JP4103959B2 - Production method of Al-Si alloy - Google Patents

Description

  The present invention relates to an aluminum-silicon alloy having excellent heat resistance, wear resistance, and workability.

Conventionally, Al-Cu-Mg heat-resistant aluminum alloy (JlS A2618) is often used for products whose operating temperature is about 180 ° C.
However, a general heat-resistant aluminum alloy is a type of alloy that ensures strength by an aging heat treatment (for example, about 190 ° C. × 15 hours). Therefore, the strength decreases when heated for a long time in a temperature range exceeding the heat treatment temperature. There is a problem of doing.
Moreover, although Al-Si type aluminum alloy (JlS A4032) is often used for the product which requires abrasion resistance and a low thermal expansion coefficient, heat resistance is not enough.
Therefore, Al-Si-Fe-based aluminum alloys are used for applications that require wear resistance, a low coefficient of thermal expansion, and heat resistance. This alloy is a combination of spray forming and hot extrusion (non-patent literature). It is necessary to obtain a fine and uniform structure by 1) or a new thermomechanical processing method (Patent Documents 1 to 3) by the present inventors. However, the use temperature is limited to a product of about 150 ° C.
For higher temperature applications, Al—Fe—X (X: Mn, Ce, Mo, Co, Ni, V, Zr) aluminum alloys are rapidly solidified and combined with rapidly solidified powder and sintered solidification method, or A method for improving the properties of an aluminum alloy by combining spray forming and a hot extrusion method to obtain a finer and more uniform structure has been proposed (see, for example, Non-Patent Document 2, Patent Documents 4 and 5). .) In these manufacturing methods, a rapidly solidified powder having a predetermined composition is prepared by a gas atomization method, and this is sintered and solidified or extruded into a billet, which requires complicated and technically difficult process conditions. There is a problem that the cost is inevitably high.

Patent No. 3005673 Patent No. 3005672 Patent No. 3111214 JP2000-161071 JP-A-10-26002 Materia 34 (1995) 736-740 Light metal 39 (1989) 147-166

Since Al-Fe-X alloys containing transition metals that crystallize stable compounds even at high temperatures contain hard and brittle compounds, both those obtained by casting and powder sintering are ductile. The forging process is difficult. For this reason, the balance between strength and ductility is improved by combining spray forming (rapidly solidified powder) and hot extrusion, but the distance between uniformly dispersed particles is small and cracks occur during secondary processing. It is necessary to take measures such as pre-processing (Patent Document 4).
Furthermore, when this Al—Fe—X alloy is applied to an engine piston or the like, excellent wear resistance and low thermal expansion are required. When satisfying such requirements, it is effective to disperse Si crystals, and high Si concentration aluminum alloys are used. However, Si crystals are very brittle, and Al-Si alloys have poor workability and mechanical properties. The physical properties are not sufficient. For this reason, as described above, the powder sintering method or the combination of spray forming (rapidly solidified powder) and hot extrusion method is used to improve the balance of strength and ductility by uniformly and finely dispersing Si particles. Since it is necessary to achieve high-density dispersion in order to maintain wearability, the interparticle distance is small, and cracks occur at the ends during secondary processing.
Therefore, aluminum alloys containing high concentrations of both Si and transition metals to improve both heat resistance and wear resistance are typical difficult-to-process materials, such as spray forming (quickly solidified powder) and hot A combination with the extrusion method is applied in part, but the cost is high.
Furthermore, in order to reduce environmental burden and improve energy efficiency, there is a demand for lighter and thinner members, and there is a problem of imparting workability that satisfies high strength and near net shape molding.

The present inventor has intensively studied to solve such problems and apply an Al—Si—Fe—Mn based normally solidified aluminum alloy to a forged part.
The present inventors have already proposed a new heat treatment method applied to Al-Si-based and Al-Si-Fe-based cast alloy wrought materials (Patent Documents 1 to 3).
That is, multi-pass low-temperature processing is applied to a multiphase bulk material containing an aluminum matrix and multiple second phases (AlSiFe-based intermetallic compounds, Si crystals, etc.) that are difficult to deform, and microcracks are formed into metal. By actively introducing it into intermetallic compounds and Si crystals, crushing and dividing each, and repeating in combination with recovery heat treatment, the second phase is refined and spheroidized to make it uniform and dense in the aluminum matrix It is a manufacturing method of the aluminum alloy characterized by dispersing. In addition, it demonstrates the effectiveness of cryogenic processing when crushing and dividing the second phase. An aluminum alloy having a hard second phase finely dispersed structure by these methods exhibits a cold work rate of 90% or more and an excellent balance between strength and ductility.
However, in the Al—Si—Fe—Mn based alloy, the recrystallization temperature of the aluminum parent phase increases due to the appropriate addition of Fe and Mn, and the ductility decreases with increasing strength. Accordingly, the deformation resistance of the aluminum matrix is increased even at room temperature processing, and the same effect as the low temperature processing shown by the above method is produced, but it is difficult to ensure the workability in the low temperature processing.

In order to solve such a problem, the present inventor combines the cold working and the heat treatment to crush and divide the hard second phase, thereby to provide wear resistance, heat resistance, workability (plastic working). The present inventors have found that an Al—Si based alloy excellent in forging, etc.) can be obtained and completed the present invention.
In the hard second phase finely dispersed structure thus obtained, a dynamic recovery structure in submicron units is obtained in the aluminum matrix by cold working (Patent Document 3).
In the case of an Al—Si binary alloy having a fine dispersion of ordinary Si crystals, the dynamic recovery structure remains even in the 99% cold hard working, and this is re-applied even if heat treatment is performed at a temperature of 180 ° C. or less. A crystal structure cannot be obtained. On the other hand, at 180 ° C. or higher, solute Si precipitates and the parent phase crystal grains begin to coarsen. Above 200 ° C., even if rapid heating is performed for a short time, the mother phase crystal grains become several μm or more due to coarsening.
However, when a substitutional solid solution element such as Fe, Mn, or Cu, particularly Fe or Mn, is contained in a total amount of 1% or more, a dynamic recrystallization structure can be obtained in cold working. Moreover, precipitation of Si crystals is suppressed even at a temperature of 180 ° C. or higher, and the growth of continuous recrystallized grains is suppressed.
Therefore, in an alloy manufactured by the method of the present invention, a product manufactured by cold or warm forging has a fine crystal structure and is excellent in strength characteristics.

In other words, the present invention performs at least two steps including a step of performing hot working at 400 to 550 ° C. on a cast material at least once, then performing a cold working in a low temperature region, and then performing a heat treatment in a high temperature region. A process for producing an Al-Si based alloy consisting of repetition, wherein the composition of the cast material and the Al-Si based alloy is 2 to 24% by weight of Si, 1 to 5% by weight of Fe, and 1 to 4% by weight of Mn. %, Ce, Mo, Co, V, Zr and Ti at least one of 0 to 2 wt%, Cu and Mg of at least one of 0 to 5 wt%, and the remaining Al, the low temperature range from room temperature to 100 ° C., the high temperature range is 400 to 550 ° C., Ru preparation der of Al-Si based alloy cumulative area reduction of cold rolling is 60% or more.

  The composition of the Al—Si based alloy of the present invention is 2-24 wt% Si, at least one of Fe and Ni, preferably 1-5 wt% of Fe, at least one of Mn and Cr, preferably 1 to Mn. 4% by weight, as well as the balance Al. Furthermore, optionally, at least one of Ce, Mo, Co, V, Zr and Ti may be contained in an amount of 0 to 2 wt%, and at least one of Cu and Mg may be contained in an amount of 0 to 5 wt%, preferably 0 to 3 wt%. Good. In addition to these components, inevitable impurities may be contained.

  The Si concentration in the Al—Si alloy needs to be at least 2% by weight in order to obtain eutectic Si crystallization. Furthermore, in order to obtain crystallization of primary Si effective for wear resistance, a content of 12% by weight or more is required. However, in the solidification process by continuous casting or die casting, which is a general alloy manufacturing method, as the Si amount increases, the primary Si becomes coarse and the volume increases, and the hot workability also decreases. Therefore, 24% by weight is the upper limit of the Si concentration.

  Room temperature strength and high temperature strength are improved by moderate addition of Fe or Ni. Addition of Fe crystallizes out as a primary intermetallic compound during solidification. Further, the addition of Fe has the effect of increasing the recrystallization temperature of the aluminum matrix by the dispersion of the intermetallic compound. When the added amount of Fe is less than 1%, a eutectic compound is formed, and a sufficient volume of crystallized material cannot be obtained. On the other hand, when the amount of Fe added to the Al-Si alloy is 5% or more, the workability is poor with a normal cast material, and the solidification structure becomes fine due to rapid solidification or semi-solidification (including ultrasonic vibration). It is difficult to impart processing without making it.

When Mn or Cr, preferably Mn is added, the recrystallization temperature of the aluminum matrix is increased, and the normal temperature strength and high temperature strength can be improved. Moreover, creep strength and fatigue strength can be improved. Mn forms a solid solution in the intermetallic compound and also forms a solid solution in the aluminum matrix, making it difficult to recover the work structure and recrystallize, and contribute to improving the creep strength and fatigue strength of the alloy. . Therefore, together with the addition of Fe, the deformation resistance of the aluminum matrix during cold working is increased, which effectively acts on the crushing of coarse crystals. However, the addition of Mn also promotes the deterioration of ductility and toughness, so it is necessary to make it preferably 4% or less.
By adding an appropriate amount of Mn (or Cr), the shape of a needle-like AlSiFe-based compound having a length of several tens of μm to several hundreds of μm can be controlled into a massive AlSiFeMn-based compound having a diameter of several tens of μm. In the case of coarse needle-shaped compounds, it is oriented in the processing direction when it is divided and arranged in the hot and subsequent room temperature processing steps, so the problem of inferior delamination properties and cracking during molding processing Have In the case of a massive compound, the orientation in the processing direction is not remarkable, and quality improvement during molding such as forging can be reliably performed.
In the present invention, the amount of Fe (or Ni) + Mn (or Cr) is preferably 2 to 8% by weight, and the weight ratio Mn (or Cr) / Fe (or Ni) is preferably 0.2 to 2.

The addition of Ce, Co, and V refines the dispersion of the intermetallic compound and has an effect of increasing the high temperature strength. Further, the addition of Mo, Zr, and Ti makes the dispersion state of the intermetallic compound uniform, and at the same time promotes the refinement of the aluminum matrix. However, even if added in a large amount, effects such as an increase in high-temperature strength are saturated, so the total amount added is 2% by weight or less.
Addition of Cu or Mg contributes to normal temperature strength due to aging precipitation and strength increase at 150 ° C. or lower. At a high temperature of 180 ° C. or higher, the precipitation strengthening mechanism is lost due to the solid solution of the precipitate, but a slight improvement in the high temperature strength and an increase in the recrystallization temperature are obtained by the solid solution strengthening. The effective addition amount of Cu / Mg alone for such aging precipitation is 5% by weight or less. However, it is cold due to the reduction of the Cu / Mg solid solubility limit due to partial dissolution in the aluminum matrix such as Si, Fe and Mn, and the manifestation of non-uniform deformation of the aluminum matrix due to fine precipitation of the Cu / Mg compound. Since the workability is lowered, the upper limit of the Cu / Mg addition amount is preferably 3% by weight.

In order to perform the processing of the present invention, it is necessary that the elongation at break of the Al—Si alloy is 1% or more, and preferably 4% or more. The elongation at break decreases as the Si content increases. When the elongation at break is 1 to 4%, it is necessary to increase the number of repetitions of processing because the area of reduction in one cold processing cannot be made so large. This breaking elongation refers to, for example, a round bar test piece having a parallel part diameter of φ3.5 mm and a parallel part length of 25 mm measured at room temperature at a crosshead displacement speed of 0.05 to 500 mm / min.
An Al-Si alloy having an elongation at break of 1% or more is obtained by, for example, subjecting a continuous casting material or a die casting material to hot extrusion, hot rolling, hot forging or the like at least once. Obtainable.
This hot working is preferably performed at 400 to 550 ° C. Below 400 ° C., recrystallization of the material does not occur sufficiently and the ductility is small. In order to impart workability for homogenizing the material and processing at room temperature, and to reduce deformation resistance, it is preferable to heat to 400 ° C. or higher. On the other hand, at 550 ° C. or higher, the temperature difference to the eutectic temperature is 50 ° C. or lower, and there is a risk of partial melting of the material surface due to processing heat generation, and there is a risk of seizure in hot extrusion.
A material generally called an extruded material or a rolled material, or a material obtained by performing a similar treatment on a cast material can be used.

In the method of the present invention, the Al-Si alloy is subjected to cold working in a low temperature range of room temperature to 100 ° C, and then maintained in a high temperature range of 400 to 550 ° C. Repeat the steps at least twice.
Cold working refers to processes such as extrusion, rolling, forging (forging, swaging, etc.), forging, and the like. Cold working is performed at room temperature to 100 ° C., preferably at room temperature.
The cumulative area reduction after processing in all stages of this cold processing must be 60% or more, or an effect equivalent to this.
The alloy subjected to such cold working is heat-treated while being kept at 400 to 550 ° C. The holding time is suitably about 10 minutes to 1 hour.
This repeating process may be completed by either cold working or heat treatment. Cold-finished materials are called processed materials, while heat-treated materials are called solution-treated materials or annealed materials. The latter is soft and preferable for subsequent forging.
By repeating such cold working and heat treatment, micro cracks are actively introduced into intermetallic compounds and Si crystals, each is crushed and divided, and the aluminum matrix is recovered by heat treatment and refined particles Spheroidized and dispersed uniformly and densely in the aluminum matrix. In such an aluminum alloy, the crystallized material becomes an appropriate size by crushing and dispersing, and the ductility necessary for forging is improved, so that cold or warm forging can be performed, and the balance between strength and ductility is achieved. Is excellent.

  In the Al—Si based alloy thus obtained, crystallized substances such as AlSiFeMn, AlSiFeMnCu, and primary crystal Si are refined (several μm to several tens μm) and dispersed almost uniformly. Therefore, it has good wear resistance and can withstand plastic working. This is understood from the excellent elongation at break. This breaking elongation becomes smaller as the Si content increases, as shown in the examples described later, but is, for example, 4% or more. This breaking elongation refers to, for example, a round bar test piece having a parallel part diameter of φ3.5 mm and a parallel part length of 25 mm measured at room temperature at a crosshead displacement speed of 0.05 to 500 mm / min.

In addition, the shape of the crystallized product in the Al—Si based alloy obtained by the method of the present invention is significantly different from the Al—Si based alloy obtained by spray forming (for example, Patent Document 4) performed for the same purpose. . That is, according to spray forming, the crystallized substance is generally spherical with a diameter of about 1 to 3 μm, and the diameters are almost uniform (Tokiso, Sano, Shibue, Okubo: Al-Si manufactured by spray forming. According to the method of the present invention, the crystallized material has a variety of shapes such as rods, lumps and spheres, in contrast to the structure and mechanical properties of alloys, powder and powder metallurgy, 41 (1994), 927-932). Yes, if these crystals are observed within a certain range (for example, 50 to 200 μm square of the alloy surface), the equivalent circle diameter of the crystals (for example, the particle diameter in the case of a sphere, the long axis in the case of a rod) The difference between the smallest one and the largest one is 5 times or more.

The following examples illustrate the invention but are not intended to limit the invention.

An Al alloy (hereinafter referred to as “7Si”) containing 7% by weight of Si, 2% by weight of Fe, and 2% by weight of Mn was cast into a copper mold (φ40) to obtain a cast material (hereinafter, in this example) This casting is called “AC”).
After this AC was heated to 520 ° C., hot swaging (about 5% reduction in area / time, 8 passes) and reheating (520 ° C.-600 s / time) were repeated to form about 21 mm (hereinafter referred to as “φ21 mm”). In this example, this hot-worked material is referred to as “HW”). In the swaging process, a round bar is inserted into a swaging machine, the surface is reduced by striking a die, and further, the die interval is adjusted or the die is changed, and the bar is processed into a smaller diameter bar. The cumulative area reduction was about 70%.
Subsequently, this HW was formed into about φ11 mm by repeating the cold swaging process (about 10% reduction in area / time) and the intermediate heat treatment (500 ° C.-1.8 ks, water cooling) 7 times (hereinafter, this example) This thermomechanical material is called “RTMT”.) The cumulative reduction in cold work was about 70%.

Fig. 1 shows an optical micrograph of a 7Si alloy AC material. X-ray diffraction, differential thermal analysis, and SEM-EDS analysis of phase identification and structural morphology revealed that eutectic Si (eutectic Si) and primary AlSiFeMn-based compounds exist in the 7Si alloy. The needle shape and the compound were found to be in a large lump shape (several tens of μm to 100 μm).
The 7Si alloy HW material subjected to the swaging process is significantly crushed and divided, and the size of the dispersed particles is several μm to several tens of μm.
Since the aluminum alloy having an Si concentration of 11% or less does not contain primary crystal Si, the hot workability is relatively good. Therefore, it is not limited to the extrusion method that is hydrostatic processing, but by forging or rolling, The crystallized material can be actively crushed and divided.

2 and 3 show structural photographs (secondary electron images) of the 7Si alloy RTMT material by a scanning electron microscope.
By repeating the cold swaging process and the intermediate heat treatment, the Si crystal and the compound are cracked and further divided by the plastic flow of the base, so that the compound is finely dispersed to a size of several tens of micrometers or less. .
Particularly in the vicinity of the surface, the diameter is several μm or less (FIG. 2), and this provides excellent formability in application to forging of RTMT material.
In the central part, since the cooling rate of the cast material is low and the working strain is small by applying the swaging method for both hot processing and room temperature processing, the primary crystal compound having a diameter of about 10 μm remains (see FIG. 3).

For each material of 7Si alloy, a crosshead displacement control tensile test was performed at room temperature (in the atmosphere) and 180 ° C. (in an oil bath).
In the tensile test, a round bar test piece having a parallel part diameter of φ3.5 mm and a parallel part length of 25 mm was used, and the crosshead displacement speed was constant within a range of 0.05 to 500 mm / min with an Instron type tensile tester. Experiments were performed at different displacement rates.
Table 1 shows the tensile properties of the 7Si alloy.

In the RTMT material, the improvement in ductility is remarkable. The elongation at break is 18% at 20 ° C. and 36% at 180 ° C., which is more than 10 times that of the AC material.
The AC material easily cracks and propagates in the second phase, and thus exhibits an early fracture and poor ductility. Therefore, it has almost no workability in cold (0 degreeC-100 degreeC) and warm (100 degreeC-400 degreeC). On the other hand, dimples are observed on the fracture surface of the RTMT material, exhibiting a ductile fracture appearance.

4 and 5 show the true stress-true strain curves at 20 ° C. and 180 ° C. of the 7Si alloy RTMT material. This gives an indication of deformation resistance in secondary processing.
At 20 ° C. (293 K, FIG. 4), there is almost no influence of the high strain rate on the deformation stress (increase in work hardening rate), and sufficient plastic deformability and low deformation resistance are obtained even in forging processing at a high strain rate. That is, the cold formability is high.
At 180 ° C. (453 K, FIG. 5), the strength decreases due to the temperature increase, but if the strain rate is large, the work hardening rate increases and the deformation resistance increases. However, there is no change in uniform elongation and it has sufficient plastic deformability. The increase in deformation resistance due to work hardening is smaller than the decrease in deformation resistance due to temperature rise, and is sufficiently low. Accordingly, the warm formability is also high, the work hardening structure after the secondary processing is maintained, and the strength is increased.

Si: 14 wt%, Fe: 2 wt%, Mn: 2 wt%, Cu: 2 wt%, Zr: 0.5 wt% Al alloy (hereinafter referred to as "14Si") and Si: 17 wt%, Fe : 2 wt%, Mn: 2 wt%, Cu: 2 wt%, Zr: 0.5 wt% Al alloy (hereinafter referred to as “17Si”) was cast into a copper mold (φ40) to obtain a cast material (Hereinafter, this cast material is referred to as “AC” in this example).
Since this aluminum alloy of 12% Si or more contains primary Si, the cold workability is very poor, and it is difficult to apply free forging or the like to a normal casting material even in a hot temperature range. Hot extrusion was applied.
The 14Si alloy and the 17Si alloy AC material were extruded to φ20 mm while being heated to 500 ° C. (extrusion ratio 3.45). The larger the extrusion ratio, the better. It is desirable to ensure at least 2.0 or more. This is because an extrusion ratio of 2 or less may cause cracking during subsequent cold working, which is not preferable.
Next, in the same process as in Example 1, cold swaging (about 10% area reduction / time) and intermediate heat treatment (500 ° C.-1.8 ks, water cooling) were repeated 7 times to form about φ11 mm. (Hereinafter, this heat-treated material is referred to as “RTMT” in this example.) Processing could be performed without visible scratches. The cumulative reduction in cold work was about 70%.

FIG. 6 shows an optical micrograph of the AC material. As a result of X-ray diffraction, differential thermal analysis, and SEM-EDS analysis, eutectic Si (eutectic Si), primary Si (primary Si), and AlSiFeMnCu based compounds were observed in 14Si and 17Si. The eutectic Si was acicular, and the primary crystal Si and the intermetallic compound were coarse lumps (several tens μm to several hundreds μm).
FIG. 7 and FIG. 8 show structural photographs (secondary electron images) of the RTMT material by a scanning electron microscope.
By repeating the cold swaging process and the intermediate heat treatment, the Si crystal and the compound are cracked, and further, they are divided by the plastic flow of the base. Si crystals and compounds having a diameter of about 10 μm are dispersed on both the surface and the center.

Table 2 summarizes the tensile properties of 14Si alloy and 17Si alloy. The test temperature is 20 ° C. and the crosshead speed is 0.5 mm / min.

It can be seen that the tensile properties of the HW material are improved and the RTMT method can be applied. Compared with the AC material structure, coarse primary crystal Si and AlSiFeMnCu-based compounds are divided perpendicular to the processing direction.
The elongation of high Si concentration 14Si and 17Si alloy RTMT materials is about 5%, and can be forged.

In the Al—Si based alloy of the present invention, the crystallized material is refined and dispersed, and the workability necessary for the development of a forged product can be imparted. Furthermore, the Al—Si based alloy of the present invention is lightweight, excellent in wear resistance, has low thermal expansion, is further provided with heat resistance and formability, and can be provided at low cost as a high value-added material.
The Al—Si based alloy of the present invention makes it possible to produce a forged product from a cast material at a low cost and improve the performance of various machine parts. Because secondary metal can be used, high addition that brings about effective use of Si, Fe and Mn (elements that are difficult to remove), which is a problem in the recycling process of aluminum alloys, and application of secondary alloys to wrought materials Value-added cascade recycling becomes possible.
The method for producing an Al—Si alloy of the present invention can be applied to a cast material and a secondary metal, and can be produced and mass-produced using a production line in a current industrial facility.
In addition, since the Al—Si based alloy of the present invention is a dispersion strengthened alloy composed of a transition metal compound and Si crystal, an alloy that ensures strength by heat treatment, such as a conventional aging precipitation strengthened alloy, In contrast, even in a temperature region exceeding 200 ° C., there is no rapid decrease in strength, and high temperature strength characteristics are improved.

It is a figure which shows the optical microscope photograph of the cross-sectional structure | tissue of 7Si alloy AC material. It is a figure which shows the optical microscope photograph of the longitudinal cross-section structure | tissue (position of 1/4 depth from the surface) of 7Si alloy RTMT material. RD indicates the processing direction. It is a figure which shows the optical microscope photograph of the cross-sectional structure | tissue (center part position) of 7Si alloy RTMT material. It is a figure which shows the influence of the strain rate which acts on the tensile deformation behavior in 293K of 7Si alloy RTMT material. It is a figure which shows the influence of the strain rate which acts on the tensile deformation behavior in 453K of 7Si alloy RTMT material. It is a figure which shows the cross-sectional structure | tissue (optical microscope) of 14Si alloy AC material. It is a figure which shows the longitudinal cross-section structure | tissue (surface vicinity position) of 14Si alloy RTMT material. It is a figure which shows the longitudinal cross-section structure | tissue (center part position) of 14Si alloy RTMT material.

Claims (2)

Al- consisting of performing at least one hot working at 400 to 550 ° C. on a cast material, and then repeating at least two steps comprising a step of cold working in a low temperature region and a step of heat treatment in a high temperature region thereafter. A process for producing a Si-based alloy, wherein the composition of the cast material and the Al-Si based alloy is 2 to 24 wt% Si, 1 to 5 wt% Fe, 1 to 4 wt% Mn, Ce, Mo, At least one of Co, V, Zr, and Ti is 0 to 2% by weight, at least one of Cu and Mg is 0 to 5% by weight, and the remaining Al, and the low temperature range is room temperature to 100 ° C., the high temperature range Is 400-550 degreeC, and the manufacturing method of the Al-Si type alloy whose cumulative reduction area of this cold working is 60% or more. The manufacturing method according to claim 1, wherein the cold working is extrusion, rolling, forging, or forging.