JP7183626B2 - RFeB-based sintered magnet and its manufacturing method - Google Patents

Description

The present invention relates to an RFeB-based sintered magnet containing a rare earth element (hereinafter referred to as "R"), iron (Fe) and boron (B) as main constituent elements, and a method for producing the same.

RFeB-based sintered magnets were discovered by Masato Sagawa et al. Therefore, RFeB sintered magnets are used in various products such as motors for automobiles such as hybrid and electric vehicles, motors for industrial machinery, speakers, headphones, and permanent magnet magnetic resonance diagnostic equipment. .

In general, sintered magnets are manufactured by preparing alloy powder (raw material powder) as a raw material, orienting the raw material powder by applying a magnetic field, and then heating and sintering the raw material powder in that state. be.

In order to increase the residual magnetic flux density, which is one of the most important magnetic properties of RFeB sintered magnets, the easy magnetization axis (crystal axis in the direction of easy magnetization) of each crystal grain that constitutes the RFeB sintered magnet should be aligned in the same direction as much as possible. The degree of orientation is an index that indicates to what extent the axes of easy magnetization of crystal grains are aligned. The degree of orientation is defined as a value B _r /I _s obtained by dividing the residual magnetic flux density B _r of the obtained magnet by the saturation magnetization I _s .

In Patent Document 1, an RFeB-based raw material powder having an average particle size of 3 μm is added with an organic lubricant such as methyl caprylate or methyl myristate, and the raw material powder is oriented by applying a magnetic field. In addition, it is described that an RFeB system sintered magnet having a maximum degree of orientation of 96.5% can be obtained by sintering the raw material powder. By setting the average particle diameter of the raw material powder to a small value of 3 μm in this way, the ratio of particles composed of single crystal grains among the particles of the raw material powder increases, and the crystal grains are also easily aligned in one direction. Further, by adding an organic lubricant to the raw material powder, the coefficient of dynamic friction between particles is reduced, so that individual particles are easily rotated when a magnetic field is applied to the raw material powder. These effects increase the degree of orientation in the RFeB-based sintered magnet described in Patent Document 1.

WO2013/146781 Japanese Patent Application Laid-Open No. 2006-019521

As described above, the RFeB-based sintered magnet described in Patent Document 1 has a maximum degree of orientation of 96.5%, but in order to further increase the residual magnetic flux density, it is necessary to increase the degree of orientation. The problem to be solved by the present invention is to provide an RFeB-based sintered magnet having a higher degree of orientation than conventional magnets, and a method for producing the same.

The RFeB-based sintered magnet according to the present invention, which has been made to solve the above problems,
a) the degree of orientation is 97% or more,
b) The aspect ratio index value, which is the value obtained by dividing the average aspect ratio of the crystal grains on the plane perpendicular to the maximum magnetization direction by the average aspect ratio of the crystal grains on the plane parallel to the maximum magnetization direction, is 0.96 or more and 1.5 or less,
c) have a nitrogen content of 400 ppm or more and 1000 ppm or less;
d) The D50 value, which is the median value of the crystal grain size determined by the equivalent circle diameter on the plane perpendicular to the maximum magnetization direction, is 3.5 μm or less.

In the present invention, the term "perpendicular to the maximum magnetization direction" refers to a plane perpendicular to the maximum magnetization direction, which is the direction in which magnetization is maximized in an RFeB sintered magnet. A plane parallel to the direction of magnetization. The maximum magnetization direction is usually parallel to the magnetic field applied when orienting the raw material powder during the production of RFeB based sintered magnets. In the obtained RFeB system sintered magnet, if all the crystal grains are aligned in the same direction, the RFeB system sintered magnet whose magnetic pole (N pole and S pole) surface is flat usually has the maximum magnetization The direction perpendicular plane is a plane parallel to the magnetic pole plane, and the maximum magnetization direction parallel plane is a plane perpendicular to the magnetic pole plane. The planes parallel to the direction of maximum magnetization may be oriented in various directions rotated about the axis of maximum magnetization, but may be arbitrarily determined.

The "average aspect ratio" is obtained as follows. First, the aspect ratios of 700 or more crystal grains are obtained in the observed plane (the plane perpendicular to the maximum magnetization direction or the plane parallel to the maximum magnetization direction), and the average value is obtained to obtain the average aspect ratio. The aspect ratio of individual crystal grains can be obtained by dividing the length of the direction in which the crystal grains are longest on the observation surface by the length of the direction perpendicular to that direction. Therefore, the aspect ratio and average aspect ratio of individual crystal grains are 1 or more.

The method for producing an RFeB-based sintered magnet according to the present invention includes:
Raw material powder for RFeB-based sintered magnets having a median D50 value of 3.0 μm or less as measured by laser diffraction is dispersed in nitrogen gas for 30 to 300 minutes. a surface-nitrided raw material powder producing step of producing a surface-nitrided raw material powder in which the surfaces of the particles are nitrided;
an orienting step of placing the surface-nitriding raw material powder in a container and applying a magnetic field to orient the surface-nitriding raw material powder without compression molding the surface-nitriding raw material powder;
and a sintering step of sintering the surface-nitriding raw material powder oriented in the orientation step without compression molding.

In the RFeB-based sintered magnet and the method for producing the RFeB-based sintered magnet according to the present invention, the raw material powder of the RFeB-based sintered magnet (raw material powder having the same composition as the RFeB-based sintered magnet to be produced) is placed in nitrogen gas. A surface-nitrided raw material powder having a nitrided surface is prepared by maintaining the dispersed state for a certain length of time (30 to 300 minutes). Here, when the raw material powder is produced, by pulverizing the raw material in nitrogen gas using a jet mill, it is possible to form a state in which the raw material powder is dispersed in nitrogen gas. By sintering the surface-nitrided raw material powder prepared in this way in a magnetic field without compression molding, an RFeB sintered magnet with a high degree of orientation of 97% or more can be obtained for the following two reasons. can be obtained.

The first reason is that the surfaces of the particles of the raw material powder are nitrided, so that the coefficient of dynamic friction of the surfaces of the individual particles is reduced, and the particles become easier to rotate when a magnetic field is applied during orientation. it is conceivable that. In addition, using a press-less process (PLP) method (see Patent Document 2) that does not compress and mold the surface-nitriding raw material powder during orientation also makes it easier for the particles to rotate when a magnetic field is applied. contribute to

The second reason is thought to be that the surface of the particles of the raw material powder is nitrided, thereby suppressing the disturbance of orientation that occurs during sintering. RFeB sintered magnets generally undergo a phenomenon called sintering shrinkage in which the overall volume becomes smaller during sintering. On the other hand, during sintering, the individual crystal grains that make up the RFeB system sintered magnet grow slightly in the direction parallel to the axis of easy magnetization (the c-axis direction in the RFeB system), but in the direction perpendicular to the axis, (A direction in which crystal grains grow more easily than other directions is called a "preferred growth direction."). Therefore, sintering shrinkage occurs largely in the c-axis direction where crystal grains are relatively difficult to grow. If sintering shrinkage occurs significantly in this way, individual crystal grains may move during sintering, changing their orientation and lowering the degree of orientation. (so that individual grains can move). On the other hand, in the RFeB-based sintered magnet according to the present invention, since the surfaces of the particles of the raw material powder are nitrided, the crystal grains grow in the direction perpendicular to the axis of easy magnetization (c-axis) during sintering. This suppresses the occurrence of sintering shrinkage in the direction of the axis of easy magnetization, thereby suppressing a decrease in the degree of orientation. Therefore, it is considered that the RFeB-based sintered magnet according to the present invention has a higher degree of orientation than the conventional RFeB-based sintered magnet.

It is confirmed by the aspect ratio index value that the growth of crystal grains in the direction perpendicular to the axis of easy magnetization is suppressed during sintering in the RFeB-based sintered magnet according to the present invention. In conventional RFeB sintered magnets in which crystal grains grow in the direction perpendicular to the axis of easy magnetization, the average aspect ratio of crystal grains is larger in the plane parallel to the maximum magnetization direction than in the plane perpendicular to the maximum magnetization direction. resulting in a small aspect ratio index value (typically less than 0.96). On the other hand, in the RFeB-based sintered magnet according to the present invention, the growth of crystal grains in the direction perpendicular to the axis of easy magnetization is suppressed, so that the average aspect ratio of the crystal grains in the plane parallel to the maximum magnetization direction is is suppressed, resulting in an aspect ratio index value of 0.96 or more. In the RFeB system, the direction perpendicular to the easy axis of magnetization is the preferential growth direction, so even if the growth of crystal grains in the direction perpendicular to the easy axis of magnetization is suppressed, the , the crystal grains do not grow in the direction parallel to the easy axis of magnetization. Therefore, the maximum aspect ratio index value is 1.5.

In the RFeB-based sintered magnet according to the present invention, the surfaces of the particles of the raw material powder are nitrided during production as described above, so the RFeB-based sintered magnet also contains nitrogen. If the amount of nitrogen contained in the RFeB-based sintered magnet is less than 400 ppm, the nitriding during fabrication is insufficient.

On the other hand, when the nitrogen content increases, the coercive force decreases. Therefore, in the RFeB system sintered magnet according to the present invention, the nitrogen content is set to 1000 ppm or less. If the nitrogen content is 1000 ppm or less, nitrogen hardly affects the value of residual magnetic flux density.

In addition, since the smaller the crystal grain size in the RFeB sintered magnet, the higher the coercive force, the RFeB sintered magnet according to the present invention has D50, which is the median value of the crystal grain size in the plane perpendicular to the maximum magnetization direction. Make sure the value is 3.5 μm or less. Here, the crystal grain size of each crystal grain is defined by the diameter of a circle having the same area as the area of the crystal grain in the plane perpendicular to the maximum magnetization direction obtained by image analysis or the like. In order to obtain an RFeB-based sintered magnet having such a median grain size, in the method for producing a RFeB-based sintered magnet according to the present invention, the median grain size (D50 value ) is 3.0 μm or less. Here, the “laser diffraction method” refers to a method of obtaining the particle size of particles based on the intensity distribution of diffracted light/scattered light generated by irradiating the particles with a laser beam.

According to the present invention, it is possible to obtain an RFeB system sintered magnet having a degree of orientation of 97% or more, which is higher than conventional ones.

FIG. 1 is a schematic diagram showing steps of an embodiment of a method for producing a RFeB-based sintered magnet according to the present invention; Schematic diagram showing a jet mill used in the method for producing a RFeB-based sintered magnet according to the present embodiment. Optical micrographs of the RFeB-based sintered magnet of the present embodiment in a plane perpendicular to the maximum magnetization direction (a) and a plane parallel to the maximum magnetization direction (b). Optical micrographs of an RFeB-based sintered magnet of a comparative example in a plane perpendicular to the maximum magnetization direction (a) and a plane parallel to the maximum magnetization direction (b). 4 is a graph showing the results of obtaining the crystal grain size and the aspect ratio in the plane perpendicular to the maximum magnetization direction (a) and the plane parallel to the maximum magnetization direction (b) for individual crystal grains of the RFeB-based sintered magnet of the present embodiment. 7 is a graph showing the results of obtaining the crystal grain size and aspect ratio in a plane perpendicular to the maximum magnetization direction (a) and a plane parallel to the maximum magnetization direction (b) for individual crystal grains of an RFeB-based sintered magnet of a comparative example.

An embodiment of an RFeB based sintered magnet and a method for producing the same according to the present invention will be described with reference to FIGS. 1 to 6. FIG.

(1) One embodiment of the method for producing a sintered RFeB magnet according to the present invention An embodiment of the method for producing a sintered RFeB magnet according to the present invention will be described with reference to FIGS. 1 and 2. FIG. First, a composition corresponding to the composition of the RFeB system sintered magnet to be produced (however, when the grain boundary diffusion treatment described later is performed, the elements that diffuse into the RFeB system sintered magnet at that time are excluded) A raw material alloy ingot 11 having The raw material alloy ingot 11 can be produced by a method such as strip casting, which is the same as that for the raw material alloy ingot for conventional RFeB sintered magnets. Next, the raw material alloy ingot 11 is made to absorb hydrogen gas to embrittle the raw material alloy ingot 11 (FIG. 1(a)) and then mechanically pulverized (coarse pulverization) to obtain a coarse raw material alloy. A powder 12 ((b) in the figure) is prepared.

Next, the coarse powder 12 of the raw material alloy is measured by the jet mill 20 shown in FIG. By pulverizing so that the surface-nitriding raw material powder 13 is produced (fine pulverization: FIG. 1(c)). The jet mill 20 includes a pulverizing tank 21, a coarse powder inlet 22 for introducing coarse powder 12 into the pulverizing tank 21, a pulverizing gas inlet 231 for introducing pulverizing gas G into the pulverizing tank 21 from below, and A fluidizing gas introduction port 232 for introducing the fluidizing gas G′ from the side, a collision plate 24 provided above the crushing gas introduction port 231 in the crushing tank 21, and a collision plate 24 provided above the crushing tank 21. and a discharge port 26 for discharging the crushed surface-nitriding raw material powder 13 from the crushing tank 21 .

The fluidizing gas G′ is a gas for sucking and accelerating the coarse powder 12 introduced from the coarse powder inlet 22 into the crushing tank 21 . It is a gas that has a role of collision. In this embodiment, nitrogen gas is used for both the pulverizing gas G and the fluidizing gas G'. The collision plate 24 has a role of crushing the particles in the crushing tank 21 by making them collide with it. The classifying rotor 25 is a rotary body that rotates at high speed (up to 6000 revolutions per minute) around the vertical axis in FIG. It has a role to introduce. Particles having a particle size larger than this predetermined particle size descend inside the pulverizing tank 21 without being classified by the classifying rotor 25, and collide with the collision plate 24 again by the pulverizing gas G. As shown in FIG. Through these operations, a surface-nitrided raw material powder 13 having a D50 value, which is the median value of particle diameters measured by a laser diffraction method, of 3.0 μm or less and having nitrided surfaces of the raw material powder particles is obtained (surface-nitrided raw material powder production process).

In this embodiment, the pressure of the pulverizing gas G in the pulverizing tank 21 is set to 0.45-0.60 MPa. Further, by adjusting the rotational speed of the classifying rotor 25, the surface-nitriding raw material powder 13 pulverized in the pulverizing tank 21 was kept in the pulverizing tank 21 for 30 to 300 minutes. In the comparative example described later, the pressure of the pulverizing gas G is 0.45 to 0.60 MPa, and the time of staying in the pulverizing tank 21 is 5 minutes or more, which are the conditions conventionally used when producing RFeB sintered magnets. A raw material powder whose surface is less nitrided than the surface-nitrided raw material powder 13 is used, which is produced for less than 30 minutes.

Next, the surface-nitriding raw material powder 13 is placed in a container 14 having a shape corresponding to the RFeB system sintered magnet to be produced, and the surface-nitriding raw material powder 13 in the container 14 is not subjected to mechanical pressure. A magnetic field is applied (that is, without compression molding) to orient the surface-nitriding raw material powder 13 (FIG. 1(d): orientation step). In this embodiment, the container 14 having a rectangular parallelepiped interior shape is used, but the shape inside the container 14 is not limited to this example. In addition, although FIG. 1(d) shows only one space for filling the surface-nitriding raw material powder 13, a plurality of spaces for filling the surface-nitriding raw material powder 13 may be provided in one container.

After the orientation step, the surface-nitriding raw material powder 13 is sintered by heating without performing compression molding while being housed in the container 14 (FIG. 1(e): sintering step). . The heating temperature in the sintering step is preferably 900-1100°C. The container 14 is made of a material having heat resistance to this heating temperature. Through the steps up to this point, the sintered body 15 of the RFeB magnet is obtained. The above-described PLP method is used in each step described so far.

The sintered body 15 thus obtained may be used as a finished product of the RFeB system sintered magnet as it is, but it is desirable to perform the grain boundary diffusion treatment described below in order to increase the coercive force. In the grain boundary diffusion treatment, a deposit 16 containing Dy (dysprosium) and/or Tb (terbium) is attached to the surface of the sintered body 15, and then heated to 850 to 930 ° C. (Fig. 1 (f) ). As a result, Dy and/or Tb in the deposit 16 are supplied to the vicinity of the surface of the crystal grain through the grain boundary of the sintered body 15 . The presence of Dy and/or Tb near the surface of the crystal grains of the RFeB sintered magnet makes it difficult for magnetization reversal to occur when a reverse magnetic field is applied to the RFeB sintered magnet, improving coercive force. . In addition, although the presence of Dy and/or Tb is a factor that reduces the residual magnetic flux density of the RFeB sintered magnet, the amount of Dy and/or Tb introduced by the grain boundary diffusion treatment is very small, so the residual magnetic flux A decrease in density can be suppressed.

Through the above steps, the RFeB system sintered magnet 17 of the present embodiment is produced (FIG. 1(g)).

(2) One embodiment of RFeB based sintered magnet according to the present invention Next, an embodiment of the RFeB based sintered magnet according to the present invention produced by the manufacturing method of the above embodiment will be described. In addition, instead of the surface-nitriding raw material powder 13, the aforementioned raw material powder prepared by finely pulverizing the coarse powder 12 by a conventional method was used, and the other conditions were the same as those of the present embodiment. A system sintered magnet will be described and compared with the RFeB system sintered magnet of the above embodiment.

Specific conditions for producing the RFeB based sintered magnets of the present embodiment and the comparative example will be described (below, the producing conditions of the present embodiment and the comparative example are the same except as noted). The raw material alloy ingot 11 contains, as standard values, 25.65% by mass of Nd (neodymium), 4.55% by mass of Pr (praseodymium), 0.9% by mass of Co (cobalt), 0.99% by mass of B, and 0.2% by mass of Al (aluminum). %, 0.1% by mass of Cu (copper), 0.1% by mass of Zr (zirconium), and the balance of Fe. The actual composition of the raw material alloy ingot 11 may slightly deviate from the above standard values, and may contain trace amounts of elements such as Dy, Tb, Ni, and Ga. In addition, the raw material alloy ingot 11 may contain O (oxygen), C (carbon), N (nitrogen), and H (hydrogen) as impurities. N was less than 300 ppm, and H was less than 10 ppm.

The conditions for fine pulverization are as follows. In this embodiment, the rotation speed of the classifying rotor 25 is 4700 times/min, the pressure of the pulverizing gas G is 0.45 MPa, and the surface-nitriding raw powder 13 stays in the pulverizing tank 21 (pulverizing gas G) for 30 to 300 minutes. did. In the comparative example, the number of revolutions of the classifying rotor 25 was 3700 times/min, the pressure of the pulverizing gas G was 0.45 MPa, and the surface-nitriding raw powder 13 remained in the pulverizing tank 21 (pulverizing gas G) for 5 minutes or more and less than 30 minutes. and

The heating temperature in the sintering process was 980°C. A mixture of TbCuAl alloy powder containing 75.3% by mass of Tb, 18.8% by mass of Cu and 5.9% by mass of Al and silicone grease was used as the deposit 16 in the grain boundary diffusion treatment. The heating temperature in the grain boundary diffusion treatment was 885°C.

Table 1 shows the results of measuring the magnetic properties of the obtained RFeB system sintered magnets of this embodiment and the comparative example at room temperature. The squareness ratio SQ in _Table 1 is the _{ratio H k90 /} Defined by H _cj .

As shown in Table 1, the RFeB-based sintered magnet of the present embodiment has a higher residual content than the comparative example, although the raw materials (the raw material alloy ingot 11 and the deposit 16) having almost the same composition are used. High magnetic flux density and orientation. In particular, a high degree of orientation of 97% or more, which has been difficult in the past, has been obtained. Although the coercive force and the squareness ratio in this embodiment are lower than those in the comparative example, they are within the practically permissible range for automobile motors and the like.

Next, for the RFeB-based sintered magnets of the present embodiment and the comparative example, cross-sectional optical micrographs were taken to observe the microstructure. The cross-sections for taking optical micrographs were the plane perpendicular to the maximum magnetization direction and the plane parallel to the maximum magnetization direction. Here, the plane perpendicular to the maximum magnetization direction refers to the plane perpendicular to the maximum magnetization direction, which is the direction in which the magnetization is maximized, as described above. It was regarded as the plane perpendicular to the magnetization direction. Also, any one of the planes parallel to the magnetic field applied during the orientation process (there can be many such planes) was regarded as the plane parallel to the maximum magnetization direction.

Optical microscope photographs of the cross section of the RFeB-based sintered magnet of this embodiment are shown in FIG. 3(a) for the plane perpendicular to the maximum magnetization direction and in FIG. 3(b) for the plane parallel to the maximum magnetization direction. Optical microscope photographs of the cross section of the RFeB-based sintered magnet of the comparative example are shown in FIG. 4(a) for the plane perpendicular to the maximum magnetization direction and in FIG. 4(b) for the plane parallel to the maximum magnetization direction.

About 850 crystal grains were targeted for each of these optical micrographs, and the crystal grain size (equivalent circle diameter) and aspect ratio of each crystal grain were determined by image analysis. These values obtained for individual crystal grains are plotted on a graph in which the horizontal axis is the crystal grain size and the vertical axis is the aspect ratio. The graphs are shown in FIG. 5(a) for the plane perpendicular to the maximum magnetization direction in this embodiment, in FIG. 5(b) for the plane parallel to the maximum magnetization direction in this embodiment, and in FIG. 6 ( FIG. 6(b) shows a plane parallel to the maximum magnetization direction in the comparative example.

From this result, the median value (D50 value) of the crystal grain size in the plane perpendicular to the maximum magnetization direction of this embodiment was 3.25 μm, which was 3.5 μm or less. The D50 values of the other crystal grains were 2.93 μm in the plane parallel to the maximum magnetization direction in this embodiment, 3.30 μm in the plane perpendicular to the maximum magnetization direction in the comparative example, and 3.05 μm in the plane parallel to the maximum magnetization direction in the comparative example.

Further, the average aspect ratio in the plane perpendicular to the maximum magnetization direction of this embodiment obtained from the results of FIG. there were. On the other hand, the average aspect ratio in the plane perpendicular to the maximum magnetization direction of the comparative example obtained from the results of FIG. Met. As described above, the average aspect ratio was less than 0.96 in the comparative example, whereas the present embodiment obtained a value of 0.96 or more, which is higher than that in the comparative example.

Furthermore, the length, width, and height (the height direction is the direction parallel to the magnetic field) of the rectangular parallelepiped in the container 14 (dimensions of the portion occupied by the surface-nitriding raw material powder 13 before sintering) are The shrinkage ratio divided by the length, width and height of the sintered body 15 was obtained. In this embodiment, the shrinkage rate in both the vertical and horizontal directions was 16.2%, and the shrinkage rate in the height direction was 32.2%. In the comparative example, the shrinkage rate in both the vertical and horizontal directions was 15.4%. %, and the shrinkage in the height direction was 36.4%. Thus, this embodiment has a lower shrinkage rate in the height direction than the comparative example, and shrinkage in this direction is suppressed.

When the nitrogen content of the RFeB based sintered magnet of this embodiment was measured, it was 460 ppm, which was within the aforementioned range of 400 ppm to 1000 ppm. On the other hand, the measured value of the nitrogen content in the RFeB system sintered magnet of the comparative example was 225 ppm, which was lower than the above range.

Based on the degree of orientation, the aspect ratio index value, the shrinkage rate, and the nitrogen content in the present embodiment and the comparative example described so far, the following can be considered. One of the reasons why the degree of orientation is higher in the present embodiment than in the comparative example is that the surface of the particles of the surface-nitrided raw material powder 13 of the present embodiment is more nitrided than the raw material powder of the comparative example. Therefore, it is considered that the dynamic friction coefficient of the surface of each particle becomes small during orientation, and the particles become easier to rotate. Another reason is that the surfaces of the particles of the surface-nitrided raw material powder 13 of the present embodiment are more nitrided than the raw material powder of the comparative example, so that the crystal grains have an easy magnetization axis (c-axis) during sintering. ), which suppresses the sintering shrinkage in the direction of the easy axis of magnetization, thereby suppressing the decrease in the degree of orientation. Nitrogen content is relevant for both of these reasons. Aspect ratio index values and shrinkage also support the latter reason.

The RFeB based sintered magnet of the present embodiment has been described above in comparison with the comparative example, but the present invention is of course not limited to the above embodiment, and various modifications are possible.

REFERENCE SIGNS LIST 11 Raw alloy lump 12 Coarse powder 13 Surface nitriding raw powder 14 Container 15 Sintered body 16 Deposits 17 RFeB sintered magnet 20 Jet mill 21 Crushing tank 22 Coarse powder inlet 231 Pulverization gas introduction port 232 Fluid gas introduction port 24 Collision plate 25 Classifying rotor 26 Discharge port G Pulverization gas (nitrogen gas)
G'... Fluid gas (nitrogen gas)

Claims (3)

a) the degree of orientation is 97% or more,
b) The aspect ratio index value, which is the value obtained by dividing the average aspect ratio of the crystal grains on the plane perpendicular to the maximum magnetization direction by the average aspect ratio of the crystal grains on the plane parallel to the maximum magnetization direction, is 0.96 or more and 1.5 or less,
c) have a nitrogen content of 400 ppm or more and 1000 ppm or less;
d) An RFeB-based sintered magnet characterized by having a D50 value, which is the median value of the crystal grain size determined by the equivalent circle diameter in the plane perpendicular to the maximum magnetization direction, of 3.5 μm or less. Raw material powder for RFeB-based sintered magnets having a median D50 value of 3.0 μm or less as measured by laser diffraction is dispersed in nitrogen gas for 30 to 300 minutes. a surface-nitrided raw material powder producing step of producing a surface-nitrided raw material powder in which the surfaces of the particles are nitrided;
an orienting step of placing the surface-nitriding raw material powder in a container and applying a magnetic field to orient the surface-nitriding raw material powder without compression molding the surface-nitriding raw material powder;
and a sintering step of sintering the surface-nitrided raw material powder oriented in the orientation step without compression molding. 3. The method for producing a RFeB-based sintered magnet according to claim 2, wherein in the surface-nitriding raw material powder producing step, the surface-nitriding raw material powder is produced by pulverizing the raw material in nitrogen gas using a jet mill. .

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