Method for manufacturing coated member and coated member

Abstract

Provided is a coated member that maintains electrical insulation even when electric discharge processing is performed thereon and that can suppress melt-adhesion of said members when being laminated and heated.　This method is for manufacturing a coated member and comprises: an application step for applying a magnesium hydroxide solution on a surface of a Fe-Co-based alloy base material; and a baking step for baking the base material after the application step at 600-900°C to form a magnesium oxide coating on the base material. This coated member has, on a Fe-Co-based alloy base material, a baked coating of magnesium oxide having a lattice constant of 4.20-4.23 Å.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a coated member and a coated member. [Background technology]

[0002] In recent years, growing awareness of environmental conservation has led to active efforts toward the electrification of automobiles and hybrid aircraft, and key technologies for these include increasing the output, miniaturization, and reducing loss of electric motors. Laminated cores, which have a structure in which multiple thin plates of soft magnetic alloys are stacked, are used as the motor core shape used in these electric motors because they have a large amount of magnetization per unit volume and are advantageous for miniaturizing the core.

[0003] One effective way to further miniaturize this laminated core is to use a soft magnetic material with a high saturation magnetic flux density, and one effective way to further reduce loss is to improve the electrical insulation (hereinafter simply referred to as insulation) between the laminated sheets. For example, Patent Document 1 discloses a laminated core made of a single layer of permendur (an Fe-Co alloy) with a high saturation magnetic flux density, and proposes forming a ceramic layer of magnesium oxide, zirconium oxide, aluminum oxide, or the like on the surface of the single layer as an insulating coating. Patent Document 2 also discloses a method of subjecting a cold-rolled Fe-Co alloy to a two-stage heat treatment, and fabricating a laminated core from the heat-treated alloy material, and also describes the possibility of forming a magnesium oxide coating by heat treatment to prevent welding of the alloy material. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2012-521649 [Patent Document 2] US Patent Application Publication No. 2020 / 0299820 Summary of the Invention [Problem to be solved by the invention]

[0005] The laminated cores made of laminated permendur coated with a ceramic layer, as disclosed in Patent Documents 1 and 2, have excellent insulation properties. Patent Document 1 also discloses a method for shaping a ceramic-coated permendur single-layer material (hereinafter also referred to as a coated member) by electrical discharge machining. However, the inventors' research confirmed that the insulating layer discolors when the coated member is subjected to electrical discharge machining. This discoloration is due to leaching of the insulating ceramic layer, which may result in an insufficient insulation and increased loss in the laminated core. This problem is not recognized in Patent Documents 1 and 2, and there is room for further investigation. Another issue that can occur with laminated cores is welding between the single-layer materials. This welding occurs during magnetic annealing and significantly increases iron loss. Furthermore, peeling the welded single-layer materials can cause shape defects in the single-layer materials, which can lead to concerns about the accompanying degradation of magnetic properties. Therefore, it is necessary to prevent welding. Therefore, an object of the present invention is to provide a coated member and a manufacturing method thereof that maintain insulation properties even when shaping by electrical discharge machining and suppress welding during stacking and heat treatment. [Means for solving the problem]

[0006] The inventors discovered that if the baking temperature is not appropriate, the lattice that constitutes magnesium oxide becomes unstable, and they believed that this instability causes the coating to dissolve. They then conducted extensive research into the appropriate baking temperature, and arrived at the present invention. That is, one aspect of the present invention is a method for producing a coated member, comprising: a coating step of coating a surface of an Fe—Co-based alloy substrate with a magnesium hydroxide solution; and a baking step of baking the substrate after the coating step at 600 to 900°C to form a magnesium oxide coating on the substrate. Another aspect of the present invention is a coated member having a baked coating of magnesium oxide having a lattice constant of 4.20 to 4.23 Å ​​on an Fe—Co alloy substrate. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a coated member that maintains its insulating properties even when subjected to electrical discharge machining and that can suppress welding when laminated and heat treated. DETAILED DESCRIPTION OF THE INVENTION

[0008] First, a method for producing a coated member of the present invention will be described. In the present invention, an Fe-Co alloy substrate is used as the substrate that serves as the soft magnetic material. The Fe-Co alloy in the present invention refers to an alloy material that contains, by mass %, 95% or more of Fe+Co and 25 to 60% of Co. This allows the material to exhibit high magnetic flux density.

[0009] Next, elements that may be contained in the Fe-Co alloy of the present invention will be described. To improve magnetic properties and cold workability, the Fe-Co alloy of the present invention may contain one or more of the following elements: V, Si, Mn, Al, Zr, B, Ni, Ta, Nb, W, Ti, Mo, and Cr, in a total amount of up to 5.0% by mass. Examples of other impurity elements that are inevitably contained include C, S, P, and O, and the upper limit of each of these elements is preferably set to 0.1%.

[0010] In the method for producing a coated member of the present invention, a coating step is first performed to form a coating of magnesium oxide (hereinafter also referred to as MgO) on the surface of an Fe-Co alloy substrate. The MgO coating was selected because of its excellent electrical insulation properties and adhesion to the Fe-Co alloy substrate. Furthermore, when magnetically annealing the coated member to obtain the desired magnetic properties, the insulating film is exposed to a reducing atmosphere such as hydrogen at approximately 850°C. However, MgO is not easily reduced by hydrogen, does not evaporate even at high temperatures, and is not susceptible to deterioration of properties due to thermal diffusion. Therefore, MgO offers excellent insulation and resistance to welding.

[0011] In the present invention, the coating liquid for forming the MgO coating described above uses an Mg(OH)2 solution (hereinafter also referred to as a slurry) in which MgO precursor powder, Mg(OH)2, is dispersed in a solvent as a solute. By using this slurry as a coating liquid, the slurry can be applied uniformly to the surface of a metal substrate with a stable film thickness. The Mg(OH)2 used as the slurry solute thermally decomposes to MgO when heated, making it easy to form an MgO coating. Furthermore, the thermal decomposition temperature of Mg(OH)2 is low, approximately 500°C, allowing stable formation of an MgO coating at low temperatures. Magnesium carbonate (MgCO3) can also be used as a precursor other than Mg(OH)2. Water, amphoteric solvents such as alcohol, and organic solvents can be used as the slurry solvent. Methods for applying the slurry to a metal substrate include roll coating, in which the slurry is applied to the substrate with a roller; dip coating, in which the substrate is immersed in the slurry and then lifted off; and screen printing.

[0012] In the present invention, after the coating step, a baking step is carried out in which the substrate coated with the slurry is baked at 600 to 900°C to form an MgO coating on the substrate (hereinafter, the step of thermally decomposing the precursor to form an MgO coating will also be simply referred to as "baking"). Electrical discharge machining (EDM), a shaping method used to fabricate laminated cores, involves machining the workpiece in water. This process tends to cause leaching of the MgO coating, potentially resulting in a thinner MgO coating and potentially impaired electrical insulation. In the present invention, the baking temperature of Mg(OH)2 is set to 600 to 900°C, thereby suppressing the leaching of the MgO coating that occurs when the coated member is subjected to EDM, resulting in the formation of an MgO coating with excellent insulation and welding resistance. One of the reasons for the leaching suppression effect of the present invention is thought to be the improved stability of the lattice constituting the MgO coating due to the appropriate baking temperature. When the baking temperature is below 600°C, leaching of the MgO coating occurs. Furthermore, when the baking temperature exceeds 900°C, fine crystal grains resulting from the precipitation of gamma phases precipitate at the grain boundaries of the Fe—Co-based metal substrate. These fine crystal grains inhibit domain wall motion, increasing coercivity and potentially degrading soft magnetic properties. The upper limit of the baking temperature is preferably 850° C., more preferably 800° C., and even more preferably 700° C. The dissolution of the MgO coating described above appears as color unevenness, which can be easily observed from the appearance of the coated member.

[0013] The baking time in the baking process of the present invention can be appropriately set in relation to the baking temperature, as long as it does not impair the effects of the present invention. A higher baking temperature allows for a shorter baking time. For example, when the baking temperature is 600°C, the baking time can be set to 1 to 30 minutes. Furthermore, a heating rate of 200°C / h to 300°C / h during the baking process is preferred, as this tends to improve the stability of the lattice that constitutes the MgO coating. Furthermore, furnace cooling is preferred, with slow cooling from the baking temperature to room temperature over a period of 90 to 180 minutes. The baking atmosphere is preferably an inert gas atmosphere or a vacuum atmosphere. This is because baking in air would cause excessive oxidation of the coating, which would then oxidize the Fe-Co-based metal substrate, thereby preventing deterioration of the soft magnetic properties of the Fe-Co-based metal substrate. Examples of inert gas atmospheres include a nitrogen gas atmosphere and an Ar gas atmosphere.

[0014] In the present invention, the slurry is applied to the surface of the metal substrate and then baked. However, a drying step for evaporating the solvent may be performed before baking. The drying step can be performed at a low temperature near the boiling point of the solvent, which allows the slurry to dry quickly, preventing dripping and forming a uniform film thickness. Another advantage of the drying step is that it can be performed in the atmosphere because it can be performed at a low temperature.

[0015] The coated member of the present invention obtained by the above-described manufacturing method of the present invention has a baked magnesium oxide coating on a substrate with a lattice constant of 4.20 to 4.23 Å. By approaching the theoretical value of 4.213 Å for the lattice constant of MgO, leaching of the MgO coating after electrical discharge machining tends to be suppressed. This is because the MgO precursor, Mg(OH)2, is hexagonal, while the MgO formed by baking is cubic. In the present invention, MgO is produced by the thermal decomposition reaction of Mg(OH)2 during the baking process. However, if the baking temperature is low, for example, at 500°C, the thermal decomposition reaction does not proceed sufficiently, resulting in the production of MgO with a lattice constant larger than the theoretical value. This large lattice constant MgO is likely to dissolve in water, which is thought to result in color unevenness during electrical discharge machining in water. The lattice constant of MgO can be measured by X-ray diffraction. Whether the coating of the present invention is a baked coating formed by applying a solution and baking it can be determined from a cross-sectional photograph of the coating (for example, a cross-sectional photograph of the coating observed at 1,000,000 times magnification with a transmission electron microscope). That is, in a baked coating, voids with an equivalent circle diameter of about 10 to 50 nm are observed in the coating structure, but such voids are not observed in a coating formed by physical vapor deposition, and it is from this difference that a baked coating can be identified.

[0016] The thickness of the coating formed on the covering member may be set within a range that ensures electrical insulation. While electrical insulation improves with a thicker coating, excessively thick coatings may reduce the space factor of the laminated core, potentially leading to performance degradation. Therefore, the thickness of the coating should be set taking into consideration the insulation and space factor. For example, a preferred thickness is 10 to 1,000 nm. [Example]

[0017] Example 1 Three cold-rolled Fe-Co alloy substrates (110 mm long x 60 mm wide x 0.2 mm thick) with the composition shown in Table 1 were prepared as metal substrates and subjected to alkaline degreasing. Next, a slurry was prepared by dispersing Mg(OH)2 powder as a solute in water as a solvent, and the slurry was applied to the substrates by dip coating. After the application of the slurry, the substrates were dried by heating at 110°C for 5 minutes in the atmosphere. <Example 1 of the present invention> The dried substrate was heated in a nitrogen atmosphere at a heating rate of 250°C / h until it reached 600°C, held at 600°C for 30 minutes, and then subjected to a baking step in which it was furnace-cooled to room temperature over 160 minutes, thereby producing a coated member of Example 1 of the present invention coated with an MgO coating having a thickness of 0.1 μm. <Example 2 of the present invention> The dried substrate was heated in a nitrogen atmosphere at a heating rate of 250°C / h until it reached 700°C, held at 700°C for 30 minutes, and then subjected to a baking process in which it was furnace-cooled to room temperature over 170 minutes, thereby producing a coated member of Example 2 of the present invention coated with an MgO coating having a thickness of 0.1 μm. <Comparative Example 1> The dried substrate was heated in a nitrogen atmosphere at a heating rate of 250°C / h until it reached 500°C, and then held at 500°C for 30 minutes. After that, a baking step was performed in which the substrate was furnace-cooled to room temperature over 150 minutes, thereby producing a coated member of Comparative Example 1 coated with an MgO coating having a thickness of 0.1 μm.

[0018] [Table 1]

[0019] Each of the coated members prepared above was machined by electrical discharge machining into a ring-shaped specimen with an outer diameter of 45 mm and an inner diameter of 33 mm, and the presence or absence of color unevenness was visually evaluated. To evaluate the lattice constant, a RINT2500V X-ray diffractometer from Rigaku Corporation was used with a CoKα source to determine the lattice constant a of the MgO coating formed on the surface of the metal substrate by the thin-film method. The results are shown in Table 2. The results in Table 2 indicate that the lattice constant a of the MgO coating of the coated member of Comparative Example 1 was larger than that of the inventive examples, and color unevenness occurred after electrical discharge machining. In contrast, the lattice constant a of MgO in the coated members of Inventive Examples 1 and 2 was close to the theoretical value, confirming that no color unevenness occurred in any of the samples after electrical discharge machining.

[0020] [Table 2]

[0021] Example 2 Next, the welding resistance of the coated member of the present invention was confirmed. Two cold-rolled Fe-Co alloy materials (substrate A) measuring 50 mm in length, 50 mm in width, and 0.2 mm in thickness and having the composition shown in Table 1 of Example 1 were prepared, and two cold-rolled Fe-Co alloy materials (substrate B) measuring 40 mm in length, 50 mm in width, and 0.2 mm in thickness and having the composition shown in Table 1 of Example 1 were prepared. <Example 3 of the present invention> An MgO coating was formed on both substrate A and substrate B under the same conditions as in Example 1 of Invention Example 1 to produce coated members A and B. Next, coated member B was placed on top of coated member A so that it did not protrude from coated member A, and the overlapping coated members were sandwiched between alumina plates and heated in a heat treatment furnace at 850°C for 3 hours to produce a sample of Invention Example 3. At this time, the surface pressure applied to the overlapping coated members A and B was 0.1 g / cm. 2 It was. <Comparative Example 2> Substrate A and substrate B were laminated together without being coated, and the other conditions were the same as in Example 3 of the present invention to prepare a sample of Comparative Example 2.

[0022] Observation of the samples after heating confirmed that welding had occurred in Comparative Example 2, and that the A and B substrates were bonded to each other. On the other hand, no welding occurred after heating in Invention Example 3. This result was also obtained when the surface pressure applied to the coated members A and B was 0.075 g / cm 2 From the above results, it was confirmed that the covering member of the present invention does not undergo welding even when laminated and subjected to magnetic annealing, and is a member suitable for use in laminated cores.

[0023] Example 3 To confirm the effect of baking temperature on the soft magnetic properties of Fe-Co alloys, evaluations were conducted using cold-rolled Fe-Co alloys without MgO coating. A 0.2 mm thick cold-rolled Fe-Co alloy with the composition shown in Table 3 was prepared as the metal substrate, and a ring-shaped sample with an outer diameter of 45 mm and an inner diameter of 33 mm was prepared by electrical discharge machining. The sample was then heated at a rate of 250°C / h in a nitrogen atmosphere, held at 700°C for 30 minutes, and then furnace-cooled to room temperature over 120 minutes, thereby performing a heat treatment simulating baking. This was followed by magnetic annealing at 850°C for 3 hours in a hydrogen atmosphere, yielding a sample for Example 4 of the present invention. The shape of the sample and the processing method for the ring sample were the same, and the sample was heated in a nitrogen atmosphere at a heating rate of 250°C / h, held at 950°C for 30 minutes, and then furnace cooled to room temperature over 180 minutes, thereby carrying out a heat treatment simulating baking, and then magnetic annealing was carried out in a hydrogen atmosphere at 850°C for 3 hours, to obtain a sample for Comparative Example 3. Five ring-shaped samples were produced for each of the inventive example and comparative example.

[0024] [Table 3]

[0025] Next, for the evaluation of magnetic properties, five ring-shaped samples each of Example 4 of the present invention and Comparative Example 3 were stacked and wound with 100 turns of primary winding and 50 turns of secondary winding. Then, a DC magnetic field with a maximum applied magnetic field of 5000 A / m was applied to the ring-shaped samples, and the coercive force H c , maximum relative permeability μ mThe results are shown in Table 4. From Table 4, it can be seen that the sample of Inventive Example 4 had a H c is small, and μ m On the other hand, the sample of Comparative Example 3 has excellent soft magnetic properties. c is large, and μ m From the above, it was confirmed that baking at temperatures above 900°C is inappropriate as the soft magnetic properties of Fe-Co alloys deteriorate.

[0026] [Table 4]

Claims

[Claim 1] Fe—C containing 95% or more Fe+Co by mass and 25 to 60% Co. a coating step of coating a magnesium hydroxide solution on the surface of a o-based alloy substrate; Baking process in which the material is baked at 600 to 800°C to form a magnesium oxide coating on the substrate. The method for manufacturing a coated member includes the steps of:

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