Method for producing r-t-b sintered magnet

Abstract

The present invention provides a new R-T-B sintered magnet manufacturing method that can further improve yield. The present invention includes: a workpiece preparation step in which a powder compact of a rare earth alloy for an R-T-B sintered magnet is prepared; a workpiece cutting step in which the workpiece is cut to divide the workpiece into a plurality of compact pieces; and a sintering step in which the compact pieces are sintered. The workpiece cutting step includes: a workpiece cutting step in which the workpiece is cut using a wire saw having a plurality of abrasive grains with an average particle diameter of 1 μm or more and 24 μm or less and a wire in which the plurality of abrasive grains are fixed, and a powder particle recovery step in which powder particles of the rare earth alloy that are shaved from the workpiece are recovered.

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 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, they are used in a wide variety of applications, including voice coil motors (VCMs) for hard disk drives, motors for electric vehicles (EVs, HVs, PHVs), motors for industrial equipment, and various other motors and 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 can be manufactured, for example, by the following method.

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

[0005] The sintered body obtained by sintering the powder molding body is then subjected to machining processes such as grinding and cutting to achieve a single sheet shape and size. More specifically, firstly, R-Fe-B rare-earth magnet powder is compressed into a molding body using a pressing device, thereby creating a molding body larger than the final magnet product. Then, after the molding body is sintered, it is ground using, for example, a carbide saw or a rotary grinding stone, to achieve the desired shape. For example, a block-shaped sintered body is first created, and then sliced ​​using a saw to cut multiple sheet-like sintered body portions.

[0006] However, the sintered bodies of rare earth alloy magnets, such as R-Fe-B sintered magnets, are very hard and brittle, and subject to heavy machining loads. Therefore, high-precision grinding operations are extremely difficult and time-consuming. Furthermore, material loss due to machining is inevitable. Consequently, machining processes become a significant factor contributing to increased manufacturing costs.

[0007] For example, to address the former problem, Patent Document 1 describes a technique for processing magnet molded bodies using a wire saw before sintering. A wire saw is a processing technique that uses a saw wire moving in one or both directions to press against the molded body to be processed, and utilizes abrasive grains located between the saw wire and the molded body to grind or cut the body. Using this technique, powder molded bodies, which 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

[0011] However, Patent Document 1 does not provide a solution to the increased costs caused by material loss during processing. R-Fe-B sintered magnets contain expensive and rare rare earth elements, thus requiring further improvements in material utilization efficiency (yield). Summary of the Invention

[0012] The embodiments of the present invention provide a new method for manufacturing R-T-B sintered magnets that can further improve the yield.

[0013] The manufacturing method of the R-T-B sintered magnet of the present invention includes, in an exemplary embodiment, a step of preparing a workpiece of a rare earth alloy powder molded body for an R-T-B sintered magnet; a step of cutting the workpiece and dividing it into a plurality of molded body pieces; and a step of sintering the molded body pieces. The step of dividing the workpiece into the plurality of molded body pieces includes cutting the workpiece using a wire saw having a plurality of abrasive grains with an average particle size of 1 μm or more and 24 μm or less and wires with the plurality of abrasive grains attached thereto, and recovering the rare earth alloy powder particles cut off from the workpiece.

[0014] In one embodiment, the process of cutting the workpiece using the wire saw is performed while the workpiece is immersed in a liquid.

[0015] In one embodiment, the process of preparing the workpiece includes a process of preparing the rare earth alloy powder and a process of forming the powder by wet pressing.

[0016] In one embodiment, the rare earth alloy is an R-T-B series sintered magnet rare earth alloy, and the workpiece has a content of 4 g / cm³. 3 Above 5g / cm 3 The following densities.

[0017] In one embodiment, in the process of recovering the rare earth alloy powder particles cut off from the workpiece, the abrasive grains that have fallen off the wire are recovered together with the rare earth alloy powder particles cut off from the workpiece.

[0018] In one embodiment, the abrasive grains are diamond abrasive grains.

[0019] In one embodiment, the diameter of the wire is 140 μm or more and 350 μm or less.

[0020] In one embodiment, the manufacturing method of the R-T-B sintered magnet of the present invention includes: a step of cutting a workpiece of a rare earth alloy powder molded body for R-T-B sintered magnets using a wire saw having a plurality of abrasive grains with an average particle size of 1 μm to 24 μm and wires with the plurality of abrasive grains fixed thereon; when the workpiece is divided into a plurality of molded body pieces, a step of recovering the rare earth alloy powder particles cut off from the workpiece; a step of preparing a recovered powder mixed powder molded body containing the recovered powder particles; and a step of sintering the recovered powder mixed powder molded body.

[0021] In one embodiment, the process of preparing the above-mentioned recycled powder mixed powder molded body includes the process of mixing the recycled powder particles in powder formed from rare earth alloys for R-T-B sintered magnets.

[0022] In one embodiment, the process of preparing the above-mentioned recycled powder mixed powder molded body is carried out in a manner that does not separate the abrasive particles that fall off the above-mentioned wire from the above-mentioned recycled powder particles.

[0023] In one embodiment, the recycled powder mixed powder molded body comprises the aforementioned abrasive particles.

[0024] In one embodiment, the mass of the abrasive particles contained in the recycled powder mixture molded body is less than 0.3% of the total mass of the recycled powder mixture molded body.

[0025] Invention Effects

[0026] According to embodiments of the present invention, when the powder particles removed by wire sawing before the sintering process are recovered and reused, the decline in magnet properties, especially corrosion resistance, can be suppressed. Therefore, the properties of high-performance magnets can be maintained, and manufacturing costs can be reduced. Attached Figure Description

[0027] Figure 1A This is a flowchart illustrating the main steps of the manufacturing method in an embodiment of the present invention.

[0028] Figure 1B This is a flowchart illustrating the steps of another method of manufacturing in an embodiment of the present invention.

[0029] Figure 2 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.

[0030] Figure 3 This is a schematic cross-sectional view showing the cross-section of saw line 40.

[0031] Figure 4A This is a cross-sectional photograph of the sintered magnet of sample A.

[0032] Figure 4B This is a cross-sectional photograph of the sintered magnet of sample C.

[0033] Figure 4C This is a cross-sectional photograph of the sintered magnet of sample E.

[0034] Symbol Explanation

[0035] 10: Workpiece; 20: Fixing base; 30a, 30b, 30c: Roller; 40: Saw wire; 100: Wire saw device. Detailed Implementation

[0036] The following describes an embodiment of the manufacturing method of the R-T-B system sintered magnet of the present invention. Figure 1A As shown in the flowchart, the manufacturing method of the R-T-B system sintered magnet in this embodiment includes:

[0037] • Process for preparing powder molded parts of rare earth alloys for R-T-B sintered magnets (S10).

[0038] • The process of cutting the workpiece and dividing it into multiple shaped pieces (S20); and

[0039] • The process of sintering the molded sheet (S30).

[0040] The process of dividing the workpiece into multiple shaped pieces (S20) includes a process of cutting the workpiece with a wire saw having multiple abrasive grains with an average particle size of 1 μm to 24 μm and wire with the multiple abrasive grains fixed thereon, and recovering the rare earth alloy powder particles cut off from the workpiece (S25).

[0041] Furthermore, the manufacturing method of the R-T-B sintered magnet of the present invention can be implemented in other ways, such as... Figure 1BAs shown in the flowchart, after the step (S25) of recovering the rare earth alloy powder particles cut off from the workpiece, the process may further include:

[0042] • The process of preparing a recycled powder mixture containing the recycled powder particles (S40); and

[0043] • The process of sintering the recycled powder mixture into a molded body (S50).

[0044] According to the manufacturing method of the R-T-B sintered magnet of the present invention, it is easy to recover and reuse the powder particles that are cut off from the workpiece of the powder molded body by wire sawing before the sintering process. Furthermore, in the R-T-B sintered magnet obtained by sintering the recycled powder mixture containing the recovered powder particles, the decline in magnet properties, especially corrosion resistance, can be suppressed.

[0045] Reference Figure 2 An example of the configuration of a wire saw device that can be used in the above manufacturing method will be described. Figure 2 This is a perspective view showing an example configuration of the wire saw apparatus 100 according to an embodiment of the present invention. For reference, the figure shows an xyz coordinate system comprising mutually orthogonal x-axis, y-axis, and z-axis. In this example, the xy plane is horizontal, and the z-axis points in the vertical direction.

[0046] Figure 2 The wire saw device 100 has rollers 30a, 30b, and 30c arranged parallel to each other with a central axis of rotation, and a continuous saw wire 40. The workpiece 10, a powder-molded body prepared in step (S10), is supported on a fixed base 20. A specific example of the step for preparing the powder-molded workpiece 10 will be described later. It should be noted here that the workpiece 10 is not a sintered body, but rather a green compact (a compact of powder before sintering). The powder-molded body can be obtained, for example, by molding R-T-B sintered magnets with rare earth alloy powder (hereinafter referred to as "rare earth alloy") in an orientation magnetic field using wet pressing or dry pressing.

[0047] The fixing base 20 moves up and down along the z-axis while the workpiece 10 is fixed. This up-and-down movement can be performed using a drive device (not shown). The drive device can be a hydraulic cylinder or an electric motor.

[0048] Viewed from a direction parallel to the x-axis, rollers 30a, 30b, and 30c are arranged at predetermined intervals with their rotational center axis located at the apex of a triangle. Multiple grooves are provided on the side of each of rollers 30a, 30b, and 30c. A saw wire 40 is sequentially wound around the multiple grooves of rollers 30a, 30b, and 30c. The center-to-center spacing (pitch) of the multiple grooves specifies the width of the element to be cut by the wire saw. The two ends of the saw wire 40 are, for example, wound onto a recovery drum (not shown).

[0049] During cutting, rollers 30a, 30b, 30c and the recovery drum rotate. The rotation direction of rollers 30a, 30b, and 30c depends on their configuration and the way the saw wire 40 is attached. Figure 2 In the wire saw device 100 shown, rollers 30a, 30b, and 30c rotate in the same direction.

[0050] A saw wire 40 of a specified length is wound onto a take-up drum, which then rotates in opposite directions along with rollers 30a, 30b, and 30c. This reverse movement of the saw wire 40, and the reciprocating motion (movement) of the saw wire 40, is achieved by repeating this operation.

[0051] For example, saw wire 40 uses a fixed abrasive saw wire. Specifically, a saw wire can be obtained by electrodepositing high-hardness abrasive grains suitable for cutting high-hardness materials onto the wire.

[0052] Figure 3 The 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 and 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 1 μm to 24 μm. Regarding the abrasive grains 44, a known wire saw with an average grain size in the range of 1 μm to 24 μm can be used. Alternatively, it can be determined by observing the saw wire and measuring the abrasive grains of the wire saw. For example, the average grain size can be determined by observing the saw wire and randomly measuring 50 abrasive grains 44.

[0053] The process of cutting the workpiece 10 using a wire saw is preferably performed while the workpiece 10 is immersed in a liquid. When the workpiece 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).

[0054] When a workpiece 10 is machined using such a wire saw device 100, the powder particles constituting the workpiece 10 fall off as cutting powder from the portion cut by the abrasive grains 44 of the saw wire 40. The inventors of this invention have studied the reuse of this cutting powder. When cutting a hard sintered body obtained by sintering a powder molded body, the cutting powder consists of particles that have undergone particle growth due to sintering, or particles whose composition has changed due to chemical reactions, or aggregates of particles. Therefore, even if it is mixed with rare earth magnet powder and reused, the magnet properties are likely to deteriorate. In contrast, if the cutting powder is obtained from the powder molded body before sintering, its composition and size are the same as other particles contained in the powder molded body, and it can be fully reused.

[0055] However, further research by the inventors of this invention has shown that when rare earth alloy powder particles removed from workpiece 10 are recycled and sintered magnets are made from a rare earth alloy powder molded body containing the recycled powder particles, the magnet properties sometimes deteriorate. This is because the recycled powder contains abrasive grains 44 that have detached from the saw wire 40. A typical example of the material of the abrasive grains 44 is diamond, which is composed of carbon. It is known that the inclusion of diamond particles can create pores (voids) during the sintering process, deteriorating the magnet properties (especially corrosion resistance). The inventors of this invention have conducted in-depth research and found that if the average particle size of the abrasive grains 44 is set to between 1 μm and 24 μm, even when the recycled powder (cutting powder) contains abrasive grains 44 detached from the saw wire 40 as foreign matter, the pores formed during the sintering process are small enough to have almost no adverse effect on magnet properties such as corrosion resistance.

[0056] Furthermore, it has been confirmed that when the workpiece 10 is a powder molded body obtained by wet pressing and wire sawing is performed in a liquid, superior magnetic properties can be obtained. By performing wire sawing in a liquid, even when the size of the abrasive grains 44 is limited to a small size as in the embodiment of the present invention, cutting performance can be maintained and the cut surface can be smoothed.

[0057] The manufacturing method of the R-T-B sintered magnet of this embodiment will be described in detail below.

[0058] S10: Process for preparing rare earth alloy powder molded parts

[0059] <Composition of Rare Earth Alloys>

[0060] R is a rare earth element, and 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.

[0061] In R, Dy and Tb are particularly effective at enhancing H. cJ The effect is that, in addition to the elements mentioned above, other rare earth elements such as La can also be included, and cerium alloys (mischmetal) or Nd:praseodymium mixtures can also be used. Furthermore, R does not need to be a pure element and can 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, preferably 29% by mass or more and 31% by mass or less). By making the R content of R-T-B sintered magnets 31% by mass or less, and the oxygen content 500ppm by more and 3500ppm by less (preferably 500ppm by more and 3200ppm by less, more preferably 500ppm by more and 2500ppm by less), higher magnetic properties can be obtained.

[0062] T contains iron (including cases where T is substantially composed of iron), and up to 50% by mass can 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, and the alloy powder may contain up to 10% by mass of Co. The content of T may be the balance of R and B, or R and B and M (described later).

[0063] Regarding the content of B, a known content can be used, for example, 0.9% by mass to 1.2% by mass is a preferred range. When it is less than 0.9% by mass, it is sometimes impossible to obtain high H content. cJ When it exceeds 1.2% by mass, sometimes B r It will decrease. Part of B can be replaced by C (carbon).

[0064] In addition to the elements mentioned above, in order to improve H cJ Furthermore, element M can be added. Element M is selected from one or more of Al, Si, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Zr, Nb, Mo, In, Sn, Hf, Ta, and W. The amount of element M added is preferably 5.0% by mass or less. This is because when it exceeds 5.0% by mass, sometimes B... r It will decrease. Furthermore, unavoidable impurities are also permissible.

[0065] The nitrogen (N) content in R-T-B sintered magnets is preferably 50 ppm to 600 ppm. Furthermore, the carbon (C) content in R-T-B sintered magnets is preferably 50 ppm to 1000 ppm.

[0066] <Manufacturing process of R-T-B series sintered magnet alloys>

[0067] An example is given of the manufacturing process for R-T-B series sintered magnet alloys. A metal or alloy pre-adjusted to the above composition is melted, and an alloy ingot is obtained using an ingot casting method that involves placing the melt into a mold. Alternatively, the molten liquid 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 to produce a thinner solidified alloy than that obtained by ingot casting.

[0068] In embodiments of the present invention, materials obtained by either ingot casting or quenching can be used, with quenching methods such as strip casting being preferred. The thickness of quenched alloys obtained by quenching is typically in the range of 0.03 mm to 1 mm, and they are in sheet form. The alloy molten liquid begins to solidify from the contact surface of the cooling roller (roller contact surface), and crystals grow columnarly from the roller contact surface in the thickness direction. Compared to conventional alloys obtained by ingot casting (mold casting), quenched alloys are cooled in a short time, resulting in a finer microstructure and smaller grain size. Furthermore, the grain boundary area is large. The R-rich phase extends extensively within the grain boundaries, thus exhibiting excellent dispersion when using quenching. Therefore, it is easy to fracture at the grain boundaries using hydrogen pulverization. By hydrogen pulverizing the quenched alloy, the size of the hydrogen-pulverized powder (coarse powder) can be reduced to, for example, 1.0 mm or less. For example, the coarse powder obtained in this way can be finely pulverized using a jet mill.

[0069] <Process for preparing rare earth alloy powder>

[0070] The rare earth alloy powder used in R-T-B series sintered magnets is reactive and easily oxidized. Therefore, to avoid the risk of overheating and ignition, and to reduce the oxygen content as an impurity to achieve high magnet performance, inert gases such as nitrogen, argon, and helium can be used as the gas used in the jet mill.

[0071] 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 particle size: d50) of 2.0 μm to 4.5 μm, and then moved to a cyclone collector. The cyclone collector is used to separate the powder from the gas stream carrying the powder. Specifically, coarse powder of R-T-B series sintered magnet alloy is pulverized by the preceding jet mill, and the resulting fine powder, along with the gas used for pulverization, is supplied to the cyclone collector. The inert gas (pulverizing gas) and the pulverized fine powder form a high-speed gas stream, which is sent to the cyclone collector. The cyclone collector is used to separate these pulverizing gases and fine powder. The fine powder separated from the pulverizing gas is recovered by a powder collector.

[0072] <Process for manufacturing powder molded bodies>

[0073] 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 by pressing in an inert gas atmosphere or by wet pressing. In particular, wet pressing coats the surface of the particles constituting the powder molded body with a dispersant such as an oil, suppressing contact with oxygen or water vapor in the atmosphere. Therefore, it is possible to prevent or suppress the oxidation of the particles by the atmosphere before, during, or after the pressing process.

[0074] In wet pressing under a magnetic field, a slurry containing a dispersion medium mixed in micro-powder is prepared and supplied to the mold cavity of a wet pressing device, where it is pressed and formed under a magnetic field. The resulting powder-molded workpiece has, for example, a density of 4 g / cm³. 3 Above 5g / cm 3 The following densities.

[0075] • Dispersion medium

[0076] A dispersion medium is a liquid that can disperse alloy powder within it to obtain a slurry.

[0077] Mineral oil or synthetic oil can be cited as a preferred dispersion medium used in this invention. The type of mineral oil or synthetic oil is not specific, but when the kinematic viscosity at room temperature exceeds 10 cSt, the increased viscosity strengthens the bonding force between alloy powders, which can sometimes adversely affect the orientation of the alloy powders during wet molding in a magnetic field. Therefore, the kinematic viscosity of the mineral oil or synthetic oil at room temperature is preferably below 10 cSt. Furthermore, when the fractionation temperature of the mineral oil or synthetic oil exceeds 400°C, degreasing after obtaining the molded body becomes difficult, resulting in increased residual carbon content in the sintered body and a decrease in magnetic properties. Therefore, the fractionation temperature of the mineral oil or synthetic oil is preferably below 400°C. Alternatively, vegetable oil can also be used as a dispersion medium. Vegetable oil refers to oil extracted from plants, and the type of plant is not limited to a specific plant.

[0078] • Preparation of slurry

[0079] The obtained alloy powder is mixed with a dispersion medium to obtain a slurry.

[0080] The mixing ratio of alloy powder and dispersion medium is not particularly limited, but the concentration of alloy powder in the slurry is preferably 70% or more by mass (i.e., 70% by mass or more). This is because in the range of 20–600 cm⁻¹… 3 The slurry is characterized by its efficient supply of alloy powder to the mold cavity at a flow rate of [flow rate] / second, resulting in excellent magnetic properties. The concentration of alloy powder in the slurry is preferably 90% or less by mass ratio. The method of mixing the alloy powder and the dispersion medium is not particularly limited. The alloy powder and dispersion medium can be prepared separately, weighed in predetermined amounts, and mixed together. Alternatively, when obtaining alloy powder by dry grinding of coarse powder using a jet mill or similar device, a container filled with the dispersion medium can be placed at the alloy powder discharge outlet of the jet mill or similar grinding device. The pulverized alloy powder can be directly recycled into the dispersion medium within the container to obtain a slurry. In this case, it is preferable to also create an atmosphere composed of nitrogen and / or argon within the container, directly recycling the obtained alloy powder into the dispersion medium without contacting the atmosphere to produce a slurry. Alternatively, a vibratory mill, ball mill, or grinding mill can be used to wet grind the coarse powder while maintaining it in the dispersion medium to obtain a slurry composed of alloy powder and dispersion medium.

[0081] By molding the slurry obtained in this way using a known wet pressing device, a workpiece with a powder molded body of a specified size and shape can be obtained. In the prior art, the workpiece of the powder molded body is usually sintered to obtain a sintered body, but in this embodiment, as will be explained below, the workpiece of the powder molded body is cut using a wire saw before sintering.

[0082] S20: The process of cutting the workpiece using a wire saw to divide it into multiple shaped pieces.

[0083] The cutting of the workpiece in this process, for example, utilizes Figure 2 The wire saw device shown is used.

[0084] Wire 42 of saw wire 40 (reference) Figure 3 The diameter of the wire 42 is, for example, 140 μm or more and 350 μm or less. When the diameter of the wire 42 is less than 140 μm, there is a problem that the wire 42 will elongate during cutting due to insufficient strength. The larger the diameter of the wire 42, the higher the discharge of cutting powder, but the amount of cutting powder increases. Therefore, it is preferable to have a diameter of 350 μm or less.

[0085] The moving speed of the saw wire 40 (saw wire speed) can be set, for example, in the range of 100 m / min to 500 m / min. On the other hand, the workpiece conveying speed ( Figure 2 The workpiece movement speed in the z-axis direction can be set, for example, in the 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.

[0086] From the viewpoint of rapidly removing cutting powder, it is preferable to perform wire sawing in a state where the workpiece 10 is immersed in the dispersion medium (mineral oil or synthetic oil) used when the workpiece is made into a powder molded body by wet pressing (oil cutting). In the case of wire sawing in the atmosphere, it is preferable to spray the part of the workpiece 10 that contacts the saw wire 40 (the cutting part) with the same oil as the dispersion medium.

[0087] By cutting with a wire saw, the workpiece 10 can be divided into shaped pieces, for example, with a thickness of about 1 to 10 mm.

[0088] S25: Process for recovering rare earth alloy powder particles removed from workpieces.

[0089] In this embodiment, it is not necessary to separate the abrasive grains 44 that fall off the wire 42 from the recovered powder particles, and the recovered powder particles are used to prepare other workpieces. Therefore, the operation of separating the small abrasive grains 44 is unnecessary. However, it is also possible to separate the abrasive grains 44 that fall off the wire 42 from the recovered powder particles. When performing this separation, it is not necessary to remove all the abrasive grains 44 that fall off the wire 42; a portion of the abrasive grains 44 can be mixed in the recovered powder particles. The mass ratio of abrasive grains 44 in the recovered powder is typically, for example, more than 0.01% and less than 5%. For example, by intermittently observing the wire saw, the total mass of the fallen abrasive grains 44 can be estimated based on the number of fallen abrasive grains 44, and an estimated value of the mass ratio can be calculated based on the proportion relative to the total mass of the recovered powder particles. It is also known that even with the same weight ratio, when the average particle size of the abrasive grains 44 is larger than 24 μm, pores are formed after the sintering process, which leads to a decrease in corrosion resistance.

[0090] S30: The process of sintering the molded sheet.

[0091] Next, the shaped sheets cut by the wire sawing process described above are sintered to obtain an R-T-B sintered magnet (sintered body). The sintering process of the shaped sheets can, for example, be carried out at 0.13 Pa (10... -3 Torr) below, preferably 0.07 Pa (5.0 × 10) -4 The process is carried out under pressures below Torr and at temperatures ranging from, for example, 1000°C to 1150°C. To prevent oxidation caused by sintering, residual gases in the atmosphere can be replaced with inert gases such as helium or argon. It is preferable to subject the resulting sintered body to additional heat treatment, such as aging. Such heat treatment improves magnetic properties. Known conditions can be used for heat treatment temperature, heat treatment time, and other heat treatment conditions. For the R-T-B sintered magnet obtained in this manner, grinding, surface treatment, and magnetization processes are performed as needed to complete the final R-T-B sintered magnet.

[0092] In a preferred embodiment, the method for manufacturing the R-T-B sintered magnet of the present invention further includes a diffusion step in which a heavy rare earth element RH (RH being at least one of Tb, Dy, and Ho) diffuses from the surface of the sintered body into its interior. When the heavy rare earth element RH diffuses from the surface of the sintered body into its interior, the coercivity can be effectively improved. The method of the diffusion step is not particularly limited; known methods can be used.

[0093] In embodiments of the present invention, a step (S40) for preparing a recycled powder mixture molded body containing the powder particles recovered in the above-described step (S25) and a step (S50) for sintering the recycled powder mixture molded body can be performed. This will be explained below. S40: Prepare a recycled powder mixture containing the recycled powder particles for powder molding. Body process

[0094] The powder particles recovered in the rare earth alloy powder mixing process (S25) described in process (S10) "Process for preparing rare earth alloy powder" are used to prepare "recovered powder mixed powder". The process for making a molded body of the recovered powder mixed powder is as described in process (S10) "Process for making powder molded body".

[0095] Process (S40) can be performed without separating the abrasive grains that have detached from the wire from the recovered powder particles. Without separation, there is a possibility that the recovered powder mixture may contain abrasive grains, but the properties of the sintered magnet will not deteriorate as a result. However, if the mass of abrasive grains contained in the recovered powder mixture molded body exceeds 0.3% of the total mass of the recovered powder mixture molded body, the properties may deteriorate. Therefore, the mass of abrasive grains contained in the recovered powder mixture molded body is preferably 0.3% or less of the total mass of the recovered powder mixture molded body. More preferably, it is 0.1% or less of the total mass of the recovered powder mixture molded body. Furthermore, regarding the recovered powder mixture molded body, other molded body sheets can be prepared by preparing it in the form of a workpiece and cutting it, or the recovered powder mixture molded body can be prepared as other molded body sheets.

[0096] S50: The process of sintering the molded body of recycled powder mixture.

[0097] The process of sintering the molded body of the recycled powder mixture can be performed in the same way as the process of sintering the molded body sheet described above (S30). Alternatively, as described above, other molded sheets prepared by cutting the molded body of the recycled powder mixture can also be sintered.

[0098] According to an embodiment of the present invention, even if voids (pores) are formed at locations where diamond abrasive grains are mixed in, their size is small enough that the corrosion resistance of the sintered magnet does not decrease.

[0099] (Example)

[0100] Raw materials were weighed to achieve a composition of Nd: 22.6%, Pr: 7.8%, B: 0.9%, Co: 0.5%, Al: 0.1%, Cu: 0.2%, and Ga: 0.4% (all by mass%), and an alloy was fabricated using a strip casting method. The resulting alloy was hydrogen-milled to obtain coarse powder. Next, 0.04% by mass of zinc stearate as a lubricant was added to the coarse powder, and the mixture was then dry-milled using a jet mill under a nitrogen atmosphere to obtain a particle size D. 50The powder was a 4μm fine powder (alloy powder). The fine powder was impregnated in a mineral oil with a fractionation temperature of 250°C and a kinematic viscosity of 2 cSt at room temperature under a nitrogen atmosphere to prepare a slurry. The slurry concentration was 85% by weight. The resulting slurry was formed in a magnetic field (wet forming) to produce a powder-molded workpiece. The workpiece dimensions were 50mm × 40mm × 20mm. The workpiece was cut into 200 molded pieces using a wire saw, and the molded pieces were sintered to produce sintered magnets. The cutting was performed while the workpiece was immersed in a liquid (the same substance used in the molding process as the mineral oil). In sample A, no recycled powder was mixed into the powder-molded workpiece, and no diamond abrasive particles were mixed in. In sample B, recycled powder containing diamond abrasive particles with an average particle size of 5μm was reused. In sample C, recycled powder containing diamond abrasive particles with an average particle size of 10μm was reused. In sample D, recycled powder containing diamond abrasive grains with an average particle size of 20 μm was reused. In sample E, recycled powder containing diamond abrasive grains with an average particle size of 30 μm was reused. The mass of abrasive grains contained in the workpieces (recycled powder mixed powder molded bodies) used for reuse in samples B to E was 0.1% of the total mass of the recycled powder mixed powder molded body. Regarding the average particle size, the particle size of the diamond abrasive grains contained in 50 recycled powder samples was measured, and the average value was calculated.

[0101] Figure 4A , Figure 4B and Figure 4C These are cross-sectional photographs of the sintered magnets from samples A, C, and E, respectively. Figure 4A It can be seen that in sintered magnets made from ordinary powder without the reuse of wire saw cutting powder, a normal magnet structure without pores was observed. Similarly, in sample C, from Figure 4B It can be seen that normal magnet tissue without pores was also observed. On the other hand, in sample E, such as Figure 4C As shown, pores with a diameter of more than 30 μm were observed in several locations.

[0102] Next, the corrosion resistance of the sintered magnets prepared from samples A to E was evaluated. Table 1 shows the results of the 72-hour PCT (pressure cooking test) for samples A to E. The larger the negative value of the loss in Table 1, the lower the corrosion resistance is evaluated.

[0103] Table 1

[0104]

[0105] Sample A is a sintered magnet completely free of diamond abrasive particles. Using Sample A as a benchmark, the increase in wear is compared to evaluate corrosion resistance.

[0106] As shown in Table 1, the loss of samples B through D was the same as that of sample A, and the decrease in corrosion resistance was suppressed. In contrast, the corrosion resistance of sample E decreased significantly.

[0107] These results show that by using a wire saw with multiple abrasive grains having an average particle size of 1 μm to 24 μm and wire with the aforementioned multiple abrasive grains fixed, the powder particles that are removed from the workpiece of the powder molded body due to the wire sawing process are recovered, and the recovered powder mixed powder molded body containing the recovered powder particles is sintered, the decrease in corrosion resistance of the obtained R-T-B system sintered magnet can be suppressed.

Claims

1. A method for manufacturing an R-T-B system sintered magnet, characterized in that, include: The process of preparing powder molded parts of rare earth alloys for R-T-B series sintered magnets. The process of cutting the workpiece and dividing it into multiple shaped pieces; and The process of sintering the molded sheet, The process of dividing the workpiece into the plurality of shaped pieces includes cutting the workpiece using a wire saw having a plurality of abrasive grains with an average particle size of 1 μm to 24 μm and wires with the plurality of abrasive grains attached, and recovering the rare earth alloy powder particles cut off from the workpiece. In the process of recovering the rare earth alloy powder particles cut off from the workpiece, the abrasive grains that have fallen off the wire are recovered together with the rare earth alloy powder particles cut off from the workpiece.

2. The method for manufacturing an R-T-B sintered magnet as described in claim 1, characterized in that, The process of cutting the workpiece using the wire saw is performed while the workpiece is immersed in liquid.

3. The method for manufacturing an R-T-B system sintered magnet as described in claim 1 or 2, characterized in that, The process of preparing the workpiece includes: The process of preparing the rare earth alloy powder; and The process of forming the powder by wet pressing.

4. The method for manufacturing an R-T-B sintered magnet as described in claim 1 or 2, characterized in that, The rare earth alloy is an R-T-B series rare earth alloy used in sintered magnets. The workpiece has a content of 4g / cm 3 Above 5g / cm 3 The following densities.

5. The method for manufacturing an R-T-B sintered magnet as described in claim 1 or 2, characterized in that, The abrasive grains are diamond abrasive grains.

6. The method for manufacturing an R-T-B system sintered magnet as described in claim 1 or 2, characterized in that, The diameter of the wire is between 140μm and 350μm.

7. A method for manufacturing an R-T-B system sintered magnet, characterized in that, include: A process of cutting a workpiece of rare earth alloy for R-T-B sintered magnets into multiple molded pieces by using a wire saw with multiple abrasive grains having an average particle size of 1 μm to 24 μm and wires with the multiple abrasive grains fixed thereon, and recycling the rare earth alloy powder particles cut off from the workpiece when dividing the workpiece into multiple molded pieces. The process of preparing a recycled powder mixture containing the recycled powder particles; and The process of sintering the molded body of the recycled powder mixture. In the process of recovering the rare earth alloy powder particles cut off from the workpiece, the abrasive grains that have fallen off the wire are recovered together with the rare earth alloy powder particles cut off from the workpiece.

8. The method for manufacturing an R-T-B sintered magnet as described in claim 7, characterized in that, The process of preparing the recycled powder mixed powder molded body includes the process of mixing the recycled powder particles in powder formed from rare earth alloys for R-T-B sintered magnets.

9. The method for manufacturing an R-T-B sintered magnet as described in claim 7 or 8, characterized in that, The process of preparing the recycled powder mixture is carried out in a manner that does not separate the abrasive grains that have detached from the wire from the recycled powder particles.

10. The method for manufacturing an R-T-B sintered magnet as described in claim 7 or 8, characterized in that, The recycled powder mixed powder molded body contains the abrasive particles.

11. The method for manufacturing an R-T-B system sintered magnet as described in claim 10, characterized in that, The mass of the abrasive particles contained in the recycled powder mixture molded body is less than 0.3% of the total mass of the recycled powder mixture molded body.

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