JP2020053536A - Manufacturing method of dust core - Google Patents

Abstract

To provide a manufacturing method of dust core capable of producing a dust core having excellent strength.SOLUTION: A manufacturing method of a dust core includes a step of forming an aluminum oxide film on an Fe-Si-Al-based soft magnetic powder, a step of preparing a powder for a magnetic core by mixing an Fe-Si-Al-based soft magnetic powder having an aluminum oxide film formed thereon, a low-melting glass, and a lubricant, a step of compacting the powder for the magnetic core to form a compact, a step of annealing the compact, and a step of performing heat treatment at a temperature of 200°C or less for 500 hours or more after the annealing.SELECTED DRAWING: None

Description

  The present disclosure relates to a method for manufacturing a dust core.

  Dust cores have been manufactured by compacting powder for magnetic cores. It is important for the dust core to ensure magnetic properties suitable for the application while securing the insulating property between the soft magnetic powders constituting the powder for the magnetic core, and many researches and developments have been made.

  A dust core is required to not only exhibit high magnetic properties in an alternating magnetic field but also to have a small iron loss when used in an alternating magnetic field. Iron loss includes eddy current loss, hysteresis loss, residual loss, and the like. Particularly, reduction of eddy current loss that increases with the frequency of the alternating magnetic field is strongly demanded.

  In view of this, a powder magnetic core obtained by press-molding a soft magnetic powder covered with an insulating layer made of an oxide film has been developed. In such a dust core, a high specific resistance and a low iron loss can be obtained due to the presence of the insulating layer. For example, Patent Document 1 discloses a powder for a magnetic core made of a soft magnetic powder coated with such an insulating layer. The first powder made of a soft magnetic powder and an aluminum oxide coating the surface of the soft magnetic powder is disclosed. A coating layer, and a second coating layer made of a low-melting glass having a softening point lower than the annealing temperature of the soft magnetic powder and covering at least a part of the surface of the first coating layer. A dust core is disclosed.

JP-A-2015-103770

  According to the technique of Patent Document 1, excellent specific resistance can be exhibited, but there is room for improvement from the viewpoint of strength (particularly radial crushing strength).

  An object of the present disclosure is to provide a method for manufacturing a dust core that can manufacture a dust core having excellent strength.

  This embodiment is as follows.

Forming an aluminum oxide film on the Fe-Si-Al soft magnetic powder;
Mixing the Fe-Si-Al-based soft magnetic powder having the aluminum oxide film formed thereon, a low-melting glass, and a lubricant to prepare a magnetic core powder;
A step of compacting the powder for a magnetic core to form a compact;
Annealing the green compact;
After annealing, performing a heat treatment at a temperature of 200 ° C. or less for 500 hours or more,
A method for producing a dust core.

  According to the present embodiment, it is possible to provide a method of manufacturing a dust core that can manufacture a dust core having excellent strength.

It is a graph which shows the radial crushing strength improvement rate measured about the dust core manufactured by the Example and the comparative example.

  The present embodiment includes a step of forming an aluminum oxide film on an Fe-Si-Al-based soft magnetic powder, an Fe-Si-Al-based soft magnetic powder having the aluminum oxide film formed thereon, a low-melting glass, and a lubricant. , A step of preparing a powder for a magnetic core, a step of compacting the powder for a magnetic core to form a compact, a step of annealing the compact, and a step of annealing. Performing a heat treatment at a temperature of 200 ° C. or less for at least a time.

  When preparing a powder for a magnetic core, a low-melting glass is added to obtain a dust core having a high specific resistance and a high strength. At this time, by adding a lubricant, it is possible to reduce abrasion and plastic strain of particles at the time of compacting, so that the coercive force of the dust core and the hysteresis loss can be reduced. However, this lubricant volatilizes or burns during the annealing process. However, since the annealing process is generally performed in an extremely low oxygen atmosphere, it is difficult for the lubricant to sufficiently burn. Therefore, the lubricant does not decompose to carbon dioxide, and a part of the lubricant remains in the dust core as an inclusion such as carbon. When inclusions such as carbon are present in the dust core, the inter-particle adhesion of glass is hindered, which causes a reduction in strength. Therefore, in this embodiment, after the annealing step, heat treatment is performed at a temperature of 500 ° C. or more and 200 ° C. or less. By this heat treatment, inclusions such as carbon remaining in the dust core can be oxidized to carbon dioxide and sufficiently released from the dust core. As a result, the adhesive strength between the particles can be improved, and a dust core having excellent strength can be obtained.

  Hereinafter, the present embodiment will be described in detail.

<Insulating layer forming step>
This embodiment includes a step of forming an aluminum oxide film as an insulating layer on the Fe—Si—Al-based soft magnetic powder.

  In this embodiment, an Fe—Si—Al-based soft magnetic powder is used as the soft magnetic powder. The Fe-Si-Al soft magnetic powder is an iron alloy containing Al and Si. When Si is contained together with Al in the iron alloy, an insulating layer (aluminum oxide film) made of aluminum oxide is easily formed. A part of Fe as the main component may be replaced by a ferromagnetic element such as Co or Ni. Due to the presence of Al, an oxide film containing aluminum oxide can be formed near the surface of the soft magnetic powder. Si can contribute to improvement of the electric resistivity of the soft magnetic powder, improvement of the specific resistance of the dust core (reduction of eddy current loss), improvement of the strength, and the like.

  Although the composition of the Fe-Si-Al soft magnetic powder is not particularly limited, it is taken into consideration in consideration of the magnetic properties of the dust core, the moldability of the powder for the core, the formability of the oxide film containing aluminum oxide, and the like. Can be adjusted appropriately. When the total soft magnetic powder is 100% by mass, the content of Al is preferably 0.5 to 5.0% by mass, 1.0 to 4.5% by mass, or 1.5 to 4.0% by mass. It is. When the total soft magnetic powder is 100% by mass, the content of Si is preferably 0.5 to 9.0% by mass, 1.0 to 7.0% by mass, or 1.5 to 6.5% by mass. It is. When the total soft magnetic powder is 100% by mass, the content of Fe is preferably 70.0% by mass or more, 75.0% by mass or more, 80.0% by mass or more, 86.0% by mass or more, 90% by mass or more. 0.0% by mass or more. Further, when the entire soft magnetic powder is 100% by mass, the content of Al is 0.5 to 5.0% by mass, 1.0 to 4.5% by mass, or 1.5 to 4.0% by mass. It is preferable that the content of Si is 0.5 to 9.0% by mass, 1.0 to 7.0% by mass, or 1.5 to 6.5% by mass, and the balance is mainly composed of Fe. . Any of the lower limits listed can be combined with any upper limit.

  In addition, the soft magnetic powder may naturally contain unavoidable impurities as a residue other than Fe. Further, the soft magnetic powder may contain one or more kinds of modifying elements capable of improving the magnetic properties and specific resistance of the dust core, the moldability of the powder for the magnetic core, the formability of the oxide film, and the like. As such a modifying element, for example, Mn, Cr, Mo, Ti, Ni and the like can be considered. Usually, the amount of the modifying element is very small, and the total amount is preferably 3.0% by mass or less, or 1.0% by mass or less.

  The method for producing the soft magnetic powder is not particularly limited. The soft magnetic powder may be, for example, an atomized powder or a pulverized powder. The atomized powder may be any of water atomized powder, gas atomized powder, and gas water atomized powder. When atomized powder composed of substantially spherical particles is used, the aggressiveness between the particles is reduced, and a decrease in the specific resistance value due to breakage of the insulating layer or the like can be suppressed. The pulverized powder can be obtained, for example, by pulverizing an alloy ingot with a ball mill or the like.

The particle diameter (median diameter D ₅₀ ) of the soft magnetic powder is not particularly limited, but is preferably 30 to 250 μm. When the particle size is 30 μm or more, it is easy to suppress an increase in hysteresis loss of the dust core. Further, when the particle size is 250 μm or less, it is easy to suppress an increase in eddy current loss of the dust core and a decrease in strength of the dust core.

  Although the reason is not clear, when an aluminum oxide film is present on the surface of the soft magnetic particles, the diffusion of the constituent elements between the low-melting glass and the soft magnetic particles hardly occurs during annealing of the green compact. That is, the aluminum oxide coating (hereinafter, also referred to as the first coating layer) interposed between the soft magnetic particles and the low melting point glass (hereinafter, also referred to as the second coating layer) forms a structure between the soft magnetic particles and the low melting point glass. Functions as a barrier layer for suppressing the diffusion of elements. For this reason, in the present embodiment, even if the annealing treatment is performed, the second coating layer made of the low-melting glass and the first coating layer made of the aluminum oxide coating are less likely to deteriorate or have defects.

  Furthermore, the low-melting glass has good wettability with respect to the first coating layer made of the aluminum oxide coating. For this reason, the low-melting glass softened or melted during the annealing uniformly wets and spreads on the first coating layer, and further flows into minute gaps between the soft magnetic particles, and serves as a fracture starting point. Can be reduced. Then, the low-melting glass after annealing solidifies while maintaining such a state.

The aluminum oxide contained in the aluminum oxide film is not limited to a specific composition, and may be a single type of aluminum oxide or a mixture of a plurality of types of aluminum oxide. For example, in addition to aluminum oxide (III) represented by Al ₂ O ₃ , aluminum oxide (I) represented by Al ₂ O and aluminum oxide (II) represented by AlO can be given. Further, aluminum oxide (III) may be either spinel-type aluminum oxide (γ-Al ₂ O ₃ ) or corundum-type aluminum oxide (α-Al ₂ O ₃ ) having a different crystal structure. Usually, the main component of aluminum oxide is considered to be α-Al ₂ O ₃ .

  The aluminum oxide film may contain a trace amount of an oxide (silicon oxide, iron oxide, or the like) other than aluminum oxide as a main component. In addition, in addition to the case where the aluminum oxide has a perfect crystal structure, the case where the aluminum oxide has an incomplete crystal structure in which oxygen deficiency has occurred in a part of the crystal structure, or a case where they are mixed, The specific crystal structure does not matter. In short, the specific composition and structure of the aluminum oxide film are not limited as long as the aluminum oxide film has a diffusion barrier property for suppressing the diffusion of constituent elements between the soft magnetic particles and the low melting point glass and an insulating property. The first coating layer (aluminum oxide coating) or the second coating layer (low-melting glass) is preferably present uniformly or uniformly on the outer surface of all the soft magnetic particles, but is not partially coated. There may be parts or non-uniform or non-uniform parts.

  The method for forming the aluminum oxide film on the Fe-Si-Al-based soft magnetic powder is not particularly limited, and various methods can be considered. As an example of the method of forming the aluminum oxide film, a method of heating an Fe—Si—Al-based soft magnetic powder in an oxidizing atmosphere is given. In addition, as another example of the method of forming the aluminum oxide film, a method of heating an Fe—Si—Al-based soft magnetic powder having iron oxide in the vicinity of the surface of the particles in a non-oxidizing atmosphere can be used. Hereinafter, this method will be described.

The iron oxide present in the Fe-Si-Al-based soft magnetic powder is not limited to a specific composition, and may be a single oxide or a mixture of a plurality of oxides. For example, iron oxide (II) represented by FeO, iron oxide (II, III) represented by Fe ₃ O ₄ , iron oxide represented by Fe ₂ O ₃ (including α-type, β-type, etc.) In addition to III), any one or more of oxyhydroxides such as FeOOH (including α-type and β-type) and iron hydroxides such as Fe (OH) ₂ and Fe (OH) ₃ may be used. However, usually, it is any one or more of FeO, Fe ₃ O _{4 and} Fe ₂ O ₃ .

The non-oxidizing atmosphere may be an atmosphere in which oxygen is substantially not present, and may be an inert gas atmosphere or a vacuum atmosphere. Examples of the inert gas include N ₂ , He, Ar, and a mixture thereof. The dew point of the non-oxidizing atmosphere is preferably -40C or lower, or -50C or lower. The heating temperature is preferably 650 to 1000C or 700 to 950C. The heating time is preferably 0.3 to 2 hours or 0.5 to 1.5 hours from the viewpoint of efficiency.

The reason why the aluminum oxide film (insulating layer containing aluminum oxide) is formed by performing the non-oxidation treatment step on the oxidized particles is not necessarily clear, but is presumed as follows. First, O ₂ adheres or adsorbs to the surface of soft magnetic particles placed in an oxidizing atmosphere where oxygen (O ₂ ) is present, and the surface of the soft magnetic particles is softened or spontaneously heated. A large amount of iron oxide (Fe ₂ O ₃ , Fe ₃ O ₄ , FeO, etc.) reacted with Fe, which is the main component of the magnetic particles, can be generated. At this time, in addition to iron oxide, oxides of Al and Si may be partially generated near the surface of the soft magnetic particles. Next, when such soft magnetic particles having iron oxide on the surface are placed in a non-oxidizing atmosphere substantially free of O ₂ and heated in a state where O is deficient, oxides are formed more than Fe does. Al with low energy deprives iron oxide existing in the vicinity of the surface of the soft magnetic particles of O, and generates aluminum oxide which is more stable than iron oxide. The aluminum oxide film thus formed exhibits high heat resistance and high specific resistance. The change from iron oxide to aluminum oxide, which occurs near the surface of the soft magnetic particles, is likely to occur when the soft magnetic particles as the base particles are an iron alloy containing Si (Fe-Al-Si alloy). It has also been found that it does not easily occur when the magnetic particles are an iron alloy containing no Si. Therefore, it is considered that Si in the soft magnetic particles acts as a catalyst that promotes the change from iron oxide to aluminum oxide.

The oxidized particles (oxidized powder) in which iron oxide is present on the surface of the particles may be obtained by, for example, an oxidation treatment in which the soft magnetic particles are heated in an oxidizing atmosphere, regardless of the generation process or method. As a result, a sufficient amount of iron oxide that changes to aluminum oxide is stably generated on the surface of the soft magnetic particles. The oxidizing atmosphere may be an environment containing an appropriate amount of oxygen (O ₂ ), and may be a mixed gas atmosphere or an air atmosphere. For example, when using a mixed gas (air flow) of O ₂ and an inert gas (N ₂ , Ar, etc.), the O ₂ amount is preferably 0.1 to 30% by volume or 0.5 to 25% by volume. is there. Furthermore, the oxidation treatment can be performed in the air, and the dew point of the oxidizing atmosphere at that time is preferably −40 ° C. or lower, or −50 ° C. or lower. The heating temperature of the soft magnetic powder depends on the gas composition (especially the oxygen concentration) in the oxidizing atmosphere, but is preferably 800 to 1100 ° C or 850 to 1050 ° C. The heating time depends on the oxygen concentration in the oxidizing atmosphere and the heating temperature, but is preferably 0.5 to 10 hours, or 1 to 3 hours.

  In addition, when Al in the soft magnetic particles becomes excessive, aluminum oxide is easily generated on the surface of the soft magnetic particles from the beginning, and even when the non-oxidizing treatment step is performed, the change from iron oxide to aluminum oxide hardly occurs. is there.

  When agglomeration occurs due to the above treatment, the powder can be crushed by an appropriate method.

<Process for preparing powder for magnetic core>
This embodiment includes a step of mixing the Fe-Si-Al-based soft magnetic powder on which the aluminum oxide film is formed, a low-melting glass, and a lubricant to prepare a powder for a magnetic core.

  The low melting point glass has a softening point lower than the annealing temperature of the soft magnetic powder when the dust core is annealed. Examples of the low-melting glass include silicate glass, borate glass, bismuth silicate glass, borosilicate glass, vanadium oxide glass, and phosphate glass.

The silicate-based glass, for _example, a main component _{SiO 2 -ZnO, SiO 2 -Li 2} O, SiO 2 -Na 2 O, SiO 2 -CaO, SiO 2 -MgO, a SiO 2 _-Al 2 _{O 3,} etc. There is something to do. Bismuth silicate glass includes, for example, SiO ₂ —Bi ₂ O ₃ —ZnO, SiO ₂ —Bi ₂ O ₃ —Li ₂ O, SiO ₂ —Bi ₂ O ₃ —Na ₂ O, SiO ₂ —Bi ₂ O _Some include _3- CaO or the like as a main component. The borate-based glass, for _{example, B 2 O 3 -ZnO, B} 2 O 3 -Li 2 O, B 2 O 3 -Na 2 O, B 2 O 3 -CaO, B 2 O 3 -MgO, B 2 Some include O ₃ —Al ₂ O ₃ or the like as a main component. The borosilicate based glass, for _{example, SiO 2 -B 2 O 3 -ZnO} , SiO 2 -B 2 O 3 -Li 2 O, SiO 2 -B 2 O 3 -Na 2 O, SiO 2 -B 2 O _Some include _3- CaO or the like as a main component. The vanadium oxide-based glass, for _{example, V 2 O 5 -B 2 O} 3, V 2 O 5 -B 2 O 3 -SiO 2, V 2 O 5 -P 2 O 5, V 2 O 5 -B 2 O there is mainly composed of ₃ -P ₂ O ₅ or the like. The phosphoric acid-based glass, for _{example, P 2 O 5 -Li 2 O} , P 2 O 5 -Na 2 O, P 2 O 5 -CaO, P 2 O 5 -MgO, P 2 O 5 -Al 2 O 3 And the like as a main component. These low-melting glass, in addition to the aforementioned components _may contain _{SiO 2, ZnO, Na 2 O} , B 2 O 3, Li 2 O, SnO, BaO, CaO, at least one of _Al 2 _{O 3} or the like as appropriate .

  The content of the low-melting glass is preferably 0.05 to 5.0% by mass when the magnetic core powder is 100% by mass. When the content of the low-melting glass is 0.05% by mass or more, it becomes easy to form a sufficient low-melting glass coating, and it is easy to obtain a dust core having high specific resistance and high strength. When the content of the low-melting glass is 5.0% by mass or less, it is possible to effectively suppress a decrease in the magnetic properties of the dust core.

  The low melting point glass coating may be a layer adhered to the surface of the soft magnetic powder as fine particles having a smaller particle diameter than the soft magnetic powder, or may be a layer continuously adhered to the surface of the soft magnetic powder. For example, when forming a low-melting glass coating, a powder of fine particles made of low-melting glass and a soft magnetic powder may be mixed in a dispersion medium and dried, and the low-melting glass softened by heating may be dried. You may make it adhere to soft magnetic powder. Further, the powder of the fine particles made of the low melting point glass and the soft magnetic powder may be bound by a binder (binder) such as PVA or PVB. When the low-melting-point glass coating is composed of a layer adhered to the surface of the soft magnetic powder as fine particles having a smaller particle size than the soft magnetic powder, a continuous coating can be obtained through compacting and annealing in a later step.

  The low melting point glass becomes a second coating layer surrounding the first coating layer of the soft magnetic particles in the dust core, but it is necessary to completely surround the insulating layer of the soft magnetic particles at the stage of the powder for the magnetic core. There is no. That is, the low-melting glass may be in a state where the low-melting glass becomes fine particles having a smaller particle diameter than the soft magnetic particles and is scattered on the surface of the insulating layer of the soft magnetic particles. The particle size of such low melting point glass (glass fine particles) depends on the particle size of the soft magnetic powder, but is preferably 0.1 to 100 μm or 0.5 to 50 μm. If the particle size of the glass particles is too small, it becomes difficult to manufacture and handle the glass particles. If the particle size is too large, it becomes difficult to form a uniform second coating layer. Incidentally, the method for specifying the particle size of the glass fine particles includes a wet method, a dry method, a method obtained from a scattering pattern of irradiated laser light, a method obtained from a difference in sedimentation velocity, a method obtained by image analysis, and the like.

  The powder preparation step for a magnetic core (hereinafter, also referred to as a glass deposition step) is a step of depositing a low-melting glass on an insulating layer (aluminum oxide film) formed on the surface of a soft magnetic powder. Along with the agent, it is mixed with the Fe-Si-Al-based soft magnetic powder. For example, in the case of attaching fine particles (glass fine particles) made of low-melting glass, the glass attaching step may be performed by a wet method or a dry method. For example, when a wet method is used, the glass attaching step can be a wet attaching step in which glass fine particles and soft magnetic powder are mixed in a dispersion medium and then dried. When a dry method is used, the glass attaching step can be a dry attaching step of mixing glass fine particles and soft magnetic powder without using a dispersion medium. The wet method makes it easy to uniformly adhere the glass particles to the surface of the insulating layer of the soft magnetic powder. The dry method is efficient because the drying step can be omitted.

  The use of a lubricant (also referred to as an internal lubricant) makes it easier to reduce wear due to friction between particles during compacting. Further, by adding a lubricant to the soft magnetic powder, slip between particles is improved, and plastic strain of the particles is suppressed. Therefore, by manufacturing a dust core by adding an appropriate amount of lubricant to the soft magnetic powder, it is possible to reduce the coercive force and the hysteresis loss of the dust core. Examples of the lubricant include, but are not particularly limited to, zinc stearate, calcium stearate, lithium stearate, stearic acid amide, behenyl alcohol, erucic acid monoamide, oleic acid monoamide, and ethylenebis-stearic acid amide. No.

  The content of the lubricant is preferably 0.05 to 3.0% by mass, more preferably 0.1 to 2.0% by mass, assuming that the magnetic core powder is 100% by mass. It is preferably from 2 to 1.0% by mass. Any of the lower limits listed can be combined with any of the upper limits listed.

<Compacting process>
The present embodiment includes a step of compacting the magnetic core powder to form a compact.

  In the powder compaction from the powder for the magnetic core to the powder compact, the higher the pressure, the higher the density of the powder core and the higher the magnetic flux density. As such a high-pressure molding method, for example, a mold lubrication warm high-pressure molding method may be mentioned. The mold lubrication warm high-pressure molding method comprises a filling step of filling the core powder into a mold having a higher fatty acid-based lubricant applied to the inner surface thereof, and a higher fatty acid-based lubricant interposed between the core powder and the inner surface of the mold. And a warm high-pressure forming step of press-forming at a forming temperature and a forming pressure at which a metallic soap film is formed.

  Here, the term “warm” refers to, for example, a molding temperature of 70 ° C. to 200 ° C. and further 100 to 180 ° C. in consideration of the influence on the surface film (or the insulating film) and the deterioration of the higher fatty acid-based lubricant. To do. Details of this mold lubrication warm high pressure molding method are described in publications such as Japanese Patent No. 3309970 and Japanese Patent No. 4024705. According to this mold lubrication warm high-pressure molding method, ultra-high pressure molding can be performed while extending the life of the mold, and a high-density dust core can be easily obtained.

<Annealing process>
This embodiment includes a step of annealing the green compact.

  The green compact is annealed, for example, at a temperature of 500 ° C. or higher. Thereby, residual strain and residual stress introduced into the soft magnetic particles in the dust core can be removed, and the coercive force and hysteresis loss of the dust core can be reduced. Further, in the present embodiment, the low melting point glass is softened during the annealing, so that a low melting point glass layer can be interposed between the soft magnetic particles.

  The annealing temperature can be appropriately selected according to the type of the soft magnetic particles and the low-melting glass, but is preferably 550 ° C or higher, 650 ° C or higher, 700 ° C or higher, or 750 ° C or higher. Since the aluminum oxide film has excellent heat resistance, the insulating layer maintains high insulating properties and high barrier properties even when annealed at a high temperature. From the viewpoint of efficiency, the annealing temperature is preferably 1000 ° C. or lower, 950 ° C. or lower, or 920 ° C. or lower. The heating time is, for example, 0.1 to 5 hours and 0.5 to 2 hours. The heating atmosphere is preferably an inert atmosphere.

<Heat treatment process>
The present embodiment includes a step of performing a heat treatment at a temperature of not less than 500 hours and not more than 200 ° C. after annealing (also referred to as a strength improving heat treatment step).

  In the annealing treatment, the lubricant added together with the low-melting glass is volatilized and burned. However, since the annealing treatment is generally performed in an extremely low oxygen atmosphere, it is difficult for the lubricant to burn sufficiently. Therefore, the lubricant does not decompose to carbon dioxide, and a part of the lubricant remains in the dust core as an inclusion such as carbon. When inclusions such as carbon are present in the dust core, the inter-particle adhesion of glass is hindered, which causes a reduction in strength. Therefore, in this embodiment, after the annealing step, heat treatment is performed at a temperature of 500 ° C. or more and 200 ° C. or less. By this heat treatment, inclusions such as carbon remaining in the dust core can be oxidized to carbon dioxide and sufficiently released from the dust core. As a result, the adhesive strength between the particles can be improved.

The temperature of the heat treatment is preferably 70 ° C or higher, 80 ° C or higher, 100 ° C or higher, 125 ° C or higher, or 150 ° C or higher. Further, the heat treatment is preferably performed in an oxidizing atmosphere. The oxidizing atmosphere may be an environment containing appropriate oxygen (particularly O ₂ ), and is preferably an air atmosphere. Further, the oxidizing atmosphere may be under a high humidity condition. In the high humidity condition, the relative humidity is preferably 50% or more, more preferably 80% or more. The dew point of the oxidizing atmosphere is preferably −40 ° C. or lower, and −50 ° C. or lower.

<Powder core>
The obtained dust core has soft magnetic particles, an aluminum oxide coating covering the surface of the soft magnetic particles, and a low-melting glass layer covering at least a part of the surface of the aluminum oxide coating.

  Regardless of its form, the dust core can be used for various electromagnetic devices, for example, motors, actuators, transformers, induction heaters (IH), speakers, reactors, and the like. Specifically, it is preferably used for a magnetic field of an electric motor or a generator or an iron core constituting an armature. Above all, it is suitably used for an iron core for a driving motor which requires low loss and high output (high magnetic flux density).

  Hereinafter, the present embodiment will be specifically described with reference to examples. This embodiment is not limited by the following examples.

(Example 1)
In Example 1, first, an Fe—Si—Al-based soft magnetic powder having an aluminum oxide film as an insulating layer on its surface was prepared (insulating layer forming step). Next, a low-melting glass and a lubricant were added to and mixed with the soft magnetic powder on which the aluminum oxide film was formed to prepare a powder for a magnetic core (a powder preparing step for a magnetic core). Next, the powder for a magnetic core was pressed (a powder compacting step). Next, the obtained green compact was annealed (annealing step). Next, the above-described heat treatment was performed on the annealed dust compact to obtain a dust core (strength improving heat treatment step). Hereinafter, each step will be described in detail.

<Insulating layer forming step>
As the soft magnetic powder, an Fe—Si—Al-based soft magnetic powder (atomized powder) was prepared.

Oxidation treatment step The soft magnetic powder was placed in a rotary furnace and heated at 900 ° C for 2 hours in an oxidizing atmosphere in which an atmospheric gas flowed at a rate of 0.5 L / min. The dew point in the air atmosphere was -60C. As a result, a soft magnetic powder having an aluminum oxide film formed on the particle surface was obtained.

<Process for preparing powder for magnetic core>
And the soft magnetic powder 98 parts by mass of _the resultant and SiO ₂ -B ₂ O 3 -ZnO glass frit having a low melting point 1.5 parts by weight of the system, behenyl alcohol, erucic acid monoamide and stearic acid monoamide of 1: 1: 3 0.5 parts by mass (lubricant) of the mixture mixed at a mass ratio were mixed with a rotary ball mill to prepare a magnetic core powder.

<Compacting process>
Using the obtained powder for a magnetic core, a ring-shaped (30 mm inner diameter, 39 mm outer diameter, 5 mm thick) green compact was obtained by mold lubrication warm high pressure molding.

<Annealing process>
The obtained green compact was placed in a heating furnace and heated at 750 ° C. for 30 minutes in a non-oxidizing atmosphere in which nitrogen gas flows at a rate of 0.5 L / min.

<Strength improving heat treatment process>
The annealed green compact was placed in a heater and heated at 170 ° C. for 500 hours in an air atmosphere. Through the above steps, a dust core was manufactured.

<Evaluation of dust core>
The radial crushing strength of each of the obtained dust cores was measured. Specifically, the maximum load was measured using an autograph, and the radial crushing strength was calculated. The radial crushing strength test was performed using a method based on JIS Z2507. From the obtained radial crushing strength, the radial crushing strength improvement rate was calculated. Table 1 shows the results.

(Example 2)
A dust core was manufactured in the same manner as in Example 1 except that the heating time was changed to 1000 hours. Table 1 shows the results.

(Example 3)
A dust core was manufactured in the same manner as in Example 1 except that the heating temperature was set to 200 ° C. Table 1 shows the results.

(Example 4)
A dust core was manufactured in the same manner as in Example 1 except that the heating temperature was 200 ° C. and the heating time was 1000 hours. Table 1 shows the results.

(Example 5)
A dust core was manufactured in the same manner as in Example 1 except that the heating temperature was set to 85 ° C. and the conditions of high humidity (85% RH) were used. Table 1 shows the results. “85 ° C., 85% RH” refers to an atmosphere containing 0.85 times the amount of saturated steam at 85 ° C.

(Example 6)
A dust core was produced in the same manner as in Example 1 except that the heating temperature was 85 ° C., the heating time was 1000 hours, and the conditions were high humidity (85% RH). Table 1 shows the results.

(Comparative Example 1)
A dust core was manufactured in the same manner as in Example 1 except that the heating time was set to 1 hour. Table 1 shows the results.

(Comparative Example 2)
A dust core was manufactured in the same manner as in Example 1 except that the heating time was changed to 100 hours. Table 1 shows the results.

  FIG. 1 shows a graph of the results of Table 1. From FIG. 1, it is confirmed that a saturation (maximum) value of the strength improvement effect can be obtained if the time is 500 hours or more.

Claims (1)

Forming an aluminum oxide film on the Fe-Si-Al soft magnetic powder;
Mixing the Fe-Si-Al-based soft magnetic powder with the aluminum oxide film formed thereon, the low-melting glass, and a lubricant to prepare a magnetic core powder;
A step of compacting the powder for a magnetic core to form a compact;
Annealing the green compact;
After annealing, performing a heat treatment at a temperature of 200 ° C. or less for 500 hours or more,
A method for producing a dust core.