JPH05179413A - Production of aluminum alloy sheet excellent in stress corrosion cracking resistance - Google Patents

Abstract

PURPOSE:To obtain an aluminum alloy sheet excellent in stress corrosion cracking resistance. CONSTITUTION:At the time of producing a sheet of an aluminum alloy which has a composition containing, as essential elements, 3.5-4.2% Mg, 0.1-0.5% Cu, and 0.03-0.5% Mn and having the balance Al with inevitable impurities and further containing, if necessary, 0.03-0.20% Cr and/or 0.03-0.20% Zr, the aluminum alloy is subjected, as soaking treatment after casting, to heating up to 400-550 deg.C, held for 1-20hr, hot-rolled at 300-520 deg.C, cold-rolled at >=30% draft, and finish-annealed by means of slow cooling at <=5 deg.C/min cooling rate. This sheet also has formability and strength required of automobile parts member.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aluminum alloy plate suitable for automobile parts, and more particularly to various parts such as automobile frames, chassis and drive trains, which are excellent even in severe corrosive environments. The present invention relates to a method for manufacturing an aluminum alloy plate having stress corrosion cracking resistance.

[0002]

2. Description of the Related Art Conventionally, materials having excellent formability have been generally required for parts such as automobile chassis, and Al--Mg alloys have been used. Among them, in order to enhance the weight reduction effect for many parts, high strength material with Mg content over 3.5% (eg A518
2, A5083, A5086, etc.) are used.

Chassis and the like are often used in places where the corrosive environment is severe, and parts using Al-Mg alloy exceeding 3.5% Mg are apt to cause stress corrosion cracking, and remarkably improve vehicle safety. May be damaged.

The occurrence of stress corrosion cracking is affected by the corrosive environment and stress, but is greatly affected by the type of material and its manufacturing method.

By the way, Al-Mg alloys conventionally used for parts such as chassis are A5182, A5083, A5.
086 and the like, the stress corrosion cracking resistance of these alloys is significantly inferior to that of Al-Mg based alloys having Mg of 3.5% or less, which has no practical problem.

[0006] Further, the "aluminum alloy for baking coating excellent in yarn rust resistance" of Japanese Patent Application No. 63-43870 previously proposed by the applicant as a panel material is excellent in stress corrosion cracking resistance, but is annealed. At that time, it was necessary to cool rapidly when cooling to room temperature, which made it difficult to manufacture in a batch furnace.

[0007] For chassis parts and the like, a material having a plate thickness of about 2 to 5 mm is required, and for these thick plates, the rapid cooling at the time of finish annealing remarkably deteriorates mass productivity as compared with a plate having a plate thickness of 1 mm.

Further, as for automobiles, a corrosive environment is becoming more and more severe, since snow-melting agents such as CaCl ₂ are often sprayed on roads as anti-freezing agents. In reality, it is required to develop a material having excellent stress corrosion cracking resistance under such a severe corrosive environment.

The present invention has been made in order to meet such a demand, and an object thereof is to provide a method for producing an aluminum alloy sheet capable of exhibiting excellent stress corrosion cracking resistance even in a severe corrosive environment. It is what

[0010]

In order to solve the above-mentioned problems, the present inventor made efforts to develop an aluminum alloy plate capable of imparting excellent stress corrosion cracking resistance, and as a result, in particular, the composition of the composition including addition of Cu. This is made possible by optimizing the manufacturing process of the aluminum alloy plate.

That is, according to the present invention, Mg: 3.5-4.2.
%, Cu: 0.1 to 0.5%, Mn: 0.03 to 0.5% as essential elements, and if necessary, Cr: 0.03.
~ 0.20% and Zr: 0.03 to 0.20% of 1 or 2
In producing an aluminum alloy plate containing seeds and the balance Al and unavoidable impurities, after soaking after casting, the aluminum alloy plate is heated to a temperature of 400 to 550 ° C. and held for 1 to 20 hours, and then 300 to 520 ° C. Hot rolling at the temperature of
After that, cold rolling at a working rate of 30% or more is performed, and further finish annealing is performed by slow cooling at a cooling rate of 5 ° C./min or less, and a method for producing an aluminum alloy sheet having excellent stress corrosion cracking resistance is provided. It is a summary.

The present invention will be described in more detail below.

[0013]

[Action]

First, the reasons for limiting the chemical composition of the aluminum alloy in the present invention are as follows.

Mg is an element that imparts strength, but 3.5
If it is less than%, the strength and elongation will be low, and if it exceeds 4.2%, the strength will be high but the stress corrosion cracking resistance will be inferior. Therefore, the amount of Mg is 3.5-4.
The range is 2%.

Cu is an element that contributes to the improvement of stress corrosion cracking resistance. However, if it is less than 0.1%, there is no effect, and if it exceeds 0.5%, the effect of improving the stress corrosion cracking resistance begins to saturate, and further the weldability becomes remarkably inferior, which is not preferable. Therefore, the Cu content is 0.1 to 0.5.
The range is%.

Mn is an element that improves the strength, but
If it is less than 03%, the effect is not exerted. On the contrary, if it exceeds 0.5%, the effect of improving the strength is saturated and the molding processability (protrudability and elongation) is deteriorated, which is not preferable. Therefore, the amount of Mn is set in the range of 0.03 to 0.5%.

Since Cr and Zr are elements that improve the strength, at least one of them can be added in an appropriate amount. However, if both Cr and Zr are less than 0.03%, there is no effect, and conversely, if each exceeds 0.20%, the moldability (bulging property and elongation) becomes poor, which is not preferable. Therefore, when adding Cr
The amount of Zr and the amount of Zr are each in the range of 0.03 to 0.20%.

Although Fe, Si and the like may be contained as unavoidable impurities, they are permissible as long as the effects of the present invention are not impaired. For example, if Fe ≦ 0.4% and Si ≦ 0.2%, the stress corrosion cracking resistance is not affected.

Further, in order to refine the crystal grains,
Ti and Ti + B are added, but Ti ≦ 0.1% and B, respectively.
If it is ≦ 0.02%, it does not affect the stress corrosion cracking resistance.

Further, impurities such as Zn, Ni, and Na are Zn≤.
If 0.5%, Ni ≦ 0.1%, Na ≦ 0.001%,
Does not affect stress corrosion cracking resistance.

The reasons for limiting the manufacturing conditions in the present invention are as follows.

The aluminum alloy having the composition adjusted as described above is melted and cast by a conventional method and then the normal chamfering is carried out.
In the present invention, each condition of soaking, hot rolling, cold rolling, and finish annealing after chamfering is regulated.

Specifically, if the soaking temperature is lower than 400 ° C., MnAl _{6 which} is effective for refining the crystal grains is not sufficiently precipitated, and if it exceeds 550 ° C., burning occurs and the characteristics are deteriorated. Soaking temperature is 40
It shall be in the range of 0 to 550 ° C. Further, the holding time at that time is appropriately 1 to 20 hours. Under conditions outside these ranges, the above-mentioned soaking effect cannot be obtained, and the heating and holding time is 2
It is a waste of energy to go beyond 0 hours.

After the soaking treatment, hot rolling is carried out. If the temperature is less than 300 ° C., the deformation resistance becomes large and the hot rolling becomes difficult, and in order to carry out hot rolling at a temperature above 520 ° C. It is necessary to set the heat temperature to a temperature higher than 550 ° C., which causes burning during soaking and deteriorates the characteristics. Therefore, the hot rolling temperature is set in the range of 300 to 520 ° C.

Next, cold rolling after hot rolling is carried out to adjust recrystallized grains at the time of finish annealing. In cold rolling with a working rate of less than 30%, the past rate is small. As a result, the recrystallized grains become coarse during the finish annealing, which may lead to a decrease in yield strength and roughening of the product during molding. For this reason,
The cold rolling working rate is 30% or more.

Further, finish annealing is performed after cold rolling.
If the average cooling rate at this time is 5 ° C./min or less, not the β phase (Al ₂ Mg ₃ ) that causes stress corrosion cracking during cooling, but
Al-Mg-Cu based precipitates are deposited to improve the sensitivity to stress corrosion cracking. However, when the cooling rate exceeds 5 ° C / min, the amount of Cu dissolved in the mother phase increases, the Al-Mg-Cu system precipitate is not sufficiently deposited during cooling, and the amount of 2m
Aluminum alloy plate with a thickness exceeding m is 5 ℃ / m
In order to cool at a cooling rate in excess of in, it is necessary to attach a cooling device on the manufacturing equipment, and since slow cooling has the effect of improving stress corrosion cracking resistance, it is necessary to cool at a cooling rate in excess of 5 ° C / min. Is useless.

Next, examples of the present invention will be described.

[0029]

【Example】

[Table 1] 5 in a 50 mm thick ingot of Al alloy having the chemical composition shown in
After soaking at 10 ° C. for 4 hours, hot rolling was performed at 500 to 300 ° C. to a plate thickness of 6 mm, and then cold rolling was performed to 3 mm. This Al alloy plate having a thickness of 3 mm was heated at an average rate of 0.7 ° C./min and then held at 340 ° C. for 2 hours to give 0.1 to 4
Finishing annealing was performed to cool at an average rate of ° C / min to obtain a test material.

With respect to the obtained test materials, the mechanical properties of the materials were examined, and the 50 mmφ ball head overhang height, stress corrosion cracking resistance and TIG weldability were also examined. The results are also shown in Table 1.

For the accelerated test of stress corrosion cracking, test material
(O material, plate thickness 3 mm) was subjected to a sensitization treatment. That is, a plate thickness of 3 mm (O material) is cold-rolled to a plate thickness of 2.1 mm, and the temperature is 120 ° C.
× Low temperature annealing was performed for 7 days. This test material is 2.1 mmt
After processing into a test piece of × 180 mmw × 15 mml and degreasing,
U-shaped bending was performed to apply stress to the test piece. afterwards,
The test piece was immersed in 3.5% NaCl and tested by an energization method. Here, the time from the start of the test to the occurrence of cracking was measured to evaluate the stress corrosion cracking resistance. The energization time is 720 minutes at maximum, and if no crack occurs in this maximum time, crack does not occur in practical use after 720 minutes Mg 3.5%
It was determined to be similar to the following Al-Mg alloys.

As is clear from Table 1, all of the examples of the present invention have excellent resistance to stress corrosion cracking, as well as excellent formability and strength. On the other hand, it is understood that the comparative examples having chemical components outside the scope of the present invention are inferior in each characteristic as compared with the inventive examples.

That is, Comparative Examples No. 8 and No. 9 are inferior in stress corrosion cracking resistance, and Nos. 10 to 12 are Mn, Cr, and
Since the amount of each Zr added is large, the moldability to be provided as a component material is significantly deteriorated. Further, No. 13 has good resistance to stress corrosion cracking due to the addition of Cu, but the weldability is significantly deteriorated because the Cu content is too large. No. 14 had a chemical composition within the range of the present invention, but had a low cold rolling workability, and thus was inferior in stress corrosion cracking resistance and roughened during overhanging work.

[0034]

As described in detail above, according to the present invention,
Since the addition of Cu and other components were adjusted in a proper balance, even when the cooling process of finish annealing was slow cooling, compared with the conventional Al-Mg alloy containing more than 3.5% Mg. It has excellent stress corrosion cracking resistance, and also has the formability and strength to be provided as a component member.

Claims (1)

[Claims] 1. In weight% (hereinafter the same), Mg: 3.5-
4.2%, Cu: 0.1 to 0.5%, Mn: 0.03 to 0.5
% As an essential element, and if necessary, Cr:
0.03 to 0.20% and Zr: 0.03 to 0.20% of 1
In producing an aluminum alloy plate containing Al or unavoidable impurities, the balance of Al and unavoidable impurities is 1 to 2 and is heated to a temperature of 400 to 550 ° C. as a soaking treatment after casting.
After holding for ~ 20 hours, hot rolling at a temperature of 300 ~ 520 ° C, then cold rolling with a working rate of 30% or more,
Furthermore, a method for producing an aluminum alloy sheet having excellent resistance to stress corrosion cracking, characterized in that finish annealing is performed by slow cooling at a cooling rate of 5 ° C./min or less.