JP7439613B2 - Manufacturing method of RTB based sintered magnet - Google Patents

Description

The present application relates to a method for manufacturing an RTB-based sintered magnet.

RTB system sintered magnet (R is a rare earth element and always contains at least one selected from the group consisting of Nd, Pr, and Ce, T is at least one transition metal and always contains Fe) , B is boron) is a compound formed by the main phase of a compound having a R ₂ Fe ₁₄ B type crystal structure, the grain boundary phase located at the grain boundary part of this main phase, and the influence of trace additive elements and impurities. It is composed of phases. RTB-based sintered magnets have a high residual magnetic flux density B _r (hereinafter sometimes simply referred to as “B _r ”) and a high coercive force H _cJ (hereinafter simply referred to as “H _cJ ”). It is known as the highest performance magnet among permanent magnets because of its excellent magnetic properties. For this reason, RTB-based sintered magnets are used in a wide variety of applications such as voice coil motors (VCMs) for hard disk drives, motors for electric vehicles (EV, HV, PHV), motors for industrial equipment, and home appliances. It is used for various purposes.

Such RTB-based sintered magnets are manufactured through, for example, a process of preparing alloy powder, a process of press-molding the alloy powder to produce a powder compact, and a process of sintering the powder compact. Ru. The alloy powder is produced, for example, by the following method.

First, an alloy is manufactured from molten metals of various raw material metals by a method such as an ingot method or a strip casting method. The obtained alloy is subjected to a pulverization process to obtain an alloy powder having a predetermined particle size distribution. This pulverization process usually includes a coarse pulverization process and a fine pulverization process. be exposed.

The sintered body obtained by the process of sintering the powder compact is then subjected to mechanical processing such as grinding and cutting, and is separated into pieces having a desired shape and size. More specifically, first, R--Fe--B rare earth magnet powder is compression-molded using a press machine to produce a compact that is larger in size than the final magnet product. After the molded body is made into a sintered body through a sintering process, the sintered body is ground into a desired shape using, for example, a cemented carbide blade saw or a rotary grindstone. For example, after first producing a block-shaped sintered body, the sintered body is sliced with a blade saw or the like to cut out a plurality of plate-shaped sintered body parts.

However, the sintered bodies of rare earth alloy magnets such as R-Fe-B sintered magnets are extremely hard and brittle, and the processing load is large, so high-precision grinding is a difficult task and the processing time is long. It takes. Furthermore, some material parts are inevitably lost during processing. For this reason, the processing process has been a major cause of increased manufacturing costs.

For example, in order to solve the former problem, Patent Document 1 describes a technique in which a molded magnet is processed using a wire saw before sintering. A wire saw is a processing technique in which a wire running in one direction or both directions is pressed against a molded body to be processed, and the molded body is ground or cut using abrasive grains between the wire and the molded body. According to this technique, since the powder compact is cut which is much softer and easier to process than the sintered body, the time required for the cutting process is significantly shortened.

JP2003-303728A

However, Patent Document 1 does not provide a solution to the cost increase due to material loss during processing. Since R--Fe--B sintered magnets contain expensive and rare rare earth elements, there is a need to further improve the material utilization efficiency (yield).

Embodiments of the present disclosure provide a new method for manufacturing RTB-based sintered magnets that enables further increase in yield.

In an exemplary embodiment, the method for manufacturing an RTB-based sintered magnet of the present disclosure includes a step of preparing a workpiece of a powder compact of a rare earth alloy for an RTB-based sintered magnet; and dividing the workpiece into a plurality of molded body pieces, and sintering the molded body pieces, and the step of dividing the workpiece into the plurality of molded body pieces has an average particle size of Cutting the workpiece with a wire saw having a plurality of abrasive grains of 1 μm or more and 24 μm or less and a wire to which the plurality of abrasive grains are fixed, and collecting powder particles of the rare earth alloy scraped off from the workpiece. .

In one embodiment, the step of cutting the workpiece with the wire saw is performed while the workpiece is submerged in a liquid.

In one embodiment, the step of preparing the workpiece includes the steps of preparing the rare earth alloy powder and molding the powder by wet pressing.

In one embodiment, the rare earth alloy is a rare earth alloy for RTB-based sintered magnets, and the workpiece has a density of 4 g/cm ³ or more and 5 g/cm ³ or less.

In one embodiment, in the step of collecting rare earth alloy powder particles scraped off from the workpiece, the abrasive grains that have come off the wire are collected together with the rare earth alloy powder particles scraped off from the workpiece.

In one embodiment, the abrasive grains are diamond abrasive grains.

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

In an embodiment, the method for manufacturing a sintered RTB magnet of the present disclosure includes a workpiece of a powder compact of a rare earth alloy for a sintered RTB magnet with an average particle size of 1 μm or more and 24 μm or less. Powder particles of the rare earth alloy that are scraped off from the workpiece when the workpiece is divided into a plurality of molded body pieces by cutting with a wire saw having a plurality of abrasive grains and a wire to which the plurality of abrasive grains are fixed. , a step of preparing a recovered powder mixed powder compact containing the recovered powder particles, and a step of sintering the recovered powder mixed powder compact.

In one embodiment, the step of preparing the recovered powder mixed powder compact includes a step of mixing the recovered powder particles with powder formed from a rare earth alloy for an RTB-based sintered magnet.

In one embodiment, the step of preparing the recovered powder mixed powder compact is performed without separating the abrasive grains separated from the wire from the recovered powder particles.

In one embodiment, the collected powder mixed powder compact contains the abrasive grains.

In one embodiment, the mass of the abrasive grains contained in the recovered powder mixed powder compact is 0.3% or less of the entire mass of the recovered powder mixed powder compact.

According to the embodiment of the present disclosure, when collecting and reusing powder particles scraped off by wire sawing performed before the sintering process, it is possible to suppress deterioration of magnetic properties, particularly corrosion resistance. Therefore, it is possible to reduce manufacturing costs while maintaining the characteristics of a high-performance magnet.

FIG. 1A is a flowchart showing the main steps of the manufacturing method in the embodiment of the present disclosure. FIG. 1B is a flowchart showing steps in another aspect of the manufacturing method according to the embodiment of the present disclosure. FIG. 2 is a perspective view showing a configuration example of a wire saw device that can be used in the embodiment of the present disclosure. FIG. 3 is a cross-sectional view schematically showing a cross section of the wire 40. As shown in FIG. FIG. 4A is a diagram showing a cross-sectional photograph of the sintered magnet of Sample A. FIG. 4B is a diagram showing a cross-sectional photograph of the sintered magnet of Sample C. FIG. 4C is a diagram showing a cross-sectional photograph of the sintered magnet of Sample E.

Hereinafter, embodiments of a method for manufacturing an RTB-based sintered magnet according to the present disclosure will be described. The method for manufacturing the RTB-based sintered magnet in this embodiment is as shown in the flowchart of FIG. 1A.
・Preparing a workpiece of a powder compact of a rare earth alloy for an RTB-based sintered magnet (S10);
- Cutting the workpiece and dividing the workpiece into a plurality of molded body pieces (S20);
・Includes a step (S30) of sintering the molded body piece,
The step of dividing the workpiece into a plurality of molded body pieces (S20) involves cutting the workpiece with a wire saw having a plurality of abrasive grains having an average grain size of 1 μm or more and 24 μm or less, and a wire to which a plurality of abrasive grains are fixed, The process includes a step (S25) of recovering rare earth alloy powder particles scraped off from the workpiece.

Further, in another aspect of the method for manufacturing an RTB-based sintered magnet according to the present disclosure, as shown in the flowchart of FIG. 1B, a step (S25) of collecting rare earth alloy powder particles scraped from the workpiece After the,
・Preparing a recovered powder mixed powder compact containing recovered powder particles (S40);
- May include a step of sintering the recovered mixed powder compact (S50).

According to the method for manufacturing an RTB-based sintered magnet of the present disclosure, it is possible to easily collect and reuse powder particles scraped off from a powder compact workpiece by wire saw processing performed before the sintering process. Become. Further, in an RTB-based sintered magnet obtained by sintering a recovered powder mixed powder compact containing recovered powder particles, it is possible to suppress deterioration of magnetic properties, particularly corrosion resistance.

A configuration example of a wire saw device that can be used in the above manufacturing method will be described with reference to FIG. 2. FIG. 2 is a perspective view showing a configuration example of the wire saw device 100 in the embodiment of the present disclosure. For reference, the figure shows an xyz coordinate system including an x-axis, a y-axis, and an x-axis that are orthogonal to each other. In this example, the xy plane is horizontal and the z axis is oriented vertically.

The wire saw device 100 in FIG. 2 includes rollers 30a, 30b, and 30c arranged so that their central axes of rotation are parallel to each other, and a single continuous wire 40. The powder molded workpiece 10 prepared in the step (S10) is supported by the fixing base 20. A specific example of the process of preparing the powder compact workpiece 10 will be described later. It should be noted here that the workpiece 10 is not a sintered body but a powder compact (green compact) before being sintered. The powder compact is obtained, for example, by molding powder of a rare earth alloy for RTB-based sintered magnets (hereinafter simply referred to as "rare earth alloy") by wet pressing or dry pressing in an orienting magnetic field. .

The fixing base 20 moves up and down in the z-axis direction while the workpiece 10 is fixed. This vertical movement may be performed by a drive device (not shown). The drive device may obtain driving force from a hydraulic cylinder or may be operated by a motor.

The rollers 30a, 30b, and 30c are arranged at predetermined intervals so that the axis of rotation is located at the vertex of the triangle when viewed from a direction parallel to the x-axis. A plurality of grooves are provided on each side surface of the rollers 31a, 31b, and 31c. The wire 40 is wound in order around a plurality of grooves of the rollers 30a, 30b, and 30c. The center spacing (pitch) of the plurality of grooves defines the width of the element divided by wire saw cutting. Both ends of the wire 40 are wound around a collection bobbin (not shown), for example.

During cutting, the rollers 30a, 30b, 30c and the collection bobbin rotate. The rotation direction of the rollers 30a, 30b, and 30c depends on their arrangement and how the wire 40 is hung. In the wire saw device 100 shown in FIG. 2, rollers 30a, 30b, and 30c rotate in the same direction.

Once a predetermined length of wire 40 has been wound onto one of the collection bobbins, the collection bobbin and rollers 30a, 30b, and 30c are rotated in the opposite direction. This causes the wire 40 to move in the opposite direction, and by repeating this, the wire 40 can be reciprocated (moved).

For example, a fixed abrasive wire is used as the wire 40. Specifically, it is possible to use a material in which high-hardness abrasive grains suitable for cutting high-hardness materials are fixed to the strands by electrodeposition.

FIG. 3 schematically shows a cross section of the wire 40. The wire 40 includes a wire (core wire) 42, abrasive grains 44 located on the outer peripheral surface of the wire 42, and a fixed layer 46. The fixed layer 46 is made of a plated metal such as Ni, for example. The abrasive grains 44 are located on the surface of the wire 42, and a fixing layer 46 covers the entire surface of the wire 42 around the abrasive grains 44 and the abrasive grains 44, thereby fixing the abrasive grains 44 to the wire 42. can be done. Fixation of the abrasive grains 44 may be realized by other methods. The average particle size of the abrasive grains 44 is 1 μm or more and 24 μm or less. For the abrasive grains 44, a known wire saw having an average grain size in the range of 1 μm or more and 24 μm or less may be used. It can also be determined by observing the wire and measuring the abrasive grains of the wire saw. For example, the average grain size may be determined by observing the wire and randomly measuring 50 abrasive grains 44.

The process of cutting the workpiece 10 with a wire saw is preferably performed with the workpiece 10 submerged in liquid. When the workpiece 10 is a powder compact formed by wet pressing, a preferred example of this liquid is a dispersion medium such as an oil agent (mineral oil or synthetic oil) used in wet pressing.

When the workpiece 10 is machined by such a wire saw device 100, the powder particles constituting the workpiece 10 fall off as cutting dust from the part cut by the abrasive grains 44 of the wire 40. The inventor of this application considered reusing this cutting powder. When a hard sintered body obtained by sintering a powder compact is cut, the cutting powder is particles whose grains have grown due to sintering or whose composition has changed due to a chemical reaction, or a combination of particles. Therefore, even if they are mixed with rare earth magnet powder and reused, the magnetic properties are likely to deteriorate. On the other hand, cutting powder obtained from a powder compact before sintering has the same composition and size as other particles contained in the powder compact, and is fully recyclable. .

However, according to further study by the present inventor, when the rare earth alloy powder particles scraped off from the workpiece 10 are collected and a sintered magnet is made from a rare earth alloy powder compact containing the collected powder particles, the magnetic properties change. It was found that it may deteriorate. The cause of this was that the collected powder contained abrasive grains 44 that had fallen off from the wire 40. A typical example of the material of the abrasive grains 44 is diamond, which is made of carbon. It has been found that the inclusion of diamond particles can generate cavities (cavities) during the sintering process and deteriorate magnetic properties (particularly corrosion resistance). As a result of intensive study by the present inventor, if the average particle size of the abrasive grains 44 is set to 1 μm or more and 24 μm or less, even if the collected powder (cutting powder) contains the abrasive grains 44 that have fallen off from the wire 40 as foreign matter. It was found that even when the porosity is formed during the sintering process, the cavities formed during the sintering process are sufficiently small and have almost no negative effect on the magnetic properties such as corrosion resistance.

It was also confirmed that better magnetic properties could be obtained when the workpiece 10 was a powder compact produced by wet pressing and wire sawing was performed in liquid. By wire sawing in liquid, even when the size of the abrasive grains 44 is limited to a small size as in the embodiment of the present disclosure, cutting performance can be maintained and the cut surface can be smoothed.

The method for manufacturing the RTB sintered magnet of this embodiment will be described in detail below.

S10: Step of preparing a workpiece of rare earth alloy powder compact <composition of rare earth alloy>
R is a rare earth element and always contains at least one selected from the group consisting of Nd, Pr, and Ce. Preferably, 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 combinations of rare earth elements are used.

Among R, Dy and Tb are particularly effective in improving H _cJ . In addition to the above elements, other rare earth elements such as La may be contained, and misch metal and didymium may also be used. Further, R may not be a pure element, and may contain impurities that are unavoidable in production within an industrially available range. The content is, for example, 27% by mass or more and 35% by mass or less. Preferably, the R content of the RTB-based sintered magnet 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 setting the R content of the RTB-based sintered magnet to 31% by mass or less and the oxygen content to 500 ppm or more and 3500 ppm or less (preferably 500 ppm or more and 3200 ppm or less, and more preferably 500 ppm or more and 2500 ppm or less). , higher magnetic properties can be obtained.

T contains iron (including cases where T consists essentially of iron), and 50% or less of it by mass ratio may be replaced with cobalt (Co) (T consists essentially of iron and cobalt). (including cases). Co is effective in improving temperature characteristics and corrosion resistance, and the alloy powder may contain 10% by mass or less of Co. The content of T may occupy the remainder of R and B, or R and B and M, which will be described later.

The content of B may be any known content, and for example, a preferable range is 0.9% by mass to 1.2% by mass. If it is less than 0.9% by mass, high H _cJ may not be obtained, and if it exceeds 1.2% by mass, _Br may decrease. Note that a part of B can be replaced with C (carbon).

In addition to the above elements, an M element can be added to improve H _cJ . The M element is one or more selected from the group consisting of Al, Si, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Zr, Nb, Mo, In, Sn, Hf, Ta, and W. . The amount of the M element added is preferably 5.0% by mass or less. This is because if it exceeds 5.0% by mass, Br may decrease. Also, unavoidable impurities can be tolerated.

The content of N (nitrogen) in the RTB-based sintered magnet is preferably 50 ppm or more and 600 ppm or less. Further, the content of C (carbon) in the RTB-based sintered magnet is preferably 50 ppm or more and 1000 ppm or less.

<Production process of RTB alloy for sintered magnets>
The manufacturing process of an RTB alloy for sintered magnets will be exemplified. An alloy ingot can be obtained by an ingot casting method in which a metal or alloy that has been adjusted in advance to have the composition described above is melted and placed in a mold. In addition, molten metal is brought into contact with a single roll, twin rolls, rotating disk, or rotating cylindrical mold to rapidly cool it to produce a solidified alloy that is thinner than the alloy made by the ingot method, typically the strip casting method or centrifugal casting method. Alloy flakes can be produced by a rapid cooling method.

In the embodiments of the present disclosure, materials manufactured by either the ingot method or the quenching method can be used, but it is preferable that the material be manufactured by a quenching method such as a strip casting method. The thickness of the rapidly solidified alloy produced by the rapid cooling method is usually in the range of 0.03 mm to 1 mm, and has a flake shape. The molten alloy begins to solidify from the surface in contact with the cooling roll (roll contact surface), and crystals grow in columnar shapes in the thickness direction from the roll contact surface. Rapidly solidified alloys are cooled in a shorter time than alloys (ingot alloys) produced by conventional ingot casting methods (mold casting methods), so they have finer structures and smaller crystal grain sizes. Also, the area of grain boundaries is wide. Since the R-rich phase widely spreads within the grain boundaries, the rapid cooling method has excellent dispersibility of the R-rich phase. For this reason, it is easy to fracture at grain boundaries by hydrogen pulverization. By subjecting the rapidly solidified alloy to hydrogen pulverization, the size of the hydrogen pulverized powder (coarsely pulverized powder) can be reduced to, for example, 1.0 mm or less. The coarsely pulverized powder thus obtained is finely pulverized using, for example, a jet mill.

<Process of preparing rare earth alloy powder>
The rare earth alloy powder for RTB sintered magnets is active and easily oxidized. For this reason, gases used in jet mills include nitrogen, argon, helium, etc., in order to avoid the risk of heat generation and ignition, and to reduce the oxygen content as an impurity and improve the performance of the magnet. An inert gas is used.

The material to be pulverized (coarsely pulverized powder) fed into a jet mill is, for example, pulverized into a fine powder having a particle size distribution with an average particle size (median diameter: d50) of 2.0 μm or more and 4.5 μm or less, and then is captured in a cyclone. It will be moved to a collection device. Cyclone collectors are used to separate powder from the airflow that carries it. Specifically, coarsely pulverized powder of RTB-based sintered magnet alloy is pulverized in a jet mill in the previous stage, and the fine powder generated by pulverization is sent to a cyclone collection device along with the gas used for pulverization. Supplied. A mixture of inert gas (grinding gas) and pulverized fine powder forms a high-speed airflow and is sent to a cyclone collector. A cyclone collector is used to separate these grinding gases and fine powders. The fine powder separated from the grinding gas is collected by a powder collector.

<Process of producing a powder compact>
Next, a powder compact is produced from the above fine powder by pressing in a magnetic field. In pressing in a magnetic field, from the viewpoint of suppressing oxidation, it is preferable to form a powder compact by pressing in an inert gas atmosphere or wet pressing. In particular, in wet pressing, the surfaces of the particles constituting the powder compact are coated with a dispersant such as an oil agent to suppress contact with oxygen and water vapor in the atmosphere. Therefore, it is possible to prevent or suppress the particles from being oxidized by the atmosphere before, during or after the pressing process.

When performing wet pressing in a magnetic field, a slurry of fine powder mixed with a dispersion medium is prepared, and the slurry is supplied to a cavity in a mold of a wet pressing device and press-molded in a magnetic field. The powder compact work thus formed has a density of, for example, 4 g/cm ³ or more and 5 g/cm ³ or less.

- Dispersion medium A dispersion medium is a liquid in which a slurry can be obtained by dispersing alloy powder.

Preferred dispersion media for use in the present disclosure may include mineral or synthetic oils. The type of mineral oil or synthetic oil is not specified, but if the kinematic viscosity at room temperature exceeds 10 cSt, the increased viscosity will strengthen the bonding force between the alloy powders, which will affect the orientation of the alloy powder during wet compaction in a magnetic field. may have a negative impact. Therefore, the kinematic viscosity of the mineral oil or synthetic oil at room temperature is preferably 10 cSt or less. Furthermore, if the fractionation point of mineral oil or synthetic oil exceeds 400° C., it becomes difficult to remove oil after obtaining a molded body, and the amount of residual carbon in the sintered body may increase, resulting in a decrease in magnetic properties. Therefore, the fractionation point of mineral oil or synthetic oil is preferably 400°C or lower. Furthermore, vegetable oil may 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.

- Preparation of slurry A slurry can be obtained by mixing the obtained alloy powder and a dispersion medium.

Although the mixing ratio of the alloy powder and the dispersion medium is not particularly limited, the concentration of the alloy powder in the slurry is preferably 70% or more (ie, 70% by mass or more) in terms of mass ratio. This is because at a flow rate of 20 to 600 cm ³ /sec, the alloy powder can be efficiently supplied into the cavity and excellent magnetic properties can be obtained. The concentration of the alloy powder in the slurry is preferably 90% or less in terms of mass ratio. The method of mixing the alloy powder and the dispersion medium is not particularly limited. The alloy powder and the dispersion medium may be prepared separately, and a predetermined amount of both may be weighed and mixed together. In addition, when obtaining alloy powder by dry-pulverizing coarsely ground powder with a jet mill or the like, a container containing a dispersion medium is placed at the alloy powder outlet of the jet mill or other grinding device, and the alloy powder obtained by pulverizing may be collected directly into a dispersion medium in a container to obtain a slurry. In this case, it is preferable that the atmosphere in the container is also made of nitrogen gas and/or argon gas, and the obtained alloy powder is directly collected into a dispersion medium without being exposed to the atmosphere to form a slurry. Furthermore, it is also possible to obtain a slurry consisting of the alloy powder and the dispersion medium by wet-pulverizing the coarsely pulverized powder while it is held in a dispersion medium using a vibration mill, a ball mill, an attritor, or the like.

By molding the slurry thus obtained using a known wet press device, a powder compact workpiece having a predetermined size and shape can be obtained. Conventionally, this powder compact workpiece is usually sintered to obtain a sintered compact, but in this embodiment, the powder compact workpiece is divided with a wire saw before sintering, as explained below. do.

S20: Step of cutting the workpiece with a wire saw and dividing the workpiece into a plurality of molded body pieces The cutting of the workpiece in this step is performed by, for example, a wire saw device shown in FIG. 2.

The diameter of the wire 42 (see FIG. 3) of the wire 40 is, for example, 140 μm or more and 350 μm or less. When the diameter of the strand 42 is less than 140 μm, there is a problem that the strand 42 stretches during cutting due to insufficient strength. The larger the diameter of the strand 42, the better the cutting powder discharge performance, but the amount of cutting powder increases, so it is desirable that the diameter is 350 μm or less.

The running speed of the wire 40 (wire linear speed) may be set, for example, in a range of 100 m/min or more and 500 m/min or less. On the other hand, the workpiece feed speed (workpiece movement speed in the z-axis direction in FIG. 2) may be set, for example, in a range of 100 mm/min or more and 600 mm/min or less. The tension applied to the wire 40 is, for example, 2.0 kg or more and 3.0 kg or less.

Wire saw processing is performed with the workpiece 10 immersed in the dispersion medium (mineral oil or synthetic oil) used when producing the powder compact workpiece by wet pressing, in order to quickly discharge chips. (cutting in oil) is desirable. When wire sawing is performed in the atmosphere, it is desirable to spray oil similar to the dispersion medium to the portion (cutting portion) where the workpiece 10 and the wire 40 come into contact.

By wire saw cutting, the workpiece 10 can be divided into molded body pieces having a thickness of about 1 to 10 mm, for example.

S25: Step of recovering the rare earth alloy powder particles scraped off from the workpiece In this embodiment, the abrasive particles 44 that have come off the strands 42 are collected without separating the abrasive particles 44 from the recovered powder particles. Use this to prepare other workpieces. Therefore, there is no need to separate the small abrasive grains 44. However, the abrasive grains 44 that have come off the strands 42 may be separated from the recovered powder particles. During this sorting, some of the abrasive grains 44 may be mixed with the collected powder particles without being removed from all of the abrasive grains 44 that have come off the strands 42. The mass ratio of the abrasive grains 44 contained in the collected powder is usually, for example, 0.01% or more and 5% or less. The mass ratio can be estimated by, for example, estimating the total mass of the abrasive grains 44 that have fallen off based on the number of abrasive grains 44 that have fallen off by intermittently observing the wire saw, and calculating from the ratio of the collected powder particles to the total mass. is possible. It is known that even if the weight ratio is the same, if the average particle diameter of the abrasive grains 44 exceeds 24 μm, cavities are formed after the sintering process, resulting in a decrease in corrosion resistance.

S30: Step of sintering the molded body pieces Next, the individual molded body pieces cut by the above-mentioned wire saw process are sintered to obtain an RTB type sintered magnet (sintered body). The sintering process of the molded body piece is performed at a temperature of, for example, 1000°C to 1150°C under a pressure of, for example, 0.13 Pa (10 ^-3 Torr) or less, preferably 0.07 Pa (5.0 × 10 ^-4 Torr) or less. This can be done within the range of To prevent oxidation due to sintering, residual gases in the atmosphere may be replaced by inert gases such as helium, argon, etc. It is preferable to perform additional heat treatment such as aging treatment on the obtained sintered body. Such heat treatment can improve magnetic properties. Heat treatment conditions such as heat treatment temperature and heat treatment time can employ known conditions. The thus obtained RTB-based sintered magnet is subjected to a grinding/polishing process, a surface treatment process, and a magnetization process, as necessary, to produce the final RTB-based sintered magnet. The magnet is completed.

In a preferred embodiment, the method for manufacturing an RTB-based sintered magnet of the present disclosure includes introducing a heavy rare earth element RH (RH is at least one of Tb, Dy, and Ho) from the surface of the sintered body into the inside. The method further includes a step of diffusing. By diffusing the heavy rare earth element RH from the surface of the sintered body into the interior, the coercive force can be efficiently increased. The method of the diffusion step is not particularly limited. A known method can be adopted.

In an embodiment of the present disclosure, a step (S40) of preparing a recovered powder mixed powder compact containing the powder particles recovered in the above step (S25), a step of sintering the recovered powder mixed powder compact ( S50) can be performed. This point will be explained below.

S40: Step of preparing a recovered powder mixed powder compact containing recovered powder particles. Mix powder particles to prepare "recovered mixed powder". The step of producing a molded body of the recovered powder mixed powder is as described in the step (S10) of "the process of producing a powder molded body".

The step (S40) can be performed without separating the abrasive grains that have come off the wire from the collected powder particles. If separation is not performed, the recovered mixed powder may contain abrasive grains, but this will not deteriorate the characteristics of the sintered magnet. However, if the mass of abrasive grains contained in the recovered powder mixed powder compact exceeds 0.3% of the entire mass of the recovered powder mixed powder compact, there is a risk that the characteristics will deteriorate. The mass of the abrasive grains contained in the body is preferably 0.3% or less of the total mass of the collected powder mixed powder compact. More preferably, it is 0.1% by mass or less of the total mass of the recovered mixed powder compact. In addition, the recovered powder mixed powder molded body may be prepared as a workpiece and other molded body pieces may be prepared by cutting the recovered powder mixed powder molded body, or the collected powder mixed powder stagnation may be prepared as another molded piece. You may prepare.

S50: Step of sintering the recovered powder mixed powder compact The step of sintering the recovered powder mixed powder compact can be performed in the same manner as the step of sintering the compacted body pieces described above (S30). Note that, as described above, other molded pieces prepared by cutting the recovered powder mixed powder molded body may be sintered.

According to the embodiment of the present disclosure, even if voids (cavities) are formed at the positions of mixed diamond abrasive grains, the size thereof is sufficiently small and the corrosion resistance of the sintered magnet does not deteriorate.

(Example)
Nd: 22.6%, Pr: 7.8%, B: 0.9%, Co: 0.5%, Al: 0.1%, Cu: 0.2%, Ga: 0.4% (both The raw materials of each element were weighed so as to have a composition of (mass%), and an alloy was produced by a strip casting method. The obtained alloy was subjected to hydrogen pulverization to obtain coarsely pulverized powder. Next, 0.04% by mass of zinc stearate was added as a lubricant to the obtained coarsely pulverized powder based on 100% by mass of the coarsely pulverized powder, and after mixing, dry pulverization was carried out in a nitrogen stream using a jet mill. A finely pulverized powder (alloy powder) having a particle size _D50 of 4 μm was obtained. The finely pulverized material was immersed in mineral oil having a fractionation point of 250° C. and a kinematic viscosity of 2 cSt at room temperature in a nitrogen atmosphere to prepare a slurry. The slurry concentration was 85% by weight. The obtained slurry was molded in a magnetic field (wet molding) to produce a powder compact workpiece. The size of the workpiece was 50 mm x 40 mm x 20 mm. The workpiece was divided into 200 molded body pieces using a wire saw, and the molded body pieces were sintered to produce a sintered magnet. Note that cutting with a wire saw was performed with the workpiece submerged in a liquid (the same liquid as the mineral oil used during molding was used). In sample A, the collected powder was not mixed into the powder compact of the workpiece, and no diamond abrasive grains were mixed in the workpiece. In sample B, recovered powder containing diamond abrasive grains with an average particle size of 5 μm was reused. In sample C, recovered powder containing diamond abrasive grains with an average particle size of 10 μm was reused. In sample D, recovered powder containing diamond abrasive grains with an average particle size of 20 μm was reused. In sample E, recovered powder containing diamond abrasive grains with an average particle size of 30 μm was reused. The mass of abrasive grains contained in the samples B to E used for reuse (recovered powder mixed powder compacts) was 0.1% of the total mass of the recovered powder mixed powder compacts. Note that the average particle size was determined by measuring the particle size of 50 diamond abrasive grains contained in the recovered powder, and calculating the average value.

4A, 4B, and 4C are cross-sectional photographs of sintered magnets of Sample A, Sample C, and Sample E, respectively. As can be seen from FIG. 4A, a normal magnet structure without cavities was observed in the sintered magnet made from ordinary powder without reusing cutting powder from a wire saw. Similarly, in sample C, as can be seen from FIG. 4B, a normal magnetic structure without foci was observed. On the other hand, in sample E, as shown in FIG. 4C, nests with a diameter of 30 μm or more were observed in several places.

Next, the corrosion resistance of the sintered magnets produced from each of Samples A to E was evaluated. Table 1 shows the 72-hour PCT test (pressure cooker test) results for each of Samples A to E. The larger the negative value of the amount of wear in Table 1, the lower the corrosion resistance.

Sample A is a sintered magnet that does not contain any diamond abrasive grains. Corrosion resistance can be evaluated by comparing how much the amount of wear increases with sample A as a standard.

As shown in Table 1, Samples B to D have the same amount of wear as Sample A, and the decrease in corrosion resistance is suppressed. On the other hand, in sample E, there was a significant decrease in corrosion resistance.

From these results, by using a wire saw having a plurality of abrasive grains with an average grain size of 1 μm or more and 24 μm or less and a wire to which the plurality of abrasive grains are fixed, it is possible to scrape off a powder compact from a workpiece by wire saw processing. It can be seen that it is possible to suppress the decrease in corrosion resistance of the RTB-based sintered magnet obtained by collecting the collected powder particles and sintering the recovered mixed powder compact containing the collected powder particles.

DESCRIPTION OF SYMBOLS 10... Workpiece, 20... Fixing base, 30a, 30b, 30c... Roller, 40... Wire, 100... Wire saw device

Claims (10)

A workpiece of a powder compact of a rare earth alloy for RTB-based sintered magnets has a plurality of diamond abrasive grains with an average grain size of 1 μm or more and 24 μm or less, and a wire to which the plurality of diamond abrasive grains are fixed. a step of collecting powder particles of the rare earth alloy scraped off from the work when dividing the work into a plurality of molded body pieces by cutting with a wire saw;
preparing a recovered powder mixed powder compact containing the recovered powder particles;
Sintering the recovered powder mixed powder compact;
A method for manufacturing an RTB-based sintered magnet , comprising : The method for manufacturing an RTB-based sintered magnet according to claim 1, wherein the workpiece is submerged in a liquid when the workpiece is divided into a plurality of molded body pieces by cutting with the wire saw. . In order to prepare the workpiece of the rare earth alloy powder compact for the RTB sintered magnet,
a step of preparing rare earth alloy powder for the RTB-based sintered magnet ;
shaping the powder by wet pressing;
3. The method for producing an RTB based sintered magnet according to claim 1 or 2. The method for manufacturing an RTB-based sintered magnet according to any one of claims 1 to 3, wherein the workpiece has a density of 4 g/cm ³ or more and 5 g/cm ³ or less. Claims 1 to 4, wherein in the step of collecting the rare earth alloy powder particles scraped off from the workpiece, the diamond abrasive grains detached from the wire are collected together with the rare earth alloy powder particles scraped off from the workpiece. A method for producing an RTB-based sintered magnet according to any one of the above. The method for manufacturing an RTB based sintered magnet according to any one of claims 1 to 5 , wherein the diameter of the wire is 140 μm or more and 350 μm or less. Claims 1 to 6, wherein the step of preparing the recovered powder mixed powder compact includes the step of mixing the recovered powder particles with powder formed from a rare earth alloy for RTB-based sintered magnets. A method for producing an RTB-based sintered magnet according to any one of the above . R according to any one of claims 1 to 6 , wherein the step of preparing the recovered powder mixed powder compact is performed without separating the diamond abrasive grains separated from the wire from the recovered powder particles. - A method for manufacturing a TB-based sintered magnet. The method for manufacturing an RTB based sintered magnet according to any one of claims 1 to 8 , wherein the recovered mixed powder compact contains the diamond abrasive grains. The RTB system according to claim 9 , wherein the mass of the diamond abrasive grains contained in the recovered powder mixed powder compact is 0.3% or less of the entire mass of the recovered powder mixed powder compact. Method of manufacturing sintered magnets.

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