EP0354666B1

EP0354666B1 - Alloy steel powders for injection molding use, their commpounds and a method for making sintered parts from the same

Info

Publication number: EP0354666B1
Application number: EP89307117A
Authority: EP
Inventors: Minoru Nitta; Yoshisato Kiyota; Yukio Makiishi; Hiroshi Ohtsubo; Toshio Watanabe; Yasuhiro Habu
Original assignee: Kawasaki Steel Corp
Current assignee: JFE Steel Corp
Priority date: 1988-07-13
Filing date: 1989-07-13
Publication date: 1995-11-02
Anticipated expiration: 2009-07-13
Also published as: AU637538B2; DE68924678D1; KR900001447A; EP0354666A1; DE68924678T2; AU8892391A; CA1335759C; AU3802489A; KR930002523B1

Description

The present invention relates to metal powders for injection molding use, the compounds and a method for producing sintered parts from the same.

(1) Sintered steel, which is a type of sintered metal body, is partially replacing ingot alloy, since the former offers advantages over the latter with respect of improved yields and reduced machining costs.

It is hoped that injection molding methods for sintered steel will readily enable the molding of parts having complex three-dimensional configurations and replace the method of compression molding which suffers the limitation that the producible parts are restricted to those of two-dimensional designs.
However, since the manufacture of sintered steel bodies by injection molding is only a recent development, there is still a variety of technical problems that remain unsolved. In particular there is plenty of room for development of the raw material powder.
It has been found that the raw material powder used in injection molding shall be in the form of fine particles of spherical shape and with an average particle diameter of a 20 µm or less. An advantage of spherical powder is that it imparts good slip between the particles, and thus has excellent injection moldability. For instance, comparison between spherical powder and an irregular shaped powder, both of which have had an identical organic binder added in the same quantity, illustrates that the former offers a lower viscosity and demonstrates better injection moldability. Furthermore, the same degree of injection moldability can be achieved with a lower quantity of the binder. For the same reasons, it becomes possible to shorten the debinding cycle, and also achieve a high density owing to the finer particle size of the powder.
Conventionally the required properties of the raw material powder have been obtained by modification of operational parameters for the atomizing apparatus e.g. the pressure and flow rate of the atomising medium and adjustment of the diameter of the metallic melt injection nozzle. However, alternative means of effecting improvement by altering the chemical composition of the raw material powder have not been used. On the contrary, raw material powder has conventionally been similar to that intended to be used for compression molding, which has an average particle diameter of about 80 µm and has been purified by reduction of those impurities which may interfere with compressibility to the bare minimum.
Nonetheless, problems have been experienced in that it is not possible to achieve satisfactory injection moldability using fines of the conventional chemical composition since spherical particle formation does not take place to a sufficient extent in that powder.
Iron-Cobalt-type alloy is known as a soft magnetic material having the highest saturated magnetic flux density amongst all magnetic materials. This means Iron-Cobalt-type alloy exhibits a higher magnetic energy for a given volume than any other magnetic material. On account of its excellent magnetic characteristics there are great hopes for this material in applications associated with electric motors, magnetic yokes, and the like which require a high magnetic energy from small parts.
However, the industrial production of small-size parts from ingot Iron Cobalt type alloy is virtually impossible due to its poor cold workability.
It has been considered that this inferior workability may be overcome using powder metallurgy and a variety of methods has been proposed. For instance, Japanese Patent Laid Open No. 291934/86, Japanese Patent Laid Open No. 54041/87, and Japanese Patent Laid Open No. 142750/87 refer to Iron Cobalt type sintered materials, and Japanese Patent Publication No. 38663/82 (Japanese Patent Laid Open No. 85649/80) refers to Iron Cobalt type sintered materials containing phosphorus and Japanese Patent Laid Open No. 85650/80 to Iron Cobalt type sintered materials containing boron. Additionally, Japanese Patent Laid Open No. 75410/79 discloses Iron Cobalt Vanadium type sintered materials.
However, all of the hitherto proposed methods, which depend on the principle of compression molding, suffer the disadvantage that the so-called mixed powder (a powder prepared by admixing iron-cobalt alloy powder, cobalt-vanadium alloy powder, iron-phosphorus alloy powder, and/or iron boron alloy powder with iron powder and cobalt powder), is required to have the admixing or blending ratio limited to an extent that does not deteriorate the compressibility so that the raw material powder may be molded by the compression molding process. For this reason, it has been the aim of conventional techniques to overcome the low sintered density and poor magnetic characteristics attributable to these limitations. The method proposed in Japanese Patent Laid Open No. 291934/86 aims to improve the compressibility by utilizing a rapidly quenched iron-cobalt alloy, in which no regular lattice structure is formed, and to improve the sinterability by blending the rapidly quenched iron cobalt alloy powder with cobalt powder. The method proposed by Japanese Patent Laid Open No. 54041/87 to improve the sintered density is by the HIP (hot isostatic press) method, and the method proposed by Japanese Patent Laid Open No. 142750/87 aims at improvement in the magnetic characteristics by means of improved green density (compressed powder density) and sintered density by combination of a coarse Iron-Cobalt-type alloy powder with cobalt fines.
The methods proposed in Japanese Patent Publication No. 38663/82 (Japanese Patent Laid Open No. 85649/80) and Japanese Patent Laid Open No. 85650/80, both intend to improve the magnetic characteristics by achieving high sintered densities in unblended powders. The former method comprises sintering a pulverised iron-phosphorus alloy (26.5% by weight of P) so that the phosphorus content will be 0.05 to 0.7%. The latter method comprises sintering a pulverised iron-boron alloy (19.9% by weight of B) so that the boron content will be 0.1 to 0.4%.
Furthermore, the sintered material disclosed in Japanese Patent Laid Open No. 75410/79 is intended to improve the magnetic characteristics by increasing the sintered density of an Iron-Cobalt-Vanadium-type alloy through liquid phase sintering of a composition prepared by blending pulverised vanadium-cobalt alloy powder consisting of 35 to 45% by weight of vanadium, comprising 38% vanadium eutectic composition, with iron powder and cobalt powder. The conventional methods proposed above are, however, intended for compression molding, and are not applicable to injection molding, since the raw material powder is essentially a mixture of various single-element coarse metal powders having inferior sintering characteristics and two-element alloy powders, and said powders differ from one another in particle size and particle shape due to a difference in the manufacturing methods employed.
Currently, Iron-Cobalt-type sintered material is replacing part of the ingot iron-cobalt alloy material on account of the former's advantage with respect to the yield and the machining cost. In particular, future development of the injection molding method which is capable of readily giving three dimensional profile parts, in contrast to the compression molding method which is merely capable of producing two dimensional parts, is expected to lead to compression molding being superseded by the injection molding.
Nevertheless, the manufacture of Iron-Cobalt-type sintered material by the injection molding technique is a recent development and there are still various remaining technical problems which are yet to be resolved. In particular, the raw material powder could be much improved.
Thus chemical compositions which include impurities that deteriorate the compressibility and the processability in the compression molding step have been conventionally used. However, there were problems in that these chemical compositions were not necessarily satisfactory in their injection moldability and sintering characteristics, since there was insufficient knowledge and experience available about the method of obtaining spherical shaped particles of fine powder and of conventional composition (wherein the average particle diameters is 20µm or less) particles with oxides on the surface for injection molding use.
The object of the present invention is to provide Iron-Cobalt-type fine powder having spherical shaped particles, to give suitable injection moldability and excellent sintering characteristics, by virtue of the reducible oxides on the surface of the particle. Furthermore, it is intended to provide a sintered iron-cobalt material having useful magnetic characteristics, in particular a high saturation magnetic flux density, by injection molding said fine powder, sintering the injection molded part and where necessary, using HIP treatment on the sintered part.
According to the present invention there is provided an iron-cobalt based steel powder for injection-molding use, and subsequent sintering at a sintering temperature of from not less than 1100°C to not more than 1350°C,
said alloy steel powder comprising spherically-shaped particles having an average particle diameter of 20µm or less and consisting of
not more than 1.00% by weight of carbon;
not more than 1.00% by weight of silicon;
not more than 2.00% by weight of manganese,
wherein the manganese/silicon ratio is 1.00 or greater, and
15.0 to 60.0 % by weight of cobalt;
the iron-cobalt based alloy steel powder optionally further including at least one of the following elements;
1.0 to 4.0% by weight of vanadium,
0.02 to 1.00% by weight of boron,
0.05 to 1.00% by weight of phosphorus, and
the balance of the material being iron and incidental impurities. The invention also encompasses a method for manufacturing a sintered iron-cobalt based steel article as given in claim 5.
For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made by way of example only to Figure 1, which is a graph showing the relationship between the relative density ratio of sintered material and the relative density ratio after HIP (hot isostatic press) treatment.
The present inventors have carried out elaborate experiments aimed at accomplishing the above-mentioned objects, and have realised the present invention.

(1) In accordance with the invention it is possible to manufacture fine iron-cobalt powder with an average particle diameter of 20 µm or less, a shape suitable for injection molding and with a surface which imparts excellent sintering characteristics (the surface containing a special oxide composition). This may be achieved by producing metal fines from an iron-cobalt melt comprising 2.00% or less by weight of manganese, 15 to 60% by weight of cobalt, the balance being substantially iron and impurities, using the atomizing method. Accordingly, by sintering the above-mentioned alloy fine powder it is possible to obtain sintered material containing closed pores, superior magnetic characteristics, a relative density ratio (the ratio to true density) of 92% or higher and with a carbon content of 0.02% or less by weight.

Also, it is possible to manufacture Iron-Cobalt-type or Iron-Cobalt-Vanadium-type powders with an average particle diameter of 20 µm or less, a particle shape suitable for injection molding and with a surface which imparts excellent sintering characteristics (the surface containing a special oxide composition). This may be achieved by producing metal fines from Iron-Cobalt-type or Iron-Cobalt-Vanadium-type, melts comprising 1.00% or less by weight of carbon, 1.00% or less by weight of silicon, 2.00% or less by weight of manganese, and a Manganese/Silicon ratio of 1.00 or higher, using the atomizing method. Accordingly, by sintering the above-mentioned alloy powder it is possible to obtain sintered material having closed pores, superior magnetic characteristics, a relative density ratio of 92% or higher and with a carbon content of 0.02% or less by weight.
Moreover, improvements in the apparent density, the tap density and the magnetic characteristics of the alloy powder (having an average particle diameter of 20 µm or less and a carbon content of 0.02% or lower by weight) can be achieved in accordance with increase in sintered density alloying one or both of 0.02 to 1.00% by weight of boron and 0.05 to 1.00% by weight of phosphorus with the above-mentioned melt and atomizing the alloy into fine powder.

(A) Metal fine powder for the injection molding use

The Iron-Cobalt alloy powder, used in the manufacture of sintered iron-cobalt alloy material by injection molding according to the present invention, comprises 2.0% or less by weight of manganese and 15 to 60% by weight of cobalt, the balance being substantially iron except impurities and has an average particle diameter of 20 µm or less.
Furthermore, the above-mentioned composition may include either one of 0.02 to 1.00% by weight of boron and 0.05 to 1.00% by weight of phosphorus.
More preferably, the composition may contain 1.0% or less by weight of carbon, 1.0% or less by weight of silicon, 2.0% or less by weight of manganese, a Manganese/Silicon ratio of 1.0 or higher, and 15 to 60% by weight of cobalt, the balance being substantially iron except impurities, and has an average particle diameter of 20 µm or less. Furthermore, the above-mentioned composition may contain at least one of 1.0 to 4.0% by weight of vanadium, 0.02 to 1.00% by weight of boron and 0.05 to 1.00% by weight of phosphorus.
The manganese content is limited to 2.00% or less by weight, since the saturated magnetic flux density of the sintered material becomes lower than that of an Fe-single constituent sintered material for manganese contents higher than 2.00% by weight. However, Iron-Cobalt type melt or Iron-Cobalt-Vanadium-type melt having an increased manganese content produces low melting MnO-FeO on the surface of the particle at the atomizing stage, which lowers the melting point of the surface layer of the atomized particle before it solidifies and enhances spherical particle formation as a result of the increased surface tension and decrease in viscosity.
The reasons for limiting the carbon content to 1.00% or less by weight are set forth below. Generally, it is necessary to reduce the carbon content of Iron-Cobalt-type or Iron-Cobalt-Vanadium-type high saturation magnetic flux density sintered material to the bare minimum to ensure acceptable magnetic characteristics.
In particular, raw material powders used to manufacture Iron-Cobalt or Iron-Cobalt-Vanadium sintered material by compression molding are required to have their carbon contents reduced to an extent which is lower than that of ingot steel, to ensure compressibility in the compression molding as well as safeguarding the magnetic characteristics.
On the other hand, it was found that the use of low carbon raw materials in the manufacture of Iron-Cobalt-type or Iron-Cobalt-Vanadium-type sintered stainless steel does not lead to improved injection moldability. Nor does it provide any improvement in the corrosion resistance, due to contamination with carbon produced from organic binder at the debinding stage. Furthermore, it was found that both the carbon resulting from raw material powder and the carbon resulting from the organic binder can be removed by vacuum sintering.
Thus, it was intended to improve the powder properties of the powder by increasing the carbon content of the powder, rather than by reducing the same. It was found by experiment that increasing the carbon content improves the compactness of the atomized powder which was obtained using a high pressure medium to form spherical particles.
It is inferred that the atomized particle attains a spherical shape owing to the decline in the oxygen content of the melt which is caused by alloying of carbon with the constituents of stainless steel in the melt, and also the decline in viscosity and melting point of the melt. For example, it is observed that in stainless steel powders such as those shown in Table 1, obtained by atomizing the melt a circular water jet injected at a water pressure of 1,000 Kgf/cm² (100 MPa) and having an average particle diameter of 9.0 to 10.0µm the apparent density and tap density increase in accordance with the increase in the alloyed carbon content; hence spherical particle formation has taken place in powder.
Furthermore, it is recognised that the viscosity temperature of a compound decreases as a result of an increase in the alloyed carbon content of a 50% iron-containing cobalt fine powder, even in the case of compounds having equivalent powder-to-binder ratios.
However, the viscosity temperature of the compound increases remarkably, if the alloyed carbon content of 50% iron-containing cobalt powder exceeds 1.00% by weight, since the limit of deoxidation resulting from the reaction of carbon with oxygen is lowered to below the limit of deoxidation corresponding to amounts of silicon and manganese alloyed in the melt, which are limited. This causes the apparent density and tap density to drop due to production of bubble-like particles in which carbon monoxide gas is encapsulated.
Moreover, the alloyed carbon content of Iron-Cobalt-type alloy powder or Iron-Cobalt-Vanadium-type alloy powder is limited to 1.00% or less by weight, since the carbon content of the sintered material cannot be reduced to 0.02% or less by weight, and consequently the magnetic characteristics are deteriorated, when this limit is exceeded and the compound undergoes vacuum sintering under normal industrial conditions in which the maximum sintering time is ordinarily 4 hours.
The silicon content and the manganese content are limited to 1.00% or less by weight and 2.00% or less by weight, respectively, which correspond to the limits within which the saturated magnetic flux density of Iron-Cobalt-type or Iron-Cobalt-Vanadium-type sintered material is higher than that of sintered single constituent-iron material. When an alloy powder, as shown in Table 1, is obtained by atomizing the melt with a water jet, its apparent density and tap density are observed to increase and the viscosity temperature to decrease when the Manganese/Silicon ratio is 1.00 or higher; hence spherical particle formation is known to have occurred in the powder.
Furthermore, it is observed that the sintered density increases and the surface condition of the particle becomes fair, in cases where the Manganese/Silicon ratio is 1.00 or higher. Therefore, the Manganese/Silicon ratio is limited to 1.00 or higher.
It is inferred that if the manganese content of the melt increases, MnO, which has a low melting point, is produced on the surface of the particle in the atomizing stage, the melting point of the particle's surface layer drops before the particle solidifies, thus increasing the surface tension and lowering the viscosity of the atomized particle, thus causing spherical particle formation.
It is thought that the MnO is reduced to carbon monoxide by the carbon content of the compound or the alloyed carbon content of the melt. Thus sintering is not obstructed providing that the compound undergoes vacuum sintering at about 1,400°C.
On the contrary, silicon produces viscous silicon dioxide (SiO₂) on the surface of the particle in the atomizing stage making the particle shape irregular. Furthermore, silicon dioxide can hardly undergo reaction with carbon to produce carbon monoxide at a temperature of about 1,400°C in vacuum, hence sintering is obstructed. Therefore, the Manganese/Silicon ratio is limited to 1.00 or higher to achieve spherical particle formation and to produce a particle surface in the atomizing stage which imparts fair sinterability.
As is the case with ingot steel, cobalt has the effect of increasing the saturated magnetic flux density (B_s) by replacing iron. However, the cobalt content is limited to 15 to 60% by weight, since the effect is meagre if the cobalt content is less than 15% by weight or in excess of 60% by weight.
While the Iron-Cobalt-type alloy powder consists of the above-mentioned specified composition, the effect can be further enhanced by adding the constituents described hereinafter.
As is the case with ingot steel, vanadium increases the specific resistance of the sintered material. However, the vanadium content is limited to 1.0 to 4.0% by weight, since the effect is small if the vanadium content is less than 1.0% by weight and the coercive force (H_c) sharply increases (deteriorating the soft magnetism of the material) if the vanadium content exceeds 4.0% by weight.
Although a melt alloyed with vanadium causes clogging of the tundish nozzle with vanadium oxide (V₂0₃), which precipitates on the tundish nozzle due to a drop in the melt temperature, it is possible to adjust the oxygen content of the melt to below the V-0 deoxidation limit, which reaches equilibrium at the melt temperature when the melt passes through the tundish nozzle, thus nozzle clogging can be avoided.
In this sense, it is economically beneficial when manufacturing alloy powder by atomizing to alloy carbon, silicon and manganese with the melt either singularly or in any combination in the proportions of 1.00% or less by weight for carbon, 1.00% or less by weight for silicon, and 2.00% or less by weight for manganese.
Even better Iron-Cobalt-type alloy powders can be obtained by adding the constituents described below.
Although boron and phosphorus produce atomized particles with a spherical particle shape when added, either singularly or in combination, to a melt, the effect is small if the boron content is less than 0.02% by weight and if the phosphorus content is less than 0.05% by weight and the magnetic characteristics, in particular, the maximum magnetic permeability (µ_max) and the coercive force (H_c) of the sintered material deteriorate. Therefore, the boron content and the phosphorus content are limited to 0.02 to 1.00% by weight and 0.05 to 1.00% by weight, respectively.
It can be inferred that the spherical particle formation which is due to the alloying of boron and phosphorous with the melt in the atomizing stage can be attributed, as was the case with manganese, to the drop in melting point and decrease in surface viscosity caused by the production of boron oxide and phosphorus oxide on the surface of the particle; the increase in the sintered density is attributed to the diffusion promoting effect due to the alloying of boron and phosphorus with the melt; and the presence of excessive amounts of boron oxide and phosphorus oxide on the surface of the particle obstruct sintering.
As Table 1 indicates, the density and the magnetic characteristics of the final sintered material produced from the alloy powders are strongly influenced by the average particle diameter of the said alloy powders.
The average particle diameter is limited to 20 µm or less, since it is not possible to produce sintered material with closed pores and a relative sintered density ratio of 92% or higher for powders having a particle diameter greater than this. Furthermore, there is a remarkable deterioration in magnetic characteristics (maximum saturation magnetic flux density, maximum permeability, and coercive force) if the average particle diameter exceeds 20 µm.

(B) Injection molding compositions

The compositions of the present invention comprise iron-cobalt alloy steel powder and a binder. The compositions have excellent injection moldability. The binders of the present invention are organic binders whose principal constituents are thermoplastics, or waxes, or mixtures thereof, and may include plasticizer, lubricant, debinding promoting agents and/or inorganic binders, as the case may require.
The thermoplastics of the present invention may be one or more chosen from; acrylic, polyethylene, polypropylene and polystyrene.
The waxes of the present invention may be one or more chosen from; natural waxes (for example beeswax), Japan wax, and montan wax, and synthetic waxes (for example low-molecular weight polyethylene), microcrystalline wax and paraffin wax.
The plasticizer may be selected on the basis of combination with such wax or waxes which constitute the substantial part. Di-2-ethylhexyl phthalate (DOP), diethyl phthalate (DEP), di-n-butyl phthalate (DHP), and the like may be used.
The lubricants of the present invention may be higher fatty acids, fatty acids amides, fatty acid esters and the like. Waxes may be used as substitute lubricants depending on the need.
Subliming substances, such as camphor, may be added as debinding promoting agents.
Although there is no specific limit to the ratio of iron-cobalt alloy powder to binder, a binder content of 40 to 50% by volume of the total volume of the compositions is preferable.
A batch-type kneader or a continuous-type kneader may be used to mix and knead the alloy powder and the binder. Advantageously, the batch kneader may be pressurised kneader, a Banbury mixer, and the like. Preferably, the continuous kneader may be a twin-screw extruder and the like.
The composition used for injection molding in the present invention is obtained by pelletizing the kneaded material by a pelletizer or a crusher (grinder).

(C) Sintered materials obtained by sintering the metallic powder of the present invention

The high magnetic flux density sintered Iron-Cobalt alloy material of the present invention, obtained by sintering the Iron-Cobalt alloy powder described above, has a carbon content of 0.02% or less by weight and a bulk density ratio to true density of 92% or higher.
The presence of carbon, which is an impurity, has an adverse effect on magnetic characteristics, in particular, the maximum magnetic permeability and the coercive force. The carbon content of the sintered material is limited to 0.02% or less by weight, since the maximum magnetic permeability and the coercive force are remarkably deteriorated when the carbon content exceeds 0.02% by weight.
The relative sintered density ratio is an important property which influences the saturated magnetic flux density (B_s), the maximum magnetic permeability (µ_max), and the coercive force (H_c) of the sintered material.
The saturated magnetic flux density, the maximum magnetic permeability and the coercive force deteriorate remarkably when the relative sintered density ratio is less than 92%.
Based on the finding that the density does not increase when the relative sintered density ratio is less than 92% (according to experiments relating to increases in the density by HIP as shown in Fig. 1) the above-mentioned tendency is attributed to the formation of the particle with closed pores. Therefore, the relative sintered density ratio is limited to 92% or higher, since the sintered material is formed with closed pores.

(D) The method of manufacturing sintered material

The above-mentioned sintered material of the present invention is preferably obtained by the method as set forth below.
A composition is obtained by mixing Iron-Cobalt alloy powder with a binder. The resulting composition is injection molded and the injection molded part is sintered after it is dewaxed.
In the above-mentioned steps, at least the first-stage of the sintering step is carried out in a reduced pressure atmosphere.
The injection molding is ordinarily carried out by an injection molding apparatus designed to handle plastics. However, provisions can be made against contamination or for the extension of the machine life as required, by carrying out an anti-abrasion treatment of the internal surface of the machine which the raw material comes in contact with.
The resulting injection molded part is subjected to a debinding treatment in an open atmosphere or a neutral or reducing gas atmosphere.
In the steps of injection molding the composition, debinding the injection molded part and sintering the debound part, it is necessary that at least the first-stage of the sintering step is carried out in a reduced pressure atmosphere. Here "the first-stage of the sintering step" means the process prior to which the density ratio of the sintered material reaches about 90%. The reason is that when the density ratio of the sintered material exceeds 90%, a great majority of pores in the sintered material become closed pores and it becomes difficult to remove the carbon monoxide gas (generated by reduction and decarbonizing reactions which occur in a reduced pressure atmosphere) from within the pores in the sintered material, and thus the decarbonizing reaction is kept from progressing efficiently.
The atmosphere in which sintering is carried out must be capable of enabling reduction of oxides of manganese, etc., which obstruct the diffusion of atoms during the sintering step, and must also be capable of removing the large quantities of carbon contained in the debound parts after the debinding treatment.
Hydrogen and a reduced pressure atmosphere are conditions cited as being those meeting the above-mentioned requirements, as is the case in the manufacture of ordinary sintered stainless steel material.
Nevertheless, since reduction and decarbonization in a hydrogen atmosphere progress according to the following equations, respectively:

MO + H₂ → M + H₂O Reduction

(M: Metal)

C + H₂O → CO + H₂ Decarbonization

(C: Solid solution carbon)
The lower

is the faster the reaction progresses.
The higher

is the faster the decarbonization progresses.
Therefore, it is difficult to make both reactions simultaneously progress efficiently.
On the other hand, reduction and decarbonization in a reduced pressure atmosphere simultaneously progress as shown by the following equation, and by removing carbon monoxide gas as an exhaust gas, the reaction can be made to progress efficiently.

MO + C → M + CO Reduction and decarbonization

Moreover, since the amounts of oxygen and carbon contained in the final sintered material tend to be lower when sintering under reduced pressure, compared with under a hydrogen atmosphere, sintering according to the present invention is performed under reduced pressure.
In order to make the reduction and deoxidation progress efficiently, the pressure of the reduced pressure atmosphere is preferably 0.01 torr (133x10⁻² Pa) or lower, and the temperature range is preferably between 1,100°C and 1,350°C.
It is preferable that the reduced pressure atmosphere be replaced by an nonoxidizing atmosphere, such as an inert gas (e.g. nitrogen, argon) atmosphere, and a low dew point hydrogen atmosphere as a protective atmosphere in the stages following completion of the reactions since the reduced pressure atmosphere is only needed during the stage in which reduction and decarbonization are occurring.
The Examples of the present invention given below are by way of illustration only and are not intended to limit the scope of the invention.
Table 1 shows an example of a high saturation magnetic flux density sintered material of the present invention, together with a Comparative Example, prepared by sintering Iron-Cobalt-type alloy powder and Iron-Cobalt-Vanadium-type alloy powder. Both of these powders are used for high saturation magnetic flux density sintering and are obtained by the water atomizing method.
Iron-Cobalt-type and Iron-Cobalt-Vanadium-type alloy powder, having the respective chemical compositions shown in Table 1, were prepared by dripping a melt of ingot Iron-Cobalt-type and Iron-Cobalt-Vanadium-type steel manufactured by a high frequency induction furnace, perpendicularly through an orifice nozzle constructed of a refractory material provided on the bottom of a tundish and atomizing the dripping melt by applying a conical water jet, which encircles the axis of the drip and narrows in the downward direction, at a pressure of 1000 Kgf/cm² (100MPa).
The resulting alloy powder was analyzed on a Microtrack grading analyzer to determine the average particle diameter, the apparent density and the tap density.
The viscosity temperature (the temperature at which the viscosity reaches 100 poise (10Pa.s)) was measured by extruding a composition prepared by kneading each of the alloy powders with wax-type organic binders in a pressurised kneader through a die of 1 mm diameter and 1 mm length under a 10 kg load on a flow tester. The blending ratio of the binder was 46% by volume.
The composition was then injection molded into rings of 53 mm outer diameter, 41 mm inner diameter, and 4.7 mm height using an injection molding apparatus at a temperature of 150°C. The injection molded part was subjected to a dewaxing treatment in nitrogen atmosphere in which it was heated to 600°C at a rate of 7.5°C rise per hour and left to stand for 30 minutes.
Following the dewaxing step, the dewaxed material was sintered in hydrogen atmosphere in which it was heated to 700°C at a rate of 5°C rise per minute and left to stand for 1 hour at 700°C, for another hour at 950°C and the following 2 hours at 1,350°C. The dew point was limited to +30°C, until the end of the 950°C stage and beyond the end of this stage the dew point was limited to -20°C or lower.
The specific gravity of the resulting sintered material was measured by weighing samples submerged in water, and the relative sintered density ratios were calculated in each case.
A self-registering magnetic flux recorder was used to measure the magnetic characteristics of samples which had been prepared under identical conditions and had wires wound around them. The results of the measurements are shown in Table 1.
It is clear from Nos. 1 to 18 in Table 1, that the apparent density and tap density increases with increases in the manganese content, the carbon content and the manganese/silicon ratio, in the case of Iron-Cobalt-type alloy powder of the present invention; this powder having an average particle diameter of 20 micron or less and a composition comprising of 10 to 60% by weight of cobalt, 1.00% or less by weight of carbon, 1.00% or less by weight of silicon, 2.00% or less by weight of manganese, and a Manganese/Silicon ratio of 1.00 or higher.
The compositions prepared from the above-mentioned powders exhibit low viscosity values (the viscosity decreases as the temperature drops), thus spherical particle formation has been achieved in the powders and they have excellent injection moldability.
Sintered material which has a carbon content of 0.02% or less by weight and a relative sintered density ratio of 95% was obtained. Hence, sintered material having excellent magnetic characteristics (saturation magnetic flux density, maximum magnetic permeability, and coercive force) can be prepared.
It is clear from Nos. 19 to 23 in Table 1, that atomized Iron-Cobalt-Vanadium-type alloy powders which have spherical particles and exhibit excellent injection moldability can be manufactured according to the present invention by increasing the silicon and the manganese contents and controlling the Manganese/Silicon ratio to 1.00 or higher and with a vanadium content of 1.0 to 4.0% by weight so as to prevent clogging of the nozzle with the melt, due to the inclusion of vanadium.
A sintered material can be obtained which has a carbon content of 0.01% by weight, a relative sintered density ratio of 95% and which exhibits excellent magnetic characteristics (Bs, µmax, Hc).
It is clear from Nos. 24 to 33 in Table 1 that the apparent density and tap density increase due to alloying of boron and phosphorus with the melt in the case of Iron-Cobalt-type and Iron-Cobalt-Vanadium-type alloy powders of the present invention, the viscosity of the composition prepared therefrom drops, and that spherical particle formation and injection moldability are further improved, in comparison with the case in which boron and phosphorus are not added (No. 3). The composition of these powders includes 0.02 to 1.0% by weight of boron and 0.05 to 1.00% by weight of phosphorus. A sintered material can be obtained which has even better magnetic characteristics (Bs, µmax, Hc) with a high compactness, i.e. a relative sintered density ratio of 96% and has a carbon content as sintered of 0.01% by weight.
It is clear from Nos. 34 to 43 in Table 1 that the apparent density and tap density of the iron-cobalt-type alloy powder of the present invention increase with an increase in the average particle diameter, and that the viscosity of the composition made therefrom decreases with an increase in the average particle diameter, although the relative sintered density ratio and, accordingly, magnetic characteristics (Bs, µmax, Hc) decrease with an increase in the average particle diameter.
The same tendency is true for Iron-Cobalt-Vanadium-type alloy powder.
Sintered material having excellent magnetic characteristics can be obtained when the average particle diameter is 20 µm or lower.
Fig. 1 shows the relationship between the relative density ratio of sintered material, which has undergone an HIP treatment at 1,350°C for 1 hour in argon atmosphere maintained at 100 kgf/cm² (100MPa), and the relative density ratio after HIP treatment, which was measured for samples prepared by injection molding compositions made from Iron-Cobalt-type alloy powder shown in No. 3 in Table 1, which is an example of the present invention. It is clear from Figure 1 that pores in the sintered material become closed pores and the relative density ratio after HIP treatment is further improved when the relative density ratio is 92% or higher.

Claims

An iron-cobalt based steel powder for injection-molding use, and subsequent sintering at a sintering temperature of from not less than 1100°C to not more than 1350°C,
said alloy steel powder comprising spherically-shaped particles having an average particle diameter of 20µm or less and consisting of
not more than 1.00% by weight of carbon;
not more than 1.00% by weight of silicon;
not more than 2.00% by weight of manganese,
wherein the manganese/silicon ratio is 1.00 or greater, and
15.0 to 60.0 by weight of cobalt;
the iron-cobalt based alloy steel powder optionally further including at least one of the following elements;
1.0 to 4.0% by weight of vanadium,
0.02 to 1.00% by weight of boron,
0.05 to 1.00% by weight of phosphorus, and
the balance of the material being iron and incidental impurities.
A method of producing the iron-cobalt based steel powder of claim 1 the method comprising the steps of
a) providing a melt of the steel, and

b) atomizing the melt by means of a water jet.
A composition for injection molding use comprising the powder of claim 1 and an organic binder.
A composition as claimed in claim 3 further including one or more of a lubricant, a plasticiser, a debinding agent and an inorganic binder.
A method of manufacturing a sintered iron-cobalt based steel article from the composition of claim 3 or 4, the method comprising the steps of;
i) preparing said alloy steel powder by providing a melt of the steel and atomising the melt by means of a water jet,

ii) blending said alloy steel powder with an organic binder and optionally one or more of a lubricant, a plasticizer, a debinding agent and an inorganic binder to produce a composition suitable for injection molding;

iii) injection molding the compound into a mould to form a molded article;

iv) removing the molded article from the mold;

v) debinding the molded article; and

vi) sintering the debound article at a temperature of from not less than 1100°C to not more than 1350°C, to produce a sintered steel article,
wherein the sintered article has a relative sintered density of not less than 92% and the carbon content is not more than 0.02% by weight.
A method of manufacturing a sintered iron-cobalt based steel article as claimed in claim 5, wherein at least part of the sintering stage is performed under an atmosphere whose pressure is not more than 133 x 10⁻² Pa.