HIGH-STRENGTH ALUMINUM ALLOY AND MANUFACTURING METHOD THEREOF

- UACJ Corporation

An aluminum alloy suitable for anodizing contains, in mass percent, Zn: 5.0% or more and 7.0% or less, Mg: more than 2.2% and 3.0% or less, Cu: 0.01% or more and 0.10% or less, Zr: 0.10% or less, Cr: 0.02% or less, Fe: 0.30% or less, Si: 0.30% or less, Mn: 0.02% or less, and Ti: 0.001% or more and 0.05% or less, the remainder being composed of Al and unavoidable impurities. The aluminum alloy has a Zn/Mg ratio of 1.7 or more and 3.1 or less, a proof stress of 350 MPa or more and a metallographic structure composed of a recrystallized structure.

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Description
TECHNICAL FIELD

The present invention relates to a high-strength aluminum alloy that can be used in parts where at least both appearance characteristics and stress-corrosion-cracking resistance are considered to be important.

BACKGROUND ART

Aluminum alloys are being increasingly employed as materials for use in sports equipment, transportation equipment, machine parts, and other applications wherein at least strength properties and appearance characteristics are considered to be important. Because durability is required for these applications, it is desirable to use high-strength aluminum alloys having a proof stress of 350 MPa or more. For example, the aluminum-alloy extruded material described in Patent Document 1 has been proposed as an aluminum alloy for use in applications wherein both strength properties and appearance characteristics are considered to be important.

PRIOR ART LITERATURE Patent Documents

Patent Document 1

Japanese Laid-open Patent Publication 2012-246555

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In the 7000-series aluminum alloy described in Patent Document 1, there is a problem in that, when a T6 process is performed with an aging treatment, stress corrosion cracking tends to occur. In addition, there is a problem in that, if an overaging treatment is performed as a corrective measure therefor, then strength decreases even though stress-corrosion-cracking resistance can be improved.

Thus, even though the previously-existing 7000-series aluminum alloy described in, for example, Patent Document 1 has high proof stress, it cannot be said that Patent Document 1 takes corrective measures with regard to the stress-corrosion-cracking characteristic. Consequently, this alloy is not suited to applications in which the alloy is used over an extended period of time in a state wherein the alloy is continuously subject to stress in a corrosive environment.

The present invention was conceived against this background, and an object of the present invention is to provide a high-strength aluminum alloy that excels in surface quality and stress-corrosion-cracking resistance after an anodization treatment, and a manufacturing method therefor.

Means for Solving the Problems

In a first aspect of the invention, a high-strength aluminum alloy to be subjected to an anodization treatment comprises:

    • a chemical composition containing, in mass %, Zn: 5.0% or more and 7.0% or less, Mg: more than 2.2% and 3.0% or less, Cu: 0.01% or more and 0.10% or less, Zr: 0.10% or less, Cr: 0.02% or less, Fe: 0.30% or less, Si: 0.30% or less, Mn: 0.02% or less, and Ti: 0.001% or more and 0.05% or less, the remainder being composed of Al and unavoidable impurities and a Zn/Mg ratio being 1.7 or more and 3.1 or less;
      wherein,
    • the proof stress is 350 MPa or more; and
    • the metallographic structure is composed of a recrystallized structure.

In another aspect of the present invention, a method of manufacturing the high-strength aluminum alloy comprises the steps of:

    • preparing an ingot having a chemical composition containing, in mass %, Zn: 5.0% or more and 7.0% or less, Mg: more than 2.2% and 3.0% or less, Cu: 0.01% or more and 0.10% or less, Zr: 0.10% or less, Cr: 0.02% or less, Fe: 0.30% or less, Si: 0.30% or less, Mn: 0.02% or less, and Ti: 0.001% or more and 0.05% or less, the remainder being composed of Al and unavoidable impurities and a Zn/Mg ratio being 1.7 or more and 3.1 or less;
    • performing a homogenization treatment that heats the ingot at a temperature above 540° C. and 580° C. or lower for 1-24 h;
    • hot working the ingot, in a state wherein the temperature of the ingot at the start of the working has been set to 440-560° C., thereby making it a wrought material;
    • performing a quenching treatment that cools by controlling, after cooling has started while the temperature of the wrought material is 400° C. or higher, an average cooling rate while the temperature of the wrought material is in the range of 400° C. to 150° C. such that it is 1° C./s or more and 300° C./s or less;
    • cooling the temperature of the wrought material to room temperature by the quenching treatment or by cooling thereafter; and
    • subsequently performing an artificial-aging treatment on the wrought material.

Effects of the Invention

The above-mentioned high-strength aluminum alloy has the above-mentioned specific chemical composition, the proof stress being 350 MPa or more and the metallographic structure being composed of a recrystallized structure. Thereby, the above-mentioned high-strength aluminum alloy is high strength, excels in stress-corrosion-cracking resistance, and excels in surface quality after the anodization treatment, and can be suitably used in a part in which the strength properties, the appearance characteristics, and stress-corrosion-cracking resistance are considered to be important.

That is, the above-mentioned high-strength aluminum alloy has the above-mentioned specific chemical composition, and thereby an excellent stress-corrosion-cracking resistance characteristic can be ensured; thereby, the high-strength aluminum alloy can exhibit excellent durability even when used in a corrosive environment.

In addition, the above-mentioned high-strength aluminum alloy has a proof stress equal to or greater than that of the above-mentioned, previously-existing 7000-series aluminum alloy, that is, a proof stress of 350 MPa or more. Consequently, it is possible to relatively easily meet the requirements for strength, such as ensuring strength properties that can support, for example, a reduction in wall thickness in order to reduce weight.

In addition, because the above-mentioned high-strength aluminum alloy has the above-mentioned specific chemical composition and a metallographic structure composed of a recrystallized structure, the formation of streak patterns, caused by fibrous structures after the anodization treatment, and the like can be inhibited, making it possible to obtain a satisfactory surface quality.

Next, in the above-mentioned high-strength aluminum alloy material manufacturing method, the above-mentioned high-strength aluminum alloy is manufactured based on the above-mentioned specific treatment temperatures, treatment times, and treatment procedures. Consequently, the above-mentioned excellent high-strength aluminum alloy can be easily obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph, as a substitute for a drawing, that shows the metallographic structure of sample 4 according to Working Example 1.

FIG. 2 is a photograph, as a substitute for a drawing, that shows the metallographic structure of sample A19 according to Working Example 1.

 MODE(S) FOR CARRYING OUT THE INVENTION

The above-mentioned high-strength aluminum alloy has a chemical composition that contains, in mass %, Zn: 5.0% or more and 7.0% or less, Mg: more than 2.2% and 3.0% or less, Cu: 0.01% or more and 0.10% or less, Zr: 0.10% or less, Cr: 0.02% or less, Fe: 0.30% or less, Si: 0.30% or less, Mn: 0.02% or less, and Ti: 0.001% or more and 0.05% or less, wherein the remainder is composed of Al and unavoidable impurities, and the Zn/Mg ratio is 1.7 or more and 3.1 or less. First, the reasons for the range limits of the content of each element will be explained.

Zn: 5.0% or More and 7.0% or Less

Zn is an element that coexists with Mg in the aluminum alloy and thereby precipitates the η′ phase. By containing Zn together with Mg, it is possible to obtain an increase in strength due to enhanced precipitation. If the Zn content is 5.0% or less, then the η′ phase precipitated amount becomes small, and consequently the strength improving effect is lowered. Consequently, the Zn content is preferably greater than 5.0%, and more preferably 5.2% or more. On the other hand, if the Zn content exceeds 7.0%, then stress-corrosion-cracking resistance deteriorates. Consequently, the Zn content is preferably 7.0% or less and more preferably 6.8% or less.

Mg: More than 2.2% and 3.0% or Less

Mg is an element that coexists with Zn in the aluminum alloy and thereby precipitates the η′ phase. By containing Mg together with Zn, it is possible to obtain an increase in strength due to the enhancement of precipitation. If the Mg content is 2.2% or less, then the η′ phase precipitated amount becomes small, and consequently the strength improving effect is lowered. On the other hand, if the Mg content exceeds 3.0%, then hot workability decreases, productivity decreases, and stress-corrosion-cracking resistance deteriorates.

Zn/Mg Ratio: 1.7 or More and 3.1 or Less

The contents of Zn and Mg are selected such that they are each within the limit ranges described above and such that the value of the above-mentioned Zn quantity/Mg quantity ratio is definitely in the range of 1.7-3.1. If the Zn/Mg ratio is less than 1.7, then strength tends to become low; on the other hand, if it exceeds 3.1, then stress-corrosion-cracking resistance deteriorates. Furthermore, the Zn/Mg ratio means the Zn content (mass %)/Mg content (mass %) value.

Cu: 0.01% or More and 0.10% or Less

If a recycled material is used as the raw material of the above-mentioned high-strength aluminum alloy material, then Cu might be intermixed therein. If the Cu content exceeds 0.10%, then it leads to a reduction in surface quality, such as a decrease in luster after chemical polishing, a change in the color tone to yellow caused by the anodization treatment, and the like; and if the Cu content is less than 0.01%, then stress-corrosion-cracking resistance deteriorates. Such a deterioration in stress-corrosion-cracking resistance and surface quality can be avoided by restricting the Cu content to 0.01% or more and 0.10% or less.

Zr: 0.10% or Less

If the Zr content exceeds 0.10%, then the formation of a recrystallized structure is inhibited and, instead, fibrous structures tend to be formed. If the above-mentioned fibrous structures are present, then after the anodization treatment is performed, streak patterns caused by the fibrous structures tend to appear on the surface, and consequently there is a risk that surface quality will decrease. Consequently, the Zr content is set to 0.10% or less.

Cr: 0.02% or Less

If the Cr content is 0.02% or more, then there is a risk that the surface after the anodization treatment will be tinged with a yellow color tone. Such a decrease in surface quality due to a change in color tone or the like can be prevented by restricting the Cr content to less than 0.02%.

Fe: 0.30% or Less, Si: 0.30% or Less, Mn: 0.02% or Less

Fe and Si are components that might be mixed into the aluminum ore as impurities, and Mn is a component that might be mixed in if a recycled material is used. Fe, Si, and Mn have the effect of inhibiting recrystallization by forming AlMn-based, AlMnFe-based, or AlMnFeSi-based intermetallic compounds with Al. Consequently, if the above-mentioned three components are excessively mixed into the above-mentioned high-strength aluminum alloy material, then the formation of the recrystallized structure is inhibited and, instead, fibrous structures tend to be formed. If a fibrous structure is present, then after the anodization treatment is performed, a streak pattern caused by the fibrous structure tends to appear on the surface, and consequently there is a risk of a reduction in surface quality. Such a reduction in surface quality due to the streak pattern can be prevented by restricting Fe to 0.30%% or less, Si to 0.30% or less, and Mn to 0.02% or less.

Ti: 0.001% or More and 0.05% or Less

By being added to the aluminum alloy, Ti has the effect of making the ingot structure fine. The finer the ingot structure, the more a surface state of high luster without spots is obtained, and consequently incorporating Ti makes it possible to improve surface quality. If the Ti content is less than 0.001%, then the ingot structure is not made sufficiently fine, and consequently there is a risk that spots and streak patterns will arise on the surface of the above-mentioned high-strength aluminum alloy material. In addition, if the Ti content is greater than 0.05%, then an AlTi-based intermetallic compound or the like will be formed with the Al, and dot-like or streak pattern defects will tend to be generated, and consequently there is a risk that surface quality will decrease.

Next, as described above, the metallographic structure of the above-mentioned high-strength aluminum alloy material comprises a granular recrystallized structure. Because an aluminum alloy prepared by performing hot working normally has a metallographic structure composed of fibrous structures, there is a risk that streak patterns will arise on the surface and, as a result, that the surface quality will decrease. On the other hand, in the above-mentioned high-strength aluminum alloy, the metallographic structure comprises a recrystallized structure, and consequently streak patterns are not formed on the surface and therefore surface quality is satisfactory.

In addition, in the above-mentioned high-strength aluminum alloy, a b* value (chromaticity of blue to yellow), which is part of the L*a*b* color system stipulated in JIS Z8729 (ISO 7724-1), measured after the anodization treatment using a sulfuric-acid bath is preferably 0 or more and 0.8 or less. After the anodization treatment, an aluminum-alloy material having the b* value within the above-mentioned range has a suitable yellow-color density and becomes an aluminum-alloy material that excels in design characteristics.

By having at least the above-mentioned specific chemical composition, the above-mentioned high-strength aluminum alloy material can achieve a color tone with a b* value of 0.8 or less. If the b* value exceeds 0.8, then the color tone after the anodization treatment will be tinged yellow, and consequently there is a risk of a deterioration in design characteristics. Furthermore, if the anodization treatment is performed on the aluminum-alloy material having the above-mentioned chemical composition, then it is problematic to obtain the aluminum-alloy material having a b* value of less than 0.

In addition, in the above-mentioned recrystallized structure, the average grain diameter of the crystal grains is preferably 500 μm or less, and the crystal length in the direction parallel to the hot-working direction is preferably 0.5 times or more and 4 times or less than that of the crystal length in the direction perpendicular to the hot-working direction.

If the above-mentioned average grain diameter of the crystal grains exceeds 500 μm, then the crystal grains become excessively coarse, and consequently spots tend to form on the surface after a surface treatment, such as the anodization treatment, is performed, and therefore there is a risk that surface quality will decrease. Consequently, the smaller the average grain diameter of the crystal grains, the better.

In addition, if the aspect ratio of the above-mentioned crystal grains, that is, the ratio of the crystal length in the direction parallel to the hot-working direction with respect to the crystal length in the direction perpendicular to the hot-working direction, exceeds 4, then there is a risk that streak patterns will appear on the surface after the anodization treatment has been performed. On the other hand, crystal grains having an aspect ratio of less than 0.5 are difficult to obtain with generally used manufacturing equipment.

Furthermore, it is possible to confirm whether the above-mentioned metallographic structure is a recrystallized structure by, for example, electrolytic polishing the surface of the aluminum-alloy material and then observing the resulting surface using a polarizing microscope. That is, if the above-mentioned metallographic structure is composed of a recrystallized structure, then a uniform metallographic structure composed of granular crystals will be observed, and a solidified structure, which could be formed during casting, as represented by coarse intermetallic compounds, floating crystals, and the like, will not be seen. Similarly, a stripe-shaped structure (a so-called worked structure) formed by plastic working, such as extrusion or rolling, will not be seen in a metallographic structure composed of a recrystallized structure.

In addition, the average grain diameter of the crystal grains in the above-mentioned recrystallized structure can be calculated by sectioning, in accordance with the sectioning method stipulated in JIS G0551 (ASTM E 112-96, ASTM E 1382-97), the metallographic image obtained by observation using the polarizing microscope described above. That is, the average grain diameter can be calculated by drawing, at an arbitrary position in the above-mentioned metallographic image, one sectioning-plane line in each of the longitudinal, transverse, and diagonal directions, and then dividing the length of each sectioning-plane line by the number of crystal-grain boundaries that intersect the sectioning-plane line.

In addition, the above-mentioned aspect ratio, that is, the ratio of the crystal length in the direction parallel to the hot-working direction with respect to the crystal length in the direction perpendicular to the hot-working direction, can be calculated in accordance with the method described above. That is, as in the method described above, sectioning-plane lines are drawn at an arbitrary position in the above-mentioned metallographic image in the direction parallel to and the direction perpendicular to the hot-working direction, and the average grain diameter is calculated in the direction parallel to and the direction perpendicular to the hot-working direction from each of the sectioning-plane lines. Furthermore, the above-mentioned aspect ratio can be calculated by dividing the average grain diameter in the direction parallel to the hot-working direction by the average grain diameter in the direction perpendicular to the hot-working direction.

In addition, the above-mentioned recrystallized structure is preferably one that is formed during hot working. The recrystallized structure can be classified, depending on the manufacturing process, into a dynamic recrystallized structure and a static recrystallized structure; a recrystallized structure that is formed through the performance of repetitive recrystallization simultaneous with deformation during the hot working is called a dynamic recrystallized structure. On the other hand, a static recrystallized structure means one formed by first performing hot working or cold working, and then adding a heat-treatment process, such as a solution heat treatment or an annealing treatment. The problem to be solved by the present invention described above can be solved for either recrystallized structure; however, in the case of the dynamic recrystallized structure, the production process is simple, and therefore the structure can be manufactured easily.

Next, in the above-mentioned high-strength aluminum alloy material manufacturing method, a homogenization treatment is performed wherein an ingot having the above-mentioned chemical composition is heated at a temperature above 540° C. and 580° C. or lower for 1 h or more and 24 h or less. If the heating temperature in the above-mentioned homogenization treatment is 540° C. or lower, then the homogenization of the ingot segregation layer will be insufficient. As a result, the crystal grains will become coarse, a nonuniform crystalline structure will be formed, and the like, consequently reducing the surface quality of the alloy material ultimately obtained. On the other hand, if the heating temperature is higher than 580° C., then there is a risk that the ingot will melt locally, consequently making manufacture difficult. Accordingly, the temperature of the above-mentioned homogenization treatment is preferably above 540° C. and 580° C. or lower.

In addition, if the heating time for the above-mentioned homogenization treatment is less than 1 h, then the homogenization of the ingot segregation layer will be insufficient, and consequently the final surface quality will decrease in the same manner as described above. On the other hand, if the heating time exceeds 24 h, then a state will result wherein the ingot segregation layer has been sufficiently homogenized, and consequently no further effect can be expected. Accordingly, the time for the above-mentioned homogenization treatment is preferably 1 h or more and within 24 h.

The ingot subjected to the above-mentioned homogenization treatment undergoes hot working and thereby is made into a wrought material. The temperature of the ingot at the start of the hot working is set to 440° C. or higher and 560° C. or lower. If the heating temperature of the ingot before the hot working is lower than 440° C., then the deformation resistance will be high, making it difficult to work using generally used manufacturing equipment. On the other hand, if the hot working is performed after the ingot has been heated to a temperature that exceeds 560° C., then the ingot locally melts owing to the inclusion of the heat generated during the working; as a result, there is a risk that hot cracking will occur. Accordingly, the temperature of the ingot before the hot working is preferably 440° C. or higher and 560° C. or lower. Furthermore, extruding, rolling, and the like can be employed as the above-mentioned hot working.

In addition, after the above-mentioned hot working, a quenching treatment is performed wherein cooling is started while the temperature of the wrought material is 400° C. or higher, and the temperature of the wrought material is then cooled until it becomes 150° C. or lower. If the temperature of the wrought material before the above-mentioned quenching treatment is below 400° C., then the quench-hardening effect will be insufficient and there is a risk that the proof stress of the wrought material obtained as a result will be less than 350 MPa. In addition, in the case wherein the temperature of the wrought material after the quenching treatment exceeds 150° C., too, the quench-hardening effect will be insufficient and there is a risk that the proof stress of the wrought material obtained as a result will be less than 350 MPa.

Furthermore, the above-mentioned quenching treatment means a treatment that cools the wrought material by a forcible means. For example, methods such as forcible quenching using a fan, shower cooling, water cooling, or the like can be employed as the above-mentioned quenching treatment.

In addition, in the above-mentioned quenching treatment, while the temperature of the wrought material is in the range of from 400° C. to 150° C., the average cooling rate is controlled such that it is 1° C./s or more and 300° C./s or less. If the average cooling rate exceeds 300° C./s, then excessively robust equipment will be needed and, moreover, a commensurate effect cannot be obtained. On the other hand, if the average cooling rate is less than 1° C./s, then the quench-hardening effect will be insufficient, and consequently there is a risk that the proof stress of the wrought material obtained will fall below 350 MPa. Accordingly, a faster average cooling rate is better, preferably 1° C./s or more and 300° C./s or less, and more preferably 3° C./s or more and 300° C./s or less.

In addition, after the above-mentioned quenching treatment has been performed, the temperature of the wrought material is brought to room temperature. The temperature may be brought to room temperature by the above-mentioned quenching treatment or by performing an additional cooling treatment after the quenching treatment. Because the effect of room-temperature aging arises by virtue of bringing the temperature of the wrought material to room temperature, the strength of the wrought material increases. Furthermore, for example, methods such as fan air cooling, mist cooling, shower cooling, water cooling, or the like can be employed as the above-mentioned additional cooling treatment.

Here, if the above-mentioned wrought material is stored in the state wherein it is maintained at room temperature, then the strength of the wrought material will further increase owing to the effect of the room-temperature aging. In the initial stage, the longer the room-temperature aging time, the greater the increase in strength; however, when the room-temperature aging time becomes 24 h or more, the effect of room-temperature aging reaches its maximum.

Next, an artificial-aging treatment is performed wherein the above-mentioned wrought material, which has been cooled to room temperature as described above, is heated. The performance of the artificial-aging treatment finely and uniformly precipitates MgZn2 into the above-mentioned wrought material, and consequently the proof stress of the wrought material can easily be set to 350 MPa or more. Any of the aspects below can be applied as specific conditions of the above-mentioned artificial-aging treatment.

First, a first artificial-aging treatment, wherein the above-mentioned wrought material is heated at a temperature of 80-120° C. for 1-5 h, and thereafter a second artificial-aging treatment, which is performed following the first artificial-aging treatment and wherein the wrought material is heated at a temperature of 145-200° C. for 2-15 h, can be performed as the above-mentioned artificial-aging treatment.

Here, successively performing the first artificial-aging treatment and the second artificial-aging treatment means completing the first artificial-aging treatment and thereafter performing the second artificial-aging treatment while maintaining the temperature of the wrought material. That is, the wrought material should not be cooled between the first artificial-aging treatment and the second artificial-aging treatment; as a specific method, there is a method wherein, after the first artificial-aging treatment, the second artificial-aging treatment is performed without removing the wrought material from the heat-treatment furnace.

Thus, by successively performing the above-mentioned first artificial-aging treatment and the above-mentioned second artificial-aging treatment, the artificial-aging treatment time can be shortened. In addition, the treatment temperature of the second artificial-aging treatment is preferably 145-200° C. If the heating in the second artificial-aging treatment is performed in the range of 170-200° C., then the ductility of the wrought material increases, and consequently the workability can be further improved. Furthermore, if the conditions in the second artificial-aging treatment deviate from the above-mentioned temperature range or time range, then there are risks that stress corrosion cracking will tend to occur in the wrought material obtained, the proof stress will become less than 350 MPa, and the like.

In addition, a treatment wherein the wrought material is heated at a temperature of 145-170° C. for 1-24 h can also be performed as the above-mentioned artificial-aging treatment. In this case, the manufacturing process becomes simplified, and consequently manufacture can be performed easily. If the above-mentioned artificial-aging treatment deviates from the above-mentioned temperature range or time range, then there is a risk that stress corrosion cracking will occur in the wrought material obtained, the proof stress will become less than 350 MPa, it will become difficult to obtain a wrought material having sufficient strength properties, and the like.

WORKING EXAMPLES Working Example 1

Working examples according to the above-mentioned high-strength aluminum alloy material will be explained, making use of Table 1 to Table 3. In the present example, as shown in Table 1, samples (Nos. 1-30) having varying chemical compositions of the aluminum-alloy material were prepared under the same manufacturing conditions, and the strength of each sample was measured and the metallographic structure of each sample was observed. Furthermore, after each sample was subjected to a surface treatment, a surface-quality evaluation was performed.

Below, the manufacturing conditions, the strength measuring method, the metallographic structure observing method, as well as the surface treating method and the surface-quality-evaluating method of the samples will be explained.

TABLE 1 Sample Chemical Composition (mass %) Zn/Mg No. Zn Mg Cu Fe Si Mn Cr Zr Ti Al Ratio 1 5.2 2.4 0.01 0.11 0.09 0.01 0.01 0.01 0.018 bal. 2.1 2 6.8 2.4 0.06 0.12 0.08 0.01 0.01 0.00 0.021 bal. 2.9 3 5.9 2.2 0.01 0.12 0.09 0.01 0.01 0.00 0.017 bal. 2.7 4 5.9 2.8 0.08 0.12 0.08 0.01 0.01 0.00 0.007 bal. 2.1 5 5.2 2.9 0.04 0.11 0.09 0.01 0.01 0.00 0.018 bal. 1.8 6 6.9 2.2 0.06 0.12 0.08 0.01 0.01 0.01 0.021 bal. 3.1 7 6.1 2.4 0.01 0.10 0.09 0.01 0.01 0.00 0.008 bal. 2.5 8 6.0 2.5 0.09 0.11 0.08 0.01 0.01 0.00 0.008 bal. 2.4 9 5.9 2.4 0.06 0.29 0.08 0.01 0.01 0.00 0.020 bal. 2.5 10 5.9 2.4 0.06 0.12 0.28 0.00 0.01 0.00 0.011 bal. 2.5 11 5.9 2.5 0.06 0.13 0.09 0.02 0.00 0.00 0.009 bal. 2.4 12 5.8 2.5 0.05 0.12 0.08 0.01 0.02 0.03 0.013 bal. 2.3 13 5.8 2.5 0.05 0.12 0.02 0.01 0.00 0.05 0.012 bal. 2.3 14 5.8 2.5 0.05 0.12 0.08 0.01 0.01 0.00 0.002 bal. 2.3 15 5.8 2.5 0.05 0.12 0.09 0.00 0.01 0.00 0.043 bal. 2.3

TABLE 2 Sample Chemical Composition (mass %) Zn/Mg No. Zn Mg Cu Fe Si Mn Cr Zr Ti Al Ratio 16 4.9 2.3 0.08 0.12 0.09 0.01 0.01 0.01 0.012 bal. 2.1 17 7.1 2.3 0.08 0.13 0.08 0.01 0.01 0.00 0.017 bal. 3.1 18 5.9 2.1 0.07 0.13 0.08 0.01 0.01 0.00 0.020 bal. 2.8 19 5.9 3.2 0.07 0.12 0.08 0.01 0.01 0.00 0.022 bal. 1.8 20 4.9 3.2 0.07 0.12 0.09 0.01 0.01 0.00 0.023 bal. 1.5 21 7.1 2.1 0.07 0.13 0.09 0.01 0.01 0.00 0.021 bal. 3.4 22 6.0 2.3 0.00 0.11 0.09 0.01 0.01 0.00 0.010 bal. 2.6 23 5.9 2.4 0.13 0.12 0.08 0.01 0.01 0.00 0.012 bal. 2.5 24 6.1 2.4 0.07 0.34 0.08 0.01 0.01 0.00 0.015 bal. 2.5 25 6.1 2.4 0.07 0.12 0.33 0.01 0.01 0.00 0.015 bal. 2.5 26 6.1 2.4 0.07 0.13 0.09 0.04 0.00 0.00 0.017 bal. 2.5 27 6.1 2.4 0.07 0.12 0.09 0.01 0.04 0.00 0.016 bal. 2.5 28 6.1 2.4 0.06 0.12 0.09 0.01 0.01 0.07 0.018 bal. 2.5 29 5.9 2.4 0.06 0.12 0.08 0.01 0.01 0.00 0.000 bal. 2.5 30 6.0 2.4 0.06 0.07 0.08 0.01 0.01 0.01 0.070 bal. 2.5

<Manufacturing Conditions of Samples>

Ingots having a diameter of 90 mm and the chemical compositions listed in Table 1 and Table 2 were cast by semi-continuous casting. Subsequently, a homogenization treatment was performed wherein the ingots were heated at a temperature of 550° C. for 6 h. Subsequently, the ingots were hot extruded in the state wherein the temperature of the ingots was 520° C., thereby forming wrought materials having a width of 35 mm and a thickness of 7 mm. Subsequently, a quenching treatment was performed in which, in the state wherein the temperature of the wrought materials was 505° C., the wrought materials were cooled to 100° C. at an average cooling rate of 60° C./s. Furthermore, the wrought materials subjected to the above-mentioned quenching treatment were cooled to room temperature and then subjected to room-temperature aging for 24 h at room temperature. Subsequently, the first artificial-aging treatment was performed wherein the above-mentioned wrought materials were heated using a heat-treatment furnace at a temperature of 100° C. for 4 h. Thereafter, the second artificial-aging treatment was performed wherein the furnace temperature was raised to 160° C. without removing the heating wrought materials from the heat-treatment furnace, and the wrought materials were heated at 160° C. for 8 h, thereby making the samples.

<Strength Measuring Method>

Test pieces were collected from the samples using the method in accordance with JIS Z2241 (ISO 6892-1), and tension tests that measure tensile strength, proof stress, and elongation were performed. The strength characteristic of those exhibiting a proof stress of 350 MPa or more in the tension test results was judged to be acceptable.

<Metallographic Structure Observing Method>

After the samples were subjected to electrolytic polishing and electrolytic etching, micrographs of the sample surfaces were acquired using a polarizing microscope having a magnification of 50-100 times. Image analysis was performed on the micrographs and, as described above, the average grain diameter of the crystal grains constituting the metallographic structure of each of the samples was derived in accordance with the sectioning method stipulated in JIS G0551. In addition, as described above, each of the aspect ratios (indicating the ratio of the crystal length in the direction parallel to the hot-working direction with respect to the crystal length in the direction perpendicular to the hot-working direction) was calculated by dividing the average grain diameter in the direction parallel to the hot-working direction by the average grain diameter in the direction perpendicular to the hot-working direction. As a result, those having an average grain diameter of 500 μm or less and those having an aspect ratio within a range of 0.5-4.0 were judged to be preferable results.

<Surface Treating Method>

After buffing the surfaces of the samples that were subjected to the artificial-aging treatment, the samples were etched with an aqueous solution of sodium hydroxide and afterward subjected to a desmutting treatment. The samples subjected to the desmutting treatment were chemically polished using a phosphoric acid—nitric acid method at a temperature of 90° C. for 2 min. Furthermore, the samples subjected to the chemical polishing were subjected to an anodization treatment at an electric current density of 150 A/m2 in a 15% sulfuric-acid bath, thereby forming 10-μm anodic oxide films. Lastly, the anodic oxide films were subjected to a sealing treatment by immersing the samples, after they were subjected to the anodization treatment, in boiling water.

<Surface-Quality-Evaluating Method>

The surfaces of the samples subjected to the above-mentioned surface treatment were visually observed. In the visual observation, those wherein a streak pattern, a spotting pattern, a dot-like defect, or the like did not appear on the surface were judged to be acceptable.

Subsequently, the color tone of the surfaces of the samples were measured using a color-difference meter to obtain the coordinate values in the L*a*b* color system described in JIS Z8729. As a result, those having a b* value (chromaticity of blue to yellow) within a range of 0-0.8 were judged to be acceptable.

<Stress-Corrosion-Cracking Testing Method>

Tests were performed in accordance with JIS H8711 (ISO 9591). A C-ring shaped test piece was cut out from each sample, the test piece being provided with a notched part in a portion along the circumference of the ring shape having an outer diameter of 20 mm, an inner diameter of 17 mm, and an axial-direction thickness of 7 mm. The direction that connects the center of the C-ring shape with the notched part is aligned with the extrusion direction during sample preparation. In stress loading the test piece, a stress of 330 MPa is loaded in the direction that compresses the C-ring shape in the direction orthogonal to the above-mentioned extrusion direction. In this loaded state, alternating immersion is performed for 720 h in an atmosphere having a temperature of 25° C., during which each test piece is alternately immersed in a 3.5% aqueous solution of NaCl for 10 min and then dried for 50 min. The test results were judged based on the presence or absence of crack generation. Those without cracks were assigned “good” (∘), and those with cracks were assigned “bad” (x).

The evaluation results of the samples listed in Table 1 and Table 2 are shown in Table 3. Furthermore, those evaluation results in Table 3 not judged to be acceptable or not judged to be a preferable result are underlined.

TABLE 3 Average Stress- Tensile Crystal Grain Presence Corrosion- Sample Strength Proof Stress Elongation Diameter Aspect of Streak b* Cracking No. (MPa) (Mpa) (%) (μm) Ratio Pattern Value Resistance 1 396 367 21 120 1.8 No 0.3 ∘ 2 468 448 14 110 1.5 No 0.2 ∘ 3 388 355 18 100 1.2 No 0.3 ∘ 4 475 460 16 100 1.2 No 0.3 ∘ 5 390 435 21 100 1.3 No 0.4 ∘ 6 461 438 16 100 1.3 No 0.4 ∘ 7 455 425 17 110 1.4 No 0.4 ∘ 8 460 431 18 110 1.2 No 0.6 ∘ 9 467 438 14 120 1.2 No 0.3 ∘ 10 464 435 15 120 1.2 No 0.2 ∘ 11 461 432 15 130 1.2 No 0.3 ∘ 12 466 438 16 100 1.4 No 0.2 ∘ 13 465 435 16 120 1.4 No 0.7 ∘ 14 463 431 16 120 3.5 No 0.2 ∘ 15 468 438 16 130 1.3 No 0.3 ∘ 16 379 348 22 110 1.2 No 0.3 ∘ 17 471 452 13 110 1.3 No 0.3 x 18 382 347 21 110 1.2 No 0.2 ∘ 19 481 450 12 120 1.5 No 0.2 x 20 379 349 21 110 1.3 No 0.3 ∘ 21 462 431 15 110 1.2 No 0.3 x 22 468 438 16 120 1.3 No 0.4 x 23 467 438 16 100 1.2 No 1.1 ∘ 24 466 435 16 — >4   Yes 0.3 ∘ 25 467 437 16 — >4   Yes 0.4 ∘ 26 469 438 15 — 1.3 Yes 0.4 ∘ 27 462 433 16 150 1.5 Yes 1.3 ∘ 28 467 437 16 — >4   Yes 0.3 ∘ 29 467 438 15 >500  1.8 Yes 0.4 ∘ 30 464 435 15 100 1.5 Yes 0.3 ∘

As can be understood from Table 3, samples 1-15 were acceptable for all evaluation items and exhibited excellent characteristics for strength, surface quality, and stress-corrosion-cracking resistance.

As a representative example of a sample having excellent surface quality, FIG. 1 shows the metallographic structure observation result of sample 4. As can be understood from the same figure, samples having an excellent surface quality have a metallographic structure composed of a granular recrystallized structure and, simultaneously, no streak pattern is observed even by visual confirmation and the samples have a high luster without any spots.

In sample 16, the Zn content was too low, and consequently a sufficient strength improving effect was not obtained and therefore the proof stress was judged to be unacceptable.

In sample 17, the Zn content was too high, and consequently stress-corrosion-cracking resistance was poor and was judged to be unacceptable.

In sample 18, the Mg content was too low, and consequently a sufficient strength improving effect was not obtained and the proof stress was judged to be unacceptable.

In sample 19, the Mg content was too high, and consequently cracks formed in portions during extrusion; furthermore, stress-corrosion-cracking resistance was poor and was judged to be unacceptable.

In sample 20, the Zn/Mg ratio was too low, and consequently strength was poor and was judged to be unacceptable.

In sample 21, the Zn/Mg ratio was too high, and consequently stress-corrosion-cracking decreased and was judged to be unacceptable.

In sample 22, the Cu content was too low, and consequently stress-corrosion-cracking resistance was poor and judged to be unacceptable.

In sample 23, the Cu content was too high, and consequently the surface color tone was tinged yellow and judged to be unacceptable.

In sample 24, the Fe content was too high, and consequently a fibrous structure was formed; as a result, a streak pattern was visually confirmed on the surface and judged to be unacceptable.

In sample 25, the Si content was too high, and consequently a fibrous structure was formed; as a result, a streak pattern was visually confirmed on the surface and judged to be unacceptable.

In sample 26, the Mn content was too high, and consequently a fibrous structure was formed; as a result, a streak pattern was visually confirmed on the surface and judged to be unacceptable.

In sample 27, the Cr content was too high, and consequently the surface color tone was tinged with yellow and judged to be unacceptable.

In sample 28, the Zr content was too high, and consequently a fibrous structure was formed; as a result, a streak pattern was visually confirmed on the surface and judged to be unacceptable.

In sample 29, the Ti content was too low, and consequently a streak pattern caused by the coarse ingot structure appeared and was judged to be unacceptable.

In sample 30, the Ti content was too high, and consequently the Ti formed an intermetallic compound with the Al; as a result, stripe shapes and dot-like defects were visually confirmed on the surface and judged to be unacceptable.

Working Example 2

Next, a working example according to the above-mentioned high-strength aluminum alloy manufacturing method will be explained, making use of Table 4 to Table 6.

In the present example, samples (No. A1-A29) were prepared, using an aluminum alloy (material No. A) containing the chemical composition listed in Table 4, under varying manufacturing conditions as listed in Table 5 and Table 6, after which the strength of each sample was measured and the metallographic structure of each sample was observed. Furthermore, after each sample was subject to a surface treatment, a surface-quality evaluation was performed.

Below, the manufacturing conditions of each sample will be explained. Furthermore, the strength measuring method, the metallographic structure observing method, the surface treating method, and the surface-quality-evaluating method for each sample were performed using the same methods as those in Working Example 1.

<Manufacturing Conditions of Samples>

An ingot having a diameter of 90 mm and the chemical composition listed in Table 4 was cast by semi-continuous casting. Subsequently, using combinations of the temperatures, times, and average cooling rates listed in Table 5 and Table 6, the above-mentioned ingot was subjected to, in order, a homogenization treatment, hot extrusion, a quenching treatment, a first artificial-aging treatment, and a second artificial-aging treatment, and thereby the samples were obtained. Furthermore, “room-temperature aging time” in Table 5 and Table 6 means the time from when the wrought material reaches room temperature after the performance of the quenching treatment until the performance of the first artificial-aging treatment.

TABLE 4 Sample Chemical Composition (mass %) Zn/Mg No. Zn Mg Cu Fe Si Mn Cr Zr Ti Al Ratio A 6.0 2.3 0.04 0.11 0.06 0.00 0.01 0.00 0.012 bal. 2.6

TABLE 5 Quenching Room- Homogenization Hot Working Treatment Temp. First Artificial Second Artifical Treatment Extruding Cooling Final Aging Aging Aging Sample Temp. Time Temp. Rate Temp. Time Temp. Time Temp. Time No. (° C.) (h) (° C.) (° C./s) (° C.) (h) (° C.) (h) (° C.) (h) A1 540 6 520 60 100 24 100 4 160 8 A2 577 6 520 60 100 24 100 4 160 8 A3 550 1 520 60 100 24 100 4 160 8 A4 550 24 520 60 100 24 100 4 160 8 A5 550 6 440 60 100 24 100 4 160 8 A6 550 6 560 60 100 24 100 4 160 8 A7 550 6 520 1 100 24 100 4 160 8 A8 550 6 520 300 100 24 100 4 160 8 A9 550 6 520 60 150 24 100 4 160 8 A10 550 6 520 60 100 None 100 4 160 8 A11 550 6 520 60 100 240 100 4 160 8 A12 550 6 520 60 100 24 80 5 160 8 A13 550 6 520 60 100 24 120 1 160 8 A14 550 6 520 60 100 24 100 4 145 15 A15 550 6 520 60 100 24 100 4 200 2 A16 550 6 520 60 100 24 145 15 — — A17 550 6 520 60 100 24 170 2 — —

TABLE 6 Quenching Room- Homogenization Hot Working Treatment Temp. First Artificial Second Artifical Treatment Extruding Cooling Final Aging Aging Aging Sample Temp. Time Temp. Rate Temp. Time Temp. Time Temp. Time No. (° C.) (h) (° C.) (° C./s) (° C.) (h) (° C.) (h) (° C.) (h) A18 535 6 520 60 100 24 100 4 160 8 A19 550   0.25 520 60 100 24 100 4 160 8 A20 550 6 570 — — — — — — — A21 550 6 520    0.25 100 24 100 4 160 8 A22 550 6 520 60 100 24  80 3 140 14  A23 550 6 520 60 100 24 120 5 210 2 A24 550 6 520 60 100 24 100 4 145 1 A25 550 6 520 60 100 24 100 4 200 16  A26 550 6 520 60 100 24 140 23  — — A27 550 6 520 60 100 24 180 2 — — A28 550 6 520 60 100 24 165   0.25 — — A29 550 6 520 60 100 24 165 30  — —

The evaluation results of the samples prepared as described above are listed in Table 7. Furthermore, those measurement results in Table 7 that were not judged to be acceptable or were not judged to have a preferable result are underlined.

TABLE 7 Average Stress- Tensile Crystal Grain Presence Corrosion- Sample Strength Proof Stress Elongation Diameter Aspect of Streak b* Cracking No. (MPa) (Mpa) (%) (μm) Ratio Pattern Value Resistance A1 398 366 21 130 1.2 No 0.3 ∘ A2 488 479 14 110 1.3 No 0.3 ∘ A3 386 355 20 120 1.2 No 0.3 ∘ A4 489 468 13 100 1.1 No 0.2 ∘ A5 398 368 22 120 1.3 No 0.3 ∘ A6 486 471 14 120 1.2 No 0.3 ∘ A7 389 358 19 120 1.2 No 0.2 ∘ A8 486 472 14 130 1.3 No 0.3 ∘ A9 393 363 19 120 1.2 No 0.3 ∘ A10 484 462 14 110 1.2 No 0.2 ∘ A11 483 463 14 110 1.2 No 0.3 ∘ A12 399 369 21 110 1.2 No 0.2 ∘ A13 451 421 15 120 1.2 No 0.3 ∘ A14 482 461 12 120 1.2 No 0.2 ∘ A15 391 362 21 120 1.3 No 0.3 ∘ A16 479 458 13 120 1.1 No 0.2 ∘ A17 392 363 21 110 1.1 No 0.4 ∘ A18 377 349 22 210 >4   Yes 0.3 ∘ A19 376 346 20 510 1.3 Yes 0.3 ∘ A20 — — — — — — — — A21 360 330 22 120 1.1 No 0.3 ∘ A22 480 452 14 120 1.1 No 0.2 x A23 372 340 21 140 1.2 No 0.3 ∘ A24 375 345 20 130 1.2 No 0.2 x A25 369 341 20 120 1.3 No 0.3 ∘ A26 479 451 15 120 1.4 No 0.2 x A27 370 341 21 120 1.2 No 0.3 ∘ A28 371 342 21 110 1.2 No 0.2 ∘ A29 360 349 20 120 1.3 No 0.3 ∘

As can be understood from Table 7, samples A1-A17 were acceptable for all evaluation items and exhibited characteristics excelling in both strength and surface quality.

In sample A18, the heating temperature in the homogenization treatment was too low, and consequently the proof stress was less than 350 MPa and judged to be unacceptable. Simultaneously, the crystal grains became coarse and a spotting pattern was also visually confirmed on the surface.

In sample A19, the treatment time of the homogenization treatment was too short, and consequently the proof stress was less than 350 MPa and judged to be unacceptable. Simultaneously, the crystal grains became coarse and a spotting pattern was also visually confirmed on the surface.

In sample A20, the heating temperature of the ingot, before the hot extrusion, was too high, and consequently the ingot partially melted during the extrusion; as a result, hot-working cracks formed and therefore treatments subsequent to the quenching treatment could not be performed.

In sample A21, the average cooling rate of the quenching treatment was too low, and consequently the quench-hardening effect was insufficient and the proof stress was less than 350 MPa and judged to be unacceptable.

In sample A22, the treatment temperature of the second artificial-aging treatment was too low, and consequently stress-corrosion-cracking resistance was insufficient and judged to be unacceptable.

In sample A23, the treatment temperature of the second artificial-aging treatment was too high, and consequently overaging occurred and the proof stress was less than 350 MPa and judged to be unacceptable.

In sample A24, the treatment time of the second artificial-aging treatment was too short, and consequently the age hardening was insufficient, the proof stress was less than 350 MPa, and stress-corrosion-cracking resistance was also insufficient and judged to be unacceptable.

In sample A25, the treatment time of the second artificial-aging treatment was too long, and consequently overaging occurred and the proof stress was less than 350 MPa and judged to be unacceptable.

In sample A26, only one stage of artificial-aging treatment was performed, and the treatment temperature of that artificial-aging treatment was too low, and consequently stress-corrosion-cracking resistance was insufficient and judged to be unacceptable.

In sample A27, only one stage of artificial-aging treatment was performed, and the treatment temperature of that artificial-aging treatment was too high, and consequently overaging occurred and the proof stress was less than 350 MPa and judged to be unacceptable.

In sample A28, the treatment time of the first artificial-aging treatment was too short, and consequently the age hardening was insufficient and the proof stress was less than 350 MPa and judged to be unacceptable.

In sample A29, the treatment time of the first artificial-aging treatment was too long, and consequently overaging occurred and the proof stress was less than 350 MPa and judged to be unacceptable.

FIG. 2 shows the result of the observation of the metallographic structure of sample A19 as a representative example of a sample, from among the samples where the surface quality was unacceptable, in which a streak pattern was visually confirmed. As can be understood from the same figure, samples wherein streak patterns were visually confirmed have a metallographic structure composed of fibrous structures.

Claims

1.-5. (canceled)

6. An aluminum alloy, comprising in mass percent:

Zn: 5.0% or more and 7.0% or less,
Mg: more than 2.2% and 3.0% or less,
Cu: 0.01% or more and 0.10% or less,
Zr: 0.10% or less,
Cr: 0.02% or less,
Fe: 0.30% or less,
Si: 0.30% or less,
Mn: 0.02% or less, and
Ti: 0.001% or more and 0.05% or less,
the remainder being composed of Al and unavoidable impurities,
wherein the aluminum alloy has: a Zn/Mg ratio of 1.7 or more and 3.1 or less, a proof stress of 350 MPa or more, and a metallographic structure composed of a recrystallized structure.

7. The aluminum alloy according to claim 6, wherein:

the recrystallized structure includes crystal grains having an average grain diameter of 500 μm or less, and
a crystal grain length in a direction parallel to a hot working direction is 0.5 to 4 times as long as a crystal grain length in a direction perpendicular to the hot working direction.

8. The aluminum alloy according to claim 7, wherein Zn is more than 5.2% and 6.8% or less.

9. The aluminum alloy according to claim 8, wherein the aluminum alloy has an anodized surface that has a b* value of 0 or more and 0.8 or less.

10. The aluminum alloy according to claim 9, wherein the recrystallized structure is a granular recrystallized structure.

11. The aluminum alloy according to claim 6, wherein Zn is more than 5.2% and 6.8% or less.

12. The aluminum alloy according to claim 6, wherein the aluminum alloy has an anodized surface that has a b* value of 0 or more and 0.8 or less.

13. The aluminum alloy according to claim 6, wherein the recrystallized structure is a granular recrystallized structure.

14. A process for producing a wrought aluminum alloy material, which comprises:

preparing an ingot having a chemical composition comprising, in mass percent, Zn: 5.0% or more and 7.0% or less, Mg: more than 2.2% and 3.0% or less, Cu: 0.01% or more and 0.10% or less, Zr: 0.10% or less, Cr: 0.02% or less, Fe: 0.30% or less, Si: 0.30% or less, Mn: 0.02% or less, and Ti: 0.001% or more and 0.05% or less, the remainder being composed of Al and unavoidable impurities and a Zn/Mg ratio being 1.7 or more and 3.1 or less;
performing a homogenization treatment that heats the ingot at a temperature of higher than 540° C. and 580° C. or lower for 1 hour to 24 hours;
subsequently, forming a wrought material by performing hot working on the ingot in a state where the temperature of the ingot at the beginning of the working is 440° C. to 560° C.;
while the wrought material is still at 400° C. or higher, starting to cool it and subsequently performing a quenching treatment such that, while the wrought material is cooling down from 400° C. to 150° C., the average cooling rate is 1° C./s or more and 300° C./s or less;
cooling the temperature of the wrought material to room temperature by said quenching treatment or by an additional cooling treatment; and
thereafter, performing an artificial aging treatment on the wrought material.

15. The process according to claim 14, wherein the artificial aging treatment comprises performing a first artificial aging treatment at a temperature of 80° C. to 120° C. for 1 hour to 5 hours, and continuously after the first artificial aging treatment, performing a second artificial aging treatment that heats the wrought material at a temperature of 145° C. to 200° C. for 2 hours to 15 hours.

16. The process according to claim 14, wherein the artificial aging treatment comprises heating the wrought material at a temperature of 145° C. to 170° C. for 1 hour to 24 hours.

17. The process according to claim 14, wherein the hot working involves extrusion or rolling.

18. The process according to claim 14, wherein during the quenching step, the average cooling rate is 3° C./s or more and 300° C./s or less

19. The process according to claim 14, wherein the second artificial aging treatment is performed at a temperature of 170° C. to 200° C.

20. The process according to claim 14, further comprising anodizing the wrought material after the artificial aging treatment.

21. The process according to claim 14, wherein:

the homogenization treatment is performed at 550° C. for 6 hours,
the hot working comprises hot extruding the ingot while the temperature of the ingot is at 520° C.,
the quenching treatment is initiated while the temperature of the wrought material is at 505° C. and the average cooling rate of the quenching treatment is 60° C./sec, thereafter the wrought material is subjected to room temperature aging for 24 hours,
the first artificial aging treatment involves heating the wrought material at 100° C. for 4 hours, and
the second artificial aging treatment involves heating the wrought material at 160° C. for 8 hours.

22. The process according to claim 21, further comprising anodizing the wrought material after the second artificial aging treatment.

23. A process for producing the aluminum alloy of claim 6, comprising:

homogenizing an ingot having the elemental composition recited in claim 6 at a temperature of higher than 540° C. and 580° C. or lower for at least 1 hour;
hot working on the homogenized ingot while the temperature of the ingot at the beginning of the hot working is 440° C. to 560° C., thereby forming a wrought material,
while the wrought material is still at 400° C. or higher, starting to cool it and subsequently performing a quenching treatment such that, while the wrought material is cooling down from 400° C. to 150° C., the average cooling rate is 1° C./s or more and 300° C./s or less;
cooling the temperature of the wrought material to room temperature by said quenching treatment or by an additional cooling treatment; and
thereafter, performing an artificial aging treatment on the wrought material.

24. The process according to claim 23, wherein the artificial aging treatment comprises performing a first artificial aging treatment at a temperature of 80° C. to 120° C. for 1 hour to 5 hours, and continuously after the first artificial aging treatment, performing a second artificial aging treatment that heats the wrought material at a temperature of 145° C. to 200° C. for 2 hours to 15 hours.

25. The process according to claim 23, wherein the artificial aging treatment comprises heating the wrought material at a temperature of 145° C. to 170° C. for 1 hour to 24 hours.

Patent History
Publication number: 20160186302
Type: Application
Filed: Aug 5, 2014
Publication Date: Jun 30, 2016
Applicant: UACJ Corporation (Tokyo)
Inventors: Hidenori HATTA (Aichi), Satoshi UDAGAWA (Aichi), Takero WATANABE (Aichi)
Application Number: 14/911,537
Classifications
International Classification: C22F 1/053 (20060101); C25D 11/18 (20060101); C21D 1/18 (20060101); C25D 11/16 (20060101); C22C 21/10 (20060101); C21D 1/56 (20060101);