Method of manufacturing rare earth magnet

Abstract

A manufacturing method includes: manufacturing a sintered compact having a composition of (Rl).sub.x(Rh).sub.yT.sub.zB.sub.sM.sub.t; manufacturing a precursor by performing hot deformation processing on the sintered compact; and manufacturing a rare earth magnet by performing an aging treatment on the precursor in a temperature range of 450 C. to 700 C. In this method, a main phase thereof is formed of a (RlRh).sub.2T.sub.14B phase. A content of a (RlRh).sub.1.1T.sub.4B.sub.4 phase in a grain boundary phase thereof is more than 0 mass % and 50 mass % or less. Rl represents a light rare earth element. Rh represents a heavy rare earth element. T represents a transition metal. M represents at least one of Ga, Al, Cu, and Co. x, y, z, s, and t are percentages by mass of Rl, Rh, T, B, and M. x, y, z, s, and t are expressed by the following expressions: 27x44, 0y10, z=100xyst, 0.75s3.4, 0t3.

Claims

1. A method of manufacturing a rare earth magnet comprising: manufacturing a sintered compact having a structure represented by a composition of (Rl).sub.x(Rh).sub.yT.sub.zB.sub.sM.sub.t, wherein a main phase of the structure is formed of a (RlRh).sub.2Ti.sub.4B phase, and a content of a (RlRh).sub.1.1T.sub.4B.sub.4 phase in a grain boundary phase of the structure is more than 0 mass % and less than 25 mass %; manufacturing a rare earth magnet precursor by performing hot deformation processing on the sintered compact; and manufacturing a rare earth magnet by performing an aging treatment on the rare earth magnet precursor in a temperature range of 450 C. to 700 C., wherein Rl represents at least one light rare earth element or Y, Rh represents Dy or Tb, T represents a transition metal and T is at least one of Fe and Co, B represents boron, M represents at least one of Ga, Al, Cu, and Co, x, y, z, s, and t respectively represent percentages by mass of Rl, Rh, T, B, and M in the sintered compact, and x, y, z, s, and t are expressed by the following expressions:
27x44, 0y10, z=100xyst, 0.96s1.02, 0t3.

2. The method according to claim 1, wherein during the aging treatment, a modified alloy containing a light rare earth element and at least one of a transition metal element, In, Zn, Al, and Ga is diffused and infiltrated into the grain boundary phase.

Description

BRlEF DESCRlPTION OF THE DRAWINGS

(1) Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

(2) FIGS. 1A and 1B are schematic diagrams sequentially illustrating a first step of a method of manufacturing a rare earth magnet according to an embodiment of the invention, and FIG. 1C is a schematic diagram illustrating a second step thereof;

(3) FIG. 2A is a diagram illustrating a microstructure of a sintered compact illustrated in FIG. 1B, and FIG. 2B is a diagram illustrating a microstructure of a rare earth magnet precursor illustrated in FIG. 1C;

(4) FIGS. 3A and 3B, are schematic diagrams illustrating a third step of the method of manufacturing a rare earth magnet according to the embodiment;

(5) FIG. 4 is a diagram illustrating a microstructure of a crystal structure of the manufactured rare earth magnet;

(6) FIG. 5 is a diagram illustrating a heating path of the third step during the manufacture of test pieces of Examples 1 to 5 and Comparative Examples 1 to 7;

(7) FIG. 6 is a diagram illustrating a relationship between the B content after hot deformation processing and remanent magnetization and coercive force;

(8) FIG. 7 is a diagram illustrating a relationship between the B content after an aging treatment and remanent magnetization and coercive force;

(9) FIG. 8 is a diagram illustrating a relationship between the B content and variations in remanent magnetization and coercive force before and after hot deformation processing and illustrating the optimum content of a Nd.sub.1.1T.sub.4B.sub.4 phase;

(10) FIG. 9 is a diagram illustrating variations in magnetization after a heat treatment in a case where the aging treatment and the diffusion and infiltration treatment of the modified alloy were simultaneously performed and a case where the above-described treatments were not simultaneously performed;

(11) FIG. 10 is a diagram illustrating variations in coercive force after a heat treatment in a case where the aging treatment and the diffusion and infiltration treatment of the modified alloy were simultaneously performed and a case where the above-described treatments were not simultaneously performed;

(12) FIG. 11 is a diagram illustrating variations in magnetization after a heat treatment in a case where the aging treatment and the diffusion and infiltration treatment of the modified alloy were simultaneously performed while changing the boron content (B content) and a case where the above-described treatments were not simultaneously performed while changing the boron content (B, content);

(13) FIG. 12 is a diagram illustrating variations in coercive force after a heat treatment in a case where the aging treatment and the diffusion and infiltration treatment of the modified alloy were simultaneously performed while changing the boron content (B content) and a case where the above-described treatments were not simultaneously performed while changing the boron content (B content); and

(14) FIG. 13 is a diagram illustrating changes in magnetization and coercive force during a heat treatment depending on the change in the boron content (B content).

DETAILED DESCRIPTION OF EMBODIMENTS

(15) (Embodiment of Method of Manufacturing Rare Earth Magnet)

(16) FIGS. 1A and 1B are schematic diagrams sequentially illustrating a first step of a method of manufacturing a rare earth magnet according to an embodiment of the invention, and FIG. 1C is a schematic diagram illustrating a second step thereof. In addition, FIGS. 3A and 3B are schematic diagrams illustrating a third step of the method of manufacturing a rare earth magnet according to the embodiment. In addition, FIG. 2A is a diagram illustrating a microstructure of a sintered compact illustrated in FIG. 1B, and FIG. 2B is a diagram illustrating a microstructure of a rare earth magnet precursor illustrated in FIG. 1C. Further, FIG. 4 is a diagram illustrating a microstructure of a crystal structure of the manufactured rare earth magnet;

(17) As illustrated in FIG. 1A, in a furnace (not illustrated) of an Ar gas atmosphere in which the pressure is reduced to, for example, 50 kPa or less, an alloy ingot is melted by high-frequency induction heating using a single-roll melt spinning method, and molten metal is injected to a copper roll R to prepare a rapidly-solidified ribbon B, and this rapidly-solidified ribbon B is crushed. Here, the molten metal has a composition constituting a rare earth magnet.

(18) As illustrated in FIG. 1B, the crushed rapidly-solidified ribbon B is filled into a cavity which is partitioned by a cemented carbide die D and a cemented carbide punch P sliding in a hollow portion of the cemented carbide die D. Next, the crushed rapidly-solidified ribbon B is heated by causing a current to flow therethrough in a compression direction while being compressed with the cemented carbide punch P (X direction). As a result, a sintered compact S having a composition represented by (Rl).sub.x(Rh).sub.yT.sub.zB.sub.sM.sub.t is manufactured. Here, Rl represents at least one of light rare earth elements containing Y. Rh represents at least one of heavy rare earth elements containing Dy and Tb. T represents a transition metal containing at least one of Fe and Co. B represents boron. M represents at least one of Ga, Al, Cu, and Co. x, y, z, s, and t respectively represent percentages by mass of Rl, Rh, T, B, and M in the sintered compact. x, y, z, s, and t are expressed by the following expressions: 27x44, 0y10, z=100xyst, 0.75s3.4, 0t3. The sintered compact S has a structure including a main phase and a grain boundary phase, and the main phase has a grain size of about 50 nm to 300 nm (hereinabove, the first step).

(19) The grain boundary phase contains at least one of Ga, Al, Cu, and Co in addition to Nd or the like and is in a Nd-rich state. In addition, the grain boundary phase contains a Nd phase and a Nd.sub.1.1T.sub.4B.sub.4 phase as major components, in which the content of the Nd.sub.1.1T.sub.4B.sub.4 phase is controlled to be in a range of more than 0 mass % and 50 mass % or less.

(20) As illustrated in FIG. 2A, the sintered compact S has an isotropic crystal structure in which a grain boundary phase BP is filled between nanocrystalline grains MP (main phase). In order to impart magnetic anisotropy to the sintered compact S, as illustrated in FIG. 1C, the cemented carbide punch P is brought into contact with an end surface of the sintered compact S in a longitudinal direction thereof (in FIG. 1B, the horizontal direction is the longitudinal direction) such that hot deformation processing is performed on the sintered compact S while being compressed with the cemented carbide punch P (X direction). As a result, a rare earth magnet precursor C which includes a crystal structure having the anisotropic nanocrystalline grains MP as illustrated in FIG. 2B is manufactured (hereinabove, the second step).

(21) When the processing degree (compressibility) by the hot deformation processing is high, for example, when the compressibility is about 10% or higher; this processing may be called high hot deformation or simply high deformation. However, it is preferable that high deformation be performed at a compressibility of about 60% to 80%.

(22) In a crystal structure of the rare earth magnet precursor C illustrated in FIG. 2B, the nanocrystalline grains MP have a flat shape, and the boundary surface which is substantially parallel to an anisotropic axis is curved or bent and is not configured of a specific surface.

(23) Next, as illustrated in FIGS. 3A and 3B, the third step may be performed mainly using two methods.

(24) In a method of manufacturing a rare earth magnet using a first embodiment of the third step, as illustrated in FIG. 3A, the rare earth magnet precursor C is put into a high-temperature furnace H, and only the aging treatment is performed on the rare earth magnet precursor C in a temperature range of 450 C. to 700 C.

(25) The grain boundary phase constituting the rare earth magnet precursor C contains at least one of Ga, Al, Cu, and Co in addition to Nd or the like. As a result, the grain boundary phase BP can be melted and flow in a low temperature range of 450 C. to 700 C., and Nd or the like and Ga, Al, Cu, Co, or the like can be alloyed. That is, by alloying a transition metal element or the like and a light rare earth element contained in the grain boundary phase in advance, the same modification effects as in the case where the modified alloy is diffused and infiltrated can be exhibited without the necessity of diffusing and infiltrating the modified alloy into the surface of a magnet.

(26) Further, by the grain boundary phase BP containing Nd.sub.1.1Fe.sub.4B.sub.4 in a content range of 50 mass % or less, that is, by controlling the boron content (B content) in the grain boundary phase BP to be in a predetermined range, a decrease in the content of the main phase during the aging treatment is suppressed and thus a decrease in magnetization is suppressed.

(27) As a result, the coercive force can be improved by the aging treatment, and a decrease in magnetization caused by the aging treatment can be suppressed. Accordingly, a rare earth magnet which is superior in both coercive force performance and magnetization performance can be manufactured.

(28) On the other hand, in a method of manufacturing a rare earth magnet using a second embodiment of the third step, as illustrated in FIG. 3B, modified alloy powder SL is sprayed on the surface of the rare earth magnet precursor C, the rare earth magnet precursor C is put into a high-temperature furnace H, and the modified alloy SL is diffused and infiltrated while performing the aging treatment on the rare earth magnet precursor C in a temperature range of 450 C. to 700 C.

(29) Regarding the modified alloy powder SL, a plate-shaped modified alloy powder may be placed on the surface of the rare earth magnet precursor, or a slurry of the modified alloy powder may be prepared and coated on the surface of the rare earth magnet precursor.

(30) Here, a modified alloy which contains a light rare earth element and at least one of a transition metal element, In, Zn, Al, and Ga and has a low eutectic point of 450 C. to 700 C. is used as the modified alloy powder SL. As the modified alloy powder SL, any one of a NdCu alloy (eutectic point: 520 C.), a PrCu alloy (eutectic point: 480 C.), a NdPrCu alloy, a NdAl alloy (eutectic point: 640 C.), a PrAl alloy (eutectic point: 650 C.), a NdPrAl alloy, a NdCo alloy (eutectic point: 566 C.), a PrCo alloy (eutectic point: 540 C.), and a NdPrCo alloy is preferably used. Among these, alloys having a low eutectic point of 580 C. or lower, for example, a NdCu alloy (eutectic point: 520 C.), a PrCu alloy (eutectic point: 480 C.), a NdCo alloy (eutectic point: 566 C.), and a PrCo alloy (eutectic point: 540 C.) are more preferably used.

(31) By diffusing and infiltrating the modified alloy into the grain boundary phase in this way, the grain boundary phase BP of, particularly, the surface region of the rare earth magnet precursor C can be further modified. That is, the grain boundary phase BP of the entire region of the rare earth magnet precursor C can be modified by the alloying of a transition metal element or the like and a light rare earth element in the grain boundary phase BP. Therefore, it is not necessary that the non-magnetic modified alloy SL be diffused and infiltrated into the center region of the rare earth magnet precursor C to modify the grain boundary phase BP. In this way, the modification of the grain boundary phase BP by the modified alloy SL is only necessary for the surface region of the rare earth magnet precursor C. Therefore, it is sufficient that the amount of the modified alloy SL to be diffused and infiltrated be less than 5 mass % with respect to the rare earth magnet precursor C. In addition, the high-temperature holding time during the aging treatment can be made to be short, for example, in a range of 5 minutes to 180 minutes and preferably in a range of 30 minutes to 180 minutes. Since the infiltration amount of the modified alloy SL can be made to be small, the material cost can be reduced as compared to the diffusion and infiltration treatment method of the modified alloy of the related art. In addition, since the holding time during the aging treatment can be made to be short, the manufacturing time can be reduced.

(32) No matter which method is used among the methods according to the first embodiment or the second embodiment of the third step, Nd or the like and at least one of Ga, Al, Cu, and Co present in the grain boundary phase of the rare earth magnet precursor C in advance are alloyed by the aging treatment to modify the grain boundary BP. Further, by a predetermined amount of boron being present in the grain boundary phase BP, the crystal structure of the rare earth magnet precursor C illustrated in FIG. 2B is changed, and the boundary surface of the crystal grains MP is cleared as illustrated in FIG. 4. Therefore, the crystal grains MP are magnetically isolated from each other, and a rare earth magnet RM having an improved coercive force is manufactured (third step). In an intermediate step of the structure modification by the modified alloy illustrated in FIG. 4, a boundary surface which is substantially parallel to an anisotropic axis is not formed (is not configured of a specific surface). On the other hand, in a step in which the modification by the modified alloy sufficiently progresses, a boundary surface (specific surface) which is substantially parallel to an anisotropic axis is formed, and a rare earth magnet in which the shape of the crystal grains MP is rectangular or substantially rectangular when seen from a direction perpendicular to the anisotropic axis is manufactured.

(33) [Experiment for Verifying Magnetic Properties of Rare Earth Magnet While Changing Content of (RlRh).sub.1.1T.sub.4B.sub.4 Phase in Grain Boundary Phase to Specify Optimal Content Range of (RlRh).sub.1.1T.sub.4B.sub.4 Phase, and Results Thereof]

(34) The present inventors performed an experiment for specifying an optimal content range of the (RlRh).sub.1.1T.sub.4B.sub.4 phase, in which various rare earth magnets containing a Nd.sub.1.1T.sub.4B.sub.4 phase as a specific example of the (RlRh).sub.1.1T.sub.4B.sub.4 phase and containing a Nd phase were manufactured, and magnetic properties of each test piece were measured.

Examples 1 to 5

(35) A liquid rapidly-solidified ribbon having a composition represented by Nd.sub.28.9Pr.sub.0.4Fe.sub.balB.sub.0.96+aGa.sub.0.4Al.sub.0.1Cu.sub.0.1 was prepared in a single-roll furnace (a=0, 0.03, 0.04, 0.05, 0.06), the obtained rapidly-solidified ribbon was sintered to prepare a sintered compact (sintering temperature: 650 C.; 400 MPa), and high deformation (processing temperature: 750 C.; processing degree: 75%) was performed on the sintered compact, thereby preparing a rare earth magnet precursor. Next, an aging treatment was performed on the obtained rare earth magnet precursor according to a heating path illustrated in FIG. 5.

Comparative Examples 1 to 7

(36) A liquid rapidly-solidified ribbon having a composition represented by Nd.sub.28.9Pr.sub.0.4Fe.sub.balB.sub.0.96+aGa.sub.0.4Al.sub.0.1Cu.sub.0.1 was prepared in a single-roll furnace (a=0, 08.0.07, 0.06, 0.05, 0.03, 0.14, 0.24), the obtained rapidly-solidified ribbon was sintered to prepare a sintered compact (sintering temperature: 650 C.; 400 MPa), and high deformation (processing temperature: 750 C.; processing degree: 75%) was performed on the sintered compact, thereby preparing a rare earth magnet precursor. Next, an aging treatment was performed on the obtained rare earth magnet precursor according to a heating path illustrated in FIG. 5. The magnetic properties were evaluated using a vibrating sample magnetometer (VSM) and a pulsed high field magnetometer (TPM).

(37) (Experiment Results)

(38) The experiment results are shown in FIGS. 6 to 8. Here, FIG. 6 is a diagram illustrating a relationship between the B content after hot deformation processing and remanent magnetization and coercive force, and FIG. 7 is a diagram illustrating a relationship between the B content after an aging treatment and remanent magnetization and coercive force. In addition, FIG. 8 is a diagram illustrating a relationship between the B content and variations in remanent magnetization and coercive force before and after hot deformation processing and illustrating the optimum content of a Nd.sub.1.1T.sub.4B.sub.4 phase.

(39) In this experiment, the content of the main phase was 95 mass %, and thus the content of the grain boundary phase was 5 mass %. It was found from FIG. 8 that, when the content range of the Nd.sub.1.1T.sub.4B.sub.4 phase in the grain boundary phase was in a range of more than 0 mass % and 50 mass % or less, there was no change in remanent magnetization before and after the hot deformation processing, that is, the remanent magnetization was not reduced by the aging treatment, and the coercive force increased.

(40) On the other hand, it was found that, when the grain boundary phase did not contain Nd.sub.1.1T.sub.4B.sub.4 phase and the grain boundary phase contained a Nd phase and a Nd2Fe.sub.17 phase, the content of the main phase decreased due to the absence of boron in the grain boundary phase, and the remanent magnetization decreased. In addition, when the content of the Nd.sub.1.1T.sub.4B.sub.4 phase was more than 50 mass %, the remanent magnetization did not decrease, and both the remanent magnetization and the coercive force did not increase.

(41) Based on these experiment results, the content of the (RlRh).sub.1.1T.sub.4B.sub.4 phase in the grain boundary phase was defined to be in a range of more than 0 mass % and 50 mass % or less.

(42) [Experiment for Verifying Effects when Aging Treatment and Diffusion and Infiltration Treatment of Modified Alloy were Simultaneously Performed, and Results Thereof]

(43) The present inventors performed an experiment for verifying effects when aging treatment and diffusion and infiltration treatment of a modified alloy were simultaneously performed.

Examples 6 and 7

(44) A liquid rapidly-solidified ribbon having a composition represented by Nd.sub.28.9Pr.sub.0.4Fe.sub.balB.sub.0.96+aGa.sub.0.4Al.sub.0.1Cu.sub.0.1 was prepared in a single-roll furnace (a=0, 0.04). In this case, when a=0, the B content was 0.96% and the Nd.sub.1.1Fe.sub.4B.sub.4 content was 0%; and when a=0.4, the B content was 1.00% and the Nd.sub.1.1Fe.sub.4B.sub.4 content was 14.3%. Next, the obtained rapidly-solidified ribbon was sintered to prepare a sintered compact (sintering temperature: 650 C.; 400 MPa), and high deformation (processing temperature: 750 C.; processing degree: 75%) was performed on the sintered compact, thereby preparing a rare earth magnet precursor. Next, a heat treatment was performed on the obtained rare earth magnet precursor according to Method A such that 3.5 mass % of a NdCu alloy was diffused and infiltrated thereinto (as the modified alloy, a Nd70Cu30 alloy was used).

(45) Here, Method A refers to a method in which the aging treatment and the diffusion and infiltration treatment of the modified alloy are simultaneously performed. In this method, the rare earth magnet precursor is cut into a block having a size of 1 mm1 mm1 mm, and magnetic properties thereof are evaluated using a VSM and a TPM. Then, in a state where 3.5 mass % of a NdCu alloy is in contact with the surface of the block, the block is put into a high-temperature furnace and is extracted after being held at 580 C. for 300 minutes in an atmosphere of 10.sup.3 Pa, and then magnetic properties thereof are evaluated again.

Comparative Examples 8 and 9

(46) A liquid rapidly-solidified ribbon having a composition represented by Nd.sub.28.9Pr.sub.0.4Fe.sub.balB.sub.0.96+aGa.sub.0.4Al.sub.0.1Cu.sub.0.1 was prepared in a single-roll furnace (a=0, 0.04, 0.20). In this case, when a=0, the B content was 0.96% and the Nd.sub.1.1Fe.sub.4B.sub.4 content was 0%; when a=0.4, the B content was 1.00% and the Nd.sub.1.1Fe.sub.4B.sub.4 content was 14.3%; and when a=0.20, the B content was 1.16% and the Nd.sub.1.1Fe.sub.4B.sub.4 content was 71.5%. Next, the obtained rapidly-solidified ribbon was sintered to prepare a sintered compact (sintering temperature: 650 C.; 400 MPa), and high deformation (processing temperature: 750 C.; processing degree: 75%) was performed on the sintered compact, thereby preparing a rare earth magnet precursor. Next, a heat treatment was performed on the obtained rare earth magnet precursor according to Method B such that 3.5 mass % of a NdCu alloy was diffused and infiltrated thereinto (as the modified alloy, a Nd70Cu30 alloy was used).

(47) Here, Method B refers to a method in which the aging treatment and the diffusion and infiltration treatment of the modified alloy are not simultaneously performed. In this method, the rare earth magnet precursor is cut into a block having a size of 1 mm1 mm1 mm, and magnetic properties thereof are evaluated using a VSM and a TPM. Then, the block is put into a high-temperature furnace and is extracted after being held at 580 C. for 30 minutes in an atmosphere of 10.sup.3 Pa for an aging treatment. Next, in a state where 3.5 mass % of a NdCu alloy is in contact with the surface of the block subjected to the aging treatment, the block is put again into a high-temperature furnace and is extracted after being held at 580 C. for 300 minutes in an atmosphere of 10.sup.3 Pa, and then magnetic properties thereof are evaluated again.

(48) (Experiment Results)

(49) FIGS. 9 and 10 are diagrams illustrating variations in magnetization and variations in coercive force, respectively, after a heat treatment in a case where the aging treatment and the diffusion and infiltration treatment of the modified alloy were simultaneously performed and a case where the above-described treatments were not simultaneously performed. In addition, FIGS. 11 and 12 are diagrams illustrating variations in magnetization and variations in coercive force, respectively, after a heat treatment in a case where the aging treatment and the diffusion and infiltration treatment of the modified alloy were simultaneously performed while changing the boron content (B content) and a case where the above-described treatments were not simultaneously performed while changing the boron content (B content).

(50) First, it was verified from FIGS. 9 and 10 that, in Examples 6 and 7 in which the aging treatment and the diffusion and infiltration treatment of the modified alloy were simultaneously performed, a decrease in remanent magnetization was significantly decreased to about to and the coercive force was increased by about 50% as compared to those of Comparative Examples 8 and 9 in which the above-described treatments were not simultaneously performed.

(51) In addition, it was verified from FIGS. 11 and 12 that, in the method in which the aging treatment and the diffusion and infiltration treatment of the modified alloy were simultaneously performed, the effect of suppressing a decrease in remanent magnetization by the heat treatment was higher and the effect of improving the coercive force was higher as compared to those of the method in which the above treatments were separately performed or the method in which only the diffusion and infiltration of the modified alloy was performed.

(52) FIG. 13 is a diagram illustrating changes in magnetization and coercive force during a heat treatment depending on the change in the boron content (B content). In this experiment, the content of the main phase was 95 mass %, and the content of the grain boundary phase was 5 mass %.

(53) It was found from FIG. 13 that, when the B content was in a range of 0.95 mass % to 1.05 mass %, both the coercive force and the remanent magnetization were increased by the aging treatment. When the B content was less than 0.95 mass %, magnetic properties decreased due to the appearance of soft-magnetic Nd.sub.2Fe.sub.17, and when the B content was more than 1.05 mass %, magnetic properties also decreased due to an excessively large amount of Nd.sub.1.1Fe.sub.4B.sub.4.

(54) The reason why the coercive force is improved and a decrease in magnetization is suppressed by simultaneously performing the aging treatment and the diffusion and infiltration treatment of the modified alloy is presumed to be as follows: the coarsening of crystal grains is suppressed due to a short heating history; and when the NdCu alloy is infiltrated in a state where the grain boundary phase before the heat treatment is incomplete (in a Fe rich state), a gradient of the Nd concentration is large, and thus the NdCu alloy is easily infiltrated.

(55) Hereinabove, the embodiments of the invention have been described with reference to the drawings. However, a specific configuration is not limited to the embodiments, and design changes and the like which are made within a range not departing from the scope of the invention are included in the invention.