Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
  • B22F1/10—Metallic powder containing lubricating or binding agents; Metallic powder containing organic material
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
  • B22F1/10—Metallic powder containing lubricating or binding agents; Metallic powder containing organic material
  • B22F1/102—Metallic powder coated with organic material
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
  • B22F1/16—Metallic particles coated with a non-metal
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
  • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
  • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
  • H01F1/12—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials
  • H01F1/14—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials metals or alloys
  • H01F1/147—Alloys characterised by their composition
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
  • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
  • H01F1/03—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity
  • H01F1/12—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials
  • H01F1/14—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials metals or alloys
  • H01F1/20—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials characterised by their coercivity of soft-magnetic materials metals or alloys in the form of particles, e.g. powder

Definitions

• the present invention relates to iron-based soft magnetic powders for dust cores, insulating-coated soft magnetic powders for dust cores, and mixed powders for dust cores.
• Magnetic cores used in motors, transformers, etc. require high magnetic flux density and low iron loss.
• motor cores have been made from laminated electromagnetic steel sheets, but in recent years, powder magnetic cores have been attracting attention.
• dust cores are their ability to form three-dimensional magnetic circuits.
• the magnetic core is formed by laminating layers, which limits the freedom of shape.
• steel sheets with insulated surfaces are laminated, the magnetic properties differ between the surface of the steel sheet and the direction perpendicular to the surface, with the disadvantage that the magnetic properties are poor in the direction perpendicular to the surface.
• dust cores are made by pressing soft magnetic particles with an insulating coating, and their shape can be changed by changing the mold, allowing for greater freedom of shape than electromagnetic steel sheets.
• each particle is covered with an insulating coating, the magnetic properties are uniform in all directions, which is advantageous for forming three-dimensional magnetic circuits.
• press molding requires fewer steps and is less expensive than the process of laminating electromagnetic steel sheets, and this, combined with the low cost of the base powder, means that dust cores offer excellent cost performance.
• dust cores not only enable the design of three-dimensional magnetic circuits, but also offer excellent cost performance.
• research and development into motors with three-dimensional magnetic circuits that use dust cores is being actively conducted in order to meet recent demands for smaller motors, rare-earth-free motors, and lower costs.
• Iron loss is primarily comprised of two types: eddy current loss and hysteresis loss. Compared to conventional iron core materials such as electromagnetic steel sheets, dust cores have higher hysteresis loss, and there is a demand for reducing this loss.
• Patent Documents 1 and 2 disclose technology for obtaining dust cores with low hysteresis loss by reducing the strain in soft magnetic powder measured by X-ray diffraction.
• Patent Document 3 discloses a technology for obtaining a dust core with low hysteresis loss by coarsening the crystal grains in soft magnetic powder.
• Patent Document 4 discloses an iron-based soft magnetic powder for dust cores that reduces the number density of inclusions of 0.1 ⁇ m or larger.
• the present invention was made to solve the above problems, and its gist is as follows:
• Mass concentration S: 20 ppm or less, Sn and Sb: 200 ppm or less in total, and An iron-based soft magnetic powder for dust cores has a component composition that satisfies at least one of S: 10 ppm or less and Sn and Sb: 5 ppm or more in total.
• An insulating-coated soft magnetic powder for dust cores comprising the iron-based soft magnetic powder for dust cores described in 1 above, having an insulating coating on the surface of the particles that make up the powder cores.
• the present invention provides an iron-based soft magnetic powder for dust cores that can be used to produce dust cores with reduced hysteresis loss.
• iron powder refers to a powder consisting of Fe and unavoidable impurities, and is generally referred to as “pure iron powder” in this technical field.
• iron-based alloy powder refers to an alloy powder containing 50% or more Fe by mass.
• the alloying elements of the iron-based alloy powder are not particularly limited, and in addition to S, Sn, and Sb, at least one element selected from the group consisting of Al, Si, Co, Cr, Mn, Mo, and Ni can be used.
• the iron-based alloy powder may be, for example, a powder of an Fe-Al alloy, an Fe-Si alloy, Sendust, or Permalloy.
• the Fe-Si alloy may be, for example, an Fe-3% Si alloy or an Fe-6.5% Si alloy.
• the crystalline state of the iron-based soft magnetic powder is not particularly limited, and the iron-based soft magnetic powder may be, for example, an amorphous powder.
• the iron-based soft magnetic powder may be an atomized powder, and may be at least one selected from the group consisting of gas atomized powder, water atomized powder, and centrifugal atomized powder.
• gas atomized powder, water atomized powder, and centrifugal atomized powder refer to powders obtained by the gas atomization method, water atomization method, and centrifugal atomization method, respectively, as described below.
• water atomized powder is preferred, and water atomized iron powder is more preferred.
• the iron-based soft magnetic powder has a mass concentration of: S: 20 ppm or less, Sn and Sb: 200 ppm or less in total, and The composition satisfies at least one of S: 10 ppm or less, and Sn and Sb: 5 ppm or more in total.
• the composition satisfies the following mass concentrations:
• the composition is such that S is 10 ppm or less, and Sn and Sb are 200 ppm or less in total, or S is more than 10 ppm and 20 ppm or less, and Sn and Sb are 5 ppm or more and 200 ppm or less in total.
• the component composition of the iron-based soft magnetic powder will be described in detail below. In the following description, the unit of content "ppm" refers to a mass concentration unless otherwise specified.
• S content S inhibits grain growth by grain boundary segregation and the formation of MnS. If the S content exceeds 20 ppm, the hysteresis loss of the manufactured dust core will increase. Therefore, the S content is set to 20 ppm or less. From the viewpoint of ease of manufacturing, the S content may exceed 10 ppm, but from the viewpoint of further promoting grain growth, the S content is preferably 10 ppm or less, and more preferably 5 ppm or less. On the other hand, there is no restriction on the lower limit of the S content, but it may be 0 ppm, and S may not be contained at all. The S content is measured by a combustion-infrared absorption method.
• the component composition satisfies at least one of S: 10 ppm or less and Sn and Sb: 5 ppm or more in total.
• S: 10 ppm or less and Sn and Sb: 5 ppm or more in total if the S content exceeds 10 ppm, the total content of Sn and Sb is 5 ppm or more. In this way, even if the S content exceeds 10 ppm, by containing Sn and Sb in a total of 5 ppm or more, crystal grain growth can be promoted.
• the component composition preferably satisfies at least one of S: 10 ppm or less and Sn and Sb: 10 ppm or more in total. Note that if the S content is 10 ppm or less, there is no lower limit on the total content of Sn and Sb; it may be 0, or neither Sn nor Sb may be present.
• the Sn content and Sb content must both be 200 ppm or less in order to satisfy the above-mentioned conditions for the total Sn and Sb content. However, there are no lower limits for the Sn content and Sb content.
• the Sn content may be 0, and Sn may not be included, but 5 ppm or more is preferred.
• the Sb content may be 0, and Sb may not be included, but 5 ppm or more is preferred.
• the composition may further include 50% or more by mass of Fe and alloying elements, with the remainder consisting of unavoidable impurities.
• the composition may further include 50% or more by mass of Fe and at least one element selected from the group consisting of Al, Si, Co, Cr, Mn, Mo, and Ni, with the remainder consisting of unavoidable impurities.
• the composition further include 50% or more by mass of Fe, with the remainder consisting of unavoidable impurities.
• the iron-based soft magnetic powder may contain, for example, Al, Si, Mn, and Cr as unavoidable impurities. When each element is contained as an impurity, it is preferable to reduce the content as much as possible.
• Al content Al is an easily oxidizable metal element, and if it is contained, a large amount of oxide-based inclusions may be generated during atomization, which may inhibit the growth of crystal grains. Therefore, from the viewpoint of suppressing the generation of oxide-based inclusions, when Al is contained as an impurity, the Al content is preferably 0.010% by mass or less.
• the lower limit of the Al content is not limited, and it may be 0%, or Al may not be contained. However, from the viewpoint of production costs, 0.001% by mass or more is preferable.
• Al may be used as an alloy component.
• the Al content may be, for example, more than 0.010% by mass and not more than 50% by mass.
• Si content Si is an easily oxidizable metal element, and if it is contained, a large amount of oxide-based inclusions may be generated during atomization, inhibiting the growth of crystal grains. Therefore, from the viewpoint of suppressing the generation of oxide-based inclusions, when Si is contained as an impurity, the Si content is preferably 0.010% by mass or less.
• the lower limit of the Si content is not limited and may be 0%, or Si may not be contained at all. However, from the viewpoint of production costs, 0.001% by mass or more is preferable.
• Si may also be used as an alloying component.
• the Si content can be, for example, more than 0.010% by mass and 50% by mass or less.
• the Si content is preferably 10% by mass or less from the viewpoint of easily ensuring magnetic flux density, and preferably 7% by mass or less from the viewpoint of further reducing iron loss.
• the Si content is preferably 1% by mass or more from the viewpoint of further reducing iron loss.
• Cr content Cr is an easily oxidizable metal element, and if it is contained, a large amount of oxide-based inclusions may be generated during atomization, inhibiting the growth of crystal grains. Therefore, from the viewpoint of suppressing the generation of oxide-based inclusions, when Cr is contained as an impurity, the Cr content is preferably 0.050% by mass or less, and more preferably 0.040% by mass or less. Since the lower the Cr content, the better, there is no lower limit for the Cr content; it may be 0%, or Cr may not be contained at all. However, from the viewpoint of manufacturing costs, a content of 0.001% by mass or more is preferable. Cr may also be used as an alloy component. The Cr content may be, for example, more than 0.050% by mass but not more than 50% by mass.
• the apparent density of the iron-based soft magnetic powder is not limited, but is preferably 2.8 Mg/m or more from the viewpoint of easily producing a high-density dust core.
• the upper limit of the apparent density is also not particularly limited, but can usually be 5.0 Mg/m or less .
• the apparent density can be measured in accordance with JIS Z 2504.
• the maximum particle size of the iron-based soft magnetic powder is not particularly limited, but is preferably 600 ⁇ m or less in order to suppress an increase in eddy current loss.
• the maximum particle size can be measured by a sieving method.
• the insulating coating may be, for example, either or both of an inorganic insulating coating and an organic insulating coating.
• the inorganic insulating coating can use one or both of an amorphous material and a crystalline material as the inorganic insulating material.
• the amorphous material include metal phosphates and metal borates.
• the metal phosphate is preferably condensed aluminum phosphate.
• the crystalline material include metal oxides and SiO2 .
• the metal oxides include MgO, forsterite, talc, and Al2O3 .
• the inorganic insulating coating is a coating containing the inorganic insulating material, and is preferably made of the inorganic insulating material.
• Condensed aluminum phosphates include aluminum dihydrogen tripolyphosphate, aluminum metaphosphate, and mixtures of these, all of which can be obtained by heating aluminum monophosphate to cause a dehydration reaction. Of these, aluminum dihydrogen tripolyphosphate is preferred. Hereinafter, aluminum dihydrogen tripolyphosphate may be referred to as aluminum tripolyphosphate.
• the organic insulating coating is a coating containing an organic resin, and is preferably made of an organic resin.
• Silicone resin is preferably used as the organic resin. Silicone resin forms Si-O bonds with excellent heat resistance when heat treated, so excellent insulation properties can be maintained even when the compact is subjected to stress relief heat treatment at high temperatures (e.g., 600°C) during the production of the dust core.
• silicone resins include resin-based silicone resins, such as those containing 60 mol% or more of T units (trifunctional siloxane units).
• silicone resins in which 50 mol% or more of the functional groups on the Si are methyl groups are preferred, such as methylphenyl silicone resins (KR-255, KR-311, KR-300, etc., manufactured by Shin-Etsu Chemical Co., Ltd.) and methyl silicone resins (KR-251, KR-400, KR-220L, KR-220LP, KR-242A, KR-240, KR-500, KC-89, etc., manufactured by Shin-Etsu Chemical Co., Ltd.).
• SR2400 and Trefil R-910 manufactured by Dow Corning Toray Co., Ltd. can also be used.
• the insulating coating comprises, from the surface of the particle toward the outside, a first coating containing condensed aluminum phosphate and a second coating containing a silicone resin, in this order. That is, the insulating coated soft magnetic powder preferably comprises, from the inside, the iron-based soft magnetic powder, the first coating, and the second coating, in this order. It is more preferable that the insulating coating be composed of the first coating and the second coating.
• the content of condensed aluminum phosphate in the first coating is not limited and may be, for example, 50% by mass or more. However, the content is preferably 99% by mass or more, and it is more preferable that the first coating consists of condensed aluminum phosphate. It is preferable that the first coating is a coating using condensed aluminum phosphate.
• the amount of silicone resin contained in the second coating is not limited and may be, for example, 50% by mass or more. However, the amount is preferably 99% by mass or more, and it is more preferable that the second coating be made of silicone resin. It is preferable that the second coating be a coating using silicone resin.
• the insulating coated soft magnetic powder contains the insulating coating and the iron-based soft magnetic powder, and is preferably composed of the insulating coating and the iron-based soft magnetic powder.
• the content of the insulating coating is preferably 0.2 parts by mass or more per 100 parts by mass of the iron-based soft magnetic powder.
• the content of the insulating coating is preferably 1.0 part by mass or less per 100 parts by mass of the iron-based soft magnetic powder.
• the mixed powder for dust cores of the present invention contains the insulating coated soft magnetic powder and a lubricant in a predetermined ratio.
• the mixed powder for dust cores may be composed of the insulating coated soft magnetic powder and the lubricant.
• the mixed powder of the present invention contains a lubricant in addition to the insulating-coated soft magnetic powder.
• the lubricant content is expressed as 100 parts by mass of the insulating-coated soft magnetic powder.
• the mixed powder contains 0.2 parts by mass or more of the lubricant.
• the mixed powder contains 0.8 parts by mass or less of the lubricant.
• lubricant materials include organic lubricants such as fatty acid amides, and inorganic lubricants such as MoS 2 , WS, BN, and talc.
• organic lubricants such as fatty acid amides
• inorganic lubricants such as MoS 2 , WS, BN, and talc.
• fatty acid amides include stearamide, EBS (ethylene bisstearamide), erucamide, and oleamide. These may be used alone or in combination.
• Powders obtained by the oxide reduction method and the electrolytic deposition method have low apparent densities, and even if additional processing such as crushing is performed to increase the apparent density, sufficient apparent density cannot be achieved. Therefore, from the perspective of improving the density of the resulting powder core, it is preferable to manufacture the above iron-based soft magnetic powder using an atomization method.
• the atomization method involves pulverizing molten metal and cooling it to solidify it. Any of water atomization, gas atomization, and centrifugal atomization can be used. Water atomization involves spraying water onto molten metal to pulverize it, while gas atomization involves spraying gas to pulverize it. Powdering can also be performed by spraying both water and gas.
• Molten steel is used as the molten metal.
• the molten steel There are no restrictions on the molten steel, as long as it is primarily composed of iron. However, if it contains a large amount of S, desulfurization in the subsequent process may be difficult. Therefore, in order to easily obtain iron-based soft magnetic powder having the above-mentioned composition, it is preferable that the S content of the molten steel be less than 50 ppm.
• the powder obtained by the atomization method is then annealed to reduce the amount of S in the powder, thereby obtaining the iron-based soft magnetic powder.
• the annealing may include decarburization and reduction annealing, which will be described later.
• the reduction annealing is preferably carried out in one or more heat treatment stages. Reduction annealing reduces the amount of S in the powder and coarsens the crystal grain size in the resulting iron-based soft magnetic powder. Particle size adjustment, such as by crushing, may be carried out before or after each heat treatment stage.
• the temperature of the heat treatment is preferably 700°C or higher, more preferably 800°C or higher, and even more preferably 900°C or higher.
• the temperature of the heat treatment is preferably less than 1200°C, more preferably less than 1100°C.
• the holding time of the heat treatment is preferably 1 hour or longer.
• the holding time of the heat treatment is preferably 7 hours or shorter, more preferably 5 hours or shorter.
• the atmosphere used for the heat treatment is typically a reducing atmosphere, preferably an atmosphere containing hydrogen.
• the upper limit of the dew point of the atmosphere is not particularly limited, but is preferably 30°C or lower.
• the lower limit of the dew point of the atmosphere is not particularly limited, but may be, for example, -40°C or higher.
• the decarburization is preferably a heat treatment carried out in a wet hydrogen atmosphere.
• This decarburization can reduce not only the C content in the powder but also the S content. Because desulfurization occurs through oxidation, the S content is more likely to be reduced by decarburization than by reduction annealing.
• a wet hydrogen atmosphere refers to an atmosphere containing water vapor and hydrogen, specifically an atmosphere containing hydrogen with a dew point above 30°C.
• the dew point of the heat treatment in the wet hydrogen atmosphere is preferably 60°C or lower.
• the temperature and holding time of the heat treatment in the wet hydrogen atmosphere can be the same as those for the reduction annealing.
• the heat treatment for the reduction annealing at least once in an atmosphere containing hydrogen and having a dew point of 30°C or less, at a temperature of 800°C or higher, for a holding time of 1 hour or more.
• water vapor generates fine inclusions. Therefore, by subsequently performing heat treatment for a predetermined time in an atmosphere with a low dew point and high temperature to decompose the inclusions, the number density of inclusions in the iron-based soft magnetic powder produced can be reduced, thereby further reducing the hysteresis loss of the dust core produced.
• the annealing can be performed in two stages, with the first stage being a heat treatment in a wet hydrogen atmosphere, and the second stage being a heat treatment in an atmosphere containing hydrogen and having a dew point of 30°C or less, at a temperature of 800°C or higher, for a holding time of 1 hour or more.
• the iron-based soft magnetic powder can be crushed and sieved as needed to adjust the apparent density and particle size distribution.
• the content of metal elements such as Sn, Sb, Al, Si, Cr, and Mn in the final iron-based soft magnetic powder is equivalent to the chemical composition of the molten steel.
• the insulating coated soft magnetic powder can be produced by forming the insulating coating on the iron-based soft magnetic powder.
• the insulating coating can be formed by any method, including wet and dry methods.
• the wet method is a method in which the material used for the insulating coating is mixed with the iron-based soft magnetic powder using a solvent such as water or an organic solvent.
• the dry method is a method in which the material used for the insulating coating is mixed with the iron-based soft magnetic powder without using a solvent.
• the mixing can be carried out using a mixer, which may be, for example, a rotary blade mixer. Examples of rotary blade mixers include the FM Mixer series (manufactured by Nippon Coke) and the High Speed Mixer series (manufactured by EarthTechnica).
• the insulating coating comprises, from the surface of the particle outward, a first coating made of condensed aluminum phosphate and a second coating made of silicone resin, in that order.
• a first coating made of condensed aluminum phosphate is formed on the iron-based soft magnetic powder.
• the first coating can be formed, for example, by a dry method, specifically by mixing the iron-based soft magnetic powder and the condensed aluminum phosphate powder, preferably by heat mixing. Using a dry method avoids the problem of oxidation of the iron-based soft magnetic powder and eliminates the need for a solvent dissolution step, which is advantageous in terms of equipment and workability. Furthermore, forming the first coating by heat mixing allows the first coating to be formed as a layered, continuous coating.
• a continuous coating can be formed as a complete or partial coating, but it is distinguished from a state in which the powder is fused together to form a continuous coating, as opposed to a state in which the powder is simply attached in a scattered manner. It is preferable that the continuous coating covers most of the surface of the iron-based soft magnetic powder, and more preferably, it covers substantially the entire surface. Furthermore, the heat mixing can achieve excellent adhesion of the first coating to the surface of the iron-based soft magnetic powder. This is presumably due to a reaction occurring at the interface between the condensed aluminum phosphate continuous coating and the iron-based soft magnetic powder.
• the average particle size of the condensed aluminum phosphate powder may be 1 ⁇ m or more, preferably 1.5 ⁇ m or more.
• the average particle size may be 10 ⁇ m or less, preferably 7.5 ⁇ m or less.
• the mixer described above can be used for the mixing.
• the rotation speed of the mixer is preferably 100 rpm or higher, and more preferably 200 rpm or higher.
• the rotation speed is preferably 1000 rpm or lower, and more preferably 800 rpm or lower.
• the maximum temperature reached during mixing refers to the highest temperature of the powder during mixing, and can be the highest temperature measured by the thermocouple of the powder containing the iron-based soft magnetic powder and the condensed aluminum phosphate powder.
• the mixing is preferably performed in an inert gas atmosphere, such as a nitrogen atmosphere.
• the powder having the first coating is discharged from the mixer.
• the temperature of the powder during the discharge is preferably 80°C or less, and more preferably 60°C or less.
• the lower limit of the powder temperature during discharge is not particularly limited, and can be, for example, room temperature or higher, 0°C or higher, or 30°C or higher.
• a second coating made of silicone resin is formed on the powder obtained by the above-mentioned method.
• the second coating can be formed by, for example, a wet method using an organic solvent or a dry method without using a solvent, either of which can be used to form a layer of silicone resin coating.
• the dry method is preferred because it does not require safety measures associated with the use of organic solvents and is advantageous in terms of cost, equipment, and workability.
• the silicone resin can be attached by kneading a solution in which the silicone resin is dissolved in an organic solvent with the insulating coated soft magnetic powder obtained by the method described above, followed by drying.
• organic solvent include petroleum-based organic solvents such as alcohols, xylene, and toluene.
• the solids concentration of the silicone resin in the solution can be 1 to 10% by mass. Drying can be carried out in the air.
• the drying temperature can be a temperature at which the organic solvent used volatilizes and below the curing temperature of the silicone resin.
• At least one silicone resin selected from the group consisting of SR2400 manufactured by Dow Corning Toray Co., Ltd., and KR-311 and LR-220L manufactured by Shin-Etsu Chemical Co., Ltd.
• the silicone resin can be attached by mixing the solid silicone resin with the powder having the first coating obtained by the method described above.
• the mixer described above can be used for this mixing. While the mixer's rotation speed is not particularly limited, a rotation speed of 100 rpm or higher is preferred, and 200 rpm or higher is more preferred, in order to efficiently form the second coating. On the other hand, excessively high-speed stirring can cause plastic deformation of the iron-based soft magnetic powder, resulting in reduced compressibility during pressure molding and increased hysteresis loss. Therefore, the rotation speed is preferably 2000 rpm or lower, and more preferably 1500 rpm or lower.
• the solid silicone resin is not particularly limited, and either or both of powder and flake silicone resins can be used. It is preferable to use a solid silicone resin that softens when heated. When using the dry method, it is preferable to use at least one silicone resin selected from the group consisting of, for example, Toray Dow Corning Toray's Trefil R-910 and Shin-Etsu Chemical's KR-220LP.
• the silicone resin After the silicone resin has been applied by a wet or dry method, it may be subjected to a heat treatment to increase the hardness of the applied silicone resin.
• the heat treatment temperature may be, for example, 150°C or higher and 250°C or lower.
• the heat treatment may be performed in air or in an inert gas atmosphere (e.g., a nitrogen atmosphere).
• the mixed powder can be produced by adding, for example, the lubricant to the insulating coated soft magnetic powder.
• the type and amount of the lubricant to be added are as described above.
• the iron-based soft magnetic powder is coated with the insulating coating and, if necessary, mixed with a lubricant, and then loaded into a mold and pressure-molded to the desired dimensions and shape.
• the pressure-molding method is not particularly limited, and any conventional molding method, such as room-temperature molding or die-lubricated molding, can be used.
• the die-lubricated molding method is a method in which a lubricant is applied to the mold wall surface before pressure molding, thereby improving ejection properties. This method can also be used effectively when no lubricant is added to the insulating-coated soft magnetic powder.
• Suitable lubricants for application to the wall surface include metal soaps such as lithium stearate, zinc stearate, and calcium stearate, and waxes such as fatty acid amides.
• the molding pressure is determined appropriately depending on the application. Since increasing the molding pressure increases the density of the resulting dust core, the molding pressure is preferably 10 t/ cm2 or higher, more preferably 15 t/ cm2 or higher.
• the heat treatment temperature is not particularly limited.
• the holding time for the heat treatment is also not particularly limited, but is preferably 5 to 120 minutes. A stage of holding the temperature at a constant temperature during temperature increase or decrease during the heat treatment may be provided.
• the atmosphere for the heat treatment is not particularly limited, and may be, for example, air, an inert atmosphere, a reducing atmosphere, or a vacuum.
• the dew point of the atmosphere is not particularly limited, and may be determined appropriately depending on the application.
• Silicone resin was dissolved in toluene to prepare a diluted resin solution with a silicone resin solids concentration of 2.0% by mass.
• the heated and mixed powder and the diluted resin solution were mixed, dried, and then heat-treated in air at 200°C for 120 minutes.
• the insulating coating content was adjusted so that 0.3 parts by mass of aluminum tripolyphosphate coating and 0.3 parts by mass of silicone resin coating per 100 parts by mass of the iron-based soft magnetic powder. Ring-shaped test specimens were fabricated by pressure molding using these powders and heat treatment.
• this core is particularly suitable for use in motors, for example.

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