HK1000642B

HK1000642B - Rare earth permanent magnet and method for producing the same

Info

Publication number: HK1000642B
Application number: HK97102142.7A
Authority: HK
Inventors: 内田公穗; 高桥昌弘; 谷口文丈
Original assignee: 日立金属株式会社
Priority date: 1995-07-12
Filing date: 1997-11-10
Publication date: 2004-10-29

Abstract

A rare earth permanent magnet consisting essentially, by weight, of 27.0-31.0% of at least one rare earth element including Y, 0.5-2.0% of B, 0.02-0.15% of N, 0.25% or less of O, 0.15% or less of C, at least one optional element selected from the group consisting of 0.1-2.0% of Nb, 0.02-2.0% of Al, 0.3-5.0% of Co, 0.01-0.5% of Ga and 0.01-1.0% of Cu, and a balance of Fe, and a production method thereof. The contents of rare earth element, oxygen, carbon and oxygen in the magnet are regulated within the specific ranges.

Description

Rare earth permanent magnet and method for producing same

The present invention relates to R-Fe-B based rare earth permanent magnets and methods of making the same, wherein R is one or more rare earth elements including yttrium.

Rare earth permanent magnets, particularly R-Fe-B based sintered permanent magnets, are widely used in various fields due to their superior properties.

The R-Fe-B based sintered permanent magnet has a composition consisting essentially of R₂Fe₁₄Phase B (major phase), BFe₇B₆Phase (boron-rich phase) and R₈₅F₁₅The metallic structure of the phase (rare earth-rich phase), generally the R-Fe-B based sintered permanent magnet, has the performance inferior to that of the Sm-Co based sintered permanent magnet in corrosion resistance because it has a rare earth-rich phase and a three-phase metallic structure. From its development to the present, its inferior corrosion resistance is one of the drawbacks of the known R-Fe-B based sintered permanent magnets.

Although the corrosion mechanism of the R-Fe-B based sintered permanent magnet has not been established, there are reports that corrosion proceeds with anodic oxidation of the rare earth-rich phase because corrosion generally starts from the rare earth-rich phase. In fact, the amount of the rare earth-rich phase decreases with decreasing content of the rare earth element, and as a result, the corrosion resistance of the R-Fe-B based sintered permanent magnet is improved. Therefore, one method of improving the corrosion resistance is to reduce the content of rare earth elements.

Sintered rare earth permanent magnets are generally produced by a powder metallurgy method, for example, by melting and casting an alloy metal for magnets to form an alloy ingot, pulverizing the alloy ingot into alloy powder, pressurizing the alloy powder to form a green body, sintering the green body, heat-treating the green body, and then processing. Since the alloy powder is obtained by crushing an alloy ingot having high chemical activity due to its high content of rare earth elements, the rare earth elements are oxidized when exposed to the atmosphere, resulting in an increase in the oxygen content in the alloy powder. Therefore, a part of the rare earth element is consumed to form the rare earth oxide, so that the amount of the magnetic rare earth element in the sintered magnet, which functions magnetically as the sintered magnet, is reduced. In order to compensate for the rare earth element and obtain a practically sufficient amount of magnetism, for example, a coercive force (iHc) of 13kOe or more, the content of the rare earth element in the R-Fe-B based sintered permanent magnet must be increased. In practice, the amount of the rare earth element added exceeds 31% by weight.

As described above, in order to improve the corrosion resistance, the addition amount of the rare earth element should be reduced; meanwhile, the amount of the rare earth element to be added should be increased in order to obtain practically sufficient magnetism. Due to this conflicting requirement, a rare earth permanent magnet having both sufficient corrosion resistance and sufficient magnetism has not been produced.

The present invention aims to provide an R-Fe-B based sintered permanent magnet having significantly improved corrosion resistance and excellent magnetic properties.

As a result of intensive studies to achieve the above object, the present inventors have found that a rare earth permanent magnet excellent in both corrosion resistance and magnetic properties can be obtained by adjusting the content of each of the rare earth elements, oxygen, carbon and nitrogen, within specific ranges. The present invention has been completed based on this finding.

In a first aspect, the present invention provides a rare earth permanent magnet, which mainly comprises (by weight): 27.0 to 31.0% of at least one rare earth element including yttrium, 0.5 to 2.0% of B, 0.02 to 0.15% of N, 0.25% or less than 0.25% of O, 0.15% or less than 0.15% of C, at least one optional element selected from 0.1 to 2.0% of Nb, 0.02 to 2.0% of Al, 0.3 to 5.0% of Co, 0.01 to 0.5% of Ga and 0.01 to 1.0% of Cu, and the balance being Fe.

In a second aspect of the present invention, there is provided a method for producing a rare earth permanent magnet, comprising the steps of: (a) in nitrogen gas containing substantially 0% of oxygen or argon gas containing substantially 0% of oxygen and 0.0001 to 0.1 vol% of nitrogen, at a pressure of 5 to 10% kgf/cm²Then, adding R-Fe-B based alloy coarse powder, wherein R is at least one rare earth element including yttrium, into a pulverizer at a feed rate of 3-20 kg/hr, and pulverizing; (b) recovering the fine powder in the form of slurry in a nitrogen atmosphere or an argon atmosphere in the solvent; (c) wet-pressing the slurry into a blank under the action of an applied magnetic field; (d) heat-treating the body in a vacuum furnace to remove the solvent contained therein; and (e) sintering the heat-treated green body in a vacuum furnace.

In a third aspect of the present invention, there is provided a method for producing a rare earth permanent magnet, comprising the steps of: (a) strip casting (strip casting) an R-Fe-B based alloy melt wherein R is at least one rare earth element including yttrium into an alloy strip of 1mm or less than 1 mm; (b) carrying out heat treatment on the alloy strip at 800-1100 ℃ in an inert gas atmosphere or vacuum; (c) crushing the alloy strip subjected to heat treatment into coarse powder; (d) pulverizing the coarse powder into fine powder; (e) recovering the fine powder in the form of a slurry in a solvent under an inert gas atmosphere; (f) wet-pressing the slurry into a blank under the action of an applied magnetic field; (g) heat-treating the green body in a vacuum furnace to remove the solvent contained therein; and (h) sintering the heat-treated green body in a vacuum furnace.

In a fourth aspect of the present invention, there is provided a method for producing a rare earth permanent magnet, comprising the steps of: (a) wherein R is at least one rare earth element including yttrium, consisting essentially of R₂Fe₁₄Mixing first alloy coarse powder and second alloy coarse powder consisting of phase B according to a weight ratio of 70-99: 1-30, wherein the chemical components (by weight) of the first alloy are as follows: 26.7-32% R, 0.9-2.0% B, 0.1-3.0% M, wherein M is at least one of Ga, Al and Cu, and the balance Fe; the chemical composition (by weight) of the second alloy is as follows: 35-70% of R, 5-50% of Co, 0.1-3.0% of M, and the balance of Fe; (b) pulverizing the coarse powder mixture into fine powder; (c) recovering the fine powder in the form of a slurry in a solvent under an inert gas atmosphere; (d) wet-pressing the slurry into a blank under the action of an applied magnetic field; and (e) sintering the heat-treated green body in a vacuum furnace.

Description of the drawings:

FIG. 1 is a micrograph showing a metal structure of a rare earth permanent magnet having a main phase in which the total area of grains having a grain size of 10 μm or less than 10 μm is 96%; the total area of grains having a grain size of 13 μm or more is 1%, and the total area of each grain is calculated based on the total area of the grains of the main phase.

FIG. 2 is a micrograph showing a metal structure of a rare earth permanent magnet having a main phase in which the total area of grains having a grain size of 10 μm or less is 64%; the total area of grains having a grain size of 13 μm or more was 17%, and the total area of each grain was calculated based on the total area of the grains of the main phase.

FIG. 3 is a scanning electron micrograph of a cross-sectional view of the rare earth permanent magnet shown in FIG. 1 after 5000 hours of corrosion testing.

FIG. 4 is a scanning electron micrograph of a cross-sectional view of the rare earth permanent magnet shown in FIG. 2 after 2000 hours of corrosion testing.

First, the contents of each element in the rare earth permanent magnet of the present invention are explained as follows:

the rare earth element used in the present invention is at least one selected from lanthanoid elements and yttrium. The content of the rare earth element is 27.0 to 31.0 wt% based on the total weight of the rare earth permanent magnet. When the rare earth element content exceeds 31.0% by weight, the amount and size of the rare earth-rich phase in the sintered magnet are unfavorably large to lower the corrosion resistance; on the contrary, when the rare earth amount is less than 27% by weight, a dense sintered magnet cannot be obtained because the amount of the liquid phase required for densification during sintering is insufficient. As a result, the magnetic properties, particularly the remanent flux density (Br) and the coercive force (iHc) are reduced.

Preferred rare earth elements may include Nd, Pr and Dy. The content of Pr in the rare earth permanent magnet is preferably 0.1 to 10 wt%, and the content of Dy in the rare earth permanent magnet is preferably 0.5 to 15 wt%. Dy is preferably contained in an amount of 0.8 to 10% by weight because Dy improves coercive force (iHc).

The oxygen content is 0.05 to 0.25 wt%, preferably 0.2 wt% or less, based on the total weight of the rare earth permanent magnet. When the oxygen content is more than 0.25 wt%, the amount of the rare earth element directly functioning as ferromagnetism is reduced due to the conversion of a part of the rare earth element into its oxide, and the coercive force (iHc) is also reduced. Since the sintered alloy powder is derived from an alloy ingot which inevitably contains 0.04 wt.% oxygen during the production thereof, it is practically difficult to reduce the oxygen content in the final sintered magnet to a level lower than 0.05 wt.%.

The carbon content is 0.01 to 0.15 wt%, 0.12 wt% or less, preferably 0.1 wt% or less, based on the total weight of the rare earth permanent magnet. When the carbon content is more than 0.15 wt%, carbide is formed due to the consumption of a part of the rare earth element; thereby reducing the amount of rare earth elements directly contributing to ferromagnetism and also reducing the coercive force (iHc). Since the sintered alloy powder is derived from an alloy ingot, 0.008 wt.% of carbon is inevitably contained in the manufacturing process of the alloy ingot, and therefore, it is practically difficult to reduce the carbon content in the final sintered magnet to a level lower than 0.01 wt.%.

The inventors have found that, in addition to the rare earth element content being adjusted to 27.0 to 31.0 wt%, the nitrogen content should be strictly controlled to improve the corrosion resistance of the R-Fe-B based sintered permanent magnet. By controlling the nitrogen content to 0.02 to 0.15 wt%, preferably 0.03 to 0.13 wt%, based on the total weight of the R-Fe-B based sintered permanent magnet, and controlling the contents of the rare earth element, oxygen and carbon in the above ranges, both excellent corrosion resistance and high magnetic properties can be obtained. The mechanism by which corrosion resistance is improved by the presence of 0.02 to 0.15 wt.% nitrogen has not been understood to date. However, it is confirmed that nitrogen in the R-Fe-B based sintered permanent magnet exists mainly in the rare earth-rich phase in the form of a rare earth nitride. Therefore, it can be assumed that the corrosion resistance is improved because the rare earth nitride suppresses the anodic oxidation of the rare earth-rich phase. A nitrogen content of less than 0.02 wt.% is not significantly improved, probably due to the lack of rare earth nitride formation. When the amount of nitrogen is 0.02 wt% or more, the corrosion resistance is more effectively improved as the content of nitrogen increases. However, when the nitrogen content exceeds 0.15 wt%, the coercive force (iHc) decreases abruptly. This can be assumed to be caused by reducing the amount of rare earth elements due to the formation of rare earth nitrides.

The rare earth permanent magnet of the present invention may further contain one or more of niobium (Nb), aluminum (Al), cobalt (Co), gallium (Ga) and copper (Cu).

Niobium is converted into boride of niobium during sintering, and the niobium boride prevents abnormal growth of grains. The content of niobium is 0.1 to 2.0 wt%, preferably 0.2 to 1.5 wt% based on the total amount of the R-Fe-B based sintered permanent magnet. When the content is less than 0.1% by weight, it is not sufficient to effectively prevent abnormal grain growth; when the content exceeds 2.0% by weight, it is not desirable because the residual magnetic flux density (Br) is lowered due to the increase in the boride content of Nb.

Al is effective for increasing coercive force (iHc), and the content thereof may be 0.02 to 2% by weight, preferably 0.04 to 1.8% by weight, based on the total weight of the R-Fe-B-based sintered permanent magnet. When the content thereof is less than 0.02 wt%, the coercive force (iHc) cannot be effectively increased; when the amount exceeds 2.0% by weight, the residual magnetic flux density (Br) is abruptly decreased.

Co increases the Curie point, i.e., increases the temperature coefficient of saturation magnetization, and may be contained in an amount of 0.3 to 5.0 wt%, preferably 0.5 to 4.5 wt%, based on the total weight of the R-Fe-B-based sintered permanent magnet. When the content is less than 0.3% by weight, the temperature coefficient is not sufficiently increased; when the content exceeds 5% by weight, both the residual magnetic flux density (Br) and the coercive force are abruptly decreased. The corrosion resistance and thermal stability of rare earth permanent magnets increase with increasing volume while the residual magnetic flux density (Br) coercivity (iHc) decreases. Therefore, when high magnetic properties are required, the amount of Co is preferably 2.5% by weight or less, particularly preferably 2% by weight or less. Since, in the present invention, the corrosion resistance can also be improved by the uniform and fine grain structure described below, a sufficiently high corrosion resistance can be obtained even when the amount of Co is 2.5 wt% or less.

Ga is effective for increasing coercive force (iHc) and may be contained in an amount of 0.01 to 0.5% by weight, preferably 0.03 to 0.4% by weight based on the total weight of the R-Fe-B based sintered permanent magnet. When the content is less than 0.01 wt%, the coercive force (iHc) cannot be increased. When the content exceeds 0.5% by weight, both the residual magnetic flux density (Br) and the coercive force (iHc) are lowered.

Cu is also effective for enhancing coercive force (iHc), and its content may be 0.01 to 1.0% by weight, preferably 0.01 to 0.8% by weight, based on the total weight of the R-Fe-B based sintered permanent magnet. When the content is less than 0.01 wt%, the coercive force (iHc) is not improved. When the content exceeds 1.0% by weight, no further improvement is observed.

In the present invention, the corrosion resistance and the magnetic properties of the rare earth permanent magnet are improved by adjusting the rare earth element, oxygen, carbon and nitrogen in each specific range. In addition, the corrosion resistance can be further improved by uniformly refining the metal structure of the rare earth permanent magnet. By "uniformly refined metal structure" is meant a metal structure containing a main phase in which the total area of crystal grains having a grain size of 10 μm or less is 80% or more and the total area of crystal grains having a grain size of 13 μm or more is 10% or less, each of the total areas of crystal grains being calculated based on the total area of crystal grains in the main phase.

FIG. 1 is a photomicrograph of a metal structure of a rare earth permanent magnet having a main phase in which the total area of grains having a grain size of 10 μm or less is 96%; the total area of crystal grains having a grain size of 13 μm or more is 1%, and the total area of each crystal grain is calculated based on the total area of crystal grains in the main phase. FIG. 2 is a photomicrograph of the metal structure of a rare earth permanent magnet having a main phase in which the total area of crystal grains having a grain size of 10 μm or less is 64%; the total area of crystal grains having a grain size of 13 μm or more was 17%, and the total area of each crystal grain was calculated based on the total area of crystal grains in the main phase. The above two rare earth permanent magnets have the same alloy composition: 27.5 wt.% Nd, 0.5 wt.% Pr, 1.5 wt.% Dy, 1.1 wt.% B, 0.1 wt.% Al, 2.0 wt.% Co, 0.08 wt.% Ga, 0.16 wt.% O, 0.06 wt.% C, 0.040 wt.% N, and the balance Fe.

The area ratio is obtained by processing each metal structure image (about 1000 times) by an image processing method under a microscope (product name: VANOX, Olympus Optical Co., Ltd.) by using an image processing apparatus [ LUIEX II (trade name), Nireco).

In order to evaluate the corrosion resistance of the rare earth permanent magnet of FIGS. 1 and 2, the surface of each sample (8 mm. times.8 mm. times.2 mm) was plated with an Ni layer of about 20 μm thickness, and the nickel-plated sample was placed in the air under conditions of 2 atmospheres, 120 ℃ and a relative temperature of 100% to observe the degree of exfoliation of the nickel-plated layer occurring with time. No abnormality or change was observed in the rare earth permanent magnet shown in fig. 1 having a uniform fine grain structure even after 2500 hours on the nickel plating layer. In contrast, in the rare earth permanent magnet shown in FIG. 2 having a coarser grain size. Although there was no peeling after the lapse of 1000 hours, significant peeling of the nickel plated layer was observed after the lapse of 2000 hours. Since the above corrosion test is performed in an acceleration mode, both of the rare earth permanent magnets can be put into practical use without any problem in corrosion resistance. However, the above test results clearly demonstrate that corrosion resistance can be further improved by the above uniform and refined grain structure.

FIG. 3 is a scanning electron micrograph showing a cross-sectional view of the rare earth permanent magnet shown in FIG. 1 after a corrosion test for 5000 hours. FIG. 4 shows a scanning electron micrograph of a cross-sectional view of the rare earth permanent magnet of FIG. 2 after 2000 hours of corrosion testing. In fig. 3, although slight peeling of the nickel plated layer partially occurred from the substrate (permanent magnet), the adhesion between the nickel plated layer and the substrate was good from the practical viewpoint, and further, it can be seen that the metal structure of the rare earth permanent magnet hardly cracked by the corrosion test. Fig. 4 is a structure with coarse grains, and it can be seen that large spalling of the nickel plating occurs due to intergranular fracture within the metallic structure of the substrate. From this result, it was found that intergranular fracture by the accelerated corrosion test was greatly dependent on the size of crystal grains in the main phase of the permanent magnet.

The intergranular fracture of the coarse-grained structure can be considered to occur in the following manner. In the main phase having a coarser grain structure as shown in fig. 2, the intergranular voids, mainly the grain boundary triple points, are occupied by an increased amount of the high neodymium phase which is very easily oxidized. Factors causing corrosion cracking, such as moisture in the accelerated corrosion test described above, intrude into the magnet from an intergranular path, and the neodymium-rich phase is oxidized. Such oxidation of the neodymium-rich phase is considered to be a cause of intergranular fracture.

As described above, the corrosion resistance of the R-Fe-B based sintered permanent magnet can be further improved by the uniform and refined grain structure of the main phase defined as: in the main phase, the total area of grains having a grain size of 10 μm or less is 80% or more and the total area of grains having a grain size of 13 μm or more is 10% or less, based on the total area of grains in the main phase.

The R-Fe-B based sintered permanent magnet of the present invention can be produced by the following method.

Although the R-Fe-B-based raw material coarse powder can be obtained by crushing an alloy ingot, it is more preferable to obtain the coarse powder by crushing an alloy strip obtained by strip-casting (strip-casting method). The "strip casting method" referred to in the present invention is a method of producing an alloy strip by spraying an alloy melt onto the surface of a chill roll or the like to quench the molten alloy to form an alloy strip on the surface thereof. Sintering of fine powder having a uniform metal structure and a narrow particle size distribution is important for obtaining a rare earth permanent magnet having a fine and uniform metal structure. In order to obtain such fine powder having an average particle size of 1 to 8 μm, preferably 3 to 5 μm, it is preferable to subject the ingot or strip to a heat treatment, coarsely pulverize the heat-treated ingot or strip into coarse powder, and then finely pulverize the coarse powder.

Since an R-Fe-B based alloy ingot generally contains a deposited α -Fe phase in its alloy structure, the alloy ingot should be subjected to: performing solution heat treatment at 1000-1200 ℃ for 1-10 hours to eliminate the alpha-Fe phase.

According to the strip casting method, the alloy melt is rapidly quenched on a cold surface to produce an alloy strip having a fine metallic structure. However, fine powders with narrow particle size distributions have not been obtained with simple powdered alloy strips because of the hard surface on the strip that is formed during strip casting where the molten metal is rapidly quenched on chill rolls. The present inventors have found that when a ribbon is heat-treated at 800 to 1100 ℃, preferably 950 to 1050 ℃ for 10 minutes to 10 hours in an inert atmosphere or vacuum before pulverization, the ribbon can be pulverized into fine powder having a narrow particle size distribution.

Although mechanical pulverization can be used in the present invention, the rough pulverization is preferably carried out by autogenously decomposing the heat-treated alloy ingot or strip by absorbing and removing hydrogen therein. The hydrogen absorption operation is performed by holding the alloy strip in a hydrogen-filled furnace at a pressure of 1 atmosphere or less at normal temperature until the alloy strip is sufficiently cracked. The occluded hydrogen embrittles the rare earth-rich phase of the alloy strip, thereby making the alloy strip susceptible to cracking into coarse powder having a narrow particle size distribution. Then, the furnace is evacuated and heated to 150-550 ℃ and the cracked alloy strip is held therein for 30 minutes to 10 hours for complete hydrogen removal. When coarsely pulverized by a hydrogen absorption method, the coarse powder may be further subjected to mechanical coarse pulverization by a known method. The coarse powder thus obtained preferably has a particle size of 32 mesh or less.

The raw material coarse powder is prepared by the method. Further, the raw material coarse powder may be a mixture of the first alloy coarse powder and the second alloy coarse powder, both of which are obtained by heat-treating the alloy strip produced by the strip casting method and roughly pulverizing the heat-treated alloy strip by the hydrogen absorption method as described above. The first alloy consists essentially of R₂Fe₁₄B phase (main phase) and an alloy composition of 26.7 to 31 wt% R (where R is one or more rare earth elements including yttrium), 0.9 to 2.0 wt% B, 0.1 to 3.0 wt% M (where M is one or more elements of Ga, Al and Cu) and the balance Fe. The alloy composition of the second alloy is: 35 to 70 wt% of R, 5 to 50 wt% of Co, 0.1 to 3.0 wt% of M and the balance Fe. The mixing ratio of the first alloy coarse powder to the second alloy coarse powder is 70-99: 1-30 by weight. These coarse powders should also be mixed so that the alloy composition (by weight) of the final sintered permanent magnet is: 27.0 to 31.0% of at least one rare earth element including yttrium, 0.5 to 2.0% of B, 0.02 to 0.15% of N, 0.05 to 0.25% of O, 0.01 to 0.15% of C, 0.3 to 5.0% of Co, at least one optional element selected from 0.02 to 2.0% of Al, 0.01 to 0.5% of Ga and 0.01 to 1.0% of Cu, and the balance Fe.

Next, the R-Fe-B based raw material powder thus obtained is finely pulverized while adjusting the nitrogen content so that the nitrogen content in the final rare earth permanent magnet falls within the range specified in the present invention. For example, after the R-Fe-B based coarse raw material coarse powder is fed into a pulverizer such as a jet mill or the like, the atmosphere inside the pulverizer is replaced with nitrogen gas so that the oxygen content in the nitrogen gas is reduced to a level of substantially 0%. In the nitrogen atmosphereWherein the coarse powder is finely pulverized and simultaneously subjected to nitrogen pressure of 5 to 10% kgf/cm²Then, the feed is carried out at a rate of 3 to 20 kg/hr. The nitrogen content in the raw meal is preferably adjusted by varying the amount added and the feed rate in order to ensure the specified nitrogen content range of the present invention. Since the amount of nitrogen introduced into the raw powder also depends on the type, size, etc. of the pulverizer, it is preferable to experimentally determine the amount to be added and the feed rate before the actual operation.

Alternatively, the nitrogen content in the raw material powder may be adjusted by charging a predetermined amount of the R-Fe-B-based coarse powder into a pulverizer, replacing the atmosphere in the pulverizer with argon (Ar) gas so as to reduce the oxygen content in the Ar gas to substantially 0%, introducing nitrogen gas into the argon gas in an amount such that the nitrogen content in the argon atmosphere becomes, for example, 0.0001 to 0.1% by volume, and then finely pulverizing the coarse powder in such an atmosphere. During the pulverization, nitrogen is mainly combined with the rare earth element in the coarse powder to obtain a fine powder containing a predetermined amount of nitrogen.

The "substantially 0%" oxygen content in the present invention means that the oxygen content in the internal atmosphere of the pulverizer is preferably 0.01 vol% or less, more preferably 0.005 vol% or less, most preferably 0.002 vol% or less.

The finely divided powder is directly recovered in the solvent under an inert gas atmosphere. The solvent is selected from mineral oil, vegetable oil and synthetic oil, each of which has a flash point of 70 ℃ or higher and less than 200 ℃ at one atmosphere, a fractionation point of 400 ℃ or less, and a kinematic viscosity of 10cSt or less at normal temperature. The fine powder slurry thus obtained is wet-pressed in a magnetic field to form a green body, preferably by a compression molding method. The conditions of the compression molding process can be selected according to the actual operating parameters. The molding method is preferably carried out at a molding pressure of 0.3 to 4.0 ram the loose soil with a stone-roller after sowing/cm²The simultaneous application of the orienting magnetic field is carried out at 7kOe or more, preferably at 10kOe or more.

Then, the green body was placed in a vacuum furnace at a vacuum degree of 10^-1～10^-3Heating to 100-300 deg.C for enough time to remove the solvent in the blankAn agent so as to adjust the final carbon content to be in the range of 0.15 wt% or less based on the total weight of the rare earth permanent magnet. Secondly, the temperature of the vacuum furnace is raised to 1000-1200 ℃, and the blank is heated to 10 ℃ in the vacuum degree in the temperature range^-3～10^-6Sintering under the support for 30 minutes to 5 hours.

The sintered product thus obtained may be further subjected to annealing treatment, preferably two-stage heat treatment by heating at 800 to 1000 ℃ for 1 to 3 hours in an inert gas atmosphere and further heating at 400 to 650 ℃ for 30 minutes to 3 hours. Finally, if necessary, the sintered product is machined to obtain the rare earth permanent magnet of the present invention.

The invention will be further described with reference to the following examples, which are to be construed as illustrating various preferred aspects of the invention.

Example 1

Raw material coarse powder of 32 mesh or less is obtained by crushing an alloy ingot having the following chemical composition (by weight): 24.0% Nd, 3.0% Pr, 2.0% Dy, 1.1% B, 1.3% Nb, 1.0% Al, 3.3% Co, 0.1% Ga, 0.01% O, 0.005% C, 0.007% N and the balance Fe. The raw material coarse powder prepared in the way has the following chemical components (by weight): 23.9% Nd, 2.9% Pr, 2.0% Dy, 1.1% B, 1.2% Nb, 1.0% Al, 3.3% Co, 0.1% Ga, 0.14% O, 0.02% C, 0.007% N and the balance Fe.

When 50Kg of the raw material coarse powder was charged into the jet mill, the internal atmosphere of the jet mill was replaced with argon while controlling the oxygen content in the argon atmosphere to be substantially 0%. By adding N₂The gas was introduced into an argon atmosphere, and the nitrogen content in the argon gas was adjusted to 0.003 vol%. Then, the pressure was set at 7.5kgf/cm²Next, the coarse powder was finely pulverized by feeding the coarse powder into a jet mill at a rate of 8kg/hr simultaneously.

After the completion of the fine pulverization, the fine powder was directly recovered from the jet mill into mineral oil (product of Idemitsu Kosan Co., Ltd., product name of ldemitsu Super Sol PA-30) under an argon atmosphere. The recovered fine powder was made into a slurry having a solid content of 75 wt% by adjusting the amount of mineral oil, and the average particle size of the fine powder was 4.7 μm.

Then, the slurry was subjected to wet-pressing in a mold cavity while applying an oriented magnetic field of 14kOe and molding 1.0 ton/cm². The direction of the applied orienting magnetic field and the embossing are perpendicular to each other. To form a green body. During wet pressing, part of the mineral oil was discharged from the upper punch equipped with a die cavity through a 1mm thick cloth filter.

The thus-formed green body was placed in a vacuum furnace at 3.0X 10^-2Heating at 200 deg.C for 1 hr to remove residual mineral oil, and heating to 4.0 × 10^-4And (3) under the support, raising the temperature to 1070 ℃ at the speed of 15 ℃/min, and keeping the temperature at 1070 ℃ for 3 hours to finish the sintering of the green body, thereby obtaining the rare earth permanent magnet.

The composition of the rare earth permanent magnet is shown in Table 1. Further, the rare earth permanent magnet was heat-treated at 900 ℃ for 2 hours and at 530 ℃ for 1 hour, both of which were carried out in an argon atmosphere. When the magnetic properties (residual magnetic flux density Br, coercive force iHc, and maximum energy product (BH) max) thereof were measured after machining, it was found that the rare earth permanent magnet had excellent magnetic properties, as shown in table 1.

In order to evaluate the corrosion resistance of the rare earth permanent magnet, the surface of an 8mm × 8mm × 2mm sample obtained by processing the rare earth permanent magnet was nickel-plated to a 10 μm-thick nickel-plated layer. The nickel plated samples were placed in air at 2 atmospheres at 120 ℃ and 100% relative humidity. The degree of exfoliation of the nickel-plated layer from the surface of the rare earth permanent magnet was observed. As shown in table 1, the rare earth permanent magnet has good corrosion resistance because no change in the nickel plating layer is observed even after the 1000-hour corrosion test.

Example 2

Coarse powder of the same raw material as used in example 1 was finely pulverized in the same manner as in example 1 except that the nitrogen content in an argon atmosphere was adjusted to 0.006 vol% to obtain a slurry containing a fine powder having an average particle size of 4.8 μm, and the slurry was subjected to the same operation as in example 1 to obtain rare earth permanent magnets having the chemical compositions shown in Table 1.

The results of the same corrosion test as that of example 1 are shown in Table 1, and it can be seen from Table 1 that the rare earth permanent magnet has excellent magnetic properties and no change in the nickel plated layer is observed even after the corrosion test at 1200 ℃.

Example 3

Coarse powder of the same raw material as used in example 1 was finely pulverized in the same manner as in example 1 except that the nitrogen content in an argon atmosphere was adjusted to 0.015% by volume to obtain a slurry containing a fine powder having an average particle size of 4.7 μm. The slurry was further processed in the same manner as in example 1 to obtain rare earth permanent magnets having the compositions shown in Table 1.

The magnetic properties and the results of the corrosion test conducted in the same manner as in example 1 are shown in Table 1. As can be seen from Table 1, the rare earth permanent magnet has excellent magnetic properties, and no change in the nickel plating layer was observed even after the 1500-hour corrosion test.

Comparative example 1

Coarse powder of the same raw material as used in example 1 was finely pulverized in the same manner as in example 1 except that the nitrogen content in an argon atmosphere was adjusted to 0.00005% by volume to obtain a fine powder slurry having an average particle size of 4.7, and the slurry was subjected to the same operation as in example 1 to obtain rare earth permanent magnets whose components are shown in Table 1.

The magnetic properties and the results of the same corrosion test as in example 1 are shown in Table 1. As can be seen from Table 1, although the rare earth permanent magnet has excellent magnetic properties, its corrosion resistance is extremely poor because the nickel plated layer starts to peel off after the 120-hour corrosion test.

Comparative example 2

Coarse powder of the same raw material as used in example 1 was finely pulverized in the same manner as in example 1 except that the nitrogen content in an argon atmosphere was adjusted to 0.13% by volume to obtain a slurry of fine powder having an average particle size of 4.6, and the slurry was subjected to the same operation as in example 1 to obtain rare earth permanent magnets whose components are shown in Table 1.

The magnetic properties and the results of the corrosion test conducted in the same manner as in example 1 are shown in Table 1. As can be seen from Table 1, the rare earth permanent magnet has excellent corrosion resistance because no change in the nickel plating layer was observed after the corrosion test for 1800 hours. However, the rare earth permanent magnet is inferior in magnetic properties, and particularly, the coercive force (iHc) is too low to be practical.

Comparative example 3

Raw material coarse powder of 32 mesh or less is prepared by pulverizing an alloy ingot whose alloy composition (by weight) is as follows: 26.8% Nd, 3.5% Pr, 2.0% Dy, 1.1% B, 1.3% Nb, 1.0% Al, 3.3% Co, 0.1% Ga, 0.01% O, 0.005% C, 0.007% N and the balance Fe. The raw meal thus prepared had the following composition (by weight): 26.7% Nd, 3.5% Pr, 2.0% Dy, 1.1% B, 1.3% Nb, 1.0% Al, 3.3% Co, 0.1% Ga, 0.18% O, 0.03% C, 0.009% N, and the balance Fe.

The raw material coarse powder was finely pulverized in the same manner as in example 1 to obtain a slurry containing a fine powder having an average particle size of 4.5 μm, from which a rare earth permanent magnet having the chemical composition shown in Table 1 was obtained in the same manner as in example 1.

The magnetic properties and the results of the same corrosion test as in example 1 are shown in Table 1. As can be seen from Table 1, although the rare earth permanent magnet has excellent magnetic properties, the corrosion resistance is extremely poor because the nickel plating layer starts to peel off only after the 24-hour corrosion test.

Comparative example 4

Coarse powder of the same raw material as used in example 1 was finely pulverized in the same manner as in example 1 except that the nitrogen content in the argon atmosphere was adjusted to 0.05 vol% and 0.06 vol%, respectively, to obtain a slurry containing a fine powder having an average particle size of 4.6 μm. Then, the slurry was subjected to the same operation as in example 1 to obtain rare earth permanent magnets whose components are shown in Table 1.

The magnetic properties and the results obtained by the same corrosion test as in example 1 are shown in Table 1, and from Table 1, it can be seen that the rare earth permanent magnet exhibits excellent corrosion resistance because no change is observed in the nickel plated layer even after the 1200-hour corrosion test. However, the rare earth permanent magnet is inferior in magnetic properties, and particularly, the coercive force (iHc) is too low to be practical.

Comparative example 5

Coarse powder of the same raw material as used in example 1 was finely pulverized in the same manner as in example 1 except that the nitrogen contents in the argon atmosphere were adjusted to 0.007% by volume, respectively, to obtain a slurry containing a fine powder having an average particle size of 4.7 μm, from which a green body was formed in the same manner as in example 1.

Heating rate of 15 deg.C/min and vacuum degree of 5.0 × 10 without heating for removing mineral oil^-4The green body was heated from room temperature to 1070 c under torr and held at 1070 c for 3 hours to complete the sintering. The sintered product was heat-treated in the same manner as in example 1 to obtain rare earth permanent magnets having the chemical compositions shown in Table 1.

The magnetic properties and the results of the corrosion test conducted in the same manner as in example 1 are shown in Table 1. As can be seen from Table 1, the rare earth permanent magnet exhibited excellent corrosion resistance, since variation in the nickel plating layer was observed even after the corrosion test for 1200 hours. However, the rare earth permanent magnet is inferior in magnetic properties, and particularly, the coercive force (iHc) is too low to be practical.

Comparative example 6

The same green body as obtained in comparative example 4 was sintered and heat-treated in the same manner as in comparative example 5 to obtain rare earth permanent magnets having the chemical compositions shown in Table 1.

The magnetic properties and the results of the corrosion test conducted in the same manner as in example 1 are shown in Table 1. As can be seen from Table 1, the rare earth permanent magnet exhibited excellent corrosion resistance, since no change was observed in the nickel plated layer even after the 1200-hour corrosion test. However, the rare earth permanent magnet is inferior in magnetic properties, and particularly, the coercive force (iHc) is too low to be practical.

TABLE 1

Chemical composition (by weight) of magnet

Number Nd Pr DyB Fe Nb Al Co Ga Cu N O C

Examples

123.92.92.01.1 balance 1.21.03.30.1-0.030.170.06

223.92.92.01.1 balance 1.21.03.30.1-0.050.160.06

323.92.92.01.1 balance 1.21.03.30.1-0.120.160.06

Comparative example

123.92.92.01.1 balance 1.21.03.30.1-0.010.180.06

223.92.92.01.1 balance 1.21.03.30.1-0.200.180.06

326.73.52.01.1 balance 1.31.03.30.1-0.040.200.07

423.92.92.01.1 balance 1.21.03.30.1-0.050.300.06

523.92.92.01.1 balance 1.21.03.30.1-0.060.160.18

623.92.92.01.1 balance 1.21.03.30.1-0.050.290.17

TABLE 1 (continuation)

Magnetic corrosion resistance

Number Br (kG) iHc (kOe) (BH) max (MGOe)

Examples

113.714.545.51000 hours later, the nickel plating layer has no change

213.714.445.51200 hours later, the nickel plating layer has no change

313.714.245.51500 hours later, the nickel plating layer has no change

Comparative example

113.714.645.5120 hours later, the nickel plating layer peeled off

213.711.044.81800 hours later, the nickel plating layer has no change

313.017.040.524 hours later, the nickel plating layer peeled off

413.710.544.11200 hours later, the nickel plating layer has no change

513.710.844.31200 hours later, the nickel plating layer has no change

613.77.542.51200 hours later, the nickel plating layer has no change

Example 4

An alloy strip with the following chemical components (by weight) and the thickness of 0.2-0.5 mm is prepared by a strip casting method: 27.0% Nd, 0.5% Pr, 1.5% Dy, 1.05% B, 0.35% Nb, 0.08% Al, 2.5% Co, 0.09% Ga, 0.08% Cu, 0.03% O, 0.005% C, 0.004% N, and the balance Fe. After the alloy strip was heat-treated at 1000 ℃ for 2 hours in an argon atmosphere, the alloy strip was naturally cracked at room temperature in a furnace by a hydrogen absorption method. The furnace was then evacuated and the strip was heated to 550 c and held at that temperature for 1 hour to effect hydrogen removal.

The cracked tape was mechanically crushed in a nitrogen atmosphere to obtain 32 mesh raw material coarse powder having the following chemical composition (by weight): 27.0% Nd, 0.5% Pr, 1.5% Dy, 1.05% B, 0.35% Nb, 0.08% Al, 2.5% Co, 0.09% Ga, 0.08% Cu, 0.12% O, 0.02% C, 0.008% N, and the balance Fe.

When 50kg of raw material coarse powder was charged into the jet mill, the internal atmosphere in the jet mill was replaced with nitrogen while controlling the oxygen content in the nitrogen atmosphere to be substantially 0% (0.001 (volume)%, as analyzed in an oxygen analyzer). Then, the mixture was pressed at a pressure of 7.0kgf/cm²Next, the coarse powder was finely pulverized by simultaneously feeding the coarse powder into a jet mill at a rate of 10 kg/hr.

When the fine pulverization was completed, the fine powder was directly recovered into mineral oil (trade name Idemitsu Super Sol PA-30.Idemitsu Ko-sen Co., Ltd.) by a jet mill under a nitrogen atmosphere. The recovered fine powder was made into a slurry having a solid content of 80% by weight by adjusting the amount of the mineral oil. The average particle size of the fine powder was 3.9 μm.

The slurry was then wet-pressed in a mold cavity while applying a 12kOe directional magnetic field and 0.8 ton/cm²And (4) molding. The direction of the applied orienting magnetic field and the direction of the die are perpendicular to each other to form the blank. During wet pressing, part of the mineral oil was discharged from the upper punch equipped with a die cavity through a 1mm thick cloth filter.

The green body thus obtained was placed in a vacuum furnace at 5.0X 10^-2Vacuum degree of the tray and temperature of 200 DEG CAnd heated for 1 hour to remove residual mineral oil. Then the vacuum degree is 4.0 multiplied by 10^-4The temperature of the vacuum furnace was raised to 1070 ℃ at a rate of 15 ℃/min. The temperature was maintained at 1070 c for 3 hours to complete sintering of the green body, thereby obtaining rare earth permanent magnets having the chemical compositions shown in table 2.

The ratio of the area of the grains in the main phase of the rare earth permanent magnet, that is, the ratio of the total area of the grains having a grain size of 10 μm or less and the ratio of the total area of the grains having a grain size of 13 μm or more based on the total area of the crystal grains in the main phase, is also shown in Table 2.

The rare earth permanent magnet was heat-treated at 900 ℃ for 2 hours and 480 ℃ for 1 hour in an argon atmosphere. The rare earth permanent magnet was found to have excellent magnetic properties shown in Table 2 when the magnetic properties were measured after the working.

The corrosion resistance of the rare earth permanent magnet was evaluated in the same manner as in example 1. As shown in Table 2, the rare earth permanent magnet has excellent corrosion resistance, because no change in the nickel plating layer was observed even after the corrosion test for 2500 hours. The rare earth permanent magnet obtained above showed excellent corrosion resistance as compared with examples 8 and 9 described below. Therefore, in the above comparison, it is apparent that the corrosion resistance can be further improved by the uniform and refined grain structure of the main phase, that is, adjusting the ratio of grains having a grain size of 10 μm or less to 80% or more and the ratio of grains having a grain size of 13 μm or more to 10% or less.

Example 5

The alloy strip with the thickness of 0.2-0.4 mm prepared by the strip casting method comprises the following chemical components (by weight): 22.3% Nd, 2.0% Pr, 5.5% Dy, 1.0% B, 0.5% Nb, 0.2% Al, 2.0% Co, 0.09% Ga, 0.1% Cu, 0.02% O, 0.005% C, 0.003% N, and the balance Fe. After heat treatment at 1100 c for 2 hours under an argon atmosphere, the alloy strip was subjected to hydrogen absorption, dehydrogenation and mechanical pulverization as in example 4 to obtain raw material coarse powder of 32 mesh or less having the following chemical composition (by weight): 22.3% Nd, 2.0% Pr, 5.5% Dy, 1.0% B, 0.5% Nb, 0.2% Al, 2.0% Co, 0.09% Ga, 0.1% Cu, 0.11% O, 0.02% C, 0.006% N, and the balance Fe.

After 100kg of raw material coarse powder was charged into a jet mill, the atmosphere in the jet mill was replaced with nitrogen while controlling the oxygen content in the nitrogen atmosphere to substantially 0% (0.002 (vol)%, as analyzed in an oxygen analyzer). Under a pressure of 8.0kgf/cm²Next, the coarse powder was finely pulverized by feeding the coarse powder into a jet mill at a feed rate of 12kg/hr simultaneously.

After the completion of the fine pulverization, the fine powder was directly recovered from the jet mill into mineral oil (trade name Idemitsu Super Sol PA-30. manufactured by Idemitsu Co., Ltd.) under a nitrogen atmosphere. The recovered fine powder was made into a slurry having a solid content of 77 wt% by adjusting the amount of the mineral oil. The average particle size of the fine powder was 3.8. mu.m.

Then, the slurry was wet-pressed in a mold cavity while applying an oriented magnetic field of 10kOe and molding 1.5 tons/cm². The applied orienting magnetic field and the direction of the embossing are perpendicular to each other to produce a green body. During wet pressing, part of the mineral oil was discharged from the upper punch equipped with a die cavity through a 1mm thick cloth filter.

The green body thus obtained was placed in a vacuum furnace at 5.0X 10^-2Heating at 200 deg.C for 2 hr to remove residual mineral oil, and vacuum-maintaining at 5.0 × 10^-4The temperature of the vacuum furnace was raised to 1090 ℃ at a rate of 15 ℃/min and maintained at 1090 ℃ for 3 hours under the torr to complete sintering of the green body, thereby obtaining rare earth permanent magnets having the chemical compositions shown in table 2.

The grain area ratio in the main phase of the rare earth permanent magnet obtained in the same manner as in example 4 is shown in Table 2.

The rare earth permanent magnet was further heat-treated at 900 ℃ for 2 hours and at 460 ℃ for 1 hour. Both under an argon atmosphere.

The magnetic properties and the results of the same corrosion test as in example 1 are shown in Table 2. As shown in Table 2, the rare earth permanent magnet has excellent magnetic properties and no change in the nickel plating layer is observed even after the corrosion test for 2500 hours.

Example 6

An alloy strip with the following chemical components (by weight) and the thickness of 0.1-0.5 mm is prepared by a strip casting method: 20.7% Nd, 8.6% Pr, 1.2% Dy, 1.05% B, 0.08% Al, 2.0% Co, 0.09% Ga, 0.1% Cu, 0.03% O, 0.006% C, and 0.004% N, with the balance Fe. After heat treatment at 900 ℃ for 3 hours under an argon atmosphere, the alloy strip was subjected to the same hydrogen absorption, dehydrogenation and mechanical pulverization as in example 4 to obtain raw material coarse powder of 32 mesh or less having the following chemical composition (by weight): 20.7% Nd, 8.6% Pr, 1.5% Dy, 1.05% B, 0.08% Al, 2.0% Co, 0.09% Ga, 0.1% Cu, 0.13% O, 0.03% C, 0.009% N, and the balance Fe.

After 50kg of raw material coarse powder was charged into a jet mill, the atmosphere in the jet mill was replaced with argon while controlling the oxygen content in the argon atmosphere to substantially 0% (0.002 vol% in the oxygen analyzer). The nitrogen content in the argon atmosphere was adjusted to 0.005 (vol)%, by introducing nitrogen gas into the argon atmosphere. Then, the mixture was pressed at a pressure of 7.5kgf/cm²Next, while feeding the coarse powder into a jet mill at a feed rate of 8kg/hr, the coarse powder was finely pulverized.

After the completion of the fine pulverization, the fine powder was directly recovered from the jet mill into mineral oil (trade name: Idemitsu Super Sol PA-30. manufactured by Idemitsu co., ltd.) under an argon atmosphere, and the recovered fine powder was made into a slurry having a solid content of 75 wt% by adjusting the amount of the mineral oil. The average particle size of the fine powder was 4.0. mu.m.

Then, the slurry was wet-pressed in a mold cavity while applying an oriented magnetic field of 13kOe and molding 0.6 ton/cm². The applied orienting magnetic field and the direction of the embossing are perpendicular to each other. To produce a green body. During wet pressing, part of the mineral oil is supplied with the mould cavityIs discharged through a cloth filter having a thickness of 1 mm.

The green body thus obtained was placed in a vacuum furnace at 6.0X 10^-2The residue mineral oil was removed by heating at 180 ℃ for 4 hours under vacuum. Then the degree of vacuum was 3.0X 10^-4The temperature of the vacuum furnace was raised to 1070 c at a rate of 15 c/min. The temperature was maintained at 1070 c for 2 hours to complete sintering of the green body, thereby producing rare earth permanent magnets having the chemical compositions shown in table 2.

The grain area ratio in the main phase of the rare earth permanent magnet produced in the same manner as in example 4 is shown in Table 2.

The rare earth permanent magnet was heat-treated at 900 ℃ for 2 hours and 510 ℃ for 1 hour in an argon atmosphere.

The results of the corrosion test conducted in the same manner as in example 1 are shown in Table 2. As can be seen from Table 2, the rare earth permanent magnet has excellent magnetic properties and no change in the nickel plating layer is observed even after the corrosion test for 2500 hours.

Example 7

An alloy strip with the following chemical components (weight) and the thickness of 0.1-0.4 mm is prepared by a strip casting method: 22.0% Nd, 5.0% Pr, 1.5% Dy, 1.1% B, 1.0% Al, 2.5% Co, 0.02% O, 0.005% C, 0.005% N and the balance Fe. After heat treatment at 1000 ℃ for 2 hours under an argon atmosphere, the alloy strip was mechanically pulverized under a nitrogen atmosphere to obtain a raw material coarse powder having the following chemical composition (by weight), 32 mesh or less: 22.0% of Nd, 5.0% of Pr, 1.5% of Dy, 1.1% of B, 1.1% of Al, 2.5% of Co, 0.1% of O, 0.01% of C, 0.009% of N and the balance of Fe.

After 50kg of raw material coarse powder was charged into a jet mill, the internal atmosphere of the jet mill was replaced with argon while controlling the oxygen content in the argon atmosphere to be substantially 0% (0.002 (vol)%, as analyzed in an oxygen analyzer). Then, the mixture was pressed at a pressure of 7.0kgf/cm²Simultaneously feeding the meal into a jet mill at a feed rate of 10kg/hrAnd the coarse powder is finely pulverized.

After completion of the fine pulverization, the fine powder was recovered directly from the jet mill into mineral oil (trade name Idemitsu Super Sol PA-30. manufactured by Idemitsu Co., Ltd.) under nitrogen atmosphere. The recovered fine powder was made into a slurry having a solids content of 78 wt% by adjusting the amount of mineral oil. The average particle size of the fine powder was 4.2. mu.m.

Then, the slurry was wet-pressed in a mold cavity while applying an oriented magnetic field of 11kOe and 0.5 ton/cm²And (5) molding. The applied orienting magnetic field and the direction of the embossing are perpendicular to each other. To form a blank. During wet pressing, part of the mineral oil was discharged from the upper punch equipped with a die cavity through a 1mm thick cloth filter.

The thus-formed green body was placed in a vacuum furnace at 5.0X 10^-2The vacuum of the tray was heated at 180 ℃ for 2 hours to remove residual mineral oil. Then, the vacuum degree was 2.0X 10^-4The vacuum oven temperature was ramped to 1080 ℃ at a rate of 15 ℃/min. The temperature was maintained at 1080 ℃ for 2 hours to complete sintering of the green body, thereby obtaining a rare earth permanent magnet having a chemical composition shown in Table 2.

The area ratio of crystal grains in the main phase of the rare earth permanent magnet obtained in the same manner as in example 4 is shown in Table 2.

Then the rare earth permanent magnet is heat-treated at 900 ℃ for 2 hours and 600 ℃ for 1 hour in an argon atmosphere.

The magnetic properties and the results of the corrosion test according to example 1 are shown in Table 2. As can be seen from Table 2, the rare earth permanent magnet has excellent magnetic properties and no change in the nickel plating layer is observed even after 2000 hours of the corrosion test.

Example 8

The same alloy strip as obtained in example 4 was subjected to the same coarse crushing operation as in example 4, except that the heat treatment was omitted, to obtain a raw coarse powder of 32 mesh having the following chemical composition (by weight): 27.0% Nd, 0.5% Pr, 1.5% Dy, 1.05% B, 0.35% Nb, 0.08% Al, 2.5% Co, 0.09% Ga, 0.08% Cu, 0.10% O, 0.02% C, 0.007% N, and the balance Fe.

A slurry containing a fine powder having an average particle size of 4.4 μm was prepared in the same manner as in example 4, except that the raw material coarse powder was finely pulverized in the same manner as in example 1. The slurry was formed into a green body, sintered and heat-treated in the same manner as in example 4 to obtain a rare earth permanent magnet having the chemical composition shown in Table 2.

Further, the results of the corrosion test conducted in the same manner as in example 1 are shown in Table 2. As can be seen from Table 2, the rare earth permanent magnets had slightly smaller magnetic properties (Br and iHc) than those of example 4, and no change in the nickel plating layer was observed even after the 1200-hour corrosion test.

Example 9

An alloy ingot having a chemical composition substantially the same as that of the alloy strip of example 5 (22.3% Nd, 2.0% Pr, 5.5% Dy, 1.0% B, 0.5% Nb, 0.2% Al, 2.5% Co, 0.09% Ga, 0.1% Cu, 0.01% O, 0.004% C, 0.002% N, and the balance Fe) was prepared. To eliminate the alpha-Fe phase precipitated in the alloy structure, the alloy ingot was solution heat-treated at 1100 ℃ for 6 hours under an argon atmosphere. The thus-treated alloy ingot was coarsely pulverized in the same manner as in example 5 to obtain raw coarse powder of 32 mesh having the following chemical composition (by weight): 22.3% Nd, 2.0% Pr, 5.5% Dy, 1.0% B, 0.5% Nb, 0.2% Al, 2.5% Co, 0.09% Ga, 0.1% Cu, 0.10% O, 0.02% C, 0.005% N, and the balance Fe.

A slurry containing a fine powder having an average particle size of 4.7 μm was prepared in the same manner as in example 4, except that the raw material coarse powder was finely pulverized in the same manner as in example 5. The slurry was shaped into a green body, sintered and heat-treated in the same manner as in example 4, to obtain a rare earth permanent magnet having the chemical composition shown in Table 2.

The results of the magnetic properties and the corrosion test conducted in the same manner as in example 1 are shown in Table 2. As can be seen from Table 2, the magnetic properties of the rare earth permanent magnet were almost equal to those of example 5, and no change in the nickel plating layer was observed even after the 1000-hour corrosion test.

Comparative example 7

In the same manner as in example 6 except that nitrogen gas was not introduced into the argon atmosphere. Rare earth permanent magnets having the chemical compositions shown in Table 2 were obtained, and the average particle size of the fine powder was 4.0. mu.m.

The results of the magnetic properties and the corrosion test conducted in the same manner as in example 1 are shown in Table 2. As can be seen from Table 2, although the rare earth permanent magnet has almost the same magnetic properties as those of example 6, the corrosion resistance is extremely poor because the nickel plating layer starts to peel off only after 192 hours.

Comparative example 8

Preparing an alloy strip with a thickness of 0.2-0.5 by a strip casting method, wherein the alloy strip comprises the following chemical components in parts by weight: 30.0% Nd, 0.5% Pr, 1.5% Dy, 1.05% B, 0.8% Nb, 0.2% Al, 3.0% Co, 0.08% Ga, 0.1% Cu, 0.02% O, 0.005% C, 0.005% N, and the balance Fe. After heat treatment at 950 ℃ for 4 hours in an argon atmosphere, the alloy strip was subjected to the same hydrogen absorption, dehydrogenation and mechanical pulverization as in example 4 to obtain raw material coarse powder of 32 mesh or less having the following chemical composition (by weight): 30.0% Nd, 0.5% Pr, 1.5% Dy, 1.05% B, 0.8% Nb, 0.2% Al, 3.0% Co, 0.08% Ga, 0.1% Cu, 0.12% O, 0.02% C, 0.009% N, and the balance Fe.

When 100kg of raw material coarse powder was charged into the jet mill, the internal atmosphere of the jet mill was replaced with nitrogen while controlling the oxygen content in the nitrogen atmosphere to be substantially 0% (at oxygen content)The analysis in the analyzer was 0.001 vol%). Then, the mixture was pressed at a pressure of 7.5kgf/cm²Next, the coarse powder was finely pulverized while feeding the coarse powder into a jet mill at a feed rate of 10 kg/hr.

After completion of the fine pulverization, the fine powder was recovered directly from a jet mill in mineral oil (trade name Idemitsu Super Sol PA-30. manufactured by Idemitsu Co., Ltd.) under a nitrogen atmosphere. The recovered fine powder was made into a slurry having a solid content of 70 wt% by adjusting the amount of mineral oil, and the average particle size of the fine powder was 4.1 μm.

Then, the slurry was wet-pressed in a mold cavity while applying a directional magnetic field of 14kOe and 0.8 ton/cm²And (5) molding. The applied directional magnetic field and the direction of the die pressing are perpendicular to each other to form a blank. During wet pressing, part of the mineral oil was discharged from the upper punch equipped with a die cavity through a 1mm thick cloth filter.

The thus-formed green body was placed in a vacuum furnace at 5.0X 10^-2The residue mineral oil was removed by heating at 180 ℃ for 2 hours. Then, the vacuum degree was 3.0X 10^-4The temperature of the vacuum furnace was raised to 1080 ℃ at a rate of 15 ℃/min under the support, and the temperature was maintained at 1080 ℃ for 3 hours to complete the sintering of the green body, thereby obtaining rare earth permanent magnets having the chemical compositions shown in table 2.

Further, the rare earth permanent magnet was heat-treated at 900 ℃ for 2 hours and 550 ℃ for 1 hour in argon gas.

The results of the same corrosion test as in example 1 are shown in Table 2, and from Table 2, it can be seen that although the rare earth permanent magnet is excellent in magnetic properties, it is extremely poor in corrosion resistance because the nickel plated layer starts to peel off in only 48 hours.

TABLE 2

Chemical composition of magnet (% by weight)

Number Nd Pr DyB Fe Nb Al Co Ga Cu N O C

Examples

427.00.51.51.05 balance 0.350.082.50.090.080.050.160.07

522.32.05.51.00 balance 0.500.202.00.090.100.040.140.06

620.78.61.21.05 balance-0.082.00.090.100.070.180.07

722.05.01.51.10 balance-1.002.5- -0.060.170.07

827.00.51.51.05 balance 0.350.082.50.090.080.040.140.06

922.32.05.51.0 balance 0.500.202.00.090.100.030.120.06

Comparative example

720.78.61.21.05 balance-0.082.00.090.100.010.180.07

830.00.51.51.50 balance-0.203.00.080.100.060.150.07

Table 2 (continuation)

Magnetic crystal grain area ratio (%) corrosion resistance

The number Br iHc (BH) max is less than or equal to 10 mu m and more than or equal to 13 mu m

(kG) (kOe) (MGOe)

Examples

413.814.045.99342500 hours later, the nickel plating layer has no change

512.723.039.09532500 hours later, the nickel plating layer has no change

613.615.545.09052500 hours later, the nickel plating layer has no change

713.913.646.68872000 hours later, the nickel plating layer has no change

813.613.544.678121200 hours later, the nickel plating layer has no change

Slight flaking off after 2000 hours

912.722.538.850441000 hours later, the nickel coating layer has no change

After 2000 hours, the part is peeled off

Comparative example

713.615.745.0924192 hours later, the nickel plating layer peeled off

813.216.542.192448 hours later, the nickel plating layer peeled off

Example 10

An alloy strip having a thickness of 0.1 to 0.3mm and having a chemical composition (alloy A) shown in Table 3 was prepared by a strip casting method in which a mixture containing Nd, Pr, B, Ga, Cu, and Fe powders each having a purity of 95% or more was melted by induction heating under an argon atmosphere, an alloy melt was injected on the circumferential surface of a copper rotary chill roll under an argon atmosphere to form an alloy strip thereon, and the alloy strip (alloy A) was placed in a vacuum furnace under a vacuum degree of 5X 10^-2Heat treatment was carried out at 1000 ℃ for 4 hours under torr.

In addition, a melt obtained by induction heating a mixture containing Nd, Pr, Dy, and Co powders having a purity of 95% or more in an argon atmosphere was cast into alloy B having the chemical composition shown in table 3.

TABLE 3

Chemical composition of alloy (% by weight)

Alloy Nd Pr DyB Nb Co Ga Cu O N C Fe

A27.50.45-1.17- -0.090.110.0100.0040.005 remainder

B31.50.5015- -20- -0.0120.0060.003 in balance

The alloy A and the alloy B were allowed to respectively absorb hydrogen in a vacuum furnace, heated to 500 ℃ while evacuating the furnace, cooled to room temperature, and coarsely pulverized to obtain coarse powders of 32 mesh or smaller.

The coarse powders of alloy A and B were uniformly mixed in a V-type mixer to prepare a mixed raw material powder containing 90 wt% of alloy A and 10 wt% of alloy B.

When the mixed raw material powder was charged into the jet mill, the internal atmosphere of the jet mill was replaced with nitrogen while controlling the oxygen content in the nitrogen atmosphere to be substantially 0% (0.001 (volume)%, as analyzed in an oxygen analyzer). Then, at 7.0kgf/cm²The mixed raw material powder was finely pulverized while being charged into a jet mill under pressure at a feed rate of 10 kg/hr.

After completion of the fine pulverization, the fine powder was recovered directly from a jet mill in a mineral oil (trade name Idemitsu Super Sol PA-30. manufactured by Idemitsu Co., Ltd.) under a nitrogen atmosphere. The recovered fine powder was made into a slurry having a solids content of 78 wt% by adjusting the amount of mineral oil, and the average particle size of the fine powder was 4.5 μm.

Then, the slurry was wet-pressed in a mold cavity while applying an oriented magnetic field of 12kOe and 0.8 ton/cm²And (5) molding. The applied directional magnetic field and the direction of the die pressing are perpendicular to each other to form a blank. During wet pressing, part of the mineral oil was discharged from the upper punch equipped with a die cavity through a 1mm thick cloth filter.

The thus-formed green body was placed in a vacuum furnace at 5.0X 10^-2The residue mineral oil was removed by heating at 200 ℃ for 1 hour under vacuum. Then, the vacuum degree was 5X 10^-5And (3) under the support, heating the vacuum furnace to 1070 ℃ at the speed of 15 ℃/min, and preserving the temperature for 2 hours at 1070 ℃ to finish the sintering of the blank.

The sintered product was then heat-treated at 900 ℃ for 2 hours and 550 ℃ for 1 hour in an argon atmosphere. Rare earth permanent magnets having the chemical compositions shown in Table 4 were obtained.

The magnetic properties after processing and the corrosion resistance evaluated in the same manner as in example 1 are shown in Table 5. As can be seen from Table 5, the rare earth permanent magnet has good magnetic properties. As is clear from comparison of the magnetic properties of example 10 with those of example 11 described below, it is preferable that the raw material powder be a mixed powder of different alloys because the magnetic properties can be further improved. In addition, it can be seen from the results of corrosion tests that the rare earth permanent magnet produced above shows good corrosion resistance.

Comparative example 9

The same mixed powder as used in example 10 (alloy a: alloy B: 90: 10 (by weight)) was finely pulverized in the same manner as in example 10 except that the fine powder was recovered from the jet mill into an empty container without using a solvent. In such dry recovery, since the fine powder may be burned by contacting with air when the oxygen content in the jet mill is too low, fine pulverization is carried out with oxygen being supplied so that the oxygen content in the nitrogen atmosphere is maintained at 0.1 (vol)%. The mean particle size of the thus prepared fine powder was 4.5. mu.m.

The dry fine powder was dry-pressed in a mold cavity while applying an oriented magnetic field of 12kOe and 0.8 ton/cm²And (5) molding. The applied orienting magnetic field and the embossing direction are perpendicular to each other.

At 5.0X 10^-5The thus-obtained green body was held at 1070 ℃ for 2 hours under a degree of vacuum to be sintered, and then subjected to a two-stage heat treatment in the same manner as in example 10; thus, rare earth permanent magnets having chemical compositions shown in Table 4 were obtained. The chemical composition of the rare earth permanent magnet thus obtained was almost equal to that of example 10 except that the oxygen content (0.612%) and the carbon content (0.045%) were different.

As shown in Table 5, the rare earth permanent magnet was inferior in magnetic properties (Br, iHc and (BH) max) to example 10. The reasons for such deterioration of magnetic properties are as follows: in that the fine powder is oxidized at the time of recovery. As a result, the amount of liquid phase generated for sintering is insufficient at the time of sintering. The lack of a liquid phase during sintering results in a low density of the sintered product, resulting in failure to provide a sintered magnet having excellent magnetic properties. Therefore, although the mixed powder is used as a raw material, high magnetic properties cannot be obtained because the fine powder is recovered and dry-pressed. On the other hand, in example 10, the fine powder prepared in the low oxygen atmosphere was recovered in the form of slurry and wet-pressed to be formed into a green body. Therefore, it can be seen that the rare earth permanent magnet having high magnetic properties can be obtained by the process of the present invention which comprises wet recovery of fine powder and wet pressing of slurry.

Example 11

A rare earth permanent magnet having almost the same chemical composition as that of example 10 was prepared from a single alloy raw material powder as follows.

A mixture of Nd, Pr, Dy, B, Co, Ga, Cu, and Fe metal powders each having a purity of 95% or more was strip-cast under the same conditions as in example 10 to prepare an alloy strip having the following chemical composition (by weight): 27.9% Nd, 0.46% Pr, 1.5% Dy, 1.05% B, 2.0% Co, 0.08% Ga, 0.10% Cu, 0.2% O, 0.005% C, 0.003% N, and the balance Fe.

According to the same operation as in example 10, rare earth permanent magnets having chemical compositions shown in Table 4 were prepared. The chemical composition of the rare earth permanent magnet thus obtained was almost equal to that of comparative example 9 except that the oxygen content was 0.170% and the carbon content was 0.063%.

As shown in table 5, both the magnetism and the corrosion resistance of the rare earth permanent magnet are sufficiently excellent.

TABLE 4

Chemical composition of magnet (% by weight)

Number Nd Pr Dy B Nb Co Ga Cu O C N Fe

Examples

1027.90.461.51.05-2.00.080.100.0960.0630.067 balance

1127.90.461.51.05-2.00.080.100.1700.0630.065 balance

Comparative example

927.90.461.51.05-2.00.080.100.6120.0450.065 balance

TABLE 5

Preparation method of magnetic density corrosion resistance

Number Br iHc (BH) max (g/cc)

Pressing of raw materials (kG) (kOe) (MGOe)

Examples

10 mixed powder wet method is unchanged after 14.116.347.57.602500 hours

11 single powder wet method is unchanged after 13.915.046.07.582500 hours

Comparative example

9 the mixed powder is unchanged after 13.511.543.37.422500 hours of dry method

Example 12

In the same manner as in example 10, a slurry having a fine powder with an average particle size of 4.1 μm was prepared from a mixed raw powder composed of 85% by weight of alloy C and 15% by weight of alloy D, and the chemical compositions of both alloys C and D are shown in Table 6.

TABLE 6

Chemical composition of alloy

Alloy Nd Pr DyB Nb Co Ga Cu O N C Fe

C27.00.40-1.18- -0.100.120.0110.0040.004 for the rest

D5.50.5040- -20- -0.0130.0060.003 as the rest

The slurry was wet-pressed to be formed into a green body in the same manner as in example 10. In a vacuum furnace at 5.0X 10^-2Vacuum degree of 5.0X 10 after heating at 200 deg.C for 1 hr to remove residual mineral oil^-5The green body was heated to 1080 ℃ at a heating rate of 15 ℃/min and sintered at 1080 ℃ for 2 hours. The sintered product was further heat-treated at 900 ℃ for 2 hours and 480 ℃ for 1 hour under an argon atmosphere to obtain rare earth permanent magnets having the chemical compositions shown in Table 7.

The magnetic properties after processing and the corrosion resistance evaluated in the same manner as in example 1 are shown in Table 8. As can be seen from Table 8, the rare earth permanent magnet has good magnetic properties. From the comparison of the magnetic properties of example 12 with example 13 described below, it can be seen that the raw material powder is preferably a mixed powder of different alloys because the magnetic properties can be further improved. From the results of the corrosion test, the rare earth permanent magnet prepared as described above showed good corrosion resistance.

Comparative example 10

The same mixed powder as used in example 12 was processed in the same manner as in comparative example 9 to obtain a fine powder having an average particle size of 4.1. mu.m. The fine powder was subjected to dry pressing and sintering in the same manner as in comparative example 9 except that the sintering temperature was 1080 ℃. The sintered product was subjected to the same heat treatment as in example 12 to obtain a rare earth permanent magnet having a chemical composition shown in Table 7, which was almost equal to that of example 12 except that the oxygen content and the carbon content were different.

The magnetic properties after processing and the corrosion resistance evaluated in the same manner as in example 1 are shown in Table 8. For the same reason as described in example 9, the rare earth permanent magnet was inferior in magnetic properties (Br, iHc and (BH) max) to those of example 12.

Example 13

A rare earth permanent magnet having a chemical composition almost equal to that of example 12 was prepared from the following single alloy raw material powder.

Under the same conditions as in example 12, alloy tapes having the following chemical compositions (by weight) were prepared by tape casting metal powder mixtures of Nd, Pr, Dy, B, Co, Ga, Cu and Fe each having a purity of 95% or more: 23.8% Nd, 0.42% Pr, 6.0% Dy, 1.00% B, 3.0% Co, 0.09% Ga, 0.09% Cu, 0.18% O, 0.006% C, 0.002% N, and the balance Fe.

In the same manner as in example 12, rare earth permanent magnets having chemical compositions shown in Table 7 were prepared. The chemical composition of the rare earth permanent magnet thus obtained was almost equal to that of example 12 except that the oxygen content was 0.182%.

As shown in table 8, both the magnetic properties and the corrosion resistance of the rare earth permanent magnet were sufficiently excellent.

TABLE 7

Chemical composition (by weight) of magnet

Number Nd Pr Dy B Nb Co Ga Cu O C N Fe

Examples

1223.80.426.01.00-3.00.090.090.0940.0640.066 balance

1323.80.426.01.00-3.00.090.090.1820.0650.064 balance

Comparative example

1023.80.426.01.00-3.00.090.090.6120.0470.064 balance

TABLE 8

Preparation method of corrosion resistance of magnetic density

Number Br iHc (BH) max (g/cc)

Raw material pressing (kG) (kOe) (MGOe)

Examples

12 mixing wet method for 12.626.237.77.602500 hours without change

13 single material wet method is unchanged after 12.425.036.57.572500 hours

Comparative example

10 mixing material and dry method for 12.124.134.97.472500 hours without change

Claims

1. A rare earth permanent magnet consisting essentially of, by weight: 27.0 to 31.0% of at least one rare earth element including yttrium, 0.5 to 2.0% of B, 0.02 to 0.15% of N, 0.25% or less of O, 0.15% or less of C, at least one optional element selected from 0.1 to 2.0% of Nb, 0.02 to 2.0% of Al, 0.3 to 5.0% of Co, 0.01 to 0.5% of Ga and 0.01 to 1.0% of Cu, and the balance of Fe.

2. A rare earth permanent magnet according to claim 1 having a coercive force (iHc) of 13.0kOe or more.

3. A rare earth permanent magnet according to claim 1 or 2 having a main phase in which the total area of grains having a grain size of 10 μm or less is 80% or more and the total area of grains having a grain size of 13 μm or more is 10% or less, each area percentage being based on the total area of the grains in the main phase.

4. The method for manufacturing the rare earth permanent magnet comprises the following steps:

in a nitrogen atmosphere containing substantially 0% of oxygen or in an argon atmosphere containing substantially 0% of oxygen and 0.0001 to 0.1 vol% of nitrogen at 5 to 10kgf/cm²Finely pulverizing a coarse powder in a pulverizer under pressure, the coarse powder consisting essentially of: 27.0 to 31.0% by weight of at least one rare earth element including yttrium, 0.5 to 2.0% of B, at least one optional element selected from Nb, Al, Co, Ga and Cu, and the balance Fe; simultaneously supplying the coarse powder into a pulverizer at a feed rate of 3-20 kg/hr;

recovering the fine powder in the form of a slurry in a solvent selected from the group consisting of mineral oil, vegetable oil and synthetic oil in a nitrogen atmosphere or an argon atmosphere;

under the condition of simultaneously applying a magnetic field, wet-pressing the slurry to form a blank;

heat treating the green body in a vacuum furnace to remove the solvent therefrom;

sintering the heat-treated green body in the vacuum furnace.

5. The process of claim 4 wherein said meal is prepared by the steps of:

casting an R-Fe-B based alloy melt strip into an alloy strip having a thickness of 1mm or less;

carrying out heat treatment on the alloy strip at 800-1100 ℃ in an inert gas atmosphere or vacuum; and

coarsely crushing the heat-treated alloy strip.

6. A method according to claim 5, characterized in that said coarse comminution of said heat-treated alloy strip is carried out by natural cracking of said alloy by hydrogen absorption followed by dehydrogenation of said cracked alloy.

7. The method of claim 6, wherein the slurry is wet pressed by compression molding.

8. The method according to claim 4, characterized in that the flash point of the solvent for the slurry at 1 atmosphere is 70 ℃ or more and less than 200 ℃, the fractionation point is 400 ℃ or less, and the kinematic viscosity at ordinary temperature is 10cSt or less.