R-T-B-based magnet material alloy and method for producing the same

Abstract

Provided is an R-T-B-based magnet material alloy including an R.sub.2T.sub.14B phase which is a principal phase and R-rich phases which are phases enriched with the R, wherein the principal phase has primary dendrite arms and secondary dendrite arms diverging from the primary dendrite arms, and regions where the secondary dendrite arms have been formed constitute a volume fraction of 2 to 60% of the alloy, whereby excellent coercive force can be ensured in R-T-B-based sintered magnets even when the amount of heavy rare earth elements added to the alloy is reduced. The inter-R-rich phase spacing is preferably at most 3.0 μm, and the volume fraction of chill crystals is preferably at most 1%. Furthermore, the secondary dendrite arm spacing is preferably 0.5 to 2.0 μm, and the ellipsoid aspect ratio of R-rich phase is preferably at most 0.5.

Claims

1. A method for producing an R-T-B-based magnet material alloy, comprising: casting a ribbon by supplying a molten R-T-B-based alloy (where R is at least one element selected from rare earth metals including Y, and T is one or more transition metals with Fe being an essential element) to an outer peripheral surface of a chill roll and solidifying the molten alloy; and crushing the ribbon, the casting of the ribbon being performed in such a manner that an average cooling rate on the chill roll is 3400 to 4500° C./second and a temperature T.sub.I (° C.) of the ribbon at a position where the ribbon separates from the chill roll satisfies the following formula (1),
400≤T.sub.M−T.sub.I≤600  (1) where T.sub.M is a melting point (° C.) of the R-T-B-based alloy.

2. A method for producing an R-T-B-based magnet material alloy, comprising: casting a ribbon by supplying a molten R-T-B-based alloy (where R is at least one element selected from rare earth metals including Y, and T is one or more transition metals with Fe being an essential element) to an outer peripheral surface of a chill roll and solidifying the molten alloy; and crushing the ribbon, the casting of the ribbon being performed in such a manner that an average cooling rate on the chill roll is 2000 to 4500° C./second and a temperature T.sub.I (° C.) of the ribbon at a position where the ribbon separates from the chill roll satisfies the following formula (1),
550≤T.sub.M−T.sub.I≤600  (1) where T.sub.M is a melting point (° C.) of the R-T-B-based alloy.

3. A method for producing an R-T-B-based magnet material alloy where R is at least one element selected from rare earth metals including Y, and T is one or more transition metals with Fe being an essential element, the method comprising: casting a ribbon by supplying a molten R-T-B-based alloy to an outer peripheral surface of a chill roll and solidifying the molten alloy; and crushing the ribbon, the casting of the ribbon being performed in such a manner that an average cooling rate on the chill roll is 2000 to 4500° C./second and a temperature T.sub.I (° C.) of the ribbon at a position where the ribbon separates from the chill roll satisfies the following formula (1),
400≤T.sub.M−T.sub.I≤600  (1) where T.sub.M is a melting point (° C.) of the R-T-B-based alloy, the R-T-B-based magnet material alloy comprising an R.sub.2T.sub.14B phase, which is a principal phase, and R-rich phases, which are phases enriched with the R, the principal phase having primary dendrite arms and secondary dendrite arms diverging from the primary dendrite arms, regions where the secondary dendrite arms have been formed constituting a volume fraction of 2 to 60% of the alloy, and wherein an average spacing between adjacent R-rich phases is at most 3.0 μm.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic diagram of a casting apparatus that is used for casting of ribbons using a strip casting method.

(2) FIG. 2 is a photograph showing an example of a magnet material alloy according to the present invention.

(3) FIG. 3(a) and FIG. 3(b) are illustrations of a procedure for measuring the ellipsoid aspect ratio of R-rich phase, with FIG. 3(a) showing a binary backscattered electron image of a cross section of the alloy and FIG. 3(b) showing an image in which the positions of the center of gravity of R-rich phases have been located.

DESCRIPTION OF EMBODIMENTS

(4) The following are descriptions of an R-T-B-based magnet material alloy according to the present invention and a method for producing the same.

(5) 1. R-T-B-Based Magnet Material Alloy of the Present Invention

(6) FIG. 2 is a photograph showing an example of a magnet material alloy according to the present invention. FIG. 2 is a photograph of a cross section, in the thickness direction, of a magnet material alloy obtained in Inventive Example 1 described below in the Example section. It is a backscattered electron image examined at a magnification of 1000× with a scanning electron microscope (SEM). In FIG. 2, the principal phase is shown in gray and the R-rich phases are shown in white.

(7) A magnet material alloy of the present invention is an R-T-B-based magnet material alloy and includes an R.sub.2T.sub.14B phase which is a principal phase and R-rich phases which are phases enriched with the R. The principal phase has primary dendrite arms and secondary dendrite arms diverging from the primary dendrite arms. Regions where the secondary dendrite arms have been formed constitute a volume fraction of 2 to 60% of the alloy.

(8) In FIG. 2, the areas surrounded by solid lines show part of the primary dendrite arms and the area surrounded by a dashed line shows part of the regions where secondary dendrite arms have been formed. In the magnet material alloy shown in FIG. 2, primary dendrite arms (trunks) composed of the principal phase have been formed and secondary dendrite arms (branches) have been formed in such a manner as to diverge from the primary dendrite arms (trunks). As shown in FIG. 2, the regions where the secondary dendrite arms have been formed are constituted by a plurality of secondary dendrite arms composed of the principal phase and R-rich phases formed in spaces between the secondary dendrite arms.

(9) In such regions where secondary dendrite arms have been formed, the R-rich phases therein are present with very small inter-R-rich phase spacings, and therefore it is possible to refine the microstructure of the magnet material alloy. In production of sintered magnets using the magnet material alloy in which secondary dendrite arms have been formed, the alloy is pulverized, in the pulverizing step, into a fine powder with a particle size of not greater than 3 μm and forming is performed, in the forming step, using the fine powder with a particle size of not greater than 3 μm while inhibiting oxidation of the fine powder, forming failures, and the like. This facilitates breaking of exchange coupling between grains for the resulting sintered magnets because of the refined grains. Thus, it is possible to improve the anisotropy field Ha and reduce the local demagnetizing factor Neff, and consequently, it is possible to improve the coercive force Hc as specified in the formula (2).

(10) Accordingly, with the magnet material alloy of the present invention, even in the case where the amount of heavy rare earth elements added to the magnet material alloy is reduced, a decrease in coercive force associated therewith can be inhibited, and therefore it is possible to ensure excellent coercive force in R-T-B-based sintered magnets.

(11) If the volume fraction of regions where secondary dendrite arms have been formed is less than 2%, the microstructure of the magnet material alloy is not sufficiently refined, and therefore the coercive force in the sintered magnet will become insufficient. On the other hand, if the volume fraction of regions where secondary dendrite arms have been formed is greater than 60%, the fine powder to be obtained by pulverization in the pulverizing step of the sintered magnet production process has an increased surface area and therefore is inevitably oxidized. In addition, the crystal orientation is unfavorable for pressing in a magnetic field in the forming step, and therefore the coercive force in the sintered magnet will become insufficient. An explanation of how the volume fraction of regions where secondary dendrite arms have been formed is measured will be provided later.

(12) Preferably, the magnet material alloy of the present invention has an inter-R-rich phase spacing of not greater than 3.0 μm in order to obtain a refined microstructure. As a result, the microstructure of the alloy as a whole, not only of the regions where secondary dendrite arms have been formed, will be in a state of being refined, and therefore the coercive force in the sintered magnet will be improved further.

(13) In the meantime, the inter-R-rich phase spacing is preferably not less than 1.4 μm. The particle size of the fine powder to be obtained in the pulverizing step of the sintered magnet production process is about 2 μm at best, and it is difficult to obtain a fine powder having a particle size smaller than that. It is preferred that the inter-R-rich phase spacing is about the same as the particle size of the fine powder to be obtained in the pulverizing step. If the inter-R-rich phase spacing is less than 1.4 μm, it is too small with respect to the lower limit of 2 μm of the particle size of the fine powder to be obtained. In such a case, part of the fine powder particles will have multiple magnetic domains by including R-rich phases (including more than one principal phase), and this results in a decreased coercive force of the sintered magnet. An explanation of how the inter-R-rich phase spacing is measured will be provided later.

(14) It is to be noted that a magnet material alloy sometimes include chill crystals, which are fine equiaxed grains that may form in the vicinity of the surface that was in contact with the chill roll. If the formation of chill crystals occurs, the chill crystal portions form an extremely fine powder in the pulverizing step in the sintered magnet production process, which results in a non-uniform particle size distribution of the fine powder and thus degradation of magnetic properties. In order to prevent the problem, in the magnet material alloy of the present invention, the volume fraction of chill crystals is preferably at most 1%, and more preferably the volume fraction of chill crystals is 0%, i.e., no chill crystals are included. An explanation of how the volume fraction of chill crystals is measured will be provided later.

(15) In the magnet material alloy of the present invention, the secondary dendrite arm spacing is preferably 0.5 to 2.0 μm. When the secondary dendrite arm spacing is not greater than 2.0 μm, the coercive force in the sintered magnet will be improved further because of the refinement of the regions where secondary dendrite arms have been formed. In the meantime, if the secondary dendrite arm spacing is less than 0.5 μm, the degree of refinement of the regions where secondary dendrite arms have been formed is too great, and as a result, oxidation of the fine powder may occur in the pulverizing step or the crystal orientation may be unfavorable for the forming step, in the sintered magnet production process. An explanation of how the secondary dendrite arm spacing is measured will be provided later.

(16) In the magnet material alloy of the present invention, the R-rich phases preferably have an ellipsoid aspect ratio of not greater than 0.5. As used herein, the ellipsoid aspect ratio of R-rich phase is an index associated with the shape, particularly the thickness (width), of an R-rich phase. An explanation of how it is measured will be provided later. The ellipsoid aspect ratio r of R-rich phase satisfies the relationship 0<r≤1 based on its definition. The closer the value is to 1, the closer the shape of the R-rich phase is to a true circle or a regular polygon, and the closer the value is to 0, the thinner the shape of the R-rich phase is (the width is narrower).

(17) When the ellipsoid aspect ratio of R-rich phase is not greater than 0.5, thin (narrow width) R-rich phases are formed in spaces between secondary dendrite arms, and thus the microstructure is placed in a state of being refined. As a result, the coercive force in the sintered magnet is improved further. The lower limit of the ellipsoid aspect ratio r of R-rich phase is expressed as 0<r based on its definition.

(18) 2. Measurement Method

(19) In the present invention, the volume fraction of regions where secondary dendrite arms have been formed, the inter-R-rich phase spacing, the secondary dendrite arm spacing, and the ellipsoid aspect ratio of R-rich phase, as described above, are measured using images taken with a scanning electron microscope. Furthermore, in the present invention, the volume fraction of chill crystals is measured using an image taken with a polarizing microscope.

(20) In the present invention, specimens to be subjected to photographing with a scanning electron microscope are prepared by the following procedures (a) to (c). In the present invention, specimens to be subjected to photographing with a polarizing microscope are prepared by the following procedures (a) and (b).

(21) (a) Ten pieces of magnet material alloy (alloy flakes) were taken and they are embedded in a thermosetting resin and fixed.

(22) (b) Polishing is performed to expose the cross section along the thickness direction of each alloy flake fixed in the resin and place the cross section in a mirror surface condition.

(23) (c) Carbon is deposited on the cross section of each alloy flake in a mirror surface condition.

(24) [Volume Fraction of Regions where Secondary Dendrite Arms have been Formed]

(25) In the present invention, the volume fraction of regions where secondary dendrite arms have been formed is measured by the following procedure.

(26) (1) Using the specimens prepared by the above procedures (a) to (c), a backscattered electron image of a cross section of each alloy flake is taken at a magnification of 1000× with a scanning electron microscope. The backscattered electron image is taken in such a manner that, assuming that the cross section of the alloy flake is equally divided into three regions in the thickness direction, the region located in the center can be entirely included in the image.

(27) (2) The image is fed into an image analyzer, and binarization based on the luminance to discern between the R-rich phases and the principal phase is performed for each of the ten taken images.

(28) (3) For each of the ten binary images, secondary dendrite arms diverging from primary dendrite arms are extracted, so that the regions where secondary dendrite arms have been formed, which are constituted by secondary dendrite arms and the R-rich phases in the spaces therebetween, are distinguished.

(29) (4) For each of the ten images, the area of the regions where secondary dendrite arms have been formed and the cross-sectional area of the alloy are calculated, and the area of the regions where secondary dendrite arms have been formed is divided by the cross sectional area of the alloy, whereby the area fraction (%) of secondary dendrite arms of the alloy flake is calculated.

(30) (5) The area fractions of secondary dendrite arms of the ten alloy flakes are averaged, and the average value is designated as the volume fraction of secondary dendrite arms of the magnet material alloy because it can be assumed that each phase is uniformly distributed in the direction perpendicular to the cross section of each alloy flake.

(31) The reason that the backscattered electron image of the central region among the three divided regions is to be taken as described in above (I) is as follows. The region close to the surface that contacted the chill roll during casting may include some portions in which the microstructure is excessively fine. On the other hand, the region close to the surface on the opposite side may include some portions in which the microstructure is excessively coarse. Such excessively fine portions and excessively coarse portions correspond to so-called statistical outliers. Thus, by obtaining the backscattered electron image of the central region among the three divided regions, it is possible to measure representative values excluding outliers for the volume fraction of regions where secondary dendrite arms have been formed. By the term “surface on the opposite side” as used herein is meant the surface located opposite from the surface that contacted the chill roll during casting (the naturally cooled surface).

(32) [Inter-R-Rich Phase Spacing]

(33) In the present invention, the inter-R-rich phase spacing is measured by the following procedure.

(34) (1) Using the specimens prepared by the above procedures (a) to (c), a backscattered electron image of a cross section of each alloy flake is taken at a magnification of 1000× with a scanning electron microscope. The backscattered electron image is taken in such a manner that, assuming that the cross section of the alloy flake is equally divided into three regions in the thickness direction, the region located in the center can be entirely included in the image.

(35) (2) The ten taken images are each fed into an image analyzer, and binarization based on the luminance to discern between the R-rich phases and the principal phase is performed on them.

(36) (3) A line parallel to the surface that contacted the chill roll is drawn at a thickness center for each of the ten binary images, and the spacings between adjacent R-rich phases on the line are measured and the average value of them is designated as the inter-R-rich phase spacing of the alloy flake.

(37) (4) The inter-R-rich phase spacings of the ten alloy flakes are averaged, and the average value is designated as the inter-R-rich phase spacing of the magnet material alloy.

(38) The reason that the backscattered electron image of the central region among the three divided regions is to be taken as described in above (1) is the same as in the case of measuring the volume fraction of regions where secondary dendrite arms have been formed. By obtaining the backscattered electron image of the central region among the three divided regions, it is possible to measure representative values excluding outliers for the inter-R-rich phase spacing.

(39) [Volume Fraction of Chill Crystals]

(40) In the present invention, the volume fraction of chill crystals is measured by the following procedure.

(41) (1) Using the specimens prepared by the above procedures (a) and (b), an image of a cross section of each alloy flake is taken at a magnification of 85× with a polarizing microscope.

(42) (2) The taken ten images are each fed into an image analyzer, and the chill crystal portions are extracted based on the region of very fine equiaxed crystals.

(43) (3) For each of the ten images in which chill crystal portions have been extracted, the area of the chill crystal portions and the cross-sectional area of the alloy are calculated, and the area of the chill crystal portions is divided by the cross sectional area of the alloy, whereby the area fraction (%) of chill crystals of the alloy flake is calculated.

(44) (4) The area fractions of chill crystals of the ten alloy flakes are averaged, and the average value is designated as the volume fraction (%) of chill crystals of the magnet material alloy because it can be assumed that chill crystal portions and the remaining alloy portions are uniformly distributed in the direction perpendicular to the cross section of each alloy flake.

(45) [Secondary Dendrite Arm Spacing]

(46) In the present invention, the secondary dendrite arm spacing is measured by the following procedure.

(47) (1) Using the specimens prepared by the above procedures (a) to (c), a backscattered electron image of a cross section of each alloy flake is taken at a magnification of 1000× with a scanning electron microscope. The backscattered electron image is taken in such a manner that, assuming that the cross section of the alloy flake is equally divided into three regions in the thickness direction, the region located in the center can be entirely included in the image.

(48) (2) The ten taken images are each fed into an image analyzer, and binarization based on the luminance to discern between the R-rich phases and the principal phase is performed on them.

(49) (3) For each of the ten binary images, secondary dendrite arms diverging from primary dendrite arms are extracted.

(50) (4) A line perpendicular to the surface that contacted the chill roll during casting is drawn on a portion where secondary dendrite arms were observed in each image, and the secondary arm spacing was measured at 20 points thereon and the average value of them is designated as the secondary dendrite arm spacing of the alloy flake.

(51) (5) The secondary dendrite arm spacings of the ten alloy flakes are averaged, and the average value is designated as the secondary dendrite arm spacing of the magnet material alloy.

(52) The reason that the backscattered electron image of the central region among the three divided regions is to be taken as described in above (1) is the same as in the case of measuring the volume fraction of regions where secondary dendrite arms have been formed. By obtaining the backscattered electron image of the central region among the three divided regions, it is possible to measure representative values excluding outliers for the secondary dendrite arm spacing.

(53) [Ellipsoid Aspect Ratio of R-Rich Phase]

(54) FIG. 3 is an illustration of a procedure for measuring the ellipsoid aspect ratio of R-rich phase, with FIG. 3(a) showing a binary backscattered electron image of a cross section of the alloy and FIG. 3(b) showing an image in which the positions of the center of gravity of R-rich phases have been located. In FIG. 3, the principal phase 8 is shown in dark gray and the R-rich phases 9 are shown in light gray.

(55) In the present invention, the ellipsoid aspect ratio of R-rich phase is measured by the following procedure.

(56) (1) Using the specimens prepared by the above procedures (a) to (c), a backscattered electron image of a cross section of each alloy flake is taken at a magnification of 1000× with a scanning electron microscope. The backscattered electron image is taken in such a manner that, assuming that the cross section of the alloy flake is equally divided into three regions in the thickness direction, the region located in the center can be entirely included in the image.

(57) (2) The taken images are each fed into an image analyzer, and binarization based on the luminance to discern between the R-rich phases and the principal phase is performed on them, so as to obtain 10 images as shown in FIG. 3(a).

(58) (3) For each of the ten binary images, the center of gravity 9a of each R-rich phase in the image is determined using image analysis software as shown in FIG. 3(b).

(59) (4) For each R-rich phase in each image, a Cartesian coordinate system is set such that the origin is the center of gravity 9a of each R-rich phase, the X-axis is parallel to the surface that contacted the chill roll during casting, and the Y-axis is parallel to the thickness direction, and then, the second moment of area (Ix, Iy) is calculated for each of them using the above-mentioned image analysis software.

(60) (5) For each R-rich phase in each image, the greater one of the second moments of area (Ix, Iy) is specified as a major axis, and the smaller one is specified as a minor axis, and then the ratio r of the minor axis to the major axis is calculated. Specifically, the ratio r is calculated by the following formula (3).
r=Min{Ix,Iy}/Max{Ix,Iy}  (3)

(61) where Max{a, b} is a function for comparing input values a and b and outputting the greater one of them, and Min{a, b} is a function for comparing input values a and b and outputting the smaller one of them.

(62) (6) For each image, the ratios r for all R-rich phases calculated by the above formula (3) are averaged, and the average value is designated as the ellipsoid aspect ratio of R-rich phase of the alloy flake.

(63) (7) The ellipsoid aspect ratios of R-rich phase of the ten alloy flakes are averaged, and the average value is designated as the ellipsoid aspect ratio of the magnet material alloy.

(64) The reason that the backscattered electron image of the central region among the three divided regions is to be taken as described in above (1) is the same as in the case of measuring the volume fraction of regions where secondary dendrite arms have been formed. By obtaining the backscattered electron image of the central region among the three divided regions, it is possible to measure representative values excluding outliers for the ellipsoid aspect ratio of R-rich phase.

(65) 3. R-T-B-Based Magnet Material Alloy Production Method of the Present Invention

(66) The method for producing a magnet material alloy of the present invention is a method for producing an R-T-B-based magnet material alloy, the method including: casting a ribbon by supplying a molten R-T-B-based alloy to the outer peripheral surface of a chill roll and solidifying the molten alloy; and crushing the ribbon. The conditions for casting the ribbon is as follows: the average cooling rate on the chill roll is 2000 to 4500° C./second, and the temperature T.sub.1(° C.) of the ribbon at a position where it separates from the chill roll (hereinafter also simply referred to as “rapid cooling end temperature”) satisfies the above formula (1).

(67) In casting operations in general, not limited to casting of a magnet material alloy, formation of secondary dendrite arms is a technique sometimes used for the purpose of improving mechanical strength of the ingot. In such a case, secondary dendrite arms are typically formed by increasing the cooling rate for casting or adding heterogeneous nuclei to the molten alloy. For a magnet material alloy, addition of heterogeneous nuclei to the molten alloy is not appropriate from the standpoint of the influence on the mechanism by which magnetic properties are exhibited. For this reason, in the method for producing a magnet material alloy of the present invention, secondary dendrite arms are formed by increasing the cooling rate as described above.

(68) Specifically, according to the method for producing a magnet material alloy of the present invention, ribbons are cast in such a manner that the average cooling rate on the chill roll is 2000 to 4500° C./second and the temperature T.sub.1(° C.) of the ribbon at the time when it separates from the chill roll (rapid cooling end temperature) satisfies the above formula (1). Consequently, the resulting magnet material alloy includes primary dendrite arms made of the principal phase and secondary dendrite arms formed therewith in such a manner that they diverge from the primary dendrite arms. In addition, the volume fraction of regions where secondary dendrite arms have been formed as described above is consequently 2 to 60%. By using such a magnet material alloy having a refined microstructure as a material for sintered magnets, it is possible to improve the coercive force of the sintered magnet as stated above.

(69) If the average cooling rate on the chill roll is less than 2000° C./second, secondary dendrite arms is not formed in some cases. Even in the case where secondary dendrite arms have been formed, the volume fraction thereof is reduced and therefore refinement of the microstructure cannot be achieved. On the other hand, if the average cooling rate is higher than 4500° C./second, the volume fraction of regions where secondary dendrite arms have been formed becomes excessively large and therefore the microstructure becomes excessively refined.

(70) Also, secondary dendrite arms may not be formed in the case where the rapid cooling end temperature T.sub.1 is increased so that the difference between the melting point Tu of the alloy and the rapid cooling end temperature T.sub.1 falls below 400° C. and does not satisfy the condition specified by the above formula (1). Even if secondary dendrite arms have been formed, the volume fraction thereof is reduced and therefore refinement of the microstructure cannot be achieved. In the meantime, if the rapid cooling end temperature T.sub.1 is decreased so that the difference between the melting point T.sub.M of the alloy and the rapid cooling end temperature T.sub.1 exceeds 600° C. and does not satisfy the condition specified by the above formula (1), the volume fraction of regions where secondary dendrite arms have been formed becomes excessively large and therefore the microstructure becomes excessively refined.

(71) In the present invention, the average cooling rate V.sub.T (° C./second) on the chill roll is calculated by the following formula (4).
V.sub.T=(T.sub.0−T.sub.1)×V.sub.C/S  (4)

(72) where T.sub.0 is a temperature (° C.) of the molten alloy at a position immediately before it contacts the chill roll, T.sub.1 is a temperature (° C.) of the ribbon at a position where it separates from the chill roll (see dashed arrow in FIG. 1), V.sub.C is a circumferential speed (mm/s) of the chill roll, and S is a length (mm) of contact between the molten alloy (ribbon) and the chill roll.

(73) In the case where the casting apparatus shown in FIG. 1 is used, the temperature T.sub.1 (° C.) of the ribbon at a position where it separates from the chill roll may be determined by measuring, using a radiation pyrometer, the temperature of the naturally cooled surface of the ribbon at a position where it separates from the chill roll. The temperature T.sub.0 of the molten alloy at a position immediately before it contacts the chill roll may be determined by measuring, using a radiation pyrometer, the temperature of the molten alloy at a rear end of the tundish (see solid arrow).

EXAMPLES

(74) To verify the advantages of the magnet material alloy of the present invention and the method for producing the same, the following test was conducted.

(75) [Test Method]

(76) In this test, a thin ribbon was cast from a molten R-T-B-based alloy by the above-mentioned procedures (A) to (C) using the casting apparatus shown in FIG. 1. The cast ribbon was crushed into alloy flakes at a stage subsequent to the chill roll. The alloy flakes were cooled to room temperature for about 8 hours, thereby producing a magnet material alloy. In the casting of the ribbon, the amount of the molten alloy to be poured and the rotational speed of the chill roll were adjusted so that the cast ribbon had a thickness of about 0.3 mm. The condition of the atmosphere was an inert gaseous atmosphere of argon at a pressure of 200 torr.

(77) In this test, the average cooling rate on the chill roll was adjusted by varying the surface temperature and the atmosphere condition. In the casting of the ribbon, the temperature of the naturally cooled surface of the ribbon (rapid cooling end temperature) at a position where it separates from the chill roll (see dashed arrow in FIG. 1) was measured using a radiation pyrometer. As the temperature of the molten alloy at a position immediately before it contacts the chill roll, the temperature of the molten alloy at a rear end of the tundish (see solid arrow in FIG. 1) was measured using a radiation pyrometer. Using these measured temperatures, the average cooling rate V.sub.T was calculated by the above formula (4).

(78) In this test, the contents of the raw materials were varied to obtain magnet material alloys having chemical compositions A to C. The chemical compositions of the alloys are shown in Table 1. In addition, melting point temperatures of the alloys having the chemical compositions A to C are also shown in Table 1.

(79) TABLE-US-00001 TABLE 1 Chemical Composition (Unit: atomic %, Balance is Fe) Alloy Melting Symbol Nd Pr Dy B Al Co Cu Point (° C.) A 10.7 2.2 1.3 6.2 0.5 1.0 0.1 1150 B 10.7 2.3 1.2 6.1 0.5 1.0 0.1 1150 C 11.0 2.3 1.0 6.0 0.5 1.0 0.1 1150

(80) In Inventive Examples 1 to 4, the average cooling rate on the chill roll was adjusted to 2500 to 3400° C./second, and in Comparative Examples 1 to 3, the average cooling rate on the chill roll was adjusted to 1500 to 1900° C./second.

(81) In both the inventive examples and comparative examples, measurements were made on the obtained magnet material alloys for the volume fraction of regions where secondary dendrite arms have been formed, the inter-R-rich phase spacing, the volume fraction of chill crystals, the secondary dendrite arm spacing, and the ellipsoid aspect ratio of R-rich phase, by the procedures described in the above “2. Measurement Method” section.

(82) [Test Results]

(83) Table 2 shows, for each experiment, the chemical compositions of the obtained magnet material alloys, and regarding the casting of the ribbon, the average cooling rate on the chill roll, the temperature of the ribbon at a position where it separates from the chill roll (rapid cooling end temperature), and the difference (T.sub.M−T.sub.1) between the melting point T.sub.M of the alloy and the rapid cooling end temperature T.sub.1. In addition, Table 2 shows the volume fraction of regions where secondary dendrite arms have been formed, the secondary dendrite arm spacing, the inter-R-rich phase spacing, the ellipsoid aspect ratio of R-rich phase, and the volume fraction of chill crystals, of the magnet material alloy obtained in each experiment. In Table 2, the symbol “-” in the column of volume fraction of regions where secondary dendrite arms have been formed and the column of secondary dendrite arm spacing indicates that no secondary dendrite arms were observed (formed) in the obtained magnet material alloy.

(84) TABLE-US-00002 TABLE 2 Secondary Chill Casting conditions dendrite arm crystals Rapid cooling Volume R-rich phase Volume Cooling rate end temperature T.sub.M-T.sub.1 fraction Spacing Spacing Ellipsoid fraction Classification Composition (° C./sec) (° C.) (° C.) (%) (μm) (μm) aspect ratio (%) Inv. Ex. 1 A 3400 650 500 28.5 0.7 2.12 0.32 0 Inv. Ex. 2 A 3000 710 440 21.3 1.0 2.16 0.43 0 Inv. Ex. 3 B 3000 600 550 29.2 1.2 1.76 0.34 0 Inv. Ex. 4 C 2500 700 450 13.6 1.4 2.86 0.47 0 Comp. Ex. 1 A 1500 770 380 — — 3.22 0.75 0 Comp. Ex. 2 A 1900 750 400  1.5 1.5 3.10 0.63 0 Comp. Ex. 3 A 1800 760 390 — — 3.20 0.69 0

(85) In Comparative Examples 1 to 3, the average cooling rate on the chill roll was less than 2000° C./second, and in some experiments, secondary dendrite arms were not formed in the obtained magnet material alloy and even in experiments in which they were formed, the volume fraction of regions where they were formed was 1.5%. As a result, the microstructure was not sufficiently refined and the inter-R-rich phase spacing exceeded 3 μm. In addition, the shape of the R-rich phase was relatively thick (wide-width) with the ellipsoid aspect ratio thereof exceeding 0.5.

(86) In contrast, in Inventive Examples 1 to 4, the average cooling rate on the chill roll was 2000° C./second or higher, and in all experiments, secondary dendrite arms were formed in the obtained magnet material alloy and the volume fraction of regions where they were formed was not less than 2%. In Inventive Examples 1 to 4, the difference between the melting point T.sub.M of the alloy and the rapid cooling end temperature T.sub.1 was 400 to 600° C. These results demonstrate that: by casting a ribbon in such a manner that the average cooling rate on the chill roll is 2000° C./second or higher and the temperature T.sub.1(° C.) of the ribbon at the time when it separates from the chill roll satisfies the above formula (1), it is possible to form secondary dendrite arms such that the volume fraction of regions where they have been formed is at least 2%.

(87) Furthermore, in Inventive Examples 1 to 4, secondary dendrite arms were formed, and as a result, the inter-R-rich phase spacing was not more than 3.0 μm and the alloy as a whole had a refined microstructure. In addition, the shape of the R-rich phase was elongated (narrow-width) with the ellipsoid aspect ratio thereof falling below 0.5, and the microstructure was refined.

(88) Using the magnet material alloys obtained in the test as a material, sintered magnets were produced by the production process as described above. In the production of sintered magnets, pulverization was performed in the pulverizing step in such a manner that the resultant fine powder had a particle size about the same as the inter-R-rich phase spacing of the magnet material alloy, and in the forming step, forming was performed using the fine powder, while inhibiting oxidation, forming failures, and the like of the fine powder. Consequently, sintered magnets produced from the magnet material alloys of Comparative Examples 1 to 3 exhibited a decreased coercive force due to the reduced amount of heavy rare earth elements added, whereas sintered magnets produced from the magnet material alloys of Inventive Examples 1 to 4 were able to maintain a coercive force comparable to that in the case where the amount of heavy rare earth elements to be added is not reduced.

(89) These results demonstrate that: the magnet material alloy of the present invention is capable of ensuring a sufficient coercive force, i.e., capable of improving the coercive force of sintered magnets by having secondary dendrite arms formed therein and thus having a refined microstructure, even in the case where the amount of heavy rare earth elements added to the alloy is reduced.

(90) When the magnet material alloy of the present invention is used as a material for sintered magnets, the coercive force can be improved, and therefore it is possible to ensure a sufficient coercive force of the sintered magnets even in the case where the amount of heavy rare earth elements added to the magnet material alloy is reduced. With the method for producing a magnet material alloy of the present invention, it is possible to produce the above-described magnet material alloy of the present invention. Consequently, the magnet material alloy of the present invention and the method for producing the same are capable of greatly contributing to improvement of the coercive force of sintered magnets and also greatly contributing to steady supply of sintered magnets by achieving the reduction of the amount of heavy rare earth elements to be added to the alloy.

REFERENCE SIGNS LIST

(91) 1: crucible, 2: tundish, 3: chill roll,

(92) 4: ingot, 5: chamber, 6: molten alloy, 8: principal phase,

(93) 9: R-rich phase, 9a: center of gravity of R-rich phase