Manufacturing method for RTB-type sintered magnets

Abstract

This process suppresses the formation of burrs on the powder molded body between the upper punch and the die during the wet pressing process. [Solution] The method for manufacturing an RTB-type sintered magnet according to the present disclosure includes a slurry preparation step of preparing a slurry containing powder of an RTB-type sintered magnet alloy and a dispersion medium, and a pressing step of forming a powder molded body from the slurry using a wet press molding apparatus. The wet press molding apparatus comprises a mold having a cavity and an upper surface located around the upper end surface of the opening of the cavity, an upper punch having a lower surface that applies pressure to the upper surface of the mold, and a lower punch inserted into the cavity from below, wherein the lower surface of the upper punch has a convex portion that protrudes toward the interior of the cavity and a surrounding surface that surrounds the convex portion.

Description

[Technical Field]

[0001] This disclosure relates to a method for manufacturing RTB-type sintered magnets. [Background technology]

[0002] RTB-type sintered magnets (where R is at least one rare earth element, T is mainly iron, and B is boron) are known as the highest-performing permanent magnets and are used in various motors in fields such as electric vehicles (EVs, HVs, PHVs), renewable energy such as wind power generation, home appliances, and industrial equipment.

[0003] RTB-type sintered magnets are primarily R2T 14 It is composed of a main phase made of compound B and a grain boundary phase located at the grain boundaries of this main phase. The main phase is R2T 14 Compound B is a ferromagnetic material with high saturation magnetization and anisotropic magnetic field. The grain boundary phase contains a nonmagnetic, low-melting-point R-rich phase with concentrated rare earth elements (R). As a method to improve the magnetic properties of RTB-based sintered magnets, (1) R2T 14 (2) Miniaturization of phase B, R2T 14 (3) Increase the degree of orientation of phase B, (4) Reduce the amount of oxygen, (5) R2T 14 An increase in the proportion of phase B is known.

[0004] In the manufacture of rare-earth sintered magnets such as RTB-type sintered magnets, alloy powder with a predetermined particle size is obtained by crushing raw material alloy castings having a desired composition, such as ingots obtained by casting molten metal or other raw materials into a mold, or flakes obtained by the strip casting method. This alloy powder is then compressed in an oriented magnetic field to produce a powder compact, and this powder compact is then sintered to produce a rare-earth sintered magnet. Oxidation of the powder particles during crushing and molding inhibits the improvement of magnetic properties.

[0005] There are two molding methods for producing powder molded bodies: dry and wet. Patent Document 1 discloses a wet molding method. This wet molding method makes it possible to suppress the oxidation of powder particles, and is therefore considered to be less likely to hinder the improvement of magnetic properties compared to the dry molding method.

[0006] Patent Document 1 discloses a method for manufacturing RTB-type sintered magnets, in which a fine powder is pulverized in a low-oxygen atmosphere, and the resulting fine powder is mixed with an oily solvent with a low dissolved oxygen content, such as mineral oil, synthetic oil, or vegetable oil, without being exposed to oxygen, to form a slurry. In the manufacturing method of Patent Document 1, the slurry is supplied into the mold cavity of a wet molding machine, and wet press molding is performed while the machine is oriented in a magnetic field. Figure 2 of Patent Document 2 shows an example of a wet press molding machine for manufacturing rare-earth sintered magnets. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 9-289127 [Patent Document 2] Japanese Patent Publication No. 2009-206511 [Overview of the initiative] [Problems that the invention aims to solve]

[0008] According to the inventors' research, it was found that when manufacturing RTB-type sintered magnets as disclosed in Patent Document 1, powder molded bodies with burrs between the upper punch and the mold are easily formed. These burrs on the powder molded body can cause chipping or cracking after removal from the mold.

[0009] This disclosure provides a novel method for manufacturing RTB-based sintered magnets that can solve the above-mentioned problems. [Means for solving the problem]

[0010] The present disclosure provides an invention described in each of the following items.

[0011] [Item 1] A slurry preparation step of preparing a slurry containing a powder of an alloy for an R-T-B sintered magnet (where R is a rare earth element, necessarily including at least one selected from the group consisting of Nd, Pr, and Ce, T is at least one of transition metals and necessarily includes Fe, and B is boron) and a dispersion medium, A pressing step of forming a powder compact from the slurry by a wet pressing forming apparatus, including The wet pressing forming apparatus has a cavity and a mold having an upper surface located around the upper end surface of the opening of the cavity, an upper punch having a lower surface that applies pressure to the upper surface of the mold, a lower punch inserted into the cavity from below, and is provided with A method for manufacturing an R-T-B sintered magnet, wherein the lower surface of the upper punch has a convex portion protruding toward the inside of the cavity and a peripheral surface located around the convex portion.

[0012] [Item 2] In the pressing step, the lower surface of the upper punch presses the upper surface of the mold through a filter. The method for manufacturing an R-T-B sintered magnet according to Item 1.

[0013] [Item 3] A plurality of holes for discharging the dispersion medium from the cavity are provided on the lower surface of the upper punch. The method for manufacturing an R-T-B sintered magnet according to Item 2.

[0014] [Item 4] The plurality of holes are provided on the peripheral surface of the lower surface of the upper punch. The method for manufacturing an R-T-B sintered magnet according to Item 3.

[0015] [Item 5] The height difference between the convex portion and the peripheral surface on the lower surface of the upper punch is 0.05 mm or more and 0.50 mm or less. The method for manufacturing an R-T-B-based sintered magnet according to any one of Items 1 to 4.

[0016] [Item 6] The convex portion has an inclined region at a position facing the upper surface edge between the upper surface of the mold and the upper end surface of the opening of the cavity. The method for manufacturing an R-T-B-based sintered magnet according to any one of Items 1 to 5.

[0017] [Item 7] The inclined region on the lower surface of the upper punch extends along the upper surface edge of the mold and surrounds the convex portion. The method for manufacturing an R-T-B-based sintered magnet according to Item 6.

[0018] [Item 8] The width of the inclined region is 2 mm or more and 4 mm or less. The method for manufacturing an R-T-B-based sintered magnet according to Item 7. [Effect of the Invention]

[0019] According to the embodiment of the present disclosure, when producing a powder compact from a slurry with a wet pressing forming device, it is possible to suppress the formation of burrs between the upper punch and the mold, so that the occurrence of cracks and fissures in the powder compact caused by burrs can be reduced, and it becomes possible to produce R-T-B-based sintered magnets with good yield. [Brief Description of the Drawings]

[0020] [Figure 1A] FIG. 1A is a diagram showing an example of the configuration of the wet pressing forming device in a certain state in the present embodiment. [Figure 1B] FIG. 1B is a diagram showing an example of the configuration of the wet pressing forming device in another state in the present embodiment. [Figure 2] FIGS. 2(a) and 2(b) are perspective views schematically showing an example of the configuration of the mold included in the wet pressing forming device. [Figure 3]Figure 3 is a schematic cross-sectional view showing an example of the configuration of the upper punch and die of the wet press molding apparatus used in this embodiment. [Figure 4] Figure 4 is a schematic plan view showing an example of the configuration of the lower surface of the upper punch of the wet press molding apparatus used in this embodiment. [Figure 5] Figure 5 is a schematic cross-sectional view showing the upper punch pressing against the upper surface of the die via the filter cloth. [Figure 6] Figure 6 is a schematic plan view showing another example of the configuration of the lower surface of the upper punch of the wet press molding apparatus used in this embodiment. [Figure 7] Figure 7 is a schematic plan view showing yet another configuration example of the lower surface of the upper punch of the wet press molding apparatus used in this embodiment. [Modes for carrying out the invention]

[0021] Embodiments of this disclosure will be described below with reference to the drawings.

[0022] <Basic configuration example> First, with reference to Figures 1A, 1B, and 2, an example of the basic configuration of a wet press molding apparatus suitably used in the method for manufacturing RTB-type sintered magnets according to this disclosure will be described. Figures 1A and 1B are schematic cross-sectional views illustrating different configuration examples of the wet press molding apparatus 100 in this embodiment, respectively. Figure 2 is a schematic perspective view illustrating an example of the configuration of the mold 10 provided in the wet press molding apparatus 100. For reference, the figure shows mutually orthogonal X, Y, and Z axes. In the example shown, the Z axis is parallel to the vertical direction, and the Y axis is perpendicular to the plane of the paper. The XY plane, including the X and Y axes, is horizontal.

[0023] The wet press molding apparatus 100 includes a die 10 having a cavity 16, as illustrated in Figure 2(a). The die 10 has an upper surface 10T and a lower surface 10U, and the cavity 16 penetrates the die 10 in the Z-axis direction from the upper surface 10T to the lower surface 10U. In the example in Figure 2, the cavity 16 is a rectangular parallelepiped space. Hereinafter, the upper end of this space will be referred to as the opening upper end surface 16T of the cavity 16. The upper surface 10T of the die 10 is located around and surrounds the opening upper end surface 16T. The opening upper end surface 16T of the cavity 16 is located at the same height as the upper surface 10T of the die 10. The edge of the upper surface 10T of the die 10 that is located between it and the opening upper end surface 16T of the cavity 16 will be referred to as the "upper edge". The top edge 10E defines the contour of the opening upper end face 16T of the cavity 16. In the example in Figure 2, the opening upper end face 16T of the cavity 16 is rectangular, and the top edge 10E defines the sides of this rectangle. The opening upper end face 16T of the cavity 16 is not limited to a rectangle and can have a circle, ellipse, polygon, or any other arbitrary shape.

[0024] The mold 10 may be made of a magnetic material and / or a non-magnetic material such that a magnetic field formed by an electromagnetic coil 20 (described later) is applied to the slurry 30 (Figure 1B) filled in the cavity 16. The mold 10 has an inner wall 10W that defines the cavity 16. The cross section of the cavity 16 perpendicular to the Z-axis has a constant shape and size along the Z-axis direction. In this example, the cavity 16 has a rectangular parallelepiped shape, but the shape of the cavity 16 is not limited to this example.

[0025] The wet press forming apparatus 100 comprises an upper punch 14 and a lower punch 12, as shown in Figures 1A and 1B. The upper punch 14 has a lower surface 14U that applies pressure to the upper surface 10T of the die 10. The lower punch 12 is inserted into the cavity 16 from below. As shown in Figure 2(b), the lower part of the cavity 16 is closed by the lower punch 12, and the lower end surface of the cavity 16 is defined by the upper surface 12T of the lower punch 12. The cavity 16 is filled with slurry. The slurry includes, for example, alloy powder (RTB alloy powder) particles containing rare earth elements, iron, and boron, and a dispersion medium such as an oil for dispersing the alloy powder particles. In the example in Figure 1A, the die 10 has an inlet 10P for supplying slurry to the cavity 16 from the outside. There does not have to be one inlet 10P, but there may be multiple. Furthermore, the number of cavities 16 in a single mold 10 is not limited to one, but may be multiple. When a single mold 10 has multiple cavities 16, the wet press molding apparatus 100 is equipped with multiple sets of lower punches 12 assigned to each cavity 16. The injection port 10P is connected to a slurry supply device (a hydraulic device having a hydraulic cylinder), and the slurry pressurized by the hydraulic cylinder or the like is supplied to the inside of the cavity 16 through the injection port 10P. The configuration and position of the injection port 10P are not limited to the example shown in the figure.

[0026] The wet press molding apparatus 100 includes an electromagnetic coil 20 for forming a magnetic field inside the cavity 16. This magnetic field is applied to the slurry filling the cavity 16, orienting the alloy powder particles in the slurry in a predetermined direction. In the state shown in Figure 1B, the slurry 30 is filled inside the cavity 16. The magnetic field formed by the electromagnetic coil 20 is applied to the slurry 30 in this state.

[0027] As shown in Figure 1A, the lower surface 14U of the upper punch 14 has a convex portion U1 that protrudes toward the cavity 16 and a peripheral surface U2 located around the convex portion U1. The lower surface 14U of the upper punch 14 is also provided with a plurality of holes (discharge holes) 14H for discharging the dispersion medium from the slurry filled in the cavity 16. In the example shown, the plurality of discharge holes 14H each have a simplified structure that extends in a linear direction, but the plurality of discharge holes 14H may communicate with a single outlet hole. The lower surface 14U of the upper punch 14 will be described in detail later.

[0028] The lower punch 12 moves up and down relative to the mold 10 with at least its tip inserted into the cavity 16. The upper punch 14 moves up and down relative to the lower punch 12. In this disclosure, “up and down movement” means movement in the vertical direction. Also, “A moves up and down relative to B” means that the distance between A and B in the vertical direction increases or decreases. Therefore, forms in which the lower punch 12 moves up and down relative to the mold 10 include the case where the lower punch 12 moves up and down while the mold 10 is stationary, the case where the mold 10 moves up and down while the lower punch 12 is stationary, and the case where the mold 10 and the lower punch 12 move up and down in the same or opposite directions. In the state of Figure 1B, compared to the state of Figure 1A, the lower punch 12 remains stationary while the mold 10 and the upper punch 14 descend. As a result, the lower punch 12 rises relative to the mold 10.

[0029] In the state shown in Figure 1A, the upper punch 14 is located above the cavity 16, but the top surface of the cavity 16 is open. In other words, a portion of the lower punch 12 is inserted into the lower part of the cavity 16 of the mold 10, but the cavity 16 is not blocked by the upper punch 14. Figure 2(b) schematically shows the state in which the cavity 16 is formed by the wall 10W of the mold 10 and the top surface 12T of the lower punch 12. The lower punch 12 inserted into the cavity 16 of the mold 10 and the inner wall 10W of the mold 10 are in contact in a slidable manner. The inner wall 10W and the lower punch 12 are in contact in such a way that the cavity 16 can hold the liquid component of the slurry without leakage. The above describes the case of magnetic field injection, where a magnetic field is applied after the slurry is injected. However, when a magnetic field is injected before the slurry is injected, the mold 10 and the upper punch 14 move up and down relative to the lower punch 12 while the cavity 16 is blocked by the upper punch 14.

[0030] Refer to Figure 1B. In the state shown in Figure 1B, the lower surface 14U of the upper punch 14 is descending so as to press downwards against the upper surface 10T of the mold 10. As a result, the cavity 16 is blocked by the upper punch 14. In the example shown in Figure 1B, a filter cloth 32 is placed between the lower surface 14U of the upper punch 14 and the upper surface 10T of the mold 10. The filter cloth 32 is a cloth-like filter material made by weaving synthetic fibers, for example, and may be called a filter. Examples of filters are not limited to filter cloth, but may include filter paper, porous filters, metal filters, or laminates thereof. The woven fabric constituting the filter cloth 32 may be formed from, for example, a multifilament system or a spun system. By providing a filter such as the filter cloth 32, it is possible to more reliably prevent alloy powder particles from entering the discharge hole 14H and allow only the dispersion medium to pass through. The size of the numerous pores in the filter cloth 32 is determined so that almost no rare earth alloy powder particles contained in the slurry 30 can pass through. The filter cloth 32 is positioned to block a plurality of discharge holes 14H provided on the lower surface 14U of the upper punch 14. Although only a portion of the filter cloth 32 is shown in the figure for simplicity, the filter cloth 32 can be extended along the X-axis and used wrapped around a roller. Rotating such a roller makes it possible to switch the portion of the filter cloth 32 that is in contact with the lower surface 14U of the upper punch 14 and the upper surface 10T of the die 10. As a result, it becomes easy to sequentially switch the soiled area of ​​the filter cloth 32 in the pressing process to a new area and perform the next pressing process.

[0031] In the example shown in Figure 1B, compared to the state in Figure 1A, not only the upper punch 14 but also the mold 10 has descended. As a result, the injection port 10P of the mold 10 is blocked by the lower punch 12. Furthermore, by shortening the distance between the upper surface 12T of the lower punch 12 and the lower surface 14U of the upper punch 14, the volume of the cavity 16 can be reduced. After filling the cavity 16 in Figure 1A with slurry, in the process of reaching the state in Figure 1B, the liquid components in the slurry 30 are discharged from the inside to the outside of the cavity 16 through the filter cloth 32 and the discharge hole 14H of the upper punch 14.

[0032] The wet press molding apparatus 100 includes an electromagnetic coil 20 that applies a transverse magnetic field to the cavity 16 of the mold 10 in a direction perpendicular to the direction in which the lower punch 12 moves up and down (Z-axis direction, i.e., vertical direction) (horizontal direction). In the example in Figure 1, the electromagnetic coil 20 can form a transverse magnetic field in the cavity 16 with magnetic flux extending in the X-axis direction. As will be described later, in this embodiment, when the slurry 30 is injected into the cavity 16 from the injection port 10P, the upper punch 14 is located away from the mold 10 as shown in Figure 1A, and no magnetic field is applied. The above describes the case of injection without a magnetic field, where the magnetic field is applied after the slurry is injected. However, when the magnetic field is applied before the slurry is injected, the upper punch 14 is first positioned to block the cavity 16 as shown in Figure 1B, and then the slurry is injected after the magnetic field is applied.

[0033] The wet press forming apparatus in the embodiments of this disclosure includes a control device that controls the operation of an upper punch 14, a lower punch 12, a die 10, and an electromagnetic coil 20. Such a control device may be implemented by a computer that operates according to a program stored in a memory device.

[0034] <Underside of the upper punch> Next, the configuration of the lower surface 14U of the upper punch 14 will be described in detail with reference to Figures 3, 4, and 5. Figure 3 is a schematic cross-sectional view showing an example of the configuration of the upper punch 14 and die 10 of the wet press molding apparatus 100. Figure 4 is a schematic plan view showing an example of the configuration of the lower surface 14U of the upper punch 14. Figure 5 is a schematic cross-sectional view showing the state in which the lower surface 14U of the upper punch 14 is pressing against the upper surface 10T of the die 10 via the filter cloth 32.

[0035] As described above, the lower surface 14U of the upper punch 14 has a convex portion U1 that protrudes toward the cavity 16 and a surrounding surface U2 located around the convex portion U1. The convex portion U1 has an inclined region U10 located opposite the upper surface edge 10E between the upper surface 10T of the mold 10 and the upper end surface 16T of the opening of the cavity 16. As shown in Figure 4, the inclined region U10 connects the convex portion U1 and the surrounding surface U2 and surrounds the convex portion U1. The inclined region U10 has a slope that mitigates the height difference (indicated by reference numeral "H" in Figure 5) between the convex portion U1 and the surrounding surface U2. This slope does not need to be constant, and the surface of the inclined region U10 may include a gently curved surface.

[0036] As shown in Figure 5, when the lower surface 14U of the punch 14 presses against the upper surface 10T of the mold 10 via the filter cloth 32, the inclined region U10 on the lower surface 14U of the upper punch 14 faces the upper edge 10E of the mold 10. More specifically, the inclined region U10 extends along the upper edge 10E of the mold 10 and completely encloses the protrusion U1.

[0037] Multiple discharge holes 14H are provided on the peripheral surface U2 of the lower surface 14U of the upper punch 14. In addition to the peripheral surface U2, the multiple discharge holes 14H may also be provided on the protrusion U1 and / or the inclined region U10. However, considering the suppression of strength reduction of the upper punch 14, it is preferable to provide the multiple discharge holes 14H only on the peripheral surface U2. Specifically, as shown in Figure 4, on the peripheral surface U2, they are arranged at approximately equal intervals along the inclined region U10 so as to surround the protrusion U1. In the state shown in Figure 5, the discharge holes 14H are located at a position shifted outward in the horizontal direction from the upper surface edge 10E by, for example, 2 mm to 6 mm.

[0038] According to the inventors' studies, when a press process is performed with an upper punch 14 that does not have a protrusion U1 on its lower surface 14U, and when a filter cloth 32 is placed between the lower surface 14U of the upper punch 14 and the upper surface 10T of the die 10, and press molding is performed while removing the dispersion medium of the slurry 30 from the discharge hole 14H provided in the upper punch 14, it was found that a small amount of alloy powder particles leaked between the upper surface 10T of the die 10 and the filter cloth 32 from between the upper surface edge 10E of the die 10 and the filter cloth 32, resulting in the formation of burrs on the upper punch 14 side of the powder molded body. Furthermore, it was found that by providing a protrusion U1 having the above configuration on the lower surface 14U of the upper punch 14, the leakage of alloy powder particles from the slurry 30 during press molding can be reduced, and the formation of burrs on the powder molded body can be suppressed.

[0039] To fully obtain these effects, the height difference H (see Figure 5) between the protrusion U1 and the surrounding surface U2 on the lower surface 14U of the upper punch 14 is preferably, for example, 0.05 mm or more and 0.50 mm or less. If this height difference H is less than 0.05 mm, there is a high possibility that burrs will be formed. If the height difference H becomes too large, exceeding 0.50 mm, problems such as partial tearing of the filter cloth 32 may occur.

[0040] By providing an inclined region U10 on the periphery of the protrusion U1, the distance between the protrusion U1 and the upper edge 10E of the mold 10 can be reduced during the pressing process, thereby locally increasing the degree of pressure applied to the filter cloth 32. This effectively suppresses the flow of alloy powder particles beyond the upper edge 10E as the dispersion medium moves. The width of the inclined region U10 (indicated by the reference numeral "L" in Figure 5) is preferably 2 mm or more and 4 mm or less.

[0041] By providing the convex portion U1 having the above configuration on the lower surface 14U of the upper punch 14, the formation of burrs between the upper punch 14 and the mold 10 can be suppressed. As a result, the occurrence of cracks and fissures in the powder molded body caused by burrs can be reduced, making it possible to produce RTB-type sintered magnets with a high yield.

[0042] The protrusion U1 provided on the lower surface 14U of the upper punch 14 can take on various shapes to match the shape of the cavity 16 of the mold 10. Figure 6 is a schematic plan view showing another example of the configuration of the lower surface 14U of the upper punch 14 of the wet press molding apparatus 100, and Figure 7 is a schematic plan view showing yet another example of the configuration. Thus, the shape of the protrusion U1 and the surrounding surface U2 provided on the lower surface 14U of the upper punch 14 is arbitrary.

[0043] <Manufacturing method> The method for manufacturing RTB-type sintered magnets according to embodiments of this disclosure will be described below. In this embodiment, a wet pressing process is performed using a wet press molding apparatus 100 equipped with an upper punch 14 having the bottom shape shown in Figure 7.

[0044] In the manufacturing method of the RTB-type sintered magnet in this embodiment, the following steps are performed.

[0045] (1): Preparation of slurry For example, the process involves preparing a slurry containing an alloy powder containing a rare earth element (preferably an alloy powder containing a rare earth element, iron, and boron) and a dispersion medium.

[0046] • Composition of alloy powder The composition of the alloy powder may have the composition of known rare-earth sintered magnets, including, for example, RTB-based sintered magnets (where 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) and samarium-cobalt-based sintered magnets.

[0047] RTB-type sintered magnets are preferred because they exhibit the highest magnetic energy product among various types of magnets and are relatively inexpensive.

[0048] The following shows preferred compositions for RTB-type sintered magnets.

[0049] R is a rare earth element and necessarily contains at least one selected from the group consisting of Nd, Pr, and Ce. Further, R preferably contains either Nd or Pr. More preferably, a combination of rare earth elements represented by Nd-Dy, Nd-Tb, Nd-Pr-Dy, or Nd-Pr-Tb is used.

[0050] Among R, Dy and Tb are particularly effective in improving H cJ . In addition to the above elements, it may contain a small amount of other rare earth elements such as Ce or La, and mischmetal or didymium can also be used. Also, R may not be a pure element and may contain inevitable impurities in the manufacturing process within the range available industrially. The content can adopt the conventionally known content. For example, a range of 25% by mass or more and 35% by mass or less is a preferable range. If it is less than 25% by mass, high magnetic properties, particularly high H cJ may not be obtained, and if it exceeds 35% by mass, B r may decrease.

[0051] T contains iron (including the case where T consists substantially of iron), and up to 50% of it by mass ratio may be replaced with cobalt (Co) (including the case where T consists substantially of iron and cobalt). Co is effective in improving temperature characteristics and corrosion resistance, and the alloy powder may contain up to 10% by mass of Co. The content of T may occupy the remainder of R and B or R, B, and M described later.

[0052] The content of B can also be a known content, for example, a range of 0.8% by mass to 1.2% by mass is a preferable range. If it is less than 0.8% by mass, high H cJ may not be obtained, and if it exceeds 1.2% by mass, B r may decrease. Note that a part of B can be replaced with C (carbon). Substitution with C may improve the corrosion resistance of the magnet. When it is B + C (including both B and C), the total content is preferably set within the above B concentration range by converting the number of substitution atoms of C into the number of atoms of B.

[0053] In addition to the above elements, HcJ For improvement, element M can be added. Element M 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 amount of element M added is preferably 5.0% by mass or less. If it exceeds 5.0% by mass, B r This is because the quality may decrease. Furthermore, unavoidable impurities can be tolerated.

[0054] • Method for manufacturing alloy powder For example, alloy powder is produced by a melting method to create ingots or flakes of a rare-earth-based magnet raw material alloy (preferably an RTB-based magnet raw material alloy) having a desired composition, and then hydrogen is absorbed (intercalated) into these alloy ingots and flakes to perform hydrogen pulverization and produce coarse pulverized powder.

[0055] Then, the coarsely ground powder can be further ground using a jet mill or the like to produce finely ground powder (alloy powder).

[0056] A method for manufacturing raw material alloys for RTB magnets is provided as an example.

[0057] Alloy ingots can be produced by ingot casting, a method in which metals that have been pre-adjusted to the final required composition are melted and poured into a mold.

[0058] Furthermore, alloy flakes can be produced by rapid cooling methods such as strip casting or centrifugal casting, which involve rapidly cooling the molten metal by contacting it with a single roll, double roll, rotating disk, or rotating cylindrical mold, thereby producing a solidified alloy thinner than that produced by the ingot method.

[0059] In this invention, alloys produced by either the ingot method or the rapid quenching method can be used, but alloys produced by the rapid quenching method are preferred.

[0060] The thickness of raw material alloys for RTB magnets produced by the rapid cooling method (rapidly cooled alloys) is typically in the range of 0.03 mm to 10 mm, and they are in flake or plate form. The molten alloy begins to solidify from the surface in contact with the cooling roll (roll contact surface), and crystals grow columnarly in the thickness direction from the roll contact surface. Compared to alloys produced by the conventional ingot casting method (die casting method), rapidly cooled alloys have a finer structure and smaller grain size because they are cooled in a shorter time. They also have a larger grain boundary area. Since the R-rich phase spreads widely within the grain boundaries, the rapid cooling method offers excellent dispersibility of the R-rich phase.

[0061] Therefore, it is prone to fracture at the grain boundaries by hydrogen pulverization. By hydrogen pulverizing the rapidly cooled alloy, the size of the hydrogen pulverized powder (coarse pulverized powder) can be reduced to, for example, 1.0 mm or less.

[0062] The coarsely ground powder obtained in this way can be further ground using a jet mill or the like, for example, by D2 by airflow dispersion laser analysis. 50 RT-B alloy powders with a particle size of 2 to 7 μm can be produced.

[0063] Jet milling is preferably performed in (a) an atmosphere consisting of nitrogen gas and / or argon gas (Ar gas) with a substantially 0% by mass oxygen content, or (b) an atmosphere consisting of nitrogen gas and / or Ar gas with an oxygen content of 0.005 to 0.5% by mass.

[0064] To control the amount of nitrogen in the resulting sintered body, it is more preferable to use Ar gas as the atmosphere inside the jet mill and introduce a small amount of nitrogen gas into it to adjust the concentration of nitrogen gas in the Ar gas.

[0065] ·Dispersion medium A dispersion medium is a liquid that can be used to create a slurry by dispersing alloy powder within it.

[0066] Examples of preferred dispersion media used in the present invention include mineral oil or synthetic oil.

[0067] While the type of mineral or synthetic oil is not specified, if the kinematic viscosity at room temperature exceeds 10 cst, the increased viscosity can strengthen the bonding force between alloy powders, potentially adversely affecting the orientation of the alloy powders during wet molding in a magnetic field.

[0068] Therefore, the kinematic viscosity of the mineral oil or synthetic oil at room temperature is preferably 10 cst or less. Furthermore, if the fractional distillation point of the mineral oil or synthetic oil exceeds 400°C, de-oiling after obtaining the powder molded body becomes difficult, and the amount of residual carbon in the sintered body increases, which may reduce the magnetic properties.

[0069] Therefore, the fractionation point of mineral oil or synthetic oil is preferably 400°C or lower.

[0070] Additionally, vegetable oil may be used as a dispersion medium. Vegetable oil refers to oil extracted from plants, and the type of plant is not limited to a specific plant. Examples include soybean oil, rapeseed oil, corn oil, safflower oil, or sunflower oil.

[0071] • Preparation of slurry A slurry can be prepared by mixing the obtained alloy powder with a dispersion medium.

[0072] The mixing ratio of alloy powder to dispersion medium is not particularly limited, but the concentration of alloy powder in the slurry is preferably 70% or more by mass (i.e., 70% by mass or more). 20-600 cm 3 This is because, at a flow rate of [number] / second, alloy powder can be efficiently supplied into the space, and excellent magnetic properties can be obtained.

[0073] Furthermore, the concentration of alloy powder in the slurry is preferably 90% or less by mass ratio. This is to ensure the smoothness of the slurry.

[0074] More preferably, the concentration of alloy powder in the slurry is 75% to 88% by mass. This is because it allows for more efficient supply of alloy powder and more reliable assurance of slurry fluidity. Even more preferably, the concentration of alloy powder in the slurry is 84% ​​or more by mass. The method of mixing the alloy powder and the dispersion medium is not particularly limited. The mixture may be prepared by separately preparing the alloy powder and the dispersion medium, weighing predetermined amounts of both, and mixing them together. Alternatively, when producing 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 collected into the dispersion medium in the container to obtain a slurry. In this case, it is preferable to maintain an atmosphere of nitrogen gas and / or argon gas inside the container, and to directly collect the obtained alloy powder into the dispersion medium without exposing it to the atmosphere to obtain a slurry. Furthermore, it is also possible to produce a slurry consisting of alloy powder and dispersion medium by wet grinding using a vibratory mill, ball mill, or attritor while the coarsely ground powder is held in the dispersion medium.

[0075] (2): Pressurized injection of slurry As shown in Figure 1A, the inside of the cavity 16 of the mold 10 in the wet press molding apparatus 100 is in communication with the injection port 10P of the mold 10, and slurry is injected into the inside of the cavity 16 from the injection port 10P. The amount of slurry supplied is, for example, 20 to 150 cm³. 3 It can be set in the range of / second. The slurry supply rate can be controlled by changing the flow rate of oil supplied to the hydraulic cylinder of the hydraulic device by adjusting the flow control valve of the hydraulic device which functions as a slurry supply device, thereby changing the speed of the hydraulic cylinder. The inlet 10P of the slurry 30 is, for example, a hole with a diameter of 2 mm to 30 mm.

[0076] When the slurry 30 is injected into the cavity 16, no magnetic field is applied to the cavity 16 (magnetic field-free injection). Such magnetic field-free injection has the effect of reducing density variations in each part of the powder molded body obtained after press molding. Preferably, after filling the cavity 16 with slurry, the descent of the upper punch 14 is started, and the lower surface 14U of the upper punch 14 closes the cavity 16. A filter cloth 32 is placed between the upper surface 10T of the mold 10 and the lower surface 14U of the upper punch 14. Note that when the slurry 30 is injected into the cavity 16, a magnetic field may be applied to the cavity 16 (magnetic field injection). In this case as well, the filter cloth 32 is placed between the upper surface 10T of the mold 10 and the lower surface 14U of the upper punch 14, but it is preferable to start the descent of the upper punch 14 before magnetic field injection, and to close the cavity 16 with the lower surface 14U of the upper punch 14. This prevents the slurry 30 from overflowing from inside the cavity 16 during magnetic field injection.

[0077] In the above description, both magnetic field-free injection and magnetic field injection have been described as necessary. According to the embodiments of this disclosure, in either case, when producing a powder molded body from slurry in a wet press molding apparatus, the formation of burrs between the upper punch and the mold can be suppressed. This reduces the occurrence of cracks and fissures in the powder molded body caused by burrs, making it possible to produce RTB-type sintered magnets with a high yield.

[0078] (3): Press forming in a transverse magnetic field Next, with a "transverse magnetic field" applied to the cavity 16 in a direction perpendicular to the direction in which the lower punch 12 moves up and down, the gap between the lower surface 14U of the upper punch 14 and the upper surface 12T of the lower punch 12 is reduced. As the gap between the lower surface 14U of the upper punch 14 and the upper surface 12T of the lower punch 12 is reduced, the dispersion medium contained in the slurry 30 is discharged through multiple discharge holes 14H in the upper punch 14, and a powder molded body of alloy powder is formed.

[0079] The magnitude of the magnetic field formed inside the cavity 16 is, for example, between 1.0T and 1.5T. If the magnetic field strength is 1.0T or higher, the magnetization direction of the alloy powder contained in the slurry 30 is more reliably oriented in the direction of the magnetic field, resulting in a high degree of orientation. Below 1.0T, the degree of orientation of the alloy powder decreases, or the orientation of the alloy powder becomes easily disrupted during press forming.

[0080] The electromagnetic coil 20 is positioned near the side of the mold 10 and can form a uniform magnetic field perpendicular to the pressing direction inside the cavity 16.

[0081] To describe the state inside the cavity 16 in more detail, during the magnetic field press forming process, the dispersion medium in the slurry 30 is filtered out through the discharge hole 14H from the part of the upper punch 14 closest to the discharge hole 14H, but the alloy powder contained in the slurry 30 remains inside the cavity 16. As a result, a "cake layer" is formed from the part closest to the discharge hole 14H. As mentioned above, the cake layer is a layer in which the dispersion medium in the slurry has been discharged to the outside of the cavity 16, resulting in a higher concentration of alloy powder. As the pressing process progresses, the cake layer eventually spreads throughout the entire cavity 16, and a powder molded body is obtained in which the alloy powder particles are in contact with each other.

[0082] After the powder molded body is formed, the mold 10 is lowered to expose the powder molded body to the outside of the mold 10. After this, the upper punch 14 is raised, making it possible to remove the powder molded body. According to this embodiment, no burrs were formed on the powder molded body. For comparison, it was confirmed that when the same pressing process was performed with an upper punch 14 that does not have a protrusion U1 on its lower surface 14U, burrs were formed on all the powder molded bodies.

[0083] Furthermore, the powder molded body obtained by the process described above retains residual dispersion medium. If the powder molded body in this state is rapidly heated from room temperature to a sintering temperature of, for example, 950 to 1150°C, the internal temperature of the powder molded body will rise rapidly, and the residual dispersion medium and the rare earth elements in the powder molded body may react to form rare earth carbides. When rare earth carbides are formed in this way, the generation of a sufficient amount of liquid phase for sintering is hindered, and a sintered body with sufficient density may not be obtained, potentially leading to a decrease in magnetic properties. For this reason, it is preferable to de-oil the powder molded body before sintering, as this allows for sufficient removal of the residual dispersion medium.

[0084] The above explanation describes the case of a transverse magnetic field, but a "longitudinal magnetic field" parallel to the direction in which the lower punch 12 moves up and down may also be applied to the cavity 16.

[0085] (4): Step of dividing into powder molded pieces In this embodiment, a powder molded body produced by press molding in a transverse magnetic field may be divided into multiple powder molded body pieces. In this embodiment, the powder molded body can be sliced ​​into plate-shaped powder molded body pieces using a wire saw. The number, size, and shape of the powder molded body pieces are arbitrary. Instead of a wire saw, a known cutting blade may be used for cutting. The larger the dimensions of the powder molded body, the more sintered magnets can be produced from a single powder molded body.

[0086] (5): Sintering process (process for producing sintered body material) Next, the powder molded body (cut powder molded body pieces) is sintered to produce a rare earth sintered magnet body. In this disclosure, if the sintered body of the powder molded body pieces is further cut, the sintered body of the powder molded body pieces will be referred to as the "sintered body material." Hereinafter, for simplicity, the powder molded body pieces may be simply referred to as the "powder molded body."

[0087] The sintering of the powder molded body is preferably carried out at 0.13 Pa (10 -3 Torr) or less, more preferably 0.07 Pa (5.0 × 10⁻⁶ Pa) -4The process is carried out at a pressure of 1000°C to 1150°C under a pressure of Torr or less. To prevent oxidation due to sintering, residual gases in the atmosphere may be replaced with inert gases such as helium or argon.

[0088] Furthermore, to obtain various effects, a diffusion process may be performed in which rare earth elements R are diffused from the surface to the interior of the sintered body material. In this case, by diffusing heavy rare earth elements RH (where RH is at least one of Tb, Dy, and Ho) from the surface to the interior of the sintered body as the rare earth element R, the coercivity can be increased particularly efficiently.

[0089] After the sintering process, it is preferable to heat-treat the sintered body (including the sintered material or sintered piece) at a temperature lower than the sintering temperature. 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 rare earth sintered magnet body thus obtained is then subjected to grinding / polishing processes, surface treatment / coating processes as needed, and finally completed as a rare earth sintered magnet through a magnetization process. [Industrial applicability]

[0090] The manufacturing method for RTB-based sintered magnets described herein is suitably used for the manufacture of rare-earth-based sintered magnets. Such rare-earth-based sintered magnets can be used in various motors such as voice electromagnetic coil motors (VCMs) for hard disk drives, motors for electric vehicles (EVs, HVs, PHVs, etc.), motors for industrial equipment, and home appliances. [Explanation of Symbols]

[0091] 10...Mold, 10H...Through hole, 10W...Inner wall, 12...Lower punch, 12T...Top surface of lower punch, 14...Upper punch, 14H...Discharge hole of upper punch, 14U...Bottom surface of upper punch, 20...Electromagnetic coil, 16...Space, 30...Slurry, 100...Wet press molding apparatus

Claims

1. A slurry preparation step to prepare a slurry containing powder and a dispersion medium of an R-T-B alloy for sintered magnets (where 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 pressing step in which a powder molded body is formed from the slurry using a wet press molding apparatus, Includes, The aforementioned wet press molding apparatus, A mold having a cavity and an upper surface located around the upper end surface of the opening of the cavity, An upper punch having a lower surface for applying pressure to the upper surface of the mold, A lower punch is inserted into the cavity from below, It is equipped with, A method for manufacturing an R-T-B type sintered magnet, wherein the lower surface of the upper punch has a convex portion that protrudes toward the interior of the cavity and a peripheral surface located around the convex portion.

2. The method for manufacturing an R-T-B type sintered magnet according to claim 1, wherein in the pressing step, the lower surface of the upper punch presses against the upper surface of the mold via a filter.

3. The method for manufacturing an R-T-B sintered magnet according to claim 2, wherein the lower surface of the upper punch is provided with a plurality of holes for discharging the dispersion medium from the cavity.

4. The method for manufacturing an R-T-B sintered magnet according to claim 3, wherein the plurality of holes are provided on the circumferential surface of the lower surface of the upper punch.

5. The method for manufacturing an R-T-B sintered magnet according to any one of claims 1 to 4, wherein the height difference between the protrusion on the lower surface of the upper punch and the surrounding surface is 0.05 mm or more and 0.50 mm or less.

6. The method for manufacturing an R-T-B type sintered magnet according to claim 5, wherein the protrusion has an inclined region at a position facing the upper edge located between the upper surface of the mold and the upper end surface of the opening of the cavity.

7. The method for manufacturing an R-T-B type sintered magnet according to claim 6, wherein the inclined region on the lower surface of the upper punch extends along the upper edge of the mold and surrounds the protrusion.

8. The method for manufacturing an R-T-B sintered magnet according to claim 7, wherein the width of the inclined region is 2 mm or more and 4 mm or less.

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