R-t-b sintered magnet

Abstract

An R-T-B sintered magnet according to the present disclosure comprises: a magnet body having a first surface, a second surface on the side opposite from the first surface, and a through-opening that reaches the second surface from the first surface; and a slit defined by two opposing inner wall surfaces of the through-opening. In a plan view seen from the normal direction of the first surface, the slit includes a first portion extending in a first direction and a second portion extending in a second direction different from the first direction and connected to the first portion.

Description

RTB system sintered magnet

[0001] This application relates to R-T-B type sintered magnets.

[0002] R-T-B sintered magnets (where R is at least one rare earth element, T is mainly iron, and B is boron) are R ２ Fe １４ It is composed of a main phase of a compound having a type B crystal structure, a grain boundary phase located at the grain boundaries of this main phase, and a compound phase formed by the influence of trace added elements and impurities. R-T-B type sintered magnets have a high residual magnetic flux density B ｒ (Hereafter, simply referred to as "B") ｒ (Sometimes it is written as ") and high coercivity H ｃＪ (Hereafter, simply "H ｃＪ (Sometimes described as "R-T-B") exhibits excellent magnetic properties and is known as the highest-performing magnet among permanent magnets. For this reason, R-T-B sintered magnets are used in various motors in fields such as automobiles (EVs, HVs, PHVs), renewable energy (wind power generation, etc.), home appliances, and industrial equipment.

[0003] Such R-T-B type sintered magnets are manufactured, for example, through a process of preparing alloy powder, press-molding the alloy powder to produce a powder molded body, and sintering the powder molded body.

[0004] The sintered body obtained by the process of sintering a powder molded body is then subjected to mechanical processing such as grinding and cutting to separate it into pieces of the desired shape and size. More specifically, first, a molded body larger than the final magnet product is produced by compression molding R-Fe-B type rare earth magnet powder with a press device. Then, after the molded body is sintered in the sintering process, the sintered body is ground using, for example, a cemented carbide blade saw or a rotary grinding wheel to give it the desired shape. For example, a sintered body with a block shape is first produced, and then multiple plate-shaped sintered body portions are cut out by slicing the sintered body with a blade saw or the like.

[0005] However, sintered bodies of rare-earth alloy magnets, such as R-Fe-B sintered magnets, are extremely hard and brittle, and the processing load is high, making high-precision grinding a difficult and time-consuming task. In addition, material loss is inevitable during processing. For this reason, the processing steps have been a major cause of increased manufacturing costs.

[0006] Patent Document 1 describes a method for processing molded articles that does not require the preparation of an inert atmosphere with controlled oxygen concentration, by submerging the powder molded article in a liquid and cutting it with a moving wire.

[0007] On the other hand, when R-T-B sintered magnets are used, for example, in the rotor of a motor, it is necessary to reduce losses caused by eddy currents. It is known that providing slits in R-T-B sintered magnets is preferable to suppress the generation of eddy currents (Patent Documents 2-5).

[0008] Japanese Patent Publication No. 2022-054683, Japanese Patent Publication No. 2018-157667, Japanese Patent Publication No. 2000-295804, Japanese Patent Publication No. 2007-053351, Japanese Patent Publication No. 2007-305818

[0009] As mentioned above, R-Fe-B sintered magnets are extremely hard and brittle, and the processing load is high, making high-precision grinding difficult. Furthermore, the high processing load makes it difficult to form deep slits. For this reason, R-Fe-B sintered magnets are provided with slits of a simple shape. As a result of the inventor's investigation, it was found that conventional slits with simple shapes do not necessarily have a high effect in suppressing eddy currents.

[0010] This disclosure provides an R-T-B type sintered magnet that can solve the above problems.

[0011] This application discloses an R-T-B type sintered magnet as described in the following items.

[0012] [Item 1] An R-T-B sintered magnet (where R is a rare earth element and always includes at least one selected from the group consisting of Nd, Pr and Ce, T is at least one transition metal and always includes Fe, and B is boron), comprising: a magnet body portion having a first surface, a second surface opposite to the first surface, and a through opening extending from the first surface to the second surface; and a slit defined by two opposing inner wall surfaces of the through opening, wherein in a plan view taken from the direction normal to the first surface, the slit includes a first portion extending in a first direction and a second portion extending in a second direction different from the first direction and connected to the first portion.

[0013] [Item 2] The R-T-B sintered magnet according to Item 1, wherein in the plan view, one end of the first portion of the slit and one end of the second portion are connected to form an L-shaped bend.

[0014] [Item 3] The R-T-B sintered magnet according to Item 2, wherein in a plan view, the slit includes a third portion extending in the first direction and connected to the second portion, and one end of the third portion and the other end of the second portion are connected to form another L-shaped bend.

[0015] [Item 4] The R-T-B sintered magnet according to Item 2, wherein in a plan view, the slit includes a third portion extending in the first direction and connected to the second portion, and one end of the third portion is connected to a part of the second portion to form a T-shaped bend.

[0016] [Item 5] The R-T-B sintered magnet according to Item 1, wherein in the plan view, one end of the first portion of the slit and a part of the second portion are connected to form a T-shaped bend.

[0017] [Item 6] The R-T-B sintered magnet according to Item 5, wherein in a plan view, the slit includes a third portion extending in the first direction and connected to the second portion, and one end of the third portion is connected to another part of the second portion to form another T-shaped bend.

[0018] [Item 7] The R-T-B sintered magnet according to any one of items 1 to 6, wherein the minimum width of the slit is 350 μm or less.

[0019] [Item 8] The R-T-B sintered magnet described in Item 7, wherein the minimum width of the slit is 200 μm or less.

[0020] [Item 9] The R-T-B type sintered magnet according to any one of items 1 to 8, wherein the inner wall surface of the through-opening of the magnet body portion is an unprocessed sintered surface.

[0021] According to this disclosure, it is possible to provide an R-Fe-B sintered magnet having a slit shape that has a superior eddy current suppression effect compared to conventional slits with simple shapes.

[0022] Figure 1 is a schematic perspective view showing a basic example of an R-T-B sintered magnet with a slit formed therein. Figure 2 is a schematic diagram showing an example of the shape of the slit in an R-T-B sintered magnet in an embodiment. Figure 3 is a schematic diagram showing another example of the shape of the slit in an R-T-B sintered magnet in an embodiment. Figure 4 is a schematic diagram showing yet another example of the shape of the slit in an R-T-B sintered magnet in an embodiment. Figure 5 is a schematic diagram showing yet another example of the shape of the slit in an R-T-B sintered magnet in an embodiment. Figure 6 is a schematic diagram showing yet another example of the shape of the slit in an R-T-B sintered magnet in an embodiment. Figure 7 is a flowchart showing the main steps of the manufacturing method of an R-T-B sintered magnet in an embodiment. Figure 8 is a schematic perspective view showing the configuration of a wire saw device that may be used in an embodiment. Figure 9A is a front view illustrating the process of cutting a powder molded body submerged in liquid with a wire. Figure 9B is a front view illustrating the process of cutting a powder molded body submerged in a liquid with a wire. Figure 10A is a side view illustrating the process of cutting a powder molded body submerged in a liquid with a wire. Figure 10B is a side view illustrating the process of cutting a powder molded body submerged in a liquid with a wire. Figure 10C is a side view illustrating the process of cutting a powder molded body submerged in a liquid with a wire. Figure 11A is a cross-sectional view showing an example of a wire formed from metal stranded wire. Figure 11B is a side view showing an example of a wire formed from metal stranded wire. Figure 12 is a schematic perspective view illustrating an example of the overall configuration of a slit formed in a powder molded body in an embodiment. Figure 13 is a schematic perspective view illustrating an example of the shape of a slit appearing on the surface of an R-T-B sintered magnet in an embodiment. Figure 14 is a schematic perspective view illustrating a plurality of molded body pieces formed from a powder molded body having slits. Figures 15(a) to (j) are diagrams schematically illustrating the process of forming slits in a powder molded body. Figure 16 schematically shows yet another example of an R-T-B sintered magnet in which a slit having a T-shaped bend is formed. Figure 17 schematically shows an example of an R-T-B sintered magnet in which a slit having multiple T-shaped bends is formed.Figure 18 schematically shows another example of an R-T-B sintered magnet with a slit having a T-shaped bend. Figure 19 schematically shows another example of an R-T-B sintered magnet with a slit having multiple T-shaped bends. Figure 20 schematically shows an example of an R-T-B sintered magnet with a slit having multiple L-shaped bends. Figure 21 schematically shows yet another example of an R-T-B sintered magnet with a slit having multiple T-shaped bends. Figure 22 is a schematic perspective view showing the external shape of an R-T-B sintered magnet in an embodiment. Figure 23 is a graph showing the relationship between the eddy current loss reduction effect (vertical axis) and the volume reduction rate due to the slit (horizontal axis) calculated by simulation for Embodiments 1 and 2. Figure 24 schematically shows a reference example of an R-T-B sintered magnet with a slit formed on one side. Figure 25 is a schematic diagram showing another reference example of an R-T-B type sintered magnet with slits formed on both sides. Figure 26A is a photograph of the R-T-B type sintered magnet of sample No. 1. Figure 26B is a photograph of the R-T-B type sintered magnet of sample No. 2. Figure 26C is a photograph of the R-T-B type sintered magnet of sample No. 3. Figure 26D is a photograph of the R-T-B type sintered magnet of sample No. 4. Figure 26E is a photograph of the R-T-B type sintered magnet of sample No. 5. Figure 27 is a graph showing the relationship between the eddy current loss measured in the R-T-B type sintered magnet and frequency.

[0023] <R-T-B Sintered Magnet> According to embodiments of the present disclosure, the following R-T-B sintered magnet is provided. Here, R is a rare earth element and must include at least one selected from the group consisting of Nd, Pr, and Ce. T is at least one transition metal and must include Fe. B is boron.

[0024] First, a basic configuration example of the R-T-B sintered magnet in this embodiment will be described with reference to Figures 1 to 6. For reference, the figures show mutually orthogonal X, Y, and Z axes. In this example, the XY plane is horizontal, and the Z axis is oriented vertically. However, this does not in any way limit the orientation of the R-T-B sintered magnet when it is used in this embodiment.

[0025] The R-T-B sintered magnet 12 shown in Figure 1 has a magnet body portion 13 that is roughly rectangular in shape and has a slit 15. The shape of the magnet body portion 13 shown is merely an example, and the shape of the magnet body portion 13 is arbitrary. The magnet body portion 13 in Figure 1 has a first surface 13A on the front, a second surface 13B on the opposite side (back) from the first surface 13A, a third surface 13C on the top, and a fourth surface 13D on the bottom. The magnet body portion 13 also has a through opening 14 that extends from the first surface 13A to the second surface 13B. The slit 15 is defined by two opposing inner wall surfaces 14A and 14B of the through opening 14. In other words, the slit 15 is the gap between the inner wall surfaces 14A and 14B. The width W of the slit 15 is the distance between the two opposing inner wall surfaces 14A and 14B, and the length L of the slit 15 is the distance from the first surface 13A to the second surface 13B. Thus, the slit 15 of the R-T-B sintered magnet 12 corresponds to the through-opening 14 formed in the magnet body portion 13, so without distinguishing between the two, the "through-opening" may be referred to as the "slit" below.

[0026] In the example shown in Figure 1, the depth D of the slit 15 is the distance from the third surface (top surface) 13C to the end of the slit 15. The distance R from the fourth surface (bottom surface) 13D to the slit 15 corresponds to the thickness of the area where no slit is formed. For this reason, the distance R may be called the "non-slit area thickness". If the non-slit area thickness R is less than 4 mm, for example, the strength will decrease and cracks or chips may occur. For this reason, it is preferable that the non-slit area thickness R is 4 mm or more.

[0027] As will be described later, the slit 15 can be formed by moving a wire, for example, running horizontally (in the X-axis direction) in an arbitrary cutting direction perpendicular to the direction of travel (for example, in the Z-axis direction, Y-axis direction, or other directions) to a powder molded body submerged in a liquid. Since the slit formed in the powder molded body remains as a slit even after sintering, the term "slit" is used to refer to the same part (through-opening) in both the powder molded body and the R-T-B type sintered magnet which is its sintered body.

[0028] In the example shown in Figure 1, the slit 15 is formed by cutting a wire into the upper surface of the powder molded body before sintering. In the example in Figure 1, the direction of the cut is initially in the negative direction of the Z axis, but changes to the positive direction of the Y axis midway through the cut. As will be described later, by performing a slit formation process using a moving wire on the powder molded body (Green Compact) before it hardens into a sintered body, slits of various shapes can be realized. Furthermore, as a result of the inventor's research, it was found that the slits formed in the powder molded body remain in the sintered magnet during the sintering process, exhibiting an effect of reducing losses due to eddy currents.

[0029] According to this embodiment, in a plan view (hereinafter simply referred to as "plan view") taken from the normal direction of the first surface 13A, the slit 15 includes a first portion 15a extending in a first direction (the Z-axis direction in the example of Figure 2) and a second portion 15b extending in a second direction different from the first direction (the Y-axis direction in the example of Figure 2) and connected to the first portion 15a. In the example of Figure 2, the first and second directions are orthogonal, but these directions only need to intersect and do not necessarily need to be orthogonal.

[0030] In the example of FIG. 2, in a plan view, one end of the first portion 15a of the slit 15 and one end of the second portion 15b are connected to form an L-shaped bent portion. By providing at least one slit 15 having such a bent portion in the R-T-B sintered magnet 12, it is possible to enhance the effect of reducing the loss due to eddy current as compared with the case of providing a conventional straight slit. The reason for this is considered to be that by bending the slit 15 within the R-T-B sintered magnet 12, it is possible to effectively divide the region where eddy current is generated using the slit 15 having the same volume and increase the resistance to eddy current.

[0031] The shape of the slit 15 is not limited to the shape having an L-shaped bent portion as shown in FIGS. 1 and 2. For example, as shown in FIG. 3, in a plan view, one end of the first portion 15a of the slit 15 and a part of the second portion 15b may be connected to form a T-shaped bent portion.

[0032] Furthermore, the slit 15 may include a third portion 15c that extends in the first direction (the Z-axis direction in the example of the figure) and is connected to the second portion 15b in a plan view. For example, as shown in FIG. 4, in a plan view, one end of the third portion 15c of the slit 15 and the other end of the second portion 15b may be connected to form another L-shaped bent portion. Also, as shown in FIG. 5, in a plan view, one end of the third portion 15c of the slit 15 and a part of the second portion 15b may be connected to form a T-shaped bent portion. In this example, the slit 15 has an L-shaped bent portion and a T-shaped bent portion. As shown in FIG. 6, in a plan view, one end of the third portion 15c of the slit 15 and another part of the second portion 15b may be connected to form another T-shaped bent portion. In this example, the slit 15 has a plurality of T-shaped bent portions. The number of T-shaped bent portions may be three or more, or the orientation of the T may be reversed. According to the embodiment of the present disclosure, various combinations of the illustrated L-shaped bent portion and / or T-shaped bent portion are possible, and it is also possible to realize a slit having a complex shape by combining other bent portions.

[0033] Although the angle (bending angle) of the bent portion shown in FIGS. 2 to 6 is 90°, the bending angle is not limited to 90°. Also, each part of the slit 15 does not have to be linear and may be curved. The "bent portion" may also include a portion where the direction of the slit 15 changes gradually rather than discontinuously.

[0034] Thus, according to this embodiment, in a plan view, different from a linear slit extending straight, various shapes of slits can be realized because, as described above, a slit forming step using a wire is performed on a powder compact (green compact) before it becomes a hard sintered body.

[0035] The width W of the slit 15 formed in the powder compact depends on the diameter of the wire used in the slit forming step performed on the powder compact. Also, in the sintering step performed after the slit forming step, due to the sintering shrinkage of the powder compact that occurs during heat treatment, the width W of the slit 15 decreases by, for example, about 10%. According to the study of the present inventor, the minimum value of the width W of the slit 15 in the state of the R-T-B-based sintered magnet is preferably 350 μm or less, and more preferably 200 μm or less. Therefore, it is preferable to perform the slit forming step so that the minimum value of the width W of the slit 15 after sintering becomes a desired value.

[0036] When a slit (through-opening) is formed in the R-T-B-based sintered magnet by mechanical processing or discharge plasma processing, etc., the inner wall surface in the through-opening naturally constitutes the processed surface. In such a processed surface, the magnetic properties deteriorate. In particular, when a slit (through-opening) is formed in the R-T-B-based sintered magnet using discharge plasma processing, it is considered that the two inner wall surfaces facing each other in the through-opening are melted and solidified to become a region that does not function properly as a magnet. On the other hand, according to the embodiment of the present disclosure, since the through-opening 14 defining the slit 15 is formed in the powder compact before sintering, the inner wall surfaces 14A, 14B in the through-opening 14 of the R-T-B-based sintered magnet are unprocessed sintered surfaces. Therefore, even if a slit 15 having a complex bending pattern is provided in the R-T-B-based sintered magnet, deterioration of the magnetic properties due to processing does not occur.

[0037] <Method for Manufacturing R-T-B Sintered Magnets> Next, an outline of the method for manufacturing R-T-B sintered magnets according to this disclosure will be described. The method for manufacturing R-T-B sintered magnets in this embodiment includes the following steps, as shown in the flowchart of Figure 7: - A molding step (S10) to produce a powder molded body from powder of an R-T-B sintered magnet alloy (R is a rare earth element and must contain at least one selected from the group consisting of Nd, Pr and Ce, T is at least one transition metal and must contain Fe, and B is boron); - A slit forming step (S20) to form at least one slit in the powder molded body; and - A sintering step (S30) to sinter the powder molded body with the slit formed thereon.

[0038] Furthermore, the slit formation step (S20) includes a step (S25) in which a slit is formed in the powder molded body submerged in a liquid by moving a wire that travels horizontally in an arbitrary cutting direction perpendicular to the direction of travel.

[0039] The following describes examples of the configuration of an apparatus that can be used in the slit formation process (S20), and a basic example of the process of forming slits in a powder molded body, with reference to Figures 8 to 11B.

[0040] First, with reference to Figure 8, an example of the configuration of a wire saw apparatus that can be used to manufacture R-T-B sintered magnets in this embodiment will be described. Figure 8 is a schematic perspective view showing an example of the configuration of the wire saw apparatus 100.

[0041] The wire saw device 100 in Figure 8 has rollers 30a, 30b, and 30c arranged so that their rotational axes are parallel to each other, and a single continuous wire 40. The wire 40 is a metal wire, and no abrasive grains are fixed to its surface. A single metal wire may be used as the metal wire, or a stranded metal wire made by twisting multiple wires together may be used.

[0042] Each of the rollers 30a to 30c is rotatably supported by a support device 50. The rotation axes of the rollers 30a to 30c are parallel to the Y-axis. The rotation of the rollers 30a to 30c causes the wire 40 to run under tension. The wire 40 is wound onto a bobbin or the like (not shown). The wire saw device 100 may also be equipped with other rollers for adjusting the tension, etc. The tension of the wire 40 can be set to, for example, 40N or more.

[0043] During slit formation, rollers 30a, 30b, 30c and the retrieval bobbin rotate. The direction of rotation of rollers 30a, 30b, and 30c depends on their arrangement and how the wire 40 is wound. In the wire saw device 100 shown in the illustration, rollers 30a, 30b, and 30c rotate in the same direction. Once a predetermined length of wire 40 has been wound onto one of the retrieval bobbins to a length of, for example, 250 m or more, the retrieval bobbin and rollers 30a, 30b, and 30c are rotated in opposite directions. This causes the wire 40 to move in the opposite direction, and by repeating this process, the wire 40 can be made to reciprocate (move).

[0044] A specific example of the process for producing the powder molded body 10 will be described later. It should be noted that the powder molded body 10 is not a sintered body, but a molded body of powder before sintering (green compact). The powder molded body is obtained by molding powder of R-T-B type sintered magnet alloy in an oriented magnetic field using a wet press or a dry press.

[0045] In the example shown in Figure 8, the portion of the wire 40 located between rollers 30a and 30b contacts the powder molded body 10. The support device 50 has a shape that allows it to move in the Y-axis direction without interfering with the powder molded body 10 when the wire 40 running between rollers 30a and 30b is forming a slit in the powder molded body 10. In the example shown in Figure 8, the support device 50 has an opening 51 that allows the powder molded body 10 to move in the Y-axis direction. Specifically, rollers 30a and 30b are located on both sides of the opening 51 of the support device 50. The support device 50 illustrated in Figure 8 has a roughly U-shape or C-shape that defines the opening 51. The size (width) of the opening 51 in the X-axis direction is larger than the size (width) of the powder molded body 10 in the X-axis direction. Note that the R-T-B sintered magnet in this disclosure may also be manufactured by a wire saw device having a configuration different from that of the wire saw device 100 illustrated in Figure 8.

[0046] The powder molded body 10 produced in the molding process (S10) is fixed to a fixing base 20 by a clamp (not shown) and placed inside a tank 62 that stores liquid 60. In Figure 8, the tank 62 is shown by a dashed line, and the surface height of the liquid 60 is shown by a dotted line. In the example in Figure 8, the entire powder molded body 10 is immersed in the liquid 60.

[0047] The direction in which the wire 40 travels when it contacts the powder molded body 10 (hereinafter sometimes simply referred to as the "wire travel direction") is parallel to the X-axis.

[0048] The wire saw device 100 in this embodiment includes a drive device 70 that moves the relative position of the powder molded body 10 with respect to the wire 40 in the vertical direction (Z-axis direction) and the horizontal direction (Y-axis direction). In the example in Figure 4, the drive device 70 includes a support stage 72 on which the powder molded body 10 is placed, a Z-axis drive unit 74 configured to reciprocate the support stage 72 in the Z-axis direction, and a Y-axis drive unit 76 configured to reciprocate the support stage 72 in the Y-axis direction. The Z-axis drive unit 74 and the Y-axis drive unit 76 each have actuators such as motors. These actuators can move the support stage 72 and the powder molded body 10 fixed to the support stage 72 in response to drive signals from the control device. In a plan view of the powder molded body 10 from a direction parallel to the wire running direction (X-axis direction), the position of the powder molded body 10 can be defined by coordinates on the YZ coordinate system.

[0049] By moving the powder molded body 10 using the Z-axis drive unit 74 and the Y-axis drive unit 76 while the wire 40 is running, the wire 40 can be moved relative to the powder molded body 10 in any direction perpendicular to the running direction (hereinafter referred to as the "cutting direction"). In particular, by adjusting the movement speed in the Z-axis direction by the Z-axis drive unit 74 and the movement speed in the Y-axis direction by the Y-axis drive unit 76, the cutting direction of the wire 40 can be freely changed.

[0050] In the above example, the position of the wire 40 relative to the YZ coordinate system is fixed, and the powder molded body 10 is in a movable state. However, alternatively, the position of the powder molded body 10 may be fixed, and the position of the wire 40 relative to the YZ coordinate system may be movable. In this case, the support device 50 is driven to move in the Y-axis direction and the Z-axis direction. Alternatively, for example, the support device 50 may move in the Y-axis direction, and the powder molded body 10 may move in the Z-axis direction. With the above configuration, in a plan view of the powder molded body 10 from a direction parallel to the wire travel direction (X-axis direction), the position of the wire 40 relative to the powder molded body 10 (coordinates in the YZ coordinate system) can be moved in any direction.

[0051] In the following, prioritizing clarity, the details of the slit formation process will be explained using an example where the relative position of the wire 40 with respect to the fixed powder molded body 10 is changed.

[0052] First, refer to Figures 9A and 9B. In the following description, the process of forming slits in the powder molded body 10 is performed by a wire saw device 100, which is schematically shown. Figures 9A and 9B are front views illustrating the process of forming slits in the powder molded body 10, which is submerged in liquid 60, using a wire 40. Figure 9A shows the state before the slit formation process begins, and Figure 9B shows the state during the slit formation process. The dashed lines within the powder molded body 10 shown in Figure 9B schematically indicate the position of the wire 40 when forming slits in the powder molded body 10.

[0053] In the illustrated example, the wire 40 can move in a direction perpendicular to the direction of travel of the wire 40 (any direction in the YZ plane) while traveling at a predetermined speed in the X-axis direction. In the example shown in Figure 9B, the traveling wire 40 is shown moving, for example, in the negative direction of the Z axis relative to the stationary powder molded body 10, but as mentioned above, the powder molded body 10 may be lifted in the positive direction of the Z axis together with the fixing base 20.

[0054] As the moving wire 40 cuts into the powder molded body 10, and the wire 40 is moved in the cutting direction, the through-opening 14 (slit 15) in the powder molded body 10 shown in Figure 1 can be enlarged in the cutting direction. By changing the cutting direction midway through the cutting process, a bent portion can be formed in the through-opening 14 (slit 15).

[0055] Figures 10A, 10B, and 10C are side views illustrating the process of forming slits in a powder molded body 10 submerged in liquid 60 using a wire 40. Figure 10A shows the state before slit formation begins, Figure 10B shows the state during slit formation, and Figure 10C shows the state after slit formation is completed.

[0056] In this embodiment, the slit formation process is performed with the powder molded body 10 submerged in the liquid 60. If the powder molded body 10 is a powder molded body formed by a wet press, a preferred example of the liquid 60 is an oil of the same type as the dispersion medium (mineral oil or synthetic oil) used in the wet press.

[0057] When a slit formation process is performed on a powder molded body 10 using such a wire saw device 100, the powder particles constituting the powder molded body 10 fall off as chips from the parts cut by the wire 40. These chips are powder particles that have fallen off the powder molded body 10, and the individual particles do not have rough fracture surfaces like metal chips (cutting shavings). The shape and size of the particles constituting the chips cut off by the wire from the powder molded body before sintering are the same as the shape and size of the powder particles used to manufacture the powder molded body 10, making them easy to recover and reuse. Furthermore, if the powder molded body 10 is manufactured by a wet press, if the wire sawing is performed in an oil of the same type as the dispersant, the recovered powder (chips) can be used directly in the wet press, increasing production efficiency.

[0058] Figure 10C schematically shows a slit 15 formed in a powder molded body 10. In this example, the slit 15 has an L-shaped bend formed by connecting one end of a first portion 15a and one end of a second portion 15b. The first portion 15a of the slit 15 is formed by cutting a wire 40 in the negative Z-axis direction from the starting position S of the cut. By changing the cutting direction of the wire 40 by 90° at a position 15M midway through the cut, a second portion 15b extending in the negative Y-axis direction from position 15M is formed. When the wire 40 reaches the end 15E of the slit 15, the cutting direction of the wire 40 reverses from the negative Y-axis direction to the positive Y-axis direction. After that, the wire 40 moves through the formed slit 15 in the opposite direction to when the slit was formed, returning to the starting position S and exiting the powder molded body 10. In this disclosure, the movement of the wire 40 through an already formed slit 15 is also included in "cutting". Therefore, the third portion of the slit 15 may be formed by changing the cutting direction while the wire 40 is returning from the end 15E of the slit 15 to the starting position S (in the middle of the cut).

[0059] The wire saw device 100 can not only form one slit 15 in the powder molded body 10, but also form multiple slits 15. In addition to forming at least one slit 15 in the powder molded body 10, it is also possible to cut the powder molded body 10 and divide it into multiple molded body pieces.

[0060] According to the embodiments of this disclosure, the slits 15 formed in the powder molded body 10 by the wire saw device 100 also exist as slits 15 in the final R-T-B sintered magnet.

[0061] Figure 11A is a cross-sectional view showing an example of a wire 40 made of stranded metal wire, and Figure 11B is a side view of the wire 40. In the illustrated example, no abrasive grains are fixed to the surface of the wire 40. The wire 40 may also be a metal strand (metal single wire) without abrasive grains fixed to its surface.

[0062] In the examples shown in Figures 11A and 11B, the wire 40 is made by twisting together seven single metal wires 40a, 40b, 40c, 40d, 40e, 40f, and 40g. These single metal wires 40a to 40g are all made from the same material and have the same diameter. However, the material and / or diameter of the central single metal wire (core wire) 40g does not need to be the same as the material and / or materials of the other single metal wires 40a to 40f. Also, each of the single metal wires 40a to 40g may be made from a different material and have a different diameter. The number of single metal wires constituting the metal strand of wire 40 is not limited to seven.

[0063] As shown in Figures 11A and 11B, the surface of the wire 40, which is made of stranded metal wire, has roughly spirally extending recesses and protrusions. In other words, the surface of the wire 40 has spirally extending grooves and ridges. The recesses (grooves) are formed between adjacent metal strands, and the protrusions (ridges) are the exposed surfaces of each metal strand.

[0064] According to the inventors' research, the high-speed liquid flow (jet flow) generated when the wire 40, which is made of stranded metal wire, travels through the liquid is thought to form a helical fluid flow that moves in the direction of the swirl axis while swirling. Therefore, as the travel speed of the wire 40 increases, the high-speed liquid flow (jet flow) scrapes off the powder particles constituting the powder molded body 10, and the scraped-off powder particles can be efficiently discharged to the outside of the cutting region by the helical swirling flow.

[0065] Suitable metal single wires for use in metal stranded wires include, for example, piano wire and high-tensile steel wire. The surface of each metal single wire or the surface of the metal stranded wire may be plated. The diameter D of the wire 40 is preferably in the range of 100 μm to 350 μm, and preferably in the range of 200 μm to 300 μm. If the diameter D of the wire 40 is less than 100 μm, the wire 40 may stretch during the slit formation process due to insufficient strength. While a larger diameter D of the wire 40 improves the discharge of cutting chips, it can increase the cutting allowance and reduce the number of molded pieces obtained, so it is desirable that the diameter be 350 μm or less.

[0066] The diameter D of the wire 40 determines the minimum width of the slit 15 in the final R-T-B sintered magnet. The width of the slit 15 in the R-T-B sintered magnet is the width of the slit 15 formed in the powder molded body that has been reduced by the sintering process. The width of the slit 15 formed in the powder molded body is influenced not only by the diameter D of the wire 40, but also by the deflection of the wire 40 that occurs during slit formation. Furthermore, the cutting speed of the wire 40 during the slit formation process may be zero, or the cutting direction may change. For this reason, even when using wires of the same diameter D, variations in the width W of the slit 15 may occur depending on its position. However, the minimum width W of the slit 15 is largely determined by the diameter D of the wire 40 used in the slit formation process and the shrinkage rate in the sintering process.

[0067] The pitch P of one single metal wire (e.g., single metal wire 40a) contained in the metal stranded wire making one turn around the axis of the wire 40 is preferably in the range of 5 to 15 times (e.g., 10 times) the diameter D of the wire 40. For example, when the diameter D is about 300 μm, the pitch P of turns is, for example, about 3 mm. If the pitch P relative to the diameter D falls outside the above range, the efficiency of the swirling flow formed by the spiral recesses and protrusions on the surface of the metal stranded wire in discharging cutting chips (chips) to the outside may decrease. Note that the above mechanism for cutting a powder molded body with a metal stranded wire cannot be realized in a gas, but is only possible in a liquid. Examples of liquids that can be used in the embodiments of this disclosure are oils such as mineral oil or synthetic oil. As will be described later, when a powder molded body is manufactured by a wet press, it is preferable to perform wire saw cutting in the same or the same type of liquid as the liquid (e.g., oil) mixed with the powder. This is because the liquids such as oils used in wet pressing have components and properties that have been confirmed to volatilize during the sintering process and not adversely affect the properties of the final sintered magnet.

[0068] The metal stranded wire constituting the wire 40 preferably used in the embodiments of this disclosure is made by twisting together N single metal wires (where N is an integer of 2 or more), but it is desirable that N is between 3 and 20 (for example, N = 5 to 8). Even when N is 2, it is possible to form slits in the powder molded body, but the ability to discharge the scraped powder particles is relatively small. When N exceeds 20, the depth of the spiral recesses and the height of the protrusions formed on the surface by twisting together a large number of single metal wires become smaller than the diameter D of the wire, so the ability to discharge the scraped powder particles is likely to decrease. The surface of the wire 40 may be treated in some way (surface treatment).

[0069] In the slit formation process, the running speed of the wire 40 is preferably 100 m / min or more, as this is a sufficient flow velocity necessary to form slits in the powder molded body. As the running speed of the wire increases, the velocity of the jet flow generated in the liquid also increases, thus increasing the efficiency of discharging the removed powder particles. In other words, clogging by so-called chips in the cutting process becomes less likely. For this reason, the running speed of the wire 40 is more preferably 300 m / min or more, and even more preferably 800 m / min or more (for example, 900 m / min). The running direction of the wire 40 can be maintained in one direction for as long as the metal wire travels 250 m or more. For example, when the wire is traveled 250 m at a running speed of 1000 m / min, the wire can be traveled in one direction for a period of 15 seconds or more. Switching the running direction of the wire 40 in a short time (for example, less than 15 seconds) will cause the running speed to become zero at the time of the switch, which will temporarily stop slit formation and reduce work efficiency. For this reason, it is preferable that the distance over which the wire 40 is traveled in one direction is 250 m or more. When using the stranded metal wire 40 in a liquid, even if the wire is continuously moved over a long distance in a constant direction, clogging with chips is unlikely to occur for the reasons mentioned above. Furthermore, after moving 250m or more, the metal wire may be moved another 250m or more in the opposite direction.

[0070] Furthermore, in the slit formation process, the cutting speed (slit formation speed or workpiece feed speed) in the direction perpendicular to the wire's running direction is preferably 300 mm / min or more. If the cutting speed is less than 300 mm / min, the time required for the slit formation process increases, reducing production efficiency. According to the embodiments of this disclosure, by increasing the wire's running speed, the removed powder particles can be discharged efficiently, thus suppressing wire deflection due to load even when the cutting speed is increased. As a result, the cut surface maintains the desired shape, and high dimensional accuracy can be achieved. However, in the embodiments of this disclosure, the cutting direction is changed during slit formation, so the cutting speed may be temporarily reduced or cutting may be stopped. For this reason, the above preferred range of cutting speed applies during the cutting operation, except when the cutting direction is changed or cutting is temporarily stopped.

[0071] Furthermore, by forming slits in the powder molded body in a liquid, the temperature rise due to frictional heat at the point of contact between the powder molded body and the wire saw is suppressed, and the generated heat is also more easily dissipated into the liquid. In the atmosphere, the powder molded body, which becomes hot due to the generated frictional heat, reacts with oxygen or water vapor in the atmosphere, leading to an increase in the oxygen concentration in the final sintered magnet and a deterioration of its magnetic properties. However, in this embodiment, such problems can be avoided.

[0072] Another advantage of forming slits in a powder molded body in a liquid is that the powder particles scraped off the powder molded body by the wire saw settle in the liquid, making them easy to recover. In a preferred embodiment, the step of preparing the powder molded body includes a step of forming the powder by wet pressing. In this case, it is desirable to add the same type of liquid used in the slitting step to the powder during the wet pressing. This is because it makes it easier to recover and reuse the powder particles scraped off the powder molded body by the slitting step.

[0073] Furthermore, even if the wire 40, which is made of stranded metal wire, is cut horizontally, a slit can be easily formed in the powder molded body if it is in a liquid. The surface of the powder molded body may have irregularities on at least a portion (for example, the top surface) depending on the powder pressing process, and it was necessary to cut or polish it by processing after the sintering process. According to the embodiment of this disclosure, such cutting or polishing steps can be eliminated, making it possible to reduce manufacturing costs while maintaining the properties of high-performance magnets.

[0074] Next, with reference to Figures 12 to 15, an example of a cutting method in the slit formation process will be explained.

[0075] Figure 12 is a schematic perspective view showing an example of the overall configuration of a slit 15 (through-opening 14) formed in the powder molded body 10 in this embodiment. For clarity, Figure 12 schematically shows the shape of the slit 15 so as to be transparent to the inside of the powder molded body 10. The slit 15 formed in the powder molded body is a void defined by two opposing inner wall surfaces of the through-opening formed in the powder molded body. However, for simplicity, these figures represent the slit 15 with surfaces or lines.

[0076] Figure 13 is a schematic perspective view showing an example of the shape of the slits that appear on the surface of an R-T-B type sintered magnet obtained by sintering a powder molded body 10 in which the slits 15 described above are formed.

[0077] Alternatively, the powder molded body 10 shown in Figure 12 may be divided into multiple molded body pieces using a wire saw device 100 before the sintering process is carried out.

[0078] Figure 14 is a schematic perspective view showing four molded body pieces 11 formed from a powder molded body having slits. The cutting process for dividing the powder molded body having slits into a plurality of molded body pieces 11 can be performed by running the wire in a direction different from the wire running direction when forming the slits 15 (the X-axis direction in Figure 14) (the Y-axis direction in Figure 14). This can be done by rotating the orientation of the powder molded body 10 by 90° relative to the wire saw device 100. Note that the cutting method for obtaining a plurality of molded body pieces 11 from a single powder molded body 10 is not limited to the example shown in Figure 14. In the example in Figure 14, the cutting surface is parallel to the YZ plane, but the cutting surface may include surfaces parallel to the XZ plane, XY plane, or other planes.

[0079] Next, the process of forming the slit 15 in the powder molded body 10 will be explained with reference to Figures 15(a) to (j). Figures 15(a) to (j) show the shape of the slit 15 formed in the powder molded body 10 in a plan view, indicated by dashed lines. In the figures, the thick solid arrows schematically indicate the cutting direction of the wire during slit formation and the slit 15 being formed. The dashed lines indicate the position (planned cutting path) where the slit 15 is formed.

[0080] First, as shown in Figure 15(a), the wire is used to make an incision from the center of the upper surface of the powder molded body 10. In this example, the starting position S of the incision is located on the upper surface of the powder molded body 10. However, the starting position S of the incision may be located on any other surface of the powder molded body 10. After the incision is made, the wire is moved in the incision direction, as shown in Figure 15(b). By changing the incision direction midway through, a bent portion can be formed in the slit 15. At the stage shown in Figure 15(b), two L-shaped bent portions are formed in the slit 15 (indicated by solid arrows).

[0081] As shown in Figure 15(c), the cutting direction of the wire is reversed, and the formed slit 15 is moved in the opposite direction. As shown in Figure 15(d), by changing the cutting direction in the middle of the slit 15, a T-shaped bend is formed that bends perpendicular to the slit 15.

[0082] Subsequently, by moving the wire while changing the cutting direction as shown in Figures 15(c), 15(d), 15(e), 15(f), 15(g), 15(h), 15(i), and 15(j), the wire can be returned to the starting position S of the cut, and the formation of the slit 15 can be completed.

[0083] The method of cutting that can be used to form the slit 15 with the shape shown in Figure 15(j) is not limited to the above example. Also, the slit 15 that can be formed in the powder molded body by the wire saw device is not necessarily limited to having an L-shaped bend and / or a T-shaped bend. For example, it is possible to form multiple parallel slits 15 by repeatedly reversing the cutting direction at the end of the slit 15.

[0084] The slit formation process allows for a high degree of freedom in the shape of the slits 15 that can be formed in the powder molded body 10. For example, an R-T-B type sintered magnet 12 can be obtained that has slits 15 of the shapes shown in Figures 16 to 21.

[0085] In the R-T-B sintered magnet 12 shown in Figures 16 and 17, the slit 15 has a portion that extends along the X-axis direction near the center of the R-T-B sintered magnet 12. Therefore, in the Z-axis direction of the figures, the conductive portion of the R-T-B sintered magnet 12 is roughly divided into two parts. Since the slit 15 functions as an insulating portion, the width of the eddy currents in the R-T-B sintered magnet 12, at least in the Z-axis direction, is reduced by about half, and the electrical resistance is thought to increase effectively.

[0086] Furthermore, eddy currents tend to concentrate at the tip of the slit 15, leading to losses. To reduce such losses, it is preferable to add a portion extending along the Z-axis to the tip of the slit 15 extending along the X-axis, as shown in Figures 16 and 17, thereby mitigating the concentration of eddy currents. In the examples of Figures 16 and 17, a T-shaped bend is provided at the tip of the slit 15 extending along the X-axis, but an L-shaped bend or other shaped portion may also be provided. In addition, to reduce the number of times the wire cutting direction is changed during the slit formation process, a bend may not be provided at the tip of the slit 15, as shown in Figure 18.

[0087] In the R-T-B sintered magnet 12 shown in Figures 19 and 20, the slit 15 has two portions that extend along the X-axis direction of the R-T-B sintered magnet 12. Therefore, in the Z-axis direction of the figure, the conductive portion of the R-T-B sintered magnet 12 is roughly divided into three parts. The width of the eddy currents in the R-T-B sintered magnet 12, at least in the Z-axis direction, is reduced to about 1 / 3, and it is thought that the electrical resistance effectively increases further.

[0088] The R-T-B sintered magnet 12 shown in Figure 21 has a slit 15 extending from the starting position S1 of the cut and a slit 15 extending from the starting position S2 of the cut. Thus, the slit 15 may be multiple slits extending from different starting positions of the cut. In the R-T-B sintered magnet 12 shown in Figure 21, the conductive part is divided into approximately three parts, so it is thought that the width of the eddy current in the Z-axis direction is at least about 1 / 3. In a plan view, the R-T-B sintered magnet 12 has slits 15 with T-shaped bends arranged symmetrically, but the arrangement of the slits 15 does not necessarily have to be symmetrical.

[0089] The manufacturing method of the R-T-B type sintered magnet of this embodiment will be described in more detail below.

[0090] S10: Molding Process In the molding process (S10), a powder molded body is produced from the powder of the R-T-B sintered magnet alloy. Here, the composition of the R-T-B sintered magnet alloy, the alloy manufacturing process, and the process of preparing the alloy powder will be explained in order.

[0091] <Composition of Rare Earth Alloy for R-T-B Sintered Magnets> R is a rare earth element and must contain at least one selected from the group consisting of Nd, Pr, and Ce. Preferably, a combination of rare earth elements represented by Nd-Dy, Nd-Tb, Nd-Dy-Tb, Nd-Pr-Dy, Nd-Pr-Tb, Nd-Pr-Dy-Tb, Nd-Ce-Dy, Nd-Ce-Tb, Nd-Ce-Dy-Tb, Nd-Pr-Ce-Dy, Nd-Pr-Ce-Tb, Nd-Pr-Ce-Dy-Tb is used.

[0092] Of R, Dy and Tb are particularly H ｃＪ This is effective in improving [the above]. In addition to the above elements, other rare earth elements such as La may be included, and mischmetal or didymium can also be used. Furthermore, R does not have to be a pure element, and may contain impurities that are unavoidable in manufacturing, within the range of industrially available materials. The content is, for example, 27% by mass or more and 35% by mass or less. Preferably, the R content of the R-T-B sintered magnet is 31% by mass or less (27% by mass or more and 31% by mass or less, preferably 29% by mass or more and 31% by mass or less). By setting the R content of the R-T-B sintered magnet to 31% by mass or less and the oxygen content to 500 ppm or more and 3500 ppm or less (preferably 500 ppm or more and 3200 ppm or less, more preferably 500 ppm or more and 2500 ppm or less), higher magnetic properties can be obtained.

[0093] T contains iron (including cases where T is substantially composed of iron), and may be replaced by cobalt (Co) at a mass ratio of 50% or less (including cases where T is substantially composed of iron and cobalt). Co is effective in improving temperature characteristics and corrosion resistance, and the alloy powder may contain 10% by mass or less of Co. The content of T may occupy the remainder of R and B, or R and B and M described later.

[0094] The content of B can also be a known content, for example, the range of 0.85% by mass to 1.2% by mass is preferred. If it is less than 0.85% by mass, high H ｃＪ may not be obtained, and if it exceeds 1.2% by mass, B ｒ may decrease. Note that part of B can be replaced by C (carbon).

[0095] In addition to the above elements, H ｃＪ For improvement, an M element can be added. The M element is one or more selected from the group consisting of Al, Si, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Zr, Nb, Mo, In, Sn, Hf, Ta and W. The addition amount of the M element is preferably 5.0% by mass or less. If it exceeds 5.0% by mass, B ｒ may decrease. Also, inevitable impurities can be tolerated.

[0096] The content of N (nitrogen) in the R-T-B system sintered magnet is preferably 50 ppm or more and 600 ppm or less. Also, the content of C (carbon) in the R-T-B system sintered magnet is preferably 50 ppm or more and 1000 ppm or less.

[0097] <Manufacturing process of alloy for R-T-B system sintered magnet> The manufacturing process of the alloy for the R-T-B system sintered magnet is exemplified. An alloy ingot can be obtained by an ingot casting method in which a metal or alloy preliminarily adjusted to have the above-described composition is melted and put into a mold. Also, the molten metal is brought into contact with a single roll, double roll, rotating disk or rotating cylindrical mold, etc. and rapidly cooled, and alloy flakes can be produced by a rapid cooling method typified by a strip casting method or a centrifugal casting method for producing a solidified alloy thinner than the alloy made by the ingot method.

[0098] In the embodiments of this disclosure, materials manufactured by either the ingot method or the rapid quenching method can be used, but it is preferable that they be manufactured by a rapid quenching method such as the strip casting method. The thickness of the rapidly quenched alloy produced by the rapid quenching method is usually in the range of 0.03 mm to 1 mm and is in flake shape. The molten alloy begins to solidify from the surface in contact with the cooling roll (roll contact surface), and the crystals grow columnar in the thickness direction from the roll contact surface. Compared to alloys (ingot alloys) produced by the conventional ingot casting method (die casting method), rapidly quenched alloys are cooled in a shorter time, resulting in a finer structure and smaller grain size. They also have a larger grain boundary area. The R-rich phase, which has more R than the target composition and readily bonds with hydrogen, spreads widely within the grain boundaries, so the rapid quenching method offers excellent dispersibility of the R-rich phase. For this reason, it is easy to fracture at the grain boundaries by the hydrogen pulverization method. By hydrogen pulverizing the rapidly quenched alloy, the size of the hydrogen pulverized powder (coarse pulverized powder) can be reduced to, for example, 1.0 mm or less. The coarsely ground powder obtained in this way is then finely ground, for example, using a jet mill.

[0099] <Process for preparing powder for R-T-B type sintered magnet alloy> Rare earth alloy powder for R-T-B type sintered magnets is active and easily oxidized. For this reason, inert gases such as nitrogen, argon, and helium are used as the gas in the jet mill to avoid the risk of heat generation and ignition, and to improve the performance of the magnet by reducing the oxygen content as an impurity.

[0100] The material to be ground (coarsely ground powder) fed into the jet mill is ground into fine powder with a particle size distribution of, for example, an average particle size (median diameter: d50) of 2.0 μm to 4.5 μm before being moved to the cyclone collection device. The cyclone collection device is used to separate the powder from the airflow carrying the powder. Specifically, coarsely ground powder of R-T-B sintered magnet alloy is ground in the preceding jet mill, and the fine powder generated by the grinding is supplied to the cyclone collection device along with the gas used for grinding. A mixture of inert gas (grinding gas) and the ground fine powder is sent to the cyclone collection device in a high-speed airflow. The cyclone collection device is used to separate these grinding gases from the fine powder. The fine powder separated from the grinding gas is recovered in a powder collector.

[0101] Next, a powder molded body is prepared using the powder obtained in the grinding process.

[0102] In this embodiment, a powder molded body is produced from the above powder by pressing in a magnetic field. In pressing in a magnetic field, it is preferable to form the powder molded body by pressing in an inert gas atmosphere or by wet pressing, from the viewpoint of suppressing oxidation. In particular, with wet pressing, the surface of the particles constituting the powder molded body is coated with a dispersant such as an oil, and contact with oxygen and water vapor in the atmosphere is suppressed. Therefore, oxidation of the particles by the atmosphere before, during, or after the pressing process can be prevented or suppressed.

[0103] When performing wet pressing in a magnetic field, a slurry is prepared by mixing fine powder with a dispersion medium, and this slurry is supplied to the cavity of the mold in the wet press apparatus and pressed in a magnetic field. The resulting powder molded body has a viscosity of, for example, 4 g / cm³. ３ 5g / cm or more ３ It has the following density:

[0104] • Dispersion medium: A dispersion medium is a liquid that can be used to obtain a slurry by dispersing alloy powder within it.

[0105] Preferred dispersion media for use in this disclosure include mineral oil or synthetic oil. While the type of mineral oil or synthetic oil is not specified, if the kinematic viscosity at room temperature exceeds 10 cSt, the increased viscosity strengthens the bonding force between alloy powders, which may adversely affect the orientation of the alloy powders during wet molding in a magnetic field. Therefore, the kinematic viscosity at room temperature of the mineral oil or synthetic oil is preferably 10 cSt or less. Furthermore, if the fractional distillation temperature of the mineral oil or synthetic oil exceeds 400°C, de-oiling after obtaining the molded body becomes difficult, and the amount of residual carbon in the sintered body increases, potentially reducing the magnetic properties. Therefore, the fractional distillation temperature of the mineral oil or synthetic oil is preferably 400°C or less. Additionally, vegetable oil may be used as the dispersion medium. Vegetable oil refers to oil extracted from plants, and the type of plant is not limited to a specific plant.

[0106] - Slurry preparation: A slurry can be obtained by mixing the obtained alloy powder with a dispersion medium.

[0107] The mixing ratio of the alloy powder and the dispersion medium is not particularly limited, but the concentration of the alloy powder in the slurry is preferably 70% or more by mass (i.e., 70% by mass or more). 20 to 600 cm ３ This is because, at a flow rate of 1 / second, the alloy powder can be efficiently supplied into the cavity, and excellent magnetic properties can be obtained. The concentration of alloy powder in the slurry is preferably 90% or less by mass ratio. The method of mixing the alloy powder and the dispersion medium is not particularly limited. The alloy powder and dispersion medium may be prepared separately, and the mixture may be produced by weighing predetermined amounts of both and mixing them. Alternatively, when obtaining alloy powder by dry grinding coarsely ground powder with a jet mill or the like, a container containing the dispersion medium may be placed at the alloy powder discharge port of the grinding device such as a jet mill, and the alloy powder obtained by grinding may be directly recovered into the dispersion medium in the container to obtain a slurry. In this case, it is preferable that the atmosphere inside the container is composed of nitrogen gas and / or argon gas, and that the obtained alloy powder is directly recovered into the dispersion medium without being exposed to the atmosphere to obtain a slurry. Furthermore, it is also possible to obtain a slurry consisting of alloy powder and dispersion medium by wet grinding using a vibratory mill, ball mill or attritor, etc., while the coarsely ground powder is held in the dispersion medium.

[0108] By molding the slurry obtained in this way with a known wet press, a powder molded body having a predetermined size and shape can be obtained. Conventionally, this powder molded body is usually sintered to obtain a sintered body, but in this embodiment, as will be explained below, at least one slit is formed in the powder molded body before sintering.

[0109] S20: Slit Forming Process In the slit forming process (S20), at least one slit is formed in the powder molded body using the wire saw device described above. This slit forming process includes a step (S25) in which at least one slit is formed in the powder molded body by moving a wire that travels horizontally in an arbitrary cutting direction perpendicular to the direction of travel with respect to the powder molded body submerged in liquid.

[0110] The slits to be formed may have the various shapes described above, or other shapes. Here, after forming the slits 15 as explained with reference to Figure 15, multiple molded pieces are produced from the powder-formed body as shown in Figure 14.

[0111] S30: Sintering process In the sintering process (S40), a sintered body is produced by sintering multiple molded body pieces. That is, an R-T-B type sintered magnet (sintered body) is obtained by sintering the individual molded body pieces cut by the slit formation process described above. The sintering process can be performed simultaneously on multiple molded body pieces. When the sintering of multiple molded body pieces is performed in the same sintering process, each molded body piece may be a molded body piece divided from a single powder molded body, or it may be a collection of molded body pieces obtained from different powder molded bodies. Note that some of the multiple molded body pieces may include molded body pieces obtained by cutting the powder molded body with a device different from the wire saw device in the slit formation process described above.

[0112] The sintering process is, for example, 0.13 Pa (10 －３ Torr) Less than or equal to, preferably 0.07 Pa (5.0 × 10⁻⁶ －４ The process can be carried out under a pressure of Torr or less, for example, at a temperature in the range of 1000°C to 1150°C. To prevent oxidation due to sintering, residual gas in the atmosphere can be replaced with an inert gas such as helium or argon. It is preferable to perform additional heat treatment, such as aging treatment, on the obtained sintered body. Such heat treatment can improve the magnetic properties. Known conditions can be used for heat treatment conditions such as heat treatment temperature and heat treatment time. The R-T-B sintered magnet thus obtained can be subjected to grinding and polishing, surface treatment, and magnetization processes as needed to complete the R-T-B sintered magnet with a slit.

[0113] In one preferred embodiment, the method for manufacturing an R-T-B sintered magnet according to the present disclosure further includes a diffusion step of diffusing a heavy rare earth element RH (RH being at least one of Tb, Dy, and Ho) from the surface to the interior of the sintered body. Diffusing the heavy rare earth element RH from the surface to the interior of the sintered body can efficiently increase the coercivity. The method of the diffusion step is not particularly limited; known methods can be employed.

[0114] The following describes examples of R-T-B type sintered magnets in this disclosure.

[0115] Figure 22 is a schematic perspective view showing the external shape of the R-T-B sintered magnet in the embodiment. In this embodiment, the size S in the X-axis direction Ｘ , size S in the Y-axis direction Ｙ , size S in the Z-axis direction Ｚ For a rectangular parallelepiped R-T-B sintered magnet having a slit, the effect of reducing eddy current loss by the slit was evaluated by simulation using commercially available electromagnetic field analysis software (JMAG®: manufactured by JSOL Corporation). The composition, density, magnetic properties, and electrical properties of the R-T-B sintered magnet are based on the values ​​of an R-T-B sintered magnet manufactured by the method described in the above embodiment.

[0116] In this example, an R-T-B sintered magnet (Example 1) having a slit 15 with the shape shown in Figure 18 and an R-T-B sintered magnet (Example 2) having a slit 15 with the shape shown in Figure 19 were evaluated. The size S of the R-T-B sintered magnets in Example 1 and Example 2. Ｘ S Ｙ S Ｚ The dimensions are 65.5 mm, 4.7 mm, and 14.6 mm, respectively, and the width of the slit 15 (the size indicated by "W" in Figure 1) is 250 μm.

[0117] In Example 1, the portion of the slit 15 extending along the X-axis direction was located in the center of the R-T-B sintered magnet in the Z-axis direction, and its length was varied to 30, 40, 50, and 60 mm. On the other hand, in Example 2, the portions of the slit 15 extending along the X-axis direction were arranged with a gap of 4.8 mm between them, flanking the center of the R-T-B sintered magnet in the Z-axis direction, and their respective lengths were varied to 30, 40, 50, and 60 mm. Since the slit 15 is an air gap in the R-T-B sintered magnet, the volume of the R-T-B sintered magnet decreases as the volume of the slit 15 increases. Therefore, the rate of volume reduction due to the slit increases in proportion to the length of the slit 15.

[0118] Figure 23 is a graph showing the relationship between the eddy current loss reduction effect (vertical axis) and the volume reduction rate due to the slit (horizontal axis), calculated by simulation, for Examples 1 and 2. The eddy current loss reduction effect is set to 0% when there is no slit and no reduction in eddy current loss, and to 100% when eddy current loss is completely eliminated. The values ​​for Example 1 are shown as white circles, and the values ​​for Example 2 are shown as black circles. The four points in each example show the values ​​when the length of the portion of the slit 15 extending along the X-axis is 30, 40, 50, and 60 mm.

[0119] In Example 1, an eddy current loss reduction effect of 48-74% was obtained, and in Example 2, an eddy current loss reduction effect of 50-80% was obtained.

[0120] According to the embodiment of the present invention, since the slit has portions in which it extends in different directions, it becomes possible to efficiently enhance the eddy current loss reduction effect.

[0121] Figures 24 and 25 schematically show reference examples of R-T-B sintered magnets with 15 slits 15 formed therein. The depth D of the slits 15 is 3.62 mm, and the size S Ｚ This corresponds to approximately 1 / 4 of the original volume. In the reference example, the width of the slit 15 was changed, and the same simulation as in the example was performed in the range of volume reduction rates from 0.6% to 1.7%. The volume reduction rate due to the slit 15 is about the same as the volume reduction rate due to the slit 15 in the example.

[0122] In the reference examples, the eddy current loss reduction effect of the slit was 40-47% in all cases.

[0123] Below, we fabricated several R-T-B sintered magnets and measured their eddy current losses.

[0124] Specifically, we prepared an R-T-B sintered magnet without a slit (Sample No. 1), an R-T-B sintered magnet made by dividing an R-T-B sintered magnet in half and joining the half-magnet pieces with an insulating adhesive layer (Sample No. 2), an R-T-B sintered magnet with a T-shaped slit as shown in Figure 18 (Sample No. 3), an R-T-B sintered magnet with overlapping T-shaped slits as shown in Figure 19 (Sample No. 4), and an R-T-B sintered magnet with two inverted T-shaped slits as shown in Figure 21 (Sample No. 5). All of the R-T-B sintered magnets had a size S in the X-axis direction. Ｘ 30.0 mm, size S in the Y-axis direction Ｙ 4.8 mm, size S in the Z-axis direction Ｚ It has a rectangular parallelepiped shape with dimensions of 14.7 mm. The electrical discharge machining speed was 2.3 mm / min. The width of the slits in samples No. 3-5 was approximately 0.44 mm, with the narrowest point being 0.39 mm and the widest point being 0.49 mm. Furthermore, comparing the total slit length of samples No. 2 and No. 3, sample No. 3 showed a smaller volume reduction rate.

[0125] Figures 26A, 26B, and 26C, and Figures 26D and 26E are photographs showing R-T-B sintered magnets of sample No. 1, sample No. 2, sample No. 3, sample No. 4, and sample No. 5, respectively.

[0126] Figure 27 is a graph showing the relationship between eddy current loss measured in these R-T-B sintered magnets and frequency. The vertical axis represents eddy current loss (unit: W / kg), and the horizontal axis represents the frequency of the AC magnetic field applied to the R-T-B sintered magnet (unit: Hz). In this graph, the values ​​for sample No. 1, sample No. 2, sample No. 3, sample No. 4, and sample No. 5 are indicated by black diamonds, black squares, white triangles, X marks, and white diamonds, respectively. There is no significant difference between the values ​​for sample No. 2 and sample No. 3, and the curves for sample No. 2 and sample No. 3 overlap in the graph. This indicates that, compared to sample No. 2, sample No. 3, with its bidirectional slits, achieved an eddy current loss reduction effect equivalent to that of a unidirectional slit, despite a smaller volume reduction rate. There is also no significant difference between the values ​​for sample No. 4 and sample No. 5, and the curves for sample No. 4 and sample No. 5 overlap in the graph.

[0127] The table below shows the relationship between the measured eddy current loss and frequency.

[0128]

[0129] As can be seen from Table 1 and the graph, the R-T-B sintered magnet without slits (Sample No. 1) exhibited the highest eddy current loss at all frequencies. On the other hand, the R-T-B sintered magnets with two T-shaped slits (Sample No. 4 and Sample No. 5) showed a significant reduction in eddy current loss regardless of the orientation of the two T-shaped slits. Sample No. 3, with only one slit, achieved a similar eddy current loss reduction effect to Sample No. 2, which was created by dividing a sintered magnet in half and joining the half-magnet pieces with an insulating adhesive layer.

[0130] The frequency dependence of eddy current losses was measured using an unmagnetized R-T-B sintered magnet sample by applying an alternating magnetic field. Specifically, the sample was placed in a C-type magnetizer with a 5 mm gap, and an alternating current of a predetermined frequency was passed through it using an excitation coil, thereby applying an alternating magnetic field with an alternating flux density of 0.05 T to the sample.

[0131] The AC magnetic field frequencies were set to 100 Hz, 300 Hz, 500 Hz, 1 kHz, 3 kHz, and 5 kHz. The input power supplied to the excitation coil was measured using a power meter at each frequency. The iron loss, including that of the sample, was calculated by subtracting the loss of the magnetizer itself and the copper loss of the excitation coil, which were measured in advance under the same magnetic flux density conditions, from the obtained input power.

[0132] Furthermore, the eddy current loss attributable to the sample was calculated by subtracting the hysteresis loss of the sample, determined by DC magnetic measurement, from the iron loss. The frequency dependence of the eddy current loss was evaluated by normalizing the obtained eddy current loss by the sample mass and organizing it as loss per unit mass (W / kg).

[0133] As described above, the slits in the R-T-B sintered magnets of the embodiments of this disclosure were formed by electrical discharge machining, and it was confirmed that these slits have a superior eddy current suppression effect compared to conventional slits with simple shapes. The method for forming the slits in the R-T-B sintered magnets of this disclosure is not limited to electrical discharge machining. As mentioned above, if the slits are formed by a moving wire on a powder molded body (green compact) before it becomes a hard sintered body, it is also possible to obtain further effects such as a reduction in processing costs and a reduction in raw material costs through recycling.

[0134] The R-T-B sintered magnets of this disclosure can be used as permanent magnets in a wide variety of applications, including motors for electric vehicles (EVs, HVs, PHVs), motors for industrial equipment, and home appliances.

[0135] 10... Powder molded body, 12... R-T-B sintered magnet, 13... Magnet body, 14... Through opening, 15... Slit, 20... Fixing base, 30a, 30b, 30c... Roller, 40... Wire, 50... Support device, 60... Liquid, 70... Tank, 100... Wire saw device

Claims

1. An R-T-B sintered magnet (where R is a rare earth element and always includes at least one selected from the group consisting of Nd, Pr and Ce, T is at least one transition metal and always includes Fe, and B is boron), comprising: a magnet body portion having a first surface, a second surface opposite to the first surface, and a through opening extending from the first surface to the second surface; and a slit defined by two opposing inner wall surfaces of the through opening, wherein in a plan view taken from the direction normal to the first surface, the slit includes a first portion extending in a first direction and a second portion extending in a second direction different from the first direction and connected to the first portion.

2. The R-T-B sintered magnet according to claim 1, wherein, in a plan view, one end of the first portion of the slit and one end of the second portion are connected to form an L-shaped bend.

3. In a plan view, the slit includes a third portion extending in the first direction and connected to the second portion, and one end of the third portion and the other end of the second portion are connected to form another L-shaped bend, as described in claim 2.

4. In a plan view, the slit includes a third portion extending in the first direction and connected to the second portion, and one end of the third portion is connected to a part of the second portion to form a T-shaped bend, as described in claim 2.

5. The R-T-B sintered magnet according to claim 1, wherein, in a plan view, one end of the first portion of the slit and a part of the second portion are connected to form a T-shaped bend.

6. The R-T-B sintered magnet according to claim 5, wherein, in a plan view, the slit includes a third portion extending in the first direction and connected to the second portion, and one end of the third portion is connected to another part of the second portion to form another T-shaped bend.

7. The R-T-B sintered magnet according to claim 1, wherein the minimum width of the slit is 350 μm or less.

8. The R-T-B sintered magnet according to claim 7, wherein the minimum width of the slit is 200 μm or less.

9. The R-T-B sintered magnet according to any one of claims 1 to 8, wherein the inner wall surface of the through-opening of the magnet body portion is an unprocessed sintered surface.

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