Preparation method of high-performance Cu-Ni-Si alloy based on microstructure design

Abstract

The application belongs to the technical field of copper alloy material, and relates to a preparation method of high-performance Cu-Ni-Si alloy based on microstructure design. The preparation method comprises the following steps: non-vacuum HCCM continuous casting, solid solution, cold rolling in sequence, discontinuous aging, secondary cold rolling, and continuous aging process, so that the high-performance Cu-Ni-Si alloy based on microstructure design is obtained. The preparation method can realize the comprehensive optimization of the alloy performance by using the Cu-Ni-Si alloy capable of continuous precipitation and discontinuous precipitation transformation. In the cold rolling and aging combination process, the number and percentage of the precipitated phase in the alloy are greatly improved, and the conductivity of the alloy is improved. Meanwhile, the cross-scale combination of the discontinuous precipitated phase and the continuous precipitated phase is constructed, the strengthening efficiency is improved, and the strength of the alloy is improved. The method shortens the process flow, reduces the production cost, improves the production efficiency, and further improves the mechanical and electrical comprehensive performance of the material.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of copper alloy materials, and relates to a high-performance Cu-Ni-Si alloy preparation method based on microstructure design. BACKGROUND

[0002] High-strength and high-conductivity copper alloys are mainly applied to integrated circuit lead frame strips, rail transit contact wires, high-voltage electrical contact components and the like. Among them, copper beryllium alloys are widely applied to the above-mentioned fields due to their good strength, electrical conductivity and elastic properties. However, although the Cu-Be alloy has such excellent properties, the beryllium element is a highly toxic element, and the Cu-Be alloy produces smoke containing beryllium in the smelting, processing and heat treatment processes, which seriously affects the life and health safety of production workers. Moreover, the Cu-Be alloy also has the disadvantages of long production cycle, low material yield, high energy consumption and high product price. Therefore, materials capable of replacing the copper beryllium alloy are currently being developed. High-strength and high-conductivity copper alloy materials represented by Cu-Ni-Si alloys have good mechanical properties and electrical conductivity, and have been widely applied to various important components such as lead frames, contacts, relays and brushes in the power electronics industry which require high strength and high electrical conductivity. For example, the KLF series and C70250 of Cu-Ni-Si alloys. However, with the continuous development of application equipment in the power electronics industry, higher functional and structural integration requirements are put forward for the mechanical properties and electrical conductivity of such alloys.

[0003] There are various preparation methods for Cu-Ni-Si alloys, among which two widely used processes are semi-continuous casting and spray forming. Among them, the semi-continuous casting method mainly adopts the process of "semi-continuous casting-hot rolling breakdown-solution treatment-cold rolling deformation-ageing treatment". Although this method is mature, easy to operate and stable in product quality, it has a series of problems such as long process flow, high energy consumption, low material yield and high cost due to multiple cold rolling passes, milling, intermediate annealing and pickling processes. The other method is spray forming, in which the metal liquid is first atomized into dispersed droplets by high-pressure inert gas, and then sprayed into a collector by high-speed gas to form a continuous and dense casting blank with a certain shape. The material or ingot prepared in this way has fine grains, uniform composition and low segregation degree. However, this method still has the disadvantages of high preparation cost, complex process and low production efficiency, which limits its application in the preparation of Cu-Ni-Si alloys.

[0004] Hot-cold combined mold continuous casting process is a near-net shape short process continuous casting process. The copper magnetic induction coil and water-cooled copper sleeve constitute a hot-cold combined mold, the induction coil is used to forcibly heat the near-melt liquid section, and the water-cooled copper sleeve is used to forcibly cool the near-outlet section, so that a high temperature gradient along the casting direction is established between the hot mold section and the cold mold section, the organization directional growth along the casting direction is realized, and the columnar crystal organization along the axial orientation is obtained. The copper and copper alloy produced by the HCCM continuous casting preparation process has good surface quality and mechanical properties, and improves the product yield.

[0005] At present, the strengthening methods of Cu-Ni-Si alloy mainly include the following: first, through continuous precipitation strengthening to form dispersed and fine δ-Ni2Si in the matrix, and through the Orowan bypass mechanism to hinder dislocation movement, so as to achieve the purpose of aging strengthening. Second, a large amount of deformation is applied to the alloy to strengthen the matrix, which will reduce the electrical conductivity and plasticity of the alloy, and is often used in combination with aging strengthening. Third, by adding alloying elements such as Co, Cr, Mg, Zn, P, Al, etc. to inhibit the growth of precipitated phase, and form a certain solid solution strengthening. But this method will significantly reduce the electrical conductivity and increase the smelting difficulty, and the composition is difficult to control. Fourth, by forming all cellular structure, and then applying a large amount of deformation to make the discontinuous precipitated phase directional distribution, so as to strengthen the matrix. This method is currently only suitable for wire rods, and the work hardening is obvious, and the plasticity is poor.

[0006] The general heat treatment process of copper-nickel-silicon alloy at present is: solid solution → cold rolling → aging. Through the combination of cold rolling and aging, the relationship between alloy precipitation kinetics and strengthening effect is optimized. The articles and patents reported at present all use the mutual combination of continuous precipitation transformation and rolling deformation to achieve the goal of optimizing the distribution of precipitated phase and strengthening effect. For example, in patent CN108330320A (Zhang Yi, An Junchao, Gao Zhi, etc., a high-performance Cu-Ni-Si alloy lead frame material and its preparation method, CN108330320A[P], 2018), the design range of Ni and Si content is expanded, the number of in-situ generated second phase during solidification is increased, and the strength is improved by relying on the precipitation of precipitated phase in solid-state phase transformation in heat treatment. In patent CN110951990A (Wang Chen, Meng Xiang, Deng Zhixiong, etc., a Cu-Ni-Co-Fe-Si-Zr-Zn copper alloy material and its preparation method, CN110951990A[P], 2020.), a CuNiCoFeSiZrZn copper alloy material and its preparation method are disclosed. The copper alloy material is composed of the following components: 2.0-3.0wt% Ni, 0.3-0.8wt% Co, 0.3-0.8wt% Fe, 0.5-1.1wt% Si, 0.1-0.2wt% Zr, 0.1%-0.3wt% Zn, and the rest is Cu. The preparation method includes the following steps: alloy casting → homogenization treatment → hot rolling → solid solution treatment → cryogenic rolling → pulse magnetic field aging treatment. By adjusting the size and distribution of the continuous precipitated phase, the strength and electrical conductivity of the copper alloy material can be improved. In patent CN102822364A (Daikubo Mitsuharu. Cu-Ni-Si alloy for electronic materials:, CN102822364A[P]. 2012.), the distribution state of Ni-Si compound particles is controlled to improve the comprehensive performance of Corson copper alloy. The copper alloy contains Ni: 0.4-6.0 mass%, Si: 0.1-1.4 mass%, and the rest is Cu and unavoidable impurities. The copper alloy contains Ni-Si compound small particles with a particle size of 0.01m or more and less than 0.3m, and Ni-Si compound large particles with a particle size of 0.3m or more and less than 1.5m. The number density of the above-mentioned small particles is 1-2000 / m, and the number density of the above-mentioned large particles is 0.05-2 / m. The above-mentioned patent represents a technical invention, which optimizes the content of Ni and Si elements and the deformation and aging process, improves the content, size and distribution of dispersed δ-Ni2Si continuous precipitated phase, and optimizes the mechanical and electrical properties of the alloy. At present, this process has certain contradictions in theory, that is, continuous precipitation strengthening requires precipitated phase to be in metastable state and cannot grow, but the improvement of electrical conductivity requires the number and proportion of precipitated phase to be as high as possible. SUMMARY

[0007] This invention discloses a method for preparing high-performance Cu-Ni-Si alloys based on microstructure design, in order to solve any of the above-mentioned and other potential problems in the prior art.

[0008] To achieve the above objectives, the technical solution of the present invention is: a method for preparing high-performance Cu-Ni-Si alloys based on microstructure design, which specifically includes the following steps:

[0009] S1) Weigh the alloy raw materials and perform continuous casting under non-vacuum conditions using the HCCM continuous casting process to obtain...

[0010] Alloy ingots;

[0011] S2) Solution treatment of S1) yields alloy ingots to obtain alloy billets;

[0012] S3) The alloy billet obtained from S2) is subjected to a cold rolling deformation process to obtain a sheet metal;

[0013] S4) After heating the board obtained in S3), perform discontinuous precipitation aging treatment and cool it to room temperature;

[0014] S5) The sheet material after the treatment in S4) is subjected to a second cold rolling deformation treatment to obtain the sheet material.

[0015] S6) The plate obtained in S5) is subjected to continuous precipitation aging treatment to obtain a high-performance Cu-Ni-Si alloy with microstructure design.

[0016] Furthermore, the atomic percentages of each component in the Cu-Ni-Si alloy are: 3.0-6.0 wt% Ni, 0.70-1.5 wt% Si, 0-0.1 wt% Ca, 0-0.1 wt% Cr, 0-0.15 wt% Mg, with the remainder being Cu and unavoidable impurities.

[0017] Furthermore, the alloy raw materials in S1) are pure copper with a purity greater than 99.9%, pure nickel with a purity greater than 99.9%, pure silicon with a purity greater than 99.9%, pure boron with a purity greater than 99.9%, 10% magnesium copper alloy and 10% calcium copper alloy.

[0018] Furthermore, in S1), the smelting temperature of the HCCM continuous casting process is 1200-1400℃, the continuous casting speed is 0.5-2mm / s, and the cooling water flow rate is 400-800L / h.

[0019] Furthermore, the solution treatment temperature in S2) is 940–1000℃, the solution treatment time is 1–3h, and the solution is then water-quenched to room temperature.

[0020] Furthermore, the reduction amount of the first cold rolling deformation in S3) is 30–90%.

[0021] Further, the S4) discontinuous precipitation aging treatment temperature is 470-530℃, and the time is 0.25-2h, air cooling to room temperature.

[0022] Further, the S5) secondary cold rolling deformation is a reduction of 50-90%.

[0023] Further, the S6) continuous precipitation aging treatment temperature is 400-460℃, and the time is 0.25-6h, air cooling to room temperature.

[0024] Further, the Cu-Ni-Si alloy has a 600-1000MPa, a yield strength of 550-900MPa, a conductivity of 30-63% IACS, and an elongation of 3.0-9.2%.

[0025] The method of the present application mainly utilizes continuous and discontinuous precipitates with different sizes, shapes and phase structures, optimizes the combination in the precipitation thermodynamic sequence and geometric space, and further improves the content and strengthening effect of the precipitates, and realizes the comprehensive goal of simultaneously improving the strength and electrical conductivity. The continuous precipitation refers to δ-Ni2Si precipitates; the discontinuous precipitation refers to long strip and spherical δ-Ni2Si, β-Ni3Si phase and DO22 phase.

[0026] The HCCM continuous casting method is used to efficiently prepare alloy cast blanks, to reduce the segregation area in the as-cast alloy structure, to form as-cast columnar crystals, and to shorten the solid solution time.

[0027] Through the combination of primary cold rolling deformation + discontinuous precipitation aging treatment + secondary cold rolling deformation + continuous precipitation aging treatment, the number and percentage of precipitates in the alloy are greatly improved, and the electrical conductivity of the alloy is improved; at the same time, the cross-scale combination of discontinuous and continuous precipitates is constructed, the strengthening efficiency is improved, and the strength of the alloy is improved.

[0028] The primary cold rolling refers to the cold rolling deformation of the as-cast alloy blank after solid solution treatment, which mainly regulates the nucleation number and rate of discontinuous precipitation, accelerates the occurrence of discontinuous precipitation, and forms various spherical discontinuous precipitates, and the reduction is between 30-99%.

[0029] The discontinuous precipitation refers to the precipitates that occur in the Cu-Ni-Si alloy, which preferentially form in the deformation band, dislocation aggregation area, grain boundary or other crystallographic interface as the nucleation site, and the discontinuous precipitation aging treatment temperature is between 470-530℃, and the time is 0.25h-2h.

[0030] The aforementioned secondary cold rolling deformation refers to the rolling deformation after discontinuous precipitation aging treatment. Its function is to elongate and refine the microstructure, while controlling the morphology and distribution of discontinuous precipitates to make them more conducive to the refinement of continuous precipitates.

[0031] The continuous precipitation refers to the uniform and continuous formation of precipitates within the polycrystalline material through diffusion-type phase transformations such as amplitude modulation decomposition or ordering. The continuous precipitation aging treatment temperature is between 400-460℃, and the time is between 0.25h and 12h. (3) The Cu-Ni-Si alloy involved in the invention undergoes deformation combined aging to regulate its microstructure, thereby obtaining excellent comprehensive properties, with a tensile strength of 600–1000MPa, a yield strength of 550–900MPa, and an electrical conductivity of 30-63% IACS.

[0032] The specific content includes two aspects: First, the HCCM continuous casting technology is used to form slabs in one step, eliminating processes such as hot forging, hot rolling, and milling, shortening the solution treatment time, and reducing the preparation cost. Second, through two-stage aging treatment, the size, type, and morphology of precipitated phases are adjusted, thereby controlling the microstructure of the alloy and achieving a good match between its mechanical and electrical properties.

[0033] The beneficial effects of the present invention are as follows: by adopting the above technical solution, the method of the present invention forms a slab in one step through HCCM continuous casting technology, omitting hot forging, hot rolling, milling and other processes, shortening the solution treatment time and reducing the preparation cost; the Cu-Ni-Si alloy slab prepared by using a combination of cold and hot casting molds has a metastable second phase that is finely dispersed, which can significantly shorten the solution treatment time. Attached Figure Description

[0034] Figure 1 This is a flowchart illustrating the preparation method of high-performance Cu-Ni-Si alloy based on microstructure design according to the present invention.

[0035] Figure 2(a) is a schematic diagram of the microstructure of the precipitated material after discontinuous precipitation aging treatment in the preparation method of the present invention.

[0036] Figure 2(b) is a schematic diagram of the precipitated microstructure after continuous precipitation aging treatment using the preparation method of the present invention. Detailed Implementation

[0037] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0038] like Figure 1 As shown, this invention discloses a method for preparing a high-performance Cu-Ni-Si alloy based on microstructure design. The preparation method specifically includes the following steps:

[0039] S1) According to the components of the Cu-Ni-Si alloy, the alloy raw materials are weighed respectively, and the HCCM continuous casting process is used for continuous casting under non-vacuum conditions to obtain alloy ingots;

[0040] S2) The alloy ingots obtained in S1) are subjected to solid solution treatment to obtain alloy blanks;

[0041] S3) The alloy blanks obtained in S2) are subjected to primary cold rolling deformation treatment to obtain plates;

[0042] S4) The plates obtained in S3) are heated and then subjected to discontinuous precipitation aging treatment, and cooled to room temperature;

[0043] S5) The plates subjected to the treatment in S4) are subjected to secondary cold rolling deformation treatment to obtain plates;

[0044] S6) The plates obtained in S5) are subjected to continuous precipitation aging treatment, thereby obtaining a microstructure designed high-performance Cu-Ni-Si alloy.

[0045] The atomic percentage of each component of the Cu-Ni-Si alloy is: 3.0-6.0wt% Ni, 0.70-1.5wt% Si, 0-0.1wt% Ca, 0-0.1wt% Cr, 0-0.15wt% Mg, and the rest is Cu and unavoidable impurities, wherein the atomic ratio of Ni / Si is 1.5-3.0.

[0046] In the alloy raw materials in S1), pure copper with a purity of more than 99.9%, pure nickel with a purity of 99.9%, pure silicon with a purity of 99.9%, pure boron with a purity of 99.9%, 10% magnesium-copper alloy, and 10% calcium-copper alloy are used as raw materials.

[0047] In the HCCM continuous casting process in S1), the melting temperature of the continuous casting is 1200-1400℃, the continuous casting speed is 0.5-2mm / s, and the cooling water flow is 400-800L / h.

[0048] In the solid solution treatment in S2), the temperature is 940-1000℃, the solid solution time is 1-3h, and the treatment is water quenched to room temperature.

[0049] In S3), the reduction of the primary cold rolling deformation is 30-90%.

[0050] In S4), the aging treatment temperature for discontinuous precipitation aging is 470-530℃, the time is 0.25-2h, and the air cooling is to room temperature.

[0051] In S5), the reduction of the secondary cold rolling deformation is 50-90% reduction.

[0052] The temperature of the continuous precipitation aging treatment in S6) is 400-460°C, and the time is 0.25-6h, and air cooling to room temperature.

[0053] The Cu-Ni-Si alloy has a tensile strength of 600-1000 MPa, a yield strength of 550-900 MPa, an electrical conductivity of 30-63% IACS, and an elongation of 3.0-9.2%.

[0054] Embodiments:

[0055] A high-performance Cu-Ni-Si alloy material based on a novel microstructure design and a preparation method thereof, specific embodiments include:

[0056] HCCM continuous casting process

[0057] Pure copper with a purity of greater than 99.9%, pure nickel with a purity of 99.9%, pure silicon with a purity of 99.9%, pure boron with a purity of 99.9%, 10% magnesium copper alloy, and 10% calcium copper alloy are used as raw materials, and are formulated according to 3.0-6.0wt% Ni, 0.56-1.9wt% Si, 0-0.1wt% Ca, 0-0.1wt% Cr, and 0-0.15wt% Mg, with a Ni / Si (atomic ratio) of 1.5-3.0. The HCCM continuous casting equipment is used for continuous casting under non-vacuum conditions, with a hot type temperature of 1300°C, a continuous casting speed of 1mm / s, and a cooling water flow of 600L / h. A slab with a thickness of 12mm is prepared by continuous casting under non-vacuum conditions.

[0058] Solution treatment

[0059] Embodiments 1-32: After solution treatment at 940-1000°C for 1-3h, water quenching to room temperature, the specific parameters of each embodiment are shown in Table 1 below.

[0060] Primary cold rolling deformation

[0061] Embodiments 1-32: The alloy billet after solution treatment is rolled into a plate at room temperature with a reduction of 30-90%, and the specific parameters of each embodiment are shown in Table 1 below.

[0062] Discontinuous precipitation aging treatment

[0063] Embodiments 1-32: The plate after primary cold rolling deformation is subjected to discontinuous precipitation aging treatment at 470-530°C for 0.25-2h, and air cooling to room temperature, and the specific parameters of each embodiment are shown in Table 1 below, and the precipitates are shown in Figure 2(a).

[0064] Secondary cold rolling deformation

[0065] Example 1-32, the plate after discontinuous precipitation aging treatment, was rolled into plate at room temperature with a reduction of 50-90%, and the specific parameters of each example are shown in Table 1 below.

[0066] Continuous precipitation aging treatment

[0067] Example 1-32, the plate after discontinuous precipitation aging treatment, was rolled into plate at room temperature with a reduction of 50-90%, and the specific parameters of each example are shown in Table 1 below, and the precipitated phase is shown in Figure 2(b).

[0068] Example 1-32, continuous precipitation aging treatment, time is 0.25-6h, air cooling to room temperature, and the corresponding continuous precipitation aging treatment time and mechanical properties of each example are shown in Tables 2-33.

[0069] Table 1 Copper-nickel-silicon alloy composition and deformation heat treatment process

[0070]

[0071] Table 2

[0072]

[0073] Table 3

[0074]

[0075] Table 4

[0076]

[0077] Table 5

[0078]

[0079] Table 6

[0080]

[0081] Table 7

[0082]

[0083] Table 8

[0084]

[0085]

[0086] Table 9

[0087]

[0088] Table 10

[0089]

[0090] Table 11

[0091]

[0092] Table 12

[0093]

[0094] Table 13

[0095]

[0096]

[0097] Table 14

[0098]

[0099] Table 15

[0100]

[0101] Table 16

[0102]

[0103] Table 17

[0104]

[0105] Table 18

[0106]

[0107] Table 19

[0108]

[0109] Table 20

[0110]

[0111] Table 21

[0112]

[0113] Table 22

[0114]

[0115] Table 23

[0116]

[0117] Table 24

[0118]

[0119]

[0120] Table 25

[0121]

[0122] Table 26

[0123]

[0124] Table 27

[0125]

[0126] Table 28

[0127]

[0128] Table 29

[0129]

[0130]

[0131] Table 30

[0132]

[0133] Table 31

[0134]

[0135] Table 32

[0136]

[0137] Table 33

[0138]

[0139] The above describes in detail the preparation of the high-performance Cu-Ni-Si alloy based on microstructure design provided by the embodiments of the present application. The above description of the embodiments is only used to help understand the method of the present application and its core idea; meanwhile, for those skilled in the art, according to the idea of the present application, the specific implementation and application range will be changed, and the above description should not be understood as the limitation of the present application.

[0140] As used in the specification and claims, certain terms have particular meanings. One skilled in the art will understand that different manufacturers can refer to a component by different names. The specification and claims should not be construed as limited to components by a particular name, but should be construed by the component's function. As used in the specification and claims, "comprising" and "including" are meant to be interpreted as specifying open-ended claims that are not limited to the listed elements. "Approximately" means within an acceptable error range for the corresponding function, which will vary from one context to another. The description that follows is intended to provide a better understanding of the preferred embodiments of the present application, and is not intended to be a complete description of all possible embodiments of the present application. The description serves only to illustrate the general principles of the present application, and is not meant to limit the present application to specific embodiments.

[0141] It should also be noted that the terms "comprising," "including," and "having" or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, product, or composition that comprises a list of elements is not necessarily limited to only those elements but can include other elements not expressly listed or inherent to such process, product, or composition. An element proceeded by "comprises... a" does not, without further recitation, preclude the existence of additional elements of the same type in the process, product, or composition.

[0142] It should be understood that the term "and / or" as used herein is merely an open-ended descriptive term indicating that three conditions exist, for example, A and / or B can mean: A exists alone, A and B exist together, and B exists alone. In addition, the character " / " as used herein generally represents an "or" relationship between the front and rear associated objects.

[0143] The above specification and description of various preferred embodiments of the present application has been presented for the purpose of illustration and description. It is not intended to be exhaustive or to limit the application to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching or knowledge of the skilled artisan, or logical adaptations from the teaching or knowledge described. The embodiments were chosen and described in order to provide the best illustration of the principles of the application and the practical application to thereby enable one skilled in the art to best utilize the application. It is intended that various alternatives within the scope of the application should be considered as described within the scope of the claims appended hereto.

Claims

1. A method for preparing high-performance Cu-Ni-Si alloys based on microstructure design, characterized in that, The preparation specifically includes the following steps: S1) According to the composition of Cu-Ni-Si alloy, the alloy raw materials are weighed separately and continuously cast under non-vacuum conditions using HCCM continuous casting process to obtain alloy ingots. The atomic percentages of each component in the Cu-Ni-Si alloy are: 3.0-6.0 wt% Ni, 0.70-1.5 wt% Si, 0-0.1 wt% Ca, 0-0.1 wt% Cr, 0-0.15 wt% Mg, with the remainder being Cu and unavoidable impurities; S2) Solution treatment of S1) yields alloy ingots to obtain alloy billets; S3) The alloy billet obtained in S2) is subjected to a cold rolling deformation process to obtain a cold-rolled sheet; The reduction in the first cold rolling deformation is 30–90%; S4) After heating the board obtained in S3), perform discontinuous precipitation aging treatment and cool it to room temperature; The discontinuous precipitation aging treatment is carried out at a temperature of 470–530℃ for 0.25–2 hours, followed by air cooling to room temperature. S5) The sheet material after the treatment of S4) is subjected to a second cold rolling deformation treatment to obtain a second cold rolled sheet material. The reduction in the secondary cold rolling deformation is 50-90%; S6) The plate obtained in S5) is subjected to continuous precipitation aging treatment to obtain a high-performance Cu-Ni-Si alloy with microstructure design; The continuous precipitation aging treatment is carried out at a temperature of 400–460℃ for 0.25–6 hours, followed by air cooling to room temperature. The Cu-Ni-Si alloy has a strength of 600–1000 MPa, a yield strength of 550–900 MPa, a conductivity of 30–63% IACS, and an elongation after fracture of 3.0–9.2%.

2. The preparation method according to claim 1, characterized in that, The alloy raw materials in S1) are pure copper with a purity greater than 99.9%, pure nickel with a purity greater than 99.9%, pure silicon with a purity greater than 99.9%, pure boron with a purity greater than 99.9%, 10% magnesium copper alloy and 10% calcium copper alloy.

3. The preparation method according to claim 1, characterized in that, The smelting temperature of the HCCM continuous casting process in S1) is 1200-1400℃, the continuous casting speed is 0.5-2mm / s, and the cooling water flow rate is 400-800L / h.

4. The preparation method according to claim 1, characterized in that, The solution treatment in S2) is performed at a temperature of 940–1000℃ for 1–3 hours, followed by water quenching to room temperature.

Images