Method for producing r-t-b sintered magnet

Abstract

The present invention provides a manufacturing method of R-T-B sintered magnet capable of reducing the machining allowance of the sintered magnet by suppressing the size deviation of the sintered magnet. The solution is that the manufacturing method of R-T-B sintered magnet includes: a crushing process of preparing the powder of the alloy for R-T-B sintered magnet; a forming process of making the powder compact using the powder; a cutting process of cutting the powder compact into multiple compact pieces; and a sintering process of sintering each of the multiple compact pieces to make multiple sintered bodies. In the cutting process, the cutting interval of the powder compact is set based on the powder physical property data of the powder used in the making of the powder compact and the powder compact physical property data of the powder compact.

Description

Technical Field

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

[0002] R-T-B system sintered magnets (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; 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 remanent magnetic flux density B. r (Hereinafter sometimes abbreviated as "B") r ") and high coercivity H cJ (Hereinafter sometimes abbreviated as "H") cJ R-T-B sintered magnets possess excellent magnetic properties and are therefore known as the highest-performing permanent magnets. Consequently, R-T-B sintered magnets have been used in a wide variety of applications, including voice coil motors (VCMs) for hard disk drives, motors for electric vehicles (EVs, HVs, PHVs), motors for industrial equipment, and various household appliances.

[0003] Such R-T-B sintered magnets can be manufactured, for example, by 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, for example, by the following method.

[0004] First, alloys are manufactured from molten 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 for example using hydrogen embrittlement, and the latter for example using an air jet mill.

[0005] The sintered body obtained by sintering a powder molded body is then subjected to machining processes such as grinding and cutting, and is individually sheeted to achieve the desired shape and size. More specifically, firstly, R-T-B based rare-earth magnet powder is compressed into a powder molded body using a pressing device, thereby creating a powder molded body larger than the final magnet product. Then, after the powder molded body is formed into a sintered body through a sintering process, the sintered body is ground using, for example, a carbide saw or a rotary grinding stone, to give it the desired shape. For example, after first creating a block-shaped sintered body, multiple plate-shaped sintered body portions are cut out by slicing the sintered body using a saw or similar tool.

[0006] However, the sintered bodies of rare-earth alloy magnets, such as R-Fe-B series magnets, are very hard and brittle, and the processing load is high, making high-precision grinding a difficult and time-consuming operation. Therefore, the machining process has become a major reason for the increase in manufacturing costs.

[0007] To address this problem, Patent Document 1 describes a technique for processing magnetic powder molded bodies using a wire saw before sintering. A wire saw is a processing technique that involves pressing a unidirectional or bidirectional saw wire onto the powder molded body to be processed, and using abrasive grains located between the saw wire and the powder molded body to grind or cut it. This technique allows for the cutting of powder molded bodies that are significantly softer and easier to process than sintered bodies, 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] R-Fe-B sintered magnets contain expensive and scarce rare earth elements, thus requiring further improvements in material utilization efficiency (yield). If the sintered powder molded body is sintered, the size of the sintered body decreases to, for example, about 60-70% of the size of the powder molded body due to shrinkage. This shrinkage rate is inconsistent; therefore, even powder molded bodies of the same size will have different dimensional variations after sintering.

[0013] The embodiments of the present invention provide a method for manufacturing R-T-B sintered magnets that can solve this technical problem.

[0014] Technical solutions for solving technical problems

[0015] In an exemplary embodiment, the method for manufacturing an R-T-B sintered magnet of the present invention includes: a pulverizing step of preparing powder of an alloy for R-T-B sintered magnets (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); 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 make multiple sintered bodies. In the cutting step, the cutting interval of the powder molded body is set based on the powder property data of the powder used in the making of the powder molded body and the powder molded body property data of the powder molded body.

[0016] In one embodiment, the powder physical property data refers to the composition and particle size of the powder.

[0017] In one embodiment, the above-mentioned powder molded body physical property data is the data on the density of the molded body.

[0018] The effects of the invention

[0019] According to embodiments of the present invention, dimensional deviations in the sintered body after sintering can be suppressed, making the size of the sintered body close to the target value. Therefore, a method for manufacturing R-T-B system sintered magnets with further improved material utilization efficiency can be provided. Attached Figure Description

[0020] Figure 1 This is a flowchart illustrating the main steps of the manufacturing method according to an embodiment of the present invention.

[0021] Figure 2 This is a perspective view schematically representing an example of a powder-forming body before cutting.

[0022] Figure 3 It is a perspective view schematically showing the relationship between the pre-sintered shaped sheet 14 and the sintered body 18 after sintering.

[0023] Figure 4 This is a perspective view showing an example of the configuration of a wire saw device that can be used in an embodiment of the present invention.

[0024] Figure 5 It is a cross-sectional diagram schematically representing the cross-section of the saw wire.

[0025] Symbol Explanation

[0026] 10: Workpiece; 20: Fixing base; 30a, 30b, 30c: Roller; 40: Saw wire; 100: Wire saw device. Detailed Implementation

[0027] When a powder molded body is cut into multiple molded body pieces, and these pieces are then sintered to produce a sintered body, shrinkage occurs as described above. The difference in size between the molded body and the sintered body, relative to the size of the molded body, is defined as the "shrinkage rate." The shrinkage rate is, for example, around 30%, but its specific value can vary depending on parameters such as the powder composition, particle size, and molding density of the powder molded body. The dimensions of the resulting sintered body may sometimes deviate by, for example, around 2 to 5%, depending on variations in the shrinkage rate.

[0028] When sintering powder-formed magnets into multiple pieces using a wire saw or similar tool, the shrinkage rate caused by sintering is currently collected as basic data. The cutting width (cutting interval) of the powder-formed magnet is set based on the maximum value of the collected shrinkage rate. In other words, assuming maximum shrinkage occurs, a margin is added to the cutting interval. This way, even with deviations in the shrinkage rate, the sintered body that has experienced maximum shrinkage (the smallest sintered body) still has a size greater than the target size. For larger sintered bodies exceeding the target size, more grinding is performed during machining, thereby allowing for size adjustment. However, adjusting the size of the sintered body through such machining leads to a decrease in material utilization efficiency or yield, resulting in poor mass production. Since R-T-B sintered magnets contain valuable rare earth elements, it is undesirable to increase the amount of grinding, i.e., the machining allowance.

[0029] In the manufacturing method of the R-T-B sintered magnet of the present invention, when cutting and dividing the powder molded body, the shrinkage rate of the powder molded body is estimated or predicted based on "powder property data" and "powder molded body property data", and the cutting width is set according to the shrinkage rate. This suppresses deviations in the sintered body size and reduces the machining allowance of the sintered body. In other words, using the manufacturing method of the R-T-B sintered magnet of the present invention, an optimal width can be set for each powder molded body, thus eliminating the need to set the cutting interval based on the maximum shrinkage rate and suppressing the addition of margin to the cutting interval. Furthermore, since the cutting width is set for each powder molded body, sometimes a cutting residue (powder molded body remaining after the step of cutting into multiple molded body pieces) is generated after the powder molded body is cut. Even in this case, the cutting residue and the cutting powder generated during cutting have the same composition and size as other particles contained in the powder molded body, and therefore can be fully reused. Therefore, by reusing the cut-off residue and cut-off powder as R-Fe-B rare earth magnet powder for obtaining powder molded bodies, it is possible to prevent a decrease in material utilization efficiency or material yield.

[0030] Powder property data can be collected by sampling a portion of the powder used in the fabrication of the powder molded body and measuring its properties. Furthermore, powder molded body property data can be collected at each molding stage or for each batch (every few hundred pieces). In a preferred embodiment, the powder property data includes data on the powder composition and particle size. The powder molded body property data also includes data on the molded body density. The powder composition and particle size, as well as the molded body density, are preferred examples of parameters defining the shrinkage rate of the powder molded body caused by sintering. By preparing data representing the relationship between this parameter and the shrinkage rate based on actual measurements, the shrinkage rate of the powder molded body can be estimated or predicted based on the set or measured values ​​of the parameters. For example, by comparing past actual data representing the relationship between powder property data (e.g., powder composition and particle size) and powder molded body property data (e.g., powder molded body density) and the shrinkage rate with these property data of the powder molded body being cut, an optimal width of the powder molded body can be set.

[0031] When the values ​​of the parameters included in the above data change or have already changed, the shrinkage rate of the powder molded body can be estimated or predicted from the relationship between the changed parameters and the shrinkage rate. For example, during mass production, when the density of the powder molded body taken from the powder pressing device is measured and a change is detected from the initial set value, the size of the molded body sheet, i.e., the cutting interval of the powder molded body, can be corrected based on the shrinkage rate corresponding to the new measured value.

[0032] In addition, when the composition and particle size of the powder are changed, the shrinkage rate after the design change can be predicted based on the above data, and the cutting interval corresponding to the shrinkage rate can be changed.

[0033] This data can be updated during mass production by obtaining the measured values ​​of the aforementioned parameters.

[0034] The following describes an embodiment of the method for manufacturing the R-T-B sintered magnet of the present invention. The method for manufacturing the R-T-B sintered magnet of this embodiment is as follows: Figure 1 The flowchart shown includes:

[0035] • The pulverization process for preparing powder of R-T-B series sintered magnet alloy (S10);

[0036] • Molding process for producing powder molded articles using powder (S20);

[0037] • Cutting process (S30) to cut the powder molded body into multiple molded body pieces;

[0038] • A sintering process (S40) in which each of the multiple molded body pieces is sintered to produce multiple sintered bodies.

[0039] In addition, the cutting process (S30) includes a process (S35) of setting the cutting interval of the powder molded body based on the powder property data of the powder used in the production of the powder molded body and the powder molded body property data of the powder molded body.

[0040] Next, refer to Figure 1 Examples of the above-mentioned processes S10 to S40 will be explained.

[0041] First, in step S10, powder for R-T-B sintered magnet alloys is prepared. Details regarding the composition of the R-T-B sintered magnet alloy and the powder preparation method will be described later.

[0042] In step S20, the powder prepared in step S10 is used to make a powder molded body. The powder molded body can be made by, for example, a wet or dry powder pressing device.

[0043] Figure 2 This is a perspective view schematically representing an example of a powder-forming body. Figure 2 For reference, an XYZ coordinate system including mutually orthogonal X, Y, and Z axes is shown in the figure. The powder molded body 10 shown in the figure has a cuboid block shape. Figure 2 In the diagram, the direction M of the orientation magnetic field is indicated by an arrow. This direction M is called the "magnetic field orientation direction." When the powder body 10 is fabricated by pressing powder of an R-T-B system sintered magnet alloy, the orientation magnetic field is applied to the powder, causing the orientation of each powder particle to be aligned along the magnetic field orientation direction M. Finally, the powder is magnetized in a direction parallel to this magnetic field orientation direction M.

[0044] Refer again Figure 1 .

[0045] Next, in step S30, the powder molded body 10 is cut into multiple molded body pieces 14. The powder molded body 10 is not a sintered body, but a molded body (green body) of powder before sintering. In a preferred embodiment, the powder molded body 10 can be cut using a wire saw. The details of the wire saw will be described later.

[0046] exist Figure 2 In this example, the powder molded body 10 can first be cut into multiple plate-shaped portions extending along a plane (YZ plane) containing the magnetic field orientation direction M of the powder molded body 10. Then, each plate-shaped portion is cut in a manner that cuts across the magnetic field orientation direction M of the powder molded body, ultimately dividing it into multiple molded body pieces 14. Figure 2In the diagram, dashed lines represent cut sections for reference. The order or method of cutting is not limited to one example; other orders may be used. Figure 2 In the diagram, the dimensions of the molded sheet 14 are specified as follows: dimension W in the X-axis direction, dimension T in the Y-axis direction, and dimension L in the Z-axis direction. Dimensions W, T, and L correspond to the "width," "thickness," and "length" of the molded sheet 14, respectively. Furthermore, the shape of the powder molded body 10 is not limited to a cuboid; it can be a cylinder or other shapes. Figure 2 In this example, the cross-sections of the powder molded body 10 are orthogonal to each other, but this is not a limitation. The final molded body sheet 14 can be plate-shaped or rod-shaped extending in one direction.

[0047] In embodiments of the present invention, when the powder molded body 10 is cut into multiple molded body pieces 14, the cutting interval of the powder molded body 10 is set based on the powder property data of the powder used in the production of the powder molded body 10 and the powder molded body property data of the powder molded body 10. Figure 1 (Process S35). The details of process S35 will be described later.

[0048] Next, in step S40, each of the multiple molded body sheets 14 is sintered to produce multiple sintered bodies 16. The sintering conditions can be specified according to the sintering temperature and sintering time. Since the sintering conditions affect the shrinkage rate of the powder molded body, they can be used together with the above data to estimate or predict the shrinkage rate.

[0049] Figure 3 A perspective view schematically showing the relationship between the pre-sintered sheet 14 and the sintered body 18. Figure 3 In the right-hand section, the dimensions of the sintered body 18 are recorded as follows: dimension Ws in the X-axis direction, dimension Ts in the Y-axis direction, and dimension Ls in the Z-axis direction. Dimensions Ws, Ts, and Ls correspond to the "width," "thickness," and "length" of the sintered body 18, respectively. Figure 3 As shown, the molded sheet 14 shrinks during sintering, becoming a sintered body 18 with reduced dimensions.

[0050] In the embodiments of the present invention, during mass production, by sampling and measuring the composition and particle size of the powder and the density of the molded body, managing the variation of shrinkage rate, and correcting the cutting interval at any time, the size of the sintered body can be made close to the target value.

[0051] The manufacturing method of the R-T-B sintered magnet according to this embodiment will be described in detail below. In this embodiment, an implementation method for manufacturing the R-T-B sintered magnet will be described.

[0052] S10: Process for preparing powder of R-T-B series sintered magnet alloy

[0053] <Composition of R-T-B series sintered magnet alloys>

[0054] R is a rare earth element, and must contain at least one element selected from Nd, Pr, and Ce. Combinations of rare earth elements shown in Nd-Dy, Nd-Tb, Nd-Dy-Tb, Nd-Pr-Dy, Nd-Pr-Tb, and Nd-Pr-Dy-Tb are preferred.

[0055] In R, Dy and Tb are particularly effective in improving HcJ. Besides the elements mentioned above, other rare earth elements such as La can be included, and cerium alloys (misch metals) and Nd:praseodymium mixtures can also be used. Furthermore, R does not need to be a pure element; it can contain unavoidable manufacturing impurities within industrially available limits. The content is, for example, 27% by mass or more and 35% by mass or less. Preferably, the R content in R-T-B sintered magnets is 31% by mass or less (27% by mass or more and 31% by mass or less, preferably 29% by mass or more and 31% by mass or less). In R-T-B sintered magnets, the R content is 31% by mass or less, and the oxygen content is 500 ppm or more and 3500 ppm or less.

[0056] T must be at least one of the transition metals and must contain Fe. By mass ratio, less than 50% of Fe may be replaced by cobalt (Co) (including cases where T is substantially composed of iron and cobalt). Co is effective in improving temperature characteristics and corrosion resistance. The alloy powder may contain less than 10% by mass of Co. The content of T may occupy the remainder of R and B or R, B and M (described later).

[0057] Regarding the content of B, it can also be a known content, for example, a preferred range of 0.85% by mass to 1.2% by mass. If it is less than 0.85% by mass, sometimes a high HcJ is not obtained, and if it exceeds 1.2% by mass, sometimes Br decreases. In addition, it is possible to replace part of B with C (carbon).

[0058] 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 if it exceeds 5.0% by mass, Br may sometimes decrease. Furthermore, unavoidable impurities are permissible.

[0059] <Manufacturing process of R-T-B series sintered magnet alloys>

[0060] This example illustrates the manufacturing process of R-T-B series sintered magnet alloys. By melting a metal or alloy pre-adjusted to the above composition and using an ingot casting method involving casting into a mold, alloy ingots can be obtained. Alternatively, the molten metal can be rapidly cooled by contacting it with a single-roll, double-roll, rotating disk, or rotating cylindrical mold, and alloy sheets can be manufactured using rapid cooling methods such as strip casting or centrifugal casting, which produce solidified alloys thinner than those produced by ingot casting.

[0061] In embodiments of the present invention, materials manufactured using either ingot casting or quenching methods can be used, with quenching methods such as 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 surface in contact with the cooling roller (roller contact surface), and crystals grow columnarly from the roller contact surface in the thickness direction. Compared to alloys produced using conventional ingot casting methods (mold casting methods), quenched alloys have a finer microstructure and smaller grain size due to rapid cooling. Furthermore, the grain boundary area is large. The dispersion of the R-rich phase is excellent due to the extensive expansion of the R-rich phase within the grain boundaries. Therefore, the grain boundaries are easily broken using hydrogen pulverization. By hydrogen pulverizing the quenched alloy, the size of the hydrogen-pulverized powder (coarse powder) can be, for example, 1.0 mm or less. The coarse powder thus obtained is then pulverized using a jet mill.

[0062] <Process for preparing powder of R-T-B series sintered magnet alloy>

[0063] The alloy powder used in R-T-B series sintered magnets is reactive and easily oxidized. Therefore, in order to avoid the danger of overheating and fire, and to reduce the oxygen content as an impurity to achieve high magnet performance, inert gases such as nitrogen, argon, and helium are used as the gas used in the jet mill.

[0064] Regarding the material (coarse powder) fed into the jet mill, for example, fine powder with an average particle size (median particle size: d50) of 2.0 μm to 4.5 μm is collected using a cyclone collector. The cyclone collector is used to separate the powder from the gas stream transporting the powder. Specifically, coarse powder of R-T-B series sintered magnet alloy is pulverized in the upstream jet mill, and the resulting fine powder, along with the gas used in the pulverization process, is fed to the cyclone collector. The inert gas (pulverizing gas) and the pulverized fine powder form a high-speed gas stream, which is then sent to the cyclone collector. The cyclone collector separates these pulverizing gases and fine powder. The fine powder separated from the pulverizing gas is recovered in a powder collector.

[0065] S20: Process for producing powder molded parts

[0066] Next, a powder molded body is formed from the aforementioned micro-powder by pressing in a magnetic field. From the viewpoint of suppressing oxidation, it is preferable to form the powder molded body using pressing or wet pressing in a neutral gas atmosphere during pressing in a magnetic field. Particularly in wet pressing, the surface of the particles constituting the powder molded body is covered by a dispersant such as an oil, suppressing contact with oxygen and water vapor in the atmosphere. Therefore, it is possible to prevent or suppress atmospheric oxidation of the particles before, during, or after the pressing process.

[0067] In wet pressing under a magnetic field, a slurry containing a dispersion medium in a fine powder is prepared and fed into the mold cavity of a wet pressing device, where it is pressed and shaped under a magnetic field. The resulting powder molded body, for example, has a particle size of 4 Mg / m³. 3 Above 5Mg / m 3 The following are the densities of the molded parts.

[0068] • Dispersion medium

[0069] A dispersion medium is a liquid in which alloy powder can be dispersed to form a slurry.

[0070] Mineral oil or synthetic oil can be cited as a preferred dispersion medium used in this invention. There is no specific type of mineral oil or synthetic oil, but if the kinematic viscosity at room temperature exceeds 10 cSt, the increased viscosity strengthens the bonding force between the alloy powders, which can sometimes negatively 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, if the fractionation temperature of the mineral oil or synthetic oil exceeds 400°C, degreasing after obtaining the powder molded body becomes difficult, and sometimes the residual carbon content in the sintered body increases, leading to 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.

[0071] • Preparation of slurry

[0072] By mixing the obtained alloy powder with a dispersion medium, a slurry can be obtained.

[0073] 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. 3At a flow rate of [flow rate] / second, alloy powder can be effectively supplied into the mold cavity, and excellent magnetic properties can be obtained. The concentration of alloy powder in the slurry is preferably 90% or less by mass ratio. There is no particular limitation on the mixing method of the alloy powder and the dispersion medium. It can be produced by preparing alloy powder and dispersion medium separately, weighing a specified amount of both and mixing them. Alternatively, when obtaining alloy powder by dry grinding of coarse powder using a jet mill or the like, a container with added dispersion medium can be placed at the alloy powder discharge port of the jet mill or the like 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, it is preferable that the container is also filled with an atmosphere formed by nitrogen and / or argon, so that the obtained alloy powder does not come into contact with the atmosphere and is directly recycled into the dispersion medium to form a slurry. Alternatively, a vibratory mill, ball mill, or atritor 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.

[0074] 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 described below, the powder molded body is cut into multiple molded body pieces before sintering.

[0075] S30: Cutting process to cut the powder molded body into multiple molded body pieces.

[0076] In an embodiment of the present invention, before the sintering process, a step (S30) is performed to cut the powder molded body into multiple molded body pieces. This cutting can be appropriately performed using a wire saw. The wire saw includes a multi-wire saw that simultaneously cuts the powder molded body into three or more molded body pieces along multiple cross-sections, and a single-wire saw that sequentially cuts along each cross-section. In an embodiment of the present invention, any wire saw can be used.

[0077] Here, refer to Figure 4 Here is an example of the configuration of a wire saw device (multi-wire saw device) that can be used in the cutting process. Figure 4 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. The wire saw apparatus 100 in this example is a multi-wire saw apparatus. For reference, an xyz coordinate system including mutually orthogonal x-axis, y-axis, and z-axis is shown in the figure. In this example, the xy plane is horizontal, and the z-axis faces the vertical direction.

[0078] Figure 4 The wire saw device 100 has rollers 30a, 30b, and 30c arranged parallel to each other on a central axis of rotation, and a continuous saw wire 40. The powder molded body 10 prepared in process (S10) is fixed and supported by a base 20.

[0079] The fixing base 20 moves up and down in the z-axis direction while the powder molded body 10 is fixed. This up and down movement can be achieved using a drive device (not shown). The drive device can obtain driving force through a hydraulic cylinder or be operated by an electric motor.

[0080] Rollers 30a, 30b, and 30c, when viewed from a direction parallel to the x-axis, are arranged at predetermined intervals such that the axis of rotation is located at the vertex of a triangle. Each of rollers 30a, 30b, and 30c has multiple grooves on its side. 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 (cutting interval) of the element to be divided by a wire saw.

[0081] As described above, the cutting process (S30) includes a process (S35) of setting the cutting interval of the powder molded body based on the powder property data of the powder used in the production of the powder molded body and the powder molded body property data of the powder molded body. Therefore, when using the wire saw apparatus 100 of this embodiment, the center interval of multiple grooves is determined in a way that allows the cutting interval set based on the powder property data of the powder used in the production of the powder molded body and the powder molded body property data of the powder molded body. When changing the cutting interval, a roller with grooves at the center interval corresponding to the cutting interval is replaced. In addition, when using a single wire saw apparatus, the position of the saw wire 40 in the X-axis direction relative to the powder molded body 10 is moved in units of cutting interval, and then the cutting is performed sequentially.

[0082] The two ends of the saw wire 40 are wound onto a recovery drum (not shown). During cutting, rollers 30a, 30b, 30c and the recovery drum rotate. The rotation direction of rollers 30a, 30b, 30c depends on their configuration and the way the saw wire 40 is attached. Figure 4 In the wire saw device 100 shown, rollers 30a, 30b, and 30c rotate in the same direction.

[0083] When the saw wire 40 of a specified length is wound onto a take-up drum on one side, the take-up drum and rollers 30a, 30b, and 30c are rotated in opposite directions. As a result, the saw wire 40 moves in the opposite direction. By repeating this operation, the saw wire 40 can be made to move back and forth.

[0084] The saw wire 40, for example, uses a fixed abrasive saw wire. Specifically, it is possible to use a saw wire formed by electrodepositing high-hardness abrasive grains suitable for cutting high-hardness materials onto the wire. High-hardness abrasive grains are also known as ultra-abrasive grains, a typical example being diamond abrasive grains.

[0085] Figure 5The cross-section of the saw wire 40 is schematically shown. The saw wire 40 includes a wire (core wire) 42, abrasive grains 44 located on the outer peripheral surface of the wire 42, and an adhesion layer 46. The adhesion layer 46 is formed, for example, by a plated metal such as Ni. The abrasive grains 44 are located on the surface of the wire 42, and the adhesion layer 46 covers the surface of the wire 42 surrounding the abrasive grains 44 as a whole, thereby enabling the abrasive grains 44 to be adhered to the wire 42. The adhesion of the abrasive grains 44 can also be achieved by other methods. The average grain size of the abrasive grains 44 is, for example, 1 μm or more and 24 μm or less.

[0086] The process of cutting the powder molded body 10 using a wire saw is preferably performed while the powder molded body 10 is immersed in a liquid. When the powder molded body 10 is a powder molded body formed by wet pressing, the preferred example of the liquid is a dispersion medium such as an oil used in wet pressing (mineral oil or synthetic oil).

[0087] When the powder molded body 10 is processed using such a 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 abrasive grains 44 of the saw wire 40. The cutting powder obtained from the powder molded body 10 before sintering is recovered and directly mixed with the micro powder used to make the powder molded body, thereby enabling easy reuse.

[0088] In a preferred embodiment, the wire 42 of the saw wire 40 (refer to...) Figure 5 The diameter of the wire 42 is, for example, 140 μm or more and 240 μm or less. When the diameter of the wire 42 is less than 140 μm, the strength is insufficient, and there is a problem of wire 42 stretching during cutting. The larger the diameter of the wire 42, the better the discharge of cutting powder, but the amount of cutting powder increases. Therefore, it is preferable to be 240 μm or less.

[0089] The moving speed of the saw wire 40 (saw wire speed) can be set, for example, to a range of 100 m / min to 500 m / min. On the other hand, the workpiece conveying speed ( Figure 4 The moving speed of the fixed base 20 in the z-axis direction can be set, for example, to a range of 100 mm / min to 600 mm / min. The tension applied to the saw wire 40 is, for example, 2.0 kg to 3.0 kg.

[0090] From the viewpoint of rapidly removing cutting powder, it is desirable to perform wire sawing (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 by wet pressing. When performing wire sawing in the atmosphere, it is desirable to spray the same oil as the dispersion medium onto the part of the powder molded body 10 that contacts the saw wire 40 (the cutting part).

[0091] In this embodiment, the cutting interval is set based on the aforementioned pre-prepared data. Therefore, it is possible to produce a molded sheet with dimensions corresponding to the shrinkage that occurs during the subsequent sintering process. Furthermore, if the composition and particle size of the powder used in the production of the powder molded body change, or if the density of the molded body changes, the setting value of the cutting interval can be changed based on the aforementioned data. As a result, it is possible to produce a molded sheet with dimensions corresponding to the actual shrinkage rate.

[0092] During the aforementioned cutting process, the powder particles constituting the powder molded body become cutting powder and fall off. When the hard sintered body obtained by sintering the powder molded body is cut, the cutting powder consists of particles that have undergone grain growth during sintering, or particles whose composition has changed through chemical reactions, or particles that have aggregated together. Therefore, these cannot be mixed with the powder of R-T-B system sintered magnet alloys for reuse. In contrast, if the cutting powder is obtained from the powder molded body before sintering, its composition and size are similar to other particles contained in the powder molded body, thus allowing for sufficient reuse. Therefore, it is preferable to recover and reuse the cutting powder.

[0093] S40: The process of sintering individual molded sheets to produce a sintered body.

[0094] Next, the shaped sheet obtained through the above cutting process is sintered to obtain an R-T-B system sintered magnet (sintered body). The sintering process of the shaped sheet can be carried out at, for example, 0.13 Pa (10 －3 Torr) or less, preferably 0.07 Pa (5.0 × 10⁻⁶ Pa). －4 The process is carried out at pressures below Torr and at temperatures ranging from, for example, 1000°C to 1150°C. To prevent oxidation caused by sintering, the residual gases in the atmosphere can be replaced by inert gases such as helium or argon.

[0095] During sintering, the molded sheet shrinks, resulting in a sintered body with a smaller size than the molded sheet. However, in this embodiment, it is possible to produce a molded sheet with a size corresponding to the shrinkage that occurs during the sintering process, thus the size of the sintered body after shrinkage is close to the target value. For example, the error between the size of the sintered body and the target value can be suppressed to less than 1%.

[0096] Furthermore, for the sintered body after process S40, elements such as heavy rare earth elements can diffuse from the surface of the sintered body into its interior. Additional heat treatments such as aging can also be performed. Through this diffusion and heat treatment, the magnetic properties can be improved. The heat treatment conditions, such as heat treatment temperature and time, can be well-known. The sintered body obtained in this way is then subjected to finishing and surface treatment processes such as grinding and polishing as needed, followed by a magnetization process to complete the final R-T-B system sintered magnet.

[0097] <Example>

[0098] Prepare powder of an R-T-B system sintered magnet alloy with the composition of Nd: 24.5 wt%, Pr: 4.5 wt%, B: 0.90 wt%, Cu: 0.1 wt%, Ga: 0.4 wt%, Co: 1.0 wt%, and the remainder Fe. The particle size D of the powder is... 50 The particle size was 4 μm. Additionally, the particle size D was determined using airflow dispersion laser diffraction (according to JIS Z 8825: 2013 revised version). 50 Powder molded bodies were fabricated using these powders via a wet pressing device. The resulting powder molded bodies had dimensions of 100 mm × 60 mm × 90 mm (90 mm being the magnetic field orientation direction). The molded body density was 4.5 Mg / m³. 3 Left and right. The obtained powder molded body is cut at the cutting interval set by conditions A and B as described below to obtain multiple molded body sheets. The size of the molded body sheets is 100mm × 60mm × 7.5mm (7.5mm is the magnetic field orientation direction). The obtained molded body sheets are sintered (selecting a temperature that fully causes densification due to sintering) to produce a sintered body.

[0099] (Condition A)

[0100] Condition A is the current setting method. As a previous experiment, multiple powder molded bodies were made, sintered, and their shrinkage rates were calculated separately. This data was used as the basis for data collection. The width of the cut of the powder molded body was set according to the maximum value of the collected shrinkage rate.

[0101] (Condition B)

[0102] Condition B is the setting method of the present invention. First, powder property data of the powder used in the production of the powder molded body are collected. Specifically, the composition and particle size of the powder are measured. Next, powder molded body property data are collected. Specifically, the molded body density of the powder molded body is measured. Based on these measured data and past actual data representing the relationship between powder property data and powder molded body property data and shrinkage rate, the width of the powder molded body is set.

[0103] For sintered bodies produced under conditions A and B, 20 measurements were taken along the magnetic field orientation direction (7.5 mm) for each condition. The difference between the maximum and minimum dimensions was calculated, thus determining the dimensional deviation of the sintered body. Furthermore, the dimensional deviation due to shrinkage was the largest among all dimensional locations along the magnetic field orientation direction. The difference from the target dimensional value of the sintered body (target dimensional value along the magnetic field orientation direction of 7.5 mm) was also calculated. Under condition A, the dimensional deviation of the sintered body was 0.22 mm, a difference of 3% from the target value. In contrast, under condition B, the dimensional deviation of the sintered body was 0.07 mm, a difference of 1% from the target value. Thus, under condition B, the dimensional deviation of the sintered body was suppressed, and the difference from the target value was small.

Claims

1. A method for manufacturing an R-T-B system sintered magnet, characterized in that, include: The process of pulverizing powder for preparing R-T-B series sintered magnet alloys, wherein 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 A sintering process is performed on each of the multiple molded body pieces to produce multiple sintered bodies. Prior to the cutting process, the composition and particle size of the powder used in the fabrication of the powder molded body, as well as the measured values ​​of the molded body density, are obtained. In the cutting process, the cutting interval of the powder molded body is set by comparing the measured value with past actual data showing the relationship between the powder physical property data, which includes data on the composition and particle size of the powder, and the powder molded body physical property data, which includes data on the density of the molded body, and the shrinkage rate caused by sintering.

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