Patent Application
20140247100
The present invention relates to an R-T-B sintered magnet and a method for its production, and to a rotary machine.
For driving motors used in a variety of different fields, there is increasing demand for smaller sizes and lighter weights, as well as increased efficiency, in line with the goal of reducing installation space and lowering cost. Along with this demand there is a desire for techniques that allow further improvement in, for example, the magnetic properties of sintered magnets to be used in driving motors.
R-T-B rare earth sintered magnets have been used in the past as sintered magnets with high magnetic properties. It has been attempted to improve the magnetic properties of R-T-B sintered magnets using heavy rare earth metals such as Dy and Tb, which have large anisotropic magnetic fields HA. However, with the rising costs of rare earth metal materials in recent years, there has been a strong desire to reduce the amount of usage of expensive heavy rare earth elements. In light of this situation, it has been attempted to improve magnetic properties by micronizing the structures of R-T-B sintered magnets.
Incidentally, R-T-B sintered magnets are produced by powder metallurgy methods. In production methods by powder metallurgy, first the starting material is melted and cast, to obtain an alloy strip containing the R-T-B based alloy. Next, the alloy strip is ground to prepare alloy powder having particle diameters of between several μm and several tens of μm. The alloy powder is then molded and sintered to produce a sintered compact. Next, the obtained sintered compact is worked to the prescribed dimensions. In order to improve the corrosion resistance, the sintered compact may be subjected to plating treatment if necessary to form a plating layer. It is thus possible to obtain an R-T-B sintered magnet.
In the production method described above, melting and casting of the starting material are usually accomplished by a strip casting method. A strip casting method is a method in which the molten alloy is cooled with a cooling roll to form an alloy strip. In order to improve the magnetic properties of R-T-B sintered magnets, it has been attempted to control the alloy structure by adjusting the cooling rate in the aforementioned strip casting method. For example, PTL 1 proposes obtaining an alloy strip comprising chill crystals, particulate crystals and columnar crystals with prescribed particle diameters, by a strip casting method.
With an alloy strip such as described in PTL 1, however, the shape and size variation of the alloy powder obtained by grinding the alloy strip is considerable. When such alloy powder is used to produce a sintered magnet, the non-uniform shapes and sizes of the alloy powder make it difficult to significantly improve the magnetic properties. Consequently, it is desirable to establish techniques that allow further improvement in the magnetic properties of R-T-B sintered magnets.
The coercive force (HcJ) and residual flux density (Br) of a sintered magnet have established relationships represented by the following formulas (I) and (II).
HcJ=α·H
A
−N·Ms (I)
Br=Ms·(ρ/ρ0)·f·A (II)
In formula (I), α is a coefficient representing the independence of the crystal grains, HA represents the anisotropic magnetic field that is dependent on the structure, N represents the local demagnetizing field dependent on shape, etc., and Ms represents the saturation magnetization of the main phase. Also, in formula (II), Ms represents the saturation magnetization of the main phase, ρ represents the sintered density, ρ0 represents the true density, f represents the volume ratio of the main phase, and A represents the degree of orientation of the main phase. Of these coefficients, HA, Ms and f are dependent on the structure of the sintered magnet, and N is dependent on the shape of the sintered magnet. As clearly seen from formula (I), increasing α in formula (I) can increase the coercive force. This suggests that controlling the structure of the alloy powder used in the compact for a sintered magnet allows the coercive force to be increased. On the other hand, from the viewpoint of restrictions on resources and production costs, there is a demand for an R-T-B sintered magnet that allows high magnetic properties to be realized without using heavy rare earth elements.
The present invention has been accomplished in light of these circumstances, and its object is to provide an R-T-B sintered magnet having sufficiently excellent coercive force without using expensive and scarce heavy rare earth elements, as well as a method for its production.
The present inventors have conducted much research centered on alloy strip structures with the aim of increasing the magnetic properties of R-T-B sintered magnets. As a result, we have found that by micronizing the structure of the alloy strip and increasing its homogeneity, the finally obtained R-T-B sintered magnet structure is micronized and R-rich phase segregation is inhibited, so that high magnetic properties can be stably obtained.
Specifically, the invention provides an R-T-B sintered magnet comprising particles containing an R2T14B phase, obtained using an R-T-B alloy strip containing crystal grains of an R2T14B phase, wherein the R-T-B alloy strip has crystal grains extending in a radial fashion from the crystal nuclei in a cross-section along the thickness direction, the following inequality (1) being satisfied, where the average value of the lengths of the crystal grains on one side in the direction perpendicular to the thickness direction and the average value of the lengths on the other side opposite the one side are represented as D1 and D2, respectively, the mean particle diameter of particles comprising the R2T14B phase in the R-T-B sintered magnet is 0.5 to 5 μm, and essentially no heavy rare earth elements are present. R represents a light rare earth element, T represents a transition element, and B represents boron.
0.9≦D2/D1≦1.1 (1)
The R-T-B sintered magnet of the invention employs an R-T-B alloy strip having the following structure, as a starting material. Specifically, the shapes of the R2T14B phase crystal grains in the R-T-B alloy strip do not extend in the direction perpendicular to the thickness direction of the R-T-B alloy strip, and variation in the shapes and widths of the crystal grains is sufficiently reduced. Usually when an R-T-B alloy strip is ground, the grain boundary phase, such as the R-rich phase at the grain boundaries of the R2T14B phase crystal grains, are preferentially fractured. The form of the alloy powder therefore depends on the shapes of the crystal grains of the R2T14B phase. The crystal grains of the R2T14B phase in the R-T-B alloy strip of the invention have sufficiently reduced variation in the columnar crystal shapes and widths, and it is thus possible to obtain an R-T-B alloy powder with sufficiently reduced variation in form and size. Thus, using such an R-T-B alloy strip allows an R-T-B sintered magnet to be obtained having minimized segregation of the R-rich phase as well as increased homogeneity of the microstructure.
In other words, the present invention does not employ a method of control by simply micronizing the crystal grains of the R2T14B phase in the R-T-B alloy strip, but rather controls the variation in the sizes and shapes of the R2T14B phase crystal grains to obtain a sharp structural distribution, and to increase the coercive force of the finally obtained R-T-B sintered magnet.
The R-T-B alloy strip preferably satisfies the following inequalities (2) and/or (3), where DAVE and DMAX are, respectively, the average value and maximum value for the lengths of the crystal grains in the direction perpendicular to the thickness direction, in the aforementioned cross-section.
1.0 μm≦DAVE<3.0 μm (2)
1.5 μm≦DMAX≦4.5 μm (3)
Since such an R-T-B alloy strip has sufficiently small widths of the crystal grains of the R2T14B phase and also sufficiently reduced variation in shapes, it can yield R-T-B alloy powder that is micronized and has sufficiently increased homogeneity of form and size. This further increases the homogeneity of the microstructure of the finally obtained R-T-B sintered magnet. As a result, the coercive force of the R-T-B sintered magnet can be further increased.
The R-T-B alloy strip of the invention contains an R-rich phase in which the R content is higher than the R2T14B phase based on mass, and the percentage of the number of R-rich phases with lengths of no greater than 1.5 μm in the direction perpendicular to the thickness direction in the cross-section, with respect to the total number of R-rich phases, is preferably 90% or greater. This allows an R-T-B alloy powder to be obtained that is even more micronized and has increased size homogeneity. As a result, the coercive force of the finally obtained R-T-B sintered magnet can be even further increased. An R-rich phase is a phase with a higher R content based on mass than the R2T14B phase.
The crystal grains of the R-T-B alloy strip are dendritic crystals, and preferably on at least one surface of the R-T-B alloy strip, the average value for the widths of the dendritic crystals is no greater than 60 μm, and the number of crystal nuclei in the dendritic crystals is at least 500 per 1 mm square area. The R-T-B alloy strip has at least a prescribed number of crystal nuclei per unit area on at least one surface. Such dendritic crystals have minimal growth in the in-plane direction of the R-T-B alloy strip. Therefore, R2T14B phases grow in a columnar fashion in the thickness direction. An R-rich phase is produced surrounding the R2T14B phases that have grown in a columnar fashion, and the R-rich phase fractures preferentially during grinding. Thus, grinding of an R-T-B alloy strip having such a structure can yield alloy powder in a uniformly dispersed state without segregation of the R-rich phase, compared to the prior art. Thus, firing such an alloy powder can minimize aggregation of the R-rich phase and abnormal grain growth of the crystal grains, to obtain an R-T-B sintered magnet having high coercive force.
The invention also provides a method for production of an R-T-B sintered magnet comprising particles containing an R2T14B phase, which has a step of grinding, molding and firing an R-T-B alloy strip, wherein the R-T-B alloy strip has crystal grains extending in a radial fashion from the crystal nuclei in a cross-section along the thickness direction, the following inequality (1) being satisfied, where the average value of the lengths of the crystal grains on one side in the direction perpendicular to the thickness direction and the average value of the lengths on the other side opposite the one side are represented as D1 and D2, respectively, the mean particle diameter of particles is 0.5 to 5 μm, and essentially no heavy rare earth elements are present. R represents a light rare earth element, T represents a transition element, and B represents boron.
0.9≦D2/D1≦1.1 (1)
In this production method there is employed an R-T-B alloy strip having the following structure, as a starting material. Specifically, the R-T-B alloy strip is such that the shapes of the R2T14B phase crystal grains do not extend in the direction perpendicular to the thickness direction of the R-T-B alloy strip, and variation in the shapes and widths of the crystal grains is sufficiently reduced. Consequently, it is possible to obtain an R-T-B alloy powder with sufficiently reduced variation in shapes and sizes. By using such R-T-B alloy powder it is possible to obtain an R-T-B sintered magnet having minimized segregation of the R-rich phase as well as increased homogeneity of the microstructure, and sufficiently high coercive force.
According to the invention it is possible to provide an R-T-B sintered magnet having sufficiently excellent coercive force without using expensive and scarce heavy rare earth elements, as well as a method for its production.
Preferred embodiments of the invention will now be explained with reference to the accompanying drawings where necessary. For the drawings, identical or corresponding elements will be referred to by like reference numerals and will be explained only once.
The term “rare earth element”, for the purpose of the present specification, refers to scandium (Sc), yttrium (Y) and lanthanoid elements belonging to Group 3 of the long Periodic Table, the lanthanoid elements including, for example, lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu). Of these, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu are heavy rare earth elements, and Sc, Y, La, Ce, Pr, Nd, Sm and Eu are light rare earth elements.
The R-T-B sintered magnet 100 in this embodiment comprises a light rare earth element, but comprises essentially no heavy rare earth elements. Even essentially without using heavy rare earth elements, since an R-T-B alloy strip with a specific structure is used as the starting material, the homogeneity of the structure is improved and it exhibits sufficiently high magnetic properties.
The R-T-B sintered magnet 100 preferably comprises at least Fe as a transition element (T), and more preferably it comprises a combination of Fe and a transition element other than Fe. Transition elements other than Fe include Co, Cu and Zr. However, the R-T-B sintered magnet 100 may contain heavy rare earth elements as impurities of the starting material or impurities introduced as contaminants during production. The content is preferably no greater than 0.01 mass % based on the total R-T-B sintered magnet 100. The upper limit for the content is 0.1 mass %, as a range that has virtually no influence on the object and effect of the invention. Thus, the phrase “comprising essentially no heavy rare earth elements” used throughout the present specification includes cases where heavy rare earth elements are included in impurity-level amounts.
The R-T-B sintered magnet 100 may contain about 0.001 to 0.5 mass % of unavoidable impurities such as Mn, Ca, Ni, Si, Cl, S and F, in addition to the elements mentioned above. However, the content of these impurities is preferably less than 2 mass % and more preferably less than 1 mass % in total.
The oxygen content of the R-T-B sintered magnet 100 is preferably 300 to 3000 ppm and more preferably 500 to 1500 ppm, from the viewpoint of achieving an even higher level for the magnetic properties. The nitrogen content of the R-T-B sintered magnet 100 is 200 to 1500 ppm and preferably 500 to 1500 ppm, from the same viewpoint explained above. The carbon content of the R-T-B sintered magnet 100 is 500 to 3000 ppm and preferably 800 to 1500 ppm, from the same viewpoint explained above.
The R-T-B sintered magnet 100 comprises particles containing an R2T14B phase as the main component. The mean particle diameter of the particles is 0.5 to 5 μm, preferably 2 to 5 μm and more preferably 2 to 4 μm. Thus, the R-T-B sintered magnet 100 contains particles with a small mean particle diameter as the main component, and the structure is fine. In addition, variation in the particle diameters and shapes of the particles is very low. Thus, the R-T-B sintered magnet 100 not only contains particles with small particle diameters but also has low variation in particle diameters and shapes, and therefore the structural homogeneity is sufficiently improved. Consequently, segregation of phases different from the R2T14B phase, such as an R-rich phase, is minimized. The R-T-B sintered magnet 100 of this embodiment therefore has high magnetic properties. The mean particle diameter of the particles containing the R2T14B phase in the R-T-B sintered magnet 100 can be determined in the following manner. A cut surface of the R-T-B sintered magnet 100 is polished, and then a metallographic microscope is used for observation of an image of the polished surface. Upon image processing, the particle diameters of the individual particles are measured and the arithmetic mean of the measured values is recorded as the mean particle diameter.
The crystal grains 150 of the R-T-B sintered magnet 100 preferably comprise an R2T14B phase. On the other hand, the triple point regions 140 include a phase with a higher R content ratio than the R2T14B phase, based on mass compared to the R2T14B phase. The average value of the area of the triple point regions 140 in a cross-section of the R-T-B sintered magnet 100 is no greater than 2 μm2 and preferably no greater than 1.9 μm2, as the arithmetic mean. Also, the standard deviation for the area distribution is no greater than 3 and preferably no greater than 2.6. Since the R-T-B sintered magnet 100 thus has minimal segregation of the phase with a higher R content than the R2T14B phase, the area of the triple point regions 140 is low and the variation in area is also reduced. It is thus possible to maintain high levels for both Br and HcJ.
The average value for the area of the triple point regions 140 in the cross-section, and the standard deviation for the area distribution, can be calculated in the following manner. First, the R-T-B sintered magnet 100 is cut and the cut surface is polished. The polished surface image is observed with a scanning electron microscope. Image analysis is performed and the area of the triple point regions 140 is calculated. The arithmetic mean value for the calculated area is the mean area. Also, the standard deviation for the area of the triple point regions 140 can be calculated based on the area of each of the triple point regions 140 and their average value.
The rare earth element content in the triple point regions 140 is preferably 80 to 99 mass %, more preferably 85 to 99 mass % and even more preferably 90 to 99 mass %, from the viewpoint of obtaining an R-T-B sintered magnet with sufficiently high magnetic properties and sufficiently excellent corrosion resistance. From the same viewpoint, the rare earth element contents of each of the triple point regions 140 are preferably equal. Specifically, the standard deviation for the content distribution in the triple point regions 140 of the R-T-B sintered magnet 100 is preferably no greater than 5, preferably no greater than 4 and more preferably no greater than 3.
The R-T-B sintered magnet 100 comprises dendritic crystal grains containing an R2T14B phase, and grain boundary regions containing a phase with a higher R content than the R2T14B phase, and preferably it is obtained by molding and firing a ground product of an R-T-B alloy strip having an average value of no greater than 3 μm for the spacing between the phases with a higher R content than the R2T14B phase in a cross-section. Since such an R-T-B sintered magnet 100 is obtained using a ground product that is sufficiently micronized and has a sharp particle size distribution, it is possible to obtain an R-T-B based sintered compact composed of fine crystal grains. In addition, since the phase with a higher R content than the R2T14B phase will be present in a higher proportion at the outer periphery than in the interior of the ground product, the state of dispersion of the phase with a higher R content than the R2T14B phase after sintering will tend to be more satisfactory. Thus, the structure of the R-T-B based sintered compact will be micronized and the homogeneity will be improved. It will thereby be possible to further increase the magnetic properties of the R-T-B based sintered compact.
An R-T-B alloy strip to be used as the starting material for the R-T-B sintered magnet 100 of this embodiment will now be described.
As shown in
The R-T-B alloy strip used for this embodiment does not have significant spread of the R2T14B phase crystal grains 2 in the direction perpendicular to the thickness direction (the left-right direction in
The R-T-B alloy strip to be used for this embodiment satisfies the following inequality (1), where D1 and D2 are, respectively, the average value for the lengths of the crystal grains 2 on one (the lower) surface side, in the direction perpendicular to the thickness direction of the R-T-B alloy strip, i.e. the left-right direction in
0.9≦D2/D1≦1.1 (1)
Throughout the present specification, D1, D2 and D3 are determined as follows. First, a cross-section such as shown in
Since D2/D1 for the R-T-B alloy strip used for this embodiment satisfies inequality (1) above, the widths and shapes of the crystal grains 2 have low variation and high homogeneity in the thickness direction. From the viewpoint of further increasing the homogeneity, the value of D2/D1 preferably satisfies the following inequality (4) and more preferably satisfies the following inequality (5). The lower limit of D2/D1 may be 1.0.
0.95≦D2/D1<1.05 (4)
0.98≦D2/D1≦1.02 (5)
The R-T-B alloy strip used for this embodiment may be produced by a strip casting method using a cooling roll as described below. In this case, R2T14B phase crystal nuclei 1 of the R-T-B alloy strip are deposited on the contact surface with the cooling roll (the casting surface). The R2T14B phase crystal grains 2 grow in a radial fashion from the casting surface side of the R-T-B alloy strip toward the side opposite the casting surface (the free surface). Thus, in the R-T-B alloy strip shown in
The values of D1, D2 and D3 are, for example, 1 to 4 μm, preferably 1.4 to 3.5 μm, and more preferably 1.5 to 3.2 μm. If the values of D1, D2 and D3 are large, it will tend to be difficult to sufficiently micronize the alloy powder that is obtained by grinding. On the other hand, an R-T-B alloy strip with excessively low values for D1, D2 and D3, while maintaining the crystal grain shapes, will generally be difficult to produce.
The R-T-B alloy strip of this embodiment preferably satisfies the following inequalities (2) and/or (3), where DAVE and DMAX are, respectively, the average value and maximum value for the lengths of the crystal grains 2 in the direction perpendicular to the thickness direction, in the cross-section shown in
1.0 μm≦DAVE<3.0 μm (2)
1.5 μm≦DMAX≦4.5 μm (3)
Throughout the present specification, DAVE is the average value for D1, D2 and D3 as determined from results of observation of the aforementioned SEM-BEI image (magnification: 1000×), and DMAX is the value for the image with the maximum lengths of the crystal grains 2, among a total of 45 images, taken in 15 visual fields each on one surface side, the other surface side and the center section.
Specifically, inequality (2) specifies that the sizes (widths) of the crystal grains 2 are in a prescribed range, and inequality (3) specifies that the variation in the sizes (widths) of the crystal grains 2 is within a prescribed range. An R-T-B alloy strip satisfying inequalities (2) and (3) is composed of crystal grains 2 that are further micronized and have sufficiently reduced variation in shapes and sizes, and an R-rich phase 4 that is further micronized and has sufficiently reduced variation in shapes and sizes. Consequently, using alloy powder obtained by grinding such an R-T-B alloy strip can yield an R-T-B sintered magnet with further inhibited segregation of the R-rich phase and further increased microstructural homogeneity. If DAVE and DMAX are too small, ultrafine powder will increase during fine grinding, and the amount of oxygen will tend to increase. Also, chill crystals, which are equiaxial crystals, will also increase, and when a sintered magnet is formed the residual flux density (Br) will tend to be lowered.
From the viewpoint of obtaining an R-T-B sintered magnet that is even more micronized and has a uniform structure, DAVE preferably satisfies the following inequality (6). From the same viewpoint, DMAX preferably satisfies the following inequality (7). The R-T-B alloy strip will thus be one that can yield an R-T-B sintered magnet having an even more micronized structure, while also facilitating production of the R-T-B alloy strip.
1.0 μm≦DAVE≦2.4 μm (6)
1.5 μm≦DMAX≦3.0 μm (7)
From the viewpoint of obtaining an R-T-B sintered magnet that has an even more micronized structure and facilitating production of the R-T-B alloy strip, DAVE preferably satisfies the following inequality (8). From the same viewpoint, DMAX preferably satisfies the following inequality (9).
1.5 μm≦DAVE≦2.4 μm (8)
2.0 μm≦DMAX≦3.0 μm (9)
In the cross-section shown in
The width M of the columnar crystal grains 2 of the R-T-B alloy strip having the cross-section shown in
The R-T-B sintered magnet 100 of this embodiment can be produced by the following procedure. The method for producing the R-T-B sintered magnet 100 comprises a melting step in which a molten R-T-B based alloy is prepared, a cooling step in which the molten alloy is poured onto the roll surface of the cooling roll rotating in the circumferential direction, cooling the molten alloy by the roll surface, to obtain an R-T-B alloy strip, a grinding step in which the R-T-B alloy strip is ground to obtain an R-T-B alloy powder, a molding step in which the alloy powder is molded to form a compact, and a firing step in which the compact is fired to obtain an R-T-B sintered magnet.
In the melting step, a starting material comprising at least one rare earth metal or rare earth alloy, or pure iron, ferroboron or an alloy thereof, for example, and containing no heavy rare earth elements, is introduced into a high-frequency melting furnace. In a high-frequency melting furnace, the starting material is heated to 1300° C. to 1500° C. to prepare a molten alloy.
The average value for the spacings a and b is preferably 40 to 100 μm. If the average value is too large, the number of crystal nuclei generated during cooling will be too low, and it will tend to be difficult to obtain crystal grains with sufficiently small widths M. However, it is not easy to form recesses 32, 34 having spacings with an average value of 40 μm or smaller.
The surface roughness Rz of the roll surface 17 is preferably 3 to 5 μm, more preferably 3.5 to 5 μm and even more preferably 3.9 to 4.5 μm. If Rz is too large the thickness of the strip will vary, tending to increase variation in the cooling rate, whereas if Rz is too small, adhesiveness between the molten alloy and the roll surface 17 will be insufficient, and the molten alloy or alloy strip will tend to detach from the roll surface earlier than the target time. In this case, the molten alloy migrates to the secondary cooling section without sufficient progression of heat loss of the molten alloy. Therefore, the alloy strips will tend to inconveniently stick together at the secondary cooling section.
The surface roughness Rz, for the purpose of the present specification, is the ten-point height of irregularities and is the value measured according to JIS B 0601-1994. Rz can be measured using a commercially available measuring apparatus (SURFTEST by Mitsutoyo Corp.).
The angle θ formed by the first recesses 32 and second recesses 34 is preferably 80-100° and more preferably 85-95°. By specifying such an angle θ, it will be possible for greater columnar growth of the crystal nuclei of the R2T14B phase deposited on the raised sections 36 of the roll surface 17 to proceed toward the thickness direction of the alloy strip.
Throughout the present specification, the average value H of the heights of the raised sections 36 and the average value W of the spacing between raised sections 36 are calculated in the following manner. Using a laser microscope, a profile image (magnification: 200×) was taken of a cross-section of the cooling roll 16 near the roll surface 17, as shown in
Also, in the same image, 100 points were measured for both spacings w1 and spacings w2 of arbitrarily selected raised sections 36. Measurement of the spacings was conducted considering only heights h1 and h2 of 3 μm and greater as raised sections 36. The arithmetic mean value of measurement data for a total of 200 points was recorded as the average value W for the spacings of the raised sections 36. When it is difficult to observe a concavoconvex pattern on the roll surface 17 with a scanning electron microscope, a replica may be formed by replicating the concavoconvex pattern of the roll surface 17, and the surface of the replica observed with a scanning electron microscope and measured as described above. A replica can be formed using a commercially available kit (SUMP SET by Kenis, Ltd.).
The concavoconvex pattern of the roll surface 17 can be adjusted by working the roll surface 17 with a short wavelength laser, for example.
The average value H of the heights of the raised sections 36 is preferably 7 to 20 μm. This will cause the recesses 32, 34 to be thoroughly saturated with the molten alloy and allow adhesiveness between the molten alloy 12 and roll surface 17 to be sufficiently increased. The upper limit for the average value H is more preferably 16 μm and even more preferably 14 μm, from the viewpoint of more thoroughly saturating the recesses 32, 34 with the molten alloy. The lower limit for the average value H is more preferably 8.5 μm and even more preferably 8.7 μm, from the viewpoint of obtaining R2T14B phase crystals with sufficiently high adhesiveness between the molten alloy and the roll surface 17, while also having more uniform orientation in the thickness direction of the alloy strip.
The average value W of the spacing between raised sections 36 is 40 to 100 μm. The upper limit for the average value W is preferably 80 μm, more preferably 70 μm and even more preferably 67 μm, from the viewpoint of further reducing the widths of the R2T14B phase columnar crystals and obtaining magnet powder with a small particle diameter. The lower limit for the average value W is preferably 45 μm and more preferably 48 μm. This will allow an R-T-B sintered magnet to be obtained having even higher magnetic properties.
For this embodiment, a cooling roll 16 having a roll surface 17 such as shown in
The role surface 17 of the cooling roll 16 has raised sections 36 that have prescribed heights and have arranged in a prescribed spacing. Numerous R2T14B phase crystal nuclei 1 are generated on the roll surface 17, after which the columnar crystals 2 grow in a radial fashion with the crystal nuclei 1 as origins. During this time, growth of the columnar crystals 2 proceeds in the thickness direction of the R-T-B alloy strip, forming R2T14B phase columnar crystals 2 with small widths and low variation in width and shape, and R-rich phases 4 that are even more micronized and have sufficiently reduced variation in shape and size.
The cooling rate can be controlled, for example, by adjusting the temperature or flow rate of cooling water flowing through the interior of the cooling roll 16. The cooling rate can also be adjusted by varying the material of the roll surface 17 of the cooling roll 16.
The cooling rate is preferably 1000° C. to 3000° C./sec and more preferably 1500° C. to 2500° C./sec, from the viewpoint of adequately micronizing the structure of the obtained alloy strip while inhibiting generation of heterophases. If the cooling rate is below 1000° C./sec, an α-Fe phase will tend to be readily deposited, and if the cooling rate exceeds 3000° C./sec, chill crystals will tend to be readily deposited. Chill crystals are isotropic microcrystals with particle diameters of 1 μm and smaller. High generation of chill crystals tends to impair the magnetic properties of the finally obtained R-T-B sintered magnet.
Cooling with the cooling roll may be followed by secondary cooling in which cooling is carried out by a method such as blowing gas. There are no particular restrictions on the method of secondary cooling, and any conventional cooling method may be employed. For example, it may be one provided with a gas tube 19 having a gas blow hole 19a, wherein cooling gas is blown through the gas blow hole 19a onto the alloy strip accumulated on a rotating table 20 rotating in the circumferential direction. The alloy strip 18 can be sufficiently cooled in this manner. The alloy strip is recovered after sufficient cooling with the secondary cooling section 20. It is thus possible to produce an R-T-B alloy strip having a cross-sectional structure such as shown in
The thickness of the R-T-B alloy strip of this embodiment is preferably no greater than 0.5 mm and more preferably 0.1 to 0.5 mm. If the thickness of the alloy strip becomes too large, the difference in cooling rate will tend to roughen the structure of the crystal grains 2 and impair the homogeneity. Also, the structure near the surface on the roll surface side (the casting surface) and the structure near the surface on the side opposite the casting surface (the free surface) of the alloy strip will differ, and the difference between D1 and D2 will tend to increase.
As shown in
On the other hand, as shown in
The width P of the dendritic crystal 40 is determined as the maximum distance among the distances between tips of two different filler-like crystal grains 2. Normally, the width P is the distance between the tips of two filler-like crystal grains 2 present at roughly opposite ends across the crystal nucleus 1. Throughout the present specification, the average value for the width P of a dendritic crystal 40 is determined in the following manner. In an image of one surface of the metal foil strip enlarged 200× with a metallographic microscope, 100 dendritic crystals 40 are arbitrarily selected and the width P of each of the dendritic crystals 40 is measured. The arithmetic mean value of the measured values is recorded as the average value for the widths P of the dendritic crystals 40.
The average value for the width P of the dendritic crystal 40 is preferably no greater than 60 μm and more preferably 25 to 60 m. The upper limit for the average value for the width P is preferably 55 μm, more preferably 50 μm and even more preferably 48 μm. This can reduce the sizes of the dendritic crystals 40 and yield even finer alloy powder. The lower limit for the average value of the width P is preferably 30 μm, more preferably 35 nn and even more preferably 38 μm. Growth of the R2T14B phase in the thickness direction of the alloy strip will thus be even further accelerated. It will thus be possible to obtain alloy powder with small particle diameters and low particle diameter variation.
The surface of the R-T-B alloy strip shown in
As shown in
For the purpose of the present specification, the average value for the aspect ratio was determined in the following manner. In an image of one surface of the metal foil strip enlarged 200× with a metallographic microscope, 100 crystal groups are arbitrarily selected, and the lengths C1 of the long axes and the lengths C2 of the short axes of each of the crystal groups are measured. The arithmetic mean value for the crystal group ratio (C2/C1) is the average value of the aspect ratio.
For one surface of the R-T-B alloy strip, the number of dendritic crystal nuclei 1 generated is 500 or greater, preferably 600 or greater, more preferably 700 or greater and even more preferably 763 or greater, per 1 mm square. Since the number of crystal nuclei 1 generated is thus high, the size per single crystal nucleus 1 is small, and an R-T-B alloy strip having a micronized structure can be obtained.
The R-T-B alloy strip used for this embodiment may have the structure described above on at least one surface. If at least one surface has such a structure, it will be possible to obtain alloy powder having small particle diameters and a uniformly dispersed R-rich phase.
There are no particular restrictions on the grinding method in the grinding step. The grinding can be carried out in the order of coarse grinding followed by fine grinding. Coarse grinding is preferably carried out in an inert gas atmosphere using, for example, a stamp mill, jaw crusher, Braun mill or the like. Hydrogen storage grinding may also be carried out, in which grinding is performed after hydrogen has been stored. By coarse grinding it is possible to prepare alloy powder with particle diameters of about several hundred μm. The alloy powder prepared by coarse grinding is subjected to fine grinding to a mean particle diameter of 1 to 5 μm, for example, using a jet mill or the like. Grinding of the alloy strip does not necessarily need to be carried out in two stages of coarse grinding and fine grinding, and may instead be carried out in a single step.
In the grinding step, the sections of the grain boundary phases 4 such as the alloy strip R-rich phase sections preferentially undergo fracturing. Consequently, the particle diameters of the alloy powder depend on the spacing of the grain boundary phase 4. The alloy strip to be used in the method for producing for this embodiment has lower variation in widths of the R2T14B phase crystal grains than in the prior art, as shown in
In the molding step, the alloy powder is molded in a magnetic field to obtain a compact. Specifically, first the alloy powder is packed into a die situated in an electromagnet. A magnetic field is then applied by the electromagnet and the alloy powder is pressed while orienting the crystal axes of the alloy powder. Molding is thus carried out in a magnetic field to prepare a compact. The molding in a magnetic field may be carried out in a magnetic field of 12.0 to 17.0 kOe, for example, at a pressure of about 0.7 to 1.5 ton/cm2.
In the firing step, the compact obtained by the magnetic field molding is fired in a vacuum or in an inert gas atmosphere to obtain a sintered compact. The firing conditions are preferably set as appropriate for the conditions including the composition, the grinding method and the particle size. For example, the firing temperature may be set to 1000° C. to 1100° C. for a firing time of 1 to 5 hours.
Since the R-T-B sintered magnet obtained by the production method of this embodiment employs alloy powder comprising highly homogeneous R2T14B phase crystals and an R-rich phase, it can yield an R-T-B sintered magnet with a more homogeneous structure than the prior art. Consequently, the production method of this embodiment allows production of an R-T-B sintered magnet having sufficiently high coercive force while maintaining residual flux density.
The R-T-B sintered magnet obtained by the process described above may also be subjected to aging treatment if necessary. By carrying out aging treatment, it is possible to further increase the coercive force of the R-T-B sintered magnet. Aging treatment is preferably carried out in two stages, for example, under two different temperature conditions such as near 800° C. and near 600° C. Aging treatment under such conditions will tend to result in particularly excellent coercive force. When aging treatment is carried out in a single step, it is preferably at a temperature of near 600° C.
The R-T-B sintered magnet comprises an R2T14B phase as the main phase and an R-rich phase as the heterophase. Since the R-T-B sintered magnet is obtained using alloy powder with low variation in shape and particle diameter, it has increased structural homogeneity and sufficiently excellent coercive force.
A preferred embodiment of a rotary machine (motor) comprising the R-T-B sintered magnet 110 of this embodiment will now be described.
The SPM motor 200 is provided with an R-T-B sintered magnet 110 according to the embodiment described above, in the rotor 120. The R-T-B sintered magnet 110 exhibits high levels in terms of both high magnetic properties and excellent corrosion resistance. Thus, the SPM motor 200 comprising the R-T-B sintered magnet 110 can continuously exhibit high output for prolonged periods.
The embodiment described above is only a preferred embodiment of the invention, and the invention is in no way limited thereto. For example, the R-T-B alloy strip had the crystal nuclei 1 of the R2T14B phase only on one side, but it may also have the crystal nuclei 1 on the other side of the R-T-B alloy strip. In this case, both sides have crystal nuclei 1 such as shown in
The nature of the invention will now be further explained through the following examples and comparative examples. However, the invention is not limited to the examples described below.
An apparatus for production of an alloy strip as shown in
The roll surface 17 of the cooling roll 16 had a concavoconvex pattern comprising straight linear first recesses 32 extending along the rotational direction of the cooling roll 16, and straight linear second recesses 34 perpendicular to the first recesses 32. The average value H for the heights of the raised sections 36, the average value W for the spacings between the raised sections 36, and the surface roughness Rz, were as shown in Table 2. Measurement of the surface roughness Rz was carried out using a measuring apparatus by Mitsutoyo Corp. (trade name: SURFTEST).
The alloy strip obtained by cooling with the cooling roll 16 was further cooled with a secondary cooling section 20 to obtain an alloy strip having an R-T-B based composition. The composition of the alloy strip was as shown in Table 1.
A SEM-BEI image was taken of a cross-section along the thickness direction of the obtained alloy strip (magnification: 350×). The thickness of the alloy strip was determined from the image. The thickness was as shown in Table 2.
In addition, SEM-BEI images of cross-sections along the thickness direction of the alloy strip were for 15 visual fields on the casting surface side, the free surface side and at the center section, for a total of 45 SEM-BEI images (magnification: 1000×). Using the images, 0.15 mm straight lines were drawn to a position 50 μm on the center section side from the casting surface, a position 50 μm on the center section side from the free surface, and to the center section. The values of D1, D2 and D3 were determined from the length of the straight line and the number of crystal grains transected by the straight line.
Incidentally, D1 is the average value for the lengths of the crystal grains on the casting surface side in the direction perpendicular to the thickness direction, D2 is the average value for the lengths of the crystal grains on the free surface side in the direction perpendicular to the thickness direction, and D3 is the average value for the lengths of the crystal grains at the center section in the direction perpendicular to the thickness direction. The average value DAVE was calculated for D1, D2 and D3. Also, DMAX was the value in the image with the maximum crystal grain length among the crystal grain lengths in the direction perpendicular to the thickness direction in the 45 images. The measurement results were as shown in Table 2.
Also, the 45 SEM-BEI images were used to determine the percentage α of the number of R-rich phases with lengths of up to 1.5 μm on the straight line, with respect to the total number of R-rich phases through which the straight line crossed. The results were as shown in Table 2.
The casting surface of the alloy strip was observed with a metallographic microscope, to determine the average value for the widths P of the dendritic crystals, the ratio of the lengths C2 of the short axes with respect to the lengths C1 of the long axes of the dendritic crystal groups (aspect ratio), the area occupancy of the R2T14B phase crystals with respect to the total visual field, and the number of dendritic crystal nuclei generated per unit area (1 mm2). The results are shown in Table 3. The area occupancy of the R2T14B phase crystals is the area ratio of dendritic crystals with respect to the total image, in a metallographic microscope image of the casting surface of the R-T-B alloy strip. In
The alloy strip was then ground to obtain alloy powder with a mean particle diameter of 2.3 to 2.6 μm. The alloy powder was packed into a die situated in an electromagnet, and molded in a magnetic field to produce a compact. The molding was accomplished by pressing at 1.2 ton/cm2 while applying a magnetic field of 15 kOe. The compact was then fired at 930° C. to 1030° C. for 4 hours in a vacuum and rapidly cooled to obtain a sintered compact. The obtained sintered compact was subjected to two-stage aging treatment at 800° C. for 1 hour and at 540° C. for 1 hour (both in an argon gas atmosphere), to obtain an R-T-B sintered magnet for Example 1.
A B-H tracer was used to measure the Br (residual flux density) and HcJ (coercive force) of the obtained R-T-B sintered magnet. The measurement results are shown in Table 3. Also, the mean particle diameter was determined for the particles containing the R2T14B phase in the R-T-B sintered magnet. Specifically, a cut surface of the R-T-B sintered magnet was polished, and then a metallographic microscope was used for observation of an image of the polished surface (magnification: 1600×). Also, upon image processing, the particle diameters of the individual particles were measured and the arithmetic mean of the measured values was recorded as the mean particle diameter. The values of the mean particle diameters are shown in Table 3.
R-T-B sintered magnets for Examples 2 to 6 and Examples 15 to 17 were obtained in the same manner as Example 1, and evaluated, except that the roll surface of the cooling roll was worked to change the average value H for the heights of the raised sections, the average value W for the spacings between the raised sections and the surface roughness Rz, as shown in Table 2, and the structure of the R-T-B alloy strip was changed as shown in Tables 2 and 3. The results are shown in Table 3.
R-T-B sintered magnets for Examples 7 to 14 and Examples 18 to 22 were obtained in the same manner as Example 1, and evaluated, except that the roll surface of the cooling roll was worked to change the average value for the heights of the raised sections, the average value for the spacings between the raised sections and the surface roughness Rz, as shown in Table 2, and the starting materials were changed to change the compositions of the alloy strip as shown in Table 1. The results are shown in Table 3.
An R-T-B alloy strip was obtained for Comparative Example 1 in the same manner as Example 1, except that there were used cooling rolls having only straight linear first recesses on the roll surfaces extending in the rotational direction of the rolls, and the structure of the R-T-B alloy strip was changed as shown in Tables 2 and 3. These cooling rolls did not have second recesses. The average value H for the heights of the raised sections, the average value W for the spacings between the raised sections and the surface roughness Rz, for the cooling rolls, were determined in the following manner. Specifically, the cross-sectional structure near the roll surface was observed with a scanning electron microscope at the cut surface, when the cooling roll was cut on a plane parallel to the axial direction running through the axis of the cooling roll. The average value H for the heights of the raised sections is the arithmetic mean value for the heights of 100 raised sections, and the average value W for the spacings between the raised sections is the arithmetic mean value for the values of spacings between adjacent raised sections measured at 100 different locations.
The alloy strip of Comparative Example 1 was evaluated in the same manner as Example 1. An R-T-B sintered magnet for Comparative Example 1 was fabricated in the same manner as Example 1 and evaluated. The results are shown in Table 3.
R-T-B sintered magnets for Comparative Examples 2 and 3 were obtained in the same manner as Example 1, and evaluated, except that the roll surface of the cooling roll was worked to change the average value H for the heights of the raised sections, the average value W for the spacings between the raised sections and the surface roughness Rz, as shown in Table 2. The results are shown in Table 3.
An R-T-B alloy strip was obtained for each of Comparative Examples 4 and 5 in the same manner as Example 1, except that the starting materials were changed to change the compositions of the alloy strips as shown in Table 1, there were used cooling rolls having only straight linear first recesses on the roll surfaces extending in the rotational direction of the rolls, and the structure of the R-T-B alloy strip was changed as shown in Tables 2 and 3. These cooling rolls did not have second recesses. The average value H for the heights of the raised sections, the average value W for the spacings between the raised sections and the surface roughness Rz, for the cooling rolls, were determined in the same manner as Comparative Example 1. The alloy strips of Comparative Examples 4 and 5 were evaluated in the same manner as Example 1. R-T-B sintered magnets for Comparative Examples 4 and 5 were fabricated in the same manner as Example 1 and evaluated. The results are shown in Table 3.
Based on the results shown in Table 3, it was confirmed that the R-T-B sintered magnets of Examples 1 to 22 have excellent coercive force without containing essentially any heavy rare earth elements such as Dy, Tb and Ho, and have coercive force equivalent to Comparative Example 4 which contains Dy.
For the R-T-B sintered magnet of Example 10 there was used an electron beam microanalyzer (EPMA: JXA8500F Model FE-EPMA), and element map data were collected. The measuring conditions were: an acceleration voltage of 15 kV, an irradiation current of 0.1 μA and a count-time of 30 msec, the data acquisition region was X=Y=51.2 μm, and the number of data points was X=Y=256 (0.2 μm-step). In the element map data, first triple point regions surrounded by 3 or more crystal grains are colored black, and by image analysis thereof, the average value for the area of the triple point regions and the standard deviation for the area distribution were calculated.
The EPMA was used for structural observation of the R-T-B sintered magnets of Example 5, Example 9, Examples 11 to 14, Examples 18 to 22, Comparative Example 4 and Comparative Example 5, in the same manner as the R-T-B sintered magnet of Example 10.
Each of the examples and comparative examples was subjected to image analysis in the same manner as Example 10, and the average value for the area of the triple point regions and the standard deviation for the area distribution were calculated. The results are shown in Table 4. As shown in Table 4, the R-T-B sintered magnets of the examples had sufficiently smaller values for the average value and standard deviation for the area of the triple point regions, compared to the comparative examples. These results confirmed that in the examples, segregation of the phase with a higher R content than the R2T14B phase was sufficiently inhibited.
An EPMA was used to determine the mass contents of rare earth elements in the triple point regions of the R-T-B sintered magnets of the examples and comparative examples. The measurement was conducted for 10 triple point regions, and the range and standard deviation for the rare earth element content was determined. The results are shown in Table 4.
A common gas analysis apparatus was used for gas analysis of the R-T-B sintered magnets of the examples and comparative examples, and the oxygen, nitrogen and carbon contents were determined. The results are shown in Table 4.
As shown in Tables 3 and 4, although Example 10 and Comparative Example 5 both used alloy powder having about the same mean particle diameter, the R-T-B sintered magnet obtained in Example 10 had a higher HcJ value. This is presumably because the R-T-B sintered magnet of Example 10 not only had a finer crystal grain particle diameter, but also had more uniform particle diameters and shapes of the crystal grains, and therefore reduced segregation of the triple point regions.
According to the invention it is possible to provide an R-T-B sintered magnet having sufficiently excellent coercive force without using expensive and scarce heavy rare earth elements, as well as a method for its production.
1: Crystal nuclei, 2: crystal grain (columnar crystal), 4: grain boundary phase (R-rich phase), 10: high-frequency melting furnace, 12: molten alloy, 14: tundish, 16: cooling roll, 17: roll surface, 18: alloy strip, 19: gas tubing, 19a: gas blow hole, 20: table, 32, 34: recesses, 36: raised section, 40: dendritic crystal, 100, 100: R-T-B sintered magnets, 120: rotor, 122: core, 130: stator, 132: coil, 140: triple point region, 150: crystal grain, 200: motor.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-226040 | Oct 2011 | JP | national |
| 2011-226042 | Oct 2011 | JP | national |
| 2011-248978 | Nov 2011 | JP | national |
| 2011-248980 | Nov 2011 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2012/076327 | 10/11/2012 | WO | 00 | 4/9/2014 |
