A low potassium doped molybdenum powder, its preparation method and use

Abstract

This invention belongs to the field of new material preparation technology, and discloses a low-potassium doped molybdenum powder, its preparation method, and its applications. The method employs an atomization doping process in the molybdenum dioxide stage, introducing potassium nitrate and lanthanum oxide as synergistic dopants to control the total potassium content to 80-98 mg / kg. A two-stage hydrogen reduction and programmed temperature rise strategy are used to achieve gas-phase transport of potassium and grain boundary control. This invention reduces energy consumption through a compensated atomization doping process while strictly controlling the total potassium content. The resulting molybdenum powder has a narrow particle size distribution, uniform morphology, and good flowability, superior to products obtained by conventional low-potassium reduction methods. Composite doping with lanthanum oxide effectively improves the density of the molybdenum billet. The eddy current testing pass rate and yield of the molybdenum wire meet the entry standards for high-end molybdenum wires in the magnetron and wire EDM fields. The process is simple, requiring no new specialized equipment; only an atomization doping unit needs to be added to the existing reduction production line, resulting in low modification costs and ease of large-scale implementation.

Description

Technical Field

[0001] This invention belongs to the field of new material preparation technology, and relates to a low-potassium doped molybdenum powder, its preparation method and application. Background Technology

[0002] Molybdenum and its alloys hold an irreplaceable position in the fields of electronic devices, vacuum devices, and precision machining due to their excellent high-temperature strength, good thermal and electrical conductivity, and low coefficient of thermal expansion, especially in high-end applications such as magnetron cathode supports and high-precision wire cutting of molybdenum wire.

[0003] As a precursor raw material, the particle size distribution, bulk density, particle morphology, and impurity element content of molybdenum powder directly determine the densification behavior of the subsequent sintered green body and the final product's process yield. Traditional industrial-grade molybdenum powder is mostly prepared by a two-stage hydrogen reduction method using molybdenum trioxide, that is, first reducing it to molybdenum dioxide at 450-600℃, and then further reducing it to metallic molybdenum powder at 900-1100℃.

[0004] In this process, trace amounts of potassium are often intentionally introduced to regulate grain growth and inhibit sintering shrinkage. The typical doping amount is controlled in the range of 100-300 mg / kg. Through the vapor migration of potassium at high temperature, a dynamic adsorption layer is formed on the surface of molybdenum particles, which effectively hinders the rapid migration of grain boundaries, thereby obtaining coarse, loose and highly fluid spherical or near-spherical molybdenum powder.

[0005] However, with the increasing demands for material purity in high-end electronic devices, low-potassium raw materials (K < 100 mg / kg) are gradually becoming a new industry trend to avoid the potential impact of potassium residue on the vacuum performance and long-term stability of devices. However, when using low-potassium molybdenum trioxide directly for conventional two-stage hydrogen reduction, the lack of sufficient potassium vapor pressure in the system makes it impossible to effectively activate the aforementioned gas-phase transport catalytic mechanism. This results in excessively high surface energy of molybdenum particles during the reduction process, making them prone to local melting, adhesion, and dense agglomeration. The resulting molybdenum powder not only has a small particle size and low bulk ratio but also exhibits severe agglomeration and morphological inhomogeneity, leading to a significant decrease in sieve pass rate.

[0006] Meanwhile, these structural defects are difficult to completely eliminate in subsequent powder metallurgy processes: the molybdenum billet obtained by pressing and sintering contains micropores and stress concentration areas, which induce macroscopic defects such as hollow wires, brittle fracture, and bamboo-like burrs on the surface during rolling and stretching. This results in low eddy current testing pass rates and yields, far from meeting the ≥95% flaw detection standard for molybdenum wire for magnetrons and the continuous processing stability requirements for wire-cut molybdenum wire. If potassium salts are directly doped at the molybdenum trioxide stage, the molybdenum trioxide undergoes violent sublimation at low temperatures, leading to uneven distribution of the dopant or even volatilization loss, which in turn exacerbates compositional fluctuations and process uncontrollability. Summary of the Invention

[0007] To achieve the aforementioned objectives, this invention provides a low-potassium doped molybdenum powder, its preparation method, and its applications. The method addresses the problems in existing technologies where low-potassium molybdenum trioxide, due to the lack of an effective grain control mechanism during conventional hydrogen reduction, suffers from severe molybdenum powder agglomeration, uneven morphology, low bulk density, and deteriorated performance in subsequent deep processing. By constructing a compensating atomization doping process, a precisely metered potassium nitrate solution is introduced into the molybdenum dioxide intermediate product stage, combined with lanthanum oxide as a synergistic dopant, reconstructing the spatiotemporal distribution and action pathway of potassium during the reduction process. This allows for precise control of the microstructure and macroscopic properties of the molybdenum powder while maintaining a total potassium content of less than 100 mg / kg in the raw materials.

[0008] The preparation method of low-potassium doped molybdenum powder according to the present invention includes the following steps: First, industrial-grade molybdenum trioxide is placed in a first-stage reduction furnace and initially reduced at 450-600℃ under a hydrogen atmosphere to obtain molybdenum dioxide powder; the initial potassium content of the molybdenum trioxide is controlled within the range of 50-90 mg / kg; second, the obtained molybdenum dioxide powder is transferred to an atomizing doping device, and a potassium nitrate aqueous solution with a concentration of 0.5-2.0 g / L is uniformly sprayed onto the surface of the molybdenum dioxide powder in an atomized form through a pressure nozzle, controlling the spraying amount of potassium nitrate solution so that the total potassium content in the final molybdenum powder product is 80-98 mg / kg; simultaneously, lanthanum oxide micro powder is pre-dissolved in the potassium nitrate solution, and the addition amount is 0.05-0.20 wt% based on the mass of molybdenum dioxide, wherein the average particle size of the lanthanum oxide micro powder is 50-200 nm and the specific surface area is 15-30 m². 2 / g; Next, the atomized doped molybdenum dioxide powder is placed in the second-stage reduction furnace, and a second reduction is carried out at 850-950℃ under the conditions of hydrogen flow rate of 2-5L / min and furnace pressure of atmospheric pressure or slightly positive pressure, and the holding time is 2-4 hours to complete the generation of metallic molybdenum powder; Finally, the obtained molybdenum powder is sieved and impurity removed to obtain the target product.

[0009] The atomizing doping device is a closed rotating fluidized bed doper, which has a central air inlet pipe and an array of annular atomizing nozzles inside. Molybdenum dioxide powder is in a suspended fluidized state under the drive of nitrogen or argon carrier gas. A potassium nitrate-lanthanum oxide mixed solution is delivered to the atomizing nozzle by a high-pressure pump, forming atomized clouds with droplet diameters of 10-50 μm under a pressure of 0.2-0.5 MPa, ensuring uniform adhesion and penetration of the dopant on the powder surface. After doping, the material is dried at 60-80℃ for 1-2 hours to completely remove moisture without triggering nitrate decomposition.

[0010] In a preferred embodiment of the present invention, the second reduction process employs a programmed temperature rise strategy: in the initial stage, the temperature is increased from room temperature to 600°C at a rate of 5-10°C / min, and maintained in this temperature range for 30-60 minutes, causing potassium nitrate to undergo thermal decomposition to generate potassium oxide and nitrogen oxide gases. Among them, potassium oxide partially reacts with the surface of molybdenum dioxide to form a potassium-molybdenum oxide intermediate phase. Subsequently, the temperature is further increased to the main reduction zone of 850-950°C. In this stage, potassium migrates to the surface of newly formed molybdenum particles in the form of K2O(g) or KO2(g) through a gas-phase transport mechanism, dynamically adsorbing at high-energy grain boundary positions, effectively inhibiting rapid grain boundary migration and particle melting and adhesion. At the same time, lanthanum oxide, as a high-melting-point rare earth oxide (melting point > 2300°C), maintains chemical stability in the reducing atmosphere and is uniformly dispersed in the molybdenum matrix. On the one hand, it hinders dislocation movement and grain boundary slip through the pinning effect, and on the other hand, it works with potassium vapor to construct a gas-solid two-phase interface control system, extending the grain growth time window and promoting the formation of equiaxed large grain structures with a size of 3-8 μm.

[0011] This invention also provides the application of the low-potassium doped molybdenum powder in the preparation of molybdenum wire for magnetron cathode support and high-precision wire-cut molybdenum wire. The molybdenum powder is cold isostatically pressed into a cylindrical blank at a pressure of 150-200 MPa for 5-10 minutes. Subsequently, it is subjected to medium-frequency induction sintering at 1800-2000℃ under hydrogen protection for 2-3 hours to obtain a sintered molybdenum blank with a relative density ≥92%. This molybdenum blank is then hot-rolled, subjected to multiple warm rolling passes and intermediate annealing, and drawn into fine wires with a diameter of 0.10-0.30 mm. Throughout the entire processing, no broken wires, hollow wires, or surface bamboo-like defects were observed. The final product undergoes eddy current testing, with a pass rate ≥96% and a yield ≥96%, meeting the stringent requirements of high-end electronic devices for the stability and surface integrity of continuous molybdenum wire processing.

[0012] The intermediate annealing process is carried out in a pure hydrogen atmosphere at an annealing temperature of 1100-1300℃ and a holding time of 30-60 minutes. After annealing, the molybdenum wire grains are oriented with a uniform axial height, with an average grain length of 200-400μm and a transverse grain size of 10-20μm. This indicates that the molybdenum powder prepared by this invention exhibits excellent microstructure inheritance and thermal stability during sintering and deformation.

[0013] This invention breaks through the dependence on the traditional high-potassium doping-high-temperature reduction path, achieving for the first time the functional reactivation of potassium in a low-potassium raw material system. Its core lies in delaying the introduction of the potassium source from the molybdenum trioxide precursor stage to the molybdenum dioxide intermediate stage, and employing atomized doping technology to ensure doping uniformity and dosage controllability. Simultaneously, by introducing lanthanum oxide as a structural stabilizer, a synergistic mechanism is formed with potassium vapor, avoiding potassium volatilization loss at low temperatures and enhancing the ability to regulate grain boundary dynamics at high temperatures. This strategy fundamentally solves the dual problems of morphology loss and processing brittleness in the preparation of low-potassium molybdenum powder.

[0014] Regarding equipment compatibility, the process described in this invention is fully compatible with existing four-tube reduction furnace production lines. The four-tube reduction furnace is a horizontal resistance heating furnace with four parallel tubes, each with an inner diameter of 80-120mm and an effective heating zone length of 1500-2000mm, and can be independently temperature-controlled. The first stage of reduction is completed in tubes 1-2, and the second stage of reduction is carried out in tubes 3-4. The atomization doping process is located between the two reduction stages, employing an online transfer and closed feeding design to prevent powder exposure to air and subsequent oxidation. The entire process achieves continuous and automated operation, with a single furnace capacity of 50-80kg / batch and a batch-to-batch composition fluctuation coefficient of less than 3%, demonstrating significant industrialization potential.

[0015] In another preferred embodiment of the present invention, a trace amount of ammonium fluoride, with a concentration of 0.01-0.05 g / L, may be added to the potassium nitrate solution to generate HF gas in the early stage of reduction, to etch the oxide film on the surface of molybdenum particles in situ, to promote hydrogen permeation and reduction reaction kinetics, and to further improve the purity of the powder and the completeness of reduction; however, the amount added is strictly controlled to avoid introducing fluorine residue that may affect the performance of the vacuum device.

[0016] Compared with the prior art, the beneficial effects of the present invention are:

[0017] 1. By using a compensated atomization doping process, the gas-phase transport catalytic cycle mechanism of potassium was successfully activated under the premise of strict control of total potassium content, thereby reducing the reduction temperature and energy consumption.

[0018] 2. The obtained molybdenum powder has a narrow particle size distribution, uniform morphology, and good flowability, with a sieve passing rate of over 95%, which is significantly better than the product obtained by the conventional low-potassium reduction method.

[0019] 3. Composite doping of lanthanum oxide effectively suppresses abnormal grain growth and micropore aggregation during sintering, thereby increasing the density of the molybdenum billet;

[0020] 4. During subsequent rolling and drawing processes, the uniformity of the mechanical properties of molybdenum wire is significantly improved, the wire breakage rate decreases, and the eddy current testing pass rate and yield are high, meeting the access standards for high-end molybdenum wire in the fields of magnetron and wire EDM.

[0021] 5. The process is simple and does not require new special equipment. It only requires adding an atomization doping unit to the existing reduction production line, which has low modification costs and is easy to implement on a large scale. Detailed Implementation

[0022] This invention provides a low-potassium doped molybdenum powder, its preparation method, and its applications. The core of this method lies in introducing a compensated atomization doping process at the molybdenum dioxide intermediate stage, and synergistically using lanthanum oxide as a structural stabilizer to reconstruct the spatiotemporal distribution and action mechanism of potassium in the hydrogen reduction process. This allows for precise control of the microstructure, particle size distribution, and bulk density of the molybdenum powder while strictly controlling the total potassium content of the raw materials to below 100 mg / kg, significantly improving its subsequent deep processing performance. This method completely avoids the risk of impurity residue associated with traditional high-potassium doping pathways, and simultaneously solves key technical bottlenecks such as severe powder caking, agglomeration, low sieving rate, and deterioration of the mechanical properties of the final molybdenum wire product caused by the direct reduction of low-potassium molybdenum trioxide.

[0023] The technical solution of the present invention will be described in detail below with reference to specific embodiments and comparative examples, so as to ensure that those skilled in the art can fully understand and implement the present invention.

[0024] Example 1: Raw material: Industrial grade molybdenum trioxide, initial potassium content 70 mg / kg;

[0025] Potassium nitrate aqueous solution: concentration 1.2 g / L, containing 0.12 wt% lanthanum oxide micro powder based on molybdenum dioxide mass, and 0.03 g / L ammonium fluoride;

[0026] Lanthanum oxide micro powder: average particle size 120 nm, specific surface area 22 m² 2 / g, purity 99.99%;

[0027] First stage reduction: hydrogen atmosphere, temperature 520℃, four-tube parallel horizontal resistance heating furnace, the inner diameter of each of the first and second tubes is 100mm, the effective heating zone length is 1800mm.

[0028] Atomized doping: A closed rotating fluidized bed doper was used, with nitrogen as the carrier gas, a potassium nitrate-lanthanum oxide mixed solution, and high-pressure pump delivery. The atomization pressure was 0.3 MPa, and the droplet diameter was 30 μm. After doping, the solution was dried at 65°C for 1.5 hours, reducing the water content to 0.08 wt%.

[0029] Second stage reduction: Hydrogen flow rate 3.5L / min, atmospheric pressure, four-tube parallel horizontal resistance heating furnace, tubes 3-4 are programmed to rise to 600℃ at 5℃ / min and hold for 45 minutes, then rise to 900℃ and hold for 3 hours.

[0030] Molybdenum wire preparation: Molybdenum powder is cold isostatically pressed at a pressure of 180 MPa for 8 minutes, sintered at 1900℃ in a hydrogen atmosphere for 2.5 hours, hot rolled, warm rolled, intermediate annealed (held at 1200℃ in a pure hydrogen atmosphere for 45 minutes), and drawn into molybdenum wire with a diameter of 0.20 mm.

[0031] Preparation process: molybdenum trioxide first-stage reduction → molybdenum dioxide atomization doping → drying → second-stage programmed temperature rise reduction → molybdenum powder sieving and impurity removal → molybdenum powder cold isostatic pressing → sintering → hot rolling → warm rolling → intermediate annealing → drawing → finished molybdenum wire.

[0032] Example 2: Potassium nitrate aqueous solution concentration 0.5 g / L, other formulations and processes are the same as in Example 1;

[0033] Preparation process: Same as in Example 1 (doping concentration adjusted).

[0034] Example 3: Potassium nitrate aqueous solution concentration 2.0 g / L, other formulations and processes are the same as in Example 1;

[0035] Preparation process: Same as in Example 1 (doping concentration adjusted).

[0036] Example 4: Lanthanum oxide micro powder was added at a rate of 0.05 wt%, and the rest of the formulation and process were the same as in Example 1;

[0037] Preparation process: Same as in Example 1 (doping ratio adjusted).

[0038] Example 5: Lanthanum oxide micro powder was added at a rate of 0.20 wt%, with the remaining formulation and process the same as in Example 1;

[0039] Preparation process: Same as in Example 1 (doping ratio adjusted).

[0040] Example 6: The second stage reduction temperature was 850℃ and held for 4 hours. The remaining formula and process were the same as in Example 1.

[0041] Preparation process: Same as in Example 1 (reduction temperature adjusted).

[0042] Example 7: The second stage reduction temperature was 950℃ and held for 2 hours. The remaining formula and process were the same as in Example 1.

[0043] Preparation process: Same as in Example 1 (reduction temperature adjusted).

[0044] Example 8: The potassium nitrate solution contained no ammonium fluoride; the rest of the formulation and process were the same as in Example 1.

[0045] Preparation process: Same as in Example 1 (without auxiliary additives).

[0046] Comparative Example 1: Without atomization doping, molybdenum trioxide with an initial potassium content of 70 mg / kg was directly reduced by hydrogen in two stages. The first stage was at 520°C and the second stage was at 900°C for 3 hours. No lanthanum oxide or ammonium fluoride was added. The rest of the process was the same as in Example 1.

[0047] Preparation process: two-stage direct reduction of molybdenum trioxide → molybdenum powder sieving → molybdenum wire preparation.

[0048] Comparative Example 2: The atomized doping contains only potassium nitrate, no lanthanum oxide, and a total potassium content of 90 mg / kg. The rest of the formulation and process are the same as in Example 1.

[0049] Preparation process: Same as in Example 1 (without lanthanum oxide addition).

[0050] Test method:

[0051] Molybdenum powder performance testing: Fisher particle size was measured using a laser particle size analyzer; density was measured using a loose / tapered density analyzer; sieve passing rate was measured using a standard sieve; potassium, lanthanum, and impurity element content were determined using inductively coupled plasma mass spectrometry; particle morphology and agglomeration were observed using a scanning electron microscope.

[0052] Molybdenum wire processing performance testing: statistical analysis of wire breakage rate during drawing process; eddy current flaw detection pass rate; calculation of yield; observation of surface defects and internal structure of molybdenum wire; metallographic microscopy analysis of grain size and orientation.

[0053] The test data comparisons are shown in Table 1 and Table 2.

[0054] Table 1 Comparison of Fisher particle size, bulk density, and sieve passing rate of molybdenum powder

[0055]

[0056] Table 2 Comparison of Total Potassium Content, Wire Breakage Rate, Eddy Current Testing Pass Rate, and Finished Product Rate

[0057]

[0058] Examples 1-8: Molybdenum powder sieve passing rate ≥95%, loose packing density ≥0.85 g / cm³ 3 The molybdenum wire breakage rate is ≤0.5%, which is far superior to the comparative example. Comparative example 1 shows that traditional direct reduction molybdenum powder has severe agglomeration and poor processing performance. Comparative example 2 has insufficient morphology and processing stability due to the lack of lanthanum oxide synergy. This confirms that atomized doping + potassium nitrate-lanthanum oxide synergy + programmed temperature reduction is the key to the high performance of low potassium molybdenum powder.

[0059] Increased potassium nitrate concentration (Examples 2→1→3) improved molybdenum powder particle size and bulk density; increased lanthanum oxide addition (Examples 4→1→5) optimized molybdenum wire processing stability; increased second-stage reduction temperature (Examples 6→1→7) improved molybdenum powder compactness; ammonium fluoride (Examples 8 vs 1) reduced impurity content and slightly improved processing performance.

[0060] The molybdenum powder in the example is spherical with uniform particle size distribution and good flowability, making it suitable for subsequent powder metallurgy processes; the molybdenum wire breakage rate is less than 0.5%, and the pass rate and yield are both ≥95%, meeting the needs of high-end applications such as magnetrons and wire cutting; the process is compatible with existing four-tube reduction furnaces, requiring only the addition of an atomization doping unit, resulting in low modification costs and easy industrialization.

[0061] Compared to traditional low-potassium direct reduction (Comparative Example 1), Example 1 showed that the loose packing density of molybdenum powder increased by 54%, the sieving rate increased by 18%, and the molybdenum wire breakage rate decreased by 91%. Compared to lanthanum oxide-free doping (Comparative Example 2), the wire breakage rate decreased by 75%, and the yield increased by 6.6%, solving the industry problems of low-potassium molybdenum powder agglomeration, uneven morphology, and poor processing performance.

[0062] In summary, the preparation method described in this invention, through multi-parameter synergistic design, can achieve the production of low-potassium, high-performance molybdenum powder and molybdenum wire with different parameter combinations, and is suitable for high-end electronic device molybdenum-based material applications.

[0063] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing low-potassium doped molybdenum powder, characterized in that, Includes the following steps: Industrial grade molybdenum trioxide with an initial potassium content of 50-90 mg / kg was placed in the first-stage reduction furnace and initially reduced at 450-600℃ under a hydrogen atmosphere to obtain molybdenum dioxide powder. The obtained molybdenum dioxide powder was transferred to a closed rotating fluidized bed doper and potassium nitrate solution was sprayed to make the total potassium content in the final molybdenum powder product 80-98 mg / kg. The atomized doped molybdenum dioxide powder was placed in the second-stage reduction furnace and subjected to secondary reduction at 850-950℃ under conditions of hydrogen flow rate of 2-5 L / min and furnace pressure of atmospheric pressure or slightly positive pressure. The resulting molybdenum powder was then sieved and impurity removed to obtain the target product. The potassium nitrate solution is a potassium nitrate aqueous solution with a concentration of 0.5-2.0 g / L, which is uniformly sprayed onto the surface of molybdenum dioxide powder in an atomized form through a pressure nozzle. Lanthanum oxide micro powder is pre-dissolved in the potassium nitrate solution, and the amount added is 0.05-0.20 wt% based on the mass of molybdenum dioxide.

2. The method for preparing low-potassium doped molybdenum powder according to claim 1, characterized in that, The closed rotating fluidized bed doper is equipped with a central air inlet pipe and an array of annular atomizing nozzles. Molybdenum dioxide powder is in a suspended fluidized state under the drive of nitrogen or argon carrier gas. A potassium nitrate-lanthanum oxide mixed solution is delivered to the atomizing nozzles by a high-pressure pump, forming atomized clouds with droplet diameters of 10-50 μm under a pressure of 0.2-0.5 MPa.

3. The method for preparing low-potassium doped molybdenum powder according to claim 1, characterized in that, The second stage of reduction employs a programmed temperature rise strategy: the temperature is raised from room temperature to 600℃ at a rate of 5-10℃ / min and held for 30-60 minutes to thermally decompose potassium nitrate into potassium oxide; then the temperature is raised to 850-950℃ and held for 2-4 hours to allow potassium to migrate to the surface of newly formed molybdenum particles in the form of K2O(g) or KO2(g) via gas-phase transport and dynamically adsorb at the grain boundary.

4. The method for preparing low-potassium doped molybdenum powder according to claim 1, characterized in that, The first and second reduction furnaces are the first and second tubes and the third and fourth tubes of a four-tube parallel horizontal resistance heating furnace, respectively. The inner diameter of each tube is 80-120mm and the effective heating zone length is 1500-2000mm.

5. The method for preparing low-potassium doped molybdenum powder according to claim 1, characterized in that, The potassium nitrate solution also contains ammonium fluoride at a concentration of 0.01-0.05 g / L, which is used to generate HF gas in situ during the 600℃ heat preservation stage to etch the oxide film on the surface of the molybdenum particles.

6. The method for preparing low-potassium doped molybdenum powder according to claim 1, characterized in that, After atomization and doping are completed, the material is dried at 60-80℃ for 1-2 hours to reduce the moisture content of the powder to below 0.1wt%.

7. The molybdenum powder prepared by the method for preparing low-potassium doped molybdenum powder according to claim 1, characterized in that, The molybdenum powder has a Fisher particle size of 4.0-6.5 μm and a loose packing density of 0.8-1.2 g / cm³. 3 The tap density is 2.0-2.5 g / cm³. 3 The sieve passing rate is ≥95%, the total potassium content is 80-98 mg / kg, the lanthanum content is 400-1600 mg / kg, and the total amount of impurity elements Fe, Ni, Cu, and Na is less than 50 mg / kg.

8. The application of molybdenum powder according to claim 7 in the preparation of molybdenum wire for magnetron cathode support, characterized in that, The molybdenum powder is cold isostatically pressed into a cylindrical blank at a pressure of 150-200 MPa, and then sintered in a hydrogen atmosphere at 1800-2000℃ for 2-3 hours to obtain a sintered molybdenum blank, which is then hot rolled, warm rolled and intermediate annealed before being drawn into molybdenum wire.

9. The application according to claim 8, characterized in that, The intermediate annealing is carried out in a pure hydrogen atmosphere at 1100-1300℃ for 30-60 minutes.

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