JP2022141094A - High-strength aluminum alloy extruded material and manufacturing method therefor - Google Patents

Abstract

To provide an 7000-series aluminum alloy extruded material having high strength and high SCC resistance and a manufacturing method therefor.SOLUTION: This aluminum alloy extruded material comprises: one or more components selected from the group consisting of 7.5 to 9.2 mass% of Zn, 1.3 to 2.0 mass% of Mg, 0.1 to 0.7 mass% of Cu, 0.30 mass% or less of Mn, 0.25 mass% or less of Cr, and 0.25 mass% or less of Zr, these components totaling 0.1 to 0.5 mass%; and 0.005 to 0.20 mass% of Ti, with the remainder being Al and inevitable impurities. The average interval between grain boundary precipitates is 0.8 to 1.4 μm, the average particle length of the grain boundary precipitates is 0.3 to 0.5 μm, and the yield strength is 440 N/mm2 or greater.SELECTED DRAWING: Figure 1

Description

The present disclosure relates to high-strength aluminum alloy extruded materials and methods of manufacturing the same.

High-strength 7000-series aluminum alloys have a problem of cracking, ie, SCC (stress corrosion cracking), which occurs at locations where tensile stress is continuously applied in a corrosive environment. SCC is strongly avoided because once a crack occurs, it progresses quickly and breaks in a short period of time. SCC is generally more likely to occur in high-strength materials. Due to the problem of SCC, the use of 7000 series aluminum alloys is sometimes postponed.

For this reason, conventional efforts have been made to improve the SCC resistance.
Patent Document 1 discloses that 5 ≤ [Zn] ≤ 7 and [Zn] + 4.7 [Mg] ≤ 14, where [Zn] is the Zn content expressed in mass% and [Mg] is the Mg content. and a Mg content in excess of the stoichiometric ratio of _MgZn2 . In addition to Zn and Mg in the above ranges, this aluminum alloy extruded material has Cu: 0.1 to 0.6 mass%, Ti: 0.005 to 0.05 mass%, and Mn: 0.1 to 0.3 % by mass, Cr: 0.05 to 0.2 mass %, Zr: 0.05 to 0.2 mass %. This aluminum alloy extruded material is manufactured by air-cooling die quenching (on-line forced cooling of the extruded material immediately after extrusion using a die, also called press quenching), and has high strength and excellent SCC resistance after aging treatment. and can be suitably used as a material for automotive members such as door beams and bumper reinforcements.

In Patent Document 2, Zn: 5.5 to 9.0% by mass, Mg: 1.0 to 2.0% by mass, Cu: 0.1 to 1.0% by mass, Ti: 0.005 to 0.00% by mass. A 7000 series aluminum alloy extruded material containing 2% by mass and 0.1 to 0.5% by mass of one or more of Zr, Cr, Mn, etc. is described. In this aluminum alloy extruded material, the average length of grain boundary precipitates (MgZn ₂ ) is 5 μm or less, and the number of grain boundary precipitates with a length of more than 5 μm is regulated to 3 or less per 100 μm of grain boundary length. . This aluminum alloy extruded material is produced by die quenching by water cooling, has high strength and excellent energy absorption properties after aging treatment, and can be suitably used as a material for automotive members such as door beams and bumper reinforcements. Further, Patent Document 2 describes that Zr, Cr, Mn, etc. have the effect of making the crystal structure of the 7000 series aluminum alloy extruded material into a fibrous structure and improving the SCC resistance.

JP 2011-144396 A JP 2015-221924 A

The SCC resistance required for aluminum alloy extruded materials has become more severe in recent years. For this reason, even the 7000 series aluminum alloy extruded materials described in Cited Documents 1 and 2 may not be able to meet the requirements for SCC resistance, and a high strength 7000 series aluminum alloy extruded material having higher SCC resistance is required. ing.

The present disclosure has been made to meet such demands, and aims to provide a 7000 series aluminum alloy extruded material having high strength and high SCC resistance, and a method for producing the same.

Aspect 1 of the present invention is
Zn: 7.5 to 9.2% by mass,
Mg: 1.3 to 2.0% by mass,
Cu: 0.1 to 0.7% by mass,
One or more selected from the group consisting of Mn: 0.30% by mass or less, Cr: 0.25% by mass or less, and Zr: 0.25% by mass or less, totaling 0.1 to 0.5 mass %,
Ti: 0.005 to 0.20% by mass,
containing, the balance consisting of Al and inevitable impurities,
The average spacing of grain boundary precipitates is 0.8 to 1.4 μm,
The average grain length of grain boundary precipitates is 0.3 to 0.5 μm,
An aluminum alloy extruded material having a yield strength of 440 N/mm ² or more.

Aspect 2 of the present invention is the aluminum alloy extruded material according to Aspect 1, wherein the average spacing of grain boundary precipitates is 1.2 μm or less.

Aspect 3 of the present invention is
Zn: 7.5 to 9.2% by mass,
Mg: 1.3 to 2.0% by mass,
Cu: 0.1 to 0.7% by mass,
Mn: 0.30% by mass or less, Cr: 0.25% by mass or less, Zr: 0.25% by mass or less, one or more selected from the group consisting of 0.1 to 0.5 mass in total %,
Ti: 0.005 to 0.20% by mass,
A step of soaking an aluminum alloy in which the balance is Al and unavoidable impurities;
A step of performing hot extrusion after performing the soaking;
A step of cooling between 400 ° C. and 300 ° C. at an average cooling rate of 100 ° C./min or more and 600 ° C./min or less when cooling after the extrusion process;
A step of performing artificial aging treatment after the cooling;
A method for producing an aluminum alloy extruded material containing

A fourth aspect of the present invention is the method for producing an aluminum alloy extruded material according to claim 3, wherein the cooling after the extrusion is performed by die quenching.

One embodiment of the present invention can provide a 7000 series aluminum alloy extruded material having high strength and high SCC resistance and a method for producing the same.

1 is a SEM (scanning electron microscope) photograph showing an example of observation results of grain boundary precipitates in a 7000 series aluminum alloy extruded material according to an embodiment of the present invention.

The inventors have studied from various angles. Then, in the 7000 series aluminum alloy extruded material having predetermined components, the average spacing of grain boundary precipitates is 0.8 to 1.4 μm, and the average grain length of grain boundary precipitates is 0.3 to 0.5 μm. As a result, it has been found that high SCC resistance can be obtained even with a high yield strength of 440 N/mm ² or more.

In addition, such an aluminum alloy extruded material is obtained by using an aluminum alloy having a predetermined composition, (a) performing soaking, (b) performing hot extrusion after soaking, and (c) extruding. Cooling between 400 ° C. and 300 ° C. at an average cooling rate of 100 ° C./min or more and 600 ° C./min or less during subsequent cooling, (d) performing artificial aging treatment after cooling. I found that
Details of embodiments of the present invention are given below.

<1. Aluminum alloy composition>
The 7000 series aluminum alloy extruded material according to the embodiment of the present invention has Zn: 7.5 to 9.2 mass%, Mg: 1.3 to 2.0 mass%, and Cu: 0.1 to 0.7. % by mass, and one or more selected from the group consisting of Mn: 0.30 mass % or less, Cr: 0.25 mass % or less, and Zr: 0.25 mass % or less, in a total amount of 0.1 to 0.5% by mass and Ti: 0.005 to 0.20% by mass.
Each element will be described in detail below.

(Zn: 7.5 to 9.2% by mass)
Zn forms _MgZn2 with Mg and improves the strength of the 7000 series aluminum alloy extruded material. In order to increase the strength represented by proof stress (0.2% proof stress) etc. after aging treatment (artificial aging treatment) in the 7000 series aluminum alloy extruded material, the Zn content must be 7.5% by mass or more. On the other hand, when the Zn content exceeds 9.2% by mass, the material strength is improved, but the average spacing of grain boundary precipitates (MgZn ₂ ) tends to be small, and there is concern about a decrease in SCC resistance. .
Therefore, the Zn content should be in the range of 7.5 to 9.2% by mass in order to obtain a predetermined strength while ensuring SCC resistance. The lower limit of the Zn content is preferably 7.7% by mass, more preferably 8.0% by mass, still more preferably 8.1% by mass, and the upper limit is preferably 9.0% by mass, more preferably 8.0% by mass. 0.8 mass %.

(Mg: 1.3 to 2.0% by mass)
Mg forms _MgZn2 with Zn and improves the strength of the 7000 series aluminum alloy extruded material. In order to increase the strength represented by proof stress after aging treatment (artificial aging treatment) in the 7000 series aluminum alloy extruded material, the Mg content must be 1.3% by mass or more. On the other hand, when the Mg content exceeds 2.0% by mass, the average spacing of grain boundary precipitates (MgZn ₂ ) tends to become small, and there is concern about a decrease in SCC resistance. In addition, the increase in deformation resistance reduces extrudability. Therefore, the Mg content should be within the range of 1.3 to 2.0% by mass. The lower limit of the Mg content is preferably 1.4% by mass, and the upper limit is preferably 1.8% by mass.

(Cu: 0.1 to 0.7% by mass)
Cu dissolves in grain boundary precipitates (MgZn ₂ ), thereby reducing the potential difference between the grain boundary precipitates and the PFZ (precipitation-free zone), suppressing the preferential dissolution of the grain boundary precipitates in a corrosive environment, This improves the SCC resistance of the 7000 series aluminum alloy extruded material. However, if the Cu content is less than 0.1% by mass, the effect is small. On the other hand, if the Cu content exceeds 0.7% by mass, the extrudability is lowered due to an increase in deformation resistance, and the weld cracking resistance of the extruded material is also deteriorated. Therefore, the Cu content should be 0.1 to 0.7% by mass. The lower limit of the Cu content is preferably 0.2% by mass, and the upper limit is preferably 0.5% by mass.

(Mn: 0.30% by mass or less, Cr: 0.25% by mass or less, and Zr: 0.25% by mass or less, one or more selected from the group consisting of 0.1 to 0.5% in total mass%)
Mn, Cr, and Zr precipitate finely in the aluminum alloy during soaking, pin the grain boundaries to suppress recrystallization, and refine the crystal grains of the 7000 series aluminum alloy extruded material to form a fibrous structure. It has an organizational effect. Further, by refining the crystal grains, there is an effect of improving the SCC resistance of the 7000 series aluminum alloy extruded material. As one or more of Mn, Cr and Zr, (1) any one of the three elements alone, (2) a combination of two of the three elements (Mn and Cr, Mn and Zr or Cr and Zr), or (3) All three elements are considered, and any one of the above (1) to (3) may be selected.

However, when the contents of Mn, Cr and Zr exceed 0.3% by mass, 0.25% by mass and 0.25% by mass respectively, or the total content exceeds 0.5% by mass, the extrudability is poor. In addition, the quenching sensitivity of the extruded material increases. On the other hand, if the total amount of Mn, Cr and Zr is less than 0.1% by mass, the desired effect may not be obtained. Therefore, the content of each of Mn, Cr and Zr is within the ranges of Mn: 0.3% by mass or less, Cr: 0.25% by mass or less, and Zr: 0.25% by mass or less. The total is within the range of 0.1 to 0.5% by mass.
Of these, Zr has a smaller effect of increasing the quenching sensitivity of the 7000 series aluminum alloy extruded material than Mn and Cr, so it is preferentially added in the range of 0.1 to 0.25% by mass, and if necessary It is preferred to add one or both of Mn and Cr complementary accordingly. A preferable lower limit of the Zr content is 0.12% by mass, a more preferable lower limit is 0.14% by mass, and a preferable upper limit is 0.23% by mass, and a more preferable upper limit is 0.20% by mass. A preferable upper limit of the Cr content is 0.1% by mass, and a more preferable upper limit is 0.06% by mass. A preferable upper limit of the Mn content is 0.1% by mass, and a more preferable upper limit is 0.06% by mass.

(Ti: 0.005 to 0.20% by mass)
Ti has the effect of forming Al ₃ Ti in the molten metal and refining the crystal grains of the ingot. However, if the Ti content is less than 0.005% by mass, the effect is small. On the other hand, when the Ti content exceeds 0.20% by mass, coarse crystallized substances are formed in the ingot, which lowers the toughness of the 7000 series aluminum alloy extruded material. Therefore, the Ti content should be 0.005 to 0.20% by mass. The Ti content preferably has a lower limit of 0.01% by mass and an upper limit of 0.05% by mass.

The basic components are as described above, with the balance being Al and incidental impurities in one preferred embodiment of the invention.
Fe and Si are major unavoidable impurities in 7000 series aluminum alloy extruded materials. If the Fe content is too high, various properties such as the elongation and fatigue strength of the 7000-series aluminum alloy extruded material are degraded. If the Si content is too high, various properties such as the elongation and fatigue strength of the 7000 series aluminum alloy extruded material will decrease, and seizure during extrusion will likely occur. % or less.
Unavoidable impurities other than Fe and Si are, for example, the allowable range of ordinary unavoidable impurities of 7000 series aluminum alloy extruded materials. Unavoidable impurity elements each contain 0.05% by mass or less, and all unavoidable impurities excluding Fe and Si is preferably regulated to 0.15% by mass or less. Among the impurities, B is mixed into the aluminum alloy in an amount of about 1/5 of the Ti content due to the addition of Ti. 01% by mass or less.

(other selective elements)
Furthermore, in another preferred embodiment of the present invention, elements other than those described above may be added as necessary within a range that does not impair the effects of the embodiments of the present invention. The properties of the aluminum alloy can be further improved depending on the components contained.

Examples of such selective elements can include one or more selected from the group consisting of Sc, Sr, Sn, Ag, Ca and Mo. The preferable content of each element and the reason thereof are shown below.
Sc: 0.05 to 0.5% by mass
By containing Sc within this range, the effect of grain refinement can be obtained.
Sr: 0.05 to 0.5% by mass
By containing Sr within this range, the effect of improving mechanical properties can be obtained.
Sn: 0.05 to 0.5% by mass
By containing Sn within this range, the effect of improving mechanical properties can be obtained.
Ag: 0.05 to 0.5% by mass
By containing Ag within this range, effects of improving mechanical properties and improving SCC resistance can be obtained.
Ca: 0.05 to 0.5% by mass
By containing Ca within this range, the effect of improving mechanical properties and improving SCC resistance can be obtained.
Mo: 0.05 to 0.5% by mass
By containing Mo within this range, effects of improving mechanical properties and improving SCC resistance can be obtained.

<2. Average spacing and average grain length of grain boundary precipitates>
When a 7000 series aluminum alloy is hot extruded and cooled by die quenching or the like, precipitates (MgZn ₂ ) precipitate in crystal grains and grain boundaries during cooling, depending on the contents of Zn and Mg and the cooling rate. do. During cooling, the MgZn ₂ precipitated at the grain boundaries is larger in size than the MgZn ₂ precipitated inside the grains.
However, if the cooling rate is sufficiently high (for example, water cooling or mist cooling), precipitation during cooling can be suppressed. Subsequently, when the 7000 series aluminum alloy extruded material (temper: T1) after die quenching is subjected to aging treatment (artificial aging treatment) (temper: T5), Zn and Mg dissolved in the aluminum alloy become MgZn. As ₂ , it is finely precipitated in grains and grain boundaries. In this aging treatment, the size and distribution of _MgZn2 precipitated during cooling after die quenching do not change significantly.
Temper T1 means a state in which natural aging is performed after die quenching, and temper T5 means a state in which aging treatment is subsequently performed.

When a 7000 series aluminum alloy extruded material is placed in a corrosive environment to dissolve _MgZn2 and at the same time a continuous tensile stress exists in the material, cracks propagate through the material mainly through the grain boundaries, and this Cracks are observed as SCC. This is because holes formed by the dissolution of fine grain boundary precipitates (MgZn ₂ ) having a length of 1 μm or less precipitated during die quench cooling and aging treatment serve as propagation paths for cracks.

SCC is more likely to occur as the grain boundary precipitates (MgZn ₂ ) are densely distributed along the grain boundaries (the smaller the average distance between adjacent grain boundary precipitates).
On the other hand, if the average spacing of grain boundary precipitates is increased, the size (average grain length) increases, and sufficient strength (proof stress) cannot be obtained even if artificial aging treatment is performed. The inventors of the present application have found that both high strength and high SCC resistance can be achieved by setting the average spacing and average grain length of intergranular precipitates within appropriate ranges.

In an embodiment of the present invention, grain boundary precipitates in the 7000 series aluminum alloy extruded material have an average spacing of 0.8 to 1.4 μm and an average grain length of 0.3 to 0.5 μm. The average spacing of grain boundary precipitates becomes smaller as the total content of Zn and Mg increases. Further, the average interval of grain boundary precipitates becomes smaller as the cooling rate by die quenching or the like increases. There is a tendency that the smaller the average spacing of grain boundary precipitates, the smaller the size (average grain length) of grain boundary precipitates. For this reason, as described above, the Zn content and Mg content are set in appropriate ranges, and the cooling rate after hot extrusion is controlled in an appropriate range as described later. and average particle length can be controlled within appropriate ranges.
In one preferred embodiment, the upper limit of the average spacing of grain boundary precipitates is 1.2 μm or less. Thereby, higher strength can be obtained while maintaining high SCC resistance.
The term "grain length of grain boundary precipitates" means the length of the grain boundary precipitates in the direction along the grain boundary.
Also, the average spacing and average grain length of grain boundary precipitates can be determined by SEM observation as described in detail in Examples below.

<3. Strength>
As described above, high strength can be achieved by preventing the average grain length of grain boundary precipitates from becoming excessively large. The 7000 series aluminum alloy extruded material according to the embodiment of the present invention has a yield strength of 440 N/mm ² or more.

<4. Manufacturing method>
The 7000 series aluminum alloy extruded material according to the embodiment of the present invention is obtained by using an aluminum alloy having a predetermined composition, (a) performing soaking, (b) performing hot extrusion after soaking, ( c) Cooling between 400 ° C. and 300 ° C. at an average cooling rate of 100 ° C./min or more and 600 ° C./min or less when cooling after extrusion, (d) Can be manufactured by performing artificial aging treatment after cooling. .
Details of the manufacturing method according to the embodiment of the present invention are shown below.

(a) Homogenization Treatment The 7000 series aluminum alloy having the above-described predetermined composition is subjected to soaking treatment.
An ingot and a billet can be exemplified as the form of the 7000 series aluminum alloy to be soaked. Any conditions that enable hot extrusion processing may be selected as the conditions for the homogenization treatment.

As a means for improving the SCC resistance, as a preferred embodiment capable of reducing the average Cu content in the Fe-based crystallized substances, a homogenization treatment of the 7000-series aluminum alloy is performed under high-temperature conditions for a long time. By performing a homogenization treatment at a temperature of 490 to 550 ° C. for 4 hours or more, Cu, which was present in a high concentration in the Fe-based crystallized substances, diffuses into the Al base material, and Cu in the Fe-based crystallized substances content decreases. In order to reduce the average Cu content in the Fe-based crystallized substances, the homogenization temperature is preferably as high as possible. However, if the homogenization temperature is too high, the grain boundary pinning action of the elements (Zr, Cr, Mn) that suppress recrystallization may be reduced, and the structure of the extruded material may become coarse. Therefore, the homogenization temperature is preferably in the range of 500 to 540°C, more preferably in the range of 510 to 530°C. Further, the homogenization time is preferably as long as possible in order to reduce the average Cu content in the Fe-based crystallized substances, but if it is too long, the structure of the extruded material may become coarse. Therefore, the homogenization time is preferably 10 hours or less. In order to sufficiently reduce the Cu content in the Fe-based crystallized product, the above-described higher temperature conditions than the homogenization treatment conditions (470 ° C. x 6 hours) that have been generally performed for 7000-series aluminum alloys. is preferred. Cooling after homogenization treatment is not particularly limited, but may be performed at a cooling rate within the range of 100 to 200° C./hour, for example.
The homogenization time described here means the holding time at the temperature.

(b) Hot Extrusion After homogenization, the 7000 series aluminum alloy in the form of, for example, a billet or an ingot is subjected to hot extrusion.
The conditions for hot extrusion processing may be any conditions that allow processing into a desired shape.
Preferred extrusion conditions include a billet temperature (extrusion temperature) of 450-510° C. and an extrusion speed of 2-15 m/min.
Heating for hot extrusion processing may be performed by reheating the 7000 series aluminum alloy that has been cooled after the homogenization treatment.

(c) Cooling after Extrusion The extruded 7000 series aluminum alloy is cooled. Cooling may be performed immediately after extrusion, or may be performed after holding at a predetermined temperature (for example, re-solution treatment, etc.) after extrusion. Cooling may be performed by any method, but cooling is performed at an average rate of 100° C./min or more and 600° C./min or less between 400° C. and 300° C., which is the temperature range where MgZn ₂ is most likely to precipitate. As a result, the obtained 7000 series aluminum alloy can have an average spacing of grain boundary precipitates of 0.8 to 1.4 μm and an average grain length of 0.3 to 0.5 μm. If the average cooling rate is less than 100°C/min, the amount of precipitation of _MgZn2 increases during cooling, the effect of subsequent aging treatment becomes insufficient, and yield strength is not sufficiently improved. On the other hand, when the average cooling rate exceeds 600° C./min, many fine grain boundary precipitates are formed, the average spacing of the grain boundary precipitates becomes too small, and the SCC resistance deteriorates.

The average cooling rate between 400°C and 300°C is preferably between 100°C/min and 500°C/min, more preferably between 100°C/min and 400°C/min.
Also, preferably, the average cooling rate is 100°C/min or more and 600°C/min or less even in the range of 400 to 200°C.

A preferred cooling method is die quenching. Cooling of the die during die quenching may be performed by any method such as water cooling, air cooling, or air cooling. Air cooling is preferred as it is relatively easy to achieve an average cooling rate of 100-600°C/min between 400-300°C.
The cooling rate may be measured by bringing a contact thermometer such as a thermocouple into contact with the extruded aluminum alloy material. Alternatively, as a simple method, a non-contact thermometer may be used to measure the surface temperature of the aluminum alloy extruded material. Furthermore, if it is difficult to measure the temperature, it may be obtained by appropriately using a simulation.

(d) Artificial Aging Treatment The 7000 series aluminum alloy extruded material that has been cooled after hot extrusion is subjected to artificial aging treatment. By performing this treatment, the proof stress can be made 440 N/mm ² or more. The conditions for the artificial aging treatment may be any conditions as long as the proof stress can be 440 N/mm ² or more.
As a preferable artificial aging treatment condition, a two-stage aging treatment can be exemplified in which the temperature is kept at 65 to 95° C. for 2 to 6 hours, and then the temperature is kept at 120 to 170° C. for 6 to 15 hours.

As described above, the method for manufacturing the 7000 series aluminum alloy extruded material according to the embodiment of the present invention has been described. and produce 7000 series aluminum alloy extrusions having the desired properties according to embodiments of the present invention, other than the production methods described above.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and can be implemented with appropriate modifications within the scope that can match the spirit described above and below. subsumed in

Sample no. 1-6 were obtained.
A 7000 series aluminum alloy billet having a diameter of 194 mm obtained by semi-continuous casting was homogenized at 520°C for 6 hours and then cooled to room temperature. The cooling method was fan air cooling. Table 1 shows sample numbers. 1 to 6 alloy compositions are shown.

Subsequently, these billet samples were reheated, subjected to hot extrusion molding at an extrusion temperature of 500° C. and an extrusion speed of 5 m/min, and subjected to fan air cooling immediately after extrusion. Each extruded material is a hollow extruded material having a cross-sectional shape of 15 mm high×120 mm wide×3 mm thick. Each extrudate sample was held at 470° C. for 1 hour to re-solubilize to simulate a die quench, and sample no. 1 to 4 are fan-cooled, sample No. Sample No. 5 is allowed to cool. 6 was cooled to room temperature by water cooling (shower cooling).

Table 1 shows the average cooling rate of the extruded material between 400°C and 300°C. The cooling rate was measured by a thermocouple inserted into a hole provided in the extruded material sample. The time (t minutes) required for the extruded material sample to reach 300° C. from 400° C. was determined, and the cooling rate was calculated as (400-300)/t (° C./minute). The calculated cooling rate is the temperature inside the extruded material, but since the aluminum alloy has excellent thermal conductivity, the temperature difference between the surface and the inside of the extruded material is small, so the temperature may be measured by either method.

The extruded material sample after cooling was subjected to two stages of artificial aging treatment (first stage: 90°C x 3 hours → second stage: 140°C x 8 hours).
Using an extruded material sample after artificial aging treatment, the average spacing and average particle length of grain boundary precipitates were measured by the method detailed below. In addition, as evaluation of the properties of these samples, yield strength and SCC critical stress were measured by the methods detailed below. Table 2 shows the measurement results.

(Average particle spacing and average particle length of grain boundary precipitates)
A test material is cut out from the upper surface of the extruded material sample, and a point of 100 μm from the surface of the extruded material on the surface perpendicular to the extrusion direction is observed with a SEM (scanning electron microscope). Precipitates present at the grain boundaries (MgZn ₂ ) was observed.
More specifically, each sample is observed to select a representative grain boundary portion, and the crystal grain boundary observed within the field of view of that portion (field of view: 12.7 μm × 9.6 μm) grain boundary precipitates were measured. The total grain length of the grain boundary precipitates measured (the length in the direction along the grain boundary) is divided by the number of grain boundary precipitates, and the obtained value is the average grain length of the grain boundary precipitates. did. Further, the value obtained by subtracting the total grain length from the grain boundary length (total extension of the grain boundaries existing in the measured range) by the number of grain boundary precipitates is calculated as It was taken as the average interval of grain boundary precipitates.
FIG. 1 shows an example of observation results of grain boundary precipitates of sample No. 3 is an SEM photograph. It can be seen that white-looking grain boundary precipitates (MgZn ₂ ) are formed along the grain boundaries.

(proof strength)
A JIS No. 13B test piece was machined parallel to the direction of extrusion from the upper surface of each extruded material sample. Two specimens were taken from each extruded sample. Using this test piece, a tensile test was performed in accordance with JISZ2241 to measure yield strength (0.2% yield strength). The crosshead speed was 5.0 mm/min until the yield strength value was reached and then 10.0 mm/min. No. described in Table 1. Yield strength values of 1 to 6 were average values of yield strength values measured on two test pieces. Yield strength values of 440 N/mm ² or more were evaluated as acceptable.

(SCC critical stress)
SCC testing was performed by the chromic acid accelerated method. An SCC test piece having a width of 10 mm and a length of 50 mm was machined from the upper surface of the extruded material sample in the direction perpendicular to the extrusion direction, avoiding the welded portion. For each sample, two specimens were prepared for each applied stress. In the SCC test, various tensile stresses were applied by adopting the three-point loading method of the plate bending test (JISH8711:2001). Loading was applied by a constant strain method (three-point support beam method). More specifically, a tensile stress was generated on the outer surface of the test piece by tightening the bolts of the three-point bending jig, and the tensile stress value was measured with a strain gauge adhered to the outer surface of the test piece.
The etchant used in the SCC test was a Cr acid aqueous solution (NaCl: 3 g, K ₂ Cr ₂ O ₇ : 30 g, CrO ₃ : 36 g per liter of distilled water), and the temperature was kept at 90°C during the test to promote SCC. held above. A test piece (two for each applied stress) is immersed in the corrosive solution while stress is applied, and is taken out every 2 hours to visually observe the presence or absence of cracks. Re-immersion was performed. This procedure was repeated until 16 hours after the start of the SCC test. The maximum load stress at which no cracks occurred in both test pieces until the end of the test was evaluated as the SCC critical stress of the test piece. An SCC critical stress of 100 N/mm ² or more was evaluated as acceptable.

As shown in Tables 1 and 2, sample No. having a composition specified by the embodiments of the present invention and having an average cooling rate between 400 and 300° C. of the extruded material within the range of 100 to 600° C./min . In Nos. 1 to 3, the average spacing of grain boundary precipitates was within the range of 0.8 to 1.4 μm, and the average grain length was within the range of 0.3 to 0.5 μm. And sample no. 1 to 3 had a proof stress of 440 N/mm ² or more and an SCC critical stress of 100 N/mm ² or more.

On the other hand, Sample No. outside the composition range specified in the embodiment of the present invention or having an average cooling rate after hot extrusion outside the range of 100 to 600° C./min. 4-6 had a yield strength of less than 440 N/mm ² or an SCC critical stress of less than 100 N/mm ² .
More specifically, sample no. No. 4 had a low yield strength due to an insufficient Zn content.
Sample no. In No. 5, the average spacing and average grain size of intergranular precipitates were larger than the ranges defined by the embodiments of the present invention, and the SCC critical stress was high. However, the cooling rate was too low, so the yield strength was low.
Sample no. In No. 6, the average cooling rate was too high, the average spacing of grain boundary precipitates was too small, and the average grain length was also too small, resulting in a low SCC critical stress.

Claims (4)

Zn: 7.5 to 9.2% by mass,
Mg: 1.3 to 2.0% by mass,
Cu: 0.1 to 0.7% by mass,
One or more selected from the group consisting of Mn: 0.30% by mass or less, Cr: 0.25% by mass or less, and Zr: 0.25% by mass or less, totaling 0.1 to 0.5 mass %,
Ti: 0.005 to 0.20% by mass,
containing, the balance consisting of Al and inevitable impurities,
The average spacing of grain boundary precipitates is 0.8 to 1.4 μm,
The average grain length of grain boundary precipitates is 0.3 to 0.5 μm,
An aluminum alloy extruded material having a yield strength of 440 N/mm ² or more. 2. The aluminum alloy extruded material according to claim 1, wherein said average spacing of grain boundary precipitates is 1.2 [mu]m or less. Zn: 7.5 to 9.2% by mass,
Mg: 1.3 to 2.0% by mass,
Cu: 0.1 to 0.7% by mass,
Mn: 0.30% by mass or less, Cr: 0.25% by mass or less, Zr: 0.25% by mass or less, one or more selected from the group consisting of 0.1 to 0.5 mass in total %,
Ti: 0.005 to 0.20% by mass,
A step of soaking an aluminum alloy in which the balance is Al and unavoidable impurities;
A step of performing hot extrusion after performing the soaking;
A step of cooling between 400 ° C. and 300 ° C. at an average cooling rate of 100 ° C./min or more and 600 ° C./min or less when cooling after the extrusion process;
A step of performing artificial aging treatment after the cooling;
A method for producing an aluminum alloy extrusion comprising: 4. The method for producing an aluminum alloy extruded material according to claim 3, wherein the cooling after the extrusion is performed by die quenching.

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