Heat treatment process of cerium-rich neodymium-iron-boron magnet

Abstract

The invention discloses a heat treatment process of a cerium-rich neodymium iron boron magnet, and relates to the technical field of rare earth permanent magnet materials. The method comprises the following steps: carrying out first heat treatment on a sintered cerium-rich neodymium-iron-boron magnet matrix in a high-purity Ar atmosphere at 450-550 DEG C, and carrying out air cooling to room temperature by adopting inert gas shielding gas; coating the surface of the magnet with an MM heavy rare earth diffusion source, carrying out second heat treatment at 600-700 DEG C in a high-purity Ar + 5% H2 atmosphere, and carrying out air cooling to room temperature by adopting inert gas shielding gas; carrying out third heat treatment in a high-purity Ar atmosphere at 800-900 DEG C, quenching by adopting Ar gas, and rapidly cooling to room temperature; and finally, carrying out fourth heat treatment in a high-purity Ar atmosphere at 450-650 DEG C, and carrying out air cooling to room temperature by adopting inert gas shielding gas. The process is simple, new raw materials and new equipment are not needed, only the time, temperature, atmosphere, cooling mode and speed of heat treatment are changed, and the comprehensive magnetic performance of the cerium-rich neodymium-iron-boron magnet is improved under the condition that the using amount of the heavy rare earth diffusion source is smaller.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of rare earth permanent magnet materials, and particularly relates to a heat treatment process of a cerium-rich neodymium-iron-boron magnet. BACKGROUND

[0002] Rare earth permanent magnet materials are not only the fastest-growing, largest and most complete development direction in the entire rare earth field, but also the key raw materials that are irreplaceable and indispensable for the development of the national new industry. Sintered neodymium-iron-boron permanent magnets are the most widely used, fastest-growing and most comprehensive permanent magnets in the world at present, and are widely used in the fields of new energy vehicles, industrial robots, energy-saving home appliances and wind power generation.

[0003] With the rapid development of the new energy industry, heavy rare earth grain boundary diffusion technology has gradually become the most commonly used means for improving the comprehensive magnetic properties of sintered neodymium-iron-boron permanent magnets at present. The grain boundary diffusion technology uses Dy, Tb and other heavy rare earth elements as diffusion agents, and through diffusion heat treatment, the heavy rare earth elements enter the magnet interior from the magnet surface along the grain boundaries and are distributed on the grain boundaries and grain surfaces to improve the coercivity and high-temperature stability of the magnet. However, with the wide application of the grain boundary diffusion technology in sintered neodymium-iron-boron permanent magnets, the demand for rare earth elements has also rapidly and unbalancedly increased. On the one hand, heavy rare earth elements are a kind of expensive strategic resources that are strictly limited for use and export, and the production of a large number of high-coercivity sintered neodymium-iron-boron permanent magnets faces the problems of cost increase and raw material supply difficulty. On the other hand, the high-abundance cerium element in the earth's crust has not been reasonably utilized, resulting in a large amount of accumulation. The price of cerium metal is only about 1 / 20 of that of praseodymium and neodymium metal and 1 / 100 of that of heavy rare earth metal. In order to solve the problem of balanced utilization of rare earth resources, the preparation of neodymium-iron-boron permanent magnets by replacing Pr / Nd with high-abundance Ce has become an important research direction in the field of rare earth permanent magnet materials.

[0004] The saturation magnetization and anisotropy field of the cerium-iron-boron phase are far lower than those of the neodymium-iron-boron phase. If a large amount of cerium is added to the neodymium-iron-boron magnet, the comprehensive magnetic properties (coercivity, remanence, thermal stability and the like) of the magnet will deteriorate, and in particular, the remanence and coercivity will significantly decrease. In order to reduce the magnetic dilution effect and improve the coercivity of the cerium-rich neodymium-iron-boron magnet, the heavy rare earth grain boundary diffusion technology is introduced into the Ce-based neodymium-iron-boron magnet, but the coercivity of the Ce-based neodymium-iron-boron magnet is not obviously improved by this scheme. At present, the research on the Ce-based neodymium-iron-boron magnet mainly focuses on the design of the matrix composition and the selection of the diffusion source, and some positive research progress has been made. Although the research on the grain boundary diffusion technology of the Ce-based neodymium-iron-boron magnet is more and more, there are still some problems. In order to achieve the same increase in coercivity, the Ce-based neodymium-iron-boron magnet needs to diffuse more heavy rare earth elements, and the coercivity of the Ce-based neodymium-iron-boron magnet increases relatively less with the same content of heavy rare earth elements.

[0005] Grain boundary diffusion heat treatment process is a key technology to improve the coercivity of the magnet, but its interface problems (such as uneven diffusion, phase reaction, grain boundary structure evolution, etc.) directly affect the comprehensive magnetic properties of the magnet. The grain boundary diffusion heat treatment process of the Ce-based neodymium-iron-boron magnet in the prior art usually adopts high-temperature diffusion at 850-950℃ and two-stage tempering treatment, and when the cerium content is higher than 5% (wt%), it is very dependent on heavy rare earth elements to ensure the comprehensive magnetic properties of the magnet.

[0006] The invention patent with publication number CN114914046B and the name of a medium-high performance neodymium-iron-boron magnet and a preparation method discloses that by designing an HReM alloy with a melting point of 600-900℃, the diffusion temperature is changed to 600-750℃, which reduces the use amount of Dy and Tb elements, avoids the decrease of residual magnetism, and improves the intrinsic coercivity; however, this method is to develop a new type of low melting point alloy diffusing agent, which needs to use a special HReM alloy, and there is no ready-to-sell product on the market, which increases the production cost and process difficulty, increases the control point of this alloy, and is not conducive to industrialization and application; in summary, in order to further expand the development prospects of the cerium-rich neodymium-iron-boron magnet, the comprehensive magnetic properties of the cerium-rich neodymium-iron-boron magnet are improved from other aspects. SUMMARY

[0007] The purpose of the present application is to provide a heat treatment process for a cerium-rich neodymium-iron-boron magnet, which can improve the comprehensive magnetic properties of the cerium-rich neodymium-iron-boron magnet by only controlling the heat treatment process.

[0008] To achieve the above-mentioned purpose, the present application provides the following technical solution: a heat treatment process for a cerium-rich neodymium-iron-boron magnet, comprising the following steps:

[0009] S1 low-temperature annealing pretreatment: the sintered cerium-rich neodymium-iron-boron magnet substrate is subjected to a first heat treatment at 450-550℃ in a high-purity Ar atmosphere, which is used to reduce Ce / Nd grain boundary segregation and inhibit the formation of CeFe2 phase, and the heat treatment is followed by air cooling to room temperature under the protection of inert gas; wherein the chemical general formula of the cerium-rich neodymium-iron-boron magnet substrate is (PrNd) a Ce b M c Fe 100-a-b-c-d B d , wherein a, b, c and d are mass percentages, 15≤a≤30, 5≤b≤20, 0≤c≤2, 0.95≤d≤1.05; M is selected from at least one of Cu, Ga, Al, Co, Zr, Nb and Ti;

[0010] S2 medium temperature diffusion: the surface of the magnet after low temperature annealing pretreatment is coated with MM heavy rare earth diffusion source, and the second heat treatment is carried out under the condition of high-purity Ar+5% H2 atmosphere and 600-700℃, for forming low melting point liquid phase channel, so that the heavy rare earth MM diffuses into the magnet along the grain boundary, and after the heat treatment, the magnet is air-cooled to room temperature by inert gas protection gas;

[0011] S3 high temperature diffusion: the magnet after medium temperature diffusion is subjected to third heat treatment under the condition of high-purity Ar atmosphere and 800-900℃, and after the heat treatment, the magnet is rapidly cooled to room temperature by Ar quenching, so as to form (MM, Nd)2Fe 14 B hard magnetic shell layer;

[0012] S4 tempering treatment: the magnet after high temperature diffusion is subjected to fourth heat treatment under the condition of high-purity Ar atmosphere and 450-650℃, for repairing and improving the grain boundary interface of high temperature diffusion, and after the heat treatment, the magnet is air-cooled to room temperature by inert gas protection gas.

[0013] Preferably, in the above step S1, the temperature rising rate of the first heat treatment is 3-5℃ / min, the temperature holding time after reaching the temperature is 0.5-2 hours, and the cooling rate of air cooling is 3-5℃ / s.

[0014] Preferably, in the above step S2, the temperature rising rate of the second heat treatment is 3-5℃ / min, the temperature holding time after reaching the temperature is 3-10 hours, and the cooling rate of air cooling is 3-5℃ / s.

[0015] Preferably, in the above step S3, the temperature rising rate of the third heat treatment is 3-5℃ / min, the temperature holding time after reaching the temperature is 0.5-2 hours, and the cooling rate is 15-30℃ / s.

[0016] Preferably, in the above step S4, the temperature rising rate of the fourth heat treatment is 3-5℃ / min, the temperature holding time after reaching the temperature is 1-5 hours, and the cooling rate of air cooling is 1-3℃ / s.

[0017] Preferably, the grade of the above cerium-rich neodymium-iron-boron magnet substrate is selected from N38, N40, N42, N45, 38H, 40H, 42H, 38SH, 40SH.

[0018] Preferably, the preparation process of the above cerium-rich neodymium-iron-boron magnet substrate includes the steps of melting, rapid solidification, hydrogen breaking, airflow grinding, magnetic field orientation, isostatic pressing and sintering.

[0019] In a more preferable embodiment, the MM heavy rare earth diffusion source includes at least one of hydride, fluoride, oxide and alloy of Dy or Tb, and the addition amount is not more than 1.5% of the mass of the cerium-rich neodymium-iron-boron magnet substrate.

[0020] Compared with the prior art, the present application has the following advantages:

[0021] 1. The heat treatment process of the cerium-rich neodymium-iron-boron magnet significantly improves the coercivity of the Ce-based neodymium-iron-boron magnet while maintaining a high remanence by synergistic regulation of low-temperature annealing pretreatment, two-stage diffusion, and tempering, and by controlling the time, temperature, atmosphere, and cooling mode and rate of heat treatment, etc., under the premise of not changing the original process and raw materials, solves the problems of uneven diffusion, interface reaction and phase change, and degradation of grain boundary structure in the current conventional Ce-based neodymium-iron-boron magnet grain boundary diffusion process, and the upper limit of the cerium addition amount can reach 20% (mass percent).

[0022] 2. The heat treatment process of the cerium-rich neodymium-iron-boron magnet is simple and easy to operate, and the cost is controllable. Compared with the existing method, the use amount of heavy rare earth is reduced while maintaining a high remanence performance under the condition of achieving the same coercivity performance.

[0023] 3. The heat treatment process of the cerium-rich neodymium-iron-boron magnet increases the low-temperature annealing pretreatment process before the grain boundary diffusion, which can reduce the Ce / Nd grain boundary segregation, inhibit the formation of CeFe2 phase, and improve the subsequent diffusion uniformity; the present application also adopts a two-stage diffusion heat treatment process, first using medium-temperature diffusion at 600-700℃ to form a low-melting-point liquid phase channel, so that the heavy rare earth diffuses into the magnet as much as possible along the grain boundary, and then using high-temperature diffusion at 800-900℃ to form a (Dy / Tb, Nd)2Fe 14 B hard magnetic shell layer, which avoids the problems of abnormal grain growth, unnecessary phase over precipitation, and uneven diffusion of the magnet, significantly improves the coercivity of the cerium-rich neodymium-iron-boron magnet, and is beneficial to maintaining a high remanence and thermal stability. DETAILED DESCRIPTION

[0024] The current conventional Ce-based neodymium-iron-boron magnet grain boundary diffusion process mainly has problems of uneven diffusion, interface reaction and phase change, and degradation of grain boundary structure. First, the uneven diffusion is caused by the difference in grain boundary energy of Nd2Fe 14 B and Ce2Fe 14 B, and the easy combination of Ce with oxygen to form CeO2, which blocks the diffusion channel and easily causes the diffusion elements Dy, Tb, etc. to preferentially penetrate along the neodymium-rich phase grain boundary, and the diffusion in the cerium-rich phase region is blocked. Second, the interface reaction and phase change are caused by the high activity of Ce element, which easily reacts with the diffusion elements, such as the formation of low-melting-point phase TbCeFe2 by the diffusion element Tb and Ce, and the formation of Ce2Fe 14The B decomposes at a lower temperature, and the main phase grain abnormal growth phenomenon is prone to occur when diffusing at high temperature. Thirdly, the grain boundary structure is degraded, and the interface pores or cracks are formed after diffusion due to the phase separation caused by local composition supersaturation during diffusion, thermal stress concentration during cooling and other reasons, and the grain boundary phase is discontinuous, thereby reducing the demagnetization coupling effect.

[0025] In order to solve the problems existing in the grain boundary diffusion process of the Ce-based neodymium-iron-boron magnet, improve the performance of the Ce-rich neodymium-iron-boron magnet, and as far as possible retain the original preparation process framework and raw material mode, avoid designing difficult-to-apply schemes, the present application has made a lot of thinking and assumption, and after numerous tests and improvements, important progress has been made in improving the heat treatment process.

[0026] A heat treatment process of a Ce-rich neodymium-iron-boron magnet, comprising the following steps:

[0027] S1 low-temperature annealing pretreatment: the sintered Ce-rich neodymium-iron-boron magnet substrate is subjected to first heat treatment under the condition of high-purity Ar atmosphere and 450-550℃, and the holding time after the temperature reaches is preferably 0.5-2 hours, and the cooling rate after heat treatment is preferably controlled at 3-5℃ / s.

[0028] The purpose of the first heat treatment is to homogenize the grain boundary chemical composition, reduce the Ce / Nd grain boundary segregation, inhibit the formation of CeFe2 phase, and promote the continuous distribution of Nd-rich phase, thereby providing a channel for subsequent diffusion.

[0029] S2 medium-temperature diffusion: the surface of the low-temperature annealing pretreated magnet is coated with MM heavy rare earth diffusion source (the MM heavy rare earth diffusion source includes at least one of hydride, fluoride, oxide and alloy of Dy or Tb, and the addition amount is not more than 1.5% of the mass of the Ce-rich neodymium-iron-boron magnet substrate, and the specific amount can be determined according to the grade of the magnet), and the second heat treatment is carried out under the condition of high-purity Ar+5% H2 atmosphere and 600-700℃, and the holding time after the temperature reaches is preferably 3-10 hours, and the cooling rate after heat treatment is preferably controlled at 3-5℃ / s.

[0030] This temperature is higher than the phase formation temperature of CeFe2 and lower than the grain diffusion temperature of the main phase, thereby forming a low-melting-point liquid phase channel, so that the heavy rare earth diffuses into the magnet along the grain boundary as much as possible, and the replacement reaction between the heavy rare earth and the grain is almost not occurred, thereby avoiding the abnormal growth of the grain and the uneven diffusion of the magnet as much as possible.

[0031] S3 high temperature diffusion: the magnet after the medium temperature diffusion is treated at a third time under the condition of high purity Ar atmosphere and 800-900℃, the holding time after the temperature reaches is preferably 0.5-2 hours, and the magnet is rapidly cooled to room temperature by Ar quenching after the heat treatment, and the cooling rate is preferably controlled at 15-30℃ / s;

[0032] This step realizes the replacement reaction of heavy rare earth and light rare earth in the grain, the heavy rare earth MM deeply replaces the main phase grain to form (MM, Nd)2Fe 14 B hard magnetic shell layer, compared with the traditional diffusion process, due to the low temperature annealing pretreatment process and the medium temperature diffusion process in the first stage, the high temperature diffusion can greatly reduce the diffusion time, avoid the abnormal growth of the grain, unnecessary phase excessive precipitation and the uneven diffusion of the magnet;

[0033] S4 tempering treatment: the magnet after the high temperature diffusion is treated at a fourth time under the condition of high purity Ar atmosphere and 450-650℃, the holding time after the temperature reaches is preferably 1-5 hours, and the magnet is air-cooled to room temperature by inert gas protection after the heat treatment, and the cooling rate is preferably controlled at 1-3℃ / s;

[0034] The fourth heat treatment adopts the traditional tempering process, which can repair and improve the grain boundary interface after the high temperature diffusion, further plays the magnetic coupling effect, and improves the coercivity.

[0035] The heating rate of the above four heat treatments can be 3-5℃ / min.

[0036] The above cerium-rich neodymium-iron-boron magnet matrix has a chemical general formula of (PrNd) a Ce b M c Fe 100-a-b-c-d B d , in which a, b, c and d are mass percentages, 15≤a≤30, 5≤b≤20, 0≤c≤2, and 0.95≤d≤1.05; M is selected from at least one of Cu, Ga, Al, Co, Zr, Nb and Ti, and specifically can adopt the following grades: N38, N40, N42, N45, 38H, 40H, 42H, 38SH and 40SH. The preparation process includes the steps of melting, rapid solidification, hydrogen breaking, airflow grinding, magnetic field orientation, isostatic pressing and sintering; the above formula is a matrix formula suitable for the heat treatment mode of the process of the application, but the above matrix preparation process is a known technology, and the application is not improved, so it will not be described here.

[0037] The content of the application is further described through several examples and comparative examples below, but the following examples are not all the examples of the application, and should not be regarded as the absolute limitation of the application.

[0038] Example 1

[0039] The cerium-rich neodymium-iron-boron magnet substrate of the present embodiment is prepared by a series of process steps of smelting rapid solidification, hydrogen decrepitation, jet milling, magnetic field orientation, isostatic pressing, sintering, etc. according to the component ratio of Pr-Nd 20%, Ce 15%, B 0.97%, Cu 0.1wt%, Al 0.1wt%, Ti 0.15wt%, Fe balance by mass ratio, taking the N38 substrate formula as an example.

[0040] First, the sintered cerium-rich neodymium-iron-boron magnet substrate is pre-processed by low-temperature annealing in a high-purity Ar atmosphere, the heating rate is 5°C / min, the temperature is raised to 500°C, and the temperature is kept for 1 hour, then cooled to room temperature by inert gas protection gas, the cooling rate is 3°C / s, and the sample is taken out. Then the surface of the magnet after low-temperature annealing pretreatment is coated with hydrogen dysprosium for medium-temperature diffusion, in a high-purity Ar + 5% H2 atmosphere, the heating rate is 5°C / min, the temperature is raised to 650°C, and the temperature is kept for 5 hours, then cooled to room temperature by inert gas protection gas, the cooling rate is 3°C / s, and the sample is taken out, wherein the added amount of dysprosium is 0.6% of the mass of the cerium-rich neodymium-iron-boron magnet substrate. Then the magnet after medium-temperature diffusion is subjected to high-temperature diffusion in a high-purity Ar atmosphere, the heating rate is 5°C / min, the temperature is raised to 900°C, and the temperature is kept for 1 hour, then quenched to room temperature by Ar gas, the cooling rate is 20°C / s, and the sample is taken out. Finally, the magnet after high-temperature diffusion is subjected to tempering treatment in a high-purity Ar atmosphere, the heating rate is 5°C / min, the temperature is raised to 500°C, and the temperature is kept for 3 hours, then cooled to room temperature by inert gas protection gas, the cooling rate is 2°C / s, and the sample is taken out.

[0041] The prepared grain boundary diffusion cerium-rich neodymium-iron-boron magnet is placed in a PFM12cn type ultra-high coercivity permanent magnet tester, and the magnetic property results are recorded in Table 1 below.

[0042] Example 2

[0043] The cerium-rich neodymium-iron-boron magnet substrate of the present embodiment is prepared by a series of process steps of smelting rapid solidification, hydrogen decrepitation, jet milling, magnetic field orientation, isostatic pressing, sintering, etc. according to the component ratio of Pr-Nd 20%, Ce 15%, B 0.97%, Cu 0.1wt%, Al 0.1wt%, Ti 0.15wt%, Fe balance by mass ratio, taking the N38 substrate formula as an example.

[0044] Firstly, the sintered Ce-rich Nd-Fe-B magnet substrate is annealed at low temperature in high purity Ar atmosphere, the heating rate is 5°C / min, the temperature is raised to 550°C, and the temperature is kept for 2 hours, then the sample is cooled to room temperature by inert gas protection, the cooling rate is 4°C / s, and the sample is taken out. Then, the surface of the magnet after low-temperature annealing pretreatment is coated with dysprosium hydride for medium-temperature diffusion, in high purity Ar + 5% H2 atmosphere, the heating rate is 5°C / min, the temperature is raised to 650°C, and the temperature is kept for 4 hours, then the sample is cooled to room temperature by inert gas protection, the cooling rate is 4°C / s, and the sample is taken out, wherein the added amount of dysprosium is 0.4% of the mass of the Ce-rich Nd-Fe-B magnet substrate. Next, the magnet after medium-temperature diffusion is diffused at high temperature in high purity Ar atmosphere, the heating rate is 5°C / min, the temperature is raised to 850°C, and the temperature is kept for 2 hours, then the sample is rapidly cooled to room temperature by Ar quenching, the cooling rate is 25°C / s, and the sample is taken out. Finally, the magnet after high-temperature diffusion is tempered in high purity Ar atmosphere, the heating rate is 5°C / min, the temperature is raised to 600°C, and the temperature is kept for 5 hours, then the sample is cooled to room temperature by inert gas protection, the cooling rate is 1°C / s, and the sample is taken out.

[0045] The prepared grain boundary diffusion Ce-rich Nd-Fe-B magnet is placed in a PFM12cn type ultra-high coercivity permanent magnet tester, and the magnetic property results are recorded in Table 1 below.

[0046] Comparative Example 1

[0047] Comparative Example 1 is the same as Example 1, except that the low-temperature annealing pretreatment and medium-temperature diffusion process are not used.

[0048] The prepared grain boundary diffusion Ce-rich Nd-Fe-B magnet is placed in a PFM12cn type ultra-high coercivity permanent magnet tester, and the magnetic property results are recorded in Table 1 below.

[0049] Comparative Example 2

[0050] Comparative Example 2 is the same as Example 2, except that the low-temperature annealing pretreatment and medium-temperature diffusion process are not used, and in addition, in order to obtain similar coercivity performance, the added amount of dysprosium is increased to 0.7% of the mass of the Ce-rich Nd-Fe-B magnet substrate.

[0051] The prepared grain boundary diffusion Ce-rich Nd-Fe-B magnet is placed in a PFM12cn type ultra-high coercivity permanent magnet tester, and the magnetic property results are recorded in Table 1 below.

[0052] Example 3

[0053] The cerium-rich neodymium-iron-boron magnet substrate of the embodiment is prepared by a series of process steps of smelting rapid solidification, hydrogen decrepitation, jet milling, magnetic field orientation, isostatic pressing, sintering, etc. according to the component ratio of Pr-Nd 25%, Ce 10%, B 0.97%, Cu 0.3wt%, Al 0.2wt%, Co 0.5wt%, Ga 0.1wt%, Zr 0.05wt%, and Fe in residual amount by mass ratio, taking the 38SH substrate formula as an example.

[0054] First, the sintered cerium-rich neodymium-iron-boron magnet substrate is subjected to low-temperature annealing pretreatment in a high-purity Ar atmosphere, the heating rate is 3 ℃ / min, the temperature is raised to 550 ℃, the temperature is kept for 2 hours, and the sample is taken out after air cooling to room temperature at a cooling rate of 5 ℃ / s. Then, the surface of the magnet after low-temperature annealing pretreatment is coated with terbium fluoride for medium-temperature diffusion, under a high-purity Ar + 5% H2 atmosphere, the heating rate is 3 ℃ / min, the temperature is raised to 650 ℃, the temperature is kept for 6 hours, and the sample is taken out after air cooling to room temperature at a cooling rate of 5 ℃ / s, wherein the added amount of terbium is 0.25% of the mass of the cerium-rich neodymium-iron-boron magnet substrate. Next, the magnet after medium-temperature diffusion is subjected to high-temperature diffusion in a high-purity Ar atmosphere, the heating rate is 3 ℃ / min, the temperature is raised to 900 ℃, the temperature is kept for 1.5 hours, and the sample is taken out after air quenching to room temperature at a cooling rate of 30 ℃ / s. Finally, the magnet after high-temperature diffusion is subjected to tempering treatment in a high-purity Ar atmosphere, the heating rate is 3 ℃ / min, the temperature is raised to 650 ℃, the temperature is kept for 3 hours, and the sample is taken out after air cooling to room temperature at a cooling rate of 1 ℃ / s.

[0055] The prepared grain boundary diffusion cerium-rich neodymium-iron-boron magnet is placed in a PFM12cn type ultra-high coercivity permanent magnet tester, and the magnetic property results are recorded in Table 1 below.

[0056] Example 4

[0057] The cerium-rich neodymium-iron-boron magnet substrate of the embodiment is prepared by a series of process steps of smelting rapid solidification, hydrogen decrepitation, jet milling, magnetic field orientation, isostatic pressing, sintering, etc. according to the component ratio of Pr-Nd 25%, Ce 10%, B 0.97%, Cu 0.3wt%, Al 0.2wt%, Co 0.5wt%, Ga 0.1wt%, Zr 0.05wt%, and Fe in residual amount by mass ratio, taking the 38SH substrate formula as an example.

[0058] Firstly, the sintered Ce-rich Nd-Fe-B magnet matrix is annealed at low temperature in high purity Ar atmosphere, the heating rate is 3°C / min, the temperature is raised to 500°C, and the sample is kept for 1 hour. Then the sample is cooled to room temperature in inert gas atmosphere, the cooling rate is 4°C / s, and the sample is taken out. Then the magnet surface is coated with TbF3 for diffusion at medium temperature. The temperature is raised to 600°C in high purity Ar + 5% H2 atmosphere, the heating rate is 3°C / min, the sample is kept for 8 hours, and the sample is cooled to room temperature in inert gas atmosphere, the cooling rate is 4°C / s, and the sample is taken out. The addition of Tb is 0.18% of the mass of the Ce-rich Nd-Fe-B magnet matrix. Then the magnet is diffused at high temperature in high purity Ar atmosphere, the temperature is raised to 850°C, the heating rate is 3°C / min, the sample is kept for 3 hours, and the sample is quenched to room temperature in Ar atmosphere, the cooling rate is 25°C / s, and the sample is taken out. Finally, the magnet is tempered in high purity Ar atmosphere, the temperature is raised to 500°C, the heating rate is 3°C / min, the sample is kept for 5 hours, and the sample is cooled to room temperature in inert gas atmosphere, the cooling rate is 2°C / s, and the sample is taken out.

[0059] The prepared grain boundary diffusion Ce-rich Nd-Fe-B magnet is placed in a PFM12cn type ultra-high coercivity permanent magnet tester, and the magnetic property results are recorded in Table 1 below.

[0060] Comparative Example 3

[0061] Comparative Example 3 is the same as Example 3, except that the low temperature annealing pretreatment and medium temperature diffusion process are not used.

[0062] The prepared grain boundary diffusion Ce-rich Nd-Fe-B magnet is placed in a PFM12cn type ultra-high coercivity permanent magnet tester, and the magnetic property results are recorded in Table 1 below.

[0063] Comparative Example 4

[0064] Comparative Example 4 is the same as Example 4, except that the low temperature annealing pretreatment and medium temperature diffusion process are not used. In addition, in order to obtain similar coercivity performance, the addition of Tb is 0.4% of the mass of the Ce-rich Nd-Fe-B magnet matrix.

[0065] The prepared grain boundary diffusion Ce-rich Nd-Fe-B magnet is placed in a PFM12cn type ultra-high coercivity permanent magnet tester, and the magnetic property results are recorded in Table 1 below.

[0066] Table 1 Magnetic property test results of Examples and Comparative Examples

[0067]

[0068] From the above table, it can be seen from the comparison of example 1 and comparative example 1, example 3 and comparative example 3 that the present application can greatly improve the coercivity of cerium-rich neodymium-iron-boron magnet under the condition of the same amount of heavy rare earth diffusion source and without changing other processes; and it can be easily found from the comparison of example 2 and comparative example 2, example 4 and comparative example 4 that, in order to make the coercivity of the product prepared by the prior method equal to that of the product prepared by the method of the present application, more heavy rare earth diffusion source has to be added, and after the amount of the diffusion source is increased, the remanence and coercivity of comparative example 2 and comparative example 4 are not as good as those of the present application, and the method of the present application only changes the timing, number, temperature and time of heat treatment, thereby realizing the improvement of the comprehensive magnetic properties of the cerium-rich neodymium-iron-boron magnet under the condition of less amount of heavy rare earth diffusion source.

[0069] The above merely describes preferred embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can easily think of changes or replacements within the technical scope disclosed by the present application, which should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be defined by the protection scope defined by the claims.

[0070] The parts not described in the present application are the known technology of the person skilled in the art.

Claims

1. A heat treatment process for cerium-rich NdFeB magnets, characterized in that: The following steps are involved: S1 low temperature annealing pretreatment: The sintered cerium-rich NdFeB magnet matrix is ​​subjected to the first heat treatment in a high-purity Ar atmosphere at 450-550°C to reduce Ce / Nd grain boundary segregation and inhibit the formation of CeFe2 phase. After heat treatment, it is cooled to room temperature using inert gas shielding gas. The chemical formula of the cerium-rich NdFeB magnet matrix is ​​(PrNd) a Ce b M c Fe 100-a-b-c-d B d , where, in percentage by mass, 15≤a≤30, 5≤b≤20, 0≤c≤2, 0.95≤d≤1.05; M is selected from at least one of Cu, Ga, Al, Co, Zr, Nb, and Ti; S2 medium temperature diffusion: The surface of the magnet after low temperature annealing pretreatment is coated with MM heavy rare earth diffusion source, and a second heat treatment is carried out at 600-700℃ in a high-purity Ar+5% H2 atmosphere to form a low melting point liquid phase channel, so that the heavy rare earth MM diffuses into the interior of the magnet along the grain boundary. After heat treatment, the magnet is cooled to room temperature with inert gas shielding gas. S3 high temperature diffusion: The magnet after medium temperature diffusion is subjected to a third heat treatment in a high purity Ar atmosphere at 800-900°C. After heat treatment, it is rapidly cooled to room temperature by Ar gas quenching to form (MM, Nd)2Fe 14 B hard magnetic shell; S4 tempering treatment: The magnet after high-temperature diffusion is subjected to the fourth heat treatment in a high-purity Ar atmosphere at 450-650°C to repair and improve the high-temperature diffusion grain boundary interface. After heat treatment, it is cooled to room temperature using inert gas protection gas.

2. The heat treatment process of a cerium-rich NdFeB magnet according to claim 1, characterized in that: In step S1, the heating rate of the first heat treatment is 3-5°C / min, and the holding time after the temperature is reached is 0.5-2 hours; the cooling rate of air cooling is 3-5°C / s.

3. The heat treatment process of a cerium-rich NdFeB magnet according to claim 1, characterized in that: In step S2, the heating rate of the second heat treatment is 3-5°C / min, and the holding time after the temperature is reached is 3-10 hours; the cooling rate of air cooling is 3-5°C / s.

4. The heat treatment process for a cerium-rich NdFeB magnet according to claim 1, wherein: In step S3, the heating rate of the third heat treatment is 3-5°C / min, the holding time after the temperature is reached is 0.5-2 hours; and the cooling rate is 15-30°C / s.

5. The heat treatment process for a cerium-rich NdFeB magnet according to claim 1, wherein: In step S4, the heating rate of the fourth heat treatment is 3-5°C / min, and the holding time after the temperature is reached is 1-5 hours; the cooling rate of air cooling is 1-3°C / s.

6. The heat treatment process for a cerium-rich NdFeB magnet according to claim 1, wherein: The brand of the cerium-rich NdFeB magnet matrix is ​​selected from N38, N40, N42, N45, 38H, 40H, 42H, 38SH, and 40SH.

7. The heat treatment process for a cerium-rich NdFeB magnet according to claim 1, wherein: The preparation process of the cerium-rich NdFeB magnet matrix includes the steps of smelting and rapid solidification, hydrogen cracking, air flow milling, magnetic field orientation, isostatic pressing and sintering.

8. The heat treatment process for a cerium-rich NdFeB magnet according to claim 1, wherein: The MM heavy rare earth diffusion source includes at least one of hydride, fluoride, oxide and alloy of Dy or Tb, and the addition amount thereof does not exceed 1.5% of the mass of the cerium-rich NdFeB magnet matrix.

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