Rare earth sintered magnet

Abstract

The present invention provides a rare earth sintered magnet which is characterized by satisfying the relational expression (1): [RGB] = [R] - [Fe] / 7, the relational expression (2): [M1] = [Co] + [Cu] + [Ga], the relational expression (3): 1.0 < [RGB] / [M1] < 2.4, and the relational expression (4): [Cu] + [Ga] ≥ 0.28, where [R], [Fe], [Co], [Cu], [Ga], and [M1] are the respective atomic percentages of R (one or more rare earth elements essentially including Nd), Fe, Co, Cu, Ga, and M1 (two or more elements selected from among Co, Cu, and Ga, and essentially including Co), and [RGB] is the atomic percentage of R contributing to the formation of the grain boundary phase, in an arbitrary region within the depth range of 0.1-200 µm from the surface of the magnet. As a result, a rare earth sintered magnet which has high magnetic characteristics and excellent corrosion resistance can be obtained.

Description

rare earth sintered magnet

[0001] The present invention relates to a rare earth sintered magnet that has high magnetic properties and excellent corrosion resistance.

[0002] Rare earth sintered magnets are functional materials that are 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 (hereinafter referred to as B r They are used in a variety of applications and environments, such as drive motors for hybrid and electric vehicles, motors for electric power steering, motors for air conditioner compressors, and voice coil motors (VCMs) for hard disk drives.

[0003] On the other hand, rare earth sintered magnets suffer from the problem of being susceptible to corrosion due to their content of the rare earth element R. It is known that corrosion begins in certain R-rich phases and progresses as the main phase is removed. In response to this, in recent years, high magnetic properties have been required for rare earth magnets, making it necessary to minimize the amount of impurities contained in the magnets. This has resulted in an increase in the effective amount of R contained in rare earth sintered magnets, which tends to reduce their corrosion resistance. In particular, when used in automotive water pumps, the magnets built into the motor are used in harsh environments where they come into direct contact with antifreeze, so it is necessary to achieve both high magnetic properties and high corrosion resistance. Various methods have been proposed to achieve this.

[0004] For example, Japanese Patent Application Laid-Open No. 2007-300791 (Patent Document 1) proposes a method of forming an electrolytic Ni plating film on the surface of a magnet to impart high corrosion resistance that enables use in water.

[0005] Furthermore, Japanese Patent Application Laid-Open No. 2020-102551 (Patent Document 2) describes that corrosion resistance in a water pump is improved by having Cu present at the grain boundaries so that there is a concentration gradient that decreases from the magnet surface toward the interior, and by providing an oxide layer at the grain boundaries in a region 0.1 to 5 μm deep from the surface.

[0006] Furthermore, Japanese Patent Application Laid-Open No. 2021-61301 (Patent Document 3) describes that high corrosion resistance can be obtained by forming a structure in which the rare earth element content of the entire grain boundary phase of the magnet is 55 mass% or more, and Cu-rich regions containing 8 mass% or more of Cu account for 9 volume% or more of the grain boundary phase.

[0007] JP 2007-300791 A JP 2020-102551 A JP 2021-61301 A

[0008] However, when the method of forming a Ni plating layer as described in Patent Document 1 is adopted, the plating step increases costs, and there is a concern that if there is a defect in the plating, the magnet may disintegrate due to direct exposure to the usage environment.

[0009] Furthermore, in the method described in Patent Document 2, it is necessary to diffuse the Cu-containing R alloy into grain boundaries in order to form a predetermined structure, and also to form a surface oxide layer, which increases the number of steps and leads to an increase in costs.

[0010] Furthermore, in the method described in Patent Document 3, it is necessary to form a grain boundary phase containing rare earth elements including heavy rare earth elements, Cu, and Fe or T in which part of Fe is substituted with Co. However, it is preferable not to contain heavy rare earth elements, since these elements have a high resource risk and are expensive.

[0011] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a rare earth sintered magnet that has high magnetic properties and excellent corrosion resistance by forming a specific composition region on the magnet surface.

[0012] In order to achieve the above-mentioned object, the inventors conducted extensive research focusing on the composition of rare earth sintered magnets, in particular the relationship between the content of R elements and M elements (Co, Cu, Ga), which contribute to the formation of grain boundary phases in specific regions near the magnet surface. As a result, they discovered that by reducing the R content or adding a certain amount or more of the M element, R and M in the grain boundary phase form intermetallic compounds, thereby eliminating the R metal phase, thereby improving corrosion resistance, and thus completed the present invention.

[0013] Therefore, the present invention provides the following rare earth sintered magnet: 1. A rare earth sintered magnet containing R (R is one or more elements selected from rare earth elements, and Nd is essential), Fe, B, M1 (M1 is two or more elements selected from Co, Cu, and Ga, and Co is essential), M2 (M2 is one or more elements selected from Al, Si, Cr, Mn, Zn, Ge, Mo, Sn, W, Pb, and Bi), and O, wherein the O content is 0.6 atomic % or less, and R2Fe 14 and a grain boundary phase containing R, the grain boundary phase being located between at least two of the main phase particles, wherein the atomic percentages of R, Fe, Co, Cu, Ga, and M1 are set to [R], [Fe], [Co], [Cu], [Ga], and [M1], respectively, in at least a portion of a range of 0.1 to 200 μm deep from the magnet surface, and the atomic percentage of R contributing to the formation of the grain boundary phase is set to [R GB ], the rare earth sintered magnet has a region that satisfies the following relational expressions (1) to (4). GB ]=[R]-[Fe] / 7...(1) [M1]=[Co]+[Cu]+[Ga]...(2) 1.0<[R GB ] / [M1]<2.4 (3) [Cu]+[Ga]≧0.28 (4) 2. Let S be the surface area of ​​the magnet, and let S be the area obtained by projecting the region that satisfies the above relations (1) to (4) within a depth range of 0.1 to 200 μm from the magnet surface onto the adjacent magnet surface. R 1. A rare earth sintered magnet according to claim 1, characterized in that the following relational expression (5) is satisfied when: 0.53≦S R / S≦1 (5) 3. The rare earth sintered magnet of 1 or 2, characterized in that the content of R metal phase in the grain boundary phase, where the content of R is 50 to 70 atomic %, is 3 volume % or less. 4. The rare earth sintered magnet of any of 1 to 3, characterized in that it contains 0.1 to 2.0 atomic % of M3 (M3 is one or more elements selected from Ti, V, Zr, Nb, Hf, and Ta). 5. The rare earth sintered magnet of 4, characterized in that the following relational expression (6) is satisfied, where [B] is the atomic percentage of B and [M3] is the atomic percentage of M3: [B] - 2 × [M3] ≧ [Fe] / 14 (6) 6. The rare earth sintered magnet of any of 1 to 5, characterized in that the C content is 0.4 atomic % or less. 7. The rare earth sintered magnet of any of 1 to 6, characterized in that the N content is 0.4 atomic % or less. 8. 8. A rare earth sintered magnet according to any one of 1 to 7, wherein the average crystal grain size of the main phase particles, determined as the equivalent circle diameter in a plane parallel to the magnetization direction, is 4 μm or less. 9. A rare earth sintered magnet according to any one of 1 to 8, wherein the weight loss rate measured by the following method is 1% or less. [Measurement method] A sample cut into a rectangular parallelepiped shape of 5 mm x 5 mm x 2 mm is immersed in an ethylene glycol-water mixture of ethylene glycol:water = 1:1 (volume ratio), and held in a sealed state at 120°C for 480 hours, and the weight loss rate of the sample is measured.

[0014] According to the present invention, it is possible to provide a rare earth sintered magnet that is excellent in terms of cost because it is possible to impart high corrosion resistance without forming a protective film through a post-treatment process and does not necessarily require a grain boundary diffusion process using a heavy rare earth element.

[0015] [R GB ] / [M1] and the weight loss rate.

[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), Fe, B, M1 (M1 is two or more elements selected from Co, Cu, and Ga, and Co is essential), M2 (M2 is one or more elements selected from Al, Si, Cr, Mn, Zn, Ge, Mo, Sn, W, Pb, and Bi), and O.

[0017] As mentioned above, R is one or more elements selected from rare earth elements, and Nd is essential. The R content is preferably 12.5 atomic % or more, and more preferably 13.0 atomic % or more, from the viewpoint of suppressing the crystallization of α-Fe in the raw material alloy during production and achieving sufficient densification. Although it is difficult to eliminate α-Fe even by homogenization, within the above range, the coercive force (hereinafter referred to as H) of the R-T-B based sintered magnet can be reduced. cJ This can prevent a significant decrease in the R-T-B based sintered magnet's densification due to a decrease in the amount of liquid phase, which is mainly composed of the R component and plays a role in promoting densification during the sintering process described below. 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 the deterioration, the content of R is preferably 17 atomic % or less, more preferably 15.5 atomic % or less, and even more preferably 15 atomic % or less.

[0018] The proportion of Nd in R is not particularly limited, but is preferably 60 atomic % or more, more preferably 75 atomic % 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] The content of Fe is the balance other than the R, B, M1, M2, and O. r From the viewpoint of obtaining R2T, the content of Fe is preferably 70 atomic % or more, and more preferably 75 atomic % or more. 17 Deterioration of squareness due to precipitation of H cJ From the viewpoint of suppressing a decrease in the content, the content is preferably 82 atomic % or less, and more preferably 80 atomic % or less.

[0020] The B content is preferably 5.0 atomic % or more, more preferably 5.5 atomic % or more, and even more preferably 5.7 atomic % 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 7.0 atomic % or less, more preferably 6.5 atomic % or less, and even more preferably 6.3 atomic % or less. Within such a range, the R 1.1 The T4B4 compound phase is formed and B r and H cJ The above B may be partially substituted with C.

[0021] As described above, M1 is two or more elements selected from Co, Cu, and Ga, with Co being essential. By including M1, high corrosion resistance can be obtained. The content of M1 is preferably 1.0 atomic % or more, more preferably 1.5 atomic % or more. By doing so, the formation of a compound between R and M1 is sufficiently carried out, the formation of a metallic R phase with a high R concentration can be suppressed, and corrosion resistance is improved. The upper limit is preferably 3.5 atomic % or less, more preferably 3.0 atomic % or less. By doing so, B r and H cJ This can suppress the decline in

[0022] In addition, the above Co is R2T 14 It can substitute for part of the Fe contained in the B phase and the grain boundary phase. From the viewpoint of obtaining the effect of improving the Curie temperature and corrosion resistance, the Co content is preferably 0.5 atomic % or more of the entire magnet, and more preferably 1.0 atomic % or more. cJ From the viewpoint of stably obtaining the above, the Co content is preferably 3.0 atomic % or less, and more preferably 2.0 atomic % or less.

[0023] Furthermore, the Cu content is preferably 0.1 atomic % or more of the entire magnet, and more preferably 0.2 atomic % or more, from the viewpoint of obtaining an optimum temperature range in the heat treatment to ensure good productivity and obtaining an effect of improving corrosion resistance. rFrom the viewpoint of stably obtaining the above, the Cu content is preferably 1.0 atomic % or less, and more preferably 0.5 atomic % or less.

[0024] Furthermore, the Ga content is preferably 0.1 atomic % or more, and more preferably 0.2 atomic % or more, of the entire magnet from the viewpoint of obtaining an optimum temperature range in the heat treatment to ensure good productivity and from the viewpoint of obtaining an effect of improving corrosion resistance. r From the viewpoint of stably obtaining the above, the Ga content is preferably 1.0 atomic % or less, and more preferably 0.5 atomic % or less.

[0025] As described above, M2 is one or more elements selected from Al, Si, Cr, Mn, Zn, Ge, Mo, Sn, W, Pb, and Bi. The content of M2 is determined from the viewpoint of obtaining an optimum temperature range in the heat treatment to ensure good productivity, and further, from the viewpoint of H cJ From the viewpoint of suppressing the decrease in B, the content is preferably 0.1 atomic % or more, and more preferably 0.2 atomic % or more. r From the viewpoint of obtaining the above, the content is preferably 1.5 atomic % or less, and more preferably 1.0 atomic % or less.

[0026] The O content is high at room temperature cJ From the viewpoint of obtaining the above, the content is 0.6 atomic % or less, preferably 0.4 atomic % or less, and more preferably 0.3 atomic % or less. If the content exceeds 0.6 atomic %, the amount of R that can function as a liquid phase decreases due to an increase in the amount of R-OCN phase or R2O3 phase formed. cJ decreases.

[0027] Furthermore, the rare earth sintered magnet of the present invention may contain optional elements other than the above-mentioned R, Fe, B, M1, M2, and O, such as M3, C, and N described below.

[0028] M3 is one or more elements selected from Ti, V, Zr, Nb, Hf, and Ta. In rare earth sintered magnets, the M3 element forms an M3B2 compound with B, which has the effect of suppressing abnormal grain growth during sintering. The content of M3 is not particularly limited, but is preferably 0.1 atomic % or more, more preferably 0.2 atomic % or more, and even more preferably 0.3 atomic % or more. On the other hand, B, which forms the main phase, is insufficient. r , H cJ From the viewpoint of suppressing the decrease in B content, the content is preferably 2.0 atomic % or less, more preferably 1.5 atomic % or less, and even more preferably 1.0 atomic % or less. Furthermore, from the above viewpoint, when the atomic percentages of B and M3 are [B] and [M3], it is preferable that the following relational expression (6) is satisfied: [B] - 2 × [M3] ≥ [Fe] / 14 (6)

[0029] The C mentioned above originates from the raw materials and lubricants added to improve the degree of orientation of fine powder during compaction in a magnetic field, but 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 some of the C remains without being completely removed and the amount is large, the amount of compounds formed with R, O, N, etc. increases, consuming R and H. cJ Furthermore, when heavy rare earth elements are diffused into grain boundaries, the diffusion of heavy rare earth elements into the magnet interior is inhibited, thereby preventing sufficient H cJ From this viewpoint, the C content is not particularly limited, but is preferably 0.4 atomic % or less, more preferably 0.3 atomic % or less, and even more preferably 0.2 atomic % or less.

[0030] The content of N is not particularly limited, but it is preferable to use a good H cJ From the viewpoint of obtaining the above, the content is preferably 0.4 atomic % or less, more preferably 0.3 atomic % or less, and even more preferably 0.2 atomic % or less.

[0031] The structure of the rare earth sintered magnet of the present invention includes R2T 14The rare earth sintered magnet of the present invention includes a B intermetallic compound as a main phase. It also includes a plurality of main phase particles and a grain boundary phase containing R, which is located between at least two of the main phase particles. The grain boundary phase may include an M3B2 phase or the like. Furthermore, the rare earth sintered magnet of the present invention may include an R-rich phase in the grain boundary phase, or may include a phase of compounds of unavoidable impurities that are mixed in during the manufacturing process, such as R carbide, R oxide, R nitride, R halide, or R oxyhalide. However, the B r and H cJ However, among the R-rich phases, the R metal phases having an R content in the range of 50% to 70% are highly active due to the high R concentration, and become the starting point of collapse by reacting with the solvent in the corrosion resistance test described later, so the proportion of the R metal phase in the entire grain boundary phase is preferably 3% by volume or less, and more preferably 1% by volume or less.

[0032] Furthermore, the rare earth sintered magnet of the present invention contains, in at least a portion of the region from the surface to a depth of 0.1 to 200 μm, an atomic percentage of M1 of [M1] and an atomic percentage of R that contributes to the formation of the grain boundary phase of [R GB ], the composition region (hereinafter referred to as a high corrosion resistance region) satisfies the following relational expression (3): 1.0<[R GB ] / [M1] < 2.4 ... (3)

[0033] In this case, it is difficult to correctly evaluate the composition in a region shallower than 0.1 μm from the magnet surface. On the other hand, the composition in a region deeper than 200 μm from the surface does not contribute to improving corrosion resistance. From the above perspective, the region having this specific composition is set to the range of 0.1 to 200 μm from the magnet surface. If the grain boundary phase is insufficient in the region near the magnet surface, [R GB ] decreases with [R GB ] / [M1] also decreased, resulting in good H cJ Furthermore, if an excess of M1 element is present in the above-mentioned region near the magnet surface, [R GB ] / [M1] decreases and B r and H cJ From the above viewpoint, [RGB On the other hand, if excess R is present in the region near the magnet surface, [R GB ] together with [R GB ] / [M1] increases, and the R metal phase with a high R concentration precipitates in that region, resulting in a decrease in corrosion resistance. GB ] / [M1] increases, and the amount of M1 that forms a compound with R is insufficient, causing the R metal phase with a high R concentration to precipitate, resulting in a decrease in corrosion resistance. GB ] / [M1] is less than 2.4, and more preferably less than 2.2.

[0034] In addition, the above [R GB ] and [M1] can be calculated as follows: That is, when the atomic percentages of R, Fe, Co, Cu, Ga, and M1 in the above region are [R], [Fe], [Co], [Cu], [Ga], and [M1], respectively, [R GB ] is calculated by the following relational expression (1), and [M1] is calculated by the following relational expression (2). GB ]=[R]-[Fe] / 7...(1) [M1]=[Co]+[Cu]+[Ga]...(2)

[0035] Furthermore, in the present invention, from the viewpoint of improving corrosion resistance by promoting the formation of compounds between R and M1 in the high corrosion-resistant region near the magnet surface and suppressing the formation of the R metal phase, the following relational expression (4) is further satisfied in the high corrosion-resistant region: [Cu] + [Ga] ≥ 0.28 (4) From the viewpoint of obtaining an optimal temperature range in heat treatment to ensure good productivity and obtaining an improved corrosion resistance effect, the value of [Cu] + [Ga] is 0.28 or more, and more preferably 0.35 or more. There is no particular upper limit, but a high B r From the viewpoint of stably obtaining a value, it is preferably 1.50 or less, and more preferably 1.00 or less.

[0036] Note that some of the R, Fe, Co, Cu, and Ga in the highly corrosion-resistant region near the magnet surface may be introduced by grain boundary diffusion.

[0037] In the rare earth sintered magnet of the present invention, the average composition of R, Fe, Co, Cu, and Ga in the highly corrosion-resistant region extending from the surface to a depth of 0.1 to 200 μm can be determined by using compositional analysis methods such as energy dispersive X-ray spectroscopy (EDS), wavelength dispersive X-ray analysis (WDS), and glow discharge mass spectrometry (GDMS) normalized by ICP analysis. On the other hand, if the surface area of ​​the magnet is sufficiently large, a 200 μm area can be cut out from each surface and the average composition determined by ICP analysis. Furthermore, if the rare earth sintered magnet does not have a change in the concentration distribution of each element from the surface to the interior of the magnet due to the application of grain boundary diffusion, the average composition of R, Fe, Co, Cu, and Ga in the highly corrosion-resistant region extending from the surface to a depth of 0.1 to 200 μm can be substituted for the content of the metal elements contained in the entire magnet determined by ICP analysis.

[0038] As described above, the rare earth sintered magnet of the present invention has a region (high corrosion-resistant region) whose average composition satisfies the above-mentioned relational expression (3) in at least a portion of the range of 0.1 to 200 μm deep from the surface. The rare earth sintered magnet of the present invention has improved corrosion resistance by having the above-mentioned high corrosion-resistant region near the magnet surface. However, since the larger the high corrosion-resistant region relative to the magnet surface area, the higher the corrosion resistance obtained, the surface area of ​​the magnet is defined as S, and the area obtained by projecting the high corrosion-resistant region onto the adjacent magnet surface is defined as S. R In this case, it is preferable that the following relational expression is satisfied: 0.53≦S R / S≦1 (5) where S R It is more preferable that / S is 0.6 or more. R When the highly corrosion-resistant region is formed by grain boundary diffusion, / S is at most 1 when various elements are supplied from the entire surface of the magnet, and when grain boundary diffusion is not performed, / S is at most 1 when the content of metal elements contained in the entire magnet satisfies relational expression (3). RTo simplify the calculation, for example, for a magnet in which grain boundary diffusion has been performed on only two opposing surfaces under the same conditions, the average composition of any region within a depth range of 0.1 to 200 μm from the surface of the diffusion surface can be treated as a representative value including the opposing diffusion surface. For the non-diffusion surface, the ICP analysis composition of the substrate to which grain boundary diffusion treatment is performed can be used as a representative value.

[0039] S R The method for determining the above is not particularly limited, but it is preferable to use WDS (Wavelength Dispersive X-ray Spectroscopy) from the viewpoint of simplicity. The beam diameter during WDS measurement is preferably sufficiently larger than the average crystal grain size of the magnet to be measured, for example, 30 μm or more, more preferably 50 μm or more, in order to obtain the average composition of the structure including multiple main phases, grain boundary phases, and grain boundary triple junctions. In addition, to simplify the analysis, it is preferable to use a beam diameter of 30 μm or more, for example, 50 μm or more, for the magnet surface area of ​​4 mm 2 Preferably, one point in the range of 0.1 to 200 μm in depth from the surface is selected, and the magnet surface area is 1 mm 2 It is also possible to measure one point within a range of 0.1 to 200 μm deep from the surface and use the result as a representative value. In other words, the magnet can be divided into 2 mm x 2 mm x 2 mm to 1 mm x 1 mm x 1 mm by cutting or other processing, and the divided magnet can be analyzed to determine the S R In this case, the area lost due to cutting or the like can be subtracted from the magnet surface area S in advance.

[0040] Furthermore, the corrosion resistance of the present invention can be evaluated by the following method. A rectangular magnet measuring 5 mm x 5 mm x 2 mm is cut from the prepared magnet, weighed, and then immersed in an ethylene glycol-water mixture of ethylene glycol:water = 1:1 (volume ratio), sealed, and left to stand at 120°C for 480 hours. The magnet is then removed, degreased with ethanol, ultrasonically cleaned in water, dried, and weighed to obtain the post-test weight. The weight loss rate due to the corrosion resistance test obtained by the above method is preferably 1% or less, and more preferably 0.5% or less, from the viewpoint of maintaining good magnetic properties even after the test.

[0041] Furthermore, the smaller the average crystal grain size (μm) in the plane parallel to the magnetization direction of the rare earth sintered magnet of the present invention, the smaller the effect of weight loss due to shedding of the main phase that occurs with the collapse of the grain boundary phase in a corrosion resistance test, improving corrosion resistance. cJ From the viewpoint of obtaining a satisfactory degree of orientation within an appropriate range of lubricant addition, the average crystal grain size (μm) is preferably 4 μm or less, and more preferably 3.5 μm or less. Furthermore, from the viewpoint of obtaining a sufficient degree of orientation within an appropriate range of lubricant addition, the lower limit of this average crystal grain size (μm) is preferably 1.2 μm or more, and more preferably 1.8 μm or more. In the present invention, the average crystal grain size 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 for each average particle spacing of 1 μm for each circle-equivalent diameter of individual particles. 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 Villela'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, based on the obtained observation image, the cross-sectional area of ​​each particle is measured by image analysis, and the diameter as an equivalent circle is calculated. The average crystal grain size may be, for example, the average of a total of about 2,000 particles in 20 different images. The measurement device is not particularly limited, but for example, a 3D measuring laser microscope (Olympus Corporation, LEXT OLS 4000) can be used, and image analysis software (Mitani Corporation, Win ROOF) can be used for image analysis.

[0042] Furthermore, to achieve high magnetic properties, the rare earth sintered magnet of the present invention is preferably produced using a process that minimizes the amount of impurity elements contained. In this case, compound formation between the impurity elements and R is not promoted, and an R metal phase with a high R concentration is likely to form. This R metal phase has a low corrosion potential and is therefore likely to serve as a starting point for corrosion, resulting in weight loss and a deterioration in magnetic properties. Methods for suppressing the formation of the R metal phase include reducing the R content and adding an element other than the impurity element that easily forms a compound with R. In the latter method, the element added is the aforementioned M1, and by adjusting the content of this M1 within the appropriate range as described above, it is possible to achieve high magnetic properties and excellent corrosion resistance.

[0043] Next, a method for producing the rare earth sintered magnet of the present invention will be described below. The process for producing the rare earth sintered magnet of the present invention includes a melting step in which raw materials are melted to obtain a raw material alloy having a predetermined composition, a crushing step in which the raw material alloy is crushed to prepare a fine alloy powder, a compacting step in which the fine alloy powder is compressed in an applied magnetic field to obtain a green body, and a sintering step in which the green body is heat-treated to obtain a sintered body.

[0044] First, in the melting process, metals or alloys that are raw materials for each element are weighed to obtain a predetermined composition. 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 in which the alloy is poured into a flat mold or a book mold, or by a strip casting method. In addition, the main phase RFe 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 additive at the sintering temperature are separately prepared, and then weighed and mixed after coarse grinding. However, since an alloy close to the main phase composition is prone to crystallization of the α-Fe phase depending on the cooling rate and alloy composition during casting, it is preferable to perform a homogenization treatment for at least one hour at 700 to 1200°C in a vacuum or Ar atmosphere as necessary 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 additive.

[0045] The above-mentioned pulverization process is a multi-stage process including at least a coarse pulverization process and a fine pulverization process. The coarse pulverization process can be performed using any suitable method, such as a jaw crusher, a Braun mill, a pin mill, or hydrogen pulverization. However, in the present invention, from the viewpoint of reducing the amounts of O, N, and C and obtaining excellent magnetic properties, it is preferable to include a hydrogen pulverization process as at least one step of the coarse pulverization process. This hydrogen pulverization process is a hydrogen absorption pulverization process in which the alloy ingot is exposed to a hydrogen atmosphere above a certain pressure, causing the alloy to absorb hydrogen. The hydrogen pressure at this time is not particularly limited, but is preferably 100 kPa or higher to reduce the adverse effects on productivity due to the long hydrogen absorption time. Dehydrogenation treatment may be performed as needed. After the hydrogen pulverization process, the heated alloy ingot is cooled and transported to the next process. At this time, it is preferable to cool it to near room temperature to prevent oxidation. This hydrogen pulverization process can produce coarse powder typically pulverized to 0.05 mm to 3 mm, particularly 0.05 mm to 1.5 mm.

[0046] The fine pulverization step can involve pulverizing the coarse powder obtained in the coarse pulverization step 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 4.0 μm, and even more preferably 1.5 to 3.5 μm. Because the O and N in rare earth sintered magnets are primarily introduced during the fine pulverization step, controlling the jet mill atmosphere is necessary to adjust the O and N content in the rare earth sintered magnets. For example, the O content in the rare earth sintered magnet is adjusted by controlling the O content 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%.

[0047] The N content in the rare earth sintered magnet can be adjusted, for example, by (A) finely pulverizing the magnet using a jet mill with a He or Ar gas stream, (B) finely pulverizing the magnet by introducing hydrogen into a jet mill with an N gas stream, or (C) finely pulverizing hydrogen-containing coarse powder using a jet mill with an N gas stream. In these cases, with methods (B) and (C), introducing hydrogen gas or using hydrogen-containing coarse powder allows hydrogen to preferentially adsorb onto the active surfaces generated by pulverization, inhibiting nitrogen adsorption and thereby reducing the amount of N in the rare earth sintered magnet.

[0048] The hydrogen-containing coarse powder in method (C) can be prepared, for example, by omitting the dehydrogenation treatment in the hydrogen pulverization step, by absorbing hydrogen into the coarse powder after dehydrogenation treatment, or by absorbing hydrogen into the coarse powder obtained by mechanical pulverization in a He or Ar gas atmosphere. From the viewpoint of reducing the N content, the hydrogen content of the coarse powder or the fine powder after pulverization is preferably 0.1 to 1.0 mass%, more preferably 0.2 to 0.5 mass%. The hydrogen content of the coarse powder or the fine powder after pulverization can be determined by measuring the change in weight before and after hydrogen sorption.

[0049] Here, in the above-mentioned crushing step, a lubricant made of, for example, saturated fatty acid or its ester can be added as appropriate before or after the above-mentioned coarse crushing step in order to improve the orientation of the powder in the subsequent step of compacting in a magnetic field. Although adding a lubricant is effective in improving the orientation, the carbon derived from the lubricant can form many R-OCN phases in the rare earth sintered magnet, which can lead to H cJ This creates a dilemma. cJTo prevent this decrease, it is preferable to add a lubricant when using method (C) above. In rare earth sintered magnets produced by this method, when hydrogen contained inside the magnet is released during heat treatment, the hydrogen decomposes the lubricant chemically adsorbed on the powder surface through a carbonyl reduction reaction or the like, and the lubricant is further decomposed and dissociated into highly volatile lower alcohols through a cracking reaction with hydrogen gas, which is thought to reduce the residual C content in the rare earth sintered magnet. The amount of lubricant added is determined appropriately depending on the type of lubricant, and is not particularly limited, but is preferably 0.01 to 0.50 parts by mass, and more preferably 0.05 to 0.30 parts by mass, per 100 parts by mass of the coarsely pulverized powder or raw alloy.

[0050] 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 handleability, the molded body density is preferably 2.8 g / cm 3 It is preferable that the B value is equal to or greater than 1000. 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 value. r From the viewpoint of obtaining the above, the density of the compact is 4.2 g / cm 3 The following is preferable: In order to suppress oxidation of the alloy fine powder, the compaction is preferably carried out in an inert gas atmosphere such as nitrogen gas or Ar gas.

[0051] The sintering step is a step of sintering the compact obtained in the compacting step in an inert gas atmosphere such as Ar gas or in a high vacuum, and preferably includes an atmospheric heat treatment step in which heat treatment is performed in an inert gas atmosphere and a vacuum heat treatment step in which heat treatment is performed in a vacuum atmosphere. When using coarse powder containing hydrogen by the method (C) above, in order to suppress the temperature drop of the compact due to the release of hydrogen gas in the compact (endothermic reaction) and the occurrence of cracks caused by the temperature difference, the atmospheric heat treatment step is preferably held in an inert gas atmosphere at 300 to 600 °C for 1 to 10 hours, although not particularly limited. Thereafter, the vacuum heat treatment step is preferably held in a high vacuum at 950 to 1200 °C for 0.5 to 10 hours, although not particularly limited.

[0052] In the present invention, after the atmospheric heat treatment step and the vacuum heat treatment step in the sintering step are sequentially performed, the obtained sintered body is then subjected to H cJ In order to increase the sintering temperature, a heat treatment or a grain boundary diffusion step may be carried out at a temperature lower than the sintering temperature. The heat treatment at a temperature lower than the sintering temperature may be a two-stage heat treatment consisting of a high-temperature heat treatment and a low-temperature heat treatment, or only a low-temperature heat treatment may be carried out. 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, the sintered body is preferably heat-treated at a temperature of 400 to 600°C.

[0053] The grain boundary diffusion step is carried out by cJ The method may be carried out for the purpose of increasing the corrosion resistance and forming a highly corrosion-resistant region, and 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.

[0054] 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 R2 (R2 is 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 added. cJ Furthermore, by including M1 (wherein M1 is one or more elements selected from Co, Cu, and Ga), the corrosion resistance of the magnet can be improved.

[0055] 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 by hot air or vacuum, or naturally dried. Alternatively, for example, the diffusion source powder may be mixed with a resin slurry, stirred, and then formed into a sheet, which may be attached to the sintered body.

[0056] The subsequent diffusion heat treatment step preferably involves conditions under which R2 is concentrated near the grain boundaries within the magnet's main phase to achieve a sufficient increase in coercivity, or conditions under which M1 is concentrated within a range of 0.1 to 200 μm below the magnet surface to satisfy the aforementioned relations (3) and (5) to enhance corrosion resistance. While specific conditions are not particularly limited, heating to temperatures above 400°C, particularly 500°C or higher, and below 1100°C, particularly 1050°C, and especially 1000°C or lower is preferred to diffuse the R2 and / or M1 elements into the sintered compact. The heat treatment time is preferably 1 minute to 50 hours to avoid alteration of the sintered compact's structure and the adverse effects of inevitable oxidation and evaporation of components on its magnetic properties and corrosion resistance. Furthermore, the diffusion source is preferably applied to at least 53% of the magnet's total surface area so that the area of ​​the highly corrosion-resistant region satisfies relation (5).

[0057] 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.

[0058] Examples 1-3, Comparative Examples 1-3: 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 in predetermined proportions, melted, and strip-cast to obtain flaked alloy starting materials with thicknesses of 0.2-0.4 mm. The resulting flaked alloy starting materials were subjected to hydrogen embrittlement in a pressurized hydrogen atmosphere to obtain coarsely pulverized powder. Next, 0.1 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 gas stream using an airflow pulverizer (jet mill) to obtain a finely pulverized powder (alloy powder) with a pulverized particle size (D50) of 2.9 μm. The pulverized particle size (D50) was the volume-based median diameter obtained by laser diffraction analysis using an airflow dispersion method. This finely pulverized powder was filled into a 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 pressure-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 The resulting compact was subjected to atmospheric heat treatment in an Ar gas atmosphere at a temperature of 500°C, and then sintered in a vacuum at 1040-1080°C (a temperature selected for each sample at which densification by sintering would occur sufficiently) for 5 hours to obtain a Nd magnet material.

[0059] The density of the resulting Nd magnet was 7.5 g / cm 3 The results were as above. Furthermore, the central part of the obtained magnet was subjected to metal component analysis using inductively coupled plasma optical emission spectroscopy (ICP-OES), and the oxygen concentration was measured by inert gas fusion infrared absorption spectroscopy, the nitrogen concentration by inert gas fusion thermal conductivity spectroscopy, and the carbon concentration by combustion infrared absorption spectroscopy. The results are shown in Table 1. The values ​​shown in Table 1 are in atomic percentage.

[0060] The structure of the obtained magnet was also observed using an EPMA (Electron Probe Micro Analyzer), and the R metal phase was identified from the backscattered electron image and semi-quantitative analysis results. The proportion of the R metal phase in the entire grain boundary phase of the magnet was measured using image processing. The results are shown in Table 2.

[0061] Furthermore, rectangular parallelepiped test pieces measuring 5 mm L x 5 mm W x 2 mm T (magnetization direction) were cut from the resulting magnets and evaluated for corrosion resistance. For the evaluation, the cut magnets were immersed in an ethylene glycol-water mixture (ethylene glycol:water = 1:1 by volume) simulating antifreeze, sealed, and held at 120°C. After 480 hours, the samples were removed and weighed, and the weight loss rate was calculated by comparing it with the weight before immersion. The results are shown in Table 2.

[0062] The obtained magnet was cut into a rectangular parallelepiped shape with dimensions of L 15 mm x W 7 mm x T (magnetization direction) 12 mm, and the BH tracer was used to measure the B r , H cJ The results are shown in Table 2. Table 2 also shows the calculated values ​​of the relational expressions (3) and (4) defined in the present invention. Note that in Examples 1 to 3 and Comparative Examples 1 to 3, grain boundary diffusion treatment was not performed, and it is presumed that there is no change in concentration distribution from the magnet surface to the interior. Therefore, the average composition of R, Fe, Co, Cu, and Ga in the range of 0.1 to 200 μm was substituted with the content of metal elements contained in the entire magnet as determined by the ICP-OES analysis described above, and this region was defined as the entire magnet surface.

[0063] Furthermore, the average crystal grain size [μm] of the obtained magnets was measured using a laser microscope, and was found to be 3.5 μm for all of Examples 1 to 3 and Comparative Examples 1 to 3.

[0064]

[0065]

[0066] When Examples 1 to 3 and Comparative Examples 1 to 3 are compared, when the Nd content is low, the condition of formula (3) is satisfied, the R metal phase is not observed in the grain boundary phase, and almost no weight loss is observed. On the other hand, when the Nd content is high, the condition of formula (3) is no longer satisfied, the R metal phase is observed in the grain boundary phase, and the weight loss rate is also high. This is thought to be because the corrosion potential of the R metal phase is low, and corrosion progresses easily in the ethylene glycol-water mixture.

[0067] Examples 4-6, Comparative Examples 4-6 Magnets were produced using the same procedure as in Example 1. Metallic component analysis was performed on each magnet using ICP-OES. The oxygen concentration 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. The results are shown in Table 3. The proportion of R metal phase and the weight loss rate during corrosion resistance evaluation tests were calculated in the same manner as in Example 1. The results are shown in Table 4. Table 4 also shows the calculated values ​​of the relational expressions (3) and (4) defined in the present invention. Note that, since grain boundary diffusion treatment was not performed in Examples 4-6 and Comparative Examples 4-6, it is assumed that there is no change in the concentration distribution from the magnet surface to the interior. Therefore, the average composition of R, Fe, Co, Cu, and Ga in the range of 0.1 to 200 μm deep from the surface was substituted with the content of the metal elements contained in the entire magnet as determined by the ICP-OES analysis. This region was defined as the entire magnet surface.

[0068] The average crystal grain size [μm] of the obtained magnets was measured using a laser microscope, and was found to be 3.5 μm for all of Examples 4 to 6 and Comparative Examples 4 to 6.

[0069]

[0070]

[0071] When Examples 4 to 6 and Comparative Examples 4 to 6 are compared, when the condition of formula (3) is satisfied, the R metal phase is not confirmed, and there is almost no weight loss, and a high B r and H cJ On the other hand, [R GB When the value of [R] / [M1] is 2.4 or more (Comparative Example 4), the proportion of the R metal phase is high and the weight loss rate is also high, resulting in a loss of corrosion resistance. GB When the value of ] / [M1] is 1.0 or less (Comparative Examples 5 and 6), H cJ It can be seen that is decreasing.

[0072] Examples 7-12, Comparative Example 7 Magnets were fabricated using the same procedure as in Example 1. Metallic component analysis was performed on each magnet using ICP-OES. The oxygen concentration 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. The results are shown in Table 5. The proportion of R metal phase and the weight loss rate during corrosion resistance evaluation tests were calculated in the same manner as in Example 1. The results are shown in Table 6. Table 6 also shows the calculated values ​​of the relational expressions (3) and (4) defined in the present invention. Note that, since grain boundary diffusion treatment was not performed in Examples 7-12 and Comparative Example 7, it is assumed that there is no change in the concentration distribution from the magnet surface to the interior. Therefore, the average composition of R, Fe, Co, Cu, and Ga in the range of 0.1 to 200 μm deep from the surface was substituted with the content of the metal elements contained in the entire magnet as determined by the ICP-OES analysis. This region was defined as the entire magnet surface.

[0073] The average crystal grain size [μm] of the obtained magnets was measured using a laser microscope, and was found to be 3.5 μm for all of Examples 7 to 12 and Comparative Example 7.

[0074]

[0075]

[0076] A comparison of Examples 7 to 12 with Comparative Example 7 reveals that all magnets satisfy the relationship of formula (3). However, Comparative Example 7, which does not satisfy the relationship of formula (4), has a high proportion of the R metal phase and a high weight loss rate. This is thought to be the result of the formation of compounds between R, Cu, and Ga not being promoted, and the formation of the R metal phase not being suppressed.

[0077] Examples 13 and 14: Tb metal, electrolytic iron, electrolytic cobalt, and Ga metal were weighed and mixed in an atomic ratio of 1:1:1, and these raw materials were melted in an arc melting furnace to obtain TbFeGa and TbFeCo alloy ingots. These alloy ingots were then heat-treated in a vacuum atmosphere at 1050°C for 10 hours. The heat-treated alloy ingots were pulverized in a ball mill to prepare alloy powder with a D50 of approximately 10 μm. The alloy powder was mixed with a water-soluble PVA resin slurry in a ratio of 7:3, stirred, molded into a 50 μm thick sheet, and dried to produce TbFeGa and TbFeCo alloy sheets.

[0078] Next, a magnet similar to Comparative Example 1 was produced. The resulting magnet was processed to L 17 mm x W 19 mm x T 2.2 mm, and a TbFeGa alloy sheet (Example 13) was attached to the LW surface perpendicular to the magnetization direction, and a TbFeCo alloy sheet (Example 14) was attached to two surfaces perpendicular to the magnetization direction. The magnet was then held in a vacuum atmosphere at 900 °C for 20 hours, and then slowly cooled to 300 °C. The temperature was then raised to 450 °C, held for 2 hours, and then rapidly cooled to 300 °C. To determine the average composition of the diffusion surface of the resulting magnet at a depth of 0.1 to 200 μm from the surface using wavelength dispersive X-ray analysis (WDS), measurements were taken at 10 locations using WDS with a beam diameter of 50 μm on the center of the substrate of the produced magnet that had not undergone diffusion treatment. The average analytical value of these values ​​and the correction value of the WDS measurement value were determined based on the ICP analytical value of the substrate. The results are shown in Table 7.

[0079] Next, to determine the average composition at a depth of 0.1 to 200 μm from the surface of the diffusion surface of the obtained magnet, measurements were taken at 10 locations at a depth of 125 μm from the diffusion surface using WDS under the condition of a beam diameter of 50 μm, and the results were averaged. Correction was then made based on the correction values ​​in Table 7 [R GB The ratio of the R metal phase to the weight loss rate in the corrosion resistance evaluation test was calculated in the same manner as in Example 1. The results are shown in Table 8. R Regarding the calculation method for / S, S is calculated by calculating the surface area of ​​a rectangular parallelepiped of T2.2 mm at the time of grain boundary diffusion and L5 mm x W5 mm, which is the shape cut out for corrosion resistance evaluation, and S RThe measurement results for Comparative Example 1 shown in Table 2 above are also included in Table 8.

[0080] The average crystal grain size [μm] of the magnets obtained was measured using a laser microscope and was found to be 3.5 μm for both Examples 13 and 14.

[0081]

[0082]

[0083] When Comparative Example 1 is compared with Examples 13 and 14, it is found that the base material (Comparative Example 1) did not satisfy the corrosion resistance, but the corrosion resistance was improved by carrying out the grain boundary diffusion treatment. This is because the Ga and Co contained in the diffusion source are introduced into the base material, and the [R GB It is considered that the calculated value of [M1] / [M1] changed, and the corrosion resistance improved. R The value of / S satisfies the relationship of formula (5), and the magnet has a sufficiently high corrosion-resistant region relative to the magnet surface area, and therefore is considered to have sufficient corrosion resistance.

[0084] In the above Examples 1 to 14 and Comparative Examples 1 to 7, [R GB The relationship between [R / [M1] and the weight loss rate is shown in Figure 1. In this figure, examples are indicated by ● and comparative examples by ▲. GB ] / [M1] is 2.4 or more, the corrosion resistance is deteriorated. GB When the value of [R ] / [M1] was 1.0 or less, the corrosion resistance was good, but the magnetic properties were deteriorated (Comparative Examples 5 and 6). GB Even if the value of [M1] / [M2] satisfies the formula (3), when the formula (4) is not satisfied, the corrosion resistance deteriorates (Comparative Example 7).

[0085] In addition, when the grain boundary diffusion treatment was performed (Examples 13 and 14), the introduction of the M1 element contained in the diffusion source reduced the [R GB It was confirmed that sufficient corrosion resistance could be obtained by reducing the value of [M1 / M2] to 2.4 or less.

[0086] As described above, the magnets of Examples 1 to 14, which satisfy the requirements of the present invention, showed almost no weight loss in the corrosion resistance evaluation test, and possessed high magnetic properties and sufficient corrosion resistance.

Claims

1. A rare earth sintered magnet containing R (R is one or more elements selected from rare earth elements, and Nd is essential), Fe, B, M1 (M1 is two or more elements selected from Co, Cu, and Ga, and Co is essential), M2 (M2 is one or more elements selected from Al, Si, Cr, Mn, Zn, Ge, Mo, Sn, W, Pb, and Bi), and O, wherein the content of O is 0.6 atomic % or less, and R2Fe 14 and a grain boundary phase containing R, the grain boundary phase being located between at least two of the main phase particles, the grain boundary phase containing R, and the grain boundary phase being located between at least two of the main phase particles and containing R, ... GB ], the rare earth sintered magnet has a region that satisfies the following relational expressions (1) to (4). GB ]=[R]-[Fe] / 7...(1) [M1]=[Co]+[Cu]+[Ga]...(2) 1.0<[R GB ] / [M1]<2.4...(3) [Cu]+[Ga]≧0.28...(4) 2. Let S be the surface area of ​​the magnet, and let S be the area obtained by projecting the region that satisfies the above relational expressions (1) to (4) within a depth range of 0.1 to 200 μm from the magnet surface onto the adjacent magnet surface. R 2. The rare earth sintered magnet according to claim 1, wherein the following relational expression (5) is satisfied when 0.53≦S R / S ≦ 1 ... (5) 3. A rare earth sintered magnet as set forth in claim 1 or 2, characterized in that the content of R metal phase in the grain boundary phase, in which the R content is 50 to 70 atomic %, is 3 volume % or less.

4. A rare earth sintered magnet according to claim 1 or 2, characterized in that it contains 0.1 to 2.0 atomic % of M3 (M3 is one or more elements selected from the group consisting of Ti, V, Zr, Nb, Hf and Ta).

5. The rare earth sintered magnet according to claim 4, characterized in that the following relationship (6) is satisfied, where [B] is the atomic percentage of B and [M3] is the atomic percentage of M3: [B]-2×[M3]≧[Fe] / 14 (6) 6. A rare earth sintered magnet as claimed in claim 1 or 2, characterized in that the C content is 0.4 atomic % or less.

7. A rare earth sintered magnet as claimed in claim 1 or 2, characterized in that the N content is 0.4 atomic % or less.

8. A rare earth sintered magnet as recited in claim 1 or 2, wherein the average crystal grain size of the main phase grains, determined as the equivalent circle diameter in a plane parallel to the magnetization direction, is 4 μm or less.

9. The rare earth sintered magnet according to claim 1 or 2, wherein the weight loss rate measured by the following method is 1% or less. [Measurement method] A sample cut into a rectangular parallelepiped shape of 5 mm x 5 mm x 2 mm is immersed in an ethylene glycol water mixture of ethylene glycol:water = 1:1 (volume ratio) and kept sealed at 120°C for 480 hours, and the weight loss rate of the sample is measured.