JP7783008B2 - Iron-based sintered body and method for producing the same - Google Patents

Description

The present invention relates to a mixture for producing an iron-based sintered body, an iron-based sintered body, and a method for producing an iron-based sintered body.

Patent Documents 1 and 2 describe powder magnetic cores used in electromagnetic components such as motor rotors and stator cores. The powder magnetic cores described in Patent Documents 1 and 2 have excellent magnetic properties and high mechanical strength.

Japanese Patent Application Laid-Open No. 2017-4992 Patent No. 4801734

In recent years, there has been an ever-increasing demand for higher performance materials used in electromagnetic components.

An object of the present invention is to provide a mixture that can be used to produce an iron-based sintered body having superior magnetic and mechanical properties compared to conventional powder magnetic cores. Another object of the present invention is to provide an iron-based sintered body having superior magnetic and mechanical properties compared to conventional powder magnetic cores, and a method for producing the same.

The mixture for producing an iron-based sintered body described in this specification contains an iron-based soft magnetic powder whose surface is coated with a phosphoric acid-based coating, and the surface of the phosphoric acid-based coating is coated with a silicone resin coating, stearic acid amide, and an inorganic solid lubricant.

The iron-based sintered body described in this specification is an iron-based sintered body obtained by heat-treating in water vapor a compact of the mixture for producing the iron-based sintered body described above, and comprises iron-based soft magnetic powder having a phosphate-based coating formed on its surface and a silicone resin coating formed on the surface of the phosphate-based coating, and iron oxide present at the grain boundaries between the iron-based soft magnetic powder. The iron oxide is present in 58% or more of the grain boundaries.

The method for producing an iron-based sintered body described in this specification includes a molding step of molding a mixture containing an iron-based soft magnetic powder whose surface is coated with a phosphoric acid-based coating, the surface of which is coated with a silicone resin coating, stearic acid amide, and an inorganic solid lubricant, and a heat treatment step of heat-treating the compact obtained in the molding step in water vapor.

By using the mixture for producing an iron-based sintered body described above, it is possible to produce an iron-based sintered body with higher magnetic properties and mechanical properties than conventional powder cores. It is also possible to provide an iron-based sintered body with higher magnetic properties and mechanical properties than conventional powder cores. According to the method for producing an iron-based sintered body described above, it is possible to produce an iron-based sintered body with higher magnetic properties and mechanical properties than conventional powder cores.

1 is an example of a scanning electron microscope image of a cross section of an iron-based sintered body according to the present invention. 1 is an example of a scanning electron microscope image of a cross section of an iron-based sintered body according to the present invention. FIG. 1 is a graph showing the relationship between heat treatment time and bending strength. 1 is a scanning electron microscope image of a cross section of an iron-based sintered body. 1 is a scanning electron microscope image of a cross section of an iron-based sintered body. 1 is a scanning electron microscope image of a cross section of an iron-based sintered body. FIG. 1 is a diagram showing the relationship between heat treatment temperature and bending strength. FIG. 10 is a diagram showing the relationship between ΔL, electrical resistance, and iron loss. FIG. 10 is a diagram showing the relationship between heat treatment temperature, ΔL, and iron loss. 1 is a scanning electron microscope image of a cross section of an iron-based sintered body. 1 is a scanning electron microscope image of a cross section of an iron-based sintered body, in which grain boundaries are colored. 1 is a scanning electron microscope image of a cross section of an iron-based sintered body, in which voids at grain boundaries are colored. 1 is a scanning electron microscope image of a cross section of an iron-based sintered body (high strength material). 1 is a scanning electron microscope image of a cross section of an iron-based sintered body (low strength material). 8C is a TEM-EDX mapping image of a partial area of the iron-based sintered body shown in FIG. 8B. FIG. 1 is a diagram showing the relationship between bending strength and grain boundary porosity.

The following describes a preferred embodiment of the present invention.

[Mixture for producing iron-based sintered body]
The mixture for producing an iron-based sintered body according to the present invention includes an iron-based soft magnetic powder whose surface is coated with a phosphoric acid-based coating (hereinafter, may be referred to as a "phosphate coating"), and the surface of the phosphoric acid-based coating is coated with a silicone resin coating, stearic acid amide, and an inorganic solid lubricant.

[Iron-based soft magnetic powder]
The iron-based soft magnetic powder is a ferromagnetic iron-based powder, and specific examples thereof include pure iron powder, iron-based alloy powder (e.g., Fe—Al alloy, Fe—Si alloy, Sendust, Permalloy, etc.), and iron-based amorphous powder. These iron-based soft magnetic powders can be produced, for example, by atomizing molten iron (or molten iron alloy) into fine particles, reducing the particles, and then pulverizing them. In principle, the iron-based soft magnetic powder does not depend on the particle size distribution as long as it has a particle size used in conventional powder metallurgy. For example, the iron-based soft magnetic powder may contain a larger amount of iron-based soft magnetic powder with a larger particle size (e.g., 250 μm to 600 μm) than usual. In this case, the compressibility during compaction can be improved while maintaining a predetermined core loss.

[Phosphoric acid coating]
A phosphate-based coating is formed on the surface of the iron-based soft magnetic powder. The phosphate-based coating can impart electrical insulation to the iron-based soft magnetic powder, thereby suppressing eddy currents. Furthermore, since the phosphate-based coating has good wettability with the iron-based soft magnetic powder, by forming the phosphate-based coating on the surface of the iron-based soft magnetic powder, the surface of the iron-based soft magnetic powder can be uniformly coated with the insulating phosphate-based coating.

A phosphate-based coating is a glassy coating formed using a compound containing P (phosphorus), and its composition is not particularly limited. For example, a glassy coating formed using a compound containing at least one of Na (sodium) and S (sulfur) in addition to the P mentioned above is preferred. This is because these elements prevent oxygen from forming a semiconductor with Fe during heat treatment (during annealing or steam heat treatment, as described below), which would reduce the resistivity. The above compound is more preferably a compound containing Na and S.

The content of the above elements, expressed as the amount per 100 mass% of the iron-based soft magnetic powder having a phosphate coating and a silicone resin coating formed thereon, is preferably 0.005 to 1 mass% for P, 0.002 to 0.6 mass% for Na, and 0.001 to 0.2 mass% for S. When the element contained in the phosphate coating other than P is Na or S, or when it is S , or when it is both Na and S, it is preferable that each of them be within this range.

Of the above elements, P forms a chemical bond with the surface of the iron-based soft magnetic powder via oxygen. Therefore, if the P content is less than 0.005% by mass, the amount of chemical bonding between the iron-based soft magnetic powder surface and the phosphate coating will be insufficient, which is undesirable as it may not form a strong coating. On the other hand, if the P content exceeds 1% by mass, the P that is not involved in the chemical bonding will remain unreacted, which may actually reduce the bond strength, which is undesirable.

Of the above elements, Na and S inhibit the formation of semiconductors between Fe (iron) and oxygen during heat treatment (annealing and steam heat treatment, as described below), thereby preventing a decrease in resistivity. Adding Na and S in combination maximizes this effect.

The phosphate coating may further contain Mg (magnesium) and/or B (boron). The content of these elements, expressed as the amount per 100% by mass of the iron-based soft magnetic powder having a phosphate coating and a silicone resin coating formed thereon, is preferably 0.001 to 0.5% by mass for Mg and 0.001 to 0.5% by mass for B. The phosphate coating may further contain at least one element selected from the group consisting of Co (cobalt), Cs (cesium), and Al (aluminum).

The thickness of the above-mentioned phosphate coating is preferably approximately 1 to 250 nm. If the thickness is thinner than 1 nm, the insulating effect may not be achieved. On the other hand, if the thickness exceeds 250 nm, the insulating effect will saturate and it is also undesirable from the standpoint of increasing the density of the molded body. A more preferable thickness is 50 to 100 nm.

[Method for forming phosphoric acid-based coating]
There is no particular limitation on the method for forming the phosphate coating on the surface of the iron-based soft magnetic powder. For example, the phosphate coating can be formed on the surface of the iron-based soft magnetic powder by mixing a solution in which a P-containing compound is dissolved in a solvent made of water and/or an organic solvent with the iron-based soft magnetic powder, and then evaporating the solvent as necessary.

Examples of the solvent include water, hydrophilic organic solvents such as alcohols and ketones, and mixtures of these. Known surfactants may also be added to the solvent.

An example of the P-containing compound is orthophosphoric acid ( _H3PO4 ). To incorporate elements such as Co into _the phosphate coating, compounds such as _Co3 (PO4) ₂ (Co and P source), _Co3 ( _PO4 ) _2.8H2O (Co and _{P source} ), _Na2HPO4 (P and Na source), _NaH2PO4 (P and Na _source ), _NaH2PO4.nH2O (P and Na source), Al _{( H2PO4} ) _{3 (} P and Al _source ), _Cs2SO4 (Cs and S source), _H2SO4 (S source), _MgO ( _Mg source), _{and H3BO3} ( _B source) can be used. Among these, when NaH ₂ PO ₄ (sodium dihydrogen phosphate) is used as the P source or the Na source, the resulting molded body has a good balance of excellent density, strength and resistivity.

The amount of P-containing compound added to the iron-based soft magnetic powder can be adjusted so that the composition of the phosphate coating formed falls within the above range. For example, a solution of a P-containing compound prepared to have a solids content of approximately 0.01 to 10% by mass, or a compound containing an element to be included in the coating as needed, can be added in an amount of approximately 1 to 10 parts by mass per 100 parts by mass of iron-based soft magnetic powder, and mixed in a known mixer, ball mill, kneader, V-type mixer, granulator, or other mixer, to achieve a composition within the above range for the phosphate coating formed.

If necessary, after the above mixing process, the mixture may be dried at 150 to 250°C in air, under reduced pressure, or in vacuum. After drying, the mixture may be passed through a sieve with a mesh size of approximately 200 to 500 μm. By going through the above process, iron-based soft magnetic powder coated with a phosphate-based coating is obtained.

[Silicone resin coating]
The iron-based soft magnetic powder of the present invention has a silicone resin coating formed on the above-mentioned phosphate coating. The silicone resin coating has Si—O bonds with excellent heat resistance, and is an insulating film with excellent thermal stability. Furthermore, during molding of the mixture described below, the silicone resin contained in the silicone resin coating undergoes a crosslinking/curing reaction, thereby firmly bonding the iron-based soft magnetic powder together. This results in a high-strength molded product. A single layer of silicone resin coating may be formed on the phosphate coating, or two or more layers of silicone resin coating may be formed.

The phosphate-based coating formed on the surface of iron-based soft magnetic powder has high insulating properties, but is brittle and prone to cracking due to the expansion and contraction of the iron-based soft magnetic powder during heat treatment (annealing and steam heat treatment, described below). On the other hand, the silicone resin coating is more flexible than the phosphate-based coating and therefore less likely to crack during heat treatment (annealing and steam heat treatment, described below). The silicone resin coating formed on the phosphate-based coating ensures that the iron-based soft magnetic powder is covered with an insulating film even during heat treatment (annealing and steam heat treatment, described below). This results in a sintered compact with high insulating properties. Furthermore, the insulating film remains even after heat treatment, allowing for high-temperature heat treatment. High-temperature heat treatment removes defects within the iron-based soft magnetic powder, reducing iron loss and resulting in excellent magnetic properties. High-temperature heat treatment also results in excellent mechanical properties. Furthermore, because the phosphate-based coating is covered by the silicone coating, it does not come into direct contact with the inorganic solid lubricant, described below. As a result, the inorganic solid lubricant does not affect the insulating properties of the phosphate coating.

The silicone resin used in the present invention may be any of the conventionally known silicone resins, such as commercially available products such as KR261, KR271, KR272, KR275, KR280, KR282, KR285, KR251, KR155, KR220, KR201, KR204, KR205, KR206, KR225, and KR261 manufactured by Shin-Etsu Chemical Co., Ltd. 311, KR700, SA-4, ES-1001, ES1001N, ES1002T, KR3093, and SR2100, SR2101, SR2107, SR2110, SR2108, SR2109, SR2115, SR2400, SR2410, SR2411, SH805, SH806A, SH840 manufactured by Dow Corning Toray Co., Ltd.

Furthermore, the silicone resin used in the present invention preferably contains more trifunctional T units ( _RSiX3 : X is the same as above) than difunctional D units ( _R2SiX2 : X is a hydrolyzable group), since slow-curing silicone resins tend to make the powder _sticky and difficult to handle after film formation. However, if the silicone resin contains a large amount of tetrafunctional Q units ( _SiX4 : X is the same as above), the powders will adhere tightly to each other during pre-curing, making the subsequent molding process difficult. Therefore, the silicone resin preferably contains 60 mol% or more of T units, more preferably 80 mol% or more, and most preferably contains all T units.

The silicone resin most commonly used is a methylphenyl silicone resin, where R is a methyl or phenyl group, and it is believed that the more phenyl groups there are, the higher the heat resistance. However, under the high-temperature steam heat treatment conditions described below, the presence of phenyl groups was not found to be particularly effective. It is thought that the bulkiness of the phenyl groups disrupts the dense glassy network structure, thereby reducing thermal stability and inhibiting the formation of compounds with iron. Therefore, in the present invention, it is preferable to use a methylphenyl silicone resin containing 50 mol% or more of methyl groups (e.g., KR255, KR311, etc., manufactured by Shin-Etsu Chemical Co., Ltd.), more preferably a methylphenyl silicone resin containing 70 mol% or more of methyl groups (e.g., KR300, etc., manufactured by Shin-Etsu Chemical Co., Ltd.), and even more preferably a methylsilicone resin containing no phenyl groups at all (e.g., KR251, KR400, KR220L, KR242A, KR240, KR500, KC89, etc., manufactured by Shin-Etsu Chemical Co., Ltd., or SR2400, etc., manufactured by Dow Corning Toray Co., Ltd.). Of these, KR220L and SR2400 are particularly preferred. The ratio of methyl groups to phenyl groups and functionality of the silicone resin (coating) can be analyzed using FT-IR, etc.

The amount of silicone resin coating applied is preferably adjusted to 0.05 to 0.3% by mass, assuming that the iron-based soft magnetic powder with the phosphate coating and the silicone resin coating thereon is 100% by mass. If the amount of silicone resin coating applied is less than 0.05% by mass, the insulation properties will be poor and the electrical resistance will likely be low. If the amount of silicone resin coating applied is more than 0.3% by mass, it will be difficult to achieve a high density in the resulting molded product.

The thickness of the silicone resin coating is preferably 1 to 200 nm. A more preferred thickness is 50 to 150 nm, and an even more preferred thickness is 50 to 100 nm.

Furthermore, it is preferable that the total thickness of the phosphate coating and the silicone resin coating be 250 nm or less. If the total thickness exceeds 250 nm, the magnetic flux density may decrease significantly.

[Method for forming silicone resin coating]
The silicone resin coating can be formed, for example, by mixing a silicone resin solution prepared by dissolving a silicone resin in an alcohol or a petroleum-based organic solvent such as toluene or xylene with an iron-based soft magnetic powder on which a phosphate-based coating has been formed (hereinafter, for convenience, this may be simply referred to as "phosphate-based coated iron powder"), and then evaporating the organic solvent as necessary.

The amount of silicone resin added to the phosphate-coated iron powder can be adjusted so that the amount of silicone resin coating formed falls within the above range. For example, approximately 0.5 to 10 parts by mass of resin solution prepared to have a solids content of approximately 2 to 10% by mass can be added to 100 parts by mass of the phosphate-coated iron powder described above, mixed, and then dried. If the resin solution is less than 0.5 parts by mass, mixing may take a long time or the coating may become uneven. On the other hand, if the resin solution is more than 10 parts by mass, drying may take a long time or the drying may be insufficient. The resin solution may be heated as needed. The same mixer as described above can be used.

During the drying process, it is desirable to heat the mixture to a temperature at which the organic solvent used volatilizes, but below the curing temperature of the silicone resin, to fully evaporate the organic solvent. Specifically, a drying temperature of approximately 60 to 80°C is suitable for the alcohols and petroleum-based organic solvents mentioned above. After drying, it is preferable to pass the mixture through a sieve with a mesh size of approximately 300 to 500 μm to remove any aggregates.

After drying, it is recommended to heat the silicone resin-coated iron-based soft magnetic powder (hereinafter, for convenience, sometimes simply referred to as "silicone resin-coated iron powder") to pre-cure the silicone resin coating. Pre-cure is a process that completes the softening process that occurs when the silicone resin coating hardens while the powder is still in its powder state. This pre-cure process ensures the flowability of the silicone resin-coated iron powder during molding (e.g., approximately 100-250°C). A simple method is to briefly heat the silicone resin-coated iron powder near the curing temperature of the silicone resin, but a method using a chemical (curing agent) can also be used. The difference between pre-cure and curing (fully cured, not pre-cured) is that pre-cure allows for easy crushing without completely solidifying the powder particles, whereas high-temperature heat curing performed after powder molding, for example, hardens the resin and solidifies the powder particles. Full curing improves the strength of the molded product.

As mentioned above, pre-curing the silicone resin and then crushing it produces a powder with excellent fluidity, which allows the powder to be poured into the mold as easily as sand during molding. If the resin is not pre-cured, the powder may adhere to itself during molding, making it difficult to pour into the mold in a short time. In practical operations, improving handleability is extremely useful. It has also been found that pre-curing significantly improves the resistivity of the resulting powder core. The reason for this is not clear, but it is thought to be due to increased adhesion to the iron powder during curing.

When the above pre-curing is performed using a short-time heating method, it is recommended to heat the material at 100 to 200°C for 5 to 100 minutes. It is more preferable to heat the material at 130 to 170°C for 10 to 30 minutes. After pre-curing, it is also preferable to pass the material through a sieve, as described above.

[Stearic acid amide]
The mixture for producing an iron-based sintered body of the present invention contains a lubricant in addition to the iron-based soft magnetic powder having the above-mentioned phosphoric acid-based coating and the silicone resin coating formed thereon. The lubricant makes it easier to fill the mixture into a molding die when molding the mixture into a desired shape. The mixture for producing an iron-based sintered body of the present invention contains stearic acid amide, which is an organic lubricant, as the lubricant.

When the above mixture is molded and then heat-treated (by annealing or steam heat treatment, as described below), the organic lubricant is removed, but the remaining organic lubricant residue reacts with the iron in the iron-based soft magnetic powder, turning the sintered body black after heat treatment. This causes an increase in iron loss. However, research by the present inventors has shown that by using stearic acid amide as the organic lubricant, the color of the sintered body does not change when the above mixture is molded and then heat-treated (by annealing or steam heat treatment, as described below). This shows that using stearic acid amide as the organic lubricant prevents organic lubricant residue from remaining, thereby suppressing an increase in iron loss.

The mixture for producing an iron-based sintered body preferably contains 0.1% by mass or more and 0.8% by mass or less of stearic acid amide relative to 100% by mass of the mixture for producing an iron-based sintered body. If the stearic acid amide content is less than 0.1% by mass, seizure with the mold may occur during molding. On the other hand, if the stearic acid amide content exceeds 0.8% by mass, the density of the molded body will be low, making it difficult to obtain a sintered body with excellent magnetic properties. The stearic acid amide content is more preferably 0.2% by mass or more, and even more preferably 0.25% by mass or more. Furthermore, the stearic acid amide content is more preferably 0.45% by mass or less, and even more preferably 0.4% by mass or less.

The mixture for producing the iron-based sintered body of the present invention may contain other organic lubricants in addition to stearic acid amide. Examples of other organic lubricants include hydrocarbon-based, fatty acid-based, higher alcohol-based, aliphatic amide-based, and ester-based organic lubricants.

Hydrocarbon-based lubricants include liquid paraffin, paraffin wax, and synthetic polyethylene wax. Fatty acid- and higher alcohol-based lubricants include stearic acid and stearyl alcohol, which are relatively inexpensive and low-toxicity.

Aliphatic amide lubricants include fatty acid amides such as stearic acid amide, oleic acid amide, and erucic acid amide, as well as alkylene fatty acid amides such as methylene bisstearic acid amide and ethylene bisstearic acid amide.

Examples of ester-based lubricants include stearic acid monoglyceride, a fatty acid ester of alcohol.

[Inorganic solid lubricant]
The mixture for producing an iron-based sintered body of the present invention contains an inorganic solid lubricant in addition to the organic lubricant stearic acid amide. An inorganic solid lubricant is an inorganic compound and a solid lubricant. Examples of inorganic solid lubricants include zinc oxide (ZnO) and molybdenum disulfide (MoS ₂ ). The mixture for producing an iron-based sintered body of the present invention may contain one type of inorganic solid lubricant or two or more types of inorganic solid lubricants. At low temperatures, the organic lubricant mainly promotes powder particle movement, while at high temperatures and high pressures, the inorganic solid lubricant contributes to powder fluidity. Inorganic solid lubricants have a higher density than organic lubricants. By using both an organic lubricant and an inorganic solid lubricant as lubricants, good moldability can be maintained while reducing the amount of lubricant. Reducing the amount of lubricant increases the density of the compact, thereby enabling the production of a compact with high magnetic permeability. Among inorganic solid lubricants, zinc oxide is highly effective in improving powder fluidity.

The density of the inorganic solid lubricant is preferably at least twice the density of the organic lubricant, stearic acid amide. This allows for effective reduction of the total amount of lubricant. Since the density of organic lubricants is often 2.0 g/cm ^or less, the density of the inorganic solid lubricant is preferably ^4.0 g/cm or more.

The particle size of the inorganic solid lubricant is preferably 20 nm or more and 20 μm or less. If the particle size of the inorganic solid lubricant is less than 20 nm, the inorganic solid lubricant will penetrate into the irregularities on the surface of the iron-based soft magnetic powder and the gaps between the iron-based soft magnetic powder particles, making it difficult to exert its lubricating function. If the particle size of the inorganic solid lubricant exceeds 20 μm, the number of inorganic solid lubricant particles will be reduced, making it difficult to contribute to reducing friction between powder particles or between the powder and the mold.

The mixture for producing an iron-based sintered body preferably contains 0.01% by mass or more and 0.2% by mass or less of inorganic solid lubricant per 100% by mass of the mixture. If the inorganic solid lubricant content is less than 0.01% by mass, the organic lubricant stearic acid amide will not be sufficiently replaced by the inorganic solid lubricant. In other words, the proportion of high-density inorganic solid lubricant will be low and the proportion of low-density organic lubricant will be high, making it difficult to increase the density of the compact. This makes it difficult to improve the DC magnetic properties. On the other hand, if the inorganic solid lubricant content exceeds 0.2% by mass, a large amount of lubricant will be added to maintain mold removability. In this case, the density of the compact will decrease, and the saturation magnetic flux density will likely decrease. The inorganic solid lubricant content is more preferably 0.02% by mass or more, and even more preferably 0.025% by mass or more. Furthermore, the inorganic solid lubricant content is more preferably 0.2% by mass or less, and even more preferably 0.1% by mass or less.

It is preferable that the inorganic solid lubricant account for 15% by mass or more but less than 25% by mass, based on a total of 100% by mass of the organic lubricant (stearic acid amide) and the inorganic solid lubricant.

In the mixture for producing an iron-based sintered body, the total content of stearic acid amide and inorganic solid lubricant is preferably 0.1% by mass or more and 0.8% by mass or less, based on 100% by mass of the mixture. If the total content is too low, seizure with the mold is likely to occur during molding. On the other hand, if the total content is too high, the density of the molded body will be low, making it difficult to obtain a powder magnetic core with excellent magnetic properties. The total content is more preferably 0.2% by mass or more, and preferably 0.25% by mass. The total content is more preferably 0.45% by mass or less, and even more preferably 0.4% by mass or less.

[Method of manufacturing an iron-based sintered body]
The method for producing an iron-based sintered body according to the present invention includes a molding step of molding the mixture for producing the iron-based sintered body described above, and a heat treatment step of heat treating the molded body obtained in the molding step in water vapor. By molding the mixture for producing the iron-based sintered body described above and heat treating the molded body obtained in water vapor, an iron-based sintered body having higher magnetic properties and mechanical properties than conventional powder magnetic cores can be obtained. Note that, after the molding step and before the heat treatment step, an annealing step of annealing the molded body may be performed.

[Molding process]
The method for molding the mixture is not particularly limited, and known methods can be employed. The molding conditions are not particularly limited as long as a molded body with the desired density is obtained. For example, the molding pressure is preferably 390 MPa or more, more preferably 490 to 1960 MPa, and even more preferably 790 to 1180 MPa in terms of surface pressure. For example, the molding pressure may be 390 MPa or more and 1180 MPa or less in terms of surface pressure. The density of the molded body obtained by the molding step is preferably 7.20 g/cm ³ or more, and more preferably 7.50 g/cm ^{3. A molded body with such a density can provide an iron-based sintered body with high magnetic properties. For example, when compression molding is performed under conditions of 800 MPa or more, a molded body with a density of 7.20 g/cm 3 or} more is likely to be obtained. Furthermore, when compression molding is performed under conditions of 980 MPa or more, a molded body with a density of 7.50 g/cm ³ or more is likely to be obtained. The molding temperature can be room temperature or warm (for example, 80° C. or higher, more preferably 100 to 250° C.).

[Annealing process]
After the molding step, an annealing step may be performed to anneal the compact before the heat treatment step described below. By annealing the compact, the organic lubricant stearic acid amide can be removed. This results in a degreased compact. Since degreasing reduces iron loss, a sintered body with good magnetic properties can be obtained. The annealing temperature is preferably above the decomposition temperature of stearic acid amide (approximately 200°C or higher). In addition, by performing the annealing step at a high temperature, distortion can be removed. This makes it possible to produce a sintered body with high electrical insulation, i.e., high resistivity, without increasing eddy current loss (corresponding to coercive force).

The annealing process is preferably carried out at a temperature above the decomposition temperature of the organic lubricant components (approximately 200°C or higher). There is no particular upper limit to the annealing temperature for degreasing the compact, but if the annealing temperature is too high, the phosphate coating on the surface of the iron-based soft magnetic powder tends to thin as it is heated. To prevent the phosphate coating from thinning, the temperature is preferably 650°C or lower, and more preferably 600°C or lower.

The annealing time is preferably 20 minutes or more. 25 minutes or more is more preferable, and 27 minutes or more is even more preferable. A longer annealing time is preferable from the standpoint of degreasing and removing distortion, but if high-temperature heat treatment is performed for a long period of time, the phosphate coating will thin, resulting in a decrease in insulation properties. Therefore, the annealing time is preferably 180 minutes or less, more preferably 60 minutes or less, and particularly preferably 35 minutes or less.

The atmosphere during annealing is not particularly limited, but it is preferable to perform the annealing in an inert gas atmosphere such as nitrogen gas.

It is not necessary to perform an annealing process between the forming process and the heat treatment process. When the formed body is heated in the heat treatment process, it is possible to degrease and remove distortion from the formed body.

[Heat treatment process]
The method for producing an iron-based sintered body of the present invention is mainly characterized by heat-treating in water vapor a compact obtained by compacting a mixture of the iron-based soft magnetic powder with high magnetic properties described above. During heat-treatment in water vapor, iron ions in the iron-based soft magnetic powder, driven by oxygen, pass through the insulating phosphate coating and silicone resin coating, and onto the insulating film, where they react with oxygen to form iron oxide directly on the insulating film. At the grain boundaries between the iron-based soft magnetic powder particles (in the present invention, the regions between the iron-based soft magnetic powder particles are referred to as "grain boundaries"), iron oxide is gradually formed from the powder side toward the center of the grain boundary. It is believed that the bonding of the powder particles by iron oxide improves the mechanical properties of the compact.

The method for heat-treating the compact in water vapor is not particularly limited. For example, the compact can be heated in an environment with a relative humidity of 100%. Heat treatment can also be performed while supplying water vapor at a constant rate. If the compact is heat-treated in an atmosphere with a low water vapor concentration, the surface of the compact will oxidize, preventing oxygen from penetrating into the interior of the compact, and iron oxide will only form on the surface of the compact. As a result, sufficient mechanical properties will not be obtained. Heat treatment in an environment with a relative humidity of 100% makes it possible to control the oxidation rate, and the compact will oxidize slowly, allowing oxygen to penetrate into the interior of the compact and forming iron oxide on the insulating film of the powder inside the compact. This bonds the iron-based soft magnetic powder together, improving mechanical properties.

The heat treatment temperature is preferably 460°C or higher, more preferably 475°C or higher, and even more preferably 480°C or higher. Furthermore, the heat treatment temperature is preferably less than 600°C, more preferably 590°C or higher, and even more preferably 530°C or higher. Heat treatment at these temperatures results in a sintered body with high magnetic and mechanical properties. If the heat treatment temperature is below 460°C, the rate of iron oxide formation is slow, and the iron-based magnetic powder particles do not bond sufficiently. As a result, high mechanical strength is not achieved. If the heat treatment temperature is 600°C or higher, the insulating phosphate coating and silicone resin coating decompose, generating gas and creating voids at the grain boundaries. Furthermore, multiple types of iron oxide are generated as oxides, and the density difference between them causes distortion, resulting in cracks within the oxides at the grain boundaries. As a result, high mechanical strength is not achieved. Furthermore, if the heat treatment temperature is too high, the insulating film decomposes, resulting in a decrease in insulation and magnetic properties. Based on the above, the preferred heat treatment temperature is 460°C to 590°C, more preferably 460°C to 580°C, and even more preferably 480°C to 530°C.

Even if an annealing step is not performed before the heat treatment step, heat treatment at the above-mentioned temperatures removes the organic lubricant stearic acid amide, making degreasing possible. It also makes it possible to remove distortion from the compact. This results in a sintered body with high electrical insulation without increasing eddy current loss (equivalent to coercive force). Therefore, the above heat treatment step may be performed after the compacting step without performing an annealing step.

The heat treatment time is preferably 30 minutes or more, and more preferably 60 minutes or more. There is no particular upper limit to the heat treatment time, but it is preferably less than 300 minutes, and more preferably less than 200 minutes. Specifically, the heat treatment time is preferably 30 to 300 minutes, and even more preferably 60 to 200 minutes.

The sintered body obtained by the heat treatment process may be dried. There are no particular limitations on the drying conditions as long as the intended purpose is achieved. For example, after the oxidation process, the body is cooled to approximately 150-200°C, the water vapor is removed, and an inert gas is circulated through the container for 30 minutes to 2 hours while maintaining the temperature inside the container at 100-300°C.

[Iron-based sintered body]
The iron-based sintered body obtained by the above-mentioned method includes an iron-based soft magnetic powder having a phosphate-based coating and a silicone resin coating formed thereon, and iron oxide present at the grain boundaries between the iron-based soft magnetic powder. The phosphate-based coating may be formed on the entire surface of the iron-based soft magnetic powder contained in the iron-based sintered body, or may be formed only on a portion of the surface of the iron-based soft magnetic powder. The silicone resin coating may be formed on the entire surface of the phosphate-based coating, or may be formed only on a portion of the surface of the phosphate-based coating. In the above-mentioned heat treatment process, iron ions in the iron-based soft magnetic powder pass through the phosphate-based coating and silicone resin coating, which are insulating films, and migrate onto the insulating film. At this time, the phosphate-based coating and silicone resin coating, which are insulating films, may be broken. As a result, the surface of the iron-based soft magnetic powder may not be partially covered by the insulating film.

In the iron-based sintered body of the present invention, iron oxide is present in 58% or more of the grain boundaries. Here, the grain boundaries refer to the regions between iron-based soft magnetic powder particles that have a phosphate-based coating and a silicone resin coating formed thereon. Research by the inventors has shown that when iron oxide is present in 58% or more of the grain boundaries, the flexural strength of the sintered body measured in accordance with the Japanese Industrial Standard "JIS Z 2511" (2006 edition) is considered to be 100 MPa or greater. An iron-based sintered body with this flexural strength is a sintered body with better mechanical properties than conventional powder magnetic cores. When the proportion of iron oxide present in the grain boundaries is 49% or less, the flexural strength of the sintered body measured in accordance with the Japanese Industrial Standard "JIS Z 2511" (2006 edition) is considered to be less than 100 MPa. This strength is not significantly different from the mechanical properties of conventional powder magnetic cores.

There are no particular limitations on the method for confirming the presence of iron oxide at grain boundaries. For example, it can be confirmed from an electron microscope image of the cross section of the iron-based sintered body. There are no particular limitations on the conditions, such as the type of electron image or acceleration voltage, of the electron microscope image. However, to evaluate the average information on the oxide loading amount, the electron microscope image must show two or more iron-based soft magnetic powder particles, and the amount of iron oxide present is evaluated by evaluating a grain boundary range with a length of 50% or more of the average particle size.

It is believed that during the heat treatment process, the rate at which iron oxide forms on the insulating film of the iron-based soft magnetic powder and the rate at which the iron oxide grows do not vary significantly throughout the entire compact (sintered body). Therefore, it is presumed that the proportion of iron oxide present at grain boundaries is roughly the same at any position in the sintered body obtained through the heat treatment process. Therefore, if an electron microscope image of a cross-section of a sintered body confirms that iron oxide is present at 58% or more of the grain boundaries, it is presumed that iron oxide is present at 58% or more of the grain boundaries in electron microscope images of other cross-sections as well. In other words, if an electron microscope image of a cross-section of a sintered body confirms that iron oxide is present at 58% or more of the grain boundaries, it is presumed that iron oxide is present at 58% or more of the grain boundaries in that sintered body.

The magnification of the electron microscope image is not particularly limited, but may be, for example, an electron microscope image taken at a magnification of 1000x or more and 2000x or less. From this electron microscope image, the iron-based soft magnetic powder, iron oxide, and grain boundaries can be visually confirmed.

The density of the iron-based sintered body is preferably 7.50 g/cm ³ or more, and more preferably 7.51 g/cm ^3. A sintered body with such a density has high magnetic properties.

Figures 1A and 1B show examples of scanning electron microscope images (SEM images) of a cross section of the iron-based sintered body of the present invention. The SEM image shown in Figure 1B is an SEM image (magnification 2,000 times, area 43 μm long × 84 μm wide (= 2752 μm ² )) showing a part of the part shown in Figure 1A. In Figures 1A and 1B, the iron-based soft magnetic powder having a phosphate-based coating and a silicone resin coating formed thereon is simply referred to as "iron-based soft magnetic powder." As shown in Figure 1B, iron oxide is densely present at the grain boundaries.

The present invention will be described in detail below based on examples. However, the following examples do not limit the present invention, and all modifications and variations that do not deviate from the spirit of this specification are included within the technical scope of the present invention.

(Experiment 1)
Iron-based sintered bodies were produced under the conditions shown in Tables 1 and 2 by the methods described in the embodiments, and their mechanical properties and magnetic properties were evaluated. The amount [%] of lubricant shown in Tables 1 and 2 indicates the amount [% by mass] relative to 100% by mass of the mixture for producing the iron-based sintered body. Nos. 1 to 4 in Table 1 and Nos. 1 to 4 in Table 2 are the same sintered bodies.

The sintered bodies Nos. 1 to 3 in Tables 1 and 2 are sintered bodies using iron-based soft magnetic powder, in which the insulating film is a single layer of either a phosphoric acid-based coating or a silicone resin coating.
The sintered body No. 4 is a sintered body using an iron-based soft magnetic powder and has a two-layer insulating film consisting of a phosphoric acid-based coating and a silicone resin coating.
In No. 1, only stearic acid amide, an organic lubricant, was used as the lubricant.
In Nos. 2 to 4, the organic lubricant stearic acid amide and the inorganic solid lubricant zinc oxide (ZnO) were used in combination as lubricants.

Table 1 shows the flexural strength and electrical resistance of the iron-based sintered bodies No. 1 to 4. The flexural strength was measured by a flexural test conducted in accordance with the provisions of the Japanese Industrial Standard "JIS Z 2511" (2006 edition). The flexural test specimens used in the flexural test were sintered bodies manufactured using a compact obtained by filling a mixture containing an iron-based soft magnetic powder and a lubricant into a 12.7 mm x 31.75 mm mold and compressing it from above and below under a compacting pressure of 392 to 1179 MPa ( ⁴ to 12 ton/cm ² ). It should be noted that in the experiments (Experiments 3 and 4) described below, when measuring the flexural strength, sintered bodies were manufactured using the compacts manufactured by the above-mentioned molding method. The flexural strength was also measured under the same flexural test conditions as in this experiment.

Table 2 shows the iron loss and magnetic flux density of iron-based sintered bodies No. 1 to 4. The iron loss shown in Table 2 is the iron loss when the magnetic flux density is 1 T and the frequency is 400 Hz. Here, the sintered bodies used were manufactured using a compact obtained by filling a mold with an outer diameter of φ45 mm and an inner diameter of φ337 mm with a mixture containing iron-based soft magnetic powder and lubricant, and compressing it from above and below. Note that in the experiments described below (Experiments 3 and 4), when measuring iron loss and magnetic flux density, sintered bodies manufactured using compacts made using the above molding method were used.

When the flexural strength of a sintered body is 100 MPa or higher, it can be said that the mechanical properties are higher than those of conventional rolled magnetic cores. When the electrical resistance of a sintered body is 60 μΩ·m or higher, it can be said that the insulation properties are good. Furthermore, when the iron loss is 40 W/kg or less, it can be said that the magnetic properties are higher than those of conventional rolled magnetic cores.

As shown in Table 1, Nos. 1 to 3 have a flexural strength of around 100 MPa, but low electrical resistance and therefore poor insulation. This is thought to be because the iron-based soft magnetic powder used had a single layer of insulating film, either a phosphate-based or silicone resin coating, which destroyed the insulating film during heat treatment, causing adjacent iron powder particles to become conductive. Furthermore, as shown in Table 2, Nos. 1 to 3 had iron loss of 40 W/kg or more. This shows that the sintered compacts Nos. 1 to 3, which use iron-based soft magnetic powder with a single layer of insulating film, do not have better magnetic properties than conventional powder cores.

On the other hand, as shown in Table 1, No. 4 has a flexural strength of over 100 MPa and an electrical resistance of 60 μΩ·m or more. Furthermore, as shown in Table 2, No. 4 has an iron loss of 40 W/kg or less. The No. 4 sintered body uses an iron-based soft magnetic powder with a two-layer insulating film, and has better mechanical and magnetic properties than conventional powder cores.

From the above, it was found that by using an iron-based soft magnetic powder with a two-layer insulating film consisting of a phosphate-based coating and a silicone resin coating, it is possible to obtain a sintered body with both higher mechanical and magnetic properties than conventional powder cores.

(Experiment 2)
Iron-based sintered bodies were produced by the method described in the embodiment, while varying the type of lubricant under the conditions shown in Table 3. The amount of lubricant added [%] shown in Table 3 indicates the amount [unit: % (mass %)] relative to 100 mass % of the mixture for producing the iron-based sintered body. The densities of sintered bodies No. 1 to 12 were measured. In addition, the color changes before and after the heat treatment process were investigated for No. 1 to 6 and No. 12. Table 3 shows the densities of iron-based sintered bodies No. 1 to 12 and the color changes for No. 1 to 6 and No. 10.

In Nos. 1 to 6 in Table 3, metal soap, an organic lubricant known as a common lubricant, was used.
In Nos. 7 to 9, stearic acid amide, an organic lubricant, was used as the lubricant.
In Nos. 10 to 12, stearic acid amide, an organic lubricant, and zinc oxide (ZnO), an inorganic solid lubricant, were used in combination as lubricants.

Adding a lubricant can improve moldability, but as the amount of lubricant added increases, the density of the sintered body decreases and its magnetic properties tend to deteriorate. Furthermore, as the amount of lubricant added increases, the lubricant tends to remain even when heat-treated in water vapor, making the sintered body more likely to turn black after heat-treatment.

From Nos. 7 to 12 in Table 3, it can be seen that the density of the sintered body tends to decrease as the amount of lubricant added increases. However, when comparing Nos. 1 to 6 (using metal soap only) with No. 10 (using stearic acid amide and ZnO in combination), which have the same total amount of lubricant added, it is clear that even with the same total amount added, when stearic acid amide and ZnO are used in combination, sintered bodies with higher densities tend to be obtained than when metal soap alone is used.

Nos. 7 and 8 are comparative examples showing the sintered body density when the insulating film is made of only a phosphoric acid coating and only stearic acid amide is used as the organic lubricant. Even when the amount of lubricant added is increased from 0.4% by mass to 1.0% by mass, the sintered body density decreases. This is thought to be due to the increase in voids caused by degassing of the lubricant and the sintered lubricant residue, rather than the improvement in powder packing (moldability) that is achieved by adding the lubricant.
No. 9 and No. 10 show data for samples with the same amount of organic lubricant (stearic acid amide), with No. 10 also containing 0.05% ZnO. From No. 9 and No. 10, it can be seen that the addition of ZnO increases the sintered body density.

From the above, it was found that when the organic lubricant stearic acid amide is used in combination with an inorganic solid lubricant, a sintered body with a higher density tends to be obtained than when only the organic lubricants metal soap or stearic acid amide are used. This suggests that by using the organic lubricant stearic acid amide in combination with an inorganic solid lubricant as a lubricant, it is possible to obtain good moldability due to the lubricant while also improving magnetic properties.

Regarding the change in color before and after the heat treatment process, sintered bodies No. 1 to 6 (which used only metal soap) turned black, but the color of sintered body No. 10 (which used both stearic acid amide and ZnO) remained unchanged. This suggests that stearic acid amide is easily removed by steam heat treatment and is unlikely to remain after the heat treatment process.

From the above, it was found that by using stearic acid amide as an organic lubricant and combining it with stearic acid and an inorganic solid lubricant, a sintered body with high magnetic properties and no blackening after steam heat treatment can be obtained.

(Experiment 3)
The mechanical and magnetic properties were investigated when the conditions of the heat treatment process in water vapor were changed.

Using the method described in the embodiment, iron-based sintered bodies were produced by varying the steam heat treatment conditions shown in Table 4A, and their mechanical and magnetic properties were evaluated. The amount of lubricant added [%] shown in Table 4A indicates the amount [% by mass] relative to 100% by mass of the mixture used to produce the iron-based sintered body. Table 4A also shows the flexural strength of the sintered bodies.

<Heat treatment time and mechanical properties>
The sintered bodies Nos. 5 to 7 and 12 in Table 4A were obtained by setting the heat treatment temperature to 550°C and varying the heat treatment time between 10 and 120 minutes. Figure 2 shows the relationship between the heat treatment time and the flexural strength for Nos. 5 to 7 and 12 in Table 4. As mentioned above, iron-based sintered bodies with a flexural strength of 100 MPa or more are sintered bodies with higher mechanical properties than conventional powder magnetic cores.

Figure 2 shows that the flexural strength increases as the heat treatment time increases, and that the increase in flexural strength stops at around 60 minutes of heat treatment, with the flexural strength remaining roughly the same for heat treatments of 60 minutes or longer. Figure 2 also shows that the flexural strength reaches 100 MPa at around 20 minutes of heat treatment, and exceeds 100 MPa at 30 minutes of heat treatment. It can be inferred from Figure 2 that the flexural strength will exceed 100 MPa for heat treatments of 20 minutes or longer.

Based on the above, in order to improve the mechanical properties compared to conventional powder magnetic cores, it is preferable to set the heat treatment time to 20 minutes or more, and even more preferable to set it to 30 minutes or more. There is no particular upper limit on the heat treatment time, but it is preferable that it be 300 minutes or less, for example.

<Cross section of sintered body>
FIG. 3A shows an example of a scanning electron microscope (SEM) image (magnification: 100x) of a wide cross section of a sintered body. As shown in FIG. 3A, the sintered body contains a plurality of iron-based soft magnetic powders. FIGS. 3B and 3C show example scanning electron microscope (SEM) images of a narrow cross section of the sintered body. The image shown in FIG. 3B is magnified at 20,000x, and the image shown in FIG. 3C is magnified at 5,000x, and FIG. 3C shows an image of an area measuring 17 μm long x 25 μm wide (= 425 μm ² ). As shown in FIGS. 3B and 3C, grain boundaries exist between the iron-based soft magnetic powders. In the image shown in FIG. 3B, iron oxide is present in most of the grain boundaries. In the image shown in FIG. 3C, there is little iron oxide present at the grain boundaries. From FIGS. 3B and 3C, it can be seen that iron oxide fills the grain boundaries from the surface side of the iron-based soft magnetic powder toward the center of the grain boundaries.

<Heat treatment temperature and mechanical properties>
Nos. 1 to 4 in Table 4A represent comparative examples using iron powder materials with a single insulating layer consisting of either an organic silicone coating or an inorganic phosphate coating. Figure 4 shows, with white triangles (△), the relationship between the heat treatment temperature and flexural strength of sintered bodies obtained using iron-based soft magnetic powder with a single silicone-coated insulating layer. Furthermore, the relationship between the heat treatment temperature and flexural strength of sintered bodies obtained using iron-based soft magnetic powder with a single phosphate-coated layer (Nos. 2 to 4 in Table 4) is shown with white diamonds (◇), and the correlation line between the heat treatment temperature and flexural strength for Nos. 2 to 4 is shown with a dashed line.

Sintered compacts No. 8 to 19 in Table 4A are made from iron powder material with a two-layer insulating film consisting of an inorganic phosphate coating and an organic silicone coating on top of it. They were obtained by heat treating for 120 minutes at temperatures ranging from 450°C to 650°C. Figure 4 shows the relationship between heat treatment temperature and flexural strength for No. 8 to 19 in Table 4 with black circles (●). Figure 4 also shows the correlation line between heat treatment temperature and flexural strength for No. 8 to 19 in Table 4 with a solid line.

The following was found from FIG.
When the insulating film has a two-layer structure, it can be seen from Nos. 8 to 19 (●) and their correlation lines that the flexural strength increases as the heat treatment temperature increases, reaching 100 MPa or more at a heat treatment temperature of around 460°C, and reaching a very high flexural strength of 110 MPa or more at a heat treatment temperature of around 480°C to 530°C. When the heat treatment temperature exceeds around 540°C, the flexural strength decreases, but remains 100 MPa or more up to a heat treatment temperature of around 600°C.

It is believed that when the heat treatment temperature is below 460°C, the iron oxide does not grow sufficiently, and the grain boundaries are barely filled with iron oxide, as in the case of a short heat treatment time, as shown in the image in Figure 3C. Therefore, it is believed that when the heat treatment temperature is below 460°C, the flexural strength is not high.

From the above, it was found that in order to increase the flexural strength compared to conventional powder magnetic cores, it is preferable to set the heat treatment temperature to 460°C or higher but lower than 600°C. For example, the heat treatment temperature may be 460°C or higher but 590°C. It was also found that setting the heat treatment temperature to 480°C or higher but 530°C or lower is more preferable, as it results in a high-strength sintered body with a flexural strength of 110 MPa or higher.

On the other hand, in the case of a single insulating layer (single layer of phosphate-based coating), it was found that the flexural strength rapidly decreased as the heat treatment temperature increased above 475°C. It is believed that the temperature range in which a high-strength sintered body can be obtained is narrower for both the single layer of phosphate-based coating and the single layer of organic silicone coating than for a two-layer insulating film. This is presumably because, in the case of a single insulating layer, the insulating coating decomposes upon heating, resulting in an increase in grain boundary voids due to peeling and degassing, which reduces the flexural strength.

<Heat treatment temperature and magnetic properties>
A common method for evaluating magnetic properties is to prepare a ring-shaped test piece and wind a wire around it, but this method requires multiple test pieces. As explained below, information correlating with iron loss can be easily obtained from the rate of change in inductance at different frequencies (hereinafter referred to as "ΔL"), so we decided to evaluate magnetic properties using ΔL here. Below, we will explain ΔL before explaining the relationship between heat treatment temperature and magnetic properties.

The inductance L is expressed by the following formula:
Inductance L is a function of magnetic permeability μ. When the number of turns of a coil is N and the cross-sectional area of the coil is S, if the value obtained by multiplying ^N2 by the absolute value of S is equal to the length of the winding l, then from the above formula 1, inductance L is equal to measuring magnetic permeability μ. The following formula shows the relationship between magnetic permeability μ, magnetic field (excitation magnetic field) H, and magnetic flux density B.
The excitation magnetic field H and magnetic flux density B are expressed by the following equations.
From the above formulas 3, 4 and 5, the AC permeability (complex permeability) is expressed by the following formula:
Here, δ represents the delay of the magnetic flux density, which means the loss. Therefore, by measuring the inductance L at low frequency (100 Hz) and the inductance L at high frequency (100 kHz) and calculating the change (ΔL), the magnitude of iron loss can be measured indirectly.

To evaluate magnetic properties from ΔL, we investigated the relationship between ΔL and iron loss, as well as the relationship between electrical resistance and iron loss. Figure 5 shows the correlation between ΔL, iron loss, and electrical resistance. The larger ΔL, the lower the electrical resistance. Low electrical resistance means poor insulation, which results in high iron loss. To improve magnetic properties compared to conventional rolled cores, it is desirable to keep iron loss below 40 W/kg, and from Figure 5, we can determine that in order to keep iron loss below 40 W/kg, ΔL should be kept below 5%.

Table 4B shows the ΔL and electrical resistance of iron-based sintered bodies No. 1 to 19 in Table 4A. Table 4B also shows the iron loss and magnetic flux density of No. 1 to 4, 8 to 9, and 11 to 14. The ΔL and electrical resistance of No. 1 to 19 in Table 4B were measured using the same sintered bodies as in Table 4A. The iron loss and magnetic flux density were measured using the same iron-based soft magnetic powder as the sintered bodies No. 1 to 19 in Table 4A, and the iron loss was measured when the magnetic flux density was 1 T and the frequency was 400 Hz.

Figure 6 shows the relationship between heat treatment temperature and ΔL value for Nos. 8 to 16 in Table 4 with black circles (●), and the relationship between heat treatment temperature and iron loss for Nos. 8, 9, and 11 to 14 with black triangles (▲). Furthermore, for reference, Figure 6 also shows the relationship between heat treatment temperature and ΔL value for sintered bodies obtained using iron-based soft magnetic powder with a single-layer phosphate-based insulating film for Nos. 2 to 4 in Table 4 with white circles (○), and the relationship between heat treatment temperature and iron loss for sintered bodies obtained using iron-based soft magnetic powder with a single-layer phosphate-based insulating film for Nos. 2 to 4 in Table 4 with white triangles (△).

The following was found from FIG.
For sintered bodies using iron-based soft magnetic powder with a single-layer phosphate-based insulating film, the relationship between heat treatment temperature and ΔL value (◯) shows that ΔL increases with increasing heat treatment temperature above approximately 450°C. Figure 5 shows that the larger ΔL, the lower the electrical resistance. Low electrical resistance indicates poor insulation. For sintered bodies using iron-based soft magnetic powder with a single-layer phosphate-based insulating film, the insulation film is likely to have been destroyed by thermal history at heat treatment temperatures above approximately 450°C, resulting in a decrease in insulation and high iron loss. Figure 6 shows that at a heat treatment temperature of 470°C, ΔL exceeds 5%, and iron loss is high at approximately 50 W/kg. At heat treatment temperatures above 470°C, ΔL increases even further, which is likely to lead to increased iron loss.

Figure 4 shows that when using iron-based soft magnetic powder with a double-layer insulating layer, a heat treatment temperature of 460°C or higher but less than 600°C is preferable to increase flexural strength compared to conventional powder cores. However, when using iron-based soft magnetic powder with a single-layer phosphate-based insulating film, when the heat treatment temperature is 460°C or higher, ΔL exceeds 5% and the iron loss is large, at approximately 50 W/kg, as shown above. It is believed that the iron loss also increases as the heat treatment temperature increases. Therefore, when using iron-based soft magnetic powder with a single-layer phosphate-based insulating film, a sintered body with poor magnetic properties is likely to be obtained when the heat treatment temperature for the water vapor heat treatment is 460°C or higher but less than 600°C.

On the other hand, in the case of the sintered body of the present invention using the iron-based soft magnetic powder on which the insulating film of the two-layer structure is formed, the following can be considered from FIG.
The relationship between heat treatment temperature and ΔL (●) shows that ΔL increases as the heat treatment temperature increases, but the increase in ΔL is small at heat treatment temperatures below 600°C (e.g., 590°C). Furthermore, at heat treatment temperatures below 600°C, ΔL is 5% or less. From this, it is believed that when the insulating film has a two-layer structure, when the heat treatment temperature is below 600°C, there is little breakdown of the insulating film and little deterioration in insulation. Note that at heat treatment temperatures below 600°C, there is little deterioration in insulation, but the insulation gradually deteriorates, which is thought to increase iron loss. However, the relationship between heat treatment temperature and iron loss (▲) in Figure 6 shows that iron loss decreases as the heat treatment temperature increases. It is believed that heat treatment removes defects in the iron-based soft magnetic powder. In the present invention, the amount of defects removed in the iron-based soft magnetic powder increases as the heat treatment temperature increases, and the insulation deterioration is also small. This is thought to be the combined effect of these factors, resulting in a decrease in iron loss as the heat treatment temperature increases. From this, it was found that when an iron-based soft magnetic powder having a two-layer insulating film is used, an iron-based sintered body having high magnetic properties can be obtained when the heat treatment temperature of the water vapor heat treatment is set to less than 600°C (for example, less than 590°C).

Figure 4 shows that when using iron-based soft magnetic powder with a double-layer insulating film, a heat treatment temperature of 460°C or higher but lower than 600°C is preferable to increase flexural strength compared to conventional powder cores. From the above, it was found that when using iron-based soft magnetic powder with a double-layer insulating film, ΔL is 5% or lower when the heat treatment temperature is lower than 600°C, and that iron loss decreases as the heat treatment temperature increases. Therefore, when using iron-based soft magnetic powder with a double-layer insulating film, a sintered body with excellent mechanical and magnetic properties can be obtained by setting the heat treatment temperature for the water vapor heat treatment to 460°C or higher but lower than 600°C (for example, 460°C or higher but lower than 590°C).

<Phosphate coating thickness>
The sintered compacts No. 10 and No. 17 to No. 19 in Table 4B were obtained by heat treating at a temperature of 500°C and varying the thickness of the phosphate coating (shown as "phosphate coating" in Table 4B) between 60 nm and 25 nm. The phosphate coating thicknesses (phosphate coating thickness) and ΔL for No. 10 and No. 17 to No. 19 indicate that when the phosphate coating thickness (phosphate coating thickness) is 40 nm or less, ΔL exceeds 5%. This suggests that when the phosphate coating thickness (phosphate coating thickness) is 40 nm or less, the insulation is insufficient and the magnetic properties are poor. Therefore, it is preferable that the phosphate coating thickness be 50 nm or more.

(Experiment 4)
Under the conditions shown in Table 5, iron-based soft magnetic powder with a two-layer insulating film was used to prepare iron-based sintered bodies (high-strength materials) with a flexural strength of 100 MPa or more and iron-based sintered bodies (low-strength materials) with a flexural strength of less than 100 MPa, and the state of the grain boundaries was examined. Here, iron-based soft magnetic powder with a phosphate coating (60 nm thick) and a silicone resin coating (100 nm thick) formed thereon was used, and stearic acid amide (0.25 mass%) and zinc oxide (0.05 mass%) were used as lubricants. Furthermore, iron-based sintered bodies were prepared using iron-based soft magnetic powder with a single-layer insulating film structure. Here, iron-based soft magnetic powder with a phosphate coating (60 nm thick) was used, and stearic acid amide (0.4 mass%) was used as the lubricant. Table 5 shows the preparation conditions and the state of the grain boundaries of the iron-based sintered bodies.

The "type and amount of oxides formed at grain boundaries" in Table 5 was measured by X-ray diffraction (XRD).
The "porosity of grain boundaries" in Table 5 was determined from SEM images of the cross sections of the sintered bodies taken at a magnification of 2,000 times (measurement range: region of 43 μm length × 64 μm width) so that a cross section of a grain boundary where two or more iron powder particles are adjacent to each other could be photographed, using the following procedure.
FIG. 7A shows an example of an SEM image (original image) of a cross section of a sintered body. FIG. 7B shows a diagram in which the entire grain boundary is colored. For example, when calculating the "grain boundary porosity" from the SEM image (original image) of the cross section of the sintered body shown in FIG. 7A, the area (grain boundary area) of the grain boundary (colored portion) is determined from FIG. 7B by image analysis. FIG. 7C shows the grain boundary voids (portions where iron oxide is not present) colored. The area (void area) of the grain boundary voids (colored portion) is determined from FIG. 7C by image analysis. The "grain boundary porosity" in Table 5 was calculated by (void area/grain boundary area) × 100 [unit: %]. The "abundance of iron oxide at the grain boundary" in Table 5 was calculated by 100 - grain boundary porosity [unit: %].

As shown in Table 5, in a high-strength material (hereinafter sometimes simply referred to as "high-strength material") using an iron-based soft magnetic powder with a two-layer insulating film, the "abundance of iron oxide at grain boundaries" was 58%. In a low-strength material (hereinafter sometimes simply referred to as "low-strength material") using an iron-based soft magnetic powder with a two-layer insulating film, the "abundance of iron oxide at grain boundaries" was 49%. The difference in the "abundance of iron oxide at grain boundaries" between the high-strength material and the low-strength material is thought to be due to the type of iron oxide produced by heat treatment in water vapor and the decomposition of the insulating film.
In addition, in the iron-based sintered body using the iron-based soft magnetic powder with a single-layer insulating film, the "abundance rate of iron oxide at grain boundaries" was 39%.

Figure 8A shows a cross-sectional SEM image of the grain boundary of the high-strength material. Figure 8B shows a cross-sectional SEM image of the grain boundary of the low-strength material. Figures 8A and _8B show enlarged images of oxides to compare the oxides at the grain boundaries. Only one type of oxide ( _Fe3O4 ) was detected at the grain boundaries in the high-strength material. From the image in Figure _8A , it can be seen that the oxide ( _Fe3O4 ) is densely formed at the grain boundaries. It can also be seen that an insulating film is uniformly formed at the interface of the iron-based soft magnetic powder.

On the other hand, two types of oxides ( _Fe3O4 and _FeO ) were detected at the grain boundaries in the low-strength material. The image in Figure 8B confirms that many cracks have occurred in the oxides. This is thought to be due to distortion caused by the difference in density between the two types of oxides, which in turn caused cracks to occur in the oxides at the grain boundaries. This is presumably what led to the decrease in strength. It can be seen that oxides with many cracks do not contribute to the bonding of the iron-based soft magnetic powder together.

FIG. 9 shows a TEM-EDX mapping image of a portion of the area shown in FIG. 8B (TEM: transmission electron microscope, EDX: energy dispersive X-ray spectroscopy). FIG. 9 reveals that peeling of the insulating film has occurred at the interface of the iron-based soft magnetic powder. Furthermore, elemental analysis using TEM-EDX mapping revealed that phosphorus (P) derived from the first phosphate coating layer covering the iron-based magnetic powder was partially detected near the powder interface. Furthermore, silicon (Si) derived from the second silicone resin coating layer was detected throughout the grain boundary oxide. From this, it is believed that the phosphate coating layer covering the iron-based magnetic powder was decomposed and destroyed by the steam heat treatment. It is also believed that the second silicone resin coating layer was completely decomposed by the steam heat treatment. It is believed that gases such as CO ₂ , CO 2 , and H ₂ O were generated when these two insulating layers decomposed, which caused the insulating film to peel off at most of the interface of the iron-based soft magnetic powder, resulting in a decrease in strength.

Fig. 10 shows the relationship between the flexural strength and the grain boundary porosity of the sintered body shown in Table 5. Fig. 10 also shows the correlation line between the flexural strength and the grain boundary porosity. From Fig. 10, it is believed that when the grain boundary porosity is 42% or less, the flexural strength is 100 MPa or more. When the grain boundary porosity is 42% or less, the proportion of iron oxide in the grain boundaries is 58% or more.
From the above, when the proportion of iron oxide present at grain boundaries in an iron-based sintered body is 58% or more, the iron-based sintered body is considered to be a high-strength sintered body having a bending strength of 100 MPa or more .

From the above, it was found that by setting the heat treatment temperature during steam heat treatment to less than 600°C, an iron-based sintered body with excellent mechanical properties, including a grain boundary iron oxide content of 58% or more and a flexural strength of 100 MPa, can be obtained.

The above describes embodiments of the present invention based on examples, but the specific configurations should not be considered limited to these embodiments. The scope of the present invention is indicated by the claims, not the above description, and includes all modifications that are equivalent in meaning to and within the scope of the claims.

Claims (3)

The iron-based soft magnetic powder has a phosphate-based coating formed on its surface and a silicone resin coating formed on the surface of the phosphate-based coating, and iron oxide present at grain boundaries between the iron-based soft magnetic powder,
The iron-based sintered body is characterized in that the iron oxide consists of one type of oxide and is present in 58% or more of the grain boundaries. The iron-based sintered body according to claim 1, characterized in that it has a flexural strength of 100 MPa or more. a molding step of molding a mixture containing an iron-based soft magnetic powder whose surface is coated with a phosphoric acid-based coating, the surface of which is coated with a silicone resin coating, stearic acid amide, and an inorganic solid lubricant;
a heat treatment step of heat treating the compact obtained in the molding step in water vapor,
In the heat treatment step, the compact is heat-treated in water vapor at 460°C or higher and lower than 600°C for 30 minutes or longer and 300 minutes or shorter,
In the iron-based sintered body obtained by the heat treatment step, iron oxide exists at the grain boundaries between the iron-based soft magnetic powders,
The method for producing an iron-based sintered body is characterized in that the iron oxide comprises one type of oxide and is present in 58% or more of the grain boundaries.