Neodymium-iron-boron magnet without heavy rare earth and preparation method thereof

Abstract

The invention belongs to the technical field of rare earth permanent magnet materials, and particularly discloses a neodymium-iron-boron magnet without heavy rare earth and a preparation method of the neodymium-iron-boron magnet. The neodymium-iron-boron magnet comprises main-phase neodymium-iron-boron coarse powder and auxiliary-phase praseodymium-titanium-aluminum coarse powder, wherein the auxiliary-phase praseodymium-titanium-aluminum coarse powder accounts for 0.5-3.5 wt% of the main-phase neodymium-iron-boron coarse powder; the main phase comprises the following components in percentage by weight: 29.5 to 32.5 percent of PrNd; 0.88 to 1 wt% of B; 0 to 0.3 wt% of Al; 0.1 to 0.4 wt% of Cu; 0 to 0.2 wt% of Ti; 0.1 to 0.4 wt% of Zr; ga is 0.2 to 0.6 wt%; 0.5 to 1.8 wt% of Co; and the balance of Fe. The auxiliary phase comprises the following components: 75-85 wt% of Pr; 5 to 10 weight percent of Ti; and 10 to 15 wt% of Al. The neodymium-iron-boron magnet has the advantages of being free of heavy rare earth, high in magnetic performance, excellent in mechanical performance, simple in process, suitable for mass production and the like.

Description

Technical Field

[0001] This invention relates to the field of rare earth permanent magnet materials technology, and in particular to a neodymium iron boron magnet without heavy rare earth elements and its preparation method. Background Technology

[0002] In recent years, sintered NdFeB magnets have played an indispensable role in new energy vehicles, humanoid robots, and consumer electronics due to their superior magnetic properties. On the one hand, the coercivity of traditional sintered NdFeB materials decreases significantly with increasing temperature in high-temperature applications, severely limiting their widespread use in fields with extremely high temperature stability requirements, such as new energy vehicle drive motors and 3C electronic products. While heavy rare earth elements Dy and Tb can effectively improve the coercivity of magnets, they are expensive and their resources are extremely unevenly distributed. On the other hand, due to the brittleness of sintered NdFeB magnets, they are prone to chipping and breakage during processing, affecting the yield rate of finished products and indirectly increasing production costs. Therefore, developing NdFeB materials without heavy rare earth elements that possess both high magnetic properties and good mechanical properties has become crucial for the rapid development of the rare earth permanent magnet industry and a focus of attention in the global rare earth materials field.

[0003] Existing technologies disclose several methods for preparing heavy rare earth-free sintered NdFeB magnets. For example, patent application number 202210830754.4 discloses a method of adding titanium hydride micron powder before air jet milling, i.e., grain boundary addition method, to prepare high coercivity heavy rare earth-free magnets; patent application number CN201610039324.5 discloses a method of grain refinement to prepare heavy rare earth-free magnets. None of the above methods have studied or reported on the mechanical properties of the magnets. Summary of the Invention

[0004] In view of the above-mentioned problems, the present invention provides a heavy rare earth-free NdFeB magnet and its preparation method. The NdFeB magnet of the present invention has the advantages of being free of heavy rare earth elements, having high magnetic properties, excellent mechanical properties, simple processing, and suitability for mass production.

[0005] To address the aforementioned problems, this invention provides a heavy rare earth-free NdFeB magnet, comprising a primary NdFeB coarse powder and a secondary praseodymium-titanium-aluminum coarse powder. The secondary praseodymium-titanium-aluminum coarse powder constitutes 0.5-3.5 wt% of the primary NdFeB coarse powder. The primary NdFeB coarse powder, by mass percentage, comprises: PrNd 29.5-32.5 wt%; B 0.88-1 wt%; Al 0-0.3 wt%; Cu 0.1-0.4 wt%; Ti 0-0.2 wt%; Zr 0.1-0.4 wt%; Ga 0.2-0.6 wt%; Co 0.5-1.8 wt%; with the balance being Fe. The secondary praseodymium-titanium-aluminum coarse powder comprises: Pr 75-85 wt%; Ti 5-10 wt%; Al 10-15 wt%.

[0006] Based on the same inventive concept, the present invention also provides a method for preparing the above-mentioned heavy rare earth-free NdFeB magnet, comprising the following steps:

[0007] S1. Prepare NdFeB main phase alloy by batching according to the composition of NdFeB coarse powder; prepare rare earth alloy steel by batching according to the composition of praseodymium-titanium-aluminum coarse powder.

[0008] S2. The NdFeB main phase alloy is subjected to hydrogen absorption, incomplete dehydrogenation, and cooling to obtain NdFeB main phase coarse powder; the rare earth alloy steel is subjected to hydrogen absorption, complete dehydrogenation, and cooling to obtain praseodymium-titanium-aluminum auxiliary phase coarse powder.

[0009] S3. The main phase NdFeB coarse powder and the auxiliary phase praseodymium titanium aluminum coarse powder are mixed and air-jet milled to obtain fine powder, which is then successively formed, sintered and tempered to obtain NdFeB magnets without heavy rare earth elements.

[0010] Preferably, in step S1, the method for preparing the NdFeB main phase alloy is as follows: the raw material is added to a vacuum rapid solidification furnace and melted at 1400~1500℃, and then cast and cooled to obtain a rapid solidification NdFeB main phase alloy sheet.

[0011] Preferably, in step S1, the rare earth alloy steel is prepared by adding the raw materials into a vacuum melting furnace and melting them at 900~1200℃, followed by casting and cooling to obtain rare earth alloy steel.

[0012] Preferably, in step S2, the hydrogen absorption temperature of the neodymium iron boron main phase alloy is 560~600℃, and the hydrogen absorption temperature of the rare earth alloy steel is 560~600℃.

[0013] Preferably, in step S3, the particle size D50 of the fine powder is 3.7~4.5μm.

[0014] Preferably, in step S3, the molding includes orientation pressing and cold isostatic pressing; the magnetic field strength of the orientation pressing is 1.6~2.0T, and the molding time is 30~60s; the pressure of the cold isostatic pressing is 150~240MPa, and the holding time is ≥25s.

[0015] Preferably, in step S3, the sintering is segmented continuous sintering, including a first-stage sintering and a second-stage sintering; the temperature of the first-stage sintering is 1020~1050℃, and the time is 2~5h; the temperature of the second-stage sintering is 1040~1080℃, and the time is 3~6h.

[0016] Preferably, in step S3, the tempering heat treatment includes a first-stage tempering heat treatment and a second-stage tempering heat treatment; the temperature of the first-stage tempering heat treatment is 880~920℃, and the time is 2~4h; the temperature of the second-stage tempering heat treatment is 460~600℃, and the time is 3~7h.

[0017] Preferably, the thickness of the rapidly solidified NdFeB main phase alloy sheet is 0.25~0.35mm.

[0018] Preparation mechanism of the present invention:

[0019] This invention is based on a main phase alloy with low B content, high low melting point Ga content, and high melting point Zr content, and introduces an auxiliary phase alloy with low melting point Al and high melting point Ti. By employing incomplete dehydrogenation technology, grain refinement technology, and segmented sintering technology, a sintered NdFeB permanent magnet with high magnetic properties and excellent mechanical properties is obtained.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] (1) This invention reduces the content of B in the main phase, causing some rare earth elements and Fe to dissolve into a rare earth-rich phase, thereby increasing the proportion of the rare earth-rich phase. At the same time, Al, Cu, and Ga in the main phase, together with rare earth elements and Fe, form a non-ferromagnetic Re6Fe magnet. 13 The M phase, a nonferromagnetic phase, acts as a pinning center, pinning the rotation of the antimagnetizing domains and thus improving the coercivity of the magnet. As the B content in the main phase decreases, Ga and Cu elements begin to move from the triangular grain boundaries into the grain boundary phase. They can improve the wettability of the rare earth-rich phase, which is conducive to the uniform diffusion of PrNd into the grain boundary phase, forming a continuous grain boundary phase, weakening the exchange coupling between the main phase grains, and thus improving the coercivity.

[0022] (2) After Al is added to the auxiliary phase of the present invention, it preferentially occupies the grain boundary phase, which is beneficial to refining Nd2Fe. 14 The B main phase grains reduce the resistance to the movement of magnetic domain walls, thereby improving coercivity; the Al in the auxiliary phase improves the grain size, and the fine and uniform grains can improve the strength and toughness of the material, while avoiding the cracking problem caused by stress concentration, which significantly improves the mechanical properties of the magnet; the addition of Ti in the auxiliary phase forms a Ti-B compound precipitate phase in the grain boundary, which can further refine the grains, enhance the strength and toughness of the magnet, and further improve the mechanical properties of the magnet.

[0023] (3) The main phase of the present invention adopts incomplete dehydrogenation technology, which can improve the brittleness and oxidation resistance of the main phase coarse powder, and prepare for grain refinement;

[0024] (4) The present invention refines the grains by air jet milling, which can reduce the grain size of the main phase, reduce the stray magnetic field around the grains, increase the nucleation field of the antimagnetization domains, and help improve the coercivity;

[0025] (5) The present invention adopts segmented sintering technology. In the first stage of sintering (low temperature stage), low melting point Al, low melting point high proportion Cu and Ga are uniformly dispersed into the grain boundary phase. In the second stage of sintering (high temperature stage), the aggregation of high melting point Zr and Ti elements is reduced, promoting the uniform distribution of the grain boundary phase and further improving the coercivity. Detailed Implementation

[0026] To make the present invention easier to understand, specific embodiments are described below to further illustrate the invention. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Unless otherwise defined, the technical terms used below have the same meaning as understood by those skilled in the art; unless otherwise specified, the raw materials and reagents involved herein can be purchased commercially or obtained by known methods.

[0027] To address the problems mentioned in the background art, this invention provides a heavy rare earth-free NdFeB magnet and its preparation method. The NdFeB magnet of this invention has advantages such as being free of heavy rare earth, having high magnetic properties, excellent mechanical properties, simple processing, and suitability for mass production.

[0028] The following examples and comparative models further illustrate this point.

[0029] Example 1

[0030] A method for preparing a neodymium iron boron magnet without heavy rare earth elements includes the following steps:

[0031] (1) Preparation of NdFeB main phase alloy: The main phase alloy is composed of the following components by mass percentage: 29.8% PrNd, 0.94% B, 0.15% Al, 0.25% Cu, 0.10% Ti, 0.12% Zr, 0.35% Ga, 0.80% Co, and balance Fe. After the components are prepared according to this composition, they are added to a vacuum melting furnace, melted at 1450℃, cast, and cooled to obtain rapidly solidified sheets with an average sheet thickness of 0.28 mm.

[0032] (2) Preparation of rare earth auxiliary phase alloy: The auxiliary phase alloy is composed of the following components by mass percentage: 80% Pr, 5% Ti, 15% Al; after the ingredients are added to a vacuum melting furnace, it is melted at 1130℃ and cast to obtain rare earth alloy steel.

[0033] (3) Powdering: The main phase alloy is subjected to hydrogen absorption, incomplete dehydrogenation and cooling to obtain coarse powder; the auxiliary phase alloy is subjected to hydrogen absorption, complete dehydrogenation and cooling to obtain coarse powder; the auxiliary phase alloy coarse powder is added according to the mass percentage of the main phase alloy, namely 0.5wt%, 1.2wt%, 2wt% and 3wt% respectively; wherein, the dehydrogenation temperature is 580℃ and the incomplete dehydrogenation time is 2 / 3 of the complete dehydrogenation time; the mixed coarse powder is subjected to air jet milling to obtain fine powder with a particle size D50 of 4.25μm;

[0034] (4) Molding: The fine powder is placed in a magnetic field press and oriented under the protection of inert gas. The magnetic field strength is 1.8T (forming time is 45s); the oriented green body is subjected to cold isostatic pressing. The cold isostatic pressing pressure is 180MPa (holding time is 30s).

[0035] (5) High-temperature sintering: The cold isostatic pressed billet is placed in a vacuum sintering furnace for sintering. The process of segmented sintering is adopted. The temperature of the first high-temperature stage is 1050℃ and the time is 3h; the temperature of the second high-temperature stage is 1065℃ and the time is 3h; after the end, argon air cooling is performed.

[0036] (6) Tempering heat treatment: The blank after high temperature sintering is subjected to first-stage tempering treatment at a temperature of 900℃*3h, and second-stage tempering is carried out at 460℃ for 5h, 500℃ for 5h, 540℃ for 5h, and 580℃ for 5h respectively; and cooled to room temperature before being taken out of the furnace.

[0037] The magnetic and mechanical properties of neodymium iron boron magnets with auxiliary phase alloy coarse powder added at amounts of 0.5 wt%, 1.2 wt%, 2 wt%, and 3 wt% in Example 1 were tested, and the results are shown in Tables 1-4.

[0038] Table 1. Magnetic and mechanical properties of NdFeB magnets with an auxiliary phase alloy coarse powder addition of 0.5 wt% in Example 1.

[0039]

[0040] Table 2. Magnetic and mechanical properties of NdFeB magnets with an auxiliary phase alloy coarse powder addition of 1.2 wt% in Example 1.

[0041]

[0042] Table 3. Magnetic and mechanical properties of NdFeB magnets with an auxiliary phase alloy coarse powder addition of 2 wt% in Example 1.

[0043]

[0044] Table 4. Magnetic and mechanical properties of NdFeB magnets with an auxiliary phase alloy coarse powder addition of 3 wt% in Example 1.

[0045]

[0046] As shown in Table 1-4, when the secondary tempering temperature is 540℃ and the time is 5h, the intrinsic coercivity Hcj of the magnet is the highest, the hardness is the greatest, and the bending strength is the best. The coercivity and bending strength of the magnet increase with the increase of the auxiliary phase alloy, and the increase is more obvious at 2%, and the increase of magnetic properties and bending strength is smaller thereafter. The hardness of the magnet decreases with the increase of the auxiliary phase alloy, that is, the greater the total rare earth content of the magnet, the greater the wear of the magnet, indicating that the magnet is easier to process.

[0047] Example 2

[0048] A method for preparing a neodymium iron boron magnet without heavy rare earth elements includes the following steps:

[0049] (1) Preparation of NdFeB main phase alloy: The main phase alloy is composed of the following components by mass percentage: 30.8% PrNd, 0.10% Al, 0.25% Cu, 0.10% Ti, 0.12% Zr, 0.30% Ga, 0.80% Co, and B content of (0.88%, 0.90%, 0.92%, 0.94%, 0.96%) respectively, with the balance being Fe. After each component is prepared according to this composition, it is added to a vacuum melting and spinning furnace, melted at 1455℃, cast, and cooled to obtain rapidly solidified thin sheets with an average sheet thickness of 0.27 mm.

[0050] (2) Preparation of auxiliary phase alloy: Same as in Example 1.

[0051] (3) Powdering: The main phase alloy is subjected to hydrogen absorption, incomplete dehydrogenation and cooling to obtain coarse powder; the auxiliary phase alloy is subjected to hydrogen absorption, complete dehydrogenation and cooling to obtain coarse powder; the auxiliary phase alloy coarse powder is added at 2wt% of the mass of the main phase alloy, wherein the dehydrogenation temperature is 580℃ and the incomplete dehydrogenation time is 2 / 3 of the complete dehydrogenation time; the mixed coarse powder is subjected to air jet milling to obtain fine powder with a particle size D50 of 4.25μm;

[0052] (4) Molding: Same as in Example 1.

[0053] (5) High-temperature sintering: The cold isostatic pressed billet is placed in a vacuum sintering furnace for sintering. A segmented continuous sintering process is adopted. The temperature of the first high-temperature stage is 1058℃ and the time is 3h; the temperature of the second high-temperature stage is 1075℃ and the time is 5h; after the end, argon air cooling is performed.

[0054] (6) Tempering heat treatment: The blank after high temperature sintering is subjected to first-stage tempering treatment at a temperature of 900℃ for 3 hours; the second-stage tempering treatment is carried out at a temperature of 540℃ for 5 hours; and then cooled to room temperature before being removed from the furnace.

[0055] The magnetic and mechanical properties of the neodymium iron boron magnets with B contents of 0.88wt%, 0.90wt%, 0.92wt%, 0.94wt%, and 0.96wt% in Example 2 were tested, and the results are shown in Table 5.

[0056] Table 5. Magnetic and mechanical properties of NdFeB magnets with different boron contents in Example 2

[0057]

[0058] As shown in Table 5, the remanence of the magnet decreases with decreasing boron content, while the coercivity increases, reaching its maximum at 0.90%. Further reduction of boron content leads to a decrease in coercivity. The mechanical properties of the magnet gradually deteriorate with decreasing boron content. Overall, the magnetic and mechanical properties of the magnet can be maintained at a relatively good level when the boron content is between 0.90% and 0.92%.

[0059] Example 3

[0060] A method for preparing a neodymium iron boron magnet without heavy rare earth elements includes the following steps:

[0061] (1) Preparation of NdFeB main phase alloy: The main phase alloy is composed of the following components by mass percentage: 30.8% PrNd, 0.92% B, 0.10% Al, 0.25% Cu, 0.10% Ti, 0.12% Zr, 0.30% Ga, 0.80% Co, and balance Fe. After the components are prepared according to this composition, they are added to a vacuum melting furnace, melted at 1455℃, cast, and cooled to obtain rapidly solidified sheets with an average sheet thickness of 0.29 mm.

[0062] (2) Preparation of auxiliary phase alloy: Same as in Example 1;

[0063] (3) Powdering: The main phase alloy is subjected to hydrogen absorption, incomplete dehydrogenation and cooling to obtain coarse powder; the auxiliary phase alloy is subjected to hydrogen absorption, complete dehydrogenation and cooling to obtain coarse powder; the auxiliary phase alloy coarse powder is added at 2wt% of the mass of the main phase alloy; wherein, the dehydrogenation temperature is 580℃ and the incomplete dehydrogenation time is 2 / 3 of the complete dehydrogenation time; the mixed coarse powder is subjected to air jet milling to obtain fine powder with particle size D50 of 4.25μm, 4.10μm, 3.95μm, 3.80μm and 3.70μm respectively;

[0064] (4) Molding: Same as in Example 1;

[0065] (5) High-temperature sintering: The cold isostatic pressed billet is placed in a vacuum sintering furnace for sintering. The process of segmented continuous sintering is adopted. The temperature of the first high-temperature stage is 1045℃ and the time is 3h; the temperature of the second high-temperature stage is 1060℃ and the time is 5h; after the end, argon air cooling is performed.

[0066] (6) Tempering heat treatment: The blank after high temperature sintering is subjected to first-level tempering treatment at a temperature of 890℃ for 3 hours; second-level tempering is carried out at 460℃ for 5 hours, 490℃ for 5 hours, 520℃ for 5 hours and 550℃ for 5 hours respectively; and then cooled to room temperature before being taken out of the furnace.

[0067] The magnetic and mechanical properties of NdFeB magnets with fine powder particle sizes D50 of 4.25 μm, 4.10 μm, 3.95 μm, 3.80 μm, and 3.70 μm obtained by air jet milling in Example 3 above were tested, and the results are shown in Table 6-10:

[0068] Table 6. Magnetic and mechanical properties of the NdFeB magnet with a fine powder particle size D50 of 4.25 μm in Example 3.

[0069]

[0070] Table 7. Magnetic and mechanical properties of the NdFeB magnet with a fine powder particle size D50 of 4.10 μm in Example 3.

[0071]

[0072] Table 8. Magnetic and mechanical properties of the NdFeB magnet with a fine powder particle size D50 of 3.95 μm in Example 3.

[0073]

[0074] Table 9. Magnetic and mechanical properties of the NdFeB magnet with a fine powder particle size D50 of 3.80 μm in Example 3.

[0075]

[0076] Table 10. Magnetic and mechanical properties of NdFeB magnets with a fine powder particle size D50 of 3.70 μm in Example 3.

[0077]

[0078] As shown in Table 6-10, when the secondary tempering temperature is between 490-520℃, the coercivity, hardness, and flexural strength of the magnet are all at a relatively high level. The finer the powder particle size, the better the magnetic properties and flexural strength of the magnet, reaching a peak when D50 reaches 3.80μm.

[0079] Further reducing the powder particle size weakens the magnet's oxidation resistance, and the magnet's magnetic and mechanical properties begin to deteriorate; the magnet's hardness increases with decreasing powder particle size, indicating that the magnet is more wear-resistant.

[0080] In summary, the neodymium iron boron magnets produced by this invention can achieve high magnetic properties and good mechanical properties without the addition of heavy rare earth elements. The process equipment is simple, the operation is convenient, and it is suitable for mass industrial production.

[0081] The above embodiments merely illustrate several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the inventive concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A neodymium iron boron magnet free of heavy rare earth elements, characterized in that, It comprises primary NdFeB coarse powder and secondary praseodymium-titanium-aluminum coarse powder, wherein the secondary praseodymium-titanium-aluminum coarse powder accounts for 0.5-3.5 wt% of the primary NdFeB coarse powder; and the primary NdFeB coarse powder comprises, by mass percentage, 29.5-32.5 wt% PrNd. B 0.88~1wt%; Al 0~0.3wt%; Cu 0.1~0.4wt%; Ti 0~0.2wt%; Zr 0.1~0.4wt%; Ga 0.2~0.6wt%; Co 0.5~1.8wt%; balance Fe; the composition of the auxiliary phase praseodymium-titanium-aluminum coarse powder is: Pr 75~85wt%; Ti 5~10wt%; Al 10~15wt%.

2. The method for preparing a heavy rare earth-free NdFeB magnet according to claim 1, characterized in that, Includes the following steps: S1. Prepare NdFeB main phase alloy by batching according to the composition of NdFeB coarse powder; prepare rare earth alloy steel by batching according to the composition of praseodymium-titanium-aluminum coarse powder. S2. The NdFeB main phase alloy is subjected to hydrogen absorption, incomplete dehydrogenation, and cooling to obtain NdFeB main phase coarse powder; the rare earth alloy steel is subjected to hydrogen absorption, complete dehydrogenation, and cooling to obtain praseodymium-titanium-aluminum auxiliary phase coarse powder. S3. The main phase NdFeB coarse powder and the auxiliary phase praseodymium titanium aluminum coarse powder are mixed and air-jet milled to obtain fine powder, which is then successively formed, sintered and tempered to obtain NdFeB magnets without heavy rare earth elements.

3. The method for preparing a heavy rare earth-free NdFeB magnet according to claim 2, characterized in that, In step S1, the preparation method of the NdFeB main phase alloy is as follows: the raw materials are added to a vacuum rapid solidification belt spinning furnace and melted at 1400~1500℃, and then cast and cooled to obtain rapid solidification NdFeB main phase alloy sheets.

4. The method for preparing a heavy rare earth-free NdFeB magnet according to claim 2, characterized in that, In step S1, the rare earth alloy steel is prepared by adding the raw materials into a vacuum melting furnace and melting them at 900~1200℃, then casting and cooling to obtain rare earth alloy steel.

5. The method for preparing a heavy rare earth-free NdFeB magnet according to claim 2, characterized in that, In step S2, the hydrogen absorption temperature of the neodymium iron boron main phase alloy is 560~600℃, and the hydrogen absorption temperature of the rare earth alloy steel is 560~600℃.

6. The method for preparing a heavy rare earth-free NdFeB magnet according to claim 2, characterized in that, In step S3, the particle size D50 of the fine powder is 3.7~4.5μm.

7. The method for preparing a heavy rare earth-free NdFeB magnet according to claim 2, characterized in that, In step S3, the molding process includes orientation pressing and cold isostatic pressing; the magnetic field strength of the orientation pressing is 1.6~2.0T, and the molding time is 30~60s; the pressure of the cold isostatic pressing is 150~240MPa, and the holding time is ≥25s.

8. The method for preparing a heavy rare earth-free NdFeB magnet according to claim 2, characterized in that, In step S3, the sintering is segmented continuous sintering, including a first-stage sintering and a second-stage sintering; the temperature of the first-stage sintering is 1020~1050℃ and the time is 2~5h; the temperature of the second-stage sintering is 1040~1080℃ and the time is 3~6h.

9. The method for preparing a heavy rare earth-free NdFeB magnet according to claim 2, characterized in that, In step S3, the tempering heat treatment includes a first-stage tempering heat treatment and a second-stage tempering heat treatment; the temperature of the first-stage tempering heat treatment is 880~920℃ and the time is 2~4h; the temperature of the second-stage tempering heat treatment is 460~600℃ and the time is 3~7h.

10. The method for preparing a heavy rare earth-free NdFeB magnet according to claim 3, characterized in that, The thickness of the rapidly solidified NdFeB main phase alloy sheet is 0.25~0.35mm.

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