JP2002246215A - Sintered magnet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an Nd2Fe14B-based sintered magnet which is high in mechanical strength and has an excellent magnetic characteristic. SOLUTION: This sintered magnet contains an R (R is at lest one kind of rare earth element), an Fe and B. A grain boundary phase exists in a particle grain boundary. The scanning type microscopic image of the magnet cross section is divided like a vertically crossed lines at a specified interval by straight lines extending respectively in a direction of magnetization easy axial direction and a direction of magnetization hard axial direction, and one straight line extending in the magnetization easy axial is called an easy axial and the other is called a hard axial line. The ratio of total length of a part of the easy axial that crosses the grain boundary is assumed to be A, and the ratio of total length of a part of the hard axial line that crosses the grain boundary is assumed to be B. In this case, an equation, 0.8<=B/A<=1.2, can be established.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a rare earth sintered magnet having an Nd ₂ Fe ₁₄ B-based composition.

[0002]

2. Description of the Related Art As a rare earth magnet having high performance, for example, Nd described in Japanese Patent No. 1431617 is disclosed.
₂ Fe ₁₄ B-based magnets are known.

The demand for Nd ₂ Fe ₁₄ B-based magnets has been increasing in recent years because they are relatively inexpensive and have high magnetic properties as rare earth magnets. When an Nd ₂ Fe ₁₄ B-based magnet is used for a motor, not only high magnetic properties are required, but also high mechanical strength is required. For example, in a motor of an electric vehicle, a rotor is configured by fixing a magnet to a yoke made of a silicon steel plate or the like. The magnet is fixed by driving the magnet into the yoke so that the magnet does not come off when the rotor rotates at high speed. Therefore, a high mechanical strength is required for the magnet in order to prevent breakage at the time of driving.

In order to increase the mechanical strength of the Nd ₂ Fe ₁₄ B-based magnet, for example, Japanese Patent Application Laid-Open No. 5-283220 discloses
It has been proposed to include a boride such as TiB ₂ in a magnet. However, in this proposal, there is a problem that magnetic properties are deteriorated because a nonmagnetic boride is contained.

[0005]

FIG. 3A shows a conceptual diagram of a three-point bending strength test. The present inventors have found that in such a three-point bending strength test, the bending strength in the easy axis (c-axis) direction of the Nd ₂ Fe ₁₄ B crystal is relatively low.

An object of the present invention is to provide an Nd ₂ Fe ₁₄ B-based sintered magnet having high mechanical strength and excellent magnetic properties.

[0007]

This and other objects are attained by the present invention which is defined below as (1). (1) A sintered magnet containing R (R is at least one kind of rare earth element), Fe and B, wherein a grain boundary phase exists in a crystal grain boundary, and an easy axis direction and a hard axis direction The scanning electron microscope image of the magnet cross section is divided into grids at regular intervals by straight lines extending in each direction, the straight line extending in the easy axis direction is defined as the easy axis, and the straight line extending in the hard axis direction is defined as the hard axis. When the ratio of the total length of the portion crossing the grain boundary phase in the easy axis is A, and the ratio of the total length of the portion crossing the grain boundary phase in the hard axis is B, 0.8 ≦ B / A A sintered magnet with ≦ 1.2.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the present invention, the mechanical strength of an R ₂ Fe ₁₄ B-based magnet is improved without deteriorating its magnetic properties by controlling its structure. Note that R
Is a rare earth element.

The R ₂ Fe ₁₄ B-based sintered magnet has crystal grains composed of the R ₂ Fe ₁₄ B phase, which is a hard magnetic phase, and has a grain boundary phase at a crystal grain boundary. The grain boundary phase is an R-rich phase, R
_1.1 B-rich phase such as Fe ₄ B ₄ , R oxide phase such as R ₂ O ₃ ,
An R-rich phase is generally composed of an R carbide phase such as R ₂ C _{3 and} has the highest abundance ratio. The presence of the R-rich phase is essential for the coercive force development.

In the present invention, the mechanical strength of the magnet is improved by controlling the shape of the grain boundary phase mainly composed of the R-rich phase. Regarding the shape control of the grain boundary phase in the present invention,
This will be described with reference to FIGS.

FIGS. 1 and 2 are scanning electron micrographs (BEI mode, CO2) of the polished surface of an R ₂ Fe ₁₄ B-based sintered magnet.
(MPO image). In this photograph, the grain boundary phase is a region having a lower concentration than the crystal grains and exhibiting white. That is,
This is a region other than crystal grains and pores (voids).

In the present invention, the scanning electron microscope image of the cross section of the magnet is divided at regular intervals by straight lines extending in the easy axis direction and the hard axis direction, respectively. In this specification,
The straight line extending in the easy axis direction is called an easy axis, and the straight line extending in the hard axis direction is called a hard axis. Then, when the ratio of the total length of the portion crossing the grain boundary phase in the easy axis is A, and the ratio of the total length of the portion crossing the grain boundary phase in the hard axis is B, in the magnet of the present invention, 0.8 ≦ B / A ≦ 1.2, and preferably 0.85 ≦ B / A ≦ 1.15. If B / A is too small or too large, c
The bending strength in the axial direction decreases. That is, in the present invention, the mechanical strength of the magnet is improved by controlling the shape anisotropy of the grain boundary phase correlated with B / A. In measuring A and B, it is preferable that the length of the object to be measured on both the easy axis and the difficult axis is 4 mm or more, and that the length of the object to be measured on both lines is the same. If the length to be measured is too short, the correlation between B / A and the shape anisotropy of the grain boundary phase will be low. However, even if the length of the object to be measured is significantly increased, the above-mentioned correlation is not significantly improved. Therefore, the length of the object to be measured does not need to exceed 10 mm.

The above-mentioned fixed interval when the magnet section is sectioned in a grid pattern by the easy axis and the hard axis is not particularly limited. However, if the separation interval is extremely narrow, the correlation between B / A and the shape anisotropy of the grain boundary phase may decrease. Also,
If the separation interval is extremely wide, a scanning electron micrograph must be taken for a large area. Therefore, the separation interval is preferably set to about 10 to 15 μm. Usually, the separation interval of 12.5 μm in Examples described later is used. It may be used.

In the present invention, the direction of the axis of easy magnetization is the direction in which the magnetic field is applied when the powder of the raw material alloy is molded in a magnetic field during the production of the magnet. The hard axis direction is a direction orthogonal to the easy axis direction.

As described above, the R ₂ Fe ₁₄ B sintered magnet has a relatively low bending strength in the c-axis direction of the R ₂ Fe ₁₄ B crystal. That is, in the three-point bending strength test as shown in FIG. 3A, as shown in FIG. 3C, the bending strength when pressing in the direction of folding the c-axis is as shown in FIG. Are relatively lower than the bending strength in the b-axis direction shown in FIG. 3A and the bending strength in the a-axis direction shown in FIG. Note that R ₂ Fe ₁₄
Since the B crystal is tetragonal, the a-axis and the b-axis are equivalent. Shape anisotropy control of the grain boundary phase in the present invention improves the bending strength of the c-axis direction, as a result, to realize a high R ₂ Fe _{1 4} B based sintered magnet mechanical strength in all directions Can be. Moreover, in the present invention, the shape of the R-rich phase, which plays an important role in expressing the coercive force, is made isotropic, so that the coercive force is also improved. Also, unlike the above-mentioned Japanese Patent Application Laid-Open No. 5-283220, since the nonmagnetic grain boundary phase is not increased, the residual magnetic flux density does not decrease.

In the present invention, means for controlling the B / A, which represents the shape anisotropy of the grain boundary phase, is not particularly limited, but control of sintering conditions and / or aging conditions is particularly effective. Control of the aging conditions is most effective.
Since preferable specific conditions for sintering and aging treatment vary depending on the magnet composition, particularly the content of the element R, it is preferable to determine experimentally for each target magnet composition.

[0017] The sintered magnet of the present invention has R (where R is Y
At least one rare earth element containing Fe), Fe and B.

It is preferable that the contents of R and B satisfy 12 ≦ R ≦ 16 and 4 ≦ B ≦ 8 in terms of mole percentage. The balance is substantially Fe.

As the rare earth element R, Nd, Pr, D
It is preferable to use at least one of y and Tb. In addition, in addition to these, La, Ce, Sm, Pm,
One or more of Eu, Gd, Ho, Er, Tm, Yb, Lu, and Y may be used. When two or more elements are used as R, a mixture such as misch metal can be used as a raw material. If the R content is too small, a high coercive force cannot be obtained because the crystal structure becomes a cubic structure having the same structure as α-Fe. On the other hand, if the R content is too large,
The number of R-rich non-magnetic phases increases, and the residual magnetic flux density decreases. If the Fe content is too low, the residual magnetic flux density will be low, and if it is too high, the coercive force will be low. If the B content is too small, the coherence becomes insufficient due to the rhombohedral braiding, and if it is too large, the B-rich nonmagnetic phase increases and the residual magnetic flux density decreases.

By replacing part of Fe with Co, the temperature characteristics can be improved without impairing the magnetic characteristics. In this case, if the amount of Co substitution exceeds 50% of Fe, the magnetic properties deteriorate, so the amount of Co substitution is preferably set to 50% or less.

Further, by replacing a part of B with one or more of C, P and S, it is possible to realize an improvement in productivity and a reduction in cost. In this case, the substitution amount is preferably 4 mol% or less of the whole. In addition, improvement of coercive force,
Al, Ti, V,
Cr, Mn, Bi, Nb, Ta, Mo, W, Sb, G
e, Sn, Zr, Ni, Si, Hf, Ga, Zn, Cu
And the like may be added. In this case, the amount of addition is preferably 10 mol% or less in total.

The sintered magnet has a main phase having a substantially tetragonal crystal structure. The particle size of this main phase is 3-50 μm
It is preferred that it is about. And usually, the volume ratio is 1
15% non-magnetic phase. The non-magnetic phase is the above-mentioned R-rich phase, B-rich phase, R-oxide phase, R-carbide phase and the like.

Next, an example of a method for producing the sintered magnet of the present invention will be described.

First, an alloy is cast to obtain an ingot.
The obtained ingot is placed in a disc mill or the like for 10 to 1
The material is roughly pulverized to a particle size of about 00 μm, and then finely pulverized to a particle size of about 0.5 to 5 μm by a jet mill or the like.
In addition, hydrogen occlusion grinding can also be performed. In the hydrogen storage pulverization, the ingot is coarsely pulverized to about 30 mm square, and finely pulverized by performing hydrogen storage and hydrogen release at least once. The hydrogen storage pulverization and the mechanical pulverization may be used in combination. The element R becomes a hydride when the hydrogen absorbing and pulverizing is performed, but is dehydrogenated at around 800 ° C. in the temperature rising process of the sintering step.

The obtained powder is molded in a magnetic field to obtain a molded body. The magnetic field strength is preferably 800 to 8000 kA / m, and the molding pressure is preferably about 50 to 500 MPa.

Next, the compact is sintered. The atmosphere gas during sintering is preferably composed of a rare gas such as Ar or an inert gas such as nitrogen, and particularly preferably composed of a rare gas. Sintering temperature (stable temperature) is 1000-1
Preferably, the temperature is set to 150 ° C. The stable temperature is a temperature in a stable temperature range between a temperature rising process and a temperature falling process. The time for maintaining the temperature at a stable temperature is preferably 0.1 to 100 hours. If the sintering temperature is too low or the sintering time is too short, sintering does not proceed sufficiently and the residual magnetic flux density tends to be low. On the other hand, if the sintering temperature is too high or the sintering time is too long, the crystal grains become coarse and the coercive force tends to decrease.

After sintering, an aging treatment is usually performed. The aging treatment is preferably performed in an inert gas atmosphere, preferably at a temperature of 500 ° C. or higher and a sintering temperature or lower, more preferably 5 ° C.
It is performed by heating at 00 to 950 ° C. for 0.1 to 100 hours. The coercive force is further improved by the aging treatment. Note that the aging treatment may be constituted by a multi-step heat treatment.
For example, in the aging treatment consisting of two stages of heat treatment, the first stage heat treatment is performed at a temperature of 700 ° C. or more and less than the sintering temperature by 0.1 to 50 °
And heat-treating the second stage at 500-700 ° C. for 0.1
It is preferably performed for up to 100 hours.

The use of the sintered magnet of the present invention is not particularly limited, and can be applied to various devices such as a motor and a speaker. However, the sintered magnet of the present invention is particularly suitable for devices requiring high strength. It is.

[0029]

EXAMPLES Sample No. 1-1 An alloy powder was obtained by grinding a cast alloy ingot in a nitrogen gas atmosphere. The composition (molar percentage) of the alloy powder was composition I: 14.59Nd-0.42Dy-0.53Co-0.77Al-Fe.

Next, the alloy powder was compacted at a pressure of 150 MPa in a static magnetic field having a strength of 1200 kA / m to obtain a compact. Next, the molded body was sintered at 1080 ° C. for 4 hours in a vacuum to obtain a sintered body. This sintered body was designated as Sample No. 1-1.

Sample No. 1-2 Sample No. 1-1 was compared with Sample No. 1-1 in an Ar atmosphere.
Sample No. 1-2 was subjected to an aging treatment consisting of a heat treatment at 50 ° C. for 1 hour and a subsequent heat treatment at 540 ° C. for 5 hours.

Sample No. 1-3 Sample No. 1-1 was compared with Sample No. 1-1 in an Ar atmosphere.
Sample No. 1-3 was subjected to an aging treatment consisting of a heat treatment at 00 ° C. for one hour.

Sample No. 1-4 Compared to Sample No. 1-1 , 7
Sample No. 1-4 was subjected to an aging treatment consisting of a heat treatment at 00 ° C. for 1 hour and a heat treatment at 600 ° C. for 1 hour.

Sample No. 2-1 An alloy powder was obtained by grinding the cast alloy ingot in a nitrogen gas atmosphere. The composition (molar percentage) of the alloy powder was composition II: 11.49Nd-3.43Dy-0.59Co-0.77Al-0.29Sn-Fe.

Next, the alloy powder was compacted under a pressure of 150 MPa in a static magnetic field having a strength of 1200 kA / m to obtain a compact. Next, the compact was sintered at 1100 ° C. for 4 hours in a vacuum to obtain a sintered body. This sintered body was designated as Sample No. 2-1.

Sample No. 2-2 Sample No. 2-1 was compared with Sample No. 2-1 in an Ar atmosphere.
Sample No. 2-2 was subjected to an aging treatment consisting of a heat treatment at 50 ° C. for 1 hour and a heat treatment at 540 ° C. for 1 hour.

Sample No. 2-3 Sample No. 2-1 was compared with Sample No. 2-1 in an Ar atmosphere.
Sample No. 2-3 was subjected to an aging treatment consisting of a heat treatment at 00 ° C. for 5 hours.

Evaluation After polishing the cross section of each sample, the B
A photograph was taken at a magnification of 1000 in the EI (COMPO) mode. FIG. 1 shows a photograph of Sample No. 1-1, and FIG. 2 shows a photograph of Sample No. 1-2. An easy axis and a difficult axis are drawn at intervals of 1 cm (actual size: 12.5 μm) in the photographed image, and the ratio A of the grain boundary phase in the easy axis
And the ratio B of the grain boundary phase at the hard axis is measured, and B /
A was calculated. In addition, the number of photographs of the measurement object is six for each of the easy axis and the difficult axis of each sample.
The length to be measured on the easy axis and the difficult axis was 4.725 mm. Also, for each sample,
Using a universal testing machine AGS-1000A manufactured by Shimadzu Corporation, a three-point bending strength test was performed according to the provisions of JIS R1601. However, when measuring the bending strength, the test piece was not rounded, the dimensions of the test piece were 10 mm × 10 mm × 1.5 mm, and the crosshead speed was 10 mm / min. The bending strength is shown in FIG.
As shown in FIG. 3 (C) and FIG. 3 (D), measurements were made in the b-axis direction, the c-axis direction, and the a-axis direction. Also,
The coercive force (HcJ) of each sample was measured. Table 1 shows the results.

[0039]

[Table 1]

From Table 1, the effect of the present invention is clear.
That is, if B / A is within the range defined by the present invention,
It can be seen that the bending strength in the c-axis direction is improved to the same level as the bending strength in other directions, and a sintered magnet with high mechanical strength can be obtained.

The density of the sample of composition I was 7.53.
Mg / m ³ , and the density of the composition II sample was 7.63 Mg / m ³
, And none of them was changed by the aging treatment. Therefore, the improvement in bending strength according to the present invention is not due to the improvement in density.

[0042]

According to the present invention, the mechanical strength of the R ₂ Fe ₁₄ B-based sintered magnet is controlled by controlling the shape of the grain boundary phase mainly composed of the R-rich phase to be isotropic. Can be improved.

[Brief description of the drawings]

FIG. 1 is a drawing substitute photograph showing a crystal structure, which is a scanning electron microscope photograph of a polished surface of a magnet.

FIG. 2 is a drawing substitute photograph showing a crystal structure, and is a scanning electron micrograph of a polished surface of a magnet.

FIG. 3A is a conceptual diagram of a three-point bending strength test.
(B) shows b in the three-point bending strength test shown in (A).
It is a figure which shows the direction of the crystal axis of a magnet when measuring the bending strength in an axial direction. (C) is a diagram showing the direction of the crystal axis of the magnet when measuring the bending strength in the c-axis direction in the three-point bending strength test shown in (A). (D) is a diagram showing the direction of the crystal axis of the magnet when measuring the bending strength in the a-axis direction in the three-point bending strength test shown in (A).

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H01F 1/053 H01F 1/04 H

Claims (1)

[Claims] 1. A sintered magnet containing R (R is at least one kind of rare earth element), Fe and B, wherein a grain boundary phase exists at a crystal grain boundary, The scanning electron microscope image of the magnet cross section is divided into grids at regular intervals by straight lines extending in the hard axis direction, and the straight line extending in the easy axis direction is defined as the easy axis, and the straight line extending in the hard axis direction is difficult. When the ratio of the total length of the portion crossing the grain boundary phase in the easy axis is A, and the ratio of the total length of the portion crossing the grain boundary phase in the hard axis is B, 0.8 ≦ B /A≦1.2 sintered magnet.