Iron-based soft magnetic powder for dust core, insulation-coating soft magnetic powder for dust core, and mixed powder for dust core

Abstract

Provided is an iron-based soft magnetic powder for a dust core, that allows manufacturing of a dust core with reduced hysteresis loss. More specifically, this ion-based soft magnetic powder for a dust core contains at least one selected from the group consisting of Al, Si, Cr, and Mn, and the number density of inclusions having a particle diameter of 150 nm or less is 500 pieces / μm3 or less.

Description

Iron-based soft magnetic powder for powder magnetic cores, insulation-coated soft magnetic powder for powder magnetic cores, and mixed powder for powder magnetic cores

[0001] The present invention relates to an iron-based soft magnetic powder for dust cores, an insulating-coated soft magnetic powder for dust cores, and a mixed powder for dust cores.

[0002] Magnetic cores used in motors, transformers, etc. are required to have high magnetic flux density and low iron loss. Traditionally, motor cores have been made by laminating electromagnetic steel sheets, but in recent years, dust cores have been attracting attention.

[0003] The greatest feature of dust cores is their ability to form three-dimensional magnetic circuits. When magnetic steel sheets are used as the material, the magnetic core is formed by laminating layers, which limits the degree of freedom in shape. Furthermore, because steel sheets with insulated surfaces are laminated, the magnetic properties differ between the surface and the direction perpendicular to the surface of the steel sheet, and the magnetic properties are poor in the direction perpendicular to the surface. On the other hand, dust cores are press-molded from soft magnetic particles with an insulating coating, and their shape can be changed by changing the mold, allowing for greater freedom in shape than with magnetic steel sheets. Furthermore, because 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.

[0004] Furthermore, press molding requires fewer steps and is less expensive than laminating electromagnetic steel sheets, and this, combined with the low cost of the base powder, means that dust cores offer excellent cost performance.

[0005] In this way, dust cores enable the design of three-dimensional magnetic circuits and at the same time offer excellent cost performance. For these reasons, research and development of motors with three-dimensional magnetic circuits that utilize dust cores has been actively conducted in order to realize recent demands for motors that are smaller, use rare earth-free materials, and reduce costs.

[0006] However, there are several issues to be resolved. The biggest obstacle to the practical application of dust cores is iron loss. 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 need to reduce this loss.

[0007] Against this background, studies have been conducted to reduce the hysteresis loss of dust cores.

[0008] Patent Documents 1 and 2 disclose techniques for obtaining a dust core with low hysteresis loss by reducing the strain in soft magnetic powder measured by X-ray diffraction.

[0009] Patent Document 3 discloses a technique for obtaining a dust core with low hysteresis loss by increasing the size of crystal grains in soft magnetic powder.

[0010] Patent Document 4 discloses an iron-based soft magnetic powder for dust cores in which the number density of inclusions of 0.1 μm or more is reduced.

[0011] JP 2022-123416 A JP 2014-49643 A JP 2008-63652 A JP 2012-140679 A

[0012] However, even when the soft magnetic powders proposed in Patent Documents 1 to 4 are used, the hysteresis loss of the dust core is not sufficiently reduced, and further improvements in performance are desired.

[0013] The present invention has been made in view of the above circumstances, and has an object to provide an iron-based soft magnetic powder for dust cores that can be used to manufacture dust cores with reduced hysteresis loss.

[0014] The present invention has been made to solve the above problems, and the gist of the present invention is as follows.

[0015] 1. An iron-based soft magnetic powder for dust cores, which contains at least one element selected from the group consisting of Al, Si, Cr, and Mn, and has a number density of inclusions having a particle diameter of 150 nm or less of 500 pieces / μm ３ The iron-based soft magnetic powder for dust cores is as follows:

[0016] 2. An insulating-coated soft magnetic powder for dust cores, comprising the iron-based soft magnetic powder for dust cores according to 1 above, having an insulating coating on the surfaces of the particles constituting the powder.

[0017] 3. The insulating coated soft magnetic powder for dust cores according to 2 above, wherein the insulating coating has, 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.

[0018] 4. The insulating coated soft magnetic powder for dust cores according to 2 or 3 above, wherein the content of the insulating coating is 0.2 to 1.0 parts by mass per 100 parts by mass of the iron-based soft magnetic powder for dust cores.

[0019] 5. A mixed powder for dust cores, comprising the insulating coated soft magnetic powder for dust cores according to any one of 2 to 4 above, and 0.2 to 0.8 parts by mass of a lubricant per 100 parts by mass of the insulating coated soft magnetic powder for dust cores.

[0020] According to the present invention, it is possible to provide an iron-based soft magnetic powder for dust cores that can be used to manufacture dust cores with reduced hysteresis loss.

[0021] A method for carrying out the present invention will be specifically described below. Note that the following description shows an example of a preferred embodiment of the present invention, and the present invention is not limited thereto. Furthermore, hereinafter, the iron-based soft magnetic powder for dust cores, the insulating-coated soft magnetic powder for dust cores, and the mixed powder for dust cores may be simply referred to as "iron-based soft magnetic powder," "insulating-coated soft magnetic powder," and "mixed powder," respectively.

[0022] [Iron-based soft magnetic powder for dust cores] The iron-based soft magnetic powder for dust cores of the present invention contains at least one element selected from the group consisting of Al, Si, Cr, and Mn, and has a number density of inclusions having a particle size of 150 nm or less of 500 pieces / μm ３ In the present invention, the term "iron-based soft magnetic powder" refers to a soft magnetic powder containing 50% by mass or more of Fe.

[0023] The iron-based soft magnetic powder is not particularly limited, and for example, iron powder or iron-based alloy powder can be used. However, iron powder is preferred because it has good compressibility during compaction and is easy to achieve high density. Here, "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. Furthermore, "iron-based alloy powder" refers to an alloy powder containing 50 mass% or more of Fe. The alloying element of the iron-based alloy powder can be, for example, at least one selected from the group consisting of Ni, Co, and Cu. The iron-based alloy powder can be, for example, a powder of permalloy. Furthermore, the crystalline state of the iron-based soft magnetic powder is not particularly limited, and the iron-based soft magnetic powder can be, for example, an amorphous powder.

[0024] 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. Here, 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. For reasons described below, water atomized powder is preferred, and water atomized iron powder is more preferred.

[0025] (Component Composition) A preferred component composition of the iron-based soft magnetic powder will be described below. In the following description, "%" as a unit of content is a value expressed in mass % unless otherwise specified.

[0026] The iron-based soft magnetic powder may be composed of 50% or more Fe, 0 to 50% in total of alloying elements, and unavoidable impurities. The iron-based soft magnetic powder preferably contains 50% or more Fe, 0 to 50% in total of at least one selected from the group consisting of Ni, Co, and Cu, and unavoidable impurities. However, it is more preferable that the powder be composed of Fe and unavoidable impurities, as this provides good compressibility during pressure molding and facilitates high density.

[0027] The iron-based soft magnetic powder may contain, as unavoidable impurities, for example, Al, Si, Mn, Cr, C, P, B, and Ti. However, it is preferable to reduce the amount of the unavoidable impurities as much as possible. The content of each element is preferably within the following ranges: Al: 0-0.010% Si: 0-0.010% Mn: 0-0.100% Cr: 0-0.050% C: 0-0.010% P: 0-0.015% B: 0-0.005% Ti: 0-0.005%

[0028] (Al Content) Al is an easily oxidizable metal element, and if contained in excess, the number density of oxide-based inclusions may become excessively high. Therefore, when Al is contained, the Al content is preferably 0.010% or less. Since the lower the Al content, the better, there is no lower limit for the Al content, and it may be 0%, or Al may not be contained at all. However, from the viewpoint of production costs, an Al content of 0.001% or more is preferable.

[0029] (Si Content) Si is an easily oxidizable metal element, and if contained in excess, the number density of oxide-based inclusions may become excessively high. Therefore, when Si is contained, the Si content is preferably 0.010% or less. Since the lower the Si content, the better, there is no lower limit for the Si content, and it may be 0%, or Si may not be contained at all. However, from the viewpoint of production costs, a content of 0.001% or more is preferable.

[0030] (Mn Content) Mn is an easily oxidizable metal element, and if contained in excess, the number density of oxide-based inclusions may become excessively high. Therefore, when Mn is contained, the Mn content is preferably 0.100% or less, and more preferably 0.070% by mass or less. Since the lower the Mn content, the better, there is no restriction on the lower limit of the Mn content, and it may be 0%, or Mn may not be contained at all. However, from the viewpoint of production costs, a Mn content of 0.010% or more is preferable.

[0031] (Cr Content) Cr is an easily oxidizable metal element, and if contained in excess, the number density of oxide-based inclusions may become excessively high. Therefore, when Cr is contained, the Cr content is preferably 0.050% or less, and more preferably 0.040% by mass or less. Since the lower the Cr content, the better, there is no restriction on the lower limit of the Cr content, and it may be 0%, or Cr may not be contained at all. However, from the viewpoint of production costs, a Cr content of 0.001% or more is preferable.

[0032] (C Content) If excessive C is contained, the desired magnetic properties may not be obtained for the manufactured dust core. Therefore, when C is contained, it is preferable that the C content be 0.010% or less. The lower the C content, the better, so there is no lower limit for the C content; it may be 0%, and no C may be contained. The C content is measured by a combustion-infrared absorption method.

[0033] (P content) Since P is an easily oxidizable metal element, when P is contained, the P content is preferably 0.015% or less. Since the lower the P content, the better, the lower limit of the P content is not limited, and it may be 0%, and P may not be contained. However, from the viewpoint of production costs, it is preferably 0.001% or more.

[0034] (B Content) Since B is an easily oxidizable metal element, when B is contained, the B content is preferably 0.005% or less. Since the lower the B content, the better, the lower limit of the B content is not limited, and it may be 0%, or B may not be contained.

[0035] (Ti Content) Since Ti is an easily oxidizable metal element, when Ti is contained, the Ti content is preferably 0.005% or less. Since the lower the Ti content, the better, the lower limit of the Ti content is not limited, and it may be 0%, or Ti may not be contained.

[0036] (Number Density of Inclusions) The present inventors have conducted extensive research into the reason why the hysteresis loss of a dust core is not reduced when conventional soft magnetic powders are used, and have come to the following findings.

[0037] Coarsening the crystal grains in the microstructure of a powder magnetic core is effective in reducing hysteresis loss. Here, during stress relief heat treatment after compaction, recrystallization behavior is observed, in which nuclei of recrystallized grains (recrystallized nuclei) are generated in the microstructure, followed by growth of recrystallized grains from the recrystallized nuclei. For example, the conventional methods proposed in Patent Documents 1 to 3 are presumed to be aimed at coarsening the crystal grains in the resulting powder magnetic core by suppressing the generation of recrystallized nuclei. However, even if the generation of recrystallized nuclei in the microstructure is suppressed, if the growth of recrystallized grains is inhibited, the crystal grains remain fine, and sufficient hysteresis loss reduction is not achieved.

[0038] Furthermore, inclusions containing at least one element selected from the group consisting of Al, Si, Cr, and Mn and having a particle size of 150 nm or less inhibit the growth of recrystallized grains.

[0039] Al, Si, Cr, and Mn are all more easily oxidized than Fe and therefore more likely to form oxide-based inclusions. Furthermore, oxide-based inclusions tend to suppress the growth of crystal grains, among other inclusions. Therefore, the number density of inclusions containing at least one element selected from the group consisting of Al, Si, Cr, and Mn has a significant effect on the size of recrystallized grains.

[0040] Furthermore, when considering two types of metals in which the same volume of inclusions are dispersed, the finer the dispersion, the greater the pinning force, i.e., the greater the effect of inhibiting grain growth. This can be explained by the Zener model given by the following equation: Here, P ｓ is the pinning force, F ｖ is the volume of the inclusion, γ is the grain boundary energy, and d is the radius of the inclusion. ｖ When γ is considered as a constant, the larger the denominator d, the faster the pinning force increases, following a hyperbola. For the above reasons, inclusions with a particle size of 150 nm or less have a greater effect on suppressing grain growth than coarse inclusions with a particle size of more than 150 nm, and therefore have a greater impact on hysteresis loss.

[0041] Furthermore, the inclusions not only hinder the growth of recrystallized grains but also themselves cause high hysteresis loss. Therefore, it is necessary to reduce the number density of the inclusions in the iron-based soft magnetic powder for dust cores.

[0042] The number density of the inclusions is 500 pieces / μm ３ If the number density of the inclusions exceeds 500 pieces / μm, the hysteresis loss of the dust core manufactured from the iron-based soft magnetic powder increases. ３ Preferably 300 or less / μm ３ On the other hand, the lower limit of the number density of the inclusions may be 0, and the inclusions may not be included.

[0043] The number density of the inclusions is determined based on mapping data of Al, Si, Cr, and Mn obtained by elemental mapping using a scanning transmission electron microscope (STEM) and an energy dispersive X-ray fluorescence analyzer (EDX). More specifically, the measurement is performed as follows. First, approximately 900 parts by mass of conductive resin is mixed with 100 parts by mass of iron-based soft magnetic powder and cured to produce a resin solid in which the iron-based soft magnetic powder is embedded. Next, the resin solid is polished to expose the cross-section of the particles constituting the iron-based soft magnetic powder, and a sample for measurement is obtained using a focused ion beam (FIB) method. The sample has a surface area of ​​1 μm x 1 μm or more and a thickness of 200 nm or less. Next, elemental mapping is performed on the sample using a scanning transmission electron microscope and an energy dispersive X-ray fluorescence analyzer to obtain mapping data of Al, Si, Mn, and Cr. The elemental mapping resolution is 5 nm or less. To achieve this resolution, an STEM such as a Talos F200X manufactured by FEI can be used. The accelerating voltage during observation is approximately 200 kV. Next, the mapping data is superimposed and binarized, and image analysis is performed to calculate the circle-equivalent diameter of each particle corresponding to an inclusion. The binarization and image analysis can be performed using image analysis software such as ImageJ. The volume of the observation area is calculated from the mapping area in the STEM and film thickness data of the observation area measured by electron energy loss spectroscopy (EELS). The number of particles with a circle-equivalent diameter of 150 nm or less is then divided by the volume of the observation area to determine the number density of inclusions containing at least one element selected from the group consisting of Al, Si, Cr, and Mn and having a particle diameter of 150 nm or less.

[0044] (Apparent Density) The apparent density of the iron-based soft magnetic powder is not limited, but from the viewpoint of easily producing a high-density dust core, it is preferably 2.8 Mg / m ３ The upper limit of the apparent density is not particularly limited, but is usually 5.0 Mg / m ３ The apparent density can be measured in accordance with JIS Z 2504.

[0045] (Average particle size) The average particle size of the iron-based soft magnetic powder is not particularly limited, but is preferably within a predetermined range from the viewpoint of improving fluidity and facilitating filling into a mold in the production of a dust core. Specifically, the volume-based median diameter D ５０ The average particle size on a mass basis measured by sieving is preferably 10 μm or more. The average particle size on a mass basis measured by sieving is preferably 60 μm or more, and the average particle size on a mass basis measured by sieving is preferably 200 μm or less.

[0046] (Maximum particle size) The maximum particle size of the iron-based soft magnetic powder is not particularly limited, but is preferably 600 μm or less in order to suppress an increase in eddy current loss. The maximum particle size can be measured by a sieving method.

[0047] [Insulating-coated soft magnetic powder for dust core] The insulating-coated soft magnetic powder for dust core of the present invention has an insulating coating on the surface of the particles constituting the iron-based soft magnetic powder. The insulating coating ensures insulation between particles and can suppress short circuits between particles.

[0048] (Insulating Coating) Any coating can be used as the insulating coating. The insulating coating may consist of one coating, or two or more coatings. When the insulating coating consists of two or more coatings, the coatings may be of the same type or different types. The insulating coating may be layered. That is, the insulating coating may be a single-layer coating, or a multi-layer coating consisting of two or more layers. The multi-layer coating may be a multi-layer coating consisting of the same type of coating, or a multi-layer coating consisting of different types of coatings.

[0049] As the insulating coating, for example, one or both of an inorganic insulating coating and an organic insulating coating can be used.

[0050] The inorganic insulating coating can use either or both of an amorphous material and a crystalline material as the inorganic insulating material. Examples of the amorphous material include metal phosphates and metal borates. The metal phosphate is preferably condensed aluminum phosphate. Examples of the crystalline material include metal oxides and SiO２ Examples of the metal oxide include MgO, forsterite, talc, and Al. ２ O ３ The inorganic insulating coating is a coating containing the inorganic insulating material, and is preferably made of the inorganic insulating material.

[0051] Condensed aluminum phosphates include aluminum dihydrogen tripolyphosphate, aluminum metaphosphate, and mixtures thereof, all of which can be obtained by heating aluminum monophosphate to cause a dehydration reaction. Among these, aluminum dihydrogen tripolyphosphate is preferred. Hereinafter, aluminum dihydrogen tripolyphosphate may be referred to as aluminum tripolyphosphate.

[0052] 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 that excellent insulating properties can be maintained even when the compact is subjected to a high-temperature (e.g., 600°C) stress relief heat treatment during production of the dust core.

[0053] Examples of silicone resins include resin-based silicone resins, such as silicone resins containing 60 mol% or more of T units (trifunctional siloxane units). Among these, 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.

[0054] In a preferred embodiment of the present invention, 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 is composed of the first coating and the second coating.

[0055] 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 the first coating is more preferably made of condensed aluminum phosphate. The first coating is preferably a coating using condensed aluminum phosphate.

[0056] The content of the silicone resin in the second 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 the second coating is more preferably made of a silicone resin. The second coating is preferably a coating using a silicone resin.

[0057] 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. From the viewpoint of further improving insulation, 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. On the other hand, if the amount of coating is excessively large, the magnetic flux density may decrease, impairing the function as an iron core, and the density of the iron core may also decrease. Therefore, 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.

[0058] [Mixed Powder for Dust Core] The mixed powder for dust core of the present invention contains the insulating coated soft magnetic powder and a lubricant in a predetermined ratio. The mixed powder for dust core may be composed of the insulating coated soft magnetic powder and a lubricant.

[0059] (Lubricant) The dust core is manufactured by pressure-molding soft magnetic powder using a mold. In order to improve releasability from the mold, the mixed powder of the present invention contains a lubricant in addition to the insulating coated soft magnetic powder. Hereinafter, the content of the lubricant is expressed as 100 parts by mass of the insulating coated soft magnetic powder. In order to ensure sufficient lubrication, the mixed powder contains 0.2 parts by mass or more of the lubricant. Furthermore, if the lubricant content exceeds 0.8 parts by mass, the density of the dust core decreases, so the mixed powder contains 0.8 parts by mass or less of the lubricant. Materials for the lubricant include organic lubricants such as fatty acid amides, MoS ２ Examples of the fatty acid amide include stearic acid amide, EBS (ethylene bisstearic acid amide), erucic acid amide, oleic acid amide, etc. These may be used alone or in combination of two or more.

[0060] [Method for producing iron-based soft magnetic powder] Next, a method for producing the iron-based soft magnetic powder according to one embodiment of the present invention will be described. Note that the following description shows one example of the production method, and the present invention is not limited to the following description.

[0061] Powders obtained by oxide reduction and electrolytic deposition 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 produce the iron-based soft magnetic powder using an atomization method. The atomization method involves pulverizing a 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 the molten metal to pulverize it, while gas atomization involves spraying gas onto it to pulverize it. Powdering can also be achieved by spraying both water and gas. Centrifugal atomization involves powdering a molten metal using centrifugal force generated by rotation. Powders obtained using two or more of these methods can also be combined. When using water atomization, the particle surfaces have numerous irregularities, which makes particle entanglement more likely, and when molded into a powder core, the strength of the powder core can be improved. Furthermore, water atomization is inexpensive. Therefore, from the viewpoints of the strength and cost of the resulting dust core, water atomization is preferable. In contrast, gas atomization allows for relatively large-scale production, so gas atomization is preferable from the viewpoint of mass production. Below, a manufacturing method using water atomization will be described as an example.

[0062] Molten steel is used as the molten metal. The molten steel is not limited as long as it contains iron as its main component. The molten steel may have a composition consisting of Fe, alloying elements, and unavoidable impurities. The composition of the molten steel preferably consists of Fe and unavoidable impurities. However, excessive amounts of easily oxidizable metal elements may result in the formation of a large amount of oxide-based inclusions during atomization. Therefore, the Al, Si, Mn, and Cr contents of the molten steel are preferably 0 to 0.010 mass% Al, 0 to 0.010 mass% Si, 0 to 0.100 mass% Mn, and 0 to 0.050 mass% Cr, respectively. The contents of each element are more preferably 0 to 0.010 mass% Al, 0 to 0.010 mass% Si, 0 to 0.070 mass% Mn, and 0 to 0.040 mass% Cr.

[0063] It is also preferable to reduce as much as possible the contents of easily oxidizable metal elements other than Al, Si, Cr, and Mn. Specifically, the contents of P, B, and Ti in the above-mentioned composition are preferably P: 0 to 0.015 mass%, B: 0 to 0.005 mass%, and Ti: 0 to 0.005 mass%, respectively.

[0064] (Annealing) The powder obtained by the atomization method is then annealed to obtain the iron-based soft magnetic powder. The annealing may consist of decarburization and reduction annealing, which will be described later.

[0065] The reduction annealing is preferably carried out in one or more heat treatment stages. Reduction annealing can reduce the number density of the inclusions and coarsen the crystal grain size in the resulting iron-based soft magnetic powder. Particle size adjustment, such as pulverization, may be carried out before or after each heat treatment stage.

[0066] 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 for the heat treatment is usually a reducing atmosphere, and an atmosphere containing hydrogen is preferred. 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.

[0067] The decarburization is preferably a heat treatment performed in a wet hydrogen atmosphere. Here, the wet hydrogen atmosphere refers to an atmosphere containing water vapor and hydrogen, specifically an atmosphere containing hydrogen and having a dew point of more than 30° C. The dew point of the heat treatment in the wet hydrogen atmosphere is preferably 60° C. or less. The temperature and holding time of the heat treatment in the wet hydrogen atmosphere can be the same as those of the heat treatment for reduction annealing.

[0068] After the heat treatment in the wet hydrogen atmosphere, it is preferable to perform 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 more, and for a holding time of 1 hour or more. When the heat treatment is performed in a wet hydrogen atmosphere, fine inclusions are generated by water vapor. Therefore, by subsequently performing a heat treatment for a predetermined time in a low dew point, high temperature atmosphere, it is possible to reduce the number density of the inclusions in the produced iron-based soft magnetic powder to 500 pieces / μm unless the inclusions are decomposed. ３ For example, the decarburization and reduction 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 with a dew point of 30°C or less, at a temperature of 800°C or higher, and for a holding time of 1 hour or longer. As long as the above conditions are met, the order of the heat treatments for decarburization and reduction annealing is not limited.

[0069] After the annealing, the iron-based soft magnetic powder can be crushed and sieved as needed to adjust the apparent density and particle size distribution.

[0070] In general, the contents of Al, Si, Cr, Mn, P, B and Ti in the finally obtained iron-based soft magnetic powder are equivalent to the composition of the molten steel.

[0071] [Method for producing insulating coated soft magnetic powder] Next, a method for producing the insulating coated soft magnetic powder according to one embodiment of the present invention will be described. Note that the following description shows one example of the production method, and the present invention is not limited to the following description.

[0072] 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 a wet method and a dry method. The wet method is a method of mixing the material used for the insulating coating with the iron-based soft magnetic powder using a solvent such as water or an organic solvent. On the other hand, the dry method is a method of mixing the material used for the insulating coating with the iron-based soft magnetic powder without using a solvent. The mixing can be performed 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).

[0073] Below, we will explain in detail the case where the insulating coating has, from the surface of the particle toward the outside, a first coating made of condensed aluminum phosphate and a second coating made of silicone resin in this order.

[0074] (First Coating) First, 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. Here, a continuous coating can be formed as a complete or partial coating. The continuous coating refers to a state in which the powder particles are fused together to form a continuous coating, as opposed to a state in which the powder particles are directly 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 method 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.

[0075] The average particle size of the condensed aluminum phosphate powder may be 1 μm or more, preferably 1.5 μm or more. On the other hand, the average particle size may be 10 μm or less, preferably 7.5 μm or less. By setting the average particle size to 1 μm or more and 10 μm or less, sufficient fluidity can be ensured to improve workability, and the first coating can be easily formed uniformly as a continuous film. The average particle size is determined by the volume-based median diameter D measured by laser diffraction. ５０ is.

[0076] The above-mentioned mixer can be used for the mixing. The rotation speed of the mixer is not particularly limited, but in order to efficiently form the first coating and make the first coating a continuous film, it is preferably 100 rpm or higher, and more preferably 200 rpm or higher. On the other hand, excessively high-speed stirring may cause plastic deformation of the iron-based soft magnetic powder, resulting in a decrease in compressibility during compaction and an increase in hysteresis loss. Therefore, the rotation speed is preferably 1000 rpm or lower, and more preferably 800 rpm or lower.

[0077] The maximum temperature reached during mixing is not particularly limited. However, in order to facilitate the formation of the first coating as a continuous film, the maximum temperature is preferably 100°C or higher, more preferably 130°C or higher, and even more preferably 150°C or higher. On the other hand, since condensed aluminum phosphate may be degraded at high temperatures, the maximum temperature is preferably 200°C or lower. Note that the temperature referred to here refers to the temperature of the powder during mixing. When a rotary blade mixer is used, it refers to the temperature indicated by a thermocouple protruding from the wall of the stirring tank to a degree that does not contact the rotary blades. The maximum temperature reached during mixing refers to the highest temperature among the temperatures of the powders during mixing, and can be the highest temperature among the temperatures of the powder containing the iron-based soft magnetic powder and the condensed aluminum phosphate powder measured with the thermocouple. The mixing is preferably performed in an inert gas atmosphere, for example, a nitrogen atmosphere, in order to suppress oxidation of the iron-based soft magnetic powder.

[0078] After the mixing, the powder having the first coating is discharged from the mixer. In order to prevent oxidation, the temperature of the powder during the discharge is preferably 80° C. or lower, more preferably 60° C. or lower. The lower limit of the temperature of the powder during the discharge is not particularly limited, and may be, for example, room temperature or higher, 0° C. or higher, or 30° C. or higher.

[0079] (Second Coating) Next, 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 any safety measures associated with the use of organic solvents and is advantageous in terms of cost, equipment, and workability.

[0080] In the case of a wet method, 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 above-mentioned method and drying. Examples of the 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. In the case of a wet method, it is preferable to use at least one silicone resin selected from the group consisting of SR2400 manufactured by Dow Corning Toray Co., Ltd., KR-311, and LR-220L manufactured by Shin-Etsu Chemical Co., Ltd.

[0081] In the dry method, the silicone resin can be attached by mixing a solid silicone resin with the powder having the first coating obtained by the above-mentioned method. The above-mentioned mixer can be used for this mixing. While the mixer's rotation speed is not particularly limited, it is preferably 100 rpm or higher, more preferably 200 rpm or higher, 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 compaction and increased hysteresis loss. Therefore, the rotation speed is preferably 2000 rpm or lower, more preferably 1500 rpm or lower. Mixing can be performed, for example, by starting mixing at room temperature and discharging the powder from the mixer when the powder temperature reaches a temperature of 40°C to 70°C. 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. In the case of the dry method, it is preferable to use at least one silicone resin selected from the group consisting of, for example, Trefil R-910 manufactured by Toray Dow Corning Co., Ltd. and KR-220LP manufactured by Shin-Etsu Chemical Co., Ltd.

[0082] After the silicone resin is applied by a wet or dry method, heat treatment may be performed 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).

[0083] [Method for Producing Mixed Powder] The mixed powder can be produced by adding, for example, the lubricant to the insulating coated soft magnetic powder. The type and amount of lubricant to be added are as described above.

[0084] [Dust core] Next, a method for producing a dust core using the iron-based soft magnetic powder for dust cores, the insulating-coated soft magnetic powder for dust cores, or the mixed powder for dust cores of the present invention will be described. Note that the following description shows one example of the production method, and the present invention is not limited to the following description.

[0085] (Pressure Molding) After forming the insulating coating and, if necessary, mixing with a lubricant, the iron-based soft magnetic powder is loaded into a mold and pressure molded to the desired dimensions and shape. The pressure molding method is not particularly limited, and any of the usual molding methods, such as room temperature molding and die lubrication molding, can be applied. The die lubrication molding method is a technique that can improve ejection properties by applying a lubricant to the mold wall surface and then performing pressure molding, and can also be used suitably even when no lubricant is added to the insulating coated soft magnetic powder. Suitable lubricants to be applied 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, but since increasing the molding pressure can increase the density of the resulting dust core, it is preferably 10 t / cm. ２ More preferably, 15 t / cm ２ That's all.

[0086] (Strain relief heat treatment) After pressure molding, the powder is subjected to a heat treatment (strain relief heat treatment) for the purpose of reducing hysteresis loss by strain relief and improving the strength of the compact. The heat treatment temperature is not particularly limited. The holding time of 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.

[0087] The present invention will be specifically described below based on examples.

[0088] Example 1 First, an iron-based powder was produced from molten steel by water atomization. Next, the obtained iron-based powder was subjected to a two-stage heat treatment. First, heat treatment was performed in a wet hydrogen atmosphere (dew point 40°C) for decarburization. The holding temperature was 950°C and the holding time was 1 hour. The obtained agglomerates were pulverized. Next, in order to reduce fine inclusions, heat treatment was further performed in a hydrogen atmosphere (dew point 20°C). The holding temperature was 950°C and the holding time was 1 hour. The obtained iron-based soft magnetic powder had a maximum particle size measured by the above-mentioned method adjusted to less than 250 μm, and a volume-based median diameter D measured by laser diffraction. ５０ The average particle size on a mass basis, as measured by a sieving method, was set to 100 to 180 μm. The iron-based soft magnetic powder had a composition containing the components shown in Table 1, with the remainder consisting of Fe and other unavoidable impurities. The C content was measured by the above-mentioned method using a CS844 manufactured by LECO Corporation. It was also confirmed that the contents of Al, Si, Mn, and Cr were the same as the contents of each component in the iron-based powder before the heat treatment. The number density of inclusions containing at least one element selected from the group consisting of Al, Si, Cr, and Mn and having a particle size of 150 nm or less was also evaluated for the obtained iron-based soft magnetic powder using the above-mentioned method. The results are also shown in Table 1.

[0089] Next, an insulating coating was formed on these iron-based soft magnetic powders. First, aluminum tripolyphosphate powder with an average particle size of 5 μm was heated and mixed to form a coating made of aluminum tripolyphosphate. A high-speed mixer (LFS-GS-2J model, manufactured by Earth Technica (formerly Fukae Powtec Co., Ltd.)) was used for mixing. The atmosphere inside the stirring tank was nitrogen, the heater temperature of the stirring tank was set to 190°C, and the rotating blade rotation speed was 500 rpm, and heating and mixing were performed while stirring for 20 minutes. The mixture was then cooled to 60°C in the stirring tank and the powder was removed.

[0090] Next, a silicone resin coating was formed on the aluminum tripolyphosphate coating. 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 kneaded, dried, and then heat-treated in air at 200°C for 120 minutes. The insulating coating contents were 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. These powders were pressure-molded and heat-treated to produce ring-shaped test specimens. The pressure-molding was performed using a die-lubricated molding method at a molding pressure of 980 MPa, resulting in a shape with an outer diameter of 38 mm, an inner diameter of 25 mm, and a height of 6 mm. The heat treatment was performed in nitrogen at 600°C for 45 minutes.

[0091] The produced ring-shaped test specimen was wound (100 turns for the primary winding, 20 turns for the secondary winding), and the hysteresis loss was measured using a DC magnetization measuring instrument (SK-110 manufactured by Metron Giken Co., Ltd.). The magnetization curve was evaluated using a DC power supply at a frequency of 1 Hz and an excitation magnetic flux density of 1.0 T, and the hysteresis loss at 400 Hz was calculated by multiplying the loop area at 1 Hz by 400. A hysteresis loss of 30.0 W / kg or less was considered acceptable, and a loss of more than 30.0 W / kg was considered unacceptable. It is said that dust cores have high hysteresis loss at 400 Hz compared to conventional iron cores made of electromagnetic steel sheets. Therefore, by setting the hysteresis loss to 30.0 W / kg or less, iron loss is at a level equivalent to that when an electromagnetic steel sheet with a thickness of 0.35 mm is used, making the core suitable for use in, for example, motors.

[0092] The evaluation results are also shown in Table 1. All of the dust cores made of powders that satisfied the requirements of the present invention had a hysteresis loss of 30.0 W / kg or less, and were therefore acceptable.

[0093]

[0094] Example 2: For an iron-based powder produced by water atomization using the same molten steel as Sample No. 4 used in Example 1, the reduction annealing conditions were changed from those in Example 1. The first heat treatment was performed in a wet hydrogen atmosphere (dew point 40°C), as in Example 1. The holding temperature was 950°C, and the holding time was 1 hour. The conditions for the subsequent heat treatment were changed to the holding temperature and dew point shown in Table 2 below. The obtained iron-based soft magnetic powder was evaluated for C content and number density of inclusions containing at least one element selected from the group consisting of Al, Si, Cr, and Mn and having a particle size of 150 nm or less, using the above-mentioned method. The measurement results are also shown in Table 2. Furthermore, ring-shaped test pieces were prepared using these powders using the same method as in Example 1, and hysteresis loss was evaluated.

[0095] The evaluation results are also shown in Table 2.

[0096]

[0097] Example 3: The iron-based soft magnetic powder No. 2 in Example 1 was used, and the amount of aluminum tripolyphosphate and silicone resin added when forming the insulating coating was changed from that in Example 1 to adjust the content of the insulating coating. Table 3 shows the contents of the aluminum tripolyphosphate coating and the silicone resin coating in parts by mass per 100 parts by mass of the iron-based soft magnetic powder. Ring-shaped test pieces were prepared from the obtained insulating-coated soft magnetic powder using the same method as in Example 1. In addition to evaluating the hysteresis loss of the obtained ring-shaped test pieces using the method described above, the density was calculated from the dimensions and mass, and the resistivity was measured using the four-terminal method. From the perspective of reducing eddy current loss, a resistivity of 100 μΩm or more is preferable. If the resistivity is less than this, even if the hysteresis loss is low, the iron loss may exceed 30.0 W / kg due to increased eddy current loss. Furthermore, the density of the ring-shaped test pieces was set to 7.30 g / cm to ensure a practical saturation magnetic flux density (2 T or more) for use as an iron core for a motor. ３ It is preferable that this is equal to or greater than this.

[0098] The results are also shown in Table 3. All of the dust cores made of powders that satisfied the requirements of the present invention had a hysteresis loss of 30.0 W / kg or less, and were therefore passable. In the five conditions 2, 2-1, 2-2, 2-4, and 2-5, the content of the insulating coating was 0.2 to 1.0 part by mass per 100 parts by mass of the iron-based soft magnetic powder, and the resistivity and density were within the preferred ranges described above.

[0099]

[0100] (Example 4) An insulating coating was formed in the same manner as in Example 1 using the iron-based soft magnetic powder No. 2 in Example 1. The types of lubricants shown in Table 4 were added to the obtained insulating-coated soft magnetic powder, and the contents (parts by mass per 100 parts by mass of the insulating-coated soft magnetic powder) shown in Table 4 were adjusted to obtain mixed powders. Ring-shaped test pieces were produced using the mixed powders by performing pressure molding and heat treatment under the same conditions as in Example 1. The hysteresis loss of the obtained test pieces was evaluated using the method described above, and all were 30.0 W / kg or less.

[0101] Furthermore, tablet-shaped test pieces with an outer diameter of 25 mm and a height of 20 mm were prepared using the mixed powder. The ejection energy was calculated from the history of the ejection load when the test piece was ejected, and the density was measured from the dimensions and mass of the resulting tablet-shaped test piece. Here, the ejection energy refers to the integral value of the load history curve obtained when the horizontal axis represents the sliding distance during ejection and the vertical axis represents the ejection load. The density of the tablet-shaped test piece was 7.30 g / cm. ３ If the magnetic flux density is less than 300 kJ / m, it is difficult to ensure a practical saturation magnetic flux density (2 T or more) for use as an iron core for a motor. ２ If the temperature exceeds this value, the mold will be severely damaged, and mass productivity will decrease.

[0102] The results are also shown in Table 4. All of the tablets that satisfied the requirements of the present invention had a density of 7.30 g / cm ３ or more, and extraction energy 300 kJ / m ２ The following was met:

[0103]

Claims

1. An iron-based soft magnetic powder for dust cores, The number density of inclusions containing at least one selected from the group consisting of Al, Si, Cr, and Mn and having a particle size of 150 nm or less is 500 pieces / μm ３ The iron-based soft magnetic powder for dust cores is as follows:

2. 2. An insulating-coated soft magnetic powder for dust cores, comprising the iron-based soft magnetic powder for dust cores according to claim 1, wherein the surfaces of the particles constituting the iron-based soft magnetic powder for dust cores are coated with an insulating coating.

3. 3. The insulating coated soft magnetic powder for dust cores according to claim 2, wherein the insulating coating has, 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.

4. The insulating coated soft magnetic powder for dust cores according to claim 2, wherein the content of the insulating coating is 0.2 to 1.0 parts by mass per 100 parts by mass of the iron-based soft magnetic powder for dust cores.

5. An insulating coated soft magnetic powder for dust cores as described in claim 3, wherein the content of the insulating coating is 0.2 to 1.0 parts by mass per 100 parts by mass of the iron-based soft magnetic powder for dust cores.

6. A mixed powder for powder magnetic cores, comprising: the insulating coated soft magnetic powder for powder magnetic cores according to any one of claims 2 to 5; and 0.2 to 0.8 parts by mass of a lubricant per 100 parts by mass of the insulating coated soft magnetic powder for powder magnetic cores.