SHEET MAGNET HAVING MICROCRYSTALLINE STRUCTURE AND METHOD OF MANUFACTURING THE SAME, AND METHOD OF MANUFACTURING ISOTROPIC PERMANENT MAGNET POWDER

Abstract: A sheet magnet having a residual magnetic flux density Br of not less than 10kG, a cost performance equal to that of a hard ferrite magnet, and a thickness of 70∼300 µm contributing to the miniaturization and thinning of a magnetic circuit, and a method of manufacturing the same. When a molten alloy of a predetermined structure having a small content of a rare earth element is subjected to continuous casting using a cooling roll in a vacuum inert gas atmosphere of not more than 30 kPa at a predetermined peripheral speed of the roll, it turns into a crystalline structure substantially not less than 90 % of which comprises a FE 3 B type compound and a compound phase having α-Fe and Nd 2 Fe 14 B type crystalline structures compatible with the former. A continuous sheet magnet of 70∼300 µm in thickness comprising a microcrystalline structure of 10∼50 nm in average crystal grain diameter having magnetic characteristics of iHc ≧ 2kOe, Br ≧ 10kG and practically usable as a permanent magnet can be obtained. A sheet magnet which has heretofore been difficult to be industrially produced can be mass-produced at a low price by a simple method.

Description: TECHNICAL FIELD The present invention relates to a thin plate magnet optimal for use in various types of small motors, actuators, and magnetic circuits for magnetic sensors and a method for its manufacture. More particularly, the present invention relates to a thin-plate magnet having magnetic properties of iHc≧160kA/m (2kOe) and Br≧1T (10kG) and a crystal structure wherein 90% of the crystal structure comprises Fe 3 B compounds coexisting with compound phases having α-Fe and Nd 2 Fe 14 B crystal structures; the present invention also relates to the method for manufacturing the thin-plate magnet directly by continuous casting of thin sheets of alloy, with thicknesses of 70 pm to 300 pm, through the casting of an alloy bath with a specific composition, including 5 at% or less of rare earth elements and 15 to 30 at% of boron, in an inert gas atmosphere at a specific reduced pressure on rotating cooling rollers. BACKGROUND ART Current requirements for household electronics, office equipment, and other electronics equipment include higher capacities, as well as smaller sizes and lighter weights. Designs for magnetic circuits using permanent magnets are being studied for the purpose of maximizing cost performance. In particular, permanent magnets with residual magnetic flux densities Br of 0.5T (5kG) to 0.7T (7kG) are optimal for use in the structure of dc brush motors, which make up more than half of current production figures. Such residual magnetic flux densities cannot be achieved with conventional hard ferrite magnets. For example, Nd-Fe-B sintered magnets and Nd-Fe-B bonded magnets, which consist mainly of Nd 2 Fe 14 B, do have the requisite magnetic properties. However, these include 10 to 15 at% of Nd, for which the isolation and refining of the metal and reducing reactions require many steps and large scale facilities. These are therefore considerably more expensive than hard ferrite magnets and, in view of cost performance, only replace hard ferrite magnets in some instruments. Inexpensive permanent magnet materials having Br of 0.5T 5kG or more have yet to be discovered. Also, small, thin magnetic circuits require that the permanent magnet itself be in the form of a thin sheet, 100 µm to 500 µm thick. With Nd-Fe-B sintered magnets, it is difficult to attain bulk material with thicknesses of 500 µm or less. These can therefore only be prepared by grinding, which greatly increases costs. Nd-Fe-B bonded magnets are made by bonding powder with thicknesses of 30 µm and diameters of 10 µm to 500 µm in resin, making it difficult to form magnets with thicknesses of 100 µm to 300 µm. Meanwhile, magnetic materials comprising mainly Fe 3 B compounds in compositions approaching Nd 4 Fe 77 B 19 (at%) have been proposed as Nd-Fe-B magnets in recent years (R. Coehoorn et al., J. de Phys, C8, 1988, p. 669-670). The technical details thereof were disclosed in U.S. Patent No. 4,935,074. Prior to that, Koon proposed in U.S. Patent No. 4,402,770 a method for manufacturing permanent magnets comprising fine crystals by crystallizing heat treatment of La-R-B-Fe amorphous alloys, including La as an essential element. More recently, as disclosed in European Patent No. 558691B1, Richter et al. reported that thin fragments having hard magnetic properties were attained by heat treatment at 700 °C of amorphous flakes attained by spraying an Nd-Fe-B-V-Si alloy bath, including 3.8 to 3.9 at% of Nd, on a rotating Cu roller. These permanent magnet materials are permanent magnet materials with metastable crystal structures including a blend of a soft magnetic Fe 3 B phase and a hard magnetic R 2 Fe 14 B phase, attained by crystallizing heat treatment of amorphous flakes with thicknesses of 20 µm to 60 µm. Such permanent magnet materials have iHc of 160kA/m (2kOe) to 240kA/m (3kOe) and Br of 1T (10kG). Because the composition includes only 4 at% of expensive Nd, the raw materials are cheaper than those for an Nd-Fe-B magnet comprising mostly Nd 2 Fe 14 B. On the other hand, the liquid quenching conditions are restricted because the amorphous alloying of the raw materials is a necessary condition. Also the heat treatment conditions for attaining hard magnetic materials are narrowly restricted. This is therefore not practical in an industrial manufacturing setting and these magnets cannot be provided as inexpensive replacements for hard ferrite magnets. Because such permanent magnet materials are attained by crystallizing heat treatment of amorphous flakes with thicknesses of 20 µm to 60 µm, it is not possible to attain permanent magnets with the 70µm to 300 µm thicknesses required of thin-film magnets. On the other hand, for the quenched Nd-Fe-B magnetic materials in U.S. Patent No. 508,266 and so forth, the crystal structures are attained directly by quenching methods, so crystallizing heat treatment is not necessary. However, the optimum peripheral speed of the rollers disclosed in those reports was 20 m/s; the thickness of the quenched alloy flakes attained under those conditions is only about 30 µm and these cannot be used as thin-film magnets. These are therefore pulverized to diameters of several dozen to 500 µm and used in the bonded magnets discussed above. It is already known from European Patent Application No 0 542 529, Sumitomo Special Metals Company Limited, to manufacture a thin-plate rare earth magnet having magnetic properties of iHc ≥ 240kA/m and Br ≥ 0.9T with a fine crystal structure composed of fine crystals with a mean crystal grain size of 5 nm to 100 nm wherein the Fe 3 B compounds exist with compound phases having α-Fe and Nd 2 Fe 14 B crystal structures, expressed by the composition formula Fe 100-x-y B x R y R being one or more types of Pr and Nd the symbols x and y, which specify the range of the composition, satisfying the following values: 16 ≤ x ≤ 22 at% and 3 ≤ y ≤ 6 at%, wherein said thin-plate magnet has a thickness of 30 µm to 40 µm. There is also disclosed a method of manufacturing a thin-plate magnet of the kind indicated above using an alloy bath and continuous casting employing a single Cu cooling roller turning at a specific speed of 5 to 50 m/s with subsequent heat treatment to yield a permanent magnet possessing the properties set forth above. The present invention differs in that it provides a thin-plate magnet having a greater thickness than that previously disclosed. Whilst using a similar technique to that as embodied in the EPA 0 542 529 the range of composition in the present invention as assigned by the x and y symbols coupled with the fine crystal diameter lying as it does in a select range manifestly distinguish the thin-plate magnet over the prior art as discussed in the preamble and overcome the shortcomings of the various instances of that prior art. DISCLOSURE OF THE INVENTION It is an object of the present invention to provide a thin-plate magnet and a method for the production thereof; such a thin-plate magnet has a fine crystal structure with a thickness of 100µm to 300 µm, contributes to the miniaturization of magnetic circuits, has a cost performance rivaling that of hard ferrite magnets and a residual magnetic flux density Br of 1T (10kG) or more, and resolves the abovementioned problems with Nd-Fe-B magnets having fine crystal structures and containing 5 at% or less of rare earth elements. The inventors performed various studies with the object of creating thin-plate magnets of Nd-Fe-B permanent magnets with low concentrations of rare earth elements and wherein soft magnetic phases and hard magnetic phases are blended. As a result, they made the following discoveries and arrived at the present invention. They found that they could use continuous casting to attain thin plate magnet of continuously cast alloy with thicknesses of 70 µm to 300 µm and having fine crystal structures with the mean crystal grain diameter of each component phase being 50 nm or less, wherein 90% of the crystal structure comprises Fe 3 B compounds coexisting with compound phases having α-Fe and Nd 2 Fe 14 B crystal structures. The method involves continuous casting of an alloy bath, having a specific composition with low content of rare earth elements, in an inert gas atmosphere with reduced pressures of 30 kPa or less on a single cooling roller or between a pair of rollers rotating at a specific peripheral speed. Furthermore, they found that magnetic properties of iHc ≥ 160 kA/m (2kOe) and Br ≥ 1T (10kG) were achieved when the mean crystal grain diameters of each phase composing the continuously cast alloy attained were 10 nm to 50 nm. Moreover, they found that, in the case of crystalline metal structures with mean crystal grain diameters of less than 10 nm, they could use a specific heat treatment with the object of grain growth to attain mean crystal grain diameters of 10 nm to 50 nm for each phase and magnetic properties of iHc ≥ 160 kA/m (2kOe) and Br ≥ 1T (10kG). Specifically, the present invention is a thin-plate magnet with magnetic properties of iHc≥160kA/m (2kOe) and Br≥1T (10kG), and having a fine crystal structure comprising fine crystals with mean crystal grain diameter of 10 nm to 50 nm, wherein 90% of the crystal structure comprises Fe 3 B compounds coexisting with compound phases having α-Fe and Nd 2 Fe 14 B crystal structures, expressed by composition formula Fe 100-x-y B x R y , R being one or two types of Pr, Nd or Dy, the symbols x and y, which specify the range of the composition, satisfying the following values: 15 ≤ x≤ 30 at% 1 ≤ y ≤ 5 at% wherein said thin-plate magnet has a thickness of 70 µm to 300 µm. Also, the present invention is method of manufacturing a thin-plate magnet with a fine crystal structure, comprising the steps of: providing an alloy bath, the composition of which is expressed as Fe 100-x-y B x R y , R being one or two types of Pr, Nd or Dy, the symbols x and y, which specify the range of composition, satisfying the following values: 15 ≤ x≤ 30 at% 1 ≤ y ≤ 5 at%, continuous casting of said alloy bath in an inert gas atmosphere at reduced pressures of 30kPa or less, on a single cooling roller or between a pair of rollers rotating at a specific peripheral speed, wherein the peripheral speed of the roller is 2 to 10 m/s for Cu rollers and 2 to 7 m/s for Fe rollers, during continuous casting using a single cooling roller, or wherein the distance between opposing rollers is 0.005 to 0.5 mm and the peripheral speed is 1.5 to 5 m/s in the case of using a pair of cooling rollers, thereby directly attaining a permanent magnet having magnetic properties of iHc≥160kA/m (2kOe) and Br≥1T (10kG), a thickness of 70 µm to 300 µm, and comprising fine crystals with mean crystal grain diameter of 50 nm or less, wherein 90% of the crystal structure comprises Fe 3 B compounds coexisting with compound phases having α-Fe and Nd 2 Fe 14 B crystal structures. Also, the present invention is the abovementioned manufacturing method for a thin-plate magnet having a fine crystal structure, for attaining a permanent magnet having magnetic properties of iHc ≥ 160 kA/m (2kOe) and Br ≥ 1T (10kG) and comprising a fine crystalline alloy with mean crystal grain diameters of 10 nm to 50 nm; such a method comprising heat treatment in the temperature range of 500 °C to 700 °C with the object of crystal growth, when the alloy bath forms fine crystals with mean crystal grain diameters of 10 nm or less, following the quenching of the cast alloy with a thickness of 70µm to 300 µm. Also, the inventors propose combination with a manufacturing method for a thin-plate magnet machined to a prescribed form by die cutting the thin-plate magnet, being a permanent magnet with a thickness of 70µm to 300 µm and having the abovementioned magnetic properties of iHc ≥ 160 kA/m (2kOe) and Br ≥ 1T (10kG). Furthermore, the inventors propose a manufacturing method for an isotropic permanent magnet powder having the magnetic properties of iHc ≥ 160 kA/m (2kOe) and Br ≥ 1T (10kG) and which is prepared by pulverizing the scraps of the thin-plate magnet to mean grain diameters of 3 µm to 500 µm, after die cutting of the abovementioned thin-plate magnet to a prescribed form. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph showing the characteristic Cu-Kα X-ray diffraction pattern for samples in the embodiment. BEST MODE FOR CARRYING OUT THE INVENTION The specifics of the structure of the thin-plate magnet relating to the present invention are explained. Strong magnetic characteristics are attained only when a rare earth element R comprises a specific quantity of one or two types of Pr, Nd, or Dy. Properties such as iHc of greater than 160kA/m (2kOe) are not attained with other rare earth materials such as Ce and La. Also, except for Tb and Dy, medium and heavy rare earth elements beyond Sm are not preferable because they add to costs and bring about deteriorating magnetic properties. R is preferably in a range of 1 at% to 5 at%. If less than 1 at%, iHc of greater than 160kA/m (2kOe) cannot be attained; and if greater than 5 at%, Br of greater than 1T (10kG) is not attained. A range of 2 at% to 5 at% is more desirable. When B is less than 15 at%, a-Fe precipitation is marked in metal structures following liquid quenching and only iHc of less than 80 kA/m (1kOe) is attained because the precipitation of compounds having the Nd 2 Fe 14 B crystal structure necessary for the manifestation of coercive force is blocked. When greater than 30 at%, the angularity of the demagnetization curve decreases markedly and Br of greater than 800 kA/m (10kOe) is not attained. Therefore, B is preferably in the range of 15at% to 30 at%. A range of 17 at% to 20 at% is better. Fe makes up the remainder of the abovementioned elements. Replacing part of the Fe with Co makes the metal structure finer; moreover, the angularity of the demagnetization curve is improved, as well as the maximum energy product (BH)max and heat resistance. Such effects are not attained if the substitution amount is less than 0.1% of Fe and if greater than 50%, Br of 1T (10kG) is not attained. The substitution amount of Co in relation to Fe is therefore in the range of 0.1% to 50%, and preferably 0.5% to 10%. The iHc is improved by substituting 0.1 to 7 at% of Cr. The angularity of the demagnetization curve is improved and Br and (BH)max can be increased by substituting Fe with one or two types of the following with the total substitution amount being 0.05 at% to 2 at%: Al, Si, S, Ni, Cu, Zn, Ga, Ag, Pt, Au, Pb. The specifics of the conditions for manufacturing the thin-plate magnet according to the present invention are explained. In the present invention, an alloy bath with the abovementioned specific composition is cast in an inert gas atmosphere with reduced pressures of 30 kPa or less, on a rotating, single cooling roller or between a pair of rollers at a specific peripheral speed. This casting process produces a continuously cast alloy, with thicknesses of 70 µm to 300 µm and wherein 90% of the crystal structure comprises Fe 3 B compounds coexisting with compound phases having α-Fe and Nd 2 Fe 14 B crystal structures. Moreover, it is most important for manufacturing thin-plate magnets that the mean crystal grain diameter of each component phase be the 10 nm to 50 nm microcrystalline structure necessary to attain magnetic properties of iHc≧160 kA/m (2kOe) and Br≧1T (10kG). During the continuous casting of the alloy bath, the pressure of the atmosphere is specified as characteristic to the present invention. When the casting atmosphere is 30 kPa or greater, gas gets between the cooling roller and the alloy bath and causes a loss of uniformity in the quenching conditions for the cast alloy, which then has a metal structure including coarse α-Fe and with which magnetic properties of iHc≧160kA/m (2kOe) and Br ≧1T (10kG) are not attained. The alloy quenching atmosphere is therefore 30 kPa or less and preferably 10 kPa or less. The atmospheric gas is an inert gas in order to prevent oxidation of the alloy bath and is preferably argon. In the abovementioned continuous casting method, heat treatment with the object of grain growth may be carried out when the mean crystal grain diameter of the cast alloy is not the 10 nm to 50 nm mean crystal grain diameter necessary to attain the magnetic properties of iHc≧160kA/m (2kOe) and Br≧1T (10kG). If the heat treatment temperature is less than 500°C, grain growth does not occur and mean crystal grain diameters of greater than 10 nm are not attained. If greater than 700°C, grain growth is marked, but iHc, Br, and the angularity of the demagnetization curve deteriorate and the abovementioned magnetic properties are not attained. The heat treatment temperature must therefore be 500°C to 700 °C. Moreover, the heat treatment temperature for maximizing the magnetic properties depends on the composition. In heat treatment, the atmosphere is preferably an inert gas atmosphere of argon gas or N 2 gas or vacuum of 1.33 Pa or less in order to prevent oxidation. The magnetic properties are not dependent on heat treatment time, but Br tends to drop slightly over the passage of time if heat treatment is longer than six hours. Therefore the heat treatment time is preferably less than six hours. Continuous casting methods using single cooling rollers or double rollers can be employed in the casting process for the alloy bath. When the thickness of the cast alloy exceeds 300 m, the magnetic properties of iHc≧160kA/m (2kOe) and Br≧1T (10kG) cannot be attained because of the precipitation of coarse α-Fe and Fe 2 B, on the order of several hundred nanometers. Moreover, when the thickness of the cast alloy is less than 70 µm, the crystal structure in the cast alloy is reduced and the amorphous phase increases. This makes it necessary to crystallize the amorphous phase using heat treatment. The heating for the crystallization of the amorphous phase increases the temperature of the alloy; this heating of the alloy directly after quenching causes grain growth in the already precipitated crystal structure. The result is a metal structure having mean grain diameters greater than the 10 nm to 50 nm mean grain diameters necessary to attain the magnetic properties of iHc ≧160kA/m (2kOe) and Br ≧1T (10kG). Br of greater than 10kG is therefore not attained. Consequently the casting conditions require that the cast alloy thickness be 70 µm to 100 µm. In consideration of heat conductivity, the cooling rollers used in the continuous casting process can be made of aluminum alloys, Cu, Fe, brass, W, or bronze. However, Cu or Fe (alloys including Cu or Fe are acceptable) are preferable in view of their mechanical strength and economy. Materials other than those above are not preferable because of poor heat conductivity; they cannot sufficiently cool the alloy bath and the magnetic properties of iHc ≧160kA/m (2kOe) and Br ≧1T (10kG) cannot be attained because of the precipitation of coarse α-Fe and Fe 2 B on the order of several hundred nm. The following apply when the cooling roller is a Cu roller having a surface roughness characterized by center line roughness Ra ≦ 0.8 µm, maximum height Rmax ≦ 3.2 µm, and mean roughness at ten points of Rz ≦ 3.2 µm. When the peripheral speed of the roller exceeds 10 m/s, the thickness of the cast alloy becomes less than 70 µm, crystal structures included in the cast alloy are reduced, and the amorphous phase increases. When the peripheral speed of the roller is 2 m/s or less, the cast alloy is greater than 300 µm thick. As a result, the magnetic properties of iHc≧160kA/m(2kOe) and Br≧1T(10kG) cannot be attained because of the precipitation of coarse α-Fe and Fe 2 B on the order of several hundred nm. Consequently, the peripheral speed of a Cu roller must be within the range of 2 m/s to 10 m/s. The following apply when the cooling roller is an Fe roller having the same surface roughness as the abovementioned Cu roller. The Fe roller has better wettability for the alloy bath than the Cu roller. Therefore, when the peripheral speed of the roller exceeds 7 m/s, the cast alloy becomes less than 70 µm thick, crystal structures included in the cast alloy are reduced, and the amorphous phase increases. This is not preferable because the magnetic properties of iHc≧160 kA/m (2kOe) and Br≧1T (10kG) cannot be attained. Consequently, the peripheral speed for an Fe roller must be in the range of 2 m/s to 7 m/s, and preferably 2.5 m/s to 5.5 m/s. Furthermore, the following apply in the case of quenching with a pair of rollers, wherein the Fe rollers discussed above are arranged in opposition and the alloy bath is cooled with the two cooling rollers. When the distance between the two rollers is 0.5 mm or greater, the bath passing between the rollers does not contact the cooling rollers and is not cooled, resulting in a metal structure including coarse α-Fe. When the distance between the rollers is less than 0.005 mm, the bath overflows from between the rollers, making continuous casting impossible. Consequently the distance between the two rollers must be in the range of 0.005 mm to 0.5 mm, and preferably 0.05 mm to 0.2 mm. When the peripheral speed of the two Fe rollers exceeds 8 m/s, the crystal structures included in the cast alloy are reduced and the amorphous phase increases. Also, when the peripheral speed is less than 1 m/s, the magnetic properties of iHc≧160 kA/m (2kOe) and Br≧1T (10kG) cannot be attained because of the precipitation of coarse α-Fe and Fe 2 B on the order of several hundred nm. Consequently, the peripheral speed of the rollers must be within the range of 1 m/s to 8 m/s, and preferably 1.5 m/s to 5 m/s. The methods used to finish the continuous thin-plate magnet attained in the continuous casting process into a prescribed form include etching, for finishing thin film metal materials manufactured by rolling, and ultrasonic machining. Die cutting with ultrasound is preferable because it is possible to finish a thin-plate magnet into a prescribed form without cracking the thin-plate magnet. Also, when manufacturing thin-plate magnets with a prescribed form, die cutting results in scraps of thin-plate magnet. These can be pulverized and used as isotropic permanent magnet powder having iHc≧160 kA/m (2kOe) and Br≧0.7T (7kG). Powder grain sizes are preferably 20 µm to 300 µm for use as magnetic powder for compression-molded, bonded magnets, and less than 50 µm for use as magnetic powder for injection-molded bonded magnets. The crystal phase of the thin-plate magnet according to the present invention comprises a fine crystalline aggregate wherein the mean crystal grain diameter of each component phase is in the range of 10 nm to 50 nm, and wherein soft magnetic Fe 3 B compounds and soft magnetic compound phases having α-Fe and Nd 2 Fe 14 B crystal structures coexist within the same structures. When the mean crystal grain diameter of the structures comprising the thin-plate magnet exceeds 50 nm, the angularity of the demagnetization curve and Br deteriorate and it becomes impossible to attain magnetic properties of Br≧1T (10kG). Also, while smaller mean crystal grain diameters are preferable, 10 nm is the lower limit because of the consequent drop in iHc if less than 10 nm. Embodiments Embodiment 1 The following metals with purities of 99.5% or better were used to form the compositions No. 117 in Table 1: Fe, Al, Si, S, Ni, Cu, Co, Ni, Cr, Zn, Ga, Ag, Pt, Au, Pb, B, Nd, Pr, Dy. These were weighed out to form total masses of 30 g, placed in a quartz crucible having an 0.8 mm diameter orifice in the bottom, and melted using high frequency heating in an Ar atmosphere maintained at the quenching pressures in Table 1. After the bath reaches a melting temperature of 1300°C, the bath surface was pressurized with Ar gas; at room temperature, the bath was then continuously cast from a height of 0.7 mm on the surfaces of Cu cooling rollers rotating at the peripheral speeds shown in Table 1. Continuous thin-plate magnets with a width of 2 mm to 3 mm and thickness of 100 µm to 300 µm were thereby prepared. Figure 1 shows a Cu-Kα X-ray diffraction pattern for the example No. 2. As shown in Figure 1, the thin-plate magnet attained was found to have a metal structure wherein Fe 3 B compounds and compound phases having α-Fe and Fe 14 B crystal structures coexist. Also, all of the samples, excluding No. 4, No. 16, and No. 17, had fine crystal structures, wherein the mean crystal grain diameters were 10 nm to 50 nm. Table 2 shows the magnetic properties of the thin-plate magnet attained. These properties were measured by VSM using thin-plate magnets with a prescribed form cut into 5 mm disks using ultrasonic machining. Embodiment 2 No. 4, No. 16, and No. 17 in Table 1 had mean crystal grain diameters of less than 10 nm. The thin-plate magnets were therefore maintained at 600 °C for ten minutes in Ar gas and heat treated so that mean crystal grain diameters would become greater than 10 nm. Table 2 shows the magnetic properties of the thin-plate magnets attained. These properties were measured by VSM using thin-plate magnets machined to a prescribed form as in Embodiment 1. Moreover, Al, Si, S, Ni, Cu, Co, Ni, Cr, Zn, Ga, Ag, Pt, Au, and Pb replaced part of the Fe in each component phase in No. 5 to No. 17. Embodiment 3 For No. 2, No. 3, No. 5, No. 6, and No. 13, an isotropic permanent magnet powder was prepared by pulverizing the scraps of the thin-plate magnet in a grinder after die cutting for VSM measurement; grain diameters were 25 µm to 300 µm and mean grain diameters were 150 µm. Table 3 shows the magnetic properties of the magnetic powder measured using VSM. Comparison 1 In the same way as Embodiment 1, a continuously cast alloy was prepared under the casting conditions in Table 1 using Fe, B, and R with purities of 99.5% or better, so as to form the compositions in No. 18 to No. 21 in Table 1. Figure 1 shows the Cu-Kα X-ray diffraction pattern for the samples attained. As determined from the results of the X-ray diffraction patterns, the sample No. 18 was an amorphous alloy which exhibited no magnetic force. The sample No. 19 was a metal structure comprising mainly the α-Fe phase. Moreover, the sample No. 20 was a structure comprising α-Fe and Nd 2 Fe 23 B 3 , being non-magnetic phases; the sample No. 21 was a metal structure comprising mainly α-Fe, like in No. 19. Table 2 shows the magnetic properties of No. 18 21 measured with VSM. Composition (at %) Peri-pheral speed quen-ching atmosphere pressure Fe B R M m/s kPa Embodiment 1 79.5 18.5 Nd2 - 2 1.3 2 78.5 18.5 Nd3 - 5 1.3 3 78.5 17 Nd3.5+Pr1 - 3 5.0 4 75 20 Nd4+Dy1 - 7 10.0 5 78 15 Nd4 Co3 2 2.0 6 71.5 18.5 Nd5 Cr5 3 3.0 7 79.5 17 Nd3 Al0.5 4 4.0 8 76.5 18.5 Nd4 Si1 5 5.0 9 69 18.5 Nd4.5 S1 5 6.0 10 76 20 Nd5 Ni1.5 5 7.0 11 75 20 Nd2.5+Pr1 Cu1.5 4 8.0 12 76.5 18.5 Nd2+Pr2 Zn1 2 9.0 13 75.5 18.5 Nd2+PR3 Ga1 3 10.0 14 76.5 18.5 Nd4 Ag1 3 1.3 15 78 18 Nd3.5 Pt0.5 5 2.0 16 76.5 18.5 Nd4.5 Au0.5 8 0.5 17 76.5 18.5 Nd5 Pb1 10 5.0 Comparison 18 78.5 18.5 Nd3 - 20 1.3 19 78.5 18.5 Nd3 - 5 75.0 20 75.5 18.5 Nd6 - 5 1.3 21 86 10 Nd4 - 5 1.3 Magnetic properties Br T (kG) iHc (kOe) kA/m (BH)max (MGOe) kJ/m 3 Embodiment 1 1.5 (15.0) 160 (2.0) 108 (13.5) 2 1.37 (13.7) 216 (2.7) 120 (15.0) 3 1.14 (11.4) 336 (4.2) 117.6 (14.7) 4 1.03 (10.3) 400 (5.0) 105.6 (13.2) 5 1.23 (12.3) 344 (4.3) 122.4 (15.3) 6 1.01 (10.1) 552 (6.9) 103.2 (12.9) 7 1.34 (13.4) 240 (3.0) 149.6 (18.7) 8 1.26 (12.6) 280 (3.5) 138.4 (17.3) 9 1.20 (12.0) 320 (4.0) 136.8 (17.1) 10 1.15 (11.5) 352 (4.4) 132.8 (16.6) 11 1.3 (13.0) 264 (3.3) 140 (17.5) 12 1.22 (12.2) 288 (3.6) 134.4 (16.8) 13 1.16 (11.6) 360 (4.5) 135.2 (16.9) 14 1.24 (12.4) 280 (3.5) 137.6 (17.2) 15 1.29 (12.9) 256 (3.2) 140.8 (17.6) 16 1.21 (12.1) 328 (4.1) 137.6 (17.2) 17 1.17 (11.7) 368 (4.6) 133.6 (16.7) Comparison 18 - - - 19 ·72 (7.2) 128 (1.6) 29.6 (3.7) 20 ·65 (6.5) 72 (0.9) 4.8 (0.6) 21 ·65 (6.5) 246 (3.7) 33.6 (4.2) Magnetic properties Br T (kG) iHc (kOe) kA/m (BH)max (MGOe) kJ/m 3 Embodiment 2 1.16 (11.6) 216 (2.7) 70.4 (8.8) 3 ·97 (9.7) 336 (4.2) 76.8 (9.6) 5 1.05 (10.5) 344 (4.3) 79.2 (9.9) 6 ·86 (8.6) 552 (6.9) 67.2 (8.4) 13 ·99 (9.9) 360 (4.5) 88 (11.0) INDUSTRIAL APPLICABILITY The present invention can provide thin-plate magnets with thickness of 70 µm to 300 pm at low costs. Such thin-plate magnets, which could not heretofore be mass produced at low costs in an industrial setting, can contribute to the miniaturization of magnetic circuits and have a cost performance comparable to hard ferrite magnets. This manufacturing is accomplished through continuous casting of an alloy bath with a specific composition and comprising low quantities of rare earth elements, under an inert gas atmosphere at reduced pressures of 30 kPa or less, with a single rotating cooling roller or between a pair of rollers at a specific peripheral speed. A continuously cast, thin-plate magnet is attained directly thereby. This thin-plate magnet has a crystal structure wherein Fe 3 B compounds coexist with compound phases comprising α-Fe and Nd 2 Fe 14 B crystal structures; other characteristics include: thicknesses of 70 µm to 300 µm, comprising microcrystalline structures with mean crystal grain diameters of 10 nm to 50 nm, having magnetic properties of iHc≧160 kPa/m (2kOe) and Br≧1T (10kG) and being able to withstand use as a permanent magnet. Furthermore, in the case where fine structures with mean crystal grain diameters of less than 10 nm are attained, heat treatment is carried out with the object of specific grain growth; the mean crystal grain diameter of each component phase thereby becomes 10 nm to 50 nm and the thin-plate magnet attained has magnetic properties of iHc≧160kA/m (2kOe) and Br≧1T (10kG)

Claims: A thin-plate magnet with magnetic properties of iHc≥160kA/m (2Koe) and Br≥1T (10kG), and having a fine crystal structure comprising fine crystals with mean crystal grain diameter of 10 nm to 50 nm, wherein 90% of the crystal structure comprises Fe 3 B compounds coexisting with compound phases having α-Fe and Nd 2 Fe 14 B crystal structures, expressed by composition formula Fe 100-x-y B x R y , R being one or two types of Pr, Nd or Dy, the symbols x and y, which specify the range of the composition, satisfying the following values: 15 ≤ x≤ 30 at% 1 ≤ y ≤ 5 at% wherein said thin-plate magnet has a thickness of 70 µm to 300 µm, directly obtained by continuous casting. A method of manufacturing a thin-plate magnet with a fine crystal structure, comprising the steps of: Providing an alloy bath, the composition of which is expressed as Fe 100-x-y B x R y , R being one or two types of Pr, Nd or Dy, the symbols x and y, which specify the range of composition, satisfying the following values: 15 ≤ x≤ 30 at% 1 ≤ y ≤ 5 at%, continuous casting of said alloy bath in an inert gas atmosphere at reduced pressures of 30kPa or less, on a single cooling roller or between a pair of rollers rotating at a specific peripheral speed, wherein the peripheral speed of the roller is 2 to 10 m/s for Cu rollers and 2 to 7 m/s for Fe rollers, during continuous casting using a single cooling roller, or wherein the distance between opposing rollers is 0.005 to 0.5 mm and the peripheral speed is 1.5 to 5 m/s in the case of using a pair of cooling rollers, thereby directly attaining a permanent magnet having magnetic properties of iHc≥160kA/m (2kOe) and Br≥1T (10kG), a thickness of 70 µm to 300 µm, and comprising fine crystals with mean crystal grain diameter of 50 nm or less, wherein 90% of the crystal structure comprises Fe 3 B compounds coexisting with compound phases having α-Fe and Nd 2 Fe 14 B crystal structures. The manufacturing method according to Claim 2 for a thin-plate magnet having a fine crystal structure, for attaining a permanent magnet having magnetic properties of iHc ≥ 160 KA/m (2kOe) and Br ≥ 1T (10kG) and comprising a fine crystalline alloy with mean crystal grain diameters of 10 nm to 50 nm, such a method comprising heat treatment in the temperature range of 500°C to 700°C with the object of crystal growth, when the alloy bath forms fine crystals with mean crystal grain diameters of 10 nm or less, following the quenching of the cast alloy with a thickness of 70µm to 300 µm. The manufacturing method, according to Claim 2 or 3, for thin-plate magnet having a fine crystal structure and machined to a prescribed form by die cutting the permanent magnet. A manufacturing method for an isotropic permanent magnet powder having a fine crystal structure to attain an isotropic permanent magnetic powder having magnetic properties of IHc ≥ 16DkA/m (2kOe) and Br ≥ 0.7T (7kG) the method comprising pulverizing die cutting scraps to mean particle diameters of 3µm to 500µm following the die cutting of the 70 µm to 300 µm thick thin-plate magnet, obtained in a manufacturing method according to claim 2, said thin-plate magnet comprising fine crystals with mean crystal grain diameters of 50 nm or less wherein 90% of the crystal structure comprises Fe 3 B compounds coexisting with compound phases having α-Fe and Nd 2 Fe 14 B crystal structures; such a thin-plate magnet being manufactured by continuous casting in an inert gas atmosphere at reduced pressures of 30 kPa or less, on a single cooling roller or between a pair of rollers rotating at a specific peripheral speed, of an alloy bath, the composition of which is expressed as Fe100-x-yBxRy (R is one or two types of Pr, Nd, or Dy) where the symbols x and y which specify the range of the composition satisfy the following values: 15 ≤ x ≤ 30 at % and 1 ≤ y ≤ 5 at %.

Inventors: HIROSAWA SATOSHI, KANEKIYO HIROKAZU, HIROSAWA, SATOSHI, KANEKIYO, HIROKAZU
Applicants: SUMITOMO SPEC METALS, SUMITOMO SPECIAL METALS COMPANY LIMITED
Application year: 1997
Main classification: H01F1
IPC codes: H01F1, C22C38