JP2000054052A - High strength 6000 series aluminum alloy excellent in stress corrosion cracking resistance and heat treatment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the stress corrosion cracking resistance in a 6000 series aluminum alloy while maintaining its high strength. SOLUTION: In an Al-Mg-Si alloy or an Al-Mg-Si-Cu alloy, intergranular precipitates are present at the intervals of >=110 nm, and, furthermore, the dimensions of the precipitates in the intergranular direction are controlled to >=70 nm. Moreover, this precipitating form can be obtd. by subjecting a 6000 series aluminum alloy ingot to primary aging treatment of holding it at 150 to 200 deg.C for 3 to 80 hr after solution treatment, next executing restoring treatment of performing heating at a temp. rising rate of >=300 deg.C/min, holding it at 200 to 270 deg.C for 5 to 20 min and thereafter performing cooling at a rate of >=500 deg.C/min and moreover executing reaging treatment of holding it at 150 to 190 deg.C for 8 to 80 hr.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a 6000 series aluminum alloy having high strength and excellent stress corrosion cracking resistance, and a heat treatment method thereof.

[0002]

2. Description of the Related Art In recent years, applications of aluminum alloys have been expanding as various structural members for vehicles, buildings, and the like by utilizing its light weight. Generally, as a high-strength aluminum alloy used as a structural member, Al-Cu-based, Al-C
u-Mg system, Al-Mg system, Al-Zn-Mg system, Al
-Mg-Si type is available. This kind of aluminum alloy tends to cause stress corrosion cracking as the strength increases, and there is a problem that the stress corrosion cracking susceptibility increases particularly in the T6 tempered state where the maximum strength is obtained. Therefore, there was no satisfactory alloy in applications requiring high strength, high stress corrosion cracking resistance, and workability.

Under these circumstances, in order to improve various properties of the above-mentioned alloy system, studies have conventionally been made from the viewpoint of alloy composition. Since the stress corrosion cracking property is deteriorated, there is an attempt to improve the stress corrosion cracking resistance by adopting T76 and T73 as aging treatment conditions. However, when the stress corrosion cracking resistance is improved, the strength is reduced, and it is difficult to achieve both the strength and the stress corrosion cracking resistance.

In order to solve such a problem, US Pat.
No. 56,584, a 7000 series aluminum alloy such as 7075 is subjected to a T6 treatment at about 121 ° C. for about 24 hours after the soaking treatment, and then at 200 to 260 ° C.
Perform a restorative heat treatment for 2-3 seconds to 2-3 minutes, and
It has been reported that by performing re-aging treatment at 15 to 125 ° C. for 16 to 48 hours, stress corrosion cracking resistance equivalent to that of T7 material can be obtained while maintaining strength equivalent to T6 material. This heat treatment method, called reversion reaging (RRA) treatment, is one of the factors affecting stress corrosion cracking resistance, in particular, control of grain boundary and intragranular precipitation morphology in the structure, specifically, the material after heat treatment. In the structure, G. P. The purpose is to promote fine precipitation contributing to the strength of zones and the like, and to coarsen precipitates at the grain boundaries to reduce the susceptibility to grain boundary cracking. However, although its effectiveness has been recognized at the experimental level, it is not enough to improve both the strength and the stress corrosion cracking resistance, and it has not been put to practical use yet. Further, Japanese Patent Application Laid-Open Nos. 62-142753 and 6-41669 disclose processing methods in which the restoration processing time of the second stage of the RRA processing is lengthened.

[0005]

However, any of the above-mentioned heat treatment methods is a method applied to a 7000 series or 2000 series aluminum alloy, and there is no report on application to other alloy systems. For example, it has been pointed out that, even in a 6000 series aluminum alloy, the addition of Mg and Si due to the increase in strength, particularly the decrease in stress corrosion cracking resistance with the increase in the addition of Si, is pointed out. No effective means has been found for balancing cracking.

The present invention has been made in view of such technical background,
Equipped with both high strength and excellent stress corrosion cracking resistance,
An object of the present invention is to provide a 6000 series aluminum alloy and a heat treatment method thereof.

[0007]

Means for Solving the Problems The present inventor has proposed the above-mentioned RR.
As a result of diligent research into the application of the A treatment to the 6000 series aluminum alloy, a grain boundary precipitation morphology having both high strength and excellent stress corrosion cracking resistance has been found, and a heat treatment for having both. I found a condition.

[0008] That is, the 6000 series aluminum alloy of the present invention is an Al-Mg-Si based alloy or Al-Mg-
In the Si-Cu alloy, the grain boundary precipitates are present at intervals of 110 nm or more, and the size of the precipitates in the grain boundary direction is 70 nm or more.

Further, the heat treatment method of the present invention has
150% after incorporation treatment
A primary aging treatment is carried out at a temperature of 200 to 200 ° C. for 3 to 80 hours.
After holding at 0 to 270 ° C for 0.5 to 20 minutes, 500 ° C /
Perform a restoration process to cool at a speed of at least
The present invention is characterized in that a re-aging treatment is performed in which the temperature is kept at -190 ° C for 8-80 hours.

[0010] In the alloy structure, the precipitation form of the precipitates at the crystal grain boundaries contributes to the stress corrosion cracking resistance. The precipitates at the grain boundaries tend to be large and the cracking susceptibility decreases as the precipitation interval increases. In the present invention, the precipitates are grown so that the length in the length direction of the grain boundary is 70 nm or more, and the precipitates are separated so that the interval between the precipitates is 110 nm or more, so that stress corrosion cracking is most unlikely to occur. High stress corrosion cracking resistance. In order to improve the stress corrosion cracking resistance, a preferred lower limit of the size of the grain boundary precipitate is 90 nm, and a preferred lower limit of the precipitation interval is 130 nm.

An Al-Mg-Si alloy or A
The following composition is preferable in the l-Mg-Si-Cu alloy, and the reasons for limiting the content of each element are as follows.

Mg, Si, and Cu are all elements that contribute to improving the strength of the alloy. Si content is 0.8 to 1.5
% Is preferred. If it is less than 0.8%, the effect of improving strength is poor, and if it exceeds 1.5%, hot workability is significantly reduced. A particularly preferred lower limit of the Si content is 0.9.
%, A particularly preferred upper limit is 1.2%. In addition, Mg
The content is preferably 0.8 to 1.4%. If it is less than 0.8%, the effect of improving the strength is poor, and if it exceeds 1.4%, the hot workability decreases and the elongation decreases. Mg
A particularly preferred lower limit of the content is 1.0%, and a particularly preferred upper limit is 1.2%. The Cu content is 0.4 to
0.8% is preferred. If it is less than 0.4%, the strength improving effect is poor, and if it exceeds 0.8%, the corrosion resistance is deteriorated and the hot workability is reduced.

Fe, Mn, Cr, Zn, and Ti are elements that arbitrarily select two or more of them to improve various properties of the alloy. That is, Fe has an effect of suppressing recrystallization, the content is preferably 0.1 to 0.3%, a particularly preferred lower limit is 0.2%, and a particularly preferred upper limit is 0.1%.
3%. Mn has an effect of suppressing recrystallization and improving strength, and its content is preferably 0.2% or less, and a particularly preferred upper limit is 0.1%. Cr has an effect of suppressing recrystallization, and its content is preferably 0.2% or less, and a particularly preferred upper limit is 0.1%. Zn has an effect of improving the strength by solid solution strengthening, and the content is preferably 0.1% or less, and particularly preferably the upper limit is 0.0%.
8%. Ti has the effect of improving strength by solid solution strengthening or precipitation strengthening, and its content is preferably 0.03% or less, and a particularly preferred upper limit is 0.02%.

In the heat treatment method of the present invention, in the first-stage primary aging treatment, the aging treatment is carried out after the soaking treatment to harden and increase the strength. The method of the encapsulation treatment is not particularly limited, and for example, T5 treatment performed after hot working such as extrusion or T6 treatment for reconstitution after extrusion can be mentioned. The aging condition is preferably such that a maximum strength is obtained, and the aging treatment is performed at 150 to 200 ° C. for 3 to 80 hours. A particularly preferred lower limit of the treatment temperature is 165 ° C, a particularly preferred upper limit is 180 ° C, a particularly preferred lower limit of the treatment time is 6 hours, and a particularly preferred upper limit is 24 hours. However, for alloys having an Mg ₂ Si content of 1% or more, the high-temperature aging has a negative effect on the strength, so that the natural aging time after the embedding treatment is preferably short, and is preferably about 1 to 72 hours.

In the second stage of the restoration treatment, the temperature is raised to a temperature at which a part of the precipitated phase in the material is dissolved, held for a predetermined time, and then cooled at a temperature at which the precipitated phase once dissolved does not precipitate. By doing so, the structure hardened by the primary aging treatment at the preceding stage is restored and the hardness is reduced. If the rate of temperature rise is too slow, the rate of precipitation is faster than the rate of solid solution of the precipitated phase. For this reason, the temperature raising rate needs to be 300 ° C./min or more, and particularly preferably 600 ° C./min or more. The subsequent heating and holding is performed at 200 to 270 ° C. for 0.5 to 20 minutes. 200
If the temperature is lower than ℃, solid solution itself of the precipitated phase for obtaining restoration hardening hardly occurs. If the temperature is higher than 270 ° C, coarse precipitation is promoted, and the recovery of strength cannot be expected even if re-aging treatment is performed at a later stage. A particularly preferred lower limit of the treatment temperature is 220 ° C., a particularly preferred upper limit is 260 ° C., a particularly preferred lower limit of the treatment time is 2 minutes, and a particularly preferred upper limit is 7 minutes. Cooling after the restoration treatment needs to be performed at a rate of 500 ° C./min or more, particularly 900 ° C./min or more, because the strength is reduced due to coarse precipitation sometimes occurring during the cooling. During this restoration process, the precipitates at the grain boundaries are coarsened, and as described above, large precipitates having a length of 70 nm or more precipitate at intervals of 110 nm or more.

The third-stage re-aging treatment is a re-aging treatment for re-precipitating the precipitate phase dissolved in the solid solution by the previous restoration treatment to restore the first-stage hardness. The re-aging condition is based on the first-stage aging condition, but the processing time is at least 8 hours in order to restore to a sufficient strength. Therefore, the temperature is kept at 150 to 190 ° C. for 8 to 80 hours. A particularly preferred lower limit of the processing temperature is 160
° C, a particularly preferred upper limit is 180 ° C, a particularly preferred lower limit of the treatment time is 6 hours, and a particularly preferred upper limit is 2 hours.
4 hours.

By performing the above-described three-stage heat treatment on the 6000 series aluminum alloy, the strength can be recovered while maintaining the grain boundary precipitation state obtained in the second stage, and high strength and excellent strength can be obtained. Both stress corrosion cracking resistance can be obtained.

[0018]

Next, specific embodiments of the present invention will be described.

Al-Mg-Si alloy test material and Al
As the test material for the -Mg-Si-Cu-based alloy, one having the composition shown in Table 1 was used.

[0020]

[Table 1]

For the test materials having these compositions, 550 ° C.
× 1 hour, and then left at room temperature for 24 hours. Further, Table 2 (Al-Mg-Si alloy) and Table 3 (Al
-Mg-Si-Cu-based alloy). The heat treatment was a first-stage primary aging treatment, a second-stage restoration treatment, and a third-stage re-aging treatment, and each alloy was subjected to only the first-stage primary aging treatment as a comparative example. And
For the treated material, the tensile strength, proof stress, and elongation were measured by an ordinary method, and a stress corrosion cracking test was performed. The stress corrosion cracking test was as follows: CrO ₃ : 36 g / l, K ₂ Cr ₂ O ₇ :
Using a test solution containing 30 g / l and NaCl: 3 g / l, a test temperature of 95 ° C. and a material strength of 10 in the manner of three-point bending were used.
The test was performed under the condition of giving 0% load. The evaluation criterion was a time until the material surface was cracked by observation with a stereoscopic microscope at a magnification of 40 times. In addition, the spacing and size of precipitates at the grain boundaries were examined for each treatment material. Table 2 shows these results.
And Table 3 together.

[0022]

[Table 2]

[0023]

[Table 3]

From the results shown in Tables 2 and 3, it was confirmed that the precipitation form of the crystal grain boundaries of the present invention was obtained by the second-stage restoration treatment, and the stress corrosion cracking resistance was improved by such a precipitation form. did it. In addition, by performing the re-aging treatment in the third stage, the strength equal to or higher than that of the primary aging material in the first stage is obtained, and the stress corrosion cracking resistance in the second stage obtained by the restoration process is maintained. Thus, a 6000 series alloy having both high strength and resistance to stress corrosion cracking was obtained.

[0025]

As described above, the high-strength 6000 series aluminum alloy excellent in stress corrosion cracking resistance according to the present invention is an Al--Mg--Si series alloy or an Al--Mg--Si alloy.
-In the Cu-based alloy, the grain boundary precipitates are present at intervals of 110 nm or more, and since the size of the precipitates in the grain boundary direction is 70 nm or more, the sensitivity to stress corrosion cracking is low,
It has both the high strength of a 6000 series aluminum alloy and excellent stress corrosion cracking resistance. In particular, the Al-Mg-
The composition of the Si-based alloy is as follows: Si: 0.8 to 1.5%, Mg:
0.8-1.4%, and further Fe: 0.1-0.4%.
3%, Mn: 0.2% or less, Cr: 0.2% or less, Z
n: 0.1% or less, Ti: 2 out of 0.035% or less
When it contains more than one kind of element and the balance is made of Al and inevitable impurities, high strength and excellent stress corrosion cracking resistance can be obtained. Further, in the Al-Mg-Si-Cu alloy, the Cu content is 0.4 to 0.8% in addition to the above composition.
In this case, particularly high strength and excellent stress corrosion cracking resistance can be obtained.

Further, the above-mentioned precipitation form at the grain boundary is determined by the heat treatment method of the present invention, that is, the high strength and the stress corrosion cracking resistance of the 6000 series aluminum alloy ingot at 150 to 200 ° C. Perform primary aging treatment for 3 to 80 hours, then heat at a heating rate of 300 ° C / min or more, hold at 200 to 270 ° C for 0.5 to 20 minutes, and then cool at a rate of 500 ° C / min or more. Perform the restoration process,
Furthermore, it can be obtained by performing re-aging treatment at a temperature of 150 to 190 ° C. for 8 to 80 hours, and can have both high strength and excellent stress corrosion cracking resistance.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22F 1/00 630 C22F 1/00 630A 640 640A 691 691B 691C 691A 692 692A (72) Inventor Yoshitomo Kato Sakai City 6,224, Showa Aluminum Co., Ltd., Showa Aluminum Co., Ltd. (72) Inventor Masashi Sakaguchi 6,224 Kaiyamacho, Sakai City Showa Aluminum Co., Ltd.

Claims (5)

[Claims] 1. An Al—Mg—Si based alloy in which grain boundary precipitates are present at intervals of 110 nm or more, and the size of the precipitates in the grain boundary direction is 70 nm or more. High strength 6000 series aluminum alloy with excellent properties. 2. The method according to claim 1, wherein the aluminum alloy is Si: 0.8.
-1.5% and Mg: 0.8-1.4%, Fe: 0.1-0.3%, Mn: 0.2% or less, C
r: 0.2% or less, Zn: 0.1% or less, Ti: 0.0
Contains at least two elements of 35% or less, with the balance being Al
And a high-strength 6000 series aluminum alloy having excellent resistance to stress corrosion cracking, comprising an unavoidable impurity. 3. An Al—Mg—Si—Cu alloy in which grain boundary precipitates are present at intervals of 110 nm or more, and the size of the precipitates in the grain boundary direction is 70 nm or more. High strength 60 with excellent corrosion cracking
00 aluminum alloy. 4. The method according to claim 1, wherein the aluminum alloy is Si: 0.8.
１．５1.5%, Mg: 0.8-1.4% and Cu: 0.
4 to 0.8%, and further Fe: 0.1 to 0.3
%, Mn: 0.2% or less, Cr: 0.2% or less, Zn:
0.1% or less, Ti: 0.035% or less of two or more elements, and the balance consists of Al and inevitable impurities. High strength 6 excellent in stress corrosion cracking resistance.
000 series aluminum alloy. 5. A 6000 series aluminum alloy ingot is subjected to a primary aging treatment at a temperature of 150 to 200 ° C. for 3 to 80 hours after the soaking process, and then heated at a rate of 300 ° C./min or more. It is characterized by performing a restoring process of cooling at a rate of 500 ° C./min or more after holding at 270 ° C. for 0.5 to 20 minutes, and further performing a re-aging process of holding at 150 to 190 ° C. for 8 to 80 hours. A heat treatment method for a high-strength 6000 series aluminum alloy having excellent stress corrosion cracking resistance.