A method for preparing Ce-containing small cylindrical NdFeB magnetic material capable of improving coercivity

Abstract

The present invention discloses a method for preparing a Ce-containing small cylindrical NdFeB magnetic material capable of improving coercivity. The method introduces CaH2 as a strong reducing agent into the preparation system of the small cylindrical NdFeB magnetic material. The CaH2 decomposes into Ca and H2 at 600°C. When used in powder metallurgy, when heated to 600-1000°C, the CaH2 has a stronger reducing effect on metal oxides than sodium hydride or lithium hydride, thereby reducing the oxidation rate of the Ce magnet main phase and partially reducing incompletely oxidized rare earth metals during the sintering process. Simultaneously, the method controls the particle size of the Ce-containing NdFeB alloy powder and the particle size of the added CaH2 powder, so that the final product has a yield of 80%, a remanent magnetization Br in the range of 11.1-12.53 kgs, and an intrinsic coercivity of 10.3-12.8 kgs, meeting the requirements for use of civilian magnetic materials.

Description

Technical Field

[0001] The invention relates to the technical field of preparation of NdFeB magnetic materials, and in particular to a preparation method of Ce-containing small cylindrical NdFeB magnetic materials capable of improving coercivity. Background Art

[0002] Neodymium iron boron magnetic materials have excellent performance and are widely used in all aspects of life. As of 2021, the national output of neodymium iron boron magnetic materials has exceeded 240,000 tons. However, due to the national total amount of rare earth mining restrictions, there is a serious shortage of praseodymium and neodymium rare earth metals used to manufacture magnetic materials on the market. Since June 2020, the price of praseodymium and neodymium rare earth metals has risen from 420,000 yuan / t to 1.28 million yuan / t in March 2022, an increase of more than two times. Therefore, the raw material cost of manufacturing 1kg of pure neodymium iron boron magnetic material blanks has reached 450 yuan / kg, which seriously restricts the use of magnetic materials in lower-performance products.

[0003] Currently, there are two methods for reducing the amount of praseodymium-neodymium metal in alloys. The first is to reduce the total rare earth content in the alloy from 32% to 29.5%. However, NdFeB alloys require at least 26.68% neodymium, plus a 3% rare earth phase for liquid-phase sintering. The second method is to replace praseodymium-neodymium metal with cheaper rare earth metals, such as Ce. The market price of Ce metal is 40,000 yuan per ton, which means that every 1% reduction in praseodymium-neodymium metal can save 12 yuan in raw material costs.

[0004] In most civil magnetic materials, especially magnetic materials for toys, luggage buckles, etc., the performance is concentrated between N30-N38, and the remanence Br range is 11-12.3KGs. This part of magnetic materials is mainly made of materials such as praseodymium and neodymium and metal cerium. 14 The theoretical remanence of Br can reach 16.1KGs, Ce2Fe 14The theoretical remanence Br of B4 can reach 11.7 kgs, and any combination of the two can meet the requirements of the remanence Br range for civilian magnetic materials. However, in specific use, the material is required to have an intrinsic coercivity Hcj of 10-12.5 kOe. However, as a cerium-iron-boron magnet, its intrinsic coercivity Hcj is very low, only reaching 3 kOe, making it unusable. To achieve this, it is necessary to increase the total amount of rare earths and the amount of praseodymium-neodymium metal to improve the coercivity to meet the use requirements, which will inevitably result in a waste of valuable elements. There are also reports in the prior art that use heavy rare earth elements Gd and Ho to replace praseodymium-neodymium metal to improve coercivity. However, heavy rare earth elements are themselves scarce resources, and Ho, in particular, is more expensive than praseodymium-neodymium. Although the price of Gd has decreased with the decline in the mining volume of southern mines in recent years, it is still 75% of that of praseodymium-neodymium metal, so there is no significant cost reduction effect. In particular, the NdFeB magnetic materials containing Ce in small cylindrical shapes with diameters of 3mm, 5mm, and 7mm, respectively, from N30 to N38, can be obtained by making the magnet into a large blank (64*54*34mm), cutting it into square bars using wire cutting, and then making it into cylinders, commonly known as square rounding. However, in actual operation, most products will turn into mud, and the yield rate is too low, only about 55%. The second method is to directly press the alloy powder into a cylindrical blank, and then sinter it into small cylinders of standard size. This method has two difficulties: (1) The small cylinders are very easy to oxidize during the pressing process, resulting in reduced performance, especially low coercive force; (2) The alloy powder cannot be too fine. If the powder is too fine, the small cylinders are very easy to deform, resulting in a decrease in product yield. Summary of the Invention

[0005] The purpose of the present invention is to solve the technical problems of low yield and low coercivity in the existing preparation method of Ce-containing small cylindrical NdFeB magnetic materials, and to provide a preparation method of Ce-containing small cylindrical NdFeB magnetic materials that can improve coercivity.

[0006] To achieve its purpose, the present invention adopts the following technical solutions:

[0007] The present invention provides a method for preparing a Ce-containing small cylindrical NdFeB magnetic material capable of improving coercivity, comprising the following steps:

[0008] S1, after evacuating the hydrogen blast furnace and flushing the furnace with argon, add metallic calcium and heat to 220°C, introduce 99.99% hydrogen, and maintain the hydrogen blast furnace temperature below 250°C. When the hydrogen blast furnace pressure no longer increases, stop the reaction, cool to room temperature, and obtain coarse calcium hydride powder, which is then placed in a steel cylinder and filled with argon for storage;

[0009] S2, the calcium hydride coarse powder in S1 is subjected to a jet mill to obtain calcium hydride fine powder with a fineness SMD of 4±0.5 μm;

[0010] S3, weighing the determined formula of 4±0.5μm Ce-containing NdFeB alloy powder, adding 0.3-3% by mass of the calcium hydride fine powder in S2, and adding 1‰ of lubricant, mixing well to obtain a mixed powder;

[0011] S4: Press the mixed powder in S3 into green blocks in a strong magnetic press, encapsulate and isostatically press to obtain a density of ≥4.1g / cm 3 of compacts;

[0012] S5, sintering the S4 medium compact in a high-temperature vacuum sintering furnace at 1030-1065°C for 4-5 hours, cooling with argon gas to below 85°C, and obtaining a sintered green body;

[0013] S6, tempering the sintered green body material in S5 in a vacuum furnace, and then cooling it with argon gas until the temperature is below 40°C and taking it out of the furnace to obtain a tempered green body material;

[0014] S7, using a grinder to remove the black skin of the blank after tempering in S6 to obtain a small cylindrical NdFeB magnetic material of standard size.

[0015] As a further preferred embodiment of the technical solution of the present invention, in S3, the composition of the Ce-containing neodymium iron boron alloy is as follows, by mass percentage: PrNd (20:80) 10-22.3%, Ce 9.2-22%, Fe≥64%, Al 0.7-1.5%, Cu 0.15-0.2%, B 0.92-0.98%, and Zr 0.3%; preferably, the mass fraction of Ce is 9.2-15.7%.

[0016] Furthermore, in S6, the primary tempering temperature of the sintered green body material is 660-690°C.

[0017] Furthermore, the primary tempering and holding time is 3-5 hours.

[0018] The main reasons for the low coercivity of metallic cerium, the main component of civilian magnetic materials, are: (1) metallic cerium is easily oxidized, and (2) its liquid phase sintering ability is poor. The oxidation of cerium iron boron magnets mainly comes from two aspects. The first is that the nitrogen purity in the airflow milling process cannot reach absolute purity, and it is easily oxidized in the high-speed airflow. The second is that the vacuum sintering process cannot achieve absolute vacuum. The residual air in the furnace and the released organic additives, such as lubricants and antioxidants, will consume rare earth metals and cause oxidation.

[0019] The present invention introduces CaH2 as a strong reducing agent into the preparation system of small cylindrical NdFeB magnetic materials. CaH2 decomposes into Ca and H2 at 600°C and is used in powder metallurgy. When heated to 600-1000°C, its reducing effect on metal oxides is stronger than that of sodium hydride or lithium hydride, which can reduce the oxidation rate of the main phase of the Ce magnet and reduce some incompletely oxidized rare earth metals during the sintering process. At the same time, the present invention controls the particle size of the Ce-containing NdFeB alloy powder and the particle size of the added CaH2 powder, so that the final product has a yield of 80%, a remanence Br range of 11.1-12.53 kgs, and an intrinsic coercive force of 10.3-12.8 kgs, meeting the requirements for use of civilian magnetic materials. DETAILED DESCRIPTION

[0020] The present invention will be further described below through specific examples.

[0021] The lubricant in the embodiment of the present invention comprises a solute and a solvent, wherein the solute is 2-aminobenzothiazole and the solvent is chloroform.

[0022] The mass ratio of solute to solvent is 0.5:5.

[0023] Example 1

[0024] The composition of the Ce-containing NdFeB alloy in this embodiment is as follows by mass percentage: PrNd (20:80) 22.3%, Ce 9.2%, Fe 66.43%, Al 0.7%, Cu 0.15%, B 0.92%, Zr 0.3%;

[0025] This embodiment provides a method for preparing a Ce-containing small cylindrical NdFeB magnetic material capable of improving coercivity, comprising the following steps:

[0026] S1, evacuate the hydrogen blast furnace and purge the furnace with argon, add metallic calcium and heat to 220°C, introduce 99.99% hydrogen, and maintain the hydrogen blast furnace temperature at 240°C. Stop the reaction when the hydrogen blast furnace pressure no longer increases, cool to room temperature, and obtain coarse calcium hydride powder, which is then placed in a steel cylinder and filled with argon for storage;

[0027] S2, the calcium hydride coarse powder in S1 is subjected to air jet milling to obtain calcium hydride fine powder with a fineness SMD of 3.8 μm;

[0028] S3, weighing the Ce-containing NdFeB alloy powder with a particle size of 3.8 μm of the above-determined formula, adding 1% by mass of the calcium hydride fine powder in S2, and adding 1‰ of a lubricant, mixing well to obtain a mixed powder;

[0029] S4: The mixed powder in S3 was pressed into green blocks in a strong magnetic press, and then isostatically pressed after packaging to obtain a density of 4.1 g / cm3 of compacts;

[0030] S5, sintering the S4 medium pressed green body in a high temperature vacuum sintering furnace at 1045°C for 4 hours, and cooling it with argon gas to below 85°C to obtain a sintered green body;

[0031] S6, tempering the sintered green body in S5 once in a vacuum furnace at a tempering temperature of 660°C for 4 hours, then cooling with argon gas to a temperature below 40°C and taking it out of the furnace to obtain a tempered green body;

[0032] In step S7, the black skin of the blank after tempering in step S6 was removed by a grinder to obtain a small cylindrical NdFeB magnetic material containing Ce of standard size. The density, remanence, intrinsic coercivity, magnetic energy product and squareness performance tests were carried out. The results are shown in Table 1.

[0033] Comparative Example 1

[0034] Steps S1 and S2 are omitted, and calcium hydride fine powder is not added in step S3;

[0035] The sintering temperature in S5 is 1065°C;

[0036] In S6, two tempering steps were performed: the first tempering temperature was 920°C for 2.5 hours; the second tempering temperature was 525°C for 4 hours. Other conditions and steps were the same as in Example 1. Standard-sized small cylindrical NdFeB magnetic materials containing Ce were obtained and tested for density, remanence, intrinsic coercivity, magnetic energy product, and squareness. The results are shown in Table 1.

[0037] Comparative Example 2

[0038] In this comparative example, the Ce-containing NdFeB alloy contained 23.5% PrNd (20:80) and 8% Ce; other conditions and procedures were the same as in Comparative Example 1. Standard-sized small cylindrical Ce-containing NdFeB magnetic materials were obtained and tested for density, remanence, intrinsic coercivity, magnetic energy product, and squareness. The results are shown in Table 1.

[0039] Example 2

[0040] The composition of the Ce-containing NdFeB alloy in this embodiment is as follows by mass percentage: PrNd (20:80) 16%, Ce 15.7%, Fe 66.23%, Al 0.7%, Cu 0.15%, B 0.92%, Zr 0.3%;

[0041] The particle size of calcium hydride fine powder in S2 is 4 μm;

[0042] The particle size of Ce-containing NdFeB alloy powder in S3 is 4 μm, and the amount of calcium hydride fine powder added is 2%;

[0043] The sintering temperature in S5 is 1055°C;

[0044] In S6, the primary tempering temperature is 670°C and the tempering holding time is 4 hours;

[0045] Other conditions and steps were the same as those in Example 1. Standard-sized small cylindrical NdFeB magnetic materials containing Ce were obtained and tested for density, remanence, intrinsic coercivity, magnetic energy product and squareness. The results are shown in Table 1.

[0046] Comparative Example 3

[0047] Steps S1 and S2 are omitted, and calcium hydride fine powder is not added in step S3;

[0048] The sintering temperature in S5 is 1055°C;

[0049] In S6, two tempering steps were performed: the first tempering temperature was 920°C for 2.5 hours; the second tempering temperature was 525°C for 4 hours. Other conditions and steps were the same as in Example 2. Standard-sized small cylindrical NdFeB magnetic materials containing Ce were obtained and tested for density, remanence, intrinsic coercivity, magnetic energy product, and squareness. The results are shown in Table 1.

[0050] Comparative Example 4

[0051] In S6, two tempering steps were performed, one at 670°C for 4 hours. Other conditions and steps were the same as in Comparative Example 3. Standard-sized small cylindrical NdFeB magnetic materials containing Ce were obtained and tested for density, remanence, intrinsic coercivity, magnetic energy product, and squareness. The results are shown in Table 1.

[0052] Example 3

[0053] The composition of the Ce-containing NdFeB alloy in this embodiment is as follows by mass percentage: PrNd (20:80) 13%, Ce 20%, Fe 64.35%, Al 1.2%, Cu 0.2%, B 0.95%, Zr 0.3%;

[0054] The particle size of calcium hydride fine powder in S2 is 4.5 μm;

[0055] The particle size of Ce-containing NdFeB alloy powder in S3 is 4.5 μm, and the amount of calcium hydride fine powder added is 0.3%;

[0056] The sintering temperature in S5 is 1030°C;

[0057] In S6, the primary tempering temperature is 685°C and the tempering holding time is 5 hours;

[0058] Other conditions and steps were the same as those in Example 1. Standard-sized small cylindrical NdFeB magnetic materials containing Ce were obtained and tested for density, remanence, intrinsic coercivity, magnetic energy product and squareness. The results are shown in Table 1.

[0059] Comparative Example 5

[0060] The composition of the Ce-containing NdFeB alloy in this embodiment is as follows by mass percentage: PrNd (20:80) 14%, Ce 19%, Fe 64.35%, Al 1.2%, Cu 0.2%, B 0.95%, Zr 0.3%;

[0061] Steps S1 and S2 are omitted, and calcium hydride fine powder is not added in step S3;

[0062] In S6, two tempering steps were performed: the first tempering temperature was 920°C for 2.5 hours; the second tempering temperature was 525°C for 4 hours. Other conditions and steps were the same as in Example 3. Standard-sized small cylindrical NdFeB magnetic materials containing Ce were obtained and tested for density, remanence, intrinsic coercivity, magnetic energy product, and squareness. The results are shown in Table 1.

[0063] Example 4

[0064] The composition of the Ce-containing NdFeB alloy in this embodiment is as follows by mass percentage: PrNd (20:80) 10%, Ce 22%, Fe 65.02%, Al 1.5%, Cu 0.2%, B 0.98%, Zr 0.3%;

[0065] The particle size of calcium hydride fine powder in S2 is 3.5 μm;

[0066] The particle size of Ce-containing NdFeB alloy powder in S3 is 3.5 μm, and the amount of calcium hydride fine powder added is 3%;

[0067] In S5, the sintering temperature is 1015°C and the sintering time is 5 hours;

[0068] In S6, the primary tempering temperature is 690°C and the tempering holding time is 3 hours;

[0069] Other conditions and steps were the same as those in Example 1. Standard-sized small cylindrical NdFeB magnetic materials containing Ce were obtained and tested for density, remanence, intrinsic coercivity, magnetic energy product and squareness. The results are shown in Table 1.

[0070] Comparative Example 6

[0071] Steps S1 and S2 are omitted, and calcium hydride fine powder is not added in step S3;

[0072] The amount of calcium hydride fine powder added in S3 is 3%;

[0073] Other conditions and steps were the same as those in Example 4. Standard-sized small cylindrical NdFeB magnetic materials containing Ce were obtained and tested for density, remanence, intrinsic coercivity, magnetic energy product and squareness. The results are shown in Table 1.

[0074] Comparative Example 7

[0075] The composition of the Ce-containing NdFeB alloy in this embodiment is as follows by mass percentage: PrNd (20:80) 12%, Ce 20%, Fe 65.02%, Al 1.5%, Cu 0.2%, B 0.98%, Zr 0.3%;

[0076] Steps S1 and S2 are omitted, and calcium hydride fine powder is not added in step S3;

[0077] In S6, two tempering steps were performed: the first tempering temperature was 920°C for 2.5 hours; the second tempering temperature was 525°C for 4 hours. Other conditions and steps were the same as in Example 4. Standard-sized small cylindrical NdFeB magnetic materials containing Ce were obtained and tested for density, remanence, intrinsic coercivity, magnetic energy product, and squareness. The results are shown in Table 1.

[0078] Table 1 Performance data of small cylindrical NdFeB magnetic materials containing Ce

[0079]

[0080] As previously mentioned, the remanence Br of civilian magnetic materials ranges from 11 to 12.3 kgs, and in specific applications, the material's intrinsic coercivity Hcj is required to reach 10 to 12.5 kOe. As shown in Table 1, the Ce-containing small cylindrical NdFeB magnetic material prepared using the present invention has a remanence Br range of 11.1 to 12.53 kgs and an intrinsic coercivity of 10.3 to 12.8 kgs, fully meeting the requirements for civilian magnetic material use.

Claims

1. A method for preparing Ce-containing small cylindrical NdFeB magnetic materials capable of improving coercivity, characterized in that: The following steps are involved: S1, after evacuating the hydrogen blast furnace and flushing the furnace with argon, add metallic calcium and heat to 220°C, introduce 99.99% hydrogen, and maintain the hydrogen blast furnace temperature below 250°C. When the hydrogen blast furnace pressure no longer increases, stop the reaction, cool to room temperature, and obtain coarse calcium hydride powder, which is then placed in a steel cylinder and filled with argon for storage; S2, the calcium hydride coarse powder in S1 is subjected to a jet mill to obtain calcium hydride fine powder with a fineness SMD of 4±0.5 μm; S3, weighing the Ce-containing NdFeB alloy powder with a particle size of 4±0.5 μm, adding 0.3-3% by mass of the calcium hydride fine powder in S2, and adding 1‰ of a lubricant, mixing well to obtain a mixed powder; The composition of the Ce-containing NdFeB alloy is as follows by mass percentage: PrNd 10-22.3%, Ce 9.2-22%, Fe≥64%, Al 0.7-1.5%, Cu 0.15-0.2%, B 0.92-0.98%, Zr 0.3%; wherein the PrNd ratio is 20:80; S4: Press the mixed powder in S3 into green blocks in a strong magnetic press, encapsulate and isostatically press to obtain a density of ≥4.1g / cm 3 of compacts; S5, sintering the S4 medium compact in a high-temperature vacuum sintering furnace at 1030-1065°C for 4-5 hours, cooling with argon gas to below 85°C, and obtaining a sintered green body; S6, tempering the sintered green body material in S5 in a vacuum furnace, and then cooling it with argon gas until the temperature is below 40°C and taking it out of the furnace to obtain a tempered green body material; S7, using a grinder to remove the black skin of the blank after tempering in S6 to obtain a small cylindrical NdFeB magnetic material of standard size.

2. The method for preparing a Ce-containing small cylindrical NdFeB magnetic material capable of improving coercivity according to claim 1, characterized in that: In S3, the mass fraction of Ce in the Ce-containing NdFeB alloy is 9.2-15.7%.

3. A method for preparing a Ce-containing small cylindrical NdFeB magnetic material capable of improving coercivity as claimed in claim 1, characterized in that: In S6, the primary tempering temperature of the sintered green body is 660-690°C.

4. A method for preparing a Ce-containing small cylindrical NdFeB magnetic material capable of improving coercivity as claimed in claim 3, characterized in that: The primary tempering and heat preservation time is 3-5 hours.

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