R-T-B series permanent magnets

Abstract

To provide an R-T-B based permanent magnet having a high coercivity and a high residual magnetic flux density.SOLUTION: An R-T-B based permanent magnet 2 contains a rare-earth element R, a transition metal element T, and B. The R-T-B based permanent magnet 2 contains at least Nd as R. The R-T-B based permanent magnet 2 contains at least Fe as T. The R-T-B based permanent magnet 2 includes a plurality of main phase grains 4 and a plurality of voids 8. The plurality of main phase grains 4 contain at least R, T, and B. An area ratio of the plurality of voids 8 in an arbitrary cross section of the R-T-B based permanent magnet 2 is larger than 0.2% and 2% or less.SELECTED DRAWING: Figure 2

Description

Technical Field

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

Background Art

[0002] R-T-B based permanent magnets contain rare earth element R (such as Nd), transition metal element T (such as Fe), and boron (B). R-T-B based permanent magnets have excellent magnetic properties and are widely used. As R-T-B based permanent magnets, sintered magnets manufactured by powder metallurgy and hot-worked magnets manufactured by hot plastic working are known. The alloy ribbon which is the raw material of the hot-worked magnet is obtained by the rapid solidification method. In the rapid solidification method, the molten R-T-B alloy is rapidly cooled on the surface of a cooling roll. As a result, the molten metal solidifies and an alloy ribbon is formed. The alloy ribbon obtained by the rapid solidification method contains microcrystals (and amorphous alloys) of the alloy. Therefore, the crystal grains (main phase particles) constituting the hot-worked magnet are finer than those of the sintered magnet. As shown in the Kronmuller's formula, it is known that the coercive force (HcJ) increases as the crystal particle size of the R-T-B based permanent magnet becomes finer. Therefore, the hot-worked magnet should have a higher coercive force than the sintered magnet. However, the coercive force of the conventional hot-worked magnet is equivalent to that of the sintered magnet having the same composition, and the high coercive force expected from the fine crystal particle size has not been obtained.

[0003] The factor for the decrease in the coercive force of the R-T-B based permanent magnet is the reverse magnetic domain generated in the R-T-B based permanent magnet. With the application of a reverse magnetic field to the R-T-B based permanent magnet, the reverse magnetic domain becomes the nucleus of magnetization reversal, and the magnetic wall propagates from the reverse magnetic domain to the whole of the R-T-B based permanent magnet. Due to the propagation of the magnetic wall, the magnetization of each main phase particle in the R-T-B based permanent magnet is reversed. The magnetization reversal of each main phase particle is suppressed by the pinning of the magnetic wall at pinning sites such as grain boundaries. (See Non-Patent Documents 1 to 3 below.)

Prior Art Documents

Non-Patent Documents

[0004] [Non-Patent Document 1] M.Soderznik et al, Magnetization reversal process of anisotropic hot-deformed magnets observed by magneto-opticalKerr effect microscopy, Journal of Alloysand Compounds 771 (2019) 51 - 59. [Non-Patent Document 2] J. Li etal, Angular dependence and thermal stability of coercivity of Nd-rich Ga doped Nd-Fe-Bsintered magnet, Acta Materialia 187 (2020) 66-72. [Non-Patent Document 3] DIPAUL, APPLICATION OF SOLITONTHEORY TO FERROMAGNETIC DOMAIN WALL PINNING, PHYSICS LETTERS, 23 January 1978, Volume 64A, number 5. [Overview of the project] [Problems that the invention aims to solve]

[0005] The greater the difference in anisotropic magnetic field strength between the pinning site and the main phase particles, the easier it is for pinning to suppress the movement of the magnetic domain walls. However, the grain boundary phase in conventional hot-worked magnets does not function adequately as a pinning site. For example, conventional hot-worked magnets contain an R-rich phase as a grain boundary phase (subphase) in which the concentration of rare earth elements R (such as Nd) is higher than that of the main phase particles. The composition of the R-rich phase is Nd 30 Fe 70 The R-rich phase is both ferromagnetic and soft magnetic. Therefore, the difference in anisotropic magnetic field strength between the R-rich phase and the main phase particles is small, and the R-rich phase does not function adequately as a pinning site.

[0006] The inventors discovered that intentionally creating multiple voids as pinning sites within a hot-worked magnet increases its coercivity. The anisotropic magnetic field strength in the voids is virtually zero, and the difference in anisotropic magnetic field strength between the voids and the main phase particles is large, effectively suppressing the movement of magnetic domain walls through pinning in the voids. However, as the number of voids in the hot-worked magnet increases, the proportion of the volume of main phase particles in the hot-worked magnet (volume ratio of the main phase) decreases, leading to a decrease in the residual magnetic flux density of the hot-worked magnet. Therefore, it is necessary to suppress the decrease in residual magnetic flux density associated with the formation of voids and to increase the coercivity through the voids.

[0007] One aspect of the present invention is to provide an R-T-B permanent magnet having high coercivity and high residual magnetic flux density. [Means for solving the problem]

[0008] For example, one aspect of the present invention relates to the following R-T-B permanent magnets. [1] An R-T-B permanent magnet containing a rare earth element R, a transition metal element T, and B, wherein the R-T-B permanent magnet contains at least Nd as R, the R-T-B permanent magnet contains at least Fe as T, the R-T-B permanent magnet comprises a plurality of main phase particles and a plurality of voids, the plurality of main phase particles contain at least R, T, and B, and the area ratio of the plurality of voids in any cross-section of the R-T-B permanent magnet is greater than 0.2% and less than or equal to 2%. R-T-B permanent magnet.

[0009] [2] The average area of ​​each of the multiple voids in any cross-section of an R-T-B permanent magnet is 0.1 (μm). 2 More than 7(μm) 2 The following is: [1] The R-T-B type permanent magnet described above.

[0010] [3] The standard deviation of the area of ​​each of the multiple voids in any cross-section of an R-T-B permanent magnet is 0 (μm). 2 More than 5 (μm) 2 The following is: The R-T-B permanent magnet described in [1] or [2].

[0011] [4] The arbitrary cross-section of the R-T-B permanent magnet is rectangular, one of the four sides of the rectangle is the x-axis, one of the four sides of the rectangle perpendicular to the x-axis is the y-axis, the intersection of the x-axis and y-axis is the origin, and in the coordinate system formed by the origin, the x-axis and the y-axis, the position of the geometric center of each of the multiple gaps is represented as (x,y), and the absolute value of the correlation coefficient r of x and y calculated from the positions of the geometric centers of each of the multiple gaps is between 0 and 0.2. A permanent magnet of the R-T-B type as described in any one of the items [1] to [3].

[0012] [5] In a cross-section of an R-T-B permanent magnet that is approximately parallel to the easy magnetization axis, the main phase particles are flattened, and the average length of the short axis of the main phase particles is between 20 nm and 200 nm. A permanent magnet of the R-T-B type as described in any one of the items [1] to [4].

[0013] [6] The R content in the R-T-B permanent magnet is 28% by mass or more and 33% by mass or less, and the B content in the R-T-B permanent magnet is 0.75% by mass or more and 1.20% by mass or less. A permanent magnet of the R-T-B type as described in any one of the items [1] to [5].

[0014] [7] Multiple main phase particles are stacked along the easy magnetization axis of the R-T-B permanent magnet. A permanent magnet of the R-T-B type as described in any one of the items [1] to [6].

[0015] [8] Hot-worked magnets, The R-T-B-based permanent magnet according to any one of [1] to [7].

[0016] [9] Further including a plurality of R-rich phases, the concentration of R in the plurality of R-rich phases is higher than the concentration of R in the plurality of main phase particles, and the unit of the concentration of R is atomic%. The R-T-B-based permanent magnet according to any one of [1] to [8].

[0017]

[10] The length in the major axis direction of each of the plurality of voids observed in any cross-section of the R-T-B-based permanent magnet is L L and is represented as such, and the length in the minor axis direction of each of the plurality of voids observed in any cross-section of the R-T-B-based permanent magnet is L S and is represented as such, and the aspect ratio of each of the plurality of voids observed in any cross-section of the R-T-B-based permanent magnet is L L / L S and is represented as such, L L / L S and the average value of is 1 or more and 2 or less. The R-T-B-based permanent magnet according to any one of [1] to [9].

Advantages of the Invention

[0018] According to the present invention, an R-T-B-based permanent magnet having a high coercive force and a high residual magnetic flux density is provided.

Brief Description of the Drawings

[0019] [Figure 1] FIG. 1(a) is a schematic perspective view of an R-T-B-based permanent magnet 2 according to an embodiment of the present invention, and FIG. 1(b) is a schematic diagram of a cross-section 2cs of the R-T-B-based permanent magnet 2 (a view in the direction of the arrow along the b-b line in the R-T-B-based permanent magnet 2). [Figure 2] FIG. 2 is an enlarged view of a part (region II) of the cross-section 2cs shown in FIG. 1. [Figure 3] FIG. 3 is a perspective view of a cavity formed in a mold used in a method for manufacturing an R-T-B-based permanent magnet according to an embodiment of the present invention. [Figure 4]Figures 4(a) and 4(b) are images relating to a cross-section of Embodiment 4 of the present invention. [Figure 5] Figures 5(a) and 5(b) are images relating to a cross-section of Embodiment 4 of the present invention. [Figure 6] Figure 6 is an image showing a cross-section of Embodiment 4 of the present invention. [Figure 7] Figures 7(a) and 7(b) are images of the cross-section of Comparative Example 2. [Figure 8] Figures 8(a) and 8(b) are images of the cross-section of Comparative Example 2. [Modes for carrying out the invention]

[0020] Preferred embodiments of the present invention will be described below with reference to the drawings. In the drawings, equivalent components are denoted by equivalent reference numerals. The present invention is not limited to the embodiments described below. The term "permanent magnet" as used below means an R-T-B system permanent magnet. The units of concentration of each element in the permanent magnet described below are atomic percent. X, Y, and Z in Figure 3 represent three mutually orthogonal coordinate axes. The coordinate axes in Figure 3 are independent of the coordinate axes (x and y axes) in Figure 5(b) and the coordinate axes (x and y axes) in Figure 8(b).

[0021] (Permanent magnet) The permanent magnet according to this embodiment contains at least a rare earth element (R), a transition metal element (T), and boron (B). The permanent magnet according to this embodiment is a hot-worked magnet. However, the permanent magnet according to the present invention may also be a sintered magnet.

[0022] The permanent magnet contains at least neodymium (Nd) as a rare earth element R. In addition to Nd, the permanent magnet may also contain other rare earth elements R. The other rare earth elements R contained in the permanent magnet may be at least one selected from the group consisting of scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). The permanent magnet 2 does not have to contain heavy rare earth elements (e.g., both Dy and Tb).

[0023] A permanent magnet contains at least iron (Fe) as the transition metal element T. A permanent magnet may contain only Fe as the transition metal element T. A permanent magnet may contain both Fe and cobalt (Co) as the transition metal element T.

[0024] Figure 1(a) is a perspective view of the permanent magnet 2 according to this embodiment, and Figure 1(b) is a schematic diagram of the cross-section 2cs of the permanent magnet 2. The cross-section 2cs of the permanent magnet 2 is substantially parallel to the easy magnetization axis direction C of the permanent magnet 2. The easy magnetization axis direction C is the direction parallel to the straight line connecting the pair of magnetic poles of the permanent magnet 2. In other words, the easy magnetization axis direction C is the direction from the south pole of the permanent magnet 2 to the north pole of the permanent magnet 2. The easy magnetization axis direction C may be determined based on the measurement of the magnetic flux distribution of the permanent magnet 2. The easy magnetization axis direction C may also be determined based on the measurement of the magnetic flux distribution of an analytical sample separated from the permanent magnet 2. The direction substantially perpendicular to the easy magnetization axis direction C is denoted as the "AB direction".

[0025] The permanent magnet 2 according to this embodiment is a rectangular parallelepiped (plate). However, the shape of the permanent magnet 2 is not limited to a rectangular parallelepiped. For example, the shape of the permanent magnet 2 may be a cube, polygonal prism, arc segment, annular sector, sphere, disc, cylinder, tube, or ring. The shape of the cross-section 2cs of the permanent magnet 2 may be a polygon, arc (chole chord), bow-shaped, arch-shaped, C-shaped, or circle.

[0026] Figure 2 is an enlarged view of a portion (region II) of the cross-section 2cs shown in Figure 1(b). As shown in Figure 2, the permanent magnet 2 contains a plurality of main phase particles 4. The permanent magnet 2 may further contain a plurality of R-rich phases 6 as subphases. The R-rich phases 6 are located between the plurality of main phase particles 4. The R-rich phases 6 may be a type of grain boundary phase contained in the grain boundaries between the plurality of main phase particles 4. The grain boundary containing the R-rich phases 6 may be a grain boundary multipoint surrounded by three or more main phase particles 4. The grain boundary containing the R-rich phases 6 may also be a two-particle grain boundary between two main phase particles 4.

[0027] As shown in Figure 2, multiple voids 8 are formed in the permanent magnet 2. These multiple voids 8 may be rephrased as multiple pores. At least some of these multiple voids 8 may be formed at grain boundary multipoints. At least some of these multiple voids 8 may be formed at two-particle grain boundaries.

[0028] The area ratio (pr) of multiple air gaps 8 in any cross-section of the permanent magnet 2 is greater than 0.2% and less than or equal to 2%. The area ratio (pr) of multiple air gaps 8 in any cross-section of the permanent magnet 2 may be expressed as Av / Acs (unit: %). Av is the sum of the areas (opening areas) of each of the multiple air gaps 8 measured in any cross-section of the permanent magnet 2. Acs is the area of ​​any cross-section of the permanent magnet 2 (the cross-section in which Av is measured). "Any cross-section of the permanent magnet 2" may be a cross-section 2cs that is approximately parallel to the easy magnetization axis C of the permanent magnet 2. "Any cross-section of the permanent magnet 2" may also be a cross-section that is approximately parallel to the AB direction of the permanent magnet 2. "Any cross-section of the permanent magnet 2" may also be a cross-section that is not parallel to the easy magnetization axis C and the AB direction of the permanent magnet 2.

[0029] When the area ratio pr of the multiple air gaps 8 is greater than 0.2%, the movement of the magnetic domain walls is sufficiently suppressed by pinning of the magnetic domain walls in the air gaps 8, and the permanent magnet 2 can have a sufficiently high coercivity. When the area ratio (pr) of the multiple air gaps 8 is 2% or less, the decrease in residual magnetic flux density associated with the formation of the air gaps 8 is sufficiently suppressed. In other words, when the area ratio (pr) of the multiple air gaps 8 is greater than 0.2% and 2% or less, high coercivity and high residual magnetic flux density can be achieved simultaneously. Since high coercivity and high residual magnetic flux density can be easily achieved simultaneously, the area ratio (pr) of the multiple air gaps 8 may be between 0.255% and 1.967%. Even when the R content in the permanent magnet 2 is low and there are few grain boundary phases (R-rich phases, etc.) that function as pinning sites, according to this embodiment, high coercivity and high residual magnetic flux density can be achieved simultaneously. Even if the permanent magnet 2 does not contain heavy rare earth elements (both Dy and Tb), this embodiment achieves both high coercivity and high residual magnetic flux density. For example, the coercivity (HcJ) of permanent magnet 2 at 23°C may be between 1103 kA / m and 1314 kA / m, or between 997 kA / m and 1512 kA / m. For example, the residual magnetic flux density (Br) of the permanent magnet 2 at room temperature may be 1282 mT or more and 1377 mT or less, or 1281 mT or more and 1385 mT or less. For example, the aspect ratio (Hk / HcJ) of permanent magnet 2 may be between 86.3% and 100%, or between 87.6% and 95.1%. Hk is the demagnetizing field strength corresponding to 90% of the remanent magnetic flux density in the second quadrant of the magnetization curve.

[0030] The average value (p_avg) of the area of ​​each of the multiple air gaps 8 in any cross-section of the permanent magnet 2 is 0.1 (μm). 2 More than 7(μm) 2 Less than or equal to 1.665(μm) 2 More than 6.658(μm) 2 The following is acceptable: The standard deviation (p_std) of the area of ​​each of the multiple air gaps 8 in any cross-section of the permanent magnet 2 is 0 (μm). 2 More than 5 (μm) 2 Less than or equal to 1.919(μm) 24.988(μm) or more 2 The following conditions may apply: That is, the standard deviation of the area of ​​each of the multiple air gaps 8 present in any cross-section may be small. The standard deviation becomes small when the size (area) of each of the multiple air gaps 8 present in any cross-section is equal. When the standard deviation is within the above numerical range, high coercivity and high residual magnetic flux density are easily achieved simultaneously, and the square aspect ratio Hk / HcJ tends to be high. Any cross-section of permanent magnet 2 may be rectangular. A rectangle implies a square. One of the four sides of the rectangle is considered the x-axis. One of the four sides of the rectangle that is perpendicular to the x-axis is considered the y-axis. The intersection of the x-axis and the y-axis is considered the origin. The correlation coefficient between x and y is expressed as follows. In a coordinate system consisting of the origin, x-axis, and y-axis, the position (coordinates) of the geometric center of each of the multiple air gaps 8 is expressed as (x,y). The geometric center can be rephrased as the barycenter. From the coordinates (x,y) of the geometric centers of all the air gaps 8 present in any cross-section, the covariance of x and y Cov(x,y) and the standard deviation of coordinate x σ are obtained. x , and the standard deviation of coordinate y σ y The following is calculated. The covariance Cov(x,y) is expressed by formula 1 below. Covariance Cov(x,y), standard deviation σ x and standard deviation σ y Based on this, the correlation coefficient r between x and y is expressed by the following formula 2. Cоv(x,y)=E[(xE[x])(yE[y])] (1) In equation 1, E[x] is the expected value (mean value) of x. In equation 1, E[y] is the expected value (mean value) of y. In equation 1, E[(xE[x])(yE[y])] is the expected value (mean value) of (xE[x])(yE[y]). r = Cov(x,y) / σ x σ y (2) The absolute value of the correlation coefficient r between the coordinates (x,y) of the geometric centers of each of the multiple air gaps 8 may be between 0 and 0.2, or between 0.005 and 0.197. In other words, the correlation coefficient r between x and y may be close to 0. In other words, the correlation between x and y may be weak. The less the positional bias of all the air gaps 8 present in any cross-section, the closer the correlation coefficient r will be to 0. When the absolute value of the correlation coefficient r between x and y falls within the above numerical range, the positional bias of each air gap 8 is small, making it easier to achieve both high coercivity and high residual magnetic flux density, and the angular ratio Hk / HcJ tends to be high.

[0031] When multiple air gaps 8 are distributed within the permanent magnet 2 such that the average area of ​​each air gap 8, the standard deviation of the areas of each air gap 8, and the correlation coefficient r are within the above range, the decrease in residual magnetic flux density associated with the formation of multiple air gaps 8 is easily suppressed, and the permanent magnet 2 tends to have high coercivity due to the pinning of magnetic domain walls in the multiple air gaps 8.

[0032] The dimensions of any cross-section on which the areas of multiple voids 8, the average value of the areas of each of the multiple voids 8, the standard deviation of the areas of each of the multiple voids 8, and the absolute value of the correlation coefficient r between x and y are measured may be, for example, 423 μm vertically × 317 μm horizontally.

[0033] As shown in Figure 2, in a cross-section 2cs of the permanent magnet 2 that is substantially parallel to the easy magnetization axis C, a plurality of flattened main phase particles 4 may be observed. In other words, each main phase particle 4 observed in the cross-section 2cs may be plate-shaped. The plurality of flattened main phase particles 4 may be stacked along the easy magnetization axis C. The permanent magnet 2 may further include secondary particles composed of a plurality of bonded main phase particles 4. The permanent magnet 2 may contain a plurality of secondary particles. At least some of the voids 8 may be located at the grain boundaries between the plurality of secondary particles. At least some of the R-rich phase 6 may be located at the grain boundaries between the plurality of secondary particles.

[0034] Each main phase particle 4 contains at least R (Nd, etc.), T, and B. Each main phase particle 4 can be rephrased as a single crystal grain (i.e., a primary particle). Each main phase particle 4 contains R2T 14Contains crystals of B (single crystal or polycrystalline). R2T 14 B is a ferromagnetic ternary intermetallic compound. The main phase particle 4 is R2T 14 It may consist only of crystals B. R2T 14 The crystal of B may be tetragonal. That is, R2T 14 The crystal axes of B are the a-axis, b-axis, and c-axis, and the a-axis, b-axis, and c-axis are orthogonal to each other, R2T 14 The lattice constant of B in the a-axis direction is R²T 14 The lattice constant in the b-axis direction of B may be equal to R²T. 14 The lattice constant in the c-axis direction of B may be different from the lattice constants in the a-axis and b-axis directions, respectively. R2T 14 The c-axis direction of B may be approximately parallel to the easy magnetization axis C of the permanent magnet 2.

[0035] The main phase particles 4 may contain other elements in addition to R, T, and B. For example, R2T may be the element that constitutes the main phase particles 4. 14 B is (Nd 1-x Pr x )2(Fe 1-y Co y ) 14 It may be represented as B. x may be between 0 and 1 (inclusive). y may be between 0 and 1 (inclusive). The main phase particle 4 may contain heavy rare earth elements such as Tb and Dy in addition to light rare earth elements as R. R2T 14 A portion of B in B may be substituted with other elements such as carbon (C). The composition within the main phase particles 4 may be uniform. The composition within the main phase particles 4 may be non-uniform. For example, the concentration distributions of R, T, and B in the main phase particles 4 may have gradients.

[0036] The main phase particle 4 may consist of a surface layer and a central part covered by the surface layer. The surface layer may be referred to as a shell, and the central part as a core. The surface layer of the main phase particle 4 may contain at least one heavy rare earth element from among Tb and Dy. The surface layer of each of the main phase particles 4 may contain at least one heavy rare earth element from among Tb and Dy. The surface layer of some of the main phase particles 4 may contain at least one heavy rare earth element from among Tb and Dy. The inclusion of heavy rare earth elements in the surface layer makes it easier for the anisotropic magnetic field to increase locally near the grain boundaries, and makes it difficult for magnetization reversal nuclei to be generated near the grain boundaries. As a result, the coercivity of the permanent magnet 2 increases at high temperatures (e.g., 100-200°C). Since the residual magnetic flux density (Br) and coercivity of permanent magnet 2 are easily compatible, the total concentration of heavy rare earth elements in the surface layer may be higher than the total concentration of heavy rare earth elements in the center.

[0037] The volume ratio of the main phase (the ratio of the volume of all main phase particles 4 in the permanent magnet 2) is not particularly limited. For example, the volume ratio of the main phase may be 80% or more and less than 99.8% by volume, 90% or more and less than 99.8% by volume, or 95% or more and less than 99.8% by volume. As the volume ratio of the main phase increases, the residual magnetic flux density of the permanent magnet 2 increases.

[0038] The R-rich phase 6 may be ferromagnetic and soft magnetic. The R-rich phase 6 contains at least R. For example, the R-rich phase 6 may contain Nd as R. In addition to Nd, the R-rich phase 6 may further contain one or more other rare earth elements as R. In addition to R, the R-rich phase 6 may further contain one or more elements other than R. The R-rich phase 6 may contain at least one component selected from the group consisting of metals, alloys, intermetallic compounds, and oxides. For example, part or all of the R-rich phase 6 may consist of only at least one component from elemental R, alloys containing R, and metal compounds containing R. Part or all of the R-rich phase 6 may contain an oxide of R. For example, the oxide of R may be an oxide of Nd. The oxidized surface of the main phase particles 4 may be an oxide of R. Part of the R-rich phase 6 may consist only of an oxide of R.

[0039] The concentration of R in the R-rich phase 6 may be higher than the average value of the R concentration in the main phase particles 4. The concentration of R in the R-rich phase 6 may be higher than the average value of the R concentration in the cross-section 2cs described above. If the permanent magnet 2 contains multiple types of R, the concentration of R may be the sum of the concentrations of the multiple types of R.

[0040] The average length of the short axis of the main phase particles 4 (primary particles) observed in the above cross-section 2cs may be 20 nm or more and 200 nm or 110 nm or more and 147 nm or less. If the average length of the short axis of the main phase particles 4 is within the above range, each main phase particle 4 (R2T 14 The anisotropic growth of the crystals of B is sufficient, and each main phase particle 4 is easily oriented in the easy magnetization axis C, which easily increases coercivity, remanent magnetic flux density, and angular ratio. The average length of the major axis of the main phase particles 4 (primary particles) observed in the above cross-section 2cs may be, for example, between 100 nm and 1000 nm. The minor axis of each principal phase particle 4 observed in cross-section 2cs may be approximately parallel to the easy magnetization axis direction C. The major axis of each principal phase particle 4 may be approximately perpendicular to the easy magnetization axis direction C. The shape of the principal phase particle 4 in cross-section 2cs is not limited to a rectangle. The shape of the principal phase particle 4 in cross-section 2cs may be distorted. The shape of the principal phase particle 4 in cross-section 2cs does not have to be uniform. If the shape of the principal phase particle 4 in cross-section 2cs is distorted, the shape of the principal phase particle 4 may be approximated by the quadrilateral with the smallest area that circumscribing the principal phase particle 4. The quadrilateral may be a rectangle. The length of the short side of this quadrilateral may be considered as the length of the minor axis of the principal phase particle 4, and the length of the long side of the above quadrilateral may be considered as the length of the major axis of the principal phase particle 4. The average value of the minor axis length of the principal phase particle 4 may be calculated from the measured values ​​of the minor axis lengths of all principal phase particles 4 present in the backscattered electron image of cross-section 2cs taken with a scanning electron microscope (SEM). The average length of the major axis of the main phase particle 4 may also be calculated from the measured lengths of the major axes of all main phase particles 4 present in the backscattered electron image. However, the dimensions of main phase particles 4 that extend beyond the backscattered electron image are excluded from the calculation of the average value. The maximum dimensions of the backscattered electron image used to measure the lengths of the minor axis and major axis of the main phase particle 4 may be, for example, 120 μm vertically × 80 μm horizontally, or 80 μm vertically × 120 μm horizontally. Several representative locations within these low-magnification backscattered electron images may be selected, and backscattered electron images of each location may be taken at high magnification. Then, the average values ​​of the major axis and minor axis may be calculated from the lengths of the major axis and minor axis of all main phase particles 4 measured in the high-magnification backscattered electron image. Commercially available image analysis software may be used to identify the shape (contour) of the main phase particle 4 and to measure the dimensions of the main phase particle 4 (the rectangle circumscribing the main phase particle 4).

[0041] The dimensions of the permanent magnet 2 in the easy magnetization axis direction C may be, for example, several millimeters to several hundred millimeters, or tens of millimeters to several hundred millimeters. The dimensions of the permanent magnet 2 in the AB direction may be, for example, several millimeters to several hundred millimeters, or tens of millimeters to several hundred millimeters.

[0042] Other grain boundary phases besides the R-rich phase 6 may be included in the grain boundary. For example, the grain boundary may include a grain boundary phase containing elements introduced into the permanent magnet 2 by the grain boundary diffusion process described later. The elements introduced into the permanent magnet 2 by the grain boundary diffusion process may be at least one heavy rare earth element from among Tb and Dy. The elements introduced into the permanent magnet 2 by the grain boundary diffusion process may be heavy rare earth elements and light rare earth elements, and the light rare earth element may be at least one from among Nd and Pr. The elements introduced into the permanent magnet 2 by the grain boundary diffusion process may be heavy rare earth elements, light rare earth elements and copper.

[0043] The length in the longitudinal direction of each gap 8 observed in any cross-section of the permanent magnet 2 is L L It is expressed as follows. The length of each gap 8 in the short axis direction is L S It is expressed as follows. The aspect ratio of each gap 8 is L L / L S It is expressed as L L / L S The average value may be between 1 and 2, between 1.124 and 1.786, or between 1.124 and 1.435. L L / L S Gaps 8 where the ratio exceeds 2 have an elongated shape in the axial direction. L / L S A gap of 8 greater than 2 does not contribute much to an increase in coercivity. The reason for this is not clear, but the inventors believe that L L / L S The length L in the short axis direction of the gap 8 where the value exceeds 2 S It tends to be smaller than the size of the magnetic domain, L L / L S It is speculated that a gap 8 greater than 2 is less likely to hinder the magnetization reversal process. On the other hand, L L / L S Since the air gap 8, where is 2 or less, has a shape that extends substantially uniformly in both the easy magnetization axis C and the AB direction, it is presumed that the magnetization reversal process in both the easy magnetization axis C and the AB direction can be inhibited. Therefore, L L / L S A gap 8 where is 2 or less is likely to contribute to an increase in coercivity. For the reasons above, LL / L S The average value of L is preferably between 1 and 2. L / L S A value closer to 1 for the average is preferable.

[0044] The main phase particles 4, voids 8, and R-rich phase 6 can each be identified based on the contrast of images of the cross-section 2cs of the permanent magnet 2 taken with a scanning electron microscope (SEM) or scanning transmission electron microscope (STEM). The composition of the main phase particles 4 and the R-rich phase 6 can be analyzed by an electron probe microanalyzer (EPMA) equipped with an energy-dispersive X-ray spectroscopy (EDS) instrument.

[0045] The overall composition of permanent magnet 2 is described below. However, the composition of permanent magnet 2 is not limited to the composition described below. The content of each element in permanent magnet 2 may be outside the ranges described below.

[0046] The R content in the R-T-B permanent magnet may be between 28.00% by mass and 33.00% by mass. When the R content is within the above range, the residual magnetic flux density and coercivity of the permanent magnet 2 tend to increase. When the R content is 28.00% by mass or more, it is easier to suppress the formation of cracks in the permanent magnet 2 during the hot deforming process. When the R content is 28.00% by mass or more, the R2T constituting the main phase particles 4 14 B is easily formed, and the α-Fe phase, which has soft magnetism, is difficult to form. As a result, the coercivity tends to increase. On the other hand, when the R content is 33.00 mass% or less, segregation of the liquid phase (R-rich phase) on the surface of the permanent magnet 2 is suppressed during the hot plastic working process, and seizure of the mold and the permanent magnet 2 is suppressed. When the R content is 33.00 mass% or less, the formation of the R-rich phase 6 is moderately suppressed, and the residual magnetic flux density tends to increase. Since the residual magnetic flux density and coercivity tend to increase, the total proportion of Nd and Pr in the total rare earth element R may be 80 atomic% or more and 100 atomic% or less, or 95 atomic% or more and 100 atomic% or less.

[0047] The total content of Tb and Dy in the permanent magnet 2 may be between 0.00 mass% and 5.00 mass%. The presence of at least one heavy rare earth element among Tb and Dy in the permanent magnet 2 tends to increase its magnetic properties (especially its coercivity at high temperatures). However, the permanent magnet 2 does not necessarily have to contain Tb and Dy.

[0048] The B content in the R-T-B permanent magnet may be 0.75% by mass or more and 1.20% by mass or less. If the B content is 0.75% by mass or more, R2Fe 17 The formation of equivalent different phases is suppressed, and coercivity and residual magnetic flux density tend to increase. When the content of B is 1.20 mass% or less, R 1+ε The formation of different phases such as Fe4B4(Boride) is suppressed, and coercivity and residual magnetic flux density tend to increase. When the B content is within the above range, the square ratio of permanent magnet 2 tends to approach 1.0. If the total content of rare earth element R in permanent magnet 2 is 28.00% by mass or more and 33.00% by mass or less, and the content of B in permanent magnet 2 is 0.75% by mass or more and 1.20% by mass or less, then the content of rare earth element R in permanent magnet 2 is R2T 14 The stoichiometric ratio of B is large. As a result, a liquid phase is easily formed at the grain boundaries during the hot plastic deformation process described later. The liquid phase within the grain boundaries is formed at the crystal grain (R2T 14 B) promotes anisotropic growth, grain boundary sliding, and grain rotation. As a result, the c-axis of the grains is more likely to be oriented in the stress direction, the volume ratio of the main phase is more likely to increase, and the coercivity and residual magnetic flux density of permanent magnet 2 are more likely to increase.

[0049] The permanent magnet 2 may contain gallium (Ga). The Ga content may be between 0.03% by mass and 1.00% by mass, or between 0.20% by mass and 0.80% by mass. When the Ga content is within the above range, the formation of subphases (e.g., phases containing R, T, and Ga) is moderately suppressed, and the residual magnetic flux density and coercivity of the permanent magnet 2 tend to increase. However, the permanent magnet 2 does not have to contain Ga.

[0050] The permanent magnet 2 may contain aluminum (Al). The Al content in the permanent magnet 2 may be between 0.01% by mass and 0.2% by mass, or between 0.04% by mass and 0.07% by mass. Having an Al content within the above range tends to improve the coercivity and corrosion resistance of the permanent magnet. However, the permanent magnet 2 does not need to contain Al.

[0051] The permanent magnet 2 may contain copper (Cu). The Cu content in the permanent magnet 2 may be 0.01% by mass or more and 1.50% by mass or less, or 0.04% by mass or more and 0.50% by mass or less. Having a Cu content within the above range tends to improve the coercivity, corrosion resistance, and temperature characteristics of the permanent magnet 2. However, the permanent magnet 2 does not need to contain Cu.

[0052] The permanent magnet 2 may contain cobalt (Co). The Co content in the permanent magnet may be 0.30% by mass or more and 6.00% by mass or less, or 0.30% by mass or more and 4.00% by mass or less. The presence of Co in the permanent magnet 2 tends to increase the Curie temperature of the permanent magnet 2. Furthermore, the presence of Co in the permanent magnet 2 tends to improve the corrosion resistance of the permanent magnet 2. However, the permanent magnet 2 does not have to contain Co.

[0053] The remainder of the permanent magnet 2 after removing the above elements may consist only of Fe, or Fe and other elements. For the permanent magnet 2 to have sufficient magnetic properties, the total content of elements other than Fe in the remainder may be 5% by mass or less of the total mass of the permanent magnet 2.

[0054] The permanent magnet 2 may contain, as other elements (e.g., unavoidable impurities), at least one selected from the group consisting of silicon (Si), titanium (Ti), manganese (Mn), zirconium (Zr), vanadium (V), chromium (Cr), nickel (Ni), niobium (Nb), molybdenum (Mo), hafnium (Hf), tantalum (Ta), tungsten (W), bismuth (Bi), tin (Sn), calcium (Ca), carbon (C), nitrogen (N), oxygen (O), chlorine (Cl), sulfur (S), and fluorine (F). The total content of other elements in the permanent magnet 2 may be 0.001% by mass or more and 0.50% by mass.

[0055] The overall composition of the permanent magnet 2 may be analyzed by methods such as X-ray fluorescence (XRF) analysis, inductively coupled plasma (ICP) emission spectrometry, inert gas fusion-nondispersive infrared absorption (NDIR) spectrometry, combustion in an oxygen stream-infrared absorption spectrometry, and inert gas fusion-thermal conductivity spectrometry.

[0056] The permanent magnet 2 may be applied to motors, generators, actuators, etc. For example, the permanent magnet 2 is used in various fields such as hybrid vehicles, electric vehicles, hard disk drives, magnetic resonance imaging (MRI) machines, smartphones, digital cameras, flat-screen TVs, scanners, air conditioners, heat pumps, refrigerators, vacuum cleaners, washer-dryers, elevators, and wind turbines.

[0057] (Method of manufacturing permanent magnets) The method for manufacturing a permanent magnet according to this embodiment includes at least a thin strip manufacturing step, a hot pressing step, a hot deforming step, and a cooling step. The method for manufacturing a permanent magnet may further include other steps such as a grain boundary diffusion step following the cooling step. However, the grain boundary diffusion step is not essential.

[0058] To suppress oxidation of permanent magnets and their work-in-progress during the manufacturing process, the manufacturing method of permanent magnets may be carried out under a non-oxidizing atmosphere. For example, the non-oxidizing atmosphere may be an inert gas such as argon (Ar) gas. In addition to the inert gas, the non-oxidizing atmosphere may further contain a reducing gas such as hydrogen gas (H2).

[0059] The thin-strip manufacturing process is a process for producing alloy strips from raw metals using an ultra-rapid solidification method. In the ultra-rapid solidification method, molten metal in a container is sprayed from a nozzle located at the front of the container onto the surface of a cooling roll. The molten metal comes into contact with the surface of the cooling roll and is instantly ejected by the rapidly rotating cooling roll, forming numerous elongated ribbon-like shapes. The molten metal is rapidly cooled and solidified upon contact with the surface of the cooling roll. As a result, numerous elongated alloy strips are formed. A container is placed in the direction from which the alloy strips are ejected by the cooling roll, and the alloy strips are collected into the container.

[0060] The molten metal is a metal (raw material) containing each element that makes up the permanent magnet. The raw material may be, for example, a rare earth element (elementary metal), an alloy containing a rare earth element, pure iron, ferroboron, or an alloy containing these. These raw material metals are weighed to match the composition of the desired permanent magnet.

[0061] The molten metal may be obtained by heating the raw metal in a container using high-frequency induction heating. The temperature of the molten metal sprayed from the nozzle (injection temperature) may be, for example, about 1400°C. The heating rate at which the temperature of the raw metal reaches the injection temperature may be, for example, about 20 to 100°C / second.

[0062] The surface of the cooling roll may be made of a metal with high thermal conductivity, such as Cu. The temperature of the surface of the cooling roll may be controlled by a coolant flowing inside the cooling roll. For example, if the cooling rate of the molten metal on the surface of the cooling roll is about 10 5 ~10 6 The surface temperature of the cooling roll may be controlled to be °C / second. The higher the cooling rate, the more the crystals (R2T) contained in the alloy strip will be affected. 14In B), the particle size tends to be finer, and the coercivity of the permanent magnet tends to increase. The less molten metal is sprayed onto the surface of the cooling roll per unit time, the thinner the molten metal adhering to the surface of the cooling roll, the higher the cooling rate, and the thinner the alloy strip. The higher the peripheral speed of the cooling roll, the thinner the molten metal adhering to the surface of the cooling roll, the higher the cooling rate, and the thinner the alloy strip. The thickness of the main phase particles in the easy magnetization axis direction (length of the short axis of the main phase particles) depends on the thickness of the alloy strip (and the crushing and classification of the alloy strip). The thinner the alloy strip, the smaller the thickness (particle size) of the main phase particles, and the higher the coercivity of the permanent magnet tends to be. The thickness of the alloy strip may be, for example, 20 μm to 60 μm, or 30 μm to 50 μm. The width of the alloy strip may be, for example, 1.0 mm to 5.0 mm.

[0063] After the thin strip manufacturing process, a crushing / classification process may be carried out. The crushing / classification process involves crushing the alloy thin strip using a crushing device to produce coarse powder, and then classifying the coarse powder to recover alloy powder having a predetermined particle size and aspect ratio. The alloy powder is a precursor of the main phase particles contained in the permanent magnet. The shape of each alloy particle constituting the alloy powder may be plate-shaped or flake-shaped. The crushing method for the alloy thin strip may be at least one of, for example, a cutter mill and a propeller mill. The means for classifying the coarse powder is a sieve. The particle size and particle size distribution of the alloy powder obtained by classification may be measured, for example, by laser diffraction scattering. The particle size of the alloy powder obtained by classification may be, for example, 60 μm to 2800 μm, or 150 μm to 2800 μm.

[0064] The hot forming process is a process of forming a molded body by heating and pressurizing an alloy strip (alloy powder). For example, the alloy powder may be heated in a mold and compressed in the mold. Pressurizing the alloy powder reduces the voids between the alloy powder particles, resulting in a dense molded body. Furthermore, the heating of the alloy powder accompanying the pressurization causes a liquid phase (R-rich phase such as an Nd-rich phase) to form from the surface of the alloy powder. This liquid phase fills the voids (grain boundaries) between the alloy powder particles, and the liquid phase also lubricates the alloy powder, resulting in a dense molded body. A cold forming process may be performed before the hot forming process. In the cold forming process, a molded body may be formed by pressurizing the alloy powder at room temperature. The molded body obtained in the cold forming process may be densified by heating and pressurizing it in the hot forming process. The temperature of the alloy powder in the hot forming process (hot forming temperature) may be, for example, 550°C or higher and 800°C or lower. If the hot forming temperature is too low, a sufficient liquid phase will not form from the surface of the alloy powder, making it difficult to densify the molded body. If the hot forming temperature is too high, the crystals (R2T) that make up the alloy powder will not form. 14 In B), excessive grain growth can easily reduce the coercivity of the permanent magnet. The pressure applied to the alloy powder during the hot forming process (hot forming pressure) may be between 50 MPa and 200 MPa. The time during which the hot forming temperature and hot forming pressure are maintained within the above range (hot forming time) may be, for example, between several tens of seconds and several hundred seconds.

[0065] After the hot forming process, a hot plastic deformation process is carried out. In the hot plastic deformation process, multiple main phase particles (R2T) whose c-axis (easy magnetization axis) are oriented in a predetermined direction are formed by hot extrusion molding of the molded body obtained in the hot forming process. 14This is a process to obtain a magnetic substrate containing crystal grains of B. For example, in the hot plastic deformation process, the molded body is heated and extruded from the mold. Inside the mold, the grain boundary phase in the heated molded body liquefies to generate a liquid phase (R-rich phase), and stress acts on the molded body in a predetermined direction, causing each alloy particle constituting the molded body to deform. Along with the generation of the liquid phase and the deformation of the alloy particles, anisotropic growth of the crystal grains in the direction perpendicular to the c-axis of the crystal grains progresses. The liquid phase also lubricates each crystal grain, and a force acts on each crystal grain according to the stress. As a result, the crystal grains rotate due to grain boundary sliding, and the c-axis of each crystal grain (main phase particle) is oriented approximately parallel to the stress direction. In other words, multiple flattened main phase particles extending in a direction approximately perpendicular to the c-axis are stacked along the stress direction. The temperature of the molded body during the hot plastic deformation process (hot plastic deformation temperature) may be, for example, 700°C or more and less than 900°C, or 700°C or more and 850°C or less. If the hot plastic deformation temperature is too low, the liquid phase (R-rich phase such as Nd-rich phase) is less likely to form at the grain boundaries within the molded body, making it difficult for crystal grains to grow and reducing the likelihood of grain rotation due to grain boundary sliding. As a result, the average length of the short axis of the main phase particles tends to be less than 20 nm, and the c-axis of each main phase particle (crystal grain) is less likely to be oriented approximately parallel to the stress direction. If the hot plastic deformation temperature is too high (for example, if the hot plastic deformation temperature is 900°C or higher), the liquid phase (R-rich phase) seeps excessively from each alloy particle and segregates onto the surface of each alloy particle and the interfaces between alloy particles, and most of the liquid phase is consumed for grain growth of the crystal grains. Because most of the liquid phase is consumed for grain growth of the crystal grains, the grain growth of the main phase particles (crystal grains) proceeds abnormally, making it easy for coarse main phase particles to form, and the average length of the short axis of the main phase particles tends to exceed 200 nm. Coarse main phase particles are difficult to orient in the easy magnetization axis direction. The extrusion speed for hot extrusion molding is 10 -2The extrusion speed may be between 9.9 mm / second and 10 mm / second. If the extrusion speed is too high (for example, if the extrusion speed is 10 mm / second or higher), the anisotropic growth of the main phase particles (crystal grains) in the molded body does not proceed sufficiently, and the average length of the short axis of the main phase particles (primary particles) tends to be less than 20 nm. In other words, if the extrusion speed is too high, the molded body is pushed out of the mold before the anisotropic growth of the crystal grains in the molded body has progressed sufficiently. As a result, the c-axis of each main phase particle (crystal grain) is unlikely to be oriented approximately parallel to the stress direction. The pressure applied to the molded body during the hot plastic deformation process (hot plastic deformation pressure) may be between 50 MPa and 200 MPa. The time during which the hot plastic deformation temperature and hot plastic deformation pressure are maintained within the above range (hot plastic deformation time) may be, for example, several tens of seconds.

[0066] The mold used in the hot plastic deformation process is cylindrical. That is, the cavity formed inside the mold penetrates the mold from the end face of the mold where the inlet for the molded body opens (start face) to the end face of the mold where the extrusion port for the molded body opens (end face). The start face and the end face are parallel planes to each other. The direction from the start face to the end face is the extrusion direction of the molded body, and the extrusion direction is perpendicular to the start face and the end face. The opening area of ​​the extrusion port of the molded body is smaller than the opening area of ​​the inlet for the molded body. A specific example of a cavity formed in a mold is shown in Figure 3. The cavity 10 is divided into an inlet-side region 10A, an intermediate region 10B, and an extrusion-side region 10C along the extrusion direction (Z-axis direction). The inlet-side region 10A opens at the starting end face. The extrusion-side region 10C opens at the ending face. The intermediate region 10B is located between the inlet-side region 10A and the extrusion-side region 10C in the extrusion direction. The shape of the cavity 10 in the cross-section of the mold perpendicular to the extrusion direction (the cross-section of the mold parallel to the start and end faces) is a quadrilateral with all four corners being right angles. In this quadrilateral, opposite pairs of sides are referred to as the first side, and the other pair of opposite sides in the above quadrilateral are referred to as the second side. The length xa of the first side in the inlet-side region 10A is constant. The length ya of the second side in the inlet-side region 10A is also constant. In other words, the opening area of ​​the inlet-side region 10A in a cross-section perpendicular to the extrusion direction is constant. In the intermediate region 10B, the length xa of the first side gradually decreases along the extrusion direction and eventually coincides with the length xc of the first side in the extrusion port-side region 10C. Therefore, the first side in the extrusion port-side region 10C is shorter than the first side in the inlet-side region. Also in the intermediate region 10B, the length ya of the second side gradually increases along the extrusion direction and eventually coincides with the length yc of the second side in the extrusion port-side region 10C. Therefore, the second side in the extrusion port-side region 10C is longer than the second side in the inlet-side region 10A. Furthermore, the opening area of ​​the intermediate region 10B in a cross-section perpendicular to the extrusion direction gradually decreases along the extrusion direction and eventually coincides with the opening area of ​​the extrusion port-side region 10C in a cross-section perpendicular to the extrusion direction. Therefore, the opening area of ​​the extrusion port side region 10C in a cross-section perpendicular to the extrusion direction is smaller than the opening area of ​​the inlet side region 10A in a cross-section perpendicular to the extrusion direction. The length of the first side xc in the extrusion port side region 10C is approximately constant. The length of the second side yc in the extrusion port side region 10C is also approximately constant. In other words, the opening area of ​​the extrusion port side region 10C in a cross-section perpendicular to the extrusion direction is approximately constant. As described above, the opening area of ​​the extrusion port side region 10C in a cross section perpendicular to the extrusion direction is smaller than the opening area of ​​the inlet side region 10A in a cross section perpendicular to the extrusion direction, and the first side in the extrusion port side region 10C (end face) is shorter than the second side in the extrusion port side region 10C (end face). In the intermediate region 10B, the length xa of the first side gradually decreases along the extrusion direction, and in the intermediate region 10B, the length ya of the second side gradually increases along the extrusion direction. Therefore, in the intermediate region 10B and the extrusion port side region 10C, a stress approximately parallel to the first side acts on the molded body, causing grain boundary sliding and rotation of the main phase particles. As a result, the c-axis of the main phase particles is oriented along the stress direction (direction of the first side). In other words, the easy magnetization axis direction C of the magnet substrate obtained by hot extrusion molding approximately coincides with the direction of the first side (X-axis direction) in the extrusion port side region 10C (end face).

[0067] As the pressure applied to the magnet substrate decreases or disappears after the hot plastic deformation process, the magnet substrate expands. The main phase particles 4 and grain boundary phases (R-rich phase 6, etc.) in the magnet substrate differ in their degree of springback. In other words, the main phase particles 4 and grain boundary phases (R-rich phase 6, etc.) in the magnet substrate differ in their degree of expansion in response to pressure reduction. Furthermore, the main phase particles 4 and grain boundary phases (R-rich phase 6, etc.) in the magnet substrate also differ in their coefficients of thermal expansion. In other words, the main phase particles 4 and grain boundary phases (R-rich phase 6, etc.) in the magnet substrate differ in their degree of contraction in response to cooling. Due to these factors, multiple voids 8 are formed in the magnet substrate. In the cooling process following the hot plastic deformation process, the magnet substrate is not heated, and the magnet substrate is continuously pressurized for a predetermined time (pressure time t). In other words, the cooling process is a process in which the pressure applied to the magnet substrate is released after the magnet substrate has been allowed to cool naturally for a predetermined pressure Pc over the pressure time t. The pressure Pc in the cooling process is approximately parallel to the easy magnetization axis C of the magnet substrate. As the pressure Pc in the cooling process increases, the area ratio (pr) of the multiple voids 8 tends to decrease. For example, the pressure Pc in the cooling process may be between 20 MPa and 50 MPa. When the pressure Pc in the cooling process is within the above range, the area ratio (pr) of the multiple voids 8 is easily controlled to be greater than 0.2% and less than or equal to 2%. When the pressure Pc in the cooling process is within the above range, the average value (p_avg) of the area of ​​each of the multiple voids 8, the standard deviation (p_std) of the area of ​​each of the multiple voids 8, and the absolute value of the correlation coefficient r of x and y are easily controlled to be within the above range. As the pressurization time t in the cooling process increases, the area ratio (pr) of the multiple voids 8 tends to decrease. For example, the pressurization time t in the cooling process may be between 60 seconds and 240 seconds. When the pressurization time t in the cooling process is within the above range, the area ratio (pr) of the multiple voids 8 is easily controlled to be greater than 0.2% and less than or equal to 2%. When the pressurization time t in the cooling process is within the above range, the average value (p_avg) of the area of ​​each of the multiple voids 8, the standard deviation (p_std) of the area of ​​each of the multiple voids 8, and the absolute value of the correlation coefficient r between x and y are easily controlled to be within the above range. If the pressure Pc in the cooling process is too high, the area ratio (pr) of the voids 8 decreases due to the disappearance of fine cavities, but the degree of local plastic deformation within the magnet substrate differs significantly from the degree of plastic deformation in other areas. Due to localized excessive plastic deformation, a small number of large voids 8 or a small number of cracks are easily formed. Therefore, if the pressure Pc in the cooling process is too high, the average area of ​​each of the multiple voids 8 tends to increase.

[0068] The magnetic substrate obtained through the above process may be a finished permanent magnet. The magnetic substrate that has undergone the grain boundary diffusion process described below may also be a finished permanent magnet.

[0069] After the cooling process, the following grain boundary diffusion process may be performed. The grain boundary diffusion process involves attaching a diffusion material containing heavy rare earth elements to the surface of a magnetic substrate and heating the diffusion material and the magnetic substrate. Heating the magnetic substrate to which the diffusion material is attached causes the heavy rare earth elements in the diffusion material to diffuse from the surface of the magnetic substrate into the interior of the magnetic substrate. Inside the magnetic substrate, the heavy rare earth elements diffuse through the grain boundaries to the vicinity of the surface of the main phase particles. Near the surface of the main phase particles, some light rare earth elements (such as Nd) are replaced by heavy rare earth elements. Due to the localization of heavy rare earth elements near the surface of the main phase particles and at the grain boundaries, the anisotropic magnetic field becomes locally larger near the grain boundaries, making it difficult for magnetization reversal nuclei to be generated near the grain boundaries. As a result, a permanent magnet with high coercivity is obtained.

[0070] The temperature of the diffusion material and the magnetic substrate in the grain boundary diffusion process (diffusion temperature) may be, for example, 550°C to 900°C. The time during which the diffusion temperature is maintained within the above range (diffusion time) may be, for example, 1 minute to 1440 minutes.

[0071] The diffusion material may contain at least one heavy rare earth element from among Tb and Dy. In addition to the heavy rare earth element, the diffusion material may further contain at least one light rare earth element from among Nd and Pr. In addition to the heavy rare earth element and the light rare earth element, the diffusion material may further contain Cu. The diffusion material may be, for example, a metal made of one of the above elements, a hydride of one of the above elements, an alloy containing multiple of the above elements, or a hydride of the alloy. The diffusion material may be a powder. In the grain boundary diffusion process, a slurry containing the diffusion material and an organic solvent may be applied to the surface of the magnet substrate. In the grain boundary diffusion process, the surface of the magnet substrate may be covered with a sheet containing the diffusion material and a binder. In the grain boundary diffusion process, the surface of the magnet substrate may be covered with an alloy foil (ribbon) made of the diffusion material.

[0072] To promote the diffusion of the diffusion agent, the surface of the magnetic substrate may be polished before the grain boundary diffusion process. After the grain boundary diffusion process, the surface of the magnetic substrate may be polished to remove any remaining diffusion agent.

[0073] The dimensions and shape of the magnetic substrate may be adjusted by cutting and polishing. A passive layer may be formed on the surface of the magnetic substrate by oxidation or chemical treatment. The surface of the magnetic substrate may be covered with a protective film such as a resin film. The passive layer or protective film improves the corrosion resistance of the permanent magnet.

[0074] The present invention is not necessarily limited to the embodiments described above. Various modifications to the present invention are possible without departing from the spirit of the invention, and such modifications are also included in the present invention. [Examples]

[0075] The present invention will be described in detail by the following examples and comparative examples. The present invention is not limited to the following examples.

[0076] <Manufacturing of permanent magnets> (Example 1) Each step in Example 1 below was carried out in a non-oxidizing atmosphere (Ar gas).

[0077] In the thin-strand fabrication process, alloy powder (alloy strips) was produced from the raw metals by an ultra-rapid solidification method. The raw metals (molten metal) used in the thin-strand fabrication process contained Nd, Fe, Co, Ga, Al, and B. The Nd content in the raw metal was 30.17% by mass. The Co content in the raw metal was 3.96% by mass. The Ga content in the raw metal was 0.59% by mass. The Al content in the raw metal was 0.04% by mass. The content of B in the raw metal was 0.97% by mass. The remaining raw material metal after Nd, Co, Ga, Al, and B was Fe.

[0078] In the hot forming process, a molded body was produced by heating the alloy powder in the mold and compressing it within the mold. The molded body was cubic. The dimensions of the molded body were 10 mm × 10 mm × 10 mm. The hot forming temperature was 660°C. The hot forming pressure was 100 MPa. The hot forming time was 240 seconds.

[0079] A hot plastic deformation process was carried out following the hot forming process. In the hot plastic deformation process, a magnet base material was manufactured by hot extrusion molding of the molded body using the mold described above (a mold in which the cavity 10 shown in Figure 3 was formed). The length of the first side xc in the extrusion port side region 10C (end face) was 7 mm. The length of the second side yc in the extrusion port side region 10C (end face) was 30 mm.

[0080] The degree of plastic deformation D, defined by Equation 3 below, was 70%. In Equation 3, Lf is the dimension of the magnet substrate in the direction of the easy magnetization axis. In Equation 3, Li is the dimension of the molded body in the direction corresponding to the easy magnetization axis of the magnet substrate (i.e., 10 mm). D = (Li - Lf) / Li (3) The mold temperature during the hot plastic deformation process was 750°C. The hot plastic deformation pressure (maximum pressure) was 60 MPa. The extrusion speed during hot extrusion molding was 0.1 mm / second.

[0081] A cooling process was carried out following the hot plastic deformation process. In the cooling process, a press machine was used to apply a nearly constant pressure Pc to the magnet substrate, which was approximately parallel to the easy magnetization axis C of the magnet substrate. The pressure Pc in the cooling process was the value shown in Table 1. The pressurization time t (time during which the pressure Pc was held) in the cooling process was the value shown in Table 1 below. The ram speed Vc (limit ram speed) of the press machine used in the cooling process was the value shown in Table 1 below.

[0082] The permanent magnet of Example 1 was fabricated using the method described above.

[0083] (Examples 2-10, Comparative Examples 1 and 2) The pressure Pc in the cooling process for Examples 2-10 and Comparative Example 2 was the value shown in Table 1. The pressurization time t in the cooling process for Examples 2-10 and Comparative Example 2 was the value shown in Table 1 below. The ram velocity Vc in the cooling process for Examples 2-10 and Comparative Example 2 was the value shown in Table 1 below. In the case of Comparative Example 1, no pressure Pc was applied to the magnet substrate during the cooling process, and the magnet substrate was allowed to cool naturally. Permanent magnets for Examples 2-10 and Comparative Examples 1 and 2 were manufactured in the same manner as in Example 1, except for the cooling process.

[0084] <Analysis of permanent magnets> (Composition and microstructure of permanent magnets) The cross-sections of the permanent magnets from Examples 1-10 and Comparative Examples 1 and 2 were observed using a scanning electron microscope (SEM). The observed cross-sections of each permanent magnet were parallel to the easy magnetization axis of the permanent magnet. The composition of the cross-sections of each permanent magnet was analyzed using an electron probe microanalyzer (EPMA) and an energy-dispersive X-ray spectroscopy (EDS) system. In all of Examples 1-10 and Comparative Examples 1 and 2, the permanent magnets had the following characteristics. A permanent magnet consists of numerous main phase particles (Nd2Fe 14 It contained crystal grains of B. R-rich phase (Nd-rich phase) was formed at the grain boundaries. Multiple voids were formed within the permanent magnet. Each principal phase particle observed in the cross-section was flattened. Numerous main phase particles were stacked along the easy magnetization axis.

[0085] (Dimensions of the main phase particles) Backscattered electron images of the cross-sections of the permanent magnets in Examples 1-10 and Comparative Examples 1 and 2 were taken using a scanning electron microscope (SEM). The cross-sections from which the backscattered electron images were taken were parallel to the easy magnetization axis. The dimensions of the backscattered electron images were 88 μm vertically and 126 μm horizontally. Several representative locations within the backscattered electron images were selected, and backscattered electron images of each location were taken at high magnification. The lengths of the major and minor axes of the main phase particles (primary particles) present in the high-magnification backscattered electron images were measured. The shape of each main phase particle was approximated by the rectangle with the smallest area that circumscribing the main phase particle. The length of the long side of this rectangle was considered to be the length of the major axis of the main phase particle, and the length of the short side of the rectangle was considered to be the length of the minor axis of the main phase particle. The average value Lc of the minor axis lengths of all main phase particles present in the high-magnification backscattered electron images was calculated. The average value Lc of the minor axis lengths of the main phase particles (primary particles) in Examples 1-10 and Comparative Examples 1 and 2 are shown in Table 1 below.

[0086] (Measurements related to voids) As shown in Figure 4(a), a backscattered electron image i4a of a portion of the cross-section of the permanent magnet of Example 4 was captured by SEM. The cross-section from which the backscattered electron image i4a of Example 4 was captured was parallel to the easy magnetization axis. Images i4b in Figure 4(b), i4c in Figure 5(a), and i4d in Figure 5(b) are images obtained from the backscattered electron image i4a of Example 4. A backscattered electron image of a portion of the cross-section of the permanent magnet of Example 4, captured at a higher magnification than the backscattered electron image i4a, is shown in Figure 6. The brightness of backscattered electron beams in any part of the backscattered electron image i4a (in arbitrary units) increases with increasing atomic weight of the element present in that part. Therefore, relatively bright areas in the backscattered electron image i4a correspond to areas with relatively high concentrations of elements with relatively large atomic weights (e.g., Nd). Conversely, the darkest areas in the backscattered electron image i4a correspond to voids where no elements are present. A monochrome image i4b was obtained by thresholding (binarization) of the backscattered electron image i4a based on the RGB color model (Red-Green-Blue color model). The black areas in the monochrome image i4b represent voids. The area of ​​each void in the monochrome image i4b was measured. Based on the measured area of ​​each void, the area ratio (pr) of multiple voids in the cross-section of the permanent magnet (backscattered electron image i4a), the average value of the areas of each void in the same cross-section (p_avg), and the standard deviation of the areas of each void in the same cross-section (p_std) were calculated. The area ratio (pr), average value of the area (p_avg), and standard deviation of the area (p_std) for Example 4 are shown in Table 1. Image processing of the monochrome image i4b identified the contours of each void in image i4b. Each of the multiple closed curves contained in image i4c corresponds to the contour of each void in image i4b. The coordinates (x,y) of the geometric center of each void were determined from the contour of each void in image i4c. Each dot in image i4d represents the coordinates (x,y) of the geometric center of each void. The position of each void in image i4c corresponds to the coordinates of each dot in image i4d. The correlation coefficient r of x and y was calculated from the coordinates (x,y) of the geometric center of each void. The correlation coefficient r for Example 4 is shown in Table 1 below. The image processing described in Example 4 above was performed using ImageJ, a public domain image processing software. The image processing of Example 4 described above revealed the aspect ratio (L) of the void in Example 4. L / L S The average value of L in Example 4 was measured. L / L S The average values ​​are shown in Table 1 below.

[0087] Using the same method as in Example 4, the area ratio (pr), mean area (p_avg), standard deviation of area (p_std), correlation coefficient r, and L were obtained for Examples 1-3, Examples 5-10, and Comparative Examples 1 and 2, respectively. L / L SThe average value was calculated. For Examples 1-3, Examples 5-10, and Comparative Examples 1 and 2, the area ratio (pr), mean area (p_avg), standard deviation of area (p_std), correlation coefficient r, and L were calculated respectively. L / L S The average values ​​are shown in Table 1 below. A backscattered electron image i2a of a portion of the cross-section of the permanent magnet of Comparative Example 2 is shown in Figure 7(a). The cross-section from which the backscattered electron image i2a of Comparative Example 2 was taken was parallel to the easy magnetization axis. Images i2b in Figure 7(b), image i2c in Figure 8(a), and image i2d in Figure 8(b) are images obtained from the backscattered electron image i2a of Comparative Example 2. The monochrome image i2b was obtained by thresholding the backscattered electron image i2a of Comparative Example 2. Each of the multiple closed curves in image i2c corresponds to the contour of each void in image i2b. Each dot in image i2d represents the coordinates (x,y) of the geometric center of each void. The position of each void in image i2c corresponds to the coordinates of each dot in image i2d.

[0088] (Magnetic properties of permanent magnets) The remanent magnetic flux density (Br), coercivity (HcJ), and aspect ratio (Hk / HcJ) of the permanent magnets in Examples 1-10 and Comparative Examples 1 and 2 were measured. The remanent magnetic flux density, coercivity, and aspect ratio were measured using a BH tracer. Coercivity was measured at 23°C. Br was measured at room temperature. The aspect ratio was measured at 23°C. The measured values ​​of the magnetic properties of each permanent magnet are shown in Table 1 below.

[0089] [Table 1] [Industrial applicability]

[0090] One aspect of the present invention, the R-T-B permanent magnet, is suitable, for example, as a material for motors mounted in electric vehicles or hybrid vehicles. [Explanation of Symbols]

[0091] 2…R-T-B permanent magnet, 2cs…Cross-section of the permanent magnet, 4…Main phase particles, 6…R-rich phase, 8…Air gap, C…Easy magnetization axis direction, AB…Direction approximately perpendicular to the easy magnetization axis direction.

Claims

1. A permanent magnet containing rare earth elements R, transition metal elements T and B, The R-T-B permanent magnet contains at least Nd as R, The R-T-B permanent magnet contains at least Fe as T, The R-T-B permanent magnet comprises a plurality of main phase particles and a plurality of voids, The plurality of main phase particles contain at least R, T, and B, The area ratio of the multiple voids in any cross-section of the R-T-B permanent magnet is greater than 0.2% and less than or equal to 2%. The average area of ​​each of the multiple voids in any cross-section of the R-T-B permanent magnet is between 0.1 (μm)² and 7 (μm)². R-T-B series permanent magnet.

2. A permanent magnet containing rare earth elements R, transition metal elements T and B, The R-T-B permanent magnet contains at least Nd as R, The R-T-B permanent magnet contains at least Fe as T, The R-T-B permanent magnet comprises a plurality of main phase particles and a plurality of voids, The plurality of main phase particles contain at least R, T, and B, The area ratio of the multiple voids in any cross-section of the R-T-B permanent magnet is greater than 0.2% and less than or equal to 2%. The standard deviation of the area of ​​each of the multiple gaps in any cross-section of the R-T-B permanent magnet is 0 (μm)² or more and 5 (μm)² or less. R-T-B series permanent magnet.

3. A permanent magnet containing rare earth elements R, transition metal elements T and B, The R-T-B permanent magnet contains at least Nd as R, The R-T-B permanent magnet contains at least Fe as T, The R-T-B permanent magnet comprises a plurality of main phase particles and a plurality of voids, The plurality of main phase particles contain at least R, T, and B, The area ratio of the multiple voids in any cross-section of the R-T-B permanent magnet is greater than 0.2% and less than or equal to 2%. The arbitrary cross-section of the R-T-B system permanent magnet is rectangular. One of the four sides of the aforementioned rectangle is the x-axis, Of the four sides of the rectangle, one side perpendicular to the x-axis is the y-axis. The intersection of the x-axis and the y-axis is the origin. In the coordinate system formed by the origin, the x-axis, and the y-axis, the position of the geometric center of each of the plurality of gaps is represented as (x, y), The absolute value of the correlation coefficient r between x and y, calculated from the positions of the geometric centers of each of the aforementioned multiple voids, is between 0 and 0.

2. R-T-B series permanent magnet.

4. A permanent magnet containing rare earth elements R, transition metal elements T and B, The R-T-B permanent magnet contains at least Nd as R, The R-T-B permanent magnet contains at least Fe as T, The R-T-B permanent magnet comprises a plurality of main phase particles and a plurality of voids, The plurality of main phase particles contain at least R, T, and B, The area ratio of the multiple voids in any cross-section of the R-T-B permanent magnet is greater than 0.2% and less than or equal to 2%. The length of each of the multiple gaps observed in the arbitrary cross-section of the R-T-B permanent magnet is expressed as L L, The lengths of the minor axis directions of each of the multiple gaps observed in the arbitrary cross-section of the R-T-B permanent magnet are denoted as L and S. The aspect ratio of each of the multiple air gaps observed in the arbitrary cross-section of the R-T-B permanent magnet is expressed as L L / L S, The average value of L L / L S is between 1 and 2. R-T-B series permanent magnet.

5. In a cross-section of the R-T-B permanent magnet that is substantially parallel to the easy magnetization axis direction of the R-T-B permanent magnet, the plurality of main phase particles are flattened. In the cross-section of the R-T-B permanent magnet, which is substantially parallel to the easy magnetization axis direction, the average length of the short axis of the plurality of main phase particles is 20 nm or more and 200 nm or less. An R-T-B permanent magnet according to any one of claims 1 to 4.

6. The R content is 28% by mass or more and 33% by mass or less. The content of B is 0.75% by mass or more and 1.20% by mass or less. The R-T-B permanent magnet according to any one of claims 1 to 4.

7. The plurality of main phase particles are stacked along the easy magnetization axis of the R-T-B system permanent magnet. The R-T-B permanent magnet according to any one of claims 1 to 4.

8. Hot-worked magnets, The R-T-B permanent magnet according to any one of claims 1 to 4.

9. It further comprises multiple R-rich phases, The concentration of R in the plurality of R-rich phases is higher than the concentration of R in the plurality of main phase particles. The unit of concentration for R is atomic percent. The R-T-B permanent magnet according to any one of claims 1 to 4.

Images