Method of production rare-earth magnet

Abstract

A production method includes producing a rare-earth magnet precursor (S′) by performing first hot working in which, in two side surfaces of a sintered body, which are parallel to a pressing direction and are opposite to each other, one side surface is brought to a constrained state to suppress deformation, and the other side surface is brought to an unconstrained state to permit deformation; and producing a rare-earth magnet by performing second hot working in which, in two side surfaces (S′1, S′2) of the rare-earth magnet precursor (S′), which are parallel to the pressing direction, a side surface (S′2), which is in the unconstrained state in the first hot working, is brought to the constrained state to suppress deformation, and a side surface (S′1), which is in the constrained state in the first hot working, is brought to the unconstrained state to permit deformation.

Claims

1. A method of producing a rare-earth magnet, comprising: accommodating a sintered body, which is obtained by sintering a rare-earth magnet material, in a forming mold which is constituted by upper and lower punches and a die and in which at least one of the upper and lower punches is slidable in a hollow inside of the die, and producing a rare-earth magnet precursor by performing a first hot working in which, in two side surfaces of the sintered body, which are parallel to a pressing direction and are opposite to each other, one side surface is caused to come into contact with an inner surface of the die and is brought to a constrained state to suppress deformation, and the other side surface is not caused to come into contact with the inner surface of the die and is brought to an unconstrained state to permit deformation when upper and lower surfaces of the sintered body are pressed by using the upper and lower punches; and moving the rare-earth magnet precursor in the forming mold, and producing a rare-earth magnet by performing second hot working in which, in two side surfaces of the rare-earth magnet precursor, which are parallel to the pressing direction, a side surface, which is in the unconstrained state in the first hot working, is caused to come into contact with the inner surface of the die and is brought to the constrained state to suppress deformation, and a side surface, which is in the constrained state in the first hot working, is brought to the unconstrained state to permit deformation when upper and lower surfaces of the rare-earth magnet precursor are pressed by using the upper and lower punches.

2. The method according to claim 1, wherein in each of the sintered body and the rare-earth magnet precursor, the side surface, which is brought to the constrained state, is maintained in the constrained state from start to end of pressing.

3. The method according to claim 1, wherein in each of the sintered body and the rare-earth magnet precursor, the side surface, which is to be brought to the constrained state, is not caused to come into contact with the inner surface of the die and is brought to an unconstrained state at an initial stage of pressing, and is caused to come into contact with the inner surface of the die and is brought to the constrained state in a course of the pressing.

4. The method according to claim 1, wherein a shape of the sintered body is a rectangular parallelepiped.

5. The method according to claim 4, wherein in each of the sintered body and the rare-earth magnet precursor, two side surfaces, which are perpendicular to the two side surfaces parallel to the pressing direction, are maintained in a constrained state from start to end of pressing.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

(2) FIGS. 1A and 1B are explanatory diagrams of a first step in a method of producing a rare-earth magnet according to a first embodiment of the invention, and FIG. 1C is a diagram illustrating a strain distribution of a rare-earth magnet precursor after the first step is performed;

(3) FIGS. 2A and 2B are explanatory diagrams of a second step according to the first embodiment, and FIG. 2C is a diagram illustrating a strain distribution of a rare-earth magnet after the second step is performed;

(4) FIGS. 3A to 3C are explanatory diagrams of a first step in a method of producing a rare-earth magnet according to a second embodiment of the invention;

(5) FIGS. 4A to 4C are explanatory diagrams of a second step according to the second embodiment;

(6) FIG. 5 is a graph illustrating residual magnetization in a thickness direction at a width-direction and longitudinal-direction center of each of rare-earth magnets of Example and Comparative Example;

(7) FIG. 6 is a graph illustrating residual magnetization in a longitudinal direction at a width-direction center of an upper surface of each of the rare-earth magnets of Example and Comparative Example;

(8) FIG. 7 is a graph illustrating residual magnetization in a longitudinal direction at a width-direction and thickness-direction center of each of the rare-earth magnets of Example and Comparative Example;

(9) FIG. 8A is a perspective diagram illustrating a sintered body before working in related art, and FIG. 8B is a perspective diagram illustrating a rare-earth magnet after the working in related art; and

(10) FIG. 9A is an explanatory diagram of a relationship between a frictional force and a plastic flow at a section CS shown in FIG. 8B, and FIG. 9B is a diagram illustrating a strain distribution at the same section of the rare-earth magnet in the related art.

DETAILED DESCRIPTION OF EMBODIMENTS

(11) Hereinafter, a method of producing a rare-earth magnet according to an embodiment of the invention will be described with reference to the attached drawings. The following embodiment describes the method of producing the rare-earth magnet that is a nanocrystal magnet. However, the method of producing the rare-earth magnet according to the invention is not limited to the production of the nanocrystal magnet, and is applicable to production of a sintered magnet having a relatively large grain size (for example, a sintered magnet having a particle size of approximately 1 μm).

First Embodiment of Method of Producing Rare-Earth Magnet

(12) In a method of producing a rare-earth magnet according to this embodiment, a sintered body, which is solidified by sintering a rare-earth magnet material such as a magnet powder produced by, for example, a liquid quenching method, is subjected to hot working to obtain a desired shape, and to give magnetic anisotropy to the sintered body.

(13) In this embodiment, for example, the sintered body which is subjected to the hot working is produced as follows. First, an alloy ingot is high-frequency melted in a furnace (not shown) under an Ar gas atmosphere decompressed to, for example, 50 kPa or lower according to a melt spinning method using a single roll, and a molten metal having a composition for producing a rare-earth magnet is sprayed onto a copper roll to prepare a quenched thin band (a quenched ribbon), and this quenched ribbon is coarsely crushed.

(14) Next, the quenched ribbon that is coarsely crushed is filled in a cavity defined by a cemented carbide die and a cemented carbide punch that slides in a hollow inside of the cemented carbide die, and is electrically heated by allowing a current to flow in a pressing direction while being pressed by the cemented carbide punch, thereby preparing a molded body that is constituted by a Nd—Fe—B-based main phase (grain size: approximately 50 nm to 200 nm) having a nanocrystalline structure and a grain boundary phase of a Nd—X alloy (X represents a metal element) at the periphery of the main phase.

(15) The molded body, which is obtained, is filled in the cavity defined by the cemented carbide die and the cemented carbide punch that slides in the hollow inside of the cemented carbide die, and is electrically heated by allowing a current to flow in a pressing direction while being pressed by the cemented carbide punch, thereby preparing a sintered body that is constituted by a RE-Fe—B-based main phase having a nanocrystalline structure (RE represents at least one kind of element selected from a group consisting of Nd, Pr, and Y) (having a grain size of approximately 20 nm to 200 nm), and a grain boundary phase of a Nd—X alloy (X represents a metal element) at the periphery of the main phase through hot press processing.

(16) The Nd—X alloy, which constitutes the grain boundary phase, is constituted by an alloy of Nd and at least one kind of element selected from a group consisting of Co, Fe, Ga, and the like. The Nd—X alloy is constituted by, for example, any one kind or two or more kinds selected from among Nd—Co, Nd—Fe, Nd—Ga, Nd—Co—Fe, and Nd—Co—Fe—Ga, and the Nd—X alloy is in an Nd-rich state.

(17) The sintered body has an isotropic crystalline structure in which the grain boundary phase is filled between a plurality of the nanocrystal grains (main phases). Accordingly, the hot working is performed on the sintered body to provide anisotropy thereto. In this embodiment, two-stage hot working is performed, that is, first hot working is performed at a first step to be described below, and second hot working is performed at a subsequent second step.

(18) (First Step)

(19) In the first step, the first hot working is performed on the sintered body to produce a rare-earth magnet precursor. FIGS. 1A and 1B are process diagrams of the first step, and are also sectional diagrams parallel to a sintered body pressing direction. FIG. 1C is a diagram illustrating a strain distribution in a section of the rare-earth magnet precursor shown in FIG. 1B. Each of FIGS. 1A to 1C illustrates a section along a central line parallel to front and rear side surfaces of the sintered body and the rare-earth magnet precursor.

(20) As shown in FIG. 1A, in the first step, first, a sintered body S is accommodated in a cavity C of a forming mold 1. The shape of the sintered body S is a hexahedron such as a cube and a rectangular parallelepiped. The forming mold 1 is constituted by a pair of cemented carbide punches 2, 3 that is vertically disposed to face each other, and a cemented carbide die 4 that is disposed around the cemented carbide punches 2, 3. The cavity C of the forming mold 1 is a space defined by the pair of punches 2, 3 and the die 4. At least one of the pair of punches 2, 3 is configured to slide in the hollow inside of the die 4. In this embodiment, the upper punch 2 is configured to slide upward and downward in the hollow inside of the die 4 so as to press an upper surface S3 and a lower surface S4 of the sintered body S that is placed on the lower punch 3.

(21) When accommodating the sintered body S in the cavity C of the forming mold 1, as shown in FIG. 1A, in the two side surfaces S1, S2 of the sintered body S, which are parallel to the pressing direction and are opposite to each other, one side surface S1 is caused to come into contact with an inner surface of the die 4 and is brought to a constrained state, and the other side surface S2 is not caused to come into contact with the inner surface of the die 4 and is brought to an unconstrained state. In this embodiment, front and rear side surfaces, which are perpendicular to the right and left side surfaces S2, S1 shown in FIG. 1A, are caused to come into contact with the inner surface of the die 4 and are brought to the constrained state. Thus, the left side surface S1 and the front and rear side surfaces of the sintered body S, which are brought to the constrained state, are maintained in contact with the inner surface of the die 4 and are maintained in the constrained state from start to end of the process of pressing the sintered body S.

(22) Next, as shown in FIG. 1B, the upper punch 2 is caused to descend toward the lower punch 3, and the upper and lower punches 2, 3 press the upper and lower surfaces S3, S4 of the sintered body S to perform compression in an upper-lower pressing direction. At this time, the left side surface S1 of the sintered body S is apt to be deformed in the leftward direction toward the outside of the sintered body S, and the right side surface S2 is apt to be deformed in the rightward direction toward the outside of the sintered body due to a plastic flow. However, the plastic flow in the leftward direction is restrained in the vicinity of the left side surface S1 which is in contact with the inner surface of the die 4 and is in the constrained state. Accordingly, in the sintered body S, deformation of the left side surface S1, which is in the constrained state, in the leftward direction is suppressed, and deformation of, the right side surface S2, which is in the unconstrained state, in the rightward direction is permitted. In addition, deformation of the front and rear side surfaces, which are in the constrained state, is suppressed.

(23) At this time, a frictional, force, which acts between the upper and lower surfaces S3, S4 of the sintered body S and the upper and lower punches 2, 3, respectively, increases toward the left side surface S1 of the sintered body S which is brought to the constrained state. In addition, the frictional force decreases in the rightward direction from the left side surface S1, that is, toward the right side surface S2 that is brought to the unconstrained state. Accordingly, the plastic flow is hindered to a larger degree by the frictional force at a location closer to the left side surface S1 in the constrained state. In addition, since the left side surface. S1 of the sintered body S is in the constrained state, the vicinity of the left side surface S1 is compressed in a state in which the plastic flow in the leftward direction is suppressed due to contact with the inner surface of the die 4. Accordingly, the vicinity of the left side surface S1 of the sintered body S, which is in the constrained state, is uniformly compressed in the pressing direction, and thus a rare-earth magnet precursor S′ is produced.

(24) As shown in FIG. 1C, a strain distribution of the rare-earth magnet precursor S′, which is produced through the first step, is more uniform than a strain distribution of the rare-earth magnet of the related art described below. In FIG. 1C, in the rare-earth magnet precursor S′, a strain of a right side surface S′2 brought to the unconstrained state is larger than a strain in the vicinity of a left side surface S′1 brought to the constrained state.

(25) (Second Step)

(26) In a second step, second hot working is performed on the rare-earth magnet precursor S′ that is produced in the first step, thereby producing a rare-earth magnet. FIGS. 2A and 2B are process diagrams of the second step, and are also sectional diagrams parallel to a rare-earth magnet pressing direction. FIG. 2C is a diagram illustrating a strain distribution in a section of the rare-earth magnet shown in FIG. 2B. As is the case with FIGS. 1A to 1C, each of FIGS. 2A to 2C illustrates a section along a central line parallel to front and rear side surfaces of the rare-earth magnet precursor S′ and the rare-earth magnet.

(27) As shown in FIG. 2A, in the second step, first, the rare-earth magnet precursor. S′ is moved in the cavity C of the forming mold 1. At this time; the left side surface S′1, which is brought to the constrained state during the pressing in the first step, is not caused to come into contact with the inner surface of the die 4 and is brought to an unconstrained state, and the right side surface S′2, which is brought to the unconstrained state during the pressing in the first step, is caused to come into contact with the inner surface of the die 4 and is brought to the constrained state. The front and rear side surfaces perpendicular to the right and left side surfaces S′2, S′1 in FIG. 2A are caused to come into contact with the inner surface of the die 4 and are brought to the constrained state as in the first step. In this embodiment, the same forming mold 1 as that used in the first step is used in the second step, but a forming mold different from that used in the first step may be used in the second step.

(28) Next, as shown in FIG. 2B, the upper punch 2 is caused to descend toward the lower punch 3, and the upper and lower punches 2, 3 press upper and lower surfaces S′3, S′4 of the rare-earth magnet precursor S′ to perform compression in the upper-lower pressing direction. At this state, the left side surface S′1 of the rare-earth magnet precursor S′ is apt to be deformed in the leftward direction toward the outside of the sintered body S due to the plastic flow, and the right side surface S′2 is apt to be deformed in the rightward direction toward the outside of the sintered body S. However, the plastic flow in the rightward direction is restrained in the vicinity of the right side surface S′2 which is in contact with the inner surface of the die 4 and is in the constrained state. Accordingly, in the rare-earth magnet precursor S′, deformation of the right side surface S′2, which is in the constrained state, in the rightward direction is suppressed, and deformation of the left side surface S′1, which is in the unconstrained state, in the leftward direction is permitted. In addition, deformation of the front and rear side surfaces, which are in the constrained state, is suppressed.

(29) As described above, the right side surface S′2, which is brought to the unconstrained state in the first step and in which the deformation is permitted in the first step, is brought to the constrained state and deformation is suppressed in the second step. Similarly, the left side surface S′1, which is brought to the constrained state in the first step and in which the deformation is suppressed in the first step, is brought to the unconstrained state and deformation is permitted in the second step.

(30) Accordingly, a frictional force, which acts on the upper and lower surfaces S′3, S′4 of the rare-earth magnet precursor S′ in the second step, increases toward the right side surface S′2 that is in the constrained state conversely to the first step. The frictional force decreases in the leftward direction from the right side surface S′2, that is, toward the left side surface S′1 that is in the unconstrained state. Accordingly, the plastic flow is hindered to a larger degree due to the frictional force at a location closer to the right side surface S′2 in the constrained state. In addition, since the right side surface S′2 of the rare-earth magnet precursor S′ is brought to the constrained state, the vicinity of the right side surface S′2 is compressed in a state in which the plastic flow in the rightward direction is suppressed. Thus, the vicinity of the right side surface S′2 of the rare-earth magnet precursor S′ is uniformly compressed in the pressing direction, and thus a rare-earth magnet M is produced.

(31) As described above, in the method of producing the rare-earth magnet of this embodiment, the first hot working is performed in the first step, and the second hot working is performed in the second step. Accordingly, the strain distribution of the rare-earth magnet M becomes uniform by the two-stage hot working in which the second hot working is performed in the second step. That is, the side surfaces of the sintered body S, Which are brought to the constrained state in the first hot working, are different from the side surfaces of the rare-earth magnet precursor S′, which are brought to the constrained state in the second hot working.

(32) Thus, a region, in which the plastic flow is most unlikely to occur during the plastic deformation of the sintered body S or the rare-earth magnet precursor S′, can be changed from one end to the other end, that is, from, the vicinity of the left side surface S1 to the vicinity of the right side surface S′2. On the other hand, a region, in which the plastic flow is most likely to occur during the plastic deformation of the sintered body S or the rare-earth magnet precursor S′, can be changed from the vicinity of the right side surface S2 to the vicinity of the left side surface S′1. In addition, the rare-earth magnet M is produced by compressing the sintered body S and the rare-earth magnet precursor S′ in the pressing direction in a state in which the deformation of the side surface S1 of the sintered body S or the side surface S′2 of the rare-earth magnet precursor S′ in a lateral direction is suppressed at least one time due to contact with the die 4.

(33) Accordingly, a material flow becomes more uniform through the first step and the second step as compared to the related art. As a result, as shown in FIG. 2C, the strain distribution in the section of the produced rare-earth magnet M is more uniform than the strain distribution in the section of the rare-earth magnet X in the related art shown in FIG. 9B. As described above, since the strain distribution in the section of the rare-earth magnet M is more uniform as compared to the related art, magnetic properties in the vicinity of a, surface of the rare-earth magnet M are improved, and the overall magnetic properties are improved. As a result, a low-magnetization portion of the rare-earth magnet M decreases, and thus a yield ratio of the rare-earth magnet M is also improved.

(34) The side surface S1 of the sintered body S, which is brought to the constrained state, and the side surface S′2 of the rare-earth magnet precursor S′, which is brought to the constrained state, are maintained in contact with the inner surface of the die 4 from start to end of pressing, and thus are maintained in the constrained state. Accordingly, in the first hot working, the region of the sintered body S, in which the plastic flow is most unlikely to occur, is constant without being changed in the course of the pressing. Then, a region in which the plastic flow is less likely to occur is changed due to movement of the rare-earth magnet precursor S′. In the second hot working, a region of the rare-earth magnet precursor S′, in which the plastic flow is most unlikely to occur, is constant without being changed from start to end of pressing.

(35) Thus, a relationship between the magnitude and direction of frictional force vector in the first hot working is inverted by 180° to that in the second hot working. Accordingly, the region of the sintered body S, in which the plastic flow is most unlikely to occur, is inverted to the region of the rare-earth magnet precursor S′ in which the plastic flow is most unlikely to occur, and thus a material flow becomes more uniform through the entirety of the process. Accordingly, the strain distribution in the first hot working and the strain distribution in the second hot working cancel each other, and thus the strain distribution in the same section of the rare-earth magnet M becomes even more uniform.

(36) As described above, according to the method of producing the rare-earth magnet relating to the first embodiment, hot working is performed in multiple stages, and a portion in which a force hindering the plastic flow of the material becomes maximum is changed each time the stage is changed. Accordingly, it is possible to improve the residual magnetization of the rare-earth magnet M by making the strain distribution of the produced rare-earth magnet M uniform while giving desired magnetic anisotropy to the sintered body S during the hot working. As a result, it is possible to produce the rare-earth magnet M, which is excellent in magnetic properties in the vicinity of a surface and the overall magnetic properties, with a high yield ratio.

Second Embodiment of Method of Producing Rare-Earth Magnet

(37) Hereinafter, a method of producing the rare-earth magnet according to a second embodiment of the invention will be described with reference to the attached drawings. The method of producing the rare-earth magnet according to this embodiment is different from the first embodiment in that side surfaces of the sintered body and the rare-earth magnet precursor, which are to be brought to the constrained state, are not caused to come into contact with the inner surface of the die and are brought to the unconstrained state at an initial stage of the pressing, and are caused to come into contact with the inner surface of the die and are brought to the constrained state in the course of the pressing. The other configurations are the same as the first embodiment, and the same reference numerals are given to the same configurations and a description thereof will not be repeated.

(38) FIGS. 3A to 3C are process diagrams of a first step of this embodiment, and are also sectional diagrams parallel to a sintered body pressing direction. Each of FIGS. 3A to 3C illustrates a section along a central line parallel to front and rear side surfaces of a sintered body and a rare-earth magnet precursor.

(39) (First Step)

(40) As shown in FIG. 3A, in a first step, first, the sintered body S is accommodated in the cavity C of the forming mold 1. At this time, the sintered body S is disposed with a predetermined distance D1 between the left side surface S1 of the sintered body S and the inner surface of the die 4 so that the left side surface S1 of the sintered body S, which is to be brought to the constrained state, is deformed in the leftward direction and comes into contact with the inner surface of the die 4 in the course of the pressing. That is, the left side surface S1 of the sintered body S is not caused to come into contact with the inner surface of the die 4, and is brought to the unconstrained state at an initial stage of the pressing of the sintered body S. As is the case with the first embodiment, the right side surface S2 of the sintered body S is maintained in the unconstrained state from start to end of pressing in the first step. As is the case with the first embodiment, the front and rear side surfaces are also maintained in the constrained state from start to end of pressing in the first step.

(41) For example, the distance D1 between the left side surface S1 of the sintered body S and the inner surface of the die 4 is set to be less than a half of a deformation amount in the first step in a direction in which the right and left side surfaces S2, S1 of the sintered body S are opposite to each other. In other words, the distance D1 is set to be equal to or less than a half of a difference between a distance between the right and left side surfaces S′2, S′1 of a rare-earth magnet precursor S′ that is produced by the first hot working in the first step and a distance between the right and left side surfaces S2, S1 of the sintered body S before the first hot working.

(42) Next, as shown in FIG. 3B, the upper punch 2 is caused to descend toward the lower punch 3, and the upper and lower punches 2, 3 press the upper and lower surfaces S3, S4 of the sintered body S to perform compression in an upper-lower pressing direction. In this case, the left side surface S1 of the sintered body S is deformed in the leftward direction toward the outside of the sintered body S due to a plastic flow, and the right side surface S2 is deformed in the rightward direction toward the outside of the sintered body S. At this time, the left side surface S1, which is in the unconstrained state, is deformed toward the leftward direction, and is caused to come into contact with the inner surface of the die 4 and is brought to the constrained state in the course of the pressing.

(43) As described above, the right and left side surface S2, S1 of the sintered body S are in the unconstrained state until the left side surface S1 comes into contact with the inner surface of the die 4 due to deformation of the left side surface S1 after start of pressing of the sintered body S. Accordingly, as shown in FIG. 3B, the left side surface S1 of the sintered body S is deformed in the leftward direction, and the right side surface S2 is deformed in the rightward direction.

(44) At this time, the frictional force that acts on the upper surface S3 and the lower surface S4 of the sintered body S is largest at the central portions of the upper and lower surfaces S3, S4 of the sintered body S in the right-left direction, and decreases toward the two side surfaces S1, S2 of the sintered body S which are opposite to each other. Accordingly, the plastic flow is most unlikely to occur at the central portions of the upper and lower surfaces S3, S4 of the sintered body S until the left side surface S1 is brought to the constrained state after start of pressing of the sintered body S.

(45) When the upper and lower surfaces S3, S4 of the sintered body S are further pressed by the upper and lower punches 2, 3, after the left side surface S1 is caused to come into contact with the inner surface of the die 4 and is brought to the constrained state in the course of the pressing of the sintered body S, deformation of the left side surface S1 of the sintered body S, which is in the constrained state, in the leftward direction is suppressed, and deformation of the right side surface S2, which is in the unconstrained state, in the rightward direction is permitted and compression in the pressing direction is performed as shown in FIG. 3C, as is the case with the first step of the first embodiment. In addition, deformation of the front and rear side surfaces, which are in the constrained state, is suppressed.

(46) At this time, as is the case with the first embodiment, the frictional force, which acts on the upper surface S3 and the lower surface S4 of the sintered body, increases toward the left side surface S1 of the sintered body S which is in the constrained state. The frictional force decreases toward the right side surfaces S2 that is in the unconstrained state. Accordingly, after the left side surface S1 is brought to the constrained state in the course of the pressing of the sintered body S, the plastic flow is most unlikely to occur in the vicinity of the left side surface S1 in the constrained state.

(47) That is, in this embodiment, it is possible to change the region of the sintered body S in which the plastic flow is most unlikely to occur, in the course of the pressing of the sintered body S in the first hot working in the first step. Thus, as is the case with the first embodiment, the strain distribution of the rare-earth magnet precursor S′ that is produced through the first step is more uniform than the strain distribution of the rare-earth magnet X in the related art.

(48) (Second Step)

(49) In a second step, second hot working is performed on the rare-earth magnet precursor S′ that is produced in the first step, thereby producing a rare-earth magnet M. FIGS. 4A to 4C are process diagrams of the second step, and are also sectional diagrams parallel to the pressing direction of the rare-earth magnet precursor S′. As is the case with FIGS. 3A to 3C, each of FIGS. 4A to 4C illustrates a section along a central line parallel to front and rear side surfaces of the rare-earth magnet precursor S′ and the rare-earth magnet M.

(50) As shown in FIG. 4A, in the second step, first, the ‘rare-earth magnet precursor S’ is moved in the cavity C of the forming mold 1. At this time, the rare-earth magnet precursor S′ is disposed with a predetermined distance D2 between the right side surface S′2 of the rare-earth magnet precursor S′ and the inner surface of the die 4 so that the right side surface S′2 of the rare-earth magnet precursor S′, which is to be brought to the constrained state, is deformed in the rightward direction and comes into contact with the inner surface of the die 4 in the course of the pressing. That is, the right side surface S′2 of the rare-earth magnet precursor S′ is not caused to come into contact with the inner surface of the die 4, and is brought to the unconstrained state at an initial stage of the pressing of the rare-earth magnet precursor S′. As is the case with the first embodiment, the left side surface S′1 of the rare-earth magnet precursor S′ is maintained in the unconstrained state from start to end of pressing in the second step. As is the case with the first embodiment, the front and rear side surfaces are also maintained in the constrained state from start to end of pressing in the second step.

(51) For example, the distance D2 between the right side surface S′2 of the rare-earth magnet precursor S′ and the inner surface of the die 4 is set to be less than a half of a deformation amount in the second step in a direction in which the right and left side surfaces S′2, S′1 of the rare-earth magnet precursor S′ are opposite to each other. In other words, the distance D2 is set to be less than a half of a difference between a distance between the right and left side surfaces M2, M2 of the rare-earth magnet M that is produced by the second hot working in the second step and a distance between the right and left side surfaces S′2, S′1 of the rare-earth magnet precursor S′ before the second hot working.

(52) Next, as shown in FIG. 4B, the upper punch 2 is caused to descent toward the lower punch 3, and the upper and lower punches 2, 3 press the upper and lower surfaces S′3, S′4 of the rare-earth magnet precursor S′ to perform compression in an upper-lower pressing direction. In this case, the right side surface S′2 of the rare-earth magnet precursor S′ is deformed in the rightward direction toward the outside of the rare-earth magnet precursor S′ due to a plastic flow, and the left side surface S′1 is deformed in the leftward direction toward the outside of the rare-earth magnet precursor S′. At this time, the right side surface S′2, which is in the unconstrained state, is deformed in the rightward direction, and is caused to come into contact with the inner surface of the die 4 and is brought to the constrained state in the course of the pressing.

(53) As described above, the right and left side surfaces S′2, S′1 of the rare-earth magnet precursor S′ are in the unconstrained state until the right side surface S′2 comes into contact with the inner surface of the die 4 due to deformation of the right side surface S′2 after start of pressing of the rare-earth magnet precursor S′. Accordingly, as shown in FIG. 4B, the left side surface S′1 of the rare-earth magnet precursor S′ is deformed in the leftward direction, and the right side surface S′2 is deformed in the rightward direction. Accordingly, as is the case with the sintered body S in the first step, the plastic flow is most unlikely to occur at the central portions of the upper and lower surfaces S′3, S′4 due to an effect of the frictional force which acts on the upper and lower surfaces S′3, S′4 of the rare-earth magnet precursor S′ until the right side surface S′2 is brought to the constrained state after start of pressing of the rare-earth magnet precursor S′.

(54) When the upper and lower surfaces S′3, S′4 of the rare-earth magnet precursor S′ are further pressed by the upper and lower punches 2, 3 after the right side surface S′2 is caused to come into contact with the inner surface of the die 4 and is brought to the constrained state in the course of the pressing of the rare-earth magnet precursor S′, deformation of the right side surface S′2 of the rare-earth magnet precursor S′, which is in the constrained state, in the rightward direction is suppressed, and deformation of the left side surface S′1, which is in the unconstrained state, in the leftward direction is permitted and compression in the pressing direction is performed as shown in FIG. 4C, as is the case with the second step of the first embodiment. Deformation of the front and rear side surfaces, which are in the constrained state, is suppressed.

(55) At this time, as is the case with the first embodiment, the frictional force, which acts on the upper surface S′3 and the lower surfaces S′4 of the rare-earth magnet precursor S′, increases toward the right side surface S′2 of the rare-earth magnet precursor S′ which is in the constrained state. The frictional force decreases toward the left side surface S′1 that is in the unconstrained state. Accordingly, as is the case with the sintered body S in the first step, after the right side surface S′2 is brought to the constrained state in the course of the pressing of the rare-earth magnet precursor S′, the plastic flow is most unlikely to occur in the vicinity of the right side surface S′2 in the constrained state.

(56) That is, in this embodiment, as is the case with the first embodiment, it is possible to change the region in which the plastic flow is most unlikely to occur during plastic deformation of the sintered body S or the rare-earth magnet precursor S′ when the first step proceeds to the second step (in other words, the region in which the plastic flow is most unlikely to occur during plastic deformation of the sintered body S in the first step is different from the region in which the plastic flow is most unlikely to occur during plastic deformation of the rare-earth magnet precursor S′ in the second step). Further, it is possible to change the region in which the plastic flow is most unlikely to occur, in the course of the pressing in the first step and in the course of the pressing in the second step. Thus, as is the case with the first embodiment, a material flow becomes more uniform through the first step and the second step, as compared to the related art.

(57) Accordingly, as is the case with the first embodiment, the strain distribution in the section of the produced rare-earth magnet M is more uniform than the strain distribution in the section of the rare-earth magnet X in the related art. Thus, since the strain distribution in the section of the rare-earth magnet M is more uniform as compared to the related art, magnetic properties in the vicinity of a surface of the rare-earth magnet M are improved, and the overall magnetic properties are improved. As a result, a low-magnetization portion of the rare-earth magnet M decreases, and thus the yield ratio of the rare-earth magnet M is also improved.

(58) As described above, according to the method of producing the rare-earth magnet according to the second embodiment, hot working is performed in multiple stages, and the portion in which the force hindering the plastic flow of the material becomes maximum is changed each time the stage is changed. Accordingly, it is possible to improve the residual magnetization of the rare-earth magnet M by making the strain distribution of the produced rare-earth magnet M uniform while giving desired magnetic anisotropy to the sintered body S during the hot working. As a result, it is possible to produce the rare-earth magnet M, which is excellent in magnetic properties in the vicinity of a surface and the overall magnetic properties, with a high yield ratio.

Example and Comparative Example

(59) Next, magnetic properties of a rare-earth magnet of Example, which was produced by the method of producing the rare-earth magnet according to the above-described first embodiment, were compared to magnetic properties of a rare-earth magnet of Comparative Example which was produced by a method in the related art.

(60) An alloy composition of the sintered body, which was used to produce the rare-earth magnet; was prepared by using raw materials mixed in proportions corresponding to, in terms of % by mass, Nd:14.6%, Fe:74.2%, Co:4.5%, Ga:0.5%, and B:6.2%. The shape of the sintered body was a rectangular parallelepiped. Dimensions of the sintered body were 15 mm (W)×14 mm (L)×20 mm (H) in which the width of the side surfaces S1, S2 shown in FIG. 1A in a depth direction was, set to W, the length in the right-left direction was set to L, and the height in the pressing direction was set to H. The dimensions of the rare-earth magnets of Example and Comparative Example after performing strong working on the sintered body were 15 mm (W)×70 mm (L)×4 mm (H). A case where a degree of working (reduction rate) due to the hot working is large, for example, a case where the reduction rate is approximately 10% or more may be called strong working.

(61) With regard to working conditions of the hot working, in Example and Comparative Example, a strain rate was set to 1.0/sec, a frictional coefficient was set to 0.2, a reduction rate in the first hot working was set to 60%, and a reduction rate in the second hot working was set to 80%.

(62) When the rare-earth magnet of Example was produced, in the first hot working, in two side surfaces of the sintered body, which were opposite to each other in a longitudinal direction (L direction), one side surface was caused to come into contact with the inner surface of the die and was brought to the constrained state to suppress deformation, and the other side surface was not caused to come into contact with the inner surface of the die and was brought to the unconstrained state to permit deformation. In the second hot working, in two side surfaces of a rare-earth magnet precursor, which were opposite to each other in the L direction, a side surface, which was in the unconstrained state in the first hot working, was caused to come into contact with the inner surface of the die and was brought to the constrained state to suppress deformation, and a side surface, which was in the constrained state in the first hot working, was brought to the unconstrained state to permit deformation. In each of the sintered body and the rare-earth magnet precursor, the two side surfaces, which were opposite to each other in a width direction (W direction), were caused to come into contact with the inner surface of the die and were brought to the constrained state in the first composition processing and the second composition processing.

(63) When a rare-earth magnet of Comparative Example was produced, in the first hot working, two side surfaces of the sintered body, which were opposite to each other in the L direction, were not caused to come into contact with the inner surface of the die and were brought to the unconstrained state to permit deformation. Similarly, in the second hot working, the two side surfaces of the rare-earth magnet precursor, which were opposite to each other in the L direction, were not caused to come into contact with the inner surface of the die and were brought to the unconstrained state to permit deformation. The two side surfaces of each of the sintered body and the rare-earth magnet precursor were caused to come into contact with the inner surface of the die in the first composition processing and the second composition processing and were brought to the constrained state, the two side surfaces being opposite to each other in the W direction.

(64) Next, the produced rare-earth magnets of Example and Comparative Example were subjected to cutting and the like to measure magnetic properties in the pressing direction, that is, in the thickness direction (H direction) at the W-direction and L-direction center, magnetic properties in the L direction at the W-direction center of an upper surface, and magnetic properties in the L direction at the W-directional and H-directional center.

(65) FIG. 5 is a graph illustrating magnetic properties in the thickness direction at the W-direction and L-direction center in each of the rare-earth magnets of Example and Comparative Example. In the graph, the horizontal axis shows a distance (mm) from the surface of each of the rare-earth magnets in the thickness direction, and the vertical axis shows residual magnetization (T) in the thickness direction using a relative value with respect to the maximum value of Comparative Example, which is set to 1. In the drawing, a black circle represents a measurement result of the rare-earth magnet in Example, and a white triangle represents a measurement result of the rare-earth magnet of Comparative Example.

(66) As shown in FIG. 5, in the rare-earth magnet of Comparative Example, as the distance in the thickness direction increases, the residual magnetization sharply decreases. In contrast, in the rare-earth magnet of Example, the residual magnetization is constant, regardless of the distance in the thickness direction. That is, in the rare-earth magnet of Example, a residual magnetization distribution in the thickness direction is more uniform as compared to the rare-earth magnet of Comparative Example.

(67) FIG. 6 is a graph illustrating magnetic properties in the L direction at the W-direction center of the upper surface of each of the rare-earth magnets of Example and Comparative Example. In the graph, the horizontal axis shows a distance (mm) from one side surface of each of the rare-earth magnets in the L direction, and the vertical axis shows residual magnetization (T) of the upper surface of each of the rare-earth magnets using a relative value with respect to the maximum value of Comparative Example, which is set to 1. In the drawing, a black circle represents a measurement result of the rare-earth magnet in Example, and a white triangle represents a measurement result of the rare-earth magnet of Comparative Example.

(68) As shown in FIG. 6, in the rare-earth magnet of Comparative Example, it is observed that the residual magnetization sharply decreases at both L-direction ends, and the residual magnetization also decreases at the L-direction central portion. In contrast, in the rare-earth magnet of Example, the decrease in the residual magnetization at the both L-direction ends is suppressed, and the decrease in the residual magnetization at the L-direction central portion is also prevented. That is, in the rare-earth magnet of Example, the residual magnetization in the vicinity of the surface is improved.

(69) FIG. 7 is a graph illustrating the magnetic properties in the L direction at the W-direction and H-direction center of each of the rare-earth magnets of Example and Comparative Example. In the graph, the horizontal axis shows a distance (mm) from one side surface of each of the rare-earth magnets in the L direction, and the vertical axis shows the residual magnetization (T) at the W-direction and H-direction center using a relative value with respect to the maximum value of Comparative Example, which is set to 1. In the drawing, a black circle represents a measurement result of the rare-earth magnet in Example, and a white triangle represents a measurement result of the rare-earth magnet of Comparative Example.

(70) As shown in FIG. 7, there is no great difference in the residual magnetization between the rare-earth magnets of Example and Comparative Example at the L-direction central portion, but the decrease in the residual magnetization of the rare-earth magnet of Example at the both L-direction ends was less in comparison to the rare-earth magnet of Comparative Example.

(71) From the above-described measurement results, it has been confirmed that the residual magnetization of the rare-earth magnet of Example in the thickness direction is more uniform, the residual magnetization in the vicinity of the surface is improved, and the overall magnetic properties of the rare-earth magnet are improved, as compared to the rare-earth magnet of Comparative Example. From the results, with regard to a yield ratio calculated in a magnetic property range of 1.4 T or more, the yield ratio of the rare-earth magnet of Comparative Example was 86%, and the yield ratio of the rare-earth magnet of Example was 91%. Accordingly, it has been confirmed that the yield ratio of the rare-earth magnet of Example is improved, as compared to the yield ratio of the rare-earth magnet of Comparative Example.

(72) The embodiments of the invention have been described in detail with reference to the attached drawings. However, specific configurations are not limited to the embodiments, and design modifications in a range that does not depart from the scope of the invention are included in the invention.

(73) For example, the shape of the sintered body does not necessarily need to be a hexahedron such as a cube and a rectangular parallelepiped. The planar shape of the sintered body may be a polygon other than a rectangular shape, and may be a circular shape or an elliptical shape. The sintered body may be a polyhedron other than the hexahedron, and the sintered body may have a shape with a rounded corner or ridge or a shape with a curved side surface.

(74) In addition, it is needless to say that a modified alloy may be subjected to grain boundary diffusion in the rare-earth magnet produced through the first step and the second step to raise a coercive force.