SINTERED MAGNETIC Fe-Co MATERIAL AND PROCESS FOR ITS PRODUCTION

Abstract: An economically advantageous process for producing sintered magnetic Fe-Co, Fe-Co-V or Fe-Co-Cr material, which comprises preparing alloy powder of at least Fe and Co, kneading it with an organic binder, subjecting the kneaded mixture to injection molding and degreasing, and conducting two-stage sintering at both low and high temperatures is disclosed. A magnetic material having excellent magnetic properties and low iron loss value and comprising a specific composition of Fe-Co, Fe-Co-V or Fe-Co-Cr is also disclosed.

Description: Technical Field: This invention relates to a process for producing sintered Fe-Co type magnetic materials having excellent dc or ac magnetic properties by injection molding. Background Art: Alloys of the Fe-Co type are known as soft magnetic materials and have the maximum saturated magnetic flux density of all magnetic materials. They are expected to have utility for motors and magnetic yokes which are required to transmit high magnetic energy in spite of small dimensions. Fe-Co type alloys in the form of ingots are however accompanied by the drawback that they do not practically permit cold working because of their brittleness. It has hence been attempted to improve the cold workability by adding vanadium. No sufficient cold workability has however been achieved yet, although some improvements are observed. Powder metallurgy is considered to be a useful means for overcoming such poor workability. This process however has difficulties in achieving sintered products of desired density, so that materials having practical magnetic properties have not been obtained. A variety of methods have hence been proposed. For example, it is attempted in Japanese Patent Application Laid-Open No. 291934/1986 to improve the compressibility and sinterability by using an Fe-Co alloy in which no ordered lattices have been formed. In Japanese Patent Applicaion Laid-Open No. 54041/1987, the sintered density has been improved by hot isostatic press (HIP) processing. In Japanese Patent Application Laid-Open No. 142750/1987, the green density and sintered density have been improved by using a coarse Fe-Co alloy powder and a fine Co powder in combination. However, all of these proposals use compression forming. They can use only poor-sinterability coarse powders of such a size that the compressibility is not impaired and the powders are not taken into mold clearances. The resulting sintered materials therefore have low magnetic properties. There has thus been a demand for sintered materials having still higher magnetic properties. Further, Japanese Patent Application Laid-Open No. 85650/1980 discloses the attempted production of a high-density sintered material by adding 0.1-0.4% of boron to an alloy of the Fe-Co type. On the other hand, Japanese Patent Publication No. 38663/1982 (Japanese Patent Application Laid-Open No. 85649/1980) discloses the attempted production of a high-density sintered material by adding 0.05-0.7% of phosphorus to an alloy of the Fe-Co type. However, all of these methods enhance densification by using the formation of a transitional liquid phase in the course of sintering, which in turn relies upon a third element. It is thus necessary to strictly control the sintering temperature within a narrow range, thereby making it difficult to achieve a high yield upon-mass production. Moreover, the elements whose addition is proposed are considered to aggravate the brittleness of Fe-Co alloys, leading to the problem that cracking or chipping may take place during the working step in which sintered products are finished to form precision parts. In addition, Japanese Patent Application Laid-Open Nos. 291934/1986 and 142750/1987 require a sintering treatment at a temperature as high as 1300-1400°C, while Japanese Patent Application Laid-Open No. 54041/1987 needs a high pressure of at least 800 atm in addition to sintering at a high temperature of about 1300°C. It is hence not only difficult to conduct mass production but also necessary to use special facilities. The methods of these publications are therefore not economical. On the other hand, materials consisting practically of Fe and Co alone have a low electrical resistivity and their core loss values increase when employed under ac power. It may hence be contemplated to add a third component to a material of the Fe-Co type. For example, materials of the Fe-Co-V type exhibit improved ac properties. However, the presence of such a third component introduces the problem that it is prone to oxidation upon sintering. This approach therefore has the disadvantage of inferior dc properties until such time as a production process capable of inhibiting oxidation is developed. In JP-A-59 125 602 a sintered magnetic material is produced by kneading a Fe-Co-Cr powder with at least two organic binders followed by injection molding, debinding and sintering. Only a single stage sintering treatment is disclosed and generally the sintering temperature is greater than 1350°C unless Si and/or B is added to improve the sintering density in which case sintering may be effected at 1200 to 1300°C. JP-A-7527010 describes the production of sintered magnetic materials by press molding a powder containing Fe, Co and V followed by a single sintering treatment at about 1400°C in an inert gas atmosphere. The sintered material is then cold worked and subjected to a heat-treatment at about 550°C. US 4721599 describes the production of sintered articles by kneading a metal powder with an organic binder, injection molding the mixture, debinding the molding and then subjecting it to a single sintering treatment. There is no disclosure of the application of the technique to Fe-Co type magnetic materials. An object of the present invention is to provide a process for the production of sintered Fe-Co type magnetic material which can be worked into intricate shapes, has excellent dc magnetic properties, a low core loss and a high saturated magnetic flux density, and also to provide an economical production process therefor. Another object of the present invention is to provide a process for the production of sintered Fe-Co type magnetic material having a small core loss value when employed under ac power and superb ac magnetic properties, and also to provide a production process therefor, said process featuring easy molding and the possibility of elimination of C, derived from an organic binder, without extreme oxidation of its components. According to a first aspect of the present invention there is provided a process for the production of a sintered Fe-Co type magnetic material, which comprises preparing an alloy powder and/or mixed powder, kneading the alloy power and/or mixed powder with at least one organic binder, subjecting the resultant composition to injection molding and debinding, and then subjecting the thus-obtained debound body to a sintering treatment characterised in that the alloy powder and/or mixed powder is a mixed powder of an Fe powder having an average particle size of 2-15 µm and a Co powder having an average particle size of 1-10 µm; an Fe-Co alloy powder having an average particle size of 3-10 µm; or a mixed powder of at least one of an Fe powder and a Co powder, both having an average particle size of 3-10 µm, and an Fe-Co alloy powder having an average particle size of 3-10 µm, said first-mentioned mixed powder, second-mentioned Fe-Co alloy powder or third-mentioned mixed powder having been prepared so as to have a final composition in which Co accounts for 15-60 wt.% and Fe substantially accounts for the remainder; and wherein the sintering treatment comprises sintering the debound body at an α-phase range temperature of 800-950°C in a reduced pressure atmosphere or a reduction gas atmosphere and then at a γ-phase range temperature of at least 1000°C in a non-oxidising atmosphere. According to a second aspect of the present invention there is provided a process for the production of a sintered Fe-Co-V type magnetic material which comprises preparing an alloy powder and/or mixed powder, kneading the alloy powder and/or mixed powder with at least one organic binder, subjecting the resultant composition injection molding and debinding, and then subjecting the thus-obtained debound body to a sintering treatment characterised in that the alloy powder and/or mixed powder has an average particle size of 3-25 µm and is prepared to have a final composition in which Co accounts for 15-60 wt.%, V for 0.5-3.5 wt.% and Fe substantially for the remainder; and wherein the sintering treatment comprises sintering the debound body at 1000-1300°C in a reduction gas atmosphere or a reduced-pressure atmosphere not higher than 3.99 kPa (30 Torr) and then at a temperature, which is at least 50°C higher than the preceding sintering temperature, in an inert gas atmosphere. According to a third aspect of the present invention there is provided a process for the production of a sintered Fe-Co-Cr type magnetic material which comprises preparing an alloy powder and/or mixed powder, kneading the alloy powder and/or mixed powder with at least one organic binder, subjecting the resultant composition to injection molding and debinding, and then subjecting the thus-obtained debound body to a sintering treatment characterised in that the alloy powder and/or mixed powder comprises an Fe powder having an average particle size of 2-15 µm, at least one powder selected from a Co powder having an average particle size of 1-10 µm or an Fe-Co alloy powder having an average particle size of 3-10 µm, and at least one powder selected from a Cr and/or Cr oxide powder having an average particle size of 1-30 µm and is prepared to have a final composition in which Co accounts for 20-50 wt.%, Cr for 0.5-3.5 wt.% and Fe substantially for the remainder; and wherein the sintering treatment comprises sintering the debound body at 1000-1350°C in a reduction gas atmosphere or a reduced-pressure atmosphere not higher than 3.99 kPa (30 Torr) and then at a temperature, which is at least 50°C higher than the preceding sintering temperature, in a non-oxidizing atmosphere. Dependent claims 4-6 relate to sintered Fe-Co, Fe-Co-V and Fe-Co-Cr type magnetic materials with particular compositions, densities and average grain sizes For a better understanding of the invention and to show how the same may be carried out, reference will now be made, by way of example, to the accompanying drawings, in which:- Fig. 1 is a graph based on Example 7 and diagrammatically illustrating the variation of magnetic flux density B 20 as a function of the sintering temperature; and Fig. 2 is a graph based on Example 7 and diagrammatically depicting the variation of electrical resistivity as a function of the sintering temperature. This invention will hereinafter be described in detail. First of all, the production process according to the first aspect of this invention will be described. In this process a metal powder is kneaded with an organic binder, followed by injection molding and debinding. The resultant debound body is then subjected to two-stage sintering treatment which is conducted under different conditions. In particular, the present invention involves the use of injection molding, which permits the formation of complex shapes, instead of compression forming which has hereto been employed generally. In contrast to the fact that raw material powders for compression forming are limited to coarse powders having poor sinterability, injection molding has the advantage that fine powders having high sinterability can be used. This has made it possible to improve the conventional low magnetic properties The subsequent two-stage sintering treatment under properly chosen different conditions can economically produce a sintered material having a high density and excellent magnetic properties. Starting material powders for forming raw material powders useful in this aspect of the present invention are metal or alloy powders prepared by a high-pressure water atomizing technique, a reduction technique, a carbonyl technique or the like. It is possible to choose a carbonyl Fe powder, water-atomized Fe powder, reduced Fe powder or the like as an iron source; an atomized Co powder, reduced Co powder, ground Co powder or the like as a cobalt source; and an atomized Fe-Co powder, ground Fe-Co powder or the like as an iron and cobalt source. They are used after adjusting their particle sizes to desired ranges by classification or grinding. The above-described starting materials may be used singly or as a mixed powder to provide a raw material powder useful in the practice of this invention. Regarding the purity of the raw material powder, it is sufficient if impurities other than C, O and N, which can be eliminated in the course of sintering, are practically negligible. In general, powders in which the sum of Fe and Co accounts for 97-99 wt.% can be used. Conventionally-known binders composed principally of one or more thermoplastic resins and waxes or a mixture thereof can be used in this invention. One or more plasticizers, lubricants, debinding promoters and the like may also be added as needed. As the thermoplastic resin, it is possible to choose from acrylic resins, polyethylene resins, polypropylene resins, polystyrene resins, vinyl chloride resins, vinylidene chloride resins, vinyl acetate resins and cellulose resins or a mixture or copolymer of two or more of these resins. As the wax, it is possible to choose and use one or more natural waxes led by bees wax, Japan wax, montan wax and the like and synthetic waxes represented by low-molecular polyethylene, microcrystalline wax, paraffin wax and the like. The plasticizer is selected depending on the resin or wax used as base-material and with which the plasticizer is combined. Dioctyl phthalate (DOP), diethyl phthalate (DEP), diheptyl phthalate (DHP) or the like can be used. As the lubricant, one or more higher fatty acids, fatty acid amides, fatty acid esters and the like can be used. In some instances, the wax may also be used as a lubricant. Further, a sublimable substance such as camphor may also be added to promote debinding. The amount of binder to be added is from 45 to 60 vol.% of the whole volume, the remaining volume being the raw material metal powder. It can be adjusted depending upon the ease of molding of the shape to be formed and the debindability. For the mixing and kneading of the powder and binder, a kneader of the batch type or continuous type can be used. After the kneading, granulation is effected using a pelletizer or grinding mill to obtain a molding raw material. The molding raw material can be molded by using a conventional plastic injection molding machine. The green body thus obtained is subjected to a debinding treatment in the atmosphere or in a surrounding gas. This is a step which is conducted to eliminate the binder after the molding. No particular limitation is imposed on it. For example, the green body may be heated at a constant rate in a non-oxidizing atmosphere such as a reduction gas atmosphere, inert gas atmosphere or reduced-pressure atmosphere and then maintained at a temperature of 400-700°C therein. It is preferable to raise the temperature at a rate of 5-100°C/hr because unduly high heating rates tend to result in the development of cracks and bulges in the final product. Although no particular limitation is necessarily imposed on the atmosphere gas for the sintering treatment, a reduced-pressure atmosphere or reduction gas atmosphere is used for the low-temperature sintering and an inert gas atmosphere is preferably used for the high-temperature sintering. Further, the material whose sintering has been completed may be subjected to magnetic annealing as needed. Magnetic annealing can be conducted at a temperature of 800-950°C or so in a non-oxidizing atmosphere. The present inventors have found that the magnetic properties of a sintered body are closely related to the particle size of the raw material powder. The average particle size of the raw material powder governs the sintered density. Particle sizes greater than a certain upper limit cannot provide a sintered material according to this invention. When a mixed powder of an Fe powder and a Co powder is used as a raw material powder, it is impossible to achieve a sintered density ratio of 95% or higher if the average particle size of the Fe powder exceeds 15 µm or the average particle size of the Co powder is greater than 10 µm. When an Fe-Co alloy powder is used, sintered densities of 95% or higher cannot be obtained should the average particle size exceed 10 µm. When a mixed powder composed of at least one of an Fe powder and a Co powder and an Fe-Co alloy powder is used, sintered densities of 95% or higher cannot be obtained should the average particle size exceed 10 µm. On the other hand, when the average particle sizes of the above-described Fe powder, Co powder, Fe-Co alloy powder, and mixed powder composed of at least one of the Fe powder and Co powder and the Fe-Co alloy powder are smaller than 2 µm, 1 µm, 3 µm and 3 µm, respectively, improvements in the magnetic properties are not substantial but the cost of the powders becomes very high. It is therefore not economical to use such fine powders. With the foregoing in view, when a mixed powder of an Fe powder and a Co powder is used as a raw material powder, the average particle sizes of the Fe powder and Co powders is limited to 2-15 µm and 1-10 µm, respectively. When an Fe-Co alloy powder is used on the other hand, the average particle size is limited to 3-10 µm. Further, when a mixed powder composed of at least one of an Fe powder and a Co powder and an Fe-Co alloy powder is used, the average particle size is limited to 3-10 µm. It is necessary to carefully control the sintering conditions, because they affect the density, void shape, crystal grain size, impurity levels, etc. of the sintered material. In the case of an injection-molded body using a raw material powder of the above-described particle size, a sintered material having better magnetic properties than conventional sintered materials can be obtained even when its sintering is conducted only at a relatively low temperature in the α-phase temperature range. However, two-stage sintering is conducted under different conditions in accordance with this first aspect of the invention. First of all, sintering is conducted at a temperature in the α-phase range. The term "a-phase" as use herein means the a phase in the composition of the sintered final poduct. This α-phase sintering is effective in increasing the sintered density ratio of the sintered final product. The present inventors have found that when a powder having a smaller average particle size like the raw material useful in the present invention is sintered, significant crystal growth takes place in a composition of the Fe-Co type if the temperature is raised immediately from the a phase, the low temperature phase, to the γ phase which is the high temperature phase. As a result of this crystal growth, voids are left over among crystal grains so that the attempted increase of the sintered density ratio is impaired. On the other hand, no crystal growth takes place in α-phase sintering, whereby crystal grain boundaries are fixed at voids. It is hence possible to easily eliminate the voids by allowing diffusion at atomic level through the crystal grain boundaries. As a result, the sintered density ratio can be increased sufficiently. Incidentally, the α-phase sintering may be repeated twice or more. The temperature range for the α-phase sinter is 800-950°C, while the holding time is 0.5-4 hr. Temperatures lower than 800°C cannot achieve sufficient sintering, whereas temperatures higher than 950°C induce transformation. The magnetic-properties are improved even by the α-phase sintering. In order to obtain still better magnetic properties, additional sintering is conducted subsequent to the α-phase sintering by raising the temperature via the α-to-γ transformation point to a temperature in the γ-phase range. The sintering in the γ-phase temperature range is very effective for the growth of crystals and also for the formation of voids into a spherical shape. In addition, it is also effective for the improvement of the sintered density ratio. These effects can each improve magnetic properties. Crystal growth takes place as mentioned above. Since the diffusion velocity of atoms in the matrix of an Fe-Co alloy at a temperature in the γ-phase range is sufficiently high, it is possible to easily form minute voids - which are formed when a fine powder like the material useful in this invention is employed - into a spherical shape and moreover even to eliminate a part of the voids. The temperature for the γ-phase sintering is at least 1000°C, while the holding time is 10-120 min. Incidentally, temperatures lower than 1000°C cannot induce any sufficient diffusion and crystal growth. No particular limitation is imposed on the atmosphere to be employed for the sintering according to this first aspect of the invention other than that the low temperature sintering is carried out in a reduced-pressure atmosphere or a a reduction gas atmosphere and the high temperature sintering is carried out in, a non-oxidizing atmosphere. It is particularly preferred for the reduction of C and O as impurities to conduct the sintering in a hydrogen atmosphere whose dew point has been controlled. The above-described holding times are merely illustrative of preferred embodiments and must not be taken as limiting the practice of this invention thereto. For example, this invention embraces a process in which α-phase sintering is carried out after conducting sintering in a γ-phase temperature range to a degree not impairing α-phase sintering, in other words, for a very short time such that no substantial crystal growth takes place. The sintered material of this invention can be economically produced by choosing the raw material powder and controlling the sintering temperature as described above. The sintered material obtained in accordance with the first aspect of this invention is preferably characterized by the following composition: Co : 15-60 wt.% O : 0.04 wt.% max. C : 0.02 wt.% max. Fe: remainder (including incidental impurities) and also by: sintered density ratio: 95% min. average crystal grain size: 50 µm min. First of all, the reasons for the above limitation of the final composition of the sintered material will be described. Co: 15-60 wt.% Substitution of Co for Fe is effective for improving the saturated magnetic flux density (Bs). However, Co amounts smaller than 15 wt.% or greater than 60 wt.% do not have the desired affect. The proportion of Co is therefore limited to 15-60 wt.%. C: 0.02 wt.% max., O: 0.04 wt.% max. C and O adversely affect magnetic properties, especially, coercive force (Hc) and maximum magnetic permeability (µ max ).As shown in Table 1, good Hc and µ max can be obtained by controlling the proportions of C and O to 0.02 wt.% max. and 0.04 wt.% max., respectively. Accordingly, the proportions of C and O are limited to 0.02 wt.% max. and 0.04 wt.% max. (C proportion ≦ 0.02 wt.%, O proportion ≦ 0.04%), respectively to improve the magnetic flux density in a low magnetic field. Incidentally, the proportions of C and O can be controlled by adjusting the sintering atmosphere. Sintered density ratio: 95% min. Sintered density ratio is a critical characteristic value, which directly governs the Bs of a sintered body and also affects its Hc and µ max . Table 2 shows measurement results of magnetic properties of sintered materials whose chemical compositions were substantially the same but whose sintered density ratios were changed by using raw material powders of different particle sizes. It can be seen from the measurement results that sintered density ratios smaller than 95% cannot improve the magnetic flux density in a low magnetic field. Accordingly, the sintered density ratio is limited to 95% min. Average crystal grain size: 50 µm min. Crystal grain size affects the energy required for the reversal of magnetic domains, so that it also affects Hc and µ max . Smaller crystal grain sizes deteriorate both Hc and µ max . Average crystal grain sizes smaller than 50 µm cannot assure magnetic properties comparable with those of ingots in a low magnetic field. The average crystal grain size is therefore limited to 50 µm min. When the average grain size increases, Hc and µ max are both improved and as a result, the magnetic properties in a low magnetic field are also improved. If the average crystal grain size however exceeds 500µm, the effects of improving the magnetic properties in a low magnetic field become smaller and the sintered body develops cracks more easily. It is therefore not preferred to make the crystal grain size unduly large. No. (wt%) Magnetic properties C O B80 (KG) B20 (KG) Hc (Oe) µmax 1 0.005 0.010 23.5 22.5 0.9 10500 2 0.005 0.015 23.3 22.4 1.0 9800 3 0.007 0.022 23.3 22.3 1.1 8700 4 0.016 0.017 23.4 22.1 1.3 7000 5 0.017 0.035 23.3 22.0 1.5 5500 6 0.046 0.019 23.2 21.7 1.7 3800 7 0.015 0.074 23.1 21.5 1.8 3700 Note: 1. Chemical composition, Fe: 51.3 ± 0.1 (wt%) Co: 48.6 ± 0.1 (wt%) 2. Sintered density ratio: 98 ± 0.3 (%) 3. Average crystal grain size: 300 ± 100 µm No. Raw material powder Sintered density ratio (%) B 80 (kG) B 20 (kG) Hc (Oe) µ max 9 Atomized Fe-Co powder(6.7µm) 98 23.3 22.6 0.7 11500 10 Atomized Fe-Co powder(9.7µm) 95 22.6 22.1 1.4 7100 11 Carbonyl Fe powder(3.5µm) + Atomized Co powder(9.3µm) 98 23.4 22.2 0.9 8400 12 Carbonyl Fe powder(5.5µm) + Reduced Co powder(6.3µm) 98 23.3 22.3 1.1 8800 13 Atomized Fe powder(14.1µm) + Reduced Co powder(5.2µm) 95 22.5 22.0 1.5 6800 14 Atomized Fe-20% Co powder(5.7µm) + Reduced Co powder (6.3µm) ⇒Average particle size of mixed powder was 5.9 µm. 98 23.3 22.4 0.8 9100 15 Atomized Fe-80% Co powder(7.7µm) + Carbonyl Fe powder(4.5µm) ⇒Average particle size of mixed powder was 6.2 µm. 97 23.0 22.3 1.2 7800 16 Atomized Fe-Co powder(10.7µm) 94 22.2 21.2 1.8 3500 17 Atomized Fe powder(16.1µm) + Reduced Co powder(6.3µm) 94 22.2 21.0 2.1 3300 18 Carbonyl Fe powder(5.3µm) + Atomized Co powder(11.7µm) 93 21.8 20.7 2.6 2200 Note: 1. Chemical composition, Fe:51.1±0.1(wt%), Co:48.8±0.1(wt%), C:0.003-0.005(wt%), 0:0.007-0.010(wt%) 2. α-phase sintering in hydrogen (dew point: +30° C) and γ-phase sintering in hydrogen (dew point: -20° C). 3. Average crystal grain size: 350±100µm 4. Figures in parentheses indicate average particle sizes. Next, the production process according to the second aspect of this invention will be described. Although the particle size of a raw material powder varies depending on the sintering method, the average particle size should range from 3 µm to 25 µm in accordance with this second aspect. Firstly, in the case of sintering by usual heating alone, an average particle size of 3-9 µm is preferred. When pressure-assisted sintering using in combination heating and pressurization by a gas pressure is applied, 10-25 µm is preferred. When sintering is conducted by heating alone, the sintered density ratio decreases as the average particle size increases. Particle sizes greater than 9 µm cannot achieve a sintered density ratio of 95%. Further, particle sizes greater than 25 µm cannot attain a sintered density ratio of 90%. When the sintered density ratio exceeds 90%, pores of the sintered body are closed so that the sintered density ratio can be increased to 95% or higher by pressure-assisted sintering. Further, average particle sizes of 10 µm and greater lead to significant improvements in the density ratio when subjected to pressure-assisted sintering and can therefore bring about rather high density ratios compared to powders having an average particle size smaller than 10 µm. On the other hand, average particle sizes greater than 25 µm can by no means achieve a density ratio of 95% or higher. Therefore, the upper limit of average particle size is limited to 25 µm. Further, powders having an average particle size smaller than 3 µm are costly and uneconomical. They are hence excluded. Sintering conditions will next be described. The first stage of the sintering has to be conducted in a hydrogen-containing gas or reduced-pressure atmosphere. The term "reduced-pressure atmosphere" as used herein means an atmosphere which is obtained by evacuating a highly-hermetic heating furnace with a vacuum pump and optionally causing a small amount of a non-oxidizing gas to flow through the furnace at the same time as the evacuation. The furnace pressure is required to be 0.00665 kPa (0.05 Torr) or less in the former case or 3.99 kPa (30 Torr) or less in the latter case. Otherwise, the reactions between the oxides on the surfaces of the raw material powder and carbon derived from the remaining binder do not proceed sufficiently, thereby failing to obtain a sintered body of high purity. The reduced-pressure atmosphere will now be described in further detail. It is the sum of the partial pressures (hereinafter abbreviated as a "product gas pressure") of CO and CO 2 gases, which are reaction products, that governs the reduction reactions between the oxides and carbon. It is thus an essential requirement to discharge the reaction gases out of the reaction system (out of the sintering furnace) in order to always maintain the product gas pressure at a level lower than the oxidation/reduction equilibrium pressures. As a method for meeting this requirement, it is possible to use a reduced-pressure atmosphere, a high-purity non-oxidizing gas such as Ar or N2, or both a reduced-pressure atmosphere and high-purity non-oxidizing gas. The first method can be conducted in a vacuum sintering furnace constructed of a heating furnace, which has high hermetic property so that the product gas pressure becomes substantially equal to the total pressure in the sintering furnace, and equipped with a vacuum pump having pumping speed sufficient to maintain the total pressure of the furnace at 0.00665 kPa (0.05 Torr) or lower. The second method is conducted while maintaining the pressure of the furnace within the range of the atmospheric pressure. To control the product gas pressure at 0.00665 kPa (0.05 Torr) or lower, it is necessary to maintain a fresh high-purity gas free of the product gases at 101 kPa (759.95 Torr) or higher as far as a simple calculation is concerned. It is however industrially impossible to feed a non-oxidizing gas in an amount as much as about 10,000 times the product gases. This method cannot therefore be considered to be preferable. In the third method, a fresh high-purity non-oxidizing gas free of the reaction product gases is introduced through a pressure control valve into the vacuum sintering furnace referred to above with respect to the first method. This method is said to be somewhat effective for the inhibition of evaporation of volatile metal elements upon heating. The total pressure of the furnace is 3.99 kPa (30 Torr) or lower. In this method, the total pressure of the furnace is expressed by the sum of the reaction product gas and the pressure of the non-oxidizing gas introduced. As long as the pumping speed of the vacuum pump remains constant, the pumping speed of the product gas out of the heating furnace remains constant whether the non-oxidizing gas is introduced or not. If the total pressure of the furnace exceeds 3.99 kPa (30 Torr), the pumping speed of the vacuum pump (especially where a mechanical booster and an oil-sealed rotary vacuum pump are combined) is lowered abruptly and the velocity of release of the reaction product gases from the surfaces of the sintered body is also reduced. The pumping speed of the product gases drops and as a result, the velocities of the reduction reactions are lowered. The upper limit of the total pressure of the furnace is therefore set at 3.99 kPa (30 Tor). It is also necessary to control the sintering temperature at 1000-1300°C. If the sintering temperature becomes lower than the lower limit, the impurity elimination reaction between the atmosphere and raw material powder does not proceed effectively. If it exceeds the upper limit, the sintering of the powder itself proceeds faster than the impurity elimination reaction so that impurities cannot be removed. Since these impurities are removed as water vapor or carbon dioxide gas, the loss of gas flow pores leads to a serious problem. In particular, the green body is formed of fine powder and gas flow pores are inherently small. Special care should therefore be exercised. In addition, the progress of the sintering begins to accelerate at these temperatures and the sintering temperature varies depending on the particle size of the raw material powder. It is therefore preferable to choose a lower temperature from the sintering temperature range of this invention where the average particle size is small or a higher temperature from the range where the average particle size is large. The sintering time is the time which is required until the proportions of C and O reach their respective equilibrium values at the sintering temperature employed. In general, it ranges from 20 minutes to 4 hours. It can be easily determined by several trial experiments. Next, the second stage of the sintering in this aspect of the invention will be described. The second-stage sintering is conducted to densify the sintered body which has been densified and pore-closed by the first-stage sintering. It is therefore no longer required to use any reactive gas. Therefore, the atmosphere gas is limited to an inert gas such as nitrogen or argon. On the other hand, the temperature has to be controlled at a level at least 50°C higher than the sintering temperature of the first-stage sintering. The lower limit of the sintering temperature is set at a level at least 50°C higher than the sintering temperature of the first-stage sintering, because the sintering temperature of the first stage is set at a temperature where the sintering speed begins to accelerate and the densification by the first-stage sintering is thus insufficient. When a reduced-pressure atmosphere is used in the first stage, differences in composition occur in the surface of the sintered body due to the differences in vapor pressure among the constituent elements. Even when a reduction gas atmosphere is used, a difference in composition occurs between the surface of the sintered body or powder exposed to the gas and the interior thereof. This distribution of composition occurs in the rate-determining step of atomic diffusion in the sintered body. It is therefore necessary to have the homogenization treatment proceed promptly at a temperature at least 50°C higher than the sintering temperature of the first stage, namely, in a temperature range of higher diffusion velocities in an atmosphere of at least the atmospheric pressure, in which the constituent elements do not evaporate, or in an atmosphere in which no chemical reaction takes place at all. The upper limit of the sintering temperature is the temperature at which the crystal grain size starts coarsening beyond necessity or melting begins. A more preferable temperature range is 1200-1400°C. The sintering time of the second stage is the time which is required until the sintered density and chemical composition distribution reach equilibrium values at the sintering temperature employed. In general, it ranges from 20 minutes to 2 hours. It can be easily chosen by several trial experiments. By limiting the sintering method as described above, sintered Fe-Co-V type materials having high magnetic properties can be produced economically by using the injection molding process. The starting material powders for forming raw material powders useful in this aspect of the invention can be selected from Fe, Co and Fe-Co powders, which have been described above, and likewise from an atomized Fe-Co-V powder, an atomized Fe-V powder, an atomized Co-V powder, a ground Fe-V powder, etc. Regarding the purity of the raw material powder, it is sufficient if the proportions of impurities other than C, O and N, which can be eliminated in the course of sintering, are so low that they can be ignored practically. In general, powders containing Fe, Co and V in a total proportion of 97-99 wt.% can be used. The raw material powder is then blended with a binder to form a molding composition which is molded by injection molding, followed by a debinding treatment. After the debinding treatment, sintering is conducted as described above in order to achieve densification and reduction of the C and O contents. Furthermore, the C and O contents of the final sintered body may be controlled as needed. As a method for controlling the C and O contents, there may be mentioned increasing or decreasing the C/O ratio of the debound body. The C content can be lowered by making the C/O ratio smaller, while the O content can be reduced by making the C/O ratio greater. This control of the C/O ratio can be achieved, for example, by adjusting the contents of C and O in the raw material powder, by adjusting the degree of removal of the binder, or by applying an oxidation treatment subsequent to the removal of the binder. Lowering of the total level of the contents of C and O, said total level being equal to the product of the C content and the O content, can be effected by modifying the sintering atmosphere of the first stage. This can be achieved by lowering the pressure when a reduced-pressure atmosphere is used or by improving the purity of the atmosphere gas when a reduction gas atmosphere is employed. The sintered material of the Fe-Co-V type obtained according to the second aspect of this invention is preferably characterized by the following composition: Co: 15-60 wt.% V : 0.5-3.5 wt.% O : 0.6 wt.% max. C : 0.4 wt% max Fe: remainder (including incidental impurities) and also by: sintered density ratio: 95% min. average crystal grain size: 50 µm min. Reasons for the above limitation of the final composition of the sintered material will be described. Co: 15-60 wt.% Substitution of Co for Fe is effective for improving the saturated magnetic flux density (Bs). However, Co amounts smaller than 15 wt.% or greater than 60 wt.% do not have the desired affect. The proportion of Co is therefore limited to 15-60 wt.%. V: 0.5-3.5 wt.% V contributes to an improvement in the electrical resistivity of an Fe-Co alloy. However, V proportions smaller than 0.5 wt.% are too small to effectively improve the electric resistivity. V amounts greater than 3.5 wt.% however result in semi-hard magnetism. O:0.6 wt.% max., C: 0.04 wt.% max. C and O adversely affect magnetic properties, especially, coercive force (Hc) and maximum magnetic permeability (µ max ).However, when a highly oxidative element like V is present, it is practically impossible to lower the content of O derived from the raw material powder and the content of C derived from the organic binder to provide the injection-molding material at the same time in the sintering atmosphere. Therefore, the primary object is to reduce the content of C which has a particularly adverse influence on the magnetic properties. In this respect, the content of C is reduced by daringly increasing the content of O which gives smaller deleterious effects to the magnetic properties. Namely, the upper limit of the C content is set at 0.04 wt.% because any C amounts greater than 0.04 wt.% lead to considerable deterioration of the magnetic properties. On the other hand, the magnetic properties are significantly deteriorated if the amount of O exceeds 0.6 wt.%. The upper limit of the O content is therefore set at 0.6 wt.%. Sintered density ratio: 95% min. Magnetic flux density is proportional to the sintered density ratio. If the sintered density ratio becomes smaller than 95%, the magnetic flux density is reduced so much that the characteristic features of the present alloy system (Fe-Co type) are lost. Accordingly, the lower limit of the sintered density ratio is set at 95%. By limiting it as mentioned above, sintered Fe-Co type materials having excellent magnetic properties can be obtained. Further, the production process according to the third aspect of this invention will next be described. When an Fe powder, Co powder, and a Cr and/or Cr oxide powder are used as raw material powders in accordance with this aspect of the invention, it is impossible to obtain a sintered density ratio of 95% or higher if the average particle size of the Fe powder exceeds 15 µm, the average particle size of the Co powder exceeds 10 µm or the average particle size of the Cr and/or Cr oxide powder becomes greater than 30 µm. When Fe-Co and Fe-Cr alloy powders are used, sintered density ratios of 95% or greater cannot be obtained if their average particle sizes exceed 10 µm and 30 µm, respectively. On the other hand, if the average particle sizes of the Fe powder, Co powder, Cr powder, Cr oxide powder, Fe-Co alloy powder and Fe-Cr alloy powder become smaller than 2 µm, 1 µm, 1 µm, 1 µm, 3 µm and 2 µm, respectively, the magnetic properties cannot be improved to any significant extent but the costs of the powders increase significantly. It is therefore not economical to use such fine powders. Sintering conditions will next be described. It is necessary to conduct the sintering in two-stages. The first stage of the sintering has to be conducted in a hydrogen-containing gas or reduced-pressure atmosphere. The term "reduced-pressure atmosphere" as used herein means an atmosphere which is obtained by evacuating a highly-hermetic heating furnace with a vacuum pump and optionally causing a small amount of a non-oxidizing gas to flow through the furnace at the same time as the evacuation. The furnace pressure is required to be 0.0133 kPa (0.1 Torr) or less in the former case or 3.99 kPa (30 Torr) or less in the latter case. Otherwise, the reactions between the oxides on the surfaces of the raw material powder and carbon derived from the remaining binder do not proceed sufficiently, thereby failing to obtain a sintered body of high purity. Matters relating to this reduced-pressure atmosphere are similar to those described above with respect to the Fe-Co-V composition. However, Cr is less oxidative than V so that the product gas pressure may be acceptable up to 0.0133 kPa (0.1 Torr). As a consequence, the furnace pressure may be 0.0133 kPa (0.1 Torr) or lower when no non-oxidizing gas is caused to flow. It is also necessary to control the sintering temperature at 1000-1350°C. If the sintering temperature becomes lower than the lower limit, the impurity elimination reaction between the atmosphere and the raw material powder does not proceed effectively and a suitable sintered density cannot be obtained. If it exceeds the upper limit, the sintering of the powder itself proceeds faster than the impurity elimination reaction so that impurities cannot be removed. Further, Cr is caused to evaporate so that the Cr content in the surface is lowered. Since these impurities are removed as water vapor or carbon dioxide gas, the loss of gas flow pores leads to a serious problem. In particular, the green body is formed of fine powder and the gas flow pores are inherently small. Special care should therefore be exercised. In addition, the progress of the sintering begins to accelerate at these temperatures and the sintering temperature varies depending on the particle size of the raw material powder. It is therefore preferable to choose a higher temperature from the sintering temperature range of this invention where the average particle size is small or a lower temperature from the range where the average particle size is large. The sintering time is the time which is required until the proportions of C and O reach their respective equilibrium values at the sintering temperature employed. In general, it ranges from 20 minutes to 4 hours. It can be easily determined by several trial experiments. Next, the second stage of the sintering in this aspect of the invention will be described. The second-stage sintering is conducted to density the sintered body which has been densified and pore-closed by the preceding sintering. It is therefore no longer required to use any reactive gas. Therefore, the atmosphere gas is limited to a non-oxidizing gas such as hydrogen gas, nitrogen gas or argon gas. On the other hand, the processing temperature has to be controlled at a level at least 50°C higher than the sintering temperature of the first stage. The lower limit of the sintering temperature is set at a level at least 50°C higher than the sintering temperature of the first-stage sintering, because the sintering temperature of the first stage is set at a temperature where the sintering speed begins to accelerate and the densification by the first-stage sintering is thus insufficient. When a reduced-pressure atmosphere is used in the first stage, differences in composition occur in the surface of the sintered body due to the differences in vapor pressure among the constituent elements. Even when a reducing gas atmosphere is used, a difference in composition occurs between the surface of the sintered body or powder exposed to the gas and the interior thereof. This distribution of composition occurs in the rate-determining step of atomic diffusion in the sintered body. It is therefore necessary to have the homogenization treatment proceed promptly at a temperature at least 50°C higher than the sintering temperature of the first stage, namely, in a temperature range of higher diffusion velocities in an atmosphere of at least the atmospheric pressure, in which the constituent elements do not evaporate, or in an atmosphere in which no chemical reaction takes place at all. The upper limit of the sintering temperature is the temperature at which the crystal grain size starts coarsening beyond necessity or melting begins. A more preferable temperature range is 1200-1350°C. The sintering time of the second stage is the time which is required until the sintered density and chemical composition distribution reach equilibrium at the sintering temperature employed. In general, it ranges from 20 minutes to 2 hours. It can be easily chosen by several trial experiments. By limiting the sintering method as described above, sintered Fe-Co-Cr type materials having high magnetic properties can be produced economically by using the injection molding process. The starting material powders for forming the raw material powders useful in this aspect of the invention can be selected from Fe, Co and Fe-Co powders, which have been described above. Likewise, an atomized Fe-Co-Cr powder or the like can be chosen as a source for iron, cobalt and chromium. Regarding the purity of the starting material powder, it is sufficient if the proportions of impurities other than C, O and N, which can be eliminated in the course of sintering, are so low that they can be ignored practically. In general, powders containing Fe, Co and Cr in a total proportion of 97-99 wt.% can be used. After molding, the resultant green body is subjected to a debinding treatment to remove the binder. This can be effected by heating the green body at a constant rate and holding it at the thus-heated temperature in a non-oxidizing atmosphere. It is desirable to raise the temperature at a rate of 5-100°C/hr because unduly high heating rates tend to result in the development of cracks and bulges in the final product. Further, oxidation of Cr takes place and the magnetic properties are impaired, unless a non-oxidizing atmosphere is used. After the debinding treatment, sintering is conducted as described above in order to achieve densification and reduction of the C and O contents. Furthermore, the C and O contents of the final sintered body may be controlled as needed. As a method for controlling the C and O contents, the same method as already described above can be used. The sintered material of the Fe-Co-Cr type obtained according to the third aspect of this invention is preferably characterized by the following composition: Co: 20-50 wt.% Cr: 0.5-3.5 wt.% O : 0.04 wt.% max. C : 0.02 wt.% max. Fe: remainder (including incidental impurities) and also by: sintered density ratio: 95% min. average crystal grain size: 50 µm min. Reasons for the above limitation of the final composition of the sintered material will be described. Co: 20-50 wt.% Substitution of Co for Fe is effective for improving the saturated magnetic flux density (Bs). However, Co amounts smaller than 20 wt.% or greater than 50 wt.% do not have the desired affect. The proportion of Co is therefore limited to 20-50 wt.%. C: 0.02 wt.% max., O: 0.04 wt.% max. C and O adversely affect magnetic properties, especially, coercive force (Hc) and maximum magnetic permeability (µ max ). By controlling the proportions of C and O to 0.02 wt.% or lower and 0.04 wt.% or lower, respectively, good Hc and µ max can be obtained. Therefore, the proportions of C and O are limited to 0.02 wt.% max. and 0.04 wt.% max (C ≦ 0.02 wt.%, O ≦ 0.04 wt.%), respectively. Incidentally, the contents of C and O can be controlled by adjusting the sintering atmosphere. Cr: 0.5-3.5 wt.% Cr is very effective for increasing electrical resistivity and decreasing iron loss (W). However, Cr amounts smaller than 0.5 wt.% are too small to produce the desired affect. On the other hand, Cr amounts greater than 3.5 wt.% cannot bring about substantial additional effects. Sintered density ratio: 95% min. Sintered density ratio is a critical characteristic value, which directly governs the Bs of a sintered body and also affects its Hc and µ max . As already demonstrated in Table 2, magnetic properties of sintered materials whose chemical compositions were substantially the same but whose sintered density ratios were changed by using raw material powders of different particle sizes were measured. As a result, it has been found that sintered density ratios smaller than 95% cannot improve the magnetic flux density in a low magnetic field. Accordingly, the requirement for sintered density ratio is the same for both sintered material of the Fe-Co type and those of the Fe-Co-Cr type. Average crystal grain size: 50 µm min. Crystal grain size affects the energy required for the reversal of magnetic domains, so that it also affects Hc and µ max . Smaller crystal grain sizes deteriorate both Hc and µ max . Average crystal grain sizes smaller than 50 µm cannot assure magnetic properties comparable with those of ingots in a low magnetic field. The average crystal grain size is therefore limited to 50 µm min. When the average grain size increases, Hc and µ max are both improved and as a result, the magnetic properties in a low magnetic field are also improved. If the average crystal grain size however exceeds 500 µm, the effects of improving the magnetic properties in a low magnetic field become smaller and the sintered body develops cracks more easily. It is therefore not preferred to make the crystal grain size unduly large. Examples: The present invention will be described more specifically by the following examples. It should however be borne in mind that the present invention is not limited to the following examples. Example 1: As raw material powders, were employed an atomized Fe-50% Co powder (Raw Material Powder A), an Fe-35% Co mixed powder (Raw Material Powder B) composed of a carbonyl Fe powder (Constituent Powder b1) and a reduced Co powder (Constituent Powder B2), and an Fe-50% Co mixed powder (Raw Material Powder C) also composed of Constituent Powders b1 and b2, and 1:1 mixed powder (Raw Material Powder D) of Raw Material Powder A and Raw Material Powder C. The compositions and average particle sizes of the raw material powders and constituent powders are summarized in Table 3. Using a pressure kneader, 49 vol.% of a wax-type binder was added to each of these raw material powders. After separately kneading the resultant mixtures, they were separately ground by a grinder into particulate injection-molding raw materials having a diameter of about 3 mm. Then, using an injection molding machine, the raw materials were separately molded at an injection temperature of 150°C into ring-shaped bodies having an outer diameter of 53 mm, an inner diameter of 41 mm and a height of 4.7 mm. The injection-molded green bodies were then subjected to a debinding treatment by heating them at 7.5°C/hr to 600°C and holding them at that temperature for 30 minutes in nitrogen. Thereafter, in hydrogen, they were heated at 5°C/min and were held at 700°C for 1 hour, at 950°C for 1 hour and then at 1250°C for 2 hours, whereby they were sintered. The atmosphere was controlled to have a dew point of +30° C until the end of the holding at 950°C, and thereafter another dew point of -20°C or lower. The sintered bodies thus obtained were measured by the underwater weight measuring method, so that their density ratios were determined. Furthermore, samples produced under the same conditions were applied with windings and their magnetic properties were determined by an automatic flux measuring and recording instrument. Properties of the sintered bodies are shown in Table 4. For the sake of comparison, an Fe-50% Co mixed powder (Conventional Powder 1) composed of an atomized Fe-20% Co powder (Constituent Powder e) and the reduced Co powder (Constituent Powder b2) was provided as a raw material powder. The compositions and average particle sizes of Constituent Powder e and Conventional Powder 1 are also shown in Table 3. Conventional Powder 1 was added and mixed with 1 wt.% of zinc stearate and was then compression-formed under a pressure of 9.57 MPa (4 tons/cm 2 ) into rings having an outer diameter of 53 mm, an inner diameter of 41 mm and a height of 4.7 mm. Next, the rings were held at 600°C for 0.5 hour in a hydrogen atmosphere, thereby conducting their debinding. They were thereafter held at 750°C for 1 hour, so that they were presintered. After conducting compression forming again under a pressure of 16.75 MPa (7 tons/cm 2 ), they were held at 1350°C for 1 hour in a hydrogen atmosphere to obtain comparative sintered bodies (Comparative Example 1-1). Further, some of the sintered bodies were heated to 1250°C in argon of 0.1 MPa (1 atm). After increasing the pressure of Ar gas to 120 MPa (1200 atm), they were held for 1 hour to conduct a pre-heated HIP treatment. The resultant sintered bodies were also used as comparative sintered bodies (Comparative Example 1-2). Their properties were measured in a similar manner as in the above example. The results are shown in Table 4. It can clearly be seen from the table that the sintered Fe-Co type materials obtained by the process of this invention had superior magnetic properties to the conventional sintered materials. Example 2: Debound bodies were prepared by conducting kneading, injection molding and debinding in exactly the same manner as in Example 1 except that Raw Material Powder B was used and the amount of the binder added was changed to 50 vol.%. They were sintered under different conditions thereby obtaining sintered bodies having different crystal grain sizes. Properties of the sintered bodies were measured in a similar manner to Example 1. It was found that soft magnetism was reduced abruptly when the crystal grain size became smaller than 50 µm. Although high properties could be obtained by γ-phase sintering only, high properties could still be obtained even at a γ-phase sintering temperature as low as 1000°C so long as the process of sintering in the γ phase subsequent to presintering in the a phase was employed. Further, a process relying upon sintering in the γ phase only could also obtain high properties compared with the conventional process, in which the raw material is shaped in a mold and then sintered, even when uneconomical high temperatures or uneconomical high temperatures and high pressures were not used. When only α-phase sintering was conducted, it was found that the average crystal grain size was as low as 15 µm and the degree of improvement was not sufficient although higher density and magnetic properties were obtained compared to the conventional process. Example 3: The raw material powders shown in Table 5 were individually added with their corresponding binders also given in Table 5. After separately kneading the resultant mixtures, they were individually ground to prepare injection-molding compositions. By means of an injection molding machine, the compositions were then molded into ring-shaped green test pieces having an outer diameter of 53 mm, inner diameter of 41 mm and a height of 5 mm. In nitrogen, the green test pieces were heated at +5°C/hr to 600°C and then held at 600°C for 30 minutes, thereby subjecting them to a debinding treatment. Thereafter, the debound test pieces were subjected to a first-stage heat treatment and a second-stage heat treatment under the respective conditions shown in Table 5. The chemical compositions, density ratios, magnetic properties and electrical resistivities of the thus-obtained sintered bodies are also shown in Table 5. Incidentally, the test pieces of Nos. 3-1 to 3-7 in Table 5 were heated at 350-650°C in a hydrogen gas atmosphere having a dew point of 0°C after the debinding. Their C and O contents were adjusted by changing the heating temperature. Thereafter, the test pieces were subjected to the first-stage and second-stage heat treatments. It can be seen from Table 5 that the magnetic properties deteriorated when the proportions of C and O exceeded 0.04 wt.% and 0.6 wt.%, respectively, in Nos. 3-1 to 3-7 (Comparative Examples 1 and 3). When the amount of O was unduly small (Comparative Example 2), it was not possible to reduce the amount of C and thus the magnetic properties were extremely deteriorated. However, excellent magnetic properties were obtained when the proportions of C and O were within their respective ranges required by the present invention (Invention Examples 1-6). When the heat treatment temperature of the first stage was too high (Comparative Example 6) or too low (Comparative Example 5) compared with the temperature range required by this invention, the C amount was higher than the range required by this invention in both examples and thus the magnetic properties were deteriorated. When the heat treatment temperature of the second stage was not higher by at least 50°C than the heating temperarture of the first stage (Comparative Example 4), a low density only was obtained and thus excellent magnetic properties were not obtained. Example 4: Using F2 Powder, Co3 Powder and Cr2 Powder shown in Table 6, powders having the various compositions given in Table 7 were prepared. Using a pressure kneader, those raw material powders were individually added and kneaded with 49 vol.% of wax type binder which was composed principally of a paraffin. The resultant mixtures were separately ground in a grinding mill, thereby forming particulate injection-molding raw materials having a diameter of about 3 mm. Using an injection molding machine, they were separately formed at an injection temperature of 150°C into ring-shaped green bodies having an outer diameter of 53 mm, an inner diameter of 41 mm and a height of 4.7 mm. In nitrogen gas, the injection-molded green bodies were heated at 7.5°C/hr to 600°C and then held at that temperature for 30 minutes, whereby they were subjected to a debinding treatment. Thereafter, they were held at 1150°C for 1 hour in a vacuum of 0.08 kPa (0.06 Torr) and further at 1300° C for 2 hours in argon gas, so that they were subjected to a sintering treatment. The sintered bodies thus obtained were individually measured by the underwater weight measuring method to determine their sintered density ratios. Furthermore, samples produced under the same conditions were applied with windings and their magnetic properties were determined by an automatic flux measuring and recording instrument. Properties of the individual sintered bodies are summarized in Table 7. The sintered bodies of the invention examples (Nos. 4-2 to 4-4) having a chemical composition within the range required by this invention showed excellent magnetic properties and high electrical resistivities. Example 5: Similar experiments to Example 4 were conducted using F3 Powder, FCo3 Powder and FCr2 Powder shown in Table 6 (Experiment No. 5-1) and F4 Powder, FCo4 Powder and FCr4 Powder in the same table (Experiment No. 5-2). The chemical compositions and properties of the resultant sintered bodies are shown in Table 8. The sintered body of the invention (Example No. 15), which had an average particle size and sintered density ratio within their corresponding ranges specified in this invention, exhibited excellent magnetic properties and high resistivity. Example 6: Similar experiments to Example 4 were conducted using F3 Powder, Co2 Powder and FCr3 Powder shown in Table 6 (Experiment No. 6-1) and F1 Powder, Co1 Powder and Cr1 Powder in the same table (Experiment No. 6-2). The chemical compositions and properties of the resultant sintered bodies are shown in Table 9. The sintered body of the invention (Example No. 16), which had an average crystal grain size within the range specified in this invention, exhibited excellent magnetic properties and high resistivity. Example 7: Similar experiments to Example 4 were conducted using F2 Powder, Cr3 Powder and FCo2 Powder shown in Table 6. However, the sintering temperature of the first stage was changed over a range of from 950 to 1400°C. Magnetic flux densities B20 and resistivities are diagrammatically shown as a function of sintering temperature in FIG. 1 and FIG. 2, respectively. Excellent properties were exhibited when using the conditions required by this invention. Incidentally, the final composition was as follows: Co: 35.2 wt.%, Cr: 2.2 wt.%, C: 0.010 wt.%, O: 0.013 wt.%, and Fe: balance. Example 8: Debound green bodies in accordance with No. 3-1 of Example 3, said bodies having the adjusted C and O contents, were provided. Debound green bodies in accordance with No. 4-2 of Example 4 were also provided. Sintering was conducted by holding the debound bodies at 1140°C for 1 hour under reduced-pressure conditions in a first stage in various atmospheres, respectively. All the bodies were then held at 1320°C for 2 hours in argon gas at atmospheric pressure to obtain sintered bodies. However, the vacuum level was adjusted and controlled during the reduced-pressure sintering by restricting a valve of a vacuum evacuation system or by leaving the vacuum evacuation system as it was and instead introducing a small amount of argon gas through a needle valve. The sintered bodies were tested in a similar manner to Example 3 or Example 4. The sintering conditions, chemical compositions, density ratios, magnetic properties, and electrical resistivities of the sintered bodies are summarized in Table 10. In Table 10, when the vacuum level was adjusted by restricting the valve of the vacuum evacuation system, the thus-adjusted pressure is indicated and when the vacuum level was adjusted by the introduction of a small amount of argon gas, "Ar" is given immediately after the pressure. As is apparent from Table 10, when the vacuum evacuation was insufficient and the vacuum level was decreased upon sintering in a vacuum (compare Example Nos. 7-1, 7-2, 7-7 and 7-8 to Comparative Example Nos. 7-3 and 7-9), the resultant sintered bodies had high C and O contents. Significant deterioration of the magnetic properties (especially Hc and µ max ) took place at a vacuum level 0.0133 kPa of (0.1 Torr)(Comparative Example No. 7-3) in Fe-Co-V type compositions or at a vacuum level of 0.066 kPa (0.5 Torr)(Comparative Example No. 7-9) in Fe-Co-Cr compositions. However low C and O contents were obtained at vacuum levels not higher than 0.0066 kPa (0.05 Torr) (Example Nos. 7-1 and 7-2) in Fe-Co-V compositions and a vacuum levels not higher than 0.0133 kPa (0.1 Torr) (Example Nos. 7-7 and 7-8) in Fe-Co-Cr compositions and thus excellent magnetic properties were obtained. On the other hand, when sufficient vacuum evacuation was conducted and a non-oxidizing gas was introduced (Example Nos. 7-4, 7-5, 7-10 and 7-11 and Comparative Example Nos. 7-6 and 7-12), no deteriorations of the magnetic properties were observed on both Fe-Co-V and Fe-Co-Cr compositions at increased furnace pressures up to less than 3.99 kPa (30 Torr) (Example Nos. 7-4, 7-5, 7-10 and 7-11), although some increases of the C and O contents were observed. When the pressure exceeded 3.99 kPa (30 Torr) (Comparative Example No. 7-6 and 7-12), the contents of C and O increased significantly. The magnetic properties were therefore deteriorated. As has been demonstrated above, a sintered body having excellent magnetic properties can be obtained in accordance with the production process of this invention by conducting evacuation thoroughly in reduced-pressure sintering, namely, 0.0066 kPa (0.05 Torr) or lower for an Fe-Co-V composition, 0.0133 kPa (0.1 Torr) or lower for an Fe-Co-Cr composition, or to vacuum levels lower than 3.99 kPa (30 Torr) irrespective of the composition when a non-oxidizing gas is introduced Possible Exploitation in Industry According to this invention, sintered Fe-Co type materials having an intricate shape and superior magnetic properties to conventional sintered materials can be obtained by an economical process without need for such extreme high temperature and/or high pressure as required in the conventional processes. When V is introduced as a third component to Fe-Co systems in accordance with this invention, sintered Fe-Co type magnetic materials having excellent ac magnetic properties can be obtained by removing C, which is derived from the organic binder, without inducing extreme oxidation. When Cr is introduced as a third component to Fe-Co system in accordance with this invention, sintered Fe-Co-Cr type materials having excellent magnetic properties and a low iron loss value can be obtained. The magnetic materials made by the process of this invention can be used widely as soft magnetic materials in motors, magnetic yokes and the like and especially as cores of printing heads of office automation machines.

Claims: A process for the production of a sintered Fe-Co type magnetic material, which comprises preparing an alloy powder and/or mixed powder, kneading the alloy power and/or mixed powder with at least one organic binder, subjecting the resultant composition to injection molding and debinding, and then subjecting the thus-obtained debound body to a sintering treatment characterised in that the alloy powder and/or mixed powder is a mixed powder of an Fe powder having an average particle size of 2-15 µm and a Co powder having an average particle size of 1-10 µm; an Fe-Co alloy powder having an average particle size of 3-10 µm; or a mixed powder of at least one of an Fe powder and a Co powder, both having an average particle size of 3-10 µm, and an Fe-Co alloy powder having an average particle size of 3-10 µm, said first-mentioned mixed powder, second-mentioned Fe-Co alloy powder or third-mentioned mixed powder having been prepared so as to have a final composition in which Co accounts for 15-60 wt.% and Fe substantially accounts for the remainder; and wherein the sintering treatment comprises sintering the debound body at an α-phase range temperature of 800-950 °C in a reduced pressure atmosphere or a reduction gas atmosphere and then at a γ-phase range temperature of at least 1000°C in a non-oxidising atmosphere. A process for the production of a sintered Fe-Co-V type magnetic material which comprises preparing an alloy powder and/or mixed powder, kneading the alloy powder and/or mixed powder with at least one organic binder, subjecting the resultant composition to injection molding and debinding, and then subjecting the thus-obtained debound body to a sintering treatment characterised in that the alloy powder and/or mixed powder has an average particle size of 3-25 µm and is prepared to have a final composition in which Co accounts for 15-60 wt.%, V for 0.5-3.5 wt.% and Fe substantially for the remainder; and wherein the sintering treatment comprises sintering the debound body at 1000-1300 °C in a reduction gas atmosphere or a reduced-pressure atmosphere not higher than 3.99 kPa (30 Torr) and then at a temperature, which is at least 50°C higher than the preceding sintering temperature, in an inert gas atmosphere. A process for the production of a sintered Fe-Co-Cr type magnetic material which comprises preparing an alloy powder and/or mixed powder, kneading the alloy powder and/or mixed powder with at least one organic binder, subjecting the resultant composition to injection molding and debinding, and then subjecting the thus-obtained debound body to a sintering treatment characterised in that the alloy powder and/or mixed powder comprises an Fe powder having an average particle size of 2-15 µm, at least one powder selected from a Co powder having an average particle size of 1-10 µm or an Fe-Co alloy powder having an average particle size of 3-10 µm, and at least one powder selected from a Cr and/or Cr oxide powder having an average particle size of 1-30 µm and is prepared to have a final composition in which Co accounts for 20-50 wt.%, Cr for 0.5-3.5 wt.% and Fe substantially for the remainder; and wherein the sintering treatment comprises sintering the debound body at 1000-1350°C in a reduction gas atmosphere or a reduced-pressure atmosphere not higher than 3.99 kPa (30 Torr) and then at a temperature, which is at least 50°C higher than the preceding sintering temperature, in a non-oxidizing atmosphere. A process as claimed in claim 1, wherein the sintered Fe-Co type magnetic material consists of 15-60 wt.% of Co, not greater than 0.04 wt.% of O, and not greater than 0.02 wt.% of C with the remainder being Fe and incidental impurities, the material having a sintered density ratio of at least 95% and an average crystal grain size of at least 50µm. A process as claimed in claim 2, wherein the Fe-Co type magnetic material is a sintered Fe-Co-V type magnetic material consisting of 15-60 wt.% of Co, 0.5-3.5 wt.% of V, not greater than 0.6 wt.% of O, and not greater than 0.04 wt.% of C with the remainder being Fe and incidental impurities, the material having a sintered density ratio of at least 95% and an average crystal grain size of at least 50µm. A process as claimed in claim 3, wherein the Fe-Co type magnetic material is a sintered Fe-Co-Cr type magnetic material consisting of 20-50 wt.% of Co, 0.5-3.5 wt.% of Cr, not greater than 0.04 wt.% of O, and not greater than 0.02 wt.% of C with the remainder being Fe and incidental impurities, the material having a sintered density ratio of at least 95% and an average crystal grain size of at least 50µm.

Inventors: HURUKIMI OSAMU, KIYOTA YOSHISATO, HURUKIMI, OSAMU, KIYOTA, YOSHISATO, HURUKIMI, OSAMU; KAWASAKI STEEL CORPORATION, KIYOTA, YOSHISATO; KAWASAKI STEEL CORPORATION
Applicants: KAWASAKI STEEL CO, KAWASAKI STEEL CORPORATION
Application year: 1989
Main classification: C22C1
IPC codes: C22C33, C22C38, H01F1