JP2007266037A - Manufacturing method of rare-earth permanent magnet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable to manufacture a rare-earth permanent magnet excellent in magnetic characteristics such as residual magnetic flux density by improving orientation degree of a rare-earth alloy powder. <P>SOLUTION: A finely pulverized rare-earth alloy powder is molded in a magnetic field, and sintered. Prior to the magnetic molding, the finely pulverized rare-earth alloy powder is subjected to heat treatment. A heat treatment temperature is preferably 600°C-900°C. The rare-earth alloy powder subjected to heat treatment may be re-pulverized. The powder may be magnetized before the heat treatment. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing a rare earth permanent magnet obtained by sintering a compact of a rare earth alloy powder to obtain a rare earth permanent magnet, and more particularly to a novel technique for realizing improvement in magnetic properties by improving orientation.

  For example, in a wide range of fields such as a hard disk drive voice coil motor and an automobile drive motor, miniaturization and high performance of the motor are required. In order to reduce the size and performance of motors, it is important to improve the performance of magnets incorporated in motors. In recent years, rare earth permanent magnets such as neodymium iron boron-based sintered magnets that exhibit extremely high magnetic properties have been used. Widely used.

  As a method for producing a rare earth permanent magnet, a powder metallurgy method is known. Specifically, a raw material alloy having a desired composition is used and manufactured through processes such as a coarse pulverization process, a fine pulverization process, a molding process, and a sintering process. Has been. That is, in order to produce the rare earth permanent magnet, the coarsely and finely ground rare earth alloy powder is filled in a mold cavity of a molding apparatus, molded into a molded body of a predetermined shape in a magnetic field, and sintered. To obtain a sintered body.

  In the rare earth permanent magnets produced by the above-mentioned powder metallurgy method, improvement of magnetic properties has been an issue, and various attempts have been made. Among them, it is considered effective to improve the orientation of the rare earth alloy powder in a molded body formed by molding while being oriented in a magnetic field.

Here, in order to improve the degree of orientation of the rare earth alloy powder, for example, it is effective to add a large amount of lubricant at the time of molding. However, if the amount of the lubricant is excessively increased, the carbon in the product (sintered body) The amount may increase and the magnetic properties may be lost. Under such circumstances, use of a lubricant that exhibits sufficient lubricity even when added in a small amount has been studied (see, for example, Patent Document 1).
JP 2006-41501 A

  However, the improvement by the lubricant has almost reached its limit. For example, it is actually difficult to improve the lubricity further only by changing the type and addition amount of the lubricant.

  As a method for improving the degree of orientation of the rare earth alloy powder, in addition to the use of a lubricant, for example, increasing the strength of the magnetic field applied during molding in a magnetic field may be considered. By increasing the current of the magnetic field generating facility and increasing the efficiency of the magnetic flux density, the strength of the applied magnetic field can be increased, and the degree of orientation of the rare earth alloy powder is improved. However, in a magnetic field generating facility, increasing the current further causes not only an increase in capital investment but also a problem that the site area becomes enormous. There is also a limit to increasing the efficiency of magnetic flux density.

  The present invention has been proposed in view of such a conventional situation, and can further improve the degree of orientation of the rare earth alloy powder, for example, a rare earth permanent having excellent magnetic properties such as residual magnetic flux density. It aims at providing the manufacturing method which can manufacture a magnet.

  In order to achieve the above-mentioned object, a method for producing a rare earth permanent magnet according to the present invention is a method for producing a rare earth permanent magnet in which a finely ground rare earth alloy powder is molded in a magnetic field and then sintered. The rare earth alloy powder is heat-treated and then molded in a magnetic field.

  The finely pulverized rare earth alloy powder has an angular shape, and when oriented as it is in a magnetic field, free orientation is hindered by frictional force, steric hindrance, etc., and the degree of orientation does not improve as expected. In contrast, when the pulverized rare earth alloy powder is heat-treated prior to molding in a magnetic field, it becomes a rounded shape to some extent, and the frictional force and steric hindrance during orientation in the magnetic field are reduced, so that it is oriented quickly. The degree of orientation is improved.

  According to the present invention, the degree of orientation can be greatly improved during molding in a magnetic field, and a rare earth permanent magnet excellent in magnetic characteristics such as residual magnetic flux density can be manufactured. Further, when the degree of orientation is improved, it is not necessary to add a lubricant excessively, and it is not necessary to increase the current of the magnetic field generating equipment, so that the magnetic characteristics are not lowered and the equipment investment is not increased.

  Hereinafter, a method for producing a rare earth permanent magnet to which the present invention is applied will be described in detail with reference to the drawings.

  The rare earth permanent magnet (rare earth sintered magnet) is composed mainly of, for example, rare earth element R, transition metal element T and boron, but the magnet composition is not particularly limited, and can be arbitrarily selected according to the application. Good. For example, the rare earth element R specifically means Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu. 1 type (s) or 2 or more types can be used. Among them, it is preferable that the main component of the rare earth element R is Nd because it is abundant in resources and relatively inexpensive. Moreover, as the transition metal element T, any conventionally used transition metal element can be used. For example, one or more of Fe, Co, Ni and the like can be used. Among these, from the viewpoint of magnetic properties, it is preferable to mainly contain Fe, and in addition to this, it is preferable to add Co that is effective in improving the Curie temperature and improving the corrosion resistance of the grain boundary phase. In addition to the rare earth element R, transition metal element T, and boron B, the inclusion of other elements is allowed. For example, elements such as Al, Cu, Zr, Ti, Bi, Sn, Ga, Nb, Ta, Si, V, Ag, and Ge can be appropriately contained. On the other hand, it is desirable to reduce impurity elements such as oxygen, nitrogen, and carbon as much as possible. In particular, the amount of oxygen that impairs magnetic properties is preferably 7000 ppm or less, more preferably 5000 ppm or less. This is because when the amount of oxygen is large, the rare-earth oxide phase, which is a nonmagnetic component, increases and the magnetic properties are deteriorated.

  In the alloying step, a raw material metal or alloy is blended according to the composition of the desired rare earth alloy powder, and melted in a vacuum or an inert gas, for example, Ar atmosphere, and cast to form an alloy. As the casting method, any method can be adopted, but the strip casting method (continuous casting method) in which a molten high-temperature liquid metal is supplied onto a rotating roll and the alloy thin plate is continuously cast is a viewpoint of productivity and the like. From the viewpoint of the form of the resulting alloy.

  As the raw material metal (alloy) used in the alloying, pure rare earth elements, rare earth alloys, pure iron, ferroboron, and alloys thereof can be used. As the alloy, a single alloy having a final magnet composition may be used, or a plurality of types of alloys having different compositions may be prepared and mixed after, for example, a pulverization process so as to have a final magnet composition. good.

  In the coarse pulverization step, the previously cast raw alloy thin plate, ingot, or the like is coarsely pulverized until the particle size becomes approximately several hundred μm. As the pulverizing means, a stamp mill, a jaw crusher, a brown mill, or the like can be used, and the pulverization is usually performed in an inert gas atmosphere. Prior to the coarse pulverization, it is effective to perform the hydrogen occlusion pulverization together. Hydrogen storage and pulverization is performed by occluding hydrogen in the raw material alloy and then releasing it. After this hydrogen storage and pulverization, dehydrogenation treatment is performed for the purpose of reducing hydrogen as an impurity in the rare earth permanent magnet. It is preferable to carry out. The hydrogen occlusion pulverization is performed at a temperature from room temperature to about 200 ° C. for 30 minutes or more, preferably 1 hour or more, and the dehydrogenation treatment is performed at 350 ° C. to 650 ° C. in a vacuum or an argon gas atmosphere. The hydrogen storage process and the dehydrogenation process are not essential processes. Moreover, this hydrogen occlusion pulverization can be positioned as coarse pulverization, and mechanical coarse pulverization can be omitted.

  After the above-described coarse pulverization step is completed, a lubricant is added to the coarsely pulverized raw material alloy powder as necessary. This is a so-called pre-adding step. The type of lubricant to be added is not limited, but it is preferable to use a fatty acid compound from the viewpoint of easy availability. As the fatty acid compound, fatty acids such as stearic acid and caprylic acid, metal soaps of fatty acids such as zinc stearate and calcium stearate, fatty acid amides or fatty acid bisamides are preferable, and among these, fatty acid amides are particularly preferable. Among fatty acid amides, it is particularly preferable to use one or more of caprylic acid amide, capric acid amide, lauric acid amide, stearic acid amide, oleic acid amide, and behenic acid amide. Alternatively, camphor or paraffin can be used as the lubricant. Furthermore, a liquid lubricant can also be used as a fine grinding lubricant. Examples of liquid lubricants include ketones, alcohols, fatty acids such as stearic acid and caprylic acid, fatty acid esters, paraffins, and aromatic compounds such as toluene and xylene. Among these, fatty acid esters are preferable from the viewpoints of lubricity and availability. Among fatty acid esters, it is preferable to use one or more of ethyl caprylate, butyl caprylate, ethyl laurate, butyl laurate, methyl oleate, ethyl oleate, and butyl oleate.

  The particle size of the lubricant at the time of addition is not particularly limited, but preferably has a particle size equal to or smaller than that of the coarsely pulverized powder. The addition amount of the lubricant is preferably as much as possible from the viewpoint of improving the grindability, but is preferably as small as possible from the viewpoint of the magnetic properties and the strength of the molded body. Therefore, it can be added in a range of 0.01 to 1% by mass in total with the molding lubricant described later.

  In the pre-addition, the mixing step after the addition of the lubricant is not essential, but the coarsely pulverized powder and the finely pulverized lubricant may be mixed after or during the addition of the finely pulverized lubricant. preferable. Thereby, the effect of improving the dispersibility of the lubricant with respect to the coarsely pulverized powder during fine pulverization described later is expected. The mixing may be performed for about 5 to 30 minutes using a screw-type mixing device (for example, trade name Nauta mixer manufactured by Hosokawa Micron Corporation).

  After the coarse pulverization step, a fine pulverization step is performed. In this fine pulverization step, a jet mill (airflow type pulverizer) is mainly used, and a rare earth alloy powder having an average particle diameter of 2.5 to 6 μm, preferably 3 to 5 μm. (Ground powder) is obtained. The jet mill releases a high-pressure inert gas from a narrow nozzle to generate a high-speed gas flow, and this high-speed gas flow accelerates the coarsely pulverized powder. It is a method of generating a collision and crushing.

  After the pulverizing step, the rare earth alloy powder is usually formed in a magnetic field in a forming step in a magnetic field. However, the rare earth alloy powder after the fine pulverization step has an angular shape as shown in FIG. 1 (a), and cannot move freely due to steric hindrance or frictional force. It becomes an obstacle to increase the degree.

  Therefore, in the present invention, the pulverized rare earth alloy powder is heat-treated to round its shape, and as much as possible to eliminate steric hindrance during molding in a magnetic field and reduce frictional force, Make it easy to align.

  FIG. 1 (b) schematically shows the shape of the rare earth alloy powder after the heat treatment. As shown in FIG. 1B, a molten liquid phase is formed on the surfaces of the particles (rare earth alloy powder) by performing an appropriate heat treatment. Utilizing this liquid phase, a plurality of particles are cooled and solidified in a state before starting densification, whereby an aggregate of spheroidized particles before neck growth is obtained. The obtained aggregate of particles is very brittle, and can be used in a molding step in a magnetic field without going through a special grinding process such as grinding with a jet mill. The heat-treated rare earth alloy powder is easily oriented due to the spheroidization, and forming this rare earth permanent magnet improves the degree of orientation and improves magnetic properties such as residual magnetic flux density.

  In the heat treatment, it is desirable to set the heating temperature appropriately. For example, if the heating temperature is too low, spheroidization by the liquid phase does not proceed and the object may not be achieved. On the other hand, when the heat treatment temperature is too high, densification proceeds too much, and it may be difficult to break up the aggregate of particles. From such a viewpoint, the heating temperature is preferably 600 ° C. or higher, and more preferably 700 ° C. to 900 ° C. In addition, the holding time during the heat treatment is preferably within 1 hour. The atmosphere during the heat treatment is preferably a reduced pressure atmosphere (vacuum) or an inert gas atmosphere. When an inert gas atmosphere is used, it is desirable to use Ar gas as the inert gas.

  Further, the heat-treated rare earth alloy powder can be reground and used. As described above, the heat-treated rare earth alloy powder becomes an aggregate of spheroidized particles, and this aggregate of particles is very brittle. However, the orientation degree can be increased by re-grinding with a jet mill or the like. Is improved and the residual magnetic flux density is improved. The aggregate of the particles is very fragile, but there are also particles in which a plurality of particles have started to be densified due to the liquid phase. If a plurality of randomly oriented particles become dense due to the liquid phase, they cannot move freely with each other, which may hinder the improvement of the degree of orientation. Therefore, by re-pulverizing such particles, as shown in FIG. 1C, the particles can be separated into particles having a single orientation axis, and the inconvenience can be solved.

  It is also effective to pre-magnetize before the heat treatment. Prior to the heat treatment, the rare earth alloy powder is preliminarily magnetically oriented, and the heat treatment is performed in a magnetized state, whereby the degree of orientation is further improved and the magnetic properties such as residual magnetic flux density are improved. This is because, in the particles partially densified after the heat treatment, each particle is not in a randomly oriented state, but in a state in which the orientation directions coincide with each other, thereby making the whole degree of orientation high. Because it can.

  FIG. 2A shows a state in which the rare earth alloy powder is magnetized before the heat treatment, and FIG. 2B shows a state in which the rare earth alloy powder is heat-treated. As shown in FIG. 2A, the orientation axes of the particles are aligned in one direction by magnetization. As shown in FIG. 2B, this state is maintained even after the heat treatment. If the orientation axes are aligned in the state of the rare earth alloy powder in this way, this state is reflected even after molding, and the degree of orientation is high. As a condition for magnetization, for example, the strength of the applied magnetic field may be 398 kA / m or more.

  Note that even when magnetization is performed before the heat treatment, re-pulverization may be performed after the heat treatment. As shown in FIG. 2 (c), there may be particles that cannot be separated into particles having a single orientation axis even after re-grinding. A higher degree of orientation can be obtained if the orientation in the particles is aligned by pre-orientation.

  For example, when the main phase alloy powder and the grain boundary phase alloy powder are mixed and used, the above-mentioned heat treatment can be effected only on the main phase alloy, but together with the grain boundary phase alloy, It is preferable to treat it, and the effect of spheroidization is high.

  As described above, in the present invention, the basic idea is to heat-treat the finely pulverized rare earth alloy powder before forming it in a magnetic field, and it is possible to combine repulverization and magnetization. FIG. 3 shows a process example from fine grinding (step A) to molding in a magnetic field (step C). As described above, in the present invention, as shown in FIG. 3A, after the fine pulverization (step A), the rare earth alloy powder is subjected to heat treatment (step B-1), and the rare earth alloy powder after the heat treatment is performed. Is used to perform molding in a magnetic field (step C). Alternatively, as shown in FIG. 3 (b), after the fine pulverization (step A), the rare earth alloy powder is subjected to heat treatment (step B-1) and re-pulverized (step B-2), and then the rare earth alloy powder. Is subjected to molding in a magnetic field (step C). Furthermore, it is possible to perform magnetization before the heat treatment. In this case, as shown in FIG. 3C, after the fine pulverization (step A), the rare earth alloy powder is magnetized (step B). -3) is performed, and in the magnetic field (step C) is performed using the rare earth alloy powder subjected to the heat treatment (step B-1). Alternatively, as shown in FIG. 3 (d), after pulverization (step A), the rare earth alloy powder is magnetized (step B-3) and subjected to heat treatment (step B-1). After pulverization (step B-2), the rare earth alloy powder is subjected to molding in a magnetic field (step C).

  Specifically, in the forming step in a magnetic field, the rare earth alloy powder after the heat treatment is filled in a mold in which an electromagnet is arranged, and forming is performed in a magnetic field in a state where crystal axes are oriented by applying a magnetic field. The forming in the magnetic field may be either a parallel magnetic field forming in which the forming pressure and the magnetic field direction are parallel, or an orthogonal magnetic field forming in which the forming pressure and the magnetic field direction are orthogonal to each other. Further, a pulse power source and an air-core coil can be employed as the magnetic field applying means. The forming in the magnetic field may be performed in a magnetic field of 700 to 1600 kA / m, for example, at a pressure of 30 to 300 MPa, preferably about 130 to 160 MPa.

  Sintering (main firing) is performed in the sintering step on the compact formed by the forming step in the magnetic field. In the sintering process, the compact is sintered in a vacuum or in an inert gas atmosphere (such as in an Ar gas atmosphere). The sintering temperature needs to be adjusted according to various conditions such as composition, pulverization method, difference in particle size and particle size distribution, etc. For example, sintering may be performed at 1000 to 1200 ° C. for about 1 to 10 hours. It is preferable to do. In addition, in a sintering process, it is preferable to perform a degreasing process prior to sintering as needed. Further, in order to reduce impurities for the purpose of improving characteristics, particularly to reduce the amount of oxygen, the oxygen concentration from hydrogen crushing to putting into a sintering furnace may be controlled to about 100 ppm.

  After the sintering treatment, the obtained sintered body is preferably subjected to aging treatment. This aging treatment is an important step in controlling the coercive force Hcj of the obtained rare earth magnet, and is performed, for example, in a vacuum or in an inert gas atmosphere. As the aging treatment, a two-stage aging treatment is preferable. The second stage aging treatment is held at a temperature of about 800 ° C. for 1 hour to 3 hours in the first stage aging treatment process, and is kept at a temperature of about 600 ° C. for 1 hour to 3 hours in the second stage aging treatment process. Just do it. Since the coercive force Hcj is greatly increased by heat treatment near 600 ° C., the aging treatment is preferably performed near 600 ° C. when the aging treatment is performed in one stage.

  After the sintering process, a machining process and a film forming process are performed to complete a rare earth permanent magnet. The machining process is a process of mechanically processing into a desired shape. The film forming step is a step performed for the purpose of suppressing oxidation of the obtained rare earth permanent magnet, and is a step of forming, for example, a plating film or a resin film on the surface of the rare earth permanent magnet.

  According to the method for producing a rare earth permanent magnet as described above, in the produced rare earth permanent magnet, the degree of orientation can be improved and the magnetic properties such as residual magnetic flux density can be improved. In addition, when the degree of orientation is improved, it is not necessary to add a lubricant excessively, so that the magnetic properties are not deteriorated. Further, since it is not necessary to increase the current of the magnetic field generating facility, it is possible to suppress the manufacturing cost without causing an increase in capital investment.

  Hereinafter, the present invention will be described based on specific experimental results.

Experiment 1
First, two types of alloys having the following compositions were produced by a strip casting method.
Alloy 1: Nd 28.0 mass%, Al 0.2 mass%, B 1.0 mass%, Zr 0.2 mass%, balance Fe
Alloy 2: Nd 32.0 mass%, Co 10.0 mass%, Cu 1.0 mass%, Al 0.2 mass%, balance Fe

  Next, a hydrogen pulverization step (coarse pulverization step) and a fine pulverization step were performed on each of the alloys. Here, in the hydrogen pulverization step, after hydrogen was occluded at room temperature, dehydrogenation was performed at 600 ° C. for 1 hour in an Ar atmosphere. In the fine pulverization step, 0.15% by mass of oleic acid amide is mixed as a lubricant at the time of pulverization, and finely adjusted to have an average particle size of 3 μm to 6 μm by using an air flow type pulverizer (jet mill) using high-pressure nitrogen gas. Grinding was performed.

Subsequently, finely pulverized fine powders of each alloy were mixed and mixed so as to have the following final composition. And the heat processing process was performed with respect to this mixed rare earth alloy powder. In the heat treatment step, the temperature was increased under a vacuum of 1 Pa or less, and cooling was started immediately after reaching a predetermined temperature (500 ° C. to 1100 ° C.) (Example 1-1 to Example 1-7).
Final composition: Nd 28.4 mass%, Co 1.0 mass%, Cu 0.1 mass%, Al 0.2 mass%, B 1.0 mass%, Zr 0.2 mass%, balance Fe

  Further, the heat-treated rare earth alloy powder was reground to an average particle size of 3 μm to 6 μm using an airflow pulverizer (jet mill) using high-pressure nitrogen gas (Example 2-1 to Example 2). Example 2-7).

  Next, the obtained rare earth alloy powder was molded in a magnetic field. Specifically, rare earth alloy powder was filled in a mold held by an electromagnet, and the crystal axis was oriented by applying a magnetic field. As molding conditions, the strength of the applied magnetic field was 1194 kA / m and the pressure was 117.6 MPa.

  The molded body was subjected to a sintering / aging process to obtain a rare earth permanent magnet. In the sintering / aging process, the compact was placed in a vacuum, sintered at 1030 ° C. to 1090 ° C. for 4 hours, and then rapidly cooled. The obtained sintered body was subjected to two-stage aging in an Ar atmosphere. In addition, each process from the above-mentioned grinding | pulverization to sintering and aging was performed by controlling oxygen concentration to 100 ppm or less.

  The magnetic characteristics (residual magnetic flux density Br) were measured for the rare earth permanent magnet produced as described above. The results are shown in Table 1. Table 1 also shows the heat treatment temperature and the presence or absence of regrinding in each example. Furthermore, an example in which a rare earth alloy powder not subjected to heat treatment is used for molding is shown as Comparative Example 1-1.

  As is apparent from Table 1, the residual magnetic flux density is improved by performing the heat treatment. This is considered to be because the rare earth alloy powder was spheroidized by the heat treatment and the degree of orientation was improved. However, when the heat treatment temperature was 1100 ° C., the aggregate after the heat treatment could not be crushed, and a molded body could not be produced. Further, at the heat treatment temperature of 500 ° C. or 1000 ° C., a slight decrease in characteristics is observed. Therefore, in the heat treatment, the heat treatment temperature is preferably set to 600 ° C to 900 ° C. By setting the temperature range, it is possible to reliably improve the magnetic characteristics. Further, looking at the difference depending on the presence or absence of re-pulverization, it can be seen that the effect of improving magnetic properties is greater when re-pulverization is performed.

Experiment 2
In this experiment, magnetization was performed before the heat treatment of the rare earth alloy powder, and a rare earth permanent magnet was prepared in the same manner as in Experiment 1 above. Magnetization was performed in a magnetic field of 1194 kA / m by placing a rare earth alloy powder in a covered container. The results are shown in Table 2.

  Comparing the results shown in Table 2 with the results shown in Table 1 above, when the heat treatment temperature is the same and the presence or absence of re-grinding is the same, the magnetic properties are further improved by performing magnetization. Recognize.

It is a schematic diagram which shows the mode of the spheroidization of the rare earth alloy powder by heat processing, (a) shows the state before heat processing, (b) shows the state after heat processing, (c) shows the state after regrinding, respectively. . It is a schematic diagram which shows the mode of the rare earth alloy powder in the case of being magnetized, (a) shows the magnetized state, (b) shows the state after heat treatment, and (c) shows the state after regrinding. It is process drawing which shows the process example from the fine crushing process at the time of applying this invention to the formation process in a magnetic field, (a) When performing only heat processing, (b) When performing heat processing and re-grinding, (C) is a case where magnetization and heat treatment are performed, and (d) is a case where magnetization, heat treatment and regrind are performed.

Explanation of symbols

A fine pulverization step, B-1 heat treatment step, B-2 re-pulverization step, B-3 magnetization step, C magnetic field forming step

Claims (5)

A method for producing a rare earth permanent magnet, in which a finely ground rare earth alloy powder is molded in a magnetic field and then sintered.
A method for producing a rare earth permanent magnet, comprising subjecting the finely pulverized rare earth alloy powder to heat treatment and then molding in a magnetic field.   The method for producing a rare earth permanent magnet according to claim 1, wherein the heat-treated rare earth alloy powder is pulverized and then molded in a magnetic field.   The method for producing a rare earth permanent magnet according to claim 2, wherein the grinding is performed by a jet mill.   4. The method for producing a rare earth permanent magnet according to claim 1, wherein the rare earth alloy powder is magnetized before the heat treatment. The method for producing a rare earth permanent magnet according to any one of claims 1 to 4, wherein the temperature of the heat treatment is 600 ° C or higher.

Images