HK1026299B - Thin plate magnet having microcrystalline structure - Google Patents

Description

Thin sheet magnet having microcrystalline structure

Technical Field

The present invention relates to a thin-sheet magnet suitable for use in various magnetic circuits for small-sized motors, actuators, magnetic sensors, and the like. The present invention is characterized in that a magnet having a microcrystalline structure is obtained by continuously casting a melt of a specific composition containing 6 at% or less of a rare earth element and 15 to 30 at% of boron on a rotating chill roll in a predetermined reduced pressure inert gas atmosphere, 90% or more of the crystal structure of the magnet in the as-cast state being substantially Fe₃B compound and alpha-Fe and Nd₂Fe₁₄B-structural compound phases coexist, and the average crystal grain size of each structural phase is straightThe diameter is 10-50nm in the as-cast state. The invention relates to a sheet magnet with a microcrystalline structure directly manufactured from an alloy melt, the thickness of the sheet magnet is 70-500 mu m, and the sheet magnet has the magnetic properties that iHc is more than or equal to 2.5kOe and Br is more than or equal to 9 kG.

Background

At present, higher performance, smaller size, and lighter weight of home appliances, OA equipment, electric fixtures, and the like are desired, and studies are being made to maximize a performance-weight ratio in an entire magnetic circuit using permanent magnets. For brush-mounted dc motors, permanent magnets with residual flux densities Br of 5-7 kG are considered ideal, which account for more than half of the motors currently manufactured, but which are not obtainable by conventional hard-magnetic ferrite magnets.

For example, Nd-Fe-B sintered magnets and Nd-Fe-B bonded magnets, in which the main phase is Nd, can realize such magnetic characteristics₂Fe₁₄B. However, the Nd-Fe-B magnet contains 10-15 at% Nd, requires a large number of processing steps and large-scale equipment for metal separation, refining, and reduction, and is very expensive compared to a hard ferrite magnet. Due to their performance-to-price ratio, these magnets can only replace hard magnetic ferrite magnets in certain types of equipment. At present, no inexpensive permanent magnet material exhibiting Br of 5kG or more has been found.

In addition, in order to realize a small-sized thin magnetic circuit, a thin permanent magnet having a permanent magnet thickness of 100 μm to 500 μm is required. Since it is difficult to obtain a bulk material having a thickness of less than 500 μm with an Nd-Fe-B sintered magnet, a thin sheet magnet can be manufactured only by grinding a sintered sheet having a thickness of several millimeters or by a method of slicing a bulk material by wire cutting or the like, there are problems of high processing cost and low productivity.

Nd-Fe-B bonded magnets are obtained by bonding powders having a thickness of about 30 μm and a diameter of several tens to 500 μm together using a resin, so that it is difficult to mold a bonded magnet having a sheet thickness of 100 μm to 300 μm.

Another one isOn the other hand, it has been proposed that the main phase is of Nd₄Fe₇₇B₁₉(at%) Fe of contiguous composition₃Nd-Fe-B permanent magnets of B compounds (R. Coehoron et al, J. de Phys, C8, 1988, page 669-670), the details of which are disclosed in US patent 4935074 et al.

Heretofore, Koon proposed a method for producing a permanent magnet composed of microcrystals in which a La-R-B-Fe amorphous alloy containing La as an essential element was subjected to a crystallization heat treatment, see US patent 4402770.

More recently, it has been reported that by melt-spraying an Nd-Fe-B-V-Si alloy containing Nd at 3.8 at% to 3.9 at% onto a rotating copper roller, amorphous flakes are produced, which are then heat-treated at a temperature of 700 ℃, thus obtaining flakes having hard magnetic properties, as disclosed in EP patent application 558691B1 to Richter et al. These permanent magnet materials obtained by subjecting amorphous flakes having a thickness of 20 μm to 60 μm to a crystallization heat treatment have a metastable structure with a crystalline aggregate structure, which is Fe₃B soft magnetic phase and R₂Fe₁₄B hard magnetic phase.

The above permanent magnet material has Br of 10kG and iHc of 2kOe-3kOe, and has a low content of expensive Nd of 4 at%, so the raw material cost is R as compared with the main phase₂Fe₁₄The Nd-Fe-B magnet of B is relatively inexpensive. However, there are limits on the liquid solidification conditions, which are essential for obtaining amorphous alloys from the starting mixture, and at the same time, there are limits on the heat treatment conditions for obtaining materials with hard magnetic properties. Therefore, such a magnet is not practical from the industrial point of view, and thus there is a problem that it cannot provide an inexpensive product replacement for the hard magnetic ferrite. Further, the permanent magnet material is obtained by subjecting amorphous flakes having a thickness of 20 μm to 60 μm to a crystallization heat treatment, so that it is impossible to obtain a permanent magnet having a thickness of 70 to 500 μm required for a thin sheet magnet.

On the other hand, US patent 508266 et al disclose a rapidly solidified Nd-Fe-B magnet material consisting of a crystal-formed structure having hard magnetic properties, which is directly obtained by rapidly solidifying an alloy melt on a roll having a peripheral speed of 20 m/s. However, the rapidly solidified alloy flakes obtained under these conditions are as thin as about 30 μm in thickness, so although they can be ground into powders having particle diameters of 10 μm to 500 μm for use in the above-mentioned bonded magnets, they cannot be used for thin sheet magnets.

Disclosure of Invention

The present invention is directed to solving the above-mentioned problems of Nd-Fe-B magnets containing 6 at% or less of rare earth elements and having microcrystals. Another object of the present invention is to obtain a magnet, which is comparable in performance-price ratio to hard magnetic ferrite and has an intrinsic coercive force iHc of 2.5kOe or more and a residual magnetic flux density Br of 9kG or more, by casting. It is still another object of the present invention to provide a thin sheet magnet having a magnet thickness of 70 to 500 μm, which can make a magnetic circuit smaller and thinner by having a microcrystalline structure.

The invention discloses a microcrystal permanent magnet with hard magnetic performance of iHc more than or equal to 2kOe and Br more than or equal to 10kG, which is directly obtained from an alloy melt and is prepared by adopting a manufacturing method that a low rare earth content Nd-Fe-B ternary alloy melt containing Nd less than 6at percent and boron of 15at percent to 30at percent is continuously cast on a cooling rotating roller with the roller peripheral speed of 2 to 10m/s in a decompressed specific inert gas atmosphere. However, this method for producing an Nd — Fe — B ternary magnet has a problem that the range of the peripheral speed of the roller in which hard magnetism can be obtained is narrow. In addition, the coercivity of 2-3 kOe can be only obtained by adopting the Nd-Fe-B ternary magnet, so that the thermal demagnetization is serious, the working point of the magnet needs to be increased as much as possible in order to realize high magnetic flux density, and consequently, the shape of the magnet and the use environment of the magnet are obviously limited.

The present inventors have conducted various experiments on the problem point of manufacturing a Nd-Fe-B microcrystalline permanent magnet having a low rare earth content, which is a mixture of a soft magnetic phase and a hard magnetic phase. As a result, it was found that the above-mentioned problems can be solved by using an alloy melt to which a specific element has been added in a process previously provided by the present inventors, by continuously casting the alloy melt on a rotating chill roll in a reduced pressure of a specific inert gas atmosphere, thereby obtaining a microcrystalline permanent magnet having a microcrystalline structure of 15nm to 50nm directly from the alloy melt. The present invention has been completed based on the finding that, by this method using an alloy melt to which specific elements have been added, the iHc of the magnet can be increased to 2.5kOe or more, the optimum roll peripheral speed range of the hard magnetic properties is obtained, the range is expanded as compared with the conventional conditions for producing Nd-Fe-B ternary magnets, and at the same time, a microcrystalline permanent magnet having a thickness of 70 μm to 500 μm can be obtained.

That is, the thin sheet magnet having a microcrystalline structure of the present invention is a permanent magnet having a thickness of 70 to 500 μm, has magnetic properties of iHc.gtoreq.2.5 kOe and Br.gtoreq.9 kG, and is composed of microcrystals having an average crystal grain diameter of 15nm to 50nm, and 90% of the crystalline structure in a cast state is Fe₃B compound and alpha-Fe and Nd₂Fe₁₄The compound phase of B crystal structure coexists, and the alloy is represented by the following composition general formula:

Fe_100-x-y-zR_xB_yM_zor (Fe)_1-mCo_m)_100-x-y-zR_xB_yM_z

Wherein R is one or more elements of Pr, Nd, Tb, and Dy, M is one or more elements of Cr, Mn, Ni, Cu, Ga, Ag, Pt, Au, and Pb, wherein symbols x, y, and z defining the composition ranges are atomic number percentages in the alloy, M represents the amount of Co substituted, x, y, z, and M satisfy the conditions of 1. ltoreq. x < 6, 15. ltoreq. y.ltoreq.30, 0.01. ltoreq. z.ltoreq.7, and 0.001. ltoreq. m.ltoreq.0.5, respectively.

Drawings

FIG. 1 is a Cu-Ka characteristic x-ray diffraction pattern of a sample representing an example.

FIG. 2 is a graph showing the dependence of coercive force on the peripheral speed of the roller at the time of rapid solidification using a rotating roller in examples and comparative examples.

Detailed Description

The alloy composition according to the present invention will be described in detail below.

High magnetic properties can be obtained only when one, two or more of Pr, Nd and Dy are contained as the rare earth element R at a specific content. It is not possible to obtain an iHc of 2.5kOe or more with other rare earth elements such as Ce and La. Besides Tb and Dy, both medium and heavy rare earth elements from Sm onwards are undesirable because of the adverse effect on magnetic properties and the magnets will be expensive.

If R is less than 1 at%, iHc of 2.5kOe or more cannot be obtained, and if R is 6 at% or more, Br of 9kG or more cannot be obtained. Therefore, the R content should be in the range of 1 at% or more and 6 at% or less. A preferred range of R is 2 at% to 5.5 at%.

If the B content is 15 at% or less, significant a-Fe is precipitated in the metal structure after rapid solidification, and Nd necessary for realizing coercive force is hindered₂Fe_i4And (4) precipitation of a compound with a B crystal structure. Therefore, only iHc less than 1kOe will be obtained. In addition, if the B content exceeds 30 at%, the squareness ratio of the demagnetization curve is significantly reduced, and therefore, the B content should be in the range of 15 at% or more and 30 at% or less. The preferable range of the B content is 16 at% to 20 at%.

The balance other than the above elements is Fe. When Fe is partially replaced by Co, and the alloy melt is rapidly solidified on a rotating roller to manufacture a microcrystal permanent magnet, a thinner metal structure can be obtained, the squareness ratio of a demagnetization curve, the maximum energy product (BH) max and the heat resistance are improved, the circumferential speed range of the optimal roller with hard magnetism can be widened, and the rapid solidification condition for obtaining the magnetic property is relaxed. If the amount of substitution of Co for Fe is less than 0.1%, these effects cannot be obtained, and if the amount of substitution exceeds 50%, Br of 9kG or more cannot be obtained. Therefore, the amount of Co substituted for Fe should be in the range of 0.1% to 50%. Preferably between 0.5% and 10%.

Cr, Mn, Ni, Cu, Ga, Ag, Pt, Au, or Pb used as the additive element M is one of the most important structural elements of the present invention. When the alloy melt is rapidly solidified on the rotating roll and the microcrystalline permanent magnet is directly manufactured from the melt, the circumferential speed range of the optimal roll for obtaining the hard magnetism can be widened, and the rapid solidification condition for obtaining the magnetic property is relaxed. In addition, the additive element M participates in obtaining a fine structure of the microcrystalline permanent magnet, improves the coercive force, improves the squareness ratio of a demagnetization curve, and improves Br and (BH) max. When the content M of the additive element is less than 0.01 at%, these effects cannot be achieved, and at a level of 7 at% or more, the magnetic properties of Br. gtoreq.9 kG cannot be achieved. Thus, the content should be 0.01 at% to 7 at%. A preferred range is 0.05 at% to 5 at%.

The following describes preferable production conditions for the permanent magnet of the present invention.

According to the invention, in a reduced-pressure inert gas atmosphere of 30kPa or less, at a roll peripheral speed of 1m/s to 10m/s (3X 10)³-1×10⁵Average cooling rate of DEG C/sec) on a rotating cooling roll, and continuously casting the alloy melt having the above composition to directly obtain a permanent magnet composed of microcrystals having an average crystal grain diameter of 50nm or less, a thickness of 70 to 500 μm, magnetic properties of iHc 2.5kOe and Br 9kG, and a crystal structure of 90% or more in a cast state of Fe₃B compound and alpha-Fe and Nd₂Fe₁₄The compound phase with B crystal structure coexists.

According to the present invention, by continuous casting on a rotating chill roll using the above alloy melt, a cast alloy having a thickness of 70 μm to 500 μm is produced from crystallites whose average crystal grain diameter is 10nm or less. Then, the heat treatment of grain growth is carried out in the temperature range of 550-750 ℃ to transform the crystal grains into microcrystalline alloy with the average grain diameter of 15nm-50nm, so that the thin sheet magnet with the magnetic properties of iHc more than or equal to 2.5kOe and Br more than or equal to 9kG can be obtained.

The most important point of the present invention is that an alloy melt having the above-mentioned specific composition is rapidly solidified by continuous casting on a rotating chill roll in a reduced pressure inert gas atmosphere of 30kPa or less to produce a thin sheet magnet having a crystal structure 90% of which is Fe in the as-cast state₃B compound and alpha-Fe and Nd₂Fe₁₄The compound phases of B crystal structure coexist, and the average grain diameter of each phase is 10nm-50nm, which is necessary for obtaining the magnetic performance of iHc ≥ 2.5kOe and Br ≥ 9 kG.

One feature of the present invention is the specific atmospheric pressure at which the alloy melt is continuously cast. The reason for this limitation is that when the casting atmosphere exceeds 30kPa, gas enters between the cooling roll and the alloy melt, thereby losing the uniformity of the conditions for rapid solidification of the cast alloy, and as a result, the metal structure contains coarse α -Fe, and magnetic properties of iHc ≧ 2.5kOe and Br ≧ 9kG cannot be obtained. Therefore, the atmosphere in which the alloy rapidly solidifies is maintained at 30kPa or less. Pressures below 10kPa are preferred. To prevent oxidation of the alloy melt, the atmosphere gas should be an inert gas. An Ar atmosphere is preferred.

The average grain diameter of the alloy cast by the continuous casting method may be heat-treated as described above to achieve grain growth if it is not within 10-50nm necessary to obtain magnetic properties of iHc.gtoreq.2.5 kOe and Br.gtoreq.9 kG. The temperature of the heat treatment to provide the best magnetic properties depends on the composition. However, grain growth does not occur at a heat treatment temperature lower than 500 deg.C, and thus an average grain diameter of 10nm to 50nm cannot be obtained. In addition, if the heat treatment temperature exceeds 750 ℃, the grain growth is significant, the squareness ratio of iHc, Br and demagnetization curve is adversely affected, and the above magnetic properties cannot be obtained. Therefore, the heat treatment temperature is limited to the range of 500-750 ℃.

In order to prevent oxidation, the heat treatment should be performed in an inert gas atmosphere such as argon or nitrogen or in a vacuum of 1.33Pa or less. Although the magnetic properties are not dependent on the heat treatment time, if the heat treatment time exceeds 6 hours, Br tends to decrease slightly with the lapse of time, so that the heat treatment time is preferably less than 6 hours.

For the casting of the alloy melt, a single cooling roll or a twin cooling roll continuous casting method may be employed. If the thickness of the cast alloy exceeds 500. mu.m, several hundred nm of coarse alpha-Fe and Fe are precipitated₃B, therefore, the magnetic properties of iHc.gtoreq.2.5 kOe and Br.gtoreq.9 kG cannot be obtained. Further, if the cast alloy thickness is 70 μm toThen, the crystal structure contained in the cast alloy decreases and the amorphous phase increases. Resulting in the necessity of crystallizing the amorphous phase by heat treatment. In addition, the increase in alloy temperature due to the heat release generated during such amorphous crystallization causes grain growth of a crystal structure that has precipitated immediately after rapid solidification, so that a coarser metal structure than a metal structure having an average grain diameter of 10nm to 50nm necessary for obtaining magnetic properties of iHc.gtoreq.2.5 kOe and Br.gtoreq.9 kG will be obtained, and Br of 10kG or more will not be obtained. The casting conditions are limited to rapid solidification so as to obtain a cast alloy with a thickness of 70-500 μm.

The material of the cooling roll for continuous casting may be aluminum alloy, copper alloy, iron, carbon steel, brass or tungsten in view of heat conductivity. It is also possible to use a cooling roll having a coating of the same or different material on the surface of a roll made of the above-mentioned material. The material of the cooling roller is preferably a copper alloy or carbon steel in view of mechanical strength and cost. Since the thermal conductivity of materials other than those described above is poor, the alloy melt cannot be sufficiently cooled, and several hundred nm of coarse α -Fe and Fe are precipitated₂B, processing can not obtain the magnetic properties of iHc more than or equal to 2.5kOe and Br more than or equal to 9 kG.

For example, a copper roll is used as the cooling roll, and the surface roughness is such that the center line roughness Ra is 0.8 μm or less, the maximum height Rmax is 3.2 μm or less, and the 10-point average roughness Rz is 3.2 μm or less, when the peripheral speed of the roll exceeds 10m/s (1X 10/s)⁵Average cooling rate c/sec), the thickness of the cast alloy will be 70 μm or less, the crystalline structure contained in the cast alloy will decrease, and the amorphous phase will increase. Moreover, if the roll peripheral speed is 1.5m/s or less, the cast alloy thickness will exceed 500 μm, and thus coarse α -Fe and Fe having several hundred nanometers will be precipitated₂B, the magnetic performance that iHc is more than or equal to 2.5kOe and Br is more than or equal to 9kG cannot be realized. The circumferential speed of the copper roll is then limited to the range of 1.5m/s to 10 m/s. A preferred range is from 2m/s to 6 m/s.

When the cooling roll uses an iron roll having the same surface roughness as the above copper roll, the compatibility between the alloy melt using the iron roll and the cooling roll is superior to that of the copper roll, and therefore if the roll peripheral speed exceeds 7m/s, the cast alloy has a thickness of 70 aThe grain size of the alloy is smaller than μm, the crystal structure contained in the cast alloy is reduced, and the amorphous phase is increased. When the peripheral speed of the roller is 1m/s or less (3X 10)³Average cooling rate per second), the thickness of the cast film will exceed 500 μm, thus precipitating several hundred nanometers of coarse alpha-Fe and Fe₂B, the magnetic performance that iHc is more than or equal to 2.5kOe and Br is more than or equal to 9kG cannot be realized. Therefore, the peripheral speed of the iron roll is limited to the range of 1m/s to 7m/s, and preferably to the range of 1.5m/s to 5.5 m/s.

In addition, when the above-mentioned twin-roll rapid solidification method facing each other is used, the alloy thickness is determined by the distance between the rolls. If the distance between the two rolls is more than 0.5mm, the melt passing between the rolls does not contact the cooling roll and therefore cannot be cooled, resulting in that a metal structure containing coarse α -Fe will be obtained. If the distance between the rolls is less than 0.005mm, the melt overflows from between the rolls, so that continuous casting cannot be performed. Thus, the distance between the two rollers is taken to be 0.005mm to 0.5 mm. Preferably 0.05-0.2 mm.

Furthermore, if the peripheral speed of the two iron rolls exceeds 8m/s, the crystalline structure in the cast alloy will decrease and the amorphous structure will increase. If the peripheral speed of the roll is less than 1m/s, several hundred nanometers of coarse alpha-Fe and Fe will precipitate₂B, the magnetic performance that iHc is more than or equal to 2.5kOe and Br is more than or equal to 9kG cannot be realized. Therefore, the peripheral speed of the roller is limited to 1m/s to 8m/s, and preferably ranges from 1.5m/s to 5 m/s.

A method of subjecting the drawn thin metal sheet to etching, ultrasonic treatment, or the like can be used as a continuous (long) thin sheet magnet molding method obtained by continuous casting. Ultrasound-based stamping is particularly suitable because the magnet can be machined to the desired shape without cracking the sheet magnet.

The crystalline phase of the microcrystalline permanent magnet alloy of the present invention is characterized by being formed by aggregation of microcrystals, and having soft magnetic Fe therein₃B compound and alpha-Fe and Nd₂Fe₁₄The hard magnetic compound phases of the B crystal structure coexist in the same structure, and the average crystal grain diameter of each structural phase is in the range of 15nm to 50 nm. If the average grain diameter constituting the microcrystalline permanent magnet exceeds 50nm, the squareness ratio of the demagnetization curveAnd Br is deteriorated, and the magnetic property of Br not less than 9kG cannot be obtained. The smaller the average crystal grain diameter, the better. However, if the average crystal grain diameter is 15nm or less, the decrease of iHc is caused, and thus the minimum average crystal grain diameter is 15 nm.

The thin sheet magnet obtained according to the present invention has a thickness of 70 to 500 μm and a surface smoothness of center line roughness Ra ≦ 5 μm, maximum height Rmax ≦ 20 μm, and 10-point average roughness Rz ≦ 10 μm.

Example 1

Fe, Co, Cr, Mn, Ni, Cu, Ga, Ag, Hf, Ta, W, Pt, Au, Pb, B, Nd, Pr, Dy and Tb metals having a purity of 99.5% or more were weighed out in a total amount of 30 g to obtain the compositions of sample Nos. 1 to 9 of Table 1 to 1. The raw material was placed in a quartz crucible having a slit of 0.3mm × 8mm in the bottom thereof, and melted by induction heating in an argon atmosphere maintained at a quenching ambient pressure shown in table 1. After reaching the melting temperature of 1300 ℃, the surface of the melt was pressurized with argon gas, and the melt was continuously cast from a height of 0.7mm on the outside of a copper cooling roll rotating at the peripheral speed of the roll shown in table 1-2, thereby obtaining a continuous thin-piece quenched alloy having a width of 8 mm.

As shown by the Cu-Ka characteristic X-ray diffraction pattern of example sample No.5 of FIG. 1, it was confirmed that the obtained flake magnet had Fe₃B compound and alpha-Fe and Nd₂Fe₁₄And a metal structure in which compound phases of B crystal structure coexist. The crystal grain diameters of all the samples, except No.8, were microcrystalline structures having an average crystal grain diameter of 15nm to 50 nm.

The thin plate magnet was punched into a predetermined shape of a disk shape having a diameter of 5mm using an ultrasonic punch and magnetized in a 60kOe pulsed magnetic field, after which the magnetic properties of the obtained thin plate magnet were determined by VSM measurement. Table 2 shows the magnetic properties and the average grain diameter. Further, Fe in each structural phase of the flake magnets of samples No.1 to 9 was partially replaced with the elements Co, Cr, Mn, Ni, Cu, Ga, Ag, Pt, Au and Pb.

As shown in FIG. 2, which is a graph relating the coercive force to the peripheral speed of the roller of example No.5, it can be seen that the magnetic properties of the rapidly solidified alloy obtained by the present invention depend on the peripheral speed of the roller at the time of rapid solidification of the melt. For example samples No.1, No.4 and No.5, table 3 gives the range of the roller peripheral speed in the rapid solidification process for obtaining iHc of 2.5kOe or more.

Example 2

Since sample No.8 of Table 1-1 had an average crystal grain diameter of less than 10nm, the rapidly solidified alloy was heat-treated in Ar gas at 670 ℃ for 10 minutes so that the average crystal grain diameter became 10nm or more. Magnetic properties were measured using VSM on a thin sheet magnet made into a predetermined shape as in example 1. The measurement results are shown in table 2.

Comparative example 1

Using the same procedure as in example 1, compositions of No.10 and No.11 as in Table 1-1 of example 1 were realized using 99.5% purity Fe, B, R and Co, and continuously cast alloys having a width of 8mm were produced. The structural phase of the obtained sample was analyzed by X-ray diffraction analysis of Cu-Ka characteristics, and as a result, sample No.10 was composed of a hard magnetic phase Nd₂Fe_i4B and soft magnetic phase Fe₃B and alpha-Fe. In sample No.11, a small amount of Nd was confirmed₂Fe₁₄B, but it was confirmed that almost no Fe was present₃B. The magnetic properties of samples No.10 and No.11 measured with VSM are shown in Table 2.

As shown by the correlation curve of coercive force with roller peripheral speed of FIG. 2, the magnetic properties of comparative sample No.10 were dependent on the roller peripheral speed during rapid solidification. The range of the peripheral speed of the roller in which the coercive force of the sample as a ternary system of Nd-Fe-B can be achieved is narrow as compared with example sample No.1 containing Co and Cr. Table 3 shows the range of the roll peripheral speed of iHc obtained by 2.5kOe or more when the alloy composition of comparative example sample No.10 was subjected to rapid solidification.

TABLE 1-1

		Composition (at%)
						Fe1-mCom	A	R	M
		Examples	1	Fe74	B18.5					Nd4.5	Cr3
2	Fe76					B18.5	Nd3.5+Dy1	Mn1
			3	Fe76	B15+C3				Nd4.5	Ni1.5
4	Fe70+Co5					B15+C5	Nd2.5+Pr1	Cu1.5
			5	Fe73+Co3	B18.5				Nd3.5+Pr1	Ga1
6	Fe76.5					B18.5	Nd4	Ag1
			7	Fe75+Co3	B18				Nd3.5	Pt0.5
8	Fe75+Co3					B18	Nd4.5	Au0.5
			9	Fe73+Co3	B18.5				Nd4+Tb0.5	Pb1
Comparative example	10					Fe77.5	B18.5	Nd4			-
		11	Fe66.0	B18.5	Nd5.5				Cr10

Tables 1 to 2

		Roll peripheral speed m/sec	Pressure of rapid solidification atmosphere kPa	Thickness of cast alloy
					Examples	1	3.5	10	250
2	3.0	10	280
				3		4.0	20	210
4	2.5	20	300
				5		4.0	20	180
6	3.5	20	240
				7		4.0	25	210
8	4.0	25	220
				9		6.5	30	80
Comparative example	10	5.0	1.3						200
				11	3.0	1.3	280

TABLE 2

		Magnetic property			Average grain diameter (nm)
						Br(kG)	iHc(kOe)	(BH)max(MGOe)
		Examples	1	10.5					5.5	13.9	20
2	10.3				5.2	13.4	20
			3	11.8				4.3	13.9	20
4	13.0				3.3	17.5	20
			5	11.9				4.5	17.0	20
6	12.4				3.5	17.2	20
			7	12.9				3.2	17.6	20
8	12.1				4.1	17.2	20
			9	11.5				5.2	16.5	20
Comparative example	10				12.3	3.3	14.9				50
		11	4.7	12.4				6.7	40

TABLE 3

		Peripheral speed of the roller (m/s)
			Examples	1	2.0～6.0
4	2.0～4.0
		5		3.0～5.0
Comparative example	12				4.2～5.4

According to the invention, Co, Cr, Mn, Ni, Cu, Ga, Ag, Pt, Au or Pb is added to the alloy melt, the alloy melt is continuously cast on a rotary cooling roller in a specific reduced pressure inert atmosphere, the coercive force of the magnet can be improved, the magnetic performance that the iHc is more than or equal to 2.5kOe and the Br is more than or equal to 9kG is realized, and the microcrystalline permanent magnet with the microcrystalline structure of 15nm-50nm is directly manufactured from the alloy melt, so that the Nd-Fe-B microcrystalline permanent magnet with low rare earth content, which is a mixture of a soft magnetic phase and a hard magnetic phase, is obtained. Meanwhile, compared with the traditional Nd-Fe-B ternary system, the peripheral speed range of the optimal roller capable of obtaining the hard magnetic performance is expanded, and the condition for manufacturing the microcrystal permanent magnet with the thickness of 70-500 mu m is relaxed, so that the stable industrial manufacturing can be carried out at low cost. The present invention can provide an inexpensive thin sheet magnet having a thickness of 70 to 500 μm, which cannot be industrially mass-produced at low cost by the conventional method, and whose performance-price ratio is comparable to that of hard ferrite, so that these thin sheet magnets can make the magnetic circuit smaller and thinner.

Claims (2)

1. A sheet magnet having a microcrystalline structure, which has a thickness of 70 to 500 μm, magnetic properties such as iHc of not less than 2.5kOe and Br of not less than 9kG, and which is composed of microcrystals having an average crystal grain diameter of 15 to 50nm, and has a cast-state grain structure in which 90% of the grain structure is Fe₃B compound and alpha-Fe and Nd₂Fe₁₄The compound phase of B crystal structure coexists, and the alloy is represented by the following composition general formula:

Fe_100-x-y-zR_xB_yM_z

wherein R is one or more elements of Pr, Nd, Tb and Dy, Pr and/or Nd are essential, M is one or more elements of Cr, Mn, Ni, Cu, Ga, Ag, Pt, Au and Pb, wherein symbols x, y and z defining the composition range are atomic number percentages in the alloy, and satisfy the following conditions, 1. ltoreq. x < 6, 15. ltoreq. y < 30, 0.01. ltoreq. z < 7, respectively.

2. A sheet magnet having a microcrystalline structure, which has a thickness of 70 to 500 μm, magnetic properties such as iHc of not less than 2.5kOe and Br of not less than 9kG, and which is composed of microcrystals having an average crystal grain diameter of 15 to 50nm, and has a cast-state grain structure in which 90% of the grain structure is Fe₃B compound and alpha-Fe and Nd₂Fe₁₄The compound phase of B crystal structure coexists, and the alloy is represented by the following composition general formula:

(Fe_1-mCo_m)_100-x-y-zR_xB_yM_z

wherein R is one or more elements of Pr, Nd, Tb and Dy, and Pr and/or Nd is essential, M is one or more elements of Cr, Mn, Ni, Cu, Ga, Ag, Pt, Au and Pb, wherein symbols x, y and z defining the composition range are atomic number percentages in the alloy, M represents the amount of Co substituted, x, y, z and M satisfy the conditions of 1. ltoreq. x < 6, 15. ltoreq. y.ltoreq.30, 0.01. ltoreq. z.ltoreq.7, and 0.001. ltoreq. m.ltoreq.0.5, respectively.