A steel surface aluminizing and vanadizing wear-resistant and corrosion-resistant coating for powder conveying and a preparation method thereof

Abstract

The present application belongs to the technical field of steel surface treatment, and particularly relates to a steel surface aluminizing and vanadizing wear-resistant and corrosion-resistant coating for powder transmission and a preparation method, which comprises the following raw materials in percentage by weight: Na2B4O7 60.2-73.7%, Na3AlF6 5-8%, Al powder 7-10%, NaF 3-5%, fluxing agent NaCl 1-3%, V2O5 10-13%, CeO 2 0.3-0.8%; the coating has good wear resistance and corrosion resistance.

Description

Technical Field

[0001] This invention belongs to the field of steel surface treatment technology, specifically relating to an aluminum-vanadium co-infiltrating wear-resistant and corrosion-resistant coating for powder transport steel surface and its preparation method. Background Technology

[0002] With the rapid development of industries such as new energy, fine chemicals, and powder metallurgy, accelerating capacity upgrades and achieving automated control have become urgent priorities. Furthermore, the increasing demands for product consistency and production efficiency from smart factories and intelligent production lines make high-speed powder transport a crucial component of modern smart factories. During high-speed mixing and transport of powder, the powder or its slurry subjected to rapid impact or continuous corrosion on steel tanks and pipes. This can easily lead to rapid wear and corrosion failure of the steel, and also introduce steel powder impurities into the powder. In short, in these service environments, steel tanks and pipes are frequently subjected to corrosion and erosion from corrosive liquids, powder transport, and high-temperature molten metal, severely reducing their service life and requiring significant manpower and resources for repair and replacement. Therefore, improving the wear and corrosion resistance of steel surfaces such as tanks and pipes is a persistent and necessary research topic.

[0003] Surface treatment of steel is considered an effective way to improve its strength and wear resistance. Currently, surface treatments for steel mainly include electroplating, infiltration casting, and thermal spraying. However, these methods all suffer from problems such as low coating hardness, poor uniformity, and easy peeling. For example, thermal spraying involves spraying a layer of chromium or nickel alloy powder onto the steel surface as a wear-resistant and corrosion-resistant coating. Although the coating preparation method is simple, the metal coating easily forms galvanic cells in water, exacerbating localized corrosion. Furthermore, the sprayed chromium or nickel coating has low hardness, poor wear resistance, and insufficient resistance to powder erosion, and the coating's protective lifespan for the steel is relatively short. High-temperature thermal diffusion salt bath method is a method for preparing coatings on steel surfaces. Compared to other surface treatments such as physical vapor deposition (PVD), chemical vapor deposition (CVD), laser surface treatment, surface metallization, carburizing, and nitriding, coatings treated with high-temperature thermal diffusion salt bath method have higher hardness and better wear resistance. Furthermore, the substrate and coating are metallurgically bonded, resulting in higher bonding strength, exceeding the E-7 structural adhesive strength limit (86MPa), and greater resistance to peeling. With the increasing demand for specialized powders and slurries in the powder handling industry and the need to improve automated handling efficiency, existing coatings are struggling to meet increasingly stringent operational requirements. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide an aluminum-vanadium co-infiltrating wear-resistant and corrosion-resistant coating for powder transport on steel surface and a preparation method thereof. The coating of the present invention has good wear and corrosion resistance.

[0005] This invention provides an aluminum-vanadium co-infiltrating wear-resistant and corrosion-resistant coating for steel surfaces used in powder transport, comprising the following raw materials by weight percentage: Na2B4O7 60.2~73.7%, Na3AlF 65~8%, Al powder 7~10%, NaF 3~5%, flux NaCl 1-3%, V2O5 10~13%, CeO2 0.3~0.8%.

[0006] Preferably, the raw materials include the following weight percentages: Na2B4O7 66~67%, Na3AlF 66~7%, Al powder 8~9%, NaF 4~4.5%, flux NaCl 1.5-2%, V2O5 11.7~12.6%, CeO2 0.3~0.4%.

[0007] This invention provides an aluminum-vanadium co-infiltrating wear-resistant and corrosion-resistant coating for steel surfaces used in powder transport. The coating is obtained by a high-temperature thermal diffusion salt bath method. The coating thickness is 20-50 μm. The coating is free of defects such as pores and obvious cracks and exhibits metallurgical bonding with the steel substrate.

[0008] This invention provides a method for preparing an aluminum-vanadium co-diffusion wear-resistant and corrosion-resistant coating on a steel surface for powder transport, comprising the following steps: placing the workpiece to be processed in a salt bath and holding it at a temperature for a period of time (generally 2-4 hours), wherein the salt bath is a mixture of Na2B4O7, Na3AlF6, Al powder, NaF and flux NaCl, and the temperature of the salt bath is 800-880℃; then adding V2O5 and CeO2, raising the temperature to 880-940℃ (holding time is 4-8 hours), and performing high-temperature thermal diffusion salt bath vanadium infiltration treatment to obtain an aluminum-vanadium co-diffusion wear-resistant and corrosion-resistant coating on a steel surface for powder transport.

[0009] Preferably, the workpiece to be processed is a steel substrate with oil and oxide layers removed. The removal is generally achieved by polishing the surface of the steel substrate with metallographic sandpaper to remove surface oil and oxide layers, followed by cleaning with alcohol and drying.

[0010] Preferably, the steel substrate includes, but is not limited to, carbon steel, high-speed steel, or 45# steel.

[0011] Preferably, the temperature of the salt bath agent is 850-870℃, and after adding V2O5 and CeO2, the temperature is raised to 930-940℃.

[0012] Preferably, the workpiece is preheated before being placed in the salt bath. The preheating temperature is 400-500℃, and the holding time is 30 minutes. Before placing the workpiece in the salt bath, the salt bath is kept at 800-880℃ for a period of time, generally 1-2 hours.

[0013] Preferably, the salt bath agent is stirred during the heat preservation process of the workpiece placed in the salt bath agent, and during the high-temperature thermal diffusion salt bath vanadium infiltration treatment, to ensure the uniformity of the coating.

[0014] Preferably, the heating rate to 880-940℃ is 2℃ / min.

[0015] The relevant reaction equations of this invention are as follows: 2Al+12F-+ 3 (1.1) 3 +2Fe=2Fe(Al)+12F-+ (1.2) (1.3) (1.4) (1.5) An aluminum-vanadium co-diffusion composite coating was prepared on the surface of steel using a high-temperature thermal diffusion salt bath method. First, Na₂B₄O₇, Na₃AlF₆, Al, NaF, and flux NaCl were heated and melted. Reactions (1.1) and (1.2) occurred between the molten Na₃AlF₆, NaF, and Al atoms. 3＋ Al generated by ionic reaction 1＋ Active ions are adsorbed onto the surface of the substrate material and then diffuse inward to form an aluminized layer.

[0016] When V2O5 and CeO2 are added, borax acts as a carrier for the presence and transport of active metal atoms. In the molten state, borax undergoes a decomposition reaction (1.3) to generate sodium metaborate and boron anhydride. Then, Al acts as a reducing agent to react (1.4) to reduce the V element in V2O5 and diffuse into the steel surface for deposition. At the same time, the free carbon in the steel matrix reacts with active vanadium atoms along the diffusion channels formed by aluminizing (1.5) to form a vanadium carbide layer.

[0017] The outer high-hardness vanadium carbide layer significantly improves the wear resistance of the steel, while the inner aluminide layer gives the steel excellent corrosion resistance. Therefore, this coating scheme improves both the corrosion resistance and wear resistance of the steel.

[0018] The beneficial effects of this invention are that it uses a high-temperature thermal diffusion salt bath method to prepare an aluminum-vanadium co-infiltration composite coating on the surface of steel. This overcomes the problems of high cost, small size, and difficulty in plating internal cavities associated with advanced surface treatment methods such as physical vapor deposition (PVD) and chemical vapor deposition (CVD). It also overcomes the problems of low coating hardness, poor uniformity, and easy peeling associated with traditional surface treatment methods such as electroplating and thermal spraying. At the same time, the aluminum-vanadium co-infiltration composite coating prepared on the steel surface not only has excellent corrosion resistance but also excellent wear resistance. This solves the problem that single-function coatings cannot simultaneously achieve wear resistance, corrosion resistance, and powder erosion resistance, effectively improving the service life of steel used for powder transport. Furthermore, this method is simple, can meet various practical production needs, and has broad prospects for industrial application. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the salt bath furnace used in the high-temperature thermal diffusion salt bath method.

[0020] Figure 2 The images shown are surface morphology diagrams (electron micrographs) of Examples 1 and 2 and the comparative examples.

[0021] Figure 3 The images shown are cross-sectional topographic images (electron micrographs) of Examples 1 and 2 and the comparative examples.

[0022] Figure 4 The images show the surface morphology (electron microscopy images) after friction and wear in Examples 1, 2, and the comparative examples.

[0023] Figure 5 The graph shows a comparison of the sliding friction coefficients of Examples 1 and 2 and the comparative example.

[0024] Figure 6 The images show the surface morphology (electron microscopy images) after static etching for Examples 1, 2, and the comparative examples. Detailed Implementation

[0025] Example 1 An aluminum-vanadium co-diffusion composite coating was prepared on the surface of a steel substrate using a high-temperature thermal diffusion salt bath method. The specific steps are as follows: 1. Cut the M35 high-speed steel into square samples of 200×200×4mm, and sand the surface of the sample to remove oil and oxide layer from the substrate surface. Then clean it with alcohol and blow it dry.

[0026] 2. Weigh 67wt.% Na2B4O7, 6wt.% Na3AlF6, 8wt.% Al powder, 4wt.% NaF and 2wt.% NaCl flux, mix them evenly and heat to 870℃ at a rate of 5℃ / min, keep at the temperature for 1.5 h, and after the salt bath reaction is complete, carry out high-temperature thermal diffusion salt bath treatment.

[0027] 3. After pretreatment in step 1, the sample is kept at 460℃ in an inert atmosphere furnace for 30 min, and then placed in a salt bath furnace and kept at 870℃ for 4 h. During the high-temperature thermal diffusion salt bath aluminizing process, the salt bath furnace is stirred every 1 h by means of electromagnetic stirring to maintain the fluidity of the salt bath to ensure a sufficient supply of active metal atoms on the sample surface and to ensure the uniformity of the aluminized coating.

[0028] 4. Add the remaining 12.6 wt.% V2O5 and 0.4 wt.% CeO2 materials to step 3, and stir the salt bath furnace using electromagnetic stirring or other methods. Continue to increase the furnace temperature to 930℃ at a rate of 2℃ / min, and hold for 6 hours. Stir the salt bath furnace every hour using electromagnetic stirring or other methods to maintain the fluidity of the salt bath and ensure a sufficient supply of active metal atoms on the sample surface, thereby ensuring the uniformity of the vanadium infiltration coating.

[0029] The equipment used, such as Figure 1 As shown, the equipment includes a furnace chamber 2, which is surrounded by furnace walls 1. A crucible 4 is installed inside the furnace chamber 2. The salt bath agent and the material to be processed are placed in the crucible 4 for processing. A furnace cover 5 is installed above the crucible 4. The temperature is measured and controlled by a thermocouple 3 that extends into the crucible 4.

[0030] The surface microstructure of the aluminum-vanadium co-diffusion composite coating prepared by the high-temperature thermal diffusion salt bath method is as follows: Figure 2 As shown, from Figure 2 It can be seen that the coating surface is smooth and flat, without obvious cracks and pores.

[0031] from Figure 3 The microstructure of the coating cross section shows that the aluminum-vanadium co-diffusion coating is divided into two distinct layers with a thickness of approximately 35 μm. There are no large cracks or pores or other defects inside the coating. The outer layer is a vanadium carbide layer, and the inner layer is mainly composed of an aluminum-rich layer. The coating is tightly bonded to the substrate and has no obvious defects. The interface between the substrate and the diffusion layer is well bonded.

[0032] The surface morphology of the coating after friction and wear is as follows Figure 4 As shown in the figure, it can be clearly seen that there is less wear debris near the wear marks, the wear marks are darker and shallower, therefore the coating sample underwent oxidative wear during the friction and wear test.

[0033] Figure 5 By comparing the friction coefficient curves, it was found that the friction coefficient of Example 1 was only 0.258, which was significantly lower than that of Comparative Example 1 (0.501), indicating that the aluminum-vanadium co-infiltration composite coating greatly improved the wear resistance of the steel.

[0034] Further 600-hour lead corrosion test was conducted, and the morphology after corrosion was as follows: Figure 6As shown in the figure, after 600 hours of lead liquid corrosion, a protective oxide film still exists on the coating surface, with corrosion pits appearing only in localized areas. This indicates that the coating can effectively improve the corrosion resistance of the steel substrate.

[0035] Finally, the treated steel plate underwent a high-speed powder impact test. After weighing using a precision electronic scale with an accuracy of 0.01g, it was placed in a high-speed powder impact test chamber. The powder spray gun pressure was 0.7MPa, and the blasting time was 1 hour. The weight was then measured after the test. Comparing the weight changes before and after the test, it was found that the mass reduction was only 0.27g, a significant improvement compared to the 4.9g weight loss of the uncoated substrate sample, representing a weight reduction of approximately 18 times.

[0036] Example 2 An aluminum-vanadium co-diffusion composite coating was prepared on the surface of a steel substrate using a high-temperature thermal diffusion salt bath method. The specific steps are as follows: 1. Cut the M35 high-speed steel into square samples of 200×200×4 mm, and sand the surface of the sample to remove oil and oxide layer from the substrate surface. Then clean it with alcohol and blow it dry.

[0037] 2. Weigh 66 wt.% Na2B4O7, 7 wt.% Na3AlF6, 9 wt.% Al powder, 4.5 wt.% NaF and 1.5 wt.% NaCl flux, mix them evenly and heat them to 850℃ at a rate of 5℃ / min, keep them at that temperature for 1.8h, and after the salt bath reaction is complete, carry out high-temperature thermal diffusion salt bath treatment.

[0038] 3. After pretreatment in step 1, the samples were kept at 450℃ in an inert atmosphere furnace for 30 minutes, and then placed in a salt bath furnace and kept at 860℃ for 3 hours. During the high-temperature thermal diffusion salt bath aluminizing process, the salt bath furnace was stirred every hour by means of electromagnetic stirring to maintain the fluidity of the salt bath and ensure a sufficient supply of active metal atoms on the sample surface, thus ensuring the uniformity of the aluminized coating.

[0039] 4. Add the remaining 11.7wt.%V2O5 and 0.3wt.%CeO2 materials to step 3, and stir the salt bath furnace using electromagnetic stirring or other methods. Continue to increase the furnace temperature to 940℃ at a rate of 2℃ / min, and hold for 8 hours. Stir the salt bath furnace every hour using electromagnetic stirring or other methods to maintain the fluidity of the salt bath and ensure a sufficient supply of active metal atoms on the sample surface, thereby ensuring the uniformity of the vanadium infiltration coating.

[0040] The surface microstructure of the aluminum-vanadium co-diffusion composite coating prepared by the high-temperature thermal diffusion salt bath method is as follows: Figure 2 As shown in the figure, the coating surface is relatively smooth and flat, with no obvious cracks or pores.

[0041] from Figure 3 The microstructure of the coating cross section shows that the aluminum-vanadium co-diffusion coating is divided into two distinct layers with no obvious separation between them. The thickness is about 30 μm. There are no large cracks or pores or other defects inside the coating. The outer layer is a vanadium carbide layer, and the inner layer is mainly composed of an aluminum-rich layer. The inner layer is tightly connected to the substrate and has no obvious defects. The interface between the substrate and the diffusion layer is well bonded.

[0042] The surface morphology of the coating after friction and wear is as follows Figure 4 As shown, there are fewer wear debris on the surface of the coating. In the depth of the wear track, there is flaking caused by shearing, tearing and other actions, and the wear track color becomes darker, indicating that the coating sample underwent oxidative wear and fatigue wear during the friction and wear test.

[0043] Figure 5 The friction coefficient curves of Example 2 were compared, and it was found that the friction coefficient of Example 2 was only 0.282, which was significantly lower than that of the comparative example of 0.501. The friction curve first increased and then gradually decreased. It is speculated that during the wear process, as the wear time increases, the wear debris generated by the wear is gradually pressed into the bottom of the grinding groove, resulting in an increase in the surface roughness of the grinding groove. The friction coefficient also increases during dry friction. However, the vanadium carbide coating has high hardness, so the amount of wear debris pressed in is small, and the change in the friction coefficient is also small.

[0044] like Figure 6 The figure shows the results of a 600-hour lead corrosion test on the coating sample. As can be seen from the figure, after 600 hours of lead corrosion, the coating surface is still relatively smooth, with some pitting and pitting in some areas. This indicates that the lead has a slight erosion effect on the coating. However, due to the continuous formation of an Al2O3 film in the aluminum-rich layer inside the coating, it plays a good protective role for the steel substrate.

[0045] Finally, the treated steel plate underwent a high-speed powder impact test. After weighing using a precision electronic scale with an accuracy of 0.01g, it was placed in a high-speed powder impact test chamber. The powder spray gun pressure was 0.7MPa, and the blasting time was 1 hour. The weight was then measured after the test. Comparing the weight changes before and after the test, it was found that the mass reduction was only 0.21g, a significant improvement compared to the 4.9g weight loss of the uncoated substrate sample, representing a weight reduction of approximately 23 times.

[0046] Comparative Example 1 An aluminum-vanadium co-diffusion composite coating was prepared on the surface of a steel substrate using a high-temperature thermal diffusion salt bath method. The specific steps are as follows: 1. Cut the M35 high-speed steel into square samples of 200×200×4mm, and sand the surface of the sample to remove oil and oxide layer from the substrate surface. Then clean it with alcohol and blow it dry.

[0047] 2. Weigh 66 wt.% Na2B4O7, 4 wt.% Na3AlF6, 11 wt.% Al powder, 6 wt.% NaF and 0.5 wt.% NaCl flux, mix them evenly and heat to 830℃ at a rate of 5℃ / min, keep at the temperature for 1 hour, and after the salt bath reaction is complete, carry out high-temperature thermal diffusion salt bath treatment.

[0048] 3. After pretreatment in step 1, the samples were kept at 450℃ in an inert atmosphere furnace for 30 minutes, and then placed in a salt bath furnace and kept at 860℃ for 3 hours. During the high-temperature thermal diffusion salt bath aluminizing process, the salt bath furnace was stirred every hour by means of electromagnetic stirring to maintain the fluidity of the salt bath and ensure a sufficient supply of active metal atoms on the sample surface, thus ensuring the uniformity of the aluminized coating.

[0049] 4. Add the remaining 11.5wt.%V2O5 and 1wt.%CeO2 materials to step 3, and stir the salt bath furnace using electromagnetic stirring or other methods. Continue to increase the furnace temperature to 870℃ at a rate of 2℃ / min, and hold for 6 hours. Stir the salt bath furnace every hour using electromagnetic stirring or other methods to maintain the fluidity of the salt bath and ensure a sufficient supply of active metal atoms on the sample surface, thereby ensuring the uniformity of the vanadium infiltration coating.

[0050] The surface microstructure of the aluminum-vanadium co-diffusion composite coating prepared by the high-temperature thermal diffusion salt bath method is as follows: Figure 2 As shown in the figure, some microcracks appear on the surface of the coating, and the surface is rough and uneven.

[0051] from Figure 3 The microstructure of the coating cross section shows that the aluminum-vanadium co-diffusion coating is divided into two distinct layers with a thickness of approximately 25 μm. Numerous pores are visible inside, and the bonding between the vanadium-diffusion layer and the aluminum-rich layer is poor. This may be due to the excessively low vanadium infiltration temperature and an unsuitable salt bath formulation.

[0052] from Figure 4 The surface morphology of the coating after friction and wear shows that there are many wear debris near the wear marks, as well as a large number of obvious cracks. Irregular pits left by adhesion and tearing are also present in some areas, indicating that adhesive wear occurred during the wear process. The cracks are due to the numerous pores in the coating. During wear, the grinding ball begins to contact the coating. At the pores, the contact area decreases, the pressure increases, and shear deformation occurs at the pore edges. When the deformation exceeds the coating's strength limit, cracks begin to initiate.

[0053] from Figure 5As can be seen from the friction coefficient curve, its friction coefficient of 0.501 is much larger than that of other coatings. This is because in the initial stage of friction, due to the small contact area, the pressure is high. After adhesive wear occurs, the contact surface becomes more uneven, and the friction coefficient also increases. Along with the large amount of heat generated by dry friction, oxidative wear occurs. The oxidized wear debris is pressed into the substrate, making the substrate more uneven. The tearing and wear debris pressing phenomena correspond to the fluctuating friction coefficient during the friction process.

[0054] Further 600-hour lead corrosion test was conducted, and the morphology after corrosion was as follows: Figure 6 As shown in the figure, after 600 hours of lead corrosion, obvious corrosion characteristics can be seen on the coating surface. Loose, fine and uniformly distributed corrosion products appear on the surface. There are two corrosion layers. The outer corrosion layer has partial breakage and peeling, while the corrosion products of the inner layer are regular granular. The numerous pores inside the aluminum-rich layer make it easy for lead to penetrate into the coating, resulting in limited protection of the steel.

[0055] Finally, the treated steel plate was subjected to a high-speed powder impact test. After weighing using a precision electronic scale with an accuracy of 0.01g, it was placed in a high-speed powder impact test chamber. The powder spray gun pressure was 0.7MPa, and the blasting time was 1 hour. The weight was then measured after the test. Comparing the weight changes before and after the test, a mass reduction of 1.2g was found, a significant improvement compared to the 4.9g weight loss of the uncoated substrate sample, representing a reduction of approximately four times.

[0056] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of protection of this application is limited to these examples; within the framework of this application, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of different aspects of one or more embodiments of this application as described above, which are not provided in detail for the sake of brevity.

[0057] One or more embodiments in this application are intended to cover all such substitutions, modifications, and variations that fall within the broad scope of this application. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of one or more embodiments in this application should be included within the protection scope of this application.

Claims

1. A wear-resistant and corrosion-resistant coating for steel surfaces using aluminum-vanadium co-infiltration for powder transport, characterized in that, The raw materials include the following weight percentages: Na₂B₄O₇ 66-67%, Na₃AlF₆ 6-7%, Al powder 8-9%, NaF 4-4.5%, flux NaCl 1.5-2%, V₂O₅ 11.7-12.6%, CeO₂ 0.3-0.4%; The preparation method of the aluminum-vanadium co-infiltrating wear-resistant and corrosion-resistant coating for steel surface used in powder transport includes the following steps: placing the workpiece to be processed in a salt bath agent and keeping it at a certain temperature for a period of time; the salt bath agent is a mixture of Na2B4O7, Na3AlF6, Al powder, NaF and flux NaCl; the temperature of the salt bath agent is 800-880℃; then adding V2O5 and CeO2, raising the temperature to 880-940℃, and performing high-temperature thermal diffusion salt bath vanadium infiltration treatment to obtain the aluminum-vanadium co-infiltrating wear-resistant and corrosion-resistant coating for steel surface used in powder transport.

2. A method for preparing an aluminum-vanadium co-infiltrating wear-resistant and corrosion-resistant coating for powder transport on a steel surface as described in claim 1, characterized in that, The process includes the following steps: placing the workpiece to be processed in a salt bath and keeping it at a temperature of 800-880℃ for a period of time; then adding V2O5 and CeO2, raising the temperature to 880-940℃, and performing a high-temperature thermal diffusion salt bath vanadium infiltration treatment to obtain an aluminum-vanadium co-infiltrating wear-resistant and corrosion-resistant coating on the surface of steel for powder transport.

3. The preparation method according to claim 2, characterized in that, The workpiece to be processed is a steel substrate with oil and oxide layers removed.

4. The preparation method according to claim 3, characterized in that, The steel substrate includes, but is not limited to, carbon steel, high-speed steel, or 45# steel.

5. The preparation method according to any one of claims 2-4, characterized in that, The temperature of the salt bath agent is 850-870℃, and after adding V2O5 and CeO2, the temperature is raised to 930-940℃.

6. The preparation method according to any one of claims 2-4, characterized in that, Before placing the workpiece in the salt bath, preheat the workpiece to a temperature of 400-500℃.

7. The preparation method according to any one of claims 2-4, characterized in that, During the process of keeping the workpiece in the salt bath and during the high-temperature thermal diffusion salt bath vanadium infiltration treatment, the salt bath is stirred to ensure the uniformity of the coating.

8. The preparation method according to any one of claims 2-4, characterized in that, The heating rate to 880-940℃ is 2℃ / min.

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