Iron-based sintered body and method for producing the same

Abstract

To provide an iron-based sintered body having high magnetic properties and mechanical properties.SOLUTION: A mixture for producing an iron-based sintered body contains: iron-based soft magnetic powder in which the 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 inorganic solid lubricant. A method for producing the iron-based sintered body includes a molding step of molding the mixture for producing the iron-based sintered body and a heat treatment step of heat-treating the molded body obtained in the molding step in water vapor. The iron-based sintered body contains: the iron-based soft magnetic powder with the phosphoric acid-based coating and the silicone resin coating thereon formed; and iron oxide present at grain boundaries between the iron-based soft magnetic powder. The iron oxide exists at 50% or more of the grain boundaries.SELECTED DRAWING: None

Description

[Technical Field]

[0001] 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. [Background technology]

[0002] Patent Documents 1 and 2 describe powder magnetic cores used in electromagnetic parts 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. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-4992 [Patent Document 2] Patent No. 4801734 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, there has been an ever-increasing demand for higher performance materials used in electromagnetic components.

[0005] An object of the present invention is to provide a mixture that allows for the production of an iron-based sintered body having higher magnetic properties and mechanical properties than conventional powder magnetic cores. Another object of the present invention is to provide an iron-based sintered body having higher magnetic properties and mechanical properties than conventional powder magnetic cores, and a method for producing the same. [Means for solving the problem]

[0006] The mixture for producing an iron-based sintered body described in this specification includes 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.

[0007] 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 includes 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 grain boundaries between the iron-based soft magnetic powder. The iron oxide is present in 58% or more of the grain boundaries.

[0008] 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 molded body obtained in the molding step in water vapor. [Effects of the Invention]

[0009] By using the mixture for producing an iron-based sintered body described above, an iron-based sintered body having higher magnetic properties and mechanical properties than conventional powder cores can be produced. Also, an iron-based sintered body having higher magnetic properties and mechanical properties than conventional powder cores can be provided. According to the method for producing an iron-based sintered body described above, an iron-based sintered body having higher magnetic properties and mechanical properties than conventional powder cores can be produced. [Brief explanation of the drawings]

[0010] [Figure 1A] 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. [Figure 1B] 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. [Figure 2] FIG. 1 is a graph showing the relationship between heat treatment time and bending strength. [Figure 3A] 1 is a scanning electron microscope image of a cross section of an iron-based sintered body. [Figure 3B] 1 is a scanning electron microscope image of a cross section of an iron-based sintered body. [Figure 3C] 1 is a scanning electron microscope image of a cross section of an iron-based sintered body. [Figure 4]FIG. 1 is a diagram showing the relationship between heat treatment temperature and bending strength. [Figure 5] FIG. 10 is a diagram showing the relationship between ΔL, electrical resistance, and iron loss. [Figure 6] FIG. 10 is a diagram showing the relationship between heat treatment temperature, ΔL, and iron loss. [Figure 7A] 1 is a scanning electron microscope image of a cross section of an iron-based sintered body. [Figure 7B] 1 is a scanning electron microscope image of a cross section of an iron-based sintered body, in which grain boundaries are colored. [Figure 7C] 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. [Figure 8A] This is a scanning electron microscope image of a cross section of an iron-based sintered body (high-strength material). [Figure 8B] This is a scanning electron microscope image of a cross section of an iron-based sintered body (low strength material). [Figure 9] 8C is a TEM-EDX mapping image of a partial area of ​​the iron-based sintered body shown in FIG. 8B. [Figure 10] FIG. 1 is a diagram showing the relationship between bending strength and grain boundary porosity. DETAILED DESCRIPTION OF THE INVENTION

[0011] Preferred embodiments of the present invention will now be described.

[0012] [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.

[0013] [Iron-based soft magnetic powder] Iron-based soft magnetic powders are ferromagnetic iron-based powders, specifically, pure iron powder, iron-based alloy powders (e.g., Fe-Al alloy, Fe-Si alloy, Sendust, Permalloy, etc.), and iron-based amorphous powders. 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, iron-based soft magnetic powders are not dependent on particle size distribution as long as they have the particle size used in conventional powder metallurgy. For example, iron-based soft magnetic powders may contain 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.

[0014] [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.

[0015] The phosphate coating is a glassy coating formed using a compound containing P (phosphorus), and its composition is not particularly limited. For example, it is preferable that the glassy coating is formed using a compound containing at least one of Na (sodium) and S (sulfur) in addition to the P. 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 compound is more preferably a compound containing Na and S.

[0016] The content of the above elements 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, based on 100 mass% of the iron-based soft magnetic powder having a phosphate coating and a silicone resin coating formed thereon. Even when the element contained in the phosphate coating other than P is Na, of Na and S, In SIn the case where Na and S are present, it is preferable that each of them is within this range.

[0017] Among 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 surface of the iron-based soft magnetic powder and the phosphate coating may be insufficient, which may result in an insufficient strong coating, which is undesirable. On the other hand, if the P content exceeds 1% by mass, the P that is not involved in the chemical bonding may remain unreacted, which may actually reduce the bond strength, which is undesirable.

[0018] Among the above elements, Na and S are elements that inhibit the formation of a semiconductor between Fe (iron) and oxygen during heat treatment (annealing or steam heat treatment, which will be described later), and have the effect of suppressing a decrease in resistivity. The combined addition of Na and S maximizes this effect.

[0019] The phosphate coating may further contain Mg (magnesium) and / or B (boron). The content of these elements is preferably 0.001 to 0.5 mass% for Mg and 0.001 to 0.5 mass% for B, based on 100 mass% of the iron-based soft magnetic powder having the phosphate coating and the silicone resin coating formed thereon. The phosphate coating may further contain at least one element selected from the group consisting of Co (cobalt), Cs (cesium), and Al (aluminum).

[0020] The thickness of the phosphate coating is preferably about 1 to 250 nm. If the thickness is thinner than 1 nm, the insulating effect may not be exhibited. On the other hand, if the thickness exceeds 250 nm, the insulating effect saturates and it is also undesirable from the viewpoint of increasing the density of the molded body. A more preferable thickness is 50 to 100 nm.

[0021] [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.

[0022] Examples of the solvent include water, hydrophilic organic solvents such as alcohols and ketones, and mixtures thereof. A known surfactant may also be added to the solvent.

[0023] Examples of the P-containing compound include 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, using NaH2PO4 (sodium dihydrogen phosphate) as the P or Na source results in a compact with a well-balanced density, strength, and resistivity.

[0024] The amount of the P-containing compound added to the iron-based soft magnetic powder may be adjusted so that the composition of the phosphate coating film formed falls within the above range. For example, a solution of the P-containing compound, or a compound containing an element to be included in the coating as needed, prepared so that the solid content is about 0.01 to 10 mass %, may be added in an amount of about 1 to 10 mass parts per 100 mass parts of the iron-based soft magnetic powder, and the mixture may be mixed in a known mixer, ball mill, kneader, V-type mixer, granulator, or other mixer, to thereby achieve a composition within the above range of the phosphate coating film formed.

[0025] If necessary, after the mixing step, the mixture may be dried at 150 to 250°C in the air, under reduced pressure, or in a vacuum. After drying, the mixture may be passed through a sieve with a mesh size of about 200 to 500 μm. By going through the steps above, an iron-based soft magnetic powder having a phosphate coating formed thereon can be obtained.

[0026] [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, when the mixture described below is molded, the silicone resin contained in the silicone resin coating undergoes a crosslinking and curing reaction, thereby firmly bonding the iron-based soft magnetic powder together. This results in a high-strength molded product. A single silicone resin coating may be formed on the phosphate coating, or two or more silicone resin coating layers may be formed.

[0027] Although the phosphate-based coating formed on the surface of the iron-based soft magnetic powder has high insulating properties, it is brittle and easily cracked due to the expansion and contraction of the iron-based soft magnetic powder during heat treatment (annealing or steam heat treatment, as described below). On the other hand, the silicone resin coating is more elastic than the phosphate-based coating and therefore less likely to crack during heat treatment (annealing or steam heat treatment, as described below). By forming a silicone resin coating on the phosphate-based coating, the iron-based soft magnetic powder is covered with an insulating film even during heat treatment (annealing or steam heat treatment, as described below). This results in a sintered compact with high insulating properties. Furthermore, the insulating film remains even after heat treatment, making high-temperature heat treatment possible. High-temperature heat treatment can remove defects in the iron-based soft magnetic powder, reducing iron loss and resulting in excellent magnetic properties. Furthermore, high-temperature heat treatment results in excellent mechanical properties. Furthermore, because the phosphate-based coating is covered by the silicone coating, the phosphate-based coating does not come into direct contact with the inorganic solid lubricant, as described below. Therefore, the inorganic solid lubricant does not affect the insulating properties of the phosphate-based coating.

[0028] The silicone resin used in the present invention may be a conventionally known silicone resin, and examples of commercially available products include 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 Toray Dow Corning Co., Ltd.

[0029] 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 (RSiX2:X is a hydrolyzable group), since slow-curing silicone resins result in sticky powders and poor handling 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.

[0030] The silicone resin is generally a methylphenylsilicone resin, in which 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 the inhibitory effect on compound formation 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 by FT-IR or the like.

[0031] The amount of the silicone resin coating is preferably adjusted to 0.05 to 0.3% by mass when the iron-based soft magnetic powder with the phosphate coating and the silicone resin coating formed thereon is taken as 100% by mass. If the amount of the silicone resin coating is less than 0.05% by mass, the insulating properties are poor and the electrical resistance is likely to be low. If the amount of the silicone resin coating is more than 0.3% by mass, it is difficult to achieve high density in the resulting molded body.

[0032] The thickness of the silicone resin coating is preferably 1 to 200 nm, more preferably 50 to 150 nm, and even more preferably 50 to 100 nm.

[0033] The total thickness of the phosphoric acid coating and the silicone resin coating is preferably 250 nm or less. If the total thickness exceeds 250 nm, the magnetic flux density may decrease significantly.

[0034] [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.

[0035] The amount of silicone resin added to the phosphate-coated iron powder may 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 a resin solution prepared to have a solids content of approximately 2 to 10% by mass may be added to 100 parts by mass of the phosphate-coated iron powder, mixed, and then dried. If the amount of resin solution is less than 0.5 parts by mass, mixing may take a long time or the coating may become non-uniform. On the other hand, if the amount of 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.

[0036] In the drying step, 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, so that the organic solvent is sufficiently evaporated. Specifically, in the case of the above-mentioned alcohols and petroleum-based organic solvents, a drying temperature of about 60 to 80°C is suitable. After drying, it is preferable to pass the mixture through a sieve with a mesh size of about 300 to 500 μm to remove aggregates.

[0037] After drying, it is recommended to heat the iron-based soft magnetic powder with the silicone resin coating (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 during curing of the silicone resin coating while the powder is still in powder form. This pre-cure process ensures the flowability of the silicone resin-coated iron powder during molding (e.g., approximately 100 to 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.

[0038] As described above, by pre-curing the silicone resin and then crushing it, a powder with excellent fluidity is obtained, which allows the powder to be poured into the molding die smoothly like sand during molding. If the resin is not pre-cured, for example, the powder particles may adhere to each other during molding, making it difficult to pour the powder into the molding die 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 magnetic core. The reason for this is unclear, but it is thought to be due to improved adhesion to the iron powder during curing.

[0039] When the pre-curing is carried out by a short-time heating method, the heat treatment is preferably carried out at 100 to 200°C for 5 to 100 minutes, more preferably at 130 to 170°C for 10 to 30 minutes. After the pre-curing, it is also preferable to pass the mixture through a sieve as described above.

[0040] [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.

[0041] When the above mixture is molded and then subjected to heat treatment (annealing or steam heat treatment, 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 subjected to heat treatment (annealing or steam heat treatment, 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.

[0042] 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 decreases, 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.

[0043] The mixture for producing the iron-based sintered body of the present invention may contain, in addition to stearic acid amide, other organic lubricants, such as hydrocarbon-based, fatty acid-based, higher alcohol-based, aliphatic amide-based, and ester-based organic lubricants.

[0044] Examples of hydrocarbon lubricants include liquid paraffin, paraffin wax, synthetic polyethylene wax, etc. Examples of fatty acid and higher alcohol lubricants include stearic acid and stearyl alcohol, which are relatively inexpensive and low in toxicity.

[0045] Examples of the aliphatic amide lubricant include fatty acid amides such as stearic acid amide, oleic acid amide, and erucic acid amide, and alkylene fatty acid amides such as methylene bisstearic acid amide and ethylene bisstearic acid amide.

[0046] Examples of ester-based lubricants include stearic acid monoglyceride, which is a fatty acid ester of alcohol.

[0047] [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 (MoS2). 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.

[0048] The density of the inorganic solid lubricant is preferably at least twice the density of the organic lubricant, stearic acid amide. This effectively reduces the total amount of lubricant. The density of the organic lubricant is 2.0 g / cm. 3 The density of inorganic solid lubricants is often less than 4.0 g / cm3 It is preferable that this is equal to or greater than this.

[0049] 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 unevenness of the surface of the iron-based soft magnetic powder and the gaps between the iron-based soft magnetic powder particles, making it difficult to exhibit its lubricating function. If the particle size of the inorganic solid lubricant exceeds 20 μm, the number of inorganic solid lubricant particles will be small, making it difficult to contribute to reducing friction between powder particles and between the powder and the die.

[0050] 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 an inorganic solid lubricant relative to 100% by mass of the mixture. If the content of the inorganic solid lubricant is less than 0.01% by mass, the organic lubricant stearic acid amide is not sufficiently replaced with the inorganic solid lubricant. In other words, the proportion of high-density inorganic solid lubricant is low and the proportion of low-density organic lubricant is 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 content of the inorganic solid lubricant exceeds 0.2% by mass, a large amount of lubricant must be added to maintain mold removability. In this case, the density of the compact decreases, and the saturation magnetic flux density tends to decrease. The content of the inorganic solid lubricant is more preferably 0.02% by mass or more, and even more preferably 0.025% by mass or more. Furthermore, the content of the inorganic solid lubricant is more preferably 0.2% by mass or less, and even more preferably 0.1% by mass or less.

[0051] The inorganic solid lubricant preferably accounts for 15% by mass or more and less than 25% by mass relative to 100% by mass of the total of the stearic acid amide as the organic lubricant and the inorganic solid lubricant.

[0052] 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 decreases, 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. Furthermore, the total content is more preferably 0.45% by mass or less, and even more preferably 0.4% by mass or less.

[0053] [Method of manufacturing 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.

[0054] [Molding process] The method for molding the mixture is not particularly limited, and known methods can be used. The molding conditions are not particularly limited as long as a molded body with a desired density can be 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. The density of the molded body obtained by the molding step is 7.20 g / cm. 3 It is preferable that the content is 7.50 g / cm or more. 3 It is more preferable that the density of the compact is 7.20 g / cm. With a compact having such a density, an iron-based sintered body with high magnetic properties can be obtained. For example, when compression molding is performed under conditions of 800 MPa or more, the density of the compact is 7.20 g / cm. 3 Furthermore, when compression molding is performed under conditions of 980 MPa or more, a molded product having a compressive strength of 7.50 g / cm or more can be easily obtained. 3The molding temperature can be room temperature or warm (for example, 80° C. or higher, more preferably 100 to 250° C.).

[0055] [Annealing process] After the compacting 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. Degreasing reduces iron loss, resulting in a sintered body with good magnetic properties. The annealing temperature is preferably above the decomposition temperature of stearic acid amide (approximately 200°C or higher). In addition, performing the annealing step at a high temperature makes it possible to remove distortion. This allows the production of a sintered body with high electrical insulation, i.e., high resistivity, without increasing eddy current loss (corresponding to coercive force).

[0056] The annealing step is preferably carried out at a temperature equal to or higher than 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 with heating. To prevent the phosphate coating from thinning, the annealing temperature is preferably 650°C or lower, and more preferably 600°C or lower.

[0057] The annealing time is preferably 20 minutes or more. The annealing time is more preferably 25 minutes or more, and even more preferably 27 minutes or more. From the viewpoint of degreasing and strain relief, a longer annealing time is preferable, but if high-temperature heat treatment is performed for a long time, the phosphate coating film becomes thinner, resulting in a decrease in insulating properties. Therefore, the annealing time is preferably, for example, 180 minutes or less, more preferably 60 minutes or less, and particularly preferably 35 minutes or less.

[0058] 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.

[0059] The annealing step does not have to be performed between the forming step and the heat treatment step. When the formed body is heated in the heat treatment step, it is possible to degrease and remove distortion from the formed body.

[0060] [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.

[0061] 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 be oxidized, preventing oxygen from penetrating into 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 be slowly oxidized, allowing oxygen to penetrate into 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.

[0062] The heat treatment temperature is preferably 460°C or higher, more preferably 475°C or higher, and even more preferably 480°C or higher. 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 excellent magnetic and mechanical properties. Heat treatment at temperatures below 460°C results in a slow rate of iron oxide formation, resulting in insufficient bonding of the iron-based magnetic powder particles. Consequently, high mechanical strength is not achieved. Heat treatment at temperatures above 600°C results in decomposition of the insulating phosphate coating and silicone resin coating, generating gas and creating voids at the grain boundaries. Furthermore, multiple types of iron oxide are generated as oxides, and the density difference between these generates distortion, leading to cracks within the oxides at the grain boundaries. Consequently, high mechanical strength is not achieved. Heat treatment at temperatures that are too high results in decomposition of the insulating film, resulting in a decrease in insulation and magnetic properties. From the above, the heat treatment temperature is more preferably 460°C to 590°C, even more preferably 460°C to 580°C, and even more preferably 480°C to 530°C.

[0063] Even if an annealing step is not performed before the heat treatment step, degreasing is possible by performing heat treatment at the above-mentioned temperature, since the organic lubricant stearic acid amide is removed. It is also possible to remove distortion from the compact. Therefore, a sintered body with high electrical insulation can be obtained without increasing eddy current loss (corresponding to coercive force). Therefore, the above heat treatment step may be performed after the compacting step without performing an annealing step.

[0064] The heat treatment time is preferably 30 minutes or more, 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, more preferably less than 200 minutes. Specifically, the heat treatment time is preferably 30 to 300 minutes, more preferably 60 to 200 minutes.

[0065] The sintered body obtained by the heat treatment step may be dried. The drying conditions are not particularly limited as long as the purpose can be achieved. For example, after the oxidation step, the sintered body is cooled to about 150 to 200°C, and then the water vapor is discharged. While maintaining the temperature inside the container at 100 to 300°C, an inert gas is circulated inside the container for 30 minutes to 2 hours.

[0066] [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.

[0067] 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 the iron-based soft magnetic powder having a phosphate-based coating and a silicone resin coating formed thereon. Research by the present 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 more. An iron-based sintered body having 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.

[0068] There are no particular limitations on the method for confirming the presence of iron oxide at the grain boundaries. For example, it can be confirmed from an electron microscope image of the cross section of the iron-based sintered body. The conditions, such as the type of electron image and acceleration voltage, of the electron microscope image are not particularly limited. However, in order to evaluate the average information on the oxide loading amount, it is necessary that two or more iron-based soft magnetic powders are shown in the electron microscope image, and furthermore, the amount of iron oxide present is evaluated by evaluating a grain boundary range having a length of 50% or more of the average particle size.

[0069] It is believed that 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 during the heat treatment process do not vary significantly throughout the compact (sintered body). Therefore, it is presumed that the proportion of iron oxide present at the grain boundaries is roughly the same at any position in the sintered body obtained through the heat treatment process. Therefore, if it is confirmed that iron oxide is present at 58% or more of the grain boundaries in an electron microscope image of a cross section of a sintered body, it is presumed that iron oxide is present at 58% or more of the grain boundaries in an electron microscope image of another cross section. In other words, if it is confirmed that iron oxide is present at 58% or more of the grain boundaries in an electron microscope image of a cross section of a sintered body, it is presumed that iron oxide is present at 58% or more of the grain boundaries in that sintered body.

[0070] 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 1000 to 2000. From this electron microscope image, the iron-based soft magnetic powder, iron oxide, and grain boundaries can be visually confirmed.

[0071] The density of the iron-based sintered body is 7.50 g / cm 3 It is preferable that the density is 7.51 g / cm or more. 3 A sintered body having such a density has high magnetic properties.

[0072] 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 FIG. 1B is an SEM image (magnification 2,000 times, length 43 μm × width 84 μm (= 2752 μm)) showing a part of the part shown in FIG. 1A. 2) region). In Figures 1A and 1B, the iron-based soft magnetic powder with a phosphate coating and a silicone resin coating formed on top of it is simply referred to as "iron-based soft magnetic powder." As shown in Figure 1B, iron oxide is densely present at the grain boundaries. [Example]

[0073] The present invention will be described in detail below with reference to examples. However, the following examples do not limit the present invention, and all modifications and variations within the scope of the present invention are included within the technical scope of the present invention.

[0074] (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.

[0075] [Table 1]

[0076] [Table 2]

[0077] 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 only a phosphoric acid-based coating or a silicone resin coating. No. 4 sintered body is a sintered body using 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.

[0078] Table 1 shows the flexural strength and electrical resistance of the iron-based sintered bodies Nos. 1 to 4. The flexural strength was measured in a flexural test conducted in accordance with the provisions of the Japanese Industrial Standards "JIS Z 2511" (2006 edition). The flexural test specimens used in the flexural test were prepared by filling a mixture containing iron-based soft magnetic powder and a lubricant into a 12.7 mm x 31.75 mm mold and compacting it under a compaction pressure of 392 to 1179 MPa (4 ton / cm). 2 ~12ton / cm 2 The sintered body was manufactured using a molded body obtained by compressing from above and below under pressure. In the experiments (Experiments 3 and 4) described below, when measuring the flexural strength, the molded body manufactured by the above molding method was used to manufacture the sintered body. The flexural strength was also measured under the same flexural test conditions as in this experiment.

[0079] Table 2 shows the iron loss and magnetic flux density of iron-based sintered bodies Nos. 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 mixture containing an iron-based soft magnetic powder and a lubricant into a mold having an outer diameter of φ45 mm and an inner diameter of φ337 mm, and compressing it from above and below. Note that in the experiments (Experiments 3 and 4) described below, when measuring the iron loss and magnetic flux density, the sintered bodies manufactured using the compacts made by the above-mentioned molding method were used.

[0080] If 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. If the electrical resistance of a sintered body is 60 μΩ·m or higher, it can be said that the insulation properties are good. Furthermore, if 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.

[0081] 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 film or a silicone resin film, and the insulating film was destroyed during heat treatment, causing electrical conduction between adjacent iron powder particles. 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.

[0082] On the other hand, No. 4 has a flexural strength exceeding 100 MPa and an electrical resistance of 60 μΩ·m or more, as shown in Table 1. Furthermore, No. 4 has an iron loss of 40 W / kg or less, as shown in Table 2. The No. 4 sintered body uses an iron-based soft magnetic powder with a double-layer insulating film, and has better mechanical and magnetic properties than conventional powder cores.

[0083] 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, a sintered body with both higher mechanical properties and magnetic properties than conventional powder cores can be obtained.

[0084] (Experiment 2) Iron-based sintered bodies were produced by the method described in the embodiment, using different types of lubricants under the conditions shown in Table 3. The amount of lubricant added [%] shown in Table 3 indicates the amount [unit: % (mass %)] relative to 100% by 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.

[0085] [Table 3]

[0086] 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, the organic lubricant stearic acid amide and the inorganic solid lubricant zinc oxide (ZnO) were used in combination as lubricants.

[0087] Although the addition of a lubricant can improve moldability, the density of the sintered body tends to decrease and the magnetic properties tend to deteriorate as the amount of lubricant added increases. Furthermore, as the amount of lubricant added increases, the lubricant tends to remain even after heat treatment in water vapor, and the sintered body tends to turn black after heat treatment.

[0088] 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 only metal soap) 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, a sintered body with a higher density tends to be obtained than when only metal soap is used.

[0089] 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 (compactibility) that is achieved by adding the lubricant. No. 9 and No. 10 are data for samples with the same amount of organic lubricant (stearic acid amide) added, with No. 10 also containing 0.05% ZnO. No. 9 and No. 10 show that the addition of ZnO increases the sintered body density.

[0090] From the above, it was found that when the organic lubricant stearic acid amide and the inorganic solid lubricant are used together as lubricants, a sintered body with a higher density tends to be obtained than when only the organic lubricant metal soap or stearic acid amide is used. From this, it is thought that by using the organic lubricant stearic acid amide and the inorganic solid lubricant together as lubricants, it is possible to obtain good moldability due to the lubricant while also improving the magnetic properties.

[0091] Regarding the change in color before and after the heat treatment process, the sintered bodies Nos. 1 to 6 (which used only metal soap) turned black, but the color of the sintered body No. 10 (which used both stearic acid amide and ZnO) remained unchanged. This suggests that stearic acid amide is easily removed by the steam heat treatment and is unlikely to remain after the heat treatment process.

[0092] From the above, it was found that by using stearic acid amide as an organic lubricant and using stearic acid in combination with an inorganic solid lubricant, a sintered body with high magnetic properties and that does not turn black after steam heat treatment can be obtained.

[0093] (Experiment 3) The mechanical and magnetic properties were investigated when the conditions of the heat treatment process in water vapor were changed.

[0094] Iron-based sintered bodies were produced by the method described in the embodiment under different steam heat treatment conditions as shown in Table 4A, and their mechanical properties 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 for producing the iron-based sintered body. Table 4A also shows the bending strength of the sintered bodies.

[0095] [Table 4A]

[0096] <Heat treatment time and mechanical properties> The sintered bodies Nos. 5 to 7 and 12 in Table 4A were obtained by heat treating at a temperature of 550°C for a heat treatment time that varied 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.

[0097] 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, and that the flexural strength remains roughly the same for heat treatments of 60 minutes or more. 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. Figure 2 suggests that the flexural strength will reach 100 MPa or more for heat treatments of 20 minutes or more.

[0098] From the above, in order to improve the mechanical properties compared to conventional powder magnetic cores, the heat treatment time is preferably 20 minutes or more, and more preferably 30 minutes or more. There is no particular upper limit to the heat treatment time, but it is preferably, for example, 300 minutes or less.

[0099] <Cross section of sintered body> FIG. 3A shows an example of a scanning electron microscope image (SEM image) (magnification 100 times) 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 examples of scanning electron microscope images (SEM images) of a narrow cross section of a sintered body. The magnification of the image shown in FIG. 3B is 20,000 times, and the magnification of the image shown in FIG. 3C is 5,000 times. The image shown in FIG. 3C is 17 μm long × 25 μm wide (= 425 μm). 2 ) are photographs of the area. As shown in Figures 3B and 3C, grain boundaries exist between the iron-based soft magnetic powder. In the image shown in Figure 3B, iron oxide is present in most parts of the grain boundaries. In the image shown in Figure 3C, there is little iron oxide present at the grain boundaries. Figures 3B and 3C show 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.

[0100] <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 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, indicated by a white triangle (△). 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 also shown by a white diamond (◇), and the correlation line between the heat treatment temperature and flexural strength for Nos. 2 to 4 is indicated by a dashed line.

[0101] The sintered compacts No. 8 to No. 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, and were obtained by heat treating for 120 minutes at temperatures ranging from 450°C to 650°C. Figure 4 shows the relationship between the heat treatment temperature and flexural strength for Nos. 8 to 19 in Table 4 with black circles (●). Figure 4 also shows the correlation line between the heat treatment temperature and flexural strength for Nos. 8 to 19 in Table 4 with a solid line.

[0102] The following was learned from Figure 4. 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 heat treatment temperatures of around 460°C, and reaching a very high flexural strength of 110 MPa or more at heat treatment temperatures 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.

[0103] It is presumed that when the heat treatment temperature is less than 460°C, the iron oxide does not grow sufficiently, and the grain boundaries are hardly 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 thought that when the heat treatment temperature is less than 460°C, the bending strength is not high.

[0104] From the above, it was found that in order to increase the bending 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 lower than 590°C. It was also found that setting the heat treatment temperature to 480°C or higher but lower than 530°C is more preferable because it results in a high-strength sintered body with a bending strength of 110 MPa or higher.

[0105] On the other hand, in the case of a single insulating layer (single layer of phosphoric acid-based coating), it was found that the bending strength rapidly decreased as the heat treatment temperature increased above 475°C. t It is believed that the temperature range in which a high-strength sintered body can be obtained is narrower for both the single-layer phosphoric acid coating and the single-layer organic silicone coating than for the double-layer insulating film. This is presumably because, in the case of a single-layer insulating film, the insulating film decomposes upon heating, resulting in peeling and an increase in grain boundary voids due to degassing, which reduces the flexural strength.

[0106] <Heat treatment temperature and magnetic properties> The magnetic properties are typically evaluated by creating a ring-shaped test piece and winding 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 the magnetic properties using ΔL here. Below, we will explain ΔL before explaining the relationship between heat treatment temperature and magnetic properties.

[0107] The inductance L is expressed by the following formula:

number

number

number

number

number

number

[0108] 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 at 40 W / kg or less, and from Figure 5 we can determine that in order to keep iron loss at 40 W / kg or less, ΔL should be kept at 5% or less.

[0109] 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 Nos. 1 to 4, 8 to 9, and 11 to 14. The ΔL and electrical resistance of Nos. 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 Nos. 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.

[0110] [Table 4B]

[0111] In Figure 6, the relationship between the heat treatment temperature and the ΔL value for Nos. 8 to 16 in Table 4 is shown by black circles (●), and the relationship between the heat treatment temperature and the iron loss for Nos. 8, 9, and 11 to 14 is shown by black triangles (▲). Furthermore, for reference, Figure 6 also shows the relationship between the heat treatment temperature and the 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, by white circles (○), and the relationship between the heat treatment temperature and the 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, by white triangles (△).

[0112] The following was learned from Figure 6. For sintered compacts 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 as the heat treatment temperature increases above approximately 450°C. Figure 5 shows that the larger ΔL, the lower the electrical resistance. Low electrical resistance indicates poor insulation. For sintered compacts 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 large 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.

[0113] 4, when using an iron-based soft magnetic powder with a double-layer insulating layer, it is preferable to set the heat treatment temperature to 460°C or higher but lower than 600°C in order to increase the flexural strength compared to conventional powder cores. However, when using an iron-based soft magnetic powder with a single-layer phosphate-based insulating film, when the heat treatment temperature is 460°C or higher, it is thought that ΔL exceeds 5% and the iron loss is large, at about 50 W / kg, and that the iron loss also increases as the heat treatment temperature increases. Therefore, when using an iron-based soft magnetic powder with a single-layer phosphate-based insulating film, it is thought that a sintered body with poor magnetic properties will be obtained when the heat treatment temperature is 460°C or higher but lower than 600°C.

[0114] 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. This suggests that when the insulating film has a two-layer structure, heat treatment temperatures below 600°C result in little breakdown of the insulating film and little degradation of insulation. Although the degradation of insulation is small at heat treatment temperatures below 600°C, it is thought that the gradual degradation of insulation increases 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 from the iron-based soft magnetic powder increases as the heat treatment temperature increases, and the degradation of insulation 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. This shows that when an iron-based soft magnetic powder having a two-layer insulating film is used, an iron-based sintered body with high magnetic properties can be obtained when the heat treatment temperature of the water vapor heat treatment is less than 600°C (for example, less than 590°C).

[0115] 4, it can be seen that when an iron-based soft magnetic powder with a double-layer insulating film is used, the heat treatment temperature should preferably be 460°C or higher but lower than 600°C in order to increase the flexural strength compared to conventional powder cores. From the above, it can be seen that when an iron-based soft magnetic powder with a double-layer insulating film is used, ΔL is 5% or lower when the heat treatment temperature is lower than 600°C, and that the iron loss decreases as the heat treatment temperature increases. Therefore, it can be seen that when an iron-based soft magnetic powder with a double-layer insulating film is used, 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).

[0116] <Phosphate coating thickness> The sintered compacts No. 10 and No. 17-19 in Table 4B were obtained by heat treating at 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. 17-19 indicate that ΔL exceeds 5% when the phosphate coating thickness (phosphate coating thickness) is 40 nm or less. This suggests that a phosphate coating thickness of 40 nm or less results in insufficient insulation and poor magnetic properties. Therefore, it is preferable for the phosphate coating thickness to be 50 nm or more.

[0117] (Experiment 4) Using the conditions shown in Table 5, iron-based soft magnetic powder with a double-layer insulating film was used to fabricate iron-based sintered bodies with a flexural strength of 100 MPa or more (high-strength material) and iron-based sintered bodies with a flexural strength of less than 100 MPa (low-strength material), and the state of the grain boundaries was investigated. 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. Iron-based sintered bodies were also fabricated 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 a lubricant. Table 5 shows the fabrication conditions and the state of the grain boundaries of the iron-based sintered bodies.

[0118] [Table 5]

[0119] 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 using the following procedure from SEM images of the cross section of the sintered body taken at a magnification of 2,000x (measurement range: 43 μm vertical × 64 μm horizontal area) so that the cross section of the grain boundary where two or more iron powder particles are adjacent to each other could be photographed. FIG. 7A shows an example of an SEM image (original image) of a cross section of a sintered body. FIG. 7B shows an image 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 of ​​the grain boundary (colored portion) (grain boundary area) is determined from FIG. 7B by image analysis. FIG. 7C shows the grain boundary voids (portions without iron oxide) colored. The area of ​​the grain boundary voids (colored portion) (void area) 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 "grain boundary iron oxide abundance rate" in Table 5 was calculated by 100 - grain boundary porosity [unit: %].

[0120] As shown in Table 5, in a high-strength material (hereinafter sometimes simply referred to as "high-strength material") that uses an iron-based soft magnetic powder with a double-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") that uses an iron-based soft magnetic powder with a double-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%.

[0121] 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 the oxides in order to compare the oxides at the grain boundaries. In the high-strength material, only one type of oxide (Fe3O4) was detected at the grain boundaries. The image in Figure 8A confirms 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.

[0122] 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 there are many cracks in the oxides. This is thought to be because the density of the two types of oxides differs, causing distortion, which in turn caused cracks to form 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 bonding between the iron-based soft magnetic powder particles.

[0123] Figure 9 shows a TEM-EDX mapping image (TEM: transmission electron microscope, EDX: energy dispersive X-ray spectroscopy) of a portion of the area shown in Figure 8B. Figure 9 reveals that the insulating film peeled off at the interface of the iron-based soft magnetic powder. Furthermore, elemental analysis using TEM-EDX mapping revealed that phosphorus (P) from the first phosphate-based coating that covered the iron-based magnetic powder was detected partially near the powder interface. Furthermore, silicon (Si) from the second silicone resin coating was detected throughout the grain boundary oxide. This suggests that the phosphate-based coating that covered the iron-based magnetic powder was decomposed and destroyed by the steam heat treatment. Furthermore, it is believed that the second silicone resin coating was completely decomposed by the steam heat treatment. It is believed that gases such as CO2, CO, and HO 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.

[0124] Figure 10 shows the relationship between the flexural strength and grain boundary porosity of the sintered body shown in Table 5. Figure 10 also shows the correlation line between flexural strength and grain boundary porosity. From Figure 10, it can be seen 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 in the 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 with a bending strength of 100 MPa or more. 。

[0125] 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, in which the proportion of iron oxide at grain boundaries is 58% or more and the bending strength is 100 MPa, can be obtained.

[0126] Although the embodiments of the present invention have been described above based on examples, it should be understood that the specific configurations are not limited to these embodiments. The scope of the present invention is defined by the claims rather than the above description, and includes all modifications within the meaning and scope of the claims.

Claims

1. The iron-based soft magnetic powder has a surface on which a phosphate-based coating is formed 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.

2. 2. The iron-based sintered body according to claim 1, wherein the flexural strength is 100 MPa or more.

3. 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.

Images