An Nd-Fe-B sintered magnet and method for producing the same

Abstract: Nd containing R-Fe-B sintered magnet which has 0.5 %/°C or more of temperature-coefficient of coercive force (iHc) and a composi­ tion that R=11-18 at% (R is one or more rare-earth elements except for Dy, with the proviso of 80 at%≦(Nd+Pr)/R≦100 at%), B=6-12 at%, and balance of Fe and Co (with the proviso of Co is 25 at% or less relative to the total of Co and Fe (including 0 % of Co)) and impurities, is improved to have 15 kOe or more of coercive force (iHc) by means of further containing 2 - 6 at% of V and modifying the minority phase such that B in excess of a stoichiometric composition of R₂Fe₁₄B compound-phase essentially does not form RFe₄B₄-compound minority phase but forms a finely dispersed V-T-B compound minority phase (T is Fe, and in a case of containing Co, T is Fe and Co).

Description: The present invention relates to a permanent magnet, more particularly an Nd-Fe-B sintered magnet, as well as to a method for producing the same. In the Nd-Fe-B magnets there are melt-quenched magnets and sintered magnets. Essentially, the melt-quenched magnet is magnetically isotropic. There is a proposed method for rendering the melt-quenched magnet anisotropic, residing in crushing a strip obtained by melt-quenching to produce powder, hot-pressing and then die-upsetting the powder. This method is however not yet industrially carried out, since the production steps are complicated. Nd-Fe-B sintered magnets were developed by the present inventor et al. They have outstanding characteristics in that they exhibit excellent magnetic properties in terms of 50 MGOe of maximum energy product (BH)max in a laboratory scale and 40 MGOe even in a mass production scale; and, the cost of raw materials is remarkably cheaper than for the rare-earth cobalt magnet, since the main components are cheap elements such as Fe and B, and Nd (neodymium) and Pr (praseodymium), whose yielding amount is relatively high in the rare earth elements. Representative patents of the Nd-Fe-B sintered magnet are Japanese Unexamined Patent Publication No. 59-89401, Japanese Unexamined Patent Publication No. 59-46008 (Japanese Examined Patent Publication No. 61-34242, Japanese Patent 14316170), Japanese Unexamined Patent Publication No. 59-217003), USP No.4597938 and European Patent EP-A-0101552. As an academic paper, there is "New Material for permanent magnets on a base of Nd and Fe (invited)", M. Sagawa et al, J. Appl. Phys., 55, No.6, Part II, p 2083/2087 (March, 1984). A permanent magnet is exposed, after magnetization, to an inverse magnetic field due to various reasons. A permanent magnet must have a high coercive force in order that irreverible demagnetization does not occur even after exposure to a strong reverse magnetic field. Recently, along with size reduction of and efficiency-increase of appliances, the inverse magnetic field applied to the appliances is increasing more and more. In a motor, for example, a magnet is exposed after its magnetization to a strong self demagnetization, until it is mounted in a yoke. After mounting, the magnet is exposed, during energization, to an inverse magnetic field from a coil and to a magnetic field which corresponds to the permeance of a magnetic circuit. The inverse magnetic field from the coil reaches the maximum at start. When a motor stops due to an excessive load and is then immediately restarted by switching on, the most severe load is applied to the magnet. In order to withstand this and suppress the irreversible demagnetization field, a permanent magnet must have a coercive force as high as possible. Under recent progress of appliances, the level of load, which is required for magnets, is unforeseen heretofore. In an appliance for extracting a strong emission light in an accelerator referred to as an undulator, there is a proposal of structure that completely magnetized plates of permanent magnets are bonded with one another in such a manner that N poles face one another and alternately S poles face one another. Obviously, for such application permanent magnets having a high coercive force are necessary. There is a trend that such use of permanent magnets is increasing more and more in future. The coercive force also has a relationship with the stability of a permanent magnet. When a permanent magnet is allowed to stand after magnetization, irreversible demagnetization occurs little by little. In order to lessen the irreversible change of magnetization with time, coercive force should be as high as possible over the inverse magnetic field under using state. Accordingly, there are more and more requests for permanent magnets having a high coercive force. In addition, when a permanent magnet is exposed to high temperature, since the coercive force lowers at a high temperature, its temperature characteristics become important. Temperature coefficient of coercive force, which exerts an influence upon the temperature-characteristics of coercive force, is from 0.3 to 0.4 %/°C for the melt-quenched strip magnets, and is slightly lower than this value for the melt-quenched and then anisotropically treated strip magnets. Temperature coefficient of coercive force is 0.5%/°C or more for sintered magnets. The temperature-coefficient of a sintered magnet varies depending upon a measurement temperature range and is greater at a lower temperature. The temperature coefficient (β) of the coercive force herein is determined by the following formula. Δ iHc: difference (kOe) of the intrinsic coercive force (iHc) at a temperature change of 20°C to 120°C iHc: intrinsic coercive force at 20°C (kOe) ΔT: temperature difference (100°C) The measuring interval of temperature coefficient of coercive force (iHc) is set from 20 to 120°C, so that the temperature interval becomes 100°C. Since the temperature coefficient of the coercive force (iHc) is 0.5%/°C and is therefore very high for Nd-Fe-B sintered magnets the intrinsic coercive force (iHc), hereinafter referred to as the coercive force (iHc), is lowered at a high temperature, which makes the magnet unusable. Specifically speaking, in case the permeance coefficient being 1, the limiting usable temperature of the Nd-Fe-B sintered magnet is approximately 80 °C. The Nd-Fe-B sintered magnets, whose temperature coefficient of coercive force (iHc) is 0.5 %/°C or more and is very high irrespective of the composition, could therefore not be used at high temperatures and as parts of automobiles and motors used at temperature raising to 120 -130 °C during use. Various devices have been made to enhance the coercive force of Nd-Fe-B sintered magnets. Coercive force (iHc) of the Nd-Fe-B sintered magnet having the standard composition Nd₁₅Fe₇₇B₈ is approximately 6 kOe. Considering that the residual magnetization (Br) of this magnet exceeds 12 kG, the coercive force (iHc)=6kOe is too low so that its application scope is extremely limited. One of the most successful methods for enhancing the coercive force was beat treating the Nd₁₅Fe₇₇B₈ sintered magnet, subsequent to sintering, at 600 °C, which increased the coercive force (iHc) to 12 kOe (M.Sagawa et al. J. Appl. Phys. vol. 55, No.6,15, March 1984). This was a great achievement but even higher coercive force is necessary from a practical point of view. Japanese Unexamined Patent Publication No. 61-295355 discloses a Nd-Fe-B sintered magnet containing a boride phase of BN, ZrB₂, CrB, MoB₂, TaB₂, NbB₂, and the like. According to the explanation in this publication, this is effective for providing a high coercive force to lessen the grain size of a sintered body as far as possible; the boride particles added to the main raw materials incur suppression of grain growth during sintering; and, the coercive force (iHc) increases by 1 - 2 kOe due to the suppressed grain growth. In addition, according to the above publication, it is indispensable for obtaining a permanent magnet having improved magnetic properties that the R₂Fe₁₄B phases are surrounded along their boundary by R rich phases and B rich phases. Japanese Unexamined Patent Publication No. 62-23960 discloses to suppress the grain growth by using such borides as TiB₂, BN, ZrB₂, HfB₂, VB₂, NbB, NbB₂, TaB, TaB₂, CrB₂, MoB, MoB₂, Mo₂B, WB, WB₂, and the like. Nevertheless, only a slight enhancement of coercive force is attained by the technique of suppressing the grain-growth due to addition of these borides. Such borides incur generation of the Nd₂Fe₁₇ phase which is magnetically detrimental. The addition amount of borides is therefore limited to a relatively small amount. Most of the borides, such as BN and TiB, impede the sintering and densification of the sintered product. Explorations have also been made for methods of enhancing the coercive force by means of additive element(s). Virtually all of the elements in the Periodic Table have been tested. The most successful method among them was the addition of heavy rare-earth elements, such as Dy. For example, when 10 % of Nd of Nd₁₅Fe₇₇B₈ is replaced to provide Nd 13.5 Dy 1.5 Fe₇₇B₈, the coercive force (iHc) amounts to ≧ 17 kOe. Because of the discovery that Dy is effective for enhancing the coercive force (iHc), Nd-Fe-B sintered magnet is at present being used in a broad field of application. Various additive elements other than the heavy rare-earth elements were also tested. For example, in Japanese Unexamined Patent Publications Nos. 59-218704 and 59-217305, V, Nb, Ta, Mo, W, Cr and Co were added and heat treatment was carried out in various ways. However, the coercive force (iHc) obtained is low and the effects obtained were exceedingly inferior to those attained by Dy. Al is effective for enhancing the coercive force (iHc), although not as prominent as Dy and Pr, but disadvantageously drastically lowers Curie point. Although Dy provides excellent coercive-force characteristics, the abundance of Dy in ores is approximately 1/20 times of Sm and is therefore very small. If Nd-Fe-B sintered magnets with Dy additive are mass-produced, Dy is used in an amount greater than the amounts of respective elements balanced in the rare-earth resources. There is a danger that the balance is destroyed and the supplying amount of Dy soon becomes tight. Tb and Ho, which belong to rare-earth elements as Dy, have the same effects as Dy, but, Tb is even more rare than Dy and is used for many applications such as opto-magnetic recording material. The effects of Ho for enhancing the coercive force (iHc) is exceedingly smaller than that of Dy. In addition, the resource of Ho is poorer than Dy. Tb and Ho therefore practically speaking cannot be used. As is described hereinabove there are two methods for producing Nd-Fe-B series magnet. According to the melt-quenching method, alloy melt is blown through a nozzle and impinged upon a roll rotating at a high speed to melt-quench the same. A high coercive force is obtained by this method by means of adjusting the rotation number of the roll and the conditions of post-heat treatment after the melt-quenching. The melt-quenched magnet has a grain size of 0.1 µm or less and is fine. Therefore, even if a melt-quenched magnet has the same composition as a Nd-Fe-B sintered magnet, the former magnet is characterized by a higher coercive force than the latter magnet. In addition, the mechanism of the coercive force of the melt-quenched magnet is of the pinning type and hence is different from the nucleation type of the sintered magnet. The temperature coefficient of coercive force (iHc) of a melt-quenched magnet is 0.3 - 0.4 %/°C and is hence lower than 0.5 %/°C or more of the sintered magnet. This is also a feature of the melt-quenched magnet. Contrary to this, the melt-quenched magnet involves a problem in the properties other than the coercive force. That is, the melt-quenched magnet is isotropic in the state as it is. A special technique is necessary for rendering the melt-quenched magnet anisotropic. The isotropic magnet exhibits Br approximately 1/2 times and (BH) max approximately 1/4 times those of an anisotropic magnet and cannot provide a high performance. The hot-pressing and then die upsetting method causes a deformation which aligns the crystal orientation. Although a high performance is obtained by this method, the process is complicated. Generally, the production method of sintered magnet is for example as follows. (a) Melting An alloy ingot having a target composition or alloy ingots having a few kinds of the compositions are obtained. (b) Rough Crushing Roughly crushed powder under 35 - 100 mesh is obtained by a jaw crusher and a disc mill or the like. (c) Fine pulverizing Fine powder having an average grain size of 3 µm or less is obtained by a jet mill or the like. (d) Press under magnetic field Compressing is carried out for example in a magnetic field of 13 kOe with a pressure of 2 ton/cm². (e) Sintering Sintering is carried out in vacuum or Ar gas at 1000 to 1160 °C for 1 - 5 hours. (f) Heat treatment Heat treatment is carried out at 600 °C for 1 hour. Nd-Fe-B sintered magnets produced by such methods as described above have already been industrially produced in large amounts and have been used in magnetic resonance imaging (MRI), office automation (OA) and factory automation (FA) appliances, such as MRI, various motors, actuators (VCM), and a driving part of a printer head. During the sintering process of Nd-Fe-B sintered magnet (hereinafter simply referred to as Nd-Fe-B magnet), the green compact powder is densified. An aim of the densification is as follows. In the well prepared powder, Nd-rich alloy powder, whose melting point is far lower than that of the Nd₂Fe₁₄B main phase, is uniformly dispersed, and by means of the low temperature melting Nd-rich phase liquid phase sintering is realized. The liquid phase of the Nd rich phase is distributed over the surface of the main-phase powder. The liquid-phase sintering enables densification at a relatively low temperature, without incurring grain growth appreciably. Another important function of the Nd rich phase is to repair defects on the surface of the main-phase powder, which defects generate during the pulvering step. The most serious defects on the surface of main-phase powder are Nd-deficient layer formed due to preferential oxidation of Nd. The Nd rich phase supplies, from its liquid phase, Nd to this layer, thereby repairing the defects on the main-phase powder and hence enhancing the coercive force. High densification of the sintered body is attained at a relatively low temperature by the liquid-phase sintering. However, it is desirable that the sintering temperature is high and close to the melting point of the main phase and sintering is carried out for a long time. However, when the sintering is carried out at high temperature and/or for a long time according to the conventional methods, in a case that 3 µm raw materials-powder is used, the crystal grains of the main phase coarsen to 15 µm or more, with the result that the coercive force of Nd-Fe-B magnet is lowered. The coercive force (iHc) of the Nd-Fe-B magnet, which is obtained by an heretofore ordinary sintering method without coarsening the crystal grains of the main phase, is approximately 12 - 13 kOe. The addition amount of borides is therefore limited to a relatively small amount. The conventional Nd-Fe-B magnets are applied for such appliances of OA and FA, where the environment is relatively moderate and the temperature and humidity are low. It is known that the Nd-Fe-B magnets are less liable to rust in dry air than the SmCo magnets (R. Blank and E. Adler: The effect of surface oxidation on the demagnetization curve of sintered Nd-Fe-B permanent magnets, 9th International Workshop on Rare Earth Magnets and Their Applications, Bad Soden, FRG. 1987). The Nd-Fe-B magnet is liable to rust in water or in a high humidity environment. As countermeasures for rusting liability of Nd-Fe-B magnet various surface-treatment methods, such as plating and resin-coating, are employed. However, since every coating of the surface has defects, such as pinholes and cracks, water can intrude through the defects of the coating to the surface of an Nd-Fe-B magnet and then vigorously oxidize the magnet. When oxidation occurs, the properties of the magnet are rapidly deteriorated and, rust, which floats on the surface of a magnet, impedes the functions of an appliance. One of the previously proposed methods for improving the corrosion resistance to water, not referring on a surface treatment, is that Al or Co is added to the Nd-Fe-B magnet. However, Al and Co can improve the corrosion resistance only slightly. The corrosion resistance of Nd-Fe-B magnet is studied also from the view point of structure. Sugimoto et al made a study on the mechanism of water-corrosion of Nd-Fe-B magnet (Corrosion mechanism of Nd-Fe-B magnet alloy. Sugimoto et al, Autumn Lecture Meeting of Japan Institute of Metals. No. 604, (October, 1987)). It has been clarified by this study that the corrosion speed in the water is in the following order of 3 ○>2 ○>1 ○, wherein 1 ○ is the Nd₂Fe₁₄B phase, 2 ○ is the Nd rich-phase (e.g., Nd-10 wt%Fe), and 3 ○ is the NdFe₄B₄ phase (B rich phase), which phases constitute the sintered alloy having a standard composition of 33.3 wt% of Nd, 65.0 wt% of Fe, 1.4 wt% of B, and 0.3 wt% of Al. The Nd-Fe-B magnet with addition of approximately 1.5 % of Dy exhibits at room temperature 17 kOe or more of coercive force (iHc) and approximately 5 kOe of coercive force (iHc) at 120 -140 °C. Although the temperature coefficient of coercive force (iHc), i.e., 0.5 %/°C or more, is not improved by the Dy addition, it is satisfactory that the coercive force (iHc) which can overcome inverse magnetic field, is obtained even at high temperature. Most of rare-earth magnets have approximately 10 kG of residual magnetization. Magnetic circuit is therefore designed under using conditions of the magnet being B/H ≧ 1 and targetting iHc ≧ 5kOe. It has been considered that the Dy addition method is employed for Nd-Fe-B magnet used for an AC motor (R.E. Tompkins and T.W. Neumann. General Electric Technical Information Series, Class 1 Report No. 84crd312. November 1984). When the Nd-Fe-B magnets are used for starter-motors and generators of automobiles as well as general high-power motors, magnetic properties must be stable at 180 - 200 °C, which is an extremely severe environment. As high as 4 % or more of Dy must therefore be added. Since an addition of Dy in such a great amount involves a problem in the supply of Dy resources, the Nd-Fe-B magnet cannot be used for high temperature-applications, such as high-power-motors, automobiles and the like. Japanese Unexamined Patent Publication No. 61-295355, supra, which teaches to suppress the grain growth by borides, recites the following coercive force (iHc). Nd₁₅Fe₈B₇₇ magnet has 14.8 kOe of coercive force (iHc). When 0.3 at % of MoB₂ is added to the above magnet, coercive force (iHc) becomes 15.2 kOe. This coercive force (iHc) is very high. Note, however, the coercive force (iHc) obtained without the addition of MoB₂ is 14.8 kOe and is also very high. Over this value the coercive force is hence increased only 0.4 kOe. In order to obtain the very high coercive force (iHc) of 14.8 kOe, various strict precautions are necessary such as the rare-earth element containing powder is not brought into contact with oxygen at the most, distribution of grain size of powder is made sharp at the most, and further the sintering condition is strictly controlled. It is not practical to set and adjust the process conditions as above. The grain growth during sintering is suppressed and hence the coercive force (iHc) can be enhanced by utilizing borides. According to the disclosure of Japanese Unexamined Patent Publication No. 61-295355 supra, the enhancement of coercive force (iHc) by the suppression of grain growth is 2 kOe at the maximum. Therefore, if the technique for suppressing the grain growth is applied to a magnet (15 at%Nd-77at%Fe-8at%B) heat-treated at 600 °C (coercive force (iHc) is 12 kOe as described above), the coercive force (iHc) obtained is presumably 14 kOe. This value is however unsatisfactory. The present invention starts from the fact that the coercive force (iHc) of the sintered and then heat-treated Nd-Fe-B magnet, whose temperature coefficient of the coercive force (iHc) is 0.5 %/°C or more, is enhanced by 3 kOe or more, by means of using another element than Dy in order to facilitate industrial production. The coercive force (iHc) of such sintered magnet decreases 60% or more upon the temperature rise of 120°C, thereby incurring decrease of the coercive force (iHc) from for example 12 kOe to 4.8kOe or less, while contrary to this, in the melt-quenched magnet, whose temperature coefficient of the coercive force (iHc) is approximately 0.3 %|°C, the decrease of coercive (iHc) force is only 36 % and from 12 kOe to approximately 7.7 kOe upon the temperature rise mentioned above. The inventor has, therefore, recognized that it is essential to enhance the coercive force (iHc) of the Nd-Fe-B sintered magnet having a high temperature-coefficient of the coercive force (iHc). It is therefore an object of the present invention to provide an Nd-Fe-B sintered magnet, in which the coercive force (iHc) is enhanced without use of, or only small use of, Dy. This object is solved by the sintered magnets as described in independent claims 1 and 5. Further advantageous features of these magnets are evident from the dependent claims. The present invention also provides an Nd-Fe-B sintered magnet having an improved corrosion resistance. It is a further object of the present invention to provide a method for producing an Nd-Fe-B sintered magnet, wherein the coercive force (iHc) is enhanced more than heretofore and further an industrial production is facilitated. This object is solved by the method of independent claim 10. Further advantageous features of the method are evident from the dependent claims. The present invention is related to the structure of Nd-Fe-B magnets. In the Nd-Fe-B magnet, the matrix or main phase is the R₂Fe₁₄B compound-phase wherein R is Nd and and the other rare-earth elements. It has been ascertained that, because of strong magnetic anisotropy of this phase, excellent magnetic properties are obtained. In the Nd-Fe-B magnet, the magnetic properties are enhanced at a compositional range, in which both Nd and B are greater than the stoichiometrical composition of R₂ Fe₁₄B compound (11.76 at % of Nd, 5.88 at% of B, and balance of Fe). As is known, the excess Nd forms a minority phase, which is referred to as the Nd-rich phase and has a composition of R= 85 - 97 at %, and Fe in balance if any rare earth element other than Nd, which is contained in the sintered body, is also contained in the composition , and which plays an important role for the sintering and for enhancing the coercive force. In addition, the excess of B forms an Nd₁Fe₄B₄ compound phase which is referred to as the B rich phase. In some documents, the B rich phase is reported as Nd₂Fe₇B₆ or Nd 1.1 Fe₄B₄. It has been made clear that every one of these compounds indicates the identical tetragonal compound. NdFe₄B₄ compound is a non-magnetic tetragonal crystal having the lattice constants of a=0.712 nm and c=0.399 nm. The compound is, however, magnetic at cryogenic temperature. In the conventional Nd-Fe-B sintered magnet, B in an amount greater than the stoichiometric composition of R₂Fe₁₄B compound-phase forms RFe₄B₄ compound phase. In the Nd-Fe-B magnet having the standard composition the formation amount of NdFe₄B₄ compound phase calculated on the phase diagram is approximately 5 %. Enhancement of coercive force by the B rich phase is slight. Dy as well as Tb and Ho enhance the magnetic anisotropy of the R₂Fe₁₄B compound-phase, thereby enhancing the coercive force (iHc) and stability at high temperatures compared with magnets free of Dy and the like. The present inventor further researched and discovered the following. That is, in a Nd-Fe-B magnet with added V having a specified composition the NdFe₄B₄ phase (B rich phase) is suppressed to a minimum amount, and a compound phase other than the NdFe₄B₄ phase, i.e., a V-Fe-B compound phase, whose presence was heretofore unknown, is formed and replaces the NdFe₄B₄ phase. The absolute value of the coercive force (iHc) is exceedingly enhanced and the stability at high temperature is improved due to the functions of both V-Fe-B compound phase and particular composition. An Nd-Fe-B series sintered magnet (Nd-Fe-B magnet) according to the present invention has a temperature-coefficient of coercive force (iHc) of 0.5 %/°C or more and a composition that R=11-18 at% R is one or more rare-earth elements except for Dy, with the proviso that 80 at% ≦ Nd+Pr /R ≦ 100 at%, B=6-12 at%, and the balance being Fe and Co, with the proviso that Co is 25 at% or less relative to the total amount of Co and Fe, including 0% of Co and impurities, and is characterized in that B in excess of a stoichiometric composition of the R₂Fe₁₄B compound-phase essentially does not form the RFe₄B₄-compound minority phase but forms a finely dispersed V-T-B compound minority phase (T is Fe, and in a case of containing Co, T is Fe and Co), and, further, the magnet exhibits 20 MGOe or more of maximum energy product and 15 kOe or more of coercive force (iHc). Another Nd-Fe-B series sintered magnet (Nd-Fe-B magnet) according to the present invention has a temperature-coefficient of coercive force (iHc) of 0.5 %/°C or more and a composition that R=11-18 at% R being rare-earth elements, R₁ =Nd+Pr, R₂=Dy, with the proviso of 80 at% ≦ R₁+R₂ /R ≦ 100 at% , 0 ≦ R₂ ≦ 4at%, B=6-12 at%, and the balance being Fe and Co with the proviso of Co being 25 at% or less relative to the total amount of Co and Fe, including 0 % of Co and impurities, and is characterized in that B in excess of a stoichiometric composition of the R₂Fe₁₄B compound-phase essentially does not form the RFe₄B₄-compound minority phase but forms a finely dispersed V-T-B compound minority phase, wherein T is Fe, and in a case of Co being present, T is Fe and Co , and, further, the magnet exhibits 20 MGOe or more of maximum energy product and 15 + 3x kOe of coercive force, x being the Dy content (at%), with the proviso that when 15 + 3x is 21 kOe or more, the coercive force is 21 kOe or more. A method for producing an Nd-Fe-B series sintered magnet (Nd-Fe-B magnet) according to the present invention is characterized by carrying out liquid-phase sintering while dispersing among the particles of the R₂Fe₁₄B compound-phase (R is one or more rare-earth elements whose main component(s) is Nd and/or Pr), fine particles of the V-T-B compound phase in such an amount that V amounts to 2-6 at % in the sintered body. In the Nd-Fe-B magnet produced according to this method, excess B being present more than the stoichiometric composition of the R₂Fe₁₄B compound-phase virtually does not form the RFe₄B₄ phase. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is an EPMA (electron-probe-micro-analysis) image of the Nd-Fe-B magnet according to the present invention. Fig. 2(A) and Fig. 2(B) show the electron diffraction of V-Fe-B compound contained in a Nd₁₅Fe bal V₄B₈ magnet. Fig. 3 shows the transmission-electron micrograph of a Nd₁₅Fe bal V₄B₈ magnet. Fig. 4 is a graph showing the influence of the presence of V-Fe-B compound upon the coercive force (iHc) and grain size. Fig. 5 is a graph illustrating the corrosion resistance of Nd-Fe-B sintered magnets. DETAILED DESCRIPTION OF THE INVENTION Microstructure The V-T-B compound (phase) may hereinafter also be referred to as V-Fe-B compound (phase). The V-Fe-B compound phase is formed in the constitutional structure of the sintered body, as long as Nd, Pr, (Dy), B, Fe and V are within the above described range. When these components are outside the above ranges, the constitutional phases of sintered magnet are the R₂Fe₁₄B compound-phase, Nd rich phase and B rich phase as in the conventional Nd-Fe-B magnet, and hence the V-T-B compound phase is not formed. Another possibility is that the amount of V-T-B compound formed is very small, or the Nd₂Fe₁₇ phase which is detrimental to the magnetic properties is formed. The V-Fe-B compound phase in the sample of No.1 in Table 1 described below turned out, as a result of the EPMA measurement, to have a composition of 29.5 at% of V, 24.5 at % of Fe, 46 at% of B, and a trace of Nd. The V-Fe-B compound turned out, as a result of electron diffraction, to have a unit cell of tetragonal structure having lattice constants of a=5.6 Å and c=3.1 Å. An electron diffraction-photograph used for analysis of the crystal structure of the V-Fe-B compound is shown in Figs. 2(A) and (B). For identification of the crystal structure, it is now compared with the one of already known compounds. At present, tetragonal V₃B₂ is the most probable compound. Presumably, a part of V of this compound is replaced with Fe. Elements other than the above mentioned can be dissolved in solid solution of that compound. Depending upon the composition, additive elements, and impurities of sintered bodies, V of that compound can be replaced with various elements having similar properties as V. B of that compound can be replaced by C which has similar properties as B. Even in these cases, an improved coercive force (iHc) is obtained, as long as in the sintered body is present the phase (possibly, (V 1-x Fe x )₃B₂ phase) of the binary V-B compound is present, part of which V is replaced with Fe and is occasionally additionally replaced with Co and the M elements described hereinbelow. The B rich phase, which is contained in most of the conventional Nd-Fe-B magnets, is gradually lessened and finally becomes zero with the increase in the amount of V-Fe-B compound phase formed. When the B rich phase, which contains approximately 11 at% of Nd, is replaced by the V-Fe-B compound, in which virtually no Nd is dissolved as solid solution, remainder of Nd constitutes the Nd rich phase, which is essential for the liquid-phase sintering, with the result that Nd is effectively used for improving the magnetic properties. That is, the Nd-Fe-B magnet according to the present invention, which is essentially free of the B rich phase, exhibits a higher coercive force (iHc) than the conventional Nd-Fe-B magnet having the same composition as the former magnet but containing B more than the stoichiometric composition of R₂Fe₁₄B. The excess boron more than the stoichiometric composition of R₂Fe₁₄B means the B which is present in an amount more than (1/17)x100 at% = 5.8 at%, for example 2.2 at% in the case of 8 at% of B. In an Nd-Fe-B magnet, whose coercive force (iHc) is particularly improved, the B rich phase is completely inappreciable or extremely slight even if partially appreciable. As is shown in the EPMA image of Fig. 1, the V-Fe-B compound phases are dispersed in the grain boundaries and triple points of grain boundaries of the R₂Fe₁₄B compound-phase. During observation by means of an electron microscope with a further higher resolving power, it turned out, as shown in Fig. 3, that a finer V-Fe-B compound phase dispersed mainly at the grain boundaries and partly within the grains. The properties of Nd-Fe-B magnets are better in cases where the V-Fe-B compound phase is dispersed mainly in the grain boundaries, than in cases where the V-Fe-B compound phase is dispersed mainly within the grains. Ideally, almost all of the crystal grains of R₂Fe₁₄B compound-phase are at their boundaries in contact with a few or more of the particles of the V-Fe-B compound phase. Inventive Method The method according to the present invention is hereinafter described in detail. According to the method of the present invention, particles of the V-T-B compound phase are dispersed uniformly and finely during the liquid-phase sintering. The V-T-B compound phase dispersed as mentioned above exerts a strong influence upon the distribution, amount and presence (absence) of the various minority phases contained in the sintered body. As a result, the Nd-Fe-B magnet having the characterizing structure is obtained. When T is Fe, the V-Fe-B compound phase must be an inter-metallic compound, in which an approximate integer ratio is established in the atom numbers of V+Fe to B. The V-Fe-B compound, which is present during sintering according to the present invention, may be such borides as V₃B₂, V₅B₆, V₃B₄, V₂B₃, VB₂ or the like, in which preferably 5 at% or more of V is replaced by Fe. The atom ratio between V+B and B occasionally deviates from the strict integer ratio. When two or more kinds of V-Fe-B compounds are mixed, the resultant mixture as a whole does not provide an interger ratio. Even such V-Fe-B compound(s) may be used in the present invention, provided that the constitutional atoms of the respective compound(s) have approximate integer ratio. The particles of the V-Fe-B compound used as an additive before sintering must be fine. If such particles are considerably coarser than the main phase particles, then the former particles do not disperse well in the latter particles, with the result that reactions of the V-Fe-B compound-phase with the other phases become unsatisfactory and hence its influence upon the various minority phases is weakened, The particles of the V-Fe-B compound must therefore be as fine as, or finer, than the main-phase particles. It is also important that the particles of the V-Fe-B compound are satisfactorily uniformly dispersed in the powder as a whole. The grain boundaries are improved at the most, when the particles of the V-Fe-B compound are dispersed in such a manner that at least one of these particles is brought into contact with every one of the sintered particles of the main phase. The amount of the V-Fe-B compound-particles must be such that V is contained from 2 to 6 at% in the sintered body. If the amount is less than 2 at%, it is impossible to realize an effect that V-Fe-B phase satisfactorily replaces the RFe₄B₄ phase. On the other hand, if the amount is more than 6 at%, the residual magnetization is lessened and the detrimental Nd₂Fe₁₇ phase, which impairs the magnetic properties, is formed. Methods for obtaining the powder for sintering, in which the above described V-Fe-B compound-particles are finely dispersed, are hereinafter described. There are two methods for obtaining the powder of V-Fe-B compound. (1) An ingot of V-Fe-B compound is pulverized. (2) An Nd-Fe-V-B alloy-ingot containing the V-Fe-B compound is formed, and then the ingot is pulverized, simultaneously pulverizing the V-Fe-B compound. The powder mixture of V-Fe-B compound-phase together with the other phases is obtained. Various devices are possible for obtaining the powder, in which the particles of V-Fe-B compound are uniformly and finely dispersed. Since the V-Fe-B compound is harder and hence more difficult to pulverize than the R₂Fe₁₄B compound-phase, the V-Fe-B compound is not satisfactorily refined even when the R₂Fe₁₄B is pulverized to fine particles of a predetermined size. Longer pulverizing time is therefore necessary for obtaining the V-Fe-B compound particles than that for obtaining the R₂Fe₁₄B particles, The powder, in which the respective phases reach a predetermined average size, is mixed for a satisfactorily long time, so as to attain uniform dispersion of the respective phases. In order to pulverize the respective phases as the separate particles as described above, the pulverizing time is varied depending upon the hardness, so that the respective phases are size-reduced to a predetermined average grain-diameter. The resultant powder is then uniformly mixed satisfactorily to obtain the starting powder of sintering according to the present invention. Depending upon the accuracy of pulverizing, composite particles may be obtained, in which the particles of V-Fe-B and R₂Fe₁₄B are not separated from but adhere to each other. Such composite particles may also be used as the starting material of sintering according to the present invention. A possible alloy or combinations of alloys used in the present invention are for example as follows. (1) An R-poor alloy, whose R is poorer than that of the R₂Fe₁₄B-compound, an R rich alloy, whose R is richer than that of the R₂Fe₁₄B-compound, and a V-Fe-B compound (2) An R-rich alloy, whose R is richer than that of the R₂Fe₁₄B compound, and a V-Fe-B compound (3) An R-rich alloy, whose R is richer than that of the R₂Fe₁₄B compound, V-Fe-B compound and an R-Fe-B-V alloy (4) Two or more kinds of an R-Fe-B-V alloys having different compositions (5) One kind of an R-Fe-B-V alloy Combinations other than above are possible but are not recommended since they are complicated, In the R-poor alloy of (1), above, the constitutional phases are, depending upon the composition, three of R₂Fe₁₄B, R₂Fe₁₇, Fe and Fe₂B. The constitutional phases of the R-rich alloy above are R₂Fe₁₄B, R-rich phase and R₁Fe₄B₄. Generally, when the phases, whose pulverizability is different from one another, are pulverized simultaneously by means of an attritor or the like, the resultant powder has a broad distribution of grain size and the magnetic properties are poor. The above mentioned alloys and alloy combination (1), (2) and (3) are superior to (4) and (5), since the respective alloys can be pulverized separately and then mixed with each other. (4) and (5) are however sometimes superior to (1), (2) and (3) in the light of productivity. The constitutional phases of cast alloys according to (4) and (5) are particles of the R₂Fe₁₄B, R rich and V-Fe-B phases having a size of several hundreds µm. In order to uniformly disperse throughout the powder the R₂Fe₁₄B compound-phase 1-5 µm in size and fine particles of the V-Fe-B compound, a method is undesirable, which has no classification effect and pulverizes every phase for identical time and to identical degree, since it is difficult to obtain the powder, in which the fine particles of V-Fe-B compound are uniformly and finely dispersed. When the crushed powder of alloys according to (4) and (5) are subjected to pulverizing by a jet mill with the use of nitrogen gas, the particles, whose average grain-diameter is reduced to a predetermined one, are successively collected in vessels attached to a cyclone. The pulverizing time is therefore automatically adjusted in accordance with the hardness and toughness of the respective phases. The powder of respective phases, which is suitable for the present invention, is therefore prepared even from the alloys according to (4) and (5) having the mixed phases. Due to the difference in the pulvarizing property of the respective phases, the respective phases tend to separate from each other and are collected separately. The powder of the alloys according to (4) and (5), as they are pulverized by a jet mill, is therefore undesirable, because a sintered Nd-Fe-B magnet produced by using such a powder contains a significant amount of the remaining B rich phase. The crystal grains of the V-Fe-B compound-phase in the alloy-ingots of (4) and (5) are desirably fine. That is, since the particles of the V-Fe-B compound are difficult to pulverize, it is desirable that the fine particles are already formed in an ingot. The alloy melt is therefore desirably rapidly cooled during solidification by means of using a small ingot or a water-cooled mold for casting the alloy after melting. It is then possible to disperse the V-Fe-B compound-particles in the powder of the R₂Fe₁₄B compound-phase having a grain-diameter of 1 - 5 µm in average. If the average grain-diameter of the R₂Fe₁₄B compound-particles is less than 1 µm, chemical activity is so high as to render their handling difficult. On the other hand, if the average grain diameter is more than 5 µm, a high coercive force is difficult to obtain after sintering. For measuring the average grain diameter of the powder a Fisher sub-sieve sizer was used. It is necessary for obtaining a high coercive force that the R rich phase is uniformly dispersed in the powder. Subsequently, the sintering is carried out. The sintering must be liquid-phase sintering in order to obtain the effect for repairing the R₂Fe₁₄B compound-phase by R-rich liquid phase. Known sintering temperatures, times and atmospheres may be used in the present invention. Heat treatment is carried out at a temperature of from 600 to 800 °C after sintering. This treatment causes an appreciable change in the crystal grain-boundaries and hence the enhancement of the coercive force (iHc) at room temperature by 7-11 kOe, and at 140 °C by 2-5 kOe. The above described inventive method is carried out irrespective of the composition of Nd-Fe-B magnet, as long as excess B being present more than the stoichiometric composition of the R₂Fe₁₄B compound is present in the Nd-Fe-B magnet. However, the R content is desirably 10 at% or more in the final alloy composition, in the light of liquid-phase sintering. The B content of 6 at% or more is necessary for obtaining a high coercive force. Coercive force Although the Nd-Fe-B magnet having a temperature-coefficient of 0.5%/°C or more of coercive force (iHc) exhibits a considerable decrease in the coercive force at a high temperature, the coercive force (iHc) obtained by the present invention is enough for using the inventive magnet for various appliances at high temperatures. The coercive force (iHc) of the permanent magnets according to the present invention is hereinafter described. Note, however, that the production conditions are ordinary, particularly, the contact of oxygen with treated articles during the production process (for example the oxygen concentration in nitrogen gas used for the pulverising in a jet mill), atmosphere in the pressing process, and the oxygen concentration of the sintering atmosphere are ordinary ones such that the Nd₁₅Fe₇₇B₈ magnet having the optimum composition exhibits a coercive force (iHc) = 12 kOe after optimum heat treatment. The coercive force (iHc) of the Nd-Fe-B magnet according to claim 1 is 15 kOe or more. Since the coercive force (iHc) is enhanced by 3 kOe by addition of 1 at% of Dy, the coercive force (iHc) is ≧ 15 + 3x (kOe) (x is Dy content by atomic %) in the Nd-Fe-B magnet, in which Dy is added. However, since the applied maximum magnetic field of an electromagnet used in the experiments for measuring the demagnetizing curves until the completion of the present invention was 21 kOe, actual values could not be measured, when the coercive force (iHc) exceeded 21 kOe. Therefore, when the coercive force (iHc) calculated according to the above formula exceeds 21 kOe, the inventive coercive force (iHc) is set at least 21 kOe or more. Aluminum, which may be added to the Nd-Pr-(Dy)-Fe-B magnet having the composition according to the present invention, furthermore enhances the coercive force (iHc), presumably because aluminum in a small amount promotes the fine dispersion of the V-T-B compound phases. One standard, which is necessary for using Nd-Fe-B magnets at a high temperature, is a coercive force (iHc) of 5 kOe or more. Now consideration is made that temperature raises up to 140 °C, as frequently seen when magnets are used for motors and the like. If the temperature-coefficient of coercive force (iHc) is, for example, 0.5 %/°C, the coercive force (iHc) at room temperature must be 12.5 kOe or more. This value of coercive force (iHc) is fulfilled in the compositional range according to claim 1. If the temperature-coefficient of coercive force (iHc) is, for example, 0.6 %/°C, the coercive force (iHc) at room temperature must be 17.8 kOe or more. This value of coercive force (iHc) is fulfilled by a compositional range according to claim 1 except for vicinities of the upper and lower limits, provided that aluminium is added to the composition of claim 1. When the temperature coefficient of coercive force (iHc) is 0.7 %/°C or more, 5 kOe or more of the coercive force (iHc) is obtained at 140 °C by a composition with Dy addition. The coercive force (iHc) at 200 °C amounting to 5 kOe or more is obtained by a composition containing 3 to approximately 5.5 at% of V, 13 at % or more of R, more than 1 at% of Dy and aluminum addition. Composition Reasons for limiting the compostions are as described above. In addition, if the contents are less than the lower limits, the coercive force (iHc) becomes low. On the other hand, if the contents exceed the upper limits, the residual magnetization becomes low. With regard to Al, there are further detrimental effects which become serious at a content of more than 3 at% or more, that is, the Curie point is lower than 300 °C, and change of residual magnetization depending on the temperature increases. Addition of V causes enhancement of the coercive force (iHc) but only slight decrease in the Curie point. When the amount of V is very high, since detrimental Nd₂Fe₁₇ phase is formed, not only is the residual magnetization reduced but also the coercive force (iHc) is reduced to impair the stability at high temperature. Nd and Pr are mainly used for the rare-earth elements (R), because both the Nd₂Fe₁₄B-compound and the Pr₂Fe₁₄B-compound have higher saturation magnetisation and higher uniaxial crystal- and magnetic-anisotropies than the R₂Fe₁₄B compound-phase of the other rare-earth elements. (Nd+Pr)/R is ≧ 80 at%, because a high saturation magnetization and high coercive force (iHc) are obtained by setting high contents of Nd and Pr except for Dy. Dy enhances coercive force (iHc) at 140 °C and 200 °C by approximately 2 kOe/% and 1 kOe/%, respectively. The content of Dy is 4 at% or less, because Dy is a rare resource and further the residual magnetization considerably lowers at more than 4 at%. Incidentally, not only highly refined rare-earth elements but also mixed raw-materials, such as dydimium, in which Nd and Pr remain unseparated, and Ce-dydimium, in which Ce remains unseparated, can be used as the raw material for rare-earth elements. Co, which may partly replace Fe, enhances the Curie point and improves the temperature-coefficient of residual magnetization. If, however, Co amounts to 25 at% or more of the total of Co and Fe, the coercive force (iHc) is lessened due to the minority phase described hereinafter. The amount of Co must therefore be 25 at% or less of the total of Co and Fe. In the Co-containing Nd-Fe-B magnet according to the present invention, Nd₂Fe₁₄B compound and V-Fe-B compound are changed to R₂(FeCo)₁₄B compound and V-(FeCo)-B compound, respectively. In addition, (Co·Fe)-Nd phase generates as a new minority phase, which lowers the coercive force (iHc). The present inventor added various elements to the above described Nd-Fe-B magnet and investigated influences of the additive elements on the coercive force (iHc). It turned out as a result that the coercive force (iHc) is slightly improved or is virtually not improved, but not decreased. M₁ enhances the coercive force (iHc), as V does but not outstandingly as V does. M₂ and M₃ have slight effect for enhancing the coercive force (iHc). However, M₂ and M₃ may be incorporated in the refining process of rare-earth elements and Fe. It is advantageous therefore in view of the cost of raw materials when the addition of M₁ , M₂ and M₃ may be permitted. M₁ = 0-4 at% (M₁= one or more of Cr, Mo and W), M₂ = 0-3 at% (one or more of Nb, Ta and Ni), and M₃ = 0-2 at% (one or more of Ti, Zr, Hf, Si and Mn). Transition elements among the above elements replace a part of T of the V-T-B compound. When the addition amount of M₁, M₂ and M₃ exceeds the upper limits, the Curie point and residual magnetization are lowered. The elements other than the above described ones are impurities. Particularly, ferroboron, which is frequently used as the raw material of boron, contains aluminum. Aluminum also dissolves from a crucible. Aluminum is therefore contained in 0.4 wt% (0.8 at%) at the maximum in the Nd-Fe-B magnet, even if aluminum is not added as an alloy element. There are other elements which are reported to be added to Nd-Fe-B magnets. For example, Ga is alleged to enhance the coercive force (iHc), when it is added together with cobalt. Ga can also be added to the Nd-Fe-B magnets of the present invention. Cu in an amount less than 0.01 % is also an impurity. Oxygen is incorporated in the Nd-Fe-B sintered magnet during the alloy-pulverizing step, the post-pulverizing, pressing step, and the sintering step. In addition, a large amount of Ca is incorporated in the Nd-Fe-B magnet as a residue of the leaching step (rinsing step for separating CaO) of the co-reducing method for directly obtaining the alloy powder of the Nd-Fe-N alloy by reduction with the use of Ca. Oxygen is incorporated in the Nd-Fe-B magnet in an amount of 10000 ppm (weight ratio) at the maximum. Such oxygen improves neither magnetic properties nor the other properties. Into the Nd-Fe-B magnets carbon is incorporated from the raw materials of the rare-earths and Fe-B, as well as carbon, phosphorus and sulfur from the lubricant used in the pressing step. Under the present technique, carbon is incorporated in the Nd-Fe-B magnet in an amount of 5000 ppm (weight ratio) at the maximum. Also, this carbon improves neither the magnetic properties nor the other properties. A high coercive force (iHc) is obtained by means of heat treating the above inventive Nd-Fe-B magnet in the temperature range of from 500 to 1000 °C, as follows. In this table, the range of heat treatment indicates the temperature range, in which the coercive force (iHc) lower than the maximum coercive force (iHc) by 1 kOe is obtained. If not specified, aluminum is contained as an impurity. Corrosion Resistance According to the present invention, all, or almost all, of the B rich phase, which has the lowest corrosion resistance, is replaced by the V-Fe-B phase, thereby enhancing the corrosion resistance against water. V forms with B a very stable compound and suppresses the formation of Nd₁Fe₄B₄. The corrosion resistance of the V-T-B compound is higher than that of the B rich phase and even higher than that of both the main phase and the Nd-rich phase. The corrosion resistance of the Nd-Fe-B magnet according to the present invention is twice as high as the one of conventional magnets when evaluated in terms of weight increase by oxidation under high-temperature and high-humidity conditions of 80 °C and 80 % of RH (test for 120 hours). That is, the weight increase of the inventive magnet is half of the conventional magnet. Since the corrosion resistance is improved as described above, it appears that problems of rust, which occur heretofore when magnets are used in appliances, can be drastically lessened. Advantages When Fe of the standard composition Nd₁₅Fe₇₇B₅ is replaced by 3.5 at% of V, the coercive force (iHc) is 15 kOe or more. This value is higher than the 12 kOe coercive force (iHc) of the heat-treated standard composition by 3 kOe. In addition, as is described in the examples hereinbelow, 18 kOe of the coercive force (iHc) is obtained. The enhancement of coercive force (iHc) by the same comparison is 6 kOe and hence is extremely high. Such enhancement of the coercive force can be explained from the following four points of view. (1) Effective utilization of R Since the B rich phase is replaced by the V-Fe-B compound-phase, in which virtually no Nd is solid-dissolved, Nd is relieved from the B rich phase and is utilized for liquid-phase sintering and for forming the main phase. As a result, the coercive force (iHc) is enhanced. (2) Control of grain-diameter Specifically speaking, the powder of the main phase, in which the R₂Fe₁₄B compound-phase particles have an average diameter of 1 to 5 µm, is liquid-phase sintered, until the average diameter falls within a range of 5 to 15 µm. Fig. 4 graphically illustrates dependence of the coercive force (iHc) and average particle-diameter of the R₂Fe₁₄B compound-phase upon the sintering temperature, with regard to the inventive composition of Example 4, in which 6 wt% of V-Fe-B compound is added, and the comparative composition without the addition. The sintering time is 4 hours. When the sintering temperature is such that the average grain-diameter is in the range of from 5 to 15 µm, the coercive force (iHc) is 13 kOe or less in the comparative case but is more than 15 kOe and hence high in the inventive case. (3) Control of sintering temperature Specifically speaking, sintering is carried out at T₂ and the sintering temperature is suppressed by 10 °C in terms of the temperature (ΔT), given below. ΔT is T₂ - T₁. T₁ is the sintering temperature, at which the average grain-diameter (d₁) is obtained under the absence of V-T-B compound. T₂ is the sintering temperature, at which the average grain-diameter (d₂=d₁) is obtained under the presence of V-T-B compound. ΔT therefore indicates the temperature which reflects the effects for suppressing the grain growth, The following table shows T₂ and ΔT obtained from Fig. 4, Average Grain-Diameter of Sintered Body (d₁, d₂, µm) Suppressing Effects of Grain Growth (ΔT,°C) Sintering Temperature (T₂, °C) 6 40 1060 7 45 1090 8 50 1130 9 53 1140 10 52 1145 12 50 1160 As shown in Table 2, the sintering temperature (T₂) can be elevated by 40 °C or more (ΔT ≧ 40 °C), over the sintering temperature T₁ while keeping the average-grain diameters equal (d₁=d₂) . (4) Modification of grain-boundaries It is known that the coercive force of Nd-Fe-B magnets is closely related to the micro structure of the grain boundaries. Presumably, the V-Fe-B compound functions in the inventive magnet to modifiy the grain boundaries. When Nd-Fe-Mo-B or Nd-Fe-Cr-B is used instead of V-Fe-B, improvement is not attained at all. This fact suggests that a function of the V-Fe-B compound other than the suppression of grain growth is important. The inventive magnets are fundamentally different from the conventional sintered Nd-Fe-B series magnets in the nature and morphology of minority phases, that is, RFe₄B₄ phase is present in the latter magnet but is essentially not present in the former magnet. It appears in the light of the morphology of minority phases that V-Fe-B compound phase is more appropriate as the phase around the R₂Fe₁₄B compound-phase (main phase) than the RFe₄B₄ phase for obtaining a high coercive force. Because of addition of V, the grain boundaries are presumably modified such that nuclei for inversion of the magnetization are difficult to generate. Incidentally, the maximum energy product of Nd-Fe-B magnet according to the present invention is 20MGOe or more. This value is the minimum one required for rare-earth magnets having a high-performance. Below this value, the rare-earth magnets cannot compete with the other magnets. The present invention is hereinafter described with reference to the examples. Example 1 Alloys were melted in a high-frequency induction furnace and cast in an iron mold. As the starting materials the following materials were used: for Fe an electrolytic iron having a purity of 99.9 wt%; for B a ferro-boron alloy and boron having a purity of 99 wt%; Pr having a purity of 99 wt%; Dy having a purity of 99 wt%; for V a ferrovanadium containing 50 wt% of V; and, Al having a purity of 99.9 wt%. The melt was stirred thoroughly during melting and casting so as to provide a uniform amount of V in the melt. The thickness of ingots was made 10 mm or less thin, and cooling was carried out quickly, so as to finely disperse the V-Fe-B compound phase in the ingots. The resultant ingots were pulverized by a stamp mill to 35 mesh. A fine pulverizing was then carried out by a jet mill with the use of nitrogen gas. As a result, the powder having a grain diameter of 2.5 - 3.5 µm was obtained, This powder was shaped under a pressure of 1.5 kg/cm² and in a magnetic field of 10 kOe. After the treatment of the powder by a jet mill, the powder was thoroughly stirred so as to uniformly and finely disperse the V-Fe-B compound in the sintered body. The green compact obtained by the pressing under a magnetic field was then sintered at 1050 to 1120 °C for 1 to 5 hours in an argon atmosphere. The sintered body was heat-treated at 800 °C for 1 hour, followed by rapid cooling by blowing argon gas. Heat treatment was subsequently carried out at 600 - 700 °C for 1 hour, followed by rapid cooling by blowing argon gas. The compositions and magnetic properties of samples are shown in Table 3. When the B content is 8 at% and V-addition amount is 2.7 at%, the V-T-B phase is 90 % relative to the total of V-T-B phase and B rich phase. When V-addition amount exceeds 3 at%, V-T-B phase is nearly 100 %. However, also in this case, fine RFe₄B₄ phase is partly seen due to compositional non-uniformity and the like. The average value (area percentage) of EPMA was converted to volume, which is the percentage of phase mentioned above. Example 2 Sheets 10x10x1 mm in size, consisting of Nd₁₄Fe bal B₈V x were prepared by the same method as Example 1. These sheets were heated at 80 °C in air having 90 % of RH up to 120 hours, and the weight increase by oxidation was measured. The results are shown in Fig. 5. It is apparent from Fig. 5 that the corrosion resistance is considerably improved by the addition of V. Example 3 The weight increase by oxidation was measured by the same method as in Example 2 for the compositions given in Table 5. The results are shown in Table 4. In the following Examples the composition is Nd₁₆Fe₇₂V₄B₈ or (Nd 0.9 Dy 0.1 )₁₆Fe₇₂V₄B₈. Example 4 A: Nd₁₀Fe₈₆B₄, B: Nd₃₀Fe₆₆B₄, and C: (V 0.6 Fe 0.4 )₃B₂ were melted in a high-frequency induction furnace, and ingots were formed. The ingots were pulverized by a jaw crusher and a disc mill to obtain powder through 35 mesh. A and B were then pulverized by a ball mill to an average particle diameter of 3 µm. C was pulverized by a ball mill to an average particle diameter of 1 µm. At this step, the powder A consisted of particles of Nd₂Fe₁₄B, Fe₂B, and α-Fe. The powder B consisted of particles of Nd₂Fe₁₄B, Nd₂Fe₁₇, and Nd-rich phase. Almost all of the powder of C was the single-phase (V 0.6 Fe 0.4 )₃B₂ powder. The A, B, and C powders were blended in weight ratio of 51:43:6 and then mixed for 3 hours by a rocking mixer. The mixed powder was pressed at a pressure of 1 t/cm² in a magnetic field of 12 kOe, and then sintered at 1100 °C for 4 hours in Ar under a pressure of 10⁻² torr. After sintering, rapid cooling was carried out. Heat treatment was then carried out at 670 °C for 1 hour. The magnetic properties were as follows. The residual magnetization Br=11.6 kG The coercive force (iHc)=18.4 kOe The maximum energy product (BH)max=31.3 MGOe The average particle-diameter of the sintered body was 5.9 µm. The B rich phase was inappreciable by measurement of the sintered body by EPMA. Example 5 A: Nd₁₈Fe₇₇B₄ and B: (V 0.6 Fe 0.4 )₃B₂ were pulverized by the same methods as in Example 4 to 3.7 µm and 1.5 µm, respectively. At this step, the powder A consisted of particles of the Nd₂Fe₁₄B-compound, the Nd rich phase and the Nd₂Fe₁₇ phase, and the powder B consisted of the particles of the single (V 0.6 Fe 0.4 )₃B₂ phase. Mixing by a rocking mixer was carried out for 1 hour to provide the weight proportion of A:B = 94:6. A sintered magnet was produced under the same conditions as in Example 4. The magnetic properties were as follows. The residual magnetization Br=11.7 kG The coercive force (iHc)=17.9 kOe The maximum energy product (BH)max=31.7 MGOe The average particle-diameter of the sintered body was 6.1 µm. The B rich phase was inappreciable by measurement of the sintered body by EPMA. Example 6 An Nd₁₆Fe₇₂V₄B₈ alloy was pulverized by a jet mill with the use of nitrogen gas to 2.5 µm in average. At this step, the powder consisted of particles of the respective single Nd₂Fe₁₄B, Nd rich alloy, and V-Fe-B phases. The dispersion state of the particles of the V-Fe-B compound were however not uniform. After the pulverizing, the crushing by a rocking mixer was carried out for 2 hours. A sintered magnet was produced under the same conditions as in Example 4. The magnetic properties were as follows. The residual magnetization Br=11.6 kG The coercive force (iHc)=17.3 kOe The maximum energy product (BH)max=31.7 MGOe The average particle-diameter of the sintered body was 6.8 µm. The B rich phase was inappreciable by measurement of the sintered body by EPMA. Example 7 A: Nd₁₆Fe₈₀B₄ and B: Nd₁₆Fe₇₀V₅B₉ were pulverized by a jet mill and a ball mill to 2.8 µm and 1.9 µm, respectively. At this step, the powder A consisted of particles of the Nd₂Fe₁₄B, Nd rich phase and Nd₂Fe₁₇ phase, and the powder B consisted of the particles of Nd₂Fe₁₄B phase, Nd rich phase, V-Fe-B compound, and Nd₂Fe₁₇ phase. Mixing by a rocking mixer was carried out for 2 hours to provide the weight proportion of A:B = 6:94. A sintered magnet was produced under the same conditions as in Example 4. The magnetic properties were as follows. The residual magnetization Br=11.5 kG The coercive force (iHc)=17.6 kOe The maximum energy product (BH)max=31.5 MGOe The average particle-diameter of the sintered body was 6.3 µm. The B rich phase was inappreciable by measurement of the sintered body by EPMA. Example 8 A: Nd 16.4 Dy 1.8 Fe 79.5 B 2.3 and B: V₃₃Fe₂₂B₄₅ were pulverised by a jet mill and a ball mill to 2.6 µm and 1.5 µm, respectively. At this step, the powder A consisted of particles of the R₂Fe₁₄B, R rich phase and R₂Fe₁₇ phase, and the powder B consisted of the particles of (V 0.6 Fe 0.4 )₃B₂ and (V 0.6 Fe 0.4 )B phases. Mixing by a rocking mixer was carried out for 2 hours to provide the mixture having weight proportion of A:B = 94:6. A sintered magnet was produced under the same conditions as in Example 3. The magnetic properties were as follows. The residual magnetization Br=11.0 kG The coercive force (iHc)=21 kOe or more The maximum energy product (BH)max=28.5 MGOe The average particle-diameter of the sintered body was 6.0 µm. The B rich phase was inappreciable by measurement of the sintered body by EPMA. Comparative Example 1 The same methods as in Example 5 were carried out except that the mixing by a rocking mixer was omitted. The magnetic properties were as follows. The residual magnetization Br=11.5 kG The coercive force (iHc)=12.8 kOe The maximum energy product (BH)max=30.7 MGOe The particle-diameter of the sintered body greatly dispersed from 10.3 µm at the minimum to 17 µm at the maximum. The B rich phase was locally observed in the sintered body under measurement of EPMA. The amount of B rich phase was 3 % in the sintered body as a whole.

Claims: An Nd-Fe-B sintered magnet having a temperature-coefficient of the coercive force (iHc) of 0.5 %/°C or more and having a composition that R=11-18 at% R is one or more rare-earth elements except for Dy, with the proviso of 80 at% ≦ Nd+Pr /R ≦ 100 at% , B=6-12 at%, and balance of Fe and Co, with the proviso of Co is 25 at% or less relative to the total of Co and Fe, including 0 % of Co and impurities, characterized in that V in an amount of from 2 to 6 at% is further contained and B in excess of a stoichiometric composition of the R₂Fe₁₄B compound-phase essentially does not form the RFe₄B₄-compound minority phase but forms a finely dispersed V-T-B compound minority phase, T is Fe, and in a case of containing Co, T is Fe and Co , and, further, the magnet exhibits 20 MGOe or more of maximum energy product (BH)max and 15 kOe or more of coercive force (iHc). An Nd-Fe-B sintered magnet according to claim 1, further containing 3 at% or less of aluminum. An Nd-Fe-B sintered magnet according to claim 1 or 2, wherein said magnet further contains at least one of M₁, M₂ and M₃ with the provisio of M₁=0-4 at% of one or more of Cr, Mo and W, M₂=0-3 at% of one or more of Nb, Ta and Ni, and M₃=0-2 at% of one or more of Ti, Zr, Hf, Si and Mn, and, further T are transition elements mainly composed of Fe or Fe plus Co in the case of containing Co. An Nd-Fe-B sintered magnet according to any one of claims 1 through 3, having 5 kOe or more of coercive force (iHc) at 140 °C. An Nd-Fe-B sintered magnet having a temperature-coefficient of coercive force (iHc) of 0.5 %/°C or more and having a composition that R=11-18 at%, R is rare-earth elements, R₁=Nd+Pr, R₂=Dy, with the proviso of 80 at% ≦ R₁+R₂ /R ≦ 100 at% , 0 ≦ R₂ ≦ 4 at%, B=6-12 at%, and balance of Fe and Co, with the proviso of Co is 25 at% or less relative to the total of Co and Fe including 0 % of Co , and impurities, characterized in that V in an amount of from 2 to 6 at% is further contained and B in excess of a stoichiometric composition of the R₂Fe₁₄B compound-phase essentially does not form the RFe₄B₄-compound minority phase but forms a finely dispersed V-T-B compound minority phase (T is Fe, and in a case of containing Co, T is Fe and Co), and, further, the magnet exhibits 20 MGOe or more of maximum energy product (BH) max and 15 + 3x (kOe) of coercive force, x is Dy content (at%), with the proviso that when 15 + 3x (kOe) is 21 kOe or more, the coercive force is 21 kOe or more . An Nd-Fe-B sintered magnet according to claim 5, further containing 3 at% or less of aluminum. An Nd-Fe-B sintered magnet according to claim 5 or 6, wherein said magnet further contains at least one of M₁, M₂ and M₃ with the proviso of M₁=0-4 at% of one or more of Cr, Mo and W, M₂=0-3 at% of one or more of Nb, Ta and Ni, and M₃= 0-2 at% of one or more of Ti, Zr, Hf, Si and Mn, and, further T are transition elements mainly composed of Fe or Fe plus Co in the case of containing Co. An Nd-Fe-B sintered magnet according to any one of claims 5 through 7, having 5 + 2x (kOe) or more of coercive force (iHc) at 140 °C. An Nd-Fe-B sintered magnet according to any one of claims 5 through 8, having 5 kOe or more of coercive force at 200 °C. A method for producing an Nd-Fe-B sintered magnet according to claims 1 or 5, by a liquid-phase sintering, characterized by dispersing in particles of R₂Fe₁₄B compound-phase, R is one or more rare-earth elements whose main component(s) is Nd and/or Pr , fine particles of V-T-B compound phase, T is Fe in such an amount that V in the sintered body amounts to 2-6 at%, thereby producing an Nd-Fe-B magnet, in which an excess B more than the stoichiometric composition of R₂Fe₁₄B compound-phase virtually does not form the RFe₄B₄ phase but forms finely dispersed V-T-B compound phase. A method according to claim 10, wherein the composition of Nd-Fe-B magnet is R=11-18 at%, R is one or more rare-earth elements except for Dy, with the proviso of 80 at% ≦ Nd+Pr /R ≦ 100 at% , B=6-12 at%, and balance of Fe and Co, with the proviso of Co is 25 at% or less relative to the total of Co and Fe, including 0 % of Co and impurities, and T is Fe and Co in the case of containing Co. A method according to claim 10, wherein the composition of said magnet is R=11-18 at%, R is rare-earth elements, R₁=Nd+Pr, R₂=Dy, with the proviso of 80 at% ≦ R₁+R₂ /R ≦ 100 at% , 0 ≦ R₂ ≦ 4at%, B-6-12 at%, and balance of Fe and Co, with the proviso of Co is 25 at% or less relative to the total of Co and Fe, including 0 % of Co and impurities. A method according to any one of claims 10 through 12, wherein said magnet further contains 3 at% or less of aluminum. A method according to any one of claims 10 through 13, wherein said magnet further contains at least one of M₁, M₂ and M₃ with the proviso of M₁=0-4 at% of one or more of Cr, Mo and W, M₂=0-3 at% of one or more of Nb, Ta and Ni, and M₃=0-2 at% of one or more of Ti, Zr, Hf, Si and Mn, and, further T is transition elements mainly composed of Fe or Fe plus Co in the case of containing Co.

Inventors: SAGAWA MASATO, SAGAWA, MASATO
Applicants: SAGAWA MASATO, SAGAWA, MASATO
Application year: 1989
Main classification: H01F1
IPC codes: H01F1, C22C1