Manufacturing method of R-T-B series sintered magnets

Abstract

This invention provides a method for manufacturing R-T-B based sintered magnets without the need for preparing an inactive atmosphere. The method comprises: 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 comprises: a first processing step of cutting the powder molded body submerged in a liquid using a traveling wire saw to form a first cut surface; and a second processing step of cutting the powder molded body submerged in a liquid that is the same as or different from the traveling wire saw using a wire saw, forming one or more second cut surfaces intersecting the first cut surface.

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, always containing at least one element selected from the group consisting of Nd, Pr, and Ce; T is at least one transition metal, always containing Fe; B is boron) are composed of R2Fe 14 The compound has a B-type crystalline structure, consisting of a main phase, grain boundary phases located at the grain boundaries of this main phase, and compound phases formed due to trace additions or impurities. R-T-B sintered magnets exhibit high residual magnetic flux density Br (hereinafter sometimes simply referred to as "Br") and high coercivity HcJ (hereinafter sometimes simply referred to as "HcJ"), and possess excellent magnetic properties; therefore, they are known as the highest-performing permanent magnets. 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, an alloy is manufactured from molten metals of various raw material metals 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, for example, hydrogen embrittlement, while the latter uses, for example, an air jet mill (jetting mill).

[0005] The sintered body obtained through the sintering powder molding process is then subjected to mechanical processing such as grinding and cutting to achieve single-piece processing and obtain the 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, the molded body is sintered to form a sintered body, which is then ground using, for example, a superhard alloy blade saw or a rotary grinding stone to give it the desired shape. For example, after first creating a block-shaped sintered body, it is cut using a blade saw or the like to create 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 subject to heavy machining loads. Therefore, high-precision grinding is difficult and time-consuming. Furthermore, material loss due to machining is inevitable. Thus, machining processes become a major reason for increased manufacturing costs.

[0007] To address the problems mentioned above, 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 blade traveling in one or both directions against the molded body to be processed, and grinding or cutting the molded body using abrasive grains located between the saw blade and the molded body. According to this technique, powder molded bodies that are much softer and easier to process than sintered bodies can be cut, thus significantly 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 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 or more and 1.0 mm or less, and abrasive grains fixed to the saw wire, for processing powder molded bodies in an inert gas atmosphere in which the oxygen concentration is adjusted to a total of 5% or more and 18% or less in a molar ratio. The equipment and management for wire sawing in such an inert atmosphere with controlled oxygen concentration become complex, resulting in poor mass production capabilities.

[0013] Embodiments of the present invention provide a new method for manufacturing R-T-B sintered magnets that allows for wire sawing processes that do not require the preparation of an inactive atmosphere.

[0014] Technical solutions for solving the problem

[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, necessarily containing at least one selected from the group consisting of Nd, Pr, and Ce; T is at least one transition metal, necessarily containing 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: a first processing step of cutting the powder molded body submerged in a liquid using a traveling wire saw to form a first cut surface; and a second processing step of cutting the powder molded body submerged in a liquid that is the same as or different from the traveling wire saw using a wire saw, to form one or more second cut surfaces intersecting the first cut surface.

[0016] In one embodiment, the first sectional surface is parallel to the horizontal plane, and the second sectional surface is orthogonal to the first sectional surface.

[0017] In one embodiment, during the cutting process described above, the traveling speed of the wire saw is 300 m / min or more.

[0018] In one embodiment, during the cutting process described above, the tension of the wire saw is 3 kgf or more.

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

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

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

[0022] In one embodiment, the wire saw used in the first processing step is different from the wire saw used in the second processing step, wherein the wire saw used in the second processing step comprises a plurality of parallel metal wires.

[0023] Invention Effects

[0024] According to embodiments of the present invention, cutting can be performed using a wire saw without the need for an inert atmosphere, resulting in excellent mass production capabilities. Furthermore, the surface of the powder-molded body can be machined to achieve a flat surface. At least a portion of the surface of the powder-molded body (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, such cutting or grinding processes can be eliminated, thus maintaining the characteristics of high-performance magnets and reducing manufacturing costs. Attached Figure Description

[0025] Figure 1 This is a flowchart illustrating the main steps of the manufacturing method according to an embodiment of the present invention.

[0026] Figure 2 This is a flowchart illustrating the cutting process of the manufacturing method according to an embodiment of the present invention.

[0027] Figure 3 This is a perspective view schematically illustrating the structure of the wire saw device used in an embodiment of the present invention.

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

[0029] Figure 4B This is a front view used to illustrate the process of cutting powder molded bodies submerged in liquid using a wire saw with a single metal wire.

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

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

[0032] Figure 6A This is a side view used to illustrate the process of cutting powder molded bodies submerged in liquid using a wire saw.

[0033] Figure 6B This is a side view used to illustrate the process of cutting powder molded bodies submerged in liquid using a wire saw.

[0034] Figure 7A This is a schematic diagram showing a cross-section formed on the powder molded body 10 using a wire saw.

[0035] Figure 7B This is a schematic diagram showing a cross-section formed on the powder molded body 10 using a wire saw.

[0036] Figure 7CThis is a schematic diagram showing a cross-section formed on the powder molded body 10 using a wire saw.

[0037] Figure 8 It is a graph showing how the saw travel speed and cutting speed affect the shape of the shaped piece.

[0038] Figure 9 It is a graph showing how the saw travel speed and cutting speed affect the shape of the shaped piece.

[0039] Symbol Explanation

[0040] 10. Powder molding body; 20. Fixing base; 30a, 30b, 30c. Rollers; 40. Saw wire; 50. Support device; 60. Liquid; 70. Tank; 100. Wire saw assembly. Detailed Implementation

[0041] 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 and Figure 2 As shown in the flowchart, the manufacturing method of the R-T-B system sintered magnet in this embodiment includes:

[0042] • 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 the group consisting of Nd, Pr and Ce, T is at least one transition metal, which must contain Fe, and B is boron).

[0043] • A molding process (S20) for making powder molded articles using the powder obtained in the pulverizing process (S10);

[0044] • Cutting process (S30) to cut powder molded body into multiple molded body pieces;

[0045] • A sintering process (S40) to produce multiple sintered bodies by sintering each of multiple shaped body sheets.

[0046] The cutting process (S30) includes a first processing step (S32) of cutting the powder molded body submerged in the liquid using a traveling wire saw to form a first cut surface;

[0047] A second processing step (S34) involves cutting a powder-formed body submerged in a liquid that is the same as or different from the aforementioned wire saw used in the process, using a wire saw that is the same as or different from the liquid used in the process, to form one or more second cut surfaces that intersect with the first cut surface.

[0048] 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 inactive atmosphere. Examples of liquids that can be used in embodiments of the present invention are oils such as mineral oil or synthetic oil.

[0049] Previously, in order to cut powder molded objects with a wire saw, it was believed that hard abrasive grains fixed to the surface of the metal wire constituting the wire saw needed to contact the powder molded object and remove a portion of it through friction. However, the results of experiments conducted by the inventors show that even when the traveling metal wire comes into contact with a powder molded object submerged in liquid, the powder molded object can be ground and cut even with only the metal wire without fixed abrasive grains. 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 object, thereby removing the powder particles constituting the powder molded object. It is believed that a portion of the powder particles removed from the powder molded object are trapped between the metal wire and the powder molded object by the high-speed flowing liquid, performing a grinding function similar to free abrasive grains, thus promoting the cutting of the powder molded object. From the mechanism of the saw wire cutting the powder molded object in liquid, it is believed that the shape and form 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.

[0050] 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 for cutting the powder molded body cannot be obtained. When the saw wire tension is lower than 3 kgf, the saw wire may flex, and the flatness of the cut surface may decrease. When the saw wire tension exceeds 15 kgf, problems such as breakage may occur. In addition, in the cutting process, the cutting speed (workpiece feed 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 cutting time required for the process increases, and production efficiency decreases.

[0051] 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 cutting 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 cutting speed can reach 150mm / min or more.

[0052] One advantage of cutting powder molded bodies in a liquid is that it suppresses the temperature rise caused by frictional heat in the part of the powder molded body that comes into contact with 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's properties. However, in this embodiment, such problems can be avoided.

[0053] Another advantage of cutting powder molded bodies in a liquid is that the powder particles removed from the powder molded body by a wire saw settle in the liquid and are easily recovered. In a preferred embodiment, the process of preparing the powder molded body includes a process of forming the powder by wet pressing. In this case, wet pressing is preferably performed by adding the powder to a liquid of the same type as the liquid used in the cutting process. This is because the powder particles removed from the powder molded body by the cutting process can be easily recovered from the liquid and reused.

[0054] Furthermore, in the manufacturing method of the R-T-B sintered magnet according to the present invention, a horizontal transverse cut is performed before the vertical longitudinal cut, thus enabling the surface of the powder molded body to be processed to achieve a flat surface. 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, such cutting or grinding processes can be eliminated, thereby maintaining the characteristics of a high-performance magnet and reducing manufacturing costs.

[0055] Reference Figure 3 Here is an example of the structure of a wire saw device that can be used in the manufacturing method described above. Figure 3 This is a perspective view showing a structural example 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.

[0056] Figure 3 The wire saw device 100 includes: rollers 30a, 30b, and 30c arranged parallel to each other to direct a central axis of rotation; and a continuous saw line 40. Each of the rollers 30a, 30b, and 30c is rotatably supported by a support device 50. The support device 50 is movable in the vertical longitudinal direction (positive and negative directions of the Z-axis) by 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.

[0057] 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 3 In the diagram, tank 70 is represented by a dashed line, and the height of the liquid 60 surface is also represented by a dashed line. Figure 3 In this example, the powder molded body 10 is entirely immersed in the liquid 60. Alternatively, instead of the support device 50, it can move in the vertical longitudinal direction and the horizontal transverse direction, thereby configuring the fixed base 20 to move in the vertical longitudinal direction and the horizontal transverse direction.

[0058] The following describes a specific example of the process for manufacturing the powder molded body 10. It is important to note that the powder molded body 10 is not a sintered body, but rather a molded body (compacted blank) of powder before sintering. The powder molded body is obtained by molding powder of an R-T-B system sintered magnet alloy (R is a rare earth element, necessarily containing at least one selected from the group consisting of Nd, Pr, and Ce; T is at least one transition metal, necessarily containing Fe; and B is boron) in an orientation magnetic field through wet or dry pressing.

[0059] 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 31a, 31b, and 31c. 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 using a wire saw. The two ends of the saw wire 40 are wound onto, for example, a reel (not shown).

[0060] The saw wire 40 of this embodiment is a single metal wire without abrasive grains fixed to its surface. In existing wire saw technology, the saw wire includes a single wire (core wire) and abrasive grains located on the outer circumference of the single 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 existing 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, 3.0 kgf or more during the cutting process. The material that can be used for the single metal wire of 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 or more and 350 μm, preferably in the range of 200 μm or more and 300 μm or less. 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 chip removal, but the amount of chips will increase. Therefore, it is preferable to have a diameter of 350 μm or less.

[0061] During cutting, rollers 30a, 30b, and 30c, as well as the take-up 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 3 In the wire saw device 100 shown, rollers 30a, 30b, and 30c rotate in the same direction.

[0062] When the entire length of the saw wire 40 is wound onto a take-up spool, the take-up spool and rollers 30a, 30b, and 30c are rotated in the opposite direction. As a result, the saw wire 40 also moves in the opposite direction. By repeating this operation, the saw wire 40 can be made to move back and forth.

[0063] In this embodiment, the process of cutting the powder molded body 10 by 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).

[0064] When processing the powder molded body 10 using this wire saw device 100, the powder particles constituting the powder molded body 10 become chips and fall off from the portion cut by the saw wire 40. These chips are the portions of the powder particles constituting the powder molded body 10 that have fallen off, and each particle may not have the rough fracture surface of metal chips (cutting chips). The shape and size of the particles constituting the chips cut off from the powder molded body before sintering by the saw wire are the same as the shape and size of the powder particles used in making the powder molded body 10. The inventors have studied the reuse of these chips. When cutting a hard sintered body obtained by sintering the powder molded body, the chips are particles that have undergone grain growth through sintering, particles whose composition has changed due to chemical reactions, or aggregates 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 chips are obtained from the powder molded body before sintering, their composition and size are the same as other particles contained in the powder molded body, and therefore, they are easy to reuse.

[0065] In addition, when the powder molded body 10 is produced by wet pressing, if the wire sawing is carried out in the same oil as the dispersant, the recovered powder (chips) can be directly used for wet pressing, thus increasing production efficiency.

[0066] The manufacturing method of the R-T-B sintered magnet of this embodiment will be described in detail below.

[0067] S10: Crushing process

[0068] In the pulverization process (S10), powder of the R-T-B series sintered magnet alloy is prepared. Hereinafter, the composition of the R-T-B series sintered magnet alloy, the alloy manufacturing process, and the process of preparing the alloy powder will be described in sequence.

[0069] <Composition of Rare Earth Alloys for R-T-B Series Sintered Magnets>

[0070] R represents a rare earth element, and it must contain at least one element selected from Nd, Pr, and Ce. Preferably, combinations of rare earth elements represented by Nd-Dy, Nd-Tb, Nd-Dy-Tb, Nd-Pr-Dy, Nd-Pr-Tb, Nd-Pr-Dy-Tb, Nd-Ce-Dy, Nd-Ce-Tb, Nd-Ce-Dy-Tb, Nd-Pr-Ce-Dy, Nd-Pr-Ce-Tb, and Nd-Pr-Ce-Dy-Tb are used.

[0071] In R, Dy and Tb, in particular, play a role in improving HcJ. Besides the aforementioned elements, other rare earth elements such as La can also be included, and mixed metals and neodymium-praseodymium 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 the R-T-B sintered magnet to 31% by mass or less and the oxygen content to 500ppm to 3500ppm or less (preferably 500ppm to 3200ppm or less, more preferably 500ppm to 2500ppm or less), higher magnetic properties can be obtained.

[0072] T contains iron (including cases where T is actually composed of iron), and can also be replaced by less than 50% cobalt (Co) by mass (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 also occupy the remainder of R and B, or R, B, and M (described later).

[0073] Regarding the content of B, known content can also be used; for example, 0.9% to 1.2% by mass is a preferred range. If it is below 0.9% by mass, high HcJ is sometimes not obtained, and when it exceeds 1.2% by mass, Br sometimes decreases. In addition, some of B can be substituted for C (carbon).

[0074] In addition to the elements mentioned above, element M can be added to improve HcJ. 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, Br sometimes decreases. Furthermore, unavoidable impurities are also permissible.

[0075] The nitrogen (N) content in R-T-B sintered magnets is preferably 50 ppm or more and 600 ppm or less. Furthermore, the carbon (C) content in R-T-B sintered magnets is preferably 50 ppm or more and 1000 ppm or less.

[0076] <Manufacturing process of R-T-B series sintered magnet alloys>

[0077] Example: Manufacturing process of R-T-B series sintered magnet alloy. Alloy ingots can be obtained by ingot casting, which involves melting a pre-adjusted metal or alloy in the manner described above and placing it 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 molten metal with a single-roll, double-roll, rotating disk, or rotating cylindrical mold.

[0078] 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 molten alloy metal begins to solidify from the surface in contact with the cooling roller (roller contact surface), and crystals grow columnarly along the thickness direction from the roller contact surface. Compared to alloys produced by conventional 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 expands significantly within the grain boundaries; therefore, quenching exhibits excellent dispersibility of the R-rich phase. Consequently, it is easy to break the R-rich phase 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. This coarse powder is then finely pulverized using, for example, a jet mill.

[0079] <Process for preparing powder of R-T-B series sintered magnet alloy>

[0080] The rare earth alloy powder used in R-T-B series sintered magnets is reactive and easily oxidized. Therefore, in order to avoid the risk of overheating 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 grinding mill.

[0081] 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 or more and 4.5 μm or less, 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 in the previous stage jet mill, and the resulting fine powder, along with the gas used in the pulverization process, is supplied to the cyclone collector. The mixture of inert gas (pulverizing gas) and pulverized fine powder forms a high-speed gas stream and is conveyed to the cyclone collector. The cyclone collector separates these pulverizing gases and fine powder. The fine powder separated from the pulverizing gas is recovered by a powder collector.

[0082] S20: Molding process

[0083] In the molding process (S20), the powder obtained in the crushing process (S10) is used to make a powder molded body.

[0084] In this embodiment, a powder molded article is formed from the above-mentioned powder by pressing in a magnetic field. From the viewpoint of inhibiting oxidation, it is preferable to form the powder molded article by pressing in an inert gas atmosphere or by wet pressing. In particular, in wet pressing, the surface of the particles constituting the powder molded article is coated with a dispersant such as an oil, inhibiting contact with oxygen and water vapor in the atmosphere. Therefore, it is possible to prevent or inhibit the oxidation of the particles by the atmosphere before, during, or after the pressing process.

[0085] In wet pressing under a magnetic field, a slurry containing a dispersion medium mixed in micro-powder is prepared, supplied to the cavity of a mold in a wet pressing apparatus, and pressed into shape under a magnetic field. The resulting powder molded body has, for example, a density of 4 g / cm³. 3 Above and 5g / cm 3 The following densities.

[0086] • Dispersion medium

[0087] A dispersion medium is a liquid in which alloy powder is dispersed to form a slurry.

[0088] 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. When the dynamic viscosity at room temperature exceeds 10 cSt, the increased viscosity strengthens the bonding force between the alloy powders, sometimes adversely affecting the orientation of the alloy powders during wet molding in a magnetic field. Therefore, the dynamic viscosity of the mineral oil or synthetic oil at room temperature is preferably below 10 cSt. Furthermore, when the separation point of the mineral oil or synthetic oil exceeds 400°C, degreasing after obtaining the molded body becomes difficult, the residual carbon content in the sintered body increases, and sometimes the magnetic properties decrease. Therefore, the separation point 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.

[0089] • Preparation of slurry

[0090] By mixing the alloy powder and the dispersion medium, a slurry can be obtained.

[0091] 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, the mixing ratio of alloy powder and dispersion medium is relatively constant. 3 At a flow rate of / second, alloy powder can be effectively supplied to the chamber, resulting in excellent magnetic properties. The concentration of alloy powder in the slurry is preferably 90% or less by mass ratio. The mixing method of the alloy powder and the dispersion medium is not particularly limited. The alloy powder and the dispersion medium can also be prepared separately, weighed and mixed in a predetermined amount, and then manufactured. Alternatively, when obtaining alloy powder by dry grinding of coarse powder using a jet mill or the like, a container containing the dispersion medium can be placed at the alloy powder discharge port of the jet mill or similar grinding device, and the pulverized alloy powder can be directly recycled into the dispersion medium in the container to obtain a slurry. In this case, the container is preferably set to an atmosphere composed of nitrogen and / or argon, so that the obtained alloy powder is not exposed to the atmosphere and is directly recycled into the dispersion medium to form a slurry. Alternatively, by wet grinding with a vibratory mill, ball mill, or grinding mill while the coarse powder is held in the dispersion medium, a slurry composed of alloy powder and dispersion medium can also be obtained.

[0092] 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. Currently, this 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.

[0093] S30: Cutting process

[0094] In the cutting process (S30), the powder molded body is cut and divided into multiple molded body pieces.

[0095] The cutting of the powder molded body in this process utilizes, for example... Figure 3 The wire saw device shown is used. Figure 4A and Figure 4B These are front views illustrating the process of cutting the powder molded body 10 submerged in liquid 60 using saw wire 40. Figure 4A This indicates the state before the cutting process begins. Figure 4B This indicates the state during the cutting process. Figure 4B The dashed lines within the powder molded body 10 schematically indicate the positions of the saw lines 40 used to cut 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.

[0096] 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 4B 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.

[0097] Figure 5A and Figure 5B These are side views illustrating the process of cutting the powder molded body 10 submerged in the liquid 60 using a saw wire 40. Figure 5A This indicates the state before the cutting process begins. Figure 5B This indicates the state midway through the cutting process. In the example shown, a powder molded body 10 is divided into 8 molded body pieces.

[0098] The diameter of the saw wire 40 is, for example, 100 μm or more and 350 μm or less. The travel speed of the saw wire 40 (saw wire speed) can be set, for example, to a range of 100 m / min or more and 800 m / min or less. On the other hand, the cutting speed ( Figure 3The feed speed of the saw wire relative to the powder molding body 10 in the negative Z-axis direction can be set, for example, to a range of 100 mm / min or more and 600 mm / min or less. The tension applied to the saw 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 cutting with a wire saw, the powder molding body 10 can be divided into molding body sheets, for example, with a thickness of 1 to 10 mm. Figure 5B 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.

[0099] By performing wire sawing in a liquid, it is also advantageous to promote chip removal. In addition, as mentioned above, by performing the process (oil cutting) 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, it is possible to recover the powder particles that have settled in the liquid during wire sawing and to directly reuse the recovered powder particles in the molding process.

[0100] Figure 6A and Figure 6B This is a side view illustrating the process of cutting a powder molded body 10 submerged in liquid 60 horizontally using a saw wire 40. In the illustrated example, during the cutting process, rollers 30a, 30b, and 30c move relative to the powder molded body 10 in a relatively horizontal direction (the direction of the rotation axis of each roller). (For reference...) Figures 4A to 5B Before the described process, a horizontal cut is made using a saw 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) may sometimes have unevenness due to the powder pressing process. For example, after filling the hole of the die in the powder pressing apparatus with powder, before pressing the powder with a punch, a "filter cloth" is placed between the punch and the powder, through which a dispersant (oil) can be expelled. In this case, unevenness can be formed on the upper surface of the resulting powder molded body through the filter cloth.

[0101] 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.

[0102] Figures 7A to 7C This is a schematic diagram illustrating a cross-section formed on the powder molded body 10 using a wire saw. (Refer to...) Figure 6A and Figure 6B The described process (first processing step) involves the traveling saw wire 40 moving relative to the powder molded body 10 submerged in the liquid 60 along... Figure 7AThe 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 referring to the reference... Figure 5A and Figure 5B The described process (second processing step) forms a plurality of second cut surfaces 12 that intersect with the first cut surface 11. In the second processing step, the second cut surfaces 12 are formed by moving a traveling saw line along the dotted line 12c. The first processing step and the second processing step can be performed using the same wire saw or different wire saws. 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.

[0103] exist Figures 7A to 7C 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.

[0104] The second cutting speed is preferably, for example, 100 mm / min or more and 800 mm / min or less.

[0105] Furthermore, the technical advantage of performing a first process of horizontal cutting before the second process of vertically cutting the powder molded body in the liquid is not limited to the case of using a wire saw made of a single metal wire; this technical advantage can also be achieved when using a wire saw with abrasive grains fixed to the surface of the single metal wire. However, when cutting the powder molded body in the liquid, the method of cutting with a single metal wire without fixed abrasive grains can avoid the problem of abrasive grains falling off, and is therefore preferred.

[0106] S40: Sintering process

[0107] In the sintering process (S40), each of the multiple shaped body pieces is sintered to produce multiple sintered bodies. That is, each shaped body piece 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 shaped body pieces can be carried out at, for example, 0.13 Pa (10 －3 Below Torr), preferably at 0.07 Pa (5.0 × 10⁻⁶). －4The 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, the residual gas in the atmosphere can be replaced with an inert gas 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 the heat treatment temperature and time. Based on the R-T-B sintered magnet thus obtained, grinding, surface treatment, and magnetization processes are performed as needed to complete the final R-T-B sintered magnet.

[0108] 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 its interior. When the heavy rare earth element RH is diffused from the surface of the sintered body to its interior, the coercivity can be effectively improved. The method of the diffusion step is not particularly limited; known methods can be used.

[0109] (Example)

[0110] The raw materials were weighed to obtain an alloy with the following composition: Nd: 22.6%, Pr: 7.8%, B: 0.9%, Co: 0.5%, Al: 0.1%, Cu: 0.2%, Ga: 0.4% (all by mass%), and the balance being Fe. The alloy was then produced by strip casting. The resulting alloy was subjected to hydrogen pulverization to obtain a coarse powder.

[0111] Next, 0.04% by mass of zinc stearate as a lubricant was added to the obtained coarsely ground powder, relative to 100% by mass of the coarsely ground powder. After mixing, the mixture was dry-milled in a nitrogen atmosphere using a jet mill to obtain micro-ground powder (alloy powder) with a particle size D50 of 4 μm. The micro-ground powder was then impregnated in mineral oil with a separation point of 250°C and a dynamic viscosity of 2 cSt at room temperature under a nitrogen atmosphere to prepare a slurry. The slurry concentration was 85% by mass. The obtained slurry was then molded in a magnetic field (wet molding) to produce a powder molded body. The dimensions of the powder molded body were 80 mm × 45 mm × 60 mm.

[0112] The powder molded body was cut into eight pieces using a 250μm diameter wire saw (a single metal wire made of piano wire). The cutting was performed while the powder molded body was submerged 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 lines (multi-saw lines). A tension of 10 kg was applied to the saw lines during cutting, and the roller spacing was 250 mm.

[0113] Figure 8This is a graph showing the effect of wire saw travel speed and cutting speed on 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 cracking occurred on the shaped sheet, allowing for the cutting of a well-shaped sheet.

[0114] Using a 250μm diameter saw wire, at a travel speed of 300m / min, a cut speed of 100-150mm / min can be achieved to obtain a crack-free shaped piece. Furthermore, at a travel speed of 500m / min, a cut speed of 250mm / min can be achieved to obtain a crack-free shaped piece. Moreover, at a travel speed of 700m / min, even at a cut speed of 400mm / min, the saw wire will not bend during cutting, resulting in a crack-free shaped piece.

[0115] 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 material travels at high speed, cracking or defects are more likely to occur during the cutting of the powder-formed body. Therefore, the diameter of the saw wire (metal single wire) is preferably 200 μm or more. Furthermore, although a larger saw wire diameter increases the cutting range, normal cutting can still be performed.

[0116] In addition, for comparison, it was found that even if only a single metal wire is used to cut a powder molded body placed in the atmosphere, the cutting cannot be carried out normally, and the contact between the traveling single metal wire and the powder molded body needs to be carried out in a liquid (preferably oil).

[0117] Figure 9 express Figure 6A and Figure 6B As shown, this is the experimental result of cutting the upper surface area of ​​a powder molded body in the horizontal direction using a saw wire. "Horizontal advance" 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 cracking. Furthermore, a travel speed of 500 m / min allows for a cutting speed of 300–500 mm / min without cracking. Even at a travel speed of 700 m / min, a cutting speed of 500 mm / min without cracking is achieved.

[0118] In order to cut near the upper surface of the powder molded body by "lateral advance", the powder molded body preferably 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 the density of the powder molded body in air is less than 4 g / cm³. 3 However, problems such as uneven cut surfaces may occur. 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 process of pulverizing powder for R-T-B series sintered magnet alloys, where R is a rare earth element and must contain at least one element 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. 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; The sintering process of producing multiple sintered bodies by sintering each of the plurality of shaped body sheets. The cutting process includes: The first processing step involves using a traveling metal wire with unfixed abrasive grains on its surface to cut the powder molded body that has sunk into the oil, thereby forming a first cut surface. The second processing step involves cutting a powder-formed body into an oil that is the same as or different from the oil in which the metal wire is embedded, using a metal wire with an unfixed abrasive grain on the same or different surface as the traveling metal wire, to form one or more second cut surfaces that intersect with the first cut surface.

2. The method for manufacturing an R-T-B system sintered magnet according to claim 1, characterized in that, The first cut surface is parallel to the horizontal plane. The second cut surface is orthogonal to the first cut surface.

3. The method for manufacturing an R-T-B system sintered magnet according to claim 1 or 2, characterized in that, In the cutting process, the traveling speed of the metal wire is 300m / minute or more.

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

5. The method for manufacturing an R-T-B system sintered magnet according to claim 1 or 2, characterized in that, The cutting speed perpendicular to the direction of travel of the metal wire is 100 mm / min or more and 800 mm / min or less.

6. The method for manufacturing an R-T-B system sintered magnet according to claim 1 or 2, characterized in that, The metal single wire has a smooth surface.

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

8. The method for manufacturing an R-T-B sintered magnet according to claim 7, characterized in that, The wet pressing is performed by mixing the same type of oil as the oil used in the cutting process with the powder.

9. The method for manufacturing an R-T-B sintered magnet according to 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 oil.

10. The method for manufacturing an R-T-B sintered magnet according to claim 1 or 2, characterized in that, The metal single wire used in the first processing step is different from the metal single wire used in the second processing step. The metal single wire used in the second processing step comprises multiple metal single wires traveling in parallel.

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