JP2009013479A - High strength aluminum alloy material having excellent stress corrosion cracking resistance, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an Al-Zn-Mg-Cu based alloy material having high strength, further exhibiting high elongation, and provided with excellent SCC resistance. <P>SOLUTION: The hot-worked or cold-worked material of an aluminum alloy comprising Mg and Zn in quantities surrounded by points A (6.0, 1.5), B (12.0, 1.5), C (12.0, 1.75), D (9.0, 2.5) and E (6.0, 2.5) in Fig.1 showing the Zn content (mass%) in the horizontal axis and the Mg content in the vertical axis, including Cu of 1.0 to 2.5 mass% and Zr of 0.08 to 0.20 mass% is subjected to solution treatment, is thereafter quenched at a prescribed temperature in the range of 150 to 350°C, is held to the prescribed temperature for 1 s to 30 min, is subsequently water-cooled, and is thereafter subjected to natural aging or artificial aging, so as to be a microstructure where the size of precipitates in the crystal grain boundary in which an orientation difference between the adjoining crystal grains is ≥10° is 50 to 250 nm by the major axis, and further, the inside of each crystal grain has microstructure provided with a GP zone and/or a η'-MgZn<SB>2</SB>intermediate phase, thus high strength and excellent SCC resistance are exhibited. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an Al—Zn—Mg—Cu based alloy material that can be used for fasteners such as bolts and rivets, has high strength and high elongation, and is excellent in stress corrosion cracking resistance, and a method for producing the same.

Al-Zn-Mg-Cu alloy materials can obtain high strength by aging treatment, but their use is limited because they tend to cause stress corrosion cracking. However, various heat treatment methods have been proposed because the stress corrosion cracking resistance can be improved by combining various heat treatments.
That is, when an Al-Zn-Mg-Cu alloy material is subjected to an artificial aging treatment (so-called T6 treatment) after solution treatment, high strength is obtained, but stress corrosion cracking (hereinafter referred to as "SCC"). ). For this reason, it cannot be used for a long time in a corrosive environment loaded with stress.

SCC resistance is improved by over-aging treatment (so-called T73 treatment) after solution treatment, but the strength is reduced compared to T6 treatment.
In order to solve this problem, a solution that has been subjected to solution treatment and then artificial aging treatment (so-called T6 treatment) is subjected to restoration treatment that performs overaging at a higher temperature for a short time, and then artificial aging treatment at a lower temperature again. A treatment method (so-called T77 treatment) that secures the same strength as T6 and the same SCC resistance as T73 has been proposed. However, the T77 treatment method described above is not suitable for mass production because it involves many heat treatment steps and is complicated.
Under such circumstances, Patent Document 1 proposes a technique for ensuring high strength and excellent SCC resistance by adopting a simple heat treatment method.
Japanese Patent No. 3246940

The technique proposed in Patent Document 1 is to heat-treat a 7000 series alloy after solution treatment. After the solution treatment, it is quenched into a temperature range of 200 to 400 ° C. and kept at that temperature for 2 hours or less. Post-water quenching followed by T6 aging treatment.
However, it cannot be said that high strength is obtained even by this heat treatment method.
The present invention has been devised to solve such problems, and is an Al-Zn-Mg-Cu alloy material that has higher strength, higher elongation, and excellent SCC resistance. An object is to provide an aluminum alloy material.

The high-strength aluminum alloy material excellent in stress corrosion cracking resistance according to the present invention shows the Zn content (mass%) on the horizontal axis and the Mg content (mass%) on the vertical axis in order to achieve the object. 1, Mg and Zn in an amount surrounded by points A (6.0, 1.5), B (12.0, 1.5), C (12.0, 1.75), D (9.0, 2.5) and E (6.0, 2.5), 1.0 to 2.5 Cu of mass% and Zr of 0.08 to 0.20 mass%, optionally further including one or more of 0.05 to 0.5 mass% of Mn, 0.05 to 0.25 mass% of Cr and 0.05 to 0.15 mass% of V, The balance consists of Al and unavoidable impurities, and has a composition in which Fe as the inevitable impurities is regulated to 0.20 mass% or less and Si is regulated to 0.20 mass% or less, and the tensile strength is 650 MPa or more, 0.2% proof stress / tensile. The strength ratio is 0.7 to 0.95, the elongation is 10% or more, and the stress corrosion cracking resistance is equal to or higher than that of the T77 heat treatment material of 7075 alloy.
And it is preferable that the size of the precipitate in the grain boundary where the orientation difference between adjacent crystal grains is 10 ° or more is a major axis of 50 to 250 nm.

  Such a high-strength aluminum alloy material excellent in stress corrosion cracking resistance is obtained by homogenizing a continuous cast material of an aluminum alloy having the above composition and using hot working or cold working, or both working together. After being processed, solution treatment is performed, after which it is quenched to a predetermined temperature in the range of 150 to 350 ° C., held at the predetermined temperature for 1 second to 30 minutes, and then cooled with water, and then natural aging or Manufactured by artificial aging.

The aluminum alloy material provided by the present invention has an unprecedented strength, excellent SCC resistance and high elongation, and is widely used as a fastener or a structural material.
In addition, it is possible to obtain an aluminum alloy material with high strength and excellent SCC resistance just by keeping the holding time after quenching to a predetermined temperature in a short time compared with the conventional method, so that the production efficiency is further improved. Can do.

The present inventors have repeatedly studied a heat treatment method capable of expressing and maintaining SCC resistance without reducing the strength of Al-Zn-Mg-Cu alloy materials represented by 7000 series.
Certainly, although the technique proposed in Patent Document 1 is an effective heat treatment method, it has a problem that the strength decreases. When examining this problem, the cause of the decrease in strength is that the retention time after quenching to a predetermined temperature is too long, so that the precipitates formed at the grain boundaries grow too large, and the alloy elements in the grains It was predicted that the amount of solid solution decreased. Therefore, by shortening the holding time after quenching, the growth of precipitates formed on the crystal grain boundaries was suppressed, and the solid solution amount was secured to prevent the strength from being lowered.
The details will be described below.

  First, the alloy material of the present invention shows the content of Mg and Zn, the abscissa indicates the Zn content (mass%), and the ordinate indicates the Mg content (mass%). ), B (12.0, 1.5), C (12.0, 1.75), D (9.0, 2.5) and E (6.0, 2.5) and the amount enclosed, 1.0 to 2.5 mass% Cu and 0.08 to 0.20 mass% Zr of 0.05 to 0.5% by mass of Mn, 0.05 to 0.25% by mass of Cr, and 0.05 to 0.15% by mass of V, if necessary, and the balance consisting of Al and inevitable impurities, It is assumed that the composition has a composition in which Fe as inevitable impurities is regulated to 0.20 mass% or less and Si is regulated to 0.20 mass% or less.

Zn and Mg
Zn is the most important element for increasing the strength by aging treatment, and if the content is less than 6.0% by mass, the desired strength cannot be obtained. On the other hand, if it exceeds 12.0% by mass, SCC will occur in a short period even if the proposed method is applied. Mg, as well as Zn, is an element important for improving the strength by aging treatment, and if its content is less than 1.5% by mass, the desired strength cannot be obtained. On the other hand, if it exceeds 2.5% by mass, SCC is likely to occur in a short period even if the proposed method is applied.
The upper part of the line segment CD connecting the points C (12.0, 1.75) and D (9.0, 2.5) with Zn and Mg contents is easy to partially melt as the Cu content increases. Since strength, elongation, etc. were lowered, it was excluded.

Cu: 1.0-2.5% by mass
Cu, like Zn and Mg, is an element important for improving the strength by aging treatment, and if its content is less than 1.0% by mass, the desired strength cannot be obtained. On the other hand, if the content exceeds 2.5% by mass, the melting point of the alloy is excessively lowered and the homogenization temperature is lowered, so that a sufficient homogenization effect cannot be obtained.

Zr: 0.08-0.20 mass%
Zr precipitates as a Zr-Al compound, and the presence of this precipitate delays the recrystallization of the alloy material, stabilizes the recovery structure, and contributes to improvement in strength and SCC resistance. If the content is less than 0.08% by mass, the effect of delaying recrystallization is small, and if the content exceeds 0.20% by mass, a coarse intermetallic compound is formed and the ductility is impaired. Therefore, the Zr content is set to 0.08 to 0.20 mass%.

Mn: 0.05 to 0.5% by mass
Cr: 0.05-0.25 mass%
V: 0.05-0.15 mass%
Since Mn, Cr, and V all have the action of delaying recrystallization as with Zr, one or more of them are contained as necessary. By containing it together with Zr, the delay effect of recrystallization is improved. In each case, the delay effect of recrystallization is not recognized unless the prescribed amount is reached. On the other hand, if it is included in an amount exceeding the specified amount, a coarse compound is crystallized to impair toughness and ductility.

Fe: 0.20 mass% or less
Si: 0.20 mass% or less
Fe and Si are the most inevitable impurities contained in the Al alloy material. If these elements are excessively contained, a coarse intermetallic compound is formed, and the ductility is impaired. Therefore, both Fe content and Si content are regulated to 0.20 mass% or less.
The other inevitable impurities are 0.10% by mass or less, preferably 0.05% by mass or less, and the total of other inevitable impurities is 0.3% by mass or less, preferably 0.15% by mass or less.
In addition, in order to prevent casting cracks in the ingot at the time of casting, 0.005 to 0.20 mass% Ti addition, 0.005 to 0.20 mass% Ti and 0.0005 to 0.20 mass% B addition, or a slight amount of TiC addition May be performed.

Next, the microstructure of the alloy material of the present invention will be described.
Size of precipitates at grain boundaries where the orientation difference between adjacent crystal grains is 10 ° or more: 50 to 250 nm
In the present invention, the position of precipitates related to resistance to stress corrosion cracking is defined as a crystal grain boundary in which the orientation difference between adjacent crystal grains is 10 ° or more. The reason for this is that even if the crystal grains adjacent to each other have a misorientation of less than 10 °, the period until stress corrosion cracking occurs is a crystal whose misorientation is 10 ° or more. This is because it is about 10 times longer than the case where it precipitates at the grain boundary, and the proportion involved in stress corrosion cracking is low.
In an Al alloy material that has been subjected to an aging treatment, it is necessary to adjust the size of precipitates formed on the grain boundaries in order to develop the required strength. In the technique proposed in Patent Document 1, since the size of the precipitate exceeded 300 nm, the strength was lowered. In the present invention, a decrease in strength can be suppressed by adjusting the size of the precipitate to a range of 50 to 250 nm in terms of major axis by combining the component composition and heat treatment under appropriate conditions.

The size of the precipitate at the grain boundary where the orientation difference between adjacent crystal grains is 10 ° or more is 50 to 250 nm in the major axis. With such a size, there is a PFZ (precipitation-free zone) around the precipitates, and SCC resistance at least equivalent to or higher than that of the T77 heat treatment material of 7075 alloy is expressed and contributes to strength. Ensure the solid solution amount of Zn, Mg, Cu, then GP zone or η'-MgZn ₂ or S'-Al ₂ CuMg or other intermediate phase or GP zone and intermediate Both phases are formed and the tensile strength is improved. If the size of the precipitate is less than the lower limit, the strength is improved by the subsequent heat treatment, but the width of the PFZ is narrow and corrosion is likely to proceed continuously, and does not contribute to SCC resistance. If the upper limit is exceeded, the width of the PFZ is wide and corrosion does not progress continuously and contributes to SCC resistance. However, the stable phase such as η-MgZn ₂ also increases in the crystal grains, and the solid phase of Zn, Mg, Cu increases. The amount of solution cannot be secured, and there is a limit to the improvement of tensile strength in the subsequent treatment.

  When the component composition and microstructure are adjusted as described above, the tensile strength is 650 MPa or more, the 0.2% proof stress / tensile strength ratio is 0.7 to 0.95, the elongation is 10% or more, and the stress corrosion cracking resistance is T77 of 7075 alloy. High-strength Al-Zn-Mg-Cu alloy material with excellent SCC resistance with characteristics equivalent to or better than heat-treated materials can be provided at low cost, and even if tension is applied, such as bolts and screws, for a long time It can be used safely.

Furthermore, a method for producing a high-strength Al—Zn—Mg—Cu alloy material having excellent SCC resistance as described above will be described.
There are no particular limitations on the steps from continuous casting, homogenization, hot working and / or cold working and subsequent solution treatment. The solution treatment is performed in the usual manner.
However, since mixing of oxide particles or coarse intermetallic compound particles is not preferable, the molten alloy having the above composition is degassed and degassed and then passed through a filter to remove the particles and cast into an ingot. It is preferable to do.

The casting is cast into a billet, slab, or sheet shape depending on the application, but any shape may be used.
The ingot is cast by a continuous casting method including DC, CC, and the like. The reason for adopting the continuous casting method is to improve productivity. The use of equipment attached to the casting apparatus, such as an ultrasonic applicator or a low frequency applicator, is also optional. What is necessary is just to select and employ | adopt suitably according to the use purpose of an ingot.

The obtained ingot is homogenized. This treatment is performed in order to homogenize intermetallic compounds and precipitates crystallized during casting or segregation during casting. The homogenization conditions vary slightly depending on the composition, but a temperature suitable for the composition is selected in the range of 450 to 470 ° C. and heated.
Holding time is homogenized by reaching the target temperature and holding for 60 minutes or more. The holding time is preferably within 24 hours. Even if kept for a long time, the effect is saturated and not economical.

After completion of the homogenization treatment, it may be kept at a high temperature or may be reheated after cooling. The ingot is processed hot. When the ingot is billet-shaped, it is extruded to obtain an extruded material having a required cross section. In the case of a slab shape, hot rolling is performed to obtain a hot rolled sheet having a required thickness, and then cold rolling is performed to obtain a cold rolled sheet having a required thickness.
An intermediate annealing treatment may be performed. In the case of a sheet shape, hot rolling may be performed, but only cold rolling may be performed, or both may be used in combination. Cold-rolled sheet with the required thickness. Intermediate annealing may be performed during cold rolling.

The obtained extruded material or cold-rolled sheet is subjected to a solution treatment to dissolve the alloy element. Since the solution treatment conditions may be obtained by dissolving the alloy element, it is not necessary to define the temperature and time in detail. When heated to a temperature range of 400 to 470 ° C. and kept at this temperature for 0.5 hours or more, the alloy elements are dissolved. Even if it is kept for a long time exceeding 24 hours, the effect is saturated and it is not economical.
Immediately after solution treatment at this temperature, it is quenched to a predetermined temperature in the temperature range of 150 to 350 ° C., kept at this quenching temperature within 30 minutes, and then cooled with water.

The present invention is characterized in that the above quenching is performed under the conditions of a quenching temperature of 150 to 350 ° C. and a holding time of 1 second to 30 minutes.
By adopting the above conditions, structurally, a part of the alloy element is preferentially precipitated on the grain boundary, and a precipitate having a major axis of 50 to 250 nm is generated. Compared with the technique proposed in Patent Document 1, since small precipitates are dispersed, the strength and elongation can be increased. Furthermore, by adopting the above conditions, PFZ (no precipitation zone) is generated around the precipitates precipitated at the grain boundaries, and SCC resistance at least equivalent to or higher than that of the T77 heat treatment material of 7075 alloy is expressed. .

In further detail regarding the size of the precipitate, since the growth of the precipitate of the present invention is suppressed as compared with that proposed in Patent Document 1, Zn, Mg, and Cu that contribute to strength are contained in the crystal grains. Many are dissolved. Therefore, by quenching under the above-mentioned conditions of the present invention, further natural aging is performed to form a GP zone formed of Zn, Mg, Cu that contributes to the strength of solid solution in the crystal grains, A tensile strength of 650 MPa or more, a 0.2% proof stress / tensile strength ratio of 0.7 to 0.95, and an elongation of 10% or more can be obtained. Alternatively, Zn, Mg, Cu that contributes to strength by artificial aging, preferably by T6 treatment, forms an intermediate layer such as η'-MgZn ₂ or S'-Al ₂ CuMg with further growth of GP zone, and tensile strength Of 650 MPa or more, 0.2% proof stress / tensile strength ratio of 0.7 to 0.95, and elongation of 10% or more.

In addition, when the quenching temperature exceeds 350 ° C. or the holding time exceeds 30 minutes, a part of the alloy element structurally precipitates and grows coarsely at a grain boundary exceeding 250 nm, While the elongation decreases, Zn, Mg, and Cu that contribute to the strength of the solid solution in the crystal grains precipitate as a compound that does not contribute to the strength, such as η-MgZn ₂ equilibrium phase. In this case, the formation amount of the GP zone or the intermediate phase such as η′-MgZn ₂ or S′-Al ₂ CuMg that contributes to the strength decreases, and there is a limit to the improvement of the tensile strength.

  The holding time at the quenching temperature is preferably 1 second or longer. The higher the quenching temperature, the shorter the holding time, and the lower the quenching temperature, the longer the holding time. If the quenching temperature after solution treatment is less than 150 ° C or the holding time is less than 1 second, the size of the precipitate at the grain boundary is less than 50 nm in the major axis and the size is small, and the width of the PFZ is narrow, so the corrosion is precipitated. It is difficult to obtain the effect of SCC resistance.

Example 1;
Next, specific examples will be described.
A molten aluminum alloy having the composition shown in Table 1 was melted, and a billet having a diameter of 200 mm was cast by a continuous casting method. The average equivalent circle diameter of the ingot crystal grains was about 40 μm. The variation in crystal grain size was ± 10 μm in the ingot cross section.
The obtained billet is homogenized at 460 ° C x 6h, extruded to a 44.5mm diameter extrusion rod with an extrusion ratio of 20, hot extruded at 400 ° C, and subjected to a solution treatment of 460 ° C x 1h, from the solution treatment temperature. It was quenched in a salt bath at 200 ° C., kept at that temperature for 20 minutes, and finally cooled with water. Thereafter, an artificial aging treatment of 120 ° C. × 24 h was performed to obtain a test material.

The specimens were evaluated by measuring mechanical properties by a tensile test and observing the structure with an optical microscope and a transmission electron microscope.
For mechanical properties, a tensile test was performed in accordance with JIS Z2241 using a JIS Z2201 No. 4 test piece, and 0.2% proof stress, tensile strength, and elongation were obtained.
In the structure observation, the presence or absence of local melting was mainly determined while the specimen was polished and etched with a 0.5% HF aqueous solution. In addition, precipitates in the crystal grains and grain boundaries were observed with a transmission electron microscope. The results are shown in Table 2.

From the results in Table 2, it can be seen that Sample Nos. 1 to 12 according to the present invention are excellent in mechanical properties and proof stress ratio and have a normal metal structure.
On the other hand, it can be seen that Sample Nos. 13 to 17 which are outside the scope of the present invention are inferior in any of the characteristics. In particular, it can be seen that Sample No. 17, which is A7075, has many coarse particles in the grain boundaries and grains, and the elongation is low.

Example 2;
A molten aluminum alloy having the alloy composition indicated by J6 and J8 in Table 1 was melted, and a billet having a diameter of 200 mm was cast by a continuous casting method. The average equivalent circle diameter of the ingot crystal grains was about 40 μm. The variation in crystal grain size was ± 10 μm in the ingot cross section.
The billet thus obtained was homogenized at 460 ° C. for 6 hours in the same manner as in Example 1. This was used as a test material.

For each specimen, measurement of mechanical properties by a tensile test, evaluation of SCC resistance by a stress corrosion cracking test, and observation of a structure by an optical microscope and a transmission electron microscope were performed.
As for the mechanical properties, 0.2% proof stress, tensile strength, and elongation were obtained in the same manner as in Example 1.
Evaluation of SCC resistance conforms to the stress corrosion cracking test method of JIS H8711 aluminum alloy, and a C-ring test piece is prepared from the extruded bar so that it can be loaded perpendicular to the direction of extrusion as shown in Fig. 2, 0.2% A stress of 75% of the proof stress was applied and immersed alternately in a 3.5% saline solution at 25 ° C. to determine whether or not stress corrosion cracking occurred and the time of occurrence. The immersion time was a maximum of 30 days.
In the structure observation, as in Example 1, the presence or absence of local melting and the precipitates in the crystal grains and the grain boundaries were observed. In Example 2, the size of precipitates at high-angle grain boundaries with an orientation difference of 10 ° or more was measured.
The results are shown in Tables 4 and 5.

From the results of Tables 4 and 5, Sample Nos. 21, 22, 23, 24 and 30, 31, 32, 33 according to the present invention were excellent in mechanical properties and proof stress ratio without local melting. It can also be seen that the size of precipitates present at high-angle grain boundaries with an orientation difference of 10 ° or more is large, and the SCC resistance is equivalent to that of the A7775 T77-treated material shown in Sample No.
On the other hand, it can be seen that sample numbers 25, 26, 27 and 34, 35, 36 outside the heat treatment condition range of the present invention are inferior in any of the characteristics. That is, if the quenching temperature after the solution treatment is too high or the holding time is too long, the precipitate grows greatly and the mechanical properties are deteriorated (sample numbers 25, 27 and 34, 36). Moreover, when the solution treatment temperature is too high (sample numbers 26 and 35), local melting occurs and elongation decreases.

Diagram showing the relationship between Zn and Mg in the aluminum alloy material of the present invention Diagram showing C-ring specimen shape for stress corrosion cracking test

Claims (4)

  In FIG. 1 showing the Zn content (mass%) on the horizontal axis and the Mg content (mass%) on the vertical axis, points A (6.0, 1.5), B (12.0, 1.5), C (12.0, 1.75), Mg and Zn in an amount surrounded by D (9.0, 2.5) and E (6.0, 2.5), 1.0 to 2.5 mass% Cu and 0.08 to 0.20 mass% Zr, with the balance consisting of Al and inevitable impurities, It has a composition in which Fe as an inevitable impurity is regulated to 0.20 mass% or less and Si is regulated to 0.20 mass% or less, tensile strength is 650 MPa or more, 0.2% proof stress / tensile strength ratio is 0.7 to 0.95, elongation is High-strength aluminum alloy material with excellent stress corrosion cracking resistance, characterized by 10% or more and stress corrosion cracking resistance equivalent to or higher than T77 heat treated material of 7075 alloy.   The high-strength aluminum alloy having excellent stress corrosion cracking resistance according to claim 1, further comprising one or more of 0.05 to 0.5 mass% of Mn, 0.05 to 0.25 mass% of Cr, and 0.05 to 0.15 mass% of V. Wood.   The stress corrosion cracking resistance according to claim 1 or 2, which has a microstructure in which the size of precipitates at grain boundaries where the orientation difference between adjacent crystal grains is 10 ° or more is 50 to 250 nm in major axis. Excellent high-strength aluminum alloy material.   The aluminum alloy continuous cast material having the composition according to claim 1 or 2 is homogenized, hot-worked or cold-worked, or processed in combination with both, then solution-treated, Quenched to a predetermined temperature in the range of 150 to 350 ° C, held at the predetermined temperature for 1 second to 30 minutes, then cooled with water, and then subjected to natural aging or artificial aging for stress corrosion cracking resistance. A method for producing an excellent high-strength aluminum alloy material.

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