Molding die, method for manufacturing sintered magnet, and sintered magnet

Abstract

To provide a mold for molding which can obtain a sintered magnet having corner radius that can sufficiently exhibit the effect of preventing chipping and cracking even after grinding the sintered body, a method for manufacturing a sintered magnet using the mold for molding, and a sintered magnet having a corner radius capable of sufficiently exhibiting the effect of preventing chipping and cracking.SOLUTION: A mold for molding for use in molding a sintered magnet is characterized in that, a corner radius is provided at a corner portion corresponding to a corner portion of a molded body molded by the mold for molding, and in a cross section orthogonal to the ridgeline direction of the corner portion the corner radius includes an arc-shaped curved portion and a straight portion connected to an end of the arc-shaped curved portion.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a molding die, a method for producing a sintered magnet using the molding die, and a sintered magnet. [Background technology]

[0002] 2. Description of the Related Art Sintered rare earth magnets and sintered ferrite magnets are widely used in a variety of applications, including rotating machines such as motors and generators.

[0003] Known examples of rare earth sintered magnets include RTB 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 examples of sintered ferrite magnets include sintered magnets such as hexagonal M-type (magnetoplumbite type) Sr ferrite and Ba ferrite (Patent Document 2).

[0004] These sintered magnets are obtained by molding magnetic powder into a predetermined shape and sintering it. Known molding methods include a dry molding method (e.g., Patent Document 3 (JP Patent Publication No. 2004-296849)) and a wet molding method (e.g., Patent Document 4 (JP Patent No. 3833861)). In the dry molding method, dried magnetic powder is pressure molded in an applied magnetic field to produce a molded body, and the resulting molded body is fired. In the wet molding method, a slurry containing magnetic powder is pressure molded while removing liquid components in an applied magnetic field to produce a molded body, and the resulting molded body is fired.

[0005] For example, a sintered ferrite magnet can be 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) coarsely pulverizing the calcined body, adding compounds such as sintering aids, and wet-pulverizing the resulting calcined body; (d) molding a slurry of fine particles of the resulting calcined body in a magnetic field to obtain a green body; (e) sintering the green body; and (f) processing the resulting sintered body into a shape suitable for its intended use.

[0006] 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., so-called radius chamfer. Usually, such radius chamfers (hereinafter also referred to as "corner radius") are formed by providing a corner of a molding die with a circular arc shape corresponding to the corner radius. However, the corner radius formed at the corner of the compact by molding using a die having the circular arc corner radius shape has a desired (ideal) arc shape before processing (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 formed corner radius is also removed by grinding, and the shape of the corner radius becomes different from the desired shape. Therefore, the obtained sintered magnets are not sufficiently effective in preventing chipping or cracking.

[0007] Patent Document 5 (JP Patent Publication 2016-188402) discloses a press device for molding 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 forming 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 molding part having a length Ld and a width Wd (Ld>Wd) and a tapered part formed on the upper side of the molding 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).

[0008] The pressing device and method described in Patent Document 5 aim to prevent cracks from occurring horizontally from the lower part of the fringe part of the molded body when the molded body is removed from the pressing device by optimizing the shape of the tapered portion, and are not intended to fully utilize the effect of the corner radius.

[0009] Patent Document 6 (JP 2020-39251 A) discloses a method for manufacturing a sintered magnet for motors, 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 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.

[0010] The method described in Patent Document 6 is intended to improve the yield of magnetic materials and shorten the grinding process time without reducing motor performance, and is not intended to fully utilize the effects of the radiused corners. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Re-tabled publication 2014 / 027638 [Patent Document 2] JP 2011-216857 A [Patent Document 3] JP 2004-296849 A [Patent Document 4] Patent No. 3833861 [Patent Document 5] JP 2016-188402 A [Patent Document 6] JP 2020-39251 A Summary of the Invention [Problem to be solved by the invention]

[0012] Therefore, an object of the present invention is to provide a molding die that can produce a sintered magnet having a corner radius that can fully prevent chipping and cracking even after the sintered body is ground, a method for producing a sintered magnet using the molding die, and a sintered magnet having a corner radius that can fully prevent chipping and cracking. [Means for solving the problem]

[0013] As a result of careful investigation in light of the above-mentioned object, the inventors have discovered that, in order to ensure that the corners of a sintered magnet are formed 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 can be formed at the corners of a molded body, and during processing, the arc-shaped curved portion can be left unground and only the portion equivalent to the straight portion can be ground, thereby leaving all or most of the arc-shaped curved portion unground, thereby producing a sintered magnet with a corner radius of the desired arc shape, and have arrived at the present invention.

[0014] 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.

[0015] The connecting point between the arcuate curved portion and the straight line portion is a point P 1 When Point P of the circular arc curve 1 It is preferable that a tangent line at the point coincides with the straight line portion.

[0016] 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.

[0017] In the production method of the present invention, The connecting portion between the arc-shaped curved portion and the straight portion of the rounded corner of the sintered body is defined as point P 3 When Point P of the circular arc curve3 It is preferable that a tangent line at the point coincides with the straight line portion.

[0018] In the production method of the present invention, The sintered magnet is preferably a rare earth sintered magnet or a ferrite sintered magnet.

[0019] 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 surfaces, and The central angle θ (deg) when the radius is approximated by a circular arc is 0.5×(180-α)≦θ<(180-α) The present invention is characterized in that:

[0020] 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.

[0021] The sintered magnet is preferably a rare earth sintered magnet or a ferrite sintered magnet. Effect of the Invention

[0022] According to the present invention, a sintered magnet having a desired, ideal arc-shaped corner radius can be obtained even after the side surface of the sintered body is ground, thereby effectively preventing chipping or cracking of the sintered magnet. [Brief description of the drawings]

[0023] [Figure 1] 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. [Diagram 2] 2 is a partial cross-sectional view of a molded product formed using the molding die shown in FIG. 1. [Figure 3(a)] 3 is a partial cross-sectional view of a sintered body obtained by sintering the compact shown in FIG. 2. [Figure 3(b)] FIG. 4 is a partial cross-sectional view of a sintered magnet obtained by grinding the sintered body shown in FIG. [Figure 3(c)] This is a partial cross-sectional view of FIG. 3(b) with an explanation of the radius angle added. [Figure 4] FIG. 4 is a schematic cross-sectional view for explaining a grinding position on a side surface of a sintered body. [Figure 5(a)] FIG. 2 is a schematic cross-sectional view showing an example of a compression molding die. [Figure 5(b)] FIG. 5(a) is a cross-sectional view taken along line AA of FIG. [Figure 6] 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. [Figure 7(a)] 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. [Figure 7(b)] FIG. 8 is a partial cross-sectional view of a sintered magnet obtained by grinding the sintered body shown in FIG. 7(a). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] [1] Molding mold (1) Conventional molding dies 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. The conventional molding die 110 is provided with a corner radius 111 having a desired arc shape at a corner portion 112 corresponding to the corner portion of a molded body (not shown) so that the corner portion (edge) 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.

[0025] Here, the desired arc-shaped corner radius formed on the corners of the molded product refers to an arc shape (as viewed in cross section in the ridge line direction) that is formed so that two adjacent faces that intersect at the corner (ridge) 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.

[0026] 6, corner radius 111 formed in molding die 110 has an arc shape formed so as to smoothly connect to two faces 113, 114 that intersect at corner portion 112. When the angle between faces 113 and 114 of molding die 110 is α (90° in the figure), it is preferable to form corner radius 111 as an arc with a central angle θ (=180-α).

[0027] In this application, a corner portion refers to a convex portion (also called a "ridge" or "convex ridge") formed by the intersection of two adjacent surfaces, and a corner portion refers to a concave portion (also called 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 are smoothly connected, and a corner radius refers to a curved portion formed by smoothly connecting two adjacent surfaces that intersect at a corner portion (concave ridge).

[0028] A green body molded using such a molding die 110 has a corner radius corresponding to the corner radius 111, and a 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 although the sintered body 130 does 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, for simplicity in this application, it will be described as having substantially the same shape as the green body.

[0029] One surface 133 connected to the corner radius 131 of this sintered body 130 is L 0By grinding to this position (see FIG. 7(a)), a sintered magnet 130' is obtained that has a machined surface 133', an unmachined surface 134, and a radiused corner 131' connected to them (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 radiused corner 131 is also removed by grinding, shortening the arc of the radiused corner 131' (reducing the central angle) and the radiused corner 131' and the machined surface 133' do not connect smoothly. This reduces the effectiveness of preventing chipping and cracking.

[0030] (2) The 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 corresponding 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 a circular arc-shaped curved portion 11a and a straight portion 11b that connects to an end of the circular arc-shaped curved portion 11a.

[0031] 2 is a partial cross-sectional view showing a corner portion 22 of a molded product 20 formed using this molding die 10. Molded product 20 formed using molding die 10 has a corner radius 21 formed at corner portion 22 corresponding to corner portion 12 of molding die 10. Corner radius 21 is made up of an arc-shaped curved portion 21a and a straight portion 21b corresponding to arc-shaped curved portion 11a and straight portion 11b of molding die 10 in a cross section perpendicular to the ridge direction of corner portion 22.

[0032] 3(a) to 3(c) are partial cross-sectional views showing a corner portion 32 of a 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 have the same shape as the molded body 20 strictly due to shrinkage caused by sintering, but for simplicity, the sintered body 30 is described as having substantially the same shape as the molded body 20. As in the case of 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 an 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, like the molded body 20. 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.

[0033] As shown in Fig. 3(a), the surface 33 of the obtained sintered body 30 connected to the straight portion 31b of the corner portion 32 is ground to position L, and the machining allowance 35 including the straight portion 31b is removed. This grinding process results in a sintered magnet 30' having a corner radius 31' consisting of only the arc-shaped curved portion 31a and a ground surface 33' formed by removing the machining allowance 35 (see Fig. 3(b)). Here, the corner radius 31' (the 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 the sintered magnet 30' can have the desired corner radius 31'.

[0034] (a) Corner radius configuration The corner radius 11 formed in the corner portion 12 of the molding die 10 is composed of an arc-shaped curved portion 11a and a straight portion 11b connected to the end of the arc-shaped curved portion 11a in a cross section perpendicular to the ridgeline direction of the corner portion. The arc-shaped curved portion 11a forms a curved surface portion of the corner radius 11, and is preferably, but not limited to, an arc shape with a radius r and a central angle θ in a cross section perpendicular to the ridgeline direction of the corner portion 12. The radius r is appropriately set depending on the size and intended use of the final sintered magnet, and is not particularly limited. The central angle θ (deg) is preferably set according to the angle α (deg) between the two faces connecting to the corner portion 12, and is preferably 0.5×(180-α)≦θ<(180-α), more preferably 0.7×(180-α)≦θ<(180-α), and most preferably 0.9×(180-α)≦θ<(180-α).

[0035] The straight line portion 11b is formed on at least one of the two surfaces 13 intersecting 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 the 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 connected to the corner portion 32 of the sintered body 30. When only one of the surfaces 33, 34 connected to the corner portion 32 of the sintered body 30 is ground out, the straight line portion 11b connected to the arc-shaped curved portion 11a may be formed only on the side of the surface 13 of the molding die 10 corresponding to the surface 13. Furthermore, when grinding both surfaces 33, 34 connected to the corner portion 32 of the sintered body 30, a straight line portion connecting to the arc-shaped curved portion is formed so as to connect to both surfaces 13, 14 that intersect at the corner portion 12 of the molding die 10.

[0036] In a cross section perpendicular to the ridge direction of the corner portion 12, the straight line portion 11b is connected to the arc-shaped curved portion 11a at a point P 1 Then, point P of the circular arc curved portion 11a 1It is preferable to set the tangent at the corner 12 so that it coincides with (overlaps with) the straight line portion 11b. In other words, it is preferable to form the corner portion 12 of the molding die 10 so that the arc-shaped curved portion 11a and the straight line portion 11b are smoothly connected.

[0037] Point P of the circular arc curved portion 11b 1 The reason why it is preferable to make the tangent (direction) at and the straight line portion 11b (direction) coincide with each other will be explained below. The molding die of the present invention is used for molding sintered magnets, and the compact molded by the molding die is then sintered to become a sintered body. Generally, the compact shrinks when sintered, but the shrinkage rate is not necessarily constant, so a certain side surface is ground to a certain extent to produce a sintered magnet product of specified dimensions. At this time, the thickness of the machining allowance removed by grinding varies depending on the individual product, so for example, as shown in Figure 4, at point P 3 beyond position L to position L 1 In some cases, grinding is performed up to point P 3 Position L just before position L 2 In some cases, grinding is completed at point P. 3 Position L just before position L 2 When grinding is completed at the point P 3 At this time, a part of the straight line portion 31b including the point P 1 If the tangent direction at the point P1 does not coincide with the direction of the straight line portion 11b and the angle between the straight line portion 11b and the processed surface is too small, the connecting portion (point P2) between the arc-shaped curved portion 31a and the straight line portion 31b that remains unground will be 3 Since the area of ​​the portion (a) and the straight portion 31b is large, they are visible and may result in poor appearance of the product, which is not preferable.

[0038] However, point P of the circular arc curved portion 11b 1 The tangent line at the corner portion 12 and the straight line portion 11b do not need to be exactly the same as each other as long as the above-mentioned problems do not occur. In a cross section perpendicular to the ridge line direction of the corner portion 12, if the difference is within ±2°, there is practically no problem. 2When the angle is smaller than -2° (when the point P of the compact is convex), it may cause chipping of the green body and sintered body. 2 If the part is concave, this may cause cracks in the molded product.

[0039] (b) Overall structure 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.

[0040] An example of a compression molding die for producing a sintered magnet is shown in Fig. 5(a) and Fig. 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 Fig. 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, the corner radius 55 is composed 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.

[0041] [2] Manufacturing method of sintered magnets The method of the present invention for producing a sintered magnet includes 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 to produce 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.

[0042] The sintered magnet that can be produced by the method of the present invention can be any magnet that can be produced by a process of molding magnet powder (also called magnetic powder) or a slurry obtained by dispersing magnet powder in water or an organic solvent, and sintering the resulting molded body. However, rare earth sintered magnets or ferrite sintered magnets are particularly preferable.

[0043] (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.

[0044] 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 30 to 392 MPa (500 to 4,000 kg / cm) in the case of RTB sintered magnet green bodies. 2 ) for ferrite magnet compacts, and 34 to 44 MPa (350 to 450 kg / cm 2 In the case of a green compact for an RTB sintered magnet, the density of the green compact is preferably 3.7 to 4.7 g / cm. 3In the case of a ferrite magnet compact, the density of the compact is preferably adjusted to about 2.6 to 3.2 g / cm. 3 It is preferable to adjust it to a certain extent.

[0045] (2) Sintering process The compact obtained in the compacting step is sintered to obtain a sintered body.

[0046] (3) Processing process 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 an 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 an end of the arc-shaped curved portion is formed in the corner portions of the compact 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.

[0047] Grinding is performed at point P as shown in Fig. 3(a). 3 In the ideal case, however, the thickness of the machining allowance removed by grinding may differ depending on the individual sintered body, as described above. For example, as shown in FIG. 4, 3 beyond position L to position L 1 In some cases, grinding is performed up to point P 3 Position L just before position L 2 In some cases, grinding may be completed up to this point, but any such case is included in the method of the present invention.

[0048] (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.

[0049] [3] Sintered magnets As shown in FIG. 3(c), the sintered magnet 30' of the present invention obtained by the above-mentioned method has an unmachined corner radius 31a formed at the corner portion, and two faces 33', 34 connected to the said corner radius, of which at least one face 33' is a machined face, and the angle α (deg) between the two faces and the central angle θ (deg) when the corner radius 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).

[0050] The sintered magnet may be either a rare earth sintered magnet or a ferrite sintered magnet. The composition of each magnet will be described below.

[0051] (a) Rare earth sintered magnet The rare earth sintered magnet preferably consists essentially of RTB. R is at least one rare earth element including Y, and preferably always contains at least one of Nd, Dy, and Pr. T is at least one transition metal, and preferably always contains Fe. The composition is preferably R: 24-34 mass%, B: 0.6-1.8 mass%, and the remainder Fe. When R is less than 24 mass%, the residual magnetic flux density B r Coercive force H cj At more than 34%, the residual magnetic flux density B r In addition, the area of ​​the rare earth-rich phase in the sintered body increases, and the morphology becomes coarse, resulting in a decrease in corrosion resistance. 2 Fe 14 The amount of B required to form the B phase is insufficient, resulting in soft magnetic properties. 2 Fe 17 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 rA part of Fe may be substituted with Co, and may contain about 3 mass % or less of elements such as Al, Si, Cu, Ga, Nb, Mo, W, and Zr.

[0052] (b) Sintered ferrite magnet The sintered ferrite magnet is preferably made of Sr ferrite, Ba ferrite, Sr-La-Co based ferrite, Ca-La-Co based ferrite, or the like having a magnetoplumbite (M-type) structure. [Explanation of symbols]

[0053] 10...Mold for molding 11 Corner radius 11a...Circular curved part 11b...Straight section 12...corner part 13,14...plane 20...Molded body 21... Corner radius 21a...Circular curved part 21b...Straight section 22...corner part 30...Sintered body 31... Corner radius 31a...Circular curved part 31b...Straight section 32...corner part 33,34...plane 35 Machining allowance 30'···Sintered magnet 31' Radius 33'...Processed surface 50...Molding mold 51 ···Die 52 Lower punch 53 Upper punch 54 Cavity 55···Corner radius 110...Mold for molding 111... Corner radius 112...corner part 113,114...planes 130 Sintered body 131... Corner radius 133,134...planes 135 Machining allowance 130'···Sintered magnet 131' Radius 133'... Machined surface

Claims

1. 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.

2. In the molding die according to claim 1, The connecting point between the arcuate curved portion and the straight line portion is a point P 1 When Point P of the circular arc curve 1 a tangent line in the mold coincides with the straight line portion.

3. 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.

4. The method for producing a sintered magnet according to claim 3, The connecting portion between the arc-shaped curved portion and the straight portion of the rounded corner of the sintered body is defined as point P 3 When Point P of the circular arc curve 3 a tangent line in the sintered magnet coincides with the straight line portion.

5. 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.

6. 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 surfaces, and The central angle θ (deg) when the radius of curvature is approximated by a circular arc is 0.5×(180-α)≦θ<(180-α) A sintered magnet characterized by satisfying the above requirements.

7. 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.

8. 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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