JP2005197299A - Rare earth sintered magnet and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress the growth of a crystal grain, and to increase density by reducing sintering time. <P>SOLUTION: A forming body is subjected to debindering treatment by resistance heating before sintering by discharge plasma sintering, when manufacturing a rare earth sintered magnet, by sintering the forming body for forming a raw material alloy fine powder containing a rare earth element, a transition metal element, and boron. In this case, the amount of oxygen contained in the raw material alloy fine is preferably set to 2,500 ppm or smaller. Preliminary sintering is preferably made until sintering density reaches 70% or higher than the degree of vacuum by resistance heating. <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 a rapid temperature increase is possible compared to resistance heating using radiant heat. However, there is a problem that it is difficult to accurately maintain the temperature range necessary for the debinding process. When sintering a rare earth sintered magnet, the binder removal process is also an important process. The compact of the raw material alloy powder is added with a lubricant such as zinc stearate for smooth pulverization and molding, but vacuum is applied so that they do not remain as carbides or oxides in the sintered body. It must be decomposed in the middle or in the Ar flow and removed outside the system. If the lubricant remains during the sintering, the amount of carbon and oxygen in the obtained rare earth sintered magnet increases, which causes deterioration of characteristics.

  The present invention has been proposed in view of such a conventional situation, and can be reliably debindered. Further, the sintering time can be shortened, and densification and grain growth can be suppressed simultaneously. An object of the present invention is to provide a method for producing a rare earth sintered magnet that can be achieved and can suppress the formation of heterogeneous phases, and to provide a rare earth sintered magnet having excellent magnetic properties such as coercive force.

  In order to solve the above-mentioned problem, the rare earth sintered magnet according to the present invention is a discharge plasma obtained by forming a compact of a raw material alloy powder containing a rare earth element, a transition metal element, and boron after debinding treatment by resistance heating. It is characterized by being sintered. In addition, the method for producing a rare earth sintered magnet according to the present invention includes the step of sintering a molded body obtained by molding a raw material alloy fine powder containing a rare earth element, a transition metal element, and boron. After the binder is removed by resistance heating, the sintering is performed by discharge plasma sintering.

In the production of rare earth sintered magnets, raw material alloy fine powder is used as a compact, and this is sintered into a sintered compact, which contains the lubricant added at the stage of pulverization. . Therefore, in the present invention, the binder removal treatment is performed by resistance heating prior to sintering. By adopting resistance heating for the debinding process, temperature control at low temperatures is facilitated, temperature uniformity is achieved, and the temperature can be stably maintained to decompose organic substances, and the lubricant can be quickly Removed outside the system. As a result, the remaining carbon (carbide) or oxygen (oxide) is suppressed. When the temperature is increased before the binder is removed from the molded body, the active Nd generates compounds such as NdC (carbide) and Nd ₂ O ₃ (oxide). On the other hand, if the temperature is too low, the binder will not be decomposed and removed from the system even if the temperature is maintained for a long time. In addition, prior to sintering by the discharge plasma sintering method, the molded body can be pre-sintered by resistance heating used for binder treatment.

  In the present invention, sintering is performed by spark plasma sintering. In the spark plasma sintering method, a direct current pulse voltage is applied to a powder compression-molded body, thereby generating several thousand to one instantaneously generated in the powder gap. This is a method of sintering using high energy of discharge plasma of about 10,000 ° C. When pulsed current is applied to the compact under pressure, rapid Joule heating of the powder surface due to the passage of a large pulse current between the raw material alloy fine powder, surface activation and cleaning action by discharge plasma generated in the raw material alloy fine powder gap, Only the surface of the raw material alloy powder rapidly generates heat and is heated by the movement of the discharge point and the Joule heating point. Therefore, the temperature can be raised and lowered much more rapidly than resistance heating using radiant heat.

  By the way, when applying the discharge plasma sintering method to the sintering of rare earth sintered magnets such as NdFeB magnets, 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. It is possible to make the most of the crystal grain growth suppressing effect which is an advantage of the above.

  The rare earth sintered magnet of the present invention is sintered by spark plasma sintering after debinding by resistance heating, and the oxygen content is controlled to a predetermined value or less, so that the remaining carbon and oxygen are suppressed. At the same time, the advantages of the spark plasma sintering method are utilized to the maximum, and high density, suppression of crystal grain growth, and generation of heterogeneous phases are simultaneously realized. 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, resistance heating is employed for the debinding process and discharge plasma sintering is employed for the sintering, and the oxygen content is controlled to a predetermined value or less. Binder is efficiently performed, and rapid sintering reaction and densification proceed due to the cleaning and activating action of the raw material alloy fine powder surface. Therefore, according to the present invention, it is possible to obtain a sintered body with a reduced sintering time and a high density without grain growth, and further, since there is little residual carbon and oxygen, the obtained rare earth sintered magnet The sintered state of can be brought close to the ideal state.

  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. The magnet 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.

Here, 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 More than a seed. 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.

  In the rare earth sintered magnet of the present invention, the oxygen content is 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 metal or alloy as a raw material is blended according to the magnet 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 raw alloy thin plate, ingot or the like is pulverized until the particle size is about several hundreds of micrometers. 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 aforementioned coarse pulverization step 2 is completed, a pulverization aid is usually added to the coarsely pulverized raw material 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 for fine pulverization may be appropriately set according to the airflow pulverizer to be used, and the raw material alloy powder is finely pulverized until the average particle size becomes about 1 to 10 μm, for example, 3 to 6 μm. 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 pulverizing step 3, in the forming step 4 in the magnetic field, the raw material alloy fine powder is formed in the magnetic field. 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.

  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, it is conceivable that the binder removal treatment is also performed by the discharge plasma sintering method. However, in the discharge plasma sintering method, stable temperature control at a low temperature suitable for the binder removal process 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, causing the characteristic deterioration of the finally obtained rare earth sintered magnet. For this reason, resistance heating using radiant heat is applied to the binder removal treatment.

  Moreover, as pre-sintering, it is preferable to perform pre-sintering to control the density of the compact after debinding. Specifically, in the binder removal step 5, after the binder removal treatment, resistance heating is performed until the sintered density reaches 70% or more, more preferably 85% or more of the true density. Thereby, the strength of the compact is improved, and finally, a high-density and high-performance rare earth sintered magnet can be stably obtained.

  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.

  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 or alloy as a raw material was blended so as to have a predetermined composition, and an alloy melted by high frequency melting in an alumina crucible was made into a thin plate alloy having a thickness of 1 mm or less by a strip casting method.

  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 raw material alloy fine powder was subjected to a forming process while oxygen was blocked. In the forming step, a green compact (formed product) in which the crystal directions of the particles of the raw material alloy fine powder obtained by the magnetic field were aligned was obtained using a magnetic field forming 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, and main phase crystal grain distribution were measured. Measurement of magnet characteristics [residual magnetic flux density Br, coercive force iHc, energy product (BH) m] was performed using a BH tracer. Regarding the main phase crystal grain distribution, after polishing the surface, a photograph was taken with a polarizing microscope, and image processing was performed in an area of about 100 μm × 100 μm to obtain the crystal grain size distribution.

<Examination of heating method>
First, after performing a binder removal process by resistance heating with the composition and conditions shown in Table 1, the sintering process was performed by spark plasma sintering (SPS) to produce Samples 1 and 2. As the discharge plasma sintering apparatus, the discharge plasma sintering apparatus shown in FIG. 2 was used. Further, in the apparatus shown in FIG. 2, a resistor for performing resistance heating was installed in the vacuum chamber, and a debinding process by resistance heating and a sintering process by discharge plasma sintering were performed in the same vacuum chamber. At this time, the die set and the resistor could be moved up and down independently, and they were moved to the position of the molded body and heated as needed. In addition, although resistance heating and discharge plasma sintering can also be performed by a separate apparatus, in that case, it is preferable to move between apparatuses so that a molded object may not oxidize.

In sintering, the formed body is placed in the vacuum chamber 11 adjusted to a vacuum atmosphere (10 ⁻⁴ Pa or less), and after confirming the degree of vacuum, resistance heating is performed at 300 ° C. for 40 minutes. (Sample 2 was held for 30 minutes). Next, the compact was moved and placed in a die set having an upper electrode 17 and a lower electrode 16. Then, the current density 100~500A / cm ^2, voltage 30～70V, adjusted in a range of pulse period 100～200Msec, while pressurized to 150 kg / cm ^2, subjected to electric current sintering, given sintered body grain distribution Samples 1 and 2 were prepared. The time from binder removal to the end of sintering was 30 minutes.

  For comparison, a sample having the same composition is subjected to both binder removal treatment and sintering by discharge plasma sintering to produce sample 3, and further sintered by resistance heating to obtain sample 4 , 5 were produced. Heating method, composition of each sample, oxygen content of raw material alloy fine powder (oxygen amount of forming powder), average particle diameter (fine pulverized particle size) of raw material alloy fine powder, binder removal condition, sintering completed after binder removal treatment Table 1 shows the time until. Moreover, sintered body crystal grain distribution (main phase crystal grain distribution), sintered body oxygen content, sintered body carbon content, magnet characteristics [residual magnetic flux density Br, coercivity iHc, energy product ( BH) m] and the sintered body density are shown in Table 2. Furthermore, FIG. 4 shows the main phase crystal grain distribution of the sintered body of Sample 2 corresponding to the example of the present invention, and FIG. 5 shows the main phase crystal grain distribution of Sample 4 corresponding to the comparative example.

  As is clear from these tables, in order to obtain a rare earth sintered magnet having high magnetic properties with suppressed grain growth and high density, it is advantageous to perform sintering by spark plasma sintering. Understand. However, in Sample 3 where the discharge plasma sintering was performed until the binder removal treatment, since the binder removal was insufficient, the amount of sintered carbon was slightly larger and the coercive force was slightly lower. On the other hand, in the sample 4 by resistance heating, since the sintering time is long, the density is high, but the grain growth proceeds and the magnet characteristics are low. In the sample 5 by resistance heating, since the sintering time was shortened, the sintering reaction was insufficient, and the magnet characteristics and density were significantly reduced.

<Examination of oxygen content>
Sintering by spark plasma sintering was tried in the same manner as in Samples 1 and 2 except that the oxygen content of the raw material alloy powder used was changed. Heating method (debinder + sintering) in each sample 6 to 9, composition, oxygen content of raw material alloy fine powder (oxygen amount of forming powder), average particle diameter (fine pulverized particle size) of raw material alloy fine powder, binder removal Table 3 shows the conditions and the time from debinding to the end of sintering. Table 4 shows the sintered body oxygen content, sintered body carbon content, magnet characteristics [residual magnetic flux density Br, coercive force iHc, energy product (BH) m], and sintered body density of the produced samples 6 to 9. .

  In Sample 6 having an oxygen content of 5500 ppm, the sintered body was deformed. Further, it can be seen that it is difficult to obtain a high density in the samples 6 and 7 having a large amount of oxygen. In contrast, when the oxygen content is 2500 ppm or less in the raw material alloy fine powder and the sintered body, extremely high characteristics are obtained. Therefore, within this range, it can be said that a sintered body having a good crystal grain distribution and a high density can be obtained.

<Examination on pre-sintering>
In accordance with the previous example, preliminary sintering by resistance heating was performed, and sintering by spark plasma sintering was attempted. The conditions were the same as those of Sample 1. The pre-sintering density and the applied pressure were changed, and the chipped and cracked sintered bodies were observed. The results are shown in Table 5. In Table 5, “x” represents “chips and cracks”, and “◯” represents “chips and no cracks”.

  As is apparent from this table, in the pre-sintered sample, even if a high pressure is applied from the beginning, no chipping or cracking is observed in the finally obtained sintered body. It can be seen that there are advantages such as production without particular attention.

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. It is a graph which shows the main phase crystal grain distribution of the sintered compact of the sample 2 (Example). It is a graph which shows the main phase crystal grain distribution of the sintered compact of the sample 4 (comparative example).

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 (9)

  A rare earth sintered magnet, wherein a compact formed of a raw material alloy fine powder containing a rare earth element, a transition metal element and boron is sintered by discharge plasma sintering after debinding treatment by resistance heating.   2. The rare earth sintered magnet according to claim 1, wherein the oxygen content 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 claim 1, wherein the rare earth sintered magnet is at most%.   Rare earth element 27.0-32.0% by weight, boron 0.5-2.0% by weight, carbon 1500ppm or less, nitrogen 200-1500ppm, the balance has a composition consisting essentially of Fe The rare earth sintered magnet according to any one of claims 1 to 3. Sintering a compact formed from a raw material alloy fine powder containing a rare earth element, a transition metal element and boron, when producing a rare earth sintered magnet,
A method for producing a rare earth sintered magnet, wherein the sintering is performed by discharge plasma sintering after debinding treatment by resistance heating.   6. The method for producing a rare earth sintered magnet according to claim 5, wherein the amount of oxygen contained in the raw material alloy fine powder is 2500 ppm or less.   7. The method for producing a rare earth sintered magnet according to claim 5, wherein preliminary sintering is performed until the sintered density reaches 70% or more of the true density by the resistance heating.   The method for producing a rare earth sintered magnet according to claim 7, wherein preliminary sintering is performed until the sintered density reaches 85% or more of the true density by the resistance heating.   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 5, wherein sintering is performed so that

Images