EP0392077B1

EP0392077B1 - Magnetically anisotropic hot-worked magnets and composition and method for their production

Info

Publication number: EP0392077B1
Application number: EP19890119269
Authority: EP
Inventors: Katsunori Iwasaki; Shigeho Tanigawa; Masaaki Tokunaga
Original assignee: Hitachi Metals Ltd
Current assignee: Proterial Ltd
Priority date: 1989-04-14
Filing date: 1989-10-17
Publication date: 1994-08-31
Anticipated expiration: 2009-10-17
Also published as: EP0392077A3; EP0392077A2; DE68917906T2; JPH02272703A; JP3047239B2; DE68917906D1

Description

Magnetically Anisotropic Hot-Worked Magnets and Compositions and Method for their Production

The present invention relates to hot-worked permanent magnets consisting substantially of rare-earth elements, transition metals and boron and provided with magnetic anisotropy by hot-working, and more particularly to hot-worked magnets having improved crystal grain orientation and thus having good magnetic properties. The present invention especially relates also to a method of producing such hot-worked magnets without crack formation by adding proper amounts of additives such as graphite powder and glass material having a low melting point to improve workability, and to compositions of matter for use in this method.
Permanent magnets consisting essentially of rare-earth elements, transition metals and boron (hereinafter referred to as "R-T-B" permanent magnets) have been receiving much attention as inexpensive permanent magnets having excellent magnetic properties. This is because intermetallic compounds expressed by R₂T₁₄B having a tetragonal crystal structure have excellent magnetic properties. Nd₂Fe₁₄B, in which Nd is employed as R, has lattice parameters of a₀=0.878 nm and c₀=1.219 nm.
The R-T-B permanent magnets are usually classified into two groups: Sintered magnets and rapidly quenched magnets. Whichever production method is utilized, it is necessary to form them to desired shapes. In this sense, they should have good workability. In order to improve the workability of the magnets, the addition of lubricating agents has conventionally been conducted. The lubricants are classified into external lubricants which are applied to die surfaces or surfaces of magnet products to be formed to reduce friction between the die surfaces and the magnet products being formed, and internal lubricants which are in the form of powder, liquid, solid, etc. and are added to the magnet products to be formed to reduce friction between the powder particles.
EP-A-133 758 discloses the coating of a die surface with graphite as an external lubricant for hot die-upsetting, to improve the workability of magnets in the hot-working process, thereby obtaining hot-worked magnets free from cracks. The effects of graphite on the inner lubrication of the magnets are not referred to. US-A-4 780 226 discloses a method of producing hot-worked magnets wherein there is used a complex additive of graphite and glass material as an external lubricant for hot die-upsetting, to improve the workability of magnets in the hot-working process. In the method, a glass powder material having a melting point which is lower than the hot-working temperature, or a mixture of glass powder and graphite powder is sprayed on the surfaces of punches and dies to form a green body of magnet material.
In the case of sintered magnets, stearic acid is widely used as an internal lubricant (JP-A-61-34101). Stearic acid is a saturated aliphatic acid having the formula CH₃(CH₂)₁₆COOH. It is also known to suppress the growth of crystal grains and simultaneously increase the density of the resulting magnet in the sintering step by adding carbon powder or a powder of carbide-forming components such as Ti, Zr, Hf, etc. to form metal carbides (JP-A-63-98105).
However, if sintered magnets are to be provided with magnetic anisotropy, a pressing step in a magnetic field must be conducted, limiting the shapes of magnets to be formed. In view of this fact, much attention has been paid to rapidly quenched magnets which do not need to be pressed in a magnetic field, particularly permanent magnets obtained by pulverizing thin ribbons or flakes produced from melts of R-T-B alloys by a rapid quenching method, hot-pressing them (high-temperature treatment) and then subjecting them to plastic working at high temperature to provide them with magnetic anisotropy, which will be called "hot-worked magnets" hereinafter (EP-A-133 758).
The individual thin ribbons or flakes produced by such a rapid quenching method usually contain innumerable fine crystal grains. Even though the thin ribbons or flakes produced by rapid quenching are in various planar shapes of 30 µm in thickness and 500 µm or less in length, the crystal grains contained therein are as fine as 0.02-1.0 µm as an average grain size, which is smaller than the average grain size of 1-90 µm in the case of sintered magnets (see, for instance, EP-A-126 179). The average grain size of the rapidly quenched magnets is close to 0.3 µm, the critical size of a single domain of the R-T-B magnet, which means that it provides essentially excellent magnetic properties.
In the case of hot-working rapidly quenched magnetic materials, it is important that there is a close relationship between the direction of plastic flow and magnetic orientation perpendicular to the direction of the plastic flow. Further, it is necessary to cause the plastic flow uniformly in the entire magnet to be worked, in order to improve the orientation of the crystal grains which strongly influence the magnetic properties. Incidentally, a non-uniform deformation may cause bulging of the magnets in the plastic working process, which in turn produces large and/or many cracks in the peripheral portions of the magnets. This is a serious problem when hot-worked magnets are to be formed into the shape of final products. Most of the force applied in hot-working is used for plastic deformation, but part of the force is consumed by friction between the particles. This may be a partial cause of the above bulging phenomenon.
Various types of internal lubricants for hot-worked magnets are known in the art. EP-A-195 219 discloses a rapidly quenched hot-worked permanent magnet of the R-T-B type in which each particle of the powder material used for the preform may be coated with an inorganic or organic lubricant. Examples of suitable lubricants given are graphite and molybdenum disulfide. JP-A-60-184 602 discloses the use of polyethylene glycol monolaurate to increase the formability of sintered magnets.
In the above-mentioned conventional techniques, external lubricants such as graphite and/or glass applied to the die surface for die lubrication to reduce friction between a work body and surfaces of tools (dies and punches) only partly, if at all, attach to the thin ribbons or flakes produced by a rapid quenching method, which are 30 µm or so in thickness and 500 µm or less in length, much less to the innumerable fine crystal grains inside the thin flakes. Hence, external lubricants do not play a role as an inner lubricant to reduce occurence of cracks in a magnet produced by hot-working.
Incidentally, in the case of adding carbon powder or powder of carbide-forming components such as Ti, Zr, Hf, etc. to sintered magnets, it is expected that such powder is relatively easily dispersed in magnet powder by appropriately selecting the powder shape and the mixing method. The same is true of stearate. This is because in the case of sintered magnets, magnetic powder particles produced by pulverizing alloy ingots are in a shape close to spherical. However, unlike the sintered magnets produced by powder metallurgy method in which compacting is conducted at room temperature, hot-working such as by die-upsetting, is usually conducted at as high a temperature as 600-850 °C. Accordingly, lubricants dispersed among thin flakes show essentially different behavior, and this has not yet been appreciated.
In addition, the conventional techniques in which an external lubricant is applied to a die surface do not show effects peculiar to the hot-working of the magnets, but they simply show effects of lubricants which slightly decrease the friction between the die surface and materials being worked. In fact, there has been no report so far with respect to the improvement of workability without significant cracking and the improvement of uniform orientation, in the field of hot-working rapidly quenched magnet ribbons or flakes.
Accordingly, it is the object of the present invention to provide hot-worked magnets made of an R-T-B alloy free from cracks and with excellent magnetic characteristics, a method of producing such hot-worked magnets, and compositions of matter to be used in this method as starting materials, including a new type of lubricant.
The above object is achieved according to the independent claims. The dependent claims relate to preferred embodiments.
The method according to the present invention for producing fine-grained magnetically anisotropic hot-worked magnets of an average grain size of ≦ 1 µm comprises the following steps:

(I) Rapidly quenching a melt of an R-T-B type alloy, wherein R is selected from the rare-earth elements including yttrium, and mixtures thereof, T is a transition metal, and B is boron, to form thin ribbons or flakes,
(II) pulverizing the rapid-quenched melt ribbons or flakes to form a magnetic powder,
(III) mixing the magnetic powder with an internal lubricant,
(IV) forming a green body from the mixture, and
(V) hot-working the green body to form a magnetically anisotropic magnet;

a glass component consisting of at least one glass material, and
a carbon-based material.

According to a preferred embodiment, the component R of the R-T-B type alloys is Nd, and the boundary layers include neodymium carbide.
In accordance with another preferred embodiment, an internal lubricant is used in step III of the method comprising
a glass component having a low melting point and particularly a softening point below the temperature at which precipitation of an R-rich phase occurs in the R-T-B alloy.
The softening point of the glass component is advantageously 500 to 800 °C, preferably less than 650 °C, and more preferably 550 °C.
The fine-grained magnetically anisotropic hot-worked magnets according to the present invention comprise an R-T-B type alloy, wherein R is selected from the rare-earth elements including yttrium, and mixtures thereof, T is a transition metal, and B is boron, each of the grains of which is at least partially surrounded by a boundary layer comprising a carbide material dispersed in a glass material.
They are obtainable according to the above-defined method.
The magnets of the present invention preferably have an average grain size of the fine crystal grains of 0.02 to 1.0 µm, the carbon content is ≦ 0.5 mass-%, and the oxygen content is ≦ 0.3 mass-%, the transition metal T preferably being the main alloy component.
The grains of the magnets preferably have an average aspect ratio greater than about 2.0.
The hot-working in step V of the method of the present invention is preferably carried out at a temperature of 600 to 850 °C and more preferably at 720 to 760 °C.
The compositions of matter of the present invention, particulary for use in the above-defined method, comprise accordingly

(A) an R-T-B type alloy powder, wherein R is selected from the rare-earth elements including yttrium, and mixtures thereof, T is a transition metal, and B is boron, and
(B) an internal lubricant;
they are characterized in that the internal lubricant (B) comprises
- (B1) a glass component consisting of at least one glass material, and
- (B2) a carbon-based material,
the lubricant preferably being homogeneously distributed throughout the R-T-B type alloy powder.

In accordance with a preferred embodiment of the invention, the glass component B1 consists of at least one glass material selected from water glasses PbO-B₂O₃-SiO₂ type glasses, B₂O₃-SiO₂-Bi₂O₃ type glasses, and Deltaglaze® conventionally used in casting processes for Ti metal or extrusion processes for Ti metal at room temperature. The Deltaglaze® is applied in a powder form together with trichloroethylene.
In the following, the invention will be explained with more details with reference to preferred embodiments and the drawings.

Fig. 1: shows magnetic properties such as 4 πI, iHc and (BH)max of a hot-worked magnet depending on the amount of glass added as an internal lubricant;
Fig. 2: shows magnetic properties such as 4 πI, iHc and (BH)max of a hot-worked magnet depending on the amounts of graphite and glass added in combination as internal lubricants;
Fig. 3: shows the relation between coercive force and the amount of oxygen in magnets comprising various amounts of glass and graphite as internal lubricants;
Fig. 4: is a graph showing the relations between the amount of graphite added, carbon content, and oxygen content, for various amounts of glass addition;
Fig. 5: shows a plane view of a conventional hot-worked magnet having cracks at the periphery;
Fig. 6: presents comparative photomicrographs of hot-worked magnets showing fracture planes observed in a direction perpendicular to the compression direction; and
Fig. 7: presents comparative photomicrographs of hot-worked magnets showing fracture planes observed in a direction parallel to the compression direction.

Magnets provided with improved magnetic properties compared with conventional hot-worked magnets can be produced as explained below according to the method of the present invention.
In the following discussion, the role of the glass additive is explained. Often, many spherical shapes or lumps having a black color can be observed in magnets containing graphite as the only internal lubricant component. Although it is difficult to confirm that all the black lumps are flakes or agglomerates of graphite rich material, the lumps increase and tend to be larger as the graphite content added to the magnet increases. This means that graphite rich material powder can become locally concentrated in the magnet without presence of a glass additive.
On the contrary, such lumps have not been observed in magnets having a second additive added in combination, namely a glass material additive, even with the same amount of graphite. It is assumed that the glass material softened by heat during the hot-working process contributes to disperse the added graphite powder uniformly in the magnet. However, this theory is not to be taken as a limitation of the scope of the invention.
Comparing the cases of the addition of only glass, the addition of only graphite, and the combined addition of glass and graphite, as an internal lubricant, the magnetic properties of the resulting magnets are "good", "better" and "best", respectively. The synergism accompanying the combined addition of proper amounts of graphite and glass has been found to provide the magnet with excellent magnetic properties.
Observations of the metallurgical microstructure of magnets produced according to the present invention suggest that the flow of the grains of the magnetic particles is remarkably improved, with the grains becoming more uniformly oriented parallel to the die-upsetting direction because of the lubrication provided by the combined addition of glass and graphite. In addition to helping disperse the graphite material, the glass material component of the combination inner lubricant also contributes to improve the workability of the magnet in and of itself. Thus, the role played by the glass material as an inner lubricant in the present invention is different from the role of the glass material used as an outer lubricant of the previously described conventional processes for hot-worked magnets.
Although the presence or nature of the chemical reaction between the low melting point glass powder and the graphite or powder is not clear at the present, it appears that a kind of catalysis is caused by the combined addition, although, again, this theory is not intended to be a limitation of the scope of this invention. It is believed those "low melting point" glass materials having rather higher softening points provide the magnet with better magnetic characteristics and better workability. It is easy for a person skilled in the art to select such better glass materials on the basis of the glass composition, the softening point and other teachings in the present disclosure, according to composition, shape and other factors of the magnet to be produced.
The graphite powder affects the residual magnetic flux density of the hot-worked magnets produced according to the present invention. However, graphite powder alone mixed with flakes of magnet material tends to reduce the iHc value of the magnet produced by hot-working. Moreover, plastic deformation of the grains in the magnet flakes tends to be hindered and even prevented because of the lumps produced as the graphite content increases.
As explained above, it is necessary to define the respective upper limits of the glass and graphite internal lubricant additives. It is necessary to adjust the amount of glass and the amount of graphite to obtain the preferable O₂ content and carbon content remaining in the magnet as explained below (for example, in Example 3).
However, general observations of the metallurgical structure of the magnets produced according to the present invention teach the following. Although 0.3 mass-% or less of glass material addition has some effect, 0.5 mass-% of glass material addition causes a remarkable effect with regard to the more uniform arrangement of the grains and the orientation of the grains perpendicular to the dieupsetting direction. On the other hand, graphite is effective to make boundaries of the flakes clear as understood by an observation of the fracture plane of the magnets containing 0.1 mass-% of glass material and various amounts of graphite. Although the flake boundaries are difficult to observe in magnets with 0.1 mass-% of glass and no graphite, the flake boundaries become clearer according to the amount of graphite added to the magnet.
The boundaries can be apparently observed in a magnet containing 0.1 mass-% of glass and 0.3 mass-% of graphite, and the boundaries are remarkably distinct in a magnet containing 0.1 mass-% of glass and 0.5 mass-% of graphite.
By observation of magnets containing 0.3 mass-% of graphite and various amounts of a glass material in a range of 0.1 to 0.5 mass-%, the following statement can be made. The flow shape of the rapid-quench magnet material varies depending on the glass content, with the boundaries of the individual flakes being clearer in magnets containing graphite than in magnets having no graphite additive. The magnets containing 0.3 mass-% of graphite and 0.3 mass-% of glass are provided with a more uniform shape of flake-flow than ones comprising 0.3-% mass of graphite and 0.1 mass-% of glass. However, some irregular flows which are not penpendicular to the die-upsetting direction are observed in magnets containing 0.5 mass-% of glass and 0.3 mass-% of graphite.
The above-stated observations of metallurgical structure of the magnets were conducted by means of an electron microscope. The examples are shown in Figs. 6 and 7. Fig. 6 shows the microstructures of the fracture planes observed in a direction perpendicular to the hot-compression direction. Fig. 7 shows the microstructures of the fracture planes observed in a direction parallel to the hot-compression direction. The photomicrographs in the upper column are magnified by 2,000 times, and the photomicrographs in the lower column are magnified by 30,000 times in Figs. 6 and 7. A uniform microstructure is observed in the case of combined addition of 0.3 mass-% of glass material and 0.3 mass-% of graphite (case (b) in the figure), compared with the microstructure in the case of no additive (case (a) in the figure). As shown in case (c) in the figures, a combined addition of 0.3 mass-% of glass and 0.5 mass-% of graphite sometimes causes coarse grains to develop.
The excellent workability and also the excellent magnetic properties of the magnets according to the present invention are affected by the oxygen content and also the carbon content remaining in the magnet. Fig. 3 shows the residual carbon content and the residual oxygen content in magnets containing glass material for various amounts of added graphite. It is believed that the slight increase of residual oxygen content according to the increase of the added graphite content is caused by absorption of water from the air during mixing of the flakes and the graphite. The residual carbon content increases linearly with the increase of added graphite and independently of the added glass content. The preferable carbon content and oxygen content remaining in a magnet are, respectively, ≦ 0.5 mass-% and ≦ 0.3 mass-% (3000 ppm) in a magnet provided with good magnetic characteristics as taught by Figs. 4 and 5 which will be explained later in relation to Example 3.
The strain rate affects the magnetic characteristics of a magnet which is hot-worked according to the present invention. Although the external appearance of the formed magnet is not affected by a strain rate of about 0.5 to 0.1 mm/s, the deformation resistance does depend on the strain rate even in the range of 0.5 to 0.1 mm/s. This tendency is pronounced when the strain rate is relatively fast. The coercive force tends to decrease somewhat as the deformation rate is reduced. The residual magnetic flux density and the saturation magnetization are sensitive to the deformation rate. These properties decrease with an increase in the deformation rate, and increase as the deformation rate is reduced. In particular, the rate of increase is enhanced in cases of a deformation rate of 0.006 (l/s) or less. As stated above, a magnet can be provided with a high saturation magnetization and a high residual magnetic flux density, resulting in a maximum energy product as high as 40 MOe, without lowering the coercive force appreciably when it is hot-worked at a low strain rate.
For example, isothermal forging makes such a preferable hot-working step easy. The high degree of orientation of the grains which causes the magnetic anisotropy contributes to improve the magnetic characteristics significantly according to the present invention. The high degree of orientation of grains is observed by X-ray diffraction analysis.
It can be effective in the present invention to add as an inner lubricant component diethylene glycol and other organic lubricants in liquid form. Organic lubricants in liquid form are inferior to the dry powder additives disclosed in this specification because of the following problem. Segregation of oxygen and carbon would occur by virtue of the time lag of the vaporization of oxygen and carbon depending on the speed of heat transfer during the hot-compression process, particularly in cases of large hot-worked magnets. In such cases the characteristics, especially the coercive force of the magnet, are not uniform in the magnet. It is a problem to produce large hot-worked magnets in an industrial scale using liquid lubricants. However, a proper amount of such liquid lubricants can be used in the present invention along with a glass material to produce magnets having excellent characteristics.
The upper limit of the average grain size in a magnet produced according to the present invention is about 1 µm, but a smaller grain size is preferable to provide the excellent magnetic characteristics. Preferably, the average grain size in a magnet according to the present invention is about 0.5 µm. Also, it is difficult at the present time to manufacture a magnet having an average grain size of less than 0.2 µm because of the tendency of the powder to rapidly oxidize. A magnet having an average grain size of more than 1 µm suffers from a reduction in its coercive force.
An excess addition of graphite powder (about 0.5 mass-% or more) can form gross grains distributed in the magnet. The determination of the average grain size can be accomplished by a "cut-method" of microphotography. The average grain size can be calculated by taking an average of about twenty or more values which are obtained using lines arbitrarily marked on the photomicrograph. Each line length is divided by the number of grain particles in that line length to obtain the value for that line, and the values are then averaged. It should be noted that the grain has a flat shape which is shorter in a direction parallel to the C-axis of the crystal, and the above stated average grain size is measured in a plane perpendicular to the C-axis of the crystal. It is instructive to consider the average grain size (a) measured in a plane parallel to the C-axis of the crystal in addition to the average grain size (b) measured in a plane perpendicular to the C-axis. For example, the average grain size of (b) is about 0.2-0.3 µm, and the average grain size of (a) is about 01. µm, giving an aspect ratio b/a of 2 or more, in cases where excellent characteristics of anisotropic bonded magnets are produced, as described in JP-A-62-37378.
An excess addition of graphite (about 0.5 mass-% or more) causes a severe reduction in the aspect ratio of magnets produced according to the present invention. The excess graphite results in an excess amount of carbon remaining in the magnet of more than 0.5 mass-% which reduces the magnetic properties of the magnet substantially. Also an excess amount of oxygen remaining in a magnet causes enhanced deformation resistance which, in turn, results in a severe reduction in the workability of the magnet.
The magnets according to the present invention comprise as main components a transition element T, a rare-earth element R, and boron B. The compositions of the magnets are similar to the compositions disclosed in JP-A-60-100 402 which discloses known hot-worked magnets. In the present invention the transition metal element T can be Co, Ni, Ru, Rh, Pd, Os, Ir and Pt in the narrow sense of the transition element definition, and also an element having an atomic number of 21-29, 39-47, 72-79 and 89 or more, in accordance with a broad definition of transition elements.
Further, the addition of Ga is effective to enhance the magnetic properties of hot-worked magnets produced according to the present invention. "R" can be Nd, Pr as the main constituent, Ce or Didymium can be used to partially substitute for Nd or Pr, and Dy or Tb can be added to enhance thermal stability.
The present invention will be explained in further detail by the following Examples.

Example 1

An alloy having the composition of 14.5 at.-% of Nd, 6 at.-% of B, 7.5 at.-% of Co, 0.75 at.-% of Ga and the balance Fe was produced by arc melting. This alloy melt was ejected onto a single roll rotating at a surface velocity of 30 m/s in an Ar atmosphere to produce irregularly shaped thin flakes of about 30 µm in thickness. As a result of X-ray diffraction measurements, it was found that the thin flakes contained a mixture of amorphous phases and crystalline phases. The thin flakes were then pulverized to produce magnetic powder of 500 µm (32 mesh) or less in particle size, and then spherically shaped particles were removed by a classifier. 150 g of the separated particles were mixed with 0.2 mass-% of graphite powder and 0.3 mass-% of a low melting point glass material in a V-shaped mixer for 10 min. The graphite was flake shaped, and the glass material was an amorphous B₂O₃-SiO₂-Bi₂O₃ type glass. The characteristics of the above-mentioned glass are shown in the following table.

Coefficient of linear expansion: 72·10⁻⁷ cm/cm·°C
Glass trans. temp.: 470 °C
Yielding point: 502 °C
Softening point: 550 °C.

The mixture was pressed in a die under a pressure of 3 t/cm² without applying a magnetic field, yielding green bodies having a density of 5.8 g/cm³, a diameter of 28.5 mm and a height of 40.5 mm.
Each of the resulting green bodies was hot-pressed and subjected to die-upsetting at 740 °C and a compression ratio of 3.90 in a hot-working machine having a capacity of 30 t, to provide magnetic anisotropy.
The obtained magnet samples were evaluated by various methods on the basis of test specimens each having a 0.5 mm x 10.5 mm rectangular shape cut out from each sample. The following are the evaluation methods and apparatus therefore.

(A) Stress(Deformation Resistance)-Strain

Stress values were calculated at a strain value of ε =0.1 on the basis that cracks would not be caused at this level of relative deformation during hot-working and die-upsetting process. A press was used to provide a measured stress value σ from about 40 MPa to about 250 MPa to generate a stress-strain curve from which the stress value σ_0.1 corresponding to a ε =0.1 strain was determined.
Workability in various conditions was compared on the basis of nominal values.

(B) Magnetic properties

The hysteresis loop in the second quadrant of the iHc v. 4 I curve was measured by a B-H tracer. A mean value was calculated as a representative value based on five samples which were cut out from a magnet. A layout of the cut portions used to obtain the samples and their dimensions are shown in Fig. 5 (depicting a prior art specimen which developed peripheral cracks), each sample having a 10.5 mm x 10.5 mm rectangular shape. In Fig. 5, the numerals indicate the cut portions, and the portions numbered as 1, 3, 5, 7, and 9 were actually used as samples. Observations by an optical microscope were conducted for sample 8.

(C) Additive Distribution

Measurements of the carbon content, oxygen content and glass content remaining in the hot-worked magnet were conducted on the basis of magnet powder produced by pulverizing the center portion of the sample magnet using concentration analyzers. The mean value of these measurements on each sample is the representative value of the content.
The distribution of glass was estimated on the basis of the distributions of the element Si and the element Bi which are contained in the low melting point glass used in the experiment. The analysis of Bi and Si was conducted by an EPMA, measuring linearly the values in a plane oriented perpendicular to the die-upsetting direction.

(D) Composition

The observation of the microstructure was conducted on a surface direction, the surface being first ground with emery paper and then mirror polished by buffing.

(E) Fracture plane

Observations of the fracture plane were conducted on the surface in a direction perpendicular to the die-upsetting direction after fracturing a hot-worked magnet, to investigate the flow of grains and grain growth intersecting the boundary of flakes produced by melt-quenching. The composition analysis was conducted by SEM-EDX.

(F) Hardness

The hardness of the hot-worked samples cut out from a magnet were measured by a Micro-Vickers testing apparatus after mirror polishing the surface to be observed. The hardness was estimated on basis of a correlation table of hardness and the length of a diagonal line of a compressed mark formed by a compression pin made of diamond under a load of 1000 g. The measurements were conducted on two surfaces parallel to the die-upsetting direction and two surfaces perpendicular to the die-upsetting direction. The mean of ten values measured on ten points each comprising five points in the two parallel planes is the representative value of the hardness in the respective direction. The evaluation results of the examples are as follows:

(A) Stress(Deformation Resistance)-Strain

The stress value was 0.48 to/cm² for the case of a strain rate of 0.1 s⁻¹. It is understood that the deformation resistance decreases with a decrease in the strain rate.

(B) Magnetic properties:

4 πI_r: = 12.3 kG
iHc: = 15.7 kOe
BH max: = 34.6 MOe

(C) Carbon content, oxygen content; carbon distribution, glass distribution Carbon content remaining = 0.32 mass-% Oxygen content remaining = 1700 ppm.

By comparing a magnet obtained using only graphite with a magnet obtained with use of graphite and glass as internal lubricants, it was confirmed that the graphite was more uniformly distributed in the latter than in the former magnet. Glass is distributed uniformly in the magnet.

(D) Metallurgical Structure

A microstructure having a uniform composition flow was observed. The Vicker's hardness of the magnet was 650 Hv.

(E) Fracture Plane

The flow of flakes by the hot-working step was confirmed.

(F) Hardness

The hardness of the magnet made in accordance with the present invention was 650 Hv measured by Vicker's hardness test. The hardness of a magnet comprising no glass and no graphite was 580 Hv. Although the magnet according to the present invention is provided with a higher hardness, it does not become brittle.

Comparative example

Magnetic characteristics were measured on magnets produced using only low melting point glasses for internal lubricant additives, that is, without graphite additives. The experimental results are shown in Fig. 1. The figure shows magnetic properties of magnets in dependence of the glass amount added. As shown in Fig. 1, the residual magnetic flux density and the maximum energy product increase as the content of the glass additive increases. The peak value of 4 πI_r and BH max can be observed at a glass amount of 0.3 mass-%. The 4 πI_r value and the BH max value are, respectively, 320 G and 2 MOe higher than the corresponding properties of a magnet having no additives. The intrinsic coercive force decreases only slightly as the glass amount increases, the iHc value remaining as high as 10900 Oe at the glass amount of 0.5 mass-%.

Example 2

Example 1 was repeated except that various amounts of graphite powder were used with various amounts of a low melting point glass material. With respect to each of the resulting magnetically anisotropic hot-worked magnets, magnetic properties were measured to evaluate the effects of the additives. In Fig. 2, the dependence of the magnetic characteristics on the glass amount are shown, measured at 0.1, 0.3 and 0.5 mass-% of glass amount, each with various amounts of graphite powder.
It becomes clear from Fig. 2 that as the amount of graphite increases, the residual magnetic flux density and the maximum energy product increase almost linearly at first, and that the maximum values of 4 I_r and BH max correspond to the 0.3 mass-% of graphite in cases of 0.1 mass-% of glass additive and 0.3 mass-% of glass additive.
Among the magnetic properties, the 4 πI_r value is improved by 910 G, and BH max is improved by 5.9 MOe as compared with the case of no additive when 0.3 mass-% of graphite and 0.3 mass-% of glass is added. When the amount of glass added was 0.5 mass-%, the residual magnetic flux density rapidly increased with increasing amounts of added graphite, but remarkably decreased when graphite amounts reached about 0.5 mass-%.
On the contrary, the iHc value decreases rapidly as graphite content increases. The tendency for iHc to decrease is pronounced when the graphite additive and glass additive amounts each were 0.5 mass-%. The iHc value was 15430 Oe in case of 0.3 mass-% of glass and 0.3 mass-% of graphite, lower by about 2590 Oe compared with the case of no additive.
The amount of graphite powder is preferably less than 0.5 mass-% because an iHc value of at least 10 kOe is necessary for a practical magnet having sufficient heat resistance. A maximum of BH max can be obtained with addition of 0.3 mass-% of graphite powder.

Example 3

It is important for providing a preferable residual O₂ content and a preferable residual carbon content in the hot-worked magnet, in order to enhance the magnetic characteristics, to add the graphite powder and the low melting point glass as inner lubricants in proper amounts. In the same hot-working process as in Example 1, the residual oxygen content and residual carbon content were changed, in order to investigate the effects on iHc. The experiments were conducted on samples in which the contents of graphite powder and of glass material were changed. The experimental results are shown in Fig. 3.
Fig. 4 shows correlations between the residual oxygen content, the residual carbon content and the amounts of graphite and glass added to the magnet. As the residual oxygen content does not strongly depend on the graphite content, the increase of oxygen content by graphite additive can be neglected, which is in contrast to the case of a complex additive of organic lubricant and glass. The residual carbon content is a strong function just of the graphite amount added. As stated above, the oxygen content is considered to depend only on the glass amount added.
The iHc value of a hot-worked magnet decreases as the graphite amount added or glass amount added increases. The tendency of iHc to decrease with increasing amounts of added graphite is not as pronounced in the case of concurrent glass addition as in case of the sole addition of graphite. Thus, it is important to consider the balance of oxygen and carbon even in case of combination additives as in the present invention. Simply stated, the decrease in iHc by carbon or oxygen can not be avoided because these elements react with e.g. the Nd component which is necessary to increase the coercive force. For example the maximum amount of glass additive is 0.4 mass-% in a case where a coercive force of 16 kOe is necessary and the graphite amount is 0.2 mass-%, according to the data in Fig. 3.

Claims

A method for producing fine-grained magnetically anisotropic hot-worked magnets of an average grain size of ≦ 1 µm by
(I) rapidly quenching a melt of an R-T-B type alloy, wherein R is selected from the rare-earth elements including yttrium, and mixtures thereof, T is a transition metal, and B is boron, to form thin ribbons or flakes,

(II) pulverizing the rapid-quenched melt material to form a magnetic powder,

(III) mixing the magnetic powder with an internal lubricant,

(IV) forming a green body from the mixture, and

(V) hot-working the green body to form a magnetically anisotropic magnet,
characterized in that in step III,
an internal lubricant is used comprising
- a glass component consisting of at least one glass material,
and

- a carbon-based material.
The method according to claim 1, characterized in that in step III, an internal lubricant is used comprising a glass component having a softening point below the temperature of which precipitation of an R-rich phase occurs in the R-T-B alloy.
The method according to claim 1 or 2, characterized in that in step III, an internal lubricant is used comprising a glass component having a softening point of 500 to 800 °C, and preferably of 550 °C.
The method according to one of claims 1 to 3, characterized in that in step III, an internal lubricant is used comprising a glass component consisting of at least one glass material selected from water glasses, PbO-B₂O₃-SiO₂ type glasses, B₂O₃-SiO₂-Bi₂O₃ type glasses, and Deltaglaze ®.
The method according to one of claims 1 to 4, characterized in that in step III, the internal lubricant is used comprising the glass component in an amount of 0.1 to 0.5 mass-% and preferably 0.1 to about 0.3-mass-%, and comprising the carbon-based material in an amount of 0.1 to 0.3 mass-% and preferably 0.1 to 0.3 mass-%, each based on the total mass of the mixture components.
The method according to one of claims 1 to 5, characterized in that step V is carried out at a temperature of 600 to 850 °C and preferably at 720 to 760 °C.
The method according to one of claims 1 to 6, characterized in that in step III, kind and amounts of the glass component and of the carbon-based material are selected in accordance with the desired residual oxygen content and residual carbon content.
The method according to one of claims 1 to 7, characterized in that in step III, graphite is used as carbon-based material.
The method according to one of claims 1 to 8, characterized in that in step I, an R-T-B type alloy is used wherein R is Nd.
Fine-grained magnetically anisotropic hot-worked magnets of an average grain size of ≦ 1 µm, comprising an R-T-B type alloy, wherein R is selected from the rare-earth elements including yttrium, and mixtures thereof, T is a transition metal, and B is boron, wherein
the grains are at least partially surrounded by a boundary layer comprising a carbide material dispersed in a glass material.
Magnets according to claim 10, wherein R is Nd, and the boundary layers include neodymium carbide.
Magnets according to claim 10 or 11, wherein the average grain size of the fine crystal grains is 0.02 to 1.0 µm, the carbon content is ≦ 0.5 mass-%, and the oxygen content is ≦ 0.3 mass-%.
Magnets according to claim 12, characterized in that they comprise an element of the glass material used.
Magnets according to one of claims 10 to 13, characterized in that the grains have an average aspect ratio greater than 2.0.
Compositions of matter for use in forming fine-grained hot-worked anisotropic magnets,
comprising
(A) an R-T-B type alloy powder, wherein R is selected from the rare-earth elements including yttrium, and mixtures thereof, T is a transition metal, and B is boron,
and

(B) an internal lubricant,
particulary for use in the method according to one of claims 1 to 9,
characterized in that the internal lubricant (B) comprises
(B1) a glass component consisting of at least one glass material, and

(B2) a carbon-based material.
The compositions according to claim 15, characterized in that the internal lubricant is homogeneously distributed throughout the R-T-B type alloy powder.
The compositions according to claim 15 or 16, characterized in that the glass component B1 has a softening point below the temperature at which precipitation of an R-rich phase occurs in the R-T-B alloy.
The compositions according to one of claims 15 to 17, characterized in that the glass component B1 has a softening point of 500 to 800 °C, preferably less than 650 °C and more preferably of 550 °C.
The compositions according to one of claims 15 to 18, characterized in that the glass component B1 consists of at least one glass material selected from water glasses, PbO-B₂O₃-SiO₂ type glasses, B₂O₃-SiO₂-Bi₂O₃ type glasses, and Deltaglaze®.
The compositions according to one of claims 15 to 19, characterized in that the internal lubricant B is comprised in such an amount that the amount of the glass component B1 is 0.1 to 0.5 mass-% and preferably 0.1 to 0.3 mass-%, and the amount of the carbon-based material is 0.1 to 0.5 mass-% and preferably 0.1 to 0.3 mass-%, each based on the total mass of the compositions.
The compositions according to one of claims 15 to 20, characterized in that the carbon-based material is graphite.
The compositions according to one of claims 15 to 21, characterized in that it comprises an R-T-B type alloy wherein R is Nd.