WO2016086397A1

WO2016086397A1 - Method for improving coercive force of sintered neodymium iron boron magnet by adding dysprosium hydrogen compound and product

Info

Publication number: WO2016086397A1
Application number: PCT/CN2014/093068
Authority: WO
Inventors: 王新华; 严密; 刘攀; 裴晓东; 周军; 孙红军; 余进
Original assignee: 浙江大学; 中钢集团安徽天源科技股份有限公司
Priority date: 2014-12-04
Filing date: 2014-12-04
Publication date: 2016-06-09

Abstract

A method for improving a coercive force of a sintered neodymium iron boron magnet by adding a dysprosium hydrogen compound, the method specifically comprises the following steps: 1) preparing a main phase alloy as a uniform fine powder by a hydrogen explosion airflow milling method, and milling a grain boundary phase alloy into a fine powder by a ball milling method; 2) sufficiently and uniformly mixing the prepared main phase alloy powder and grain boundary phase fine powder; 3) orientating the mixed main phase and grain boundary phase powders in a magnetic field, and performing press forming; and 4) sintering the pressed block in a high-vacuum sintering furnace and tempering twice to prepare a final magnet. Also disclosed is a sintered neodymium iron boron magnet prepared by the method. The sintered neodymium iron boron magnet prepared by the method is high in coercive force, simple and convenient to prepare, easy to operate, is not prone to oxidization in the preparation process, and can be applied to large-scale batch production.

Description

Method and product for improving coercivity of sintered NdFeB magnet by adding hydrazine compound

Technical field

The invention belongs to the technical field of magnetic materials, and particularly relates to a method and a product for improving the coercive force of a sintered NdFeB magnet by adding a hydrogen compound.

Background technique

Since the 1880s, Sagawa et al. have prepared NdFeB magnets by powder metallurgy, which is called the third generation rare earth permanent magnet material. Because it is the most magnetic permanent magnet in the world, with high magnetic energy product, high cost performance and other outstanding advantages, this material has been applied in such as aerospace, automotive industry, electronic appliances, medical equipment, military equipment, computers, instrumentation, etc. The field has become an indispensable part of production and life. NdFeB materials have played an important role in the modern technology industry.

The sintered NdFeB magnet is a Nd ₂ Fe ₁₄ B compound as a main phase, and is surrounded by a structure rich in a rare earth-rich phase. The main technical indicators include remanence B _r , maximum magnetic energy product (BH) _max , coercivity H _cj , Curie temperature T _c . After more than 20 years of research and development, a reasonable alloy composition and mature preparation process have been designed, so that the remanence B _r of the magnet reaches more than 96% of the theoretical value, and the magnetic energy product can reach 474kJ/m ³ , which is close to the theoretical magnetic energy. The product has 93% of 512kJ/m ³ . However, there has been no significant progress in the improvement of coercivity, which can only reach about 20% of the theoretical value, which makes the temperature stability of the magnet not high, which limits its application in the high temperature environment. Therefore, increasing the coercive force of the magnet and increasing its temperature stability has become a crucial research link.

In order to obtain high coercivity NdFeB magnets, many studies have been done by the predecessors. The first method is to add other elements such as Dy, Tb, Cu, Al, Nb, Ga, Co, Si, etc. by smelting, but since these elements are uniformly distributed in the magnet during the smelting process, the coercive force of the magnet is improved. The magnitude is not large, but the other properties of the magnet are deteriorated in the case of a large amount of introduction of other elements. The second method is to add alloying element powder by grain boundary reconstruction. This method can make the alloy powder distribute evenly around the main phase, which can improve the coercive force of the magnet to some extent, but the alloy powder is complicated to prepare and powder. Difficult, and the powder particle size is too large to be coated very uniformly around the main phase. Recently, a third method, the grain boundary diffusion method, has been invented. It is reported that a magnet having a coercive force of 319.2 kJ/m ³ of 2780.5 kA/m can be prepared by this method, but this method can only be applied to magnetic On the sheet, the operation is complicated, which greatly limits the application of its industrial production.

According to research, the coercivity is closely related to the microstructure of the magnet. It is generally believed that a 2-4 nm rare earth-rich phase is coated around each main phase of Nd ₂ Fe ₁₄ B grains, and the main bodies are isolated. The contact between the crystal grains acts as a demagnetization coupling, so that a higher coercive force can be obtained. Studies have also shown that Dy ₂ Fe ₁₄ B has higher magnetocrystalline anisotropy than Nd ₂ Fe ₁₄ B, but the remanence is lower, if the main phase grain boundary layer is coated with a thin layer of Dy ₂ Fe ₁₄ B phase and the intact Nd ₂ Fe ₁₄ B phase inside the main phase grain can get the best comprehensive magnetic properties. Therefore, based on previous studies, the method of increasing the coercive force of the magnet by the addition of the grain boundary of the rare earth metal hydride powder has been widely concerned.

For example, the patent document published as CN101521069A discloses a preparation method of a heavy rare earth hydride nanoparticle doped sintered NdFeB permanent magnet, which adopts a rapid condensation sheet process and a hydrogen explosion method to prepare NDFEB powder; and a physical vapor deposition technique to prepare a hydrogenated ruthenium Or hydrogenated cerium nano powder; mixing the two powders, magnetic field orientation and press forming; the compact is subjected to dehydrogenation treatment, sintering and heat treatment at different temperatures to obtain a sintered magnet. The ruthenium hydride and the ruthenium hydride used in the preparation method have a particle diameter of about 10 to 50 nm, a small particle size, are easily oxidized, and have a high cost. In order to obtain an ideal coercive force, it is necessary to add more hydrazine hydride. Or hydrogenated hydrazine.

Summary of the invention

The invention provides a method for increasing the coercive force of a sintered NdFeB magnet by adding an hydrazine compound. The method directly adopts a micron-sized hydrazine compound for grain boundary addition, further avoiding oxidation of the powder and improving the sintered NdFeB Magnet coercivity.

A method for increasing the coercivity of a sintered NdFeB magnet by adding a hydrogen compound, comprising:

(1) The main phase NdFeB alloy is made into a main phase ingot or a quick-setting strip by a casting process or a rapid-coagulation strip process, and a small fragment of a grain boundary phase hydrogen compound is prepared by a hydrogen explosion method in the grain boundary phase;

(2) The main phase ingot or quick-setting strip is crushed into a main phase particle powder with an average particle diameter of 2-10 μm by hydrogen explosion and jet milling, and the grain boundary phase hydrogen compound is broken into average by ball milling. Grain boundary phase particle powder having a particle diameter of 0.5-1.0 μm;

(3) The prepared grain boundary phase granule powder is added to the main phase powder in a mass percentage of 0.2% to 2%, and is thoroughly mixed in a glove box, and the gas in the glove box is nitrogen;

(4) subjecting the mixed alloy powder to orientation molding under a magnetic field of 1.2-2.0 T;

(5) The magnetic block completed by pressing is subjected to cold isostatic pressing of 150-220 MPa in liquid oil to make it pressed into a green body;

(6) The magnetic block completed by pressing is sintered under vacuum of 1050-1100 ° C for 2-4 h, and then tempered by tempering 2-4 h at 850-950 ° C and tempering at 2-4 h at 500-650 ° C for 2-4 h. The final magnet.

In the step (1), the main phase NdFeB alloy is in atomic percentage, and its composition is (Nd _a Pr _1-a ) _b Fe _100-bcd B _c M _d , wherein Nd is a lanthanum element and Pr is a lanthanum element. Fe is iron, B is boron, M is Dy (镝), Tb (铽), Ce (铈), Co (cobalt), Ni (nickel), V (vanadium), Ti (titanium), Mo ( One or more of molybdenum), Mn (manganese), Ga (gallium), Al (aluminum), Cu (copper), Zr (zirconium), Ta (germanium), Ag (silver), and Si (silicon) ; a, b, c, d satisfy the following relationship: 0.7≦a≦1, 11≦b≦16, 5.4≦c≦6.5, 0≦d≦6.

Further preferably, the a, b, c, and d satisfy the following relationship: 0.75 ≦ a ≦ 0.85, 13 ≦ b ≦ 14, 5.5 ≦ c ≦ 6.0, 1 ≦ d ≦ 2. Further preferably, a=0.80, b=13.35; c=5.96; d=1.67.

The M is a mixture of Al, Co, Cu, Zr, Ga, and Tb; or the M is a mixture of Al, Co, Cu, Zr, Ga, and Dy.

In the step (1), the grain boundary phase 镝 hydrogen compound is in atomic percentage, and its composition is DyH _x , wherein Dy is a lanthanum element and H is a hydrogen element; x satisfies the following relationship: 2 ≦ x ≦ 3; x further It is preferably 2 or 3, and still more preferably 3. The hydrazine hydride compound may be commercially available or may be prepared by itself. Preferably, the hydrazine hydride compound may be prepared by the following method: (1) The grain boundary phase metal ruthenium is fully subjected to a pressure of 16-17 bar and room temperature. Hydrogen absorption, and according to the selection step (2), vacuum dehydrogenation at 400-500 ° C for 2-4 h.

In the step (2), preferably, the main phase particle powder has an average particle diameter of 3-5 μm. The grain boundary phase particle powder has an average particle diameter of 0.8-1.0 μm; the particle size is much larger than the prior art 10-50 nm particle size, which reduces the grinding difficulty and the grinding cost, and avoids the long-term grinding process of the nanoparticles. The oxidation increases the coercive coercivity of the final sintered NdFeB magnet.

In the step (3), preferably, the percentage of the grain boundary phase particle powder is from 0.2 to 0.5%. When adopting the technical scheme, the amount of grain boundary phase particle powder is saved, and at the same time, the final sintered NdFeB magnet has higher coercive force.

In the step (6), in order to ensure that the final sintered NdFeB magnet has a high coercive force, the sintering and tempering conditions are preferably: 0.85-1090 ° C under vacuum for 3.5-4 h, and then after 900-910 ° C first return Fire 2-3h and 510-520 °C secondary tempering 3.5-4h. Further preferably, it is sintered at 1085-1090 ° C for 3.5 h under vacuum, and then subjected to tempering at 905 ° C for 2 h and 515 ° C for tempering for 3.5 h. When the sintering and tempering conditions are employed, the dehydrogenation of the hydride can more effectively inhibit the oxidation of the magnet and reduce the oxygen content of the magnet.

The present invention also provides a sintered neodymium iron boron magnet prepared by the above method.

The invention has the following beneficial effects: the process of preparing the grain boundary to add the ruthenium hydride is simple, the powder is made of micron grade, can be uniformly wrapped around the main phase, and the dehydrogenation of the hydride during the sintering process can effectively inhibit the oxidation of the magnet. The oxygen content of the magnet is reduced, and the fine powder can be effectively diffused to the outer layer of the main phase grain boundary to form an ideal magnetic hardening layer, thereby obtaining a magnet with high coercivity and high magnetic properties. The whole preparation process is simple and low in cost, and is very suitable for mass production in industry.

DRAWINGS

1 is an XRD pattern of DyH ₃ prepared in Example 1;

2 is a microstructure diagram of a sintered body of a DyH ₃ powder prepared in Example 1 after sintering;

3 is an XRD pattern of DyH ₂ prepared in Example 3;

4 is a microstructural view of a magnet to which DyH ₂ powder added in Example 3 was added after sintering.

detailed description

The main phase NdFeB alloy is in atomic percentage and its composition is (Nd _a Pr _1-a ) _b Fe _100-bcd B _c M _d , where Nd is yttrium element, Pr is yttrium element, Fe is iron element, B is Boron element, M is one or more of Dy, Tb, Ce, Co, Ni, V, Ti, Mo, Mn, Ga, Al, Cu, Zr, Ta, Ag, Si elements; a, b, c , d satisfies the following relationship: 0.9≦a≦1,11≦b≦16, 5.4≦c≦6.5,0≦d≦6. The bismuth hydrogen compound added at the grain boundary is in atomic percent and its composition is DyH _x , wherein Dy is a lanthanum element and H is a hydrogen element; x satisfies the following relationship: 2 ≦ x ≦ 3.

Method for adding coercive force to increase the coercive force of sintered NdFeB magnets The sudden is:

1) The main phase NdFeB alloy is made into an ingot or a quick-setting strip by a casting process or a rapid-coagulation process, and a small fragment of a hydrogen-based compound is prepared by a hydrogen explosion method in the grain boundary phase;

2) The main phase ingot or quick-setting strip is crushed into a main phase particle powder with an average particle diameter of 2-10 μm by hydrogen explosion and jet milling, and the grain boundary phase hydride is broken into the average particle diameter by ball milling. a grain boundary particle powder of 0.5-1.0 μm;

3) The prepared ruthenium hydride powder is added to the main phase powder in a mass fraction of 0.2% to 2%, and is thoroughly mixed in a glove box, and the gas in the glove box is nitrogen;

4) performing the orientation molding of the mixed alloy powder under a magnetic field of 1.2-2.0T;

5) The magnetic block completed by pressing is subjected to cold isostatic pressing of 150-220 MPa in liquid oil to make it into a green compact;

6) The magnetic block completed by pressing is sintered under vacuum of 1050-1100 ° C for 2-4 h, and then subjected to tempering 2-4 h at 850-950 ° C and tempering at 2-4 h at 500-650 ° C for 2-4 h. magnet.

The technical solution of the present invention is further described below in conjunction with the embodiments; the main phase alloy powder used in the embodiment may be an existing product (for example, the main phase powder of Example 1 and Example 2) purchased from Anhui Maanshan Tianyuan Tianyuan Co., Ltd. It can be made by the product of No. N48 of Anhui Maanshan Zhonggang Tianyuan Co., Ltd.), and can also be prepared by the existing method, and the metal raw materials used in the preparation can be commercially available.

Example 1:

1) The main alloy powder is prepared by using three alloy processes of rapid solidification casting, hydrogen explosion and jet milling, and the powder particles have a diameter of about 3.5 μm. The main alloy is atomic percentage, and its composition is:

Nd _10.63 Pr _2.72 Fe _79.02 Al _0.71 Co _0.50 Cu _0.14 Zr _0.14 Ga _0.09 Tb _0.09 B _5.96 ;

2) The grain boundary phase metal ruthenium is sufficiently hydrogen-absorbed at a pressure of 16-17 bar and room temperature, and is ground by mechanical ball milling into a powder having a particle diameter of approximately 0.8 μm, the alloy being in atomic percentage and having a composition of DyH ₃ , its structure is detected by XRD as shown in Figure 1, and its composition is determined as DyH ₃ , which is hcp structure;

3) uniformly mixing the main alloy powder and the DyH ₃ powder in a glove box under the protection of high purity nitrogen to obtain a mixed powder, wherein the weight of the DyH ₃ grain boundary phase alloy powder is 0.2% by weight of the total powder;

4) The mixed powder is oriented and formed under a magnetic field of 2T, and is formed into a green body under cold isostatic pressing of 20 MPa;

5) The green body was sintered at 1090 °C for 3.5 h in a high vacuum positive pressure sintering furnace, tempered at 905 ° C for 2 h, and tempered at 515 ° C for 3.5 h to obtain a neodymium iron boron magnet.

The prepared NdFeB magnet was placed in a VSM to measure its magnetic properties. The results were as follows: B _r = 1.39 T, H _cj = 12262.40 kA/m, and (BH) _max = 378.24 kJ/m ³ . The microstructure of the magnet is shown in Figure 2. The grain boundary of the magnet is continuously clear, which makes the magnet performance better.

Comparative example 1

The main phase alloy is used: Nd _29.5 Fe _68.2 Co _1.2 B _1.1 (mass percentage);

The remaining conditions are the same as in the first embodiment.

The coercive force of the finally prepared NdFeB magnet is H _cj =1106.23 kA/m. It can be seen that when the raw materials are different, the coercive force of the obtained NdFeB magnet is much lower than the same addition amount. The coercive force of the neodymium iron boron magnet obtained in the first embodiment.

Comparative example 1’

The raw materials used were the same as in Comparative Example 1, but the amount of DyH ₃ powder added was 3%;

The remaining conditions are the same as in Embodiment 1;

The coercive force of the finally prepared NdFeB magnet was H _cj = 1250.12.23 kA/m; the coercive force value was comparable to the coercive force of the product prepared in Example 1. It can be seen that the addition amount of the DyH ₃ powder can be greatly reduced by the method of the present invention; meanwhile, the particle size of the DyH ₃ powder in the first embodiment is large, which reduces the grinding difficulty and the grinding cost.

Example 2:

1) The main alloy powder is prepared by using three alloy processes of rapid solidification casting, hydrogen explosion and jet milling, the diameter of the powder particles is about 3.5 μm, and the main alloy is atomic percentage, and its composition is Nd _10.63. Pr _2.72 Fe _79.02 Al _0.71 Co _0.50 Cu _0.14 Zr _0.14 Ga _0.09 Tb _0.09 B _5.96 ;

2) The grain boundary phase metal ruthenium is sufficiently hydrogen-absorbed at a pressure of 16-17 bar and room temperature, and is ground by mechanical ball milling into a powder having a particle diameter of approximately 0.8 μm, which is in atomic percentage and has a composition of DyH. ₃ , the structure is the same as Figure 1;

3) uniformly mixing the main alloy powder and the DyH ₃ powder in a glove box under the protection of high purity nitrogen to obtain a mixed powder, wherein the weight of the DyH ₃ grain boundary phase alloy powder is 2.0% by weight of the total powder;

5) The green body was sintered at 1085 ° C for 4 h in a high vacuum positive pressure sintering furnace, tempered at 905 ° C for 2 h, and tempered at 515 ° C for 3.5 h to obtain a neodymium iron boron magnet.

The prepared NdFeB magnet was placed in a VSM to measure its magnetic properties. The results were as follows: B _r = 1.35 T, H _cj = 1640.00 kA/m, and (BH) _max = 356.56 kJ/m ³ . Its microstructure is shown in Figure 2. The grain boundary of the magnet is continuously clear, which makes the magnet performance better.

Comparative example 2

The particle size of the hydrazine hydride is ground to 3 nm;

The sintering and tempering conditions are the same as those of the embodiment 3 in the comparative document CN101521069A;

The remaining conditions are the same as in Embodiment 2;

The coercive force value of the finally prepared NdFeB magnet was H _cj = 1360.00 kA/m; this value was much lower than the coercive force value of the NdFeB magnet prepared in Example 2. From this, it is understood that the yttrium hydride polishing particle size of the present invention is combined with the sintering and tempering conditions to obtain a neodymium iron boron magnet having a higher coercive force.

Example 3:

1) The main alloy powder is prepared by using three alloy processes of rapid solidification casting, hydrogen explosion and jet milling, the diameter of the powder particles is about 3.5 μm, and the main alloy is atomic percentage, and its composition is Nd _10.63. Pr _2.72 Fe _79.02 Al _0.71 Co _0.50 Cu _0.14 Zr _0.14 Ga _0.09 Dy _0.09 B _5.96 ;

2) The grain boundary phase metal ruthenium is sufficiently hydrogen-absorbed at a pressure of 16-17 bar and room temperature, and dehydrogenated under vacuum at 450 ° C for 4 h, and ground into a powder having a particle diameter of approximately 0.8 μm by mechanical ball milling. The alloy is in atomic percentage and has a composition of DyH ₂ , and its structure is detected by XRD as shown in FIG. 3 , and its composition is determined to be DyH ₂ , which is an fcc structure;

3) uniformly mixing the main alloy powder and the DyH ₂ powder in a glove box under the protection of high purity nitrogen to obtain a mixed powder, wherein the weight of the DyH ₂ grain boundary phase alloy powder is 0.2% by weight of the total powder;

The prepared neodymium iron boron magnet was placed in a VSM to measure its magnetic properties. The results were as follows: B _r = 1.39 T, H _cj = 1277.60 kA/m, and (BH) _max = 378.64 kJ/m ³ . The microstructure of the magnet is shown in Figure 4. The grain boundary of the magnet is continuously clear, which makes the magnet performance better.

Example 4:

2) The grain boundary phase metal ruthenium is sufficiently hydrogen-absorbed at a pressure of 16-17 bar and room temperature, and dehydrogenated under vacuum at 450 ° C for 4 h, and ground into a powder having a particle diameter of approximately 0.8 μm by mechanical ball milling. The alloy is in atomic percentage, the composition is DyH ₂ , and the structure is the same as in the above Figure 1;

3) uniformly mixing the main alloy powder and the DyH ₂ powder in a glove box under the protection of high purity nitrogen to obtain a mixed powder, wherein the weight of the DyH ₂ grain boundary phase alloy powder is 2.0% by weight of the total powder;

The prepared NdFeB magnet was placed in a VSM to measure its magnetic properties. The results were as follows: B _r = 1.34 T, H _cj = 1424.80 kA/m, and (BH) _max = 352.40 kJ/m ³ . Its microstructure is as shown in Figure 4, and the grain boundaries of the magnet are continuously clear, which makes the magnet performance better.

It can be seen from the above analysis that the present invention adopts a main phase alloy doped with various metal ions, and uses micron-sized DyH ₃ and DyH ₂ particles together with optimized sintering and tempering conditions to avoid oxidation of fine particles and improve With the performance of the magnet, it is possible to obtain a neodymium-iron-boron magnet having a high coercive force; at the same time, the grinding cost is lowered.

Claims

A method for increasing the coercive force of a sintered NdFeB magnet by adding a hydrazine compound, characterized in that it comprises:

(1) The main phase of the NdFeB alloy ingot or quick-condensing ribbon is crushed by hydrogen explosion and jet milling to prepare a primary phase particle powder with an average particle diameter of 2-10 μm, and the grain boundary is entangled by ball milling. The hydrogen compound is broken into grain boundary phase particle powder having an average particle diameter of 0.5-1.0 μm;

(2) adding the prepared grain boundary phase particle powder to the main phase particle powder in a mass percentage of 0.2% to 2%, and thoroughly mixing in an oxygen-free atmosphere;

(4) subjecting the mixed alloy powder to orientation molding under a magnetic field of 1.2-2.0 T;

(5) The magnetic block completed by pressing is subjected to cold isostatic pressing of 150-220 MPa in liquid oil to make it pressed into a green body;

(6) The magnetic block completed by pressing is sintered under vacuum of 1050-1100 ° C for 2-4 h, and then tempered by tempering 2-4 h at 850-950 ° C and tempering at 2-4 h at 500-650 ° C for 2-4 h. The final magnet.
The method for increasing the coercive force of a sintered NdFeB magnet according to claim 1, wherein in the step (1), the main phase NdFeB alloy is in atomic percentage and its composition is ( Nd a Pr 1-a ) b Fe 100-bcd B c M d , wherein Nd is a lanthanum element, Pr is a lanthanum element, Fe is an iron element, B is a boron element, and M is Dy, Tb, Ce, Co, Ni, One or more of V, Ti, Mo, Mn, Ga, Al, Cu, Zr, Ta, Ag, Si elements; a, b, c, d satisfy the following relationship: 0.7≦a≦1,11≦b ≦ 16, 5.4 ≦ c ≦ 6.5, 0 ≦ d ≦ 6.
The method for increasing the coercive force of a sintered NdFeB magnet by the addition of an hydrazine compound according to claim 2, wherein the M is a mixture of Al, Co, Cu, Zr, Ga, and Tb; or the M It is a mixture of Al, Co, Cu, Zr, Ga, and Dy; the a, b, c, and d satisfy the following relationship: the a, b, c, and d satisfy the following relationship: 0.75 ≦ a ≦ 0.85, 13 ≦ b ≦14,5.5≦c≦6.0,1≦d≦2.
The method for improving the coercive force of a sintered NdFeB magnet according to any one of claims 1 to 3, wherein the grain boundary phase hydrogen compound is in atomic percent and its composition is DyH x , where Dy is a lanthanum element and H is a hydrogen element; x satisfies the following relationship: 2 ≦ x ≦ 3.
The method for increasing the coercive force of a sintered NdFeB magnet according to claim 4, wherein the hydrogen hydride compound is prepared by the following method: (1): arranging the grain boundary phase metal at 16 Full hydrogen absorption at -17 bar pressure and room temperature; optional entry step (2), step (2): vacuum dehydrogenation at 400-500 ° C for 2-4 h.
The method of increasing the coercive force of a sintered NdFeB magnet by the addition of a hydrogen hydride compound according to claim 4, wherein the main phase particle powder has an average particle diameter of 3-5 μm.
The method of increasing the coercive force of a sintered NdFeB magnet according to claim 4, wherein the grain boundary phase particle powder has an average particle diameter of 0.8 to 1.0 μm.
The method of increasing the coercive force of a sintered NdFeB magnet by the addition of a hydrogen hydride compound according to claim 4, wherein the grain boundary phase particle powder has a percentage content of 0.2 to 0.5%.
The method for improving the coercive force of a sintered NdFeB magnet according to the addition of the hydrazine compound according to claim 4, wherein in the step (6), the sintering and tempering conditions are: 0.85-1090 ° C under vacuum for 3.5-4 h, and then after 900 - 910 ° C first tempering 2-3h and 510-520 ° C secondary tempering 3.5-4h.
A sintered neodymium iron boron magnet, characterized in that the sintered neodymium iron boron magnet is prepared by adding the hydrogen hydrazine compound according to any one of claims 1 to 9 to improve the coercive force of the sintered NdFeB magnet. .