount is preferably 0.3% or less.
In the present invention, a Zr component was added to suppress the depth of the recrystallized layer.
Since Zr has a limit that can be dissolved in the molten aluminum, the range of Zr: 0.10 to 0.25% was set.
<Sr, Ti components>
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, if 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.
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 increases, the strength and bendability will decrease. 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 fan has a cooling rate of 50 to 750 ° C./min, preferably a cooling rate of 50 to 500 ° C./min immediately after extrusion. Perform air cooling.
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, air cooling makes the cooling device large.
Cool the extruded material to near room temperature below 100 ° C.
The present invention is characterized in that after the extruded material is cooled to 100 ° C. or lower, the extruded material is heat-treated so as to be in the range of 110 to 270 ° C., preferably 120 to 260 ° C.
The heating time is in the range of 30 to 800 seconds.
When the heating temperature is low, heating for a relatively long time is preferable.
Since the restoration treatment disclosed in Patent Document 1 aims at re-solid solution of the intermetallic compound folded out in the metal structure, it is necessary to heat it to a solution temperature of about 400 ° C., so that cooling after heating is performed. It becomes important.
On the other hand, the purpose of the heat treatment in the present invention is to reduce the residual stress generated when plastic working such as bending is applied to the extruded material.
As a result, the extruded material is given stickiness and stress corrosion cracking resistance is also improved.
For the purpose of reducing the residual stress in bending molding, it is necessary to heat to 110 ° C. or higher, but if the heating time is long, artificial aging proceeds, so the heating time is set to 800 seconds or less.

In the present invention, plastic working refers to applying plastic deformation such as bending to an extruded material by press molding, bending of a bender or the like.
Since the aluminum alloy according to the present invention is a natural aging type, it is preferable to perform plastic working such as bending molding within 168 hours after the heat treatment.

The cast billet of an aluminum alloy 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 when the casting speed is 50 mm / min or more, the billet structure is formed. The average crystal grain size inside is 250 μm or less, and the depth of the surface recrystallized layer during extrusion can be kept small.

In the present invention, the extruded material becomes high in strength by being artificially aged after plastic working such as bending.
For example, when the first stage 90 to 120 ° C. × 1 to 8 hours, the second stage 130 to 180 ° C. × 1 to 16 hours of two-stage artificial aging treatment (T5 treatment), 0.2% proof stress 460 MPa or more and tensile strength It becomes 480 MPa or more.
The artificial aging treatment time is preferably in the range of 2 to 24 hours in total of the first stage and the second stage.
The aim is to generate primary crystals in the first stage and grow these primary crystals in the second stage, and the productivity decreases as the total time increases.

The extruded material produced by the method for producing an 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, press edge quenching may be performed by air cooling means such as fan air cooling, and the extruded material is less likely to be distorted or deformed, which improves productivity.

The composition example (Example) of the aluminum alloy used for the 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.
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.
Here, in order to ensure hardenability, the preheating temperature of the billet is preferably 400 ° C. or higher, and the temperature immediately after extrusion of the extruded material is preferably 500 to 550 ° C.
Next, the mixture was heated at a heating rate (° C./sec) to the heating temperature (° C.) shown in the table, and the extruded material temperature was maintained for the heating time (seconds).
The conditions are set within the range where artificial aging does not progress.
For example, the heating temperature is preferably in the range of 110 to 270 ° C. and the heating time is preferably in the range of 30 to 800 seconds.
The rate of temperature rise during heating is preferably 1.8 ° C./sec or higher.
Next, after the heat treatment, a predetermined plastic working is performed within 168 hours.
For example, in the case of car parts, the shape of products such as bumper reinforcement and door beams is assumed, and bending is performed into a bow shape.
For example, bending molding with a curvature of 500 to 3000 mm.
In the table, the bending start time (Hr) indicates the elapsed time after the heat treatment.
The aluminum alloy according to the present invention is a natural aging alloy, and cracks are likely to occur after 168 hours.
After that, a two-stage artificial aging treatment was performed under the heat treatment conditions shown in the table.
The first stage is preferably in the range of 90 to 120 ° C. for 1 to 8 hours, and the second stage is preferably in the range of 130 to 180 ° C. for 1 to 16 hours.
In the heat treatment conditions (hr) in the table, the first stage indicates the heat treatment time of the first stage, the second stage indicates the heat treatment time of the second stage, and the total time indicates the total time.

The evaluation results are shown in the tables of FIGS. 5 and 6.
In the table, the value of T1 indicates the tensile strength (MPa), 0.2% proof stress (MPa), and elongation (%) before the artificial aging treatment.
If the tensile strength and proof stress become too high at the stage of T1, the stickiness described later decreases, so the target values are shown in the table.
In order to suppress the tensile strength and the like at the stage of T1, the heating time after extrusion is set to 270 ° C. or less and the heating time is set to 800 seconds or less.
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.
In the table, the small R bending test evaluates the stickiness of the extruded material, and a test piece of 20 × 150 mm is cut out after artificial aging treatment.
A test piece was placed on a test table with an interval of 7 mm, a load was applied from above with a punch having a tip R = 1.5 and an outer diameter of 3 mm, and a displacement one-load diagram was measured.
Here, it was evaluated that the tenacity target was achieved when the elongation of the tip portion was 30% or more until the tip portion of the U-shaped bent portion was cracked.

Consideration of Evaluation Results In Examples 1 to 48, the range of alloy components was set to a predetermined range, and the production conditions were also within a predetermined range, so that all the evaluation items of the extruded material were cleared.
On the other hand, in Comparative Examples 101 to 105, the heating temperature during the heat treatment exceeded 270 ° C., so that the target was not achieved in the small R bending test.
Further, in Comparative Examples 106 and 107, the heating time exceeded 800 seconds and the time was long, so that the small R bending test was not achieved.
Further, in Comparative Examples 109 to 114 and 117 to 120, since the heat treatment conditions exceeded the predetermined range, the tensile strength and the proof stress did not reach the targets.
Since Comparative Examples 115 and 116 were not heat-treated, cracks were likely to occur in the small R bending test.
In Comparative Example 121, the composition of the aluminum alloy contained a large amount of Cu and Cr was also added, so that the target of SCC property was not achieved. In Comparative Example 122, the Mg component was small and the strength was insufficient.

Claims (4)

In the following mass%, Zn: 6.0 to 8.0%, Mg: 1.50 to 3.50%, Cu: 0.25 to 1.50%, Zr: 0.10 to 0.25%, Ti: 0.005 to 0.05%, Mn: 0.3% or less, Sr: 0.25% or less, and [Mn + Zr + Sr]: 0.10 to 0.50%, the balance is composed of Al and unavoidable impurities. Extruded using cast aluminum alloy billets,
Immediately after the extrusion process, the mixture is cooled at a cooling rate of 50 to 750 ° C./min to 100 ° C. or lower, and then heat-treated at 110 to 270 ° C. within a predetermined time after the heat treatment. A method for producing an extruded aluminum alloy material, which is characterized by plastic working. The method for producing an extruded aluminum alloy according to claim 1, wherein the cast billet has an average crystal grain size of 250 μm or less. The method for producing an aluminum alloy extruded material according to claim 1 or 2, wherein the plastic working is bending molding, and artificial aging treatment is performed after the bending molding. The method for producing an aluminum alloy extruded material according to claim 3, wherein the extruded material has a tensile strength of 480 MPa or more and a proof stress of 460 MPa or more.

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