JP7658238B2 - Molding die, method for manufacturing sintered magnet, and sintered magnet - Google Patents

Description

The present invention relates to a molding die, a method for producing a sintered magnet using the molding die, and a sintered magnet.

Rare earth sintered magnets and ferrite sintered magnets are widely used in a variety of applications, including rotating machines such as motors and generators.

Known rare earth sintered magnets include R-T-B sintered magnets (R is at least one rare earth element and always includes at least one of Nd and Pr. T is at least one transition metal element and always includes iron (Fe). B stands for boron) and Sm-Co sintered magnets (Sm (samarium) may be partially replaced by other rare earth elements) (Patent Document 1). Known sintered ferrite magnets include hexagonal M-type (magnetoplumbite type) Sr ferrite, Ba ferrite, and other sintered magnets (Patent Document 2).

These sintered magnets are obtained by forming magnetic powder into a desired shape and sintering it. Known forming methods include the dry forming method (e.g., Patent Document 3 (JP Patent Publication No. 2004-296849)) and the wet forming method (e.g., Patent Document 4 (JP Patent No. 3833861)). In the dry forming method, dried magnetic powder is pressure-formed in an applied magnetic field to produce a molded body, and the resulting molded body is then fired. In the wet forming method, a slurry containing magnetic powder is pressure-formed in an applied magnetic field while removing the liquid components to produce a molded body, and the resulting molded body is then fired.

For example, sintered ferrite magnets are manufactured by the steps of (a) mixing raw materials such as compounds of Fe, Sr, Ba, Ca, La, Co, etc., (b) calcining to perform a ferritization reaction to obtain a calcined body, (c) roughly pulverizing the calcined body, adding compounds such as sintering aids, and wet-pulverizing it, (d) forming a slurry of the obtained calcined body particles in a magnetic field to obtain a molded body, (e) firing the molded body, and (f) processing the obtained sintered body into a shape appropriate for the intended use.

In order to prevent chipping or cracking during handling of the compacts and sintered bodies in the manufacturing process of sintered magnets or during use of the sintered magnet products, the corners of the compacts may be chamfered in an arc shape, i.e., chamfered with a radius. Usually, such chamfered radius (hereinafter also referred to as "corner radius") is formed by providing a corner radius of an arc shape on the corners of the molding die that corresponds to the corner radius. However, the corner radius formed on the corners of the compact by molding using a die having the corner radius of an arc shape has the desired (ideal) arc shape before processing (in the state of the compact and sintered body). However, when the surface connected to the corner radius of the sintered body is ground in the processing process, part of the corner radius formed is also removed by grinding, and the shape of the corner radius becomes different from the desired shape. This causes a problem in that the effect of preventing chipping and cracking is not fully exerted in the resulting sintered magnet.

Patent Document 5 (JP Patent Publication 2016-188402) discloses a press device for forming a rod-shaped compact such as a cylindrical one while suppressing the occurrence of cracks, and a method for manufacturing a magnet using the press device. The press device includes a die having a through hole that forms a cavity, and an upper punch and a lower punch for pressing the magnetic powder filled in the cavity, the through hole of the die has a forming part having a length Ld and a width Wd (Ld>Wd) and a tapered part formed on the upper side of the forming part, and the pressing surfaces of the upper punch and the lower punch include a concave curved surface and a flat surface connected to the concave curved surface. The cylindrical compact formed by the press device has a fringe part formed by the flat surface connected to the concave curved surface and the die, and this fringe part is removed by processing to obtain a cylindrical compact (or sintered body).

The pressing device and method described in Patent Document 5 aim to prevent cracks from occurring horizontally from the lower part of the fringe of the molded body when the molded body is removed from the pressing device by optimizing the shape of the tapered portion, but are not intended to fully utilize the effect of the corner radius.

Patent Document 6 (JP 2020-39251 A) discloses a method for manufacturing a sintered magnet for a motor, which comprises partially grinding the top of a sintered magnet having a convex first main surface, an opposite second main surface, and a side surface to form a reference plane on a part of the first main surface, and grinding at least a part of the side surface to form a surface that defines the length and/or width of the sintered magnet. The sintered magnet obtained by the method of Patent Document 6 has a reference plane formed by grinding and unmachined (unground) curved surfaces connected to both sides of the reference plane.

The method described in Patent Document 6 aims to improve the yield of magnet material and shorten the grinding process time without reducing motor performance, but is not intended to fully utilize the effects of the radiused corners.

Republished No. 2014/027638 JP 2011-216857 A JP 2004-296849 A Patent No. 3833861 JP 2016-188402 A JP 2020-39251 A

The object of the present invention is therefore to provide a molding die that can produce sintered magnets with a corner radius that can adequately prevent chipping and cracking even after the sintered body has been ground, a method for producing sintered magnets using the molding die, and a sintered magnet with a corner radius that can adequately prevent chipping and cracking.

As a result of thorough research in consideration of the above object, the inventors discovered that in order to ensure that the corners of a sintered magnet are rounded to prevent chipping or cracking and have a desired arc shape after the surface connected to the corners is ground, a corner radius consisting of an arc-shaped curved portion having the desired arc shape and a straight portion connected to the end of the arc-shaped curved portion is formed at the corners of the molded body, and during processing, the arc-shaped curved portion is left unground and only the portion corresponding to the straight portion is ground, leaving all or most of the arc-shaped curved portion unground, thereby making it possible to produce a sintered magnet with a corner radius consisting of the desired arc shape, and thus arrived at the present invention.

That is, the molding die of the present invention for use in molding a sintered magnet is
a corner radius is provided at a corner portion corresponding to a corner portion of a molded body molded by the molding die,
In a cross section perpendicular to the ridge direction of the corner portion,
The corner radius is characterized in that it comprises an arcuate curved portion and a straight portion connected to an end of the arcuate curved portion.

When the connection point between the arcuate curved portion and the straight portion is defined as point _P1 ,
It is preferable that a tangent to the circular arc curved portion at point _P1 coincides with the straight line portion.

The method of the present invention for producing a sintered magnet comprises:
A method for producing a sintered magnet, comprising the steps of: molding fine powder for sintered magnets using a molding die to produce a green body; sintering the green body to produce a sintered body; and grinding a predetermined side surface of the sintered body by machining,
The molding die has a corner portion having a rounded corner corresponding to a corner portion of the molded body, and in a cross section perpendicular to a ridge line direction of the corner portion,
The corner radius is composed of an arcuate curved portion and a straight line portion connected to an end of the arcuate curved portion,
The sintered body has corners that correspond to corners of the molding die, and the corners are rounded.
In a cross section perpendicular to the ridge direction of the corner portion,
the corner radius is composed of an arcuate curved portion and a straight portion corresponding to the arcuate curved portion and the straight portion of the molding die,
The linear portion of the radiused corner of the sintered body and the side surface connected to the linear portion are removed by grinding, thereby forming a sintered magnet having a radiused corner consisting of the arc-shaped curved portion.

In the production method of the present invention,
When a connection point between the circular arc portion and the straight portion of the rounded corner of the sintered body is designated as point _P3 ,
It is preferable that a tangent to the circular arc curved portion at point _P3 coincides with the straight line portion.

In the production method of the present invention,
The sintered magnet is preferably a rare earth sintered magnet or a ferrite sintered magnet.

The sintered magnet of the present invention is
The corner portion has an unprocessed corner radius and two faces connected to the corner radius,
At least one of the two surfaces is a machined surface;
The angle α (deg) between the two faces and the central angle θ (deg) when the radius is approximated by a circular arc are
0.5×(180-α)≦θ＜(180-α)
The present invention is characterized in that:

The angle α (deg) and the central angle θ (deg) are
0.7×(180-α)≦θ＜(180-α)
It is preferable that the following conditions are satisfied:

The sintered magnet is preferably a rare earth sintered magnet or a ferrite sintered magnet.

According to the present invention, even after grinding the side of the sintered body, it is possible to obtain a sintered magnet with the desired ideal arc-shaped corner radius, effectively preventing chipping or cracking of the sintered magnet.

FIG. 1 is a partial cross-sectional view showing a schematic diagram of an example of a molding die of the present invention used for molding a sintered magnet. 2 is a partial cross-sectional view of a molded product formed using the molding die shown in FIG. 1. 3 is a partial cross-sectional view of a sintered body obtained by sintering the compact shown in FIG. 2. FIG. 4 is a partial cross-sectional view of a sintered magnet obtained by grinding the sintered body shown in FIG. This is a partial cross-sectional view of FIG. 3(b) with an explanation of the radius angle added. FIG. 4 is a schematic cross-sectional view for explaining a grinding position on a side surface of a sintered body. FIG. 2 is a schematic cross-sectional view showing an example of a compression molding die. FIG. 5(b) is a cross-sectional view taken along line A-A in FIG. FIG. 1 is a partial cross-sectional view showing a schematic diagram of an example of a conventional molding die used for molding a sintered magnet. 7 is a partial cross-sectional view of a sintered body obtained by sintering a compact formed using the molding die shown in FIG. 6. FIG. 8 is a partial cross-sectional view of a sintered magnet obtained by grinding the sintered body shown in FIG. 7(a).

[1] Molding mold
(1) Conventional Molding Die Before describing the molding die of the present invention, a conventional molding die will be described. Fig. 6 is a partial cross-sectional view perpendicular to the ridgeline direction of the corner portion, which is a schematic illustration of an example of a corner portion of a conventional molding die 110 used for molding a sintered magnet. Conventional molding die 110 is provided with corner radius 111 having a desired arc shape at corner portion 112 corresponding to the corner portion of a molded body (not shown) so that the corner portion (ridge) of the molded body (not shown) formed by molding has a corner radius having a desired arc shape in a cross-sectional view perpendicular to the ridgeline direction of the corner portion.

Here, the desired arc-shaped corner radius formed at the corners of the molded body refers to an arc shape (when viewed in cross section in the ridge line direction) that is formed so that two adjacent faces that intersect at the corner (edge) are smoothly connected. When the angle between the two adjacent faces is θ, the corner radius is preferably formed by an arc with a central angle of θ.

Therefore, the corner radius 111 formed in the molding die 110 has an arc shape formed to smoothly connect to the two faces 113, 114 that intersect at the corner portion 112, as shown in FIG. 6. When the angle between the faces 113 and 114 of the molding die 110 is α (90° in the figure), it is preferable to form the corner radius 111 as an arc with a central angle of θ (=180-α).

In this application, a corner portion refers to a convex portion (also known as a "ridge" or "convex ridge") formed by the intersection of two adjacent surfaces, and a corner portion refers to a concave portion (also known as a "ridge" or "concave ridge") formed by the intersection of two adjacent surfaces. A corner radius refers to a curved portion formed by rounding a corner portion (convex ridge) so that two adjacent surfaces connect smoothly, and a corner radius refers to a curved portion formed by smoothly connecting two adjacent surfaces that intersect at a corner portion (concave ridge).

The green body molded using such a molding die 110 has a corner radius corresponding to the corner radius 111, and the sintered body 130 obtained by sintering this green body has a corner radius 131 corresponding to the corner radius 111, as shown in FIG. 7(a). Note that the sintered body 130 will not strictly have the same shape as the green body (i.e. the shape molded by the molding die 110) due to shrinkage caused by sintering, but for simplicity in this application, it will be described as having substantially the same shape as the green body.

By grinding one surface 133 connected to the radius 131 of this sintered body 130 down to the position _L0 (see FIG. 7(a)), a sintered magnet 130' is obtained having a machined surface 133', an unmachined surface 134, and a radius 131' connected thereto (see FIG. 7(b)). After machining, the surface 133 of the sintered magnet 130' is ground and the machining allowance 135 is removed, so that part of the radius 131 is also removed by grinding, shortening the arc of the radius 131' (reducing the central angle) and the radius 131' and the machined surface 133' do not connect smoothly. This reduces the effect of preventing chipping and cracking.

(2) Molding die of the present invention Next, the molding die of the present invention will be described. Fig. 1 is a partial cross-sectional view showing a schematic example of a corner portion 12 of a molding die 10 of the present invention used for molding a sintered magnet. The molding die 10 of the present invention is characterized in that a corner radius 11 is provided at the corner portion 12 that corresponds to a corner portion 22 of a molded body 20 (see Fig. 2) formed by molding, and in a cross section perpendicular to the ridge line direction of the corner portion 12, the corner radius 11 comprises an arcuate curved portion 11a and a straight portion 11b that connects to an end of the arcuate curved portion 11a.

Figure 2 is a partial cross-sectional view showing a corner portion 22 of a molded body 20 formed using this molding die 10. The molded body 20 formed using the molding die 10 has a corner radius 21 formed at the corner portion 22 corresponding to the corner portion 12 of the molding die 10. In a cross section perpendicular to the ridge direction of the corner portion 22, the corner radius 21 consists of an arc-shaped curved portion 21a and a straight portion 21b corresponding to the arc-shaped curved portion 11a and straight portion 11b of the molding die 10.

Figures 3(a) to 3(c) are partial cross-sectional views showing the corner portion 32 of the sintered body 30 obtained by sintering the molded body 20. As in the case of the conventional example described above, the sintered body 30 does not strictly have the same shape as the molded body 20 due to shrinkage caused by sintering, but for simplicity, it will be described here as having substantially the same shape as the molded body 20. As with the molded body 20, the sintered body 30 has a corner radius 31 consisting of an arc-shaped curved portion 31a and a straight portion 31b connected to the end of the arc-shaped curved portion 31a at the corner portion 32 in a cross section perpendicular to the ridge line direction of the corner portion 32. That is, the sintered body 30 has a corner radius 31 at the corner portion 32 corresponding to the corner portion 12 of the molding die 10, and the corner radius 31 consists of an arc-shaped curved portion 31a and a straight portion 31b corresponding to the arc-shaped curved portion 11a and the straight portion 11b of the molding die 10 in a cross section perpendicular to the ridge line direction of the corner portion 32.

As shown in FIG. 3(a), the surface 33 of the sintered body 30 that connects to the straight portion 31b of the corner portion 32 is ground to the position L, and the machining allowance 35 including the straight portion 31b is removed. This grinding process results in a sintered magnet 30' that has a corner radius 31' consisting of only the arc-shaped curved portion 31a and a ground surface 33' that is formed by removing the machining allowance 35 (see FIG. 3(b)). Here, the corner radius 31' (arc-shaped curved portion 31a) is not removed by grinding, and the shape formed by the molding die 10 remains as it is, so that a sintered magnet 30' with the desired corner radius 31' can be obtained.

(a) Configuration of Corner Radius The corner radius 11 formed in the corner portion 12 of the molding die 10 consists, in a cross section perpendicular to the ridgeline direction of the corner portion, of an arcuate curved portion 11a and a straight line portion 11b connected to the end of the arcuate curved portion 11a. The arcuate curved portion 11a forms the curved surface portion of the corner radius 11, and in a cross section perpendicular to the ridgeline direction of the corner portion 12, it is preferable that the shape is an arc with a radius r and a central angle θ, although this is not limited thereto. The radius r is set appropriately depending on the size and intended use of the final sintered magnet, and is not particularly limited thereto. It is preferable to set the central angle θ (deg) in accordance with the angle α (deg) between the two faces connecting to the corner portion 12, and it is preferable that 0.5×(180-α)≦θ<(180-α), more preferably that 0.7×(180-α)≦θ<(180-α), and most preferably that 0.9×(180-α)≦θ<(180-α).

The straight line portion 11b is formed on at least one of the two surfaces 13 that intersect at the corner portion 12 so as to connect the arc-shaped curved portion 11a and the one of the surfaces 13. In this way, by using a molding die 10 in which the arc-shaped curved portion 11a and the straight line portion 11b are formed at the corner portion 12, a molded body and a sintered body having an arc-shaped curved portion and a straight line portion at the corresponding corner portion can be obtained. At this time, the straight line portion 11b of the molding die 10 is provided so that the straight line portion 31b is formed on the side of the surface 33 to be ground out of the two surfaces 33, 34 that connect to the corner portion 32 of the sintered body 30. When only one surface 33 of the two surfaces 33, 34 that connect to the corner portion 32 of the sintered body 30 is ground out, the straight line portion 11b that connects to the arc-shaped curved portion 11a may be formed only on the side of the surface 13 of the molding die 10 that corresponds to it. Furthermore, when grinding both faces 33, 34 that connect to the corner portion 32 of the sintered body 30, a straight line portion that connects to the arc-shaped curved portion is formed so as to connect to both faces 13, 14 that intersect at the corner portion 12 of the molding die 10.

Straight line portion 11b is preferably set so that, when a connection point between arcuate curved portion 11a and straight line portion 11b is defined as point _P1 in a cross section perpendicular to the ridge direction of corner portion 12, a tangent to arcuate curved portion 11a at point _P1 coincides with (overlaps with) straight line portion 11b. In other words, corner portion 12 of molding die 10 is preferably formed so that arcuate curved portion 11a and straight line portion 11b are smoothly connected.

The reason why it is preferable to make the tangent (direction) at point _P1 of the arc-shaped curved portion 11b coincide with the straight portion 11b (direction) is explained below. The molding die of the present invention is used for molding sintered magnets, so the molded body molded by the molding die is then sintered to become a sintered body. Generally, the molded body shrinks by sintering, but the shrinkage rate is not necessarily constant, so a certain side surface is ground to a certain extent by processing to produce a sintered magnet product of a certain dimension. At this time, the thickness of the processing allowance removed by grinding varies depending on _the individual variation, so for example, as shown in FIG. 4, grinding may be performed to position _L1 beyond position L of point _P3 , or grinding may be completed to position _L2 just before position L of point P3. If grinding is completed to position _L2 just before position L of point _P3 , a part of straight portion 31b will remain, including the connection portion (point _P3 portion) between arc-shaped curved portion 31a and straight portion 31b. At this time, if the tangent direction at point _P1 of the arc-shaped curved portion 11b of the molding die 10 does not coincide with the direction of the straight portion 11b and the angle between the straight portion 11b and the processing surface is too small, the connection portion (point _P3 ) between the arc-shaped curved portion 31a and the straight portion 31b that remains unground and the area of the straight portion 31b will be large, which will be visible and may result in poor appearance of the product, which is not preferable.

However, the tangent line at point _P1 of arc-shaped curved portion 11b and straight portion 11b do not need to be exactly the same as long as the above-mentioned problems do not occur, and there is essentially no problem if it is within a range of ±2° in a cross section perpendicular to the ridge direction of corner portion 12. If it is more than +2° (if the part of the molded body at point _P2 has a convex shape), it may cause chipping of the molded body and sintered body, and if it is less than -2° (if the part of the molded body at point _P2 has a concave shape), it may cause cracks in the molded body.

(b) Overall configuration of the molding die The molding die of the present invention may be a compression molding die or an injection molding die. In the case of compression molding, dry molding and wet molding are performed. The outline of the molding die will be described below using the compression molding die as an example.

An example of a compression molding die for producing sintered magnets is shown in Figures 5(a) and 5(b). The molding die 50 has a die 51, a lower punch 52, and an upper punch 53, and the space surrounded by these constitutes a cavity 54. As shown in Figure 5(b), the cavity 54 has a corner radius 55 formed at the corner. Although not shown, in the molding die of the present invention, this corner radius 55 consists of an arc-shaped curved portion and a straight portion connected to the end of the arc-shaped curved portion. The lower punch 52 and the upper punch 53 are configured to be able to move up and down independently. For example, with the upper punch 53 removed upward, a predetermined amount of sintered magnet fine powder or slurry is filled into the cavity 54, and the upper punch 53 is lowered to perform compression molding. At this time, by applying a magnetic field to the magnetic powder or slurry in the cavity 54 while performing compression molding, an anisotropic magnet in which the magnetic powder is oriented in a certain direction can be obtained.

[2] Method for manufacturing a sintered magnet The method for manufacturing a sintered magnet of the present invention includes molding a powder for sintered magnet using a molding die to produce a green body, sintering the green body to produce a sintered body, and grinding a predetermined side surface of the sintered body to manufacture a sintered magnet,
The molding die has a corner portion having a rounded corner corresponding to a corner portion of the molded body, and in a cross section perpendicular to a ridge line direction of the corner portion,
The corner radius is composed of an arcuate curved portion and a straight line portion connected to an end of the arcuate curved portion,
the sintered body has corners that correspond to corners of the molding die,
In a cross section perpendicular to the ridge direction of the corner portion,
the corner radius is composed of an arcuate curved portion and a straight portion corresponding to the arcuate curved portion and the straight portion of the molding die,
The linear portion of the radiused corner of the sintered body and the side surface connected to the linear portion are removed by grinding, thereby forming a sintered magnet having a radiused corner consisting of the arc-shaped curved portion.

The sintered magnets that can be produced by the method of the present invention can be any magnet that can be produced by a process that involves molding a slurry of magnet powder (also called magnetic powder) or magnet powder dispersed in water or an organic solvent, and sintering the resulting molded body. However, rare earth sintered magnets or ferrite sintered magnets are particularly preferable.

(1) Molding process Molding is performed by compression molding (dry molding, wet molding) or injection molding. In the dry molding method, for example, dried magnetic powder is pressure molded while a magnetic field is applied to form a molded body. In the wet molding method, for example, a slurry containing magnetic powder is pressure molded while a magnetic field is applied to remove the liquid component to form a molded body. In the case of injection molding, the magnetic powder is heated and kneaded with a binder resin, and the formed pellets are injection molded in a mold to which a magnetic field is applied to obtain a premolded body, and the premolded body is subjected to a binder removal process to obtain a molded body.

In the case of compression molding, the fine powder or slurry for sintered magnets is filled into the molding die of the present invention described above and compression molded. The compression molding pressure is preferably 30 to 392 MPa (500 to 4,000 kg/ ^cm2 ) for RTB sintered magnet green bodies, and 34 to 44 MPa (350 to 450 kg/ ^cm2 ) for ferrite magnet green bodies. In the case of RTB sintered magnet green bodies, the density of the green bodies is preferably adjusted to about 3.7 to 4.7 g/ ^cm3 . In the case of ferrite magnet green bodies, the density of the green bodies is preferably adjusted to about 2.6 to 3.2 g/ ^cm3 .

(2) Sintering Step The compact obtained in the molding step is sintered to obtain a sintered body.

(3) Processing step In the method of the present invention, a predetermined side surface of the sintered body is removed by grinding. The molding die is provided with a corner radius consisting of an arc-shaped curved portion and a straight portion connected to the end of the arc-shaped curved portion in a cross section perpendicular to the ridge direction of the corner portion, and a corner radius consisting of an arc-shaped curved portion and a straight portion connected to the end of the arc-shaped curved portion is formed in the corner portions of the molded body and sintered body corresponding to the corner portions of the molding die in a cross section perpendicular to the ridge direction of the corner portion. In the processing step, the straight portion of the corner radius of the sintered body is removed by grinding to form a sintered magnet having a corner radius consisting of an arc-shaped curved portion.

As shown in Fig. 3(a), ideally, grinding should be performed up to position L of point _P3 . However, as described above, the thickness of the machining allowance removed by grinding may differ depending on the individual variation of the sintered body, and for example, as shown in Fig. 4, grinding may be performed beyond position L of point _P3 to position _L1 , or grinding may be completed up to position _L2 just before position L of point _P3 . Any case is included in the method of the present invention.

(4) Other processes
In the case of RTB sintered magnets, diffusion or heat treatment is usually carried out between the sintering step and the processing step, and a surface treatment is carried out after the processing step.

[3] Sintered magnet As shown in FIG. 3(c), the sintered magnet 30' of the present invention obtained by the above-mentioned method has an unmachined corner 31a formed at the corner and two faces 33', 34 connected to the said corner. At least one of the two faces 33' is a machined face. The angle α (deg) between the two faces and the central angle θ (deg) when the corner is approximated by a circular arc are
0.5×(180-α)≦θ＜(180-α)
It is characterized by satisfying the following. By configuring the corners of the sintered magnet in this way, it is possible to effectively prevent chipping or cracking during handling. It is more preferable that the angle α (deg) and the central angle θ (deg) satisfy 0.7×(180-α)≦θ<(180-α), and most preferably 0.9×(180-α)≦θ<(180-α).

The sintered magnets may be either rare earth sintered magnets or ferrite sintered magnets. The composition of each magnet is explained below.

(a) Rare Earth Sintered Magnet The rare earth sintered magnet is preferably substantially composed of RTB. R is at least one rare earth element including Y, and preferably always includes at least one of Nd, Dy, and Pr. T is at least one transition metal, and preferably always includes Fe. The composition is preferably R: 24-34 mass%, B: 0.6-1.8 mass%, and the remainder Fe. If R is less than 24 mass%, the residual magnetic flux density B _r and the coercive force H _cj decrease. If it exceeds 34%, the residual magnetic flux density B _r decreases. In addition, the area of the rare earth-rich phase inside the sintered body increases, and the morphology becomes coarse, resulting in a decrease in corrosion resistance. If B is less than 0.6 mass%, the amount of B required to form the main phase R ₂ Fe ₁₄ B phase is insufficient, and the R ₂ Fe ₁₇ phase, which has soft magnetic properties, is generated, resulting in a decrease in coercive force. On the other hand, if the B content exceeds 1.8 mass%, the non-magnetic B-rich phase increases, and the residual magnetic flux density B _r decreases. Fe may be partially replaced by Co, and may contain elements such as Al, Si, Cu, Ga, Nb, Mo, W, and Zr in amounts of up to about 3 mass%.

(b) Sintered Ferrite Magnet The sintered ferrite magnet is preferably made of Sr ferrite having a magnetoplumbite (M-type) structure, Ba ferrite, Sr-La-Co based ferrite, Ca-La-Co based ferrite, or the like.

10...Molding die
11... Corner radius
11a...Circular curved part
11b...Straight section
12... Corner part
13, 14... sides
20...Molded body
21...Rectangular
21a...Circular curved part
21b...Straight section
22... Corner part
30...Sintered body
31...Rectangular
31a...Circular curved part
31b...Straight section
32... Corner part
33, 34...faces
35... Machining allowance
30'...sintered magnet
31'・・・R
33'...Processed surface
50...Molding die
51... Die
52...lower punch
53...upper punch
54...Cavity
55... Corner radius
110...Molding die
111... Corner radius
112... Corner part
113,114...faces
130...Sintered body
131... Corner R
133,134...faces
135... Machining allowance
130'...sintered magnet
131'・・・Rectangular
133'...Processed surface

Claims (8)

A molding die for use in molding a sintered magnet, comprising:
a corner radius is provided at a corner portion corresponding to a corner portion of a molded body molded by the molding die,
In a cross section perpendicular to the ridge direction of the corner portion,
A molding die, wherein the corner radius comprises an arcuate curved portion and a straight portion connected to an end of the arcuate curved portion. In the molding die according to claim 1,
When the connection point between the arcuate curved portion and the straight portion is defined as point _P1 ,
A molding die, characterized in that a tangent to the arc-shaped curved portion at point _P1 coincides with the straight line portion. A method for producing a sintered magnet, comprising the steps of: molding fine powder for sintered magnets using a molding die to produce a green body; sintering the green body to produce a sintered body; and grinding a predetermined side surface of the sintered body by machining,
The molding die has a corner portion having a rounded corner corresponding to a corner portion of the molded body, and in a cross section perpendicular to a ridge line direction of the corner portion,
The corner radius is composed of an arcuate curved portion and a straight line portion connected to an end of the arcuate curved portion,
The sintered body has corners that correspond to corners of the molding die, and the corners are rounded.
In a cross section perpendicular to the ridge direction of the corner portion,
the corner radius is composed of an arcuate curved portion and a straight portion corresponding to the arcuate curved portion and the straight portion of the molding die,
a sintered magnet having a radius of curvature consisting of a circular arc-shaped curved portion, the sintered body having a radius of curvature and a side surface connected to the straight portion being removed by grinding. The method for producing a sintered magnet according to claim 3,
When a connection point between the circular arc portion and the straight portion of the rounded corner of the sintered body is designated as point _P3 ,
A method for producing a sintered magnet, wherein a tangent to the circularly arcuate curved portion at point _P3 coincides with the straight line portion. The method for producing a sintered magnet according to claim 3 or 4,
4. The method for producing a sintered magnet, wherein the sintered magnet is a rare earth sintered magnet or a ferrite sintered magnet. A sintered magnet having an unprocessed corner radius formed at a corner portion and two surfaces connected to the corner radius,
At least one of the two surfaces is a machined surface;
The angle α (deg) between the two faces and the central angle θ (deg) when the radius is approximated by a circular arc are
0.5×(180-α)≦θ＜(180-α)
A sintered magnet characterized by satisfying the above requirements. The sintered magnet according to claim 6,
The angle α (deg) and the central angle θ (deg),
0.7×(180-α)≦θ＜(180-α)
A sintered magnet characterized by satisfying the above requirements. The sintered magnet according to claim 6 or 7,
The sintered magnet is a rare earth sintered magnet or a ferrite sintered magnet.

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