Magnetic body and magnetic body production method

Abstract

The present invention pertains to a magnetic body that: includes a green compact that includes triiron tetraoxide and an iron-based powder that has an insulating film on at least part of the surface thereof; has a cylindrical shape; includes a region where an internal diameter magnetic path is formed on an inner diameter surface includes a surface covered in an iron oxide film that includes triiron tetraoxide; and is used for sensors used at operating frequencies of at least 100 kHz.

Description

Magnetic body and method for manufacturing the same

[0001] The present invention relates to a magnetic body and a method for manufacturing the magnetic body.

[0002] In the mechatronics field, such as industrial machinery and automobiles, measuring displacement and torque is important for achieving high-precision control. A known measurement technique is magnetostrictive sensors that utilize the magnetostrictive effect and incorporate ferromagnetic materials.

[0003] For example, Patent Document 1 discloses a torque sensor 1, in which a sensor unit 2 of the torque sensor 1 includes a coil 21 and a magnetic ring 22. The sensor unit 2 of the torque sensor 1 is attached to the periphery of a magnetostrictive torque sensor shaft 101 having magnetostrictive properties.

[0004] JP 2018-112451 A JP 2020-150135 A

[0005] Generally, the magnetic body used in a sensor is integrated with a resin bobbin around which a coil is wound by a resin mold. For example, when a sensor is operated in a high frequency band of 100 kHz or more, it is desirable for the magnetic body to have high complex permeability and low iron loss. For this reason, low iron loss powder cores or ferrite cores are used as the magnetic body material. In order to suppress eddy current loss in the magnetic body, it is effective to reduce the cross-sectional area of ​​the magnetic path. In the case of a ring-shaped magnetic body, it is possible to reduce the thickness, but if it is made thinner, the magnetic body becomes more susceptible to cracking.

[0006] One possible way to improve the strength of magnetic materials is to treat them with steam, but as described in paragraph 0030 of Patent Document 2, steam treatment forms triiron tetroxide, which has high electrical conductivity, on the particle surface, and when an oxide coating containing triiron tetroxide is formed, cracks occur in the oxide coating on the particle surface. As a result, in applications with high frequencies of 100 kHz or more, iron loss due to eddy currents inside the particles becomes high, making them unusable.

[0007] An object of the present invention is to provide a magnetic body having good magnetic properties and high strength even at operating frequencies of 100 kHz or more. Another object of the present invention is to provide a method for manufacturing a magnetic body having good magnetic properties and high strength even at operating frequencies of 100 kHz or more.

[0008] According to an embodiment of the present invention, by including a surface covered with an iron oxide film containing triiron tetroxide in the region of the inner diameter surface of the powder compact after steam treatment where a magnetic path is formed, eddy current loss in the entire powder compact is suppressed, and it is possible to realize a magnetic material suitable for use in sensors used at operating frequencies of 100 kHz or more.

[0009] Examples of embodiments of the present invention are listed below. The present invention is not limited to the following embodiments. One embodiment relates to a magnetic body for use in a sensor used at an operating frequency of 100 kHz or more, the magnetic body including a compact containing an iron-based powder having an insulating coating on its surface and triiron tetroxide, the magnetic body having a cylindrical shape, an inner diameter surface region where a magnetic path is formed including a surface covered with an iron oxide film containing triiron tetroxide. Another embodiment relates to a method for producing a magnetic body, the method including: compression-molding a raw material powder containing an iron-based powder having an insulating coating on its surface to obtain a compact; and heat-treating the compact in a water vapor atmosphere, the magnetic body having a cylindrical shape, an inner diameter surface region where a magnetic path is formed including a surface covered with an iron oxide film containing triiron tetroxide, the magnetic body being used in a sensor used at an operating frequency of 100 kHz or more.

[0010] According to the present invention, a magnetic body having good magnetic properties and high strength is provided. According to the present invention, a method for producing a magnetic body having good magnetic properties and high strength is provided.

[0011] Fig. 1 is a schematic diagram showing an example of a magnetic body, and Fig. 2 is an optical microscope photograph of a cross section of the magnetic body produced in the example.

[0012] Preferred embodiments of the present invention will be described below, but the present invention is not limited to the following embodiments.

[0013] In the present specification, numerical ranges indicated using "to" include the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In numerical ranges described in stages in the present specification, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another staged numerical range. Furthermore, in numerical ranges described in the present specification, the upper or lower limit value of that numerical range may be replaced with a value shown in the Examples. In the present specification, each component may contain multiple corresponding substances. When multiple substances corresponding to each component are present in the composition, the content or amount of each component means the total content or amount of the multiple substances present in the composition, unless otherwise specified. In the present specification, the term "coating" includes cases where, when observing an area where the coating is present, the coating is formed over the entire area, as well as cases where the coating is formed only on a portion of the area. In the present specification, the term "process" includes not only independent processes but also processes that cannot be clearly distinguished from other processes, as long as the desired effect is achieved.

[0014] <Method for manufacturing magnetic body> A method for manufacturing a magnetic body according to an embodiment of the present invention includes compression-molding a raw material powder containing an iron-based powder having an insulating coating on its surface to obtain a green compact, and heat-treating the green compact in a water vapor atmosphere. The method for manufacturing a magnetic body according to an embodiment of the present invention is a method for manufacturing a magnetic body for use in a sensor used at an operating frequency of 100 kHz or more (sometimes referred to as a "magnetic body for sensors"). The method for manufacturing a magnetic body for sensors may further include an optional step.

[0015] In some embodiments, the magnetic body has a cylindrical shape, and a region of the inner diameter surface where the magnetic path is formed may include a surface covered with an iron oxide film containing triiron tetroxide. In some embodiments, the magnetic body has a cylindrical shape, and the radial thickness of the region where the magnetic path is formed may be 3 mm or less.

[0016] [Forming Step] The method for producing a magnetic body for a sensor includes compression-molding a raw material powder containing an iron-based powder having an insulating coating on its surface to obtain a green compact (sometimes referred to as the "forming step").

[0017] (Raw material powder) The raw material powder contains an iron-based powder having an insulating coating on its surface (sometimes simply referred to as "iron-based powder"). The iron-based powder having an insulating coating on its surface contains a metal powder containing iron and an insulating coating that covers at least a portion of the surface of the metal powder. The insulating coating can suppress eddy currents. The raw material powder may contain one type of iron-based powder alone or two or more types in combination.

[0018] The metal powder may contain, for example, at least one selected from the group consisting of iron powder and iron-containing alloy powder. The metal powder may be a powder consisting of at least one selected from the group consisting of iron powder and iron-containing alloy powder. The iron-containing alloy may include at least one selected from the group consisting of a solid solution, a eutectic, and an intermetallic compound. The alloy may be, for example, stainless steel (such as an Fe—Cr alloy or an Fe—Ni—Cr alloy). The metal powder may be amorphous. The metal powder may contain multiple metal elements. For example, the metal powder may contain, in addition to iron, at least one element selected from the group consisting of a base metal element, a noble metal element, a transition metal element, and a rare earth element.

[0019] The alloy powder containing iron may be, for example, a soft magnetic alloy containing iron as a main component. The iron content in the metal powder may be, for example, 50 mass % or more, 70 mass % or more, or 90 mass % or more, based on the mass of the metal powder. Specifically, the metal powder may contain at least one selected from the group consisting of Fe—Si alloy powder, Fe—Si—Al alloy powder, Fe—Ni alloy powder, Fe—Cu—Ni alloy powder, Fe—Co alloy powder, Fe—Cr—Si alloy powder, and iron-based amorphous alloy powder. From the viewpoint of maintaining the iron oxide film that will be formed later on the inner diameter surface, pure iron, which is a material with high dimensional stability, is preferred.

[0020] The shape of each metal particle contained in the metal powder is not particularly limited, and may be, for example, flat, spherical, or acicular.

[0021] The average particle size of the iron-based powder is, for example, 60 μm or more and 250 μm or less. The average particle size of the metal powder can be measured according to the Metal Powder - Particle Size Test Method by Dry Sieving (ISO 4497:2020). When the average particle size of the iron-based powder is 60 μm or more, sufficient strength of the magnetic body tends to be ensured. The average particle size may be 80 μm or more, or 100 μm or more. When the average particle size of the iron-based powder is 250 μm or less, a magnetic body having high density and exhibiting good magnetic properties is likely to be obtained. The average particle size may be 220 μm or less, or 200 μm or less. The average particle size of the iron-based powder may be, for example, 80 to 220 μm, or 100 to 200 μm.

[0022] The insulating coating may include at least one coating selected from the group consisting of inorganic compound coatings and organic compound coatings. The insulating coating may be a single-layer coating or a coating including multiple layers. The inorganic compound coating contains at least one selected from the group consisting of phosphates such as aluminum phosphate, zinc phosphate, iron phosphate, manganese phosphate, and calcium phosphate; borates such as lithium borate, magnesium borate, and calcium borate; silicates such as aluminum silicate, potassium silicate, and calcium silicate; carbonates such as lithium carbonate, sodium carbonate, aluminum carbonate, calcium carbonate, and barium carbonate; and oxides such as silicon oxide and tungsten oxide. The organic compound coating contains at least one selected from the group consisting of silicone resin, polyimide resin, polyamide resin, polyamideimide resin, phenolic resin, epoxy resin, and fatty acid.

[0023] From the viewpoint of achieving both good magnetic properties and high strength, the insulating coating is preferably a coating containing phosphate. The insulating coating may be formed on a part of the surface of each metal particle contained in the metal powder, or on the entire surface. That is, the iron-based powder has an insulating coating on at least a part of the surface, or on the entire surface.

[0024] The raw material powder may contain optional powders in addition to the iron-based powder. Examples of optional powders include soft magnetic powders without an insulating coating; metal powders, alloy powders, and metal oxides containing chromium, copper, or the like as their main components; and additives such as powder lubricants and flowability improvers such as dry silica fine particles. Among these, it is preferable to contain a powder lubricant in order to reduce the ejection pressure after compaction.

[0025] Examples of powder lubricants include metal soaps such as zinc stearate and calcium stearate; and amide-based lubricants such as stearic acid amide, stearic acid bisamide, and ethylene bisstearic acid amide. Because of their excellent degreasing properties, it is preferable to use a compound with a small molecular weight as the powder lubricant. Powder lubricants may be used singly or in combination of two or more.

[0026] The content of the powder lubricant in the raw powder is preferably 0.1 mass% or more, more preferably 0.2 mass% or more, and even more preferably 0.3 mass% or more, based on the mass of the iron-based powder, from the viewpoint of obtaining sufficient effects when used. The content of the powder lubricant is preferably 3 mass% or less, more preferably 2 mass% or less, even more preferably 1 mass% or less, and particularly preferably 0.5 mass% or less, based on the mass of the iron-based powder, from the viewpoint of obtaining sufficient magnetic properties. The content of the powder lubricant may be, for example, 0.1 to 2 mass%, 0.2 to 1 mass%, or 0.3 to 0.5 mass%.

[0027] (Compression Molding) The raw material powder is molded by compression molding to obtain a green compact. For example, the raw material powder is compressed by upper and lower punches. During molding, a die lubricant may be applied to the die to perform die lubrication molding. The shape of the green compact is, for example, cylindrical, and the cylindrical shape may be hollow.

[0028] The compacting pressure is, for example, 400 MPa or more, 500 MPa or more, or 600 MPa or more from the viewpoint of obtaining a green compact with high precision and obtaining good magnetic properties. The compacting pressure is, for example, 1,000 MPa or less, 900 MPa or less, or 800 MPa or less from the viewpoint of mold durability, etc. The compacting pressure may be, for example, 400 to 1,000 MPa, 500 to 900 MPa, or 600 to 800 MPa.

[0029] During molding, the mold may be heated, or the raw material powder and the mold may be heated, and warm molding may be performed. The heating temperature is, for example, 80 to 140°C.

[0030] (Density of Green Compact) From the viewpoint of obtaining good magnetic properties, the density of the green compact is, for example, 6.5 g / cm ３ Above, 6.8g / cm ３ or more, or 7.2 g / cm ３ The density of the powder compact is, for example, 7.8 g / cm ３ Below, 7.6g / cm ３ or less, or 7.4 g / cm ３ The density of the green compact is 7.2 g / cm ３ If the density is above this, water vapor does not penetrate into the powder compact during the steam treatment stage, and an iron oxide film is likely to form on the surface. Eddy current loss in the powder compact as a whole is suppressed. The density of the powder compact can be measured in accordance with the method for measuring uniaxial compressibility of metal powders excluding those for cemented carbide (ISO 3927:2001). The density of the powder compact is, for example, 6.5 to 7.8 g / cm ３ , 6.8-7.6g / cm ３ , or 7.2 to 7.4 g / cm ３ It may be.

[0031] [Water Vapor Treatment Step] The method for producing a magnetic body for a sensor includes heat treating the compact in a water vapor atmosphere (sometimes referred to as a "water vapor treatment step").

[0032] By heat treating the powder compact in a water vapor atmosphere, triiron tetroxide (Fe) is formed on at least a part of the surface of the iron-based powder contained in the powder compact. ３ O ４) is formed. Triiron tetroxide is usually formed on the surface of the iron-based powder exposed to water vapor. Because the individual iron-based particles contained in the iron-based powder are in close contact with each other, the iron-based powder may also include surfaces that are not exposed to water vapor. Triiron tetroxide is usually not formed on the surface of the iron-based powder that is not exposed to water vapor.

[0033] The heat treatment temperature is, for example, 300 to 600°C. The heat treatment time is, for example, 5 to 60 minutes. The heat treatment in a water vapor atmosphere may involve exposing the powder compact to high-temperature water vapor. An example of a method for exposing the powder compact to water vapor is a method in which water vapor is sprayed onto the powder compact. The water vapor penetrates from the surface of the powder compact into the internal pores, and triiron tetroxide is produced on the surface of the powder compact and at the interface between the iron-based powder and the pores. The water vapor temperature is, for example, 300 to 600°C. The exposure time to water vapor is, for example, 5 to 60 minutes. From the viewpoint of producing an iron oxide film containing triiron tetroxide concentrated on the surface of the powder compact, it is preferable to perform the treatment at a high temperature of 500°C or higher for a short period of time of, for example, 30 minutes or less.

[0034] For the heat treatment in a water vapor atmosphere, a pressurized furnace or a normal pressure furnace can be used.Specific examples include a pot-type furnace capable of maintaining a high pressure, and a belt furnace such as a mesh belt furnace.

[0035] The magnetic body obtained by heat-treating the green compact in a water vapor atmosphere has high strength. It is believed that the strength of the magnetic body is enhanced by the triiron tetroxide produced by the oxidation of the iron-based powder. A magnetic body with high strength can withstand the pressure during resin molding and can prevent cracks from occurring during molding.

[0036] The magnetic body obtained by heat-treating the powder compact in a water vapor atmosphere has a surface covered with an iron oxide film containing triiron tetroxide. The thickness of the iron oxide film containing triiron tetroxide can be adjusted by changing the conditions, such as the atmosphere, temperature, and time, when the powder compact is heat-treated in a water vapor atmosphere.

[0037] [Optional Steps] Optional steps that may be included in the manufacturing method of a magnetic material for a sensor include preparing a raw material powder, heat-treating the compact (heat treatment step), cutting the compact into a desired shape (cutting step), etc.

[0038] (Heat Treatment Step) In the heat treatment step, heat is applied to the powder compact to impart desired properties to the powder compact depending on the application. For example, the heat treatment can improve properties such as strength, hardness, and impact resistance. The heat treatment here does not include heat treatment of the powder compact in a water vapor atmosphere.

[0039] The atmosphere in which the heat treatment is performed may be air, but a gas atmosphere in which the powder compact is less likely to be oxidized is preferred. For example, the atmosphere can be selected from non-oxidizing gases such as nitrogen gas, reducing gases such as decomposed ammonia gas, and carburizing gases (e.g., a mixed gas of hydrogen, nitrogen, and carbon monoxide with a carbon potential in the range of 0.1 to 1.2%). The temperature of the heat treatment is, for example, 300 to 600°C. The time of the heat treatment is, for example, 5 to 60 minutes.

[0040] In a preferred embodiment, the method for producing a magnetic body includes a heat treatment step between the compacting step and the steam treatment step. The method for producing a magnetic body may include: compression-molding a raw material powder to obtain a green compact; heat-treating the green compact; and heat-treating the green compact in a steam atmosphere.

[0041] (Cutting step) In the cutting step, the powder compact is cut into a desired shape. For example, the cutting may be a turning process, a milling process, or both. Examples of materials for cutting tools include cermet, ceramics, cemented carbide, high-speed tool steel, sintered diamond, and sintered cBN. In the cutting step, it is preferable to machine, for example, the outer peripheral surface or the bottom surface of the cylindrical powder compact.

[0042] As described below, the magnetic body may include a textured surface. To obtain a magnetic body including a textured surface, the manufacturing method for the magnetic body may not include cutting the powder compact, or may include cutting only a portion of the surface of the powder compact. When the powder compact is cylindrical, for example, the portion of the surface may be the outer circumferential surface of the cylindrical powder compact.

[0043] As will be described later, the magnetic body may have a cylindrical shape, and the region of the inner diameter surface where the magnetic path is formed may include a surface covered with an iron oxide film containing triiron tetraoxide. To obtain a magnetic body where the region of the inner diameter surface where the magnetic path is formed includes a surface covered with an iron oxide film containing triiron tetraoxide, the manufacturing method for the magnetic body may not include cutting the powder compact, or may include cutting only a portion of the surface of the powder compact. When the powder compact is cylindrical, for example, the portion of the surface may be the outer circumferential surface of the cylindrical powder compact.

[0044] [Magnetic body] The magnetic body obtained by the manufacturing method of the magnetic body for sensors includes a compacted powder heat-treated in a water vapor atmosphere. The magnetic body is used in sensors that operate at an operating frequency of 100 kHz or more. The description of the magnetic body for sensors according to the embodiment of the present invention, which will be described later, can also be applied to the magnetic body obtained by the manufacturing method of the magnetic body for sensors.

[0045] <Magnetic Body for Sensors> The magnetic body according to an embodiment of the present invention is a magnetic body made of iron-based powder having an insulating coating on the surface thereof and triiron tetroxide (Fe ３ O ４ The magnetic body for the sensor may further contain an optional component. The above-mentioned descriptions regarding the iron-based powder, the compact, the magnetic body, etc. can also be applied to the magnetic body of this embodiment.

[0046] The presence of triiron tetroxide in the powder compact can be confirmed, for example, by X-ray photoelectric spectroscopy (XPS). If a peak appears between binding energies of 711 eV and 708 eV in a spectrum obtained by X-ray photoelectric spectroscopy (XPS), it can be said that the powder compact contains triiron tetroxide.

[0047] The magnetic body according to an embodiment of the present invention is a magnetic body for use in a sensor used at an operating frequency of 100 kHz or more. The operating frequency may be 200 kHz or more. The upper limit of the operating frequency is not particularly limited, but is, for example, 1 MHz or less. The sensor may be a magnetostrictive torque sensor.

[0048] The magnetic body has a cylindrical shape, for example, and the cylindrical shape may be hollow. FIG. 1 is a schematic diagram showing an example of a magnetic body. (a) is a schematic front view, and (b) is a schematic cross-sectional view of the magnetic body shown in (a) taken along the line A-A. In (b), t represents the radial thickness of the region where a magnetic path is formed. The magnetic body shown in FIG. 1 is disposed outside a resin bobbin around which a coil is wound, and is used in a state where it is attached to the periphery of a shaft for a torque sensor together with the resin bobbin.

[0049] The radial thickness of the region where the magnetic path of the magnetic material is formed is, for example, 8 mm or less, or 5 mm or less. The thinner the radial thickness of the region where the magnetic path is formed, the more likely it is that eddy current loss can be suppressed. From the viewpoint of obtaining good magnetic properties, the radial thickness of the region where the magnetic path is formed is preferably 3 mm or less. From the viewpoint of strength, the radial thickness of the region where the magnetic path is formed is, for example, 1 mm or more.

[0050] At least a portion of the surface of the magnetic body may be a molded surface. Preferably, the region of the inner diameter surface of the magnetic body where a magnetic path is formed includes the molded surface. In this specification, the region where a magnetic path is formed may be a region where a magnetic path is formed by the magnetic body and a magnetostrictive material (e.g., a shaft) to which torque is applied when the magnetic body is incorporated into a sensor and used. In this specification, the region where a magnetic path is formed on the inner diameter surface refers to the portion of the inner diameter surface of the magnetic body that corresponds to the region where a magnetic path is formed.

[0051] The molded surface refers to a surface that has not been subjected to machining or other processing after being compression molded and extruded from a mold. When the region where the magnetic path is formed on the inner diameter surface is a molded surface, eddy current loss tends to be suppressed.

[0052] Whether or not a powder compact has a shaped surface can be confirmed by determining whether or not an insulating coating is present on the surface of the powder compact before the steam treatment. The presence or absence of an insulating coating can be determined, for example, by measuring the resistivity of the surface of the powder compact. The resistivity of the shaped surface is high, and the resistivity of a surface that has been subjected to machining, such as cutting, tends to be similar to that of the iron-based powder. Whether or not a magnetic body obtained after steam treatment of the powder compact has a shaped surface can be confirmed by determining whether or not triiron tetroxide is present on the surface of the magnetic body. The presence or absence of triiron tetroxide can be determined, for example, by visual observation. If triiron tetroxide is present, the surface of the magnetic body will appear dark gray or black.

[0053] The magnetic body may have a cylindrical shape, and the region of the inner diameter surface where the magnetic path is formed may include a surface covered with an iron oxide film containing triiron tetraoxide. When the magnetic body has an iron oxide film containing triiron tetraoxide in the region of the inner diameter surface where the magnetic path is formed, eddy current loss tends to be suppressed. The ratio of the area of ​​the surface covered with the iron oxide film containing triiron tetraoxide to the area of ​​the region of the inner diameter surface where the magnetic path is formed is most preferably 100%, but may be, for example, 90% or more, or 95% or more depending on the application. The ratio of the area of ​​the surface covered with the iron oxide film containing triiron tetraoxide to the area of ​​the region of the inner diameter surface where the magnetic path is formed can be calculated, for example, from the area of ​​the surface that has not been subjected to cutting or the like and the area of ​​the surface that has been subjected to cutting or the like.

[0054] The thickness of the iron oxide film containing triiron tetroxide is, for example, 1 μm or less. The thickness of the iron oxide film containing triiron tetroxide can be measured by taking an SEM photograph and enlarging it as appropriate. When the thickness of the iron oxide film containing triiron tetroxide is 2 μm or less, eddy current loss tends to be suppressed. The thickness may be 3 μm or less, or 5 μm or less. When the thickness of the iron oxide film containing triiron tetroxide is 0.1 μm or more, it has high density and is likely to obtain good magnetic properties. The thickness of the iron oxide film containing triiron tetroxide may be, for example, 0.1 to 5 μm, 0.1 to 3 μm, or 0.1 to 1 μm.

[0055] When the green compact is heat-treated in a water vapor atmosphere, triiron tetroxide, which has high electrical conductivity, is formed on the surface of the iron-based powder, and cracks may occur in the insulating coating on the surface of the iron-based powder. Therefore, it has been conventionally thought that when the resulting magnetic body is used at a high operating frequency of 100 kHz or more, iron loss due to eddy currents will be high. According to some embodiments of the present invention, the region on the inner diameter surface of the magnetic body where a magnetic path is formed includes a surface covered with an iron oxide film containing triiron tetroxide, thereby suppressing eddy currents and enabling the magnetic body to be used at a high operating frequency of 100 kHz or more.

[0056] The magnetic body for sensors can be manufactured by, for example, the manufacturing method of the magnetic body for sensors according to the above embodiment.

[0057] <Examples of Embodiments> Preferred examples of embodiments of the present invention are listed below: The embodiments of the present invention are not limited to the following examples.

[0058] (1) A magnetic body for a sensor used at an operating frequency of 100 kHz or more, comprising a compact containing an iron-based powder having an insulating coating on its surface and triiron tetroxide. (2) The magnetic body according to (1) above, having a cylindrical shape, and a region of the inner diameter surface where a magnetic path is formed includes a surface covered with an iron oxide film containing triiron tetroxide. (3) The magnetic body according to (2) above, in which the area of ​​the surface covered with the iron oxide film containing triiron tetroxide is 90% or more of the area of ​​the inner diameter surface where the magnetic path is formed. (4) The magnetic body according to (2) or (3) above, in which the thickness of the iron oxide film containing triiron tetroxide is 5 μm or less. (5) The magnetic body according to any one of (1) to (4) above, having a cylindrical shape, and a radial thickness of the region where the magnetic path is formed is 3 mm or less. (6) The magnetic body according to any one of (1) to (5) above, having a cylindrical shape, and a region of the inner diameter surface where the magnetic path is formed includes a molded surface. (7) The magnetic body according to any one of (1) to (6) above, wherein the sensor is a magnetostrictive torque sensor.

[0059] (8) A method for manufacturing a magnetic body for a sensor used at an operating frequency of 100 kHz or more, the method comprising: compression-molding a raw material powder containing an iron-based powder having an insulating coating on its surface to obtain a green compact; and heat-treating the green compact in a water vapor atmosphere. (9) The method for manufacturing a magnetic body according to (8) above, wherein the magnetic body has a cylindrical shape and a region of the inner diameter surface where a magnetic path is formed includes a surface covered with an iron oxide film containing triiron tetroxide. (10) The method for manufacturing a magnetic body according to (9) above, wherein the area of ​​the surface covered with the iron oxide film containing triiron tetroxide is 90% or more of the area of ​​the region of the inner diameter surface where the magnetic path is formed. (11) The method for manufacturing a magnetic body according to (9) or (10) above, wherein the thickness of the iron oxide film containing triiron tetroxide is 5 μm or less. (12) The method for manufacturing a magnetic body according to any of (8) to (11) above, wherein the magnetic body has a cylindrical shape and the radial thickness of the region where the magnetic path is formed is 3 mm or less. (13) The method for manufacturing a magnetic body according to any one of (8) to (12) above, wherein the magnetic body has a cylindrical shape and the region of the inner diameter surface where the magnetic path is formed includes a molded surface. (14) The method for manufacturing a magnetic body according to any one of (8) to (13) above, wherein the sensor is a magnetostrictive torque sensor.

[0060] The disclosure of this application is related to the subject matter described in PCT / JP2021 / 041569, filed on November 11, 2021, the entire disclosure of which is incorporated herein by reference.

[0061] The following examples and comparative examples will further illustrate the present invention, but the present invention is not limited to these examples.

[0062] Example 1: Strength evaluation A dust core material ("Somaloy 700 3P" manufactured by Höganäs AB, powder lubricant content: approximately 0.3 to 0.4% by mass based on the pure iron powder having a phosphate coating as an insulating coating on the surface) and a powder lubricant ("Lube 3P") was prepared. The phosphate-coated pure iron powder (spherical) had an average particle size of 150 μm. The average particle size was measured in accordance with the metal powder - particle size test method by dry sieving (ISO 4497:2020).

[0063] The powder magnetic core material was compression molded using a hydraulic press to obtain a rectangular parallelepiped green compact. The molding pressure was 686 MPa, and the density of the green compact was 7.4 g / cm. ３ The rectangular powder compact had a short side of 12 mm, a long side of 34 mm, and a thickness of 5 mm. The powder compact was heated in a nitrogen atmosphere at 500°C for 30 minutes using a belt furnace to obtain a magnetic powder core designated Sample No. 01. The powder compact was then heated in a nitrogen atmosphere at 500°C for 30 minutes using a belt furnace, and then heated in a water vapor atmosphere at 530°C for 20 minutes to obtain a powder core designated Sample No. 02. The surfaces of the powder cores were evaluated using XPS, and it was confirmed that the powder core designated Sample No. 01 did not contain triiron tetroxide, while the powder core designated Sample No. 02 contained triiron tetroxide. Specifically, peaks related to iron, the main element contained in the surfaces of the powder cores, were measured using an XPS device, and after deconvolution, the presence or absence of a peak related to triiron tetroxide was confirmed. The density of the green compact was measured in accordance with the method for measuring uniaxial compressibility of metal powders excluding those for cemented carbide (ISO 3927:2001).

[0064] In a cross section of a rectangular parallelepiped powder magnetic core parallel to the long and short sides, both ends in the long side direction were supported and a load was applied to the center. The load was increased and the load at which the powder magnetic core broke was measured. The load at which the powder magnetic core broke indicates the flexural strength (unit: MPa). The flexural strength was measured in air at room temperature (25°C). The flexural strength is shown in Table 1 below.

[0065]

[0066] Table 1 shows that the strength of the powder magnetic core is improved by including triiron tetroxide.

[0067] [Example 2: Evaluation of magnetic properties] The same powder magnetic core material as that used in Example 1 was prepared. The powder magnetic core material was compression molded using a hydraulic press to obtain cylindrical green compacts of sample numbers 03 to 07. The molding pressure was 686 MPa, and the density of the green compacts was 7.4 g / cm. ３The cylindrical powder cores were obtained by heating the powder cores of sample Nos. 03 to 07 in a nitrogen atmosphere at 500°C for 30 minutes, followed by heating them in water vapor at 530°C for 20 minutes. The surfaces of the powder cores of sample Nos. 03 to 07 were evaluated by XPS, and it was confirmed that all of the powder cores contained triiron tetroxide. Furthermore, the surfaces of the powder cores of sample Nos. 03 to 07 were molded surfaces.

[0068] A primary coil and a secondary coil were wound 12 times in the poloidal direction around the cylindrical powder magnetic core. Using an AC magnetic property measuring device, the complex permeability was measured at a frequency of 200 kHz and an excitation magnetic flux density of 0.01 T. The AC magnetic property measuring device used was the "SY-8258" manufactured by Iwasaki Electric Co., Ltd. The measurement results are shown in Table 2 below. FIG. 2 also shows an optical microscope photograph (magnification 300x) of the cross section of the powder magnetic core of sample number 06. In the photograph, the black lines indicate the insulating coating, the black areas indicate pores, the dark gray areas indicate triiron tetroxide, and the light gray areas indicate the iron-based powder.

[0069]

[0070] Table 2 shows that the magnetic properties improve as the radial thickness of the cylindrical powder magnetic core decreases.

[0071] [Example 3] A powder magnetic core material identical to that used in Example 1 was prepared. The powder magnetic core material was compression-molded using a hydraulic press to obtain cylindrical green compacts of sample numbers 08 and 09. The molding pressure was 686 MPa, and the density of the green compacts was 7.4 g / cm. ３The powder compact of sample No. 08 had an outer diameter of 24 mm, an inner diameter of 19 mm, and a total length of 5 mm. The powder compact of sample No. 09 had an outer diameter of 24 mm, an inner diameter of 20 mm, and a total length of 5 mm. Using a belt furnace, the powder compacts of sample Nos. 08 and 09 were heated in a nitrogen atmosphere at 500°C for 30 minutes, and then heated in water vapor at 530°C for 20 minutes to obtain a heat-treated body of sample No. 08 and a powder core of sample No. 09. The heat-treated body of sample No. 08 was cut using a lathe into a cylindrical shape having an outer diameter of 24 mm, an inner diameter of 20 mm, and a total length of 5 mm, to obtain a powder core of sample No. 08. The surfaces of the powder cores of sample Nos. 08 and 09 were evaluated by XPS, and it was confirmed that the powder cores contained triiron tetroxide.

[0072] A primary coil and a secondary coil were wound 12 times in the poloidal direction around a cylindrical powder magnetic core. Using an AC magnetization characteristic measuring device, the complex permeability was measured at a frequency of 200 kHz and an excitation magnetic flux density of 0.01 T. The AC magnetization characteristic measuring device used was "SY-8258" manufactured by Iwasaki Electric Co., Ltd. The measurement results are shown in Table 3 below.

[0073] Table 3 shows that in a powder magnetic core, the magnetic properties are improved by having a molded surface in the region where a magnetic path is formed on the inner diameter surface.Table 3 shows that in a powder magnetic core, the magnetic properties are improved by having a surface covered with an iron oxide film containing triiron tetroxide in the region where a magnetic path is formed on the inner diameter surface.

[0074]

Claims

1. The invention includes a powder compact including an iron-based powder having an insulating coating on a surface thereof and triiron tetroxide, The rotor has a cylindrical shape, and a region of the inner diameter surface where a magnetic path is formed includes a surface covered with an iron oxide film containing triiron tetroxide, Magnetic materials for sensors used at operating frequencies above 100 kHz.

2. 2. The magnetic body according to claim 1, wherein the area of ​​the surface covered with the iron oxide film containing triiron tetroxide is 90% or more of the area of ​​the region of the inner diameter surface where a magnetic path is formed.

3. 3. The magnetic body according to claim 1, wherein the thickness of the iron oxide film containing triiron tetroxide is 5 [mu]m or less.

4. 3. The magnetic body according to claim 1, wherein the radial thickness of the region where the magnetic path is formed is 3 mm or less.

5. The magnetic body according to claim 1 or 2, wherein the region of the inner diameter surface where a magnetic path is formed includes a molded surface.

6. 3. The magnetic body according to claim 1, wherein the sensor is a magnetostrictive torque sensor.

7. A method for manufacturing a magnetic body, comprising: A raw material powder containing an iron-based powder having an insulating coating on its surface is compression-molded to obtain a green compact; and heat treating the powder compact in a water vapor atmosphere; Including, the magnetic body has a cylindrical shape, an area of ​​the inner diameter surface where a magnetic path is formed includes a surface covered with an iron oxide film containing triiron tetroxide, and is for a sensor used at an operating frequency of 100 kHz or more; A method for manufacturing magnetic materials.

8. 8. The method for manufacturing a magnetic body according to claim 7, wherein the area of ​​the surface covered with the iron oxide film containing triiron tetroxide is 90% or more of the area of ​​the region on the inner diameter surface where the magnetic path is formed.

9. 9. The method for producing a magnetic body according to claim 7, wherein the thickness of the iron oxide film containing triiron tetroxide is 5 [mu]m or less.

10. 9. The method for manufacturing a magnetic body according to claim 7, wherein the radial thickness of the region where the magnetic path is formed is 3 mm or less.

11. The method for manufacturing a magnetic body according to claim 7 or 8, wherein the region of the inner diameter surface where a magnetic path is formed includes a molded surface.

12. The method for manufacturing a magnetic body according to claim 7 or 8, wherein the sensor is a magnetostrictive torque sensor.