JP2001335874A - Aluminum alloy sheet for structure excellent in strength and corrosion resistance and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an aluminum alloy sheet for structure excellent in strength and corrosion resistance, particularly in stress corrosion cracking resistance, and to provide its production method. SOLUTION: This aluminum alloy sheet has a composition containing 4.8 to 7% Zn, 1 to 3% Mg, 1 to 2.5% Cu and 0.05 to 0.25% Zr, and the balance Al with impurities, and, in the sheet face, a structure containing grain boundaries in which the crystal orientation difference is 3 to 10 deg. are contained by >=25% is provided. It is characterized that repeated rolling is performed in the temperature range of 400 to 150 deg.C so as to control the working degree to >=70%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aluminum alloy sheet having excellent strength and corrosion resistance, and more particularly to an aluminum alloy sheet having excellent strength and corrosion resistance suitably used for aircraft and vehicles, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, a method of manufacturing a stringer material for an aircraft has been proposed as an example of an aluminum alloy sheet for a structure, particularly an aluminum alloy sheet for an aircraft (Japanese Patent No. 13-135).
Nos. 37646 to 1337649, Patent No. 1339
927, Japanese Patent No. 1405136).

[0003] As a specific manufacturing method, for example, JI
The ingot of SA7075 alloy is heated at a temperature of around 450 ° C.
After homogenizing for 20 hours, hot rolling is started at a temperature of 400 to 450 ° C. to obtain a sheet having a thickness of about 6 mm, and then an intermediate heat treatment is performed at about 410 ° C. for about 1 hour.
Cold-rolled in a temperature range of 100 ° C. or less to obtain a cold-rolled sheet having a thickness of 3 to 4 mm.
Solution treatment by rapid heating to a temperature of 120 ° C.
A predetermined strength is obtained by performing aging treatment for several hours to about 24 hours at a temperature around ℃.

[0004] In the above process, in the aging treatment process,
Precipitation hardening can be achieved without causing a change in crystal grain size, and the obtained plate material has an average crystal grain size of 25 μm or less, and has practically sufficient characteristics in strength and formability. . However, in terms of corrosion resistance, especially stress corrosion cracking resistance, even if it is judged that the corrosion resistance evaluation at the laboratory level is sufficient, it may not always be sufficient in terms of stress corrosion cracking resistance under actual use environment, Further improvement of corrosion resistance is required.

It is well known that it is preferable to reduce the crystal grain size in terms of the mechanical strength and formability of a metal material. However, in terms of corrosion resistance, reducing the crystal grain size rather deteriorates the corrosion resistance. It has also been reported that the inventors conducted experiments and studies from various viewpoints on the relationship between grain refinement and stress corrosion cracking resistance in a 7000 series aluminum alloy containing Zn and Mg,
In the process, it was found that the misorientation of adjacent crystal grains affects the stress corrosion cracking resistance.

[0006] The azimuth difference between adjacent crystal grains indicates, as shown in FIG. 1, how much angle difference (azimuth difference θ) exists with respect to a rotation axis common to crystal grains 1 and 2. Things. As a result of investigating the crystal grains after solution treatment in the production of the aircraft stringer material, the misorientation was 20 °.
It was found that the above-described large-angle grain boundaries were formed. In this case, in the subsequent aging treatment, the segregation of the second phase compound at the grain boundaries increases, and the electrochemical properties of the intragranular and the grain boundaries are different, so that the corrosion resistance is reduced.

[0007]

DISCLOSURE OF THE INVENTION The present invention has been made on the basis of the above findings, and has as its object to solve the conventional problems in structural aluminum alloy sheets, to have excellent strength properties, to have corrosion resistance, In particular, it is an object of the present invention to provide a structural aluminum alloy sheet with further improved stress corrosion cracking resistance and a method of manufacturing the same. The structure using the aluminum alloy plate can reduce costs and improve reliability.

[0008]

According to the first aspect of the present invention, there is provided a structural aluminum alloy plate comprising: Zn: 4.8 to 7%, Mg: 1 to 3%, Cu: 1 ~
An aluminum alloy sheet containing 2.5% and Zr: 0.05 to 0.25% and having a composition consisting of the balance of Al and impurities, wherein the crystal orientation difference on the plate surface of the aluminum alloy sheet is 3 to 10%. It is characterized by having a structure containing 25% or more of crystal grain boundaries of °.

The aluminum alloy sheet having excellent strength and corrosion resistance according to claim 2 is characterized in that in claim 1, the average crystal grain size as viewed from the plate surface is 10 μm or less.

According to a third aspect of the present invention, there is provided a method for producing an aluminum alloy sheet having excellent strength and corrosion resistance, comprising: subjecting an ingot of an aluminum alloy having the composition described in the first aspect to a homogenizing treatment; In a temperature range of 400 to 150 ° C., the sheet is repeatedly rolled so that the degree of work becomes 70% or more to obtain a predetermined thickness, and then subjected to a solution treatment at a temperature of 450 to 490 ° C. for 5 minutes or more. It is characterized by cooling at a cooling rate of at least seconds.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is characterized in that high strength and high corrosion resistance are obtained by optimally combining the alloy composition of 7000 series aluminum alloy and the crystal orientation difference. Explaining the significance of the components and the reasons for limitation, Zn is Zn-M during aging treatment.
It functions to generate g-based fine precipitation and improve the material strength by precipitation hardening. The preferred content of Zn is 4.8.
If it is less than 4.8%, the JI of the conventional alloy is used.
The strength equivalent to SA7075 alloy or A7475 alloy cannot be obtained. If the content exceeds 7%, the hot workability deteriorates, and problems such as cracks occur. Zn also has the effect of suppressing crystal grain growth during the solution treatment. The more preferable content range of Zn is 5.0 to 6.5%.

Mg, like Zn, is an element that contributes to improvement in strength, and the preferable content of Mg is in the range of 1 to 3%. If it is less than 1%, it is difficult to obtain a strength equivalent to that of a conventional alloy, and if it exceeds 3%, hot workability deteriorates and problems such as cracks occur.

[0013] Cu functions to increase the material strength by precipitation hardening of the Al-Cu-Mg compound during aging treatment. The preferred content of Cu is 1 to
If it is less than 1%, it is difficult to obtain a strength equivalent to that of the conventional alloy, and if it exceeds 2.5%, the hot workability is lowered and problems such as cracks occur.

Zr is an element that suppresses the growth of crystal grains during the solution treatment, and as a result, has the effect of retaining many small-angle grain boundaries. The preferable content of Zr is 0.05 to 0.2.
If it is less than 0.05%, the effect is small, and if it exceeds 0.25%, coarse Al-Zr
Formability of the final sheet is reduced by forming a system compound. 0.2
Even if the content exceeds 5%, the effect of suppressing the growth of crystal grains during the solution treatment is saturated, and no further suppressing effect can be obtained. A more preferable content range of Zr is 0.08 to 0.2.
0%.

In the present invention, the amounts of Mn, Cr, and T generally contained in the 7000 series aluminum alloy are generally small.
i, B, Fe, Si can be contained.
From the viewpoint of formability, Si is preferably limited to 0.5% or less, and Cr is also preferably limited to 0.05% or less.

In the combination with the above composition, it has been found that when the misorientation is 10 ° or less, the segregation at the grain boundaries after aging treatment is reduced. Therefore, it was found that the grain boundary area increased, the degree of grain boundary segregation decreased, and the corrosion resistance improved. After further study, 0.02
mm ² or more ranges results of the examination of the distribution of misorientation, 3
It was recognized that when the small-angle grain boundaries of 10 ° were 25% or more of the total grain boundaries, the stress corrosion cracking resistance was significantly improved.

In a structure in which the small-angle grain boundaries having a misorientation of 3 to 10 ° account for 25% or more of all the grain boundaries, the average crystal grain size becomes fine. However, if the average crystal grain size exceeds 10 μm, the stress corrosion cracking resistance becomes poor, and the material strength also decreases. Therefore, the average crystal grain size is preferably 10 μm or less.

The azimuth difference (mis-orientation) is measured by using an automatic measuring device comprising a combination of a scanning electron microscope (SEM) and a CCD camera. This device captures the Kikuchi pattern into a CCD camera by irradiating an electron beam onto the crystal plane that appears on the sample surface, and specifies the crystal plane using a computer. This means that a common rotation axis can be specified, and an angle difference (azimuth difference = misorientation) with respect to the rotation axis can be determined. In the present invention, the lower limit of the azimuth difference is set to 3 ° in consideration of the resolution and error of the measuring device.
And

Hereinafter, a method of manufacturing a structural aluminum alloy sheet according to the present invention will be described. An aluminum alloy having the above composition is formed by, for example, ordinary DC casting, and the obtained ingot is subjected to a homogenization treatment and then hot worked according to a conventional method. The intermediate heat treatment after hot working may be performed according to a conventional method, but may be omitted.

The characteristics of the present invention are as follows:
In a temperature range of ℃, more preferably in a temperature range of 350 to 180 ° C., rolling is repeatedly performed so that the working ratio becomes 70% or more. By repeated rolling in a specific temperature range, a lower structure capable of suppressing crystal grain growth during the subsequent solution treatment can be formed. If the working ratio is less than 70%, fine precipitation of Zr becomes insufficient, and it becomes difficult to suppress the crystal grain growth during the solution treatment. When rolling is repeatedly started at a temperature exceeding 400 ° C., precipitation of Zr is inhibited, and the effect of suppressing the growth of crystal grains during the solution treatment is weakened.
If the temperature is less than 0 ° C., the precipitation of Zr is delayed, and the effect of suppressing the growth of crystal grains during the solution treatment is weakened.

After a predetermined thickness is obtained by repeated rolling of a solution, a solution treatment is performed at a temperature of 450 to 490 ° C. for 5 minutes or more, and cooled at a cooling rate of 10 ° C./sec or more. If the solution treatment temperature is lower than 450 ° C., the solid solution of the alloy element becomes insufficient, and a predetermined strength cannot be obtained after the aging treatment. 490
If the temperature exceeds ℃, the growth of crystal grains cannot be suppressed, and the proportion of small-angle grain boundaries of 10 ° or less decreases.

If the cooling rate after the solution treatment is less than 10 ° C./sec, precipitation of the second phase occurs during the cooling, and the effect of the solution treatment is weakened, and the predetermined strength after the aging treatment cannot be obtained. After the solution treatment and cooling, aging treatment is performed according to a conventional method.

In the present invention, Zr, a transition element, is added as an alloying element, so that Zr is finely precipitated during rolling in a temperature range of 400 to 150 ° C., and the crystal grain growth in the solution treatment is performed. It is important to prevent tilting.
The same transition element, Cr, may be added for grain refinement in aluminum alloys, but this is not effective in the present invention, and the combined use of Cr and Zr also suppresses crystal grain growth during solution treatment. Could not.

[0024] Further, it is known from the research so far that a structure (sub-grain structure) having a small angle grain boundary can be obtained by subjecting a strongly processed aluminum alloy to heat treatment at a medium temperature range of 100 to 300 ° C. However, in the 7000 series aluminum alloy of the present invention, a solution treatment at a temperature of 450 ° C. or more is essential, and a structure containing many small-angle grain boundaries must be maintained after the solution treatment. As a result of conducting many experiments and examinations on the manufacturing method therefor, it was found that a method of performing rolling repeatedly so that the degree of work was 70% or more in the above-mentioned temperature range of 400 to 150 ° C. was effective. It has been reached.

[0025]

EXAMPLES Examples of the present invention will be described below in comparison with comparative examples, and the effects thereof will be demonstrated based on them.
It should be noted that these examples are for describing a preferred embodiment of the present invention, and the present invention is not limited thereto.

Example 1 An aluminum alloy having a composition shown in Table 1 was ingoted by a DC casting method, and the obtained billet (diameter: 90 mm) was used as an ingot.
And cut to a length of 100 mm.
After performing the homogenizing treatment for a long time, forging was performed at a temperature of 400 ° C. to prepare a sample having a thickness of 30 mm.

The above sample was chamfered to a thickness of 20 mm.
Rolling was repeatedly performed in a temperature range of 350 to 200 ° C. to obtain a 1.5 mm-thick plate. The number of rolling repetitions is 12. Then, the plate material was placed in a salt bath at 480 ° C for 5 minutes.
Solution treatment, and water-cooled.
Aging treatment was performed for a time to obtain a test material.

With respect to the obtained test material, a crystal grain structure examination, a tensile test, and a stress corrosion cracking resistance test were carried out according to the following methods. Investigation of the crystal grain structure: The crystal grain structure on the plate surface was determined by SEM manufactured by Hitachi, Ltd. and EBSD (Electron backscatt manufactured by Oxford).
er diffraction), and the ratio of crystal grain boundaries showing a tilt angle of 3 to 10 ° was determined from a histogram showing the crystal orientation difference (misorientation) distribution.

Tensile test: 90 with respect to the rolling direction of the test material
The specimen was sampled in the ° direction, and the distance between the gauges of the specimen was 10 m.
As m, a tensile test was performed using an Instron type tensile tester, and the tensile strength (σ _B ), 0.2% proof stress (σ _0.2 ), and elongation (δ) were measured.

Stress corrosion cracking test: A test piece was sampled in a direction at 90 ° to the rolling direction of the test material, and 0.2% was added to the test piece.
Load 82% of proof stress, 3.5% Na at 30 ° C
After immersion in a Cl solution for 10 minutes, a dry-wet alternating test is repeated, in which a cycle of drying at 25 ° C. for 50 minutes is performed.
The number of breaks within 0 hours was investigated. In addition, as a test piece, five test pieces were prepared for each alloy, and a stress corrosion cracking test was performed.

Table 2 shows the results of these investigations and tests. As can be seen from Table 2, the test material No. 1-4
Each of them had excellent proof stress exceeding 500 MPa and exhibited excellent stress corrosion cracking resistance without breaking in a stress corrosion cracking test.

[0032]

[Table 1]

[0033]

[Table 2]

Comparative Example 1 An aluminum alloy having a composition shown in Table 3 was ingoted by DC casting, and the obtained billet (diameter: 90 mm) was used as an ingot.
And cut to a length of 100 mm.
After performing the homogenizing treatment for a long time, forging was performed at a temperature of 400 ° C. to prepare a sample having a thickness of 30 mm. This sample was processed in the same process as in Example 1 to produce a test material, and the obtained test material was subjected to a crystal grain structure survey, a tensile test, and a stress corrosion cracking test according to the same method as in Example 1. Was.
Table 4 shows the results.

[0035]

[Table 3] << Table Note >> Alloy I: JIS A7475

[0036]

[Table 4]

As shown in Table 4, the test material No. 5 is Z
The strength is low due to the small content of n, and the stress corrosion cracking resistance is inferior due to the low ratio of small angle grain boundaries. Test material No. No. 6 is inferior in strength because the amount of Mg and the amount of Cu are small. Test material No. Since the content of Zr is small, the effect of suppressing the growth of crystal grains during the solution treatment is small, the ratio of the small-angle crystal grain boundaries is reduced, and the stress corrosion cracking resistance is poor. Test material No. 8 has a Zn content exceeding the upper limit,
Cracks occurred during hot rolling, and the final plate could not be manufactured.
Test material No. Reference numeral 9 denotes a conventional JIS A7475 alloy, which has a low ratio of small-angle grain boundaries and is inferior in stress corrosion cracking resistance.

Example 2 The alloy A of Example 1 was used to evaluate the characteristics while changing the manufacturing conditions. The ingot, homogenization, hot forging, and facing conditions were the same as in Example 1, and the steps after repeated rolling were performed under the conditions shown in Table 5 to produce a test material. The number of rolling repetitions was 8 to 12, and the aging treatment was all performed at 120 ° C for 24 hours.

With respect to the obtained test material, the examination of the crystal grain structure, the tensile test, and the stress corrosion cracking test were performed in the same manner as in Example 1. Table 6 shows the results. As shown in Table 6, the test material No. All of Nos. 10 to 14 had excellent proof stress exceeding 500 MPa, and exhibited excellent stress corrosion cracking resistance without breaking in a stress corrosion cracking test.

[0040]

[Table 5]

[0041]

[Table 6]

Comparative Example 2 Using the alloy A of Example 1, the characteristics were evaluated under different manufacturing conditions. Ingot making, homogenization treatment, hot forging, and facing conditions were the same as in Example 1, and the steps after repeated rolling were performed under the conditions shown in Table 7, to produce test materials. The number of rolling repetitions was 8 to 12, and the aging treatment was all performed at 120 ° C. for 24 hours. With respect to the obtained test material, an examination of a crystal grain structure, a tensile test, and a stress corrosion cracking test were performed in the same manner as in Example 1. Table 8 shows the results.

[0043]

[Table 7]

[0044]

[Table 8]

As shown in Table 8, the test material No. 15 is
Since the rolling start temperature is high and the effect of Zr is not sufficient, the growth of crystal grains during the solution treatment cannot be suppressed, and the stress corrosion cracking resistance is poor. Test material No. In No. 16, since the lower limit of the temperature of the repeated rolling is low and the effect of Zr is not sufficient, the growth of crystal grains during the solution treatment cannot be suppressed, and the stress corrosion cracking resistance is poor. Test material No. No. 17 has a low degree of rolling and insufficient precipitation of Zr, so that the growth of crystal grains during the solution treatment cannot be suppressed and the stress corrosion cracking resistance is poor. Test material No. In No. 18, crystal grain growth occurred due to the high solution treatment temperature, and the stress corrosion cracking resistance was poor. Test material No. In No. 19, since the cooling rate after the solution treatment was low, precipitation of the second phase occurred during cooling,
Sufficient precipitation hardening was not obtained in the aging treatment. In addition, fracture occurred in the stress corrosion cracking test.

[0046]

According to the present invention, there is provided a structural aluminum alloy sheet having excellent strength and corrosion resistance, especially resistance to stress corrosion cracking. By using the aluminum alloy plate, the thickness of the material can be reduced, and the weight and cost of the structure can be reduced. In addition, the effect of improving the reliability of the structure can be achieved due to the excellent stress corrosion cracking resistance.

[Brief description of the drawings]

FIG. 1 is a diagram showing a difference in orientation of crystal grains.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22F 1/00 640 C22F 1/00 640A 682 682 683 683 691 691B 691C 692 692A 694 694A 694B (71) Applicant 000004743 Nippon Light Metal Co., Ltd. 2-2-2 Higashi Shinagawa, Shinagawa-ku, Tokyo (71) Applicant 000005290 Furukawa Electric Co., Ltd. 2-6-1 Marunouchi, Chiyoda-ku, Tokyo (71) Applicant 000107538 Sky Aluminum Co., Ltd. Tokyo 1-2-1 Kinshi, Sumida-ku, Tokyo (72) Inventor Hiroki Tanaka 5-11-3, Shimbashi, Minato-ku, Tokyo Sumitomo Light Metal Industries Co., Ltd. (72) Inventor Tadashi Minoda 5-chome, Shimbashi, Minato-ku, Tokyo No. 11-3 Sumitomo Light Metal Industries Co., Ltd. (72) Inventor Ezaki Hiroki 5-11-3 Shimbashi, Minato-ku, Tokyo Sumitomo Light Metal Industries Co., Ltd.

Claims (3)

[Claims] 1. Zn: 4.8 to 7% (mass%, the same applies hereinafter), Mg: 1 to 3%, Cu: 1 to 2.5%, Zr:
An aluminum alloy sheet containing 0.05 to 0.25% and having a balance of Al and impurities, wherein a crystal orientation difference on a plate surface of the aluminum alloy sheet is 3-1.
An aluminum alloy sheet having excellent strength and corrosion resistance, characterized by having a structure containing 25% or more of 0 ° crystal grain boundaries. 2. An average crystal grain size as viewed from the plate surface is 10 μm.
m or less, and the aluminum alloy plate having excellent strength and corrosion resistance according to claim 1. 3. An ingot of an aluminum alloy having the composition according to claim 1 is subjected to hot working after homogenization treatment, and
In a temperature range of 00 to 150 ° C., the sheet is repeatedly rolled to a predetermined thickness so that the working ratio becomes 70% or more.
Perform a solution treatment for 5 minutes or more at a temperature of 50 to 490 ° C,
A method for producing an aluminum alloy sheet having excellent strength and corrosion resistance, characterized by cooling at a cooling rate of 10 ° C./sec or more.