JP2021110042A - Production method for high-strength aluminum alloy extrusion material excellent in toughness and corrosion resistance - Google Patents

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

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

In the fields of vehicles and various industrial machines, weight reduction of structural members is desired due to demands such as reduction of environmental load, energy saving, and weight reduction.
Extruded materials made of aluminum alloys are being studied as one of the means, and as high-strength aluminum alloys, Al-Zn-Mg-based 7000-based alloys and Al-Mg-Si-based 6000-based alloys are typical examples. ..
Among them, the 7000 series alloy can be increased in strength without relatively lowering the extrusion processability.

For example, Patent Document 1 describes 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%, Zr: 0.01 to 0.25%, Cr: 0.01 to 0.25%, V: 0. Disclosed is an extruded aluminum alloy using an aluminum alloy containing at least one of 01 to 0.25% and Sc: 0.01 to 0.25% in total of 0.1 to 0.5%. Since rapid cooling of 200 ° C./sec or more, that is, 12000 ° C./min or more must be performed while the temperature of the material is 400 ° C. or higher, cooling strain is likely to occur in the shape of the extruded material, and energy absorption characteristics. Over-aging treatment is required to ensure (toughness), which causes a decrease in productivity.

Patent Document 2 describes 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%, Zr: 0.01 to 0.25%, Cr: 0.01 to 0.25%, Mn: 0.01 to Extruded material extruded using an aluminum alloy having at least one of 0.25% is reheated to 330 to 550 ° C. at a heating rate of 10 ° C./sec or higher, and then cooled to 50 ° C./sec or higher. Disclose a method of cooling at a rate.
However, this requires a heat treatment furnace and a cooling device for reheating, and recrystallization of the surface proceeds when the extruded material is reheated, which may result in inferior corrosion resistance.

As described above, in the conventional 7000 series high-strength aluminum alloy extruded material, when trying to obtain high strength, the stickiness (toughness) is lowered and the stress corrosion cracking resistance of the member to which stress is applied is lowered. Furthermore, there is a problem that productivity is lowered because water cooling is required for die edge quenching.

JP 2015-221924 JP-A-2017-222920

An object of the present invention is to provide a method for producing a high-strength aluminum alloy extruded material which can obtain high strength and has excellent toughness and corrosion resistance and good productivity by quenching the edge of a die by air cooling after extrusion.

The method for producing a high-strength aluminum alloy extruded material according to the present invention is as follows, in terms of 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 step of casting a billet at a casting speed of 50 mm / min or more using an aluminum alloy whose balance is Al and unavoidable impurities, and 460 of the cast billet. A step of homogenizing at ~ 540 ° C. for 1 to 14 hours, a step of cooling at a cooling rate of 50 ° C./hr or more after the homogenizing treatment, and a step of extrusion using the billet to perform the extrusion processing. It is characterized by having a step of performing air cooling at a cooling rate of 50 to 500 ° C./min as die end casting, and then performing artificial aging treatment.
Here, the artificial aging treatment comprises a first stage of 1 to 6 hours at 80 to 120 ° C. and a second stage of 1 to 14 hours at 130 to 180 ° C., and is a two-stage artificial treatment within 20 hours in total. It is preferably an aging process.
The extruded material produced in this way has a high strength with a proof stress of 260 MPa or more, a Charpy impact value of 25 J / cm ² or more, and a toughness with an axial cracking EA rate of 55% or more, and DSC (integration corresponding to a heat absorption peak). The value of (quantity) is 30 or less and the recrystallization depth of the surface portion is 150 μm or less, and the stress corrosion cracking resistance is excellent.

The reason for selecting the component range of the aluminum alloy in the present invention is as follows.
<Zn and Mg components>
Extrudability of Zn does not decrease even at a relatively high concentration, which contributes to the improvement of strength, and the addition of Mg causes MgZn ₂ to fold out in the structure to increase the strength.
However, if the amount of Mg added is large, the extrudability is lowered, and the _{amount of MgZn 2} precipitated is too large, so that the toughness may be lowered.
Therefore, a combination in the range of Zn: 5.0 to 7.0% and Mg: 0.5 to 1.60% is preferable.
_{The precipitation amount of MgZn 2} may be controlled by setting Zn: 5.5 to 7.0% and Mg: 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, but since the general corrosion resistance decreases as the addition amount increases, the range of Cu: 0.05 to 0.50% is preferable. Is in the range of Cu: 0.10 to 0.30%.
<Zr, Mn and Cr components>
All of these components are transition elements, and are effective in suppressing the recrystallization depth formed on the surface of the extruded material during extrusion processing and in refining the crystal grains.
This improves toughness and stress corrosion cracking resistance.
However, of these, the Cr component has the sharpest quenching sensitivity, and if the amount added is large, sufficient strength cannot be obtained unless quenching by cooling immediately after extrusion (die end quenching) is rapidly cooled to a water-cooled level.
Although the Mn component is not as sensitive to quenching as Cr, it has a greater effect than Zr.
Therefore, it is preferable that the present invention corresponds only to the addition of Zr: 0.10 to 0.25% without adding Cr and Mn.
When Mn is added, Mn: 0.30% or less is suppressed, and when Cr is added, Cr: 0.20% or less is suppressed, and the total of [Mn + Cr + Zr] is 0.10 to 0.65%. It is better to keep it within the range.
<Ti component>
The Ti component is effective in refining crystal grains when casting billets for use in extrusion processing, and generally B is also added in a very small amount.
Ti: A small amount of 0.005 to 0.05% may be added.
<Other ingredients>
In the casting process of 7000 series aluminum alloy, Fe component and Si component are often contained as impurities, but if the amount is large, it affects extrusion property, stress corrosion cracking resistance, etc., so Fe: It is preferable to keep it at 0.2% or less and Si: 0.1% or less.

Next, the manufacturing conditions will be described.
Cylindrical billets are cast and used in the production of extruded aluminum alloys.
The casting conditions of this billet and its homogenization treatment conditions also affect the quality of the extruded material produced.
Billet is continuously cast in a columnar shape using molten aluminum alloy by hot top casting or the like, but when the casting speed is 50 mm / min or more, the average particle size of the crystal grains of the cast structure of the billet is 250 μm. It has the following microstructure, and the crystal grains of the extruded material become finer and the toughness is improved even in the subsequent extrusion processing.
Further, the homogenization treatment condition after casting is that heat treatment is performed at 460 to 540 ° C. for 1 hour or more and 14 hours or less in order to sufficiently dissolve the precipitate, and the subsequent cooling is performed at 50 ° C./hr or more. It is better to do it at speed.

In extrusion processing, a billet is loaded in a container of an extruder and pressed by a stem or the like, so that the extruded material is extruded through a pressing die.
In this case, before loading the billet into the container, it is preheated to 400 ° C. or higher and 500 ° C. or lower.
The extruded material immediately after being extruded from the extruded die has a high temperature of 500 ° C. or higher.
Die edge quenching enables quenching by air cooling at a cooling rate of 50 to 500 ° C./min with a fan or the like using the high temperature immediately after extrusion.
When the aluminum alloy according to the present invention is used, it is not necessary to perform water cooling as in Reference 1, and it is possible to suppress deformation of the extruded material during cooling, and the air cooling device has a simpler structure than the water cooling device. Yes, it is excellent in productivity.

The extruded material that has undergone the die edge quenching process is subsequently subjected to artificial aging treatment, and the conditions also affect the quality of the extruded material.
In the present invention, the first stage is 80 to 120 ° C. × 1 to 6 hours, the second stage is 130 to 180 ° C. × 1 to 14 hours, and the two-stage artificial aging treatment is performed within 20 hours in total.
As a result, the target quality can be obtained without overaging or reheat treatment.

Conventionally, 7000 series aluminum alloys are easily cracked and it is difficult to secure toughness when trying to increase the strength, and the extrusion material becomes hot due to the increase in processing resistance during extrusion processing, and is recrystallized on the surface of the extrusion material. There is a technical problem that the stress corrosion cracking resistance is lowered due to the increase in the thickness of the crystal layer.
In the present invention, a well-balanced extruded material can be obtained by optimizing the chemical composition of the aluminum alloy and optimizing the production conditions.

The chemical composition of the aluminum alloy used for the evaluation is shown. The billet casting, homogenization (HOMO) conditions and post-extrusion artificial aging treatment conditions used for the evaluation are shown. The evaluation result is shown. An example of a DSC analysis chart is shown. An explanatory diagram of the purpose for which DSC was evaluated is shown. An example of the load-displacement curve in the shaft crush test is shown. (A) shows an example, and (b) shows an external photograph of a comparative example.

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 range of the present invention are prepared and shown in FIG. The billet was cast at the casting rate shown in Table 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 billet at that time is shown in the column of "Billette crystal grain size" in the table of FIG.
Next, the billet was preheated at the temperature shown in "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 about 3 mm.
Immediately after extrusion, the fan is air-cooled at the cooling rate shown in the table of FIG. 2, die edge quenching is performed, and then the first stage heat treatment is performed at the heat treatment temperature and heat treatment time shown in FIG. The second stage heat treatment was performed, and a two-stage artificial aging treatment was carried out.
In the table of FIG. 2, the preferable conditions and ranges in the present invention are also shown as manufacturing conditions.

The evaluation items and the evaluation results are shown in the table of FIG.
The target values for each evaluation item are shown in the table.
The evaluation conditions are as follows.
<Mechanical properties>
A JIS-5 test piece was prepared based on JIS-Z2241, and tensile strength, σ _0.2 proof stress, and elongation were measured using a tensile tester conforming to JIS standards.
<Billette crystal grain size>
The billet surface was mirror-polished and etched with Keller's reagent.
The metallographic structure was observed by observing this with an optical microscope, and a 100-fold image was image-processed to determine the average crystal grain size.
<Surface recrystallization depth of extruded material>
The cross section of the extruded material was mirror-finished and then etched with 3% NaOH.
The average thickness of the recrystallized layer was determined by image processing from a 100-fold image with an optical microscope.
<Charpy impact test>
A JIS-V notch No. 4 test piece was prepared based on JIS-Z2242, and the Charpy impact value was determined by a Charpy impact tester conforming to JIS standards.
<Stress corrosion cracking resistance (SCC property)>
With a stress of 80% of the proof stress applied to the test piece, the target was achieved when no cracks occurred in 720 cycles under the following conditions as one cycle.
If cracks occurred in the middle, the number of cycles was displayed.
[1 cycle]
It is immersed in a 3.5% NaCl aqueous solution at 25 ° C. for 10 minutes, then left at 25 ° C. for 50 minutes in a humidity of 40%, and then air-dried.
<DSC analysis>
From the chart obtained using a differential thermal analyzer (Thermo plus evo2 manufactured by Rigaku), the value of the surface integral (integral amount) corresponding to the endothermic peak was taken as the value of DSC.
An example chart is shown in FIG.
In the present invention, the purpose of evaluating the value of DSC is that if the amount of precipitates in the crystal grains becomes too large, the toughness is lowered and cracks are likely to occur, as shown in the explanatory diagram in FIG. The area (integrated amount) of the heat absorption peak at 100 to 200 ° C. at 100 to 200 ° C. where the precipitate re-dissolves was set to 30 or less.
FIG. 4 shows a portion of the endothermic peak.
<Axial crushability and its appearance>
The axial crushability test was carried out as follows.
Using a square cylinder-shaped test piece with a "field" shape, a static load is applied along the shaft length direction of the test piece at a compression rate of 50 mm / min (shaft length before test 150 mm → shaft length after test 60 mm). Was allowed to act.
FIG. 6 shows a load-displacement curve when the test piece according to Example 4 is subjected to an axial crush test.
The horizontal axis represents the stroke of the crosshead of the compression tester when a load is applied in the axial length direction of the test piece.
The vertical axis shows the magnitude of the load.
As shown in FIG. 6, when the test piece is crushed and deformed in a bellows shape, peaks of load values (P1 to P3) occur at intervals.
FIG. 7A shows the state of the test piece according to Example 4 after the shaft crush test.
After the shaft crush test, the test piece was crushed and deformed into a bellows shape.
When the bellows shape was crushed and deformed in this way, the evaluation was evaluated as ◯.
FIG. 7B shows the state of the test piece according to Comparative Example 3 after the axial crush test.
After the shaft crush test, the wall of the test piece was broken.
When the wall portion was broken in this way, the evaluation was evaluated as x.

<Discussion of evaluation results>
Examples 1 to 18 cleared all quality targets.
On the other hand, in Comparative Example 19, the amount of Zr added was small, the surface recrystallization depth was thick, and the Charpy impact value and stress corrosion cracking resistance did not reach the targets.
In Comparative Example 20, although the amount of Mg added was large and the strength was high, the toughness was lowered.
In Comparative Example 21, the artificial aging treatment conditions were not within the scope of the present invention, and the toughness did not reach the target.
In Comparative Examples 22 and 23, the toughness was poor because the amount of Zr added was small.
In Comparative Example 24, the range of [Mn + Cr + Zr] was out of range, and the heat treatment conditions were not within the predetermined range, so that the quality target was not achieved.
In Comparative Example 25, the Mg content was high and the toughness was inferior.

Claims (3)

Zn: 5.0 to 7.0%, Mg: 0.50 to 1.60%, Cu: 0.05 to 0.50%, Zr: 0.10 to 0.25% in terms of mass%. , 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 Using an aluminum alloy consisting of Al and unavoidable impurities,
The step of casting billets at a casting speed of 50 mm / min or more,
The step of homogenizing the cast billet at 460 to 540 ° C. for 1 to 14 hours, and
After the homogenization treatment, a step of cooling at a cooling rate of 50 ° C./hr or more, and
The toughness and corrosion resistance are characterized by having a step of extruding using the billet and performing air cooling at a cooling rate of 50 to 500 ° C./min as die end quenching of the extrusion processing, followed by artificial aging treatment. A method for producing an excellent high-strength aluminum alloy extruded material. The artificial aging treatment comprises a first stage of 1 to 6 hours at 80 to 120 ° C. and a second stage of 1 to 14 hours at 130 to 180 ° C., and is a two-stage artificial aging treatment within 20 hours in total. The method for producing a high-strength aluminum alloy extruded material having excellent toughness and corrosion resistance according to claim 1. It has a high strength with a proof stress of 260 MPa or more, a Charpy impact value of 25 J / cm ² or more, a toughness with an axial crushability EA rate of 55% or more, a DSC (integrated amount corresponding to a heat absorption peak) value of 30 or less, and a surface portion. The method for producing a high-strength aluminum alloy extruded material having excellent toughness and corrosion resistance according to claim 2, wherein the recrystallization depth of the above is 150 μm or less and the stress corrosion cracking resistance is excellent.

Images