Method for manufacturing magnetic powder, and method for manufacturing RFeB-based sintered magnets

Abstract

The present invention provides a method for producing magnetic powder, which is a raw material for sintered magnets, bonded magnets, etc., in a way that can minimize the loss of raw materials. [Solution] A jet mill grinding step is performed in which a lump of magnet raw material alloy (flaky alloy 11) is pulverized by a jet mill to produce a main powder (12) consisting of particles of the raw material alloy with a median particle size distribution in the range of 1 to 30 μm, and fine powder (13) consisting of fine particles of the raw material alloy that are generated as a result of producing the main powder (12) and have a median particle size distribution smaller than that of the main powder (12). A powder mixing step is performed in which magnet powder (14) is obtained by mixing the main powder (12) and the fine powder (13), in a low-oxygen atmosphere with an oxygen concentration of 1000 ppm or less.

Description

[Technical Field]

[0001] This invention relates to a method for producing magnetic powder, which is a powder used as a raw material for magnets such as sintered magnets, bonded magnets (resin bonded magnets, metal bonded magnets), and hot-formed magnets. This invention can be suitably applied to the production of RFeB-based magnetic powder, which has rare earth elements (R) such as neodymium (Nd), iron (Fe), and boron (B) as its main constituent elements, and SmFeN-based magnetic powder, which has samarium (Sm), a type of rare earth element, iron, and nitrogen (N) as its main constituent elements. The magnetic powder produced by the method according to this invention can be suitably used in the production of RFeB-based sintered magnets. [Background technology]

[0002] First, let's explain the RFeB-based sintered magnet as an example from among the various types of magnets listed above. RFeB-based sintered magnets are manufactured by sintering magnet powder obtained by crushing a raw alloy ingot having R, Fe, and B as the main constituent elements to an average particle size of several μm.

[0003] When crushing raw alloy ingots, the ingots are generally brittle by absorbing hydrogen, then coarsely crushed, and finally finely crushed using a jet mill to achieve the average particle size mentioned above. A jet mill is a device that generates an airflow by ejecting high-pressure gas of several atmospheres or more from a nozzle, accelerates the magnetic powder particles with this airflow, and crushes them by applying impact and shear force through collisions with each other. Typically, a jet mill is equipped with a classifier such as a cyclone classifier, which classifies and recovers the crushed particles that fall within a predetermined particle size range.

[0004] Conventionally, when sintering the magnetic powder for RFeB-based sintered magnets, a method has been used in which the powder is compressed using a press and then heated to a high temperature (e.g., approximately 1100°C) (press method). On the other hand, in recent years, a press-less process (PLP) method has also been used, in which the magnetic powder is filled into a mold and then heated to a slightly lower temperature (e.g., approximately 1000°C) without compression molding, as a method that can achieve higher coercivity and produce sintered magnets in a shape close to the final product (near-net shape) (see, for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2006-019521 [Patent Document 2] Japanese Patent Application Publication No. 04-214803 [Overview of the project] [Problems that the invention aims to solve]

[0006] When RFeB-based sintered magnets are manufactured using the above methods (either the press method or the PLP method), the mass of the resulting RFeB-based sintered magnet is at least a few percent less than the mass of the raw materials used. This means that some of the raw materials are lost during the manufacturing process of RFeB-based sintered magnets. In particular, since it is thought that there is little reduction in the raw materials during the compression molding process in the press method, the filling of magnet powder into the mold in the PLP method, and subsequent processes, it is thought that some of the raw materials are lost during the process of manufacturing the magnet powder. Among the elements that make up the raw materials for RFeB-based sintered magnets, rare earth elements in particular are expensive and scarce, so it is necessary to minimize the loss of raw materials as much as possible.

[0007] Up to this point, we have explained using RFeB-based sintered magnets as an example, but similar problems arise when manufacturing RFeB-based bonded magnets or hot-formed magnets, as well as when manufacturing SmFeN-based bonded magnets.

[0008] The problem that this invention aims to solve is to provide a method for producing magnetic powder that can suppress the loss of raw materials. In addition, the invention provides a method for producing RFeB-based sintered magnets that can particularly suitably use the magnetic powder produced by the method according to this invention. [Means for solving the problem]

[0009] The method for producing magnetic powder according to the present invention, which was developed to solve the above problems, A jet mill grinding process is performed to produce a main powder consisting of particles of the raw material alloy having a median particle size distribution within the range of 1 to 30 μm by grinding a lump of the raw material alloy of the magnet using a jet mill, and to recover a fine powder consisting of fine particles of the raw material alloy having a median particle size distribution smaller than that of the main powder, which is generated during the production of the main powder. A powder mixing step to obtain magnetic powder by mixing the main powder and the fine powder. This process is characterized by being carried out in a low-oxygen atmosphere with an oxygen concentration of 1000 ppm or less.

[0010] When manufacturing RFeB-based sintered magnets, bonded magnets, or hot-formed magnets, or SmFeN-based bonded magnets, fine grinding is typically performed using a jet mill method, with the particle size range to be classified determined so that the median particle size distribution falls within a specified range. Here, the particle size is measured using a laser method. The median particle size distribution is determined according to the composition of the raw material alloy (RFeB-based, SmFe-based (which becomes SmFeN-based composition by nitriding during or after grinding)), the form of the magnet to be manufactured (sintered magnet, bonded magnet, hot-formed magnet, etc.), etc. Furthermore, since SmFe-based raw material alloys include anisotropic SmFe alloys where the easy magnetization axes are aligned and isotropic SmFe alloys where the easy magnetization axes are not aligned, the difference between these two types must also be considered.

[0011] When manufacturing RFeB-based sintered magnets or SmFeN-based bonded magnets using anisotropic SmFe-based alloys as raw materials, it is best to finely grind the material so that the median particle size distribution is within the range of 1 to 10 μm in order to maximize its magnetic properties. When manufacturing RFeB-based sintered magnets from RFeB-based magnet powder, both the press method and the PLP method are equally effective (see Patent Document 2 for the press method), however, the PLP method generally results in smaller particle sizes than the press method. Magnet powder made from anisotropic SmFe-based alloys is suitably used in the manufacture of SmFeN-based resin-bonded magnets and metal-bonded magnets. For both RFeB-based and SmFeN-based magnets, the closer the particle size of the magnet powder is to the single-domain particle critical diameter, the higher the coercivity.

[0012] When manufacturing SmFeN-based resin-bonded magnets, etc., using isotropic SmFe-based alloys as raw materials, it is preferable to finely grind the material so that the median particle size distribution is within the range of 10 to 30 μm in order to increase the packing density of the magnet powder when kneading with the resin and to increase the fluidity of the mixture (compound) of magnet powder and resin.

[0013] As described above, depending on the composition of the raw alloy and the form of the magnet to be manufactured, the particle size range to be classified is determined so that the median particle size distribution is within the range of 1 to 30 μm, and fine grinding is performed using the jet mill method. In a normal jet mill, when the particle size distribution of the particles to be classified is determined, some of the material to be ground is excessively ground, resulting in the generation of fine powder consisting of fine powder particles with a smaller median particle size distribution. Such fine powder particles are carried out of the jet mill by the airflow. In conventional methods for manufacturing magnet powder, this generated fine powder was discarded without being used, but in the present invention, this fine powder is recovered. The recovery of the fine powder can be done using a filter with a fine mesh or the like. Then, in the powder mixing process, the main powder and the fine powder are mixed to produce magnet powder. Sintered magnets, bonded magnets, and hot-plastic processed magnets can be manufactured using the magnet powder obtained in this way.

[0014] According to the method for producing magnetic powder of the present invention, since the fine powder generated when the raw material alloy ingot is crushed is recovered and used as part of the magnetic powder, the loss of raw materials can be suppressed.

[0015] The method for manufacturing an RFeB-based sintered magnet according to the present invention is: A jet mill grinding step is performed to produce a main powder consisting of particles of the raw material alloy with a median particle size distribution in the range of 1 to 10 μm by grinding a lump of RFeB-based raw material alloy with a jet mill, and to recover a fine powder consisting of fine particles of the raw material alloy with a median particle size distribution smaller than that of the main powder, which is generated during the production of the main powder. A powder mixing step in which magnetic powder is produced by mixing the main powder and the fine powder, A sintering step of sintering the aforementioned magnet powder and This process is characterized by being carried out in a low-oxygen atmosphere with an oxygen concentration of 1000 ppm or less.

[0016] In other words, the method for manufacturing an RFeB-based sintered magnet according to the present invention comprises a sintering step of sintering the magnet powder obtained by the method for manufacturing magnet powder according to the present invention in a low-oxygen atmosphere with an oxygen concentration of 1000 ppm or less, wherein the raw material alloy of the magnet is an RFeB-based raw material alloy.

[0017] Generally, the particles of the main powder (main powder particles) consist of either single crystal grains or multiple crystal grains bonded together via grain boundary phases where the rare earth content is higher than that of the raw alloy composition. In contrast, the particles of the fine powder are small in size and therefore usually consist of single crystal grains. In a sintered body obtained by sintering a mixture of these main powder and fine powder raw materials, there are crystal grains contained in the fine powder particles or main powder particles, or crystal grains that have grown due to the heating during sintering from the crystal grains contained in those raw materials.

[0018] In general, in an RFeB-based sintered magnet, if there are fine crystal grains with too small particle sizes (for example, less than 1 μm) in the sintered body, while the coercive force increases, when an external magnetic field is applied to the sintered body during magnetization, it becomes difficult for the magnetization direction in the fine crystal grains to change. As a result, the magnetization rate decreases, and the magnetization of the entire sintered magnet becomes small. Therefore, in an RFeB-based sintered magnet obtained by sintering a raw material mixed powder containing fine powder particles as in the present invention, there is concern that the magnetization may become small due to the generation of fine crystal grains derived from the fine powder particles in the sintered body. However, in reality, during the sintering process, the fine crystal grains are incorporated into and disappear in crystal grains larger than them, so the overall magnetization of the RFeB-based sintered magnet manufactured by the method of the present invention does not become small.

[0019] Further, in the method for manufacturing an RFeB-based sintered magnet according to the present invention, when filling the powder of the raw material into the mold in either the pressing method or the PLP method, the filling rate can be increased by the intrusion of the fine powder particles into the gaps between the main powder particles.

[0020] When a bonded magnet is manufactured using the magnet powder according to the present invention, since it is not heated to a high temperature as in the production of a sintered magnet, fine crystal grains remain in the obtained bonded magnet. Thereby, a bonded magnet with a high coercive force can be obtained.

[0021] In the method for producing magnetic powder and RFeB-based sintered magnets according to the present invention, it is desirable that the jet mill grinding step and the powder mixing step be carried out in a low-oxygen atmosphere with an oxygen concentration of 100 ppm or less. In the method for producing RFeB-based sintered magnets, it is also desirable that the sintering step be carried out in a low-oxygen atmosphere with an oxygen concentration of 100 ppm or less. This suppresses oxidation of the main powder particles and fine powder particles during the manufacturing process of magnetic powder and RFeB-based sintered magnets, thereby suppressing a decrease in magnetic properties such as magnetization. In particular, as the particle size of the powder particles decreases, the surface area per unit mass increases, making them more susceptible to oxidation. Therefore, when producing magnetic powder and RFeB-based sintered magnets using fine powder particles as in the present invention, it is important to suppress the oxygen concentration in the atmosphere. [Effects of the Invention]

[0022] This invention makes it possible to reduce the loss of raw materials when manufacturing magnetic powder. [Brief explanation of the drawing]

[0023] [Figure 1] A schematic diagram showing one embodiment of the method for producing magnetic powder and the method for producing RFeB-based sintered magnets according to the present invention. [Figure 2] A schematic diagram showing some steps of a method for manufacturing an RFeB-based sintered magnet using a pressing method, which is a modified example of this embodiment. [Figure 3] This graph shows the results of measuring the particle size distribution of the main particles and fine particles prepared in the example using a laser method. [Figure 4] A graph showing the particle size distribution in Figure 3 as the cumulative particle size distribution. [Figure 5] Electron microscope image of the main powder prepared in the example. [Figure 6] Electron microscope image of the fine powder prepared in the example. [Figure 7] Electron microscope image of the RFeB-based sintered body prepared in the example. [Figure 8]This graph shows the relationship between the magnitude of the external magnetic field and the magnetic susceptibility measured for each of the following: an example without grain boundary diffusion treatment, an example with grain boundary diffusion treatment, a comparative example without grain boundary diffusion treatment, and a comparative example with grain boundary diffusion treatment. [Figure 9] This graph shows the results of measuring coercivity Hcj and residual magnetic flux density Br at a measurement temperature of 20°C for examples and comparative examples that underwent grain boundary diffusion treatment. [Figure 10] This graph shows the results of measuring coercivity Hcj and residual magnetic flux density Br at a measurement temperature of 130°C for examples and comparative examples that underwent grain boundary diffusion treatment. [Figure 11] This graph shows the results of measuring the temperature change rate α of the residual magnetic flux density Br and the temperature change rate β of the coercivity Hcj at a measurement temperature of 130°C for examples and comparative examples that underwent grain boundary diffusion treatment. [Figure 12] This graph shows the results of measuring the reduced magnetic field strength HD5 and squareness ratio SQ at a measurement temperature of 130°C for examples and comparative examples that underwent grain boundary diffusion treatment. [Modes for carrying out the invention]

[0024] Using Figures 1 to 12, embodiments of the method for manufacturing magnet powder and RFeB-based sintered magnets according to the present invention will be described. In the following, the embodiment of the method for manufacturing magnet powder will be described as part of the embodiment of the method for manufacturing RFeB-based sintered magnets, but the method for manufacturing magnet powder of this embodiment can also be applied when manufacturing magnet powder that is a raw material for RFeB-based bonded magnets, hot-plastic processed magnets, SmFeN-based bonded magnets, etc.

[0025] Figure 1 shows a schematic diagram of a method for manufacturing an RFeB-based sintered magnet, including the method for manufacturing the magnetic powder of this embodiment. As described above, in the present invention, either the press method or the PLP method may be used in the sintering process. Below, an embodiment using the PLP method will be described first, and then the differences when using the press method will be described. Furthermore, each of the following steps is carried out in a low-oxygen atmosphere chamber (low-oxygen atmosphere container) in which the internal oxygen concentration is 1000 ppm or less, preferably 100 ppm or less, by flowing an inert gas.

[0026] First, a flaky alloy 11 is prepared by strip casting from a molten metal in which metals have been melted to approximately equal the ratio of R, Fe, and B in the RFeB-based sintered magnet to be manufactured (Figure 1(a)). In this invention, such a flaky alloy 11, along with other materials having the same composition as the raw material alloy powder to be prepared later and having particles that are sufficiently larger than those of the raw material alloy powder, are called raw material alloy ingots. Next, the flaky alloy 11 is embrittled by contacting it with a hydrogen-containing gas (Figure 1(b)).

[0027] Next, as described below, the brittle flake alloy 11 is crushed by a jet mill 20 (jet mill crushing process; see Figure 1(c)). The jet mill 20 comprises a jet mill body 21, a hopper 22 for crushing, a gas supply source 23, a classifier 24, and a bag filter 25. The hopper 22 is a hopper for storing the flake alloy 11, which is the material to be crushed, and a crushing material supply pipe 221 extends from its lower part. The crushing material supply pipe 221 is connected to the jet mill body 21. The gas supply source 23 supplies an inert gas (nitrogen gas, argon gas, helium gas, etc.) and is connected to the jet mill body 21 by multiple gas supply pipes 231. The jet mill body 21 has a built-in classifier rotor 210. Furthermore, the jet mill body 21 is connected to the cyclone 24 by a material transfer pipe 211, and the cyclone 24 is connected to the bag filter 25 by an exhaust pipe 241.

[0028] In this jet mill 20, the material to be pulverized hopper 22 and the gas supply source 23 mix the flake alloy 11 stored in the material to be pulverized hopper 22 with the inert gas flowing through the first gas supply pipe 231 and introduce it into the jet mill body 21. The jet mill body 21 accelerates the flake alloy 11 with the airflow of the inert gas and pulverizes it by causing the flake alloy 11 particles to collide with each other, thereby applying impact and shear force. The classifier rotor 210 classifies the pulverized material into powder particles whose particle size is below a predetermined value. The classified powder particles are transferred to the classifier 24 through the material transfer pipe 211.

[0029] Cyclone 24 removes fine powder particles with a particle size below a predetermined value from the transferred powder particles and recovers those with a particle size above the predetermined value. The powder consisting of particles classified by the classification rotor 210 and recovered by the cyclone 24 is the main powder 12 in this invention. The range of particle size of the classified and recovered particles is determined by setting the operating conditions of the classification rotor 210 and cyclone 24 so that the median of the particle size distribution is between 1 and 10 μm, using the particle size measured by the laser method. When manufacturing magnets other than RFeB-based sintered magnets, the median of the particle size distribution is determined within the range of 1 to 30 μm, depending on the composition of the raw material alloy (RFeB-based, SmFe-based, etc.) and the form of the magnet to be manufactured (sintered magnet, bonded magnet, hot-plastic processed magnet, etc.). For example, when manufacturing SmFeN bonded magnets using an anisotropic SmFe alloy as a raw material, the median particle size distribution should be in the range of 1 to 10 μm, similar to the case of RFeB sintered magnets, in order to maximize the magnetic properties. On the other hand, when manufacturing SmFeN resin bonded magnets using an isotropic SmFe alloy as a raw material, the median particle size distribution should be in the range of 10 to 30 μm in order to increase the packing rate of the magnet powder when kneading with the resin, and to increase the fluidity of the compound after mixing.

[0030] Meanwhile, the fine powder particles removed by the cyclone 24 are carried by the airflow and discharged from the cyclone 24, and are transferred to the bag filter 25 through the exhaust pipe 241. The bag filter 25 captures the fine powder particles contained in the airflow and discharges the gas. The fine powder particles captured by the bag filter 25 are recovered as fine powder 13.

[0031] Next, the main powder 12 obtained in the jet milling process and the fine powder 13 are mixed to produce magnet powder 14, which will be the raw material for RFeB-based sintered magnets (powder mixing process, Figure 1(d)). The steps up to this point correspond to an embodiment of the method for producing magnet powder according to the present invention. In addition, a lubricant may be mixed during the powder mixing process.

[0032] Next, the magnetic powder 14 obtained in the powder mixing step is filled into the mold 31 (Figure 1(e)), and an external magnetic field is applied to the magnetic powder 14 in the mold 31 to orient the powder particles of the magnetic powder 14 (Figure 1(f)). Then, without removing the magnetic powder 14 from the mold 31, the magnetic powder 14 is sintered by heating it together with the mold 31 to a predetermined sintering temperature (for example, a temperature in the range of 900 to 1050°C) (sintering step using the PLP method; Figure 1(g)), thereby obtaining an RFeB-based sintered body 15 (Figure 1(h)). Since the mold 31 is heated to the sintering temperature together with the magnetic powder 14 in this way, the mold 31 is made of a material that has heat resistance at the sintering temperature.

[0033] The obtained RFeB-based sintered body 15 loses its ferromagnetism because it is heated to a temperature higher than the Curie temperature during the sintering process. Therefore, magnetization is performed by applying an external magnetic field to the RFeB-based sintered body 15 (Figure 1(i)). Through the above operation, an RFeB-based sintered magnet 1 is obtained (Figure 1(j)).

[0034] When manufacturing an RFeB-based sintered magnet 1 by the press method, each step up to the powder mixing step (Figure 1(d)) is carried out in the same manner as described above. Then, the magnet powder 14 is filled into the mold 41 of the press machine 40, and pressure is applied to the magnet powder 14 in the mold 41 while applying a magnetic field (Figure 2(a)) to produce a compacted body 14A of the magnet powder 14. After that, the compacted body 14A is removed from the mold 41, and the compacted body 14A is heated to a predetermined sintering temperature (for example, a temperature in the range of 950 to 1100°C) to sinter it, thereby obtaining an RFeB-based sintered body 15A (Figure 2(b)). Thereafter, magnetization is performed by applying an external magnetic field to the RFeB-based sintered body 15A in the same manner as the PLP method described above (Figure 2(c)), thereby obtaining an RFeB-based sintered magnet 1A (Figure 2(d)).

[0035] In addition, regardless of whether the PLP method or the pressing method is used, after the RFeB-based sintered bodies 15 and 15A are fabricated but before magnetization, a deposit consisting of Dy (dysprosium), Tb (terbium), or Ho (holmium), or a substance containing two or three of these elements, may be attached to the surface of the RFeB-based sintered bodies 15 and 15A, and then a grain boundary diffusion treatment may be performed by heating to a predetermined temperature (for example, a temperature in the range of 700 to 1000°C). By performing the grain boundary diffusion treatment, Dy, Tb, and / or Ho diffuse through the grain boundaries of the RFeB-based sintered bodies 15 and 15A to the vicinity of the crystal grain surface (without diffusion into the depths of the crystal grains), thereby increasing the coercivity of the RFeB-based sintered magnets 1 and 1A while suppressing a decrease in the residual magnetic flux density. Furthermore, after the grain boundary diffusion treatment, an aging treatment may be performed by further heating the RFeB-based sintered bodies 15 and 15A. The heating temperature during the aging process can be, for example, within the range of 440 to 560°C, and the heating time can be, for example, within the range of 30 minutes to 15 hours.

[0036] In the RFeB-based sintered magnet manufacturing method of this embodiment, by using magnet powder 14 obtained by recovering the fine powder 13 generated when crushing the flake-shaped alloy 11 with a jet mill 20 and mixing it with the main powder 12, the loss of raw materials can be reduced, thereby reducing costs.

[0037] Furthermore, when filling the molds 41 and 41A with the magnetic powder 14, the fine powder particles of the fine powder 13 can penetrate into the gaps between the main particles of the main powder 12, thereby increasing the filling rate.

[0038] The following describes an example in which an RFeB-based sintered magnet 1 was manufactured using the RFeB-based sintered magnet manufacturing method using the PLP method, as described in this embodiment. In this embodiment, a flake-shaped alloy 11 having the composition shown in Table 1 below was prepared as the raw material alloy ingot. As a comparative example, an RFeB-based sintered magnet was manufactured by sintering only the main powder 12. The raw material alloy ingot used in the comparative example was from a different manufacturing lot than that used in the embodiment, and although its composition is slightly different from that used in the embodiment as shown in Table 1, it can be considered to be almost the same. [Table 1]

[0039] Of the elements contained in the flake alloy (raw alloy ingot) 11, oxygen (O), carbon (C), and nitrogen (N) are unavoidable impurities. The concentration of O in the flake alloy is preferably 300 ppm or less. The concentration of C in the flake alloy is preferably 1000 ppm or less, more preferably 600 ppm or less, and even more preferably 200 ppm or less. The concentration of N in the flake alloy is preferably 50 ppm or less.

[0040] In the example, the raw alloy ingot was crushed in a jet mill 20 so that the median particle size distribution of the powder to be classified was 3 μm. The powder classified and recovered by the classification rotor 210 and cyclone 24 was used as the main powder 12, and the powder captured and recovered by the bag filter 25 was used as the fine powder 13. On the other hand, in the comparative example, the raw alloy ingot was crushed in a jet mill 20 so that the median particle size distribution of the powder to be classified was 3 μm. The powder classified and recovered by the classification rotor 210 and cyclone 24 was used, and the powder captured and recovered by the bag filter was discarded without being used.

[0041] Figure 3 shows the particle size distribution and Figure 4 shows the cumulative particle size distribution measured using a laser method for the main powder 12 and fine powder 13 of the example. From these figures, the median of the particle size distribution (the particle size value on the horizontal axis when the cumulative particle size distribution value on the vertical axis in Figure 4 is 50%) is approximately 3 μm for the main powder 12 and approximately 1 μm for the fine powder 13. Furthermore, Figure 5 shows an electron microscope image of the main powder 12 and Figure 6 shows an electron microscope image of the fine powder 13 of the example. The magnification is the same in Figures 5 and 6, allowing for a comparison of the sizes of the main particles and fine particles.

[0042] In the example, an RFeB-based sintered body 15 was prepared by mixing the main powder 12 and the fine powder 13 to obtain magnetic powder 14, and then sintering the resulting powder at a sintering temperature of 1010°C using the PLP method. In the comparative example, an RFeB-based sintered body 15A was prepared by sintering the powder classified by the classifier 24 under the same conditions as in the example.

[0043] Figure 7 shows an electron microscope image of the RFeB-based sintered body 15 prepared in the example, taken at the same magnification as in Figures 5 and 6. This electron microscope image shows that the RFeB-based sintered body 15 has almost no crystal grains with a particle size of less than 1 μm. This indicates that the fine crystal grains originating from the fine powder 13 have been incorporated into larger crystal grains and disappeared.

[0044] Multiple RFeB-based sintered bodies 15 in the example and 15A in the comparative example were prepared. Some were magnetized as is, while the rest underwent grain boundary diffusion treatment and aging treatment before magnetization. The grain boundary diffusion treatment and aging treatment were carried out under the same conditions for both the example and the comparative example. The deposits used in the grain boundary diffusion treatment had a composition of terbium (Tb) 75.3 mass%, copper (Cu) 18.8 mass%, and aluminum (Al) 5.9 mass%. Magnetization was performed while gradually increasing the magnitude of the external magnetic field and measuring the magnetic susceptibility.

[0045] Figure 8 shows the results of measuring the relationship between the magnitude of the external magnetic field and the magnetic susceptibility for each of the following: an example without grain boundary diffusion treatment, an example with grain boundary diffusion treatment, a comparative example without grain boundary diffusion treatment, and a comparative example with grain boundary diffusion treatment. In both the examples and comparative examples, the magnetic susceptibility was lower when grain boundary diffusion treatment was performed than when it was not performed, within a range of relatively small external magnetic fields. However, when the external magnetic field was increased to 40 kOe, 100% magnetization was achieved.

[0046] Furthermore, when comparing the examples with the comparative examples, the magnetic susceptibility was almost the same in both cases: when grain boundary diffusion treatment was not performed and when it was performed. Generally, RFeB-based sintered magnets containing crystal grains with small particle sizes (e.g., less than 1 μm) tend to have a lower magnetic susceptibility. However, corresponding to the fact that the RFeB-based sintered body 15 of the examples contains almost no crystal grains with a particle size of less than 1 μm (Figure 7), such a trend was not observed in the measurement results of the magnetic susceptibility of the examples. In other words, it can be seen that there is no problem in terms of magnetic susceptibility even when the RFeB-based sintered magnet 1 is manufactured using magnet powder 14, which is a mixture of main powder 12 and fine powder 13.

[0047] Next, for each of the examples and comparative examples, six samples were subjected to grain boundary diffusion treatment, and the coercivity H was measured at 20°C (room temperature) and 130°C (temperature equivalent to the operating environment for automobile motors). cj and residual magnetic flux density B r The coercivity H was measured. The measurement results are shown with the horizontal axis representing the coercivity H. cj The vertical axis represents the residual magnetic flux density B. r The graphs show the results for a measurement temperature of 20°C in Figure 9 and for 130°C in Figure 10. In both the examples and comparative examples, measurements were taken individually on six samples prepared from the same lot, and all the data for those six samples is shown in the graphs. In both cases, the range of data distribution for the six samples in the examples and the range of data distribution for the six samples in the comparative examples overlap, and no significant difference is observed between the examples and comparative examples.

[0048] Also, for the case where the measurement temperature is 130°C, the temperature change rate α of the residual magnetic flux density B r and the temperature change rate β of the coercive force H cj were determined. Here, these temperature change rates correspond to the slopes at the said measurement temperature in a graph where the horizontal axis is temperature and the vertical axis is the residual magnetic flux density B r or the coercive force H cj . The smaller the absolute values of these values, the better the stability with respect to temperature change. The measurement results are shown in Fig. 11 in a graph where the vertical axis is the temperature change rate α of the residual magnetic flux density B r and the horizontal axis is the temperature change rate β of the coercive force H cj . Also in this measurement result, the ranges in which the data of the six samples of the example are distributed and the ranges in which the data of the six samples of the comparative example are distributed overlap, and no significant difference is seen between the example and the comparative example.

[0049] Next, for the case where the measurement temperature is 130°C, the demagnetizing field strength HD5 and the squareness ratio SQ were determined. Here, the demagnetizing field strength HD5 refers to the reverse magnetic field when the magnetization becomes 95% of the residual magnetic flux density Br (decreases by 5%) in the second quadrant (demagnetization curve) of the magnetization curve. The squareness ratio SQ refers to the value obtained by dividing the reverse magnetic field when the magnetization becomes 90% of the residual magnetic flux density Br in the demagnetization curve by the coercive force. Also, no significant difference is seen between the example and the comparative example in these demagnetizing field strength HD5 and squareness ratio SQ.

[0050] As described above, according to this example, an RFeB sintered magnet 1 having magnetic properties equivalent to those of the RFeB sintered magnet of the comparative example can be produced by a method with less raw material loss than the comparative example.

[0051] So far, the embodiments of the method for manufacturing magnet powder and the method for manufacturing RFeB sintered magnet according to the present invention have been described. Needless to say, the present invention is not limited to the above embodiments, and various modifications are possible within the scope of the gist of the present invention.

[0052] For example, in the above embodiment, the flake-shaped alloy 11 was produced by the strip casting method. However, when producing isotropic magnet powder, which has a large number of unoriented fine crystal grains in a single powder particle and is used in bonded magnets or hot-formed magnets, the flake-shaped alloy may be produced using an ultra-rapid cooling method, which has a faster cooling rate of the molten metal than the strip casting method. [Explanation of symbols]

[0053] 1, 1A...RFeB-based sintered magnets 11…Flake-like alloy (raw material alloy lump) 12…Main powder 13…Fine powder 14…Magnetic powder 14A... Compacted powder 15, 15A...RFeB-based sintered body 20... Jetmill 21... Jet Mill main unit 210…Classification rotor 211...Powder transfer pipe 22...Hopper for crushed material 221...Crushed material supply pipe 23…Gas supply source 231...Gas supply pipe 24...Cyclone 241... Exhaust pipe 25…Bug filter 31, 41... Mold 40… Press machine

Claims

1. A jet mill grinding step is performed to produce a main powder consisting of particles of the raw material alloy having a median particle size distribution within the range of 1 to 30 μm by grinding a lump of the raw material alloy of the magnet using a jet mill, and to recover a fine powder consisting of fine particles of the raw material alloy having a median particle size distribution smaller than that of the main powder, which is generated as a result of producing the main powder. A powder mixing step to obtain magnetic powder by mixing the main powder and the fine powder. A method for producing magnetic powder, characterized by carrying out the process in a low-oxygen atmosphere with an oxygen concentration of 1000 ppm or less.

2. The method for producing magnetic powder according to claim 2, characterized in that the oxygen concentration is 100 ppm or less.

3. A jet mill grinding step is performed to produce a main powder consisting of particles of the raw material alloy with a median particle size distribution in the range of 1 to 10 μm by grinding a lump of RFeB-based raw material alloy with a jet mill, and to recover a fine powder consisting of fine particles of the raw material alloy with a median particle size distribution smaller than that of the main powder, which is generated as a result of producing the main powder. A powder mixing step in which magnetic powder is produced by mixing the main powder and the fine powder, A sintering step of sintering the aforementioned magnet powder and A method for manufacturing RFeB-based sintered magnets, characterized by carrying out the process in a low-oxygen atmosphere with an oxygen concentration of 1000 ppm or less.

4. The method for manufacturing an RFeB-based sintered magnet according to claim 3, characterized in that the oxygen concentration is 100 ppm or less.

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