Manufacturing method for RTB sintered magnets

Abstract

To efficiently manufacture a wide variety of R-T-B sintered magnets in response to user requests.SOLUTION: A manufacturing method of an R-T-B sintered magnet according to the present disclosure includes a step of preparing an R-T-B sintered magnet material block, a wire electrical discharge machining step of manufacturing a plurality of R-T-B sintered magnet material pieces from the R-T-B sintered magnet material block by cutting the R-T-B sintered magnet material block at an average machining speed of 2.0 mm / min or more by wire electrical discharge, and a surface processing step of grinding or polishing the electrical discharge machined surface of each of the plurality of R-T-B sintered magnet material pieces.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present application relates to a method for producing an RTB based sintered magnet. [Background technology]

[0002] RTB sintered magnets, a typical example of rare earth sintered magnets (R is a rare earth element and must contain at least one selected from the group consisting of Nd, Pr, and Ce, T is at least one transition metal and must contain Fe, and B is boron), are classified as RFe 14 It consists of a main phase of a compound with a B-type crystal structure, a grain boundary phase located at the grain boundaries of this main phase, and a compound phase formed by the influence of trace additive elements and impurities. RTB-based sintered magnets have a high residual magnetic flux density B r (Hereafter, simply "B r ") and high coercive force H cJ (Hereafter, simply "H cJ ") and has excellent magnetic properties, making it known as the highest performance magnet among permanent magnets.

[0003] For this reason, RTB sintered magnets are used in a variety of motors in the automotive sector, including electric vehicles (EVs, HVs, PHVs), renewable energy sectors such as wind power generation, home appliances, and industrial sectors. RTB sintered magnets are an essential material for making these motors smaller, lighter, more efficient, and more energy-efficient (improving energy efficiency). RTB sintered magnets are also used in the drive motors of electric vehicles, and the replacement of internal combustion engine vehicles with electric vehicles contributes to preventing global warming by reducing greenhouse gases such as carbon dioxide (reducing fuel and exhaust gases). In this way, RTB sintered magnets are making a significant contribution to the realization of a green energy society.

[0004] Rare earth sintered magnets such as RTB sintered magnets are manufactured, for example, through the steps of preparing an alloy powder, press-molding the alloy powder to produce a compact, and sintering the compact. The alloy powder is manufactured, for example, by the following method.

[0005] First, an alloy is produced from a molten metal of various raw material metals by a method such as the ingot method or strip casting method. The obtained alloy is subjected to a pulverization process to obtain an alloy powder with a predetermined particle size distribution. This pulverization process usually includes a coarse pulverization process and a fine pulverization process, the former of which is carried out, for example, by utilizing the hydrogen embrittlement phenomenon, and the latter of which is carried out, for example, by using an airflow pulverizer (jet mill).

[0006] The sintered body obtained by the sintering process is then mechanically processed, such as by grinding and cutting, to produce individual pieces. Specifically, RTB rare earth magnet powder is first compressed in a press to produce a compact larger than the final magnet product. The compact is then sintered in a sintering process to produce a sintered body. If necessary, the sintered body may be subjected to a diffusion process in which a diffusion source containing rare earth element R is diffused from the surface to the interior of the magnet. The sintered or diffused sintered body is then ground, for example, with a cemented carbide blade saw or a rotating grinding wheel to give it the desired shape. For example, a block-shaped sintered body may first be produced, and then the sintered body may be sliced ​​with a blade saw or the like to produce multiple plate-shaped sintered body portions. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-303728 Summary of the Invention [Problem to be solved by the invention]

[0008] As mentioned above, rare earth sintered magnets are used in a wide variety of applications, and they come in a wide variety of shapes and with a wide range of desired properties. However, when sintering or diffusion sintered bodies are singulated using conventional methods, it is difficult to efficiently produce a wide variety of sintered bodies to meet user demands.

[0009] An embodiment of the present disclosure provides a method for producing an RTB based sintered magnet that can solve these problems. [Means for solving the problem]

[0010] In an exemplary embodiment, the method for producing an RTB based sintered magnet of the present disclosure includes the steps of: preparing an RTB based sintered magnet block (where R is a rare earth element and must include at least one selected from the group consisting of Nd, Pr, and Ce; T is at least one transition metal and must include Fe; and B is boron); wire electrical discharge machining the RTB based sintered magnet block at an average machining speed of 2.0 mm / min or more to produce a plurality of RTB based sintered magnet material pieces from the RTB based sintered magnet block; and surface machining the electrical discharge machined surface of each of the plurality of RTB based sintered magnet material pieces.

[0011] In one embodiment, the surface machining step removes convex portions from the uneven surface present on the electrical discharge machined surface.

[0012] In one embodiment, the height of the convex portions removed in the surface processing step is in the range of 10 μm to 500 μm.

[0013] In one embodiment, the plurality of sintered RTB magnet material pieces produced in the wire electric discharge machining process include two or more sintered magnet material pieces that are different in at least one of size and shape.

[0014] In one embodiment, in the wire electric discharge machining process, the cutting direction by the wire electric discharge includes a first direction and a second direction perpendicular to the first direction, and the wire electric discharge is also performed during an operation of changing the cutting direction from the first direction to the second direction.

[0015] In one embodiment, the RTB based sintered magnet material block has a dimension in the thickness direction of 10 mm or more and 60 mm or less.

[0016] In one embodiment, the RTB based sintered magnet material block has a dimension in the thickness direction of 20 mm or more and 60 mm or less.

[0017] In one embodiment, the average machining speed in the wire electric discharge machining step is 3.0 mm / min or more.

[0018] In one embodiment, the surface processing step includes a barrel polishing step.

[0019] In one embodiment, the polishing time in the barrel polishing step is fixed to a constant value regardless of the average machining speed in the wire electric discharge machining step. [Effects of the Invention]

[0020] According to the embodiments of the present disclosure, sintered bodies having different shapes and sizes according to user needs can be efficiently obtained. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a photograph showing the electrical discharge machined surface of a raw piece of an RTB based sintered magnet cut by wire electrical discharge machining. [Figure 2] FIG. 2(a) is a cross-sectional view schematically showing an RTB-based sintered magnet material piece in which a deteriorated layer 14 is formed on the electrical discharge-machined surface when the average machining speed is relatively low, and FIG. 2(b) is a cross-sectional view schematically showing an RTB-based sintered magnet material piece in which a deteriorated layer is formed on the electrical discharge-machined surface when the average machining speed is relatively high. [Figure 3] FIG. 3 is a flowchart showing the main steps of a method for producing an RTB rare earth sintered magnet according to an embodiment of the present disclosure. [Figure 4] FIG. 4 is a perspective view showing an example of the configuration of a wire electric discharge machining apparatus that can be used in an embodiment of the present disclosure. [Figure 5] FIG. 5 is a perspective view that schematically shows the state in which the wire electric discharge machining device of FIG. 4 is cutting the RTB based sintered magnet material block. [Figure 6] FIG. 6 is a graph showing the relationship between the thickness T of the RTB based sintered magnet material block and the machining speed (average machining speed) of the wire electric discharge machining device in an example of the present invention. [Figure 7] FIG. 7 is a graph showing the relationship between the thickness T of the RTB-based sintered magnet material block in an example of the present invention and the measurement results of the thickness of the deteriorated layer in the thickness direction of the RTB-based sintered magnet pieces after cutting (surface roughness Rz of the cut surface). [Figure 8] FIG. 8 is a graph showing the relationship between the thickness T of the sintered RTB magnet material block and the measurement results of the thickness of the sintered RTB magnet pieces after cutting in an example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0022] The inventors have investigated cutting a block of RTB sintered magnet material by wire electric discharge machining. 14 It is composed of a main phase of compound particles having a B-type crystal structure, a grain boundary phase located at the grain boundaries of these particles, and a compound phase formed by the influence of trace additive elements and impurities. An RTB-based sintered magnet material is a high-hardness material made by sintering alloy powder particles. Research by the inventors has revealed that in a process (wire-EDM process) in which an RTB-based sintered magnet material block is cut by wire-EDM to produce multiple RTB-based sintered magnet material pieces from the RTB-based sintered magnet material block, if the average machining speed is increased to 2.0 mm / min or more, the following phenomenon occurs. A deteriorated layer forms on the surfaces (electric discharge machined surfaces) of multiple RTB sintered magnet blanks. The presence of a degraded layer causes deterioration of the magnetic properties of the final RTB-based sintered magnet, and may also prevent the desired magnet dimensions from being achieved. When the average processing speed is reduced, the deteriorated layer becomes thinner. Therefore, to suppress the formation of the deteriorated layer, it is effective to reduce the average processing speed to, for example, less than 2.0 mm / min.

[0023] However, a decrease in the average processing speed leads to an increase in processing time. When the inventors further investigated the details of the deteriorated layer, the following findings were obtained.

[0024] First, the thickness of the deteriorated layer increases with the average machining speed, but the magnet dimensions after removing the deteriorated layer were approximately the target size regardless of the average machining speed. According to the inventor's observations, the deteriorated layer had an uneven shape caused by numerous hard granular particles welding to the EDM surface.

[0025] Figure 1 is a photograph (taken at 20x magnification) showing the electrical discharge machined surface of a piece of RTB-based sintered magnet material cut by wire electrical discharge machining at an average machining speed of 2.6 mm / min. As can be seen from Figure 1, the electrical discharge machined surface has numerous granular convexities, forming irregular asperities. The roughness of the asperities increased as the average machining speed increased. According to the inventors' investigations, the reason the deterioration layer became thicker as the average machining speed increased was because sludge, which should have been discharged from the cut surface during electrical discharge machining, deposited on the surface, forming convexities. Furthermore, it was found that the deterioration layer was not located inside the intended cut surface of the magnet material cut by electrical discharge machining, but outside the cut surface. In other words, the wire can cut the desired location with high accuracy, regardless of the surface roughness. Previously, it was believed that the deterioration layer was formed not by sludge but by deterioration of the machined surface itself, which was machined by wire electrical discharge machining. Therefore, it was believed that at least a portion of the deterioration layer was located inside the cut surface. In this case, if the deteriorated layer is removed, the dimensions of the resulting magnet material pieces will change depending on the size of the deteriorated layer. Therefore, it is necessary to reduce the deteriorated layer as much as possible, which makes it difficult to achieve an average processing speed of 2 mm / min or more. However, as a result of the above-mentioned investigations by the inventors, the method disclosed herein makes it possible to accurately obtain magnet material pieces of the target dimensions while increasing the average processing speed (to 2 mm / min or more).

[0026] Fig. 2(a) is a cross-sectional view schematically showing an R-T-B sintered magnet material piece 12 with a deteriorated layer 14 formed on the electrical discharge machining surface when the average machining speed is relatively low. Fig. 2(b) is a cross-sectional view schematically showing an R-T-B sintered magnet material piece 12 with a deteriorated layer 14 formed on the electrical discharge machining surface when the average machining speed is relatively high. In the example of Fig. 2(a), the thickness of the deteriorated layer 14 is indicated by "t1", and in the example of Fig. 2(b), the thickness of the deteriorated layer 14 is indicated by "t2". Here, t1 < t2. The deteriorated layer 14 includes a plurality of convex portions arranged in an island shape, and these convex portions are firmly welded to the R-T-B sintered magnet material piece 12 by electrical discharge. The surface roughness on the electrical discharge machining surface of the R-T-B sintered magnet material piece 12 is defined by the height of the convex portions constituting the deteriorated layer 14.

[0027] As shown in Fig. 2(a) and Fig. 2(b), the dimension t0 of the R-T-B sintered magnet material piece 12 in the direction perpendicular to the electrical discharge machining surface is not affected by the thickness of the deteriorated layer 14. The deteriorated layer 14 is formed from sludge welded to the cut surface by electrical discharge and spreads outside the cut surface.

[0028] As a result of examination based on these findings, even if electrical discharge machining is performed at a high speed (2.0 mm / min or more) that is not normally performed because the unevenness of the deteriorated layer 14 becomes prominent, if a process of removing the deteriorated layer 14 is performed thereafter, it has been found that it is possible to produce an R-T-B sintered magnet having desired dimensions and magnetic properties.

[0029] Hereinafter, embodiments of a method for manufacturing an R-T-B rare earth sintered magnet according to the present disclosure will be described. The method for manufacturing an R-T-B rare earth sintered magnet in the present embodiment, as shown in the flowchart of Fig. 3, · A step (S10) of preparing an R-T-B sintered magnet material block, and · A wire electrical discharge machining step (S20) of cutting the R-T-B sintered magnet material block by wire electrical discharge at an average machining speed of 2.0 mm / min or more to produce a plurality of R-T-B sintered magnet material pieces from the R-T-B sintered magnet material block, a surface processing step (S30) of grinding or polishing the electrical discharge machined surface of each of the plurality of RTB based sintered magnet material pieces; Includes.

[0030] Here, R is a rare earth element and must include at least one selected from the group consisting of Nd, Pr, and Ce, T is at least one transition metal and must include Fe, and B is boron.

[0031] According to the manufacturing method of the present disclosure, cutting is performed in the wire electric discharge machining step at a high speed of 2.0 mm / min or more on average, thereby increasing production efficiency. The average machining speed is preferably 3.0 mm / min or more, and may be 6.0 mm / min or more. Furthermore, in the surface processing step (S30), the electric discharge machined surface of each of the multiple RTB-based sintered magnet material pieces is ground or polished to remove the deteriorated layer, thereby obtaining an RTB-based sintered magnet with the desired dimensions and magnetic properties.

[0032] Next, an example of a wire electric discharge machining apparatus that can be used in the embodiments of the present disclosure will be described.

[0033] FIG. 4 is a perspective view schematically illustrating an example of the configuration of a wire electric discharge machining apparatus according to this embodiment. The illustrated wire electric discharge machining apparatus 100 includes a conductive stage 20 on which a workpiece (a block of RTB-based sintered magnet material) to be machined is placed, main guide wheels 40 and 42 that guide a wire 30 for electric discharge, and sub-guide wheels 44, 46, and 48 that guide the wire 30. Most of the wire 30 is wound around a drum 50. The wire 30 is made of, for example, molybdenum and has a length of, for example, 200 meters. The diameter of the wire 30 is, for example, approximately 0.2 mm. The running speed of the wire 30 during electric discharge machining is, for example, in the range of 0.1 m / s to 20 m / s. The tension of the wire 30 during running is, for example, in the range of 10 N to 15 N.

[0034] The stage 20 is driven by an actuator such as a motor. With a workpiece placed on it, the stage 20 can move at a predetermined speed in any direction in a plane parallel to the XY plane. More specifically, the stage 20 can move freely in any direction by combining movement in the X-axis direction and movement in the Y-axis direction. The movement speed of the stage 20 during wire electric discharge machining corresponds to the machining speed. Note that while the wire electric discharge machine described here uses the electric discharge wire repeatedly by rotating the drum 50 forward and backward, the wire electric discharge machine may use the wire electric discharge only once by scanning it in only one direction. In this case, the wire electric discharge may be made of brass wire instead of molybdenum. Furthermore, because there are both small and large wire electric discharge machines, the running speed and tension of the wire 30 are not limited to the scope of this example.

[0035] FIG. 5 is a perspective view schematically showing a state in which an RTB-based sintered magnet material block 10 is placed on a stage 20 and wire electric discharge machining is being performed. The RTB-based sintered magnet material block 10, which moves along with the stage 20, is being cut by a wire 30. A pulsed high voltage is repeatedly applied between the stage 20 and the wire 30. As a result, an arc discharge occurs between the RTB-based sintered magnet material block 10 and the wire 30, scraping the surface of the RTB-based sintered magnet material block 10 facing the wire 30 and generating sludge. Because a machining fluid (such as a water-soluble grinding fluid) is supplied to the wire 30, most of the sludge is discharged from the cut area. However, when the average machining speed increases, the discharge of sludge cannot keep up with its generation, and some of the sludge fuses to the electric discharge machined surface. This point will be explained in detail below.

[0036] In wire electric discharge machining, a pulse voltage is applied (ON state) between the RTB-based sintered magnet material block 10 and the wire 30 via the stage 20, and a pulse voltage is not applied (OFF state) and this state is alternately repeated. For example, the ON time is fixed to a value in the range of 40 μs to 150 μs (e.g., 60 μs), and the OFF time can be variably adjusted in the range of 150 μs to 250 μs. In this case, the higher the machining speed, the shorter the OFF time. Sludge generated during electric discharge is generated when a pulse voltage is applied (ON state) and is discharged by the oil in the OFF state. Therefore, the higher the machining speed and the shorter the OFF time, the more likely it is that sludge will be hindered from being discharged.

[0037] In the surface processing step, it is preferable to remove the protrusions (fused sludge) on the uneven surface present on the electric discharge machined surface. The height of the protrusions to be removed in the surface processing step is in the range of 10 μm to 500 μm, for example, in the range of 50 μm to 200 μm. The processing conditions for the surface processing step are determined so that protrusions of such height can be removed.

[0038] The multiple RTB based sintered magnet material pieces produced in the wire electrical discharge machining process can include two or more sintered magnet material pieces that differ in at least one of size and shape. This makes it possible to quickly extract pieces of the size and shape required by the user from one or more RTB based sintered magnet material blocks. This is suitable for high-mix, low-volume production.

[0039] In the wire electric discharge machining process, the cutting direction by wire electric discharge includes a first direction (e.g., the X-axis direction) and a second direction (e.g., the Y-axis direction) perpendicular to the first direction, and the wire electric discharge is also performed during the operation of changing the cutting direction from the first direction to the second direction.

[0040] The thickness of the RTB-based sintered magnet block 10 is, for example, 10 mm to 60 mm, and may be 20 mm to 60 mm. As the thickness of the RTB-based sintered magnet block 10 increases, the average processing speed decreases and the surface roughness (Rz) also decreases. However, the surface roughness (Rz) saturated at about 0.08 mm and did not decrease any further.

[0041] According to the method for manufacturing an RTB-based sintered magnet disclosed herein, the machining speed changes depending on the thickness of the RTB-based sintered magnet material block. Even if the thickness (surface roughness) of the deteriorated layer changes as a result, the dimensional accuracy and magnetic properties of the normal portion do not fluctuate. Therefore, by removing the deteriorated layer in the surface machining process, it becomes possible to efficiently manufacture an RTB-based sintered magnet with the desired dimensions and shape. The present disclosure can also be applied to a wire electric discharge machining apparatus that cuts the RTB-based sintered magnet material block 10 by submerging it in a machining fluid tank.

[0042] The surface processing step includes a barrel polishing step, and the polishing time in the barrel polishing step can be fixed to a constant value regardless of the average machining speed in the wire electric discharge machining step.

[0043] Hereinafter, an embodiment of a method for producing an RTB based sintered magnet according to the present invention will be described in detail.

[0044] (1) Preparation of RTB sintered magnet material block <Alloy composition> In this embodiment, an alloy for an RTB-based sintered magnet is used. Here, R is a rare earth element and must contain at least one element selected from the group consisting of Nd, Pr, and Ce. Preferably, a combination of rare earth elements such as Nd-Dy, Nd-Tb, Nd-Dy-Tb, Nd-Pr-Dy, Nd-Pr-Tb, Nd-Pr-Dy-Tb, Nd-Pr-Tb-Ho, or Nd-Pr-Dy-Tb-Ho is used.

[0045] Among R, Dy and Tb are particularly cJThis is effective in improving the R content. In addition to the above elements, other rare earth elements such as La and Ho may be contained, and misch metal or didymium may also be used. The R 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).

[0046] T is at least one transition metal and must contain Fe, with up to 50% of the Fe by mass being substituted with cobalt (Co) (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 account for the remainder of R and B, or R, B, and M, which will be described later.

[0047] The content of B may be a known content, and for example, a preferred range is 0.9% by mass to 1.2% by mass. If it is less than 0.9% by mass, high H cJ If it exceeds 1.2 mass%, B may not be obtained. r In addition, a part of B can be substituted with C (carbon).

[0048] In addition to the above elements, H cJ To improve the properties, an M element can be added. The M element is one or more elements 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 it exceeds 5.0 mass%, Br may decrease. Inevitable impurities are also acceptable.

[0049] <Alloy preparation> Here is an example of a manufacturing process for an alloy for an RTB sintered magnet. An alloy ingot can be obtained by ingot casting, in which a metal or alloy previously prepared to have the composition described above is melted and poured into a mold. Alternatively, alloy flakes can be produced by a rapid cooling method, such as strip casting or centrifugal casting, in which the molten metal is brought into contact with a single roll, twin rolls, rotating disk, or rotating cylindrical mold, and quenched to produce a solidified alloy that is thinner than the alloy produced by the ingot method.

[0050] In the embodiments of the present disclosure, materials produced by either the ingot method or the quenching method can be used, but production by a quenching method such as strip casting is preferred. The thickness of a quenched alloy produced by the quenching method is typically in the range of 0.03 mm to 1 mm and is in the form of flakes. The molten alloy begins to solidify from the surface that contacts the chill roll (the roll contact surface), and crystals grow columnarly from the roll contact surface in the thickness direction. Compared to alloys (ingot alloys) produced by conventional ingot casting (mold casting), quenched alloys are cooled in a shorter time, resulting in a finer structure and smaller crystal grain size. Furthermore, the grain boundary area is larger. Because the R-rich phase spreads widely within the grain boundaries, the quenching method provides excellent dispersibility of the R-rich phase. Therefore, hydrogen pulverization easily causes fracture at the grain boundaries. By hydrogen pulverizing the quenched alloy, 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 obtained in this manner is pulverized using a jet mill.

[0051] <Alloy fine grinding process> The alloy powder for RTB sintered magnets is active and easily oxidized. For this reason, inert gases such as nitrogen, argon, and helium are used in jet mills to avoid the risk of heat generation and fire and to reduce the oxygen content as an impurity, thereby improving the performance of the magnet.

[0052] The material to be pulverized (coarsely pulverized powder) fed into the jet mill is pulverized into fine powder with a particle size distribution, for example, with an average particle size (median diameter: d50) of 2.0 μm or more and 4.5 μm or less, and then collected by a cyclone collector. The cyclone collector is used to separate the powder from the airflow that carries it. Specifically, coarsely pulverized powder of alloy for RTB-based sintered magnets is pulverized in 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 inert gas (pulverization gas) and the pulverized fine powder forms a high-velocity airflow 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.

[0053] <Step of producing a molded body> Next, a compact is produced from the fine powder by pressing in a magnetic field. When pressing in a magnetic field, it is preferable to form the compact by pressing in an inert gas atmosphere or wet pressing, from the viewpoint of suppressing oxidation. In particular, wet pressing coats the surfaces of the particles constituting the compact with a dispersant such as an oil, suppressing contact with oxygen and water vapor in the atmosphere. This prevents or suppresses oxidation of the particles by the atmosphere before, during, or after the pressing process.

[0054] When wet pressing in a magnetic field is performed, a slurry is prepared by mixing a dispersion medium with fine powder, and the slurry is supplied to a cavity in a mold of a wet pressing device and press-molded in a magnetic field.

[0055] ·Dispersion medium The dispersion medium is a liquid in which the alloy powder can be dispersed to obtain a slurry.

[0056] Preferred dispersion media used in the present disclosure include mineral oils and synthetic oils. While the type of mineral oil or synthetic oil is not limited, if the kinematic viscosity at room temperature exceeds 10 cSt, the increased viscosity may strengthen the bonding strength between the alloy powders, adversely affecting the orientation of the alloy powder during wet compaction in a magnetic field. For this reason, the kinematic viscosity of the mineral oil or synthetic oil at room temperature is preferably 10 cSt or less. Furthermore, if the distillation point of the mineral oil or synthetic oil exceeds 400°C, deoiling after obtaining a compact becomes difficult, resulting in increased residual carbon in the sintered compact and possibly degrading the magnetic properties. Therefore, the distillation point of the mineral oil or synthetic oil is preferably 400°C or less. Vegetable oil may also be used as the dispersion media. Vegetable oil refers to oil extracted from plants, and the type of plant is not limited to a specific plant.

[0057] · Preparation of slurry The obtained alloy powder is mixed with a dispersion medium to obtain a slurry.

[0058] 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% by mass or more) in terms of mass ratio. 3 This is because a flow rate of 1 / sec allows the alloy powder to be efficiently supplied into the cavity and excellent magnetic properties to be obtained. The concentration of the alloy powder in the slurry is preferably 90% or less by mass. The method for mixing the alloy powder and the dispersion medium is not particularly limited. The alloy powder and the dispersion medium may be prepared separately, and then weighed and mixed in predetermined amounts. Alternatively, when dry-pulverizing coarsely pulverized powder using a jet mill or the like to obtain alloy powder, a container containing the dispersion medium may be placed at the alloy powder outlet of the jet mill or other grinding device, and the pulverized alloy powder may be directly recovered in the dispersion medium in the container to obtain a slurry. In this case, the container is preferably also filled with a nitrogen and / or argon gas atmosphere, and the obtained alloy powder is directly recovered in the dispersion medium without being exposed to the air to form a slurry. Furthermore, it is also possible to wet-pulverize the coarsely pulverized powder in the dispersion medium using a vibrating mill, ball mill, attritor, or the like to obtain a slurry consisting of the alloy powder and the dispersion medium.

[0059] The slurry thus obtained is molded in a known wet press to obtain a molded body having a predetermined size and shape.

[0060] Next, the compact is sintered to obtain a block of RTB based sintered magnet material. The sintering process for the RTB based sintered magnet material block is carried out at a pressure of, for example, 0.13 Pa (10 -3 Torr) or less, preferably 0.07 Pa (5.0 × 10 -4 The sintering can be carried out under a pressure of 1000 Torr or less at a temperature in the range of, for example, 1000 to 1150° C. To prevent oxidation due to sintering, residual gas in the atmosphere can be replaced with an inert gas such as helium or argon.

[0061] (2) Wire EDM process for RTB sintered magnet material blocks The wire electric discharge machining process can be performed using, for example, a wire electric discharge machining apparatus 100 shown in FIG.

[0062] In this embodiment, an RTB sintered magnet material block can be machined and cut into any desired shape using an on-demand method.

[0063] The wire electrical discharge machining process produces RTB based sintered magnet material pieces of various shapes and sizes, but the machined surfaces of these pieces have deteriorated layers with surface roughnesses that may vary from piece to piece. In this embodiment, the individual RTB based sintered magnet material pieces are advanced to the next surface machining process without observing or measuring the thickness of the deteriorated layer.

[0064] (3) Surface processing of RTB sintered magnet blanks The surface processing step in this embodiment is carried out using a barrel polishing machine.

[0065] A plurality of RTB-based sintered magnet material pieces, each with a deteriorated layer of a different thickness, can be fed into the barrel polishing machine and processed simultaneously. The polishing time in the barrel polishing process can be fixed at a constant value regardless of the average machining speed in the wire electric discharge machining process, but the deteriorated layer is removed to obtain RTB-based sintered magnet material pieces with the desired dimensions and shape.

[0066] It should be noted that the method for producing an RTB based sintered magnet according to the present disclosure is not limited to the above embodiment. The compact may be produced by dry pressing, and the shape and size of the compact before and after cutting may be arbitrary. [Example]

[0067] A number of RTB based sintered magnet material blocks were fabricated using a known method. The fabricated RTB based sintered magnet material blocks had a length of 60 mm, a width of 30 mm, and thicknesses (T) of 5 mm, 10 mm, 20 mm, and 32 mm.

[0068] Each of these RTB based sintered magnets was cut using the wire electric discharge machining device shown in Figure 4. Specifically, the RTB based sintered magnet was set on stage 20 shown in Figure 4 with its thickness direction aligned with the up-down direction (vertical direction), and wire electric discharge was performed by moving stage 20 in the length direction (60 mm).

[0069] The wire running speed during wire electric discharge machining was 0.8 to 11 mm / sec, and the tension of the wire 30 during running was 9 N. The ON / OFF times of the pulse voltage were set to 60 μsec for ON time and 210 μsec for OFF time.

[0070] Fig. 6 is a graph showing the relationship between the thickness (T) of the RTB based sintered magnet material block and the machining speed (average machining speed) using the wire electric discharge machining device in this example. As shown in Fig. 6, although the average machining speed differs depending on the thickness (T), cutting was performed at a high average machining speed of 2.0 mm / min or more in all cases.

[0071] FIG. 7 is a graph showing the relationship between the thickness (T) of the RTB based sintered magnet material block in this example and the measurement results of the thickness of the deteriorated layer in the thickness direction of the RTB based sintered magnet pieces after cutting (surface roughness Rz on the cut surface). FIG. 8 is a graph showing the relationship between the thickness T of the RTB based sintered magnet material block in this example and the measurement results of the thickness of the RTB based sintered magnet pieces after cutting. As shown in FIGS. 7 and 8, as the average processing speed increases, the deteriorated layer becomes thicker (surface roughness Rz increases), and the overall thickness of the RTB based sintered magnet pieces, including the deteriorated layer, also increases. Note that R Z The measurements were taken using a digital microscope.

[0072] The RTB-based sintered magnet pieces after wire EDM were subjected to a surface processing process. Specifically, multiple RTB-based sintered magnet material pieces, each with a deterioration layer of different thickness, were placed in a barrel polishing machine containing 14 mm diameter media and processed at 20 rpm for two hours (i.e., the polishing time in the barrel polishing process was fixed at a constant value regardless of the average processing speed in the wire EDM process). The thickness dimensions (thickness dimensions of the cut cross sections) of the RTB-based sintered magnets after barrel polishing were measured, and the thicknesses were 5 mm, 10 mm, 20 mm, and 32 mm, respectively, which were confirmed to be consistent with the target dimensions for wire EDM. In other words, the deterioration layer was composed of material adhering to the cut surface formed by wire cutting. Therefore, by removing the deterioration layer, a processed product with the desired dimensions could be obtained. [Explanation of symbols]

[0073] 12 RTB sintered magnet blank 14...deterioration layer 20 Stages 30. Discharge wire 40, 42... Main guide wheel 44, 46, 48... Sub guide wheels 50··· Winding drum

Claims

1. preparing an R-T-B based sintered magnet material block (R is a rare earth element and must contain at least one element selected from the group consisting of Nd, Pr, and Ce; T is at least one transition metal and must contain Fe; and B is boron); a wire electrical discharge machining process in which the sintered R-T-B based magnet material block is cut by wire electrical discharge at an average machining speed of 2.0 mm / min to 6.0 mm / min to produce a plurality of sintered R-T-B based magnet material pieces from the sintered R-T-B based magnet material block; a surface processing step of grinding or polishing the electrical discharge machined surface of each of the plurality of sintered RTB based magnet material pieces; A method for producing an RTB based sintered magnet, comprising:

2. 2. The method for producing a sintered RTB based magnet according to claim 1, wherein said surface processing step removes convex portions from the uneven surface present on said electrical discharge processed surface.

3. 3. The method for producing a sintered RTB based magnet according to claim 1, wherein the height of the protrusions removed in the surface processing step is in the range of 10 μm to 500 μm.

4. The method for producing an R-T-B based sintered magnet according to any one of claims 1 to 3, wherein the plurality of R-T-B based sintered magnet material pieces produced in the wire electric discharge machining step include two or more sintered magnet material pieces that differ in at least one of size and shape.

5. 5. The method for producing an R-T-B based sintered magnet according to claim 1, wherein in the wire electric discharge machining step, the direction of cutting by the wire electric discharge includes a first direction and a second direction perpendicular to the first direction, and the wire electric discharge is also performed during an operation of changing the cutting direction from the first direction to the second direction.

6. The method for producing a sintered RTB based magnet according to any one of claims 1 to 5, wherein the dimension in the thickness direction of the sintered RTB based magnet material block is 10 mm or more and 60 mm or less.

7. 7. The method for producing a sintered RTB based magnet according to claim 6, wherein the dimension in the thickness direction of the sintered RTB based magnet material block is 20 mm or more and 60 mm or less.

8. The method for producing a sintered RTB based magnet according to any one of claims 1 to 7, wherein the average machining speed in the wire electric discharge machining step is 3.0 mm / min or more.

9. 9. The method for producing a sintered RTB based magnet according to claim 1, wherein the surface processing step includes a barrel polishing step.

10. 10. The method for producing a sintered RTB based magnet according to claim 9, wherein the polishing time in the barrel polishing step is fixed to a constant value regardless of the average machining speed in the wire electric discharge machining step.

Images