HK1026059A - Method of manufacturing thin plate magnet having microcrystalline structure - Google Patents

Description

Method for producing thin-sheet magnet having microcrystalline structure

The present invention relates to a method for manufacturing a thin-sheet magnet suitable for use in magnetic circuits for various small-sized motors, actuators, magnetic sensors, and the like. The present invention obtains a magnet by continuously casting a melt having a specific composition containing 6 at% or less of a rare earth element and 15 to 30 at% of boron on a rotating cooling roll or a plurality of cooling rolls in a predetermined reduced pressure inert gas atmosphere so that the magnet has a crystal structure in which 90% or more is substantially made of Fe in the as-cast state₃B compound and alpha-Fe and Nd₂Fe₁₄The compound phase of B crystal structure coexists and exhibits a microcrystalline structure in which each constituent phase has an average crystal grain diameter of 10 to 50 nm. The invention relates to a method for the immediate production of a thin-sheet magnet with a microcrystalline structure from an alloy melt, having a thickness of 70-500 [ mu ] m, exhibiting magnetic properties with an iHc of 2.5kOe or more and a Br of 9kG or more.

At present, home appliances, office automation equipment, and electric devices are expected to be higher in performance, smaller in size, and lighter in weight. In this regard, design efforts tend to maximize the performance-to-weight ratio in the overall magnetic circuit employing permanent magnets. In particular, in the structure constituting the brush type DC motor currently most of the motor manufactures, a permanent magnet having a residual magnetic flux density Br of about 5-7kG is considered to be desirable, but this cannot be obtained by the conventional hard ferrite magnet.

Such magnetic characteristics can be realized by Nd-Fe-B sintered magnets and Nd-Fe-B bonded magnets in which the main phase is, for example, Nd₂Fe₁₄B. However, since they contain 10 to 15 at% of Nd, require a large number of processing steps and large-scale equipment for metal separation refining and reduction reaction, and therefore have a cost much higher than that of hard ferrite magnets, they can only replace hard ferrite magnets in some respects from the viewpoint of performance-price ratio. At present, no inexpensive permanent magnet material exhibiting Br of 5kG or more has been found.

Furthermore, in order to achieve smaller and thinner magnetic circuits, thin permanent magnets, which have a thickness of the order of 100-500 μm, have been sought. However, with the Nd-Fe-B sintered magnet, since it is extremely difficult to obtain a bulk material having a thickness of less than 500 μm, it can be produced only by grinding a sheet-like sintered body having a thickness of several mm or by a method of slicing a bulk material by wire cutting, resulting in problems of high cost and low productivity.

The Nd-Fe-B bonded magnet is obtained by bonding powders having a diameter of several tens to 500 μm and a thickness of about 30 μm together using a resin, so it is extremely difficult to form a bonded magnet having a flake thickness of 100-300 μm.

In recent years, in the field of Nd-Fe-B magnets, magnet materials have been proposed in which Fe₃B compound is made to have Nd₄Fe₇₇B₁₉(at%) adjacent to the main phase of composition (R. Coehoorn et al, J.de Phys, C8, 1988, page 669, 670). Details of this technique are disclosed in us patent 4935074.

Earlier in US patent 4402770, Koon proposed a method of manufacturing a microcrystalline permanent magnet in which a La-R-B-Fe amorphous alloy containing La as an essential element was subjected to a crystallization heat treatment.

More recently, Richter et al have reported that amorphous flakes are produced by spraying a melt of Nd-Fe-B-V-Si alloy containing Nd in an amount of 3.8 to 3.9 at% onto a rotating copper roll, and heat-treating these flakes at a temperature of 700 ℃ to obtain flakes having hard magnetic properties, as in European patent application558691B 1. These permanent magnet materials have a metastable structure with a crystalline aggregate structure, Fe, obtained by subjecting amorphous flakes having a thickness of 20-60 μm to a crystallization heat treatment₃B soft magnetic phase and R₂Fe₁₄The B hard magnetic phase is mixed therein.

The above permanent magnet material exhibits Br of about 10kG and iHc of 2 to 3kOe, in which the expensive Nd content is low concentration of about 4 at%, so that the raw material mixed therein is R as a main phase₂Fe₁₄The Nd-Fe-B magnet of B is cheaper.

However, the above permanent magnet material is limited to the rapid solidification condition because it is a necessary condition to mix the raw materials as an amorphous alloy in this way, and at the same time, the limitation of the heat treatment required to obtain the hard magnetic material is extremely strict. This is therefore impractical from an industrial production point of view and therefore does not provide an inexpensive product replacement for hard magnetic ferrites. Further, this permanent magnet material is obtained by subjecting amorphous flakes having a thickness of 20 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.

Meanwhile, in U.S. Pat. No. 508266, there is disclosed a rapidly solidified Nd-Fe-B magnet material consisting of a structure formed of a crystalline substance exhibiting hard magnetism, which is directly obtained by rapidly solidifying an alloy melt on a rotating roll having a peripheral speed of about 20 m/s. However, the rapidly solidified alloy flakes obtained under these conditions have a thickness of about 30 μm, so although they can be ground into powders having a particle diameter of between 10 and 500 μm for use in the above-mentioned bonded magnets, they cannot be used for thin sheet magnets.

The present invention aims to solve the above-described problems in an Nd-Fe-B magnet containing 6 at% or less of a rare earth element and exhibiting microcrystals. Another object of the present invention is to obtain a magnet exhibiting a performance-price ratio comparable to that of a hard magnetic ferrite and exhibiting 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 method for manufacturing a thin sheet magnet having a microcrystalline structure, which has a thickness of 70 to 500 μm, so that a magnetic circuit can be made smaller and thinner.

The present inventors previously disclosed (in Japanese patent application No. Hei 8-355015/1996) how to obtain a microcrystalline permanent magnet exhibiting hard magnetic properties (iHc.gtoreq.2kOe and Br.gtoreq.10kG) directly from an alloy melt by a production method in which an alloy melt of a low rare earth content Nd-Fe-B ternary structure containing 6 at% or less of Nd and 15 at% to 30 at% of boron is continuously cast on a cooled rotating roll having a roll peripheral speed of 2 to 10m/s in a specific reduced-pressure inert gas or inert gas atmosphere. However, in the method for producing such an Nd — Fe — B ternary magnet, there is a problem that the range of the peripheral speed of the roll must be strictly limited in order to obtain hard magnetic properties. Furthermore, in these Nd-Fe-B ternary magnets, the maximum coercivity achievable is on the order of 2-3 kOe. As a result, not only is thermal demagnetization severe, but also the operating point of the magnet must be raised as much as possible, thereby causing a problem of limiting the shape of the magnet and the use environment.

Accordingly, the present inventors have conducted extensive studies on the problems involved in producing a Nd-Fe-B microcrystalline permanent magnet having a low rare earth content in which a soft magnetic phase and a hard magnetic phase coexist to the extent of nanometer size. As a result of this study, the present inventors found that the above-mentioned problems can be solved by using an alloy melt to which a specific element has been added, which is obtained directly from the alloy melt by continuous casting on a cooled rotating roll in a specific reduced-pressure inert gas or inert gas atmosphere in a process for producing a microcrystalline permanent magnet exhibiting a microcrystalline structure of 10 to 50nm, previously proposed by the present inventors. By this method using an alloy melt to which a specific element has been added, the iHc of the magnet can be increased to 2.5kOe or more, the optimum roll circumferential speed range of the hard magnetic properties is obtained, the range is expanded as compared with the production conditions of the conventional Nd-Fe-B ternary magnet, and at the same time, a microcrystalline permanent magnet having a thickness of 70 to 500 μm can be obtained. The present invention is therefore desirable.

More specifically, in the production of a thin sheet magnet having a microcrystalline structure according to the present inventionIn the manufacturing method, the alloy melt used is expressed as Fe_100-x-y-zR_xA_yM_zOr (Fe)_1-mCo_m)_100-x-y-zR_xA_yM_zWherein R is one or more elements of Pr, Nd, Tb and Dy, A is C or B, or C and B, M is one or more elements of Al, Si, Ti, V, Cr, Mn, Ni, Cu, Ga, Zr, Nb, Ag, Hf, Ta, W, Pt, Au and Pb, and the symbols x, y, z and M satisfy the conditions of 1. ltoreq. x < 6 at%, 15. ltoreq. y < 30 at%, 0.01. ltoreq. z. ltoreq. 7 at%, and 0.001. ltoreq. M. ltoreq.0.5, respectively, to thereby define the composition range.

In the present invention, the roll peripheral speed is 1 to 10m/s (at 3X 10) in a reduced pressure inert gas or inert gas atmosphere of 30kPa or less³～1×10⁵At an average cooling rate of c/sec) on a rotating chill roll or rolls, and continuously casting the alloy melt. Thus, a permanent magnet exhibiting magnetic properties of iHc ≥ 2.5kOe and Br ≥ 9kG, a thickness of 70-500 μm, and composed of microcrystals having an average grain diameter of 50nm or less, of which more than 90% is Fe, can be obtained directly in the as-cast state₃B compound and alpha-Fe and Nd₂Fe₁₄A crystal structure in which compound phases having the B crystal structure coexist.

Further, the present invention is a production method whereby a thin permanent magnet exhibiting magnetic properties of iHc.gtoreq.2.5 kOe and Br.gtoreq.9 kG can be obtained, wherein the above alloy melt is continuously cast on a rotating chill roll or a plurality of chill rolls to produce a cast alloy having a thickness of 70 to 500 μm and composed of crystallites having an average crystal grain diameter of 10nm or less, followed by heat treatment for growing the crystal grains at a temperature range of 550 ℃ to 750 ℃ to produce a crystallite alloy having an average crystal grain diameter of 10 to 50 nm.

FIG. 1 is a graph showing the Cu-Ka characteristic x-ray diffraction pattern of the sample of example.

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

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

Only when the rare earth element R, which is one or more elements among Pr, Nd, and Dy, is contained in a specific content, excellent magnetic properties can be obtained. iHc of 2.5kOe or more cannot be obtained with other rare earth elements such as Ce and La. In addition, medium rare earth and heavy rare earth elements from Sm downward, except Tb and Dy, both deteriorate magnetic properties and make the magnet expensive, and therefore they are not desirable.

When R is less than 1 at%, iHc of 2.5kOe or more cannot be obtained, and when it reaches 6 at% or more, Br of 9kG or more cannot be obtained, and therefore the range is set to not less than 1 at% but less than 6 at%. A preferred range is 2 at% to 5.5 at%.

A in the composition formula is carbon or boron, or carbon and boron. When the total amount of A is less than 15 at%, α -Fe is apparently present in the metal structure after liquid cooling, and Nd necessary for developing coercive force is contained₂Fe₁₄The production of the compound having a B crystal structure is impaired, so that only iHc of 1kOe or less can be obtained. On the other hand, when the total amount thereof exceeds 30 at%, the squareness ratio of the demagnetization curve sharply decreases. Therefore, the range is set to 15 at% or more but not more than 30 at%. A preferred range is 16 at% to 20 at%.

The balance other than the above elements is Fe. When Fe is partially substituted by Co, the metal structure is refined, the squareness ratio of demagnetization curve and maximum energy product (BH) max are improved, the heat resistance is improved, and when rapidly cooling the alloy melt on the rotating roller in the manufacture of the microcrystalline permanent magnet, the ideal circumferential speed range of the roller exhibiting hard magnetism is widened, so that it is possible to realize a looser cooling condition of the melt for obtaining the above-mentioned magnetic properties. These effects cannot be obtained when the content of substitutional Co is less than 0.1% with respect to Fe. On the other hand, when the substitution amount exceeds 50%, Br of 9kG or more cannot be obtained. Therefore, the substitution amount of Co for Fe is set in the range of 0.5% to 50%. The preferred range is 0.5% to 10%.

Al, Si, Ti, V, Cr, Mn, Ni, Cu, Ga, Zr, Nb, Mo, Ag, Hf, Ta, W, Pt, Au and Pb for adding the element M are favorable for endowing the microcrystalline permanent magnet with a finer microstructure, improving the coercive force, improving the squareness ratio of a demagnetization curve and improving Br and (BH) max. When an alloy melt is rapidly solidified on a rotating roll to produce a microcrystalline permanent magnet, the addition of element M widens the range of the circumferential speed of the roll which is ideal for obtaining magnetic properties, as with Co, and makes the rapid solidification conditions for obtaining the magnetic properties more relaxed. When the additive element M 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. The range is then set at 0.01 at% to 7 at%. A preferred range is 0.05 at% to 5 at%.

The reason for the limitation of the production conditions of the present invention will be described below. The most important point of the present invention is that a microcrystalline structure of 10nm to 50nm is obtained when the alloy melt of the above specific composition is continuously cast on a rotating chill roll in a reduced pressure inert gas or inert gas atmosphere of 30kPa or less. More specifically, it is most important to obtain a thin sheet magnet having an average grain diameter of 10nm to 50nm in each constituent phase by cooling, which is necessary for obtaining magnetic properties of iHc.gtoreq.2.5 kOe and Br.gtoreq.9 kG, the magnet being formed of a microcrystalline structure under casting conditions in which practically 90% or more of Fe coexisting₃B compound and alpha-Fe and Nd₂Fe₁₄B crystal structure compound phase.

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 rapid solidification condition for the cast alloy. As a result, a coarse α -Fe-containing metal structure is formed, and magnetic properties of iHc of not less than 2.5kOe and Br of not less than 9kG cannot be realized. The atmosphere for rapid solidification is set to 30kPa or less, preferably 10kPa or less. To prevent oxidation of the alloy melt, the atmosphere gas should be an inert gas or an inert gas. An Ar atmosphere is preferred.

In the above continuous casting method, when the average grain diameter of 10 to 50nm necessary for obtaining magnetic properties of iHc.gtoreq.2.5 kOe and Br.gtoreq.9 kG cannot be achieved, heat treatment may be performed to achieve grain growth. The heat treatment temperature at which the magnetic energy can be maximized depends on the composition, but the heat treatment temperature below 500 ℃ cannot cause crystallization, and thus an average crystal grain diameter of 10nm or more cannot be obtained. On the other hand, when the heat treatment temperature exceeds 750 ℃, coarsening of crystal grains is significant, the squareness ratio of iHc, Br and demagnetization curve is lowered, 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 of argon or nitrogen, or in a vacuum of 1.33Pa or less. Although the magnetic properties are not dependent on the heat treatment time, when the time exceeds 6 hours, Br tends to decrease with the lapse of time, so that the time is preferably less than 6 hours.

For the casting process of the alloy melt, a continuous casting method using a single cooling roll or a double cooling roll is suitable. However, when the thickness of the cast alloy exceeds 500. mu.m, coarse α -Fe and Fe of several hundred nm are formed₂B, therefore, the magnetic properties of iHc.gtoreq.2.5 kOe and Br.gtoreq.9 kG cannot be obtained. On the other hand, when the thickness of the cast alloy is 70 μm or less, the volume ratio of the crystalline microstructure contained in the cast alloy decreases, the amorphous phase increases, and it is necessary to crystallize the amorphous (alloy) by heat treatment. The increase in alloy temperature brought about by the exotherm associated with this amorphous crystallization causes grain growth in the crystal structure that has precipitated immediately after rapid solidification, resulting in a metal structure that is coarser than those in which the average grain diameter is 10-50nm, necessary to achieve magnetic properties of iHc ≧ 2.5kOe and Br ≧ 9kG, and Br of 10kG or more cannot be obtained. Thus, the rapid cooling casting conditions are limited to produce a cast alloy having a thickness of 70 to 500 μm.

As the material of the chill roll or rolls used in the continuous casting process, an aluminum alloy, a copper alloy, iron, carbon steel, brass or tungsten may be used in view of thermal conductivity. It is also possible to use on the surface of a roll made of the above-mentioned materialA chill roll having a coating, which may be the same or different material. The material of the cooling roller is preferably a copper alloy or carbon steel in view of mechanical strength and cost. Materials other than those described above exhibit poor thermal conductivity, so that the alloy melt cannot be sufficiently cooled, thereby forming coarse α -Fe and Fe on the order of several hundred nm₂B, magnetic properties of iHc ≧ 2.5kOe and Br ≧ 9kG cannot be achieved, so these materials are undesirable.

As an example, a copper roll was used as the cooling roll, and from the viewpoint of surface roughness, the center line roughness Ra was 0.8 μm or less, the maximum height Rmax was 3.2 μm or less, the 10-point average roughness Rz was 3.2 μm or less, and the roll peripheral speed was more than 10m/s (average cooling speed 1X 10)⁵At least 70 c/sec), the thickness of the cast alloy decreases to 70 μm or less, the crystal structure contained in the cast alloy decreases, and the amorphous phase increases. On the other hand, when the roll peripheral speed is 1.5m/s or less, the cast alloy thickness exceeds 500. mu.m, and thus α -Fe and Fe having a coarseness of several hundred nm are formed₂B, it is not desirable because the magnetic properties of iHc.gtoreq.2.5 kOe and Br.gtoreq.9 kG cannot be achieved. 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 1m/s to 6 m/s.

When a steel roll is used as the chill roll or the chill rolls, the surface roughness is the same as that of the copper roll, and the alloy melt wettability of the iron chill roll is superior to that of the copper roll, so that when the roll peripheral speed exceeds 7m/s, the thickness of the cast alloy is reduced to 70 μm or less, the crystal microstructure contained in the cast alloy is reduced, and the amorphous phase is increased. When the peripheral speed of the roller is less than 1m/s (average cooling speed 1X 10)³Per second), the cast alloy thickness exceeds 500 μm, thus precipitating several hundred nm 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, with the iron roll, the peripheral speed of the 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, with the twin roll rapid solidification method, the alloy melt is cooled against two cooled rolls of the steel roll, the thickness of the alloy being determined by the distance between the rolls. When the distance between the two rolls is more than 0.5mm, the melt passing between the rolls does not contact one of the cooling rolls, and therefore, is not cooled effectively, resulting in a metal microstructure containing coarse α -Fe, and thus, is not desirable. On the other hand, when the distance between the rolls is less than 0.005mm, the melt overflows from between the rolls, so continuous casting cannot be performed in an uninterrupted manner, and thus it is also undesirable. Thus, the distance between the two rollers is limited to the range of 0.005mm to 0.5mm, preferably 0.05 to 0.2 mm.

When the peripheral speed of the two iron rolls exceeds 8m/s, the volume ratio of the crystalline microstructure contained in the cast alloy decreases and the amorphous phase increases. At a roll peripheral speed of less than 1m/s, several hundred nm coarse alpha-Fe and Fe are formed₂B, and therefore, cannot achieve magnetic properties of iHc.gtoreq.2.5 kOe and Br.gtoreq.9 kG, so that it is not desirable. The roller peripheral speed is therefore limited to the range of 1-8 m/s. A preferred range is 1.5-5 m/s.

As a method of processing a continuous (long) thin sheet magnet obtained by continuous casting into a desired shape, a method such as etching or ultrasonic treatment, which is conventionally used for processing a thin sheet metal material produced by rolling, can be used. Ultrasound-based stamping is particularly suitable because the desired shape can be made without causing cracks in the sheet magnet.

The crystalline phase of the microcrystalline permanent magnet according to the invention is characterized by Fe exhibiting soft magnetism₃B compound and alpha-Fe and Nd₂Fe₁₄The hard magnetic compound phases of the B crystal structure coexist in the same structure and are aggregated from crystallites, wherein the average crystal grain diameter in each constituent phase is in the range of 15 to 50 nm. When the average grain diameter of the microcrystalline permanent magnet exceeds 50nm, the squareness ratio of Br to demagnetization curve is reduced, and the magnetic performance of Br more than or equal to 9kG cannot be obtained. The finer the average crystal grain diameter is, the better, but at a size below 15nm, the lower limit is 15nm, since the lower limit causes a decrease in iHc.

The thin sheet magnet obtained by the invention has the thickness of 70-500 mu m, and the surface roughness shows that the center line roughness Ra is less than or equal to 5 mu m, the maximum height Rmax is less than or equal to 20 mu m, and the 10-point average roughness Rz is less than or equal to 10 mu m.

Example 1

For the compositions of samples 1 to 19 in Table 1-1, Fe, Co, C, Al, Si, Ti, V, Cr, Mn, Ni, Cu, Ga, Zr, Nb, Mo, 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 to obtain a total weight of 30 g. 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 rapid solidification atmospheric pressure shown in table 1. After reaching the melting temperature of 1300 c, the molten surface was pressurized with argon gas, and the melt was continuously cast from a height of 0.7mm at room temperature on the outer circumferential surface of a copper cooling roll rotating at the peripheral speed of the roll shown in table 1-2. Thereby producing a continuous thin sheet of rapidly solidified alloy having a width of 8 mm.

As shown by the Cu-Ka characteristic X-ray produced for the X-ray diffraction pattern of example sample 5 given in FIG. 1, it was confirmed that the obtained thin plate magnet had Fe₃B compound and alpha-Fe and Nd₂Fe₁₄A metal structure in which compound phases having a B-crystal structure coexist. In all the test samples, except for sample Nos. 1, 3 and 17, a microcrystalline structure having an average crystal grain diameter of 15 to 50nm was exhibited.

The thin sheet 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 sheet magnet were measured using VSM. The magnetic properties and the average grain diameter are shown in Table 2. In the thin sheet magnets in test samples Nos. 1 to 19, Fe in the constituent phases was partially replaced with the elements Co, Al, Si, Ti, V, Cr, Mn, Ni, Cu, Ga, Zr, Nb, Hf, Ta, W, Mo, Ag, Pt, Au and Pb.

As shown in the graph relating the coercive force to the roller peripheral speed of sample No.5 of the example shown in FIG. 2, it can be seen that the magnetic properties of the rapidly solidified alloy obtained by the present invention depend on the roller peripheral speed at which the melt is rapidly solidified. Table 3 gives the range of the peripheral speed of the rolls in which the melt having the alloy composition of example sample Nos. 2, 4, 5, 8, 9 and 10 obtained iHc of 2.5kOe or more by rapid solidification.

Example 2

In sample Nos. 1, 3 and 17 of Table 1-1, the average grain diameter was less than 10nm, and therefore, the rapidly solidified alloys were heat-treated in Ar gas at 670 ℃ for 10 minutes so that the average grain diameter became 10nm or more. The same measurement as in example 1 was performed using VSM on the thin sheet magnet made into a predetermined shape, and the magnetic properties were determined. The measurement results are shown in table 2.

Comparative example 1

Using the same process as in example 1, the compositions of samples No.20-23 in Table 1-1 were realized using 99.5% purity Fe, B, R and Co, and continuously cast alloys having a width of 8mm were produced. When the constituent phase of the test sample obtained by X-ray diffraction detection of Cu-Ka characteristics was used, the test sample No.20 exhibited Nd containing a hard magnetic phase₂Fe₁₄B and soft magnetic phase Fe₃B and alpha-Fe. In test sample No.21, a small amount of Nd was confirmed₂Fe₁₄B, but no Fe was confirmed₃B。

Test sample No.22 exhibited a metallic structure having α -Fe as a main phase, and test sample No.23 exhibited a metallic structure including a nonmagnetic phase Nd₂Fe₂₃B₃And a structure of alpha-Fe, in both cases, the hard magnetic phase Nd is not contained in the metal structures₂Fe₁₄B. The magnetic properties measured by VSM in test sample Nos. 20-23 are shown in Table 2.

Considering the magnetic properties of the test samples of comparative example sample 20, these properties are dependent on the roller peripheral speed during rapid solidification, as shown by the coercive force-to-roller peripheral speed correlation curve of fig. 2. The range of the peripheral speed of the roller exhibiting coercive force in these Nd-Fe-B ternary test samples was narrow as compared with example samples Nos. 5 and 9 containing Co and Cr, Nb, Cu, Ga. Table 3 shows the range of roll peripheral speeds at which iHc of 2.5kOe or more was obtained when the melt having the alloy composition of comparative example No.20 was rapidly solidified.

TABLE 1-1

		Composition (at%)
						Fe1-mCom	A	R	M
		Examples	1	Fe73+Co3	B18.5					Nd4.5	Al1
2	Fe77+Co3					B18.5	Nd3+Pr0.5	Si1
			3	Fe75.5	B18.5				Nd5	Ti1
4	Fe74.0					B18.5	Nd5.5	V2
			5	Fe74.0	B18.5				Nd4.5	Cr3
6	Fe76.0					B18.5	Nd3.5+Dy1	Mn1
			7	Fe76.0	B15+C3				Nd4.5	Ni1.5
8	Fe70+Co5					B15+C5	Nd2.5+Pr1	Cu1.5
			9	Fe70+Co2	B18.5				Nd2+Dy2	Nb0.5
10	Fe73+Co3					B18.5	Nd3.5+Pr1	Ga1
			11	Fe76.5	B18.5				Nd4	Ag1
12	Fe76.5					B18.5	Nd4	Hf1
			13	Fe75+Co3	B18				Nd3.5	Pt0.5
14	Fe75+Co2					B18.5	Nd4	Ta0.5
			15	Fe75+Co3	B18				Nd4.5	Au0.5
16	Fe70+Co2					B18.5	Nd2+Dy2	W0.5
			17	Fe73+Co3	B18.5				Nd4+Tb0.5	Pb1
18	Fe77					B7+C10	Nd5	Mo1
			19	Fe75.5	B18.5				Nd4+Tb1	Ti1
Comparative example	20					Fe77.5	B18.5	Nd4			-
		21	Fe66.0	B18.5	Nd5.5				Cr10
	22					Fe74.5	B18.5	Nd4		Si3
		23	Fe74.0	B18.5	Nd6.5				Al1

Tables 1 to 2

		Peripheral speed of the roller m/s	Pressure of rapid solidification atmosphere kPa	Thickness of cast alloy
					Examples	1	6.0	1.3	100
2	4.0	1.3	200
				3		7.0	10	70
4	3.5	10	240
				5		3.5	10	250
6	3.0	10	280
				7		4.0	20	210
8	2.5	20	300
				9		2.5	20	290
10	4.0	20	180
				11		3.5	20	240
12	3.0	20	290
				13		4.0	25	210
14	3.0	20	280
				15		4.0	25	220
16	2.5	25	290
				17		6.5	30	80
18	3.5	1.3	260
				19		3.5	1.3	250
Comparative example	20	5.0	1.3						200
				21	3.0	1.3	280
	22	2.5	75.0					400
				23	3.5	1.3	280

TABLE 2

		Magnetic property			Average grain diameter (nanometer)
						Br(kG)	iHc(kOe)	(BH)max(MGOe)
		Examples	1	12.0					4.6	15.3	20
2	12.8				3.5	17.4	20
			3	11.7				4.3	16.5	20
4	11.0				5.0	13.4	20
			5	10.5				5.5	13.9	20
6	10.3				5.2	13.4	20
			7	11.8				4.3	13.9	20
8	13.0				3.3	17.5	20
			9	9.8				5.6	11.4	20
10	11.9				4.5	17.0	20
			11	12.4				3.5	17.2	20
12	12.9				3.2	17.6	20
			13	12.3				3.3	15.4	15
14	12.1				4.1	17.2	20
			15	11.7				4.2	16.5	25
16	11.5				5.2	16.5	20
			17	10.0				5.4	11.0	15
18	11.6				4.4	16.5	20
			19	10.3				5.3	12.4	20
Comparative example	20				12.3	3.3	14.9				50
		21	4.7	12.4				6.7	40
	22				6.0	0.5	0.7			1μm
		23	4.8	0.8				0.9	100

TABLE 3

		Peripheral speed of the roller (meter/second)
			Examples	2	2.0～4.5
4	2.0～5.0
		5		2.0～6.0
8	2.0～4.0
		9		2.0～4.5
10	3.0～5.0
		Comparative example		20	4.2～5.4

The invention is an improved method for making a low rare earth content Nd-Fe-B microcrystalline permanent magnet containing soft and hard magnetic phases. A microcrystalline permanent magnet having a microcrystalline structure of 15nm to 50nm is immediately produced from an alloy melt by continuous casting on a rotating cooling roll in a specific reduced pressure inert atmosphere or an inert atmosphere, and Co, Al, Si, Ti, V, Cr, Mn, Ni, Cu, Ga, Zr, Nb, Mo, Ag, Hf, Ta, W, Pt, Au and Pb are added to the alloy melt, whereby the coercive force of the magnet can be improved, and the magnetic properties of iHc not less than 2.5kOe and Br not less than 9kG are achieved. Compared with the conventional Nd-Fe-B alloy, it is also effective in expanding the range of the peripheral speed of the ideal roll in which the hard magnetic properties can be obtained, making the conditions for producing microcrystalline permanent magnets having a thickness of 70 to 500 μm relaxed, and also effective in stably producing industrially at low cost. The present invention provides an inexpensive thin sheet magnet having a thickness of 70 to 500 μm, which exhibits a performance-price ratio comparable to that of a hard magnetic ferrite, which ferrite could not be manufactured in large quantities at low cost earlier, thus facilitating a smaller and thinner magnetic circuit.

Claims (7)

1. A method of manufacturing a thin sheet magnet having a microcrystalline structure, wherein an alloy melt is expressed as Fe_100-x-y-zR_xA_yM_zOr (Fe)_1-mCo_m)_100-x-y-zR_xA_yM_zWherein R is one or more elements of Pr, Nd, Tb and Dy, A is C or B, or C and B, M is one or more elements of Al, Si, Ti, V, Cr, Mn, Ni, Cu, Ga, Zr, Nb, Ag, Hf, Ta, W, Pt, Au and Pb, and the symbols x, y, z and M satisfy the following conditions, 1. ltoreq. x < 6 at%, 15. ltoreq. y.ltoreq.30 at%0.01 to 7 at% of z and 0.001 to 0.5, in a reduced pressure inert gas or inert gas atmosphere of 30kPa or less, and continuous casting is carried out on a chill roll rotating at a roll peripheral speed of 1 to 10m/s, thereby obtaining a permanent magnet directly in the as-cast state, exhibiting magnetic properties of iHc 2.5kOe and Br 9kG, having a thickness of 70 to 500 μm, consisting of crystallites having an average grain diameter of 50nm or less, 90% or more of which are Fe₃B compound and alpha-Fe and Nd₂Fe₁₄A crystal structure in which compound phases having the B crystal structure coexist.

2. The method for producing a sheet magnet having a microcrystalline structure according to claim 1, wherein the alloy melt is continuously cast on a rotating chill roll to produce a cast alloy having a thickness of 70 to 500 μm, composed of crystallites having an average grain diameter of 10nm or less, followed by heat treatment at a temperature range of 550 ℃ to 750 ℃ for grain growth to produce a crystallite alloy having an average grain diameter of 10 to 50nm, thereby obtaining a permanent magnet exhibiting magnetic properties of iHc ≥ 2.5kOe and Br ≥ 9 kG.

3. The method of manufacturing a sheet magnet having a microcrystalline structure according to claim 1 or 2, wherein said permanent magnet obtained to have a thickness of 70 to 500 μm is formed into a predetermined shape by a punching process.

4. The method for producing a flaky magnet having a microcrystalline structure according to claim 1 or 2, wherein the cooling roller is composed of two rollers, the relative distance between the rollers is 0.005mm to 0.5mm, and the roller peripheral speed is 1m/s to 8 m/s.

5. The method of producing a flaky magnet having a microcrystalline structure according to claim 2, wherein said cooling roll is a single copper roll, and said roll peripheral speed is 1.5m/s to 5 m/s.

6. The method of producing a flaky magnet having a microcrystalline structure according to claim 2, wherein said cooling roll is a single iron roll, and said roll peripheral speed is 1m/s to 7 m/s.

7. The method for producing a flake magnet having a microcrystalline structure according to any one of claims 1 to 6, wherein an average cooling rate of the melt is 3 x 10³1X 10 ℃ per second⁵DEG C/sec.