JP7468058B2 - Manufacturing method of RTB based sintered magnet - Google Patents

Description

This application relates to a method for producing R-T-B based sintered magnets.

An R-T-B based sintered magnet (R is a rare earth element and always includes at least one selected from the group consisting of Nd, Pr, and Ce, T is at least one transition metal and always includes Fe, and B is boron) is composed of a main phase of a compound having an R ₂ Fe ₁₄ B type crystal structure, a grain boundary phase located at the grain boundary portion of this main phase, and a compound phase generated by the influence of trace additive elements and impurities. An R-T-B based sintered magnet exhibits a high residual magnetic flux density B _r (hereinafter sometimes simply referred to as "B _r ") and a high coercive force H _cJ (hereinafter sometimes simply referred to as "H _cJ ") and has excellent magnetic properties, and is known as the magnet with the highest performance among permanent magnets. For this reason, an R-T-B based sintered magnet is used in a wide variety of applications, such as voice coil motors (VCMs) for hard disk drives, motors for electric vehicles (EVs, HVs, PHVs), motors for industrial equipment, and various other motors and home appliances.

Such R-T-B based sintered magnets are manufactured, for example, through the steps of preparing alloy powder, press-molding the alloy powder to produce a powder compact, and sintering the powder compact. The alloy powder is manufactured, for example, by the following method.

First, an alloy is produced from molten metals using an ingot method, strip casting method, or other method. The alloy obtained is then subjected to a crushing process to obtain an alloy powder having a specific particle size distribution. This crushing process usually includes a coarse crushing process and a fine crushing process, the former of which utilizes, for example, the hydrogen embrittlement phenomenon, and the latter of which uses, for example, an airflow crusher (jet mill).

The sintered body obtained by the process of sintering the powder compact is then subjected to mechanical processing such as grinding and cutting to be cut into pieces having the desired shape and size. More specifically, R-T-B rare earth magnet powder is first compressed and molded in a press to produce a powder compact larger in size than the final magnet product. After the powder compact is sintered into a sintered body by the sintering process, the sintered body is ground, for example, with a cemented carbide blade saw or a rotating grindstone to give it the desired shape. For example, a block-shaped sintered body is first produced, and then the sintered body is sliced with a blade saw or the like to cut out multiple plate-shaped sintered body portions.

However, sintered bodies of rare earth alloy magnets such as R-Fe-B magnets are extremely hard and brittle, and because the processing load is large, high-precision grinding is difficult and takes a long time. For this reason, the processing process is a major cause of increased manufacturing costs.

To solve these problems, Patent Document 1 describes a technique for processing magnet powder compacts using a wire saw before sintering. A wire saw is a processing technique in which a wire traveling in one or both directions is pressed against the powder compact to be processed, and the powder compact is ground or cut by abrasive grains between the wire and the powder compact. With this technique, the powder compact is cut when it is much softer and easier to process than a sintered body, so the time required for cutting is significantly reduced.

JP 2003-303728 A

Since R-Fe-B sintered magnets contain rare earth elements, which are expensive and scarce, there is a demand for further improving the efficiency of material utilization (yield). When the powder compact after cutting is sintered, shrinkage occurs, and the dimensions of the sintered compact become smaller than the dimensions of the powder compact, for example, by about 60 to 70%. The rate of shrinkage at this time varies, so even powder compacts with the same dimensions can have variations in the dimensions of the sintered compact after sintering.

Embodiments of the present disclosure provide a method for producing R-T-B based sintered magnets that can solve these problems.

In an exemplary embodiment, the method for producing an R-T-B based sintered magnet disclosed herein includes a grinding step for preparing a powder of an alloy for an R-T-B based sintered magnet (R is a rare earth element and always includes at least one selected from the group consisting of Nd, Pr, and Ce, T is at least one transition metal and always includes Fe, and B is boron), a molding step for producing a powder compact using the powder, a cutting step for cutting the powder compact and dividing it into a plurality of compact pieces, and a sintering step for sintering each of the plurality of compact pieces to produce a plurality of sintered bodies, and in the cutting step, the cutting interval of the powder compact is set based on the powder property data of the powder used to produce the powder compact and the powder property data of the powder compact.

In one embodiment, the powder property data is data on the powder composition and particle size.

In one embodiment, the powder compact physical property data is compact density data.

According to the embodiments of the present disclosure, it is possible to suppress the dimensional variation of the sintered body after sintering and bring the dimensions of the sintered body closer to the target value. This makes it possible to provide a manufacturing method for R-T-B based sintered magnets with even higher material utilization efficiency.

FIG. 1 is a flow chart showing the main steps of a manufacturing method according to an embodiment of the present disclosure. FIG. 2 is a perspective view that illustrates an example of a powder compact before being cut. FIG. 3 is a perspective view showing a schematic relationship between a green compact piece 14 before sintering and a sintered body 18 after sintering. FIG. 4 is a perspective view showing an example configuration of a wire saw device that can be used in an embodiment of the present disclosure. FIG. 5 is a cross-sectional view that diagrammatically shows a cross section of a wire.

When the powder compact is cut to prepare multiple compact pieces, and then each of the compact pieces is sintered to produce a sintered body, shrinkage occurs as described above. The difference in dimensions between the compact and the sintered body relative to the dimensions of the compact is defined as the "shrinkage rate." The shrinkage rate is, for example, about 30%, but the specific value can vary depending on parameters such as the powder composition, particle size, and molding density of the powder compact. The dimensions of the resulting sintered body can vary, for example, by about 2 to 5%, depending on the variation in the shrinkage rate.

When cutting a powder compact into multiple compact pieces using a wire saw or the like and then sintering, the shrinkage rate due to sintering has traditionally been collected as basic data, and the width of the cut of the powder compact (cutting interval) has been set according to the maximum value of the collected shrinkage rate. In other words, a margin is added to the cutting interval, assuming the largest shrinkage. By doing so, even the sintered body that has shrunk the most (the sintered body with the smallest dimensions) will have dimensions equal to or larger than the target dimensions when the shrinkage rate varies. For relatively large sintered bodies that exceed the target dimensions, it is possible to adjust the dimensions by removing a large amount of the sintered body through mechanical processing. However, adjusting the dimensions of the sintered body through such mechanical processing leads to a decrease in the efficiency of material utilization or the material yield, and deteriorates mass productivity. Since R-T-B sintered magnets contain valuable rare earth elements, it is not desirable to increase the amount of sintered body that is removed, i.e., the machining allowance.

In the manufacturing method of the R-T-B sintered magnet according to the present disclosure, when cutting and dividing the powder compact, the shrinkage rate of the powder compact is estimated or predicted based on the "powder physical property data" and the "powder compact physical property data", and the cutting width is set according to the shrinkage rate. This makes it possible to suppress the variation in the sintered body dimensions and reduce the machining allowance of the sintered body. In other words, the manufacturing method of the R-T-B sintered magnet according to the present disclosure makes it possible to set the optimal width for each powder compact, so there is no need to set the cutting interval of the powder compact according to the maximum shrinkage rate, and the extra dimension (margin) added to the cutting interval can be suppressed. In addition, by setting the cutting interval width for each powder compact, cutting excess (powder compact remaining after cutting into multiple compact pieces) may be generated after cutting the powder compact. Even in such a case, the cutting excess and cutting powder generated during cutting are both similar in composition and size to other particles contained in the powder compact, and therefore can be fully reused. Therefore, by reusing the cutting excess and cutting powder as R-Fe-B rare earth magnet powder for obtaining the powder compact, a decrease in material utilization efficiency or material yield can be prevented.

The powder physical property data can be collected by sampling a portion of the powder used to produce the powder compact and measuring the physical property data. The powder compact physical property data can be collected by measuring each molding or each lot (every few hundred pieces). In a preferred embodiment, the powder physical property data includes data on the composition and grain size of the powder. The powder compact physical property data includes data on the density of the compact. The powder composition and grain size, as well as the compact density, are preferred examples of parameters that define the shrinkage rate of the powder compact due to sintering. By preparing data showing the relationship between such parameters and the shrinkage rate based on actual measurements, it becomes possible to estimate or predict the shrinkage rate of the powder compact based on the set parameters or the measured parameter values. For example, the optimum width of the powder compact can be set by comparing past performance data showing the relationship between the powder physical property data (e.g., the composition and grain size of the powder) and the powder compact physical property data (e.g., the powder compact density) and the shrinkage rate with these physical property data of the powder compact to be cut.

When the values of the parameters included in the above data change or have changed, the shrinkage rate of the powder compact can be estimated or predicted from the relationship between the changed parameters and the shrinkage rate. For example, during mass production, when the compact density of a powder compact removed from a powder press device is measured and it is detected that the measured value has changed from the initial set value, it becomes possible to correct the dimensions of the compact pieces, i.e., the cutting interval of the powder compact, based on the shrinkage rate corresponding to the new measured value.

In addition, when design changes are made to the powder composition or particle size, the shrinkage rate after the design change can be predicted based on the above data, and the cutting interval can be changed according to that shrinkage rate.

The content of such data can be updated during the mass production process by obtaining actual measured values of the above parameters.

Hereinafter, an embodiment of a method for producing a sintered R-T-B based magnet according to the present disclosure will be described. As shown in the flow chart of FIG. 1, the method for producing a sintered R-T-B based magnet in this embodiment includes the following steps:
A pulverization step (S10) of preparing a powder of an alloy for an R-T-B based sintered magnet;
A molding step (S20) of producing a powder compact using the powder;
A cutting step (S30) of cutting the powder compact and dividing it into a plurality of compact pieces;
A sintering step (S40) of sintering each of the plurality of compact pieces to produce a plurality of sintered bodies;
including.

The cutting process (S30) also includes a process (S35) of setting the cutting interval of the powder compact based on the powder property data of the powder used to produce the powder compact and the powder property data of the powder compact.

Next, an example of each of the above steps S10 to S40 will be described with reference to Figure 1.

First, in step S10, powder of an alloy for R-T-B sintered magnets is prepared. Details of the composition of the alloy for R-T-B sintered magnets and the method for producing the powder will be described later.

In step S20, a powder compact is produced using the powder prepared in step S10. The powder compact can be produced, for example, by a wet or dry powder pressing device.

Figure 2 is a perspective view showing a schematic example of a powder compact. For reference, an XYZ coordinate system including mutually orthogonal X-axis, Y-axis, and Z-axis is shown in Figure 2. The powder compact 10 shown in this figure has a rectangular block shape. In Figure 2, the direction M of the aligning magnetic field is indicated by an arrow. This aligning magnetic field, which is called the "magnetic field orientation direction", is applied to the powder when the powder of the rare alloy for R-T-B sintered magnet is pressed to produce the powder compact 10, and the orientation of each powder particle is oriented in the magnetic field orientation direction M. Finally, the powder is magnetized in a direction parallel to the magnetic field orientation direction M.

Refer to Figure 1 again.

Next, in step S30, the powder compact 10 is cut and divided into a plurality of compact pieces 14. The powder compact 10 is not a sintered body, but a compact (green compact) of powder before sintering. In a preferred embodiment, the powder compact 10 is cut using a wire saw. Details of the wire saw will be described later.

In the example of FIG. 2, the powder compact 10 can first be cut into a plurality of plate-shaped parts extending along a plane (YZ plane) including the magnetic field orientation direction M of the powder compact 10. After that, each plate-shaped part can be cut so as to cross the magnetic field orientation direction M of the powder compact, and eventually divided into a plurality of compact pieces 14. In FIG. 2, the cut surfaces are shown by dotted lines for reference. Such a cutting order or manner is merely an example, and cutting may be performed in another order. In FIG. 2, the dimensions of the compact piece 14 are shown as a dimension W in the X-axis direction, a dimension T in the Y-axis direction, and a dimension L in the Z-axis direction. The dimensions W, T, and L correspond to the "width," "thickness," and "length" of the compact piece 14, respectively. Note that the shape of the powder compact 10 is not limited to a rectangular parallelepiped, but may be a cylinder or another shape. In addition, in the example of FIG. 2, the cut surfaces of the powder compact 10 are mutually orthogonal, but are not limited to such an example. The final shape of the compact piece 14 may be a plate shape or a rod shape extending in one direction.

In an embodiment of the present disclosure, when cutting the powder compact 10 to divide it into a plurality of compact pieces 14, the cutting interval of the powder compact 10 is set based on the powder physical property data of the powder used to produce the powder compact 10 and the powder compact physical property data of the powder compact 10 (step S35 in FIG. 1). Details of step S35 will be described later.

Next, in step S40, each of the multiple compact pieces 14 is sintered to produce multiple sintered bodies 16. The sintering conditions may be determined by the sintering temperature and sintering time, etc. Since the sintering conditions affect the shrinkage rate of the powder compact, they may be used to estimate or predict the shrinkage rate in addition to the data described above.

Figure 3 is a perspective view that shows a schematic relationship between the green body piece 14 before sintering and the sintered body 18 after sintering. The right side of Figure 3 shows the dimensions of the sintered body 18: 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. As shown in Figure 3, the green body piece 14 shrinks during sintering, becoming the sintered body 18 with reduced dimensions.

In an embodiment of the present disclosure, even during mass production, the powder composition and particle size, as well as the green body density, are sampled and measured to control fluctuations in the shrinkage rate, and the cutting intervals can be adjusted as needed to bring the sintered body dimensions closer to the target values.

The manufacturing method of the R-T-B based sintered magnet in this embodiment will be described in detail below. In this embodiment, an embodiment of the manufacturing method of the R-T-B based sintered magnet will be described.

S10: Step of preparing powder of alloy for R-T-B based sintered magnet <Composition of alloy for R-T-B based sintered magnet>
R is a rare earth element and necessarily includes at least one selected from the group consisting of Nd, Pr and Ce. Preferably, a combination of rare earth elements represented by Nd-Dy, Nd-Tb, Nd-Dy-Tb, Nd-Pr-Dy, Nd-Pr-Tb and Nd-Pr-Dy-Tb is used.

Among R, Dy and Tb are particularly effective in improving _HcJ . In addition to the above elements, other rare earth elements such as La may be contained, and misch metal or didymium may also be used. Furthermore, R does not have to be a pure element, and may contain impurities that are unavoidable in the manufacturing process within the range of industrial availability. The content is, for example, 27% by mass or more and 35% by mass or less. Preferably, the R content of the R-T-B 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). The R content of the R-T-B based sintered magnet is 31% by mass or less, and the oxygen content is 500 ppm or more and 3500 ppm or less.

T is at least one transition metal and must contain Fe, with up to 50% of Fe being replaced by cobalt (Co) by mass (including cases where T is essentially 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 remainder of R and B, or R, B, and M, which will be described later.

The content of B may be any known content, and for example, a preferred range is 0.85% by mass to 1.2% by mass. If it is less than 0.85% by mass, a high _HcJ 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, M element can be added to improve _HcJ . 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 M element added is preferably 5.0 mass% or less. If the amount exceeds 5.0 mass%, Br may decrease. In addition, unavoidable impurities are also acceptable.

<Manufacturing process of alloy for R-T-B based sintered magnet>
The following is an example of a manufacturing process for an alloy for an R-T-B sintered magnet. An alloy ingot can be obtained by ingot casting, in which a metal or alloy previously adjusted to have the above-mentioned composition is melted and poured into a mold. Alternatively, alloy flakes can be produced by a quenching method, typically a strip casting method or a centrifugal casting method, in which the molten metal is brought into contact with a single roll, a twin roll, a rotating disk, or a rotating cylindrical mold, and quenched to produce a solidified alloy thinner than the alloy produced by the ingot method.

In the embodiment of the present disclosure, materials produced by either the ingot method or the quenching method can be used, but it is preferable to produce the alloy by a quenching method such as a strip casting method. The thickness of the quenched alloy produced by the quenching method is usually in the range of 0.03 mm to 1 mm, and it is in a flake shape. The molten alloy starts to solidify from the surface (roll contact surface) that contacts the cooling roll, and the crystals grow columnarly in the thickness direction from the roll contact surface. The quenched alloy is cooled in a short time compared to the alloy (ingot alloy) produced by the conventional ingot casting method (mold casting method), so the structure is finer and the crystal grain size is smaller. In addition, the area of the grain boundary is large. Since the R-rich phase spreads widely within the grain boundary, the quenching method has excellent dispersibility of the R-rich phase. Therefore, it is easy to break at the grain boundary by the hydrogen crushing method. By hydrogen crushing the quenched alloy, the size of the hydrogen crushed powder (coarsely crushed powder) can be reduced to, for example, 1.0 mm or less. The coarsely crushed powder obtained in this way is crushed by a jet mill.

<Step of Preparing Powder of Alloy for R-T-B-Based Sintered Magnet>
The alloy powder for R-T-B based sintered magnets is active and easily oxidized, so in order to avoid the risk of heat generation and fire and to reduce the oxygen content as an impurity and improve the performance of the magnet, inert gases such as nitrogen, argon, and helium are used as the gas used in the jet mill.

The material to be pulverized (coarsely pulverized powder) fed into the jet mill is collected by a cyclone collector as 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. The cyclone collector is used to separate the powder from the air flow that carries the powder. Specifically, coarsely pulverized powder of R-T-B sintered magnet alloy is pulverized by the upstream jet mill, and the fine powder generated by the pulverization is supplied to the cyclone collector together with the gas used for pulverization. A mixture of an inert gas (pulverization gas) and the pulverized fine powder forms a high-speed air flow and is sent to the cyclone collector. The cyclone collector is used to separate the pulverization gas from the fine powder. The fine powder separated from the pulverization gas is collected by a powder collector.

S20: Step of preparing a powder compact Next, a powder compact is prepared from the above fine powder by pressing in a magnetic field. In the case of pressing in a magnetic field, it is preferable to form the powder compact by pressing in an inert gas atmosphere or by wet pressing from the viewpoint of suppressing oxidation. In particular, in the case of wet pressing, the surface of the particles constituting the powder compact is covered with a dispersant such as an oil agent, and contact with oxygen or water vapor in the air is suppressed. Therefore, it is possible to prevent or suppress the particles from being oxidized by the air before, during, or after the pressing step.

When performing wet pressing in a magnetic field, a slurry is prepared by mixing a fine powder with a dispersion medium, and the slurry is supplied to a cavity in a die of a wet pressing device and press-molded in a magnetic field. The powder compact thus formed has a density of, for example, 4 Mg/ ^m3 or more and 5 Mg/ ^m3 or less.

Dispersion Medium The dispersion medium is a liquid in which the alloy powder can be dispersed to obtain a slurry.

A preferred dispersion medium for use in the present disclosure may be mineral oil or synthetic oil. Although the type of mineral oil or synthetic oil is not specified, if the kinetic viscosity at room temperature exceeds 10 cSt, the increased viscosity may strengthen the bonding force between the alloy powders, which may adversely affect the orientation of the alloy powder during wet molding in a magnetic field. For this reason, the kinetic viscosity of the mineral oil or synthetic oil at room temperature is preferably 10 cSt or less. Furthermore, if the fractional point of the mineral oil or synthetic oil exceeds 400°C, it becomes difficult to remove the oil after obtaining the powder compact, and the amount of residual carbon in the sintered body may increase, resulting in a deterioration of the magnetic properties. Therefore, the fractional point of the mineral oil or synthetic oil is preferably 400°C or less. Vegetable oil may also be used as the 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 The obtained alloy powder and a dispersion medium are mixed to obtain a slurry.

The mixing ratio of the alloy powder and the dispersion medium is not particularly limited, but the concentration of the alloy powder in the slurry is preferably 70% or more (i.e., 70% or more by mass). 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 by mass. 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 the alloy powder may be produced by weighing and mixing a predetermined amount of both. In addition, when dry-pulverizing the coarsely pulverized powder with a jet mill or the like to obtain the alloy powder, a container containing the dispersion medium may be placed at the alloy powder outlet of the pulverizing device such as a jet mill, and the alloy powder obtained by pulverization may be directly collected in the dispersion medium in the container to obtain the slurry. In this case, it is preferable that the atmosphere inside the container is also made of nitrogen gas and/or argon gas, and the obtained alloy powder is directly collected in the dispersion medium without being exposed to the air to obtain the slurry. Furthermore, it is also possible to wet-pulverize the coarsely pulverized powder in a dispersion medium using a vibration mill, ball mill, attritor, or the like to obtain a slurry consisting of the alloy powder and the dispersion medium.

The slurry thus obtained can be molded in a known wet press to obtain a powder compact having a desired size and shape. Conventionally, this powder compact has usually been sintered to obtain a sintered body, but in this embodiment, the powder compact is cut and divided into multiple compact pieces before sintering, as described below.

S30: Cutting step of cutting the powder compact into a plurality of compact pieces In an embodiment of the present disclosure, a step (S30) of cutting the powder compact into a plurality of compact pieces is performed before the sintering step. This cutting can be suitably performed by a wire saw. Wire saws include a multi-wire saw that simultaneously divides the powder compact into three or more compact pieces along a plurality of cut surfaces, and a single-wire saw that sequentially divides the powder compact along each cut surface. In an embodiment of the present disclosure, either wire saw may be used.

Here, with reference to FIG. 4, an example of the configuration of a wire saw device (multi-wire saw device) that can be used in this cutting process will be described. FIG. 4 is a perspective view showing an example of the configuration of a wire saw device 100 in an embodiment of the present disclosure. In this example, the wire saw device 100 is a multi-wire saw device. For reference, an xyz coordinate system including mutually orthogonal x-, y-, and x-axes is shown in the figure. In this example, the xy plane is horizontal, and the z-axis is oriented vertically.

The wire saw device 100 in FIG. 4 has rollers 30a, 30b, and 30c arranged so that their central axes of rotation are parallel to one another, and a single continuous wire 40. The powder compact 10 prepared in step (S10) is supported by a fixing base 20.

The fixing base 20 moves up and down in the z-axis direction with the powder compact 10 fixed thereto. This up and down movement can 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.

Rollers 30a, 30b, and 30c are arranged at a specified interval so that the axis of the center of rotation is located at the apex of a triangle when viewed from a direction parallel to the x-axis. Multiple grooves are provided on the side of each of rollers 31a, 31b, and 31c. Wire 40 is wound around the multiple grooves of rollers 30a, 30b, and 30c in order. The center distance (pitch) between the multiple grooves determines the width of the elements divided by cutting with the wire saw (cutting interval).

As described above, the cutting step (S30) includes a step (S35) of setting the cutting interval of the powder compact based on the powder property data of the powder used to produce the powder compact and the powder compact property data of the powder compact. Therefore, when using the wire saw device 100 in this embodiment, the center distance between the multiple grooves is determined so as to realize the cutting interval set based on the powder property data of the powder used to produce the powder compact and the powder compact property data of the powder compact. When changing the cutting interval, the roller is replaced with one having grooves at a center distance corresponding to the cutting interval. Note that, when a single wire saw device is used, the position of the wire 40 in the X-axis direction relative to the powder compact 10 is moved in units of the cutting interval, and the cutting is performed sequentially.

Both ends of the wire 40 are wound, for example, on a recovery bobbin (not shown). When cutting, the rollers 30a, 30b, and 30c and the recovery bobbin rotate. The direction of rotation of the rollers 30a, 30b, and 30c depends on their arrangement and how the wire 40 is wound. In the wire saw device 100 shown in FIG. 4, the rollers 30a, 30b, and 30c rotate in the same direction.

When a predetermined length of wire 40 has been wound onto one of the recovery bobbins, the recovery 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 process, the wire 40 can be made to reciprocate (move).

Wire 40 can be, for example, a fixed abrasive wire. Specifically, wire can be used in which high-hardness abrasive grains suitable for cutting high-hardness materials are fixed to the wire by electrochemical deposition. High-hardness abrasive grains are also called superabrasive grains, and a typical example is diamond abrasive grains.

Figure 5 shows a schematic cross section of wire 40. Wire 40 includes a wire (core) 42, abrasive grains 44 located on the outer periphery of wire 42, and adhesion layer 46. Adhesion layer 46 is formed of a plated metal such as Ni. Abrasive grains 44 are located on the surface of wire 42, and abrasive grains 44 can be adhered to wire 42 by adhesion layer 46 covering the surface of wire 42 around abrasive grains 44 and abrasive grains 44 as a whole. Adhesion of abrasive grains 44 may be achieved by other methods. The average grain size of abrasive grains 44 is, for example, 1 μm or more and 24 μm or less.

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

When the powder compact 10 is processed by such a wire saw device 100, the powder particles that make up the powder compact 10 fall off as cutting powder from the part cut by the abrasive grains 44 of the wire 40. The cutting powder obtained from the powder compact 10 before sintering can be collected and mixed directly with the fine powder used to make the powder compact, making it easy to reuse it.

In a preferred embodiment, the diameter of the strands 42 of the wire 40 (see FIG. 5) is, for example, 140 μm or more and 240 μm or less. If the diameter of the strands 42 is less than 140 μm, there is a problem that the strands 42 stretch during cutting due to insufficient strength. The larger the diameter of the strands 42, the better the discharge of cutting powder will be, but the amount of cutting powder will increase, so it is desirable for the diameter to be 240 μmm or less.

The running speed (wire linear speed) of the wire 40 can be set, for example, in the range of 100 m/min to 500 m/min. On the other hand, the work feed speed (movement speed of the fixing base 20 in the z-axis direction in FIG. 4) can be set, for example, in the range of 100 mm/min to 600 mm/min. The tension applied to the wire 40 is, for example, 2.0 kg to 3.0 kg.

From the viewpoint of quickly discharging cutting chips, wire sawing is desirably performed with the powder compact 10 immersed in the dispersion medium (mineral oil or synthetic oil) used when producing the powder compact by wet pressing (cutting in oil). When wire sawing is performed in the air, it is desirably to spray oil similar to the dispersion medium onto the area where the powder compact 10 and wire 40 come into contact (cutting area).

In this embodiment, the cutting interval is set based on the aforementioned data prepared in advance. Therefore, compact pieces having dimensions corresponding to the shrinkage that occurs in the subsequent sintering process can be appropriately produced. Furthermore, if the composition and particle size of the powder used to produce the powder compact change, or if the compact density changes, the setting value of the cutting interval can be changed based on the aforementioned data. As a result, compact pieces having dimensions corresponding to the actual shrinkage rate are produced.

During the cutting process, the powder particles that make up the powder compact fall off as cutting dust. When cutting a hard sintered body obtained by sintering a powder compact, the cutting dust is made up of particles that have grown due to sintering or whose composition has changed due to chemical reactions, or particles that have been combined, and therefore cannot be mixed with the powder of the alloy for R-T-B sintered magnets and reused. In contrast, cutting dust obtained from the powder compact before sintering has a similar composition and size to the other particles contained in the powder compact, and can be fully reused. For this reason, it is preferable to collect and reuse the cutting dust.

S40: Step of sintering each of the compact pieces to produce a sintered body Next, the compact pieces obtained by the above cutting step are sintered to obtain an R-T-B based sintered magnet (sintered body). The sintering step of the compact pieces can be carried out, for example, under a pressure of 0.13 Pa (10 ^-3 Torr) or less, preferably 0.07 Pa (5.0 x 10 ^-4 Torr) or less, at a temperature in the range of, for example, 1000°C to 1150°C. To prevent oxidation due to sintering, the residual gas in the atmosphere can be replaced with an inert gas such as helium or argon.

Since shrinkage occurs when the green body pieces are sintered, the dimensions of the resulting sintered body are reduced from the dimensions of the green body pieces. However, in this embodiment, green body pieces having dimensions corresponding to the shrinkage that occurs in the sintering process are properly produced, so that the dimensions of the sintered body after shrinkage are close to the target values. For example, the error between the dimensions of the sintered body and the target values can be suppressed to 1% or less.

In addition, elements such as heavy rare earth elements may be diffused from the surface of the sintered body to the inside of the sintered body after step S40. Additional heat treatment such as aging treatment may also be performed. Such diffusion and heat treatment can improve the magnetic properties. Heat treatment conditions such as heat treatment temperature and heat treatment time can be well-known conditions. The sintered body thus obtained is subjected to finishing processes such as grinding and polishing and surface treatment processes as necessary, and is then subjected to a magnetization process to complete the final R-T-B based sintered magnet.

<Example>
A powder of an alloy for R-T-B-based sintered magnets having a composition of Nd; 24.5 mass%, Pr: 4.5 mass%, B: 0.90 mass%, Cu: 0.1 mass%, Ga: 0.4 mass%, Co: 1.0 mass%, and the balance Fe was prepared. The particle size D ₅₀ of the powder was 4 μm. The particle size D ₅₀ was measured by an airflow dispersion type laser diffraction method (in accordance with JIS Z 8825: 2013 revised version). A powder compact was produced using these powders in a wet press device. The dimensions of the obtained powder compact were 100 mm x 60 mm x 90 mm (90 mm is the magnetic field orientation direction). The compact density of the obtained powder compact was about 4.5 Mg/m ^3. The obtained powder compact was cut at a cutting interval set under Condition A and Condition B described below to obtain a plurality of compact pieces. The dimensions of the molded piece were 100 mm x 60 mm x 7.5 mm (7.5 mm was the magnetic field orientation direction). The obtained molded piece was sintered (a temperature was selected at which sufficient densification by sintering occurred) to produce a sintered body (condition A).
Condition A is a conventional setting method. In a preliminary experiment, a plurality of powder compacts were produced, sintered, and their respective shrinkage rates were calculated and collected as basic data. The cutting width of the powder compact was set according to the maximum value of the collected shrinkage rates.
(Condition B)
Condition B is a setting method of the present disclosure. First, powder property data of the powder used to produce the powder compact was collected. Specifically, the composition and particle size of the powder were measured. Next, powder compact property data was collected. Specifically, the compact density of the powder compact was measured. From these measurement data, the width of the powder compact was set based on the powder compact property data and past performance data showing the relationship between the powder compact property data and the shrinkage rate.

The dimensions of 20 pieces each of the sintered bodies produced under condition A and condition B in the magnetic field orientation direction (7.5 mm) were measured, and the dimensional variation of the sintered bodies was calculated by determining the difference between the maximum and minimum dimensions. Note that the dimension in the magnetic field orientation direction had the largest dimensional variation due to shrinkage among the other dimensional locations. Furthermore, the difference from the target dimensional value of the sintered body (target dimensional value in the magnetic field orientation direction of 7.5 mm) was also calculated. Under condition A, the dimensional variation of the sintered body was 0.22 mm, a difference from the target value of 3%. In contrast, under condition B, the dimensional variation of the sintered body was 0.07 mm, a difference from the target value of only 1%. In this way, the dimensional variation of the sintered body under condition B is more suppressed, and the difference from the target value is smaller.

10: Workpiece, 20: Fixing base, 30a, 30b, 30c: Rollers, 40: Wire, 100: Wire saw device

Claims (1)

a grinding step of preparing a powder of an alloy for an R-T-B based sintered magnet (R is a rare earth element and necessarily contains at least one selected from the group consisting of Nd, Pr, and Ce, T is at least one transition metal and necessarily contains Fe, and B is boron);
a molding step of producing a powder compact using the powder;
a cutting step of cutting the powder compact into a plurality of compact pieces;
a sintering step of sintering each of the plurality of compact pieces to produce a plurality of sintered bodies;
Including ,
Before the cutting step, measurements of the composition and particle size of the powder used to prepare the powder compact and the compact density of the powder compact are obtained;
a cutting interval for the powder compact, the cutting step being performed by comparing powder physical property data , including data on the composition and particle size of the powder, and powder compact physical property data, including data on the density of the compact, with past performance data showing the relationship between the rate of shrinkage due to sintering.

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