Electrical steel sheet and method for producing electrical steel sheet

Abstract

An electrical steel sheet (1) comprises a base material (11) which has a crystal structure and a modified layer (12) which is formed on a part of the surface of the base material (11), includes the same components as the base material (11), and has an amorphous structure or a crystal structure finer than that of the base material (11). The thickness (T1) of the modified layer (12) is more than 1 μm and not more than 20 μm.

Description

Electrical steel sheet and method for manufacturing the same

[0001] The present disclosure relates to an electromagnetic steel sheet used for the iron core of a motor and a method for manufacturing the electromagnetic steel sheet.

[0002] Many manufacturing techniques have been developed to improve the AC magnetic properties of electromagnetic steel sheets (soft magnetic materials). For example, Patent Document 1 discloses a technique for improving the AC magnetic properties of electromagnetic steel sheets by diffusing Si (silicon) by vapor deposition of SiCl gas onto the manufactured electromagnetic steel sheets.

[0003] Japanese Unexamined Patent Publication No. 62-227078

[0004] A laminated electromagnetic steel sheet, in which multiple electromagnetic steel sheets are stacked, is sometimes used for the iron core of a motor. For the purpose of improving motor performance and controlling functions, it is conceivable to apply the technology described in Patent Document 1 to the surface of each of the electromagnetic steel sheets that make up the laminated electromagnetic steel sheet. However, if this technology is applied to the entire surface of the electromagnetic steel sheet before punching, there is a problem that the hardness of the punched portion of the electromagnetic steel sheet increases, making punching of the electromagnetic steel sheet difficult. Furthermore, if this technology is applied to the electromagnetic steel sheet after punching, there is a problem that warping occurs in the electromagnetic steel sheet due to the vapor deposition process, making the subsequent formation of the laminated electromagnetic steel sheet difficult.

[0005] This problem is not limited to cases where the electromagnetic steel sheet is used for the iron core of a motor, but also applies to cases where the electromagnetic steel sheet is used for the iron core of a generator or other magnetic parts.

[0006] The present disclosure has been made in view of the above, and aims to provide an electrical steel sheet that can achieve both improved AC magnetic properties and ease of punching.

[0007] In order to solve the above-mentioned problems and achieve the object, the electrical steel sheet according to the present disclosure comprises a base material having a crystalline structure, and a modified layer formed on a part of the surface of the base material, the modified layer containing the same components as the base material, and having a finer crystalline structure or amorphous structure than the base material. The modified layer has a thickness of more than 1 μm and not more than 20 μm.

[0008] The electrical steel sheet according to the present disclosure has the effect of achieving both improved AC magnetic properties and ease of punching.

[0009] FIG. 1 is a diagram showing a schematic example of the cross-sectional structure of an electromagnetic steel sheet according to the first embodiment; FIG. 2 is a flowchart showing an example of the procedure for a method for manufacturing an electromagnetic steel sheet according to the first embodiment; FIG. 3 is a schematic diagram showing an example of the configuration of an electric discharge modification apparatus used in the method for manufacturing an electromagnetic steel sheet according to the first embodiment; FIG. 4 is a diagram showing an example of an inverse pole figure orientation map obtained by analyzing a cross section of an electromagnetic steel sheet according to the first embodiment by electron backscatter diffraction; FIG. 5 is a flowchart showing an example of the procedure for a method for manufacturing an electromagnetic steel sheet according to the second embodiment;

[0010] Hereinafter, an electrical steel sheet and a method for manufacturing an electrical steel sheet according to an embodiment will be described in detail with reference to the drawings.

[0011] First Embodiment Figure 1 is a diagram schematically showing an example of the cross-sectional structure of an electromagnetic steel sheet 1 according to a first embodiment. The electromagnetic steel sheet 1 comprises a substrate 11 having a crystalline structure, and a modified layer 12 formed on part of the surface of the substrate 11, which contains the same components as the substrate 11 and has a finer crystalline structure or amorphous structure than the substrate 11. In the example of Figure 1, the up-down direction of the paper coincides with the vertical direction, and the modified layer 12 is formed on the top surface of the substrate 11. In this embodiment, a laminated electromagnetic steel sheet in which a plurality of electromagnetic steel sheets 1 are stacked will be described as being used for the iron core of a motor.

[0012] The substrate 11 is a metal plate. In this embodiment, the substrate 11 contains Fe (iron) as a main component, 0.1 wt % to 10.0 wt % of Si, 0.02 wt % to 4.0 wt % of Mn (manganese), 0.001 wt % to 4.0 wt % of Al (aluminum), and trace components such as C (carbon), P (phosphorus), S (sulfur), N (nitrogen), and O (oxygen), but is not limited thereto.

[0013] The modified layer 12 is a microcrystalline layer having a crystal structure with a smaller crystal grain size than the substrate 11, or a layer having an amorphous structure. Therefore, the modified layer 12 has excellent AC magnetic properties. In this embodiment, the modified layer 12 is primarily composed of Fe, Si, Mn, Al, and trace elements such as C, P, S, N, and O. However, the modified layer 12 is not limited to this as long as it contains the same components as the substrate 11. Because the portion of the surface of the substrate 11 where the modified layer 12 is formed (the portion to be modified) can be arbitrarily selected, unnecessary portions of the electromagnetic steel sheet 1 (the portion to be punched before stacking multiple electromagnetic steel sheets 1) are not modified, preventing the electromagnetic steel sheet 1 from being difficult to punch. Furthermore, the thickness T1 of the modified layer 12 is greater than 1 μm and less than 20 μm, improving the ease of punching the electromagnetic steel sheet 1. This allows for both improved AC magnetic properties and ease of punching of the electromagnetic steel sheet 1.

[0014] Next, a description will be given of the component composition of the substrate 11 according to embodiment 1. For elements whose concentration varies in the thickness direction, the average value in the thickness direction is taken as the content of this element.

[0015] In the substrate 11 according to the first embodiment, Si is an element effective in increasing the resistivity of the electrical steel sheet 1 and reducing iron loss, and is therefore added in an amount of 0.1 wt% or more. On the other hand, if Si is added in an amount exceeding 10.0 wt%, the steel becomes significantly embrittled. Therefore, the Si content is set to a range of 0.1 wt% to 10.0 wt%. The Si content is preferably in the range of 1.0 wt% to 6.5 wt%, and more preferably in the range of 1.5 wt% to 4.5 wt%.

[0016] In the base material 11 according to the first embodiment, Mn is added in an amount of 0.02 wt% or more to suppress red shortness during hot rolling. However, if the Mn content exceeds 4.0 wt%, the magnetic flux density decreases and embrittlement becomes significant. Therefore, the Mn content is set to a range of 0.02 wt% to 4.0 wt%. The Mn content is preferably set to a range of 0.02 wt% to 2.0 wt%.

[0017] In the substrate 11 according to the first embodiment, Al is an element that is effective in increasing the resistivity of the electrical steel sheet 1 and reducing iron loss, and is therefore added in an amount of 0.001 wt% or more. However, if the Al content exceeds 4.0 wt%, embrittlement becomes a problem, similar to that of Si. Therefore, the Al content is set to the range of 0.001 wt% to 4.0 wt%.

[0018] Furthermore, the substrate 11 according to the first embodiment contains trace elements of C, P, S, N, and O, but these are present at impurity levels. C is a harmful element that causes magnetic aging and degrades the magnetic properties of the finished sheet. Therefore, the C content should be 0.005 wt% or less, preferably 0.003 wt% or less. High P content severely embrittles the steel, significantly reducing productivity. Therefore, the P content should be 0.2 wt% or less, preferably 0.001 wt% or less. S is a harmful element that generates sulfides such as MnS (manganese sulfide) and increases iron loss. Therefore, the S content should be 0.005 wt% or less, preferably 0.003 wt% or less. N is a harmful element that generates nitrides and increases iron loss. Therefore, the N content should be 0.005 wt% or less, preferably 0.003 wt% or less. O is a harmful element that generates oxides and deteriorates magnetic properties, so the O content should be 0.005 wt % or less, preferably 0.003 wt % or less.

[0019] Next, a method for manufacturing the electromagnetic steel sheet 1 according to the first embodiment will be described with reference to Fig. 2. Fig. 2 is a flowchart showing an example of the steps of the method for manufacturing the electromagnetic steel sheet 1 according to the first embodiment. The method for manufacturing the electromagnetic steel sheet 1 according to the first embodiment includes a forming step.

[0020] The forming step is a step of forming, by electrical discharge, a modified layer 12 containing the same components as the substrate 11 and having a thickness of more than 1 μm and not more than 20 μm on a portion of the surface of a substrate 11 having a crystalline structure (step S1). In the forming step, the modified layer 12 is formed at an arbitrary position on the surface of the substrate 11. By the forming step, an electrical steel sheet 1 having the modified layer 12 shown in FIG. 1 is manufactured. The electrical steel sheet 1 is, for example, a non-oriented electrical steel strip defined in Japanese Industrial Standards (JIS) C 2552:2014.

[0021] Here, the discharge reforming device 2 used in the forming step of the method for manufacturing the electromagnetic steel sheet 1 and the mechanism by which the reformed layer 12 is formed will be described with reference to Fig. 3. Fig. 3 is a schematic diagram showing an example of the configuration of the discharge reforming device 2 used in the method for manufacturing the electromagnetic steel sheet 1 according to the first embodiment. As shown in Fig. 3, the discharge reforming device 2 includes a processing tank 21, a reforming electrode 22, and a power supply device 23.

[0022] The machining tank 21 is a member that supports the substrate 11 (electromagnetic steel sheet 1) and stores a machining fluid 24. The machining tank 21 is filled with the machining fluid 24. The modifying electrode 22 and substrate 11, where discharge occurs, are immersed in the machining fluid 24 in the machining tank 21. The machining fluid 24 is, for example, water or machining oil. The machining fluid 24 plays a role in cooling the modifying electrode 22 and substrate 11, preventing fires, and removing machining chips generated by discharge.

[0023] The reforming electrode 22 is a rectangular plate-shaped member containing the same components as the substrate 11. The reforming electrode 22 is housed in the processing tank 21. The reforming electrode 22 is movable in the horizontal and vertical directions. The substrate 11 and the reforming electrode 22 are arranged with a gap therebetween in the vertical direction. The reforming electrode 22 is formed, for example, from a material containing 5.0 wt % or more of Si.

[0024] The power supply device 23 is a device that applies a voltage between the substrate 11 and the modifying electrode 22. The substrate 11 and the modifying electrode 22 are electrically connected via wiring 25 and the power supply device 23.

[0025] When forming the reformed layer 12 on the substrate 11 using such an electric discharge reforming device 2, first, the reforming electrode 22 is moved so that it is sufficiently close to the substrate 11 in the processing tank 21. Next, a potential difference is applied between the substrate 11 and the reforming electrode 22 using the power supply 23, forming a discharge region 26 between the substrate 11 and the reforming electrode 22. Heat generated by discharge in the discharge region 26 locally reforms the surface of the substrate 11. Furthermore, the reforming electrode 22 becomes hot, causing a portion of the reforming electrode 22 to fall off. The fallen portion of the reforming electrode 22 becomes charged and is accelerated by the high electric field between the substrate 11 and the reforming electrode 22, colliding with the surface of the substrate 11 at high speed. As a result, the surface of the substrate 11 is locally reformed, and at the same time, the surface of the substrate 11 and the fallen portion of the reforming electrode 22 mix and are retained on the surface of the substrate 11. Through the above process, the reformed layer 12 is formed on a portion of the surface of the substrate 11. In other words, an electrical steel sheet 1 having a surface that has been reformed is manufactured. By moving the modifying electrode 22, the modified layer 12 can be formed at any position on the surface of the substrate 11.

[0026] 3 shows an example of an electrical discharge surface modification method using an electrical discharge modification device 2 as a method for forming the modified layer 12 on the surface of the substrate 11, but the method is not limited to this as long as the modifying component undergoes plastic deformation and is retained on the surface of the substrate 11. For example, a cold spray method or a thermal spray method under a reducing atmosphere may also be used as a method for forming the modified layer 12 on the surface of the substrate 11.

[0027] FIG. 4 is a diagram showing an example of an inverse pole figure orientation map obtained by analyzing the cross section of the electrical steel sheet 1 according to the first embodiment by electron backscatter diffraction. In FIG. 4 , a modified layer 12 is formed on the upper surface of a substrate 11. The substrate 11 has coarse crystal grains 11a with diameters of 10 μm or more. On the other hand, the modified layer 12 has a crystalline structure or an amorphous structure smaller than 1 μm, which can be observed by electron backscatter diffraction. The crystal grains 12a of the modified layer 12 (see FIGS. 7 and 8 ) have a smaller crystal grain size than the crystal grains 11a of the substrate 11. As described above, when a part of the detached modifying electrode 22 collides with the surface of the substrate 11, a part of the surface of the substrate 11 is locally modified. At the same time, the surface of the substrate 11 and the part of the detached modifying electrode 22 are mixed and held on the surface of the substrate 11, thereby forming a modified layer 12 on the surface of the substrate 11. Since a part of the detached reforming electrode 22 collides with the surface of the substrate 11 at high speed, the grain size of the crystal grains 12 a of the reforming layer 12 becomes smaller than the grain size of the crystal grains 11 a of the substrate 11 .

[0028] The average grain size of the crystal grains 11a constituting the substrate 11 and the average grain size of the crystal grains 12a constituting the modified layer 12 are values ​​measured using images obtained by SEM, transmission electron microscope (TEM), or metallurgical microscope. Within the observed area, not only interphase interfaces and grain boundaries but also all crystal boundaries are observed, and the diameter of the crystalline region surrounded by the crystal boundaries is taken as the crystal grain size. If the crystal boundaries are difficult to see, it is preferable to etch the crystal boundaries using a wet method using a nital solution or a dry etching method. As a general rule, the average grain size is measured in a region containing at least 100 crystal grains 11a, 12a selected from a representative portion within the observed area. Although the number of crystal grains 11a, 12a may be smaller, in this case, it is necessary to measure a portion that is statistically sufficiently representative of the entire region. The average crystal grain size can be determined by photographing the observation area, defining an appropriate rectangular region on the photograph plane, i.e., the magnified projection of the photographed surface, and applying the Jeffries method to the region. The Jeffries method is also known as the area measurement method. When observing with an SEM or metallurgical microscope, the crystal boundary width may be too small for the resolution to be observed. In this case, the measured average crystal grain size provides an upper limit for the actual crystal grain size. Specifically, an average crystal grain size measurement of up to 20 μm is acceptable. However, if phenomena such as the absence of clear diffraction peaks in X-ray diffraction (XRD) or the confirmation of superparamagnetism on the magnetic curve indicate that some or all of the magnetic material may be below the lower limit of 1 nm, the actual crystal grain size must be determined again by TEM observation.

[0029] The average crystal grain size may be determined using the results of XRD instead of the results of microscopic observation. In this case, the full width at half maximum of the diffraction peak from the (110) plane in the X-ray diffraction pattern obtained from the XRD experiment can be used to determine the average crystal grain size D using the Scherrer equation. The full width at half maximum of the diffraction peak from the (110) plane can be determined by peak resolution using a pseudo-Voigt function for the diffraction pattern. The average crystal grain size D can be determined from the Scherrer equation given by the following equation (1), where B is the full width at half maximum, θ is the Bragg angle, K is the Scherrer constant, and λ is the wavelength of the X-ray. In the first embodiment, however, it is assumed that the X-ray wavelength λ is 0.154 nm and the Scherrer constant K is 0.891. The Bragg angle is half the diffraction angle 2θ.

[0030]

[0031] Next, the effects of the electromagnetic steel sheet 1 according to the first embodiment will be described.

[0032] In this embodiment, as shown in FIG. 1 , the electromagnetic steel sheet 1 includes a substrate 11 having a crystalline structure and a modified layer 12 formed on a portion of the surface of the substrate 11. The modified layer 12 contains the same components as the substrate 11 and has a finer crystalline or amorphous structure than the substrate 11. The thickness T1 of the modified layer 12 is greater than 1 μm and less than 20 μm. That is, an electromagnetic steel sheet 1 can be obtained in which the modified layer 12, which has a fine crystalline or amorphous structure and a thickness T1 greater than 1 μm and less than 20 μm, is formed at any position on the substrate 11 before punching. This improves the AC magnetic properties of the electromagnetic steel sheet 1. Furthermore, compared to when the modified layer 12 is formed on the entire surface of the substrate 11, an increase in hardness at the punched portion of the electromagnetic steel sheet 1 can be avoided, thereby suppressing wear on the press die used in the punching process. This improves the ease of punching the electromagnetic steel sheet 1. Therefore, this embodiment can achieve both improved AC magnetic properties and ease of punching.

[0033] Next, a modification of the electromagnetic steel sheet 1 according to the first embodiment will be described.

[0034] In the first embodiment, it is assumed that the surface-modified electromagnetic steel sheet 1 is applied to the iron core of a motor, for which complex punching shape design and magnetic design are important, that is, that a laminated electromagnetic steel sheet having the surface-modified electromagnetic steel sheet 1 is used. Because the hardness of the electromagnetic steel sheet 1 can be improved by forming the modified layer 12 having a fine crystalline structure or amorphous structure, the application of the electromagnetic steel sheet 1 according to the first embodiment is not limited to motors. For example, the electromagnetic steel sheet 1 according to the first embodiment can be applied to the iron core of a generator. Furthermore, the electromagnetic steel sheet 1 according to the first embodiment can also be applied to magnetic parts such as automobile parts and aircraft drive parts, which are required to have sufficient strength due to stress and also good thermal conductivity.

[0035] Second Embodiment Next, an electrical steel sheet 1A according to a second embodiment will be described with reference to Figures 5 to 8. In this embodiment, the crystalline structure of the modified layer 12 differs from that of the first embodiment. In the second embodiment, parts that overlap with those of the first embodiment are denoted by the same reference numerals, and descriptions thereof will be omitted.

[0036] First, a method for manufacturing an electromagnetic steel sheet 1A according to the second embodiment will be described with reference to Fig. 5. Fig. 5 is a flowchart showing an example of the steps of the method for manufacturing an electromagnetic steel sheet 1A according to the second embodiment. The method for manufacturing an electromagnetic steel sheet 1A according to the second embodiment includes a forming step and a heat treatment step.

[0037] In the forming step, the modified layer 12 is formed by discharge on a part of the surface of the substrate 11 (step S11). The forming step is the same as the forming step in the first embodiment (step S1 in FIG. 2), and therefore a description thereof will be omitted here.

[0038] The heat treatment step is a step of heating the base material 11 and the modified layer 12 in a reducing atmosphere at a heating temperature of 500° C. to 800° C. after the formation step. By performing the heat treatment step, it is possible to change the crystalline structure of the modified layer 12 in the electrical steel sheet 1A (step S12).

[0039] Here, the heat treatment device 3 used in the heat treatment step of the manufacturing method of the electromagnetic steel sheet 1A and the mechanism for changing the crystalline structure of the modified layer 12 will be described with reference to Fig. 6. Fig. 6 is a schematic diagram showing an example of the configuration of the heat treatment device 3 used in the manufacturing method of the electromagnetic steel sheet 1A according to the second embodiment. As shown in Fig. 6, the heat treatment device 3 includes a heat treatment furnace 31 and a heating device 32.

[0040] The heat treatment furnace 31 accommodates the electromagnetic steel sheet 1A and a heating device 32. In this embodiment, the heat treatment furnace 31 is an atmospheric furnace, but may be, for example, a vacuum furnace. The heating device 32 heats the electromagnetic steel sheet 1A. The heating device 32 is, for example, a heater.

[0041] When using such a heat treatment device 3 to change the crystalline structure of the modified layer 12 of the electromagnetic steel sheet 1A, first, the electromagnetic steel sheet 1A having the modified layer 12 is placed in a heat treatment furnace 31. Next, a reducing atmosphere is created inside the heat treatment furnace 31, and the electromagnetic steel sheet 1A is heated in this state using a heating device 32. In one example, the reducing atmosphere is a hydrogen atmosphere, the heating temperature is 500°C or higher and 800°C or lower, and the heating time is one hour. Experiments conducted by the inventors have confirmed that the crystalline structure of the modified layer 12 does not change when the heating temperature is 450°C and the heating time is one hour. Furthermore, it has been confirmed that when the heating temperature is 850°C and the heating time is one hour, the crystal grains 12a of the modified layer 12 become coarse, and the eddy current loss coefficient becomes equivalent to that of an electromagnetic steel sheet without the modified layer 12. On the other hand, it has been confirmed that when the heating temperature is 550°C or 750°C and the heating time is 1 hour, the crystalline structure of the modified layer 12 changes, and the crystalline orientation of some of the crystal grains 12a changes continuously within the modified layer 12. The heat treatment process is performed in a reducing atmosphere at a temperature and for a time sufficient to change the crystalline structure of the modified layer 12. Note that, although the shape of the electromagnetic steel sheet 1A in the example of FIG. 6 is a plate, the shape of the electromagnetic steel sheet 1A may be a laminated structure, a punched shape, or both. The crystalline structure of the modified layer 12 can be changed by heating the electromagnetic steel sheet 1A using a heating device 32 in a heat treatment furnace 31. Through the above process, an electromagnetic steel sheet 1A with a changed crystalline structure of the modified layer 12 is manufactured.

[0042] FIG. 7 is a diagram showing an example of an inverse pole figure orientation map obtained by analyzing a cross section of an electrical steel sheet 1A according to embodiment 2 using electron backscatter diffraction. FIG. 7 illustrates a case in which the crystal structure of a modified layer 12 is changed by heating the electrical steel sheet 1A at a certain heating temperature. In FIG. 7, a modified layer 12 is formed on the upper surface of a substrate 11. The substrate 11 has coarse crystal grains 11a with a diameter of 10 μm or more. On the other hand, the modified layer 12 has crystal grains 12a with an average crystal grain size of 1 μm. The crystal grain size of the crystal grains 12a in the modified layer 12 is smaller than the crystal grain size of the crystal grains 11a in the substrate 11. By subjecting the modified layer 12 to a heat treatment process, the fine crystals in the modified layer 12 undergo grain growth.

[0043] FIG. 8 is a diagram showing another example of an inverse pole figure orientation map obtained by analyzing a cross section of the electromagnetic steel sheet 1A according to the second embodiment using electron backscatter diffraction. FIG. 8 illustrates an electromagnetic steel sheet 1A obtained by a heat treatment process performed at a higher heating temperature than that used in the heat treatment process for manufacturing the electromagnetic steel sheet 1A shown in FIG. 7 . In FIG. 8 , a modified layer 12 is formed on the upper surface of a substrate 11. The substrate 11 has coarse crystal grains 11a with a diameter of 10 μm or more. On the other hand, the crystal grains 12a of the modified layer 12 have a smaller crystal grain size than the crystal grains 11a of the substrate 11. The average crystal grain size of the crystal grains 12a constituting the modified layer 12 is less than 1 / 10 of the average crystal grain size of the crystal grains 11a constituting the substrate 11. By increasing the heating temperature during the heat treatment process of the modified layer 12, the grain growth of the fine crystals in the modified layer 12 is further accelerated, and the crystal structure of the modified layer 12 can be changed to have crystal grains 12a with a larger crystal grain size than that of the example shown in FIG. 7 . Furthermore, as interdiffusion between the substrate 11 and the modified layer 12 progresses, the crystal orientation of the crystal grains 12a located at the boundary between them changes continuously within the modified layer 12. In Fig. 8, the area where the crystal orientation of the crystal grains 12a changes continuously within the modified layer 12 is surrounded by a dashed line.

[0044] Next, the effects of the electromagnetic steel sheet 1A according to the second embodiment will be described.

[0045] First, an electromagnetic steel sheet 1B according to a comparative example will be described with reference to FIG. 9 . FIG. 9 is a diagram showing an example of an inverse pole figure orientation map obtained by analyzing a cross section of the electromagnetic steel sheet 1B according to the comparative example using electron backscatter diffraction. In the comparative example, the electromagnetic steel sheet 1B was subjected to a heat treatment process at a heating temperature higher than that of the heat treatment process in embodiment 2. In the comparative example, the heating temperature in the heat treatment process is 850°C, and the heating time is 1 hour. In FIG. 9 , a modified layer 12 is formed on the upper surface of the substrate 11. The average grain size of the crystal grains 12a constituting the modified layer 12 is at least 1 / 10 of the average grain size of the crystal grains 11a constituting the substrate 11. Furthermore, due to grain growth of the fine crystals in the modified layer 12, the modified layer 12 is expanded, and the thickness T2 of the modified layer 12 exceeds 20 μm. According to experiments and research by the present inventors, it has been found that when the thickness T2 of the modified layer 12 exceeds 20 μm, as in the comparative example, the AC magnetic properties of the electromagnetic steel sheet 1B are degraded.

[0046] In contrast, in this embodiment, as shown in Fig. 7, the crystal grains 12a constituting the modified layer 12 have an average crystal grain size of 1 µm, or as shown in Fig. 8, the average crystal grain size of the crystal grains 12a constituting the modified layer 12 is less than 1 / 10 of the average crystal grain size of the crystal grains 11a constituting the base material 11. In this way, it is possible to achieve both low loss and high magnetic flux density. This improves the AC magnetic properties of the electrical steel sheet 1A.

[0047] Next, the AC magnetic properties of the electrical steel sheet, which are an effect of the present disclosure, will be further described using examples and comparative examples.

[0048] Example 1 A test piece was used, which was an electromagnetic steel sheet having the same configuration as the electromagnetic steel sheet 1 according to the above-described embodiment 1. Using the manufacturing method for the electromagnetic steel sheet 1 according to the above-described embodiment 1, a test piece was produced in which a modified layer having a square area with a side length of 20 mm was formed on the surface of a substrate having a width of 22 mm, a length of 150 mm, and a thickness of 0.35 mm.

[0049] Example 2 A test piece of an electromagnetic steel sheet having the same configuration as the electromagnetic steel sheet 1A according to the above-described embodiment 2 was used. Using the manufacturing method of the electromagnetic steel sheet 1A according to the above-described embodiment 2, a test piece was produced in which a modified layer having a square area with a side length of 20 mm was formed on the surface of a substrate having a width of 22 mm, a length of 150 mm, and a thickness of 0.35 mm. In Example 2, the heating temperature in the heat treatment step was 750°C, and the heating time was 1 hour.

[0050] (Comparative Example 1) A test piece of an electromagnetic steel sheet having the same configuration as the electromagnetic steel sheet 1B according to the comparative example described above was used. Using the manufacturing method of the electromagnetic steel sheet 1B according to the comparative example described above, a test piece was produced in which a modified layer having a square area with a side length of 20 mm was formed on the surface of a substrate having a width of 22 mm, a length of 150 mm, and a thickness of 0.35 mm. In the heat treatment step of Comparative Example 1, the heating temperature was 850°C, and the heating time was 1 hour.

[0051] Comparative Example 2: A test piece of an electrical steel sheet having no modified layer was used. A test piece of an electrical steel sheet having a width of 22 mm, a length of 150 mm, and a thickness of 0.35 mm was produced by a known manufacturing method.

[0052] (Test Method) Magnetic hysteresis curves were obtained at multiple frequencies by AC magnetic measurement for the test pieces of the electrical steel sheets of Examples 1 and 2 and Comparative Examples 1 and 2. Iron loss was calculated from each of the obtained curves, and fitting was performed by the least squares method as a function of frequency expressed by the following equation (2) to obtain k c , k h Here, P cm is the iron loss, f is the measurement frequency, and k c is called the eddy current loss coefficient, and k h is called the hysteresis coefficient.

[0053]

[0054] Eddy current loss coefficient k c is a function P cm / f, is a proportional coefficient of the measurement frequency f, and is a value that indicates the AC magnetic properties. c The smaller the value of f, the smaller the increase in iron loss when the measurement frequency f increases.

[0055] Table 1 shows the measurement results of the eddy current loss coefficients of the electrical steel sheets according to Examples 1 and 2 and Comparative Examples 1 and 2. In Table 1, the eddy current loss coefficient of Comparative Example 2 is set to 100, and the eddy current loss coefficients of Examples 1 and 2 and Comparative Example 1 are expressed as a ratio to Comparative Example 2. The unit of the eddy current loss coefficient is %. A smaller eddy current loss coefficient means that the AC magnetic properties of the electrical steel sheet are more excellent.

[0056]

[0057] As is clear from Table 1, the eddy current loss coefficient is reduced in Example 1 compared to Comparative Example 2. This shows that forming a modified layer having a finer crystalline structure or amorphous structure than the substrate on the surface of the substrate is effective in improving the AC magnetic properties of the electrical steel sheet.

[0058] Furthermore, the eddy current loss coefficient of Example 2 is further reduced, and the reduction is the greatest, compared to Example 1. This shows that heating the modified layer in an appropriate atmosphere, at an appropriate heating temperature, and for an appropriate heating time can appropriately change the grain size or crystal orientation of the crystal grains inside the modified layer, which is effective in improving the AC magnetic properties of the electrical steel sheet.

[0059] On the other hand, the eddy current loss coefficient of Comparative Example 1 was equivalent to that of Comparative Example 2. This shows that if the crystal grains of the modified layer are allowed to grow coarsely, the excellent AC magnetic properties obtained by forming the modified layer are lost.

[0060] The configurations shown in the above embodiments are merely examples, and may be combined with other known technologies, or different embodiments may be combined with each other. It is also possible to omit or modify parts of the configurations as long as they do not deviate from the gist of the invention.

[0061] 1, 1A, 1B: electromagnetic steel sheet, 2: discharge reforming device, 3: heat treatment device, 11: substrate, 11a, 12a: crystal grains, 12: reformed layer, 21: machining tank, 22: reforming electrode, 23: power supply device, 24: machining liquid, 25: wiring, 26: discharge area, 31: heat treatment furnace, 32: heating device, T1, T2: thickness.

Claims

1. An electrical steel sheet comprising: a base material having a crystalline structure; and a modified layer formed on a portion of the surface of the base material, containing the same components as the base material and having a finer crystalline structure or amorphous structure than the base material, wherein the thickness of the modified layer is greater than 1 μm and less than 20 μm.

2. The electrical steel sheet according to claim 1, characterized in that the average grain size of the grains constituting the modified layer is less than 1 / 10 of the average grain size of the grains constituting the base material.

3. The electrical steel sheet according to claim 1 or 2, wherein the crystal orientation of the crystal grains in the modified layer changes continuously within the modified layer.

4. An electrical steel sheet according to any one of claims 1 to 3, characterized in that the base material contains 0.1 wt% to 10.0 wt% Si, 0.02 wt% to 4.0 wt% Mn, and 0.001 wt% to 4.0 wt% Al, and contains trace elements of C, P, S, N, and O, with the main element being Fe.

5. A method for manufacturing an electrical steel sheet according to any one of claims 1 to 4, comprising a forming step of forming, by electrical discharge, a modified layer on a part of the surface of a base material having a crystalline structure, the modified layer containing the same components as the base material and having a thickness of 1 μm to 20 μm.

6. The method for manufacturing an electrical steel sheet according to claim 5, further comprising, after the forming step, a heat treatment step of heating the base material and the modified layer at a heating temperature of 500°C or more and 800°C or less in a reducing atmosphere.

7. A method for manufacturing an electrical steel sheet according to claim 5 or 6, characterized in that the base material contains 0.1 wt% to 10.0 wt% Si, 0.02 wt% to 4.0 wt% Mn, and 0.001 wt% to 4.0 wt% Al, and contains trace elements of C, P, S, N, and O, with the main element being Fe.

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