JP2005197301A - Rare earth sintered magnet and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress the growth of a crystal grain, to increase density by reducing sintering time, and to reduce an assistant alloy in a two-alloys method. <P>SOLUTION: The fines of a mother alloy mainly made of R<SB>2</SB>T<SB>14</SB>B phase (R indicates at least one type of rare earth element, and T indicates a transition metal containing at least one type from Fe and Co.), and the fines of the assistant alloy containing at least one type selected from Zr, Hf, Nb, Ta, Mo, W, Si, V, Ti, and Cr are used as raw material alloy fines. The compact for forming the raw material alloy fine is sintered as the rare earth sintered magnet, where the sintering is made by discharge plasma sintering. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a rare earth sintered magnet mainly composed of a rare earth element, a transition metal element and boron, and a method for producing the same, and more particularly to a technique for suppressing crystal grain growth and improving magnetic properties during sintering.

  Rare earth sintered magnets, for example, Nd-Fe-B based sintered magnets have advantages such as excellent magnetic properties, Nd as a main component is abundant in resources, and is relatively inexpensive. In recent years, the demand has been increasing. Under these circumstances, research and development for improving the magnetic properties of Nd—Fe—B based sintered magnets and improvement of manufacturing methods for producing high quality rare earth sintered magnets are being promoted in various fields. ing.

  As a method for producing a rare earth sintered magnet, a powder metallurgy method is known and widely used because it can be produced at low cost. In the powder metallurgy method, a raw material alloy ingot is first roughly pulverized and finely pulverized to obtain a raw material alloy fine powder having a particle size of about several μm. The raw material alloy fine powder thus obtained is magnetically oriented in a static magnetic field, and press molding is performed in a state where a magnetic field is applied. After molding in a magnetic field, the compact is sintered in a vacuum or in an inert gas atmosphere.

  In the sintering process of the rare earth sintered magnet, the compact is heated in a vacuum or an inert atmosphere such as Ar to advance the sintering reaction to obtain a high-density sintered body. At this time, resistance heating is widely adopted as a heat source for heating. In resistance heating, a rod-shaped or plate-shaped resistor made of a refractory metal such as Mo or carbon is energized, and the temperature in the sintering furnace is set to a predetermined temperature.

  In addition to the resistance heating, for example, a discharge plasma sintering method is also known as a sintering method. The spark plasma sintering method is a method in which a DC pulse voltage is applied to a compression molded body and sintering is performed using a discharge phenomenon that occurs in the gap between metal powders. For example, application of the discharge plasma sintering method to an iron-based sintered body has been studied, and Patent Document 1 discloses a method of sintering a green compact made of a ferrite-based stainless powder. And the improvement of intensity | strength is mentioned as the effect.

As an application example of the discharge plasma sintering method to the rare earth sintered magnet, there are techniques described in Patent Documents 2 to 3, and the like. In Patent Document 2, in the case of NdFeB-based and RCo (magnet phase + compound phase), a fluoride and oxide (one of Li, Na, Mg, Ca, Ba, and Sr) having high electrical resistance are added. As one method for producing a high electric resistance magnet by sintering, a discharge plasma sintering method is shown. Patent Document 3 discloses a method of producing a rare earth magnet using a discharge plasma sintering method by adding one or more metals having a melting point of 200 to 700 ° C. to magnet powder. Patent Document 4 discloses a method for manufacturing an anisotropic magnet using a discharge plasma sintering method and improving mass productivity.
JP 2002-332503 A JP-A-9-186010 JP-A-8-264361 JP 2001-68366 A

  By the way, in the case of the resistance heating described above, the heating temperature depends on the energized electric power, and heat energy is supplied to the molded body mainly using radiant heat. There is no. For this reason, before reaching a desired temperature, it is exposed to a temperature lower than this for a long time. Also, in the case of a vacuum atmosphere, it is difficult to control the temperature inside the furnace because it is not possible to expect a uniform temperature due to conduction heat that can be expected when an inert gas is present. Therefore, there is a high possibility that the temperature of the molded body varies.

In the sintering of rare earth sintered magnets, the low melting point phase (subphase) that becomes a liquid phase at the sintering temperature melts, wets the surface of the main phase particles mainly composed of Nd ₂ Fe ₁₄ B compound, and voids in the molded body are formed. Densification (densification) is realized by eliminating the outside. At the same time, at the sintering temperature, the solid phase (particles) react with each other to cause the growth of main phase crystal grains. Grain growth of main phase crystal grains proceeds in such a manner that relatively large main phase particles absorb surrounding small particles. Furthermore, when the sintering reaction proceeds, large main phase particles that have absorbed small particles react with each other to generate larger particles.

  Here, the size of the crystal grains has a great influence on the characteristics of the rare earth sintered magnet, particularly the coercive force, and if the crystal grain size is large, the coercive force is reduced. It is required to suppress the growth of phase crystal grains as much as possible. That is, in the sintering of rare earth sintered magnets, it is desirable to increase the density while maintaining the size of the fine powder of the raw material alloy constituting the compact as much as possible.

  In order to achieve both the objectives of increasing the density and suppressing the grain growth, it is very important to control the temperature and time pattern during sintering. For example, if the liquid phase can sufficiently wet the surface of the main phase particles within a short period of time, the time required for densification by liquid phase transfer can be shortened and the grain growth of the main phase can be suppressed. it can. Since the temperature at which the liquid phase can freely move and the temperature at which grain growth occurs overlap, it goes without saying that the temperature that needs to be controlled is not just the temperature in the sintering furnace, but the actual temperature of the compact.

  Considering such a sintering reaction, it is difficult to control the sintering temperature by controlling the grain growth by the resistance heating described above. In resistance heating, since radiant heat is used, it is difficult to control temperature, in particular, rapid temperature increase and decrease, and a temperature difference is easily generated between the periphery and the inside of the molded body. As a result, there is a problem that it is difficult to achieve high density and suppression of grain growth at the same time. In addition, as described above, the resistance heating is exposed to a lower temperature for a long time before reaching a desired temperature, and thus there is a problem that a heterogeneous phase is likely to occur. The growth of main phase particles, the decrease in density, and the occurrence of heterogeneous phases all cause deterioration of the magnetic properties of the obtained rare earth sintered magnet, and it is necessary to suppress their control.

  On the other hand, the discharge plasma sintering method described in the above-mentioned Patent Documents 2 to 4 and the like uses a high-temperature discharge plasma generated between particles, so that it is rapidly increased as compared with resistance heating using radiant heat. The temperature is considered possible. However, when the discharge plasma sintering method is actually applied to the sintering of rare earth sintered magnets, the sintering reaction and densification do not always progress rapidly, and the growth of crystal grains becomes insufficient. It has been found that there is a problem that the advantages of the discharge plasma sintering method are impaired. Therefore, even if the techniques described in Patent Documents 2 to 4 are applied to a rare earth sintered magnet as they are, it is difficult to improve the performance of the obtained rare earth sintered magnet and refine the crystal grains. Further, for example, in the techniques described in Patent Document 2 and Patent Document 3, it is necessary to add a fluoride, an oxide, a metal having a low melting point, or the like, and there is a problem that the composition is limited.

  The present invention has been proposed in view of such conventional circumstances, and can reduce the sintering time, simultaneously achieve higher density and suppression of grain growth, and suppress the formation of heterogeneous phases. It is an object of the present invention to provide a method for producing a rare earth sintered magnet capable of performing the above, and to provide a rare earth sintered magnet having excellent magnetic properties such as coercive force.

The present inventors have made various studies in order to solve the above-described problems. As a result, when the spark plasma sintering method is applied to the sintering of rare earth sintered magnets, a synergistic effect is exhibited in combination with the two alloy method, and the progress of densification and the reduction of the amount of additive alloy are simultaneously performed. I came to the conclusion that it would be realized. The present invention has been completed based on such findings. That is, the rare earth sintered magnet according to the present invention mainly comprises an R ₂ T ₁₄ B phase (where R represents one or more rare earth elements, and T represents a transition metal containing at least one of Fe and Co). Using raw material alloy fine powder comprising fine powder of mother alloy and fine powder of auxiliary alloy containing at least one selected from Zr, Hf, Nb, Ta, Mo, W, Si, V, Ti, Cr The compact formed from the fine powder is sintered by spark plasma sintering. Also, the method for producing a rare earth sintered magnet of the present invention uses a raw material alloy fine powder containing the mother alloy fine powder and the auxiliary alloy fine powder, and the compact formed from the raw alloy fine powder is sintered by discharge plasma sintering. It is characterized by tying.

  In the spark plasma sintering method, a direct current pulse voltage is applied to a powder compact, and sintering is performed using the high energy of discharge plasma of about several thousand to 10,000 ° C. that is instantaneously generated in the powder gap. How to do it. In the present invention, by applying a pulse current to the compact of the raw material alloy fine powder, rapid Joule heating of the powder surface due to the passage of a large pulse current between the raw material alloy fine powder, surface activity by discharge plasma generated in the raw material alloy fine powder gap Only the vicinity of the surface of the raw material alloy powder rapidly generates heat and is heated by the conversion and cleaning action, the movement of the discharge point and the Joule heating point, and the like. Therefore, the temperature can be raised and lowered much more rapidly than resistance heating using radiant heat.

However, even if the discharge plasma sintering method is applied to the sintering of the rare earth sintered magnet as it is, the advantages are not necessarily fully exhibited. Therefore, in the present invention, a ferromagnetic mother alloy mainly composed of an R ₂ T ₁₄ B phase (where R represents one or more rare earth elements, T represents a transition metal containing at least one of Fe and Co), and The two-alloy method of mixing and sintering a nonmagnetic or weakly magnetic auxiliary alloy is combined with the discharge plasma sintering method.

As described in, for example, Japanese Patent No. 3143181, the 2-alloy method is a method used in a sintering method using resistance heating, but it is more effective when combined with discharge plasma sintering. It was found that The reason for this is that when the mother alloy and the auxiliary alloy are mixed and sintered, plasma discharge occurs between the powders, so that the auxiliary alloy does not diffuse into the parent phase (R ₂ T ₁₄ B phase). This is thought to be due to the progress of densification and the high concentration in the vicinity of the grain boundaries.

  When applying the spark plasma sintering method to the sintering of the raw material alloy fine powder including the mother alloy fine powder and the auxiliary alloy fine powder, it is necessary to pay attention to the amount of oxygen. If the amount of oxygen contained in the raw material alloy fine powder is large, rare earth elements present as oxides increase, so that the surface activation and cleaning action by the discharge plasma becomes insufficient, and the effect of suppressing the growth of crystal grains cannot be obtained. The advantages of the discharge plasma sintering method are impaired. Therefore, in the present invention, by setting the amount of oxygen to 2500 ppm or less, it is possible to sinter in a short time without hindering the activation and cleaning action of the raw material alloy fine powder surface by the discharge plasma, and the discharge plasma sintering method. The crystal grain growth inhibitory effect, which is the advantage of, is made to the maximum.

  The rare earth sintered magnet of the present invention is sintered by the discharge plasma sintering and uses the combination of the mother alloy fine powder and the auxiliary alloy fine powder as the raw material alloy fine powder, so the advantage of the discharge plasma sintering method is the greatest. By making the most of this, it is possible to simultaneously increase the density, suppress the growth of crystal grains, suppress the occurrence of heterogeneous phases, and further reduce the amount of additive alloy added. Therefore, according to the present invention, a rare earth sintered magnet having a high coercive force and excellent magnetic characteristics can be provided.

  In addition, according to the method for producing a rare earth sintered magnet of the present invention, since spark plasma sintering is employed for sintering, and the raw material alloy fine powder is used in combination of the master alloy fine powder and the auxiliary alloy fine powder, A rapid sintering reaction and densification proceed by the cleaning action and activation action of the alloy fine powder surface, and the auxiliary alloy is unevenly distributed in the vicinity of the grain boundary. Therefore, according to the present invention, the sintered time can be shortened and a sintered body with a high density can be obtained without grain growth, and the sintered state of the manufactured rare earth sintered magnet is brought close to the ideal state. Can do.

  Hereinafter, a rare earth sintered magnet to which the present invention is applied and a manufacturing method thereof will be described with reference to the drawings.

The rare earth sintered magnet of the present invention is mainly composed of a rare earth element, a transition metal element and boron. Here, the magnet composition (final composition) may be arbitrarily selected according to the purpose. For example, R-T-B (one or more of rare earth elements including R = Y, T = one or more of transition metal elements essential to Fe or Fe and Co, B = boron) system When a rare earth sintered magnet is used, in order to obtain a rare earth sintered magnet having excellent magnetic properties, the sintered magnet composition has a rare earth element R of 27.0 to 32.0 wt% and boron B of 0. It is preferable that the blending composition is 5 to 2.0% by weight and the balance is substantially the transition metal element T (for example, Fe). When the amount of the rare earth element R is less than 27.0% by weight, α-Fe or the like that is soft magnetic precipitates, and the coercive force decreases. Conversely, when the rare earth element R exceeds 32.0% by weight, the amount of the R-rich phase increases and the corrosion resistance deteriorates, and the volume ratio of the R ₂ T ₁₄ B crystal grains as the main phase decreases. The residual magnetic flux density is reduced. Further, when boron B is less than 0.5% by weight, a high coercive force cannot be obtained. Conversely, if boron B exceeds 2.0% by weight, the residual magnetic flux density tends to decrease.

In the composition, the rare earth element R is a rare earth element including Y, that is, one selected from Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, and Lu, or 2 or more types. Among these, Nd and Pr are preferably Nd and Pr because the balance of magnetic properties is good and they are abundant and relatively inexpensive. Dy ₂ Fe ₁₄ B and Tb ₂ Fe ₁₄ B compounds have a large anisotropic magnetic field and are effective in improving the coercive force Hcj.

  Furthermore, the rare earth sintered magnet of the present invention may be an R-TBM type rare earth sintered magnet by adding the additive element M. In this case, examples of the additive element M include Al, Ga, Cr, Mn, Mg, Si, Cu, C, Nb, Sn, W, V, Zr, Ti, Hf, and Mo. A seed | species or 2 or more types can be selected and added. For example, the addition of Nb, Zr, W or the like, which is a refractory metal, has an effect of suppressing crystal grain growth. Of course, it is needless to say that the composition of the rare earth sintered magnet is not limited to these compositions and can be applied to all known compositions.

However, the rare earth sintered magnet of the present invention is mainly composed of an R ₂ T ₁₄ B phase (where R represents one or more rare earth elements and T represents a transition metal containing at least one of Fe and Co). Using raw material alloy fine powder containing fine powder of alloy and fine powder of auxiliary alloy containing at least one selected from Zr, Hf, Nb, Ta, Mo, W, Si, V, Ti, Cr, the raw material alloy fine powder The molded body obtained by molding is sintered by spark plasma sintering.

  In the mother alloy, R is one selected from Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, and Lu, but mainly contains Nd, Pr, and Dy. It is preferable that they are 1 type, or 2 or more types of rare earth elements. T is one or more transition metals mainly composed of Fe or Fe and Co, and the content of Co is preferably 0 to 40% by weight. The master alloy may include at least one selected from Zr, Nb, and Hf, and may include at least one selected from Co, Zr, Nb, and Hf.

  The auxiliary alloy contains at least one selected from Zr, Hf, Nb, Ta, Mo, W, Si, V, Ti, and Cr. In particular, Zr and Hf are preferable because they have the effect of expanding the optimum sintering temperature range. Therefore, the auxiliary alloy preferably contains at least one selected from Zr, Nb, and Hf. In addition to these, the auxiliary alloy preferably contains B or Co. This is because Co is added as an auxiliary agent, and the corrosion resistance of the magnet is improved by being present at a high concentration in the vicinity of the grain boundary, and the compound as an inhibitor of abnormal grain growth is easily formed by containing B. by. Furthermore, the auxiliary alloy preferably contains at least one selected from Nd, Pr, Dy, Tb, Ga, Al, and Cu at a higher concentration than the mother alloy. By containing these at a high concentration, there is an advantage that they are unevenly distributed in the vicinity of the grain boundary and increase the coercive force.

  These fine powder of the mother alloy and the fine powder of the auxiliary alloy are mixed in such a ratio that the final composition is obtained and used as the raw material alloy fine powder. At this time, it is possible to further add a third alloy fine powder to form a three-alloy or a multi-component system having more than that.

  In the rare earth sintered magnet of the present invention, the oxygen content is preferably 2500 ppm or less. This is also related to the discharge plasma sintering described later. However, when the oxygen content exceeds 2500 ppm, the amount of rare earth elements present as oxides increases, and the magnetic phase that should exist in the main phase and subphase. This causes a problem that the effective rare earth elements are reduced and the coercive force is lowered. Furthermore, the generated oxide is non-magnetic and causes a decrease in magnetization of the sintered body. The relationship between the amount of oxygen and the amount of oxide produced has a linear relationship according to the stoichiometric ratio of the compounds, but in order to satisfy the coercive force and magnetization required for high performance rare earth magnets in recent magnet applications. It is required to be 2500 ppm or less, and particularly preferably 2000 ppm or less.

  Furthermore, the rare earth sintered magnet of the present invention preferably has a carbon (C) content of 1500 ppm or less and a nitrogen (N) content of 200 to 1500 ppm. When the carbon content exceeds 1500 ppm, carbon forms a carbide with a part of the rare earth element, and the magnetically effective rare earth element is reduced and the coercive force is lowered. Further, by setting the nitrogen amount in the above range, both excellent corrosion resistance and high magnetic properties can be achieved.

  The rare earth sintered magnet of the present invention is manufactured by a powder metallurgy method, and in particular, sintered by spark plasma sintering. Hereinafter, a method for producing a rare earth sintered magnet by powder metallurgy will be described.

  FIG. 1 shows an example of a process for producing a rare earth sintered magnet by powder metallurgy. This manufacturing process basically includes an alloying step 1, a coarse pulverization step 2, a fine pulverization step 3, a magnetic field forming step 4, a binder removal step 5, a sintering step 6, an aging step 7, a processing step 8, and And a surface treatment step 9. In order to prevent oxidation, most of the steps after sintering are performed in a vacuum or in an inert gas atmosphere (in a nitrogen atmosphere, an Ar atmosphere, etc.).

  In the alloying step 1, a raw material metal or alloy is blended according to the mother alloy composition and the auxiliary alloy composition, dissolved in an inert gas, for example, Ar atmosphere, and cast into an alloy. As a casting method, a strip casting method (continuous casting method) in which molten high-temperature liquid metal is supplied onto a rotating roll and an alloy thin plate is continuously cast is preferable from the viewpoint of productivity and the like. As the raw material metal (alloy), pure rare earth elements, rare earth alloys, pure iron, ferroboron, and alloys thereof can be used. When cast as an ingot, solution treatment may be performed as necessary for the purpose of eliminating solidification segregation. As a condition for the solution treatment, for example, it is kept in a 700 to 1200 ° C. region for 1 hour or more under vacuum or Ar atmosphere.

  In the coarse pulverization step 2, the previously cast mother alloy and auxiliary alloy thin plates, ingots, or the like are pulverized to a particle size of about several hundred μm. As the pulverizing means, a stamp mill, a jaw crusher, a brown mill, or the like can be used. In order to improve the coarse pulverization property, it is effective to perform coarse pulverization after occlusion of hydrogen and embrittlement.

  After the coarse pulverization step 2 is completed, a pulverization aid is usually added to the coarsely pulverized mother alloy powder and auxiliary alloy powder. As the grinding aid, for example, fatty acid compounds can be used. In particular, by using fatty acid amide as the grinding aid, a rare earth sintered magnet having good magnetic properties, particularly high orientation and high magnetization. Can be obtained. The addition amount of the grinding aid is preferably 0.03 to 0.4% by weight. If the addition amount of the grinding aid is less than 0.03% by weight, the effect on the magnetic properties of the lubricant cannot be sufficiently obtained. If the addition amount is 0.4% by weight or less, the residual after sintering The amount of carbon can be effectively reduced, which is effective in improving the magnetic properties of the rare earth sintered magnet.

  After the coarse pulverization step 2, a fine pulverization step 3 is performed. The fine pulverization step 3 is performed using, for example, an airflow pulverizer. The conditions at the time of fine pulverization may be appropriately set according to the airflow type pulverizer to be used. . A jet mill or the like is suitable as the airflow pulverizer. A jet mill opens a high-pressure inert gas (for example, nitrogen gas) from a narrow nozzle to generate a high-speed gas flow, accelerates powder particles by this high-speed gas flow, and collides powder particles with each other. Or, it is a method of crushing by generating a collision with a collision plate or a container wall. Jet mills are generally classified into jet mills that use fluidized beds, jet mills that use vortex flow, jet mills that use impingement plates, and the like. Among these jet mills, a jet mill using a fluidized bed and a jet mill using a vortex are preferable, and a jet mill using a fluidized bed is particularly preferable. For example, the specific gravity of the raw material alloy powder and the grinding aid are greatly different, but in the fluidized bed and in the vortex, the grinding and mixing are performed well regardless of the difference in specific gravity. It is because it does not become.

  After the fine pulverization step 3, in the magnetic field forming step 4, the mother alloy fine powder and the auxiliary alloy fine powder, and further, if necessary, other alloy fine powders are mixed to obtain raw material alloy fine powder. To mold. Specifically, the raw material alloy fine powder obtained in the fine pulverization step 3 is filled in a mold in which an electromagnet is arranged, and is molded in a magnetic field with a crystal axis oriented by applying a magnetic field. The forming in the magnetic field may be either a vertical magnetic field forming in which the forming pressure and the magnetic field direction are parallel, or a horizontal 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 at a pressure of about 100 to 200 MPa in a magnetic field of 700 to 1300 kA / m, for example. In addition, the mixing of the mother alloy fine powder, the auxiliary alloy fine powder, and the other alloy fine powder may be performed after the coarse pulverization. In this case, after the coarse pulverization, these are mixed, and then the fine pulverization step and the forming step in the magnetic field. I do.

  Next, the molded body formed by the molding step in the magnetic field is sintered, and the binder removal process is performed in the binder removal step 5 prior to the sintering. This binder removal process is a process for removing the lubricant added in the pulverization process and contained in the molded body, and by removing the binder, carbon remaining as a carbide, oxide, etc. after sintering. And the remaining amount of oxygen can be reduced. Further, when sintering is performed by the spark plasma sintering method in the sintering step 6, crystal grain growth due to oxygen becomes a problem, but if the lubricant is removed by this debinding process, crystal grain growth is minimized. Can also be suppressed.

  In the present invention, this debinding step is performed by resistance heating. In the debinding process, for example, the molded body is held at a temperature at which organic substances of about 200 ° C. to 500 ° C. can be decomposed, and organic substances such as a lubricant contained in the molded body are decomposed and removed. The temperature can be easily controlled and the temperature is uniform, and the binder removal efficiency is improved. Further, prior to sintering by the discharge plasma sintering method, the compact can be pre-sintered by resistance heating used for binder treatment.

  Here, considering that the sintering step 6 described later is performed by the discharge plasma sintering method, the binder removal treatment can also be performed by the discharge plasma sintering method. However, in the discharge plasma sintering method, stable temperature control at a low temperature suitable for debinding is difficult, and efficient heating is difficult. For this reason, when the discharge plasma sintering method is applied to the binder removal treatment, carbon and oxygen are likely to remain, which may cause deterioration of characteristics of the finally obtained rare earth sintered magnet.

  Next, sintering is performed in the sintering step 6. That is, after forming the raw material alloy fine powder in a magnetic field, the formed body (pre-sintered body) subjected to the binder removal treatment is sintered in a vacuum or an inert gas atmosphere.

  In the present invention, in the sintering step 6, the compact is sintered by discharge plasma sintering. FIG. 2 shows an outline of a discharge plasma sintering apparatus. In the discharge plasma sintering apparatus, for example, a so-called die set is provided in the vacuum chamber 11. The die set sandwiches the sintered die 13 that accommodates the molded body 12, the lower punch 14 and the upper punch 15 that pressurize the molded body 12 in the sintered die 13 from one axis direction, and the lower punch 14 and the upper punch 15. The lower electrode 16 and the upper electrode 17 are connected to a DC pulse power source 18. When sintering is performed using this discharge plasma sintering apparatus, a sintering die 13, a lower punch 14 and an upper punch 15 filled with a molded body 12 are placed between a lower electrode 16 and an upper electrode 17. The DC pulse power supply 18 is energized while being pressurized. Then, the surface of the raw material alloy fine powder is activated and cleaned by blowing off the adsorbed gas on the surface of the raw material alloy powder or destroying the insulating oxide film by the discharge impact pressure of the discharge plasma and the sputtering action. Further, even when no discharge plasma is generated, rapid Joule heat is generated by passing a large current, and the surface of the raw material alloy fine powder self-heats. In addition, by applying the pulse current, high-speed movement of ions occurs due to the action of an electric field, so that the discharge points and Joule heating points move and disperse in the molded body, and the entire molded body is heated uniformly. Thus, sintering is performed at a relatively low temperature and by a rapid temperature increase.

  In addition, it is not always necessary to pressurize the discharge plasma sintering of the rare earth sintered magnet. In this case, as shown in FIG. 3, the lower electrode 16 and the upper electrode 17 are arranged above and below the molded body 12. The energization may be performed from the DC pulse power supply 18. However, pressurization is more advantageous in improving the characteristics. Moreover, when pressurizing, it is preferable to pressurize in the direction parallel to the orientation direction of the molded object 12. It is also possible to incorporate the above-described forming step 4 in a magnetic field into a discharge plasma apparatus and perform from molding to sintering as a series of steps.

  By the way, in the sintering step 6, when the compact formed from the raw material alloy fine powder is sintered by spark plasma sintering, it is necessary to pay attention to the amount of oxygen contained in the raw material alloy fine powder. For example, NdFeB-based alloys are very easily oxidized, and even when pulverization is performed while controlling the oxygen atmosphere, the amount of oxygen usually reaches a level exceeding 2500 ppm. This amount of oxygen is approximately 1% by weight or more when present as Nd oxide. Further, the amount of oxygen is larger than the amount studied as an additive amount of the lubricant (up to 0.8% by weight) and much larger than the optimum amount of the lubricant (0.1% by weight). . When the amount of oxygen in the raw material alloy fine powder is large, the activation and cleaning action of the surface of the raw material alloy fine powder by the discharge plasma is hindered, and advantages such as rapid temperature rise and shortening of the sintering time cannot be obtained. As a result, crystal grains grow, leading to a decrease in magnetic properties such as a decrease in coercivity of the rare earth sintered magnet.

  Therefore, in the present invention, it is preferable to suppress the amount of oxygen contained in the raw material alloy fine powder to 2500 ppm or less. By adopting the discharge plasma sintering method in the sintering process and setting the oxygen amount to 2500 ppm or less, the activation and cleaning action of the raw material alloy powder surface by the discharge plasma is sufficiently expressed, and the rapid sintering reaction and Since densification proceeds, crystal grain growth can be effectively suppressed. For this reason, a rare earth sintered magnet having a higher coercive force than conventional resistance heating can be obtained. There is also an advantage that rare earth elements such as expensive Dy can be reduced by increasing the coercive force.

  Further, according to the present invention, the temperature can be raised and lowered in a short time as compared with the conventional case, and the sintering is completed in a short time. Specifically, when the resistance heating method requires a sintering time of 4 hours, according to the method of the present invention, a rare earth having the same density as a rare earth sintered magnet by resistance heating with a sintering time of 1/5 or less. A sintered magnet can be obtained. As described above, since the sintering time can be greatly shortened, the time required for producing the rare earth sintered magnet is shortened, and the industrial economic effect is extremely large.

  In order to suppress the amount of oxygen contained in the raw material alloy fine powder, for example, in the fine pulverization step 3, it is necessary to suppress an increase in the amount of oxygen during pulverization by a jet mill. For that purpose, for example, when pulverizing with a jet mill, it is necessary to carry out in an inert gas atmosphere and to strictly control the conditions. Further, not only the fine pulverization step 3 but also the coarse pulverization step 2 and the like, it is required to strictly control the oxygen amount in the atmosphere in all the steps up to sintering.

  The technique of applying the discharge plasma sintering method to the sintering process of the NdFeB-based sintered magnet has been proposed so far in, for example, the above-mentioned Patent Documents 2 to 4, and is a feature of the discharge plasma sintering method. One of the major reasons that the effect of suppressing the growth of crystal grains has been realized is that it has become possible to produce a compact with a low oxygen content. For example, in the invention described in Patent Document 2 described above, only a discharge plasma sintering method is shown as an example of a technique for producing a high electric resistance magnet when a fluoride or oxide having a high electric resistance is added. It is not intended to improve the magnetic properties of sintered magnets. Further, no consideration has been given to the crystal grains of the sintered body. The same applies to the inventions described in Patent Document 3 and Patent Document 4.

  Sintering conditions by the discharge plasma sintering method may be appropriately set according to the size of the compact to be sintered, the size of the raw material alloy fine powder, and the like. By making the sintering conditions appropriate, it is possible to suppress grain growth of the crystal grains and increase the density by promoting the sintering reaction. Here, as one index of the sintering conditions, the crystal grain size distribution of the sintered body after sintering can be mentioned. Specifically, the sum of the areas of the main phase crystal grains having a crystal grain size of 10 μm or less is 90% or more and the area of the main phase crystal grains having a crystal grain size of 15 μm or more with respect to the total area of the main phase crystal grains. Sintering is preferably performed so that the sum is 5% or less. By adopting the spark plasma sintering method in the sintering process and controlling the oxygen content of the compact to a specific value or less, a finer crystal grain size distribution can be realized as compared with the prior art. The sum of the areas of the main phase grains having a crystal grain size of 10 μm or less is less than 90% and the sum of the areas of the main phase grains having a grain size of 15 μm or more is 5% of the total area of the main phase grains Exceeding this means that grain growth is progressing, and the coercivity of the rare earth sintered magnet may be reduced. More preferably, the sum of the areas of the main phase crystal grains having a crystal grain size of 8 μm or less is 80% or more and the sum of the areas of the main phase crystal grains having a crystal grain size of 13 μm or more with respect to the total area of the main phase crystal grains. Is preferably 10% or less, and more preferably, the sum of the areas of the main phase crystal grains having a crystal grain size of 8 μm or less is 90% or more and the crystal grain size is 13 μm or more with respect to the total area of the main phase crystal grains. The sum of the area of the phase crystal grains is 5% or less. Furthermore, the sum of the areas of the main phase grains having a crystal grain size of 6 μm or less is 80% or more and the sum of the areas of the main phase grains having a crystal grain size of 10 μm or more is the total area of the main phase grains. It is preferable that it is 10% or less.

  Further, as another index of the sintering condition, a ratio R / r (grain growth ratio) of the average particle diameter r of the raw material alloy fine powder before sintering and the crystal grain diameter R of the sintered body after sintering can be mentioned. it can. Specifically, it is preferable to set the sintering conditions so that the ratio R / r is 1.5 or less. When the ratio R / r exceeds 1.5, it means that grain growth is progressing and the coercive force of the rare earth sintered magnet may be lowered. The average particle diameter r of the raw material alloy fine powder before sintering and the crystal grain diameter R of the sintered body after sintering have the same unit.

  After sintering, in the aging step 7, it is preferable to subject the obtained sintered body to an aging treatment. This aging treatment is an important step in controlling the coercive force Hcj of the obtained rare earth sintered magnet. For example, the aging treatment is performed in an inert gas atmosphere or in a vacuum. As the aging treatment, a two-stage aging treatment is preferable, and in the first aging treatment step, the temperature is maintained at a temperature of about 800 ° C. for 1 to 3 hours. Next, a first quenching step is provided for quenching to room temperature to 200 ° C. In the second stage aging treatment step, the temperature is maintained at about 550 ° C. for 1 to 3 hours. Next, a second quenching step for quenching to room temperature is provided. Since the coercive force Hcj is greatly increased by heat treatment at around 600 ° C., when aging treatment is performed in a single stage, it is preferable to perform aging treatment at around 600 ° C.

  After the aging step 7, a processing step 8 and a surface treatment step 9 are performed. The processing step 8 is a step of mechanically forming into a desired shape. The surface treatment step 9 is a step performed to suppress oxidation of the obtained rare earth sintered magnet. For example, a plating film or a resin film is formed on the surface of the rare earth sintered magnet.

  Next, specific examples of the present invention will be described based on experimental results.

<Preparation of rare earth sintered magnet>
A metal alloy or a raw material alloy is mixed so as to have a predetermined composition, and a mother alloy and an auxiliary alloy melted by high-frequency melting in an alumina crucible are each a thin plate-like alloy having a thickness of 1 mm or less by strip casting. It was.

  The thin plate-like alloy was dehydrogenated by Ar flow or evacuation in a fully evacuated furnace by occlusion and embrittlement of hydrogen at around room temperature, followed by heating. The embrittled thin plate alloy was coarsely pulverized to several hundred μm by mechanical pulverization in a nitrogen atmosphere, and further finely pulverized to a mean particle size of 2.6 to 2.7 μm by a jet mill in a nitrogen stream.

  The pulverized mother alloy fine powder and auxiliary alloy fine powder were subjected to a forming process while oxygen was blocked. In the molding step, a green compact (molded body) in which the crystal direction of the mother alloy fine particles was oriented by a magnetic field was obtained using a magnetic field molding machine. Also in this molding process, the amount of oxygen in the atmosphere was strictly controlled to 500 ppm or less.

  Furthermore, the molded body was transferred to a sintering apparatus with oxygen blocked, and sintered after debinding. After sintering, an aging treatment was performed. The aging treatment was a two-stage aging treatment, and the first stage was 900 ° C. for 1 hour, and the second stage was 530 ° C. for 1 hour.

<Evaluation>
About each produced rare earth sintered magnet, residual magnetic flux density Br, coercive force iHc, energy product (BH) m, squareness ratio (Hk / iHc) were measured. Measurement of magnet characteristics [residual magnetic flux density Br, coercive force iHc, energy product (BH) m] was performed using a BH tracer. The squareness ratio is the ratio of the magnetic field Hk at the point where the magnetization B is 10% lower than the residual magnetization Br and the magnetic field (the coercive force iHc) at the point where the magnetization B becomes zero in the BH loop (Hk / iHc).

<Examples and Comparative Examples>
Rare earth sintered magnets (Examples 1 to 8 and Comparative Examples 1 to 4) were prepared by changing the mother alloy composition, the auxiliary alloy composition, and the sintering method.

When the sintering method is the discharge plasma method, the discharge plasma sintering apparatus shown in FIG. 2 was used as the discharge plasma sintering apparatus. In sintering, the upper electrode 17 and the lower electrode 16 in the discharge plasma sintering apparatus (in the vacuum chamber 11) adjusted to a vacuum atmosphere (10 ⁻⁴ Pa or less) are formed from the previously produced compact. In the die set having the above, the orientation of the molded body 12 was arranged up and down, and after confirming the degree of vacuum, the temperature was raised from room temperature to about 200 to 400 ° C., and the binder removal treatment was performed first. Then, the current density 100~500A / cm ^2, by adjusting voltage 30～70V, the range of pulse period 100～200Msec, while pressurized to 150 kg / cm ^2, to obtain optimum properties make electric current sintering. The time from binder removal to the end of sintering was 30 minutes. In addition, for comparison, compacts having the same composition were sintered by resistance heating at 1020 to 1080 ° C. for 1 to 4 hours to obtain comparative examples.
The main phase crystal grain distribution was determined by taking a photograph with a polarizing microscope after polishing the surface and performing image processing in an area of about 100 * 100 μm.

  Tables 1 to 4 show the master alloy composition, the auxiliary alloy composition, the final composition, the sintering method, and the magnet characteristics in each Example and Comparative Example.

As is clear from these tables, the effect is exhibited with a small amount of fine powder of the auxiliary alloy by performing the sintering by the discharge plasma sintering method, and in particular, the improvement of the residual magnetic flux density Br and the squareness ratio is seen. . If the amount of the auxiliary alloy fine powder can be reduced, the proportion of nonmagnetic elements such as Al, Cu, Mn, Nb, Zr, and Hf can be reduced, which is advantageous from the viewpoint of magnet characteristics.
In addition, the rare earth sintered compact oxygen amount of the Example of this invention was 1200-1800 ppm.
Further, when the main phase crystal grain distribution of the sintered body of Sample 1 corresponding to the example of the present invention was measured, the crystal grains of 10 μm or less were 99%, and 15 μm or more were 0%. The crystal grains of 8 μm or less were 94%, 13 μm or more were 0%, and 6 μm or less were 84%, 10 μm or more were 1%.

It is a flowchart which shows an example of the manufacturing process of a rare earth sintered magnet. It is a schematic diagram which shows schematic structure of a discharge plasma sintering apparatus. It is a schematic diagram which shows the other example of a discharge plasma sintering apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Alloying process 2 Coarse grinding process 3 Fine grinding process 4 Magnetic field forming process 5 Sintering process 6 Aging process 7 Processing process 8 Surface treatment process 11 Vacuum chamber 12 Molded body 13 Sintering Die, 14 Lower punch, 15 Upper punch, 16 Lower electrode, 17 Upper electrode, 18 DC pulse power supply

Claims (15)

Fine powder of a master alloy mainly composed of R ₂ T ₁₄ B phase (where R represents one or more rare earth elements, T represents a transition metal containing at least one of Fe and Co), Zr, Hf, Nb, Using raw material alloy fine powder containing fine powder of auxiliary alloy containing at least one selected from Ta, Mo, W, Si, V, Ti, Cr,
A rare earth sintered magnet, wherein a compact formed from the raw material alloy fine powder is sintered by spark plasma sintering.   The rare earth sintered magnet according to claim 1, wherein the auxiliary alloy contains at least one selected from Zr, Nb, and Hf.   The rare earth sintered magnet according to claim 1, wherein the auxiliary alloy contains B.   The rare earth sintered magnet according to any one of claims 1 to 3, wherein the auxiliary alloy contains Co.   The said adjuvant alloy contains at least 1 sort (s) chosen from Nd, Pr, Dy, Tb, Ga, Al, Cu in a density | concentration higher than a mother alloy, The any one of Claim 1 thru | or 4 characterized by the above-mentioned. Rare earth sintered magnet.   The rare earth sintered magnet according to any one of claims 1 to 5, wherein an oxygen amount is 2500 ppm or less.   The sum of the areas of the main phase crystal grains having a crystal grain size of 10 μm or less is 90% or more and the sum of the areas of the main phase crystal grains having a crystal grain size of 15 μm or more is 5% of the total area of the main phase crystal grains. The rare earth sintered magnet according to any one of claims 1 to 6, wherein the sintered rare earth magnet is at least%.   The rare earth sintered magnet according to any one of claims 1 to 7, wherein the final composition is 27.0 to 32.0% by weight of a rare earth element and 0.5 to 2.0% by weight of boron. Fine powder of a master alloy mainly composed of R ₂ T ₁₄ B phase (where R represents one or more rare earth elements, T represents a transition metal containing at least one of Fe and Co), Zr, Hf, Nb , Using raw material alloy fine powder containing auxiliary powder fine powder containing at least one selected from Ta, Mo, W, Si, V, Ti, Cr,
A method for producing a rare earth sintered magnet, comprising sintering a compact formed from the raw material alloy fine powder by spark plasma sintering.   The method for producing a rare earth sintered magnet according to claim 9, wherein the auxiliary alloy contains at least one selected from Zr, Nb, and Hf.   The method for producing a rare earth sintered magnet according to claim 9 or 10, wherein the auxiliary alloy contains B.   The method for producing a rare earth sintered magnet according to any one of claims 9 to 11, wherein the auxiliary alloy contains Co.   The said auxiliary alloy contains at least 1 sort (s) chosen from Nd, Pr, Dy, Tb, Ga, Al, Cu in a density | concentration higher than a mother alloy, The any one of Claims 9 thru | or 12 characterized by the above-mentioned. Manufacturing method of rare earth sintered magnet.   The method for producing a rare earth sintered magnet according to any one of claims 9 to 13, wherein the amount of oxygen contained in the raw material alloy fine powder is 2500 ppm or less.   The sum of the areas of the main phase grains having a crystal grain size of 10 μm or less is 90% or more and the sum of the areas of the main phase grains having a crystal grain size of 15 μm or more is 5% or less of the total area of the main phase grains The method for producing a rare earth sintered magnet according to claim 9, wherein the sintering is performed so that

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