Method for manufacturing r-t-b based sintered magnet

Abstract

To provide a method for manufacturing an R-T-B based sintered magnet that does not require preparation of an inert atmosphere.SOLUTION: A method for manufacturing an R-T-B based sintered magnet includes: a pulverization step of preparing a powder of an alloy for the R-T-B based sintered magnet; a molding step of manufacturing a powder molded body using the powder; a cutting step of cutting the powder molded body to obtain a molded body piece; and a sintering step of sintering the molded body piece to manufacture a sintered body. In the cutting step, the powder molded body submerged in a liquid is cut by running a metal wire. The metal wire is a metal stranded wire.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] RTB 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 into 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 possessing excellent magnetic properties, they are known as the highest performing permanent magnets. For this reason, RTB sintered magnets are used in a wide variety of applications, including voice coil motors (VCM) for hard disk drives, motors for electric vehicles (EV, HV, PHV), motors for industrial equipment, and various other motors, as well as in home appliances.

[0003] Such an RTB based sintered magnet is manufactured, for example, through the steps of preparing an 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.

[0004] 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).

[0005] 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 of the desired shape and size. More specifically, R-Fe-B rare earth magnet powder is first compression-molded in a press to produce a compact larger in size than the final magnet product. After the compact is sintered into a sintered body by a sintering process, the sintered body is ground using, for example, 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 ​​using a blade saw or the like to cut out multiple plate-shaped sintered body portions.

[0006] However, 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, making high-precision grinding difficult and time-consuming. Furthermore, some material is inevitably lost during the processing. For this reason, the processing step is a major cause of increased manufacturing costs.

[0007] Patent Document 1 describes a method for reducing processing costs by processing a powder compact in its green compact form before sintering it. This method uses wires with abrasive grains fixed to their outer periphery, which run parallel to each other. Patent Document 1 also describes that by holding the cuttings between the two wires running along the same line, the cuttings can be efficiently removed.

[0008] Patent Document 2 describes a molding processing method in which a powder molding is submerged in a liquid and cut by a traveling metal wire, thereby eliminating the need to prepare an inert atmosphere with a controlled oxygen concentration. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-276001 [Patent Document 2] Japanese Patent Publication No. 2022-054683 Summary of the Invention [Problem to be solved by the invention]

[0010] According to the method of Patent Document 2, if the cutting speed is excessively increased in order to improve productivity, the wire may not travel as intended, resulting in deterioration of surface roughness and generation of waviness on the cut surface. Furthermore, according to the study by the present inventors, it was found that if a wire with abrasive grains fixed to its outer periphery as described in Patent Document 1 is used in the method of Patent Document 2, cutting chips are not efficiently discharged into the liquid.

[0011] The present disclosure provides a method for producing an RTB based sintered magnet that can solve the above problems. [Means for solving the problem]

[0012] The present application discloses a method for producing an RTB based sintered magnet as described in the following items. [Item 1] a milling step of preparing a powder of an alloy for an RTB-based sintered magnet (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); a molding step of producing a powder compact using the powder; a cutting step of cutting the powder compact to obtain compact pieces; a sintering step of sintering the compact pieces to produce a sintered body; Including, In the cutting step, the powder compact submerged in the liquid is cut by running a metal wire; The method for producing an RTB sintered magnet, wherein the metal wire is a stranded metal wire. [Item 2] 2. The method for producing an RTB based sintered magnet according to item 1, wherein in the cutting step, the metal wire travels in one direction for 250 m or more at a speed of 100 m / min or more. [Item 3] 3. The method for producing an RTB based sintered magnet according to item 1 or 2, wherein in the cutting step, the metal wire travels at a speed of 400 m / min or more. [Item 4] 4. The method for producing an RTB based sintered magnet according to any one of items 1 to 3, wherein in the cutting step, the cutting speed in the direction perpendicular to the running direction of the metal wire is 600 mm / min or more. [Item 5] 5. The method for producing an RTB based sintered magnet according to any one of items 1 to 4, wherein the step of preparing the powder compact includes a step of compacting the powder by wet pressing. [Item 6] 6. The method for producing an RTB based sintered magnet according to item 5, wherein the wet pressing is performed by mixing the powder with the same type of liquid as the liquid used in the cutting step. [Item 7] 7. The method for producing an RTB based sintered magnet according to any one of items 1 to 6, wherein in the cutting step, the tension of the metal wire is 40 N or more. [Item 8] 8. The method for producing an RTB based sintered magnet according to any one of items 1 to 7, further comprising the step of recovering from the liquid the powder particles that have been cut from the powder compact in the cutting step. [Effects of the Invention]

[0013] According to the present disclosure, cutting with a wire saw is possible without preparing an inert atmosphere with a controlled oxygen concentration to suppress oxidation of the powder compact, resulting in excellent mass productivity. Furthermore, according to the present disclosure, even when the cutting speed (cutting speed) is increased, the efficiency of removal of cutting chips can be improved, which reduces wire deflection due to load and suppresses deterioration of surface roughness and the occurrence of waviness on the cut surface. This shortens the process time, thereby enabling reductions in manufacturing costs.

[0014] Furthermore, the method for producing an RTB-based sintered magnet according to the present disclosure does not use a wire with abrasive grains adhered to its outer surface, which has the advantage of preventing impurities from being mixed into the cutting chips due to the abrasive grains falling off during the cutting process, making it easy to reuse the cutting chips for magnet production. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a flowchart showing the main steps of a manufacturing method according to an embodiment of the present disclosure. [Figure 2A] FIG. 2A is a cross-sectional view showing an example of a wire (metal wire) formed from a metal strand. [Figure 2B] FIG. 2B is a side view of a wire (metal wire) formed from stranded metal wire. [Figure 3A] FIG. 3A is a cross-sectional view showing an example of a wire formed from metal strands. [Figure 3B] FIG. 3B is a side view showing an example of a wire formed from metal strands. [Figure 4] FIG. 4 is a perspective view schematically illustrating the configuration of a wire saw device used in an embodiment of the present disclosure. [Figure 5A] FIG. 5A is a front view illustrating a step of cutting a powder compact submerged in a liquid with a stranded metal wire. [Figure 5B] FIG. 5B is a front view illustrating a step of cutting the powder compact submerged in liquid with a stranded metal wire. [Figure 6A] FIG. 6A is a side view illustrating a step of cutting a powder compact submerged in a liquid with a stranded metal wire. [Figure 6B] FIG. 6B is a side view illustrating a step of cutting the powder compact submerged in liquid with a stranded metal wire. [Figure 7] FIG. 7 is a perspective view showing an example of a compact piece that can be produced by the wire saw device of FIG. [Figure 8]FIG. 8 is a table showing how the cutting speed affects the shape of the compact pieces for the Reference Example and Examples 1 to 4. [Figure 9] FIG. 9 is a graph showing how the cutting speed affects the shape of the compact piece for Example 2. [Figure 10] FIG. 10 is a graph showing how the cutting speed affects the shape of the compact piece for the reference example. DETAILED DESCRIPTION OF THE INVENTION

[0016] Before describing the embodiments of the present invention, the findings of the present inventors will be described.

[0017] When cutting a powder compact using the wire saw technology described in Patent Document 1, hard abrasive grains adhered to the surface of the metal wire constituting the wire come into contact with the powder compact, scraping off a portion of the powder compact through friction. In contrast, the wire saw technology described in Patent Document 2 allows a metal wire without abrasive grains to travel and come into contact with a powder compact submerged in liquid, thereby grinding and cutting the powder compact. This is because a high-speed liquid flow (jet flow) is generated in and around the contact area between the traveling metal wire and the powder compact, which scrapes off the powder particles constituting the powder compact. It is believed that some of the powder particles (grinding debris) scraped off from the powder compact are carried by the high-speed liquid and become sandwiched between the metal wire and the powder compact, thereby exerting a grinding function similar to that of free abrasive grains and facilitating the cutting of the powder compact.

[0018] However, the inventors' investigations have revealed that when a metal wire is used, if the cutting speed (cutting speed) is increased excessively, the wire may not travel as intended, resulting in unevenness on the cut surface. One of the reasons for this is thought to be that when a metal wire is used, the discharge efficiency of the shaved powder particles is relatively low, so an increase in the cutting speed leads to an increase in the friction load on the wire.

[0019] The inventors have discovered that by using a twisted metal wire instead of a simple solid metal wire, it is possible to increase the cutting speed (cutting speed) while increasing the efficiency of discharge of cutting chips and suppressing the occurrence of unevenness on the cut surface.

[0020] An embodiment of a method for producing an RTB based sintered magnet according to the present disclosure will now be described. As shown in the flowchart of FIG. 1, the method for producing an RTB based sintered magnet in this embodiment includes the following steps: a grinding step (S10) of preparing a powder of an alloy for an RTB-based sintered magnet (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); a molding step (S20) of producing a powder compact using the powder; a cutting step (S30) of cutting the powder compact to obtain compact pieces; a sintering step (S40) of sintering the compact pieces to produce a sintered body; Including, The cutting step (S30) includes cutting the powder compact submerged in a liquid by running a metal wire. Hereinafter, the "metal wire" in this disclosure may be referred to as a "wire." This wire is a stranded metal wire. In this disclosure, the "stranded metal wire" is composed of multiple stranded metal wires.

[0021] FIG. 2A is a cross-sectional view showing an example of wire (metal wire) 40 made of twisted metal wire, and FIG. 2B is a side view of wire 40. In the example shown, no abrasive grains are adhered to the surface of wire 40. For comparison, FIGS. 3A and 3B show a cross-section and a side view, respectively, of wire 40X disclosed in Patent Document 2. Wire 40X is a metal wire (single metal wire) similar to the metal wire described in Patent Document 2. No abrasive grains are adhered to the surface of this wire 40X either.

[0022] 2A and 2B, the wire 40 is made by twisting together seven metal wires 40a, 40b, 40c, 40d, 40e, 40f, and 40g. These metal wires 40a to 40g are all made of the same material and have the same diameter. However, the material and / or diameter of the central metal wire (core wire) 40g do not need to be the same as the material and / or materials of the other metal wires 40a to 40f. Furthermore, the metal wires 40a to 40g may each be made of a different material and have a different diameter. The number of metal wires constituting the metal strand of the wire 40 is also not limited to seven.

[0023] As shown in Figures 2A and 2B, the surface of wire 40, which is made of stranded metal wires, has recesses and protrusions that extend roughly in a spiral shape. In other words, spirally extending grooves and ridges exist on the surface of wire 40. The recesses (grooves) are formed between adjacent metal wires, and the protrusions (ridges) are the exposed surfaces of each metal wire. On the other hand, in conventional metal wires or metal wires that are not stranded wires, as shown in Figures 3A and 3B, spirally extending recesses and protrusions do not exist on the surface of wire 40X.

[0024] According to the inventor's investigations, it is believed that the high-speed liquid flow (jet flow) generated when wire 40 made of stranded metal wires travels through a liquid forms a spiral fluid flow that swirls and advances in the direction of the swirl axis. Therefore, when the traveling speed of wire 40 increases, the high-speed liquid flow (jet flow) scrapes off the powder particles that make up the powder compact, and the spirally swirling flow efficiently expels the scraped off powder particles outside the cutting area.

[0025] Examples of metal solid wires that can be used for the metal strand include piano wire and high-tensile steel wire. The surface of each metal solid wire or the surface of the metal strand may be plated. The diameter D of the wire 40 is preferably in the range of 100 μm to 350 μm, and more preferably in the range of 200 μm to 300 μm. If the diameter D of the wire 40 is less than 100 μm, the wire 40 may stretch during the cutting process due to insufficient strength. A larger diameter D of the wire 40 improves the discharge of cutting chips (swarf), but may increase the cutting allowance and reduce the number of molded pieces obtained. Therefore, a diameter D of 350 μm or less is desirable.

[0026] The pitch P of one turn of a single metal wire (e.g., metal wire 40a) around the axial center of the wire 40 is preferably in the range of 5 to 15 times (e.g., 10 times) the diameter D of the wire 40. For example, when the diameter D is approximately 300 μm, the turn pitch P is approximately 3 mm. If the pitch P relative to the diameter D is outside this range, the efficiency of the swirling flow formed by the spiral recesses and protrusions on the surface of the stranded metal wire in discharging cutting chips to the outside may decrease. Note that the above mechanism for cutting a powder compact using a stranded metal wire cannot be realized in gas, but is only possible in liquid. Examples of liquids that can be used in embodiments of the present disclosure include oils such as mineral oil or synthetic oil. As described below, when producing a powder compact by wet pressing, it is preferable to perform wire saw cutting in the same or the same type of liquid (e.g., oil) mixed with the powder. This is because the liquids such as oils used in wet pressing have components and properties that have been confirmed to volatilize during the sintering process and not adversely affect the properties of the final sintered magnet.

[0027] The stranded metal wire constituting the wire 40 preferably used in the embodiments of the present disclosure is produced by twisting together N (N is an integer of 2 or more) single metal wires, with N being preferably 3 or more and 20 or less (for example, N = 5 to 8). Even when N is 2, it is possible to cut the powder compact, but the ability to discharge the scraped powder particles is relatively small. When N is greater than 20, the depth of the spiral recesses and the height of the protrusions formed on the surface by twisting together a large number of single metal wires becomes smaller than the diameter D of the wire, which increases the likelihood of a decrease in the ability to discharge the scraped powder particles.

[0028] The surface of the wire 40 may be subjected to some kind of treatment (surface treatment).

[0029] According to the method for producing an RTB-based sintered magnet disclosed herein, cutting with a wire saw is performed while the powder compact is submerged in liquid, eliminating the need for an inert atmosphere with a controlled oxygen concentration to suppress oxidation. Furthermore, the presence of spirally extending recesses and protrusions on the surface of the stranded metal wire allows it to be quickly removed into the liquid after being scraped off the powder compact, making it less likely for the wire to bend even when the wire travel speed is increased, and thus minimizing deterioration in processing accuracy and dimensional accuracy.

[0030] In the cutting process, the running speed of the wire 40 is preferably 100 m / min or more, for example, to ensure a sufficient flow rate for cutting the powder compact. As the running speed of the wire increases, the speed of the jet flow generated in the liquid also increases, thereby improving the discharge efficiency of the scraped powder particles. In other words, clogging due to cutting chips during the cutting process is less likely to occur. Therefore, the running speed of the wire 40 is more preferably 400 m / min or more, and even more preferably 800 m / min or more (e.g., 900 m / min). The running direction of the wire 40 can be maintained in one direction while the metal wire travels 250 m or more. For example, when the wire travels 250 m at a running speed of 1000 m / min, the wire can be run in one direction for 15 seconds or more. Switching the running direction of the wire 40 in a short period of time (e.g., less than 15 seconds) temporarily halts cutting because the running speed drops to zero during the switch, temporarily halting cutting and reducing work efficiency. Therefore, the distance the wire 40 travels in one direction is preferably 250 m or more. When the stranded metal wire 40 is used in a liquid, even if the wire is continuously run in one direction for such a long distance, clogging with chips is unlikely to occur for the reasons mentioned above. After running for 250 m or more, the metal wire may be run in the other direction for 250 m or more.

[0031] Furthermore, in the cutting process, the cutting speed (cutting speed or workpiece feed speed) in the direction perpendicular to the wire traveling direction is preferably 300 mm / min or more. If the cutting speed is less than 300 mm / min, the cutting process takes longer, resulting in reduced production efficiency. The cutting speed is more preferably 600 mm / min or more, and even more preferably 900 mm / min or more. According to an embodiment of the present disclosure, increasing the wire traveling speed allows for efficient discharge of the scraped powder particles, thereby suppressing wire deflection due to load even when the cutting speed is increased. As a result, the cut surface maintains the desired shape, achieving high dimensional accuracy. Therefore, it is possible to achieve a cutting speed that is, for example, 1.5 times faster than when using conventional metal wires (FIGS. 3A and 3B) in liquid. Specifically, when the wire traveling speed is, for example, 1000 m / min, the cutting speed can be increased to 900 mm / min or more.

[0032] Furthermore, cutting the powder compact in a liquid has the advantage of suppressing temperature rise due to frictional heat at the contact point between the powder compact and the wire saw, and also of making it easier for the generated heat to dissipate in the liquid. In air, the powder compact, heated to a high temperature by the generated frictional heat, would react with oxygen or water vapor in the air, resulting in an increase in the oxygen concentration in the final sintered magnet and a deterioration in its magnetic properties. However, this embodiment avoids such problems.

[0033] Another advantage of cutting the powder compact in a liquid is that the powder particles scraped off from the powder compact by the wire saw settle in the liquid, making them easy to recover. In a preferred embodiment, the step of preparing the powder compact includes a step of wet-pressing the powder. In this case, it is desirable to perform the wet-pressing by adding the same type of liquid as used in the cutting step to the powder. This is because the powder particles scraped off from the powder compact in the cutting step can be easily recovered from the liquid and reused.

[0034] Furthermore, even if the wire 40 made of stranded metal wire is cut horizontally, the powder compact can be easily cut in a liquid. Depending on the powder pressing process, at least a portion of the surface of the powder compact (e.g., the top surface) may have irregularities, which requires cutting or polishing after the sintering process. According to the embodiments of the present disclosure, such cutting or polishing steps can be eliminated, thereby reducing manufacturing costs while maintaining the properties of a high-performance magnet.

[0035] An example of the configuration of a wire saw device that can be used in the above manufacturing method will be described below with reference to Fig. 4. Fig. 4 is a perspective view that schematically shows an example of the configuration of a wire saw device 100 according to an embodiment of the present disclosure. For reference, the figure shows 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.

[0036] 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 wire 40 is the aforementioned twisted metal wire, and does not have abrasive grains fixed to its surface.

[0037] Each of the rollers 30a to 30c is rotatably supported by a support device 50. The rotation axes of the rollers 30a to 30c are parallel to the Y axis. The wire 40 runs under tension as the rollers 30a to 30c rotate. The wire 40 is wound around a bobbin (not shown). The wire saw device 100 may further include other rollers for adjusting the tension. The tension of the wire 40 can be set to, for example, 40 N or more.

[0038] During cutting, rollers 30a, 30b, and 30c and the recovery bobbin rotate. The direction of rotation of rollers 30a, 30b, and 30c depends on their arrangement and how wire 40 is wound. In the illustrated wire saw device 100, rollers 30a, 30b, and 30c rotate in the same direction. Once a predetermined length of wire 40, for example, 250 m or more, has been wound onto one recovery bobbin, the recovery bobbin and rollers 30a, 30b, and 30c are rotated in the opposite direction. This causes wire 40 to move in the opposite direction, and by repeating this process, wire 40 can be reciprocated (moved).

[0039] Specific examples of steps for producing powder compact 10 will be described later. It should be noted here that powder compact 10 is not a sintered compact, but a compact (green compact) of powder before sintering. The powder compact is obtained by wet-pressing or dry-pressing a powder of an alloy for an RTB sintered magnet (R is a rare earth element that must contain at least one selected from the group consisting of Nd, Pr, and Ce, T is at least one transition metal that must contain Fe, and B is boron) in an aligning magnetic field.

[0040] In the example of FIG. 4 , the portion of the wire 40 located between rollers 30a and 30b contacts the powder compact 10. The support device 50 has a shape that allows it to move in the Y-axis direction without interfering with the powder compact 10 when the powder compact 10 is being cut by the wire 40 running between rollers 30a and 30b. In the example of FIG. 4 , the support device 50 has an opening 51 that allows the powder compact 10 to move in the Y-axis direction. Specifically, rollers 30a and 30b are located on either side of the opening 51 of the support device 50. The support device 50 in FIG. 2 has a roughly U-shaped or C-shaped configuration that defines the opening 51. The size (width) of the opening 51 in the X-axis direction is larger than the size (width) of the powder compact 10 in the X-axis direction. Note that the stranded metal wire 40 according to the present disclosure may also be used in a wire saw device having a configuration different from that of the wire saw device 100 illustrated in FIG. 4 . For example, when cutting is performed without moving the powder compact 10 in the Y-axis direction, the support device 50 does not need to have the opening 51.

[0041] Powder compact 10 produced in the molding step (S20) is fixed to fixing base 20 by a clamp (not shown) and placed inside tank 62 that stores liquid 60. In FIG. 2, tank 62 is indicated by a dashed line, and the height of the surface of liquid 60 is indicated by a dotted line. In the example of FIG. 2, powder compact 10 is entirely immersed in liquid 60.

[0042] The running direction of the wire 40 when the wire 40 comes into contact with the powder compact 10 (hereinafter, sometimes simply referred to as the "wire running direction") is parallel to the X-axis.

[0043] The wire saw device 100 of this embodiment includes a drive unit 70 that moves the position of the powder compact 10 relative to the wire 40 in the vertical direction (Z-axis direction) and the horizontal direction (Y-axis direction). In the example of FIG. 4 , the drive unit 70 includes a support stage 72 on which the powder compact 10 is placed, a Z-axis drive unit 74 configured to reciprocate the support stage 72 in the Z-axis direction, and a Y-axis drive unit 76 configured to reciprocate the support stage 72 in the Y-axis direction. The Z-axis drive unit 74 and the Y-axis drive unit 76 each include an actuator such as a motor. These actuators can move the support stage 72 and the powder compact 10 fixed to the support stage 72 in response to drive signals from a control device. In a planar view of the powder compact 10 seen from a direction parallel to the wire traveling direction (X-axis direction), the position of the powder compact 10 can be defined by coordinates on a YZ coordinate system.

[0044] By moving the powder compact 10 by the Z-axis driver 74 and the Y-axis driver 76 while the wire 40 is running, it is possible to move the wire 40 in any cutting direction perpendicular to the running direction relative to the powder compact 10. In particular, by adjusting the speed of movement in the Z-axis direction by the Z-axis driver 74 and the speed of movement in the Y-axis direction by the Y-axis driver 76, it is possible to freely change the cutting direction of the wire 40.

[0045] In the above example, the position of the wire 40 relative to the YZ coordinate system is fixed, and the powder compact 10 is movable. However, alternatively, the position of the powder compact 10 may be fixed, and the position of the wire 40 relative to the YZ coordinate system may be movable. In this case, the support device 50 is driven to move in the Y-axis direction and the Z-axis direction. Alternatively, for example, a configuration may be adopted in which the support device 50 moves in the Y-axis direction and the powder compact 10 moves in the Z-axis direction. With the above configuration, the position of the wire 40 (coordinates on the YZ coordinate system) relative to the powder compact 10 can be moved in any direction in a plan view of the powder compact 10 seen from a direction parallel to the wire travel direction (X-axis direction).

[0046] For ease of understanding, the cutting step will be described in detail below using an example in which the relative position of the wire 40 with respect to the fixed powder compact 10 is changed.

[0047] First, reference will be made to Figures 5A and 5B. In the following description, cutting of powder compact 10 is performed using a wire saw device 100, which is shown schematically. Figures 5A and 5B are front views illustrating the process of cutting powder compact 10 submerged in liquid 60 with wire 40. Figure 5A shows the state before the cutting process begins, and Figure 5B shows the state during the cutting process. The dashed line within powder compact 10 shown in Figure 5B schematically indicates the position of wire 40 while cutting powder compact 10.

[0048] In the illustrated example, the wire 40 travels at a predetermined speed in the X-axis direction while moving in a direction perpendicular to the wire 40's traveling direction (any direction within the YZ plane). The direction perpendicular to the wire 40's traveling direction is the cutting direction, and the speed in this direction (cutting speed) is set to, for example, 300 mm / min or more. In the example shown in FIG. 5B, the traveling wire 40 is shown moving, for example, in the negative direction of the Z-axis relative to the stationary powder compact 10. However, as described above, the powder compact 10 may be lifted together with the fixing base 20 in the positive direction of the Z-axis. According to this embodiment, since the wire 40 is made of the aforementioned twisted metal wire, increasing the wire traveling speed allows for rapid discharge of the scraped powder particles and improves the cutting speed. The wire 40, indicated by the dashed line in FIG. 5B, extends in a straight line in the horizontal direction. However, even if the cutting speed is increased, deflection due to load is suppressed, resulting in less unevenness on the cut surface.

[0049] 6A and 6B are side views illustrating the process of cutting powder compact 10 submerged in liquid 60 with wire 40. Fig. 6A shows the state before the cutting process starts, and Fig. 6B shows the state during the cutting process.

[0050] 6A and 6B are side views illustrating the process of horizontally cutting powder compact 10 submerged in liquid 60 with wire 40. In the illustrated example, during the cutting process, rollers 30a, 30b, and 30c move horizontally (in the direction of the rotation axis of each roller) relative to powder compact 10.

[0051] In this embodiment, the step of cutting the powder compact 10 with the wire 40 is carried out with the powder compact 10 submerged in the liquid 60. When the powder compact 10 is a powder compact formed by wet pressing, a preferred example of the liquid 60 is the same type of oil as the dispersion medium, such as an oil (mineral oil or synthetic oil), used in the wet pressing.

[0052] When a powder compact 10 is machined using such a wire saw device 100, powder particles constituting the powder compact 10 fall off as chips from the area cut by the wire 40. These chips are powder particles that have fallen off the powder compact 10, and the individual particles do not have rough fracture surfaces like metal chips (cutting chips). The shape and size of the chips scraped off the powder compact by the wire are similar to the shape and size of the powder particles used to produce the powder compact 10. The present inventors have investigated the reuse of these chips. When a hard sintered compact obtained by sintering a powder compact is cut, the chips are particles or particle aggregates whose grains have grown or whose composition has changed due to chemical reactions during sintering. Therefore, even if these chips are mixed with rare earth magnet powder and reused, the magnetic properties are likely to deteriorate. In contrast, chips obtained from a powder compact before sintering are easily reused because they have a similar composition and size to the other particles contained in the powder compact.

[0053] Furthermore, when the powder compact 10 is produced by wet pressing, if wire saw processing is performed in the same type of oil as the dispersant, the recovered powder (chips) can be used directly for wet pressing, thereby increasing production efficiency.

[0054] Hereinafter, the method for manufacturing the R-T-B sintered magnet of the present embodiment will be described in detail.

[0055] S10: Crushing process In the crushing process (S10), powders of an alloy for an R-T-B sintered magnet are prepared. Hereinafter, the composition of the alloy for the R-T-B sintered magnet, the manufacturing process of the alloy, and the process for preparing the alloy powders will be described in order.

[0056] <Composition of alloy rare earth for R-T-B sintered magnet> R is a rare earth element and necessarily contains 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, Nd-Pr-Dy-Tb, Nd-Ce-Dy, Nd-Ce-Tb, Nd-Ce-Dy-Tb, Nd-Pr-Ce-Dy, Nd-Pr-Ce-Tb, Nd-Pr-Ce-Dy-Tb is used.

[0057] 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 mischmetal or didymium may also be used. Further, R may not be a pure element and may contain inevitable impurities in the manufacturing process within an industrially available range. The content is, for example, 27 mass% or more and 35 mass% or less. Preferably, the R content of the R-T-B sintered magnet is 31 mass% or less (27 mass% or more and 31 mass% or less, preferably 29 mass% or more and 31 mass% or less). By setting the R content of the R-T-B sintered magnet to 31 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, more preferably 500 ppm or more and 2500 ppm or less), higher magnetic properties can be obtained.

[0058] T contains iron (including the case where T consists substantially of iron), and up to 50% by mass thereof may be replaced with cobalt (Co) (including the case where T consists substantially 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 occupy the remainder of R and B or R, B, and M described later.

[0059] The content of B may also be a known content, and for example, 0.9% to 1.2% by mass is a preferable range. If it is less than 0.9% by mass, high H cJ may not be obtained, and if it exceeds 1.2% by mass, B r may decrease. Note that a part of B can be replaced with C (carbon).

[0060] 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 addition amount of the M element is preferably 5.0% by mass or less. This is because if it exceeds 5.0% by mass, B r may decrease. Also, inevitable impurities can be tolerated.

[0061] The content of N (nitrogen) in the R-T-B sintered magnet is preferably 50 ppm or more and 600 ppm or less. Also, the content of C (carbon) in the R-T-B sintered magnet is preferably 50 ppm or more and 1000 ppm or less.

[0062] <Manufacturing process of alloy for R-T-B sintered magnet> An example of the manufacturing process of an alloy for an R-T-B sintered magnet is shown. An alloy ingot can be obtained by ingot casting, in which a metal or alloy pre-adjusted to have the above-described composition is melted and poured into a mold. Also, the molten metal can be brought into contact with a single roll, double roll, rotating disk, rotating cylindrical mold, etc., and rapidly cooled, and alloy flakes can be produced by a rapid cooling method typified by a strip casting method or a centrifugal casting method for producing a solidified alloy thinner than the alloy made by the ingot method.

[0063] In the embodiments of the present disclosure, materials manufactured by either the ingot method or the rapid cooling method can be used, but it is preferable to be manufactured by a rapid cooling method such as the strip casting method. The thickness of the rapidly cooled alloy produced by the rapid cooling method is usually in the range of 0.03 mm to 1 mm and is in the form of flakes. The alloy melt starts to solidify from the surface in contact with the cooling roll (roll contact surface), and crystals grow columnarly in the thickness direction from the roll contact surface. The rapidly cooled alloy is cooled in a short time compared with an alloy (ingot alloy) produced by the conventional ingot casting method (die casting method), so the structure is refined and the crystal grain size is small. Also, the area of the grain boundaries is large. Since R is more than the target composition and the R-rich phase that easily binds to hydrogen spreads widely within the grain boundaries, the rapid cooling method is excellent in the dispersibility of the R-rich phase. For this reason, it is easy to break at the grain boundaries by the hydrogen pulverization method. By pulverizing the rapidly cooled alloy with hydrogen, the size of the hydrogen pulverized powder (coarse pulverized powder) can be made, for example, 1.0 mm or less. The coarse pulverized powder thus obtained is finely pulverized, for example, with a jet mill.

[0064] <Process of preparing powder of alloy for R-T-B sintered magnet> The powder of the rare earth alloy for the R-T-B sintered magnet is active and easily oxidized. For this reason, as the gas used in the jet mill, for example, an inert gas such as nitrogen, argon, or helium is used in order to avoid the risk of heat generation and ignition and to reduce the oxygen content as an impurity to achieve high performance of the magnet.

[0065] 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, an average particle size (median diameter: d50) of 2.0 μm to 4.5 μm, and then transferred to 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 in a powder collector.

[0066] S20: Molding process In the compacting step (S20), a powder compact is produced using the powder obtained in the pulverizing step (S10).

[0067] In this embodiment, a powder compact is produced from the above powder by pressing in a magnetic field. When pressing in a magnetic field, it is preferable to form the powder 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 powder compact with a dispersant such as an oil, suppressing contact with oxygen and water vapor in the atmosphere. Therefore, oxidation of the particles by the atmosphere before, during, or after the pressing process can be prevented or suppressed.

[0068] When wet pressing in a magnetic field is performed, a slurry is prepared by mixing a fine powder with a dispersion medium, 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 thus formed has a density of, for example, 4 g / cm. 3 More than 5g / cm 3 It has the following density:

[0069] ·Dispersion medium The dispersion medium is a liquid in which the alloy powder can be dispersed to obtain a slurry.

[0070] Preferred dispersion media for use 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 temperature at which the mineral oil or synthetic oil is fractionated 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 temperature at which the mineral oil or synthetic oil is fractionated is preferably 400°C or less. Vegetable oil may also be used as a dispersion medium. Vegetable oil refers to oil extracted from plants, and the type of plant is not limited to a specific plant.

[0071] Slurry preparation The obtained alloy powder is mixed with a dispersion medium to obtain a slurry.

[0072] 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. 3This 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.

[0073] The resulting slurry is molded in a known wet press to obtain a powder compact having a predetermined size and shape. Conventionally, this powder compact is typically sintered to obtain a sintered body, but in this embodiment, the powder compact is divided using a wire saw before sintering, as described below.

[0074] S30: Cutting process In the cutting step (S30), the powder compact is cut to obtain compact pieces. In this step, one or more compact pieces may be obtained from one powder compact.

[0075] The cutting of the powder compact in this step can be performed by, for example, the wire saw device 100 of FIG. 4 using the wire 40 having the configuration shown in FIGS. 2A and 2B.

[0076] As a result of the cutting process, in addition to typical rectangular parallelepiped green compact pieces, green compact pieces having shapes such as those shown in FIG. 7 can be produced from the block-shaped powder compact. FIG. 7 is a perspective view showing an example of a green compact piece 10P that can be produced in an embodiment of the present disclosure. The green compact piece 10P1 on the left shown in FIG. 7 has a "bow-like" shape, and the green compact piece 10P2 on the right has a "kettle-fish-shaped" shape. According to an embodiment of the present disclosure, multiple green compact pieces 10P such as those shown in FIG. 7 can be produced from a powder compact 10 having, for example, a rectangular parallelepiped block shape. Note that the shapes of the green compact pieces that can be produced are not limited to the example shown. Alternatively, multiple green compact pieces may be obtained simultaneously using, for example, a multi-wire saw device in which multiple wires are arranged in parallel.

[0077] S40: Sintering process In the sintering step (S40), the green compact pieces are sintered to produce a sintered body. That is, the individual green compact pieces cut in the cutting step are sintered to obtain an RTB-based sintered magnet (sintered body). The sintering step can be performed simultaneously on multiple green compact pieces. When multiple green compact pieces are sintered in the same sintering step, the individual green compact pieces may be green compact pieces separated from a single powder green compact, or may be a collection of green compact pieces obtained from different powder green compacts. Note that some of the multiple green compact pieces may include green compact pieces obtained by cutting the powder green compact with a device other than the wire saw device used in the cutting step.

[0078] The sintering process may be carried out at a pressure of, for example, 0.13 Pa (10 -3 Torr) or less, preferably 0.07 Pa (5.0 × 10 -4This can be done at a pressure of 1000 Torr or less, at a temperature in the range of 1000°C to 1150°C, for example. To prevent oxidation during sintering, residual gas in the atmosphere can be replaced with an inert gas such as helium or argon. The resulting sintered body is preferably subjected to additional heat treatment such as aging. Such heat treatment can improve the magnetic properties. Known conditions can be used for the heat treatment, such as the heat treatment temperature and time. The RTB-based sintered magnet obtained in this manner is then subjected to grinding and polishing, surface treatment, and magnetization as necessary to produce the final RTB-based sintered magnet.

[0079] In a preferred embodiment, the method for producing an RTB based sintered magnet according to the present disclosure further comprises a diffusion step of diffusing a heavy rare earth element RH (RH being at least one of Tb, Dy, and Ho) from the surface of the sintered body to the interior thereof. Diffusing the heavy rare earth element RH from the surface to the interior of the sintered body can efficiently increase the coercive force. There are no particular restrictions on the method for the diffusion step; any known method can be used.

[0080] (Example) The raw materials for each element were weighed to achieve a composition of 22.6% Nd, 7.8% Pr, 0.9% B, 0.5% Co, 0.1% Al, 0.2% Cu, 0.4% Ga (all by mass), with the remainder being Fe, and the alloy was fabricated by strip casting. The resulting alloy was then hydrogen-pulverized to obtain a coarsely pulverized powder.

[0081] Next, 0.04% by mass of zinc stearate was added as a lubricant to the obtained coarsely pulverized powder relative to 100% by mass of the coarsely pulverized powder, and after mixing, the mixture was dry-pulverized in a nitrogen gas stream using a jet mill to obtain a particle size D 50 A finely pulverized powder (alloy powder) with a particle size of 4 μm was obtained. The finely pulverized powder was immersed in a nitrogen atmosphere in mineral oil with a distillation point of 250°C and a kinematic viscosity at room temperature of 2 cSt to prepare a slurry. The slurry concentration was 85 mass%. The obtained slurry was compacted (wet compacted) in a magnetic field to produce a powder compact. The size of the powder compact was 80 mm × 45 mm × 60 mm.

[0082] The powder compact was divided into eight compact pieces using the wires of each of the present examples (Examples 1 to 4) and the Reference Example.

[0083] Details of the wires used in Examples 1 to 4 and the Reference Example are as follows: Wire of Example 1: A metal strand (diameter: 330 μm, Z-winding, turn pitch: 3.4 mm) made by twisting seven SUS304 (metal material) single wires, each with a diameter of 110 μm. Wire of Example 2: A metal stranded wire (diameter: 270 μm, turn pitch: 2.7 mm) made by twisting seven SUS304 single wires, each with a diameter of 90 μm. Wire of Example 3: A metal stranded wire (diameter: 240 μm, Z-winding, turn pitch: 2.52 mm) made by twisting seven SWP-EX (metal material name) single wires, each with a diameter of 80 μm. Wire of Example 4: A metal stranded wire made by twisting seven SWP-EX single wires, each with a diameter of 85 μm (diameter: 255 μm, Z-winding, turn pitch: 2.72 mm) Wire of Reference Example: A metal single wire of SWP-B (name of metal material) (diameter: 250 μm)

[0084] As described above, this cutting was carried out while the powder compact was submerged in liquid (the liquid used was the same mineral oil as used during molding). The tension applied to the wire before cutting was 80 N for the wire of Example 1, 50 N for the wire of Example 2, 80 N for the wire of Example 3, 80 N for the wire of Example 4, and 80 N for the wire of Reference Example. The wire running speed during cutting was 800 m / min.

[0085] Fig. 8 is a table showing how the cutting speed [mm / min] affects the shape of the compact pieces for the Reference Example and Examples 1 to 4. The Examples show the results of wire saw cutting using the metal wire (metal strand) shown in Fig. 2A and Fig. 2B, while the Reference Example shows the results of wire saw cutting using the metal wire (metal strand) shown in Fig. 3A and Fig. 3B.

[0086] The "x" in the table of Figure 8 indicates that relatively large irregularities occurred on the cut surface of the molded body divided by wire saw cutting, "△" indicates that relatively small irregularities occurred, and "◯" indicates that no such irregularities occurred and the molded body was able to be divided into well-shaped pieces.

[0087] As shown in Figure 8, in Example 1, a good cut surface shape was obtained when the cutting speed was in the range of 600 mm / min or more and 900 mm / min or less. Even when the cutting speed was 1000 mm / min, only slight irregularities were formed, making it practical. In Examples 2 to 4, when the wire diameter was 270 µm or less, a good cut surface shape was obtained when the cutting speed was in the range of 600 mm / min or more and 1400 mm / min or less.

[0088] FIG. 9 is a graph showing the effect of the cutting speed on the shape of the molded body piece for Example 2. Specifically, a laser displacement meter was used to measure the displacement near the center of the cut surface of the molded body as height while moving the stage carrying the molded body in the cutting direction. FIG. 9(a) shows the measurement results for an example where the cutting speed (cutting speed) was 600 mm / min, FIG. 9(b) shows the measurement results for an example where the cutting speed was 800 mm / min, and FIG. 9(c) shows the measurement results for an example where the cutting speed was 1400 mm / min. In each graph, the vertical axis represents the measured height of the actual cut surface, with the target cut surface height as the reference (height 0 mm), and the horizontal axis represents the position in the cutting direction (cutting direction). As can be seen from the graph in FIG. 9, the fluctuation range of height due to the surface roughness and waviness of the cut surface showed a small value of 0.1 mm or less within a range of 45 mm in the cutting direction. Note that even at cutting speeds of 1000 mm / min and 1200 mm / min, results similar to those shown in the graph in Fig. 9(b) were obtained. That is, in Example 2, a good cut surface shape was obtained at cutting speeds in the range of 600 mm / min to 1400 mm / min. Furthermore, for Examples 3 and 4, results similar to those shown in Fig. 9 were obtained.

[0089] In contrast, when a similar cut was made using wire 40X in Figure 3, the cut surface undulated significantly, as shown in the graph in Figure 10, and the fluctuation range of the height of the cut surface exceeded 0.1 mm within a distance of 45 mm in the cutting direction.

[0090] As described above, according to this embodiment, even if the cutting speed is increased, the efficiency of removal of cutting chips can be increased, so the bending of the wire due to the load is small, and the deterioration of the surface roughness of the cut surface and the occurrence of waviness can be suppressed. This improves the cutting processing accuracy and shortens the process time, making it possible to reduce manufacturing costs. Furthermore, by improving the dimensional accuracy of the molded body, the amount of processing after sintering is reduced, which also improves the raw material yield. [Industrial Applicability]

[0091] The RTB-based sintered magnets of the present disclosure can be used as permanent magnets in a wide variety of applications, including voice coil motors (VCMs) for hard disk drives, motors for electric vehicles (EVs, HVs, PHVs), motors for industrial equipment, and other motors, as well as in home appliances. [Explanation of symbols]

[0092] 10 Powder compact, 20 Fixing base, 30a, 30b, 30c Rollers, 40 Wire, 50 Support device, 60 Liquid, 70 Tank, 100 Wire saw device

Claims

1. a milling step of preparing a powder of an alloy for an R-T-B based sintered magnet (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); a molding step of producing a powder compact using the powder; a cutting step of cutting the powder compact to obtain compact pieces; a sintering step of sintering the compact pieces to produce a sintered body; Including, In the cutting step, the powder compact submerged in the liquid is cut by running a metal wire; The method for producing an RTB based sintered magnet includes the step of:

2. 2. The method for producing a sintered RTB based magnet according to claim 1, wherein in the cutting step, the metal wire is run in one direction for 250 m or more at a speed of 100 m / min or more.

3. 3. The method for producing a sintered RTB based magnet according to claim 2, wherein in the cutting step, the metal wire travels at a speed of 400 m / min or more.

4. 2. The method for producing a sintered RTB based magnet according to claim 1, wherein in the cutting step, the cutting speed in the direction perpendicular to the running direction of the metal wire is 600 mm / min or more.

5. 2. The method for producing a sintered RTB based magnet according to claim 1, wherein the step of preparing the powder compact includes a step of compacting the powder by wet pressing.

6. 6. The method for producing a sintered RTB based magnet according to claim 5, wherein the wet pressing is performed by mixing the powder with the same type of liquid as the liquid used in the cutting step.

7. 2. The method for producing a sintered RTB based magnet according to claim 1, further comprising the step of recovering from the liquid the powder particles that have been cut from the powder compact in the cutting step.

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