RTB series permanent magnets

Abstract

This invention provides an RTB-type permanent magnet that has high residual magnetic flux density Br, coercivity HcJ, and aspect ratio Hk / HcJ at room temperature, and also high coercivity HcJ at high temperatures. [Solution] The RTB-type permanent magnet 1 has a total content of light rare earth elements of 28.0% to 31.5% by mass, a total content of heavy rare earth elements greater than 0% by mass and 1.0% by mass or less, a B content of 0.97% to 1.05% by mass, an Al content of 0.05% to 0.52% by mass, a Zr content greater than 0.50% by mass and 0.75% by mass or less, a Ga content of 0% to 0.20% by mass or less, and an O content of 0 ppm to 1000 ppm by mass or less.

Description

Technical Field

[0001] This disclosure relates to an R-T-B-based permanent magnet.

Background Art

[0002] Patent Document 1 describes an invention related to an R-T-B-based permanent magnet in which the residual magnetic flux density Br at room temperature and the coercive force HcJ at high temperature are both increased by grain boundary diffusion of heavy rare earth elements.

[0003] Patent Document 2 describes an invention related to a Nd-Fe-B-based rare earth permanent magnet in which abnormal grain growth is suppressed, the optimum sintering temperature range is expanded, and good magnetic properties are realized even at high carbon and low oxygen concentrations due to the alloy structure having a specific structure.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] An object of this disclosure is to obtain an R-T-B-based permanent magnet having a high residual magnetic flux density Br, a high coercive force HcJ, and a high squareness ratio Hk / HcJ at room temperature, and also having a high coercive force HcJ at high temperature.

Means for Solving the Problems

[0006] The R-T-B-based permanent magnet according to this disclosure is such that the total content of light rare earth elements is 28.0 mass% or more and 31.​​​​ Al content is 0.05% by mass or more and 0.52% by mass or less. The Zr content is greater than 0.50% by mass and less than or equal to 0.75% by mass. The Ga content is 0% by mass or more and 0.20% by mass or less, The O content is between 0 ppm by mass and 1000 ppm by mass.

[0007] The Zr content may be 0.53% by mass or more and 0.75% by mass or less.

[0008] The total content of the heavy rare earth elements may be greater than 0% by mass and less than or equal to 0.20% by mass.

[0009] The Co content may be 0.50% by mass or more and 0.80% by mass or less.

[0010] The magnet may have a concentration gradient of the heavy rare earth element that decreases from the surface towards the interior.

[0011] The RTB-type permanent magnet may have grain boundaries formed by a main phase particle and two or more adjacent main phase particles. The grain boundaries may have a Zr-C phase, and the area ratio of the Zr-C phase in the cross-section of the RTB permanent magnet may be 0.50% or more and 2.60% or less. [Brief explanation of the drawing]

[0012] [Figure 1] This is a schematic diagram of an RTB-type permanent magnet. [Figure 2] This is a SEM image of a cross-section of an RTB-type permanent magnet. [Modes for carrying out the invention]

[0013] The present disclosure will be described below based on embodiments.

[0014] RTB permanent magnets are R2T 14It has main phase particles containing crystal particles having a B-type crystal structure. Further, it has grain boundaries formed by two or more adjacent main phase particles.

[0015] R-T-B-based permanent magnet and R2T 14 In the B-type crystal structure, R represents a rare earth element, T represents a transition metal element, and B represents boron.

[0016] R-T-B-based permanent magnet and R2T 14 The rare earth element contained as R in the B-type crystal structure may be Sc, Y, and lanthanoids. The transition metal element contained as T does not contain a rare earth element. The transition metal element contained as T may be an iron group element. The iron group element contained as T may be Fe alone, or a part of Fe contained as T may be substituted with Co. A part of the boron contained as B may be substituted with carbon.

[0017] In this embodiment, rare earth elements are classified into heavy rare earth elements and light rare earth elements. Heavy rare earth elements refer to Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Light rare earth elements refer to rare earth elements other than heavy rare earth elements. Iron group elements refer to Fe, Co, and Ni.

[0018] The R-T-B-based permanent magnet according to this embodiment contains at least aluminum (Al) and zirconium (Zr) in addition to rare earth elements, iron group elements, and boron. Further, it may contain gallium (Ga), copper (Cu), carbon (C), nitrogen (N), and oxygen (O). And it has the following composition.

[0019] There is no particular limitation on the total content of rare earth elements (hereinafter sometimes referred to as TRE) in the R-T-B-based permanent magnet according to this embodiment. The R-T-B-based permanent magnet may contain only one or more selected from substantially Nd, Pr, Dy, and Tb as rare earth elements, or may contain only one or more selected from substantially Nd, Pr, and Tb. Note that when the R-T-B-based permanent magnet contains only one or more selected from substantially Nd, Pr, Dy, and Tb as rare earth elements, it means that the total content of rare earth elements other than Nd, Pr, Dy, and Tb is 0.01% by mass or less. When the R-T-B-based permanent magnet contains only one or more selected from substantially Nd, Pr, and Tb as rare earth elements, it means that the total content of rare earth elements other than Nd, Pr, and Tb is 0.01% by mass or less.

[0020] Taking the R-T-B-based permanent magnet as 100% by mass, the total content of light rare earth elements (hereinafter sometimes referred to as TRL) is 28.0% by mass or more and 31.5% by mass or less. When TRL is too large, Br at room temperature tends to decrease. When TRL is too small, it is difficult to produce the R-T-B-based permanent magnet by sintering.

[0021] There is no particular limitation on the types of light rare earth elements contained in the R-T-B-based permanent magnet. For example, it may be Nd and / or Pr. Nd / Pr may be 1.9 or more in atomic ratio. Also, Nd / Pr may be 1000 or less in atomic ratio.

[0022] Taking the R-T-B-based permanent magnet as 100% by mass, the total content of heavy rare earth elements (hereinafter sometimes referred to as TRH) is more than 0% by mass and 1.0% by mass or less. It may be more than 0% by mass and 0.20% by mass or less. The less TRH is, the more likely HcJ at room temperature and HcJ at high temperature are to decrease. The more TRH is, the more likely Br at room temperature is to decrease. Also, the more TRH is, the more likely the raw material cost is to increase.

[0023] There are no particular restrictions on the Co content in RTB-type permanent magnets. Taking the RTB-type permanent magnet as 100% by mass, the Co content may be 0% to 2.00% by mass, 0.25% to 1.25% by mass, or 0.50% to 0.80% by mass.

[0024] Low CO content tends to reduce corrosion resistance. Higher CO content tends to decrease HcJ at room temperature and high temperatures.

[0025] Co is relatively expensive. If the Co content is too high, the HcJ level decreases, the effect of improving corrosion resistance plateaus, and the cost becomes high.

[0026] The RTB-based permanent magnet according to this embodiment exhibits high corrosion resistance even with a relatively low content of expensive Co. Therefore, the RTB-based permanent magnet according to this embodiment tends to have high corrosion resistance at a low cost.

[0027] Ni may be substantially omitted. Specifically, the Ni content may be 0% by mass or more and less than 0.01% by mass.

[0028] Assuming an RTB-type permanent magnet is 100% by mass, the Zr content is greater than 0.50% by mass and less than or equal to 0.75% by mass. It may be between 0.51% by mass and 0.75% by mass, or between 0.53% by mass and 0.75% by mass. The lower the Zr content, the more easily both HcJ at room temperature and HcJ at high temperatures decrease. The higher the Zr content, the more easily Br at room temperature and Hk / HcJ at room temperature decrease.

[0029] Assuming an RTB-type permanent magnet is 100% by mass, the Al content is between 0.05% by mass and 0.52% by mass. It may also be between 0.13% by mass and 0.45% by mass. The lower the Al content, the more easily the HcJ at room temperature and at high temperatures decreases. The higher the Al content, the more easily the Br at room temperature decreases.

[0030] Assuming an RTB-type permanent magnet is 100% by mass, the Ga content is between 0% and 0.20% by mass. It may also be between 0.03% and 0.14% by mass. The lower the Ga content, the more easily both HcJ at room temperature and HcJ at high temperatures decrease. The higher the Ga content, the more easily Br at room temperature and Hk / HcJ at room temperature decrease.

[0031] There are no particular restrictions on the Cu content. Taking RTB-type permanent magnets as 100% by mass, the Cu content may be between 0% and 0.50% by mass, or between 0% and 0.35% by mass. Having a Cu content within the above range tends to improve Br and HcJ at high temperatures.

[0032] With RTB-type permanent magnets as 100% by mass, the B content is between 0.97% by mass and 1.05% by mass. Both too much and too little B content tend to decrease HcJ at room temperature and Hk / HcJ at room temperature.

[0033] With RTB-type permanent magnets as 100% by mass, the carbon content may be 1100 ppm by mass or less. The nitrogen content may be 700 ppm by mass or less.

[0034] The oxygen content is 1000 ppm by mass or less, with RTB-type permanent magnets being considered as 100% by mass. If the oxygen content is too high, the HcJ at room temperature tends to decrease.

[0035] To define an RTB-type permanent magnet as 100% by mass means that the total content of all elements is 100% by mass. Furthermore, the Fe content in an RTB-type permanent magnet may be the substantial remainder of the RTB-type permanent magnet. Specifically, the content of elements other than those mentioned above, i.e., elements other than rare earth elements, Fe, Co, Ni, B, Al, Ga, Zr, Cu, C, N, and O, may be 0.20% by mass or less each, and the total may be 1.00% by mass or less.

[0036] There are no particular restrictions on the shape of RTB permanent magnets. For example, rectangular prisms are one possible shape.

[0037] RTB-type permanent magnets may have a concentration gradient in which the concentration of heavy rare earth element RH decreases from the outside to the inside of the RTB-type permanent magnet 1. There are no particular restrictions on the type of RH having the above concentration gradient. For example, it may be Dy and / or Tb, or it may be Tb.

[0038] Specifically, as shown in Figure 1, the rectangular RTB-type permanent magnet 1 has a surface and a center, and the RH content in the surface can be 2% or more higher, 5% or more higher, or 10% or more higher than the RH content in the center. The surface refers to the surface of the RTB-type permanent magnet 1. For example, POINT C and C' in Figure 1 (the centroids of the opposing surfaces in Figure 1) are the surface. The center refers to the center of the RTB-type permanent magnet 1. For example, it refers to the part that is half the thickness of the RTB-type permanent magnet 1. For example, POINT M in Figure 1 (the midpoint of POINT C and POINT C') is the center. Note that POINT C and C' in Figure 1 may be the centroid of the surface with the largest area among the surfaces of the RTB-type permanent magnet 1, and the centroid of the surface opposite to that surface.

[0039] There are no particular restrictions on the method for forming the aforementioned RH concentration gradient in an RTB-type permanent magnet. For example, an RH concentration gradient can be formed within an RTB-type permanent magnet by grain boundary diffusion of RH, as described later.

[0040] In the RTB-type permanent magnet according to this embodiment, the grain boundaries may have a Zr-C phase. Furthermore, the grain boundaries may have a Zr-B phase, or a phase containing a large amount of R and C.

[0041] Figure 2 shows an example of a SEM image (backscattered electron image obtained by SEM) of a cross-section of an RTB-type permanent magnet according to this embodiment. The measurement magnification in Figure 2 is 10,000x. Figure 2 is an SEM image of a cross-section of sample number 5, which will be described later in the example.

[0042] The RTB-type permanent magnet 1 has a main phase particle 11 and grain boundaries 13 formed by two or more adjacent main phase particles 11. Within the grain boundaries 13, the triple junctions formed by three or more main phase particles 11 contain a Zr-C phase 15 with a roughly square shape and a Zr-B phase 17 with an elongated shape.

[0043] The area ratio of the Zr-C phase in the cross-section of the RTB permanent magnet may be 0.5% or more and 2.6% or less, or 1.2% or more and 1.8% or less.

[0044] The presence of RH in the main phase particles increases the crystalline magnetic anisotropy of the main phase particles. As a result, the HcJ of the RTB-type permanent magnet improves.

[0045] RH readily combines with C. Therefore, when the grain boundaries of an RTB-type permanent magnet have a phase rich in R and C, RH is easily incorporated into the R and C-rich phase and less easily incorporated into the main phase particles. When the grain boundaries of an RTB-type permanent magnet have a phase rich in R and C, the amount of RH contained in the main phase particles decreases, and the HcJ of the RTB-type permanent magnet decreases.

[0046] There are no particular restrictions on the type of phase that contains a large amount of R and C. Examples include the RC phase, ROC phase, and ROCN phase. Furthermore, the proportion of C in the phase containing a large amount of R and C may be greater than the proportion of C in the main phase particles. The total proportion of R in the phase containing a large amount of R and C may also be greater than the total proportion of R in the main phase particles.

[0047] The grain boundaries of RTB-type permanent magnets may contain a Zr-C phase. When the grain boundaries contain a Zr-C phase, the relative proportion of phases containing a lot of R and C in the grain boundaries decreases. As a result, the main phase particles tend to have more RH. Therefore, RTB-type permanent magnets tend to have a high HcJ.

[0048] If the area ratio of the Zr-C phase in the cross-section of an RTB-type permanent magnet is too large, the volume ratio of the main phase particles in the RTB-type permanent magnet tends to decrease. When the volume ratio of the main phase particles decreases, the Br of the RTB-type permanent magnet tends to decrease.

[0049] When the Zr content in RTB-type permanent magnets is low, the area ratio of the Zr-C phase in the cross-section of the RTB-type permanent magnet tends to be low. When the Zr content in RTB-type permanent magnets is high, the area ratio of the Zr-C phase in the cross-section of the RTB-type permanent magnet tends to be high.

[0050] When an RTB-type permanent magnet contains Zr, the grain boundaries of the RTB-type permanent magnet may have a Zr-B phase in addition to the Zr-C phase mentioned above.

[0051] In particular, when the B content in RTB-type permanent magnets is low, if the grain boundaries of the RTB-type permanent magnet have a Zr-B phase, the volume ratio of the main phase particles tends to decrease. This is because there is a shortage of B used to form the main phase particles. As a result, the HcJ of the RTB-type permanent magnet tends to decrease.

[0052] In particular, when the boron content in RTB-type permanent magnets is high, if the grain boundaries of the RTB-type permanent magnet have a Zr-B phase, the area ratio of the Zr-C phase in the cross-section of the RTB-type permanent magnet tends to decrease. This is because there is a tendency for Zr to be insufficient to form the Zr-C phase.

[0053] Here, it is thought that the ease of Zr-C phase formation is related to the melting point of the grain boundaries. Therefore, in RTB-type permanent magnets, the formation of the Zr-C phase can be controlled by adjusting the content of elements mainly contained in the grain boundaries, such as Al, Co, Cu, and Ga.

[0054] When the content of O in the R-T-B series permanent magnet is high, the content ratio of the R-O-C phase tends to increase. In this case, the RH contained in the R-O-C phase also tends to increase, and the HcJ of the R-T-B series permanent magnet tends to decrease.

[0055] The area ratio of the Zr-C phase can be obtained, for example, by the following method.

[0056] Element mapping of Zr and C is performed on the cross-section of the R-T-B series permanent magnet. There are no particular restrictions on the method and apparatus for element mapping, and any apparatus that can appropriately perform element mapping may be used. For example, EPMA and EDS can be mentioned. Also, the magnification may be any magnification at which the Zr-C phase can be appropriately measured. For example, it may be 1500 times or more and 10000 times or less.

[0057] The single-element mapping image of Zr and the single-element mapping image of C are overlaid. The range where Zr is 30 at% or more and the range where C is 30 at% or more overlap among the grain boundaries is defined as the Zr-C phase. The area of the Zr-C phase with respect to the area of the R-T-B series permanent magnet is defined as the area ratio. Note that the area of the voids covered by the main phase particles and / or grain boundaries is also included in the area of the R-T-B series permanent magnet.

[0058] <Manufacturing method of R-T-B series permanent magnet> Hereinafter, an example of a method for manufacturing the R-T-B series permanent magnet according to the present embodiment will be described. The R-T-B series permanent magnet may be an R-T-B series sintered magnet obtained through a sintering process. The method for manufacturing the R-T-B series permanent magnet according to the present embodiment may include the following steps.

[0059] (a) Alloy preparation step of preparing a main phase alloy and a grain boundary phase alloy (b) Crushing step of crushing the main phase alloy and the grain boundary phase alloy (c) Mixing step of mixing the main phase alloy and the grain boundary phase alloy (d) Forming step of forming the obtained alloy powder (e) Sintering step of sintering the formed body to obtain an R-T-B series permanent magnet (f) Aging process for RTB permanent magnets (g) Cooling process for cooling RTB permanent magnets (h) Processing steps for RTB-type permanent magnets (i) Surface treatment process for RTB permanent magnets

[0060] Depending on the type of RTB-type permanent magnet obtained in the end, some of the above steps may be omitted as appropriate.

[0061] [Alloy preparation process] First, the main phase alloy and the grain boundary phase alloy are prepared (alloy preparation step). RTB-type permanent magnets having the above configuration may also be manufactured by a two-alloy method using the main phase alloy and the grain boundary phase alloy. There is no particular difference in the manufacturing method between the main phase alloy and the grain boundary phase alloy. Below, the strip casting method will be described as an example of an alloy preparation method for the main phase alloy, but the alloy preparation method is not limited to the strip casting method. Regarding the alloy preparation method for the grain boundary phase alloy, the main phase alloy should be read as the grain boundary phase alloy in the following description.

[0062] First, raw material metals corresponding to the composition of the main phase alloy are prepared, and these raw material metals are melted in a vacuum or inert gas atmosphere such as Ar gas. Then, the main phase alloy is produced by casting the molten raw material metals.

[0063] There are no particular restrictions on the type of raw material metal. For example, rare earth metals or rare earth alloys, pure iron, pure cobalt, ferroboron, and even alloys and compounds thereof can be used. There are no particular restrictions on the casting method for casting the raw material metal. Examples include ingot casting, strip casting, book molding, and centrifugal casting. If solidification segregation occurs in the resulting main phase alloy, homogenization treatment (solution treatment) may be performed as needed.

[0064] The alloy preparation method may be carried out by atomization. In this case, the grinding step described later may be omitted.

[0065] [Grinding process] The following describes the grinding method for the main phase alloy. For the grinding method of the grain boundary phase alloy, replace "main phase alloy" with "grain boundary phase alloy" in the following description.

[0066] After preparing the main phase alloy, the main phase alloy is crushed (crushing process). The crushing process may be carried out in two stages: a coarse crushing process in which the particle size is crushed to a degree of several hundred micrometers to several millimeters, and a fine crushing process in which the particle size is crushed to a degree of several micrometers. Alternatively, it may be carried out in a single stage consisting only of the fine crushing process.

[0067] (Coarse grinding process) The main phase alloy is coarsely ground until the particle size is several hundred μm to several mm (coarse grinding step). This yields coarsely ground powder of the main phase alloy. Coarse grinding may be carried out, for example, by hydrogen storage grinding. Hydrogen storage grinding can be performed by causing self-destructive grinding by allowing hydrogen to be absorbed into the main phase alloy and then releasing hydrogen based on the difference in hydrogen storage capacity between different phases. Releasing hydrogen based on the difference in hydrogen storage capacity between different phases is called dehydrogenation. There are no particular restrictions on the conditions for dehydrogenation, but for example, dehydrogenation is carried out at 300 to 650°C in an argon flow or in a vacuum.

[0068] The coarse grinding method is not limited to the hydrogen storage grinding described above. For example, coarse grinding may be performed in an inert gas atmosphere using a coarse grinding machine such as a stamp mill, jaw crusher, or brown mill.

[0069] Furthermore, in order to obtain RTB-type permanent magnets with high magnetic properties, the atmosphere in each step from the coarse grinding step to the sintering step described later may be a low-oxygen atmosphere. The oxygen concentration is adjusted by controlling the atmosphere in each manufacturing step. If the oxygen concentration in each manufacturing step is high, the rare earth elements in the alloy powder obtained by grinding the main phase alloy will oxidize, and oxides of the rare earth elements will be generated. The oxides of the rare earth elements are not reduced during sintering and precipitate as rare earth element oxides at the grain boundaries. A grain boundary is the part that exists between two or more main phase particles. As a result, the Br of the obtained RTB-type permanent magnet will decrease. For this reason, for example, each step (fine grinding step, molding step) may be carried out in an atmosphere with an oxygen concentration of 100 ppm or less.

[0070] (Fine grinding process) After coarsely grinding the main phase alloy, the resulting coarsely ground powder of the main phase alloy is finely ground until the average particle size is about a few μm (fine grinding step). This yields finely ground powder of the main phase alloy. Finely ground powder can be obtained by further fine grinding the coarsely ground powder. There are no particular restrictions on the D50 of the particles contained in the finely ground powder. For example, D50 may be between 2.0 μm and 4.5 μm, or between 2.5 μm and 3.5 μm. The smaller D50, the easier it is to improve the HcJ of the RTB-type permanent magnet according to this embodiment. However, abnormal grain growth is more likely to occur in the sintering process, and the upper limit of the sintering temperature range becomes lower. The larger D50, the less likely it is to occur abnormal grain growth in the sintering process, and the upper limit of the sintering temperature range becomes higher. However, the easier it is to decrease the HcJ of the RTB-type permanent magnet according to this embodiment.

[0071] Fine grinding is carried out by further grinding the coarsely ground powder using a fine grinding machine such as a jet mill, ball mill, vibratory mill, or wet attritor, while appropriately adjusting conditions such as grinding time. The jet mill will be described below. A jet mill is a fine grinding machine that generates a high-speed gas flow by releasing a high-pressure inert gas (for example, He gas, N2 gas, or Ar gas) from a narrow nozzle, and uses this high-speed gas flow to accelerate the coarsely ground powder of the main phase alloy, causing collisions between the coarsely ground powder particles of the main phase alloy and with a target or container wall to grind them.

[0072] When finely grinding the coarsely ground powder of the main phase alloy, a grinding aid may be added. There are no particular restrictions on the type of grinding aid. For example, organic lubricants or solid lubricants may be used. Examples of organic lubricants include oleamide, laurate amide, and zinc stearate. Examples of solid lubricants include graphite. By adding a grinding aid, it is possible to obtain a finely ground powder that is easily oriented when a magnetic field is applied during the molding process.

[0073] [Mixing process] Next, the main phase alloy and the grain boundary phase alloy are mixed to obtain alloy powder for molding (mixing step). There are no particular restrictions on the mixing method.

[0074] [Molding process] The molding alloy powder is molded into the desired shape (molding process). In the molding process, the molding alloy powder is filled into a mold placed in an electromagnet and pressurized to mold the powder and obtain a molded body. At this time, by molding while applying a magnetic field, the crystal axes of the molding alloy powder can be oriented in a specific direction. Molding aids may also be added. There are no particular restrictions on the type of molding aid. The same lubricant as the grinding aid may be used. Also, the grinding aid may also serve as a molding aid.

[0075] The pressure applied during pressurization may be, for example, 30 MPa to 300 MPa. The applied magnetic field may be, for example, 1000 kA / m to 1600 kA / m. The applied magnetic field is not limited to a static magnetic field and can also be a pulsed magnetic field. Furthermore, a static magnetic field and a pulsed magnetic field can be used in combination.

[0076] In addition to dry molding, which involves molding the alloy powder directly as described above, wet molding can also be applied, which involves molding a slurry in which the alloy powder is dispersed in a solvent such as oil.

[0077] The shape of the molded body obtained by molding the alloy powder for molding is not particularly limited and can be a rectangular parallelepiped or any other shape that suits the desired RTB-type permanent magnet.

[0078] [Sintering process] The molded body, formed in a magnetic field to the desired shape, is sintered in a vacuum or inert gas atmosphere to obtain an RTB-type permanent magnet (sintering process). The holding temperature (sintering temperature) and holding time (sintering time) during sintering need to be adjusted according to various conditions such as composition, grinding method, particle size and particle size distribution. There are no particular restrictions on the sintering temperature, but it may be between 1040°C and 1100°C. There are no particular restrictions on the sintering time, but it may be between 1 hour and 10 hours, between 2 hours and 8 hours, or between 3 hours and 6 hours. The shorter the sintering time, the higher the production efficiency. However, the magnetic properties, especially Hk / HcJ, tend to decrease. The longer the sintering time, the easier it is to improve the magnetic properties. However, the production efficiency decreases.

[0079] There are no particular restrictions on the atmosphere during sintering. For example, an inert gas atmosphere, a vacuum atmosphere of less than 100 Pa, or a vacuum atmosphere of less than 10 Pa may be used. There are no particular restrictions on the heating rate to the sintering temperature, nor on the cooling rate after the molded body has been sintered to obtain the sintered body.

[0080] [Statute of Limitations Treatment Process] After sintering the molded body, the RTB-type permanent magnet is subjected to aging treatment (aging treatment step). After sintering, the obtained RTB-type permanent magnet may be subjected to aging treatment by holding it at a lower temperature than that used during sintering. The following description will explain the case where the aging treatment is divided into two stages, a first aging treatment and a second aging treatment, but it is also possible to perform only one aging treatment or to perform three or more stages of aging treatment.

[0081] There are no particular restrictions on the holding temperature and holding time in each aging treatment. For example, the first aging treatment may be performed at a holding temperature of 800°C to 900°C for 30 minutes to 4 hours. The heating rate to the holding temperature may be 5°C / min to 50°C / min. The atmosphere during the first aging treatment may be an inert gas atmosphere (e.g., He gas, Ar gas) at a pressure of atmospheric pressure or higher. The second aging treatment may be performed under the same conditions as the first aging treatment, except that the holding temperature may be 450°C to 550°C. The magnetic properties of RTB-type permanent magnets can be improved by aging treatment. Furthermore, the aging treatment process may be performed after the processing process described later.

[0082] [Cooling process] After the RTB-type permanent magnet undergoes aging treatment (first or second aging treatment), the RTB-type permanent magnet is rapidly cooled in an inert gas atmosphere (cooling process). There are no particular restrictions on the cooling rate.

[0083] [Processing process] The resulting RTB-type permanent magnets may be processed into the desired shape as needed (processing step). Processing methods include, for example, shaping such as cutting and grinding, and chamfering such as barrel polishing.

[0084] [Grain boundary diffusion process] Further diffusion of heavy rare earth element RH into the grain boundaries of the processed RTB-based permanent magnet (grain boundary diffusion step). There are no particular restrictions on the method of grain boundary diffusion. For example, it may be carried out by applying a compound containing RH to the surface of the RTB-based permanent magnet by coating or vapor deposition, followed by heat treatment. Alternatively, it may be carried out by heat treatment of the RTB-based permanent magnet in an atmosphere containing RH vapor. Grain boundary diffusion can further improve the HcJ of the RTB-based permanent magnet. Furthermore, the aforementioned RH concentration gradient may be formed in the RTB-based permanent magnet by grain boundary diffusion.

[0085] [Surface treatment process] The RTB-type permanent magnets obtained through the above process may be subjected to surface treatments such as plating, resin coating, oxidation treatment, or chemical conversion treatment (surface treatment process). This can further improve corrosion resistance.

[0086] (Composition of the main phase alloy) There are no particular restrictions on the composition of the main phase alloy. For example, if the main phase alloy is 100% by mass, The total content of light rare earth elements is between 28.00% by mass and 32.00% by mass. The total content of heavy rare earth elements is 0% by mass or more and 1.00% by mass or less. The content of B is 0.97% by mass or more and 1.15% by mass or less. Al content is 0.04% by mass or more and 0.57% by mass or less. Ga content is 0% by mass or more and 0.25% by mass or less. Cu content is 0% by mass or more and 0.50% by mass or less. Co content is 0% by mass or more and 3.00% by mass or less. Zr content is 0.25% by mass or more and 0.74% by mass or less. That's fine.

[0087] Setting the main phase alloy to 100% by mass means that the total content of all elements is 100% by mass. Furthermore, the Fe content in the main phase alloy may be the substantial remainder of the main phase alloy. Specifically, the content of elements other than rare earth elements, Fe, B, Al, Ga, Cu, Co, and Zr may be 0.20% by mass or less each, and the total content may be 1.00% by mass or less.

[0088] (Composition of grain boundary alloy) There are no particular restrictions on the composition of the grain boundary phase alloy. For example, if the grain boundary phase alloy is 100% by mass, The content of light rare earth elements is 28.00% by mass or more and 32.00% by mass or less. The total content of heavy rare earth elements is 0% by mass or more and 1.00% by mass or less. Al content is 0% by mass or more and 0.90% by mass or less. Ga content is 0% by mass or more and 9.50% by mass or less. Cu content is 0% by mass or more and 6.00% by mass or less. Co content is 2.00% by mass or more and 9.50% by mass or less. Zr content is 0.90% by mass or more and 15.00% by mass or less. That's fine.

[0089] To define the grain boundary phase alloy as 100% by mass means that the total content of all elements is 100% by mass. Furthermore, the Fe content in the grain boundary phase alloy may be the substantial remainder of the grain boundary phase alloy. Specifically, the content of elements other than rare earth elements, Fe, Al, Ga, Cu, Co, and Zr may each be 0.20% by mass or less, and the total content may be 1.00% by mass or less.

[0090] (Comparison of main phase alloy and grain boundary phase alloy) When comparing a main phase alloy with a grain boundary phase alloy, the main phase alloy may have a higher content of B, Al, Ga, Cu, Co, and Zr.

[0091] (Mixing ratio of main phase alloy to grain boundary phase alloy) There are no particular restrictions on the mixing ratio of the main phase alloy and the grain boundary phase alloy, but a mass ratio of 88:12 to 97:3 is acceptable. When the composition of the final RTB-type permanent magnet is similar, if there is too little main phase alloy, the Hk / HcJ of the final RTB-type permanent magnet tends to decrease. When the composition of the final RTB-type permanent magnet is similar, if there is too much main phase alloy, the magnetic properties, especially HcJ at room temperature and Hk / HcJ at room temperature, tend to decrease, and HcJ at high temperatures also tends to decrease.

[0092] The RTB-based permanent magnets obtained in this manner possess excellent magnetic properties. In other words, RTB-based permanent magnets with high magnetic properties can be obtained using relatively small amounts of heavy rare earth elements.

[0093] This disclosure is not limited to the embodiments described above, and can be modified in various ways within the scope of this disclosure. [Examples]

[0094] The invention will be described in more detail below with reference to examples, but this disclosure is not limited to these examples.

[0095] (Alloy preparation process) In the alloy preparation process, main phase alloys having the compositions shown in Tables 1, 5, 9, 13, and 17 (hereinafter sometimes referred to as Table 1, etc.) and grain boundary phase alloys having the compositions shown in Tables 2, 6, 10, 14, and 18 (hereinafter sometimes referred to as Table 2, etc.) were prepared. TRL represents the total content of light rare earth elements. The content of elements other than Fe that are not listed in each table is all less than 0.01 mass%. In other words, in the alloys shown in each table, Fe is the substantial remainder.

[0096] First, raw material metals containing the specified elements were prepared. As raw material metals, for example, elemental elements listed in each table, alloys containing the elements listed in each table, and / or compounds containing the elements listed in each table were appropriately selected and prepared.

[0097] Next, these raw material metals were weighed, and the main phase alloy and grain boundary phase alloy were prepared by the strip casting method.

[0098] (Grinding process) In the grinding process, each alloy obtained in the preparation process was ground to obtain alloy powder. Grinding was performed in two stages: coarse grinding and fine grinding. Coarse grinding was performed by hydrogen storage grinding. After hydrogen was absorbed into each alloy, dehydrogenation was performed in an argon flow or vacuum at 300-600°C. Coarse grinding yielded alloy powder with particle sizes ranging from several hundred micrometers to several millimeters.

[0099] Fine grinding was performed by adding oleic acid amide as a grinding aid to the alloy powder obtained by coarse grinding, mixing, and then using a jet mill. The amount of grinding aid added was such that the carbon, nitrogen, and oxygen content in the final magnet was as shown in Tables 4, 8, 12, 16, and 20 (hereinafter sometimes referred to as Table 4, etc.). Nitrogen gas was used in the jet mill. Fine grinding was performed until the D50 of the alloy powder was approximately 3.0 μm.

[0100] (Mixing process) Next, the main phase alloy powder obtained by crushing the main phase alloys shown in each table such as Table 1, and the grain boundary phase alloy powder obtained by crushing the grain boundary phase alloys shown in each table such as Table 2, were mixed in the mixing ratios shown in Tables 3, 7, 11, 15, and 19 (hereinafter sometimes referred to as each table such as Table 3) to obtain alloy powder for molding. In each table such as Table 3, the mixing ratio is shown in the form of main phase alloy powder:grain boundary phase alloy powder.

[0101] (molding process) In the molding process, the molding alloy powder obtained from the crushing and mixing processes was molded in a magnetic field to obtain a molded body. After filling a mold with the alloy powder placed in an electromagnet, the mold was formed by applying pressure while applying a magnetic field using the electromagnet. The magnitude of the applied magnetic field was 1200 kA / m. The molding pressure was 40 MPa.

[0102] (Sintering process) In the sintering process, the obtained molded body was sintered to produce a sintered body. The holding temperature during sintering (sintering temperature) was set to 1080°C. The holding time during sintering (sintering time) was set to 4 hours. The heating rate when raising the temperature to the holding temperature was 8.0°C / min, and the cooling rate when cooling from the holding temperature to room temperature was 50°C / min. The atmosphere during sintering was either a vacuum atmosphere or an inert gas atmosphere.

[0103] (Aging process) In the aging process, the obtained sintered body was subjected to aging treatment. The aging treatment was carried out in two stages: a first aging treatment and a second aging treatment.

[0104] In the first aging treatment, the heating rate to reach the holding temperature was 8.0°C / min, the holding temperature was 900°C, the holding time was 1.0 hour, and the cooling rate to cool from the holding temperature to room temperature was 50°C / min. The atmosphere during the first aging treatment was an Ar atmosphere.

[0105] In the second aging treatment, the heating rate to reach the holding temperature was 8.0°C / min, the holding temperature was 500°C, the holding time was 1.5 hours, and the cooling rate to cool from the holding temperature to room temperature was 50°C / min. The atmosphere during the second aging treatment was an Ar atmosphere.

[0106] (Grain boundary diffusion process) In the grain boundary diffusion process, Tb was diffused as a heavy rare earth element into the sintered body after aging treatment.

[0107] First, a diffusion paste was prepared. Hydrogen gas was flowed over 99.9% pure metal Tb to absorb hydrogen. Next, the atmosphere was switched to Ar gas, and dehydrogenation treatment was performed at 600°C for 1 hour to pulverize the metal Tb by hydrogen absorption. Then, as a pulverizing aid, 0.05% by mass of zinc stearate was added per 100% by mass of metal Tb, and mixed using a Nauta mixer. After that, fine pulverization was performed using a jet mill in an atmosphere containing 3000 ppm of oxygen to obtain a finely pulverized powder of Tb hydride with an average particle size of approximately 10.0 μm.

[0108] A diffusion paste was prepared by kneading 75.0 parts by mass of finely ground Tb hydride powder, 23.0 parts by mass of alcohol, and 2.0 parts by mass of acrylic resin. The alcohol acts as a solvent, and the acrylic resin acts as a binder.

[0109] The sintered body after aging treatment was immersed for 3 minutes in a mixed solution of nitric acid and ethanol (100 parts by mass of ethanol and 3 parts by mass of nitric acid), followed by an etching treatment in ethanol for 1 minute. This etching treatment, consisting of immersion in the mixed solution for 3 minutes followed by immersion in ethanol for 1 minute, was performed twice.

[0110] Next, the diffusion paste described above was applied to the entire surface of the sintered body after etching. The amount of diffusion paste applied was adjusted so that the final composition of the RTB-type permanent magnet would match the compositions shown in Table 3 and Table 4.

[0111] Next, the sintered body coated with the diffusion paste was left in an oven at 160°C to remove the solvent from the diffusion paste. Then, it was heated at 930°C for 18 hours while flowing Ar at atmospheric pressure (1 atm). After that, it was heated at 520-560°C for 4 hours while flowing Ar at atmospheric pressure. From the above, RTB-type permanent magnets having the compositions shown in Table 3 and Table 4 were obtained.

[0112] After removing 0.1 mm from each surface of an RTB-type permanent magnet, its composition, sinterability, and magnetic properties were evaluated.

[0113] The composition of the RTB-based permanent magnets finally obtained in each example and comparative example was confirmed to be as shown in the tables in Table 3 and Table 4 by compositional analysis using X-ray fluorescence analysis, inductively coupled plasma mass spectrometry (ICP), and gas analysis. In particular, the C content was measured by combustion in an oxygen stream-infrared absorption method. The B content was measured by ICP. The O content was measured by inert gas fusion-infrared absorption method, and the N content was measured by inert gas fusion-thermal conductivity method.

[0114] Tables such as Table 3 show the content percentages of elements other than O, C, and N. Tables such as Table 4 show the content of O, C, and N. In Tables such as Table 3, TRL means the total content of light rare earth elements. TRH means the total content of heavy rare earth elements. Since no heavy rare earth elements other than Tb were used in any of the examples and comparative examples, TRH is equal to the content of Tb in all examples and comparative examples. TRE means the content of rare earth elements. The content of elements other than Fe, C, N, and O that are not listed in Tables such as Table 3 is all less than 0.01 mass%. In Tables such as Table 4, the content of C, N, and O is listed in mass ppm. That is, in the magnets shown in Tables such as Table 3 and Table 4, Fe is the substantial remainder.

[0115] (evaluation) The Br at room temperature, HcJ at 160°C, and Hk / HcJ of the RTB-type permanent magnets in each example and comparative example were measured using a BH tracer. The HcJ at room temperature of the RTB-type permanent magnets in each example and comparative example was measured using a pulse-excitation type magnetic property measuring device. The results are shown in the tables, such as Table 4.

[0116] A good Br content was defined as 1415 mT or higher at room temperature. A good HcJ content was defined as 1760 kA / m or higher at room temperature. A good Hk / HcJ ratio was defined as 95.0% or higher at room temperature. A good high-temperature HcJ content was defined as 715 kA / m or higher at 160°C.

[0117] Table 1

[0118] Table 2

[0119] Table 3

[0120] Table 4

[0121] Table 5

[0122] Table 6

[0123] Table 7

[0124] Table 8

[0125] Table 9

[0126] Table 10

[0127] Table 11

[0128] Table 12

[0129] Table 13

[0130] Table 14

[0131] Table 15

[0132] Table 16

[0133] Table 17

[0134] Table 18

[0135] Table 19

[0136] Table 20

[0137] For sample number 5, the area ratio of the Zr-C phase was measured for samples 31-39, which mainly varied the Zr content, and for samples 41-46, which mainly varied the B content. In addition, the area ratio of the Zr-C phase was measured for sample number 5.

[0138] For each sample's cross-section, elemental mapping of Zr and C was performed using EPMA (JEOL JXA-8500F) with an acceleration voltage of 15kV, irradiation current of 200nA, analysis step of 0.20nm / step, and measurement range of 51.2μm × 51.2μm. The location of the Zr-C phase was identified by superimposing the elemental mapping images of Zr and C. The area ratio of the Zr-C phase was calculated by dividing the area of ​​the Zr-C phase by the area of ​​the mapping image, i.e., 51.2μm × 51.2μm. The results are shown in Table 21.

[0139] We confirmed that the area ratio of the Zr-C phase in the examples not listed in Table 21 was between 0.50% and 2.60%.

[0140] [Table 21]

[0141] The RTB-type permanent magnets in each embodiment exhibited good Br, HcJ, and Hk / HcJ values ​​at room temperature, and also good HcJ values ​​at high temperatures.

[0142] In contrast, sample number 1, which had a TRL that was too large, showed a decrease in Br at room temperature. Sample number 10, which had a TRL that was too small, could not be sintered sufficiently. Specifically, the density of the sintered body of sample number 10 was less than 90% compared to the density of the sintered body of sample number 5.

[0143] Samples 31 and 32, which had too little Zr content, showed decreased HcJ at both room temperature and high temperature. Sample 39, which had too much Zr content, showed decreased Br at room temperature and decreased Hk / HcJ at room temperature.

[0144] Samples 31 and 32, which had too little Zr content, showed a decreased Zr-C phase area ratio. Sample 39, which had too much Zr content, showed an increased Zr-C area ratio.

[0145] Sample number 41, which had too high a B content, showed a decrease in HcJ and Hk / HcJ at room temperature. Sample number 46, which had too low a B content, also showed a decrease in HcJ and Hk / HcJ at room temperature.

[0146] Sample number 41, which had too high a B content, showed a decrease in the area ratio of the Zr-C phase. Sample number 46, which had too low a B content, showed an increase in the area ratio of Zr-C.

[0147] Sample number 51, which had too little Al content, showed a decrease in HcJ at both room temperature and high temperature. Sample number 58, which had too much Al content, showed a decrease in Br at room temperature.

[0148] Sample number 66, which had an excessively high Ga content, showed decreased Br and Hk / HcJ at room temperature. [Explanation of symbols]

[0149] 1...RTB permanent magnet 11…Main phase particles 13...Grain boundary 15…Zr-C phase 17…Zr-B phase

Claims

1. The total content of light rare earth elements is 28.0% by mass or more and 31.5% by mass or less. The total content of heavy rare earth elements is greater than 0% by mass and less than or equal to 1.0% by mass. The content of B is 0.97% by mass or more and 1.05% by mass or less. Al content is 0.05% by mass or more and 0.52% by mass or less. The Zr content is greater than 0.50% by mass and less than or equal to 0.75% by mass. The Ga content is 0% by mass or more and 0.20% by mass or less, An R-T-B type permanent magnet having an O content of 0 ppm by mass or more and 1000 ppm by mass or less.

2. The R-T-B permanent magnet according to claim 1, wherein the Zr content is 0.53% by mass or more and 0.75% by mass or less.

3. The R-T-B permanent magnet according to claim 1 or 2, wherein the total content of the heavy rare earth elements is greater than 0% by mass and 0.20% by mass or less.

4. The R-T-B permanent magnet according to claim 1 or 2, wherein the Co content is 0.50% by mass or more and 0.80% by mass or less.

5. The R-T-B permanent magnet according to claim 1 or 2, having a concentration gradient of the heavy rare earth element that decreases from the surface of the magnet toward the interior.

6. The R-T-B permanent magnet has a main phase particle and grain boundaries formed by two or more adjacent main phase particles, The R-T-B permanent magnet according to claim 1 or 2, wherein the grain boundary has a Zr-C phase, and the area ratio of the Zr-C phase in the cross-section of the R-T-B permanent magnet is 0.50% or more and 2.60% or less.

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