JPH0733554B2 - Aluminum alloy rolled sheet for forming, which has excellent resistance to stress corrosion cracking, and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rolled aluminum alloy sheet for forming which is excellent in stress corrosion cracking resistance and a method for producing the same, and particularly to applications requiring high formability and high strength, such as automobile body sheets. The present invention relates to a rolled aluminum alloy plate for forming, which is suitable for various molded parts, chassis of electric equipment, other electric parts, articles, etc., and a manufacturing method thereof.

[0002]

2. Description of the Related Art In the past, cold-rolled steel sheets were mainly used as the body sheets of automobiles, but recently, due to the demand for weight reduction of vehicle bodies, the use of rolled aluminum alloy sheets has been studied. Car body seats
Since it is used after being press-molded, it is required to have excellent moldability, in particular to have excellent elongation and bulging properties, and it is also essential to have high strength. It is also important to have excellent stress corrosion cracking resistance.

By the way, there are various aluminum alloy sheets used for the purpose of molded products requiring strength, and the main ones are classified as follows depending on the alloy component system.

(A) 5052 alloy (Mg 2.2 to 2.8%, Cr 0.15) which is a non-heat treatment type Al-Mg alloy.
˜0.35%, balance Al and unavoidable impurities) O material or 5182 alloy (Mn 0.20 to 0.50)
%, Mg 1.0 to 5.0%, balance Al and unavoidable impurities) O material.

(B) Heat treatment type Al-Mg-Zn-Cu
T4 treated material of system alloy. Examples of aluminum alloys of this type include those disclosed in JP-A-52-141409, JP-A-53-103914, and JP-A-57-9.
There are alloys such as 8648. Furthermore, 63-72 of April 7, 1986 issue of Nikkei New Material (No. 8).
Al-4.5% Mg-introduced on page 64, especially on page 64
0.38% Cu-1.46% Zn-0.18% Fe-
There is also a 0.09% Si alloy.

(C) 6009 alloy (Mg 0.4 to 0.8%, Si0.
6-1.0%, Cu 0.15-0.6%, Mn 0.2-
T4 treated material with 0.8%, balance Al and unavoidable impurities, and the same 6010 alloy (Mg 0.6 to 1.0%, Si)
0.8-1.2%, Cu 0.15-0.6%, Mn0.
2-0.8%, balance Al and unavoidable impurities) T4
Processing material. These are described in detail in Japanese Examined Patent Publication No. 59-39499, and in addition to these, Japanese Examined Patent Publication No. 61-15
There is an AC120 alloy T4 treated material proposed in No. 148.

(D) Cu is added to the Al-Mg alloy,
Al-Mg-Cu based alloy which has been subjected to T4 treatment by rapid cooling (JP-A-62-27544 or JP-A-63-6).
9952).

[0008]

As described above, various aluminum alloys have been conventionally used for molding which require high strength. Among these applications, the body for automobiles is also used. As the sheet, from the viewpoint of formability, (a) Al-Mg based alloy and (d) Al-M.
g-Cu based alloy, or (b) Al-Mg-Zn-
Cu-based alloys are often used. However, these alloys have a problem that the stress corrosion cracking resistance is lowered if the Mg content is set to about 3 to 4% or more in order to improve strength and formability.

Although Al-Mg-Cu alloys have relatively good resistance to stress corrosion cracking among these alloys,
If it exceeds 4%, the stress corrosion cracking resistance is also deteriorated. Further, when Zn is added to the Al-Mg-based alloy, if the final annealing is performed by rapid cooling such as continuous annealing, the strength and formability can be obtained, but the stress corrosion cracking resistance is significantly reduced. If Zn is added to the Al-Mg-based alloy, the final annealing is performed by quenching, and the precipitation treatment is further performed, the stress corrosion cracking resistance of the base material is improved, but when used by welding, the welded part is used. Since the heat-affected zone is rapidly cooled, there arises a problem that the stress corrosion cracking resistance of the welded zone deteriorates.

As described above, the conventional aluminum alloy for forming process provides a material having excellent formability and strength, and at the same time, excellent stress corrosion cracking resistance not only in the base metal portion but also in the welded portion. It was difficult to do.

The present invention has been made in view of the above circumstances. As an aluminum alloy rolled plate for high-strength forming used for an automobile body sheet or the like, a base material portion and a welded portion can be obtained without impairing strength and formability. It is an object of the present invention to provide an aluminum alloy rolled sheet having markedly improved stress corrosion cracking resistance of a portion.

[0012]

In order to solve the above-mentioned problems, in the aluminum alloy rolled sheet for forming according to the present invention, basically, the composition of the alloy components is appropriately specified, and at the same time, the grain of the rolled sheet is The size of the precipitates in the boundary, in particular the precipitates derived from Mg and Zn, is appropriately adjusted, and thereby excellent moldability and strength as well as excellent stress corrosion cracking resistance are obtained. Further, in the manufacturing method of the present invention, the process conditions are set so that the size of the grain boundary precipitate is appropriately adjusted as described above.

Specifically, the rolled aluminum alloy sheet for forming according to the invention of claim 1 has a Mg content of 3.0 to 7.0%, Z
n 0.1 to 1.0% and Cu 0.1% or less,
Fe 0.4% or less and Si 0.2% or less, respectively, the balance consisting of Al and unavoidable impurities, and the average size of precipitates on the grain boundaries due to Mg or Zn is 0.2 to 4.0 μm. It is characterized by being within the range.

The rolled aluminum alloy plate for forming according to the invention of claim 2 is the rolled aluminum alloy plate of claim 1 in which, in addition to the above respective components, Mn0.0
1% or more of 3% or more and 0.8% or less, Cr 0.03% or more and 0.3% or less, Zr 0.03% or more and 0.2% or less, and V0.03% or more and 0.2% or less Is included.

Further, the rolled aluminum alloy plate for forming according to the invention of claim 3 is characterized in that, in the aluminum alloy plate of claim 1 or 2, the Cu content is restricted to 0.01% or less. is there.

Further, the rolled aluminum alloy plate for forming according to the invention of claim 4 is the aluminum alloy plate according to any one of claims 1 to 4, wherein the Zn content is 0.1 to 0.5.
It is within the range of%.

On the other hand, the manufacturing method of the rolled aluminum alloy plate for forming according to the fifth aspect of the present invention is Mg 3.0 to 7.0.
%, Zn 0.1-1.0%, and Cu 0.1%
Below, Fe is controlled to 0.4% or less and Si to 0.2% or less, respectively, and Mn 0.03% or more to 0.
8% or less, Cr 0.03% or more and 0.3% or less, Zr0.
After casting an alloy containing one or two or more of 03% or more and 0.2% or less and V0.03% or more and 0.2% or less, and the balance of Al and unavoidable impurities, a predetermined plate Rolled to a thickness and then as solution annealing, 280-5
Heat to a temperature in the range of 80 ° C within 24 hours and cool,
When the cooling rate of the solution annealing is 1 ° C./minute or less, the solution annealing remains as it is, and when it exceeds 1 ° C./minute, it is heated to a temperature in the range of 180 to 280 ° C. for 30 minutes to 24 hours. The final annealing is performed to obtain a rolled sheet having an average size of precipitates on the grain boundaries due to Mg or Zn in the range of 0.2 to 4.0 μm.

According to a sixth aspect of the present invention, there is provided a method for producing a rolled aluminum alloy sheet for forming according to the fifth aspect, wherein the solution annealing is performed by using a continuous annealing furnace.
After heating to a temperature in the range of 00 to 580 ° C. without holding or holding for 5 minutes or less, it is cooled at a cooling rate of 1 ° C./sec or more, and then as the final annealing, a temperature in the range of 180 to 280 ° C. It is characterized by heating for 30 minutes to 24 hours.

Further, the manufacturing method of the aluminum alloy rolled plate for forming according to the invention of claim 7 is the method of manufacturing according to claim 5, wherein the solution annealing is performed by using a batch type solution hardening quenching furnace. It is characterized by heating to a temperature in the range of up to 500 ° C., holding for 30 minutes or more and within 24 hours, and then cooling at a cooling rate of 1 ° C./minute or less.

The method of manufacturing an aluminum alloy rolled sheet for forming according to the invention of claim 8 is the method of manufacturing according to any one of claims 5 to 7, wherein C of the alloy is C.
It is characterized in that the amount of u is regulated to 0.01% or less.

Furthermore, the manufacturing method of the aluminum alloy rolled sheet for forming according to the invention of claim 9 is the manufacturing method according to any one of claims 5 to 8, wherein the Zn content is 0.1 to 0.5.
It is characterized by being set within the range of%.

[0022]

First, the reasons for limiting the component composition in the present invention will be explained.

Mg: Mg is an alloy component which is the basis of the present invention, and contributes to improvement of strength and formability, particularly elongation and bulging property. If the amount of Mg is less than 3.0%, the strength is low and it is unsuitable for structural purposes such as automobile body sheets. On the other hand, if it exceeds 7.0%, the rolling property deteriorates and the production becomes difficult. Therefore, the amount of Mg is 3.0 to 7.0%
Within the range of. Here, when the amount of Mg is 3.0% or more, the β phase (Mg
_The precipitate mainly composed of ₂ Al ₃ ) precipitates at the grain boundary and causes stress corrosion cracking. However, in the present invention, as will be described later, by appropriately adjusting the size of the grain boundary precipitate,
Excellent stress corrosion cracking resistance can be obtained even when the amount of Mg is 3.0 to 7.0%.

Zn: Zn has a great influence on the stress corrosion cracking resistance. That is, in general, Zn promotes precipitation of Zn-based precipitates and β-phases, and Zn-based precipitates and β-phases are continuously precipitated at grain boundaries at the operating temperature of the molded product after molding, and Zn Concentration at grain boundaries accelerates the risk of stress corrosion cracking. On the other hand, by appropriately controlling the precipitation state at the stage of the rolled plate beforehand, Zn-based precipitates and β phase can be continuously precipitated at the grain boundaries at the operating temperature of the molded product after molding. If it suppresses, stress corrosion cracking resistance will improve conversely. However, since there is a thermal effect at the time of welding in the welded portion, it is difficult to improve the stress corrosion cracking resistance depending on the Zn content only by appropriately controlling the precipitation state in the stage of the rolled plate as described above. May be. That is, in the alloy containing Zn,
As described above, even if the precipitation state is properly adjusted at the rolling plate stage, Zn is re-dissolved in the heat-affected zone during welding and is solidified by rapid cooling after welding.
Depending on the amount of n, subsequent precipitation at a use temperature may be promoted, and the stress corrosion cracking resistance may be deteriorated.
If the Zn content is less than 0.1%, the effect of improving the stress corrosion cracking resistance by controlling the precipitates is insufficient, and if the Zn content exceeds 1.0%, it may occur in the rolling plate stage as described above. Even if the precipitation state is properly controlled, the stress corrosion cracking resistance of the welded portion will decrease due to the thermal effect during welding. Therefore, Zn is set within the range of 0.1 to 1.0%. In order to further improve the stress corrosion cracking resistance of the welded portion, Zn is preferably 0.5% or less (0.1% or more).

Cu: Cu is an element effective for improving strength, but in the present invention, its upper limit is regulated as a harmful element. That is, in the manufacturing method of the present invention, the cooling after solution annealing is performed at a slow cooling rate of 1 ° C./minute or less, or the final annealing (precipitation treatment) is further performed at a temperature of 180 to 280 ° C. While properly controlled conditions, if Cu is mixed into Al-Mg-Zn based alloy, CuMgAl ₂ (S-phase) at the time of annealing time or final annealing after solution annealing or AlCuMgZn
Phases precipitate and age hardening occurs, which increases strength and reduces moldability. Therefore, in the present invention, in order to obtain excellent stress corrosion cracking resistance by appropriate structure control and to secure good formability, C
It is necessary to reduce the u amount as much as possible. Cu amount is 0.1
If the content is less than 0.1%, the above object can be achieved, so the Cu content is limited to 0.1% or less. In addition, the Cu amount was set to 0.
If the content is regulated to 01% or less, better moldability can be obtained.

Fe: Fe contributes to the refinement of crystal grains and to the improvement of strength, but if it exceeds 0.4%, the formability deteriorates, so the Fe content was made 0.4% or less.

Si: Si is an element that adversely affects the formability, but if it is 0.2% or less, it has almost no effect, so Si is set to 0.2% or less.

Mn, Cr, Zr, V: These elements have the effect of improving the strength through the refinement of crystal grains, and one or more of them are added if necessary. Mn is 0.03
% Or less, Cr is 0.03% or less, Zr is 0.03% or less, and V is 0.03% or less, the above effect cannot be obtained, while Mn is 0.8% and Cr is 0.3%. Zr is 0.2
%, If V exceeds 0.2%, a coarse intermetallic compound is generated and the formability is impaired. Therefore, when these elements are added, the addition amount of Mn is 0.03 to 0.8%. , Cr is 0.03 to 0.3%, Zr is 0.03 to 0.2%, and V is 0.03 to 0.2%.

In addition to the above elements, basically Al and inevitable impurities may be used. However, in an ordinary aluminum alloy, a minute amount of Ti, or Ti and B may be added for refining the ingot structure, and the rolled aluminum alloy sheet of the present invention also contains a small amount of Ti, or Ti and B. It may be done. In that case, T
If i exceeds 0.15%, primary TiAl ₃ crystallizes and impairs formability. Therefore, Ti is preferably 0.15% or less. When B is added together with Ti, B is 5
If it exceeds 00 ppm, coarse particles of TiB ₂ are mixed and impair the formability, so the amount of B is preferably 500 ppm or less. Further, in general, a small amount of Be may be added to an aluminum alloy containing Mg, but the aluminum alloy of the present invention may also contain a small amount of Be. Be suppresses the oxidation of the molten metal particularly when melting an alloy containing Mg, and contributes to prevent oxide particles from being contained as impurities in the material. However, 500
Even if Be is added in excess of ppm, the effect is saturated and the economy is impaired. Therefore, Be is preferably set to 500 ppm or less.

In the rolled aluminum alloy plate of the present invention,
Not only the composition of the components is determined as described above, but also the microstructure state of the rolled plate, that is, the microstructure in the stage before the forming process such as press working, particularly the precipitation state of precipitates is important. In other words, in order to prevent the formation of new precipitates that may adversely affect the stress corrosion cracking resistance at the operating temperature of the molded product after molding, preliminarily precipitate them in a pre-deposited state and use them. Even if new precipitation occurs at temperature, the precipitation state of the precipitate should be adjusted in advance so that it does not form a form that adversely affects the stress corrosion cracking resistance, that is, a form that continues on the grain boundaries. It needs to be controlled. Specifically, the average size of precipitates on the crystal grain boundaries due to Mg or Zn is 0.2 μm or more and 4.0 μm.
Must be: In addition, as the precipitate on the grain boundary due to Mg or Zn in the alloy of the composition targeted by the present invention, the T phase (Mg ₃ Zn ₃ A
l ₂ ) and β phase (Mg ₂ Al ₃ ) are typical, and various other types are possible, but the amount of precipitates other than the T phase and β phase is small. In addition to the precipitates caused by Mg or Zn, other precipitates may be slightly precipitated, but they do not significantly affect the stress corrosion cracking.

Here, when the average size of precipitates on the grain boundaries due to Mg or Zn is less than 0.2 μm, M
Since precipitation of g and Zn is insufficient, new precipitation is likely to occur at the operating temperature of the molded product after the molding process, and the size of the precipitate existing from the stage of the rolled plate is small, There are few surrounding non-precipitation regions, and in the end, new precipitation at the operating temperature does not use the original precipitate as a starting point, and new fine precipitates are continuously formed. Such continuous precipitates have a significant adverse effect on stress corrosion cracking resistance. On the other hand, if the average size of the precipitates on the grain boundaries due to Mg or Zn is 0.2 μm or more, Mg and Zn have already been precipitated in a considerable amount, so the operating temperature of the molded product after molding is There is little new precipitation below, and the new precipitation at the operating temperature mainly takes the form of growth of existing precipitates, so there is a precipitate-free region in the periphery, and this precipitate-free region is the precipitate. , The size of the newly-deposited fine-precipitated portion is also blocked by this non-precipitated region, so that it becomes isolated and discontinuous. If the precipitation is discontinuous in this way, even if stress corrosion cracking occurs, the crack will be interrupted at the discontinuous part (non-precipitated part), the progress of the crack will be prevented, and the stress corrosion cracking resistance will be improved. Remarkably good.
If the size of the precipitates on the grain boundaries exceeds 4.0 μm, the formability will deteriorate. Therefore, in order to improve the stress corrosion cracking resistance while ensuring good formability, the average size of precipitates on the grain boundaries due to Mg and Zn in the rolling plate stage is 0.2 to 4. It is necessary to set it within the range of 0 μm.

Next, a method for manufacturing the rolled aluminum alloy plate for forming according to the present invention will be described.

An important point in the manufacturing method of the present invention is that
Solution annealing after rolling to a predetermined plate thickness, or further subsequent annealing (precipitation treatment), the casting and rolling steps may be similar to the conventional general method,
The preferred process will be described first.

First, the molten metal having the alloy composition as described above is semi-continuously cast (DC casting) into an ingot having a rectangular cross section.
To do. The casting speed in this case is not specified, but is usually 2
Often cast at speeds in the range of 5 mm / min to 250 mm / min. The resulting ingot is 4 prior to hot rolling.
Heat to a temperature in the range of 00-560 ° C. for 1-48 hours. The purpose of this ingot heating is to eliminate the unevenness of the ingot and improve the formability, and the heating temperature is 400 ° C.
If the heating temperature is less than 1 hour, or if the heating time is less than 1 hour, the degree of homogenization is insufficient. On the other hand, if the heating temperature exceeds 560 ° C., eutectic melting will occur, and if the heating time exceeds 48 hours, the economy will decrease. Instead of the semi-continuous casting, a thin plate continuous casting method (continuous casting rolling method) in which a molten metal is continuously supplied between a pair of cooling rolls to continuously cast a thin plate may be applied. In this case, there is no limitation on the casting speed, and normally, it is produced only by cold rolling without hot rolling, but before cold rolling, the purpose is to promote homogenization and improve moldability. so,
Preheating at 300 to 560 ° C. for 1 to 48 hours is desirable.

If necessary, the aluminum alloy sheet hot-rolled as described above is subjected to hot-rolling and then cold-rolling to a sheet thickness of about 6 to 0.5 mm. In this case, if intermediate annealing is performed during the cold rolling or between the hot rolling and the cold rolling, the formability is further improved. That is, when coarse crystal grains are generated during hot rolling, if they are subjected to cold rolling as they are, waviness called ridging or a flow line is generated during the molding process, which may impair the appearance of the molded product. In order to solve this, intermediate annealing may be performed to cause recrystallization once. When this intermediate annealing is performed in a batch-type annealing furnace, recrystallization does not occur at an intermediate annealing temperature of less than 250 ° C, while coarsening of crystal grains tends to occur if the intermediate annealing temperature exceeds 450 ° C. Intermediate annealing time is 1 to 4
8 hours is appropriate, and if it is shorter than that, the effect of intermediate annealing becomes insufficient, and if it is longer than that, the effect saturates.
Impair economics. Further, a continuous annealing furnace may be used for the intermediate annealing while continuously rewinding the coil. When the continuous annealing furnace is used as described above, the annealing temperature is 400 to 58.
A temperature of 0 ° C. is suitable, and an annealing time of 5 minutes at the longest is sufficient since there is no holding after reaching the above temperature.

After rolling to a predetermined plate thickness as described above, solution annealing is performed. That is, the stage after the hot rolling, or the stage after the hot rolling and then the cold rolling, or the stage after the final cold rolling after the intermediate annealing when performing the intermediate annealing. Then, solution annealing is performed.
This solution annealing is intended to cause recrystallization, and at the same time, to solution-solubilize the soluble element.
It suffices to maintain the temperature within the range of 80 ° C. or less for a maximum of 24 hours for cooling. Here, if the solution annealing temperature is lower than 280 ° C, recrystallization does not occur, whereas if the temperature is higher than 580 ° C, the crystal grains become coarse and the formability deteriorates. Moreover, even if the holding time exceeds 24 hours, the effect of solution annealing is saturated and is economically wasted. Further, since the cooling rate of this solution annealing has a great influence on the precipitation state, the necessity of the subsequent final annealing (precipitation treatment) is determined by the cooling rate.

That is, the cooling rate after solution annealing is 1 ° C.
In the case of less than 1 minute / minute, Zn-based precipitates and β-phase are precipitated at the grain boundaries with an appropriate size of 0.2 to 4.0 μm as described above during cooling, so that good stress corrosion resistance is maintained. Crackability is obtained. Such solution annealing at a cooling rate of 1 ° C./min or less corresponds to annealing using a batch type annealing furnace such as an air furnace or a salt bath. In the case of batch type solution annealing, a holding temperature of 300 to 500 ° C. is appropriate, and a holding time of 30 minutes to 24 hours may be used.

On the other hand, when the cooling rate after solution annealing exceeds 1 ° C./min, Zn solid-dissolved by heating hardly precipitates during the cooling process, and a solution-rolled sheet is obtained. To be Therefore, if it is left as it is, precipitates are continuously deposited at a use temperature after forming such as press working, and the stress corrosion cracking resistance is lowered. Therefore, when the cooling rate after solution annealing is set to a rate exceeding 1 ° C./minute, a final annealing as a precipitation treatment is performed again thereafter.
The solution annealing at a cooling rate exceeding 1 ° C./minute as described above can be achieved by annealing using a continuous annealing furnace. For solution annealing using a continuous annealing furnace, it is suitable to hold at 400 to 580 ° C. or not to hold it for 5 minutes or less. In this case, it is usually 1 ° C./force by forced air cooling, mist cooling, water cooling, etc.
A cooling rate of more than a second can be easily obtained.

After the solution annealing has been performed at a cooling rate exceeding 1 ° C./minute as described above, a final annealing is then performed as a precipitation treatment for appropriately precipitating Zn at the grain boundaries. This final anneal heats to a temperature in the range of 180-280 ° C for 30 minutes to 24 hours. This temperature is 18
If the temperature is lower than 0 ° C., the size of the precipitates due to Mg or Zn on the crystal grain boundaries, that is, the size of the precipitates typically composed of β phase or T phase is less than 0.2 μm on average, Insufficient improvement in stress corrosion cracking resistance. 28 on the other hand
If the temperature exceeds 0 ° C., these precipitates on the crystal grain boundaries are redissolved, which makes the purpose of the final annealing meaningless and the stress corrosion cracking resistance deteriorates. Further, if the heating time is less than 30 minutes, precipitation will not be sufficient, and if the heating time is 24 hours or more, the effect will be saturated and the economy will be impaired.

As described above, the component composition of the alloy, especially Zn
The amount of the precipitates on the crystal grain boundaries due to Mg or Zn in the state of the final plate (rolled plate in a stage before being subjected to forming) is 0.2 to 4.0 on average while appropriately adjusting the amount.
By adjusting the thickness to be μm, it is possible to obtain a plate having excellent resistance to stress corrosion cracking in the base material portion and the welded portion without impairing formability and strength.

[0041]

【Example】

Example 1: Nos. Shown in Table 1 1-No. Alloy 7 was cast by the semi-continuous casting method. Ingot size is 500 mm x 120
It was 0 mm × 300 mm, and the casting speed was 65 mm / min. Here, No. Alloy 7 has a Mg content of 3.0%
It is a reference alloy with good stress corrosion cracking resistance of less than less than this. The reference alloy of No. 7 is used as a standard for stress corrosion cracking resistance evaluation. After each of the ingots obtained as described above was chamfered, it was heated at 500 ° C. for 2 hours as a homogenizing treatment, and hot rolling was started as it was. This hot rolling gives a plate thickness of 5 mm, followed by cold rolling to give a plate thickness of 1 mm.
And After that, the manufacturing process number No. 1-No.
Under each of the conditions shown in FIG. 9, solution annealing was performed, and in some cases final annealing (precipitation treatment) was performed. In addition, each annealing was implemented using the salt bath.

For each plate after the final annealing, a tensile test was conducted to examine various mechanical properties and formability.
Further, a stress corrosion cracking test was conducted on the base metal part and the welded part. For moldability, the Erichsen value and the ball head overhang test value were examined. The Erichsen test is based on JIS-
It is performed by method B, and the ball bulge test is 10
Using a 0 mmφ ball head punch, the test piece was attached with a vinyl chloride film. On the other hand, before the stress corrosion cracking test for the base material, as a sensitizing treatment, 30% cold working was performed in advance, and then 120 ° C. × 1 week annealing was performed. It is said that this sensitization treatment corresponds to a change with time of 10 to 20 years at room temperature. For the stress corrosion resistance test of the welded portion, bead-on welding was performed after 30% cold working, and further sensitizing treatment was performed by annealing at 120 ° C. for 1 hour. After such a sensitization treatment, a stress corrosion cracking resistance test was performed by a corrosion acceleration method by applying an electric current. Specifically, while adding 13 kg / mm ² as additional stress,
The additional current of 5 mA / cm ² was added, the 25 ° C. 3.5% NaC
The stress corrosion cracking resistance test was carried out in an aqueous solution. It is said that the accelerated corrosion test by applying such an electric current well reflects the tendency of stress corrosion cracking in an actual natural environment. The value of the additional stress of 13 kg / mm ² is about 50% of the proof stress of the base metal,
It corresponds to about 90% of the yield strength of the weld. The results of these tests are shown in Table 3.

Furthermore, the average size of the precipitates on the grain boundaries of each of the plates after the final annealing as described above was examined by using a transmission electron microscope. The results are shown in Table 4. In addition, Table 4 also shows comprehensive evaluations of strength, formability, and stress corrosion cracking resistance found from the test results shown in Table 3.
For the stress corrosion cracking resistance evaluation, reference alloy No. with Mg content of less than 3.0 wt% was used. On the basis of 7, the case where it is equal to or better than that is marked with a circle, the case where it is slightly inferior, the case where it is slightly inferior, and the case where it is markedly inferior are marked with x.

[0044]

[Table 1]

[0045]

[Table 2]

[0046]

[Table 3]

[0047]

[Table 4]

As is apparent from Table 4, for alloys within the compositional range of the present invention, the average size of precipitates on grain boundaries was 0.2 by applying the process of the present invention.
Rolled plate within the range of ~ 2 μm (manufacturing process No. 1,
No. 4, No. In 5), the stress corrosion cracking resistance is excellent in both the base material portion and the welded portion, and at the same time, the strength and formability are excellent. On the other hand, the component composition is within the range specified in the present invention, but the average size of precipitates on the grain boundaries is 0.2 μm.
Less than or not precipitated (manufacturing process No.
2, No. In 3), the stress corrosion cracking resistance was extremely poor. Furthermore, when the comparative alloy (alloy No. 4) having a Cu content of more than 0.1 wt% was used (manufacturing process No. 6), the formability was poor, and the Zn content was less than 0.1%. When using a small amount of comparative alloy (alloy No. 5) (Manufacturing process N
o. In 7), even if the manufacturing process of the present invention was applied, the average size of precipitates on the grain boundaries was less than 0.2 μm, and sufficient stress corrosion cracking resistance could not be obtained. In addition, a comparative alloy having a Zn content exceeding 1.0 wt% (alloy No.
When 6) was used (manufacturing process No. 8), the formability and the stress corrosion cracking resistance of the base material were excellent, but the stress corrosion cracking resistance of the welded portion was poor. The amount of Mg is 3.0 wt
%, The reference alloy had good resistance to stress corrosion cracking and formability, but had insufficient strength.

Example 2 The alloys indicated by alloy codes A and B in Table 5 were cast by semi-continuous casting. Ingot size is 500
mm × 1200mm × 300mm, casting speed is 65mm /
It was min. After this ingot was faced, it was heated at 500 ° C. for 2 hours and hot rolling was started as it was. The plate thickness was 5 mm by this hot rolling, and the plate thickness was 1 mm by further cold rolling.
The obtained cold-rolled sheet was divided into two, solution-annealed by different methods as shown in Table 6, further divided, and finally annealed (precipitation treatment) under different conditions. Ie 2
One of the divided cold-rolled sheets was 500 ° C using a continuous annealing furnace,
Annealing was performed without holding, and the other was subjected to batch annealing at 340 ° C. for 6 hours. The cooling rate of continuous annealing is 30 ° C / sec (1
800 ° C / min), batch annealing at 20 ° C / hr (0.33
C / min). The continuous annealed plate is further divided into two,
One of them was not subjected to final annealing (precipitation treatment), and the other was subjected to final annealing (precipitation treatment) at 240 ° C. for 2 hours. Final annealing (precipitation treatment) was not performed on the batch annealed plate.

Then, each plate was subjected to a tensile test, and the formability and the stress corrosion cracking resistance of the base material portion and the welded portion were examined. For moldability, the Erichsen value and the ball head protrusion value were examined in the same manner as in Example 1. As for the stress corrosion cracking resistance of the base metal portion, a loop bending test piece was prepared by subjecting to a sensitizing treatment in advance as in Example 1, 30% cold working, and annealing at 120 ° C. for 1 week. 3.5% NaC
A stress corrosion test was carried out in which the solution was alternately dipped in an aqueous solution of n = 5 for 3 months to examine the occurrence of cracks. As for the stress corrosion cracking resistance of the welded portion, bead-on welding was performed after 30% cold working as in Example 1, and then 120 ° C. as a sensitizing treatment.
It was annealed for 1 week, a loop bending test piece was prepared centering on the welded portion, and alternately immersed for 3 months in a 3.5% NaCl aqueous solution at n = 5 to examine whether cracking occurred. The average size of the precipitates on the grain boundaries of each rolled plate was examined using a transmission electron microscope. The results are shown in Table 7.

[0051]

[Table 5]

[0052]

[Table 6]

[0053]

[Table 7]

As is apparent from Table 7, when the invention alloy A is applied with the comparative method B in which the final annealing is not carried out while the rapid cooling rate of the solution annealing is 30 ° C./sec, the formability is Although good, the stress corrosion cracking resistance of the base material was poor. Further, when Comparative Method B was applied to Comparative Alloy B having a Zn content of more than 1.0%, the formability was good, but the stress corrosion cracking resistance of the base material was poor. Further, regarding Comparative Alloy B, when Invention Method B was applied, the formability and the stress corrosion cracking resistance of the base material were good, but the stress corrosion cracking resistance of the welded portion was inferior. When Inventive Method C was applied, the stress corrosion cracking resistance of the base material was good, but the formability and the stress corrosion cracking resistance of the weld were poor.

Only when the invention method A or the invention method C was applied to the invention alloy A, the formability, the stress corrosion cracking resistance of the base material, and the stress corrosion cracking resistance of the welded portion were all excellent.

[0056]

As is apparent from the above-described embodiments, the rolled aluminum alloy sheet for forming according to the present invention has a high Mg content of 3.0 wt% or more in order to obtain high strength. By properly adjusting other alloy components, especially Zn amount and Cu amount, and adjusting the average size of the grain boundary precipitates derived from Mg or Zn to 0.2 to 4.0 μm, the formability is impaired. It is possible to remarkably improve the stress corrosion cracking resistance of not only the base metal portion but also the welded portion. Therefore, both strength and formability and stress corrosion cracking resistance of the base material portion and the welded portion are excellent. As an aluminum alloy rolled plate for forming and processing, it is most suitable for automobile body sheets and others. Further, according to the manufacturing method of the present invention, it is possible to reliably and easily obtain an appropriate precipitation state of the precipitates on the grain boundaries as described above, and as described above, the strength, the formability, and the resistance of the base metal portion and the welded portion. It is possible to actually manufacture a rolled plate for forming processing, which has both excellent stress corrosion cracking properties, on a mass production scale.

Claims (9)

[Claims] 1. Mg3.0-7.0% (wt%, the same applies hereinafter), Zn0.1-1.0%, and Cu0.1
%, Fe 0.4% or less, and Si 0.2% or less, with the balance being Al and unavoidable impurities, and the average size of precipitates on the grain boundaries due to Mg or Zn being 0.2 to 4 An aluminum alloy rolled plate for forming, which is excellent in stress corrosion cracking resistance, characterized by being in the range of 0.0 μm. 2. In addition to the above respective components, Mn0.03 is further added.
% Or more and 0.8% or less, Cr 0.03% or more and 0.3% or less, Zr 0.03% or more and 0.2% or less, and V0.03% or more and 0.2% or less An aluminum alloy rolled plate for forming, which contains the excellent stress corrosion cracking resistance according to claim 1. 3. The content of Cu in each of the components is 0.01%.
An aluminum alloy rolled sheet for forming, which is excellent in stress corrosion cracking resistance according to claim 1 or 2 and is regulated below. 4. The Zn content of each of the components is 0.1 to 0.1.
The aluminum alloy rolled plate for forming, which has excellent stress corrosion cracking resistance according to any one of claims 1 to 3, with a content of 0.5%. 5. Mg3.0-7.0%, Zn0.1-
Contains 1.0%, Cu 0.1% or less, Fe 0.4
%, Si 0.2% or less, and Mn 0.03% or more and 0.8% or less, Cr as required.
0.03% to 0.3%, Zr 0.03% to 0.
After casting an alloy containing 2% or less and one or two or more of V0.03% or more and 0.2% or less and the balance consisting of Al and unavoidable impurities, rolled to a predetermined plate thickness, After that, as solution annealing, it is heated to a temperature in the range of 280 to 580 ° C. within 24 hours and cooled, and when the cooling rate of the solution annealing is 1 ° C./minute or less, the solution annealing remains as it is, and 180 ~ 2 after 1 ℃ / min
Final annealing is carried out by heating to a temperature in the range of 80 ° C. for 30 minutes to 24 hours, and a rolled plate having an average size of precipitates on the grain boundaries due to Mg or Zn in the range of 0.2 to 4.0 μm is obtained. A method for producing an aluminum alloy rolled sheet for forming, which is excellent in stress corrosion cracking resistance, which is obtained. 6. As the solution annealing, a continuous annealing furnace is used to heat to a temperature in the range of 400 to 580 ° C. without holding or for 5 minutes or less, and at a cooling rate of 1 ° C./second or more. After cooling, the final annealing is 180-28.
The method for producing an aluminum alloy rolled sheet for forming having excellent stress corrosion cracking resistance according to claim 5, wherein heating is performed at a temperature in the range of 0 ° C for 30 minutes to 24 hours. 7. The solution annealing is carried out by heating to a temperature in the range of 300 to 500 ° C. using a batch type furnace, and
The method for producing a rolled aluminum alloy sheet for forming having excellent stress corrosion cracking resistance according to claim 5, which is characterized by cooling at a cooling rate of 1 ° C./minute or less after holding for 0 minute or more and 24 hours or less. . 8. The production of a rolled aluminum alloy plate for forming, which is excellent in stress corrosion cracking resistance according to any one of claims 5 to 7, wherein the Cu content of the alloy is regulated to 0.01% or less. Method. 9. The aluminum for forming having excellent stress corrosion cracking resistance according to claim 5, wherein the amount of Zn in the alloy is in the range of 0.1 to 0.5%. Method for manufacturing rolled alloy sheet.