JP4539585B2 - Metal powder for dust core and method for producing dust core - Google Patents

Description

  The present invention relates to a metal powder for a dust core that is suitable as a material for a magnetic core of a motor or a transformer and an inductor, and a method for manufacturing a dust core using the metal powder.

Advances in powder metallurgy technology have made it possible to manufacture parts with high dimensional accuracy and complex shapes in a near net shape, and products using powder metallurgy technology have been used in various fields.
In powder metallurgy technology, lubricant powder and alloy powder are mixed with metal powder as needed, then pressed with a mold to form a compact, then sintered and heat treated to obtain the desired dimensions. Sintered parts with shape and characteristics. In the powder metallurgy technique, a binder such as a resin is mixed with metal powder, and then pressure-molded with a mold to form a molded body, which may be a part.

Using such powder metallurgy technology, when producing parts with excellent magnetic properties and high strength, so as to obtain a high-density molded body when pressure-molded with a constant molding pressure, The metal powder used is required to have high compressibility.
In order to obtain excellent magnetic properties, it is required to reduce the iron loss (eddy current loss and hysteresis loss) of the dust core. In order to reduce eddy current loss, it is necessary to insulate metal powder and increase the specific resistance of the dust core. In order to reduce the hysteresis loss, it is necessary to remove the distortion accumulated in the green compact during pressure molding.

  In response to such a request, for example, Patent Document 1 discloses a soft magnetic metal powder having an average Si content of at least 0.5 wt% from the surface of the powder to a depth of 0.2 μm, phosphoric acid, boric acid, Or after being molded into a magnetic core shape by compression molding after insulation treatment with a solution of Na, K, Ca, Mg, Al, Si, Mn or Zn salt of phosphoric acid or boric acid, or insulation treatment with silicone resin A method of manufacturing a dust core having a low coercive force and high electric resistance by removing residual strain by strain relief annealing at 500 ° C. or higher has been proposed.

Patent Document 2 discloses a powder magnetic core formed by press-molding magnetic powder mainly composed of iron (Fe) and silicon (Si), which is covered with an insulating film, and contains Si in the magnetic powder. The amount (X: mass%) and the density ratio (ρ / ρ ₀ :%), which is the ratio of the bulk density (ρ) of the dust core to the true density (ρ ₀ ) of the magnetic powder, are ρ / ρ A dust core characterized by satisfying ₀ ≧ 94−X (%) is disclosed. According to this technology, even when using Fe-Si magnetic powder, which has been considered difficult to increase in density, by using the mold lubrication warm pressing method, a higher density than in the past has been achieved. A dust core can be obtained, and excellent magnetic properties can be obtained by the dust core of the Fe-Si magnetic powder coated with an insulating film.

JP 2003-142310 A JP 2003-297624 A

  However, the soft magnetic metal powder described in Patent Document 1 is a powder made of a hard alloy such as Fe-Si alloy, Fe-Si-Al, and Fe-Si-Co alloy, and has compressibility. Since it is bad, it is difficult to obtain a high-density green compact using these, and it is also difficult to obtain a high magnetic flux density.

  Moreover, in the above-mentioned patent document 2, even if it is Fe-Si alloy powder, although a high compaction density is obtained, this technique requires a special shaping | molding technique and it is not only necessary to shape | mold by a very high pressure. According to the example, the upper limit of the density of the green compact obtained is 95% of the true density.

The present invention advantageously solves the problems of the prior art as described above, and does not cause a decrease in saturation magnetic flux density or deterioration in compressibility by uniformly concentrating Si only on the surface layer portion of the powder. Another object of the present invention is to propose an advantageous method for producing a metal powder for a dust core having an excellent electrical insulating property and having a high bonding force between an insulating material and particles.
In addition, the present invention proposes a method of manufacturing a dust core that can obtain a high dust density of 95% or more of the true density by using the metal powder for dust core as a raw material. Objective.

As described above, when an appropriate amount of Si is present in the surface layer portion of the metal powder, the insulation treatment effect is enhanced, and as a result, a dust core having high electrical resistance is obtained. However, as in Patent Document 1, the entire powder is Fe-Si. When it is an alloy, it is difficult to obtain a high dust density and a high magnetic flux density.
However, if Si can be concentrated only in the surface layer portion of the powder, it is considered that a high dust density and a high magnetic flux density can be obtained while ensuring the above-described insulation treatment effect.
Therefore, the inventors have repeatedly studied a method of forming a Si concentrated layer only on the surface layer portion of the metal powder.

By the way, conventionally, a method for producing a high silicon steel sheet by subjecting a low Si-containing steel sheet to a siliconization treatment by a gas phase reaction method is known. In this method, for example, a steel sheet with an Si content of less than ₄ mass%, which is easy to roll, is reacted with SiCl ₄ at a temperature of about 1000 to 1200 ° C., and a reaction of SiCl ₄ + 5Fe → Fe ₃ Si + 2FeCl ₂ is applied to the steel sheet surface. This is a method of obtaining a high Si concentration steel sheet having excellent magnetic and magnetostrictive properties by forming Fe ₃ Si and further diffusing Si in the thickness direction.

Therefore, the inventors applied this method to the metal powder for a dust core that is the subject of the present invention. In the metal powder, Si diffuses to the inside of the powder within a short period of time. It was found that it was extremely difficult to concentrate Si stably only on the surface layer.
The reason is that the powder has a larger specific surface area and higher reactivity than the steel plate, and therefore Si easily penetrates to the center. When the Si concentration increases throughout the powder, the particles become hard, as well as the difficulty in rolling a steel sheet with a high Si concentration, the compressibility decreases in the subsequent molding step, and the compact density decreases. As a result, a high saturation magnetic flux density cannot be obtained.

  Therefore, as a result of further research, the inventors thought that the reason why Si penetrates into the powder within a short time by the above-described siliconization treatment is that the steel structure is a ferrite phase. . That is, the diffusion rate of Si is extremely fast in the ferrite phase. In this regard, when the steel structure is an austenite phase, the diffusion rate is much slower than that of the ferrite phase.

Therefore, next, the inventors of a material having a composition that exhibits an austenite phase in a high temperature range, i.e., pure iron or low Si-containing steel powder, in a temperature range at which the austenite phase is formed by a gas phase reaction method. When vapor deposition was attempted, the knowledge was obtained that the Si diffusion rate in the steel was remarkably suppressed, and an Si-enriched layer could be effectively formed only on the surface layer of the powder.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
(1) heating a metal powder exhibiting an austenite phase in a high temperature range to a temperature range where the austenite phase is formed, and concentrating Si in a surface layer portion of the metal powder by a gas phase reaction in this temperature range A method for producing a metal powder for a dust core.

(2) The method for producing a metal powder for a dust core according to (1), wherein the metal powder is a pure iron powder having a purity of 99% or more.

(3) The method for producing a metal powder for a dust core according to (1), wherein the metal powder is an Fe-Si alloy powder containing Si at 1 mass% or less.

(4) In any one of the above (1) to (3), the surface of the metal powder whose surface layer portion is enriched with Si is further subjected to an insulation coating treatment, whereby the metal powder for a dust core is produced. Method.

(5) A method for producing a dust core, wherein the metal powder produced by the method described in (4) above is subjected to pressure molding.

(6) A dust core characterized by subjecting a metal powder having been subjected to an insulation coating process manufactured by the method described in (4) above to heat treatment in a temperature range of 600 ° C. to 1000 ° C. after pressure forming. Manufacturing method.

ADVANTAGE OF THE INVENTION According to this invention, the metal powder for powder magnetic cores which is excellent in insulation process property and has a high saturation magnetic flux density can be obtained.
In addition, a powder magnetic core having a high electrical resistance and a high molding density can be obtained by pressure molding using the above metal powder for powder magnetic core as a raw material.
Therefore, by using the above-described powder magnetic core, a motor and a transformer having excellent magnetic characteristics, an inductor element, and the like can be obtained.

The present invention will be specifically described below.
The soft magnetic metal material targeted by the present invention is a material exhibiting an austenite phase in a high temperature range, and representative examples thereof include pure iron and a low Si concentration Fe—Si alloy. Here, pure iron refers to one having an Fe concentration of 99 mass% or more, and since it is high purity and soft, high saturation magnetic flux density and excellent compressibility can be obtained. Further, the Fe-Si alloy having a low Si concentration refers to an Si concentration of 1 mass% or less, and although it contains a small amount of Si, it is possible to increase the specific resistance and thus reduce the eddy current loss. Moreover, since Si content is as small as 1 mass% or less, there is almost no adverse effect on compressibility and saturation magnetic flux density.

  In the present invention, after pure iron or Fe-Si alloy powder is heated to a predetermined temperature range, Si is concentrated on the surface layer of the powder by a gas phase reaction, and then an insulating coating is applied to the particle surface. After that, it is a series of processes in which a desired magnetic core shape is processed by pressure molding and further heat treatment is performed to remove distortion.

As the pure iron powder used in the present invention, atomized iron powder, reduced iron powder, electrolytic iron powder and the like can be used, and are not particularly limited, but water atomized iron powder obtained by water atomizing a molten metal among them Is preferable from the viewpoint of densification.
Hereinafter, although a preferable manufacturing method will be described by taking the case of applying the water atomizing method as an example, it goes without saying that the present invention is not limited thereto.
A molten metal having a normal pure iron composition is sprayed, rapidly cooled and solidified by a water atomizing method, and crushed with high-pressure water to obtain a water atomized iron powder (raw powder). Next, the raw powder is subjected to dehydration / drying treatment and reduction treatment to obtain a product (pure iron powder) from which the oxide film on the particle surface has been removed.
Similarly, the Fe—Si alloy powder can be manufactured by using an atomizing method or a pulverizing method.

  The particle sizes of such pure iron powder and Fe—Si alloy powder (hereinafter simply referred to as metal powder) are preferably 500 μm or less from the viewpoint of high density, but are not particularly limited. The lower limit of the particle size is preferably about 1 μm. This is because fine powder has high reactivity, and therefore, when subjected to the gas phase reaction step described below, sintering may start before the gas phase reaction starts, and the particles may stick to each other. .

Next, a method for concentrating Si in the surface layer portion of the metal powder by a gas phase reaction will be described.
As described above, a method of manufacturing a high silicon steel sheet by subjecting a low Si-containing steel sheet to a siliconization process by a gas phase reaction method is known. In this method, for example, a steel sheet having a Si content of less than ₄ mass%, which is easy to roll, is reacted with SiCl ₄ at a temperature of about 1000 to 1200 ° C., whereby the surface of the steel sheet is reacted with SiCl ₄ + 5Fe → Fe ₃ Si + 2FeCl ₂ . In this method, Fe ₃ Si is formed on the surface of the steel sheet and Si is diffused in the thickness direction to obtain a steel sheet having a high Si concentration with excellent magnetic properties and magnetostrictive properties.

  However, when this siliconization method is applied to a metal powder for a dust core that is the subject of the present invention, the powder has a higher specific surface area and higher reactivity than a steel plate, so that Si penetrated and diffused to the inside, and it was not possible to concentrate Si stably only on the surface layer of the powder. As the Si concentration increases throughout the powder, the particles become harder, as well as the difficulty in rolling steel sheets with high Si concentration. As a result, a high saturation magnetic flux density cannot be obtained.

  Therefore, as a result of intensive investigations to solve this point, the inventors have determined that the Si temperature can be appropriately controlled by controlling the reaction temperature and the reaction time when vapor-depositing Si on the powder surface. They found that it was possible to effectively suppress the penetration and diffusion into the powder, and to concentrate Si stably only on the surface layer of the powder.

In the Fe-Si binary phase diagram, an austenite phase exists in the steel temperature range. For pure Fe, the temperature range is 910 to 1400 ° C, but this temperature range gradually narrows as the Si concentration increases, with 1% Si-Fe approximately 1000 to 1350 ° C, and with 2% Si-Fe, almost the entire temperature. In the range, austenite phase and ferrite phase coexist.
In the ferrite phase, the Si diffusion rate is much faster than in the austenite phase. For this reason, if the Si concentration of the steel sheet to be subjected to the gas phase reaction treatment is 3 to 4 mass%, since it is in the ferrite phase, it is possible to sufficiently infiltrate Si in the thickness direction. On the other hand, in the case of the powder that is the subject of the present invention, when the gas phase reaction treatment is performed on the same powder composition under the same conditions, Si has penetrated because the specific surface area is larger and the reactivity is higher than that of the steel plate. The Si concentration is easily increased to the center of the powder quickly due to diffusion.

On the other hand, the austenite phase has a much slower diffusion rate of Si than the ferrite phase.
Therefore, if the inventors perform a gas phase reaction in the temperature range where the diffusion rate of Si becomes a low austenite phase, Si penetration / diffusion to the inside of the powder is effectively suppressed, and only the surface layer portion has Si. As a result of conducting experiments and thinking that a thickened layer could be formed, an unexpected result was achieved with regard to achieving the intended purpose, and the present invention was completed.

Hereinafter, a preferred Si concentration method will be described by taking the case of using SiCl ₄ gas and pure iron powder as an example, but it goes without saying that the present invention is not limited to this.
An iron powder having a particle size of 10 to 200 μm is placed in a quartz container to a thickness of 5 mm or less, more preferably 3 mm or less, and 910 ° C. or higher and 1400 ° C. or lower, more preferably in a non-oxidizing atmosphere. Is heated to 910 ° C or higher and 1200 ° C or lower. Next, 0.01 to 10 NL / min / kg of SiCl ₄ gas is introduced.
By doing so, it was possible to concentrate Si stably only on the surface layer portion of the iron powder.

Incidentally, when Noseki thickness of iron powder is more than 5 mm, not spread SiCl ₄ gas in the powder mass uniformly Si is not deposited on all of the powder surface. Therefore, when a large amount of treatment is performed, it is preferable to suppress non-uniform gas phase reaction by a method of treating the powder while stirring. As a method of stirring the powder, a method of rotating the container itself containing the powder, a method of stirring using a stirring blade, and introducing a mixed gas of non-oxidizing gas and SiCl ₄ gas into the container to flow the powder Methods and the like.
The flow rate of SiCl ₄ gas is preferably about 0.01 to 10 NL / min / kg from the viewpoints of effects and economy.

It is important that the gas phase treatment temperature is a temperature range in which an austenite phase is formed. This is because the austenite phase has a much slower diffusion rate of Si than the ferrite phase, so if a gas phase reaction is performed in the temperature range where this austenite phase is used, the penetration and diffusion of Si into the powder is effective. This is because the Si concentration layer can be formed only on the surface layer portion.
Here, the temperature range of the austenite phase when the metal powder is pure iron powder is 910 to 1400 ° C. A more preferred treatment temperature is in the range of 910 to 1200 ° C.
When the metal powder is Fe-Si alloy powder, the austenite phase temperature range when the Si content is 0.5 mass% is approximately 950 to 1370 ° C, and the austenite phase temperature range when the Si content is 1 mass% is approximately 1000 to 1350. ° C.

Further, the reaction time, that is, the time for contacting with the SiCl ₄ gas, may be appropriately adjusted in accordance with the heat treatment temperature, but is preferably about 1 to 60 minutes from the viewpoints of effects and economy.

In addition, after the gas phase reaction, iron chloride generated by the reaction is likely to adhere, so it is desirable to keep the temperature at or higher than the reaction temperature by flowing an inert gas for several minutes to 1 hour. It is also advantageous to reduce the pressure of the reaction system.
In addition, after these gas phase reactions, by introducing a gas containing a small amount of oxygen into the system, the surface of the powder particles can be oxidized to form SiO ₂ . This has the effect of preventing the powder from having a particle size of 5 μm or less from being rapidly oxidized and generating heat at the moment when it is exposed to the atmosphere, and further preventing the oxidation from proceeding. Furthermore, there is an effect in increasing the adhesion between the particles and the insulating coating material in the insulating coating treatment.

In the present invention, the thickness of the Si concentrated layer formed on the surface portion of the metal powder is preferably about 0.01 to 10 μm. Because, if the thickness of the Si concentrated layer is less than 0.01μm, the improvement of the insulation treatment effect intended in the present invention cannot be expected, whereas if it exceeds 10μm, the Si concentrated layer penetrates too much into the powder. This is because the compressibility and the saturation magnetic flux density are reduced.
In the present invention, it is important that the thickness of the Si concentrated layer is ½ or less of the powder particle diameter (radius) within the thickness range of the Si concentrated layer. This is because when the thickness of the Si concentrated layer exceeds 1/2 of the powder particle diameter, the Si concentrated layer penetrates too much into the powder, resulting in a decrease in saturation magnetic flux density and a decrease in compressibility. A more preferable thickness of the Si concentrated layer is 1/5 or less of the powder particle diameter.

  Furthermore, in the present invention, the average Si concentration in the Si enriched layer is preferably about 0.5 to 32 mass%. This is because if the Si concentration is less than 0.5 mass%, the insulation effect as good as expected in the present invention cannot be obtained. On the other hand, if it exceeds 32 mass%, the Si-concentrated layer tends to peel off, and the insulation effect This is because of a decrease.

Next, a description will be given of the insulating coating treatment of the powder in which Si is concentrated in the surface layer portion.
When the metal powder of the present invention is applied to a magnetic component such as a powder magnetic core, the powder particles are subjected to an insulation coating treatment to form an insulating layer having a coating structure that covers the particle surface in a layered manner. There is a need to increase magnetic properties by increasing electrical resistance and reducing eddy current losses.
The material for the insulating coating is not particularly limited as long as it can maintain the required insulation even after the metal powder is pressure-molded and formed into a desired shape. Examples of such a material include oxides such as Al, Si, Mg, Ca, Mn, Zn, Ni, Fe, Ti, V, Bi, B, Mo, W, Na, and K. Further, a magnetic oxide such as spinel type ferrite or an amorphous material typified by water glass can also be used. Furthermore, a phosphate chemical conversion treatment film or a chromate chemical conversion treatment film can also be used. The phosphate chemical conversion coating can also contain boric acid and Mg. In addition, as an insulating material, phosphate compounds such as aluminum phosphate, zinc phosphate, calcium phosphate, and iron phosphate can be used. Moreover, you may use organic resins, such as an epoxy resin, a phenol resin, a silicone resin, and a polyimide resin.
Further, the insulating coating material described in JP-A-2003-303711, for example, a mixture of a silicone resin and a pigment such as a metal oxide, a metal nitride, a metal carbide, a mineral, and glass can be used. No problem.

  Note that a surfactant or a silane coupling agent may be added for the purpose of increasing the adhesion of the insulating material to the metal particle surface or for the purpose of increasing the uniformity of the insulating layer. In this case, the addition amount of the surfactant and the silane coupling agent is preferably about 0.001 to 1 mass% with respect to the total amount of the insulating layer.

  The thickness of the insulating layer formed by the insulating coating treatment is preferably about 10 to 10,000 nm, although it depends on the particle size of the powder. If the thickness is less than 10 nm, the insulating effect is not sufficient. On the other hand, if the thickness exceeds 10000 nm, the density of the green compact decreases and a high magnetic flux density cannot be obtained.

  The method for forming the insulating layer on the surface of the metal powder is not particularly limited, and any conventionally known film forming method (coating method) is advantageously suitable. Typical coating methods include fluidized bed method, dipping method, spray method and the like. In any method, a step of drying a solvent for dissolving or dispersing the insulating material is required after the coating step or simultaneously with the coating step. Further, a reaction layer may be formed between the insulating layer and the powder particle surface in order to prevent the insulating layer from peeling off during pressure molding. Formation of the reaction layer is preferably performed by chemical conversion treatment.

Next, the pressure molding method will be described.
A metal powder (insulating coating powder), which has been subjected to the above-described insulating coating process and formed an insulating layer on the particle surface, is pressed to form a powder magnetic core. Prior to the pressure molding, the powder may be blended with a lubricant such as a metal soap or an amide wax, if necessary. The blending amount of the lubricant is preferably about 0.5 parts by mass or less with respect to 100 parts by mass of the powder. This is because as the blending amount of the lubricant increases, the density of the dust core decreases.

  Any conventionally known method can be applied as the pressure molding method. For example, a mold forming method in which pressure molding is performed at normal temperature using a uniaxial press, a warm molding method in which pressure molding is performed warm, a mold lubrication method in which a mold is lubricated and pressure molded, These include a warm mold lubrication method to be performed, a high pressure molding method in which molding is performed at a high pressure, and a hydrostatic pressure pressing method.

Next, heat treatment for removing distortion will be described.
Since the green compact is strained during molding, the hysteresis loss is large. Therefore, in order to remove this distortion and develop the original magnetic properties, a distortion removing heat treatment is necessary. The treatment temperature is preferably about 600 ° C. or higher and 1000 ° C. or lower. If this treatment temperature is too high, the effect of removing strain increases, but the insulating coating loses its insulating effect due to crystallization and decomposition, and the electrical resistance is remarkably reduced. Also, a longer heat treatment time is preferable for removing distortion, but if it is too long, the electrical resistance is remarkably lowered as well. Therefore, the heat treatment time is preferably about 5 to 300 minutes, more preferably about 10 to 120 minutes from the viewpoints of effect and economy.

As metal powder, pure iron powder “JIP-304AS” (Fe concentration: 99.8 mass%, average particle size: 90 μm) manufactured by JFE Steel Co., Ltd., and various types of Si containing 0.1 to 3 mass% Fe-Si alloy powder (both average particle diameter is 90 μm) was used. In addition, the particle diameter of the powder was measured by a laser scattering diffraction particle size distribution measuring device, and an average value was obtained.
These powders are filled in a quartz container at a loading thickness of 3 to 10 mm, heated in argon gas at 880 to 1420 ° C. for 5 minutes, and then silicon chloride gas is flowed at a flow rate of 1 Nl / min / kg for 1 to 30. A gas phase reaction process was performed in which the temperature was maintained at a predetermined temperature while flowing for a minute, and further heat treatment was performed for 3 to 60 minutes after substitution with argon gas.
Table 1 shows the heating temperature of the metal powder, the heating time in SiCl ₄ gas, and the heating time in Ar gas. Table 1 also shows the results of examining the Si concentrated layer thickness of the metal powder after the gas phase reaction treatment and the average Si concentration in the concentrated layer.

  Subsequently, the surface of the obtained powder particles was coated with a silicone resin by the following method. “SR2400” from Toray Dow Corning was used as the silicone resin. Resin solids shown in Table 1 are sprayed on Si-concentrated powder obtained by fluidizing the coating liquid adjusted with xylene so that the resin content becomes 5 mass% in the apparatus container with a rolling fluidized bed type coating apparatus. Sprayed to minutes. After spraying, the fluidized state was maintained for 20 minutes to dry. Subsequently, a heat treatment was performed in the atmosphere at 250 ° C. for 60 minutes to heat and cure the silicone resin to obtain a coating powder.

Next, the obtained coated powder was pressure-molded to produce a ring-shaped dust core for measurement (outer diameter: 38 mm, inner diameter: 25 mm, height: 6.2 mm). Prior to molding, a 5 mass% alcohol suspension of zinc stearate was applied to the mold and the mold was lubricated, and molded at a molding pressure of 980 MPa.
Thereafter, the obtained green compact was heat-treated at 800 ° C. for 60 minutes in a nitrogen atmosphere.
The results of examining the dust density, magnetic flux density and electrical resistance of the dust core thus obtained are also shown in Table 1.
In Table 1, for comparison, 2% Si-Fe alloy powder (Comparative Example 3) and 3% Si-Fe alloy powder (Comparative Example 4) which were not subjected to Si concentration treatment according to the present invention are also shown. The results of a similar investigation are shown.

The dust density was obtained by measuring the size and weight of the dust core and calculating it.
The magnetic flux density was measured by measuring the magnetic flux density (B _10k ) at a magnetization of 10 kA / m using a DC magnetization characteristic measuring device by winding the primary side: 100 turns and the secondary side: 20 turns around the dust core. .
Furthermore, the electrical resistance was measured at an energization current of 1 A by the four probe method.

As shown in Table 1, all metal powders subjected to the gas phase reaction treatment under the conditions of the present invention had an Si thickened layer having an appropriate thickness and an appropriate concentration on the surface layer portion. Further, any dust core produced using such metal powder was able to obtain an excellent dust density of 95% or more of the true density, and was excellent in magnetic flux density and electrical resistance.
On the other hand, Comparative Examples 1 and 2 are cases where the gas phase reaction treatment temperature deviated from the appropriate temperature range of the present invention, and although high specific resistance was obtained, the dust density and magnetic flux density were low. This is probably because the heat treatment is performed in the temperature range where the ferrite phase is generated, so that the penetration of Si deeply progresses into the iron powder and the iron powder is cured.
Comparative Examples 3 and 4 are cases where the Fe-Si alloy powder was simply used instead of the iron powder whose surface of Si was concentrated. However, since the hardness of the powder itself was high, the powder density did not increase, and the specific resistance. However, high magnetic flux density could not be obtained.

  According to the present invention, when Si is vapor-deposited on the surface of the powder particles by a gas phase reaction, the diffusion of Si is delayed by performing the conditions under which an austenite phase is formed on the powder surface, and an appropriate amount of Si is applied only to the surface layer portion. Can be obtained. Furthermore, after applying an insulating coating to this powder, it is pressure-molded and heat treated at 600 ° C to 1000 ° C to obtain a dust core having a high dust density, a high magnetic flux density and a specific resistance. Can do. As a result, dust cores for motors, transformers, and inductors having excellent magnetic properties can be obtained at low cost.

Claims (6)

  A metal powder exhibiting an austenite phase in a high temperature range is heated to a temperature range where the austenite phase is formed, and Si is concentrated in a surface layer portion of the metal powder by a gas phase reaction in this temperature range. A method for producing a metal powder for a dust core.   The method for producing a metal powder for a dust core according to claim 1, wherein the metal powder is a pure iron powder having a purity of 99% or more.   The method for producing a metal powder for a dust core according to claim 1, wherein the metal powder is an Fe-Si alloy powder containing Si at 1 mass% or less.   4. The method for producing a metal powder for a dust core according to any one of claims 1 to 3, further comprising subjecting the surface of the metal powder having the surface layer portion enriched with Si to an insulation coating treatment.   A method for producing a powder magnetic core, comprising pressure-molding an insulating coating-treated metal powder produced by the method according to claim 4.   A method for producing a powder magnetic core, comprising subjecting a metal powder having been subjected to an insulation coating produced by the method according to claim 4 to heat treatment in a temperature range of 600 ° C to 1000 ° C after pressure forming.