JP5025372B2 - Sintered body manufacturing method and neodymium iron boron based sintered magnet manufactured by this sintered body manufacturing method - Google Patents

Description

  The present invention relates to a method for producing a sintered body, and more specifically, a high-performance permanent magnet is produced by preferentially evaporating rare earth elements from an Nd—Fe—B (neodymium iron boron) sintered magnet. The present invention relates to a method for manufacturing a sintered body.

  Nd-Fe-B based sintered magnets (so-called neodymium magnets) can be manufactured at low cost by being made of a combination of iron and Nd and B elements that are inexpensive and abundant in resources and can be stably supplied. In addition, since it has high magnetic properties (the maximum energy product is about 10 times that of ferrite magnets), it is used in various products such as electronic equipment, and is also used in motors and generators for hybrid cars. is increasing.

Nd-Fe-B magnets are mainly produced by powder metallurgy. In this method, Nd, Fe, and B are first blended at a predetermined composition ratio, and melted and cast to produce an alloy raw material. For example, it is roughly pulverized by, for example, a hydrogen pulverization process, and then finely pulverized by, for example, a jet mill pulverization process to obtain alloy raw material powder. Next, the obtained alloy raw material powder is oriented in a magnetic field (magnetic field orientation), and compression molded in a state where a magnetic field is applied to obtain a compact. And this sintered compact is sintered on predetermined conditions, and a sintered magnet is produced (refer patent document 1).
Japanese Unexamined Patent Application Publication No. 2004-6761 (for example, see the description of the prior art)

By the way, the R ₂ T ₁₄ B phase (main phase component) responsible for the magnetism of the Nd—Fe—B based sintered magnet is not directly generated from the liquid phase in an equilibrium state, and firstly γ iron is generated as an initial phase. It is formed by the reaction between the liquid phase and its iron (peritectic reaction) (γ iron transforms into α iron with decreasing temperature). In this case, for example, even if the alloy raw material is melted and cast by the strip casting method (SC method), which is a rapid cooling method with a rapid solidification cooling rate, if the Nd content is 28.5% or less, the production of α iron It is known that it is difficult to suppress, and it is formed in a dendrite form in the alloy.

  If alpha iron is formed in a dendritic form in the alloy and is three-dimensionally connected, the pulverizability of the alloy in the subsequent pulverization step is significantly impaired. In other words, if the grindability is poor, the particles are once coarsely pulverized by the hydrogen pulverization process, and then the particle shapes suitable for producing sintered magnets with high magnetic properties are prepared even if the pulverization is attempted by the jet mill pulverization process. It is difficult to obtain fine powder particles. In addition, coarse grains (due to α-iron produced in a dendritic form) remain in the jet mill, and composition deviation tends to occur due to an increase in the amount of fine powder collected by the bag filter, making quality control difficult. There is a problem that there is.

On the other hand, if the Nd content is more than 28.5%, it is possible to produce an ingot that does not produce α iron, but the R-rich phase increases and the volume ratio of the R ₂ T ₁₄ B phase that plays a role in magnetism. Therefore, there arises a problem that it becomes difficult to manufacture an ultra-high performance magnet having a maximum energy product ((BH) max) and a large residual magnetic flux density (Br) exhibiting magnetic characteristics.

  In view of the above, an object of the present invention is to provide a method for manufacturing a sintered body that can improve product functions such as improvement of magnetic properties of a permanent magnet.

In order to solve the above-mentioned problem, in the method for manufacturing a permanent magnet according to claim 1, after obtaining a primary sintered body by liquid phase sintering, the primary sintered body is made to have a high vapor pressure among liquid phase components. The liquid phase volume ratio is reduced or eliminated by heating in a vacuum atmosphere at a temperature higher than the temperature at which the specific element can be preferentially evaporated and lower than the sintering temperature.

According to the present invention, by preferentially evaporating an element having a high vapor pressure among liquid phase components that contribute to the promotion of sintering, the volume ratio of the liquid phase is reduced and the original characteristics of the main phase are exhibited. Can do. In particular, when the sintered body is an R ₂ T ₁₄ B phase magnet, the rare earth elements are efficiently evaporated, the volume ratio of the R rich phase is decreased, and the volume ratio of the R ₂ T ₁₄ B phase that plays a role in magnetism is increased. By doing so, the magnetic characteristics can be improved.

  In the present invention, the primary sintered body is preferably obtained by manufacturing a raw material alloy by a strip casting method or a centrifugal casting method, and then performing pulverization, magnetic field forming, and sintering processes.

The raw material alloy is for a neodymium iron boron based sintered magnet having a rare earth element including at least one of neodymium, praseodymium and terbium, and the content of the rare earth element is 28.5 wt% or more and 30 wt%. The following is preferable. According to this, in the primary sintered body, for example, the main phase component is composed of the R ₂ T ₁₄ B phase, R is at least one rare earth element mainly including Nd, and T is mainly Fe. For example, when the transition metal is a sintered magnet that has an excess of R ₂ T ₁₄ B phase stoichiometric composition as the R-rich phase, particularly a liquid phase at the time of sintering and serves to promote sintering, When melting and casting the alloy raw material, an ingot that does not produce alpha iron is produced by setting a large amount of rare earth element so that dendritic alpha iron is not produced in the alloy. The volume ratio of the Nd-rich phase is decreased by evaporating only the R-rich phase rare earth element, and as a result, the maximum energy product ((BH) max) and the residual magnetic flux density (Br) exhibiting magnetic characteristics are reduced. ) And a high performance permanent magnet.

  In this case, the rare earth element content may be reduced to less than 28.5% by evaporation of the rare earth element or the average concentration of the rare earth element may be reduced by 0.5% by weight or more.

  It is preferable that the heating temperature when heating in the vacuum atmosphere is set to 900 ° C. or higher and lower than the sintering temperature. When the temperature is lower than 900 ° C., the rare earth element cannot be preferentially evaporated when the primary sintered body is the sintered magnet, and when the temperature is higher than the sintering temperature, abnormal grain growth occurs. The characteristics are greatly reduced.

The pressure in the vacuum atmosphere is preferably set to 10 ⁻³ Pa or less. When the pressure is higher than 10 ⁻³ Pa, the rare earth element cannot be evaporated preferentially and efficiently when the primary sintered body is the sintered magnet.

  Further, it is preferable to provide and collect a mechanism for trapping the evaporated rare earth element. Thereby, especially when an expensive rare earth element such as dysprosim is evaporated, the rare earth element may be recovered.

  In addition, the sintered compact manufactured by the method of any one of Claims 1 thru | or 7 is a neodymium iron boron series sintered magnet, for example.

Referring to FIG. 1, reference numeral 1 denotes a vacuum evaporation apparatus suitable for carrying out the method for manufacturing a sintered body of the present invention to produce a permanent magnet having high magnetic properties, for example. The vacuum evaporation apparatus 1 has a vacuum chamber 12 that can be held at a reduced pressure to a predetermined pressure (for example, 10 ⁻⁵ Pa) through a vacuum exhausting means 11 such as a turbo molecular pump, a cryopump, or a diffusion pump. A cylindrical sintered body case 2 having an upper surface opened is installed in the vacuum chamber 12, and a space surrounded by the sintered body case 2 constitutes a processing chamber 20. When the sintered body case 2 is heated by the heating means 4 to be described later, when the rare earth element R such as Nd and Pr of the rare earth rich phase which is a liquid phase is preferentially evaporated from the sintered magnet S, It is made of a material that does not react with the rare earth element R.

  That is, if the evaporated rare earth element R adheres to the surface of the sintered body case 2 and forms a reaction product on the surface, it may be difficult to recover the rare earth element R. For this reason, the sintered body case 2 is manufactured from Mo or SUS from which the rare earth element R adhering to the surface is easily peeled, or Mo or the like is formed as a lining film on the surface of another heat insulating material. It is composed. In the processing chamber 20, for example, a plurality of Mo-shaped plate-like mounting portions 3 are arranged at predetermined intervals from the bottom surface to the upper surface of the sintered body case 2. Can be mounted side by side with the sintered magnets S, which are primary sintered bodies formed into a predetermined shape such as a rectangular parallelepiped and then sintered.

  The vacuum chamber 12 is provided with a heating unit 4 so as to surround the sintered body case 2. The heating means 4 is an electric heater (not shown) composed of a plurality of annular filaments arranged over the entire length so as to surround the periphery of the sintered body case 2, and each filament is a vacuum chamber. 12 is supported by a support piece (not shown) provided in the inside. In this case, the filament is also composed of Mo or the like that does not react with the rare earth element R. Then, by heating the sintered body case 2 by the heating means 4 under reduced pressure, the sintered magnet S placed on the processing chamber 20 and thus on the placement portion 3 indirectly through the sintered body case 2. Can be heated substantially evenly.

  A trap plate 5 is disposed above the sintered body case 2 so as to cover the opened upper surface. The trap plate 5 is also made of a material that does not react with the evaporated rare earth element R and that adheres to the surface easily peels off, for example, Mo. In this case, the volume of the processing chamber 20 is determined by taking into account the mean free path of the evaporated rare earth element R, and the evaporated rare earth element R repeatedly hits the inner wall of the sintered body case 2 directly or through the upper surface opening. The interval from the upper end of the sintered body case 2 to the trap plate 5 is set so that most of the discharged rare earth element R adheres. In this case, the sintered body case 2 may be provided with, for example, a cooling means by circulating a refrigerant so that the rare earth element R evaporated by the temperature difference adheres to the trap plate 5 and is deposited.

  Next, production of a permanent magnet by the method for producing a sintered body of the present invention will be described. First, the alloy raw material powder is produced as follows. That is, industrial pure iron, metallic neodymium, and low carbon ferroboron are blended and dissolved using a vacuum induction furnace so that Fe, Nd, and B have a predetermined composition ratio, and then quenched by a rapid cooling method such as a strip casting method. First, an alloy raw material of 05 mm to 0.5 mm is prepared. Alternatively, an alloy raw material having a thickness of about 5 to 10 mm may be produced by a centrifugal casting method, and Dy, Tb, Co, Cu, Nb, Zr, Al, Ga, or the like may be added during blending. . In this case, the total content of rare earth elements is set to more than 28.5%, and an ingot that does not produce α iron is obtained.

  Next, the produced alloy raw material is coarsely pulverized by a known hydrogen pulverization step, and then finely pulverized in a nitrogen gas atmosphere by a jet mill fine pulverization step to obtain an alloy raw material powder having an average particle diameter of 3 to 10 μm. Next, the alloy raw material powder is compression molded into a predetermined shape in a magnetic field using a known compression molding machine. Next, the molded body taken out from the compression molding machine is housed in a sintering furnace (not shown), sintered in a vacuum at a predetermined temperature (for example, 1050 ° C.) for a predetermined time (sintering process), and further, a predetermined temperature (500 ° C), a primary sintered body S is obtained.

Next, after the produced primary sintered body S is placed on the placement portion 3 of the vacuum evaporator 1, a vacuum chamber is operated until a predetermined pressure (for example, 10 ⁻⁵ Pa) is reached by operating the vacuum exhaust means. 12 is depressurized. After the inside of the vacuum chamber 12 reaches a predetermined pressure, the heating means 4 is operated to heat the processing chamber 20 and eventually the sintered magnet S, and after reaching the predetermined temperature, this state is maintained for a predetermined time (vacuum evaporation process). ).

In this case, the heating temperature of the processing chamber 20, and consequently the sintered magnet S, is set to 900 ° C. or higher and lower than the sintering temperature. When the temperature is lower than 900 ° C., the evaporation rate of the rare earth element R is slow, and when the sintering temperature is exceeded, abnormal grain growth occurs and the magnetic properties are greatly deteriorated. In addition, an open / close valve 11b whose opening degree can be adjusted is provided in an exhaust passage (exhaust pipe) 11a that connects the vacuum chamber 12 and the vacuum exhaust means 11, and the vacuum chamber 11, As a result, the pressure in the processing chamber 20 is set to a pressure of 10 ⁻³ Pa or less. When the pressure is higher than 10 ⁻³ Pa, the rare earth element R cannot be efficiently evaporated.

Thereby, due to the difference in vapor pressure at a constant temperature (for example, at 1000 ° C., the vapor pressure of Nd is 10 ⁻³ Pa, the vapor pressure of Fe is 10 ⁻⁵ Pa, and the vapor pressure of B is 10 ⁻¹³ Pa). Only the rare earth element R in the R-rich phase evaporates. As a result, the ratio of the Nd-rich phase is reduced, the maximum energy product ((BH) max) and the residual magnetic flux density (Br) showing magnetic characteristics can be improved, and a high-performance permanent magnet can be obtained. In this case, in order to obtain a high-performance permanent magnet, the rare earth element R content of the permanent magnet is less than 28.5 wt%, or the average concentration reduction of the rare earth element R is 0.5 wt% or more. Heat treatment.

  Then, after carrying out the vacuum evaporation process, the operation of the heating means 4 is temporarily stopped, the open / close valve 11b is fully opened, the vacuum chamber 12 is exhausted and cooled, and the temperature in the process chamber 20 is reduced to, for example, 500 ° C. Lower it once. Subsequently, the heating means 4 is actuated again, the temperature in the processing chamber 20 is set in the range of 550 ° C. to 650 ° C., and heat treatment for further improving the magnetic properties is performed. Finally, it is cooled to approximately room temperature, and the permanent magnet that is a sintered body is taken out.

  In the present embodiment, the case where the trap plate 5 is disposed above the sintered body case 2 has been described. However, the present invention is not limited to this. For example, the trap means 6 is provided in the exhaust passage 11a leading to the vacuum exhaust means. May be arranged. As shown in FIGS. 2A and 2B, the trap means 6 has a storage chamber 62 that communicates with the exhaust passage 11a via a gate valve 61. The storage chamber 62 has a substantially rectangular cross section. A support frame 63 is accommodated, and a plurality of trap plates 64 are arranged in the support frame 63. The support frame 63 is connected to a driving means 65 such as an air cylinder that allows the support frame 63 to move forward and backward with respect to the exhaust passage 11a when the gate valve 61 is open.

  Similarly to the above, the trap plate 64 is made of Mo, SUS, or the like that does not react with the rare earth element R, and is alternately arranged in two rows in an inclined manner with respect to the gas flow direction so as to block the gas flow in the exhaust passage 6. . Then, only when the vacuum chamber 12 is depressurized to a predetermined pressure and the heating means 4 is operated to heat the sintered magnet S, the trap means 6 enters the exhaust passage 11a and the evaporated rare earth element R is attached. To collect. In addition, it is preferable to provide the trap means 6 in the range which becomes a viscous flow area | region in the exhaust passage 11b.

  Further, in the present embodiment, the case where the vacuum evaporation treatment is performed on the primary sintered body S after being sintered has been described. However, the present invention is not limited to this. For example, after sintering, After the magnet S is processed into a desired shape by wire cutting or the like, the vacuum evaporation process may be performed.

  Furthermore, in the present embodiment, the manufacture of the sintered magnet S has been described as an example. However, the sintered body of the present invention can be used as long as it can improve the function of the sintered body by evaporating a specific substance from the sintered body. The manufacturing method can be applied. For example, a case of a sintered body obtained by mixing metal carbide powder into silicon carbide (SiC) powder as a sintering aid and performing liquid phase sintering at a temperature equal to or higher than the melting point of silicon after molding.

  In Example 1, a permanent magnet was obtained by subjecting the primary sintered body S to vacuum evaporation using the vacuum evaporator 1 shown in FIG. First, industrial pure iron, metallic neodymium, low carbon ferroboron, electrolytic cobalt, and pure copper are used as raw materials so that the composition is 29Nd-1B-0.1Cu-1Co-BalFe (wt%), and vacuum induction melting is performed. And a flaky ingot having a thickness of about 0.3 mm was obtained by a strip casting method. Next, it was roughly pulverized by a hydrogen pulverization step, and then finely pulverized by, for example, a jet mill pulverization step to obtain an alloy raw material powder.

  Next, a molded body was obtained using a transverse magnetic field compression molding apparatus having a known structure, and then sintered in a vacuum sintering furnace at a temperature of 1050 ° C. for 2 hours to obtain a primary sintered body S. . Then, the primary sintered body was processed into a shape of φ10 × 5 mm by wire cutting, and then finished to have a surface roughness of 10 μm or less, and then the surface was etched with dilute nitric acid.

Next, using the vacuum evaporation apparatus 1 shown in FIG. 1, after placing the 100 primary sintered bodies S on the mounting part 3 in the processing chamber 20, the pressure in the vacuum chamber 12 reaches 10 ⁻⁵ Pa. It was evacuated until. After the pressure in the vacuum chamber 12 reached a predetermined position, the heating unit 4 was activated to heat the processing chamber 20. In this case, the heating temperature was set to 975 ° C., and the opening of the opening / closing valve 11b was adjusted to set the pressure in the vacuum chamber 12 to 5 × 10 ⁻⁵ Pa. Next, heat treatment was performed to improve the coercive force. In this case, the heat treatment temperature was set to 530 ° C., and the treatment time was set to 60 minutes.

  FIG. 3 shows the amount of change in the Nd content (wt%) of the material to be treated when the above-described vacuum evaporation treatment is performed at different treatment times, and the average value of the magnetic properties after heat treatment (measured by a BH curve tracer) It is a table | surface which shows these relationships. According to this, as the evaporation process time becomes longer, the Nd content decreases, and when the vacuum evaporation process for 24 hours is performed, the Nd content can be reduced by about 2 wt%. In this case, the maximum energy product showing the magnetic characteristics is 55.1MG0e, the residual magnetic flux density is 14.88 kG, and it can be seen that the magnetic characteristics can be improved.

  In Example 2, a primary sintered body S was produced under the same conditions as in Example 1. Next, using the vacuum evaporator 1, the evaporation time was fixed at 12 hours, the processing temperature was changed, and the vacuum evaporation process was performed under the same conditions as in Example 1 for other conditions. The heat treatment after the vacuum evaporation treatment was also performed under the same conditions as in Example 1.

  FIG. 4 shows the amount of change in Nd content (wt%) of the material to be treated when the above treatment is performed by changing the evaporation treatment temperature from 875 ° C. to 1050 ° C. in increments of 25 ° C. It is a table | surface which shows the average value of a magnetic characteristic (measured with a BH curve tracer). According to this, Nd could hardly be evaporated at a temperature lower than 900 ° C., whereas Nd could be evaporated at a content exceeding 2 wt% at 1050 ° C. (sintering temperature of the sintered magnet). However, the maximum energy product and the residual magnetic flux density showing the magnetic characteristics were greatly reduced. On the other hand, when the temperature was set in the range of 900 ° C. to 1025 ° C., the Nd content decreased as the evaporation temperature increased, and in the case of 1025 ° C., 1.89 wt% could be reduced. In this case, the maximum energy product showing the magnetic characteristics is 55.1MG0e, the residual magnetic flux density is 14.88 kG, and it can be seen that the magnetic characteristics can be improved.

  In Example 3, a primary sintered body S was produced under the same conditions as in Example 1. Next, using the vacuum evaporator 1, the heating temperature is fixed at 975 ° C. and the evaporation time is fixed at 12 hours, the pressure in the vacuum chamber is changed, and the vacuum evaporation treatment is performed under the same conditions as in Example 1 for other conditions. Carried out. The heat treatment after the vacuum evaporation treatment was also performed under the same conditions as in Example 1.

FIG. 5 shows the amount of change in the Nd content (wt%) of the material to be processed and the magnetic properties after heat treatment (measured by a BH curve tracer) when the pressure is changed in the vacuum chamber and the above treatment is performed. It is a table | surface which shows an average value. According to this, Nd could not be efficiently evaporated at a high pressure of 3 × 10 ⁻² Pa or higher, but about 1.5 wt% could be reduced at a pressure of 10 ⁻³ Pa or lower. In this case, the maximum energy product showing the magnetic characteristics is about 55MG0e, the residual magnetic flux density is about 14.8 kG, and it can be seen that the magnetic characteristics can be improved.

The figure which illustrates schematically the vacuum evaporation apparatus which enforces the manufacturing method of this invention. The figure explaining the trap means which concerns on another modification. The table | surface which shows the variation | change_quantity and magnetic characteristic of content of rare earth elements of the sintered magnet material produced in Example 1. FIG. The table | surface which shows the variation | change_quantity and magnetic characteristic of content of rare earth elements of the sintered magnet material produced in Example 2. FIG. The table | surface which shows the variation | change_quantity and magnetic characteristic of content of rare earth elements of the sintered magnet material produced in Example 3. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Vacuum evaporation apparatus 11 Vacuum exhaust means 12 Vacuum evaporation apparatus 2 Sintered body case 20 Processing chamber 3 Placement part 4 Heating means 5 Trap plate S Primary sintered body (sintered magnet)
R Rare earth elements

Claims (7)

After obtaining a primary sintered body by liquid phase sintering, the primary sintered body is higher than a temperature at which a specific element having a high vapor pressure among liquid phase components can be preferentially evaporated , and a sintering temperature. A method for producing a sintered body, wherein the volume ratio of the liquid phase is reduced or eliminated by heating in a vacuum atmosphere at a lower temperature.   The primary sintered body is obtained by manufacturing a raw material alloy by a strip casting method or a centrifugal casting method, and thereafter performing pulverization, magnetic field forming, and sintering processes. A method for producing a sintered body.   The raw material alloy is for a neodymium iron boron based sintered magnet having a rare earth element including at least one of neodymium, praseodymium and terbium, and the content of the rare earth element is 28.5 wt% or more and 30 wt%. The method for producing a sintered body according to claim 1 or 2, wherein:   The method for producing a sintered body according to claim 3, wherein a heating temperature when heating in the vacuum atmosphere is set to 900 ° C or higher and lower than a sintering temperature. The method for producing a sintered body according to claim 3 or 4, wherein the pressure of the vacuum atmosphere is set in a range of ^10-3 to ^10-5Pa .   The method for manufacturing a sintered body according to any one of claims 3 to 5, wherein a mechanism for trapping the evaporated rare earth element is provided and recovered.   A neodymium iron boron-based sintered magnet manufactured by the method according to any one of claims 1 to 6.

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