Magnetic material and method for 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 manufacturing the same.

Background Art

[0002] In order to realize motors and transformers for electric vehicles, heat pumps, drones, etc. that are required to be small, highly efficient, and high-output, in the soft magnetic material used as the core material for these, in order to achieve small size and high output, the saturation magnetic flux density (Bs) needs to be improved, and for high efficiency, soft magnetization (high magnetic permeability), high electrical resistivity, and thinning are required. Soft magnetic materials that have been put into practical use so far include silicon steel sheets and Permendur, which is an Fe-Co alloy. The silicon steel sheet has a lower Bs compared to Permendur, and Permendur is advantageous for achieving small size and high output. However, Co is very expensive and not practical. Therefore, it has been necessary to reduce the ratio of Co.

[0003] It has been considered to reduce the amount of Co and introduce (solid-solve) nitrogen, which is an inexpensive light element, to expand the interatomic distance and induce a high magnetic moment to achieve a high Bs. As such a soft magnetic material, there is disclosed a soft magnetic material in the form of a plate or foil having a high saturation magnetic flux density, containing iron, carbon, and nitrogen, including martensite and γ-Fe containing carbon and nitrogen, and a phase containing nitrogen is formed in γ-Fe (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, while attempting to achieve a high Bs by introducing nitrogen, there was a problem in that nitrogen hardly enters (is difficult to dissolve) in the Fe-Co alloy, which is a ferrite-based alloy.

[0006] The present invention has been made by paying attention to such problems, and an object thereof is to provide a magnetic material that realizes high-concentration nitrogen diffusion without forming nitrogen compounds as much as possible in Fe and Fe-Co alloys, and a method for manufacturing the same.

Means for Solving the Problems

[0007] The magnetic material according to the present invention is made of a steel sheet of Fe and Fe-Co alloy containing Co at a Co concentration of 0 to 20 mass%, and has a nitrogen intrusion region where nitrogen penetrates to a depth of 20 μm or more in the plate thickness direction and has a nitrogen concentration of 0.5 mass% or more. having a thickness of 0.05 or more and 1.0 mm or less and has a nitrogen intrusion region where nitrogen penetrates to a depth of 20 μm or more in the plate thickness direction and has a nitrogen concentration of 0.5 mass% or more. and the nitrogen intrusion region includes a structure of a single-phase ferrite (α) phase in which nitrogen is supersaturatedly dissolved or a two-phase structure of a ferrite (α) phase in which nitrogen is supersaturatedly dissolved and a nitrogen compound (γ') phase.

[0008] The method for manufacturing a magnetic material according to the present invention is a method for manufacturing a magnetic material made of a steel sheet of Fe and Fe-Co alloy containing Co at a Co concentration of 0 to 20 mass% and having a thickness of 0.2 mm or more and 4 mm or less. After cold rolling at a processing rate of 75% or more, where the processing rate is the value obtained 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, nitriding treatment is performed at 650 °C or lower, and nitrogen penetrates to a depth of 20 μm or more in the plate thickness direction and has a nitrogen intrusion region with a nitrogen concentration of 0.5 mass% or more. Co is an optional component. The method for manufacturing a magnetic material according to the present invention may be a method for manufacturing a magnetic material that plastically processes a steel sheet made of Fe and Fe-Co alloy containing Co at a Co concentration of 0 to 20 mass% at a hardness increase rate of 160% or more and then performs nitriding treatment at 650 °C or lower, and has a nitrogen intrusion region where nitrogen penetrates to a depth of 20 μm or more in the plate thickness direction and has a nitrogen concentration of 0.5 mass% or more.

[0009] The magnetic material according to the present invention and the method for manufacturing the magnetic material according to the present invention realize high-concentration nitrogen diffusion without forming nitrogen compounds as much as possible in Fe and Fe-Co alloys.

[0010] former 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 alloys preferably contain one or more of C, Si, Al, V, Mn, Ni, and Ti at 3 mass% or less. In this case, soft magnetism can be realized, and a high-Bs phase can be stably formed. The method for manufacturing a magnetic material according to the present invention includes steps of melting, casting, chunking, and hot rolling a base material of the steel sheet, and preferably includes one or both of annealing and cold plastic working processes for the steel sheet one or more 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 nitrogen diffusion is realized at a high concentration without forming nitrogen compounds as much as possible in Fe and Fe-Co alloys, and a method for manufacturing the same.

Brief Description of the Drawings

[0013]

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Embodiments for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present invention will be described. In the melting process of the Fe-Co alloy base material, a raw material with a 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. Also, the Fe-Co alloy base material contains Fe at an Fe concentration of 80 mass% or more and 100 mass% or less. To achieve soft magnetism and stably generate a high Bs phase, one or more of the third elements C, Si, Al, V, Mn, Ni, Ti are added. The addition amount of one or more of C, Si, Al, V, Mn, Ni, and Ti is 3 mass% or less.

[0015] After melting, casting, chunking, and hot rolling the base material into a steel plate with a thickness of 0.2 mm or more, one or more processes of annealing and cold plastic working are performed once or multiple times. For example, (1) a hot rolling process and a cold rolling process, (2) a hot rolling process, an annealing process thereafter, and a cold rolling process, (3) a hot rolling process, an annealing process thereafter, a cold rolling process, followed by an annealing process, and further a cold rolling process, etc. are carried out to manufacture a steel plate with a thickness of 0.05 to 0.1 mm.

[0016] At this time, the value obtained 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 is defined as the processing rate, and the processing rate is measured. The processing rate of the final cold rolling process is set to be 75% or more, and a state in which processing strain is introduced into the structure (a state in which the dislocation density is increased) is obtained. Since it is only necessary to introduce processing strain and increase the dislocation density, the method of plastic processing may be, for example, pressing a plate material, and the temperature during the processing is not limited to cold.

[0017] Next, as a process of introducing nitrogen, a nitriding treatment with less nitride formation is performed. The nitriding treatment temperature is 650 °C or lower. For example, by performing radical nitriding treatment at 450 °C, high-concentration nitrogen diffusion can be realized without forming nitrogen compounds as much as possible in the soft magnetic material steel plate. Thereby, a magnetic material steel plate made of Fe and Fe-Co alloy containing Co at a Co concentration of 0 to 20 mass%, having a thickness of 0.05 to 1.0 mm, in which nitrogen penetrates to a depth of 20 μm or more in the plate thickness direction and has a nitrogen penetration region with a nitrogen concentration of 0.5 mass% or more can be manufactured.

[0018] The relationship between the nitrogen penetration depth and the working ratio for Fe and Fe-Co alloys is shown in Fig. 1, the relationship between the working ratio and the hardness is shown in Fig. 2, and further the relationship between the working ratio and the hardness increase rate is shown in Fig. 3. In the graph of Fig. 1, the nitrogen penetration depth of 0.5 wt% was used as a threshold value for determination, and it was determined that nitrogen was contained when it was 0.5 wt% or more. In Fig. 3, the hardness increase rate (%) was obtained by the formula hardness increase rate (%) = ΔH / H0×100, where the hardness at a working ratio of 0% was H0 and the change amount of hardness before and after working was ΔH. From the results of Fig. 1 and Fig. 3, it can be seen that in order to achieve a nitrogen penetration depth of 20 μm, a working ratio of 75% or more, or a hardness increase rate of 160% or more is required.

[0019] The manufactured magnetic material steel sheet contains unavoidable impurities of 0.5 mass% or less. The unavoidable impurities consist of one or more of P, Mn, and S. The nitrogen penetration region includes a structure of a single-phase 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 manufactured magnetic material steel sheet realizes high-concentration nitrogen diffusion without forming nitrogen compounds as much as possible in Fe and Fe-Co alloys. As a result, when used as a core material for motors and transformers, miniaturization, high output, and high efficiency can be expected.

Example

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

[0021]

Table 1

[0022] In the cold rolling process, the processing rate was 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. As shown in Table 2, steel plates with processing rates of 0%, 30%, 50%, 70%, 90%, and 97% were produced. The radical nitriding treatment was carried out at a temperature of 450°C for 7.5 hours in an atmosphere of hydrogen and ammonia.

[0023] [Table 2]

[0024] The steel plates after the radical nitriding treatment were evaluated by the following methods. [Analysis of nitrogen concentration distribution in the plate thickness direction] Test pieces with a thickness × width of 10 mm × length of 15 mm were cut out and embedded in hot resin so that the longitudinal section of the test piece became the observation surface. Then, the test samples embedded in hot resin were 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] Regarding the surface of the sample subjected to the radical nitriding treatment, the constituent phases 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 carried out 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 at the locations containing nitrogen] Test pieces with a thickness × width of 10 mm × length of 15 mm were cut out and embedded in hot resin so that the longitudinal section of the test piece became the observation surface. Then, the test samples embedded in hot resin were mirror-polished, and the surface of the sample was further polished using an active oxide polishing suspension. The constituent phases at the locations containing nitrogen in the polished test pieces were evaluated using EBSD (electron backscatter diffraction method). The results are shown in Figures 10 to 12.

[0027] When the working rate is 75% or more, or the hardness increase rate is 160% or more (the data are the results of 90% and 97%), it can be seen that in the region with a high nitrogen concentration in the test piece, the α-phase in which nitrogen is (supersaturated) solid-solved and the fine nitrogen compound (γ´) phase (Fe4N) coexist in a two-phase state. On the other hand, in the test piece with a working rate of 70%, in the region of the surface layer to 5 μm with a high nitrogen concentration, nitrides are formed in a needle shape, and it does not have the tissue state as in the case where the working rate is 75% or more, or the hardness increase rate is 160% or more. The part deeper than the surface layer to 5 μm is almost a single-phase ferrite (α), but the nitrogen concentration is low.

Claims

1. It is made of a steel sheet with a thickness of 0.05 mm or more and 1.0 mm or less of Fe and Fe—Co alloy containing Co at a Co concentration of 0 to 20% by mass, nitrogen penetrates at a depth of 20 μm or more in the plate thickness direction and has a nitrogen penetration region with a nitrogen concentration of 0.5% by mass or more, and the nitrogen penetration region contains a structure of a single-phase ferrite (α) phase in which nitrogen is supersaturatedly dissolved or a two-phase structure of a ferrite (α) phase in which nitrogen is supersaturatedly dissolved and a nitrogen compound (γ′) phase. A magnetic material characterized by this.

2. The Fe and Fe—Co alloy contains unavoidable impurities of 0.5% by mass or less, and the unavoidable impurities are composed of one or more of P, Mn, and S. The magnetic material according to claim 1, characterized by this.

3. The Fe and Fe—Co alloy contains one or more of C, Si, Al, V, Mn, Ni, and Ti at 3% by mass or less. The magnetic material according to claim 1, characterized by this.

4. A steel sheet made of Fe and Fe—Co alloy containing Co at a Co concentration of 0 to 20% by mass and having a thickness of 0.2 mm or more and 4 mm or less is cold-rolled at a rolling reduction rate of 75% or more, where the rolling reduction rate is a value obtained 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, and then nitrided at 650° C. or lower. A method for manufacturing a magnetic material having a nitrogen penetration region in which nitrogen penetrates at a depth of 20 μm or more in the plate thickness direction and has a nitrogen concentration of 0.5% by mass or more.

5. The method for manufacturing a magnetic material according to claim 4, including steps of melting, casting, chunking, and hot rolling the base material of the steel sheet, and including one or more times of one or both of annealing and cold plastic working on the steel sheet before the cold rolling.

6. A steel sheet made of Fe and Fe—Co alloy containing Co at a Co concentration of 0 to 20% by mass is plastically worked at a hardness increase rate of 160% or more, and then nitrided at 650° C. or lower. A method for manufacturing a magnetic material having a nitrogen penetration region in which nitrogen penetrates at a depth of 20 μm or more in the plate thickness direction and has a nitrogen concentration of 0.5% by mass or more.

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