Method for manufacturing r-t-b based sintered magnet

Abstract

A method for manufacturing an R-T-B based sintered magnet according to the present disclosure comprises: a milling step for preparing a powder of an alloy for an R-T-B based sintered magnet; a molding step for fabricating a powder molded body using the powder; a cutting step for cutting and dividing the powder molded body into a plurality of molded body pieces; and a sintering step for sintering each of the plurality of molded body pieces to fabricate a plurality of sintered bodies. The cutting step includes a step for forming one or a plurality of cut surfaces including a curved surface, by cutting the powder molded body, submerged in a liquid, with a wire running in a horizontal direction, and by causing the wire to move in an arbitrary incising direction perpendicular to the running direction.

Description

Method for manufacturing RTB based sintered magnet

[0001] The present application relates to a method for producing an RTB based sintered magnet.

[0002] R-T-B based sintered magnets (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) are R ２ Fe １４ It is composed of a main phase of a compound with a B-type 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 additive elements and impurities. R-T-B based sintered magnets have a high residual magnetic flux density B ｒ (Hereafter, simply "B ｒ ") and high coercive force H ｃＪ (Hereafter, simply "H ｃＪ ") and have excellent magnetic properties, making them known as the highest-performance permanent magnets. For this reason, R-T-B based sintered magnets are used in a wide variety of applications, including voice coil motors (VCMs) for hard disk drives, motors for electric vehicles (EVs, HVs, PHVs), motors for industrial equipment, and various other motors, as well as in home appliances.

[0003] Such an RTB based sintered magnet can be manufactured, for example, by the steps of preparing an alloy powder, press-molding the alloy powder to produce a powder compact, and sintering the powder compact. The alloy powder can be manufactured, for example, by the following method.

[0004] First, an alloy is produced from a molten metal of various raw materials by a method such as ingot casting or strip casting. The resulting alloy is then subjected to a pulverization process to obtain an alloy powder with a predetermined particle size distribution. This pulverization process typically includes a coarse pulverization process and a fine pulverization process. The former is carried out, for example, by utilizing the hydrogen embrittlement phenomenon, and the latter is carried out, for example, by using an airflow pulverizer (jet mill).

[0005] The sintered body obtained by the process of sintering the powder compact is then subjected to mechanical processing such as grinding and cutting to be individual pieces having the desired shape and size. More specifically, R—Fe—B rare earth magnet powder is first compression-molded in a press to produce a compact larger in size than the final magnet product. After the compact is sintered into a sintered body by a sintering process, the sintered body is ground using, for example, a cemented carbide blade saw or a rotating grindstone to give it the desired shape. For example, a block-shaped sintered body is first produced, and then the sintered body is sliced ​​using a blade saw or the like to cut out multiple plate-shaped sintered body portions.

[0006] 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 large, making high-precision grinding difficult and time-consuming. Furthermore, some material is inevitably lost during the processing. For this reason, the processing step is a major cause of increased manufacturing costs.

[0007] For example, to solve the former problem, Patent Document 1 describes a technique for processing a magnet compact using a wire saw before sintering. Wire sawing is a processing technique in which a wire traveling in one or both directions is pressed against the compact to be processed, and the compact is ground or cut using abrasive grains between the wire and the compact. This technique cuts a powder compact, which is much softer and easier to process than a sintered compact, and therefore significantly reduces the time required for cutting.

[0008] Japanese Patent Application Laid-Open No. 2003-303728

[0009] Patent Document 1 discloses that a powder compact is processed using a wire saw having a wire with an outer diameter of 0.1 mm to 1.0 mm and abrasive grains fixed to the wire in an inert gas atmosphere in which the oxygen concentration is adjusted to 5% to 18% by molar ratio. Performing wire saw processing in an inert atmosphere with a controlled oxygen concentration in this way requires complicated equipment and management, making it less suitable for mass production.

[0010] The embodiments of the present disclosure provide a new method for producing a sintered RTB based magnet that enables a wire saw process that does not require the preparation of an inert atmosphere.

[0011] In an exemplary embodiment, a method for producing an R-T-B based sintered magnet according to the present disclosure includes the following steps: a milling step for preparing a powder of an alloy for an R-T-B based sintered magnet (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; and B is boron); a molding step for producing a powder compact using the powder; a cutting step for cutting the powder compact into a plurality of compact pieces; and a sintering step for sintering each of the plurality of compact pieces to produce a plurality of sintered bodies. The cutting step includes a step of cutting the powder compact submerged in a liquid with a horizontally traveling wire and moving the wire in any cutting direction perpendicular to the traveling direction to form one or more cut surfaces, including curved surfaces.

[0012] In one embodiment, the cutting step includes a step of dividing the molded body into a plurality of molded body pieces surrounded by the plurality of cutting surfaces by moving the wire running horizontally in any cutting direction perpendicular to the running direction.

[0013] In one embodiment, in the cutting step, the maximum running speed of the wire is 300 m / min or more.

[0014] In one embodiment, in the cutting step, the tension of the wire is 29.4 N (3 kgf) or more.

[0015] In one embodiment, the moving speed of the wire in the cutting direction is 100 mm / min or more and 800 mm / min or less.

[0016] In one embodiment, the moving speed of the wire in the cutting direction when forming the curved surface is 100 mm / min or more and 600 mm / min or less.

[0017] In one embodiment, the surface of the wire is of metallic composition.

[0018] In one embodiment, the step of preparing the powder compact includes a step of molding the powder by wet pressing.

[0019] In one embodiment, the method further includes a step of recovering, from the liquid, the powder particles that have been cut off from the powder compact in the cutting step.

[0020] According to the embodiments of the present disclosure, cutting with a wire saw is possible without preparing an inert atmosphere, which is excellent for mass production. Furthermore, it is also possible to process powder compacts into curved surfaces. According to the embodiments of the present disclosure, the degree of freedom in designing the shape of the powder compact is improved regardless of the mold shape of the press, making it possible to reduce manufacturing costs while maintaining the properties of a high-performance magnet.

[0021] FIG. 1 is a flowchart illustrating main steps of a manufacturing method according to an embodiment of the present disclosure. FIG. 2 is a flowchart illustrating the details of a cutting step of the manufacturing method according to an embodiment of the present disclosure. FIG. 3 is a perspective view schematically illustrating the configuration of a wire saw device used in an embodiment of the present disclosure. FIG. 4A is a front view illustrating a step of cutting a powder compact submerged in liquid with a wire. FIG. 4B is a front view illustrating a step of cutting a powder compact submerged in liquid with a metal wire. FIG. 5A is a side view illustrating a step of cutting a powder compact submerged in liquid with a wire. FIG. 5B is a side view illustrating a step of cutting a powder compact submerged in liquid with a wire. FIG. 6A is a side view illustrating a step of cutting a powder compact submerged in liquid with a wire. FIG. 6B is a side view illustrating a step of cutting a powder compact submerged in liquid with a wire. FIG. 7A is a diagram schematically illustrating a cut surface formed in a powder compact 10 by a wire saw. FIG. 7B is a diagram schematically illustrating a cut surface formed in a powder compact 10 by a wire saw. FIG. 7C is a diagram schematically illustrating a cut surface formed in the powder compact 10 by a wire saw. FIG. 8A is a diagram schematically illustrating another example of a cut surface formed in the powder compact 10 by a wire saw. FIG. 8B is a diagram schematically illustrating another example of a cut surface formed in the powder compact 10 by a wire saw. FIG. 8C is a diagram schematically illustrating another example of a cut surface formed in the powder compact 10 by a wire saw. FIG. 8D is a diagram schematically illustrating another example of a cut surface formed in the powder compact 10 by a wire saw. FIG. 8E is a diagram schematically illustrating another example of a cut surface formed in the powder compact 10 by a wire saw. FIG. 8F is a diagram schematically illustrating another example of a cut surface formed in the powder compact 10 by a wire saw. FIG. 9 is a graph illustrating how the wire traveling speed and cutting speed affect the shape of the compact piece. FIG. 10 is a graph illustrating how the wire traveling speed and cutting speed affect the shape of the compact piece. Figure 11 is a graph showing how the wire running speed affects the shape of the curved compact piece. Figure 12 is a graph showing how the wire running speed affects the shape of the curved compact piece.

[0022] An embodiment of a method for manufacturing an R-T-B based sintered magnet according to the present disclosure will now be described. As shown in the flowcharts of Figures 1 and 2, the method for manufacturing an R-T-B based sintered magnet in this embodiment includes: a milling step (S10) of preparing a powder of an alloy for an R-T-B based sintered magnet (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 molding step (S20) of producing a powder compact using the powder obtained in the milling step (S10), a cutting step (S30) of cutting the powder compact into a plurality of compact pieces, and a sintering step (S40) of sintering each of the plurality of compact pieces to produce a plurality of sintered bodies.

[0023] The cutting process (S30) includes a first processing process (S32) in which the powder molding submerged in a liquid is cut with a traveling wire to form a first cut surface, and a second processing process (S34) in which the powder molding submerged in a liquid that is the same as or different from the liquid is cut with a traveling wire that is the same as or different from the wire to form one or more second cut surfaces that intersect with the first cut surface.

[0024] In a preferred embodiment, the cutting step (S30) includes a step of cutting with a horizontally traveling wire and moving the wire in any cutting direction perpendicular to the traveling direction to form one or more cut surfaces including curved surfaces. The cutting step (S30) includes a step of dividing the molded body into multiple pieces surrounded by multiple cut surfaces including curved surfaces by moving the horizontally traveling wire in any cutting direction perpendicular to the traveling direction.

[0025] According to the method for producing an R-T-B based sintered magnet of the present disclosure, cutting with a wire is performed while the powder compact is submerged in a liquid, eliminating the need to prepare an inert atmosphere. Examples of liquids that can be used in embodiments of the present disclosure include oils such as mineral oils and synthetic oils.

[0026] Conventionally, cutting powder compacts using wire saw technology has been thought to require hard abrasive grains adhered to the surface of the metal wire constituting the wire to come into contact with the powder compact and scrape off a portion of the powder compact through friction. However, experiments conducted by the inventors have shown that when a traveling metal wire comes into contact with a powder compact submerged in liquid, the metal wire alone, without any abrasive grains attached, can grind and cut the powder compact. The inventors' investigations have revealed that a high-speed liquid flow (jet flow) is generated in the area where the metal wire traveling at a predetermined speed contacts the powder compact and its vicinity, scraping off powder particles constituting the powder compact. It is believed that some of the powder particles scraped off from the powder compact are carried by the high-speed liquid and trapped between the metal wire and the powder compact, exerting a grinding function similar to that of free abrasive grains, facilitating cutting of the powder compact. Given the mechanism by which a wire cuts a powder compact in liquid, the shape and configuration of the wire surface are not particularly limited. In other words, the wire surface may be smooth, like that of a typical piano wire.

[0027] In the cutting process, the wire running speed is preferably 300 m / min or more, and the wire tension at that time is preferably 3 kgf (29.4 N) or more, for example, 15 kgf (147 N) or less. If the wire running speed is less than 300 m / min, a sufficient flow rate required to cut the powder compact cannot be obtained, and if the wire tension is less than 3 kgf, the wire may bend, potentially reducing the flatness of the cut surface. If the wire tension exceeds 15 kgf, there is a possibility of breakage. Furthermore, in the cutting process, the cutting speed (workpiece feed speed) in the direction perpendicular to the wire running direction is preferably 100 mm / min or more. This is because if the cutting speed is less than 100 mm / min, the time required for the cutting process increases, reducing production efficiency.

[0028] When the wire diameter is 200 μm or more, the wire running speed can be 500 m / min or more. The higher the wire running speed, the higher the cutting speed can be. For example, when the wire diameter is 250 μm and the wire running speed is 500 m / min or more, the cutting speed can be 150 mm / min or more. As will be described later, the wire moving speed in the cutting direction when forming a curved surface is preferably 100 mm / min or more and 600 mm / min or less.

[0029] One advantage of cutting a powder compact in a liquid is that the temperature rise due to frictional heat at the contact point between the powder compact and the wire is suppressed, and the generated heat is easily dissipated in the liquid. In air, the powder compact, heated to a high temperature by the generated frictional heat, would react with oxygen or water vapor in the air, resulting in an increase in the oxygen concentration in the final sintered magnet and a deterioration in its magnetic properties. However, this embodiment avoids such problems.

[0030] Another advantage of cutting the powder compact in a liquid is that the powder particles scraped off from the powder compact by the wire settle in the liquid, making them easy to recover. In a preferred embodiment, the step of preparing the powder compact includes a step of wet-pressing the powder. In this case, it is desirable to wet-press the powder by adding the same type of liquid as used in the cutting step to the powder. This is because the powder particles scraped off from the powder compact in the cutting step can be easily recovered from the liquid and reused.

[0031] Furthermore, according to the manufacturing method of the present disclosure for an R-T-B based sintered magnet, horizontal and transverse cutting is performed before vertical and longitudinal cutting, allowing the surface of the powder compact to be processed and flattened. Depending on the powder pressing process, at least a portion of the surface of the powder compact (e.g., the top surface) may have irregularities, which previously required cutting or polishing after the sintering process. According to the embodiments of the present disclosure, this cutting or polishing process can be eliminated, thereby reducing manufacturing costs while maintaining the properties of a high-performance magnet.

[0032] An example of the configuration of a wire saw device that can be used in the above-described manufacturing method will be described with reference to Fig. 3. Fig. 3 is a perspective view showing an example of the configuration of a wire saw device 100 according to an embodiment of the present disclosure. For reference, the figure shows an X-axis, a Y-axis, and an X-axis that are orthogonal to each other. In this example, the XY plane is horizontal, and the Z-axis is oriented vertically.

[0033] The wire saw device 100 in Fig. 3 includes rollers 30a, 30b, and 30c arranged so that their central axes of rotation are parallel to one another, and a single continuous wire 40. Each of the rollers 30a, 30b, and 30c is rotatably supported by a support device 50. The support device 50 can be moved up and down and in the longitudinal direction (positive and negative directions of the Z axis) by a drive device (not shown). The drive device may obtain driving force from a hydraulic cylinder or may be operated by a motor. Furthermore, to perform cutting along the horizontal and lateral direction (X axis direction) described below, the support device 50 may be moved horizontally and lateral.

[0034] The powder molded body 10 produced in the molding step (S20) is fixed to a fixing base 20 by a clamp (not shown) and placed inside a tank 70 that stores a liquid 60. In FIG. 3, the tank 70 is indicated by a dashed line, and the height of the surface of the liquid 60 is indicated by a dotted line. In the example of FIG. 3, the powder molded body 10 is entirely immersed in the liquid 60. Note that instead of the support device 50 moving in the vertical and horizontal directions, the fixing base 20 may be configured to move in the vertical and horizontal directions.

[0035] Specific examples of steps for producing the powder compact 10 will be described later. It should be noted here that the powder compact 10 is not a sintered compact, but a compact (green compact) of powder before sintering. The powder compact is obtained by wet-pressing or dry-pressing a powder of an R-T-B based sintered magnet alloy (R is a rare earth element that must contain at least one selected from the group consisting of Nd, Pr, and Ce; T is at least one transition metal that must contain Fe; and B is boron) in an aligning magnetic field.

[0036] The rollers 30a, 30b, and 30c are arranged at a predetermined interval so that the axes of their rotation centers are located at the vertices of a triangle when viewed parallel to the X-axis. Multiple grooves are formed on the side surfaces of each of the rollers 31a, 31b, and 31c. The wire 40 is wound around the multiple grooves of the rollers 30a, 30b, and 30c in order. The center-to-center spacing (pitch) of the multiple grooves determines the width of the elements separated by cutting with the wire saw. Both ends of the wire 40 are wound around, for example, a collection bobbin (not shown).

[0037] The wire 40 in the embodiment of the present disclosure is a metal wire without abrasive grains fixed to its surface. In conventional wire saw technology, the wire includes a wire (core wire) and abrasive grains located on the outer surface of the wire. The average particle size of the abrasive grains is, for example, several micrometers to several tens of micrometers. A typical example of such abrasive grains is synthetic diamond, which has a higher hardness than rare earth alloys. Unlike the wire portion used in such conventional wire saw technology, the wire 40 in this embodiment is formed from a metal material such as carbon steel and can be used without elongation even when a tension of, for example, 3.0 kgf or more is applied during the cutting process. Examples of metal wire materials that can be used for the wire 40 include piano wire and high-tensile steel wire. The surface of the wire 40 may be plated. The diameter of the wire 40 is, for example, in the range of 100 μm to 350 μm, preferably in the range of 180 μm to 300 μm. If the diameter of the wire 40 is less than 100 μm, the wire 40 may stretch during cutting due to insufficient strength. The larger the diameter of the wire 40, the better the discharge of chips will be, but the amount of chips will increase, so it is desirable that the diameter be 350 μm or less.

[0038] During cutting, the rollers 30a, 30b, and 30c and the recovery bobbin rotate. The direction of rotation of the rollers 30a, 30b, and 30c depends on their arrangement and how the wire 40 is wound. In the wire saw device 100 shown in Figure 3, the rollers 30a, 30b, and 30c rotate in the same direction.

[0039] When a predetermined length of wire 40 has been wound onto one of the recovery bobbins, the recovery bobbin and rollers 30a, 30b, and 30c are rotated in the opposite direction, causing wire 40 to move in the opposite direction. By repeating this process, wire 40 can be made to reciprocate (move).

[0040] In this embodiment, the step of cutting the powder compact 10 with the wire 40 is carried out with the powder compact 10 submerged in the liquid 60. When the powder compact 10 is a powder compact formed by wet pressing, a preferred example of the liquid 60 is the same type of oil as the dispersion medium, such as an oil (mineral oil or synthetic oil), used in the wet pressing.

[0041] When a powder compact 10 is machined using such a wire saw device 100, powder particles constituting the powder compact 10 fall off as chips from the portion cut by the wire 40. These chips are powder particles that have fallen off the powder compact 10, and the individual particles do not have rough fracture surfaces like metal chips (shavings). The shape and size of the chips scraped off the powder compact by the wire are similar to the shape and size of the powder particles used to produce the powder compact 10. The present inventors have investigated the reuse of these chips. When a hard sintered compact obtained by sintering a powder compact is cut, the chips are particles or particle aggregates whose grains have grown or whose composition has changed due to chemical reactions due to sintering. Therefore, even if these chips are mixed with rare earth magnet powder and reused, the magnetic properties are likely to deteriorate. In contrast, chips obtained from a powder compact before sintering are easily reused because they have a similar composition and size to the other particles contained in the powder compact.

[0042] Furthermore, when the powder compact 10 is produced by wet pressing, if wire saw processing is performed in the same type of oil as the dispersant, the recovered powder (chips) can be used directly for wet pressing, thereby increasing production efficiency.

[0043] The method for producing the RTB based sintered magnet of this embodiment will now be described in detail.

[0044] S10: Pulverization Step In the pulverization step (S10), a powder of an alloy for an RTB based sintered magnet is prepared. Below, the composition of the alloy for an RTB based sintered magnet, the process for producing the alloy, and the process for preparing the alloy powder will be described in order.

[0045] <Composition of rare earth alloy for R-T-B based sintered magnet> R is a rare earth element and must contain at least one element selected from the group consisting of Nd, Pr, and Ce. Preferably, a combination of rare earth elements such as 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, or Nd-Pr-Ce-Dy-Tb is used.

[0046] Among R, Dy and Tb are particularly ｃＪ This is effective in improving the properties of the magnet. In addition to the above elements, other rare earth elements such as La may be contained, and misch metal or didymium may also be used. Furthermore, R does not have to be a pure element; it may contain impurities unavoidable during production as long as they are industrially available. The R content is, for example, 27% to 35% by mass. The R content of the R-T-B based sintered magnet is preferably 31% by mass or less (27% to 31% by mass, preferably 29% to 31% by mass). By controlling the R content of the R-T-B based sintered magnet to 31% by mass or less and the oxygen content to 500 ppm to 3500 ppm (preferably 500 ppm to 3200 ppm, more preferably 500 ppm to 2500 ppm), better magnetic properties can be obtained.

[0047] T contains iron (including cases where T consists essentially of iron), and up to 50% by mass of iron may be substituted with cobalt (Co) (including cases where T consists essentially 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 account for the remainder of R and B, or R, B, and M, which will be described later.

[0048] The content of B may be a known content, and for example, a preferred range is 0.9 mass % to 1.2 mass %. If it is less than 0.9 mass %, high H ｃＪ If it exceeds 1.2 mass%, B may not be obtained. ｒ In addition, a part of B can be substituted with C (carbon).

[0049] In addition to the above elements, H ｃＪ To improve the properties, an M element can be added. The M element is one or more elements 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 M element added is preferably 5.0 mass% or less. This is because if it exceeds 5.0 mass%, Br may decrease. In addition, unavoidable impurities are also acceptable.

[0050] The N (nitrogen) content in the RTB based sintered magnet is preferably 50 ppm to 600 ppm, and the C (carbon) content in the RTB based sintered magnet is preferably 50 ppm to 1000 ppm.

[0051] <Manufacturing Process for Alloys for R-T-B Based Sintered Magnets> An example of a manufacturing process for alloys for R-T-B based sintered magnets is shown below. An alloy ingot can be obtained by ingot casting, in which a metal or alloy previously prepared to have the composition described above is melted and poured into a mold. Alternatively, alloy flakes can be produced by a rapid cooling method, such as strip casting or centrifugal casting, in which the molten metal is brought into contact with a single roll, twin rolls, rotating disk, or rotating cylindrical mold, and quenched to produce a solidified alloy that is thinner than the alloy produced by the ingot method.

[0052] In embodiments of the present disclosure, materials produced by either the ingot method or the quenching method can be used, but production by a quenching method such as strip casting is preferred. The thickness of quenched alloys produced by the quenching method is typically in the range of 0.03 mm to 1 mm and is in the form of flakes. The molten alloy begins to solidify at the surface that contacts the chill roll (the roll contact surface), and crystals grow columnarly from the roll contact surface in the thickness direction. Compared to alloys (ingot alloys) produced by conventional ingot casting (mold casting), quenched alloys are cooled in a shorter time, resulting in a finer structure and smaller crystal grain size. They also have a larger grain boundary area. Because the R-rich phase spreads widely within the grain boundaries, the quenching method provides excellent dispersibility of the R-rich phase. Therefore, hydrogen pulverization makes it easier for the alloy to fracture at the grain boundaries. By hydrogen pulverizing the quenched alloy, the size of the hydrogen pulverized powder (coarsely pulverized powder) can be reduced to, for example, 1.0 mm or less. The coarsely pulverized powder obtained in this manner is then finely pulverized, for example, using a jet mill.

[0053] <Step of Preparing Alloy Powder for R-T-B Based Sintered Magnets> Rare earth alloy powder for R-T-B based sintered magnets is active and easily oxidized. For this reason, the gas used in the jet mill is an inert gas such as nitrogen, argon, or helium, in order to avoid the risk of heat generation and fire and to reduce the oxygen content as an impurity, thereby improving the performance of the magnet.

[0054] The material to be pulverized (coarsely pulverized powder) fed into the jet mill is pulverized into fine powder having a particle size distribution, for example, with an average particle size (median diameter: d50) of 2.0 μm or more and 4.5 μm or less, and then transferred to a cyclone collector. The cyclone collector is used to separate the powder from the airflow carrying the powder. Specifically, coarsely pulverized powder of an R-T-B-based sintered magnet alloy is pulverized in a jet mill in the upstream stage, and the fine powder generated by the pulverization is supplied to the cyclone collector together with the gas used for pulverization. A mixture of an inert gas (pulverization gas) and the pulverized fine powder forms a high-velocity airflow and is sent to the cyclone collector. The cyclone collector is used to separate the pulverization gas from the fine powder. The fine powder separated from the pulverization gas is collected in a powder collector.

[0055] S20: Molding Step In the molding step (S20), a powder compact is produced using the powder obtained in the pulverizing step (S10).

[0056] In this embodiment, a powder compact is produced from the above powder by pressing in a magnetic field. When pressing in a magnetic field, it is preferable to form the powder compact by pressing in an inert gas atmosphere or wet pressing, from the viewpoint of suppressing oxidation. In particular, wet pressing coats the surfaces of the particles constituting the powder compact with a dispersant such as an oil, suppressing contact with oxygen and water vapor in the atmosphere. Therefore, oxidation of the particles by the atmosphere before, during, or after the pressing process can be prevented or suppressed.

[0057] When wet pressing in a magnetic field is performed, a slurry is prepared by mixing a fine powder with a dispersion medium, and the slurry is supplied to a cavity in a mold of a wet pressing device and press-molded in a magnetic field. The powder compact thus formed has a density of, for example, 4 g / cm ３ 5g / cm or more ３ It has the following density:

[0058] Dispersion Medium The dispersion medium is a liquid in which the alloy powder can be dispersed to obtain a slurry.

[0059] Preferred dispersion media used in the present disclosure include mineral oils and synthetic oils. While the type of mineral oil or synthetic oil is not limited, if the kinematic viscosity at room temperature exceeds 10 cSt, the increased viscosity may strengthen the bonding strength between the alloy powders, adversely affecting the orientation of the alloy powder during wet compaction in a magnetic field. For this reason, the kinematic viscosity of the mineral oil or synthetic oil at room temperature is preferably 10 cSt or less. Furthermore, if the distillation point of the mineral oil or synthetic oil exceeds 400°C, deoiling after obtaining a compact becomes difficult, resulting in increased residual carbon in the sintered body and possibly degrading the magnetic properties. Therefore, the distillation point of the mineral oil or synthetic oil is preferably 400°C or less. Vegetable oil may also be used as the dispersion media. Vegetable oil refers to oil extracted from plants, and the type of plant is not limited to a specific plant.

[0060] Preparation of Slurry The obtained alloy powder and a dispersion medium are mixed to obtain a slurry.

[0061] 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 (i.e., 70% by mass or more) in terms of mass ratio. ３ This is because a flow rate of 1 / sec allows the alloy powder to be efficiently supplied into the cavity and excellent magnetic properties to be obtained. The concentration of the alloy powder in the slurry is preferably 90% or less by mass. The method for mixing the alloy powder and the dispersion medium is not particularly limited. The alloy powder and the dispersion medium may be prepared separately, weighed out in predetermined amounts, and mixed together. Alternatively, when dry-pulverizing the coarsely pulverized powder using a jet mill or the like to obtain the alloy powder, a container containing the dispersion medium may be placed at the alloy powder outlet of the jet mill or other grinding device, and the alloy powder obtained by grinding may be directly recovered in the dispersion medium in the container to obtain the slurry. In this case, the container may also be filled with a nitrogen and / or argon gas atmosphere, and the obtained alloy powder may be directly recovered in the dispersion medium without being exposed to the air to form the slurry. Furthermore, the coarsely pulverized powder may be wet-pulverized in the dispersion medium using a vibrating mill, ball mill, attritor, or the like to obtain a slurry consisting of the alloy powder and the dispersion medium.

[0062] The resulting slurry is molded in a known wet press to obtain a powder compact having a predetermined size and shape. Conventionally, this powder compact is typically sintered to obtain a sintered body, but in this embodiment, the powder compact is divided using a wire saw device before sintering, as described below.

[0063] S30: Cutting Step In the cutting step (S30), the powder compact is cut and divided into a plurality of compact pieces.

[0064] First, a method for forming a flat cut surface by making vertical or horizontal cuts will be described, and then a method for forming a cut surface that expands in an oblique direction or a curved cut surface will be described.

[0065] The cutting of the powder compact in this process is performed, for example, by a wire saw device as shown in Fig. 3. Figs. 4A and 4B are front views illustrating the process of cutting a powder compact 10 submerged in a liquid 60 with a wire 40. Fig. 4A shows the state before the cutting process begins, and Fig. 4B shows the state during the cutting process. The dashed line within the powder compact 10 shown in Fig. 4B schematically indicates the position of the wire 40 while cutting the powder compact 10. When the position indicated by the dashed line of the wire 40 moves downward from the top surface of the powder compact 10 and reaches the bottom surface of the powder compact 10, the powder compact 10 is divided into multiple compact pieces.

[0066] In the illustrated example, the wire 40 travels in the Y-axis direction at a predetermined speed while moving in a direction perpendicular to the traveling direction of the wire 40 (the negative direction of the Z-axis). The direction perpendicular to the traveling direction of the wire 40 is the cutting direction, and the speed in this direction (cutting speed) is set to, for example, 100 mm / min or more. In the example shown in Fig. 4B, the traveling wire 40 moves in the negative direction of the Z-axis relative to the powder compact 10 in a stationary state, but the powder compact 10 may also be lifted together with the fixing base 20 in the positive direction of the Z-axis.

[0067] 5A and 5B are side views illustrating the process of cutting a powder compact 10 submerged in a liquid 60 with a wire 40. Fig. 5A shows the state before the cutting process starts, and Fig. 5B shows the state during the cutting process. In the illustrated example, one powder compact 10 is divided into eight compact pieces.

[0068] The diameter of the wire 40 is, for example, 100 μm or more and 350 μm or less. The running speed of the wire 40 (wire linear speed) can be set, for example, in the range of 100 m / min or more and 800 m / min or less. Meanwhile, the cutting speed (the wire feed speed relative to the powder compact 10 in the negative direction of the Z axis in FIG. 3) can be set, for example, in the range of 100 mm / min or more and 600 mm / min or less. The tension applied to the wire 40 is, for example, 3 kgf or more and 15 kgf or less. The tension can be adjusted, for example, by adjusting the distance between roller 30c and rollers 30a and 30b. By wire saw cutting, the powder compact 10 can be divided into compact pieces, for example, with a thickness of approximately 1 to 10 mm. The thickness of the compact pieces is determined by the spacing between the wires 40 and the diameter of the wires 40, as shown in FIG. 5B.

[0069] Performing wire sawing in a liquid has the advantage of facilitating the discharge of chips. Furthermore, as described above, by performing wire sawing while immersing powder compact 10 in the dispersion medium (mineral oil or synthetic oil) used to produce powder compact 10 by wet pressing (oil cutting), powder particles that have settled in the liquid during wire sawing can be recovered and reused directly in the molding process.

[0070] 6A and 6B are side views illustrating the process of horizontally cutting a powder compact 10 submerged in a liquid 60 with a wire 40. In the illustrated example, during the cutting process, rollers 30a, 30b, and 30c move horizontally (in the direction of the rotation axis of each roller) relative to the powder compact 10. Before performing the process described with reference to FIGS. 4A to 5B , horizontal cutting with the wire 40 can flatten the surface of the powder compact 10. Depending on the powder pressing process, at least a portion of the surface of the powder compact 10 (e.g., the top surface) may have irregularities. For example, after filling the die hole of a powder pressing machine with powder, a filter cloth may be placed between the punch and the powder before pressing the powder with a punch. A dispersant (oil) may be dispensed through the filter cloth. In this case, the filter cloth may form irregularities on the top surface of the resulting powder compact.

[0071] In the embodiment of the present disclosure, such uneven surfaces are removed by a wire before the sintering process, so that the process of cutting or polishing for flattening after the sintering process can be omitted.

[0072] 7A to 7C are diagrams schematically illustrating cut surfaces formed in a powder compact 10 by a wire saw. In the process (first process) described with reference to FIGS. 6A and 6B , a running wire 40 moves along the dashed lines 11 c in FIG. 7A on the powder compact 10 submerged in liquid 60, thinly cutting the roughened surface region 10T of the powder compact 10 to form first cut surfaces 11 perpendicular to the Z-axis direction. Then, in the process (second process) described with reference to FIGS. 5A and 5B , a plurality of second cut surfaces 12 intersecting the first cut surfaces 11 are formed. In the second process, the second cut surfaces 12 are formed by moving the running wire along the dashed lines 12 c. The first process and the second process may be performed using the same wire saw device or different wire saw devices. In other words, in the second processing step, the powder compact may be cut with the same wire while submerged in the same liquid as the powder compact in the first processing step, or may be cut with a different wire while submerged in a different liquid.

[0073] 7A to 7C, the first cut surface 11 is parallel to the horizontal plane, and the second cut surface 12 is perpendicular to the first cut surface 11. The orientations of the first cut surface 11 and the second cut surface 12 are not limited to this example.

[0074] The second cutting speed is preferably, for example, 100 mm / min or more and 800 mm / min or less.

[0075] The technical effect of performing the first process step of cutting the powder compact in the horizontal and transverse directions before performing the second process step of cutting the powder compact in the vertical and longitudinal directions is not limited to the case where a wire made of metal wire is used, but can also be obtained when a wire with abrasive grains fixed to the surface of the metal wire is used. However, when cutting a powder compact in a liquid, it is preferable to use a metal wire without abrasive grains fixed to the surface, because this avoids problems caused by abrasive grains falling off.

[0076] 8A to 8F are diagrams illustrating a process for dividing a powder compact 10 submerged in a liquid 60 into a plurality of compact pieces surrounded by a plurality of cut surfaces, including curved surfaces, by moving a wire 40 in an arbitrary cutting direction perpendicular to the traveling direction of the wire 40. In the illustrated example, during the cutting process, rollers 30a, 30b, and 30c that rotate the wire 40 and a support device 50 can move simultaneously in both the Z-axis direction and the X-axis direction.

[0077] 8B, the wire 40 moves horizontally in the negative direction of the X-axis, resulting in a horizontal cut surface being formed on the powder compact 10.

[0078] Next, in the process of changing the state from Fig. 8B to the state of Fig. 8C, the wire 40 moves vertically in the negative direction of the Z axis, resulting in the formation of a vertical cut surface in the powder compact 10.

[0079] Furthermore, in the process of changing from the state of Fig. 8C to the state of Fig. 8D, wire 40 moves in the positive direction of the X-axis while moving in the negative direction of the Z-axis. As a result, a curved cut surface that is convex downward is formed in powder compact 10. In this case, for example, if the moving speed in the X-axis direction is constant, an arbitrary curved surface can be formed by gradually changing the moving speed in the Z-axis direction.

[0080] 8E, wire 40 moves in the positive direction of the Z axis while moving in the positive direction of the X axis. As a result, a downwardly convex curved cut surface is formed in powder compact 10.

[0081] Finally, in the process of changing the state from Fig. 8E to Fig. 8F, wire 40 moves vertically in the positive direction of the Z axis, resulting in a vertical cut surface being formed in powder compact 10, ultimately resulting in a compact piece with a downwardly convex, semi-cylindrical shape.

[0082] 8A to 8F, one green compact piece is formed from one powder green compact 10, but the present invention is not limited to this example. By controlling the movement of the wire 40, multiple green compact pieces can be formed from one powder green compact 10. Furthermore, the shape and size of the green compact piece to be formed are not limited to the examples shown in the drawings.

[0083] S40: Sintering Step In the sintering step (S40), each of the plurality of compact pieces is sintered to produce a plurality of sintered bodies. That is, the individual compact pieces cut by the wire saw step described above are sintered to obtain R-T-B based sintered magnets (sintered bodies). The compact piece sintering step is carried out, for example, under a pressure of 0.13 Pa (10 －３ Torr), preferably 0.07 Pa (5.0 x 10 －４ The sintering process can be carried out under a pressure of 1000 Torr or less, at a temperature ranging from 1000°C to 1150°C, for example. To prevent oxidation during sintering, residual gas in the atmosphere can be replaced with an inert gas such as helium or argon. The resulting sintered body is preferably subjected to additional heat treatment such as aging. Such heat treatment can improve its magnetic properties. Known conditions can be used for the heat treatment, such as the heat treatment temperature and time. The R-T-B based sintered magnet obtained in this manner is then subjected to grinding / polishing, surface treatment, and magnetization processes as necessary to produce the final R-T-B based sintered magnet.

[0084] In a preferred embodiment, the method for producing an R-T-B based sintered magnet according to the present disclosure further comprises a diffusion step of diffusing a heavy rare earth element RH (where RH is at least one of Tb, Dy, and Ho) from the surface of the sintered body to the interior thereof. Diffusing the heavy rare earth element RH from the surface to the interior of the sintered body can efficiently increase the coercive force. There are no particular restrictions on the method for the diffusion step; any known method can be used.

[0085] (Example) The raw materials for each element were weighed to have a composition of 22.6% Nd, 7.8% Pr, 0.9% B, 0.5% Co, 0.1% Al, 0.2% Cu, 0.4% Ga (all by mass), with the balance being Fe, and an alloy was produced by strip casting. The obtained alloy was subjected to hydrogen pulverization to obtain a coarsely pulverized powder.

[0086] Next, the resulting coarsely pulverized powder was mixed with 0.04% by mass of zinc stearate as a lubricant based on 100% by mass of the coarsely pulverized powder, and then dry-pulverized in a nitrogen stream using a jet mill to obtain a finely pulverized powder (alloy powder) with a particle size D50 of 4 μm. The finely pulverized powder was then immersed in a nitrogen atmosphere in mineral oil with a distillation point of 250°C and a kinematic viscosity of 2 cSt at room temperature to prepare a slurry. The slurry concentration was 85% by mass. The resulting slurry was then compacted (wet-molded) in a magnetic field to produce a powder compact. The powder compact measured 80 mm x 45 mm x 60 mm.

[0087] The powder compact was divided into eight compact pieces using a 250 μm diameter wire (metallic wire made of piano wire). The cutting with a wire saw was performed while the powder compact was submerged in liquid (the liquid used was the same mineral oil as used during molding). Each powder compact was cut using eight wires (multi-wire) running in parallel. The tension applied to the wires during cutting was 10 kg, and the roller spacing was 250 mm.

[0088] 9 is a graph showing the effect of wire running speed and cutting speed on the shape of the compact pieces. The horizontal axis of the graph is wire running speed [m / min], and the vertical axis is cutting speed [mm / min]. An "x" in this graph indicates that a "crack" occurred in a part of the compact piece separated by wire saw cutting, and an "o" indicates that no such crack occurred in the compact piece and that the compact piece was separated into a good shape.

[0089] When a wire with a diameter of 250 μm was used, crack-free molded pieces could be obtained at a cutting speed of 100 to 150 mm / min at a running speed of 300 m / min. Furthermore, at a running speed of 500 m / min, crack-free molded pieces could be obtained at a cutting speed of 250 mm / min. Furthermore, at a running speed of 700 m / min, the wire did not bend during cutting, and crack-free molded pieces could be obtained even at a cutting speed of 400 mm / min. Therefore, it is preferable that the maximum running speed of the wire be 300 m / min or more.

[0090] When a wire with a diameter of 160 μm was used, good compact pieces could be separated when both the running speed and the cutting speed were relatively low. The smaller the wire diameter, the easier it is to stretch and bend. Therefore, applying high tension and running at high speed is thought to increase the likelihood of cracking and chipping during cutting of the powder compact. For this reason, it is preferable that the diameter of the wire (metal wire) be 200 μm or more. The larger the wire diameter, the greater the amount of cutting required, but normal cutting is possible.

[0091] For comparison, it was confirmed that even if an attempt was made to cut a powder compact placed in the air using only a metal wire, it would not be possible to cut it properly, and that contact between the traveling metal wire and the powder compact must be made in a liquid (preferably oil).

[0092] FIG. 10 shows the experimental results when the upper surface region of a powder compact was cut in oil in the horizontal direction with a single wire, as shown in FIGS. 6A and 6B. "Lateral feed" refers to the horizontal cutting speed, and "linear speed" refers to the wire running speed. When a wire with a diameter of 250 μm was used, crack-free compact pieces were obtained at a cutting speed of 100 to 300 mm / min at a running speed of 300 m / min. Furthermore, crack-free compact pieces were obtained at a cutting speed of 300 to 500 mm / min at a running speed of 500 m / min. Furthermore, crack-free compact pieces were obtained at a cutting speed of 500 mm / min even at a running speed of 700 m / min.

[0093] To cut off the upper surface of the powder compact by "transverse feeding," it is preferable that the powder compact has sufficient "hardness." The hardness of the powder compact can be evaluated, for example, by the molding pressure or density during powder molding. If the density of the powder compact in air is 4 g / cm ３ It was found that if the density of the powder compact is less than 4 g / cm3, the cut surface will not be smooth. ３ It is preferable that this is equal to or greater than this.

[0094] Figures 11 and 12 are graphs showing how the wire movement speed in the cutting direction affects the shape of a curved molded piece when forming a semi-cylindrical molded piece. The data in Figure 11 was obtained when the wire tension was 9 kgf (88.2 N), and the data in Figure 12 was obtained when the wire tension was 12 kgf (117.6 N). The wire diameter was 250 μm. In this example, when cutting the curved surface, the wire cutting was advanced in the direction of increasing position (coordinate) in the X-axis direction. It can be seen that the position of the cut surface deviates from the target position in the latter half of the cutting. This is due to wire deflection.

[0095] According to the inventor's investigation, it was found that the desired curved surface shape can be cut with a good yield by adjusting the wire tension if the wire movement speed in the cutting direction is 100 mm / min or more and 600 mm / min or less. The tension is preferably 9 kgf (88.2 N) or more. Furthermore, the wire movement speed in the cutting direction is preferably 540 mm / min or less.

[0096] 10: powder compact, 20: fixing base, 30a, 30b, 30c: rollers, 40: wire, 50: support device, 60: liquid, 70: tank, 100: wire saw device

Claims

1. a milling step of preparing a powder of an alloy for an R-T-B based sintered magnet (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 molding step of producing a powder compact using the powder; a cutting step of cutting the powder compact into a plurality of compact pieces; a sintering step of sintering each of the plurality of compact pieces to produce a plurality of sintered bodies; Including, The cutting step includes: a cutting step of cutting the powder compact submerged in a liquid with a wire traveling horizontally, and moving the wire in any cutting direction perpendicular to the wire traveling direction to form one or more cut surfaces, including curved surfaces.

2. 2. The method for producing an R-T-B based sintered magnet according to claim 1, wherein the cutting step includes a step of dividing the molded body into a plurality of pieces surrounded by the plurality of cut surfaces by moving the wire, which runs horizontally, in an arbitrary cutting direction perpendicular to the running direction.

3. 2. The method for producing a sintered RTB based magnet according to claim 1, wherein the maximum running speed of the wire in the cutting step is 300 m / min or more.

4. 2. The method for producing a sintered RTB based magnet according to claim 1, wherein the tension applied to the wire in the cutting step is 29.4 N (3 kgf) or more.

5. 2. The method for producing a sintered RTB based magnet according to claim 1, wherein the moving speed of the wire in the cutting direction is 100 mm / min or more and 800 mm / min or less.

6. 6. The method for producing a sintered RTB based magnet according to claim 5, wherein the wire is moved at a speed of 100 mm / min or more and 600 mm / min or less in the cutting direction when forming the curved surface.

7. The method for producing an RTB based sintered magnet according to any one of claims 1 to 6, wherein the surface of the wire has a metallic composition.

8. 7. The method for producing a sintered RTB based magnet according to claim 1, wherein the step of preparing the powder compact includes a step of compacting the powder by wet pressing.

9. 7. The method for producing a sintered R-T-B based magnet according to claim 1, further comprising the step of recovering from the liquid the powder particles that have been cut from the powder compact in the cutting step.