Permanent magnet alloy having improved resistance to oxidation and process for production thereof

Abstract: A permanent magnet made of an R-Fe-B-C or R-Fe-Co-B-C based alloy (where R is at least one rare-earth element) consisting of its individual magnetic crystal grains that are covered with an oxidation-resistant protective film is promising as a practicable next-generation magnet because of its having not only excellent magnetic properties inclusive of magnetic force that surpasses Sm-Co based magnets but also such highly improved oxidation resistance that may withstand use in practical applications for a prolonged time period without being coated on its outermost exposed surface with an oxidation-resistant protective film. Said protective film surrounding the individual magnetic crystal grains contains at least one, preferably substantially all, of the alloying elements of which said magnetic crystal grains are made, with 0.05 - 16 wt% of said protective film being composed of C in the R-Fe-B-C system and up to 16 wt% (not inclusive of zero wt%) of said protective film being composed of C and up to 30 wt% (not inclusive of zero wt%) being composed of Co in the R-Fe-Co-B-C system.

Description: The present invention relates to a sintered permanent magnet alloy, as well as a magnet made thereof, that is based on a rare-earth element (R), iron (Fe), boron (B) and carbon (C) and that has improved resistance to oxidation. The invention also relates to a process for producing such an alloy and a magnet. The term "permanent magnet alloy" herein used means a magnetic alloy which is adapted for making a permanent magnet. Since its first disclosure (Japanese Patent Public Disclosures Nos. 59-46008, 59-64733, 59-163803 and 61-143553), a magnet based on the R-Fe-B system has been the subject of many reports principally because it has the potential to be used as a next-generation magnet that surpasses Sm-Co based magnets in terms of magnetic force produced. However, though that magnet surpasses Sm-Co based magnets in terms of magnetic force, the heat stability of the magnetic characteristics and oxidation resistance of the new magnet are far inferior to those of said prior art magnets. For instance, the permanent magnet material described in Japanese Patent Public Disclosure No. 59-46008 is not capable of withstanding use in practical applications. Many of the reports on said new magnets that have been published to date point out their shortcomings in regard of oxidation resistance and propose various methods for improvement, which are roughly divided into two categories, one based on modifying alloy compositions and the other based on covering the surface of magnets with an oxidation-resistant protective film. As an example of the methods of the first approach, Japanese Patent Public Disclosure No. 59-64733 teaches that a magnet can be made corrosion-resistant by replacing part of Fe with Co. Japanese Patent Disclosure No. 63-114939 teaches that improved oxidation resistance can be provided by incorporating in the matrix phase a low melting metal element such as Al, Zn or Sn or a high melting metal element such as Fe, Co or Ni. Further, Japanese Patent Public Disclosure Nos. 62-133040 and 63-77103 show that C (carbon) in a magnet promotes its oxidation and hence its oxidation resistance can be improved by reducing the C content to a level below a certain limit. However, the effectiveness of these methods which solely depend upon the modification of alloy compositions for improving the resistance to oxidation is limited and it is difficult to produce magnets that reasonably withstand use in practical applications. Under these circumstances, it is necessary to manufacture a practicable magnet by coating its surface (the outermost exposed surface of the magnet) with an oxidation-resistant protective film through many complicated steps as shown in Japanese Patent Public Disclosure No. 63-114939. It has been proposed that the oxidation-resistant protective film be formed on the surface of a magnet by covering it with an oxidation-resistant material by various methods such as plating, sputtering, evaporation and coating of organic materials. However, in each of these cases, a rugged and homogeneous protective film layer must be formed in a thickness of at least several tens of µms on the outer surface of the magnet. The procedure of forming such a thick layer requires many and complicated steps, which unavoidably results in such problems as spalling, low dimensional accuracy and increased production cost. As described above, the existing R-Fe-B, R-Fe-Co-B and R-Fe-Co-B-C based magnets are not completely satisfactory in their ability to resist oxidation. As a matter of fact, these magnets have superior magnetic characteristics over Sm-Co based magnets and in addition, they have a great advantage in that they can be supplied consistently from abundant resources. However, these magnets cannot be put to practical use unless they are insulated from the operating atmosphere by means of an oxidation-resistant protective film formed on their surface and the above-described great advantage of these magnets is substantially compromised by the increased production cost and such problems as variations in dimensional accuracy. A magnet based on R-Fe-B system, e.g. a Nd-Fe-B system, is generally composed of magnetic crystal grains and a non-magnetic phase including a B-rich phase and a Nd-rich phase. A plausible explanation for the mechanism of oxidation that occurs in the magnet is that oxidation starts in the B-rich phase on either the magnet surface or in a nearby area and proceeds into the Nd-rich phase. Thus, it can be concluded that in order to improve the oxidation resistance of the magnet, it is necessary that not only the B content be reduced to the lowest possible level but also oxidation resistance be imparted to the Nd-rich phase. However, with the state of the art, the B content must inevitably be increased in order to attain magnetic characteristics of high practical levels, and no significant results have been achieved in the efforts to impart oxidation resistance to the Nd-rich phase. As already mentioned, Japanese Patent Public Disclosure No. 59-64733 proposes that corrosion resistance be imparted by replacing part of Fe with Co but it makes no mention at all of the relevancy of the B content to oxidation resistance. The only disclosure given in this patent in regard of the B content is as follows: the B content is adjusted to lie within the range of 2 - 28 at.% in order to secure a coercive force (iHc) of at least 1 kOe; in order to insure iHc of 3 kOe, the B content must be at least 4 at.%; and in order to attain high practical levels of iHc, the B content is further increased. However, if boron is to be contained in an increased amount with a view to attaining high magnetic characteristics, it is very difficult in practice to secure satisfactory oxidation resistance even if corrosion resistance is imparted by adding Co. Hence, in order to make a commercial magnet having high B content, it is essential to form a rugged oxidation-resistant protective film on the surface (the outermost exposed surface) of a magnet as taught by the inventors of the invention described in the Japanese Patent Public Disclosure mentioned at the beginning of this paragraph. Japanese Patent Public Disclosure No. 63-114939 teaches the inclusion of a low melting metal element (e.g. Al, Zn or Sn) or a high melting metal (e.g. Fe, Co or Ni) in the matrix phase in order to improve the oxidation resistance of the active Nd-rich phase According to an example shown in this patent, a weathering test (60°C x 90% RH) was conducted on a sinter and the period of time for which it could be left to stand until red rust developed noticeably on the surface of the magnet was prolonged to 100 h from 25 h which was the value for a comparative sample. However, the magnet having this level of oxidation resistance is not suitable for use in practical situations unless the surface of the magnet is protected by a rugged oxidation-resistant film. Thus, in this case, too, it is difficult to achieve a substantial improvement in the oxidation resistance of the magnet per se. It should also be noted that this Japanese Patent Public Disclosure makes no mention at all of the B content with regard to oxidation resistance and in the light of the B content which ranges from 3.5 to 6.7 at.% that is specified in the examples, one may safely conclude that the inclusion of B within the range of 2 - 28 at.% as set forth in Japanese Patent Public Disclosure No. 59-46008 is also contemplated by this publication. The principal object, therefore, of the present invention is to solve the aforementioned problems, particularly with respect to oxidation resistance, of prior art R-Fe-B-C based permanent magnets by imparting higher oxidation resistance to the magnets per se without sacrificing their high magnetic characteristics rather than by forming an oxidation-resistant protective film on the outermost exposed surface of the magnets. In order to solve the aforementioned problems of the prior art, the present inventors conducted intensive studies on the improvement of the oxidation resistance of the above-mentioned permanent magnets not by taking the conventional "macroscopic" approach which involves coating the surface of the magnet with an oxidation-resistant protective film but by taking a "microscopic" approach that is capable of improving the oxidation resistance of the magnet per se. As a result, the present inventors discovered a novel technique that was not even anticipated from the prior art and that involves coating the individual magnetic crystal grains in the magnet with an oxidation-resistant protective film. By adopting this technique, the present inventors successfully enabled the production of a new permanent magnet alloy having drastically enhanced oxidation resistance. The present inventors also found that by employment of this technique, satisfactory magnetic characteristics that enabled the magnet to withstand practical use could be imparted even when the B content was less than 2 at.%, which was previously considered as an impractical range where satisfactory magnetic characteristics could no longer be achieved by the prior art. IEEE Translation Journal on Magnetics in Japan 4(1989)May, No. 5, New York, US, discloses a permanent magnet alloy based on an R-Fe-B-C system (R is at least one of the rare-earth elements including Y), the individual magnetic crystal grains of said alloy being covered with an oxidation-resistant protective film containing C, the R content of said protective film being higher than that of said grains. One object of the present invention is to provide a permanent magnet alloy having improved resistance to oxidation which is based on an R-Fe-B-C system (R is at least one of the rare-earth elements including Y), and it is characterized by the features of claim 1. Further object of the present invention is to provide a process for producing the above-mentioned R-Fe-B-C based permanent magnet alloy. Fig. 1 shows demagnetization curves of Br (Magnetic Remanence or Retentivity) and iHc for the sintered magnets of the present invention having magnetic crystal grains covered by the C-containing oxidation-resistant protective film (Example 1, 5 and 6) and those for the sintered magnets of the prior art having no such protective layer (Comparative Example 1) when they were left to stand at 60°C and 90% RH (Relative Humidity). Fig. 2 is an electron micrograph showing the metallic structure of the magnet of the present invention prepared in Example 1; Fig. 3 is a photo showing the result of spectral line analyses for Nd, Fe and C elements in the metallic structure shown in Fig. 2; and Fig. 4 is a diagram showing the spectral lines of the respective elements as reproduced from Fig. 3. Fig. 5 shows demagnetization curves of Br and iHc for the sintered magnets of the present invention having magnetic crystal grains covered with the C-containing oxidation-resistant protective film (Examples 24, 28, 31 and 42) and those of the comparative samples having no such protective layer (Comparative Example 5) when they were left to stand at 60°C and 90% RH with the surface of the magnets being exposed; Fig. 6 is a diagram showing the spectral lines of the respective elements as reproduced from a photo showing the result of spectral line analyses for Nd, Fe and C elements in the metallic structure shown in an electron micrograph showing the metallic structure of the magnet of the present invention prepared in Example 24. The magnetic crystal grains in this magnet have a particle size in the range of 0.3 - 150 µm, preferably 0.5 - 50 µm and the oxidation-resistant protective film over these crystal grains has a thickness in the range of 0.001 - 30 µm, preferably 0.001 - 15 µm. In a preferred embodiment, the composition of the R-Fe-B-C based magnet alloy as the sum of the magnetic crystal grains and the oxidation-resistant protective film consists of 10 - 30% R (which is at least one of the rare-earth elements including Y), less than 2% (not inclusive of zero percent) of B, 0.1 - 20%, perferably 0.5 - 20% C, all percentages being on an atomic basis, with the balance being Fe and incidental impurities. In the present invention, satisfactory improvement in oxidation resistance can be achieved even if the B content is 2% or more, but particularly good results are attained at a lower B level (<2%) in that satisfactory magnetic characteristics are exhibited as accompanied by a marked improvement in oxidation resistance. Further object of the present invention is to provide a process for producing an R-Fe-B-C based alloy magnet, and it has been accomplished based on the following findings: it is possible to cover individual magnetic crystal grains of a magnet with an oxidation-resistant protective film if a proper treatment is conducted during a process of producing an alloy comprising the steps of preparing a molten mass of a crude alloy, preparing a powder of said alloy either directly from said molten mass or by casting said molten mass into an alloy ingot followed by crushing the ingot to obtain a powder of said alloy, compacting the resulting powder into a shaped product and sintering the shaped product to provide an R-Fe-B-C system alloy magnet (where R is at least one of the rare-earth element including Y). The essential points of said treatment are as follows: (1) heat treating the alloy ingot or the alloy powder at a temperature in the range of 500 - 1,100°C for a period of 0.5 h or more before the ingot or the powder is subjected to the compaction step; (2) adding part or all of the raw material as a C source or part or all of the raw material as a C source and/or Co source after the step of melting but before the step of compacting; or (3) the combination of the above steps (1) and (2). By the treatment mentioned above, an oxidation-resistant protective film having the C content higher than that of the magnetic crystal grains or an oxidation-resistant protective film having the C content higher than that of the magnetic crystal grains and also containing Co was formed surrounding the magnetic crystal grains and an R-Fe-B-C based permanent magnet alloy having an excellent oxidation resistance was produced. In either of the above magnet alloys, 0.05 - 16 wt%, preferably 0.1 - 16 wt% of the oxidation-resistant protective film formed on the surface of the individual magnetic crystal grains consists of C. Preferably, the oxidation-resistant protective film contains at least one, preferably substantially all of the alloying elements of which said magnetic crystal grains are made, with 0.05 - 16 wt%, preferably 0.1 - 16 wt% of said protective film being composed of C. The thickness of the oxidation-resistant protective film is in the range of 0.001 - 30 µm, preferably 0.001 - 15 µm and the particle size of the magnetic crystal grain is in the range of 0.3 - 150 µm, preferably 0.5 - 50 µm. According to the process of the present invention, one can obtain a permanent magnet alloy having a composition, as the sum of the crystal grains and the oxidation-resistant protective film, of 10 - 30% R, less than 2% (not inclusive of zero percent) B, 0.1 - 20%, preferably 0.5 - 20% C, all percentages being on an atomic basis, with the balance being Fe and impurities. This is a novel permanent magnet alloy which can be distinguished from the prior art permanent magnet alloy in an aspect that each of the individual magnetic crystal grains is covered with an oxidation-resistant protective film and in addition it can exhibit excellent magnetic characteristics even if the B content is less than 2%. If we guess correctly, the theory is as follows: when the heat treatment of the alloy ingot or powder mentioned above under (1) is effected, the element C contained in said alloy ingot or powder in the state of solid solution is concentrated or precipitates at the grain boundary interface, and this C is concentrated during the step of sintering at the grain boundary phase which exists surrounding magnetic crystal grains. As a result, the oxidation-resistant protective film is formed around the magnetic crystal grains. When the treatment mentioned above under (2) is effected, the element C as a raw material is added from an external source to the powder before the steps of compaction and sintering. Hence this C is concentrated, as in the case previously mentioned, during the step of sintering at the grain boundary phase which exists surrounding the magnetic crystal grains and the oxidation-resistant protective film is formed around the magnetic crystal grains. The permanent magnet of the present invention exhibits improved oxidation resistance by itself even if its outermost surface is not covered with an oxidation-resistant protective film as in the prior art. Thus, even if this magnet is left to stand in a hot and humid atmosphere (60°C x 90% RH) for 5 040 h with its surface exposed to the atmosphere, it will experience a very low level of demagnetization as evidenced by the decreases of 0.3 - 10% and 0 - 10% in Br (magnetic remanence or retentivity) and iHc, respectively. Hence, the permanent magnet of the present invention need not be protected with an oxidation-resistant surface film even if it is to be used in such a hot and humid atmosphere. This ability to resist oxidation and hence demagnetization was not achievable by the conventional magnets and in this respect. the magnet of the present invention is an entirely novel permanent magnet. The magnetic characteristics of the magnet of the present invention are such that Br ≥ 4,000 G, iHc ≥ 4,000 Oe and a capacity (BH)max ≥ 4 MG Oe if it is an isotropic sintered magnet, and Br ≥ 7,000 G, iHc ≥ 4,000 Oe, and (BH) max ≥ 10 MG Oe if it is an anisotropic sintered magnet. Thus, it is at least comparable to or even better than the existing R-Fe-B or R-Fe-Co-B based-, particularly Nd-Fe-B or R-Fe-Co-B based permanent magnets in terms of magnetic characteristics. These characteristics of the magnet of the present invention were attained by surrounding the individual magnetic crystal grains in the magnet with a non-magnetic film having an appropriate C content. To state more specifically, the present inventors found that a great ability to resist oxidation could be imparted to the non-magnetic phase of a magnet by incorporating a selected amount of C (carbon) in the grain boundary phase, i.e., the non-magnetic phase of the magnet. That is, a great ability to resist oxidation could be imparted to the non-magnetic film by incorporating therein 0.05 - 16 wt% of said film of C, preferably 0.1 - 16 wt % of said film of C. In addition, the present inventors obtained the following observations: by coating the individual magnetic crystal grains of the magnet with a non-magnetic film having the oxidation-resisting ability described above, satisfactory resistance to oxidation could be achieved even when the B content was comparable to the conventionally used level; and the formation of the C-containing protective film allowed for reduction in the B content, whereby a marked improvement in oxidation resistance could be achieved whereas the magnetic characteristics were comparable to or better than the heretofore attained level even when the B content was less than 2 at.%. One of the most characteristic aspects of the magnet of the present invention lies in the way it utilizes C (carbon). Carbon has generally been considered as an incidental impurity element that is unavoidably present in magnets of the type contemplated by the present invention and except in special cases, it has not been dealt with as an alloying element that is to be intentionally added. For instance, Japanese Patent Public Disclosure No. 59-46008 specifies the inclusion of 2 - 28 at.% B in a magnet and points out that its coercive force (iHc) will decrease below 1 kOe if the B content is less than 2 at.%. This patent merely states that part of B may be replaced with C from an economic viewpoint (i.e. reduction in production cost). Further, Japanese Patent Public Disclosure No. 59-163803 discloses an R-Fe-Co-B-C based magnet containing 2 - 28 at.% B and up to 4 at.% C. This patent teaches the combined use of B and C in a specific way but notwithstanding its use in combination with C, boron must be contained in an amount of at least 2 at.% and it is specifically mentioned that below 2 at.% B, the magnet has an iHc of less than 1 kOe as in the case described in Japanese Patent Public Disclosure No. 59-46008. In other words, as said patent points out, carbon is considered as an impurity that is detrimental to magnetic characteristics and it is unavoidable that the magnet is contaminated by C which originates from lubricants and other additives used in the compaction of powders. Since the procedure of completely eliminating this impurity increases the production cost, the patent proposes that the C content of up to 4 at.% be permissible if the Br value to be achieved is no more than 4,000 G which is comparable to that of a hard ferrite magnet. Hence, carbon produces negative effects on magnetic characteristics and it is not necessarily an essential element. Further, this patent does not suggest at all the formation of a C-containing oxidation-resistant protective film (non-magnetic phase). Japanese Patent Public Disclosure No. 62-133040 teaches that a higher C content is not desirable for the purpose of improving the oxidation resistance of R-Fe-Co-B-C based magnets and on the basis of this observation, it proposes that the C content be reduced to 0.05 wt% (ca. 0.3% on an atomic basis) or below. Japanese Patent Public Disclosure No. 63-77103 filed by a different applicant also proposes that the C content be reduced to 1,000 ppm or below to attain the same objective. Thus, in the prior art, carbon has not been dealt with as an indispensable element to be added but it has been considered to be a negative element in regard of magnetic and oxidation-resisting properties. Instead of incorporating C as a mere substituent element for B, the present inventors deliberately incorporated it in the non-magnetic phase (grain boundary phase) surrounding magnetic crystal grains and found unexpectedly that the carbon incorporated in this way made great contribution to an improvement in the oxidation resistance of the magnet. Further, it was found that this method helped improve the magnetic characteristics of the magnet. In other words, the intentional inclusion of C in the non-magnetic phase offered the advantage that even when the B content was within the known range commonly employed in the art, an improvement in oxidation resistance was achieved, with particularly good results being attained when the B content was less than 2 at.%. It was held in the prior art that iHc would become 1 kOe or below when the B content was less than 2 at.% but in accordance with the present invention, iHc values of at least 4 kOe can be achieved even if the B content is less than 2 at.%. This novel action of the present invention is brought about by the formation of a C-containing oxidation-resistant protective film that surrounds the individual magnetic crystal grains of the magnet, and compared to the conventional magnets in which carbon is considered to be a negative element because of its seemingly deleterious effects on oxidation resistance and magnetic characteristics, the magnet of the present invention is entirely novel in that it contains carbon as an essential element. The C-containing oxidation-resistant protective film which surrounds the individual magnetic crystal grains in the magnet of the present invention preferably contains not only C but also at least one, preferably substantially all of the alloying elements of which said magnetic crystal grains are made. Such a C-containing oxidation-resistant protective film can be formed by incorporating carbon or both carbon and cobalt in the grain boundary layer that exists between magnetic crystal grains in the magnet. A plausible reason for this possibility may be explained as follows: since the protective film mentioned above preferably contains at least one or substantially all of the alloying elements of which the magnetic crystal grains are made, the formation of R-Fe-C intermetallic compounds would play an important role; it is generally held that rare-earth elements will easily rust and that their carbides are highly susceptible to hydrolysis; however, in the protective film formed in accordance with the present invention, intermetallic compounds comprising R, Fe and C in unspecified proportions would be generated to minimize the occurrence of the defects described above. As described above, the present inventors found that by covering the individual magnetic crystal grains of the magnet with a C-containing oxidation-resistant protective film, its oxidation resistance could be markedly improved and that this effect was further enhanced by reducing the B content of the magnet. On the basis of these findings, the inventors succeeded in producing a high-performance permanent magnet that was hardly unattainable by the prior art technology. It is necessary for the purposes of the present invention that the C-containing oxidation-resistant protective film described above preferably contains at least one, preferably substantially all of the alloying elements of which the magnetic crystal grains in the magnet are made and that the C content of said protective film be within the range of 0.05 - 16 wt%, preferably 0.1 - 16% of the total weight of said film. The carbon in the protective film is effective not only in imparting oxidation resistance to the magnet but also in minimizing the possible decrease in iHc that may result from the lower B content. Hence, the carbon content of the protective film must be within the range of from 0.05 to 16 wt%, preferably from 0.1 to 16 wt%, more preferably from 0.2 to 12 wt%, of the protective film. If the C content of the protective film is less than 0.1 wt%, particularly less than 0.05 wt%, oxidation resistance will not be satisfactorily imparted or will not be imparted at all to the magnet and its iHc will become lower than 4 kOe. If the C content of the protective film exceeds 16 wt%, the magnet will experience such a great drop in Br that it is no longer useful in practical applications. In addition to C, the protective film preferably contains at least one, preferably substantially all of the alloying elements of which the magnetic crystal grains are made although their proportions in the protective film may differ from those in the magnetic crystal grains. The thickness of the protective film is not critical and resistance to oxidation is substantially retained as long as said film provides a uniform coating over the individual magnetic crystal grains. However, if the thickness of that film is less than 0.001 µm, iHc will drop significantly. If the thickness of the protective film exceeds 15 µm, or particularly exceeds 30 µm, Br will no longer be able to provide the value intended by the present invention. Hence, the thickness of the protective film is to be in the range of from 0.001 µm to 30 µm, preferably within the range of from 0.001 to 15 µm, more preferably within the range of from 0.005 to 12 µm. The thickness of the protective film described above should be taken as a value that includes the triple point at the grain boundary. The thickness of the protective film may be measured with a transmission electron microscope (TEM) as in the examples to be described hereinafter. The individual magnetic crystal grains which are surrounded by the oxidation-resistant protective film may have a composition similar to that of well-known R-Fe-B-(C) based permanent magnets, except that the magnet of the present invention is capable of exhibiting satisfactory magnetic characteristics even if the B content is lower than in the prior art magnets. The composition of the C-containing alloy magnet of the present invention as the sum of the magnetic crystal grains and the oxidation-resistant protective film consists of 10 - 30% R, up to 3% (not inclusive of zero percent) B, 0.1 - 20%, preferably 0.5 - 20% C, all percentages being on an atomic basis, with the balance being Fe and incidental impurities. The total C content in the magnet of the present invention is in the range of 0.1 - 20 at.%, preferably in the range of 0.5 - 20 at.%. If the total content of carbon in the magnet exceeds 20 at.%, Br will drop significantly and the values desirable for the present invention (Br ≥ 4 kG with an isotropic sintered magnet, and Br ≥ kG with an anisotropic sintered magnet) can no longer be achieved. If the total content of carbon in the magnet is less than 0.5 at.%, particularly less than 0.1 at.%, it is no longer possible to impart desired oxidation resistance. Hence, the preferred range of the total carbon content in the magnet of the present invention is from 0.1 to 20 at.%, preferably from 0.5 to 20 at.%. As already mentioned, the carbon in the oxidation-resistant protective film is effective not only in imparting oxidation resistance to the magnet but also in minimizing the possible decrease in iHc that may result from the lower B content. Hence, carbon content of this protective film must be in the range of 0.05 - 16 wt%, preferably within the range of 0.1 to 16 wt%, more preferably from 0.1 to 12 wt%, and the most preferably in the range of 0.2 - 12 wt% of the protective film. Carbon sources that may be used in the present invention include carbon black, high-purity carbon, and alloys such as Nd-C and Fe-C. The symbol R used in the present invention represents a rare-earth element which is at least one member selected from the group consisting of Y, La, Ce, Nd, Pr, Tb, Dy, Ho,Er, Sm, Gd, Eu, Pm, Tm, Yb and Lu. If desired, misch metal, didymium and other mixtures of rare-earth elements may also be used. The content of R in the magnet of the present invention is within the range of from 10 to 30 at.% since the values of Br exhibited within this range are highly satisfactory for practical purposes. Boron to be used in the present invention may be pure boron or ferroboron. Even if the B content exceeds 2 at.% which is one of the critical value conventionally used in the prior art, the magnet of the present invention has markedly improved oxidation resistance as compared with the prior art versions and the already stated objects of the present invention can be attained. Preferably, the B content is less than 2 at.% and much better results can be attained if the B content is 1.8 at.% or less. If boron is absent from the magnet, its oxidation resistance is improved but on the other hand, iHc will drop so greatly that the objectives of the present invention can no longer be attained. If ferroboron is to be used, it may contain impurities such as Al or Si. As described above, the permanent magnet alloy of the present invention has the individual magnetic crystal grains covered with the C-containing oxidation-resistant protective film whose thickness is in the range of from 0.001 to 30 µm, preferably within the range of from 0.001 to 15 µm, more preferably from 0.005 to 12 µm. The magnetic crystal grains in this alloy preferably have a grain size within the range of 0.3 - 150 µm, preferably within the range of 0.5-50 µm, more preferably in the range of 1 - 30 µm. If the size of the magnetic crystal grains is less than 0.5 µm, particularly less than 0.3 µm, the iHc of the magnet will become less than 4 kOe. If the size of the magnetic crystal grains exceeds 50 µm, particularly when it exceeds 150 µm, the iHc of the magnet will drop significantly to such an extent that the characteristic features of the magnet of the present invention will be substantially lost. The size of the magnetic crystal grains in the magnet of the present invention can be correctly measured with a scanning electron microscope (SEM) and its composition can be correctly analyzed with an electron probe microanalyzer (EPMA), as in the examples to be described hereinafter. The permanent magnet of the present invention is to be made as a sintered alloy. It can be produced by a conventional process which comprises a sequence of melting, casting, pulverizing, compacting and sintering steps, or a sequence of melting, casting, pulverizing, compacting, sintering and heat treating steps. Preferably, more advantageous results can be attained by modifying this production process in such a way that the casting operation is followed by the step of heat treating the cast alloy, or that part or all of the C source is additionally added during or after the pulverizing step. The alloy powder made of the permanent magnet alloy of the present invention can provide a bonded magnet which exhibits improved oxidation resistance as compared with the prior art product. Because of its having highly improved oxidation resistance, hardly rusting characteristic properties and excellent magnetic properties as compared with the prior art products, the permanent magnet alloy of the present invention can be advantageously used in various products in which a magnet is practically used. Examples of magnet applied products include, for example, the following: Electric motors such as a DC brushless motor and a servo-motor; actuators such as a driving actuator and a F/T actuator for optical pickup; acoustic instruments such as a speaker, a headphone and an earphone; sensors such as a rotating sensor and a magnetic sensor; a substitute for an electro-magnet such as MRI: relays such as a reed relay and a polarized relay; magnetic couplings such as a brake and a clutch; vibration oscillators such as a buzzar and a chime: adsorptive instruments such as a magnetic separator and a magnetic chuck; switching instruments such as an electromagnetic switch, a microswitch and a rodless air cylinder; microwave instruments such as a photoisolator, a klystron and a magnetron: magneto generators; health-promoting instruments; and toys, etc. The above-listed products are no more than part of the examples of the products to which a magnet alloy of the present invention can be applied. The application of the magnet alloy should not be limited thereto. The permanent magnet alloy of the present invention can be characterized by its improved resistance to rusting. It has eliminated the necessity of forming an oxidation-resistant protective film on the outermost exposed surface of the magnet which was necessary to the prior art products. Without sacrificing its high magnetic properties, higher oxidation resistance is imparted to the magnet per se. Hence, generally the protective film on the outermost exposed surface thereof need not be formed. There may be some special cases when such conventional protective film should be formed on the exposed surface of the magnet of the present invention such as in the case when they are to be used in some special circumstances. Even in such a case, the magnet of the present invention has its merits in that there will be no rust from inside the magnet and accordingly good adhesion can be obtained when the protective film is to be formed on the exposed surface of the magnet. This will eliminate the problems such as the peeling of the film due to poor adhesion and the problem of bad dimentional precision due to the variation of film thickness. Thus, we can provide the permanent magnets most suitable for uses in which oxidation resistance is required. In another aspect, the present invention is to provide a process for producing an R-Fe-B-C based permanent magnet alloy having such a characteristic structure that individual magnetic crystal grains of said alloy are covered with a non-magnetic film which has the C content higher than that of the magnetic crystal grains. Thus, the behavior of C is very important. Hence, first reference will be given to C in question. Behavior of C So far, C in the magnet of this system has been considered as follows. For instance, Japanese Patent Public Disclosure No. 59-46008 specifies the inclusion of 2 - 28 at.% B in a magnet and points out that its coercive force (iHc) will decrease below 1 kOe if the B content is less than 2 at.%. This patent merely states that if a large amount of B is to be used, part of B may be replaced with C for the reduction in production cost. Further, Japanese Patent Public Disclosure No. 59-163803 discloses an R-Fe-Co-B-C based magnet containing 2 - 28 at.% B and up to 4 at.% C. This patent teaches the combined use of B and C in a specific way but notwithstanding its use in combination with C, boron must be contained in an amount of at least 2 at.% and it is specifically mentioned that below 2 at.% B, the magnet has an iHc of less than 1 kOe as in the case described in Japanese Patent Public Disclosure No. 59-46008. In other words, as said patent points out, carbon is considered as an impurity that is detrimental to magnetic characteristics and it is unavoidable that the magnet is contaminated by C which originates from lubricants and other additives used in the compaction of powders. Since the procedure of completely eliminating this impurity increases the production cost, the patent proposes that the C content of up to 4 at.% be permissible if the Br value to be achieved is no more than 4,000 G which is comparable to that of a hard ferrite magnet. Hence, carbon produces negative effects on magnetic characteristics and it is not necessarily an essential element. Japanese Patent Public Disclosure No. 62-13304 proposes that for the purpose of improving the oxidation resistance of R-Fe-Co-B-C based magnets the C content be reduced to 0.05 wt% (ca. 0.3% on an atomic basis or below). Japanese Patent Public Disclosure No. 63-77103 filed by a different applicant also proposes that the C content be reduced to 1,000 ppm or below to attain the same objective. Thus, in the prior art, carbon has been considered to be a negative element also in regard of oxidation-resisting properties. The present inventors deliberately incorporated C, which had been considered as a negative element for the magnetic characteristics and the oxidation-resistant properties, in the grain boundary phase and found that this enabled the formation of an oxidation-resistant protective film on the surface of individual magnetic crystal grains and that this helped improve the magnetic characteristics of the magnet. In other words, the intentional inclusion of C in the grain boundary phase offered the advantage that even when the B content was within the known range commonly employed in the art, an improvement in oxidation resistance was achieved, with particularly good results being attained when the B content was less than 2 at.%. It was held in the prior art that iHc would become 1 kOe or below when the B content was less than 2 at.% but in accordance with the present invention, iHc values of at least 4 kOe can be achieved even if the B content is less than 2 at.%. This novel effect has been attained by the formation of the C-containing oxidation-resistant protective film. The present invention provides a process for drastically enhancing the oxidation resistance of the above-mentioned type magnet by positively incorporating C in the oxidation-resistant protective film which is formed on the individual magnetic crystal grains as a homogeneous and strong protective film, and as a means to form such an oxidation-resistant protective film, advantageously, the process of the invention contains one of the special treatments explained hereinbefore under (1), (2) and (3). The heat treatment explained above under (1), i.e., the heat treatment of the alloy ingot or powder before the compaction step at a temperature in the range of 500 - 1,100°C for 0.5 h or more is effective to accelerate the segregation of C into the grain boundary. If the alloy ingot or powder before the steps of compacting and sintering is heated to a temperature in the range of 500 - 1,100°C, preferably in the range of 700 - 1,050°C, the migration of C to the grain boundary interface is caused to result in the segregation of C. Japanese Patent Public Disclosure No. 61-143553 proposes the introduction of a heat-treatment step into the process of producing an alloy for the purpose of dissolving the problem of segregation in the cast alloy composition of an R-Fe-B based alloy. In contrast, the present invention does not aim at avoiding segregation but conducts heat treatment so as to positively cause the segregation of C. Thus, the object of the heat treatment and the manner in which it is effected in the process of the present invention are just the opposite of those used in the prior art process. In addition, the present invention has another merit in that the magnetic characteristics is also improved as a result of such heat treatment as mentioned under (1). In order to segregate C at the grain boundary interface by said heat treatment, the crude alloy should contain C. These elements can be the ones contained as contaminants inevitably introduced into the alloy during the melting step. It is more practical, however, that C source material is positively added to the alloy during the melting step. On the other hand, when the method previously mentioned under (2) is employed, i.e., when only the C source material is added after melting step but before compacting step, the C source material only, or C source material is secondly added to the crude alloy. Practically, it is preferred to effect this addition by incorporating a fine powder of raw material such as carbon black optionally containing cobalt in the crude alloy powder before the compaction thereof. By compacting and sintering the mixed powder of said crude alloy powder and the powder of said raw materials, the incorporation of C in the non-magnetic phase of a product magnet can be done more effectively. Whichever method may be used, the Br value of the final product magnet will be reduced significantly, if the C content of the oxidation-resistant protective film surrounding the individual magnetic crystal grains in the magnet exceeds 16 wt%. Hence, it is necessary to hold said upper limit value of 16 wt%. It is of course possible to form the oxidation-resistant protective film having the intended C content by combining the two methods previously mentioned under (1) and (2). By employing this combined method, it is possible to form a more homogeneous and stronger oxidation-resistant protective film on the surface of the magnetic crystal grains. Now, the components and the composition of the permanent magnet alloy of the present invention will be explained as follows. Components and Compositions of Alloys The composition of the magnet alloy of the present invention (as the sum of the magnetic crystal grains and the oxidation-resistant protective film) consists of 10 - 30% R, up to 3% (not inclusive of 0 at.%; but, even if less than 3%, satisfactory magnetic characteristics can be realized) B, 0.1 - 20%, preferably 0.5 - 20% C, all percentages being on an atomic basis, with the balance being Fe and incidental impurities. The symbol R used in the present invention as one of the indispensable elements of the alloy of the invention represents a rare-earth element which is one or two or more members selected from the group consisting of Y, La, Ce, Nd, Pr, Tb, Dy, Ho, Er, Sm, Gd, Eu, Pm, Tm, Yb and Lu. If desired, misch metal, didymium and other mixtures of rare-earth elements may also be used. The content of R in the magnet of the present invention is within the range of from 10 to 30 at.% since the values of Br exhibited within this range are highly satisfactory for practical purposes. B may be present in an amount exceeding 2 at.%, which has been the known upper limit of this element, and extending up to 28 at.%. Even within this range of the boron content, the oxidation resistance of the alloy can still be remarkably improved in comparison with the prior art alloy and the objectives of the present invention already mentioned could be attained. Preferably, however, the B content is less than 2 at.% and much better results can be attained if the B content is 1.8 at.% or less. If B is absent from the magnet, its oxidation resistance is improved but on the other hand, iHc will drop significantly. As a B source material pure boron or ferroboron can be used. If ferroboron is to be used, it may contain impurities such as Al or Si. The total C content of the magnet is in the range of 0.1 - 20 at.%, preferably in the range of 0.5 - 20 at.%. The presence of C in the oxidation-resistant protective film is not only effective for providing the protective film with the oxidation resistance but also for restraining the drop of iHc due to the decrease of B. Hence the content of carbon in the protective film is in the range of 0.05 - 16 wt%, preferably in the range of 0.1 - 16 wt%, more preferably 0.2 - 12 wt% in the composition of the oxidation-resistant protective film of the non-magnetic phase. If the C content of the protective film is less than 0.1 wt%, particularly less than 0.05 wt%, oxidation resistance will not be imparted to the magnet, and if then the B content of the same film is low, iHc will become lower than 4 kOe. If the C content of the protective film exceeds 16 wt%, the magnet will experience such a great drop in Br that it is no longer useful in practical applications. As regards the composition of the oxidation-resistant protective film, it preferably contains at least one, preferably substantially all of the alloying elements of which the magnetic crystal grains are made. The total C content of the magnet is set within the range of 0.1 - 20 at.%, more preferably in the range of 0.5 - 20 at.% from a practical viewpoint, because if it exceeds 20 at.%, the drop in Br will be significant, and if it is less than 0.5 at.%, particularly less than 0.1 at.%, the oxidation resistance will no longer be imparted to the magnet. As a C source material, carbon black, high purity carbon or alloys such as Nd-C, Fe-C, etc., may be used. According to the present invention a permanent magnet alloy having the above-mentioned composition is produced by the process including the following steps. Steps in the Production Process (a) Production of Crude Alloy Starting materials are weighed and mixed to obtain the mixture having the composition within the above-mentioned desired range. (If the method (2) is to be employed, decreased amount of C should be used in the raw material mixture considering the amount of C to be added in the later stage.) Then the mixture is melted under vacuum or in the atmosphere of inert gas by using a high-frequency induction furnace or an arc furnace. The resulting melt is cast into a water-cooled copper mold to form an alloy ingot, or alternatively a powder of the crude alloy is produced from the melt by means of the atomization method or the rotating disc method. (b) Heat Treatment of the Crude Alloy (Aforementioned Method (1)) The alloy ingot or the alloy powder obtained in the previous step is subjected to heat treatment to thereby cause the segregation of C as explained. This heat treatment comprises holding the product at an elevated temperature in the range of 500 - 1,100°C, preferably in the range of 700 - 1,050°C in an inert gas atmosphere for a period of 0.5 h or more. In doing this, if the temperature is less than 500°C, satisfactory segregation of C in the grain boundary phase will not be attained and the improvement of magnetic characteristics will also be unsatisfactory. On the other hand, if the temperature reaches 1,100°C, the advantage mentioned above will saturate. As regards holding time, less than 0.5 h will not bring about any significant advantage. If holding time of 0.5 h or more is given, apparent advantage will be obtained. Since extremely long time holding is economically disadvantageous, holding time of not greater than 24 h is preferred. As regards cooling rate after the heat treatment, no specific limitation will be required. After this heat treatment, grinding to the particle size of 32 mesh (500 µm)or less, preferably 100 mesh (149 µm)or less is effected by means of a jaw crusher, a roll crusher, a stamp mill or the like in an inert gas atmosphere. (c) Secondary Addition of C Source Material (Aforementioned Method (2)) According to this method, C is not added at all, or only part of C is added in the melting step and all the necessary or the supplementary amount of C is secondly added to incorporate the intended amount of this element in the alloy This secondary addition may be effected after the step of producing a crude alloy and before the step of compacting the powder. It is also possible to add this or these elements before the heat treatment for causing the segregation of C mentioned before so that the raw material containing the secondly added C may be subjected to heat treatment. By taking this method, the grain boundary phase having highly segregated C can be formed. The amount of C to be added secondly is the difference between the desired amount and the amount already added in the melting stage. In spite of whether the crude alloy is an alloy ingot or a powder, the mixture thereof with a C source material secondly added is preferably ground into fine powder by using a ball mill or a vibration mill. Alternatively, a finely powdered C source material may be added to the finely ground ingot or powder of the crude alloy before it is subjected to the compaction. Whichever method may be chosen, the C source material should be a fine powder in the range of up to 1 mm, preferably not greater than 200 µm in the particle size. (d) Compaction Stage The finely powdered material obtained in the above-mentioned stage is then formed into any desired shape by compaction. Generally, there exists a pulverizing stage for obtaining a fine powder before said compaction-shaping stage. This pulverizing is preferably effected either by a dry process which is carried out in an inert gas atmosphere or by a wet process which is carried out in an organic solvent such as toluene, etc. The average particle size of the powder is controled within the range of 1 - 50 µm, preferably 1 - 20 µm. If the raw material contains C which has been secondly added, this C will function as an agent to promote the pulverization. If the average particle size of the powder obtained by pulverizaion is less than 1 µm, particularly less than 0.3 µm, the powder is activated too much and is easy to be influenced by the oxidation. As a result, its magnetic characteristics is easy to drop. On the other hand, if the average particle size of the powder produced by pulverization exceeds 50 µm, particularly when it exceeds 150 µm, the magnet produced with this powder will fail to obtain a sufficiently high coercive force. If fine powder having an average particle size of 1 - 50 µm has been produced from a melt of a crude alloy by means of atomization, the powder can be directly subjected to the step of compaction after the heat treatment previously mentioned under (1) or after the secondary addition of C previously mentioned under (2) without being subjected to the step of pulverization stage. The fine powder thus obtained is then shaped by compaction under the molding pressure preferably in the range of 0.5 - 5 t/cm 2 . If high magnetic quality is desired, compaction may be effected under applied magnetic field (in the range of 5 - 20 kOe). This compaction may be carried out in an organic solvent such as toluene, or alternatively by a dry process using stearic acid, etc., as a lubricant. If the raw material contains the secondly added C, this C also functions as a lubricant during the compaction stage. (e) Sintering Stage The compaction product is subsequently subjected to sintering treatment which is carried out in vacuum or in an inert gas or reducing atmosphere. Sintering is carried out at a temperature in the range of 950 - 1,150°C, preferably holding the sample at this temperature range for a period of 0.5 - 4 h. If the sintering temperature is less than 950°C, satisfactorily good sintering will not be attained. If the sintering temperature exceeds 1,150°C, the formation of coarse magnetic crystal grains proceed to result in the significant drop in Br and iHc. Less than 0.5 h of holding time will fail to provide a homogeneous sinter. More than 4 h of holding time will not add the advantage. In the cooling stage after the sintering treatment, quenching or the combination of slow cooling and quenching is preferably employed. Quenching may be carried out in a gaseous atmosphere or in an oil. Slow cooling may be effected in a furnace. The combination of slow cooling and quenching is the most preferred, and when this combination is used, slow cooling, which follows the sintering stage, is conducted at a cooling rate in the range of 0.5 - 20 °C/min. until the temperature reaches 600 - 1,050 °C at which quenching starts immediately. By treating in this manner, the oxidation-resistant protective film surrounding the magnetic crystal grains is made homogeneous and strong. If slow cooling is effected at a cooling rate out of the specified range of 0.5 - 20 °C/min., the film will not become sufficiently homogeneous. If quenching is started at a temperature out of the range of 600 - 1,050 °C, homogenization of said protective film will not be fully attained. (f) Final Heat Treatment Stage By subjecting the sintered sample to post heat treatment at a temperature in the range of 400 - 1,100 °C, preferably 500 - 1,050 °C for 0.5 - 24 h, further improvement of its magnetic property is attained. If this final heat treatment is carried out at a temperature lower than 400°C, the degree of improvement in the magnetic property is small. If it is carried out at a temperature higher than 1,100°C, sintering is accompanied and the resulting magnetic crystal grains will become coarse and the values of Br and iHc will drop. If the sample is held at the above-mentioned temperature range for less than 0.5 h, the degree of improvement in the magnetic property is small. If said holding period exceeds 24 h, the addition of improvement will be small. The permanent magnet alloy of the present invention prepared by the process mentioned above comprises magnetic crystal grains having a grain size within the range of 0.3 - 150 µm, preferably in the range of 0.5 - 50 µm, more preferably in the range of 1 - 30 µm and the grains are covered with the oxidation-resistant protective film whose thickness is in the range of 0.001 - 30 µm, preferably in the range of 0.001 - 15 µm, more preferably in the range of 0.005 - 15 µm. If the particle size of magnetic crystal grains becomes less than 0.5 µm, particularly when it becomes less than 0.3 µm, iHc will drop to less than 4 kOe. If said particle size exceeds 50 µm, particularly when it exceeds 150 µm, the iHc of the magnet will drop significantly to such an extent that the characteristic features of the magnet of the present invention will substantially lost. As regards the thickness of the oxidation-resistant protective film, if the protective film uniformly covers the individual magnetic crystal grains, the oxidation resistance will be held at a satisfactory value without depending on the thickness of the protective film. If the protective film becomes less than 0.001 µm thick, iHc of the magnet will drop significantly. If it exceeds 15 µm, particularly when it exceeds 30 µm, the Br of the magnet will drop significantly to such an extent that the characteristic features of the magnet of the present invention will be substantially lost. The thickness of this oxidation-resistant protective film includes the triple point of the grain boundary. The composition of the magnet alloy of the present invention can be analyzed with an electron probe microanalyzer (EPMA), the size of the magnetic crystal grains can be measured with a scanning electron microscope (SEM), and the thickness of the oxidation-resistant protective film can be measured with a TEM (as in the examples to be described hereinafter). The following examples are provided for the purpose of further illustrating the characteristics of the magnet of the present invention. Example 1 Starting materials, which consisted of 99.9% pure electrolytic iron, a ferroboron alloy with a boron content of 19.32%, 99.5% pure carbon black, and 98.5% pure neodymium metal containing other rare-earth elements as impurities, were weighed and mixed in such proportions that a composition designated by 18Nd/71Fe/1B/3C (at.%) would be obtained. The mixture was melted under vacuum in a high-frequency induction furnace and thereafter cast into a water-cooled copper mold to form an alloy ingot. The thus obtained alloy ingot was crushed into particles of 10 - 15 mm in size with a jaw crusher and subsequently held at 700°C for 5 h, followed by cooling at a rate of 50°C/min. The crushed ingot was then coarsely ground to a size of -100 mesh (-0.149 µm) with a stamp mill in an argon gas. Thereafter, 99.5% pure carbon black was added to the coarsely ground ingot in such an amount that a composition designated by 18Nd/71Fe/1B/10C (at.%) would be obtained. Then, the mixture was finely ground to an average particle size of 5 µm by means of a vibrating mill. The thus obtained alloy powder was compacted at a pressure of 1 ton/cm 2 in a magnetic field of 10 kOe, held in an argon gas at 1,100°C for 1 h and subsequently quenched to obtain a sinter. Comparative Example 1 A sample was prepared by repeating the procedure of Example 1 except that no carbon black was used. Starting materials were weighed and mixed to provide a composition designated by 18Nd/76Fe/6B (at.%). The mixture was subsequently treated as in Example 1, i.e., it was melted (in the absence of carbon black), coarsely ground, pulverized, compacted in a magnetic field, sintered and quenched to obtain a sinter. In order to evaluate the oxidation resistance of the sinters, they were subjected to a weathering test in which they were left to stand in a hot and humid atmosphere (60°C x 90% RH) for 7 months (5 040 h). Demagnetization (drop in Br and iHc) data and curves for the respective sinters are shown in Table 1 and Fig. 1, respectively. As is clear from Fig. 1, the sinter prepared in Example 1 by coating magnetic crystal grains with a Containing protective film experienced very small degrees of demagnetization (-0.36% in Br as indicated by a solid line, and -0.1% in iHc as indicated by a dashed line) after 7 months, showing that said sinter had very high resistance to oxidation. On the other hand, the sinter prepared in Comparative Example 1 which was not protected by a C-containing film experienced significant demagnetization (-9.8% in Br and -3.0% in iHc) only after 1 month (720 h) and upon further standing, it rusted so heavily that Br and iHc measurements were impossible. Fig. 2 is a SEM micrograph showing the microstructure of the sinter of Example 1. The same sinter was subjected to spectral line analyses for C and Nd elements with EPMA and the result is shown in photo in Fig. 3. Fig. 4 shows spectral lines for the respective elements as reproduced from the photo of Fig. 3. These pictures clearly show that the magnetic crystal grains are covered with a C-containing oxidation-resistant protective film and that the greater part of C is present in the Nd-rich portion of this protective film. The C content of the protective film was 6.1 wt%. The size of magnetic crystal grains was measured for 100 grains selected from the SEM micrograph showing the microstructure of the sinter and it was found to be within the range of 0.7 - 25 µm. The thickness of the protective film as measured with TEM was 0.01 - 5.6 µm. The values of grain size and film thickness are also shown in Table 1. Magnetization measurements were conducted with a vibrating-sample magnetometer (VSM) and the values of Br, iHc and (BH)max thus measured are shown in Table 1. As the above results show, the permanent magnet alloy of the present invention is much more resistant to oxidation than the known sample of Comparative Example 1, and the magnetic characteristics of this alloy are comparable to or better than those of the known sample. Examples 2 - 6 Sinters were prepared by repeating the procedure of Example 1 except that the starting materials to be melted were weighed and mixed to provide the boron (B) contents shown in Table 1. Comparative Example 2 A sinter was prepared by the same procedure except that no boron was incorporated (B = 0 at.%). The oxidation resistance of each sinter, the C content of the protective film, the size of magnetic crystal grains, the thickness of the protective film and the magnetic characteristics of each sinter were evaluated as in Example 1 and the results are shown in Table 1. Demagnetization curves for the sinters prepared in Examples 5 and 6 are also shown in Fig. 1. The above results show that the sinters prepared in accordance with the present invention by coating magnetic crystal grains with a C-containing protective film experienced very small degrees of demagnetization over a prolonged period, indicating their great ability to resist oxidation. This effect was reasonably displayed by the sample prepared in Example 6 which contained 3 at.% B, but particularly good results were attained when the B content was less than 2 at.% as in the samples that were prepared in Examples 1 and 5 and depicted in Fig. 1. Examples 7 - 10 Additional sinters were prepared by repeating the procedure of Example 1 except that carbon black was further added just before the pulverization step in order to provide the carbon contents shown in Table 1. In Example 7, carbon black was not added to the starting materials to be melted but it was totally added just before the pulverization step. Comparative Example 3 A sinter was prepared by repeating the procedure of Comparative Example 1 except that the starting materials were weighed and mixed to provide a composition designated by 18Nd/81Fe/1B (at.%). Comparative Example 4 A sinter was prepared by repeating the procedure of the above examples except that the starting materials were weighed and mixed to provide a composition designated by 18Nd/56Fe/1B/25C. The oxidation resistance of each sinter, the C content of the protective film, the size of magnetic crystal grains, the thickness of the protective film and the magnetic characteristics of each sinter were evaluated as in Example 1 and the results are shown in Table 1. As the data in Table 1 shows, all the sinters that satisfied the requirements of the present invention for alloy composition (at. percent) and protective film experienced small degrees of demagnetization and displayed high oxidation resistance. The sample prepared in Comparative Example 3 did not contain carbon in the protective film, so it rusted too heavily to justify the measurement of oxidation resistance. The sample prepared in Comparative Example 4 contained such a great amount of carbon in the protective film that the value of Br was undesirably low. Examples 11 - 13 Sinters were prepared by repeating the procedure of Example 1 except that the starting materials were weighed and mixed to provide the neodymium contents shown in Table 2. The oxidation resistance of each sinter, the C content of the protective film, the size of magnetic crystal grains, the thickness of the protective film and the magnetic characteristics of each sinter were evaluated as in Example 1 and the results are shown in Table 2. As the data in Table 2 shows, the sinters of the present invention had excellent magnetic characteristics and their resistance to oxidation was also very satisfactory. Examples 14 - 22 Additional sinters were prepared by repeating the procedure of Example 1 except that the neodymium added to the starting materials to be melted was replaced by other rare-earth elements as set forth in Table 2. The oxidation resistance of each sinter, the C content of the protective film, the size of magnetic crystal grains, the thickness of the protective film and the magnetic characteristics of each sinter were evaluated as in Example 1 and the results are shown in Table 2. As the data in Table 2 shows, the sintered magnets of the present invention had excellent magnetic characteristics and their resistance to oxidation was also very satisfactory. Example 23 A sinter was prepared by repeating the procedure of Example 1 except that the fine alloy powder was compacted in the absence of an applied magnetic field. The oxidation resistance of the sinter, the C content of the protective film, the size of magnetic crystal grains, the thickness of the protective film and the magnetic characteristics of the sinter were evaluated as in Example 1 and the results are shown in Table 2. Examples 23a - 23d A sinter was prepared by repeating the procedure of Example 1 except that the starting materials were weighed and mixed to provide the neodymium contents shown in Table 2. The oxidation resistance of the sinter, the C content of the protective film, the size of magnetic crystal grains, the thickness of the protective film and the magnetic characteristics of the sinter were evaluated as in Example 1 and the results are shown in Table 2. The advantage of the present invention will be shown below by referring to the representative examples of the process of the present invention. Example 24 Starting materials, which consisted of 99.9% pure electrolytic iron, a ferroboron alloy with a boron content of 19.32%, 99.5% pure carbon black, and a 98.5% pure neodymium metal containing other rare-earth elements as impurities, were weighed and mixed in such proportions that a composition designated by 18Nd/76Fe/3B/3C would be obtained. The mixture was melted under vacuum in high-frequency induction furnace and thereafter cast into a water-cooled copper mold to form an alloy ingot. The thus obtained alloy ingot was heat treated at 800°C for 15 h and then was held to stand in a fumace for cooling. Then, the alloy ingot was crushed into particles with a jaw crusher and was then coarsely ground to a size of -100 mesh with a stamp mill in an argon gas and was further finely ground to an average particle size of 5 µm by means of a vibrating mill. The thus obtained alloy powder was compacted at a pressure of 1 ton/cm 2 in a magnetic field of 10 kOe. The resulting shaped product was held in an argon gas at 1,100°C for 1 h and subsequently quenched to obtain a sinter. Comparative Example 5 A sinter was prepared by repeating the procedure of Example 24 except that the heat treatment of the alloy ingot was omitted. In order to evaluate the oxidation resistance of the sinters obtained in Example 24 and in Comparative Example 5 they were subjected to an evaluation test for determining the oxidation resistance (a weathering test). This test was carried out by leaving the samples to stand in a hot and humid atmosphere (60°C x 90% RH) for 7 months (5,040 h) and then measuring the demagnetization (drop in Br and iHc). The results are shown in Table 3 and Fig. 5. As is clear from Fig. 5 and Table 3, the sinter prepared in Example 24 experienced very small degrees of demagnetization as shown by -0.98% in Br, and -0.56% in iHc after 7 months. This shows that the oxidation resistance of this sinter had been remarkably improved. In contrast, the sinter prepared in Comparative Example 5 experienced significant demagnetization as shown by -3.27% in Br and -5.8% in iHc. Demagnetization data of some other sinters prepared in the examples to be described hereinafter are also shown in Fig. 5. Fig. 6 shows spectral lines for the respective elements as reproduced from the photo of spectral line analyses for Fe, C and Nd elements with EPMA. These pictures clearly show that the magnetic crystal grains are covered with a C-containing oxidation-resistant protective film and that the greater part of C is present in the Nd-rich portion of this protective film. The C content of the protective film was 4.7 wt%. The size of magnetic crystal grains was measured for 100 grains selected from the SEM micrograph showing the microstructure of the sinter and it was found to be within the range of 1.8 - 21 µm. The thickness of the protective film as measured with TEM was 0.013 - 5.8 µm. These values are shown in Table 3 given hereinbelow. Magnetization measurements were conducted with a vibrating sample magnetometer (VSM) and the values of Br, iHc and (BH)max thus measured are shown in Table 3. As the above results show, the permanent magnet alloy of the present invention is much more resistant to oxidation than the known sample of Comparative Example, and the magnetic characteristics of this alloy are comparable to or better than those of the known sample. Examples 25 - 27 Sinters were prepared by repeating the procedure of Example 24 except that the heat treatment temperature of the alloy ingot and the holding time were, in the respective case, 600°C x 24 h (in Example 25), 1,000°C x 0.5 h (in Example 26) and 1,100°C x 0.5 h (in Example 27). The oxidation resistance of each sinter, the C content of the protective film, the size of magnetic crystal grains, the thickness of the protective film and the magnetic characteristics of each sinter were evaluated as in Example 24 and the results are shown in Table 3. Example 28 Starting materials, which consisted of 99.9% pure electrolytic iron, a ferroboron alloy with a boron content of 19.32%, 99.5% pure carbon black and a 98.5% pure neodymium metal (containing other rare-earth elements as impurities), were weighed and mixed in such proportions that a composition designated by 18Nd/76Fe/3B/1C would be obtained. The mixture was melted under vacuum in a high-frequency induction furnace and thereafter cast into a water-cooled copper mold to form an alloy ingot. The thus obtained alloy ingot was crushed with a jaw crusher and the crushed ingot was then coarsely ground to a size of -100 mesh with a stamp mill in an argon gas. Thereafter 99.5% pure carbon black was added to the coarsely ground ingot in such an amount that a composition designated by 18Nd/76Fe/3B/3C would be obtained. Then, the mixture was finely ground to an average particle size of 5 µm by means of a vibrating mill. The thus obtained alloy powder was compacted at a pressure of 1 ton/cm 2 in a magnetic field of 10 kOe, held in an argon gas at 1,100°C for 1 h and subsequently quenched to obtain a sinter. With respect to the sinter thus obtained, the C content of the protective film, the size of magnetic crystal grains, the thickness of the protective film and the magnetic characteristics were evaluated as in Example 24 and the results are shown in Table 4. Examples 29 - 30 Sinters were prepared by repeating the procedure of Example 28 except that the amount of carbon for the primary addition to be made in the melting stage and that for the secondary addition to be made either in the coarsely grinding stage or in the finely grinding stage were changed as shown in Table 4. With respect to the sinters thus obtained, the C content of the protective film, the size of magnetic crystal grains, the thickness of the protective film and the magnetic characteristics were evaluated as in Example 24 and the results are shown in Table 4. The primary composition as given in Table 4 means the composition in the melting stage, and the secondary composition as given in the same table means that in the sintering stage. Examples 31 Sinters were prepared by repeating the procedure of Example 28 except that the extra stage of subjecting the alloy ingot to heat treatment at 700°C for 18 h was added. With respect to the sinters thus obtained, the oxidation resistance, the C content of the protective film, the size of magnetic crystal grains, the thickness of the protective film and the magnetic characteristics were evaluated as in Example 27 and the results are shown in Table 4. Examples 32 - 38 Sinters were prepared by repeating the procedure of Example 24 except that the temperature of sintering, the holding time for sintering, the slow cooling rate after sintering and the temperature at which quenching was to start were changed as shown in Table 5. With respect to the sinters thus obtained, the oxidation resistance, the C content of the protective film, the size of magnetic crystal grains, the thickness of the protective film and the magnetic characteristics were evaluated as in Example 24 and the results are shown in Table 5. Examples 39 - 41 The same procedure as in Example 24 was repeated except that sinters were subjected to the final heat treatment under the conditions as shown in Table 6. With respect to the sinters thus obtained, the oxidation resistance, the C content of the protective film, the size of magnetic crystal grains, the thickness of the protective film and the magnetic characteristics were evaluated as in Example 24 and the results are shown in Table 6. Example 28 29 30 31 1st 18Nd-76Fe-3B-1C 18Nd-76Fe-3B-2C 18Nd-76Fe-3B 18Nd-76Fe-3B-1C Composition 2nd 18Nd-76Fe-3B-3C 18Nd-76Fe-3B-3C 18Nd-76Fe-3B-3C 18Nd-76Fe-3B-3C Conditions for Heat Treating Alloys Temperature (°C) - - - 700 time (hr) - - - 18 Oxidation Resistance (%) ΔBr -1.12 -1.28 -0.98 -0.86 ΔiHc -1.09 -2.15 -0.87 -0.47 Br (kG) 10.8 10.5 11.7 11.8 iHc (kOe) 10.7 10.5 11.3 11.4 (BH)max (MGOe) 26.3 25.9 28.0 30.9 C Content in Protective Film (wt.%) 5.2 4.8 6.7 5.8 Thickness of Protective Film (µm) 0.009-5.3 0.008-5.5 0.012-5.1 0.009-5.2 Size of Magnetic Crystal Grains (µm) 1.2-18 1.6-21 1.8-23 2.1-19 Examples 42 - 51 Sinters were prepared by repeating the procedure of Example 24 except that the compositions were changed as shown in Table 7. With respect to the sinters thus obtained, the oxidation resistance, the C content of the protective film, the size of magnetic crystal grains, the thickness of the protective film and the magnetic characteristics were evaluated as in Example 24 and the results are shown in Table 7. Example 52 Sinters were prepared by repeating the procedure of Example 24 except that the compaction of the alloy fine powder was conducted in the non-magnetic field. With respect to the sinters thus obtained, the oxidation resistance, the C content of the protective film, the size of magnetic crystal grains, the thickness of the protective film and the magnetic characteristics were evaluated as in Example 24 and the results are shown in Table 7. Example 53 Sinters were prepared by repeating the procedure of Example 24 except that the alloy powder produced by atomizing the molten crude alloy in the argon atmosphere was subjected to heat treatment at 800°C for 15 h followed by cooling, and the powder thus obtained was compacted in the non-magnetic field. With respect to the sinters thus obtained, the oxidation resistance, the C content of the protective film, the size of magnetic crystal grains, the thickness of the protective film and the magnetic characteristics were evaluated as in Example 24 and the results are shown in Table 7. Examples 53a - 53c Sinters were prepared by repeating the procedure of Example 24 except that the starting materials were weighed and mixed to provide the neodymium contents shown in Table 7. With respect to the sinters thus obtained, the oxidation resistance, the C content of the protective film, the size of magnetic crystal grains, the thickness of the protective film and the magnetic characteristics were evaluated as in Example 24 and the results are shown in Table 7.

Claims: A sintered permanent magnet alloy having an improved resistance to oxidation which is based on an R-Fe-B-C system, R is at least one of the rare-earth elements including Y, the individual magnetic crystal grains of said alloy being covered with an oxidation-resistant protective film containing C, the R content of said protective film being higher than that of said grains, characterized in that 0.05-16wt% of said protective film is composed of C and the composition of said magnet alloy as the sum of the magnetic crystal grains and the oxidation-resistant protective film consists of 10 - 30% R, up to 3%, not inclusive of zero percent, of B and 0.1 - 20% C, all percentages being on an atomic basis, with the balance being Fe and incidental impurities. A sintered permanent magnet alloy according to claim 1 wherein the composition of said magnet alloy as the sum of the magnetic crystal grains and the oxidation-resistant protective film consists of less than 2% of B. A sintered permanent magnet alloy according to one of the claims 1 to 2 wherein said magnetic crystal grains have a particle size in the range of 0.3-150µm and said oxidation-resistant protective film has a thickness in the range of 0.001-30µm. A sintered permanent magnet alloy according to at least one of the claims 1 to 3 wherein the C content of the oxidation-resistant protective film is higher than that of the individual magnetic crystal grains. A sintered permanent magnet alloy according to at least one of the claims 1 to 4, characterized in that the oxidation-resistant protective film contains all of the alloy elements of which said magnetic crystal grains were made. A sintered permanent magnet alloy according to claim 5, characterized in that the composition as the sum of the magnetic crystal grains and the oxidation-resistant protective film contains 0.5-20% C. A sintered permanent magnet alloy according to at least one of the claims 1 to 6, wherein 0.05-16wt%, preferably 0.1-16wt%, of said oxidation-resistant protective film of the R-Fe-B-C system is composed of C. A sintered permanent magnet alloy according to at least one of the claims 1 to 7 wherein R is at least one rare-earth element of the group comprising Y, La, Ce, Nd, Pr, Tb, Dy, Ho, Er, Sm, Gd, Eu, Pm, Tm, Yb and Lu. In a process for producing a sintered permanent magnet alloy according to at least one of the claims 1 to 8, comprising the steps of preparing a molten crude alloy R-Fe-B-C, producing a powder directly therefrom and after casting it into an alloy ingot and then grinding into the powder, compacting the thus obtained powder and sintering the compacted product, the improvement characterized in that the ingot or powder of the alloy before being sent to the compacting step is subjected to a heat treatment which is carried out at a temperature in the range of 500-1100°C for a period of 0.5h or more, and optionally in that the sintered compacted product is subjected to a final heat treatment. In a process for producing a sintered permanent magnet alloy according to at least one of the claims 1 to 8, comprising the steps of preparing a molten crude alloy, producing a powder directly therefrom or after casting it into an alloy ingot and then grinding into the powder, compacting the thus obtained powder and sintering the compacted product, the improvement characterized in that part or all of the C source material is added to the raw material mixture in a step which is given after said step of preparing a molten crude alloy but before said step of compacting the powder, and optionally in that the sintered compacted product is subjected to a final heat treatment. Process according to claim 10, characterized in that the alloy ingot or powder in a step before said compacted step is subjected to a heat treatment which is carried out at a temperature in the range of 500-1100°C for a period of 0.5h or more. Process according to at least one of the claims 9 to 11 wherein the final heat treatment is carried out at a temperature in the range of 400-1100°C. Process according to at least one of the claims 9 to 12 wherein said sintering step is carried out by holding the material at a temperature in the range of 950-1150°C for a period of 0.5-4h. Process according to at least one of the claims 9 to 13, characterized in that the sintering step is followed by slow cooling, preferably at a rate in the range of 0.5 to 20°C/min. Process according to at least one of the claim 9 to 14, characterized in that the sintering step is followed by slow cooling and then quenching from a temperature in the range of 600-1050°C.

Inventors: HISANO SEIICHI, ISOYAMA SEIJI, SATO YUICHI, SENDA MASAYASU, UEDA TOSHIO, HISANO, SEIICHI, ISOYAMA, SEIJI, SATO, YUICHI, SENDA, MASAYASU, UEDA, TOSHIO, Hisano, Seiichi, c/o Dowa Mining Co.,Ltd., Isoyama, Seiji, c/o Dowa Mining Co.,Ltd., Sato, Yuichi, c/o Dowa Mining Co.,Ltd., Senda, Masayasu, c/o Dowa Mining Co.,Ltd., Ueda, Toshio, c/o Dowa Mining Co.,Ltd.
Applicants: DOWA MINING CO, DOWA MINING CO., LTD.
Application year: 1990
Main classification: H01F1
IPC codes: H01F1