Rare earth sintered magnet and manufacturing method of the same

Abstract

SOLUTION: To provide an R-T-B system rare earth sintered magnet which contains R (Nd is essential for one or more rare earth elements), T (Fe is essential for one or more elements of Fe and Co), B, M (one or more elements of Cu, Ga, Al, Si, Cr, Mn, Zn, Ge, Mo, Sn, W, Pb, and Bi), O, C, N and inevitable impurities, and which contains a main phase which is an R2T14B intermetallic compound and a grain boundary phase, in which when an R-O-C-N phase having a higher R concentration and a higher O, C, and N concentration than the main phase is present in the grain boundary phase, and atomic ratios of O, C, and N of the R-O-C-N phase are [O], [C], and [N], a value of [O] / ([O]+[C]+[N]) is 0.4 to 1.0, and a value of [C] / ([O]+[C]+[N]) is 0 to 0.3.EFFECT: By improving a utilization efficiency of a heavy rare earth element in a grain boundary diffusion step, an amount of the heavy rare earth element to be used can be reduced, problems of cost and a risk of resource procurement of the heavy rare earth element can be solved, and a rare earth sintered magnet having excellent magnetic properties can be efficiently and stably obtained.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an RTB rare earth sintered magnet that has a low impurity concentration and that has high utilization efficiency of heavy rare earth elements when a grain boundary diffusion process is carried out, and to a method for producing the same. [Background technology]

[0002] Rare earth sintered magnets are functional materials essential for energy conservation and high performance, and their range of applications and production volume are expanding year by year. Among rare earth sintered magnets, Nd-based sintered magnets in particular have high residual magnetic flux density and are used in a variety of applications, such as drive motors for hybrid and electric vehicles, motors for electric power steering, motors for air conditioner compressors, and voice coil motors (VCM) for hard disk drives.

[0003] On the other hand, rare earth sintered magnets lose their coercive force at high temperatures, causing irreversible thermal demagnetization. For this reason, rare earth sintered magnets used in automobiles, particularly electric vehicle motors, are required to have high coercive force.

[0004] Grain boundary diffusion treatment using heavy rare earth elements is known as a method for increasing the coercivity of rare earth sintered magnets. It is known that in this grain boundary diffusion treatment, some of the heavy rare earth elements react with the grain boundary phase in the magnet, reducing the utilization efficiency of the heavy rare earth elements. In particular, it is expected that higher coercivity can be achieved with less heavy rare earth elements by reducing the amounts of impurity elements such as O, C, and N contained in the magnet and suppressing the concentration of heavy rare earth elements.

[0005] Japanese Patent Application Laid-Open No. 2019-160949 (Patent Document 1) describes a method for obtaining excellent magnetic properties and high corrosion resistance in an ROCN enrichment section by forming a structure in which the concentration of heavy rare earth elements in the core section is relatively low compared to the concentration of heavy rare earth elements in the shell section.

[0006] Furthermore, WO 2013 / 100008 (Patent Document 2) describes a method for reducing the carbon content in a magnet by absorbing hydrogen into the raw material alloy of a rare earth magnet, crushing and molding the magnet without dehydrogenation, and then controlling the conditions for subsequent heat treatment to cause the hydrogen contained in the molded body to decompose and remove carbon derived from the lubricant added during molding. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent Publication No. 2019-160949 [Patent Document 2] International Publication No. 2013 / 100008 Summary of the Invention [Problem to be solved by the invention]

[0008] Such ROCN phases do not contribute to improving coercivity, and it has been found that they reduce the utilization efficiency of heavy rare earth elements because some of the heavy rare earth elements become concentrated during the grain boundary diffusion process.It is expected that the utilization efficiency of heavy rare earth elements will be improved by suppressing this concentration and reducing the concentrations of impurities, which are constituent elements.

[0009] However, even if the concentration of heavy rare earth elements in the core of the ROCN enrichment section is adjusted to be lower than that in the shell section by the method described in Patent Document 1, the heavy rare earth elements are still concentrated in the ROCN enrichment section to a certain extent, and the utilization efficiency of the heavy rare earth elements is reduced.

[0010] Furthermore, while the method of Patent Document 2 can suppress the formation of the ROCN phase by reducing the C content in the rare earth sintered magnet and improve the utilization efficiency of the heavy rare earth elements through grain boundary diffusion, it does not sufficiently reduce the N element that constitutes the ROCN phase, and as with the above, the utilization efficiency of the heavy rare earth elements is insufficient. In any event, there is a need to further improve the utilization efficiency of heavy rare earth elements, which have high resource risks and are expensive.

[0011] The present invention has been made in view of the above-mentioned problems, and has an object to provide a rare earth sintered magnet that improves the utilization efficiency of heavy rare earth elements when subjected to grain boundary diffusion treatment using the heavy rare earth elements, and that has a higher coercive force with a lower heavy rare earth content. [Means for solving the problem]

[0012] As a result of extensive research to achieve the above object, the inventors discovered that by adjusting conditions such as the heat treatment temperature and heat treatment atmosphere in the sintering process of a rare earth sintered magnet, it is possible to reduce the impurities in the rare earth sintered magnet, resulting in a magnet containing an ROCN phase with a predetermined concentration relationship, and that such a magnet can improve the utilization efficiency of the heavy rare earth element in the grain boundary diffusion process, thereby completing the present invention.

[0013] Therefore, the present invention provides the following rare earth sintered magnet and method for producing the same. 1. R (R is one or more elements selected from rare earth elements, and Nd is essential), T (T is one or more elements selected from Fe and Co, and Fe is essential), B, M (M is one or more elements selected from Cu, Ga, Al, Si, Cr, Mn, Zn, Ge, Mo, Sn, W, Pb, and Bi), O, C, N, and unavoidable impurities are included, and R2T 14 An RTB rare earth sintered magnet containing a main phase which is a B intermetallic compound and a grain boundary phase, The RTB rare earth sintered magnet is characterized in that the grain boundary phase contains an ROCN phase having higher R, O, C, and N concentrations than the main phase, and when the atomic ratio of O in the ROCN phase is [O], the atomic ratio of C is [C], and the atomic ratio of N is [N], the value of the following relational expression (1) is 0.4 or more and 1.0 or less, and the value of the following relational expression (2) is 0 or more and 0.3 or less. [O] / ([O]+[C]+[N])···(1) [C] / ([O]+[C]+[N])···(2) 2. The RTB rare earth sintered magnet of 1, wherein the value of the following relational expression (5) is 0.4 or less: [N] / ([O]+[C]+[N]) (5) 3. The RTB sintered magnet of 1 or 2, wherein when the atomic ratio of R in the ROCN phase is [R], the value of the following relational expression (3) is 0.8 or more and 1.5 or less: ([O]+[C]+[N]) / [R]···(3) 4. The RTB sintered magnet of any one of 1 to 3, wherein the O content is 0.15 mass % or less. 5. The RTB sintered magnet of any one of 1 to 4, wherein the average crystal grain size in a plane parallel to the magnetization direction is 3 μm or less. 6. An RTB sintered magnet according to any one of 1 to 5, characterized in that it contains R' (where R' is one or more elements selected from Dy and Tb) that is introduced by grain boundary diffusion, and that within at least 500 μm from the surface of the sintered magnet, at least part of the surface vicinity of the main phase particles has a region with a higher concentration of Dy and / or Tb than the center of the main phase particles. 7. The RTB sintered magnet of any one of 1 to 6, wherein R does not contain Dy or Tb but contains R' (R' is one or more elements selected from Dy and Tb) that is an element introduced by grain boundary diffusion, and the ROCN phase also contains R', and when the concentration of R' in the ROCN phase is [R'], the value of the following relational expression (4) is greater than 0 and less than or equal to 0.1: [R'] / [R] (4) 8. A method for producing a rare earth sintered magnet, comprising: a melting step for obtaining a raw material alloy having a predetermined composition containing the R, T, B, and M; a coarse crushing step for coarsely crushing the raw material alloy to obtain a coarse powder; a fine crushing step for finely crushing the coarse powder to obtain a fine powder; a molding step for molding the fine powder in a magnetic field to obtain a compact; and a sintering step for heat-treating the compact to obtain a sintered body, The coarse pulverization step is hydrogen pulverization in which hydrogen is absorbed into the raw alloy and the raw alloy is coarsely pulverized, the sintering step includes a first heat treatment, a second heat treatment, and a third heat treatment; the first heat treatment is a heat treatment in which the temperature is increased from room temperature to a predetermined first heat treatment temperature in an inert gas atmosphere, the second heat treatment is a heat treatment in which, after the first heat treatment, the temperature is raised to a second heat treatment temperature, which is a predetermined temperature higher than the first heat treatment temperature and lower than a sintering temperature, in a gas atmosphere containing hydrogen released from a hydrogen generating source other than hydrogen released from the compact; a third heat treatment, which is a heat treatment performed after the second heat treatment, in which the temperature is raised to a sintering temperature exceeding the second heat treatment temperature in a vacuum atmosphere, and the temperature is maintained at the sintering temperature for a predetermined period of time. 9. The method for producing a rare earth sintered magnet according to claim 8, wherein the hydrogenation pulverization does not involve dehydrogenation. 10. The method for producing a rare earth sintered magnet according to claim 8 or 9, wherein hydrogen gas is introduced into the atmosphere during the second heat treatment. 11. The method for producing a rare earth sintered magnet according to any one of 8 to 10, wherein the first heat treatment temperature is 150°C or higher and 350°C or lower. 12. The method for producing a rare earth sintered magnet according to any one of claims 8 to 11, wherein the sintering step comprises heat treating the compact mixed with a hydrogen storage alloy. 13. The method for producing a rare earth sintered magnet according to claim 12, wherein the hydrogen-absorbing alloy has a hydrogen release temperature of 150°C or higher and 350°C or lower in an atmospheric pressure environment. 14. The method for producing a rare earth sintered magnet according to claim 12 or 13, wherein the hydrogen storage alloy is an alloy in which 0.1 mass % or more of hydrogen is stored in the raw material alloy. 15. A method for producing a rare earth sintered magnet according to any one of claims 8 to 14, wherein the fine pulverization step comprises adding a lubricant to the coarse powder and pulverizing the coarse powder, and the amount of lubricant added is 0.1 to 0.5 mass% relative to 100 mass% of the coarsely pulverized powder. 16. A method for producing a rare earth sintered magnet according to any one of 8 to 15, wherein the second heat treatment temperature is 400°C or higher and 600°C or lower. 17. The method for producing a rare earth sintered magnet according to any one of 8 to 16, wherein the hydrogen partial pressure in the heat treatment furnace during the second heat treatment is 20 kPa to 200 kPa. 18. A method for producing a rare earth sintered magnet according to any one of claims 8 to 17, characterized in that it includes a grain boundary diffusion treatment step in which a diffusion source is present on the surface of the obtained sintered body and heat treatment is carried out in a vacuum or inert gas atmosphere to diffuse the diffusion source into the sintered magnet. [Effects of the Invention]

[0014] According to the present invention, the utilization efficiency of heavy rare earth elements in the grain boundary diffusion process using heavy rare earth elements is improved, thereby making it possible to reduce the amount of heavy rare earth elements used, thereby resolving problems related to cost and the risk of procuring heavy rare earth elements as resources, and enabling the efficient and stable production of rare earth sintered magnets with excellent magnetic properties. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a graph showing the atomic ratio contents of O, C, and N in the ROCN phase in magnets of Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0016] The rare earth sintered magnet of the present invention contains R (R is one or more elements selected from rare earth elements, and Nd is essential), T (T is one or more elements selected from Fe and Co, and Fe is essential), B, M (M is one or more elements selected from Cu, Ga, Al, Si, Cr, Mn, Zn, Ge, Mo, Sn, W, Pb, and Bi), O, C, N, and inevitable impurities.

[0017] As mentioned above, R is one or more elements selected from rare earth elements, and Nd is essential. The R content is preferably 28% by mass or more, and more preferably 29% by mass or more, from the viewpoints of suppressing the crystallization of α-Fe in the raw alloy during production and of achieving sufficient densification. Although it is difficult to eliminate α-Fe even by homogenization, within the above range, the coercivity (hereinafter referred to as H) of the RTB sintered magnet can be reduced. cJ This can prevent a significant decrease in the R2T and squareness. This is also true when the raw material alloy is produced by strip casting, which is less likely to cause α-Fe crystallization. In addition, during the sintering process described below, the amount of liquid phase, which is mainly composed of R components and plays a role in promoting densification, decreases, preventing a decrease in sinterability and insufficient densification of the RTB-based sintered magnet. On the other hand, if the R content is too high, the R2T in the sintered magnet will decrease. 14 The proportion of B phase is low and B r Therefore, this B r From the viewpoint of preventing a decrease, the content of R is preferably 36% by mass or less, more preferably 33.5% by mass or less, and even more preferably 32.5% by mass or less.

[0018] The proportion of Nd in R is not particularly limited, but is preferably 60% or more, more preferably 75% or more, of all elements R. Furthermore, although not particularly limited, elements R other than Nd may preferably contain Pr, Dy, Tb, Ho, Er, Sm, Ce, Y, etc.

[0019] In the present invention, one or more elements selected from Dy and Tb may be contained as elements constituting at least a part of the R. The content is not particularly limited, but the higher the content, the higher the H cJ However, from the viewpoints of cost and resource risks, the amount of Dy and Tb is preferably 0 to 5 mass %, more preferably 0.1 to 2 mass %, and even more preferably 0.3 to 1 mass %, relative to the entire rare earth sintered magnet composition. Furthermore, from the viewpoint of efficiently utilizing one or more elements selected from Dy and Tb, it is preferable that these elements are introduced into the magnet by grain boundary diffusion.

[0020] The above-mentioned T is one or more elements selected from Fe and Co, and Fe is essential. The content of T is the balance other than the above-mentioned R, B, M, O, C, and N, and is not particularly limited. However, the higher the content of B, the better. r From the viewpoint of obtaining R2T, the content is preferably 60% by mass or more, and more preferably 63% by mass or more. 17 Deterioration of squareness due to precipitation of H cJ From the viewpoint of suppressing a decrease in magnetic properties, the proportion of Fe in T is preferably 72% by mass or less, and more preferably 70% by mass or less. From the viewpoint of obtaining good magnetic properties, the proportion of Fe in T is preferably 90% by mass or more, and more preferably 95% by mass or more, and from the viewpoint of obtaining good corrosion resistance, the proportion is preferably 99% by mass or less, and more preferably 98% by mass or less.

[0021] The content of B is preferably 0.80% by mass or more, more preferably 0.90% by mass or more, and even more preferably 0.93% by mass or more. 14 The proportion of B phase is low and B r A significant decline in R2T 17 On the other hand, the upper limit is preferably 1.15% by mass or less, more preferably 1.07% by mass or less, and even more preferably 1.03% by mass or less. Within this range, the R 1.1The T4B4 compound phase is formed. r and H cJ A portion of B in each phase contained in the sintered magnet may be substituted with C.

[0022] The above M is one or more elements selected from Cu, Ga, Al, Si, Cr, Mn, Zn, Ge, Mo, Sn, W, Pb, and Bi. Its content is preferably 0.3 mass% or more, more preferably 0.5 mass% or more. Within this range, the effect of improving the coercive force can be enjoyed. On the other hand, the upper limit is preferably 2.5 mass% or less, more preferably 2.0 mass% or less. In this way, B r and H cJ This can suppress the decline in

[0023] The higher the O content, the lower the relative contents of C and N in the ROCN phase (described later), and the more concentrated the heavy rare earth elements in the ROCN phase can be suppressed. On the other hand, the higher the O content, the more ROCN phase is formed, resulting in an increase in the amount of heavy rare earth elements consumed in the ROCN phase and a decrease in the utilization efficiency of the heavy rare earth elements. From the above viewpoints, the O content is not particularly limited, but is preferably 0.03% by mass or more and 0.15% by mass or less, and more preferably 0.05% by mass or more and 0.10% by mass or less.

[0024] The C mentioned above originates from the raw materials and the lubricant added to improve the orientation of fine powder during compaction in a magnetic field. In the present invention, the C derived from the lubricant is decomposed and removed in the sintering process described below, making it possible to reduce the C content compared to conventional methods. On the other hand, if a large amount of C remains without being completely removed, it will form an ROCN phase, consuming R and resulting in H. cJ Furthermore, when heavy rare earth elements are diffused into the grain boundaries, a high C concentration in the ROCN phase hinders the diffusion of heavy rare earth elements into the magnet, which makes it difficult to obtain sufficient H inside the magnet. cJFrom this viewpoint, the C content is not particularly limited, but is preferably 0.06% by mass or less, more preferably 0.05% by mass or less, and even more preferably 0.04% by mass or less.

[0025] The N content is not particularly limited, but from the viewpoint of suppressing the concentration of heavy rare earth elements in the ROCN phase due to an increase in the N concentration in the ROCN phase, similarly to the case of C, it is preferably 0.05% by mass or less, more preferably 0.04% by mass or less, and even more preferably 0.03% by mass or less.

[0026] The structure of the rare earth sintered magnet of the present invention includes R2T 14 The main phase contains a ZrB intermetallic compound. The grain boundary phase contains an ROCN phase, which has higher R, O, C, and N concentrations than the main phase. In addition to the ROCN phase, the grain boundary phase may also contain an R2O3 phase, a ZrB2 phase, a ZrC phase, etc.

[0027] In the ROCN phase, when the atomic ratio of O in the ROCN phase is [O], the atomic ratio of C is [C], and the atomic ratio of N is [N], the value of the following relational expression (1) is 0.4 or more, preferably 0.43 or more, and 1.0 or less, preferably 0.8 or less, and the value of the following relational expression (2) is 0 or more, preferably 0.1 or more, and 0.3 or less. [O] / ([O]+[C]+[N])···(1) [C] / ([O]+[C]+[N])···(2)

[0028] When [O] and [C] satisfy the conditions of the above-mentioned relational expressions (1) and (2), the value of the following relational expression (5) is 0.6 or less. The value of this relational expression (5) is not particularly limited, but from the viewpoint of further suppressing the concentration of heavy rare earth elements in the ROCN phase, it is preferably 0.4 or less, and more preferably 0.3 or less. [N] / ([O]+[C]+[N]) (5)

[0029] If the composition of the ROCN phase satisfies the conditions of the above relational expressions (1) and (2), and more preferably if the value of the above relational expression (4) is 0.4 or less, the extent of improvement in coercivity due to the grain boundary diffusion treatment using a heavy rare earth element can be increased. On the other hand, if the ROCN phase does not satisfy the conditions of the above relational expressions (1) and (2), the concentration of the heavy rare earth element contained in the ROCN phase increases, the utilization efficiency of the heavy rare earth element for improving coercivity decreases, and sufficient coercivity cannot be obtained.

[0030] From the same viewpoint as above, when the R does not contain Dy or Tb, where [R] is the atomic ratio of R contained in the ROCN phase, R' is one or more elements selected from Dy and Tb introduced into the magnet by grain boundary diffusion, and [R'] is the concentration of R' in the ROCN phase, the value of the following relational expression (4) is preferably greater than 0 and less than or equal to 0.1, and more preferably greater than 0 and less than or equal to 0.06. Note that whether the entire amount of heavy rare earth elements contained in the ROCN phase is R' introduced by grain boundary diffusion can be determined by confirming that the core of the main phase particles in the center of the sintered magnet does not contain Dy or Tb. [R'] / [R] (4)

[0031] By controlling the composition ratio of O, C, and N in the ROCN phase as described above, the rare earth sintered magnet of the present invention improves the increase in coercivity due to grain boundary diffusion using heavy rare earth elements compared to conventional rare earth sintered magnets.The reason for this is not necessarily clear, but it can be speculated as follows.

[0032] In conventional rare earth magnets with grain boundary diffusion of heavy rare earth elements, the heavy rare earth elements are concentrated in the ROCN phase, and some of the heavy rare earth elements are present in a form that does not contribute to improving coercivity. On the other hand, first-principles calculations of the structural stability of the ROCN phase when a portion of the R in the ROCN phase is replaced with a heavy rare earth element showed that, when the O, C, and N composition ratios in the ROCN phase satisfy the conditions of the above-mentioned relations (1) and (2), the substitution of a portion of the R in the ROCN phase with a heavy rare earth element results in a larger enthalpy change of mixing and greater instability than the unsubstituted ROCN phase. In other words, controlling the composition and impurity ratio of the ROCN phase within a predetermined range can suppress the concentration of heavy rare earth elements in the ROCN phase, thereby increasing the utilization efficiency of the heavy rare earth elements in the grain boundary diffusion process and enhancing the extent of the improvement in coercivity.

[0033] The composition of the ROCN grain boundary phase and the concentration distribution of heavy rare earth elements in the main phase particles due to grain boundary diffusion (described later) can be measured using EDS (energy dispersive X-ray spectroscopy) and WDS (wavelength dispersive X-ray spectroscopy). It is generally known that carbon contamination is superimposed on the analytical values ​​when carbon is analyzed using an EDS device attached to an SEM (scanning electron microscope). Therefore, in order to reduce contamination and obtain a clean surface, it is preferable to perform analysis using an EDS device after polishing the magnet surface to be analyzed using ion milling or a focused ion beam (FIB) device to remove the effects of oxidation on the outermost surface. Note that, since the effects of carbon contamination cannot be completely eliminated in EDS or WDS analyses, and it is difficult to determine the absolute value of the carbon concentration, carbon contamination may be excluded for convenience by subtracting the carbon atomic concentration in the main phase from the carbon atomic concentration in the ROCN phase. To minimize the influence of analytical errors, it is preferable to determine the composition of each phase as the average of 10 or more analytical values. Furthermore, when heavy rare earth elements are introduced by grain boundary diffusion, the distribution of the heavy rare earth elements may differ between the magnet surface where the heavy rare earth elements are deposited and the center of the magnet. Therefore, when grain boundary diffusion treatment is performed, the analytical values ​​of each phase at a depth of 150 to 250 μm from the diffusion surface should be used.

[0034] Furthermore, to identify the ROCN phase, it is preferable to confirm by obtaining an ED (electron diffraction) pattern, etc. The ROCN phase has a cubic NaCl-type structure, and the value of the following relational formula (3) is preferably 0.8 or more, more preferably 1.0 or more, and preferably 1.5 or less, more preferably 1.3 or less. ([O]+[C]+[N]) / [R]···(3)

[0035] Furthermore, the average crystal grain size (μm) in the plane parallel to the magnetization direction of the rare earth sintered magnet of the present invention is sufficient H cJFrom the viewpoint of obtaining a satisfactory degree of orientation within an appropriate range of lubricant addition, the average crystal grain size is preferably 3.0 μm or less, more preferably 2.5 μm or less. From the viewpoint of obtaining a sufficient degree of orientation within an appropriate range of lubricant addition, the average crystal grain size in the present invention is defined as the area median diameter obtained from a histogram showing the particle size distribution, in which the percentage of the area occupied by crystal grains is plotted at intervals of 1 μm for each particle diameter. That is, the area median diameter can be calculated by creating a histogram of the crystal grain area ratio from the diameter of each particle calculated as the circle-equivalent diameter and fitting it with a Gaussian function. The circle-equivalent diameter of the above-mentioned particles can be measured, for example, by the following procedure. First, the cross section of the sintered magnet is polished to a mirror finish, and then immersed in an etching solution such as Vilela's solution (a mixture of glycerin, nitric acid, and hydrochloric acid in a ratio of 3:1:2) to selectively etch the grain boundary phase. The cross section is then observed with a laser microscope. Next, the cross-sectional area of ​​each particle is measured using image analysis based on the obtained observation image, and the equivalent circle diameter is calculated. The average crystal grain size may be, for example, the average of a total of about 2,000 particles in images taken at 20 different locations. The measurement device is not particularly limited, but for example, a 3D measuring laser microscope (LEXT OLS 4000, manufactured by Olympus Corporation) can be used, and image analysis software (Win ROOF, manufactured by Mitani Corporation) can be used for image analysis.

[0036] Next, a method for producing an RTB rare earth sintered magnet of the present invention is described below. The process for producing a rare earth sintered magnet of the present invention includes a melting step for obtaining a raw material alloy having a predetermined composition containing the above-mentioned R, T, B, and M, a coarse crushing step for coarsely crushing the raw material alloy to obtain a coarse powder, a fine crushing step for finely crushing the coarse powder to obtain a fine powder, a molding step for molding the fine powder in a magnetic field to obtain a compact, and a sintering step for heat-treating the compact to obtain a sintered body.

[0037] First, in the melting step, metals or alloys that are raw materials for each element are weighed to obtain a predetermined composition including R, T, B, and M. After weighing to obtain the predetermined composition, the raw materials are melted by, for example, high-frequency melting, and cooled to produce a raw alloy. The raw alloy is generally cast by a melting casting method or a strip casting method in which the alloy is poured into a flat mold or a book mold. In addition, the main phase, R2T, is also weighed. 14 The present invention can also be applied to the so-called two-alloy method, in which an alloy close to the B compound composition and an R-rich alloy that becomes a liquid phase auxiliary at the sintering temperature are separately prepared, coarsely crushed, and then weighed and mixed. However, since the α-Fe phase is likely to crystallize in an alloy close to the main phase composition depending on the cooling rate and alloy composition during casting, it is preferable to perform a homogenization treatment in a vacuum or Ar atmosphere at 700 to 1200°C for at least one hour in order to homogenize the structure and eliminate the α-Fe phase. Note that if an alloy close to the main phase composition is prepared by strip casting, homogenization can be omitted. In addition to the above-mentioned casting method, the so-called liquid quenching method can also be applied to the R-rich alloy that becomes a liquid phase auxiliary.

[0038] In the coarse pulverization step, the raw alloy is subjected to hydrogen pulverization by exposing it to a hydrogen atmosphere above a certain pressure, causing the alloy to absorb hydrogen and then pulverize it. The hydrogen pressure is not particularly limited, but is preferably 100 kPa or higher to reduce the adverse effects on productivity caused by the time required for hydrogen absorption. In the present invention, it is preferable not to perform a dehydrogenation treatment after hydrogen absorption in order to promote decomposition of the lubricant in the heat treatment step described below. After the hydrogen pulverization step is performed, the heated alloy raw material is cooled and then subjected to the next step. In this case, it is preferable to cool it to around room temperature to prevent oxidation. This hydrogen pulverization step can produce coarse powder pulverized to 0.05 mm to 3 mm, particularly 0.05 mm to 1.5 mm.

[0039] The fine pulverization step can employ a method in which the coarse powder obtained in the coarse pulverization step is pulverized using a jet mill with a non-oxidizing gas stream, such as N2, He, or Ar. In the present invention, the coarse powder is pulverized in this fine pulverization step to a volume-based median diameter D50 of preferably 0.2 μm to 10 μm, more preferably 1.2 μm to 3.0 μm, and even more preferably 1.5 μm to 2.5 μm. Because the O and N contained in the rare earth sintered magnet are mainly mixed in during the fine pulverization step, controlling the jet mill atmosphere is necessary to adjust the O and N contents in the rare earth sintered magnet. For example, the O content in the rare earth sintered magnet can be adjusted by controlling the O amount and dew point in the jet mill atmosphere. The moisture content in the atmosphere during pulverization is preferably 100 ppm or less, and the oxygen concentration is preferably 1 ppm or less. The volume-based median diameter D50 is the particle diameter at which the cumulative volume frequency reaches 50%.

[0040] The N content in the rare earth sintered magnet can be further reduced by, for example, (A) finely pulverizing the magnet using a jet mill with a He or Ar gas stream, or (B) finely pulverizing the magnet by introducing hydrogen into a jet mill with a N gas stream. In this case, method (B) promotes hydrogen adsorption on the activated surfaces created by pulverization, inhibiting nitrogen adsorption, thereby reducing the amount of N in the rare earth sintered magnet.

[0041] In the pulverization process of the present invention, a lubricant, such as a saturated fatty acid or its ester, can be added before or after the coarse pulverization process to improve the orientation of the powder during the subsequent compaction process in a magnetic field. While adding a lubricant is effective in improving orientation and reducing the N content of the magnet by suppressing nitrogen adsorption to the fine powder, the greater the amount of lubricant added, the more difficult it becomes to remove the C from the lubricant during heat treatment. As a result, the concentration of C in the ROCN phase increases, and during grain boundary diffusion treatment using heavy rare earth elements, the heavy rare earth elements concentrate in the ROCN phase, reducing their utilization efficiency. Therefore, in the present invention, heat treatment is performed in a hydrogen gas atmosphere at a predetermined temperature in the heat treatment process described below, thereby promoting the decomposition of the lubricant and binder adsorbed to the fine powder surface. The amount of lubricant added is determined appropriately depending on the type of lubricant and is not particularly limited. However, it is preferable to add a lubricant in an amount that improves the degree of orientation and results in a high B. r From the viewpoint of obtaining the above, and from the viewpoint of maintaining a low carbon concentration in the ROCN phase of the rare earth sintered magnet obtained after sintering and increasing the utilization efficiency of the heavy rare earth element in grain boundary diffusion, the amount is preferably 0.10 to 0.50 parts by mass, and more preferably 0.15 to 0.30 parts by mass, per 100 parts by mass of the coarsely pulverized powder or raw alloy.

[0042] In the compacting step, a magnetic field of, for example, 400 to 1600 kA / m is applied to orient the alloy powder in the direction of the easy axis of magnetization, and the powder is compacted in a compression molding machine. 3 That is, from the viewpoint of ensuring the strength of the molded body and obtaining good handling properties, the density of the molded body is preferably 2.8 g / cm. 3 It is preferable that the B content is equal to or greater than 100%. In addition, a binder such as PVA or fatty acid can be added to increase the strength of the molded body after molding. On the other hand, it is possible to obtain a sufficient molded body strength while suppressing the disorder of particle orientation during pressure application, thereby achieving a suitable B content. r From the viewpoint of obtaining this, the density of the compact is 4.2 g / cm 3 It is preferable that the temperature is not more than 100°C. In order to prevent oxidation of the alloy fine powder, the compaction is preferably carried out in an inert gas atmosphere such as nitrogen gas or Ar gas.

[0043] The sintering process in the present invention is a process of sintering the molded body obtained in the molding process in an inert gas atmosphere such as Ar gas or in a high vacuum, and includes a first heat treatment, a second heat treatment, and a third heat treatment. The first heat treatment is a heat treatment in which the temperature is raised from room temperature to a predetermined first heat treatment temperature in an inert gas atmosphere. The second heat treatment is a heat treatment in which the temperature is raised after the first heat treatment to a second heat treatment temperature, which is a predetermined temperature higher than the first heat treatment temperature but lower than the sintering temperature, in a gas atmosphere containing hydrogen released from a hydrogen generating source other than hydrogen released from the molded body. The third heat treatment is a heat treatment in which the temperature is raised after the second heat treatment to a temperature higher than the second heat treatment temperature but lower than the sintering temperature in a vacuum atmosphere, and the temperature is maintained at the sintering temperature for a predetermined time.

[0044] The first heat treatment is performed in an inert gas atmosphere such as Ar to raise the temperature to the first heat treatment temperature in order to prevent temperature drops in the compact due to the release of hydrogen gas (endothermic reaction) in the compact and cracks caused by temperature variations in the furnace. The first heat treatment temperature is not particularly limited, but is preferably 150°C to 350°C, more preferably 200°C to 300°C, to prevent cracks caused by sudden changes in the amount of hydrogen absorption due to an increase in hydrogen partial pressure during the second heat treatment and to prevent reactions between R and impurity gas components, thereby promoting decomposition of the lubricant. The temperature rise rate in the first heat treatment is not particularly limited, but is preferably 0.3 to 5.0°C / min, more preferably 0.5 to 3.0°C / min.

[0045] The second heat treatment is performed after the first heat treatment in an inert gas atmosphere. The purpose of this is to promote the lubricant removal reaction and liberate nitrogen adsorbed on the fine powder. The second heat treatment temperature is a predetermined temperature above the first heat treatment temperature and below the sintering temperature. The second heat treatment temperature is not particularly limited, but is preferably 400°C to 600°C, more preferably 450°C to 550°C, to prevent cracks caused by the sudden release of hydrogen contained in the compact when the atmosphere is switched to a vacuum atmosphere during the third heat treatment. The rate of temperature increase from the first heat treatment temperature to the second heat treatment temperature is not particularly limited, but is preferably 0.3 to 5.0°C / min, more preferably 0.5 to 3.0°C / min.

[0046] The atmosphere in the second heat treatment is a gas atmosphere containing hydrogen released from a hydrogen generating source other than the hydrogen released from the compact, and partially contains the hydrogen gas released from the compact and the inert gas used in the first heat treatment. To further reduce the C and N content in the magnet, it is preferable to add hydrogen gas as a hydrogen generating source other than the hydrogen released from the compact by either (C) introducing hydrogen gas or (D) incorporating a hydrogen storage alloy.

[0047] When introducing hydrogen gas as described in (C) above, although not particularly limited, it is preferable to introduce hydrogen gas after the first heat treatment while maintaining the first heat treatment temperature, from the viewpoint of promoting the decarburization reaction. Furthermore, from the viewpoint of replacing the inert gas atmosphere during the first heat treatment and increasing the hydrogen partial pressure in the heat treatment furnace in a short time to shorten the heat treatment cycle, it is preferable to replace the gas while maintaining a constant furnace pressure. The hydrogen introduction rate is preferably 2% / min or more, more preferably 3% / min or more, relative to the volume of the heat treatment furnace. Furthermore, from the viewpoint of the cost of using hydrogen gas, it is preferably 10% / min or less, more preferably 5% / min or less. The time required for hydrogen introduction is preferably 2 to 10 hours, more preferably 4 to 8 hours, from the viewpoints of the gas introduction rate, the time required for replacement, and the time required for the decarburization reaction.

[0048] On the other hand, when incorporating a hydrogen storage alloy (D) into the molded body, the hydrogen released from the hydrogen storage alloy during the first heat treatment is absorbed by the molded body, and the hydrogen release temperature of the hydrogen storage alloy at atmospheric pressure is preferably 150°C or higher and 350°C or lower, in order to prevent cracks from occurring in the molded body. Furthermore, since incorporating a hydrogen storage alloy reduces the weight of molded bodies that can be loaded into a furnace, in order to ensure productivity, the hydrogen storage alloy preferably absorbs 0.1 parts by mass or more of hydrogen per 100 parts by mass of the hydrogen storage alloy. While the hydrogen storage alloy is not particularly limited, from the above perspectives, it is preferably a rare earth magnet alloy, and more preferably a raw material alloy for an RTB rare earth sintered magnet. Its composition may be the same as or different from that of the molded body.

[0049] The hydrogen partial pressure in the heat treatment furnace during the second heat treatment is determined from the change in furnace pressure upon introduction of hydrogen gas in the case of (C), but is calculated using the gas equation of state in the case of (D). It is calculated based on the amount of hydrogen absorbed by the raw alloy and the hydrogen storage alloy, the weight of the compacts placed in the furnace, the weight of the hydrogen storage alloy, the volume of the heat treatment furnace, and the furnace temperature, converted to room temperature. The hydrogen partial pressure in the heat treatment furnace can also be increased by increasing the load weight of the compacts, but this can be limited by the furnace volume and the packing rate of the compacts in the heat treatment vessel, which can limit the hydrogen partial pressure. Furthermore, variations in the hydrogen partial pressure due to fluctuations in the production weight can occur. Therefore, the introduction of hydrogen gas is advantageous for achieving stable reductions of carbon and nitrogen and for reducing variations in carbon concentration due to the temperature distribution in the furnace. From the above perspectives, the lower limit of the hydrogen partial pressure in the heat treatment furnace is not particularly limited, but is preferably 20 kPa or higher, more preferably 50 kPa or higher, and even more preferably 70 kPa or higher. On the other hand, the upper limit of the hydrogen partial pressure in the furnace is not particularly limited, but from the viewpoint of the cost of using hydrogen gas, it is preferably 200 kPa or less, more preferably 150 kPa or less, and even more preferably 120 kPa or less.

[0050] The third heat treatment is a heat treatment in which, after the second heat treatment, the temperature is raised to the sintering temperature in a vacuum atmosphere containing hydrogen gas and maintained for a predetermined time to sinter. The sintering temperature is generally in the range of 950 to 1200°C, and the temperature is preferably maintained for 0.5 to 15 hours. The sintered body is then cooled to a temperature of preferably 400°C or less, more preferably 300°C or less, and even more preferably 200°C or less. The cooling rate is not particularly limited, but is preferably 5 to 100°C / min, and more preferably 15 to 50°C / min, until the upper limit of the above range is reached. The heating rate from the second heat treatment temperature to the sintering temperature is also not particularly limited, but is preferably 3 to 20°C / min, and more preferably 5 to 15°C / min.

[0051] In the present invention, after the first heat treatment, the second heat treatment, and the third heat treatment in the sintering step are performed in this order, the obtained sintered body is then subjected to H cJ In order to increase the sintering temperature, heat treatment may be performed at a temperature lower than the sintering temperature. This heat treatment may be performed in two stages, high-temperature heat treatment and low-temperature heat treatment, or only low-temperature heat treatment may be performed. In the high-temperature heat treatment, the sintered body is preferably heat-treated at a temperature of 600 to 950°C, and in the low-temperature heat treatment, it is preferably heat-treated at a temperature of 400 to 600°C.

[0052] The obtained sintered body H cJ The method includes a diffusion source providing step of providing a diffusion source on the surface of the obtained sintered body, and a diffusion heat treatment step of heat treating the sintered body and the diffusion source in a vacuum or inert gas atmosphere.

[0053] The composition of the diffusion source is not particularly limited, but can be appropriately selected according to the above-mentioned purpose. In this case, for example, by including one or more elements selected from Dy and Tb, H, which improves the anisotropic magnetic field on the surface of the main phase particles of the magnet, can be obtained. cJ can be increased.

[0054] A known method can be used for the diffusion source application step. For example, a powder of the diffusion source is dispersed in an organic solvent such as alcohol or water to form a slurry, and the sintered body is immersed in this slurry and then pulled out and dried with hot air or in a vacuum, or naturally dried. Another example is a method of applying the diffusion source to the sintered body by sputtering.

[0055] Although the specific conditions for the diffusion heat treatment are not particularly limited, it is preferable to heat the sintered compact to a temperature above 400°C, particularly 500°C or higher, and not higher than 1100°C, particularly not higher than 1050°C, and especially not higher than 1000°C, so as to diffuse one or more elements selected from Dy and Tb into the sintered compact. The heat treatment time is preferably 1 minute to 50 hours to avoid alteration of the structure of the sintered compact and adverse effects on the magnetic properties due to inevitable oxidation and evaporation of components.

[0056] In the sintered magnet obtained by the above grain boundary diffusion treatment, from the viewpoint of providing a high anisotropic magnetic field by substituting the heavy rare earth element into the outer shells of the main phase particles and sufficiently improving the coercive force, it is preferable that, within a region of at least 500 μm from the surface of the sintered magnet, at least a part of the surface of the main phase particles has a region where the concentration of the R' element is higher than that of the center of the main phase particle.

[0057] Furthermore, the sintered magnet obtained by the above grain boundary diffusion treatment was also subjected to the same H cJ For the purpose of increasing the grain boundary diffusion temperature, the heat treatment may be carried out at a temperature lower than the grain boundary diffusion treatment temperature.

[0058] The rare earth sintered magnet obtained in this manner has impurity elements removed efficiently and stably, improving the utilization efficiency of heavy rare earth elements in the grain boundary diffusion treatment process. As a result, it is possible to obtain a rare earth sintered magnet that is excellent in terms of cost and resource procurement risk and is suitable for applications requiring high coercivity, such as electric vehicle motors. [Example]

[0059] EXAMPLES The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0060] [Examples 1 to 7] Nd metal, ferroboron alloy, electrolytic Co, Al metal, Cu metal, Ga metal, Zr metal, and electrolytic iron (all metals with a purity of 99% or higher) were weighed and blended to yield 31.7% by mass of Nd, 0.98% by mass of B, 1.8% by mass of Co, 0.1% by mass of Al, 0.2% by mass of Cu, 0.3% by mass of Ga, 0.2% by mass of Zr, and the remainder Fe. The alloy was melted and strip-cast to obtain flakes of the raw alloy with a thickness of 0.2–0.4 mm. A portion of the resulting flakes was subjected to hydrogen embrittlement in a pressurized hydrogen atmosphere to obtain a coarsely pulverized powder. Next, 0.2–0.5 parts by mass of stearic acid as a lubricant was added to 100 parts by mass of the resulting coarsely pulverized powder and mixed. The mixture was then dry-pulverized in a nitrogen stream using an air-flow pulverizer (jet mill) to obtain a finely pulverized powder (alloy powder). This finely pulverized powder was filled into the mold of a molding machine in a N2 gas atmosphere, and while being oriented in a magnetic field of 15 kOe (1.19 MA / m), it was pressed and molded in a direction perpendicular to the magnetic field. The density of the compact at this time was 3.0 to 4.0 g / cm3. 3 It was.

[0061] The resulting compacts and the above-mentioned flake-shaped raw alloy, in which 0.4 mass% hydrogen had been absorbed, were mixed in a weight ratio such that the hydrogen partial pressure in the furnace was 20–100 kPa. The mixture was then introduced into a heat treatment furnace. The first heat treatment was performed in an Ar gas atmosphere, where the temperature was raised at 2°C / min to 200°C. The second heat treatment was performed in a gas atmosphere containing hydrogen generated from the mixed compacts and the hydrogen-absorbing alloy, where the temperature was raised from 200°C to 500°C at 2°C / min. The atmosphere was then switched to a vacuum, where the temperature was raised at 5°C / min from 1020°C to 1060°C (a temperature selected for each sample to ensure sufficient sintering densification) and a 5-hour vacuum heat treatment. The third heat treatment was then performed in a vacuum, where the sintering process yielded a Nd magnet material. The hydrogen partial pressure generated during the second heat treatment was controlled by adjusting the amount of the mixed compacts and hydrogen-absorbing alloy. The hydrogen partial pressures at these times are shown in Table 1. The resulting Nd magnet had a density of 7.5 g / cm. 3 That was all.

[0062] [Example 8] A compact was produced using the same procedures as in Examples 1 to 7. The resulting compact was introduced into a heat treatment furnace and subjected to a first heat treatment in an Ar gas atmosphere, where the temperature was raised to 200°C at 2°C / min. Then, while maintaining the temperature at 200°C, hydrogen gas was flowed at a flow rate of 4% / min relative to the volume of the heat treatment furnace for 6 hours, while maintaining the furnace pressure at 120 kPa. Then, a second heat treatment was performed, where the temperature was raised to 500°C at 2°C / min. The atmosphere was then switched to a vacuum, and the temperature was raised to 1060°C at 5°C / min, followed by a third heat treatment in a vacuum for 5 hours, whereby a Nd magnet material was obtained. The density of the resulting Nd magnet was 7.5 g / cm. 3 That was all.

[0063] [Comparative Examples 1 to 4] A compact was produced using the same procedures as in Examples 1 to 7. The resulting compact was introduced into a heat treatment furnace alone, without being mixed with a hydrogen storage alloy, and subjected to a first heat treatment in which the temperature was raised to 200°C in an Ar gas atmosphere; a second heat treatment in which the temperature was raised from 200°C to 500°C in a gas atmosphere containing hydrogen generated only from the compact; and a third heat treatment in which the temperature was then raised in a vacuum and vacuum heat treated for 5 hours at 1020°C to 1060°C (a temperature selected for each sample at which sufficient densification by sintering occurs), thereby obtaining a Nd magnet material. The hydrogen partial pressure generated during the second heat treatment was determined by the amount of compact introduced, and the hydrogen partial pressure at that time is shown in Table 1. The density of the resulting Nd magnet was 7.5 g / cm. 3 That was all.

[0064] Next, to determine the average crystal grain size of the magnets obtained in the examples and comparative examples, first, a cross section of the sintered magnet parallel to the magnetization direction was polished to a mirror finish, then immersed in a mixed solution of glycerin:nitric acid:hydrochloric acid = 3:1:2 to selectively etch the grain boundary phase of the cross section. Then, 25 cross-sectional images of an 85 × 85 μm area were obtained using a 3D measuring laser microscope (Olympus Corporation, LEXT OLS 4000). Based on the obtained cross-sectional images, the cross-sectional area of ​​each particle was measured using an image analyzer (Mitani Corporation, Win ROOF). A histogram of the crystal grain area fraction was created from the diameter of each particle calculated as the equivalent circle diameter, and the area median diameter was calculated by fitting with a Gaussian function. The oxygen concentration of the sintered magnets was measured using inert gas fusion infrared absorption spectroscopy, the nitrogen concentration using inert gas fusion thermal conductivity spectroscopy, and the carbon concentration using combustion infrared absorption spectroscopy.

[0065] Next, the magnets obtained in the examples and comparative examples were machined to L 17 mm x W 19 mm x T 2.2 mm, and a sputtering device was used to deposit 1 wt% Dy on the two LW faces perpendicular to the magnetization direction. The magnets were then held in a vacuum atmosphere at 900°C for 20 hours for grain boundary diffusion treatment, and then slowly cooled to 300°C. The temperature was then raised to 450°C, held there for 2 hours, and then rapidly cooled to 300°C. Each of the two LW faces of the magnets thus obtained was ground by 0.1 μm, machined to a L 7 mm x W 7 mm x T 2 mm shape, and the magnetic properties were measured using a pulse tracer. The H caused by the grain boundary diffusion treatment was confirmed. cJ Growth ΔH cJ The composition of the ROCN phase was calculated by measuring the central portion of the ROCN phase at 10 locations 200 μm deep from the diffusion surface using WDS and calculating the average value. The results are shown in Tables 1 and 2. The ratios of O, C, and N elements in the ROCN phase of each magnet are also shown as triangular graphs in Figure 1.

[0066] [Table 1]

[0067] [Table 2]

[0068] When Examples 1 to 8 are compared with Comparative Examples 1 to 4, it can be seen that in Examples 1 to 8, in which the lubricant was mixed with the hydrogen storage alloy, the lubricant removal reaction proceeded more remarkably, with [C] / ([O]+[C]+[N])≦0.3 and [O] / ([O]+[C]+[N])≧0.4. cJ Growth ΔH cJ In Comparative Examples 1 to 3, [O] / ([O]+[C]+[N])<0.4, and in Comparative Examples 2 to 4, [C] / ([O]+[C]+[N])>0.3. cJ However, all of the values ​​were less than 7.0 kOe, which were inferior to Examples 1 to 8.

[0069] As shown in FIG. 1, the magnets of Examples 1 to 8 have ratios of O, C, and N elements in the ROCN phase within the ranges specified by the present invention, which increases the utilization efficiency of the heavy rare earth elements through diffusion. As shown in Table 1, cJ becomes a large magnet.

Claims

1. R (R is one or more elements selected from rare earth elements, and Nd is essential), T (T is one or more elements selected from Fe and Co, and Fe is essential), B, M (M is one or more elements selected from Cu, Ga, Al, Si, Cr, Mn, Zn, Ge, Mo, Sn, W, Pb, and Bi), O, C, N, and unavoidable impurities, and R 2 T 14 An R-T-B based rare earth sintered magnet containing a main phase which is a B intermetallic compound and a grain boundary phase, The R-T-B based rare earth sintered magnet is characterized in that the grain boundary phase has an R-O-C-N phase with higher R, O, C, and N concentrations than the main phase, and when the atomic ratio of O in the R-O-C-N phase is [O], the atomic ratio of C is [C], and the atomic ratio of N is [N], the value of the following relational expression (1) is 0.4 or more and 1.0 or less, and the value of the following relational expression (2) is 0 or more and 0.3 or less. [O] / ([O]+[C]+[N])...(1) [C] / ([O]+[C]+[N])...(2)

2. 2. The RTB rare earth sintered magnet according to claim 1, wherein the value of the following relational expression (5) is 0.4 or less: [N] / ([O]+[C]+[N])...(5)

3. The R-T-B rare earth sintered magnet according to claim 1 or 2, characterized in that, when the atomic ratio of R in the R-O-C-N phase is [R], the value of the following relational expression (3) is 0.8 or more and 1.5 or less: ([O]+[C]+[N]) / [R]...(3)

4. 3. The RTB based rare earth sintered magnet according to claim 1, wherein the O content is 0.15 mass % or less.

5. 3. The RTB rare earth sintered magnet according to claim 1, wherein the average crystal grain size in a plane parallel to the magnetization direction is 3 μm or less.

6. 3. The R-T-B rare earth sintered magnet according to claim 1, wherein the magnet contains an element R' (R' is one or more elements selected from Dy and Tb) introduced by grain boundary diffusion, and wherein, within at least 500 μm from the surface of the sintered magnet, at least a portion of the surface vicinity of the main phase particles has a region with a higher concentration of Dy, Tb, or both, than the center of the main phase particles.

7. 3. The R-T-B rare earth sintered magnet according to claim 1, wherein R is free of Dy and Tb and contains R' (R' is one or more elements selected from Dy and Tb) that is an element introduced by grain boundary diffusion, and wherein the R-O-C-N phase contains R', and where the concentration of R' in the R-O-C-N phase is [R'], the value of the following relational expression (4) is greater than 0 and 0.1 or less: [R'] / [R]...(4)

8. a melting step of obtaining a raw material alloy having a predetermined composition containing the R, T, B, and M; a coarse crushing step of coarsely crushing the raw material alloy to obtain a coarse powder; a fine crushing step of finely crushing the coarse powder to obtain a fine powder; a compacting step of compacting the fine powder in a magnetic field to obtain a compact; and a sintering step of heat-treating the compact to obtain a sintered body, The coarse pulverization step is hydrogen pulverization in which hydrogen is absorbed into the raw alloy and the raw alloy is coarsely pulverized, the sintering step includes a first heat treatment, a second heat treatment, and a third heat treatment; the first heat treatment is a heat treatment in which the temperature is increased from room temperature to a predetermined first heat treatment temperature in an inert gas atmosphere, the second heat treatment is a heat treatment in which, after the first heat treatment, the temperature is raised to a second heat treatment temperature, which is a predetermined temperature higher than the first heat treatment temperature and lower than a sintering temperature, in a gas atmosphere containing hydrogen released from a hydrogen generating source other than hydrogen released from the compact; a third heat treatment, which is a heat treatment performed after the second heat treatment, in which the temperature is raised to a sintering temperature exceeding the second heat treatment temperature in a vacuum atmosphere, and the temperature is maintained at the sintering temperature for a predetermined period of time.

9. 9. The method for producing a rare earth sintered magnet according to claim 8, wherein the hydrogenation pulverization does not involve dehydrogenation.

10. 10. The method for producing a rare earth sintered magnet according to claim 8, wherein hydrogen gas is introduced into the atmosphere during the second heat treatment.

11. 10. The method for producing a rare earth sintered magnet according to claim 8, wherein the temperature of the first heat treatment is 150°C or higher and 350°C or lower.

12. 10. The method for producing a rare earth sintered magnet according to claim 8, wherein the sintering step comprises heat treating the compact and a hydrogen storage alloy together.

13. 13. The method for producing a rare earth sintered magnet according to claim 12, wherein the hydrogen storage alloy has a hydrogen release temperature of 150° C. or higher and 350° C. or lower in an atmospheric pressure atmosphere.

14. 14. The method for producing a rare earth sintered magnet according to claim 12, wherein the hydrogen storage alloy is an alloy in which 0.1 mass % or more of hydrogen is stored in the raw material alloy.

15. 10. The method for producing a rare earth sintered magnet according to claim 8, wherein the fine pulverization step comprises adding a lubricant to the coarse powder and pulverizing the coarse powder, and the amount of the lubricant added is 0.1 to 0.5% by mass relative to 100% by mass of the coarsely pulverized powder.

16. 10. The method for producing a rare earth sintered magnet according to claim 8, wherein the second heat treatment temperature is 400°C or higher and 600°C or lower.

17. 10. The method for producing a rare earth sintered magnet according to claim 8, wherein the hydrogen partial pressure in the heat treatment furnace during the second heat treatment is 20 kPa to 200 kPa.

18. 10. The method for producing a rare earth sintered magnet according to claim 8, further comprising a grain boundary diffusion treatment step of providing a diffusion source on the surface of the obtained sintered body and diffusing the diffusion source into the sintered magnet by heat treatment in a vacuum or inert gas atmosphere.

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