JP5610643B2 - Cu-Ni-Si-based copper alloy strip and method for producing the same - Google Patents

Description

  The present invention relates to a Cu-Ni-Si-based copper alloy strip that can be suitably used for manufacturing electronic parts such as electronic materials, and a method for manufacturing the same.

  In recent years, lead frames, various terminals of electronic devices, connectors, and the like have rapidly increased in the number of leads and narrowed pitch, and high-density mounting and high reliability of electronic components are required. From the viewpoint of high-density mounting and high reliability, not only high strength and high conductivity, but also various severe bending processes such as 180 ° close contact bending and 90 ° bending after notching are used in materials used for electronic components. The required properties, such as excellent bendability that can withstand temperatures, are becoming increasingly severe. Among them, Cu-Ni-Si-based copper alloy strips are copper alloys that combine high strength, high electrical conductivity, high heat resistance, and high stress relaxation properties, as materials for lead frames, various terminals of electronic devices, connectors, etc. It has been put into practical use.

However, at present, it is difficult to combine high strength and excellent bending workability.
In particular, ultra-miniaturized terminals are subjected to severe bending such as bending after notching (box bending), and therefore, a copper alloy strip having both high bendability and high strength after notching is required.

  As measures for improving the bending workability of the Cu—Ni—Si based copper alloy strip, control of precipitates (for example, see Patent Document 1), control of crystal grain shape (for example, see Patent Document 2) Etc. have been proposed. On the other hand, it has also been proposed to improve the bending workability by controlling the crystal orientation (Patent Documents 3 and 4).

JP 2001-49369 A JP 2002-38228 A JP 2000-80428 A JP 2006-9108 A

However, in the case of the techniques described in Patent Documents 1 to 4, the bendability and strength after notching have not been sufficient.

  That is, the present invention has been made to solve the above-described problems, and it is an object of the present invention to provide a Cu—Ni—Si based copper alloy strip having improved bending workability and strength after notching and a method for producing the same. And

The Cu—Ni—Si based copper alloy strip of the present invention contains 1.0 to 4.5% by mass of Ni, contains Si at a ratio of 1/6 to ¼ with respect to the mass% of Ni, The balance is made of copper and inevitable impurities, contains 100 to 500 particles / mm ^{2 of} second phase particles having a diameter of 1.0 to 3.0 μm, and the X-ray diffraction intensity from the {200} plane on the surface is I {200 }, When the X-ray diffraction intensity from the {311} plane is I {311} and the X-ray diffraction intensity from the {220} plane is I {220}, 1.0 ≦ [I {311 } + I { 200} ] / I {220} ≦ 2.5, and when the X-resolved diffraction intensity from the (311) plane of the pure copper powder standard sample is I ₀ {311}, 1 ≦ I {311} / I ₀ {311} ≦ 2.5 meets, that is excellent in bendability.

Furthermore, it is preferable to contain 2.0 mass% or less of Zn and / or Sn in total.
Furthermore, it is preferable to contain 0.005-0.8 mass% of 1 or more types chosen from the group of Mg, Fe, P, Mn, Co, and Cr in total.
The crystal grain size determined by the cutting method is preferably 11 μm or less.
The crystal grain size determined by the cutting method is preferably 8 μm or less.

The method for producing a Cu—Ni—Si based copper alloy strip of the present invention contains 1.0 to 4.5% by mass of Ni, and Si is added at a ratio of 1/6 to 1/4 with respect to the mass% of Ni. A Cu—Ni—Si based copper alloy ingot containing copper and inevitable impurities, and after hot rolling at an end temperature of 700 to 800 ° C., an end temperature of 310 to 349 ° C., an end temperature of 310 to 349 ° C. , and Warm rolling is performed with a total processing degree of 60% or more and a processing degree per pass of 10 to 25%, and then at least solution treatment and aging treatment are performed in this order.

  According to the present invention, it is possible to obtain a Cu—Ni—Si-based copper alloy strip that has high strength, improves the bendability in the rolling parallel direction (GW direction), and does not deteriorate the bendability in the direction perpendicular to the rolling direction (BW direction). .

It is a figure which shows the test method of W bending workability after a notch process. It is another figure which shows the test method of W bending workability after notch processing. It is a figure which shows the cross section (1 mm from the end of a test piece) photograph (FIG.3 (a)) of the bending part after the notch process of Example 5, and the surface photograph (FIG.3 (b)). It is a figure which shows the cross section (1 mm from the edge of a test piece) photograph (FIG. 4 (a)) of the bending part after the notch process of the comparative example 2, and the surface photograph (FIG.4 (b)). It is a figure which shows the SEM image of the rolling parallel cross section of the comparative example 2 (FIG. 5 (a)) and Example 2 (FIG.5 (b)).

  Hereinafter, the Cu—Ni—Si based copper alloy strip according to the embodiment of the present invention will be described. In the present invention, “%” means “% by mass” unless otherwise specified.

First, the reasons for limiting the composition of the copper alloy strip will be described.
<Ni and Si>
Ni and Si form precipitation particles of intermetallic compounds mainly composed of Ni ₂ Si in which Ni and Si are fine by performing an aging treatment, and remarkably increase the strength of the alloy. Further, the conductivity is improved with the precipitation of Ni ₂ Si in the aging treatment. However, when the Ni concentration is less than 1.0% or the Si concentration is less than 0.17 (1/6 of Ni%), the desired strength cannot be obtained even if the other component is added. . Also, when the Ni concentration exceeds 4.5%, or when the Si concentration exceeds 1.2 (1/4 of Ni%), sufficient strength can be obtained, but the conductivity is lowered, and further the strength Coarse Ni—Si-based particles (crystallized substances and precipitates) that do not contribute to the improvement of the yield are generated in the matrix phase, leading to a decrease in bending workability, etching property and plating property. Therefore, the Ni content is 1.0 to 4.5 mass%. Preferably, the Ni content is 1.0 to 3.5% by mass.
The content of Si is set to a ratio of 1/6 to 1/4 with respect to mass% of Ni. If the Si content is the above-described ratio with respect to Ni mass%, both the strength after precipitation hardening and the electrical conductivity can be improved. When the Si content is less than 1/6 with respect to the mass% of Ni, both strength and conductivity are lowered, and when it exceeds 1/4, the strength is increased but the conductivity is lowered.

<Zn and Sn>
Furthermore, it is preferable to contain 2.0 mass% or less of Zn and / or Sn in total.
Zn and Sn improve the strength and heat resistance of the Cu—Ni—Si based copper alloy strip. Zn also has the effect of improving the heat resistance of the solder joint. If the total content of Zn and Sn exceeds 2.0%, the conductivity may be significantly reduced. The lower limit of the total content of Zn and Sn is not particularly limited, but is preferably about 0.1%.

<Other elements>
Furthermore, in the above alloy, for the purpose of improving the strength, heat resistance, stress relaxation resistance, etc. of the alloy, one or more selected from the group of Mg, Fe, P, Mn, Co and Cr is added in a total of 0.005 to 0.8. It can be contained by mass%. When the total amount of these elements is less than 0.005% by mass, the above effect does not occur, and when it exceeds 0.8% by mass, the desired properties can be obtained, but the conductivity and bending workability may be deteriorated.

<Group organization>
Next, the rules for the texture of the copper alloy strip will be described. The present inventors investigated and analyzed the relationship between the degree of integration and crystal grain morphology of each crystal plane, bending workability and bending anisotropy when Cu—Ni—Si based copper alloy strips were produced under various conditions. As a result, the following knowledge was obtained.
That is, if the ratio of the {311} plane is decreased and the ratio of the {200} plane and the {220} plane is increased, the bendability in the rolling parallel direction (GW direction) is improved. On the other hand, if the ratio of the (311) plane is too small, the bendability in the BW direction is deteriorated.
Therefore, the X-ray diffraction intensity from the {200} plane on the copper alloy strip surface is I {200}, the X-ray diffraction intensity from the {311} plane is I {311}, and the X-ray diffraction from the {220} plane When the strength is I {220},
1.0 ≦ [I {311] + I {200}} / I {220} ≦ 2.5 is satisfied,
And, when the X-resolved diffraction intensity from the (311) plane of the pure copper powder is I ₀ {311},
1 ≦ I {311} / I ₀ {311} ≦ 2.5
Control the texture to satisfy

When [I {311] + I {200}} / I {220} ≦ exceeds 2.5 and I {311} / I ₀ {311} exceeds 2.5, the ratio of the {311} plane increases. The bendability in the rolling parallel direction (GW direction) is not improved.
Further, if I {311} / I ₀ {311} is less than 1.0, the bendability in the direction perpendicular to the rolling direction (BW direction) is not improved.

  Here, the bending skin is an evaluation based on the “method for evaluating the bending workability of copper and copper alloy strips” defined in the technical standard T307: 1999 of the Japan Copper and Brass Association (JBMA). According to this rule, the bent skin is subjected to W-bending with a bending angle of 90 ° on the sample, and the bent processed portion that becomes a mountain is observed. E: The degree of bending skin is evaluated in 5 stages of large cracks. In general, if the material has the same composition, the smaller the crystal grain size, the better the bending skin, and the larger the crystal grain size, the worse the bending skin (approaches evaluation E). Usually, samples of evaluation D and E are not suitable for practical use.

<Second phase particle>
The alloy contains 100 to 500 particles / mm ^{2 of} second phase particles having a diameter of 1.0 to 3.0 μm. Fine precipitates having a diameter of several nanometers to several tens of nanometers, which are said to contribute to improving the strength of the alloy, and coarse crystals having a diameter of 3 μm or more crystallized during casting hardly contribute to the final texture. On the other hand, the second phase particles precipitated during warm rolling described later affect the texture. When the conditions of the warm rolling are defined as follows, the second phase particles having a diameter of 1.0 to 3.0 μm are 100. ˜500 pieces / mm ² will be contained. Moreover, since the second phase particles precipitated during the warm rolling preferentially grow during the aging treatment, they finally grow to a diameter of 1.0 to 3.0 μm.

The second phase particles are observed with a FE-SEM (Field Emission Scanning Electron Microscope) in the rolling parallel section, and the SEM image is binarized. Then, the vertical and horizontal lengths of each bright granular area were measured with image analysis software. The second phase particles were assumed to be circular, and the length obtained by averaging the vertical and horizontal lengths for each region was regarded as the (circle) diameter of the second phase particles.
The number of second phase particles having a diameter of 1.0 to 3.0 μm in the field of view of the SEM image was counted with the image analysis software.

Usually, a Cu-Ni-Si-based copper alloy strip is manufactured by performing hot rolling, cold rolling, and solution treatment, performing cold rolling as necessary, and further performing an aging treatment in this order. After the aging treatment, finish rolling and strain relief annealing may be performed as necessary. However, in this method, it is difficult to control the texture of the copper alloy strips within the above range, and the bendability in the rolling parallel direction (GW direction) deteriorates.
Therefore, as a result of the study by the present inventors, the hot temperature after the hot rolling is 300 to 450 ° C., the end temperature is 300 ° C. or more, and the total workability is 60% or more and the work degree per pass is 10 to 25%. By rolling, a heterogeneous structure is formed after dynamic recrystallization, and flat Ni—Si based particles (hereinafter, Ni—Si based precipitates are represented as “second phase particles” unless otherwise specified). Is dispersed in the matrix, and after the solution treatment, the texture at the time of recrystallization changes. Therefore, the ratio of the {311} plane can be reduced, the ratio of the {220} plane can be increased, and the texture can be controlled within the above range. Furthermore, while the bendability of GW (box bend and contact bend) is improved, the bendability of BW is not deteriorated, and it can be sufficiently used for in-vehicle applications and relay applications.

Here, when the start temperature of warm rolling exceeds 450 ° C., the structure obtained by dynamic recrystallization becomes too uneven, and the above-described texture cannot be obtained. On the other hand, when the start temperature or end temperature of warm rolling is less than 300 ° C., the desired second phase particles cannot be obtained because the diffusion rate is slow.
Further, when the total degree of warm rolling is less than 60%, the texture does not develop sufficiently, and the above-mentioned texture cannot be obtained finally.
When the degree of processing per pass during warm rolling is less than 10%, the strain rate is slow, so that precipitation is not promoted and the above-described texture cannot be obtained. On the other hand, if the degree of processing per pass exceeds 25%, the structure obtained by dynamic recrystallization becomes completely non-uniform, and thus the above-described texture cannot be obtained.
The warm rolling time is preferably 3 minutes or more.

The end temperature of hot rolling is preferably 700 to 800 ° C. If the end temperature of hot rolling is less than 700 ° C., second phase particles may precipitate and cracks may occur during hot rolling. On the other hand, if the end temperature of the hot rolling exceeds 800 ° C., the nuclei of the second phase particles that precipitate during the subsequent hot rolling are not generated, and the texture cannot be controlled within the above range and the effect of bending improvement is small. There is a case.
Moreover, after finishing hot rolling, it is good to cool to 400-450 degreeC with the cooling rate of 10-30 degree-C / sec, and to transfer to warm rolling.
The solution treatment temperature is preferably 700 to 800 ° C., and the total degree of cold rolling is preferably 85 to 99%. The aging treatment can be performed at a temperature of 400 to 500 ° C. for about 10 to 30 hours, for example. Moreover, you may finish-roll after an aging treatment, and it is good to set the processing degree of finish rolling to 0 to 30%. If the degree of finish rolling exceeds 30%, the bendability may deteriorate. Further, strain relief annealing may be performed after finish rolling.
Note that the cold rolling after the warm rolling is not necessarily required, and the rolling may be performed only to the warm rolling to the final plate thickness before the solution treatment.
The thickness of the Cu—Ni—Si based copper alloy strip of the present invention is not particularly limited, but may be 0.03 to 0.6 mm, for example.

Samples of Examples and Comparative Examples were prepared as follows.
Using copper as a raw material, copper alloys having the compositions shown in Tables 1 and 2 were melted using an atmospheric melting furnace and cast into ingots having a thickness of 20 mm and a width of 60 mm. This ingot was hot-rolled at 950 ° C. to a plate thickness of 10 mm. The start temperature and end temperature of hot rolling are shown in each table. Immediately after the hot rolling, water cooling by watering was performed at 400 to 450 ° C. for 20 to 30 seconds. The cooling rate was controlled by adjusting the sprinkling amount while measuring the temperature. Table 1 shows the cooling temperature by water cooling (after hot rolling and before warm rolling).
Then, warm rolling was performed by 6-8 passes over 3-4 minutes, and it reduced to the board thickness shown to each table | surface. Tables 1 and 2 show the start temperature, end temperature, total working degree, and working degree per pass of warm rolling. The warm rolling mill rolled while heating the work roll with a heater.
After the warm rolling, cold rolling at a working degree of 75.8 to 98.7 %, solution treatment for 5 minutes at the temperature shown in Table 1, and aging treatment (450 ° C. for 15 hours) were performed in this order. Thereafter, final finish rolling was performed at a working degree of 20%, and samples having the thicknesses shown in Table 2 were obtained.
In addition, about each comparative example, the sample of the plate | board thickness of Table 2 was obtained similarly to each Example except having performed cold rolling with a workability of 93.5-96.2 % after warm rolling.

<X-ray diffraction intensity>
The X-ray diffraction intensity I of the {200}, {311}, and {220} planes of the surface of the obtained sample was measured. For the measurement, RINT 2500 made by Rigaku was used, the X-ray irradiation conditions were a Co tube, tube voltage 25 KV, tube current 20 mA. Similarly, {200} of the pure copper powder standard sample, {311}, the X-ray diffraction intensity _{I 0} of the surface were measured.
<Tensile strength (TS)>
The tensile strength (TS) in a direction parallel to the rolling direction was measured by a tensile tester according to JIS-Z2241.
<0.2% yield strength (YS)>
The 0.2% proof stress (YS) in the direction parallel to the rolling direction was measured by a tensile tester in accordance with JIS-Z2241.

<W-bending workability after notching>
As shown in FIG. 1, a strip-shaped test piece 2 having a width of 10 mm and a length of 30 mm is prepared, and the depth is adjusted to 1/2 to 1/10 of the plate thickness on one side 2a. A notch 4 having a trapezoidal cross section was made. The specimen collection direction was GW. The mold 7 having the trapezoidal protrusion 7a described above was pressed against the test piece 2 on the base 6, and a notch 4 was formed so as to extend in the BW direction. As shown in FIG. 2, the protrusion 7a has a tapered isosceles trapezoidal shape in which the bottom of the cross section is 0.16 mm and the upper side on the side of the test piece 2 is 0.1 mm, and the angle formed by the two oblique sides to be tapered is 90 degrees. is there. Further, the distance between the bottom side and the top side is 30 μm, and the depth from the top side to the bottom side is recessed into the test piece 2 to form the notch 4.
Next, the test piece 2 was arranged on the lower die 12 so that the notch 4 faces the top portion 12a of the lower die 12 used in the JIS-H3130 W bending test. In this state, the upper mold 10 was pressed against the lower mold 12 to perform a W bending test. At this time, the external appearance of the bent portion 5 which is a crest of the opposite surface of the test piece 2 facing the notch 4 is visually observed, and “copper and copper” defined in the Japan Copper and Brass Association (JBMA) technical standard T307: 1999. The bending skin was evaluated on the basis of “bending workability evaluation method of alloy sheet strip”. Based on this rule, the bending skin was evaluated in five stages: A: no wrinkle, B: small wrinkle, C: large wrinkle, D: small crack, E: large crack. If it is a sample of evaluation A, B, C, there is no problem in practice.

<W bending workability>
A strip-shaped test piece having a width of 10 mm and a length of 30 mm was prepared and subjected to a W bending test (JIS-H3130). Specimen sampling directions were GW and BW, and evaluation was performed based on a ratio MBR / t of a minimum bend radius MBR (Minimum Bend Radius) where a crack does not occur and a sheet thickness t.
<Conductivity (% IACS) and crystal grain size>
The conductivity (% IACS) of the obtained sample was measured by the 4-terminal method, and the crystal grain size (GS) on the sample surface was determined by the cutting method specified in JIS-H0501.

<Number of second phase particles having a diameter of 1.0 to 3.0 μm>
The number of second phase particles was measured as follows. First, the rolled parallel section of the sample was mirror-polished and then immersed in a 47 ° Baume ferric chloride solution for 2 minutes while being rocked to perform etching. Immediately after that, it was washed with running pure water and ultrasonically washed with pure water for about 2 minutes.
A rolled parallel cross-sectional image of the sample after ultrasonic cleaning was taken with a FE-SEM (field emission scanning electron microscope, model number XL30SFEG manufactured by OXFORD) under the following conditions.
FE-SEM settings Magnification: 750 times, acceleration voltage: 15 kV, spot size: 4.0, detector: TLD, contrast: 35.0, brightness: 45.0

For the SEM image, the image analysis software (EDX particle / phase analysis software attached to EDX (energy dispersive X-ray analyzer)) uses vertical and horizontal lengths for each bright granular area under the following conditions. Was measured. The second phase particles were assumed to be circular, and the length obtained by averaging the vertical and horizontal lengths for each region was regarded as the (circle) diameter of the second phase particles.
EDX particle / phase analysis software settings Field size: 1024 pixels x 800 pixels, Strips (number of image divisions during capture): 10, Thresholds (greyscale threshold): Max: 255 (fixed) Min: Around 130

  The number of second phase particles having a diameter of 1.0 to 3.0 μm in the field of view of the SEM image was counted with the image analysis software.

  FIG. 5 shows SEM images of the rolling parallel sections of Comparative Example 2 (FIG. 5A) and Example 2 (FIG. 5B), which will be described later. In the SEM image, bright granular regions are second phase particles.

  The obtained results are shown in Tables 1 and 2.

As apparent from Tables 1 and 2, 100 to 500 particles / mm ^{2 of} second phase particles having a diameter of 1.0 to 3.0 μm are contained, and 1.0 ≦ [I {311] + I {200}} / In each example satisfying I {220} ≦ 2.5 and 1 ≦ I {311} / I ₀ {311} ≦ 2.5, the strength and the W bending workability after notching in the GW direction Both improved.

On the other hand, in the case of Comparative Example 1 in which the start temperature of the warm rolling exceeds 450 ° C., the second phase particles having a diameter of 1.0 to 3.0 μm are reduced to less than 100 particles / mm ² , and the warm rolling is complete. Recrystallization occurred. For this reason, [I {311] + I {200}} / I {220} exceeds 2.5 and I {311} / I ₀ {311} exceeds 2.5. W bending workability deteriorated.
In the case of Comparative Example 2 in which the start temperature of warm rolling is less than 300 ° C., the second phase particles having a diameter of 1.0 to 3.0 μm are reduced to less than 100 particles / mm ² , and [I {311] + I {200 }} / I {220} exceeded 2.5 and I {311} / I ₀ {311} exceeded 2.5, so that the W bending workability after notching in the GW direction deteriorated.

In the case of Comparative Example 3 where the end temperature of warm rolling is less than 300 ° C., the second phase particles having a diameter of 1.0 to 3.0 μm are reduced to less than 100 particles / mm ² , and [I {311] + I { Since 200}} / I {220} exceeded 2.5 and I {311} / I ₀ {311} exceeded 2.5, W bending workability after notching in the GW direction deteriorated.
In the case of Comparative Example 4 in which the degree of processing per pass during warm rolling exceeded 25%, recrystallization occurred because a large strain was introduced during warm rolling. For this reason, [I {311] + I {200}} / I {220} exceeds 2.5 and I {311} / I ₀ {311} exceeds 2.5. W bending workability deteriorated.
In the case of Comparative Example 5 in which the total degree of work during warm rolling was less than 60%, the texture did not change sufficiently during warm rolling. For this reason, [I {311] + I {200}} / I {220} exceeds 2.5 and I {311} / I ₀ {311} exceeds 2.5. W bending workability deteriorated.

Reference Examples 1 to 4 were not cold-cooled after hot rolling, further cold-rolled with a working degree of 93.7 to 96.2% without performing warm rolling, solution treatment for 5 minutes at the temperatures shown in Table 1, and An aging treatment (at 450 ° C. for 15 hours) was performed in this order. Thereafter, final finish rolling was performed at a workability of 20% to obtain a sample having a thickness of 0.15 mm.
In the case of Reference Examples 1 to 4, since warm rolling was not performed, the second phase particles having a diameter of 1.0 to 3.0 μm were reduced to less than 100 particles / mm ² , and [I {311] + I {200} } / I {220} exceeded 2.5 and I {311} / I ₀ {311} exceeded 2.5, so that the W bending workability after notching in the GW direction deteriorated.
In the case of Reference Example 4 in which the solution treatment temperature was lower than that of other Reference Examples (650 ° C.), the W bending workability after notching in the GW direction was good, but the strength was inferior to the Examples. It was.

  In addition, FIG. 3 shows the cross section (1 mm from the end of a test piece) photograph (FIG. 3 (a)) of the bending part after the notch process of Example 5, and the surface photograph (FIG.3 (b)). 4 shows a cross-section (1 mm from the end of the test piece) of the bent part after notching in Comparative Example 2 (FIG. 4A) and a photograph of the surface (FIG. 4B).

2 Test piece 4 Notch 12 Lower mold 12a Top part of lower mold 5 Bending part

Claims (6)

1.0 to 4.5 mass% of Ni is contained, Si is contained at a ratio of 1/6 to 1/4 with respect to the mass% of Ni, and the balance is made of copper and inevitable impurities,
Containing 100 to 500 particles / mm ^{2 of} second phase particles having a diameter of 1.0 to 3.0 μm,
The X-ray diffraction intensity from the {200} plane on the surface is I {200}, the X-ray diffraction intensity from the {311} plane is I {311}, and the X-ray diffraction intensity from the {220} plane is I {220 }
1.0 ≦ [I {311 } + I {200} ] / I {220} ≦ 2.5 is satisfied,
And when the X-resolved diffraction intensity from the (311) plane of the pure copper powder standard sample is I ₀ {311},
1 ≦ I {311} / I ₀ {311} ≦ 2.5
It meets the excellent Cu-Ni-Si-based copper alloy strips in bendability. Furthermore, the Cu-Ni-Si-type copper alloy strip | line of Claim 1 which contains Zn and / or Sn in total 2.0 mass% or less. The Cu-Ni-Si-based copper alloy strip according to claim 1 or 2, further comprising 0.005 to 0.8 mass% in total of at least one selected from the group consisting of Mg, Fe, P, Mn, Co and Cr. . The Cu-Ni-Si-based copper alloy strip according to any one of claims 1 to 3, wherein a crystal grain size determined by a cutting method is 11 µm or less. The Cu-Ni-Si-based copper alloy strip according to claim 4, wherein the crystal grain size determined by a cutting method is 8 µm or less. Cu-Ni-Si containing 1.0 to 4.5% by mass of Ni, containing Si at a ratio of 1/6 to 1/4 with respect to the mass% of Ni, the balance being copper and inevitable impurities After hot rolling a copper alloy ingot at an end temperature of 700 to 800 ° C., an end temperature of 310 to 349 ° C. at an end temperature of 300 to 450 ° C. %, And then at least a solution treatment and an aging treatment are carried out in this order.