Method of manufacturing rare earth-based sintered magnet and wet-type forming device

Abstract

To solve the problem of a wet-type forming process to occur in carrying out transverse magnetic-field press.SOLUTION: A method of manufacturing a rare earth-based sintered magnet in the present disclosure includes the steps of: preparing a compact by compression-forming an alloy powder containing a rare earth element and slurry containing a dispersion medium using a wet-type forming device; and sintering the compact. Without applying magnetic field when the slurry is being injected into a space forming a cavity of the wet-type forming device, the slurry is pressed to start a discharge of the dispersion medium contained in the slurry from internal of the space.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a method for manufacturing a rare earth sintered magnet and a wet forming apparatus.

Background Art

[0002] In recent years, rare earth sintered magnets have shown high demand. Among them, R-T-B sintered magnets (where R is at least one of rare earth elements, T is mainly iron, and B is boron) are known as the most high-performance magnets and are used in various motors such as voice coil motors (VCMs) of hard disk drives, motors for electric vehicles (EVs, HVs, PHVs, etc.), and motors for industrial equipment, as well as in household appliances.

[0003] The R-T-B sintered magnet mainly consists of a main phase composed of an R2T 14 B compound and a grain boundary phase located at the grain boundary portion of this main phase. The R2T 14 B compound is a ferromagnetic material having high saturation magnetization and anisotropic magnetic field. In the grain boundary phase, there exists a non-magnetic and low-melting-point R-rich phase in which rare earth elements (R) are concentrated. As methods for improving the magnetic properties of R-T-B sintered magnets, (1) refinement of the R2T 14 B phase, (2) increasing the orientation degree of the R2T 14 B phase, (3) reducing the oxygen content, and (4) increasing the ratio of the R2T 14 B phase are known.

[0004] For the production of rare earth sintered magnets such as R-T-B sintered magnets, for example, an ingot obtained by casting a molten metal or the like into a mold, or a raw material alloy casting material having a desired composition such as flakes obtained by the strip casting method is pulverized to obtain an alloy powder having a predetermined particle size. This alloy powder is compressed in an orientation magnetic field to produce a powder compact (compressed powder body), and then this powder compact is sintered to produce a rare earth sintered magnet. Note that if the powder particles are oxidized during pulverization and forming, the improvement of magnetic properties is inhibited.

[0005] By the way, there are two methods for forming a powder compact, namely a dry method and a wet method, and Patent Document 1 discloses a wet forming method. According to this wet forming method, since oxidation of powder particles can be suppressed, it is considered that improvement of magnetic properties is less likely to be inhibited compared with the dry forming method.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] In the wet forming method disclosed in Patent Document 1, a slurry containing rare earth alloy powder is pressure-injected into the cavity (space) of the mold. However, according to the study by the present inventor, even in such a case, it has been found that "density variation of the powder compact" and "disorder of orientation" generated by compression in the orientation magnetic field are likely to occur.

[0008] In the former "density variation of the powder compact", cracks and fissures may occur when the powder compact is taken out or during subsequent sintering. Also, in the latter "disorder of orientation", the magnetic properties may deteriorate. In particular, due to the relationship between the pressing direction when the slurry is pressure-injected into the cavity of the mold and the direction of the magnetic field, and the situation of the slurry in the cavity of the mold, etc., the degree of density variation and disorder of orientation of the powder compact differ, so it has been difficult to stably produce the required high magnetic properties.

[0009] The present disclosure provides a method for manufacturing a new rare earth sintered magnet and a wet forming apparatus capable of solving the above problems.

Means for Solving the Problems

[0010] The method for manufacturing a rare earth sintered magnet according to the present disclosure, in a non-limiting embodiment, is a method for manufacturing a rare earth sintered magnet in which a slurry containing an alloy powder containing a rare earth element and a dispersion medium is supplied into a space of a mold, and the formed body obtained by pressing the supplied slurry is sintered. When the slurry is supplied into the space of the mold, no magnetic field is applied. On the other hand, when the dispersion medium is discharged from the space of the mold, the application of a transverse magnetic field in a direction perpendicular to the pressing direction is started before the discharge.

[0011] In one embodiment, the dimensions of the formed body are 90 mm or more in length, 90 mm or more in width, and 90 mm or more in height.

[0012] In one embodiment, it includes a first dividing step of dividing the formed body into 10 or more formed body pieces by cutting the formed body, and a sintered body material manufacturing step of manufacturing a plurality of sintered body materials by sintering each of the plurality of formed body pieces after the first dividing step.

[0013] In one embodiment, after the sintered body material manufacturing step, it includes a second dividing step of dividing each of the plurality of sintered body materials into 100 or more sintered body pieces by cutting.

[0014] In one embodiment, a gap is formed between the device for pressing the slurry and the upper surface of the slurry before starting the application of the transverse magnetic field.

[0015] In a method for manufacturing a rare earth sintered magnet according to the present disclosure, in a non-limiting embodiment, a mold having a through-hole, a lower punch that moves vertically relative to the mold with at least the tip inserted into the through-hole, and an upper punch that moves vertically relative to the lower punch are provided. The upper punch has a lower end with a plurality of discharge holes through which a liquid passes. A cavity is formed inside the through-hole between the upper end of the lower punch and the lower end of the upper punch, and a wet forming apparatus capable of reducing the volume of the cavity by reducing the distance between the upper end of the lower punch and the lower end of the upper punch is prepared. A step of preparing a slurry containing an alloy powder containing a rare earth element and a dispersion medium, a step of forming a space by the inner wall of the through-hole in the wet forming apparatus and the upper end of the lower punch, injecting the slurry into the space, and filling the space with the slurry, a step of forming the cavity filled with the slurry by closing the space with the lower end of the upper punch, a step of reducing the distance between the lower end of the upper punch and the upper end of the lower punch while applying a transverse magnetic field in a direction perpendicular to the direction in which the lower punch moves vertically to the cavity, discharging the dispersion medium contained in the slurry through the plurality of discharge holes in the upper punch, and producing a molded body of the alloy powder, and a step of sintering the molded body. When injecting the slurry into the space, no magnetic field is applied to the space, and the space is temporarily or intermittently covered with a non-magnetic lid, and before applying the transverse magnetic field to the cavity, the non-magnetic lid is moved from the position where it covered the space.

[0016] In an embodiment, in the step of producing a molded body of the alloy powder while reducing the distance between the lower end of the upper punch and the upper end of the lower punch, a filter cloth or a filter is disposed between the slurry in the cavity and the lower end of the upper punch.

[0017] In one embodiment, after filling the space with the slurry, the non-magnetic lid is moved from the position covering the space, and before at least starting the application of the transverse magnetic field, the lower punch is lowered relative to the mold to form a gap between at least one of the lower end of the upper punch and the filter cloth and the slurry.

[0018] In one embodiment, the size of the gap is 2 mm or more and 4 mm or less. In one embodiment, after filling the space with the slurry, the non-magnetic lid is moved from the position covering the space, and before starting to discharge the dispersion medium contained in the slurry through the plurality of discharge holes in the upper punch, the application of the transverse magnetic field is started.

[0019] In one embodiment, when injecting the slurry into the space, the step of temporarily communicating the inside of the space with the outside by moving the non-magnetic lid up and down is included.

[0020] In one embodiment, the concentration of the alloy powder in the slurry at the time of injection is 75 to 88% by mass.

[0021] The wet forming apparatus of the present disclosure is a wet forming apparatus for producing a molded body of a rare earth-based alloy powder, and includes a mold having a through hole, a lower punch that moves up and down relative to the mold with at least the tip inserted into the through hole, and an upper punch that moves up and down relative to the lower punch, the upper punch having a lower end with a plurality of discharge holes through which liquid passes, and an electromagnetic coil that applies a transverse magnetic field in a direction perpendicular to the direction in which the lower punch moves up and down inside the through hole of the mold. The mold has an injection port for injecting a slurry containing the rare earth-based alloy powder into the space formed by the inner wall of the through hole and the upper end of the lower punch. The wet forming apparatus further includes a non-magnetic lid that temporarily or intermittently covers the space when injecting the slurry into the space.

[0022] In one embodiment, the wet forming device includes a control device that controls the operations of the upper punch, lower punch, mold, electromagnetic coil, and non-magnetic lid. The control device forms the space by the inner wall of the through hole and the upper end of the lower punch in the wet forming device, injects the slurry into the space, and fills the space with the slurry. Then, by closing the space with the lower end of the upper punch, a cavity filled with the slurry is formed. While applying the transverse magnetic field in a direction perpendicular to the direction in which the lower punch moves relatively up and down to the cavity, the distance between the lower end of the upper punch and the upper end of the lower punch is reduced, and the dispersion medium contained in the slurry is discharged through the plurality of discharge holes in the upper punch to produce a molded body of the rare earth-based alloy powder. Further, when injecting the slurry into the space, no magnetic field is applied to the space, and the space is temporarily or intermittently covered with the non-magnetic lid. Before applying the transverse magnetic field to the cavity, the non-magnetic lid is moved from the position where it covered the space.

Advantages of the Invention

[0023] According to the embodiment of the present disclosure, the slurry can be uniformly supplied into the space of the mold while suppressing the concentration variation. Thereby, the density variation and the disorder of the orientation of the powder molded body can be suppressed, and the occurrence of cracks and fissures caused thereby can be suppressed. Therefore, it becomes possible to stably produce the required high magnetic properties.

Brief Description of the Drawings

[0024]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

MODE FOR CARRYING OUT THE INVENTION

[0025] As a result of investigations, the present inventors have found that when supplying a slurry into the space of a mold, no magnetic field is applied, and by applying the transverse magnetic field forming method described later after the supply of the slurry, the slurry can be uniformly supplied into the space of the mold while suppressing the concentration variation.

[0026] Before explaining the embodiments of the present disclosure, the findings found by the present inventor and the technical background thereof will be explained.

[0027] Methods for producing a powder compact for a rare earth sintered magnet include a dry molding method in which a powder of a rare earth alloy is press-molded in a dry state, and a wet molding method in which a slurry in which alloy powder is dispersed in a dispersion medium such as oil is supplied into a cavity of a mold and press-molded. Further, press molding in a magnetic field is divided into a transverse magnetic field forming method in which the direction of compression by pressing (pressing direction) is orthogonal to the direction of the magnetic field applied to the alloy powder, and a longitudinal magnetic field forming method in which the pressing direction and the direction of the magnetic field applied to the alloy powder are parallel.

[0028] The dry forming method has a relatively simple structure of the press device (forming device), and processes such as the removal of the dispersion medium during press forming (removal of the dispersion medium) and the removal of the dispersion medium from the formed body after pressing are unnecessary. In particular, according to the transverse magnetic field forming method, since the press direction and the magnetic field application direction are orthogonal, it is possible to produce a formed body with a high degree of orientation without disturbing the orientation of the alloy powder oriented in the magnetic field application direction. In the longitudinal magnetic field forming method, since the press direction and the magnetic field application method are parallel, the orientation of the alloy powder is likely to be disturbed during press forming, and the degree of orientation is lower than that of the transverse magnetic field forming method. Therefore, in the dry forming method, the transverse magnetic field forming method is mainly used, and shapes such as disc-shaped, ring-shaped, and thin plate-shaped that are difficult to form by the transverse magnetic field forming method are mainly manufactured by the longitudinal magnetic field forming method.

[0029] However, in the dry forming method, when supplying the alloy powder to the cavity and during press forming, it is inevitable that the alloy powder comes into contact with the atmosphere. Also, when taking out the formed body after the press forming is completed, the formed body comes into contact with the atmosphere. For this reason, the oxygen amount of the formed body increases, leading to a decrease in magnetic properties. In addition, since it is difficult to avoid large friction between the alloy powders or between the alloy powder and the mold, the resistance when the alloy powder rotates and orientates due to the applied magnetic field increases, and there is also a limit to increasing the degree of orientation.

[0030] On the other hand, in the wet forming method, since it is necessary to supply the slurry and remove the dispersion medium, the structure of the forming device becomes relatively complex. However, the oxidation of the alloy powder and the formed body is suppressed by the dispersion medium, and the oxygen amount of the formed body can be reduced. Also, since the dispersion medium is interposed between the alloy powders during press forming in a magnetic field, the restraint due to frictional force etc. is weak, so the alloy powder can rotate more easily in the magnetic field application direction. For this reason, there is an advantage that a rare earth sintered magnet having superior magnetic properties than the dry forming method can be produced. Thus, when using the wet forming method, a higher degree of orientation and an excellent oxidation suppression effect can be obtained compared to the dry forming method, and the resulting rare earth sintered magnet tends to have higher magnetic properties.

[0031] However, such a wet forming method also has problems. In the wet forming method, when a slurry is placed in a cavity and press forming is performed in a magnetic field, it is necessary to discharge most of the dispersion medium (such as oil) in the slurry outside the cavity. For this reason, a discharge hole for the dispersion medium is provided in at least one of the upper punch or the lower punch. Then, when the volume of the cavity decreases due to the movement of the upper punch and / or the lower punch, the dispersion medium contained in the pressurized slurry is discharged from this discharge hole. At this time, since the dispersion medium in the slurry is filtered and discharged from the portion close to the discharge hole, in the initial stage of press forming, a layer called a "cake layer" with a high density of alloy powder is formed in the portion close to the discharge hole.

[0032] As the upper punch and / or the lower punch move and the press forming progresses, more dispersion medium is filtered and discharged, and the area of the cake layer in the cavity expands. Finally, the entire area within the cavity layer becomes a cake layer with a high density of alloy powder (low dispersion medium concentration), and a formed body in which the alloy powders are relatively weakly bonded is obtained.

[0033] In the initial stage of press forming, when a cake layer is formed in a portion near the discharge hole, in the transverse magnetic field forming method, the direction of the magnetic field tends to bend. This is because the cake layer has a high density of alloy powder (a large amount of alloy powder per unit volume), so the magnetic permeability is higher compared to the portion other than the cake layer of the slurry (the portion with a small amount of alloy powder per unit volume). Therefore, the magnetic field tends to converge on the cake layer. For this reason, even if the magnetic field is applied substantially perpendicular to the cavity side surface outside the cavity, inside the cavity, the magnetic field will be bent towards the cake layer. Therefore, since the alloy powder is oriented along this bent magnetic field, there may be a portion where the orientation is bent in the formed body after press forming. If there is a portion where the orientation is bent, the degree of orientation in the formed body decreases, so sufficient magnetic properties may not be obtained in the rare earth sintered magnet. The problem that the magnetic properties of the rare earth sintered magnet deteriorate due to such bending of the magnetic field becomes more prominent as the dimension of the cavity in the magnetic field application direction is larger (for example, when it exceeds 15 mm or typically exceeds 30 mm). Furthermore, when the dimension of the cavity in the press direction becomes 90 mm or more, the magnetic field bends greatly, resulting in a significant deterioration of the magnetic properties of the rare earth sintered magnet and many cracks occurring after sintering. Therefore, a formed body having such a large size could not be mass-produced by the wet forming method. Therefore, in order to make the formed body longer in the press direction, it is necessary to solve the above problems. And, it has been found by the study of the inventors that such problems are particularly likely to occur when the slurry concentration inside the cavity varies or the slurry is not supplied uniformly inside the cavity.

[0034] On the other hand, in the longitudinal magnetic field forming method, since the magnetic field is applied in a direction parallel to the press direction, that is, in a direction parallel to the direction from the upper punch to the lower punch, even if a cake layer is formed in a portion near the dispersion medium discharge port of the upper punch and / or the lower punch, the magnetic field is hardly bent and easily travels straight from the portion without the cake layer into the cake layer. For this reason, there is no restriction on the dimension of the cavity in the magnetic field application direction as in the transverse magnetic field forming method. However, in the longitudinal magnetic field forming method, during press forming, the particles of the alloy powder rotate and the orientation is likely to be disturbed, and a high residual magnetic flux density Br It is difficult to achieve uniformly.

[0035] Hitherto, compacts with large dimensions in the magnetic field application direction have mainly been manufactured by the transverse magnetic field forming method using a dry forming method. However, in the dry forming method, the oxygen content of the compact increases, leading to a deterioration of magnetic properties and there is a limit to increasing the degree of orientation.

[0036] According to the method for manufacturing a rare earth sintered magnet and the wet forming apparatus of the present disclosure, it becomes possible to solve the above problems of the wet forming method that occur when performing transverse magnetic field forming. Therefore, it is possible to stably manufacture a rare earth sintered magnet with a pressing direction of 90 mm or more by transverse magnetic field forming. Specifically, a rare earth sintered magnet having a compact size of 90 mm or more in length × 90 mm or more in width × 90 mm or more in height (either the length or the width is the magnetic field application direction, and the height is the pressing direction), preferably, a rare earth sintered magnet having a length of 100 mm × a width of 100 mm × a height of 90 mm or more. Here, "having a size of 90 mm or more in length × 90 mm or more in width × 90 mm or more" means that the size in the longitudinal direction is 90 mm or more, the size in the transverse direction is 90 mm or more, and the size in the height direction is 90 mm or more. The same applies to "a length of 100 mm × a width of 100 mm × a height of 90 mm or more". The shape of the compact is preferably a rectangular parallelepiped. If it is a rectangular parallelepiped, it is easy to divide it into a plurality of compact pieces. However, the compact may have other shapes.

[0037] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0038] <Basic configuration example> First, with reference to FIGS. 1 and 2, a basic configuration example of the wet forming apparatus in the embodiment of the present disclosure will be described. FIG. 1 is a diagram showing a basic configuration example of the wet forming apparatus 100 in the present embodiment. FIG. 2 is a perspective view schematically showing a configuration example of the mold 10 provided in the wet forming apparatus 100. In the figure, for reference, the X-axis, Y-axis, and Z-axis orthogonal to each other are shown. The Z-axis is parallel to the vertical direction, and the Y-axis is perpendicular to the plane of the drawing. The XY plane including the X-axis and Y-axis is horizontal.

[0039] The wet forming apparatus 100 in the present embodiment includes a mold 10 having a through-hole 10H as illustrated in, for example, FIG. 2(a). The mold 10 is formed of a magnetic material that allows magnetic flux to pass through. The through-hole 10H penetrates in the Z-axis direction from the upper end to the lower end of the mold 10. The through-hole 10H has an inner wall 10W. A cross-section of the through-hole 10H perpendicular to the Z-axis has a constant shape and size along the Z-axis direction. In this example, the through-hole 10H has a rectangular parallelepiped shape, but the shape of the through-hole 10H is not limited to this example. The inner wall 10W of the through-hole 10H is not limited to a flat surface and may include a curved surface partially or entirely.

[0040] The shape and dimensions of the molded body to be produced depend on the shape and dimensions of the through-hole 10H. For example, when the dimensions of the molded body are 100 mm in length × 100 mm in width × 90 mm in height, the dimensions of a cross-section of the through-hole 10H parallel to the XY plane can be 100 mm or less in length × 100 mm or less in width. When producing a larger molded body, for example, when the dimensions of the molded body are 150 mm in length × 150 mm in width × 100 mm or more in height, the dimensions of a cross-section of the through-hole 10H parallel to the XY plane can be 150 mm or less in length × 150 mm or less in width.

[0041] Referring to FIG. 1 again, the wet forming apparatus 100 includes a lower punch 12 that moves up and down relative to the mold 10 with at least the tip inserted into the through-hole 10H, and an upper punch 14 that moves up and down relative to the lower punch 12. The upper punch 14 in the present embodiment has a lower end 14U with a plurality of discharge holes 14H through which the liquid (liquid component) contained in the slurry passes. The slurry includes, for example, an alloy powder (R-T-B-based alloy powder) containing rare earth elements, iron, and boron, and a dispersion medium.

[0042] In the present disclosure, "vertical movement" means moving in the vertical direction. Further, "A moves vertically relative to B" means that the distance between A and B in the vertical direction increases or decreases. Therefore, the form in which the lower punch 12 moves vertically relative to the mold 10 includes the case where the lower punch 12 moves vertically with the mold 10 stationary, the case where the mold 10 moves vertically with the lower punch 12 stationary, and the case where the mold 10 and the lower punch 12 move vertically in the same or opposite directions. In the state of FIG. 1(b), compared with the state of FIG. 1(a), the mold 10 and the upper punch 14 have descended while the lower punch 12 remains stationary. As a result, the lower punch 12 has risen relative to the mold 10.

[0043] In the state shown in FIG. 1(a), a space 16 is formed by the inner wall 10W of the through hole 10H of the mold 10 and the upper end 12T of the lower punch 12. The space 16 has a volume for receiving the slurry. Above this space 16, the upper punch 14 is located, but the upper surface of the space 16 is open. In other words, a part of the lower punch 12 is inserted into the lower part of the through hole 10H of the mold 10, but the space 16 is not blocked by the upper punch 14. FIG. 2(b) schematically shows a state in which the space 16 is formed by the inner wall 10W of the through hole 10H of the mold 10 and the upper end 12T of the lower punch 12. The lower punch 12 inserted into the through hole 10H of the mold 10 and the inner wall 10W of the through hole 10H are in slidable contact. The inner wall 10W and the lower punch 12 are in contact so that the space 16 can hold the liquid component of the slurry without leakage.

[0044] Refer to Fig. 1(b). In the state of Fig. 1(b), the lower end 14U of the upper punch 14 has descended so as to press the mold 10 downward. As a result, the space 16 is blocked by the upper punch 14, and a cavity is formed. In the example of Fig. 1(b), a "filter cloth" 32 is disposed between the upper punch 14 and the mold 10. The filter cloth 32 is a cloth-like filtering material made by knitting synthetic fibers or the like, and may also be called a filter. Examples of filters include filter cloth, filter paper, porous filters, and metal filters. By providing such a filter, it is possible to more reliably prevent the particles of the alloy powder from entering the discharge holes 14H and to allow only the dispersion medium to pass through. The size of the small holes present in the filter cloth 32 is determined so as to hardly allow the particles of the rare earth-based alloy powder to pass through. Specifically, the filter cloth 32 is attached to the upper punch 14 so as to cover a plurality of discharge holes 14H provided at the lower end 14U of the upper punch 14. In the figure, only a part of the filter cloth is shown for simplicity, but the filter cloth extends long along the X-axis direction and can be used by being wound around a roller. By rotating such a roller, it is possible to switch the portion that contacts the lower end 14U of the upper punch 14 of the filter cloth 32. As a result, it becomes easy to switch the contaminated area in the pressing process of the filter cloth to a new area and execute the next pressing process.

[0045] In the example of Fig. 1(b), compared with the state of Fig. 1(a), not only the upper punch 14 but also the mold 10 has descended. By shortening the distance between the upper end 12T of the lower punch 12 and the lower end 14U of the upper punch 14, the volume of the cavity 10C can be reduced. After filling the slurry into the space 16 in Fig. 1(a) and changing to the state of Fig. 1(b), the liquid component in the slurry is discharged from the inside to the outside of the cavity 10C through the filter cloth 32 and the discharge holes 14H of the upper punch 14.

[0046] As shown in FIG. 1, the mold 10 has an injection port 10P for injecting slurry into the space 16 formed by the inner wall 10W of the through-hole 10H and the upper end 12T of the lower punch 12. The injection port 10P does not necessarily have to be one, and may be plural. Also, the number of through-holes 10H in one mold 10 is not limited to one, and may be plural. When one mold 10 has a plurality of through-holes 10H, the wet forming apparatus 100 includes a plurality of sets of lower punches 12 assigned to the respective through-holes 10H. The injection port 10P is connected to a slurry supply device (a hydraulic device having a hydraulic cylinder), and the slurry 30 pressurized by a hydraulic cylinder or the like is supplied into the space 16 through the injection port 10P.

[0047] The wet forming apparatus 100 includes an electromagnetic coil 20 that applies a transverse magnetic field in a direction (horizontal lateral direction) perpendicular to the direction (Z-axis direction, i.e., vertical direction) in which the lower punch 12 moves up and down inside the through-hole 10H of the mold 10. In the example of FIG. 1, the electromagnetic coil 20 can form a transverse magnetic field in which magnetic flux extends in the X-axis direction inside the cavity 10C. As will be described later, in the present embodiment, when injecting slurry into the space 16 from the injection port 10P, as shown in FIG. 1(a), the upper punch 14 is at a position separated from the mold 10 and no magnetic field is applied.

[0048] The wet forming apparatus 100 in the present embodiment further includes a "non-magnetic lid" not shown in FIG. 1. This non-magnetic lid temporarily or intermittently covers the space 16 when slurry is being injected into the space 16. FIG. 3 is a perspective view schematically showing an example of the non-magnetic lid 34. In the example of FIG. 3, the non-magnetic lid 34 completely covers the through-hole 10H of the mold 10. The broken line in FIG. 3 schematically shows the state in which the non-magnetic lid 34 is in the retracted position. The role of the non-magnetic lid 34 will be described later.

[0049] Note that, in order to implement the method for manufacturing a rare earth-based sintered magnet according to the present disclosure, the "non-magnetic lid" is not necessarily an essential element.

[0050] The wet forming apparatus according to an embodiment of the present disclosure includes a control device that controls the operations of an upper punch 14, a lower punch 12, a mold 10, an electromagnetic coil 20, and a non-magnetic lid 34. Such a control device can be realized by a computer that operates according to a program stored in a storage device.

[0051] <Manufacturing method> Hereinafter, with reference to FIGS. 4 and 5, a method for manufacturing a rare earth sintered magnet according to an embodiment of the present disclosure will be described. FIG. 4 is an explanatory diagram for explaining the method for manufacturing a rare earth sintered magnet in the present embodiment. FIG. 5 is an explanatory diagram for explaining the method for manufacturing a rare earth sintered magnet in the present embodiment. In FIG. 4, the description of the electromagnetic coil 20 is omitted.

[0052] In the method for manufacturing a rare earth sintered magnet according to the present embodiment, the following steps are executed.

[0053] (1): Preparation of slurry For example, a step of preparing a slurry containing an alloy powder containing a rare earth element (preferably an alloy powder containing a rare earth element, iron, and boron) and a dispersion medium is executed.

[0054] · Composition of alloy powder The composition of the alloy powder may have, for example, the composition of a known rare earth sintered magnet including an R-T-B sintered magnet (R is at least one of rare earth elements (a concept including yttrium (Y)), T is iron (Fe) or iron and cobalt (Co), and B means boron) and a samarium-cobalt sintered magnet.

[0055] Preferably, it is an R-T-B sintered magnet. This is because it exhibits the highest magnetic energy product among various magnets and is relatively inexpensive.

[0056] The following shows the composition of a preferred R-T-B sintered magnet.

[0057] R is selected from at least one of Nd, Pr, Dy, and Tb. However, R preferably contains either Nd or Pr. More preferably, a combination of rare earth elements represented by Nd-Dy, Nd-Tb, Nd-Pr-Dy, or Nd-Pr-Tb is used.

[0058] Among R, Dy and Tb are particularly effective in improving H cJ . In addition to the above elements, a small amount of other rare earth elements such as Ce or La may be contained, and mischmetal or didymium can also be used. Also, R may not be a pure element and may contain manufacturing-inseparable impurities within the range available industrially. The content can adopt conventionally known contents. For example, a range of 25% by mass or more and 35% by mass or less is a preferable range. If it is less than 25% by mass, high magnetic properties, particularly high H cJ may not be obtained. If it exceeds 35% by mass, B r may decrease.

[0059] T contains iron (including the case where T consists substantially of iron), and up to 50% of it by mass ratio 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.

[0060] The content of B can also be a known content without any problem. For example, a range of 0.8% by mass to 1.2% by mass is a preferable range. If it is less than 0.8% by mass, high H cJ may not be obtained. If it exceeds 1.2% by mass, B r may decrease. Note that a part of B can be replaced with C (carbon). Substitution with C may be able to improve the corrosion resistance of the magnet. When considering B + C (including both B and C), the total content is preferably set within the above B concentration range after converting the number of substitution atoms of C into the number of atoms of B.

[0061] In addition to the above elements, H cJThe M element can be added for upward direction. 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 mass% or less. This is because B may decrease when it exceeds 5.0 mass%. Also, inevitable impurities can be tolerated. r This is because B may decrease when it exceeds 5.0 mass%. Also, inevitable impurities can be tolerated.

[0062] · Method for manufacturing alloy powder The alloy powder can be produced, for example, by the melting method to produce an ingot or flake of a rare earth magnet raw material alloy (preferably an R-T-B magnet raw material alloy) having a desired composition, absorbing (occluding) hydrogen into this alloy ingot and flake, and performing hydrogen pulverization to produce a coarsely pulverized powder.

[0063] And the coarsely pulverized powder can be further pulverized by a jet mill or the like to produce a fine powder (alloy powder).

[0064] An example of the method for manufacturing an R-T-B magnet raw material alloy will be described.

[0065] The alloy ingot can be produced by the ingot casting method in which the metal preliminarily adjusted to have the finally required composition is melted and put into a mold.

[0066] Also, the molten metal is brought into contact with a single roll, double roll, rotating disk, or rotating cylindrical mold, etc., and rapidly cooled, and the alloy flakes can be produced by a rapid cooling method typified by the strip casting method or centrifugal casting method for producing a solidified alloy thinner than the alloy produced by the ingot method.

[0067] In the present invention, alloys produced by either the ingot method or the rapid cooling method can be used, but alloys produced by the rapid cooling method are preferred.

[0068] The thickness of the raw material alloy (rapidly quenched alloy) for R-T-B-based magnets produced by the rapid quenching method is usually in the range of 0.03 mm to 10 mm, and it has a flake shape or a plate shape. The molten alloy 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. Since the rapidly quenched alloy is cooled in a shorter time compared with the alloy (ingot alloy) produced by the conventional ingot casting method (die casting method), its structure is refined and the crystal grain size is small. Also, the area of the grain boundaries is large. Since the R-rich phase spreads widely within the grain boundaries, the rapid quenching method is excellent in the dispersibility of the R-rich phase.

[0069] Therefore, it is easy to break at the grain boundaries by the hydrogen pulverization method. By hydrogen-pulverizing the rapidly quenched alloy, the size of the hydrogen-pulverized powder (coarse pulverized powder) can be made, for example, 1.0 mm or less.

[0070] By pulverizing the thus obtained coarse pulverized powder with a jet mill or the like, for example, R-T-B-based alloy powder with a D 50 particle size of 2 to 7 μm can be produced by the air flow dispersion type laser analysis method.

[0071] The jet mill is preferably operated in (a) an atmosphere consisting of nitrogen gas and / or argon gas (Ar gas) with a substantially 0 mass% oxygen content, or (b) an atmosphere consisting of nitrogen gas and / or Ar gas with an oxygen content of 0.005 to 0.5 mass%.

[0072] In order to control the amount of nitrogen in the obtained sintered body, it is more preferable to use Ar gas as the atmosphere in the jet mill and introduce a small amount of nitrogen gas therein to adjust the concentration of nitrogen gas in the Ar gas.

[0073] ·Dispersion medium The dispersion medium is a liquid capable of producing a slurry by dispersing alloy powder therein.

[0074] Preferred dispersion media used in the present invention include mineral oil or synthetic oil.

[0075] The mineral oil or synthetic oil is not specified by its type, but when the kinematic viscosity at room temperature exceeds 10 cst, the binding force between alloy powders may increase due to the increased viscosity, which may have an adverse effect on the orientation of alloy powders during wet forming in a magnetic field.

[0076] Therefore, the kinematic viscosity of the mineral oil or synthetic oil at room temperature is preferably 10 cst or less. Also, when the distillation point of the mineral oil or synthetic oil exceeds 400 °C, it becomes difficult to remove the oil after obtaining the formed body, and the amount of residual carbon in the sintered body may increase, resulting in a decrease in magnetic properties.

[0077] Therefore, the distillation point of the mineral oil or synthetic oil is preferably 400 °C or less.

[0078] Also, vegetable oil may be used as the dispersion medium. Vegetable oil refers to oil extracted from plants, and the type of plant is not limited to a specific plant. For example, soybean oil, rapeseed oil, corn oil, safflower oil, or sunflower oil can be mentioned.

[0079] · Preparation of slurry A slurry can be prepared by mixing the obtained alloy powder and the dispersion medium.

[0080] 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 mass% or more) by mass ratio. This is because the alloy powder can be efficiently supplied into the space at a flow rate of 20 - 600 cm 3 / s, and excellent magnetic properties can be obtained.

[0081] Also, the concentration of the alloy powder in the slurry is preferably 90% or less by mass ratio. This is to ensure the fluidity of the slurry.

[0082] More preferably, the concentration of the alloy powder in the slurry is 75% to 88% by mass ratio. This is because the alloy powder can be supplied more efficiently and the fluidity of the slurry can be ensured more reliably. Even more preferably, the concentration of the alloy powder in the slurry is 84% or more by mass ratio. The method of mixing the alloy powder and the dispersion medium is not particularly limited. The alloy powder and the dispersion medium may be prepared separately, and both may be weighed in predetermined amounts and mixed. Also, when producing the alloy powder by dry-grinding the coarsely ground powder with a jet mill or the like, a container containing the dispersion medium may be arranged at the alloy powder discharge port of the grinding device such as a jet mill, and the alloy powder obtained by grinding may be directly recovered into the dispersion medium in the container to obtain a slurry. In this case, the inside of the container is preferably an atmosphere composed of nitrogen gas and / or argon gas, and the obtained alloy powder is directly recovered into the dispersion medium without being exposed to the atmosphere to form a slurry. Furthermore, it is also possible to produce a slurry composed of the alloy powder and the dispersion medium by wet-grinding the coarsely ground powder while holding it in the dispersion medium using a vibration mill, a ball mill, an attritor, or the like.

[0083] (2): Pressurized injection of slurry As shown in FIGS. 4(a) and 4(b), from the state where the lower punch 12 is inserted into the through hole 10H of the mold 10 in the wet forming apparatus 100, by raising the mold 10, a space 16 is formed by the inner wall 10W and the upper end 12T of the lower punch 12. As shown in FIG. 4(b), at the stage where the mold 10 has risen sufficiently, the inside of the space 16 communicates with the injection port 10P of the mold 10.

[0084] Next, as shown in FIG. 4(c), the slurry 30 is injected into the inside of the space 16 from the injection port 10P. The supply amount of the slurry 30 can be set, for example, in the range of 20 to 150 cm 3 / second. When the supply amount is less than 20 cm 3 / second, it is difficult to adjust the flow rate, and there may be a case where the slurry cannot be supplied into the space 16 due to piping resistance. On the other hand, when the supply amount is 150 cm 3If it exceeds [X] per second, variations in density occur in each part of the powder compact, and cracks may occur in the compact during removal after press molding or due to shrinkage during sintering. Also, the likelihood of disorder in orientation increases in the vicinity of the injection port 10P.

[0085] The slurry supply rate is preferably 30 - 100 cm 3 / second, and more preferably 40 - 80 cm 3 / second. The slurry supply rate can be controlled by adjusting the flow control valve of the hydraulic device that functions as the slurry supply device, changing the flow rate of the oil sent to the hydraulic cylinder of the hydraulic device, and thus changing the speed of the hydraulic cylinder. The supply pressure of the slurry 30 is, for example, 1.96 MPa - 14.71 MPa (20 kgf / cm 2 ~150 kgf / cm 2 ). The injection port 10P of the slurry 30 is, for example, a hole with a diameter of 2 mm - 30 mm.

[0086] One of the characteristic points of this embodiment is that when injecting the slurry 30 into the space 16, the space 16 is temporarily or intermittently covered with the non-magnetic lid 34.

[0087] By using the non-magnetic lid 34, the slurry 30 can be supplied uniformly to the space 16 while suppressing density variations. Thereby, it becomes possible to suppress disorder in orientation in the powder compact produced by compression in the subsequent orientation magnetic field. This will be described in detail below.

[0088] Typically, when injecting the slurry 30 into the space 16 without using a non-magnetic cover, the space 16 is covered by the upper punch 14. When filling the space 16 with the slurry 30 in such a normal method, at least a part of the slurry 30 on the upper surface of the space 16 may come into contact with the plurality of discharge holes 14H of the upper punch 14 or the filter cloth 32 disposed between the upper punch 14 and the mold 10, and the dispersion medium contained in the slurry 30 may be absorbed. As a result, the concentration of the slurry at a position close to the upper punch 14 in the space 16 may increase and concentration variations may occur. Further, even when a magnetic field is applied after the slurry injection, the powder particles in the slurry 30 on the upper surface of the space 16 are less likely to be oriented. Also, when the space 16 is not covered with an upper punch or the like, a part of the slurry 30 may jump out of the space 16 due to the injection of the slurry 30, unevenness may be formed on the upper surface of the slurry, concentration variations may occur, or the slurry may not be uniformly supplied to the cavity. In contrast, by covering the space 16 with the non-magnetic cover 34, even if at least a part of the slurry 30 comes into contact with the non-magnetic cover 34 when filling the space 16 with the slurry 30, the dispersion medium contained in the slurry is not absorbed by the non-magnetic cover 34. Further, since the space 16 is covered by the non-magnetic cover 34, a part of the slurry 30 does not jump out of the space 16 due to the injection of the slurry, and unevenness is not formed on the upper surface of the slurry. Therefore, it becomes possible to supply the slurry 30 to the space 16 uniformly while suppressing concentration variations. Thereby, it becomes possible to suppress density variations and orientation disorder in the powder compact.

[0089] The non-magnetic lid 34 is formed of, for example, rubber or resin. The non-magnetic lid 34 formed of rubber can adhere to the upper end of the mold 10. The non-magnetic lid 34 can also be formed of, for example, silicon, non-magnetic aluminum, stainless steel, etc. other than rubber. Further, the non-magnetic lid 34 does not have a through-hole through which the slurry 30 passes. This is because the dispersion medium contained in the slurry 30 may be absorbed by the through-hole, resulting in concentration variations. Furthermore, if the non-magnetic lid 34 is not non-magnetic, there is a possibility that the lid will become magnetic in the transverse magnetic field pressing process or the like and the slurry 30 will adhere to the lid. As a result, it may not be possible to supply the slurry 30 to the space 16 while suppressing concentration variations.

[0090] As described above, the non-magnetic lid 34 is not necessarily an essential element for implementing the method for manufacturing a rare earth-based sintered magnet according to the present disclosure. After injecting the slurry 30 into the space 16 without using the non-magnetic lid 34, for example, by stirring the slurry 30 using a rod-shaped member, it is possible to reduce the concentration variations of the slurry 30 and enhance the uniformity. Also, the slurry 30 is particularly difficult to enter the four corners of the space 16. Therefore, for example, by injecting the slurry 30 until it overflows from the space 16, it is also possible to reduce the concentration variations of the slurry 30.

[0091] FIG. 4(d) schematically shows a state in which the non-magnetic lid 34 is slightly lifted from the mold 10 so that the space 16 communicates with the atmosphere, and a gap is formed between the two. Through this gap, the atmospheric components contained inside the space 16 are pushed out to the outside as the slurry 30 increases. By intermittently forming a gap between the non-magnetic lid 34 and the mold 10, the pressure inside the space 16 can be maintained at approximately atmospheric pressure, enabling the slurry 30 to be supplied smoothly.

[0092] FIG. 4(e) shows the state in which the space 16 is filled with the slurry 30. At this time, the space 16 is closed by the non-magnetic lid 34, and the filling amount of the slurry 30 has reached a predetermined value. If the slurry 30 is supplied to the space 16 without the non-magnetic lid 34, as described above, unevenness may be formed on the upper surface of the slurry 30 during filling. The non-magnetic lid 34 enables the inside of the space 16 having a desired volume to be filled with the slurry 30 up to the brim.

[0093] The timing for closing the space 16 with the non-magnetic lid 34 is, for example, when about half of the space 16 is filled with the slurry 30. After that, as the amount of the slurry 30 supplied to the space 16 increases, the internal pressure of the space 16 rises. Therefore, the non-magnetic lid 34 is lifted once or a plurality of times, and the internal pressure is reduced to a level equal to the atmospheric pressure. Such an operation can be realized, for example, by attaching the upper surface of the non-magnetic lid 34 to a cylinder and driving this cylinder mechanically or electrically in the vertical direction.

[0094] When the inside of the space 16 is filled with a predetermined amount of the slurry 30, the space 16 is closed by the non-magnetic lid 34. At this time, it is desirable that the slurry 30 is in contact with the lower surface of the non-magnetic lid 34, but a slight gap (less than 1 mm) may partially exist.

[0095] Another characteristic point of this embodiment is that when injecting the slurry 30 into the space 16, no magnetic field is applied to the space 16 (non-magnetic field injection). If the slurry is injected while a magnetic field is applied (injection in a magnetic field), variations may occur in the density of each part of the powder compact obtained after pressing. This is presumably because when the slurry 30 is being injected, the alloy powder in the slurry is attracted to the mold 10 or the lower punch 12, causing the solid alloy powder and the liquid dispersion medium to separate (solid-liquid separation), and the separated dispersion medium to gather around the space 16. If the slurry 30 is supplied in such a state and the inside of the space 16 is filled with the slurry 30 and then press-molded, the density of the alloy powder (the amount of alloy powder present per unit volume) will be press-molded in a state where it is lower around the cavity 10C than at the center and bottom of the cavity 10C. As a result, the density of the upper part and the periphery may be lower than that of the center part and the bottom of the obtained compact. If the density varies in each part of the compact, it will cause a decrease in the magnetic properties of the sintered magnet obtained by sintering the compact and variations depending on the location. Also, with such density variations, cracks may occur in the compact when removing the compact after press-forming. Even if there are no cracks in the compact, cracks may occur due to shrinkage during sintering. In this embodiment, since no magnetic field is applied during slurry supply, it becomes possible to solve such problems of density variations. Conventionally, in order to obtain high magnetic properties, it has been considered necessary to inject the slurry while a magnetic field is applied. This is because non-magnetic field injection makes it particularly difficult for the central part of the magnet to be oriented compared to injection in a magnetic field. However, as a result of research, the inventors have found that by using the method of using the non-magnetic lid described above, when the slurry 30 is supplied to the space 16 while suppressing density variations and then the transverse magnetic field forming method is applied, the central part of the magnet is also uniformly oriented and the magnetic properties do not deteriorate. On the other hand, when the longitudinal magnetic field forming method is applied, the magnetic properties deteriorate due to the influence of the sagging of the orientation by press-forming alone.

[0096] Preferably, after filling the space 16 with the slurry, as shown in FIG. 4(f), the lower punch 12 is lowered relative to the mold 10, so that when the upper punch 14 is lowered as shown in FIG. 5(a) to close the space 16, a gap is formed between the lower end of the upper punch 14 or (when using the filter cloth 32) between the filter cloth and the slurry 30. Specifically, the position of the lower punch 12 is lowered relative to the mold 10 by a distance of 1 mm or more and 30 mm or less (for example, 3 mm). As a result, after the filling of the slurry 30 is completed, the space 16 expands, and an air layer gap is formed in the upper part of the space 16. The size of the gap is preferably 2 mm or more and 4 mm or less, and can be, for example, about 3 mm. In the example of FIG. 4(f), the mold 10 is raised relative to the lower punch 12, but the method of forming such an air layer gap on the upper surface of the slurry 30 is not limited to this example. For example, the lower punch 12 may be lowered with the position of the mold 10 fixed. Further, an "inlay structure" having a size and shape that fits into the through hole 10H of the mold 10 may be formed on the lower surface of the non-magnetic lid 34. Specifically, the non-magnetic lid 34 is retracted from the position covering the mold 10 before or after forming the air layer gap (FIG. 3). That is, before applying the transverse magnetic field to the cavity 10C, the non-magnetic lid 34 is moved from the position covering the space 16.

[0097] Note that in the state of FIG. 4(f), the upper punch 14 is at a position away from the mold 10, but the lowering of the upper punch 14 may start with the start of the raising of the mold 10. The important point is to ensure that the filter cloth 32 provided at the lower end of the upper punch 14 does not contact the slurry 30 even when the upper punch 14 is lowered. If the upper punch 14 and the mold 10 are separated, the filter cloth 32 will not contact the slurry 30 even if the lowering of the upper punch 14 starts at the start of the raising of the mold 10 or immediately before that.

[0098] (3): Preparation before starting press forming in a transverse magnetic field Next, by closing the space 16 with the lower end 14U of the upper punch 14, a cavity 10C filled with the slurry 30 is formed. Specifically, as shown in Fig. 5(a), the upper punch 14 is lowered with respect to the mold 10 to close the space 16. At this time, although the filter cloth 32 is disposed between the mold 10 and the upper punch 14, as described above, it is preferable to form an air layer gap between the filter cloth 32 and the slurry 30 so that the filter cloth 32 does not contact the slurry 30. Thereby, before the application of the magnetic field, the filter cloth 32 contacts the slurry 30 and the dispersion medium contained in the slurry 30 is absorbed by the filter cloth 32, so that the concentration of the alloy powder near the upper surface of the slurry 30 excessively increases and the concentration variation occurs, or even when the magnetic field is applied, the orientation of the powder particles is hardly generated. It can be surely avoided.

[0099] (4): Press forming in a transverse magnetic field Next, with a "transverse magnetic field" in a direction perpendicular to the direction in which the lower punch 12 moves up and down being applied to the cavity 10C, the distance between the lower end 14U of the upper punch 14 and the upper end 12T of the lower punch 12 is reduced. Figs. 5(b) and 5(c) show how the distance between the lower end 14U of the upper punch 14 and the upper end 12T of the lower punch 12 is reduced. The dispersion medium contained in the slurry 30 is discharged through a plurality of discharge holes 14H in the upper punch 14, and a compact 50 of the alloy powder is obtained. The magnitude of the magnetic field formed inside the cavity 10C is, for example, 1.0 T or more and 1.5 T or less. When applying the transverse magnetic field, it is preferable that there is an air layer gap G between the filter cloth 32 and the slurry 30. When the application of the transverse magnetic field is started, a part of the alloy powder particles contained in the slurry 30 moves by the magnetic force, and convex or concave portions may be formed on the upper surface of the slurry 30. However, since the direction of the magnetic field is horizontal and orthogonal to the pressing direction, the orientation directions will be aligned by the pressing process.

[0100] If the magnetic field strength is 1.0 T or more, the magnetization direction of the alloy powder contained in the slurry 30 will more surely be oriented in the direction of the magnetic field, and a high degree of orientation can be obtained. If it is less than 1.0 T, the degree of orientation of the alloy powder will decrease, or the orientation of the alloy powder will be easily disturbed during press molding. The magnetic field strength inside the cavity 10C can be measured with a gaussmeter or determined by magnetic field analysis.

[0101] The electromagnetic coil 20 is arranged near the side surface of the mold 10 and can form a magnetic field perpendicular and uniform to the press direction inside the cavity 10C.

[0102] To explain the state inside the cavity 10C in more detail, during the process of press molding in a magnetic field, when the volume of the cavity 10C becomes smaller, as described above, the dispersion medium in the slurry 30 is filtered and discharged through the discharge hole 14H from the portion near the discharge hole 14H of the upper punch 14, but the alloy powder contained in the slurry 30 remains in the cavity 10C. For this reason, a "cake layer" is formed from the portion near the discharge hole 14H. The cake layer is a layer in which the dispersion medium in the slurry is discharged to the outside of the cavity 10C and the concentration of the alloy powder becomes high, as described above. As the press process progresses, finally the cake layer spreads throughout the cavity 10C, and a powder compact in which the alloy powder particles are in contact with each other is obtained. In the present embodiment, when the application of the transverse magnetic field is started, the cake layer is not formed, and as a result, the disturbance of the orientation at the position near the upper punch 14 can be suppressed.

[0103] After the compact 50 is formed, as shown in Fig. 5(d), the mold 10 is lowered, and as shown in Fig. 5(e), the compact 50 is exposed outside the mold 10. After that, as shown in Fig. 5(f), by raising the upper punch 14, the compact 50 can be taken out.

[0104] In the molded body obtained by the above-described process, a dispersion medium such as mineral oil or synthetic oil remains. When the temperature of the molded body in this state is rapidly increased from room temperature to a sintering temperature of, for example, 950 to 1150 °C, the internal temperature of the molded body rapidly rises, and the dispersion medium remaining in the molded body may react with the rare earth elements in the molded body to form rare earth carbides. When rare earth carbides are formed in this way, the generation of a sufficient amount of liquid phase for sintering is hindered, and a sintered body with sufficient density may not be obtained, resulting in a decrease in magnetic properties. Therefore, it is preferable to perform a dewaxing treatment on the molded body before sintering. This is because the dispersion medium remaining in the molded body can be sufficiently removed.

[0105] (5): Step of dividing into molded body pieces (first dividing step) In the present embodiment, the molded body produced by press molding in a transverse magnetic field may be divided into a plurality of molded body pieces. For example, after the step of producing the above-described molded body and before the step of sintering the molded body, each molded body can be cut, and a first dividing step of dividing it into 10 or more molded body pieces can be performed.

[0106] In the present embodiment, a wire saw can be used to slice, for example, a molded body having dimensions of 100 (longitudinal) × 100 (lateral) × 90 mm (height) into plate-shaped molded body pieces having dimensions of, for example, 9.5 mm (magnetization direction) (longitudinal) × 100 mm (lateral) × 90 mm (height). The number, size, and shape of the molded body pieces are not limited to this example. Further, instead of a wire saw, a known cutting blade may be used for cutting.

[0107] The larger the dimensions of the molded body, the more sintered magnets can be produced from one molded body. In the conventional method, as the molded body becomes larger, the density of the slurry varies, making it difficult to increase its size. According to the embodiment of the present disclosure, the dimensions of the molded body can be 90 mm or more in length, 90 mm or more in width, and 90 mm or more in height (preferably 100 mm or more in length, 100 mm or more in width, 90 mm or more in height, more preferably 120 mm or more in length, 120 mm or more in width, 100 mm or more in height, and most preferably 150 mm or more in length, 150 mm or more in width, 100 mm or more in height).

[0108] (6): Sintering process (sintered body material production process) Next, the formed body (cut formed body piece) is sintered to produce a rare earth sintered magnet body. In the present disclosure, when the sintered body of the formed body piece is further cut, the sintered body of the formed body piece shall be referred to as a "sintered body material". Hereinafter, for simplicity, the formed body piece may sometimes be simply referred to as the "formed body".

[0109] The sintering of the formed body is preferably carried out under a pressure of 0.13 Pa (10 -3 Torr) or less, more preferably 0.07 Pa (5.0×10 -4 Torr) or less, in the temperature range of 1000°C to 1150°C. In order to prevent oxidation during sintering, the residual gas in the atmosphere can be replaced with an inert gas such as helium or argon. The dimensions of the sintered body obtained by sintering the formed body piece can be, for example, a size in the longitudinal direction of 4 mm or more, a size in the transverse direction of 40 mm or more, and a size in the height direction of 5 mm or more.

[0110] (7): Process of dividing into sintered body pieces (second dividing process) In this embodiment, a second dividing process is performed in which each of the sintered body materials obtained by sintering the formed body pieces is cut to divide each sintered body material into a plurality of sintered body pieces. By this second dividing process, 100 or more sintered body pieces can be produced from one sintered body material. The cutting of the sintered body material can be performed, for example, by a dicing saw or the like. According to this embodiment, 1000 (=10×100) or more rare earth sintered magnets can be produced from one large-sized formed body (90 mm or more in length, 90 mm or more in width, 90 mm or more in height), improving mass productivity.

[0111] In addition, in this embodiment, a diffusion step of diffusing a heavy rare earth element RH (RH is at least one of Tb, Dy, and Ho) from the surface to the inside of the sintered body material before cutting may be further performed. When the heavy rare earth element RH is diffused from the surface to the inside of the sintered body, the coercive force can be efficiently increased. Such a diffusion step is particularly effective when the sintered body material has a plate-like shape with a thickness of 1 mm or more and 20 mm or less. By performing diffusion from two opposing surfaces in the thickness direction, it becomes possible to efficiently diffuse the heavy rare earth element RH deep into the inside of the sintered body material. When the diffusion of the heavy rare earth element RH is performed after dividing the sintered body material into sintered body pieces, the amount of the heavy rare earth element RH consumed to obtain the required magnetic properties tends to increase. For this reason, it is desirable to perform the diffusion of the heavy rare earth element RH on the sintered body material before it is divided into sintered body pieces.

[0112] Referring to FIG. 6, summarizing the process flow from the cutting step of the formed body piece to the cutting step of the sintered body material in a certain preferred embodiment, it is as follows. In FIG. 6, the direction of the orientation magnetic field (magnetic field orientation direction) M is indicated by an arrow. Finally, magnetization is performed in a direction parallel to this magnetic field orientation direction M.

[0113] The process flow schematically shown in FIG. 6 is · A step of preparing the formed body 50 (S10); · A step of cutting the formed body 50 and dividing the formed body 50 into a plurality of formed body pieces 52 (S20); · A sintering step of sintering each of the plurality of formed body pieces 52 to produce a plurality of sintered body materials 54 (S30); · A step of bringing the powder 56 of the diffusion source containing the heavy rare earth element RH into contact with at least one of the upper surface 54a and the lower surface 54b in the thickness direction of each sintered body material 54 and performing heat treatment to diffuse at least a part of R contained in the powder of the diffusion source from the upper surface 54a and / or the lower surface 54b of each sintered body material into the inside (S40); · A step of cutting each sintered body material 54 from the upper surface 54a to the lower surface 54b to divide it into a plurality of sintered body pieces 58 (S50); is included.

[0114] After the sintering process, it is preferable to perform heat treatment on the sintered body (including the sintered body material or the sintered body piece) at a temperature lower than the sintering temperature. The magnetic properties can be improved by the heat treatment. As the heat treatment conditions such as the heat treatment temperature and the heat treatment time, known conditions can be adopted. For the rare earth sintered magnet body thus obtained, for example, if necessary, a grinding and polishing process and a surface treatment and coating process are performed, and the final rare earth sintered magnet is completed through a magnetization process.

Examples

[0115] (Example 1) The composition is Nd 22 Pr6Dy3B 0.94 Co2Al 0.25 Cu 0.1 After melting the raw material alloy in a high-frequency melting furnace so that the balance is Fe (mass %), the molten metal of the raw material alloy was rapidly cooled by the strip casting method to obtain a flaky alloy with a thickness of 0.5 mm. The alloy was coarsely pulverized by the hydrogen pulverization method and further finely pulverized by a jet mill. The particle size D of the obtained R-T-B-based alloy powder 50 was 4.7 μm. The R-T-B-based alloy powder was immersed in a mineral oil having a distillation point of 250 °C and a kinematic viscosity of 2 cSt at room temperature in a nitrogen atmosphere to prepare a slurry. The slurry concentration was 85 mass %.

[0116] For wet forming, the wet forming apparatus shown in FIG. 1 was used. As the mold 10, one having a space 16 with dimensions of 100 mm in length and 100 mm in width (magnetic field application direction) was used. The depth of the space 16 was 90 mm. From the slurry supply device, slurry was supplied into the space 16 from the supply port 15 at a slurry concentration of 85 mass % and a slurry supply rate of 50 cm 3 / second. Then, when the inside of the space 16 was filled with the slurry 30 to about half, the space 16 was covered with the non-magnetic lid 34. After that, as the amount of the slurry 30 supplied to the space 16 increased, the non-magnetic lid 34 was lifted a plurality of times by a cylinder (not shown) to maintain the internal pressure at a level equal to the atmospheric pressure. After the space 16 was filled with the slurry, the non-magnetic lid 34 was retracted from the space 16.

[0117] After that, as shown in FIG. 4(f), in order to form a gap between the filter cloth 32 and the slurry 30 when the upper punch 14 is lowered, the position of the lower punch 12 was relatively lowered by 3 mm with respect to the mold 10. Next, as shown in FIG. 5(a), the upper punch 14 was lowered with respect to the mold 10 to close the space 16 and form the cavity 10C. Then, a magnetic field of 1.5 T was applied in the lateral direction (100 mm direction) of the cavity 10C, and the distance between the lower end 14U of the upper punch 14 and the upper end 12T of the lower punch 12 was reduced to perform press forming in a transverse magnetic field.

[0118] When the density was measured at 17 different positions of the molded body produced under this condition, the variation (variance) was 0.04 g / cm 3 and was a sufficiently low value.

[0119] Next, before the step of sintering each molded body, each of these molded bodies was cut by wire processing and divided into 20 molded body pieces.

[0120] The obtained molded body pieces were heated from room temperature to 150 °C at 1.5 °C / min in a vacuum, held at that temperature for 1 hour, then heated from 150 °C to 500 °C at 1.5 °C / min to remove the mineral oil in the molded body pieces, and further heated from 500 °C to 1100 °C at 20 °C / min and held at 1100 °C for 2 hours for sintering. Thus, sintered body materials were obtained from each molded body piece. It was confirmed that there were no cracks in the obtained sintered bodies. After that, a step of dividing the sintered body material into 200 sintered body pieces was performed.

[0121] The obtained sintered body pieces were heat-treated at 900 °C for 1 hour and then further heat-treated at 600 °C for 1 hour to obtain R-T-B system sintered magnets. Machining was performed on the obtained R-T-B system sintered magnets to make them 7×7×7 (mm) in size, and the magnetic properties of 10 of them were measured using a BH tracer. By subtracting the minimum value from the maximum value of the measured B r variation was obtained, and the variation was a sufficiently low value of 0.011 T. r

[0122] ​(Example 2) The composition is Nd 30.1 Pr 0.5 Dy 1.0 B 1.0 Co 1.0 Al 0.1 Cu 0.1 After melting the raw material alloy in a high-frequency melting furnace so that the balance is Fe (mass %), the molten metal of the raw material alloy was rapidly cooled by the strip casting method to obtain a flaky alloy with a thickness of 0.5 mm. The alloy was coarsely pulverized by the hydrogen pulverization method and further finely pulverized by a jet mill. The particle size D of the obtained R-T-B-based alloy powder 50 was 4.7 μm. The R-T-B-based alloy powder was immersed in a mineral oil having a distillation point of 250 °C and a kinematic viscosity of 2 cSt at room temperature in a nitrogen atmosphere to prepare a slurry. The slurry concentration was 85 mass %.

[0123] For wet forming, the wet forming apparatus shown in Fig. 1 was used. As the mold 10, one with a space 16 having dimensions of 90 mm in length and 100 mm in width (magnetic field application direction) was used. The depth of the space 16 was 85 mm. From the slurry supply device, a slurry was supplied into the space 16 from the supply port 15 at a slurry concentration of 85 mass % and a slurry supply rate of 50 cm 3 / sec. Then, when the inside of the space 16 was filled about halfway with the slurry 30, the space 16 was covered with the non-magnetic lid 34. After that, as the amount of the slurry 30 supplied to the space 16 increased, the non-magnetic lid 34 was lifted a plurality of times by a cylinder (not shown) and maintained at a level where the internal pressure was equal to the atmospheric pressure. After the space 16 was filled with the slurry, the non-magnetic lid 34 was retracted from the space 16.

[0124] Thereafter, as shown in Fig. 4(f), in order to form a gap between the filter cloth 32 and the slurry 30, the position of the lower punch 12 was relatively lowered by 3 mm with respect to the mold 10. Next, as shown in Fig. 5(a), the upper punch 14 was lowered with respect to the mold 10 to close the space 16 and form the cavity 10C. Then, a 1.5 T magnetic field was applied in the lateral direction (100 mm direction) of the cavity 10C in the cavity 10C, and the distance between the lower end 14U of the upper punch 14 and the upper end 12T of the lower punch 12 was reduced to perform press forming in a transverse magnetic field (Condition A).

[0125] For comparison, press forming in a longitudinal magnetic field was performed under the same conditions as Condition A except that the direction of magnetic field application was the depth direction (85 mm direction) (Condition B). Further, press forming in a transverse magnetic field was performed under the same conditions except that the non-magnetic lid 34 was not used and the space 16 was covered with the upper punch (Condition C).

[0126] Under these Conditions A, B, and C, 200 formed bodies were produced respectively. The obtained formed bodies were heated in a vacuum from room temperature to 150 °C at a rate of 1.5 °C / min, held at 1100 °C for 1 hour, then heated from 150 °C to 500 °C at a rate of 1.5 °C / min to remove the mineral oil in the formed bodies, and further heated from 500 °C to 1100 °C at a rate of 20 °C / min and held at that temperature for 2 hours for sintering. The obtained sintered bodies were heat-treated at 900 °C for 1 hour and further heat-treated at 600 °C for 1 hour to obtain R-T-B system sintered magnets. Machining was performed on the obtained R-T-B system sintered magnets to make them into dimensions of 7×7×7 (mm), and the magnetic properties were measured using a BH tracer. For each of Conditions A, B, and C, 200 pieces each of B r and H cJ were measured and their average values were obtained. The results are shown in Table 1. Also, the B r variation was obtained by subtracting the minimum value from the maximum value of the measured B r , and the H cJ variation was obtained by subtracting the minimum value from the maximum value of the measured H cJ . These results are also shown in Table 1.

[0127]

Table 1

[0128] As shown in Table 1, in the examples of the present invention, the variations of B r and H cJ are small, and high magnetic properties can be stably produced. Condition B has a significantly lower B r compared to the examples of the present invention (Condition A), and Condition C has larger variations in B r and H cJ compared to the examples of the present invention (Condition B).

Industrial Applicability

[0129] The method for manufacturing a rare earth sintered magnet and the wet forming apparatus of the present disclosure are suitably used for manufacturing a rare earth sintered magnet with a reduced oxygen concentration. Such rare earth sintered magnets can be used in various motors such as voice coil motors (VCM) of hard disk drives, motors for electric vehicles (EV, HV, PHV, etc.), motors for industrial equipment, and home appliances.

Explanation of Signs

[0130] 10... Mold, 10H... Through hole, 10W... Inner wall, 12... Lower punch, 12T... Upper end of the lower punch, 14... Upper punch, 14H... Discharge hole of the upper punch, 14U... Lower end of the upper punch, 20... Electromagnetic coil, 16... Space, 30... Slurry, 100... Wet forming apparatus

Claims

1. A method for producing a rare earth sintered magnet, comprising: supplying a slurry containing an alloy powder containing a rare earth element and a dispersion medium into a space in a die; and press-molding the supplied slurry to obtain a molded body, and sintering the molded body, The space of the die has a size of 90 mm or more in the pressing direction, While the slurry is being supplied into the space of the mold, no magnetic field is applied. When discharging the dispersion medium from the space in the mold, application of a transverse magnetic field in a direction perpendicular to the pressing direction is started before the discharging; Furthermore, a first dividing step of dividing the molded body into a plurality of molded body pieces by cutting the molded body; a sintered body material preparation step of preparing a plurality of sintered body materials by sintering each of the plurality of compact pieces after the first dividing step; The method for producing a rare earth sintered magnet includes the steps of:

2. The mold has an injection port, When the slurry is supplied into the space of the mold, the slurry is injected into the space of the mold through the injection port without applying the magnetic field. A method for producing the rare earth sintered magnet according to claim 1.

3. The dimensions of the molded body are 90 mm or more in length, 90 mm or more in width, and 90 mm or more in height. A method for producing the rare earth sintered magnet according to claim 1.

4. The number of the plurality of molded bodies is 10 or more. A method for producing the rare earth sintered magnet according to any one of claims 1 to 3.

5. a second dividing step of dividing each of the plurality of sintered bodies into 100 or more sintered bodies by cutting the sintered bodies after the sintered body material preparing step; Including, A method for producing the rare earth sintered magnet according to claim 4.

6. Before starting the application of the transverse magnetic field, a gap is formed between an apparatus for pressing the slurry and an upper surface of the slurry. A method for producing a rare earth sintered magnet according to any one of claims 1 to 5.

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