JP2017050396A - Rare-earth magnet and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rare-earth magnet having a high coercive force, and a high residual magnetic flux density (or, a maximum energy product), and to provide a manufacturing method therefor.SOLUTION: A flaky powder composed of a magnetic material containing a rare earth element (R), for manufacturing a rare-earth magnet containing anisotropically-shaped grains having magnetic anisotropy, and a nonmagnetic grain boundary phase is molded to obtain a molding. Subsequently, hot exclusion of the molding is carried out, and an easy axis of magnetization of the anisotropically-shaped grains is oriented. Hot exclusion of the molding is carried out so that the angle formed by the compression direction while molding and the compression direction during hot exclusion is 5° or less.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a rare earth magnet and a method for manufacturing the same, and more particularly to a rare earth magnet having an easy axis oriented in one direction and a method for manufacturing the same.

  Magnets containing rare earth elements such as neodymium and samarium (rare earth magnets) are excellent in magnetic properties and are used in motors and generators. In particular, Nd—Fe—B magnets currently exhibit the strongest magnetic properties. Further, a magnet in which a part of the constituent elements of the Nd-Fe-B magnet is replaced with another rare earth element (R) or another transition metal element (T), or an additive element (M) is further added to these Magnets that have been made are also known to exhibit high magnetic properties. Hereinafter, Nd—Fe—B magnets (alloys) and their derivatives are collectively referred to as “RTB magnets (alloys)”.

The structure of the R-T-B magnet is composed of a main phase (magnet phase which is an R ₂ T ₁₄ B ternary intermetallic compound) and a grain boundary phase (non-magnet phase) existing so as to surround the main phase. ing. The crystal structure of the R ₂ T ₁₄ B crystal (2-14-1 crystal) is a tetragonal crystal, and the c-axis is the easy axis of magnetization. Therefore, the higher the volume fraction of the main phase and the higher the magnetic orientation, the higher the magnetic force magnet. Generally, the higher the degree of orientation, the higher the maximum energy product (BH) _max . In addition, the coercive force iHc (resistance against demagnetizing field / high temperature) increases as the crystal grains of the main phase are covered with the grain boundary phase without interruption and the microstructure is smaller (Non-patent Document 1).

Various proposals have heretofore been made regarding methods for producing such an R-T-B magnet exhibiting high magnetic properties. For example, Patent Document 1 discloses that
(A) pulverizing a super-quenched ribbon of a rare earth magnet alloy to produce a powder;
(B) A columnar body is produced by cold pressing and hot pressing the powder,
(C) A method of extruding a columnar body into a cylindrical body using a die, a punch and a mandrel is disclosed.

In Patent Document 2,
(A) A magnetic alloy ultra-quenched ribbon is crushed to produce a powder,
(B) A preform is produced by cold pressing and hot pressing the powder,
(C) A method is disclosed in which the preform is extruded so that the dimension in the X direction of the preform is reduced and the dimension in the Y direction orthogonal to the X direction is increased.

In Patent Document 3,
(A) melting and casting an iron-based alloy containing a rare earth element;
(B) A method for producing a rare earth-iron permanent magnet by hot-working (extrusion, rolling, or stamping) a cast ingot is disclosed.

Patent Document 4 includes
(A) forming a raw material alloy powder containing a rare earth element, a transition metal and boron in a magnetic field;
(B) A method for producing a rare earth sintered magnet in which a compact is sintered by discharge plasma is disclosed.

In Patent Document 5,
(A) Cold forming a raw material containing a rare earth element,
(B) Hot forming (hot pressing) the cold formed body,
(C) A method for producing a rare earth magnet is disclosed in which a hot formed body is extruded backward to obtain a cylindrical rare earth magnet.

In Patent Document 6,
(A) Cold forming the flaky alloy powder obtained by rapid solidification,
(B) After preheating the cold formed body, hot forming (hot pressing),
(C) A method for producing a magnetic anisotropic magnet material by hot extruding a hot formed body is disclosed.

In Patent Document 7,
(A) Filling the cavity with magnetic powder, pressing the magnetic powder in the cavity from the axial direction while applying a radial orientation magnetic field to the magnetic powder, and forming a ring-shaped magnet compact,
(B) A method for producing a ring-shaped sintered magnet in which a ring-shaped magnet molded body is sintered is disclosed.

In Patent Document 8,
(A) Alloy composition (% by mass): 31 Nd-3Co-1B-0.4Ga—Aluminum flakes obtained by rapidly solidifying a raw material consisting of Fe are pulverized to produce an alloy powder,
(B) Molding (hot pressing) the alloy powder to produce a bulk body,
(C) A method for manufacturing an R-T-B rare earth magnet is disclosed in which hot upsetting or hot rolling of a bulk body is performed in a processing direction different from molding.
The document describes that by performing hot upsetting or hot rolling in a processing direction different from that of forming by 90 °, magnetization can be improved and high holding force can be secured at the same time. .

Furthermore, in Patent Document 9,
(A) pressure forming a powder containing a rare earth element;
(B) producing a plate-like intermediate by extruding the molded body;
(C) A method for manufacturing a rare earth magnet that is upset in the thickness direction of the intermediate is disclosed.

The RTB-based alloy powder obtained by the ultra-quenching method has a flaky shape. Each powder is composed of an aggregate of fine main phase particles having an equiaxed shape (particle size: several tens of nanometers), and each main phase particle exhibits a non-oriented structure in a random direction. Yes. When such powder is cold-molded and hot-molded, the main phase particles grow anisotropically in the direction perpendicular to the c-axis inside each powder, and become plate-shaped anisotropic grains. At this time, the anisotropically shaped grains have a non-oriented structure in a random direction inside the compact.
Furthermore, when a molded body containing non-oriented anisotropically shaped grains is hot plastic processed, the grain boundary phase becomes liquid and grain boundary sliding occurs. As a result, the anisotropically shaped particles rotate so that the c-axis direction (thickness direction of the plate-like anisotropically shaped particles) is aligned with the compression direction.

  The hot plastic working method can reduce the crystal grains of the main phase particles and can increase the degree of orientation of the c-axis. Therefore, the hot plastic working method is effective as a method for producing a magnet having a high coercive force and a high maximum energy product (or residual magnetic flux density). However, it is desired to further improve the magnetic properties of the magnet in order to use it in applications where the usage environment is more severe.

JP 09-129463 A JP 2008-091867 A Japanese Patent Laid-Open No. 08-250312 JP 2005-197298 A JP 2010-263172 A JP 2011-042837 A JP 2006-093301 A JP 2012-174986 A JP 2014-103386 A

Keiko Hioki et al., Material, Volume 52 (2011), No. 8, pp. 19-24

  The problem to be solved by the present invention is to provide a rare earth magnet having a high coercive force and a high residual magnetic flux density (or maximum energy product) and a method for producing the same.

In order to solve the above-described problems, a manufacturing method of a rare earth magnet according to the present invention is summarized as having the following configuration.
(1) The method for producing the rare earth magnet includes:
A flaky powder made of a magnetic material containing a rare earth element (R), the powder for producing the rare earth magnet containing anisotropically shaped grains having magnetic anisotropy and a nonmagnetic grain boundary phase. A molding step of molding and obtaining a molded body;
A hot extrusion process of performing hot extrusion of the molded body and orienting the easy axis of magnetization of the anisotropically shaped grains.
(2) The angle formed by the compression direction in the molding step and the compression direction in the hot extrusion step is 5 ° or less.

  The rare earth magnet according to the present invention is obtained by the method according to the present invention.

In Patent Document 8, when raw powder of a rare earth magnet is molded and the formed body is subjected to hot upsetting or hot rolling, hot upsetting or hot in a direction different from the forming direction by 90 °. It is described that the magnetic properties are improved by rolling.
On the other hand, when the extruded product is hot extruded, if the compression direction at the time of molding and the compression direction at the time of hot extrusion are almost parallel, high holding force and high residual magnetic flux density (or maximum energy product) Can be made compatible.

  Details of the reason why the opposite result can be obtained by different hot plastic working methods are unclear, but probably the size and distribution of plastic strain during hot plastic working and the region where plastic deformation has not occurred. It is thought that the stress state is related.

It is process drawing of the manufacturing method of the rare earth magnet which concerns on this invention. It is a schematic diagram of the manufacturing method of a plate magnet. It is a schematic diagram of the manufacturing method of a ring magnet. It is a schematic diagram of the orientation mechanism of a rare earth magnet. FIG. 5A is a micrograph of a cross section of an extruded material having an angle difference of 5 ° or less during extrusion. FIG. 5B is an enlarged photograph of the area surrounded by the square in FIG. FIG.5 (c) is a cross-sectional microscope picture of the extruded material whose angle difference at the time of extrusion is 90 degrees. FIG. 5D is an enlarged photograph of the area surrounded by the square in FIG. It is a figure which shows the magnetic characteristic of the magnet obtained by carrying out the hot extrusion process on various conditions.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Rare earth magnet manufacturing method]
FIG. 1 shows a process diagram of a method for producing a rare earth magnet according to the present invention. In FIG. 1, the method for producing a rare earth magnet includes a raw material powder production process, a molding process, and a hot extrusion process.

[1.1. Raw material powder manufacturing process]
First, a powder for producing a rare earth magnet comprising a magnetic material containing a rare earth element (R) and comprising anisotropically shaped grains having magnetic anisotropy and a nonmagnetic grain boundary phase (Raw material powder production process).

[1.1.1. Composition of raw material powder]
The raw material powder is not limited as long as it is made of a magnetic material containing a rare earth element (R) and can produce a rare earth magnet containing anisotropically shaped grains having magnetic anisotropy and a nonmagnetic grain boundary phase. .
That is, the raw material powder is
(A) From the beginning, including an anisotropically shaped grain and a grain boundary phase, or
(B) Those capable of generating anisotropically shaped grains and grain boundary phases by heating,
For example, it may include crystal grains having an amorphous and / or easy axis. When the crystal grains are included, a nonmagnetic grain boundary phase is further included.

As long as these conditions are satisfied, the composition of the raw material powder, that is, the composition of the magnetic material is not particularly limited.
The magnetic material is particularly preferably made of an RTB-based alloy containing at least a rare earth element (R), an element T (T is Fe or Fe + Co), and B. When an R-T-B alloy is formed and processed under appropriate conditions, a rare earth having anisotropically shaped grains having a main phase of the R ₂ T ₁₄ B phase and a grain boundary phase surrounding the anisotropic shaped grains A magnet is obtained. In this case, the type and amount of element R, the amount of Co substituting Fe, and the type and amount of other additive element (M) are not particularly limited, and an optimum one should be selected according to the purpose. Can do.

R-T-B alloys are particularly
12.5 ≦ R ≦ 15.0 at%,
4.5 ≦ B ≦ 6.5 at%,
0.1 ≦ Ga ≦ 0.7 at%,
0 ≦ Co ≦ 6.0 at%, and
0 ≦ Cu + Al ≦ 2.0at%
In which the balance is Fe and inevitable impurities.

In the present invention, “rare earth element (R)” refers to Y and elements from La to Lu (lanthanoid elements). The element (R) is a constituent element of the R ₂ T ₁₄ B crystal (2-14-1 crystal) together with the elements (T) and B. The magnetic material may contain any one element (R), or may contain two or more kinds. In particular, the element (R) is preferably any one or more elements selected from the group consisting of Nd, Pr, Dy, and Tb.

Ga is an additive element (M) added to the RTB-based alloy and has the effect of lowering the grain boundary phase and improving the coercive force.
Co is an element (T) that substitutes a part of Fe, and has an effect of improving corrosion resistance and thermal stability. Co is added as necessary.
Cu and Al are additive elements (M) added to the RTB-based alloy, and both have the effect of lowering the grain boundary phase and improving the coercive force. Cu and Al are added as necessary. Moreover, either Cu or Al may be added, or both may be added.

[1.1.2. Raw material powder structure]
The raw material powder may contain anisotropically shaped grains and grain boundary phases from the beginning. However, when hot extrusion is performed using powder containing anisotropically shaped grains, anisotropic growth of anisotropically shaped grains may proceed excessively during hot extrusion, and the coercive force may decrease.
In order to refine the particle size of the main phase and obtain a high holding power, the raw material powder contains fine equiaxed grains consisting of the main phase (that is, the anisotropically formed grains and grain boundary phases are generated by heating). Is possible). Further, in order to generate fine anisotropically shaped particles, the smaller the particle diameter of the equiaxed particles, the better.

  For example, when an ultra-quenching method described later is used, a flaky powder composed of amorphous and / or equiaxed grains having a particle size of several tens of nanometers is obtained (see FIG. 1D). When this is heated to an appropriate temperature, equiaxed grains preferentially grow (anisotropic growth) in the direction perpendicular to the c-axis. As a result, plate-like anisotropic shaped grains having a diameter of several hundred nm × thickness: several tens of nm are generated (see FIG. 1E).

[1.1.3. Raw material powder shape]
In the present invention, the raw material powder has a flaky shape. “Flame” means that the ratio (= D / t) of the minimum dimension (D) of the surface (maximum surface) having the largest area to the thickness (t) is 2 or more. Although it does not specifically limit about a specific dimension, Thickness (t) can be 5-25 micrometers and minimum dimension (D) can be 50-300 micrometers. In addition, each dimension can be measured using an optical microscope or SEM (scanning electron microscope). Further, the minimum dimension (D) and the thickness (t) may be average values of measured values at a plurality of locations.

In the material after hot extrusion, grain boundaries (powder grain boundaries) of the raw material powder remain. The shape of the powder grain boundary varies greatly depending on the shape of the raw material powder, molding conditions, hot extrusion processing conditions, and the like. In particular, when the shape of the raw material powder is flaky, the shape of the powder grain boundary varies greatly depending on the processing conditions.
When viewed at the micro level, the magnetic properties of the entire material vary greatly due to differences in processing conditions, although there are no significant differences in the size and orientation of the main phase particles present inside the powder grain boundaries. This is probably because the shape of the powder grain boundary after hot extrusion affects the magnetic properties. Therefore, when the present invention is applied to the flaky raw material powder, a high effect is obtained.

[1.1.4. Raw material powder manufacturing method]
The method for producing the raw material powder is not particularly limited as long as the raw material powder satisfying the above-described conditions is obtained. The method for producing the raw material powder is particularly preferably a method for producing a ribbon (super quenching method) by super quenching a molten magnetic material.

  FIG. 1A shows a schematic diagram of the ultra-quenching method. When the molten metal material is sprayed on a rotating metal roll, the molten metal is super-cooled on the surface of the metal roll to obtain a ribbon. When the ribbon is pulverized, a flaky powder is obtained. The size of the powder is usually about 150 μm in particle size and about 20 μm in thickness although it depends on the production conditions. FIG. 1 (d) shows an SEM photograph of the cross section of the powder obtained by the ultra-quenching method. FIG. 1D shows that fine equiaxed grains having a particle size of about several tens of nanometers are contained in the powder.

[1.2. Molding process]
Next, the obtained powder is molded to obtain a molded body (molding step). The molding may be (a) only cold molding, (b) only hot molding, or (c) a combination of cold molding and hot molding.

[1.2.1. Cold forming]
“Cold forming” refers to forming performed at a temperature around room temperature (20 ° C. ± 15 ° C., preferably 25 ° C. ± 5 ° C.). In the present invention, the cold forming method is not particularly limited, and an optimum method can be selected according to the purpose.
For example, as shown in FIG. 1B, when a powder is filled in a die cavity and the powder is uniaxially pressed using a punch, a block-shaped molded body is obtained. In this case, the compression direction is the pressing direction by the punch.
Or although not shown in figure, a cylindrical molded object can be obtained by arrange | positioning a mandrel in the center of a cylindrical rubber container, filling the gap | interval of a rubber container and a mandrel, and performing CIP processing. In this case, the compression direction is the radial direction.

The cold forming conditions may be any conditions as long as it is possible to obtain a molded body having a strength that allows handling. Generally, the higher the density of the molded body, the higher the strength of the molded body. In order to obtain the strength to withstand handling, the relative density of the molded body is preferably 60% or more. The relative density is more preferably 70% or more, and still more preferably 80% or more.
The atmosphere at the time of cold forming is not particularly limited. Usually, cold forming is performed in the atmosphere.
The relative density in the present invention means the ratio of the actually measured density to the theoretical density.

[1.2.2. Hot forming]
“Hot molding” refers to molding performed in a temperature range in which densification of the molded body proceeds. The hot forming may be performed directly on the powder or may be performed on the cold formed body. As long as it is possible to obtain a dense molded body, the hot forming method is not particularly limited. Examples of the hot forming method include a hot press method, a HIP treatment method, and a discharge plasma sintering method.

  When a large amount of pores remain in the molded body (hot molded body) obtained in the hot forming step, pores may remain in the material (extruded material) after hot extrusion. The pores cause a decrease in the magnetic properties of the extruded material. Therefore, the higher the relative density of the hot formed body, the better. Specifically, the relative density of the hot formed body is preferably 90% or more. The relative density is more preferably 95% or more, and still more preferably 99% or more.

Generally, the higher the hot forming temperature, the higher the relative density of the hot formed body. On the other hand, if the hot forming temperature becomes too high, the growth of crystal grains (main phase particles) proceeds excessively during hot forming, and the coercive force decreases. Therefore, it is preferable to select an optimum temperature for the hot forming temperature so that the relative density of the hot formed body becomes high and the growth of the main phase particles does not proceed excessively. The optimum hot forming temperature varies depending on the hot forming method and the composition of the magnetic material. The hot forming temperature is usually 500 to 900 ° C, preferably 700 to 850 ° C.
The hot forming is preferably performed in a non-oxidizing atmosphere (for example, in Ar, vacuum, etc.) in order to prevent oxidation of the formed body.

In the case of performing both cold forming and hot forming, when using a hot press method or a discharge plasma sintering method as a hot forming method, the cold formed body is compressed in the uniaxial direction during the hot forming process. The In this case, unlike hot extrusion, the compression direction during hot forming does not necessarily have to be parallel to the compression direction during cold forming. This is because the hot forming process has much less plastic flow of the material generated during the process than the hot extrusion process.
Moreover, when the shape of the die used for the hot extrusion process is optimized, it is possible to simultaneously perform densification of the cold formed body and hot extrusion of the formed formed body (hot formed body). In such a case, an independent hot forming step can be omitted.

[1.3. Hot extrusion process]
Next, the extruded product is subjected to hot extrusion (hot extrusion process). “Hot extrusion process” refers to an extrusion process in which the crystal grains are in a solid phase and the grain boundary phase is in a liquid phase.

As shown in FIG.1 (c), a molded object is hot-extruded. As a result, the anisotropically shaped particles are rotated so that the easy magnetization axis (c-axis) is parallel to the compression direction at the same time as the generation and / or anisotropic growth of anisotropically shaped particles occurs in the compact. Further, when the shape of the die is optimized, densification of the molded body proceeds at the same time. As a result, it is possible to obtain an extruded material that is dense and has anisotropically shaped grains having easy magnetization axes oriented in one direction. FIG. 1 (e) shows an SEM photograph of a cross section of the extruded material.
The obtained extruded material is subjected to post-processing such as cutting, grinding, and chamfering as necessary.

[1.3.1. Processing conditions]
[A. Processing temperature and processing atmosphere]
The hot extrusion process is performed in a temperature range in which anisotropically shaped grains are in a solid phase and only the grain boundary phase is in a liquid phase. This is to cause grain boundary sliding during hot extrusion. The optimum processing temperature varies depending on the extrusion method and the composition of the magnetic material. Processing temperature is 500-900 degreeC normally, Preferably, it is 700-850 degreeC.
The hot extrusion process is preferably performed in a non-oxidizing atmosphere (for example, in Ar or vacuum) in order to prevent oxidation of the molded body.

[B. Compression direction]
The hot extrusion is performed so that the molded body is compressed almost in parallel with the compression direction at the time of molding. Specifically, the angle formed between the compression direction in the molding process and the compression direction in the hot extrusion process (hereinafter referred to as “angle difference”) is 5 ° or less. The angle difference is preferably 3 ° or less, more preferably 1 ° or less.
By performing the hot extrusion process as described above, the minimum principal strain is introduced in the compression direction in the hot extrusion process, and the easy magnetization axis is oriented in that direction. And it can be set as the outstanding anisotropic rare earth magnet by making the compression direction in a hot extrusion process into a magnetization direction.

Here, "the direction of compression during molding"
(A) When only cold forming is performed, the compression direction during cold forming,
(B) When only hot forming is performed, the compression direction during hot forming,
(C) When both cold forming and hot forming are performed, it refers to the compression direction during cold forming.

When the shape of the raw material powder is flaky, the cold formed body has a structure in which the flaky is laminated in the compression direction. In the case of hot extrusion of such a cold formed body, the flaky powder is compressed in a direction parallel to the maximum surface when the angle difference during hot extrusion is 90 °. As a result, an extruded material in which the powder grain boundary is bent or curved into a waveform and the powder grain boundary is complicated is obtained.
On the other hand, when the angle difference is not more than a predetermined value, an extruded material in which the powder grain boundaries are arranged substantially in parallel is obtained. This is the same when only hot forming is performed and when both cold forming and hot forming are performed.

[C. Compression strain]
Hot extrusion is performed to rotate anisotropically shaped grains by grain boundary sliding and align the c-axis in one direction. Therefore, if the compressive strain at the time of hot extrusion is too small, rotation of anisotropically shaped particles becomes insufficient. As a result, the degree of magnetic orientation of the extruded material is reduced.
In order to obtain a high degree of magnetic orientation, the compressive strain is preferably 1.0 or more, more preferably 1.1 or more, and still more preferably 1.2 or more.
Here, “compression strain” is defined as ln (t ₀ / t) when the dimensions in the compression direction during hot extrusion before and after hot extrusion are t ₀ and t, respectively. .

[1.3.2. Processing method]
As long as the above-described conditions are satisfied, the hot extrusion method is not particularly limited. For example, the hot extrusion process may be a forward extrusion process or a backward extrusion process. Further, the hot extrusion process may be an extruding process in a plate shape or an extruding process in a cylindrical shape.

[A. Manufacturing method of plate magnet]
In FIG. 2, the schematic diagram of the manufacturing method of a plate magnet is shown. When manufacturing a plate magnet, first, as shown to Fig.2 (a), the rectangular parallelepiped molded object 10 is produced. In the case of the example shown in FIG. 2, the X direction is the compression direction during molding.

Next, as shown in FIG. 2B, an extrusion processing apparatus 12 including a die 14 having a cavity 14a and a punch 16 that can be inserted into the upper portion of the cavity 14a is prepared.
The cavity 14a is
(A) The molded body 10 can be inserted from above, and
(B) When extruding the molded body 10 downward, the shape in the X direction is narrowed, and at the same time, the shape is such that plastic deformation is possible to expand the dimension in the Y direction perpendicular to the X direction.
In addition, the center figure of FIG.2 (b) is a top view (figure seen from the Z direction) of the molded object 10. FIG.

The molded body 10 is inserted into the upper portion of the cavity 14 a of the die 14, and the molded body 10 is pressed by the punch 16. As a result, the compact 10 is densified (hot molding) at the upper part of the cavity 14 a, and at the same time, the compact 10 compacted at the lower part of the cavity 14 a is pushed out below the die 14.
At this time, as shown in FIG. 2C, the dimension in the compression direction (X direction) during molding is reduced, and at the same time, the dimension in the Y direction orthogonal to the X direction is expanded. As a result, an extruded material 18 having a shape as shown in FIG.
2C is a plan view of the extruded material 18 being processed (viewed from the Z direction).

[B. Ring magnet manufacturing method]
In FIG. 3, the schematic diagram of the manufacturing method of a ring magnet is shown. When manufacturing a ring magnet, first, as shown to Fig.3 (a), the cylindrical molded object 20 is produced. In the case of the example shown in FIG. 3, the radial direction (r direction) is the compression direction during molding.

Next, as shown in FIG. 3B, an extrusion apparatus 22 including a die 24 having a cylindrical cavity 24a, a mandrel 26, and a punch 28 is prepared.
The mandrel 26 includes a small diameter part 26a having a uniform diameter and a large diameter part 26b having a uniform diameter. The diameter of the large diameter part 26b is larger than the diameter of the small diameter part 26a.
The punch 28 has a cylindrical shape. The outer diameter of the punch 28 is smaller than the inner diameter of the cavity 24 a of the die 24. Further, the inner diameter of the punch 28 is larger than the outer diameter of the small diameter portion 26 a of the mandrel 26.

The mandrel 26 is inserted into the center of the die 24 so that the small diameter portion 26a is on the top. Next, the molded body 20 is inserted between the cavity 24 a of the die 24 and the small diameter portion 26 a of the mandrel 26, and the molded body 20 is pressed by the punch 28. As a result, the molded body 20 is densified (hot-molded) in the upper portion of the cavity 24 a, and at the same time, the molded body 20 is pushed out below the die 24.
At this time, as shown in FIG. 3C, the molded body 20 is compressed in the radial direction between the cavity 24a and the large diameter portion 26b. As a result, an extruded material 30 having a shape as shown in FIG.

[2. Rare earth magnet]
The rare earth magnet according to the present invention is obtained by the method according to the present invention.
[2.1. composition]
The rare earth magnet according to the present invention is made of a magnetic material containing a rare earth element (R). Since the details of the composition of the magnetic material are as described above, the description thereof is omitted.

[2.2. Organization]
[2.2.1. Crystal grain size]
The rare earth magnet according to the present invention includes anisotropic shaped grains (magnet phase) having magnetic anisotropy and grain boundary phases (non-magnet phases) existing around the anisotropic shaped grains. The residual magnetic flux density (or maximum energy product) of the rare earth magnet mainly depends on the composition of the magnetic material and the degree of orientation of the anisotropically shaped particles, and the coercive force mainly depends on the particle size of the anisotropically shaped particles. In order to obtain a high holding force, the smaller the particle diameter of the anisotropic shaped particles, the better. Here, “particle diameter of anisotropically shaped grains” refers to the size of the maximum surface of anisotropically shaped grains.
In the case of using the method according to the present invention, when the manufacturing conditions are optimized, a rare earth magnet having an anisotropically shaped particle diameter of 150 to 500 nm is obtained.

[2.2.2. Magnetic orientation]
The anisotropically shaped particles have a plate shape, and the thickness direction is the easy magnetization axis (c-axis). Since the rare earth magnet according to the present invention is manufactured by hot extrusion, the easy magnetization axis is oriented in one direction. The degree of orientation of the easy magnetization axis (magnetic orientation degree) can be expressed by the ratio of residual magnetic flux density (B _r ) to saturation magnetic flux density (J _s ) (= B _r / J _s ).
In the case of using the method according to the present invention, when the manufacturing conditions are optimized, a rare earth magnet having a magnetic orientation degree of 0.9 or more is obtained.

[2.2.3. Grain boundary]
“Powder grain boundary” refers to an interface between flaky raw material powders, which is a region richer in rare earth elements (R) than crystal grains (main phase), and oxides of rare earth elements (R) are mainly used. included. Further, the powder grain boundary has a thickness that can be confirmed with an optical microscope.
The rare earth magnet according to the present invention can be obtained by hot extrusion so that the compression direction during molding and the compression direction during hot extrusion are substantially parallel. Therefore, the rare earth magnet according to the present invention is less disturbed in the shape of the powder grain boundary than the rare earth magnet obtained by the conventional method.
The degree of disturbance of the shape of the powder grain boundary can be represented by an average value θ _m of the angle θ formed by the compression direction during hot extrusion and the maximum length direction of the powder. θ _m is obtained from θ of 50 or more randomly selected powders.
The “maximum length direction of the powder” is a cross section parallel to the compression direction of the hot extrusion process (a surface formed by an angle between the normal direction of the surface and the compression direction of 90 °). The direction in which the length becomes the longest.
When the method according to the present invention is used, a rare earth magnet having θ _m of 70 to 90 ° can be obtained by optimizing the manufacturing conditions.

[3. Action]
[3.1. Orientation mechanism]
FIG. 4 shows a schematic diagram of the orientation mechanism of the rare earth magnet. The raw material powder produced by the ultra-quenching method includes a main phase composed of fine equiaxed grains, and exhibits a non-oriented structure in which the easy magnetization axes of the individual main phases are oriented in random directions. When such powder is cold-formed, a cold-formed body having a non-oriented structure is obtained (FIG. 4A).
When such a cold formed body is hot formed, the main phase particles (2-14-1 crystal) grow preferentially with respect to the c-axis, and anisotropically shaped grains composed of the main phase are generated. The body becomes dense. At this time, since the plastic strain applied to the compact is relatively small, the anisotropically shaped grains remain non-oriented (FIG. 4B).

  Next, when the densified hot compact is compressed at a high temperature, the growth of anisotropically shaped grains proceeds, and at the same time, the grain boundary phase becomes a liquid phase, causing grain boundary sliding. At this time, the anisotropically shaped particles rotate so that the c-axis is parallel to the compression direction due to the anisotropy of the shape (FIG. 4C). As a result, a rare earth magnet containing fine anisotropically shaped grains and having the easy axis of magnetization of the anisotropically shaped grains oriented in one direction is obtained (FIG. 4D).

When the flaky raw material powder is molded, the structure of the compact has a structure in which the flakes are laminated in the compression direction during molding. When the angle difference between the laminating direction and the compression direction during hot extrusion increases, a force that bends or curves the thin piece works. As a result, the minimum principal strain is introduced in a direction not intended locally, and the degree of orientation is lowered.
On the other hand, in the case of hot extruding a formed body in which thin pieces are laminated, if the compression direction at the time of forming coincides with the compression direction at the time of hot extruding, it is in the normal direction of the maximum surface of the thin piece. Stress is applied evenly. For this reason, local variations in strain direction and disorder of the orientation of the easy magnetization axis are unlikely to occur. The fact that the rare earth magnet according to the present invention has a higher coercive force and a higher residual magnetic flux density compared to the conventional rare earth magnet is considered to be due to such local disturbance.

[3.2. Changes in magnetic properties due to differences in hot plastic working methods]
In Patent Document 8, when raw powder of a rare earth magnet is molded and the formed body is subjected to hot upsetting or hot rolling, hot upsetting or hot in a direction different from the forming direction by 90 °. It is described that the magnetic properties are improved by rolling.
On the other hand, when the extruded product is hot extruded, if the compression direction at the time of molding and the compression direction at the time of hot extrusion are almost parallel, high holding force and high residual magnetic flux density (or maximum energy product) Can be made compatible.

  Details of the reason why the opposite result can be obtained by different hot plastic working methods are unclear, but probably the size and distribution of plastic strain during hot plastic working and the region where plastic deformation has not occurred. It is thought that the stress state is related.

(Example 1, Comparative Example 1)
[1. Preparation of sample]
Two types of powders having different compositions were prepared using the ultra-quenching method. The total amount of rare earth elements (R) in each powder was 14.0 at% or 14.2 at%. Each powder was cold formed (uniaxial pressure) at room temperature to obtain a cuboid shaped body.
Next, hot extrusion was performed using the extrusion molding apparatus 12 shown in FIG. The temperature of the extrusion process was 750 ° C., and the strain rate was changed in the range of 0.03 to 0.08 s ⁻¹ so that the compressive strain was 1.3. Moreover, the angle difference at the time of hot extrusion was set to 5 ° or less (Example 1) or 90 ° (Comparative Example 1).

[2. Test method]
[2.1. Organization]
The cross-sectional structure of the extruded material was observed using SEM.
[2.2. Magnetic properties]
The magnetic characteristics were measured by a closed magnetic circuit type and pulse excitation type BH tracer. The closed magnetic circuit type BH tracer is a direct current magnetic flux meter TRF-5BH-25 manufactured by Toei Industry Co., Ltd. The pulse excitation type BH tracer is a pulse excitation type magnetic property measuring device TPM manufactured by Toei Industry Co., Ltd. -2-08s-25VT was used.
Since the closed magnetic circuit type BH tracer is not affected by the demagnetizing field, the residual magnetic flux density is measured. The pulse excitation type BH tracer is not affected by the saturation of the magnetic pole. It is suitable for measuring the coercivity of a high coercivity magnet.
The residual magnetic flux density was measured using a closed magnetic circuit type BH tracer, and the coercive force was measured using a pulse excitation type BH tracer.

[3. result]
[3.1. Organization]
FIG. 5A and FIG. 5B show a micrograph and an enlarged photograph of a cross section of an extruded material having an angular difference of 5 ° or less during extrusion processing, respectively. FIG. 5C and FIG. 5D respectively show a cross-sectional micrograph and an enlarged photograph of an extruded material having an angle difference of 90 ° during extrusion. FIG. 5 shows that the shape of the powder grain boundary varies greatly depending on the difference in angle.

[3.2. Magnetic properties]
Table 1 shows the magnetic properties of the magnets obtained by hot extrusion under various conditions. Table 1 also shows the powder composition and the extrusion processing conditions. FIG. 6 shows the magnetic properties of the magnets obtained by hot extrusion under various conditions. Table 1 and FIG. 6 show the following.

(1) In any sample, the magnetic property of the extruded material having an angle difference of 5 ° or less (‖) was improved from the magnetic property of the extruded material having an angle difference of 90 ° (⊥). This is because, by making the compression direction of cold forming and the compression direction at the time of extrusion almost parallel, variation in local strain direction near the powder grain boundary and disorder of orientation of the easy magnetization axis were reduced. Conceivable.

(2) In any sample, the coercive force decreased as the strain rate decreased. This is presumably because the crystal grains grew as the hot working time increased.
(3) In all the samples, the residual magnetic flux density was maximized at a certain strain rate. This is because the orientation does not advance because the anisotropic growth of crystal grains becomes insufficient at high strain rates (short time), while the equiaxed abnormal grain growth occurs at low strain rates (long time). This is probably because the orientation of the film was disturbed.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the gist of the present invention.

  The rare earth magnet and the manufacturing method thereof according to the present invention can be used as a magnet used in a motor, a speaker, a microphone, a generator, and the like and a manufacturing method thereof.

DESCRIPTION OF SYMBOLS 10 Molded body 12 Extrusion apparatus 14 Die 14a Cavity 16 Punch 18 Extrusion material

Claims (7)

A method for producing a rare earth magnet having the following configuration.
(1) The method for producing the rare earth magnet includes:
A flaky powder made of a magnetic material containing a rare earth element (R), the powder for producing the rare earth magnet containing anisotropically shaped grains having magnetic anisotropy and a nonmagnetic grain boundary phase. A molding step of molding and obtaining a molded body;
A hot extrusion process of performing hot extrusion of the molded body and orienting the easy axis of magnetization of the anisotropically shaped grains.
(2) The angle formed by the compression direction in the molding step and the compression direction in the hot extrusion step is 5 ° or less.   2. The method for producing a rare earth magnet according to claim 1, wherein the hot extruding step further comprises hot extruding the molded body so that the compression strain becomes 1.0 or more.   In the hot extrusion process, the compact is hot-extruded so that the dimension in the compression direction (X direction) at the time of molding is reduced and the dimension in the Y direction orthogonal to the X direction is expanded. The method for producing a rare earth magnet according to claim 1 or 2.   The rare earth magnet according to any one of claims 1 to 3, wherein the powder is obtained by producing a ribbon by ultra-cooling the molten metal material and crushing the ribbon. Manufacturing method.   5. The magnetic material according to claim 1, wherein the magnetic material is an RTB-based alloy containing at least the rare earth element (R), the element T (T is Fe or Fe + Co), and B. 6. Method for producing rare earth magnets. The RTB-based alloy is
12.5 ≦ R ≦ 15.0 at%,
4.5 ≦ B ≦ 6.5 at%,
0.1 ≦ Ga ≦ 0.7 at%,
0 ≦ Co ≦ 6.0 at%, and
0 ≦ Cu + Al ≦ 2.0at%
The method for producing a rare earth magnet according to claim 5, wherein the balance consists of Fe and inevitable impurities.   A rare earth magnet obtained by the method according to any one of claims 1 to 6.

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