Magnetic material and method of producing the same

Abstract

To provide a magnetic material achieving high-concentration nitrogen diffusion without forming a nitride compound in Fe and an Fe-Co alloy as much as possible, and a method of producing the same.SOLUTION: A magnetic material, in which nitrogen intrudes to a depth of 20 μm or more in a thickness direction thereof and which has a nitrogen intrusion area with a nitrogen concentration of 0.5 mass% or higher, is produced by: cold-rolling a steel sheet, which has a thickness of 0.2 mm or more and comprises Fe and an Fe-Co alloy containing Co in a Co concentration of 0 or more and 20 mass% or less, at a processing rate of 75% or more, the processing rate being obtained by dividing a numeric value, which is obtained by subtracting a thickness just after cold rolling from a thickness just before cold rolling, by the thickness just before cold rolling, and then subjecting the steel sheet to nitriding at 650°C or below; or subjecting a steel sheet which comprises Fe and an Fe-Co alloy containing Co in a Co concentration of 0 or more and 20 mass% or less, to plastic processing at a hardness increase rate of 160% or more, and then subjecting the steel sheet to nitriding at 650°C or below.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a magnetic material and a method for producing the same. [Background technology]

[0002] To realize motors and transformers for electric vehicles, heat pumps, drones, and other devices that require small size, high efficiency, and high output, the soft magnetic materials used as the iron core material for these devices must have improved saturation magnetic flux density (Bs) to achieve small size and high output, and must also have soft magnetic properties (high magnetic permeability), high electrical resistance, and thin plate thickness to achieve high efficiency. Soft magnetic materials that have been put to practical use so far include silicon steel sheets and permendur, an Fe-Co alloy. Silicon steel sheets have a lower Bs than permendur, so permendur is advantageous for small-sized high-output applications. However, Co is very expensive and not practical. For this reason, it was necessary to reduce the Co ratio.

[0003] It is thought that by reducing the amount of Co and introducing (solid solution) nitrogen, an inexpensive light element, the interatomic distance can be increased to induce a high magnetic moment, thereby achieving a high Bs. As such a soft magnetic material, there has been disclosed a plate- or foil-shaped soft magnetic material having a high saturation magnetic flux density, which contains iron, carbon, and nitrogen, and which contains martensite and γ-Fe containing carbon and nitrogen, with the γ-Fe having a nitrogen-containing phase formed therein (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2020-132894 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, while it is desirable to achieve a high Bs by introducing nitrogen, there was an issue in that it is difficult for nitrogen to enter (to form a solid solution) into Fe-Co alloys, which are ferritic alloys.

[0006] The present invention has been made in light of these problems, and aims to provide a magnetic material in which high-concentration nitrogen diffusion is achieved while minimizing the formation of nitrogen compounds in Fe and Fe-Co alloys, and a method for producing the same. [Means for solving the problem]

[0007] The magnetic material according to the present invention is made of a steel plate of Fe and an Fe-Co alloy containing Co in a Co concentration of 0 to 20% by mass, and is characterized in that nitrogen penetrates to a depth of 20 μm or more in the plate thickness direction and has a nitrogen-penetrated region with a nitrogen concentration of 0.5% by mass or more. The magnetic material according to the present invention is preferably made of a steel plate having a thickness of 0.05 to 1.0 mm.

[0008] The method for producing a magnetic material according to the present invention is characterized in that a steel sheet made of Fe and an Fe-Co alloy containing Co at a Co concentration of 0 to 20% by mass and having a thickness of 0.2 to 4 mm is cold-rolled at a working ratio of 75% or more, the working ratio being the thickness immediately before cold rolling minus the thickness immediately after cold rolling divided by the thickness immediately before cold rolling, and then the steel sheet is nitrided at 650°C or less to produce a magnetic material having a nitrogen-infiltrated region where nitrogen has infiltrated to a depth of 20 μm or more in the sheet thickness direction and a nitrogen concentration of 0.5% or more by mass. Co is an optional component. The method for producing a magnetic material according to the present invention may be a method for producing a magnetic material in which a steel plate made of Fe and an Fe-Co alloy containing Co at a Co concentration of 0 to 20 mass % is plastically worked to a hardness increase rate of 160% or more, and then a nitriding treatment is performed at 650°C or less, in which nitrogen penetrates to a depth of 20 μm or more in the plate thickness direction and the magnetic material has a nitrogen-penetrated region with a nitrogen concentration of 0.5 mass % or more.

[0009] The magnetic material according to the present invention and the method for producing the magnetic material according to the present invention realize high-concentration nitrogen diffusion while minimizing the formation of nitrogen compounds in Fe and Fe--Co alloys.

[0010] The nitrogen-invaded region preferably contains a structure of a simple ferrite (α) phase in which nitrogen is supersaturated in solid solution, or a two-phase structure of a ferrite (α) phase in which nitrogen is supersaturated in solid solution, and a nitrogen compound (γ') phase. The Fe and Fe--Co alloys contain unavoidable impurities of 0.5 mass % or less, and the unavoidable impurities preferably consist of one or more of P, Mn and S.

[0011] The Fe and Fe--Co alloy preferably contain 3 mass % or less of one or more of C, Si, Al, V, Mn, Ni and Ti. In this case, soft magnetism can be achieved, and the high Bs phase can be stably generated. The method for producing a magnetic material according to the present invention includes the steps of melting, casting, blooming, and hot rolling the base material of the steel plate, and preferably includes one or both of the steps of annealing and cold plastic working the steel plate once or a plurality of times before the cold rolling. Effect of the Invention

[0012] According to the present invention, it is possible to provide a magnetic material in which high-concentration nitrogen diffusion is achieved while minimizing the formation of nitrogen compounds in Fe and Fe--Co alloys, and a method for producing the same. [Brief description of the drawings]

[0013] [Figure 1] 1 is a graph showing the relationship between nitrogen penetration depth and working rate for Fe and Fe—Co alloys. [Diagram 2] 1 is a graph showing the relationship between the working rate and hardness for Fe and an Fe—Co alloy. [Diagram 3] 1 is a graph showing the relationship between the processing rate and the rate of increase in hardness for Fe and an Fe—Co alloy. [Figure 4] 1 is a graph showing an enlarged photograph and nitrogen concentration distribution of the results of surface analysis by EPMA of a magnetic material test piece with a processing rate of 0% after radical nitriding. [Diagram 5]1 is a graph showing enlarged photographs and nitrogen concentration distribution of the results of surface analysis by EPMA of magnetic material test pieces (front) with processing rates of 30% and 50% after radical nitriding. [Figure 6] 1 is a graph showing enlarged photographs and nitrogen concentration distribution of the results of surface analysis by EPMA of magnetic material test pieces (back sides) with processing rates of 30% and 50% after radical nitriding. [Figure 7] 1 is a graph showing enlarged photographs and nitrogen concentration distribution of the results of surface analysis by EPMA of magnetic material test pieces (front) with processing rates of 70% and 90% after radical nitriding. [Figure 8] 1 is a graph showing enlarged photographs and nitrogen concentration distribution of the results of surface analysis by EPMA of magnetic material test pieces (back sides) with processing rates of 70% and 90% after radical nitriding. [Figure 9] 1 is a graph showing an enlarged photograph and nitrogen concentration distribution of the results of surface analysis by EPMA of a magnetic material test piece with a processing rate of 97% after radical nitriding. [Figure 10] 1 is a graph showing the results of XRD analysis of magnetic material test pieces with processing rates of 0 to 97% after radical nitriding. [Figure 11] 1 is a graph showing a quantitative evaluation of the constituent phases by XRD using the RIR method for magnetic material test pieces with processing rates of 0 to 97% after radical nitriding. [Figure 12] 1 shows an EBSD analysis result and a graph showing nitrogen concentration distribution of a magnetic material test piece with a processing rate of 70% after radical nitriding, and a partially enlarged photograph. [Figure 13] 1 shows an EBSD analysis result and a graph showing nitrogen concentration distribution of a magnetic material test piece with a processing rate of 90% after radical nitriding, and a partially enlarged photograph. [Figure 14] 1 shows an EBSD analysis result and a graph showing nitrogen concentration distribution of a magnetic material test piece with a processing rate of 97% after radical nitriding, and a partially enlarged photograph. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, an embodiment of the present invention will be described. In the melting process of the Fe-Co alloy base material, a raw material with high nitrogen affinity is used to increase the nitrogen content. The Fe-Co alloy base material contains Co at a Co concentration of 0 to 20 mass%. Co is an optional component. The Fe-Co alloy base material also contains Fe at a Fe concentration of 80 to 100 mass%. In order to achieve soft magnetic properties and to stably generate a high Bs phase, one or more of the third elements C, Si, Al, V, Mn, Ni, and Ti are added. The amount of one or more of C, Si, Al, V, Mn, Ni, and Ti added is 3 mass% or less.

[0015] After the base material is melted, cast, bloomed, and hot rolled to produce a steel sheet having a thickness of 0.2 mm or more, one or more of the steps of annealing and cold plastic working are carried out once or a plurality of times. For example, a steel sheet having a thickness of 0.05 to 0.1 mm is manufactured through steps such as (1) a hot rolling step and a cold rolling step, (2) a hot rolling step, a subsequent annealing step, and a further cold rolling step, and (3) a hot rolling step, a subsequent annealing step, a further cold rolling step, a subsequent annealing step, and a further cold rolling step.

[0016] At this time, the working ratio is measured by dividing the value obtained by subtracting the thickness immediately after cold rolling from the thickness immediately before cold rolling by the thickness immediately before cold rolling. The working ratio in the final cold rolling step is set to 75% or more, and a state in which working strain is introduced into the structure (a state in which dislocation density is increased) is achieved. Since it is sufficient to introduce working strain and increase the dislocation density, the method of plastic working may be, for example, pressing a plate material, and the temperature during the working is not limited to cold.

[0017] Next, as a step of introducing nitrogen, a nitriding treatment that generates little nitrides is performed. The nitriding temperature is 650°C or less. For example, by performing radical nitriding at 450°C, high-concentration nitrogen diffusion is achieved without forming nitrogen compounds in the soft magnetic steel sheet as much as possible. This makes it possible to manufacture a magnetic steel sheet that is made of Fe and an Fe-Co alloy containing Co at a Co concentration of 0 to 20 mass %, has a thickness of 0.05 to 1.0 mm, has nitrogen penetrated to a depth of 20 μm or more in the sheet thickness direction, and has a nitrogen penetration region with a nitrogen concentration of 0.5 mass % or more.

[0018] The relationship between the nitrogen penetration depth and the processing rate for Fe and Fe-Co alloys is shown in Figure 1, the relationship between the processing rate and hardness in Figure 2, and the relationship between the processing rate and the hardness increase rate in Figure 3. In the graph in Figure 1, a nitrogen penetration depth of 0.5% by weight is determined as the threshold value, and nitrogen is determined to be present when it is 0.5% by weight or more. In Figure 3, the hardness increase rate (%) is calculated by multiplying the hardness at a processing rate of 0% by H 0 When the change in hardness before and after processing is △H, the hardness increase rate (%) = △H / H 0 This was calculated using the formula: ×100. From the results of Figs. 1 and 3, it can be seen that in order to achieve a nitrogen penetration depth of 20 µm, a processing rate of 75% or more, or a hardness increase rate of 160% or more, is necessary.

[0019] The magnetic steel sheet thus produced contains unavoidable impurities of 0.5 mass % or less, which may consist of one or more of P, Mn, and S. The nitrogen-penetrated region includes a structure of a single ferrite (α) phase in which nitrogen is supersaturated in solid solution, or a two-phase structure of a ferrite (α) phase in which nitrogen is supersaturated in solid solution, and a nitrogen compound (γ´) phase. The magnetic steel sheets manufactured from this process achieve high nitrogen diffusion without forming nitrogen compounds in the Fe and Fe-Co alloys, which is expected to lead to smaller, higher output and higher efficiency when used as core materials for motors and transformers. EXAMPLES

[0020] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. A raw material containing 10% by mass of Co and 90% by mass of Fe was melted in a vacuum induction melting furnace to produce an ingot of about 7 kg. A soft magnetic steel sheet was manufactured by the process shown in Table 1 below.

[0021] [Table 1]

[0022] In the cold rolling process, the working ratio was measured by subtracting the thickness immediately after cold rolling from the thickness immediately before cold rolling, and dividing the result by the thickness immediately before cold rolling, and steel sheets with working ratios of 0%, 30%, 50%, 70%, 90%, and 97% were produced, as shown in Table 2. The radical nitriding treatment was performed in an atmosphere of hydrogen and ammonia at a temperature of 450°C for a treatment time of 7.5 hours.

[0023] [Table 2]

[0024] The steel sheets after the radical nitriding treatment were evaluated by the following methods. [Nitrogen concentration distribution analysis in the plate thickness direction] A test piece measuring 10 mm in thickness x 10 mm in width x 15 mm in length was cut out and embedded in hot resin so that the longitudinal section of the test piece was the observation surface. The hot resin-embedded sample was then mirror-polished and analyzed using an EPMA (electron probe microanalyzer). The results are shown in Figures 4 to 9.

[0025] [Evaluation of the constituent phases after radical nitriding] The constituent phases of the surface of the radical nitriding treated sample were analyzed by XRD (X-ray diffraction method). The results are shown in Figure 10. As shown in Figure 10, diffraction peaks of ferrite (α) and nitrides (γ', ε) were detected. Quantitative analysis of the constituent phases was performed using the RIR method from the obtained peak intensities. The results are shown in Figure 11. It can be seen from Figure 11 that as the processing rate increases, the proportion of ferrite (α) increases and the proportion of nitrides (γ', ε) decreases.

[0026] [Evaluation of the constituent phases of nitrogen-containing parts] A test piece measuring 10 mm thick x 10 mm wide x 15 mm long was cut out and embedded in hot resin so that the longitudinal section of the test piece was the observation surface. The hot resin-embedded sample was then mirror-polished, and the surface of the sample was further polished using an active oxide polishing suspension. The constituent phases of the polished test piece in the areas containing nitrogen were evaluated using EBSD (electron backscatter diffraction). The results are shown in Figures 10 to 12.

[0027] It can be seen that with a processing rate of 75% or more, or a hardness increase rate of 160% or more (data for 90% and 97%), in the high nitrogen concentration areas of the test piece, the α phase in which nitrogen is dissolved (supersaturated) and fine nitrogen compound (γ´) phase (Fe4N) coexist in a two-phase state. On the other hand, in the test piece with a processing rate of 70%, the area from the surface to 5 μm deep where the nitrogen concentration is high is in a state where needle-shaped nitrides have been formed, and the structure is not in the same state as in the case of a processing rate of 75% or more, or a hardness increase rate of 160% or more. The area deeper than the surface to 5 μm is almost entirely ferrite (α) phase, but the nitrogen concentration is low.

Claims

1. A magnetic material comprising a steel plate of Fe and an Fe-Co alloy containing Co at a Co concentration of 0 to 20% by mass, characterized in that nitrogen penetrates to a depth of 20 μm or more in the plate thickness direction and has a nitrogen-penetrated region with a nitrogen concentration of 0.5% by mass or more.

2. 2. The magnetic material according to claim 1, characterized in that it is made of a steel plate having a thickness of 0.05 to 1.0 mm.

3. The magnetic material according to claim 2, characterized in that the nitrogen-invaded region includes a structure of a single ferrite (α) phase in which nitrogen is supersaturated in solid solution, or a two-phase structure of a ferrite (α) phase in which nitrogen is supersaturated in solid solution and a nitrogen compound (γ') phase.

4. 4. The magnetic material according to claim 2, wherein the Fe and the Fe--Co alloy contain unavoidable impurities of 0.5 mass % or less, the unavoidable impurities being one or more of P, Mn and S.

5. 4. The magnetic material according to claim 2, wherein the Fe and Fe--Co alloy contain 3 mass % or less of one or more of C, Si, Al, V, Mn, Ni and Ti.

6. A method for producing a magnetic material having a nitrogen-penetrated region where nitrogen has penetrated to a depth of 20 μm or more in the plate thickness direction and has a nitrogen concentration of 0.5 mass % or more, the method comprising: cold-rolling a steel plate having a thickness of 0.2 mm to 4 mm and made of Fe and an Fe-Co alloy containing Co at a Co concentration of 0 to 20 mass %, at a processing ratio of 75% or more, the processing ratio being the thickness immediately before cold rolling minus the thickness immediately after cold rolling divided by the thickness immediately before cold rolling, and then nitriding the steel plate at 650° C. or less.

7. 7. The method for producing a magnetic material according to claim 6, further comprising the steps of melting, casting, blooming and hot rolling the base material of the steel plate, and further comprising one or both of the steps of annealing and cold plastic working the steel plate once or a plurality of times before the cold rolling.

8. A method for producing a magnetic material having a nitrogen-penetrated region in which nitrogen has penetrated to a depth of 20 μm or more in the plate thickness direction and has a nitrogen concentration of 0.5 mass % or more, the method comprising the steps of: plastically working a steel plate made of Fe and an Fe-Co alloy containing Co at a Co concentration of 0 to 20 mass % with a hardness increase rate of 160% or more; and then nitriding the steel plate at 650° C. or less.

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