JP2015057820A - R-t-b-based sintered magnet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a R-T-B-based sintered magnet exhibiting superior corrosion resistance, even if a wide grain boundary phase is formed as a Nd-rich composition in order to enhance magnetic decoupling effect between RTB main phase crystal particles, for higher coercive force of a sintered magnet.SOLUTION: A R-T-B-based sintered magnet contains RTB main phase crystal particles, R-rich phase crystal particles, and a grain boundary phase. R-rich phase crystal particles having crystal habit are added to the R-rich phase crystal particles.

Description

The present invention relates to an RTB-based sintered magnet, and more particularly to an RTB-based sintered magnet having both high coercive force and excellent corrosion resistance.

An RTB-based sintered magnet represented by an Nd-Fe-B-based sintered magnet (R is a rare earth element, T is one or more iron group elements having Fe as an essential element, and B is boron) Since it has a high saturation magnetic flux density, it is advantageous for miniaturization and high efficiency of equipment used, and is used for a voice coil motor of a hard disk drive. In recent years, it is being applied to various industrial motors and drive motors for hybrid vehicles, and further spread to these fields is desired from the viewpoint of energy conservation. By the way, in application of the RTB-based sintered magnet to a hybrid vehicle or the like, since the magnet is exposed to a relatively high temperature, it is important to suppress high temperature demagnetization due to heat. It is well known that a technique of sufficiently increasing the coercive force (Hcj) at room temperature of an RTB-based sintered magnet is effective for suppressing this high temperature demagnetization.

For example, as a technique for increasing the coercive force at room temperature of an Nd—Fe—B based sintered magnet, there is a technique in which a part of Nd of the main phase Nd ₂ Fe ₁₄ B compound is replaced with heavy rare earth elements such as Dy and Tb. Are known. By substituting a part of Nd with a heavy rare earth element, the magnetocrystalline anisotropy is increased, and as a result, the coercive force at room temperature of the Nd—Fe—B based sintered magnet can be sufficiently increased. In addition to substitution with heavy rare earth elements, addition of Cu element or the like is said to be effective in improving coercivity at room temperature (Patent Document 1). By adding Cu element, it is considered that the Cu element forms, for example, an Nd—Cu liquid phase at the grain boundary, thereby smoothing the grain boundary and suppressing the occurrence of reverse magnetic domains.

Patent Document 2 discloses a technique for improving the coercive force by controlling the grain boundary phase, which is a microstructure of a rare earth magnet. Although the magnet disclosed herein is a magnet having a so-called nanocomposite magnetic structure, a nonmagnetic phase (for example, an Nd—Cu alloy) is diffused between the magnetic structures to form a grain boundary phase without creating an uneven grain boundary phase. It is said that the field phase can uniformly surround the magnetic structure and improve the coercive force. In particular, in order to improve the properties as a nanocomposite magnet, the thickness of the grain boundary phase is preferably 0.5 nm to 10 nm.

JP 2002-327255 A JP 2012-234985 A

In order to improve the coercive force of the RTB-based sintered magnet, as described above, a part of R of the main phase R ₂ T ₁₄ B is replaced with a heavy rare earth element such as Tb or Dy. Is the simplest. However, heavy rare earth elements such as Dy and Tb have a resource problem because their origin and production are limited. With the replacement, for example, a decrease in the residual magnetic flux density (Br) is unavoidable due to the antiferromagnetic coupling between Nd and Dy. Furthermore, compounds containing these heavy rare earth elements are known to have a sudden decrease in the anisotropic magnetic field that becomes the source of coercive force with temperature, and a large change in coercive force at high temperatures is expected. . Although the addition of the above Cu element is an effective method for improving the coercive force, further enhancement of the coercive force is desired in order to expand the application area of the RTB-based sintered magnet.

In order to improve the coercive force of the RTB-based sintered magnet, it is important to break the magnetic coupling between the R ₂ T ₁₄ B crystal grains that are the main phase. If each main phase crystal particle can be magnetically isolated, even if a reverse magnetic domain is generated in a certain crystal particle, the adjacent crystal particle is not affected, and the coercive force can be improved.

Conventionally, an RTB-based sintered magnet includes R ₂ T ₁₄ B main phase crystal particles, an R rich phase having a higher R concentration than the main phase crystal particles, and a B concentration higher than the main phase crystal particles. It has been known that it consists of a high B rich phase and a grain boundary phase existing between these phases. Among these, the R-rich phase and the B-rich phase are not particularly important in terms of magnetic properties, and it has been considered preferable to be smaller from the viewpoint of improving the residual magnetic flux density Br. In recent years, it has been pointed out that the grain boundary phase is important for increasing the coercive force, and it is important to form a thick and homogeneous grain boundary phase in order to enhance the magnetic separation effect between adjacent main phase crystal grains. It has been recognized that.

In order to form a thick grain boundary phase between crystal grains constituting the RTB-based sintered magnet, it is conceivable to make the alloy raw material an R-rich composition. Thereby, excess R element that is not spent for forming the main phase R _{2 T} 14 _B compound is, it is expected to form a thick grain boundary phase. However, according to the experiments by the present inventors, the grain boundary phase can be increased by making the alloy raw material an R-rich composition, while a so-called particulate R-rich phase in which R elements are unevenly distributed is formed. As a result, it has been clarified that the R-rich phase contains a large amount of R element, and thus a new problem of extremely inferior corrosion resistance becomes apparent.

The present invention has been made in view of the above, and in an R-T-B system sintered magnet, by improving the corrosion resistance of the particulate R-rich phase, R having both high coercive force and excellent corrosion resistance. An object is to provide a TB sintered magnet.

Therefore, the present inventors have intensively studied to solve the above problems, and as a result, have come to complete the following invention.

That is, the RTB-based sintered magnet according to the present invention includes an R ₂ T ₁₄ B main phase crystal particle and an R rich phase crystal having a higher concentration of R element than the R ₂ T ₁₄ B main phase crystal particle. The R-rich phase crystal particles include at least particles and a grain boundary phase, and the R-rich phase crystal particles include R-rich phase crystal particles having a crystal habit.

As described above, the R-rich phase crystal particles formed in the R-T-B system sintered magnet are crystal particles having crystal habits, so that the R-rich phase crystal particles are stable with high crystallinity. It can be present as a compound. As a result, this compound is modified to have a very low reactivity with oxygen, moisture, and the like, so that the corrosion resistance can be improved. Since the oxygen, moisture, and the like that cause corrosion of the R-T-B system sintered magnet enter from the surface of the sintered body, the R-rich phase crystal particles having crystal habits are near the surface of the sintered body. Good to be. Here, the vicinity of the surface means a range within 500 μm from the surface of the sintered body.

The R-rich phase crystal particles having the crystal habit are preferably R oxide phases. By doing so, oxygen inevitably present in the R-T-B system sintered magnet is consumed in the R-rich phase crystal particles having crystal habits, thereby driving out oxygen from the main phase crystal particles, Therefore, the magnetic properties of the sintered body can be improved.

Further, the R-rich phase crystal particles having the crystal habit are preferably R—O—C—N compounds. R-T-B sintered magnets are generally manufactured by powder metallurgy. In this powder metallurgy process, for example, it is more stable to mix some oxygen, nitrogen and carbon into a part of the raw material alloy. It is known to be preferred for. The pulverization step is often performed in a nitrogen atmosphere, and here again nitrogen is mixed into the raw material alloy. In the molding process, a lubricant such as zinc stearate is added for ease of molding, and carbon is mixed into the sintered body. Therefore, if the R-rich phase crystal particle having the crystal habit is the R—O—C—N compound, inevitable mixed impurities such as C and N are fixed and consumed in the R-rich phase crystal particle. Therefore, the magnetic properties of the sintered body can be improved, and at the same time, R-rich phase crystal particles having a crystal habit can be easily generated. The R-rich phase crystal particles having the crystal habit may be R—O—C compounds or R—O—N compounds. That is, the R—O—C—N compound referred to in this specification refers to a compound in which C—N is added to a R—O oxide as a basic structure.

In the RTB-based sintered magnet according to the present invention, it is preferable that R includes Nd or Pr, or Nd and Pr. By using such an element as R, the RTB-based sintered magnet can be made at low cost. It is also possible to replace a very small part of R with heavy rare earth elements.

As used herein, “having a habit” R-rich phase crystal particle means that the cross-sectional outer shape of the R-rich phase crystal particle becomes a crystal structure of the compound constituting the R-rich phase by recrystallization after sintering. This refers to the resulting unequal polygonal shape. When a crystal grows, crystallographically equivalent surfaces are considered to have the same growth rate, but the crystal shape changes due to differences in local crystal growth conditions, resulting in a cross-sectional shape of the crystal grain. It becomes an unequal side polygonal shape. Such a growth mode of crystal grains is generally referred to as crystal grains having a crystal habit. A preferred form of the compound constituting the R-rich phase is an R oxide phase R—O compound. Although this compound exhibits a cubic crystal, the resulting R-rich crystal habit exhibits various shapes depending on the local environment during crystal growth. However, since the cross-sectional outer shape exhibits an unequal side polygonal shape including many straight portions, it can be recognized that it has crystal habit, and it can be confirmed that a stable compound is formed. Whether or not the R-rich phase crystal grains have crystal habits can also be confirmed by analyzing the crystal orientation of the growth surface of the crystal grains as described below.

The unequal polygonal shape including many straight portions means that there are no rounded corners or that the corners are less rounded, and this can be evaluated using a circularity coefficient. The circularity coefficient of the cross-sectional shape of the R-rich phase having crystal habit in the present invention is less than 0.80. If the circularity coefficient of the cross-sectional shape is less than 0.80, it can be said that the R-rich phase with high crystallinity is growing, and that the R-rich phase can be modified into a stable compound. The circularity coefficient will be described later.

According to the present invention, an RTB-based sintered magnet having a high coercive force and excellent corrosion resistance can be provided, and thus can be applied to a motor or the like used in a high temperature environment. A B-based sintered magnet can be provided.

It is a figure showing the section structure of the RTB system sintered magnet of the embodiment concerning the present invention. In the electron microscope image of the sample cross section of Example 2, it is the figure which added the tangent to the edge | side of the unequal polygon which the R rich phase crystal particle exhibits. It is a figure which shows the relationship between the tangent of the R rich phase crystal grain of FIG. 2, and a crystal orientation.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. The RTB-based sintered magnet referred to in the present invention is a sintered magnet including R ₂ T ₁₄ B main phase crystal particles, R-rich phase crystal particles, and a grain boundary phase. It contains one or more rare earth elements, T contains one or more iron group elements containing Fe as an essential element, contains B, and further includes those added with various known additive elements.

FIG. 1 is a diagram showing a cross-sectional structure of an RTB-based sintered magnet according to an embodiment of the present invention. The RTB-based sintered magnet according to the present embodiment includes R ₂ T ₁₄ B main phase crystal particles 1, R-rich phase crystal particles 2, and grains formed between two or more adjacent crystal particles. The R-rich phase crystal particles 2 including the boundary phase 3 include the R-rich phase crystal particles 2 having a crystal habit as shown in FIG.

The R-rich phase crystal particle 2 having crystal habit in the present embodiment is an oxide of R, and more specifically is composed of an R—O—C—N compound. By forming the R-rich phase crystal particles as R-rich phase crystal particles having a stable compound, that is, a crystal habit, the corrosion resistance of the sintered body can be improved.

The term “crystal grains have crystal habit” means that the cross-sectional shape of the crystal grains does not have roundness or becomes an unequal polygonal shape with little roundness. Generally, particles having a spherical shape, that is, a circular shape with a cross-sectional shape, have a smaller surface area (specific surface area) in the same volume, so that the surface energy can be kept low. Accordingly, when the liquid phase is rapidly changed to the solid phase, the formed particles are rounded. However, in this case, lattice defects, strains, and the like are included along with the rapid phase change, resulting in high chemical activity, resulting in a decrease in corrosion resistance. On the other hand, in the R-rich phase crystal particles having crystal habits in the present embodiment, O, C, and N contained in the sintered body or introduced from the surface of the sintered body by grain boundary diffusion are combined with R and formed. In addition, at least the vicinity of the surface of the R-rich phase crystal particle is remelted and recrystallized to form a stable compound, so that it can be made into an R-rich phase crystal particle having high crystallinity, and it has a corrosion resistance despite its R-rich composition. Can be improved. Conversely, it can be said that the R-rich phase crystal particles have crystal habits by making the R-rich phase crystal particles a stable compound with good crystallinity.

Here, the “circularity coefficient” in this specification will be described. As described above, a crystal grain having a crystal habit has an unequal polygonal cross-sectional shape reflecting the local difference between the crystal system to which it belongs and the crystal growth rate. That is, it becomes an unequal side polygonal shape with no rounded corners or only a small roundness at the corners. Therefore, if the degree of roundness of the crystal grains is evaluated, it is related to the degree of appearance of crystal habits of the grains. In the present specification, the degree of roundness of the crystal grain cross-sectional shape is evaluated by a circularity coefficient defined by the following equation (1).
Circularity coefficient = (L1 / L2) ² = 4πS / L2 ² (1)
Here, L1: circumference of a circle having an area equal to the cross-sectional area of the target particle
L2: Perimeter of the target particle
S: The reason why the cross-sectional area ratio of the target particles is squared is to avoid the calculation of the square root in the evaluation of the circularity coefficient. The circularity coefficient can be easily evaluated by measuring the area of the cross section of the target crystal grain and the circumference surrounding the area using an electron microscope or the like. When the cross-sectional shape of the crystal grain becomes a perfect circle, the circularity coefficient has a maximum value of 1, and becomes smaller as the crystal becomes an unequal polygonal shape by shifting from the perfect circle due to anisotropic crystal growth. In the case of the R-rich phase crystal particles of the present embodiment, when the circularity coefficient is less than 0.8, it can be said that the crystal has a crystal habit, and a chemically stable compound is formed. It can be said that.

In the R ₂ T ₁₄ B main phase crystal particles constituting the RTB-based sintered magnet according to this embodiment, the rare earth R may be any of a light rare earth element, a heavy rare earth element, or a combination of both. Although good, Nd, Pr, or a combination of both is preferred from the viewpoint of material cost. The iron group element T is preferably Fe or a combination of Fe and Co, but is not limited thereto. B represents boron. In the R-rich phase crystal particles constituting the RTB-based sintered magnet according to this embodiment, the rare earth element R is preferably Nd, Pr, or a combination of both from the viewpoint of material cost. In the sintered magnet of this embodiment, the content of each element with respect to the total mass is as follows. In this specification, mass% is regarded as the same unit as weight%.
R: 29.5 to 33% by mass,
B: 0.7-0.95 mass%,
M: 0.03-1.5 mass%,
Cu: 0.01 to 1.5% by mass, and
Fe: substantially the balance, and
The total content of elements other than Fe among the elements occupying the balance: 5% by mass or less

Hereinafter, conditions such as the content of each element and the atomic ratio will be described in more detail.

The content of R in the RTB-based sintered magnet according to this embodiment is 29.5 to 33% by mass. When heavy rare earth elements are included as R, the total content of rare earth elements including heavy rare earth elements falls within this range. The heavy rare earth element refers to a rare earth element having a large atomic number, and generally corresponds to a rare earth element from 64 Gd to 71 Lu. When the content of R is within this range, high residual magnetic flux density and coercive force tend to be obtained. If the content of R is smaller than this, the R ₂ T ₁₄ B phase, which is the main phase, is difficult to form, and an α-Fe phase having soft magnetism is easily formed, resulting in a decrease in coercive force. On the other hand, when the content of R is larger than this, the volume ratio of the R ₂ T ₁₄ B phase is lowered, and the residual magnetic flux density is lowered. The content of R is preferably 30.0 to 32.5% by mass. Within such a range, the volume ratio of the R ₂ T ₁₄ B phase, which is the main phase, is particularly high, and a better residual magnetic flux density can be obtained.

R always contains either Nd or Pr, but the ratio of Nd and Pr in R is preferably 80 to 100 atom% in total of Nd and Pr, more preferably 95 to 100 atoms. %. Within such a range, better residual magnetic flux density and coercive force can be obtained.

As described above, the RTB-based sintered magnet according to the present embodiment may include heavy rare earth elements such as Dy and Tb as R. In this case, the RT-T according to the present embodiment is included. The content of heavy rare earth elements in the total mass of the -B based sintered magnet is 1.0% by mass or less in total of heavy rare earth elements, preferably 0.5% by mass or less, and 0.1% by mass The following is more preferable. According to the RTB-based sintered magnet according to the present embodiment, even if the content of heavy rare earth elements is reduced in this way, the content and atomic ratio of other elements satisfy a specific condition. A good high coercive force can be obtained.

The RTB-based sintered magnet according to this embodiment further includes a trace amount of additive elements. Known elements can be used as the additive element. The additive element preferably has an eutectic point on the phase diagram with the R element which is a constituent element of the R ₂ T ₁₄ B main phase crystal particles and the R element which is a constituent element of the R rich phase crystal particles. In this respect, the additive element is preferably Cu or the like, but may be other elements. As addition amount of Cu, it is 0.01-1.5 mass% of the whole.

In the present invention, the R element and an additive element having a eutectic point on the phase diagram play an important role in forming R-rich phase crystal grains having crystal habit. The additive element is present in the grain boundary phase in the vicinity of the R-rich phase crystal particles, thereby causing at least the surface layer of the R-rich phase crystal particles to react with the grain boundary phase, thereby at least the surface layer of the R-rich phase crystal particles. Is recrystallized to form a stable compound such as an R—O—C—N compound. At this time, it is considered that an additive element such as Cu is not taken into the R-rich phase crystal particles but remains in the grain boundary phase. That is, it is considered that the addition of the element Cu can play a catalytic role in promoting the reaction in the crystal growth of the R—O—C—N compound. An example of a specific method for adding Cu element will be described later.

The RTB-based sintered magnet according to the present embodiment further includes an element for forming a compound containing R and T elements in the grain boundary phase. For this purpose, it is preferable to add M element such as Al, Ge, Si, Sn, and Ga. By adding these elements to the RTB-based sintered magnet in addition to the above-mentioned Cu, it is possible to consume the R element of the R-rich raw material alloy and form a wide and uniform grain boundary phase. By consuming excess R element as a compound in the grain boundary phase, a wide grain boundary phase is formed, and thereby the magnetic fragmentation effect of the main phase crystal grains can be achieved and the coercive force can be improved. At the same time, corrosion resistance can be improved by consuming part of the R element as a compound. In the RTB-based sintered magnet of this embodiment, the content of M is 0.03 to 1.5% by mass. When the content of M is smaller than this range, a wide grain boundary phase is not formed and the coercive force becomes insufficient. When larger than this range, the saturation magnetization becomes low and the residual magnetic flux density is insufficient. Become. In order to obtain better coercive force and residual magnetic flux density, the M content is preferably 0.13 to 0.8 mass%.

The RTB-based sintered magnet of this embodiment includes Fe and other elements in addition to the above-described elements, and the Fe and other elements are the total of the above elements in the total mass of the sintered magnet. Occupy the remainder excluding the content. However, in order for the RTB-based sintered magnet according to this embodiment to sufficiently function as a magnet, the total content of elements other than Fe among the elements occupying the balance is the total mass of the sintered magnet. The content is preferably 5% by mass or less.

Co, like Fe, is an element represented by T in the basic composition of R ₂ T ₁₄ B, and forms the same phase as Fe. The RTB-based sintered magnet according to the present embodiment can contain Co. In that case, the Co content is preferably more than 0% by mass and 3.0% by mass or less. By including a Co-containing phase in the RTB-based sintered magnet, the Curie temperature is improved, and by leaving Co in the grain boundary phase, the corrosion resistance of the grain boundary phase is improved. It has high corrosion resistance. In order to obtain such an effect better, the content of Co is more preferably 0.3 to 2.5% by mass.

Further, the content of C in the RTB-based sintered magnet according to the present embodiment is 0.05 to 0.3% by mass. If the C content is smaller than this range, the coercive force becomes insufficient. If it is larger than this range, the ratio of the magnetic field value (Hk) when the magnetization is 90% of the residual magnetic flux density to the coercive force. In other words, the so-called squareness ratio (Hk / coercive force) becomes insufficient. In order to obtain better coercive force and squareness ratio, the C content is more preferably 0.1 to 0.25% by mass. Part of C is taken into R—O—C—N, which is R-rich phase crystal particles, to form a stable compound.

Moreover, content of O of the RTB-type sintered magnet which concerns on this embodiment is 0.03-0.4 mass%. When the content of O is smaller than this range, the corrosion resistance of the sintered magnet becomes insufficient. When it is larger than this range, a liquid phase is not sufficiently formed in the sintered magnet, and the coercive force is lowered. In order to obtain better corrosion resistance and coercive force, the content of O is preferably 0.05 to 0.3% by mass, and more preferably 0.05 to 0.25% by mass. A part of O is taken into the R—O—C—N compound which is R-rich phase crystal particles to form a stable compound.

The RTB-based sintered magnet according to the present embodiment can contain, for example, Zr as another element. In that case, the Zr content is preferably 0.25% by mass or less in the total mass of the sintered magnet. Zr can suppress abnormal growth of crystal grains in the manufacturing process of the RTB-based sintered magnet according to this embodiment, and the structure of the obtained sintered body (sintered magnet) is uniform and fine. Thus, the magnetic characteristics can be improved. In order to obtain such an effect better, the content of Zr is more preferably 0.03 to 0.25% by mass.

The RTB-based sintered magnet according to the present embodiment contains unavoidable impurities such as Mn, Ca, Ni, Cl, S, and F as constituent elements other than those described above, about 0.001 to 0.5 mass%. May be included.

In the RTB-based sintered magnet according to this embodiment, the N content is preferably 0.15% by mass or less. A part of N is taken into the R—O—C—N compound which is R-rich phase crystal particles to form a stable compound.

Next, an example of the manufacturing method of the RTB system sintered magnet which concerns on this embodiment is demonstrated. The RTB-based sintered magnet according to the present embodiment can be manufactured by an ordinary powder metallurgy method. The powder metallurgy method includes a preparation process for preparing a raw material alloy, a raw material alloy is pulverized, and a raw fine powder There are a pulverizing step, a forming step of forming a raw material powder to form a molded body, a sintering step of firing the molded body to obtain a sintered body, and a heat treatment step of subjecting the sintered body to an aging treatment. In the present embodiment, an additive element diffusion step is included before the heat treatment step.

A preparation process is a process of preparing the raw material alloy which has each element contained in the RTB system sintered magnet concerning this embodiment. First, a raw metal having a predetermined element is prepared, and a strip casting method or the like is performed using these. Thereby, a raw material alloy can be prepared. Examples of the raw metal include rare earth metals, rare earth alloys, pure iron, pure cobalt, ferroboron, and alloys thereof. Using these raw material metals, a raw material alloy is prepared so that an RTB-based sintered magnet having a desired composition can be obtained. In the preparation of the raw material alloy, a two-alloy method in which a raw material alloy that mainly forms a main phase and a raw material alloy that mainly forms a grain boundary phase and an R-rich phase can be separately used. A part of the additive element having the eutectic point on the R element and the phase diagram described above is included in this raw material alloy. When the two-alloy method is used, a part of this additive element is preferably included in the raw material alloy that forms the grain boundary phase and the R-rich phase.

The pulverization step is a step of pulverizing the raw material alloy obtained in the preparation step to obtain a raw material fine powder. This process is preferably performed in two stages, a coarse pulverization process and a fine pulverization process, but may be performed in one stage. The coarse pulverization step can be performed in an inert gas atmosphere using, for example, a stamp mill, a jaw crusher, a brown mill, or the like. It is also possible to perform hydrogen occlusion and pulverization in which hydrogen is occluded and then pulverized. In the coarse pulverization step, the raw material alloy is pulverized until the particle size becomes several hundred μm to several mm.

In the fine pulverization step, the coarse powder obtained in the coarse pulverization step is finely pulverized to prepare a raw material fine powder having an average particle size of about several μm. The average particle size of the raw material fine powder may be set in consideration of the degree of crystal grain growth after sintering. The fine pulverization can be performed using, for example, a jet mill. A fine pulverization step can be used instead of the grain boundary diffusion method to introduce oxygen, carbon, and nitrogen useful for the formation of R-rich phase crystal grains into the material. For example, the amount of oxygen in the material can be controlled by introducing a small amount of oxygen into an inert atmosphere during pulverization. For example, the amount of nitrogen in the material can be increased by setting the inert atmosphere during pulverization to nitrogen. Furthermore, by adding grinding aids such as zinc stearate and oleic acid amide, it is possible to obtain high orientation of fine powders in the next molding step, but these grinding aids can be used as a carbon source. Good. The R-rich phase crystal particles due to the introduced oxygen, carbon, and nitrogen are produced in the subsequent sintering step after molding. If an additive for promoting recrystallization is added in advance, it is easy to obtain R-rich phase crystal particles having crystal habits in the heat treatment step.

The forming step is a step of forming a compact by forming the raw material fine powder in a magnetic field. Specifically, after forming the raw material fine powder into a mold arranged in an electromagnet, molding is performed by applying a magnetic field with an electromagnet and pressing the raw material fine powder while orienting the crystal axis of the raw material fine powder. I do. The molding in the magnetic field may be performed at a pressure of about 30 to 300 MPa in a magnetic field of 1000 to 1600 kA / m, for example.

A sintering process is a process of baking a molded object and obtaining a sintered compact. After molding in a magnetic field, the compact can be fired in a vacuum or an inert gas atmosphere to obtain a sintered compact. Firing conditions are preferably set as appropriate according to conditions such as the composition of the molded body, the method of pulverizing the raw material fine powder, and the particle size, but may be performed at 1000 to 1100 ° C. for about 1 to 10 hours, for example.

The grain boundary diffusion process is considered to be effective for forming R-rich phase crystal particles having O, C, N and crystal habits necessary for the generation of R-rich phase crystal particles from the surface of the sintered body obtained above. An additive element such as Cu is diffused into the sintered body. When Cu is used as the additive element, for example, an eutectic Nd—Cu alloy powder is produced as the diffusion material. Even if the oxygen amount and the nitrogen amount are not controlled in the fine pulverization step, the diffusion material alloy may contain oxygen and nitrogen during the fine pulverization of the diffusion material alloy powder. Further, instead of using the additive in the pulverization step as a carbon source, a binder resin for attaching the diffusing material to the surface of the sintered body may be used as the carbon source.

After adhering the diffusing material to the surface of the sintered body, it may be heated at a temperature of 550 ° C. to 650 ° C. for about 1 hour to 50 hours. As a result, oxygen, carbon, nitrogen and Cu are introduced into the sintered body through the grain boundary phase. In order to introduce the diffusing material component into the sintered body, first, the sintered body is heated to generate a sufficient amount of liquid phase from the grain boundary portion including the triple point and the two-particle grain boundary of the sintered body. The liquid phase must be oozed out on the surface of the sintered body. Diffusion components are eluted in the liquid phase and can diffuse into the sintered body. If the heating temperature is too low, the amount of liquid phase is small and the components of the diffusing material cannot be sufficiently diffused into the sintered body. For example, when an eutectic alloy of Nd—Cu is used as the diffusion material, the diffusion material itself melts at 520 ° C., so that diffusion from a relatively low temperature that lacks the liquid phase at 550 ° C. is possible. On the other hand, when heating at a temperature exceeding 650 ° C., in addition to sufficient liquid phase seepage, the surface of the sintered body becomes too R-rich due to the melted diffusion material, and R ₂ T ₁₄ B crystal particles May cause abnormal grain growth and decrease in magnetic properties.

The heat treatment step is a step of aging the sintered body. After this step, the structure and microstructure within the RTB-based sintered magnet are determined. For example, if Cu or the like that promotes recrystallization is added, R-rich phase crystal particles having crystal habits are easily formed. In order to obtain R-rich phase crystal particles having crystal habit, it is necessary to perform heat treatment at an appropriate temperature that can promote recrystallization. For this reason, the heat treatment step may be performed in a temperature range of 500 ° C. to 900 ° C., The heat treatment may be performed in two stages, such as the heat treatment at about 550 ° C. after the heat treatment at about 800 ° C.

The microstructure of the RTB-based sintered magnet according to the present embodiment, that is, the crystal habit, cross-sectional shape, etc., of the R-rich phase crystal particles can be evaluated with an electron microscope. What is necessary is just to set a magnification suitably according to the particle diameter of the observation object of observation object. In order to increase the corrosion resistance of the R-T-B system sintered magnet, it is important to increase the corrosion resistance in the vicinity of the surface of the sintered magnet in which a corrosive factor enters. Considering the penetration of oxygen or the like, corrosion resistance from the sintered magnet surface to a depth of about 500 μm is important, and in this specification, this region is referred to as the vicinity of the surface. Therefore, it is sufficient to evaluate the R-rich phase crystal particles described above in this region.

Next, the present invention will be described in more detail based on specific examples, but the present invention is not limited to the following examples.

(Production of sintered body)
The sintered body was produced by a two alloy method. First, a raw material alloy was prepared by a strip casting method so that a sintered magnet having a magnet composition shown in Table 1 was obtained. As the raw material alloys, two types of main phase alloys A mainly forming the main phase of the magnet and grain boundary phase alloys B mainly forming the grain boundaries were prepared and prepared. In Table 1, bal. Indicates the remainder when the total composition of each alloy is 100% by mass, and (T.RE) indicates the total mass% of the rare earth.

Next, hydrogen was occluded in each alloy at room temperature, and then hydrogen pulverization treatment (coarse pulverization) was performed in an Ar atmosphere for dehydrogenation at 600 ° C. for 1 hour.

In this example, each process (fine pulverization and molding) from hydrogen pulverization to sintering was performed in an Ar atmosphere having an oxygen concentration of less than 50 ppm (the same applies to the following examples and comparative examples).

Next, for each alloy, 0.1 mass% of zinc stearate was added to the coarse powder as a grinding aid before fine grinding after hydrogen grinding and mixed using a Nauta mixer. Thereafter, fine pulverization was performed using a jet mill to obtain a raw material fine powder having an average particle size of about 4.0 μm.

Thereafter, using a Nauta mixer, the raw material powder of the main phase alloy and the raw material powder of the grain boundary phase alloy are mixed at a mass ratio of 95: 5 to obtain the raw material powder of the R-T-B system sintered magnet. A mixed powder was prepared.

The obtained mixed powder was filled in a mold placed in an electromagnet, and molded in a magnetic field in which a pressure of 120 MPa was applied while applying a magnetic field of 1200 kA / m, to obtain a molded body.

Thereafter, the obtained compact was fired by holding at 1060 ° C. for 4 hours in a vacuum, and then rapidly cooled to obtain a sintered body (R-T-B system sintered magnet) having the magnet composition shown in Table 1. It was. Then, the process by vertical grinding | polishing was performed and it was set as the rectangular parallelepiped of 20.2 mm x 20.2 mm x 4.2 mm. The orientation direction of the c axis of the R ₂ T ₁₄ B crystal grains was set to be 4.2 mm thick.

(Preparation of diffusion material powder)
For the diffusion material for introducing elements such as Cu into the sintered body by the grain boundary diffusion method using the diffusion material powder, commercially available Cu reagent powder (average particle size 13 μm), and Nd and Cu in atomic ratio A 70:30 Nd-Cu eutectic alloy powder was prepared. Regarding the production of the Nd—Cu eutectic alloy powder, first, Nd and Cu simple metal were weighed and melted and casted three times in an arc melting furnace. The obtained alloy was melted by high frequency induction heating, and the molten metal was quenched and rolled to form a quenched ribbon. The obtained quenched ribbon was roughly pulverized in a nitrogen atmosphere, and then enclosed in a sealed container purged with nitrogen with a stainless steel medium and pulverized to obtain a powder having an average particle size of 10 to 20 μm. Since the obtained powder may ignite when exposed to the air as it is, it was placed in a bat in a glove box in a nitrogen atmosphere and subjected to a gradual oxidation treatment in which air was gradually introduced into the glove box. The Nd—Cu alloy powder thus obtained had an oxygen content of 3500 ppm, a carbon content of 250 ppm, and a nitrogen content of 500 ppm. On the other hand, the oxygen content of the Cu powder was 1200 ppm, the carbon content was 280 ppm, and nitrogen was not detected. A binder resin was added to Cu or the produced Nd—Cu powder, and a diffusing material paint was produced using alcohol as a solvent. As for the mixing ratio, when the mass of the diffusing material powder is 100, butyral fine powder as a binder resin is 2, and alcohol is 100. The mixture was put in a resin-made cylindrical lidded container in an Ar atmosphere, the lid was closed, and the mixture was placed on a ball mill frame and rotated at 120 rpm for 24 hours to form a paint.

The sintered body after processing was completely immersed in the diffusing material paint, and the operation of lifting and drying was repeated to attach the diffusing material to the sintered body. About the diffusing material, except for the comparative example 1, it produced by making Nd-Cu powder adhere. The adhering amount of the diffusion material to the sintered body was based on the mass of the sintered body, and was 0.72% for Cu powder and 4.5% for Nd—Cu powder. The reason why the adhesion amount differs between the two is that the same amount of Cu element is adhered to the sintered body. Thereafter, diffusion heat treatment was performed in an Ar atmosphere at the stable temperature and stable time shown in the column of diffusion heat treatment in Table 2. After quenching from the stable temperature by blowing Ar gas, it was taken out from the furnace and the residue of the diffusion material remaining on the surface of the sintered body was dropped with sandpaper.

Thereafter, the obtained sample was put into the furnace again and subjected to heat treatment under the conditions shown in the column of heat treatment 1 in Table 2 for the stable temperature and time. Those having no description of conditions in the column of heat treatment 2 in Table 2 were removed from the furnace after cooling, and those having conditions described were subsequently heat treated under the conditions of heat treatment 2 and removed from the furnace after cooling. Both heat treatment 1 and heat treatment 2 were quenched by blowing Ar gas from a stable temperature.

After finishing each of the heat treatments of diffusion heat treatment, heat treatment 1 and heat treatment 2 under the conditions shown in Table 2, all six samples of the rectangular parallelepiped were processed with as even a grinding amount as possible to obtain a rectangular parallelepiped of 20 mm × 20 mm × 4.0 mm. The shape is made so that the influence of the residue of the diffusion material does not appear. In each Example and Comparative Example, four rectangular parallelepiped samples were prepared, one for fine structure observation and the other three for corrosion resistance evaluation.

Table 3 shows the composition analysis results of each sample. The amount of Cu originally contained in the sintered body was 0.03% by mass, whereas the amount of Cu in Comparative Examples 1, 3 and 5 was not increased as much as 0.03 to 0.1% by mass. In other samples, it is increased to 0.5 to 0.6% by mass. As for the contents of O, C and N, Comparative Examples 1, 3 and 5 tend to be less than other samples. From this, it is considered that the components Cu, O, C and N of the diffusing material were sufficiently introduced into the sintered body except for Comparative Examples 1, 3 and 5. On the other hand, Nd occupies 84% by mass of the diffusing material. However, since the grain boundary phase of the sintered body originally contains a large amount of Nd, a concentration gradient serving as a driving force for diffusion cannot be obtained. I found out that I was not obsessed with it.

Table 4 shows the results of observation by EPMA in the region of 500 μm from the surface of each sample and evaluation of the shape and composition of the R-rich phase crystal particles. In each sample, five R-rich phase crystal particles in the same sample were evaluated. In the evaluation of the shape of the R-rich phase crystal particle, it is confirmed by an electron microscopic image that the shape of an unequal polygon is exhibited, and when it is judged that it has crystal habit, it is indicated in the column of crystal habit and rounded. When it is tinged, it is indicated as nothing. In Examples 1 to 4, it is recognized that any R-rich phase crystal particle has an unequal side polygon as shown in FIG. 1 and thus has a crystal habit, and the circularity coefficient is examined. Some are less than 0.8. On the other hand, the shapes of the R-rich phase crystal particles of Comparative Examples 2, 3, and 4 are rounded and no crystal habit is observed, and the circularity coefficient of the crystal particle cross section is higher than that of the example. In Comparative Example 1 and Comparative Example 5, the presence of R-rich phase crystal particles was not confirmed. In Comparative Example 3, although the presence of R-rich phase crystal particles was confirmed, the number was remarkably small as compared with the Example. The composition of the R-rich phase crystal particles is almost composed of R (Nd and Pr), O, C, and N, and no difference in composition was found between the examples and comparative examples.

Evaluation of corrosion resistance was performed by leaving it in a saturated water vapor atmosphere at 120 ° C., 2 atm and relative humidity of 100% for 200 hours, and evaluating the amount of mass loss due to corrosion. In the case of an RTB-based sintered magnet, when corrosion occurs, the mass of the sintered body decreases due to elution of grain boundary phases, detachment of crystal particles, and the like. Therefore, corrosion resistance can be evaluated by evaluating the decrease in mass. Specifically, the above-described 20 mm × 20 mm × 4.0 mm (2.0 cm × 2.0 cm × 0.40 cm) rectangular parallelepiped sample was used, and the amount of mass decrease (mg / kg) based on the total surface area of the rectangular parallelepiped shape. cm ² ). That is, it can be said that the smaller the amount of mass reduction based on the total surface area, the better the corrosion resistance. In each Example and Comparative Example, the average values of the evaluation results of the corrosion resistance tests performed on three samples are shown together in Table 4. In Examples 1-4, compared with Comparative Examples 1-5, the amount of mass reduction is reducing clearly, and corrosion resistance is improved significantly.

For confirmation, R rich phase crystal particles were also observed for the samples subjected to the corrosion resistance evaluation. As a result, in the samples having good corrosion resistance belonging to Examples 1 to 4, it was confirmed that the R-rich phase crystal particles near the surface had crystal grains having crystal habit. On the other hand, in the samples belonging to Comparative Examples 1 to 5, the presence of R-rich phase crystal particles having crystal habits could not be confirmed due to the elution and dropping of the surface layer. From the above results, it was confirmed that the corrosion resistance was significantly improved in the R-T-B system sintered magnet including the R-rich phase crystal particles having crystal habit.

In Comparative Example 1, although the heat treatment conditions were the same as in Example 1, R-rich phase crystal particles were not confirmed. It is considered that Cu powder contains less O and N and low reactivity with the liquid phase oozing out from the sintered body. In Comparative Example 3 and Comparative Example 5, introduction of Cu, O, C, and N components was insufficient despite the use of a Nd—Cu eutectic alloy diffusing material. In Comparative Example 5, it is considered that the Nd—Cu powder could not be melted because the diffusion temperature was too low, and O, C, and N necessary for forming R-rich phase crystal particles were not introduced into the sintered body. In Comparative Example 3, the diffusion temperature is appropriate, but the heat treatment time is considered to be too short. Therefore, in the samples of Comparative Examples 1, 3, and 5, it is considered that a large amount of R element is present in the grain boundary phase, and the corrosion resistance is deteriorated.

Comparative Examples 2 and 4 and Example 1 differ in the presence or absence of crystal habits, but differ only in the conditions of heat treatment 1 in the manufacturing process. The R-rich phase crystal grains are considered to have no crystal habit in the diffusion heat treatment at 570 ° C. for 10 hours, and have the crystal habit in the heat treatment 1. In Example 1, the heat treatment 1 was performed under appropriate heat treatment conditions, but the heat treatment temperature was considered to be too low in the comparative example 2 and too high in the comparative example 4.

In Examples 1, 2, and 4 including R-rich phase crystal particles having crystal habit, heat treatment 1 is performed at a temperature relatively higher than a temperature range of 550 to 650 ° C. suitable for diffusion heat treatment. Therefore, it is considered that R-rich phase crystal particles having crystal habit can be easily obtained at 750 to 900 ° C. However, in Example 3, the temperature of the heat treatment 1 is 540 ° C. for 2 hours at a low temperature in a short time, and R-rich phase crystal particles having crystal habits are obtained even if the heat treatment is not divided into two stages. From the comparison with Comparative Example 4 in which only heat treatment 1 is performed under the same conditions, it is considered that Example 3 produced R-rich phase crystal grains and acquired crystal habits in the diffusion heat treatment step. This is presumably because the diffusion heat treatment was a long time and a relatively high temperature.

The crystal plane appearing on the surface of the R-rich phase crystal grains having crystal habit was examined. An example is shown in FIGS. FIG. 2 is an electron microscopic image of the cross section of the sample of Example 2, and the tangents of each side of the unequal side polygon exhibited by the R-rich phase particle 2 are added as a solid line, a one-dot chain line, and a two-dot chain line. FIG. 3 shows these tangents without changing the direction. The cube in the center of FIG. 3 indicates the crystal orientation of the R-rich phase crystal particle 2. As described above, the R-rich phase crystal particles have a cubic shape, and the direction thereof is confirmed by an electron beam backscattering diffraction method and is indicated by a cube. Each side of the cube indicates the direction of the crystal axis. The tangent of each side of the unequal polygon formed by the R-rich crystal grains intersected either one of three arrows indicating the <111> direction of the cubic crystal or one arrow indicating <100>. Since the tangent line perpendicular to the <111> direction is included in the {111} plane, the {111} plane may appear on the surface of the R-rich phase crystal grain. When the same method was used to analyze the plurality of particles in the example, most tangents of the unequal polygons exhibited by the R-rich phase crystal particles intersected perpendicularly to the <111> direction, and perpendicular to <100> and <120>. It was rare to see anything that crossed. From these results, it is considered that {111} planes are likely to appear on the surface of the R-rich phase crystal particles having crystal habit.

The above is the confirmation of the effect when using the Nd-Cu alloy, but also for the 40 atomic% Nd-30 atomic% Pr-30 atomic% Cu alloy and the 60 atomic% Nd-10 atomic% Dy-30 atomic% Cu alloy. A diffusing material paint was prepared in the same manner as the Nd-Cu alloy. Then, it apply | coated to the sintered compact similarly to the above-mentioned Example and comparative example. Similarly, the coating amount was 4.5 wt%. Heat treatment was performed under the conditions shown in Table 5, and the removal and processing of the residue were performed in the same manner as described above. From the analysis results of each sample in Table 6, it was confirmed that Cu was introduced in the same manner as in each of the previous examples.

Table 7 shows the results of evaluation of the shape and composition of the R-rich phase crystal particles. R-rich phase crystal grains are crystallized regardless of whether 40 atomic% Nd-30 atomic% Pr-30 atomic% Cu alloy or 60 atomic% Nd-10 atomic% Dy-30 atomic% Cu alloy is used as the diffusing material. It is recognized that Also, good results were obtained with respect to corrosion resistance.

As described above based on the examples, in order to efficiently generate R-rich phase crystal grains, the raw material alloy has an R-rich composition and oxygen, carbon, and nitrogen are added to the grain boundaries in addition to the R element. It is good to introduce to. In all cases other than Comparative Example 5, it was considered that the R-rich phase crystal particles were formed because O and N contained in the diffusing material powder and C contained in the binder were appropriately supplied to the grain boundaries. Further, from the comparison of Example 1, Comparative Example 2, and Comparative Example 4, in order to obtain an R-rich phase having crystal habit, heat treatment conditions are important, and Cu or the like is added to promote recrystallization growth. Is desirable. That is, in order to control the formation of R-rich phase crystal grains and their crystallinity and chemical stability, additives that promote recrystallization growth of O, C, N and R-rich phase crystal grains are added to the grains in the sintered body. Proper introduction and heat treatment to the boundary is recommended.

The present invention has been described based on the embodiments. It will be understood by those skilled in the art that the embodiments are illustrative, and that various modifications and changes are possible within the scope of the claims of the present invention, and that such modifications and changes are also within the scope of the claims of the present invention. By the way. For example, the R-rich phase crystal particles only need to have an R oxide R—O compound as a main component, and may be a R—O—N compound or a R—O—C compound. If the R-rich phase crystal particles produced by sintering are recrystallized and grown by the heat treatment after sintering, and as a result, the crystallinity can be enhanced to the extent that the crystal habit has, the corrosion resistance can be improved. Accordingly, the description and drawings herein are to be regarded as illustrative rather than restrictive.

ADVANTAGE OF THE INVENTION According to this invention, even if a sintered compact is made into R rich composition in order to raise a coercive force, an R-T-B type sintered magnet with favorable corrosion resistance can be provided.

1 Main phase crystal particle 2 R rich phase crystal particle 3 Grain boundary phase

Claims (5)

R ₂ T ₁₄ B main phase crystal particles, R rich phase crystal particles having a higher R concentration than the R ₂ T ₁₄ B main phase crystal particles, and a grain boundary phase, and the R rich phase crystal particles Contains an R-rich phase crystal particle having a crystal habit. The R-T-B system sintered magnet according to claim 1, wherein the R-rich phase crystal particle having a crystal habit is an R-O-C-N compound. The R-T-B system sintered magnet according to claim 1 or 2, wherein the R-rich phase crystal particles having a crystal habit have an unequal polygonal shape when viewed in an arbitrary cross section. When the cross-sectional shape of the R-rich phase crystal particles having the crystal habit is evaluated by the circularity coefficient represented by the following formula (1), the R-rich phase crystal particles having a circularity coefficient of less than 0.8 are sintered. 4. The RTB-based sintered magnet according to claim 3, wherein the sintered magnet is included in a range of 500 [mu] m from the surface of the magnet.
Circularity coefficient = (L1 / L2) ² = 4πS / L2 ² (1)
Here, L1: circumference of a circle having an area equal to the cross-sectional area of the target particle
L2: Perimeter of the target particle
S: Cross-sectional area of the target particle The R ₂ T ₁₄ B main phase crystal particle and R, which is a constituent element of the R rich phase crystal particle, contain Nd or Pr, or Nd and Pr. The RTB-based sintered magnet described.