Manufacturing method of R-T-B series sintered magnets

Abstract

This invention provides a method for manufacturing R-T-B based sintered magnets that does not require the preparation of an inert atmosphere. The method includes: a pulverizing step of preparing powder for an alloy used in R-T-B sintered magnets; a molding step of using the powder to form a powder molded body; a cutting step of cutting the powder molded body into multiple molded body pieces; and a sintering step of sintering each of the multiple molded body pieces to produce multiple sintered bodies. The cutting step includes the step of cutting the powder molded body immersed in a liquid using a traveling metal wire.

Description

Technical Field

[0001] This invention relates to a method for manufacturing R-T-B sintered magnets. Background Technology

[0002] R-T-B series sintered magnets (R is a rare earth element, which must contain at least one selected from Nd, Pr, and Ce; T is at least one transition metal, which must contain Fe; B is boron) are made of R2Fe 14 The compound with a B-type crystalline structure consists of the main phase, the grain boundary phase located at the grain boundary of the main phase, and the compound phase formed due to the influence of trace added elements or impurities. R-T-B series sintered magnets exhibit high residual magnetic flux density B. r (Hereinafter, it may sometimes be simply written as "B") r ") and high coercivity H cJ (Hereinafter, it may sometimes be simply referred to as "H") cJ It possesses excellent magnetic properties and is therefore known as the highest-performing permanent magnet. Consequently, R-T-B 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 household appliances.

[0003] Such R-T-B sintered magnets are manufactured, for example, through a process of preparing alloy powder, a process of pressing the alloy powder into a powder mold to produce a powder molded body, and a process of sintering the powder molded body. The alloy powder is produced by, for example, the following method.

[0004] First, alloys are manufactured from molten metals of various raw materials using methods such as ingot casting or strip casting. The resulting alloy is then subjected to a pulverizing process to obtain alloy powder with a specified particle size distribution. This pulverizing process typically includes a coarse pulverizing process and a fine pulverizing process; the former utilizes phenomena such as hydrogen embrittlement, while the latter uses, for example, an air jet mill.

[0005] The sintered body obtained through the process of sintering powder molding is then subjected to machining such as grinding and cutting to be single-piece into a desired shape and size. More specifically, firstly, R-Fe-B based rare-earth magnet powder is compressed and molded using a pressing device to create a molded body larger than the final magnet product. Then, after the molded body is sintered, it is ground using, for example, a superhard alloy saw or a rotary grinding stone to give it the desired shape. For example, after first creating a block-shaped sintered body, it is sliced ​​using a saw or similar tool to cut out multiple plate-shaped sintered body portions.

[0006] However, the sintered bodies of rare earth alloy magnets, such as R-Fe-B sintered magnets, are extremely hard and brittle, and the machining load is high, making high-precision grinding a difficult and time-consuming operation. Furthermore, material loss due to machining is inevitable. Therefore, machining processes become a major cause of increased manufacturing costs.

[0007] To address issues such as the former, Patent Document 1 discloses a technique for processing a magnet molded body using a wire saw before sintering. The wire saw technique involves pressing a saw wire traveling in one or both directions onto the molded body to be processed, and grinding or cutting the molded body using abrasive grains located between the saw wire and the molded body. According to this technique, powder molded bodies that are in a state that is significantly softer and easier to process than sintered bodies can be cut, thus greatly reducing the time required for cutting.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 2003-303728 Summary of the Invention

[0011] The technical problem that the invention aims to solve

[0012] Patent Document 1 discloses a wire saw using a saw wire with an outer diameter of 0.1 mm to 1.0 mm and abrasive grains fixed to the saw wire, for processing powder molded bodies in an inert gas atmosphere where the oxygen concentration is adjusted to a total of 5% to 18% on a molar basis. Wire sawing in such an inert atmosphere with controlled oxygen concentration complicates equipment and management, resulting in poor mass production capabilities.

[0013] The embodiments of the present invention provide a novel method for manufacturing R-T-B sintered magnets that can perform a wire sawing process without the need for preparing an inert atmosphere.

[0014] Technical solutions for solving technical problems

[0015] This invention provides a method for manufacturing an R-T-B sintered magnet. In an exemplary embodiment, the method includes: a pulverizing step of preparing an alloy powder for an R-T-B sintered magnet (R is a rare earth element, which must contain at least one selected from Nd, Pr, and Ce; T is at least one transition metal, which must contain Fe; and B is boron); a molding step of using the powder to make a powder molded body; a cutting step of cutting the powder molded body into multiple molded body pieces; and a sintering step of sintering each of the multiple molded body pieces to produce multiple sintered bodies. The cutting step includes the step of cutting the powder molded body immersed in a liquid by a traveling metal wire.

[0016] In one embodiment, during the cutting process, the traveling speed of the metal wire is 300 m / min or more.

[0017] In one embodiment, during the cutting process described above, the tension of the metal wire is 3 kgf or more.

[0018] In one embodiment, during the cutting process described above, the cutting speed in a direction orthogonal to the traveling direction of the metal wire is 100 mm / min or more.

[0019] In one embodiment, the process of preparing the powder molded article includes the step of molding the powder by wet pressing.

[0020] In one embodiment, the wet pressing is performed by adding a liquid of the same type as the liquid used in the cutting process to the powder.

[0021] In one embodiment, the process further includes a step of recovering particles of the powder that have been removed from the powder molded body by the cutting process from the liquid.

[0022] Invention Effects

[0023] According to embodiments of the present invention, no inert atmosphere is required, and cutting can be performed using a wire saw, resulting in excellent mass production capabilities. Furthermore, it is possible to prevent abrasive particles detached from the wire saw from mixing into the cutting powder. Therefore, it is easy to reuse the cutting powder generated from the powder-molded body during the cutting process using a wire saw for use in the manufacture of magnets, maintaining the characteristics of high-performance magnets and reducing manufacturing costs. Attached Figure Description

[0024] Figure 1 This is a flowchart illustrating the main steps of the manufacturing method according to an embodiment of the present invention.

[0025] Figure 2 This is a perspective view schematically illustrating the configuration of the wire saw device used in an embodiment of the present invention.

[0026] Figure 3A This is a front view used to illustrate the process of cutting a powder molded body immersed in liquid using a wire saw.

[0027] Figure 3B This is a front view used to illustrate the process of cutting a powder molded body immersed in a liquid using a wire saw made of metal wire.

[0028] Figure 4A This is a side view used to illustrate the process of cutting a powder molded body immersed in a liquid using a wire saw.

[0029] Figure 4BThis is a side view used to illustrate the process of cutting a powder molded body immersed in a liquid using a wire saw.

[0030] Figure 5A This is a side view used to illustrate the process of cutting a powder molded body immersed in a liquid using a wire saw.

[0031] Figure 5B This is a side view used to illustrate the process of cutting a powder molded body immersed in a liquid using a wire saw.

[0032] Figure 6A This is a schematic diagram showing the cut surface of the powder molded body 10 formed by a wire saw.

[0033] Figure 6B This is a schematic diagram showing the cut surface of the powder molded body 10 formed by a wire saw.

[0034] Figure 6C This is a schematic diagram showing the cut surface of the powder molded body 10 formed by a wire saw.

[0035] Figure 7 It is a graph showing how the saw travel speed and cutting speed affect the shape of the shaped piece.

[0036] Figure 8 It is a graph showing how the saw travel speed and cutting speed affect the shape of the shaped piece.

[0037] Symbol Explanation

[0038] 10…Powder molding body; 20…Fixing base; 30a, 30b, 30c…Rollers; 40…Saw wire; 50…Support device; 60…Liquid; 70…Trench; 100…Wire saw device. Detailed Implementation

[0039] The following describes an embodiment of the manufacturing method of the R-T-B system sintered magnet of the present invention. For example... Figure 1 As shown in the flowchart, the manufacturing method of the R-T-B system sintered magnet in this embodiment includes:

[0040] • A pulverizing process (S10) for preparing powder of R-T-B system sintered magnet alloy (R is a rare earth element, which must contain at least one selected from Nd, Pr and Ce, T is at least one transition metal, which must contain Fe, and B is boron).

[0041] • Molding process for producing powder molded articles using powder (S20);

[0042] • Cutting process (S30) to cut the powder molded body into multiple molded body pieces;

[0043] • A sintering process (S40) in which each of the multiple shaped body pieces is sintered to produce multiple sintered bodies.

[0044] The cutting process (S30) includes the step of cutting the powder molded body immersed in the liquid by a traveling metal wire.

[0045] According to the method for manufacturing R-T-B sintered magnets of the present invention, the powder molded body is cut using a wire saw while immersed in a liquid, therefore, it is not necessary to prepare an inert atmosphere. Examples of liquids that can be used in embodiments of the present invention are oils such as mineral oil or synthetic oil.

[0046] Currently, to cut powder molded bodies using a wire saw, it is believed that hard abrasive grains fixed to the surface of the metal wire constituting the wire saw need to contact the powder molded body and remove a portion of it through friction. However, the results of experiments conducted by the inventors show that when the traveling metal wire comes into contact with the powder molded body immersed in liquid, even using only a metal wire without fixed abrasive grains, it is possible to grind and cut the powder molded body. The inventors' research shows that a high-speed liquid flow (jet flow) is generated in and around the area where the metal wire traveling at a specified speed contacts the powder molded body, thereby removing the powder particles constituting the powder molded body. It is believed that a portion of the powder particles removed from the powder molded body is trapped between the metal wire and the powder molded body by the high-speed flowing liquid, performing the same grinding function as free abrasive grains and promoting the cutting of the powder molded body. From the mechanism of cutting powder molded bodies with a wire saw in liquid, it is believed that the shape and morphology of the saw wire surface are not particularly limited. In other words, the surface of the saw wire can be as smooth as ordinary piano wire.

[0047] In the cutting process, the saw wire travel speed is preferably 300 m / min or higher, and the saw wire tension is preferably 3 kgf (29.4 N) or higher, for example, 15 kgf (147 N) or lower. When the saw wire travel speed is lower than 300 m / min, the sufficient flow rate required to cut the powder molded body cannot be obtained. When the saw wire tension is lower than 3 kgf, the saw wire may deflect, and the flatness of the cut surface may decrease. When the saw wire tension exceeds 15 kgf, problems such as saw wire breakage may occur. In addition, in the cutting process, the cutting speed (workpiece conveying speed) in the direction orthogonal to the saw wire travel direction is preferably 100 mm / min or higher. This is because when the cutting speed is lower than 100 mm / min, the time required for the cutting process increases, and the production efficiency decreases.

[0048] Furthermore, when the diameter of the saw wire is 200μm or more, the travel speed of the saw wire can be set to 500m / min or more. The higher the travel speed of the saw wire, the higher the cut speed can be. For example, when the diameter of the saw wire is 250μm and the travel speed of the saw wire is set to 500m / min or more, the cut speed can be set to 250mm / min or more.

[0049] One advantage of cutting powder molded bodies in a liquid is that it can suppress the temperature rise caused by frictional heat in the part where the powder molded body contacts the wire saw, and the generated heat is also easily dispersed in the liquid. If the powder molded body becomes hot due to the frictional heat generated in the atmosphere, it will react with oxygen or water vapor in the atmosphere, leading to an increase in the oxygen concentration in the final sintered magnet and a deterioration in the magnet properties. In this embodiment, this problem can be avoided.

[0050] Another advantage of cutting the powder molded body in a liquid is that the powder particles removed from the powder molded body by the wire saw settle in the liquid and are easily recovered. In a preferred embodiment, the process of preparing the powder molded body includes a step of forming the powder by wet pressing. In this case, wet pressing is preferably performed by adding a liquid of the same type as the liquid used in the cutting process to the powder. This is because it is easy to recover and reuse the powder particles removed from the powder molded body by the cutting process from the liquid.

[0051] Furthermore, it is known that even if the cutting speed of the wire saw is directed horizontally, it is still possible to cut the powder molded body in a liquid. On the surface of the powder molded body, at least a portion (e.g., the upper surface) sometimes has unevenness due to the powder pressing process, requiring cutting or grinding after the sintering process. According to embodiments of the present invention, this cutting or grinding process can be eliminated; therefore, the characteristics of the high-performance magnet can be maintained, and manufacturing costs can be reduced.

[0052] Reference Figure 2 An example of the configuration of a wire saw device that can be used in the above manufacturing method is explained. Figure 2 This is a perspective view showing an example of the configuration of a wire saw apparatus 100 according to an embodiment of the present invention. For reference, the X-axis, Y-axis, and Z-axis are shown as mutually orthogonal. In this example, the XY plane is horizontal, and the Z-axis faces the vertical direction.

[0053] Figure 2The wire saw device 100 includes rollers 30a, 30b, and 30c arranged parallel to each other about their central axes of rotation; and a continuous saw wire 40. Rollers 30a, 30b, and 30c are each rotatably supported by a support device 50. The support device 50 can move vertically (in the positive and negative directions of the Z-axis) via a drive device (not shown). The drive device can be driven by a hydraulic cylinder or by an electric motor. Furthermore, since cutting is performed along the horizontal transverse direction (X-axis direction, described later), the support device 50 can also move horizontally.

[0054] The powder molded body 10 produced in the molding process (S20) is fixed to the fixing base 20 by a clamp part (not shown) and placed inside the tank 70 for storing liquid 60. Figure 2 In the diagram, tank 70 is represented by a dashed line, and the height of the liquid 60 surface is represented by a dotted line. Figure 2 In this example, the entire powder molded body 10 is immersed in the liquid 60. Alternatively, instead of the support device 50 moving in the vertical longitudinal and horizontal directions, the fixed base 20 can be configured to move in the vertical longitudinal and horizontal directions.

[0055] The following describes a specific example of the process for fabricating the powder compact 10. It is important to note that the powder compact 10 is not a sintered body, but rather a compact of powder before sintering (green compact). The powder compact is obtained by molding powder of an R-T-B system sintering magnet alloy (R is a rare earth element, which must contain at least one selected from Nd, Pr, and Ce; T is at least one transition metal, which must contain Fe; and B is boron) in an orientation magnetic field using either wet or dry pressing.

[0056] Viewed from a direction parallel to the X-axis, rollers 30a, 30b, and 30c are arranged at predetermined intervals with their rotation centers located at the vertices of a triangle. Multiple grooves are provided on the sides of each of rollers 30a, 30b, and 30c. A saw wire 40 is sequentially wound around the multiple grooves of rollers 30a, 30b, and 30c. The center-to-center spacing (pitch) of the multiple grooves defines the width of the element to be cut by the wire saw. The two ends of the saw wire 40 are wound onto, for example, a reel (not shown).

[0057] The saw wire 40 of this embodiment is a metal wire without abrasive grains fixed to its surface. In conventional wire saw technology, the saw wire has a wire (core wire) and abrasive grains located on the outer circumferential surface of the wire. The average grain size of the abrasive grains is, for example, several μm to tens of μm. A typical example of such abrasive grains is synthetic diamond, which has a hardness higher than that of rare earth alloys. Unlike such conventional saw wires, the saw wire 40 of this embodiment is formed of a metal material such as carbon steel, and can be used without elongation even when subjected to a tension of, for example, 3 kgf or more during the cutting process. The metal wire that can be used for the saw wire 40 can be, for example, piano wire, high-tensile steel wire, etc. The surface of the saw wire 40 can also be plated. The diameter of the saw wire 40 is, for example, in the range of 100 μm to 350 μm, preferably in the range of 200 μm to 300 μm. When the diameter of the saw wire 40 is less than 100 μm, there is a problem that the saw wire 40 elongates during cutting due to insufficient strength. The larger the diameter of the saw wire 40, the better the discharge of cutting powder, but the amount of cutting powder will increase. Therefore, it is preferable to have a diameter of 350 μm or less.

[0058] During cutting, rollers 30a, 30b, 30c and the recovery spool rotate. The rotation direction of rollers 30a, 30b, and 30c depends on their configuration and the suspension method of the saw wire 40. Figure 2 In the wire saw device 100 shown, rollers 30a, 30b, and 30c rotate in the same direction.

[0059] If a cutter wire 40 of a specified length is wound onto a take-up spool, the take-up spool and rollers 30a, 30b, and 30c rotate in opposite directions. Thus, by repeatedly moving the cutter wire 40 in the opposite direction, the cutter wire 40 can be made to reciprocate (move).

[0060] In this embodiment, the process of cutting the powder molded body 10 with the saw wire 40 is performed while the powder molded body 10 is immersed in the liquid 60. When the powder molded body 10 is a powder molded body formed by wet pressing, a preferred example of the liquid 60 is an oil of the same type as the dispersing medium such as the oil used in wet pressing (mineral oil or synthetic oil).

[0061] When processing the powder molded body 10 using this wire saw device 100, the powder particles constituting the powder molded body 10 become cutting powder and fall off from the portion cut by the saw wire 40. These cutting powder particles, which are the portions of the powder particles constituting the powder molded body 10 that fall off, do not have the rough fracture surface of metal cutting powder (chips). The shape and size of the cutting powder particles that fall off the powder molded body before sintering via the saw wire are the same as the shape and size of the powder particles used in the production of the powder molded body 10. The inventors have investigated the reuse of this cutting powder. When cutting a hard sintered body obtained by sintering the powder molded body, this cutting powder is a particle that has grown through sintering or whose composition changes due to a chemical reaction, or a combination of particles. Therefore, even if they are mixed with rare earth magnet powder and reused, the possibility of magnet properties deteriorating is high. In contrast, if the cutting powder is obtained from the powder molded body before sintering, its composition and size are the same as other particles contained in the powder molded body, and therefore, it is easy to reuse.

[0062] According to the inventors' research, when using existing saw wires with abrasive particles, and recovering the rare earth alloy powder particles cut from the powder molded body 10, the magnetic properties sometimes deteriorate when using the powder molded body containing the recovered powder particles to manufacture sintered magnets. This is because the recovered powder contains abrasive particles that have detached from the saw wire 40. A typical example of abrasive material is diamond, which is composed of carbon. It is known that the inclusion of diamond particles creates pores (voids) during the sintering process, which can deteriorate the magnetic properties (especially corrosion resistance). However, when using a saw wire 40 made of metal wire without abrasive particles, the recovered powder (cutting powder) does not contain abrasive particles, allowing for the manufacture of high-performance magnets with good yield.

[0063] Furthermore, when the powder molded body 10 is produced by wet pressing, if wire sawing is performed in an oil of the same type as the dispersant, the recovered powder (cutting powder) can be directly used for wet pressing, thereby increasing production efficiency.

[0064] The manufacturing method of the R-T-B sintered magnet of this embodiment will be described in detail below.

[0065] S10: Crushing process

[0066] In the pulverization process (S10), powder of R-T-B sintered magnet alloy is prepared. Hereinafter, the composition of R-T-B sintered magnet alloy, the alloy manufacturing process, and the process of preparing alloy powder will be described in sequence.

[0067] <Composition of R-T-B series sintered magnet alloys>

[0068] R is a rare earth element and must contain at least one selected from Nd, Pr, and Ce. Preferably, combinations of rare earth elements as shown in 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, and Nd-Pr-Ce-Dy-Tb are used.

[0069] In R, Dy and Tb, in particular, improve H cJ In addition to the elements mentioned above, other rare earth elements such as La can also be included, and cerium alloys (mischmetal) or Nd:praseodymium mixtures can also be used. Furthermore, R may not be a pure element and may contain unavoidable manufacturing impurities within the range available industrially. The content is, for example, 27% by mass or more and 35% by mass or less. Preferably, the R content of R-T-B sintered magnets is 31% by mass or less (27% by mass or more and 31% by mass or less, more preferably 29% by mass or more and 31% by mass or less). By setting the R content of R-T-B sintered magnets to 31% by mass or less and the oxygen content to 500ppm by more and 3500ppm by less (preferably 500ppm by more and 3200ppm by less, more preferably 500ppm by more and 2500ppm by less), higher magnetic properties can be obtained.

[0070] T contains iron (including cases where T is actually composed of iron), and can also be replaced by cobalt (Co) at a mass ratio of less than 50% (including cases where T is actually composed of iron and cobalt). Co is effective in improving temperature characteristics and corrosion resistance, and the alloy powder can also contain less than 10% by mass of Co. The content of T can occupy the remainder of R and B or R, B and M described later.

[0071] Regarding the content of B, it can be a known content, for example, 0.9% by mass to 1.2% by mass is a preferred range. If it is below 0.9% by mass, sometimes a high H content cannot be obtained. cJ When it exceeds 1.2% by mass, B r Sometimes it decreases. Furthermore, a portion of B can be replaced by C (carbon).

[0072] In addition to the elements mentioned above, in order to improve H cJ Element M can be added. Element M is selected from one or more 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. This is because when it exceeds 5.0% by mass, B... rSometimes it is reduced. In addition, it is also possible to allow for unavoidable impurities.

[0073] The nitrogen (N) content in R-T-B sintered magnets is preferably 50 ppm to 600 ppm. Furthermore, the carbon (C) content in R-T-B sintered magnets is preferably 50 ppm to 1000 ppm.

[0074] <Manufacturing process of R-T-B series sintered magnet alloys>

[0075] Example: Manufacturing process of R-T-B series sintered magnet alloys. Alloy ingots can be obtained by ingot casting, where a metal or alloy pre-adjusted to the above composition is melted and placed into a mold. Alternatively, alloy sheets can be manufactured by rapid cooling methods, such as strip casting or centrifugal casting, which produce solidified alloys thinner than those produced by ingot casting, by contacting the molten metal with a single-roll, double-roll, rotating disk, or rotating cylindrical mold.

[0076] In embodiments of the present invention, materials manufactured by either ingot casting or quenching can be used, with quenching via thin strip casting being preferred. The thickness of quenched alloys produced by quenching is typically in the range of 0.03 mm to 1 mm, and they are in sheet form. The alloy melt solidifies from the contact surface of the cooling roller (roller contact surface), and crystals grow columnarly from the roller contact surface in the thickness direction. Compared to conventional alloys produced by ingot casting (mold casting), quenched alloys are cooled in a short time, resulting in a finer microstructure and smaller grain size. Furthermore, the grain boundary area is large. The R-rich phase extends extensively within the grain boundaries, thus exhibiting excellent dispersion when using quenching. Therefore, it is easy to fracture at the grain boundaries using hydrogen pulverization. By hydrogen pulverizing the quenched alloy, the size of the hydrogen-pulverized powder (coarse powder) can be reduced to, for example, 1.0 mm or less. For example, the coarse powder obtained by this operation can be finely pulverized using a jet mill.

[0077] <Process for preparing powder of R-T-B series sintered magnet alloy>

[0078] The rare earth alloy powder used in R-T-B series sintered magnets is active and easily oxidized. Therefore, in order to avoid the dangers of heat generation and fire, reduce the oxygen content as an impurity, and achieve high performance of the magnet, inert gases such as nitrogen, argon, and helium can be used as the gas used in the jet mill.

[0079] The material to be pulverized (coarse powder) fed into the jet mill is pulverized into fine powder, for example, having an average particle size (median diameter: d50) of 2.0 μm to 4.5 μm, and then moved to a cyclone collector. The cyclone collector is used to separate the powder from the gas stream carrying the powder. Specifically, coarse powder of R-T-B series sintered magnet alloy is pulverized by the jet mill in the preceding stage, and the resulting fine powder, along with the gas used for pulverization, is fed to the cyclone collector. The inert gas (pulverizing gas) and the pulverized fine powder form a high-speed gas stream, which is sent to the cyclone collector. The cyclone collector is used to separate these pulverizing gases and fine powder. The fine powder separated from the pulverizing gas is recovered by a powder collector.

[0080] S20: Molding process

[0081] In the molding process (S20), the powder obtained in the crushing process (S10) is used to make a powder molded body.

[0082] In this embodiment, a powder molded article is formed from the aforementioned powder by pressing in a magnetic field. From the viewpoint of suppressing oxidation, it is preferable to form the powder molded article by pressing in an inert gas atmosphere or by wet pressing. In particular, wet pressing coats the surface of the particles constituting the powder molded article with a dispersant such as an oil, suppressing contact with oxygen or water vapor in the atmosphere. Therefore, it is possible to prevent or suppress the oxidation of the particles by the atmosphere before, during, or after the pressing process.

[0083] In a wet pressing process under a magnetic field, a slurry containing a dispersion medium in a fine powder is prepared and supplied to the mold cavity of a wet pressing apparatus, where it is pressed and shaped under a magnetic field. The resulting powder-molded body, for example, has a density of 4 g / cm³. 3 Above 5g / cm 3 The following densities.

[0084] • Dispersion medium

[0085] A dispersion medium is a liquid that can disperse alloy powder within it to obtain a slurry.

[0086] Mineral oil or synthetic oil can be cited as a preferred dispersion medium used in this invention. The type of mineral oil or synthetic oil is not specific, but when the kinematic viscosity at room temperature exceeds 10 cSt, the increased viscosity strengthens the bonding force between alloy powders, which can sometimes adversely affect the orientation of the alloy powders during wet molding in a magnetic field. Therefore, the kinematic viscosity of the mineral oil or synthetic oil at room temperature is preferably below 10 cSt. Furthermore, when the fractionation temperature of the mineral oil or synthetic oil exceeds 400°C, degreasing after obtaining the molded body becomes difficult, resulting in increased residual carbon content in the sintered body and a decrease in magnetic properties. Therefore, the fractionation temperature of the mineral oil or synthetic oil is preferably below 400°C. Alternatively, vegetable oil can also 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.

[0087] • Preparation of slurry

[0088] The obtained alloy powder is mixed with a dispersion medium to obtain a slurry.

[0089] The mixing ratio of alloy powder and 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). This is because in the range of 20–600 cm⁻¹… 3 The slurry is characterized by its efficient supply of alloy powder to the mold cavity at a flow rate of [flow rate] / second, resulting in excellent magnetic properties. The concentration of alloy powder in the slurry is preferably 90% or less by mass ratio. The method of mixing the alloy powder and the dispersion medium is not particularly limited. The alloy powder and dispersion medium can be prepared separately, weighed in predetermined amounts, and mixed together. Alternatively, when obtaining alloy powder by dry grinding of coarse powder using a jet mill or similar device, a container filled with the dispersion medium can be placed at the alloy powder discharge outlet of the jet mill or similar grinding device. The pulverized alloy powder can be directly recycled into the dispersion medium within the container to obtain a slurry. In this case, it is preferable to also create an atmosphere composed of nitrogen and / or argon within the container, directly recycling the obtained alloy powder into the dispersion medium without contacting the atmosphere to produce a slurry. Alternatively, a vibratory mill, ball mill, or grinding mill can be used to wet grind the coarse powder while maintaining it in the dispersion medium to obtain a slurry composed of alloy powder and dispersion medium.

[0090] By molding the slurry obtained in this way using a known wet pressing device, a powder molded body with a specified size and shape can be obtained. In the prior art, the powder molded body is usually sintered to obtain a sintered body, but in this embodiment, as will be explained below, the powder molded body is cut using a wire saw before sintering.

[0091] S30: Cutting process

[0092] In the cutting process (S30), the powder molded body is cut into multiple molded body pieces.

[0093] The cutting of the powder molded part in this process utilizes, for example... Figure 2 The wire saw device shown is used. Figure 3A and Figure 3B These are front views illustrating the process of cutting a powder molded body 10 immersed in liquid 60 using a saw wire 40. Figure 3A This indicates the state before the cutting process begins. Figure 3B This indicates the status during the cutting process. Figure 3B The dashed lines within the powder molded body 10 schematically indicate the positions where the saw lines 40 are cut in the powder molded body 10. The positions of the saw lines 40, indicated by the dashed lines, move downwards from the upper surface of the powder molded body 10. When they reach the bottom surface of the powder molded body 10, the powder molded body 10 is divided into multiple molded body pieces.

[0094] In the illustrated example, the saw wire 40 travels at a specified speed in the Y-axis direction while simultaneously moving in a direction orthogonal to the travel direction of the saw wire 40 (the negative direction of the Z-axis). This direction orthogonal to the travel direction of the saw wire 40 is the cutting direction, and its speed (cutting speed) is set to, for example, 100 mm / min or higher. Figure 3B In the example shown, the traveling saw line 40 moves in the negative direction of the Z-axis relative to the stationary powder molding body 10, but the powder molding body 10 can also rise in the positive direction of the Z-axis together with the fixing base 20.

[0095] Figure 4A and Figure 4B These are side views illustrating the process of cutting a powder molded body 10 immersed in liquid 60 using a saw wire 40. Figure 4A This indicates the state before the cutting process begins. Figure 4B This indicates the state during the cutting process. In the example shown, a powder molded body 10 is divided into 8 molded body pieces.

[0096] The diameter of the saw wire 40 is, for example, 100μm to 350μm. The travel speed (saw wire speed) of the saw wire 40 can be set to, for example, a range of 100m / min to 800m / min. On the other hand, the cutting speed ( Figure 2The conveying speed of the saw wire relative to the powder molding body 10 in the negative Z-axis direction can be set to, for example, a range of 100 mm / min to 600 mm / min. The tension applied to the saw wire 40 is, for example, 3 kgf to 15 kgf. The tension can be adjusted, for example, by adjusting the distance between roller 30c and rollers 30a and 30b. By cutting with a wire saw, the powder molding body 10 can be divided into molding body sheets with a thickness of, for example, about 1 to 10 mm. Figure 4B As shown, the thickness of the shaped sheet is determined by the spacing of the saw lines 40 and the diameter of the saw lines 40.

[0097] By performing wire sawing in a liquid, it is also advantageous to promote the discharge of cutting powder. In addition, as described above, by performing the wire sawing process while the powder molded body 10 is immersed in the dispersion medium (mineral oil or synthetic oil) used in the production of the powder molded body 10 by wet pressing (oil cutting), it is possible to recover the powder particles that have settled in the liquid during wire sawing, and the recovered powder particles can be directly reused in the molding process.

[0098] Figure 5A and Figure 5B This is a side view illustrating the process of cutting a powder molded body 10 immersed in liquid 60 horizontally using a saw wire 40. In the illustrated example, during the cutting process, rollers 30a, 30b, and 30c move relatively horizontally relative to the powder molded body 10. (For reference only) Figures 3A to 4B Before the described process, a horizontal cut is made using a saw wire 40, thereby flattening the surface of the powder molded body 10. At least a portion of the surface of the powder molded body 10 (e.g., the upper surface) sometimes has unevenness due to the powder pressing process. For example, after filling the hole of the die of the powder pressing device with powder, before pressing the powder with a punch, a "filter cloth" is placed between the punch and the powder, through which the dispersant (oil) can be discharged. In this case, unevenness can be formed on the upper surface of the resulting powder molded body through the filter cloth.

[0099] In an embodiment of the present invention, the uneven surface is cut off by sawing before the sintering process, so that the cutting or grinding process for planarization can be omitted after the sintering process.

[0100] Figures 6A to 6C This is a schematic diagram illustrating a cut surface formed on the powder molded body 10 using a wire saw. (Refer to...) Figure 5A and Figure 5B The described process (first processing step) involves the traveling saw line 40 relative to the powder molded body 10 immersed in the liquid 60 along... Figure 6AThe dashed line 11c is moved, thereby thinning the rough surface region 10T of the powder molded body 10 to form a first cut surface 11 orthogonal to the Z-axis direction. Then, by making a reference... Figure 4A and Figure 4B The described process (second processing step) forms a plurality of second cutting surfaces 12 that intersect with the first cutting surface 11. In the second processing step, the second cutting surfaces 12 are formed by moving a traveling saw wire along the dotted line 12c. The first and second processing steps can be performed using the same wire saw device or different wire saw devices. In other words, the second processing step can be performed using the same wire saw while immersed in the same liquid as the liquid in which the powder molded body was immersed in the first processing step, or it can be performed using different wire saws while immersed in different liquids.

[0101] exist Figures 6A to 6C In the example shown, the first cut surface 11 is parallel to the horizontal plane, and the second cut surface 12 is orthogonal 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.

[0102] S40: Sintering process

[0103] In the sintering process (S40), each of the multiple molded body sheets is sintered to produce multiple sintered bodies. That is, each molded body sheet cut by the wire sawing process described above is sintered to obtain an R-T-B system sintered magnet (sintered body). The sintering process of the molded body sheets can be carried out at, for example, 0.13 Pa (10 －3 Below Torr, preferably at 0.07 Pa (5.0 × 10⁻⁶ Pa). －4 The process is carried out under pressures below Torr and at temperatures ranging from, for example, 1000°C to 1150°C. To prevent oxidation caused by sintering, residual gases in the atmosphere can be replaced with inert gases such as helium or argon. It is preferable to subject the resulting sintered body to additional heat treatment, such as aging. This heat treatment improves the magnetic properties. Known conditions can be used for heat treatment temperature, heat treatment time, and other heat treatment conditions. The resulting R-T-B sintered magnet is then subjected to grinding, polishing, surface treatment, and magnetization processes as needed to complete the final R-T-B sintered magnet.

[0104] In a preferred embodiment, the method for manufacturing the R-T-B sintered magnet of the present invention further includes a diffusion step of diffusing a heavy rare earth element RH (RH being at least one of Tb, Dy, and Ho) from the surface of the sintered body to the interior. When the heavy rare earth element RH is diffused from the surface of the sintered body to the interior, the coercivity can be effectively improved. The method of the diffusion step is not particularly limited; known methods can be used.

[0105] (Example)

[0106] The raw materials were weighed to achieve a composition of Nd: 22.6%, Pr: 7.8%, B: 0.9%, Co: 0.5%, Al: 0.1%, Cu: 0.2%, Ga: 0.4% (all by mass%), with the remainder being Fe. The alloy was then produced using a thin-strip casting method. The resulting alloy was subjected to hydrogen pulverization to obtain coarse powder.

[0107] Next, zinc stearate as a lubricant was added to the obtained coarsely ground powder at 0.04% by mass relative to 100% by mass of the coarsely ground powder. After mixing, the mixture was dry-ground using a jet mill in a nitrogen flow to obtain a particle size D. 50 The powder is a 4μm micro-pulverized powder (alloy powder). The micro-pulverized powder is impregnated in mineral oil with a fractionation temperature of 250℃ and a kinematic viscosity of 2cSt at room temperature under a nitrogen atmosphere to prepare a slurry. The slurry concentration is 85% by mass. The resulting slurry is then molded in a magnetic field (wet molding) to produce a powder molded body. The dimensions of the powder molded body are 80mm × 45mm × 60mm.

[0108] The powder molded body was cut into eight pieces using a 250μm diameter wire saw (made of piano wire). The cutting was performed while the powder molded body was immersed in a liquid (the same liquid used during molding as the mineral oil described above). Each powder molded body was cut using eight parallel saw blades (multi-saw blades). A tension of 10 kg was applied to the saw blades during cutting, and the roller spacing was 250 mm.

[0109] Figure 7 This is a graph showing how the wire saw travel speed and the cutting speed affect the shape of the shaped sheet. The horizontal axis of the graph represents the wire saw travel speed [m / min], and the vertical axis represents the cutting speed [mm / min]. An "×" in the graph indicates a "crack" occurring in a portion of the shaped sheet cut by the wire saw, while "〇" indicates that no such crack occurred on the shaped sheet, allowing for the cutting of a well-shaped sheet.

[0110] Using a 250μm diameter saw wire, at a travel speed of 300m / min, a crack-free shaped piece can be obtained at a cutting speed of 100-150mm / min. Furthermore, at a travel speed of 500m / min, a crack-free shaped piece can be obtained at a cutting speed of 250mm / min. Moreover, at a travel speed of 700m / min, even at a cutting speed of 400mm / min, the saw wire will not bend during cutting, resulting in a crack-free shaped piece.

[0111] Furthermore, when using a saw wire with a diameter of 160 μm, it is possible to cut well-formed sheets at relatively low travel and cut speeds. The smaller the saw wire diameter, the easier it is to extend and bend; therefore, it is believed that when high tension is applied and the wire travels at high speed, cracks or notches are more likely to occur during the cutting of the powder-formed body. Therefore, the saw wire (metal wire) diameter is preferably 200 μm or more. Furthermore, the larger the saw wire diameter, the greater the material cutting, but normal cutting is still possible.

[0112] In addition, it was found that, for comparison, even if one intends to cut a powder molded body placed in the atmosphere using only a metal wire, the cutting cannot be performed normally, and the contact between the traveling metal wire and the powder molded body needs to be carried out in a liquid (preferably oil).

[0113] Figure 8 Indicates as Figure 5A and Figure 5B As shown, this is the experimental result of cutting the upper surface area of ​​a powder molded body horizontally in oil using a saw wire. "Horizontal feed" refers to the horizontal cutting speed, and "linear speed" refers to the saw wire's travel speed. Using a saw wire with a diameter of 250 μm, a travel speed of 300 m / min allows for a cutting speed of 100–300 mm / min without cracks. Furthermore, a travel speed of 500 m / min allows for a cutting speed of 300–500 mm / min without cracks. Even at a travel speed of 700 m / min, a cutting speed of 500 mm / min without cracks is achieved.

[0114] In order to cut near the upper surface of the powder molded body via "lateral conveying," it is preferable that the powder molded body has sufficient "hardness." The hardness of the powder molded body can be evaluated based on, for example, the molding pressure or density during powder molding. It is known that there are powder molded bodies with a density of less than 4 g / cm³ when in air (excluding liquids such as oil). 3 This can lead to problems such as uneven cut surfaces. Therefore, the density of the powder-molded body is preferably 4 g / cm³. 3 above.

Claims

1. A method for manufacturing an R-T-B system sintered magnet, characterized in that, include: The powdering process for preparing R-T-B series sintered magnet alloy powder, where R is a rare earth element and must contain at least one selected from Nd, Pr and Ce, T is at least one transition metal and must contain Fe, and B is boron. The molding process of making powder molded articles using the powder; The cutting process of cutting the powder molded body into multiple molded body pieces; and The sintering process involves sintering each of the multiple shaped body pieces to produce multiple sintered bodies. In the cutting process, the powder-molded body immersed in the liquid is cut by the movement of a metal wire whose surface is not fixed with abrasive grains.

2. The manufacturing method of the R-T-B system sintered magnet as described in claim 1, characterized in that: In the cutting process, the metal wire travels at a speed of 300 m / min or more.

3. The method for manufacturing an R-T-B sintered magnet as described in claim 1 or 2, characterized in that: During the cutting process, the tension of the metal wire is 29.4 N or more.

4. The method for manufacturing an R-T-B sintered magnet as described in claim 1 or 2, characterized in that: In the cutting process, the cutting speed in a direction orthogonal to the traveling direction of the metal wire is 100 mm / min or more.

5. The method for manufacturing an R-T-B sintered magnet as described in claim 1 or 2, characterized in that: The process of preparing the powder molded body includes the step of molding the powder by wet pressing.

6. The method for manufacturing an R-T-B sintered magnet as described in claim 5, characterized in that: The wet pressing is performed by mixing the powder with a liquid of the same type as the liquid used in the cutting process.

7. The method for manufacturing an R-T-B sintered magnet as described in claim 1 or 2, characterized in that: It also includes a process of recovering particles of the powder that have been removed from the powder molded body by the cutting process from the liquid.

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