Grain-oriented electrical steel sheet and method for manufacturing the same

Abstract

This grain-oriented electromagnetic steel sheet is provided with a base steel sheet and an insulating coating film arranged on the base steel sheet in contact with the base steel sheet. When a cross-sectional surface of the grain-oriented electromagnetic steel sheet which is cut in a direction parallel with the thickness direction and perpendicular to the width direction is observed, the base steel sheet has an internal oxidation SiO2 in a boundary region with the insulating coating film, the insulating coating film has voids and an oxide containing iron and phosphorus, the area ratio of the voids relative to the area of the boundary region with the insulating coating film is 0.010 to 3.0%, and the area ratio of the oxide containing iron and phosphorus relative to the area of the insulating coating film is 0.10 to 5.0%.

Description

[Technical Field] 【0001】 This invention relates to grain-oriented electrical steel sheets. In particular, this invention relates to grain-oriented electrical steel sheets that have excellent coating adhesion without relying on forsterite coatings, and to a method for manufacturing the same. This application claims priority based on Japanese Patent Application No. 2022-070070, filed in Japan on April 21, 2022, and the contents of that application are incorporated herein by reference. [Background technology] 【0002】 Grain-oriented electrical steel sheets are primarily used in transformers. Since transformers are continuously energized and generate energy losses over long periods from installation to disposal, the energy loss when magnetized with alternating current, i.e., iron loss, is a major indicator of transformer performance. 【0003】 Many methods have been proposed to reduce iron loss in grain-oriented electrical steel sheets. For example, regarding the steel sheet structure, there is a method called the {110} orientation. <001> These methods include increasing the concentration of particles in a specific direction, increasing the content of solid solution elements such as Si to improve electrical resistance in steel plates, and reducing the thickness of steel plates. 【0004】 Furthermore, it is known that applying tension to steel sheets is an effective method for reducing iron loss. For this reason, a coating is usually formed on the surface of grain-oriented electrical steel sheets to reduce iron loss. This coating reduces the iron loss of the steel sheet as a single sheet by applying tension to the grain-oriented electrical steel sheet. This coating also reduces the iron loss of the core by ensuring electrical insulation between the steel sheets when grain-oriented electrical steel sheets are used in laminated form. 【0005】 In the case of coated grain-oriented electrical steel sheets, a forsterite coating, which is an oxide film containing Mg, is formed on the surface of the base steel sheet, and an insulating coating is further formed on the surface of the forsterite coating. In other words, in this case, the coating on the base steel sheet includes the forsterite coating and the insulating coating. Each of the forsterite coating and the insulating coating performs both insulating functions and tension-applying functions to the base steel sheet. 【0006】 The forsterite coating is formed during a finish annealing process that induces secondary recrystallization of the steel sheet. This process involves a reaction between an annealing separator, primarily composed of magnesia (MgO), and silicon oxide (SiO2) formed on the base steel sheet during decarburization annealing. The reaction occurs during a heat treatment at 900-1200°C for 20 hours or more. 【0007】 The insulating coating is formed by applying a coating solution containing, for example, phosphate and colloidal silica to a steel sheet after finish annealing, and then baking and drying it at 350°C to 1150°C for 5 seconds or more. 【0008】 For these coatings to perform their functions of insulation and tensioning the base steel sheet, a high degree of adhesion between the coating and the base steel sheet is necessary. 【0009】 Conventionally, the above-mentioned adhesion has been ensured primarily by the anchoring effect caused by the irregularities at the interface between the base steel sheet and the forsterite coating. However, in recent years, it has become clear that these interface irregularities hinder the movement of magnetic domain walls when grain-oriented electrical steel sheets are magnetized, thus hindering the reduction of iron loss. 【0010】 Therefore, in order to further reduce iron loss, techniques for forming an insulating coating on a smooth surface of the base steel sheet without the presence of a forsterite coating on the base steel sheet have been proposed, for example, in Patent Documents 1 to 3. 【0011】 The technology disclosed in Patent Document 1 involves removing the formed forsterite film by pickling or the like, and then smoothing the surface of the base steel sheet by chemical polishing or electropolishing. The technology disclosed in Patent Document 2 involves using an annealing separating agent containing alumina (Al2O3) to suppress the formation of the forsterite film itself and smooth the surface of the base steel sheet. The technology disclosed in Patent Document 3 involves using an annealing separating agent containing bismuth chloride to suppress the formation of the forsterite film itself and smooth the surface of the base steel sheet. 【0012】 While these technologies can smooth the surface of the base steel sheet, they have the challenge of difficulty in achieving good adhesion of the insulating coating after its formation. If the coating does not adhere well, it becomes difficult to apply tension to the base steel sheet and to ensure electrical insulation between the laminated steel sheets. 【0013】 Therefore, techniques for smoothing the surface of the base steel sheet and then improving the adhesion of the coating have been proposed, for example, in Patent Documents 4 to 6. 【0014】 The technology disclosed in Patent Document 4 involves performing finish annealing using an annealing separation agent containing alumina, followed by oxide film formation annealing to control thermal history and oxygen partial pressure, and then forming an insulating coating. In Patent Document 4, an intermediate oxide film layer of externally oxidized SiO2 is formed on the base steel sheet, and an insulating coating is formed on the intermediate oxide film layer. Patent Document 4 attempts to improve coating adhesion by solid-solving elements such as Mn in this intermediate oxide film layer. 【0015】 The technology disclosed in Patent Document 5 involves performing finish annealing using an annealing separation agent containing bismuth chloride, followed by pickling, then heat treatment to control oxygen concentration and dew point, and subsequently forming an insulating coating. In Patent Document 5, etch pits are formed on the surface of the base steel sheet, a silica-containing oxide layer and an iron-based oxide layer are formed on the base steel sheet, and an insulating coating is formed on the iron-based oxide layer. Patent Document 5 attempts to improve coating adhesion by creating etch pits on the surface of the base steel sheet. 【0016】 In the technique disclosed in Patent Document 6, finish annealing is performed using an annealing separation agent containing bismuth chloride or the like, and then an insulating film containing a metal compound and an insulating film not containing a metal compound are formed. In Patent Document 6, an intermediate layer is formed on a base steel plate, and an insulating film is formed on the intermediate layer. In Patent Document 6, attempts are made to improve the film adhesion by optimally controlling each of the manufacturing processes. 【Prior Art Documents】 【Patent Documents】 【0017】 【Patent Document 1】 Japanese Patent Application Laid-Open No. Sho 49-096920 【Patent Document 2】 Japanese Patent Application Laid-Open No. Hei 06-049534 【Patent Document 3】 Japanese Patent Application Laid-Open No. Hei 07-054155 【Patent Document 4】 International Publication No. 2020 / 012666 【Patent Document 5】 International Publication No. 2020 / 149345 【Patent Document 6】 International Publication No. 2020 / 149325 【Summary of the Invention】 【Problems to be Solved by the Invention】<​​​​​​​​​​ The present invention has been made in view of the above problems. An object of the present invention is to provide a grain-oriented electromagnetic steel sheet having excellent film adhesion without relying on a forsterite film and a method for manufacturing the same. 【Means for Solving the Problems】 【0021】 The gist of the present invention is as follows. 【0022】 (1) The grain-oriented electromagnetic steel sheet according to one aspect of the present invention has a base steel sheet and an insulating film disposed in contact with the base steel sheet, The base steel sheet has, as a chemical composition, in mass %, Si: 3.0 to 4.0%, Mn: 0.010 to 0.50%, and the balance is Fe and impurities, When viewed in a cut surface where the cutting direction is parallel to the plate thickness direction and perpendicular to the plate width direction, the base steel sheet has internal oxidized SiO2 in a base steel sheet interface region within a range of 2.0 μm from the interface with the insulating film toward the plate thickness direction, When viewed in the cut surface, the insulating film contains voids and an oxide containing iron and phosphorus, The area ratio of the voids is 0.010 to 3.0% with respect to the area of the insulating film interface region within a range of 0.5 μm from the interface with the base steel sheet toward the plate thickness direction, and The area ratio of the oxide containing iron and phosphorus is 0.10 to 5.0% with respect to the area of the insulating film. (2) In the grain-oriented electromagnetic steel sheet described in (1) above, The base steel sheet has, as the chemical composition, in mass %, further C: 0.010% or less, N: 0.010% or less, Acid-soluble Al: 0.020% or less, P: 0.040% or less, Total of S and Se: 0.010% or less, Sn: 0.50% or less, Cu: 0.50% or less, Cr: 0.50% or less, Sb: 0.50% or less, Mo: 0.10% or less Bi: 0.10% or less, It may include. (3) In the grain-oriented electrical steel sheet described in (1) or (2) above, When viewed at the aforementioned cross-section, the area ratio of internally oxidized SiO2 relative to the area of ​​the interface region of the base steel sheet may be 2.0% or more. (4) In the grain-oriented electrical steel sheet described in any one of (1) to (3) above, When viewed at the aforementioned cross-section, the internal oxidized SiO2 may have a dendritic shape. (5) In the grain-oriented electrical steel sheet described in any one of (1) to (4) above, When the cross-section is observed at 10 observation points spaced apart on the plate surface, the internal oxidized SiO2 observed within a 2 μm × 2 μm field of view may be present in 5 or more observation points. (6) In the grain-oriented electrical steel sheet described in any one of (1) to (5) above, When the cross-section is observed at 10 observation points spaced apart from each other on the plate surface, there may be 5 or more observation points where the area ratio of the void observed in a 10 μm × 10 μm field of view is 0.010 to 3.0% of the area of ​​the insulating coating interface region, and the area ratio of the iron and phosphorus-containing oxide observed in a 2 μm × 2 μm field of view is 0.10 to 5.0% of the area of ​​the insulating coating. (7) The method for manufacturing grain-oriented electrical steel sheets described in any one of (1) to (6) above is: This process includes a hot rolling process, a hot-rolled steel sheet annealing process, a cold rolling process, a decarburization annealing process, a finish annealing process, a thermal oxidation annealing process, and an insulating coating formation process. In the aforementioned finish annealing process, After the decarburization annealing process, an annealing separation agent is applied to the steel sheet, which contains 20-99.5% by mass of alumina, 0.5-20% by mass of bismuth chloride, and the remainder being magnesia and impurities, on a solid content basis. After drying, finish annealing is performed. In the aforementioned thermal oxidation annealing process, As a heating process, the steel sheet after the finish annealing process is heated from room temperature to a temperature range of 800 to 1100°C in an atmosphere where the oxygen concentration is less than 1.0 volume% and the oxygen potential PH2O / PH2 is 0.50 to 100. As a soaking process, the steel plate after the heating process is soaked for 5 to 200 seconds at a temperature range of 800 to 1100°C in an atmosphere where the oxygen concentration is less than 1.0 volume% and the oxygen potential PH2O / PH2 is less than 0.010 to 0.50. (8) In the method for manufacturing grain-oriented electrical steel sheets described in (7) above, In the aforementioned thermal oxidation annealing process, As a first surface treatment before heat treatment, the steel sheet after the finish annealing process is immersed for 3 to 60 seconds in a first treatment solution containing at least one of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid, with a total acid concentration of 1 to 20% by mass and a liquid temperature of 50 to 90°C. As the heat treatment, the heating process and the soaking process may be performed on the steel plate after the first surface treatment. (9) In the method for manufacturing grain-oriented electrical steel sheets described in (7) or (8) above, In the aforementioned thermal oxidation annealing process, As a second surface treatment after heat treatment, the steel sheet after the heating process and the soaking process may be immersed for 3 to 60 seconds in a second treatment solution containing at least one of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid, with a total acid concentration of 1 to 10% by mass and a liquid temperature of 50 to 90°C. [Effects of the Invention] 【0023】 According to the above embodiment of the present invention, it is possible to provide a grain-oriented electrical steel sheet and a method for manufacturing the same that have excellent coating adhesion without relying on a forsterite coating. In this grain-oriented electrical steel sheet, the surface of the base steel sheet is smooth because there is no forsterite coating, and the area near the surface of the base steel sheet is preferably internally oxidized and the insulating coating has a preferred form, resulting in excellent coating adhesion. Therefore, it is possible to favorably improve the iron loss characteristics. [Brief explanation of the drawing] 【0024】 [Figure 1] This is a schematic cross-sectional view showing a grain-oriented electrical steel sheet according to one embodiment of the present invention. [Figure 2] This flowchart illustrates the manufacturing method of grain-oriented electrical steel sheets according to this embodiment. [Modes for carrying out the invention] 【0025】 Preferred embodiments of the present invention are described in detail below. However, the present invention is not limited to the configuration disclosed in these embodiments, and various modifications are possible without departing from the spirit of the invention. Furthermore, the numerical limit ranges described below include both lower and upper limits. Numerical values ​​indicated as "greater than" or "less than" are not included in the numerical range. 【0026】 As mentioned above, smoothing the surface of the base steel sheet of a grain-oriented electrical steel sheet without a forsterite coating facilitates the movement of magnetic domain walls when the grain-oriented electrical steel sheet is magnetized, improving iron loss characteristics. However, this technology has the challenge of making it difficult to obtain the coating adhesion necessary for reducing iron loss. 【0027】 As a result of diligent research, the inventors have discovered a grain-oriented electrical steel sheet that exhibits excellent coating adhesion without relying on a forsterite coating. The grain-oriented electrical steel sheet according to this embodiment will be described in detail below. 【0028】 Figure 1 is a schematic cross-sectional view showing a grain-oriented electrical steel sheet according to this embodiment. As shown in Figure 1, the grain-oriented electrical steel sheet 1 according to this embodiment has a base steel sheet 11 and an insulating coating 12 arranged in contact with the base steel sheet 11 when viewed from a cross-section where the cutting direction is parallel to the thickness direction and perpendicular to the width direction. The layer structure in which the insulating coating 12 is arranged in contact with the base steel sheet 11 means that there is no forsterite coating and that the base steel sheet 11 has a smooth surface (a smooth surface equivalent to that of a cold-rolled steel sheet). Furthermore, when viewed from the above cross-section, the grain-oriented electrical steel sheet 1 according to this embodiment has internal oxidized SiO211a in the base steel sheet interface region, which is within a range of 2.0 μm from the interface with the insulating coating 12 toward the thickness direction. Furthermore, when viewed from the above cross-section, the insulating coating 12 according to this embodiment contains voids 12a and an oxide 12b containing iron and phosphorus. Specifically, when viewed on the cross-section, the area ratio of the voids 12a is 0.010 to 3.0% of the area of ​​the insulating coating interface region, which is within the range of 0.5 μm from the interface with the base steel plate 11 in the thickness direction. Also, when viewed on the cross-section, the area ratio of the iron and phosphorus-containing oxide 12b is 0.10 to 5.0% of the area of ​​the insulating coating 12. 【0029】 As described above, in order to improve iron loss characteristics, it is effective to smooth the surface of the base steel sheet to facilitate magnetic domain wall movement, and to ensure close contact between the base steel sheet and the insulating coating to apply tension to the base steel sheet and ensure electrical insulation between the steel sheets. In the grain-oriented electrical steel sheet according to this embodiment, the surface smoothness of the base steel sheet is ensured by the insulating coating being placed in contact with the base steel sheet (there is no forsterite coating), and the adhesion between the base steel sheet and the insulating coating is ensured by the internal oxidized SiO2 described above. In addition, the adhesion between the base steel sheet and the insulating coating is ensured by controlling the area ratio of the voids and the iron and phosphorus-containing oxides described above. Therefore, the grain-oriented electrical steel sheet according to this embodiment has excellent iron loss characteristics. 【0030】 The technical reason why the adhesion of the coating is improved when the aforementioned internally oxidized SiO2 is included in the base steel sheet interface region, which is within a range of 2.0 μm in the thickness direction from the interface with the insulating coating within the base steel sheet, is not yet clear, but it can be inferred as follows. 【0031】 The internal oxidized SiO2 described above is mainly composed of SiO2 and is formed when Si contained in the base steel sheet is oxidized during annealing. In the grain-oriented electrical steel sheet according to this embodiment, the internal oxidized SiO2 unique to this embodiment is created near the surface of the base steel sheet by controlling the manufacturing conditions. When the above-mentioned internal oxidized SiO2 is formed near the surface within the base steel sheet, it is thought that the Si content in the base material (matrix) surrounding the internal oxidized SiO2 decreases, resulting in a state close to pure iron. It is thought that the adhesion of the coating is enhanced by the chemical interaction between this matrix, which is close to pure iron, and the P (phosphorus) contained in the insulating coating. Therefore, it is thought that the closer the internal oxidized SiO2 is formed within the surface within the base steel sheet, the easier it is to improve coating adhesion. Specifically, if the internal oxidized SiO2 is contained within a range of 2.0 μm in the thickness direction from the interface with the insulating coating within the base steel sheet, the adhesion between the base steel sheet and the insulating coating will improve. 【0032】 Furthermore, two types of oxidation are known for the oxidation of Si contained in the base steel sheet during annealing: external oxidation and internal oxidation. For example, external oxidation is an oxidation type in which alloying elements (e.g., Si) in the base steel sheet diffuse to the surface of the base steel sheet before oxidation. This oxidation occurs in a low-oxidizing atmosphere and forms an oxide film on the surface of the base steel sheet. On the other hand, internal oxidation is an oxidation type in which alloying elements (e.g., Si) in the base steel sheet oxidize without hardly diffusing within the base steel sheet. This oxidation occurs in a high-oxidizing atmosphere and forms oxides in the form of precipitates or dendrites near the surface of the base steel sheet. 【0033】 Until now, it has generally been believed that external oxidation does not hinder magnetic domain wall movement when a steel sheet is magnetized, but internal oxidation does. For this reason, conventional technology has avoided internal oxidation of the base steel sheet. On the other hand, the grain-oriented electrical steel sheet according to this embodiment makes effective use of internal oxidation. As a result of the inventors' investigations, it was found that by controlling the manufacturing conditions to create internal oxidized SiO2 unique to this embodiment, it is possible to improve coating adhesion without hindering magnetic domain wall movement. In the grain-oriented electrical steel sheet according to this embodiment, internal oxidized SiO2 is preferentially formed near the surface of the base steel sheet, so magnetic domain wall movement is not hindered, and the matrix surrounding the internal oxidized SiO2 becomes close to pure iron, so it is thought that coating adhesion is enhanced through chemical interaction with the insulating coating. 【0034】 The presence or absence of the above-mentioned internally oxidized SiO2 can be confirmed by observing a cross-section where the cutting direction is parallel to the thickness direction and perpendicular to the width direction of the sheet. For example, the above-mentioned cross-section can be observed using a field emission transmission electron microscope (FE-TEM). In the grain-oriented electrical steel sheet according to this embodiment, it is determined that internally oxidized SiO2 is present in the base steel sheet interface region when it can be confirmed by the following method. 【0035】 Specifically, first, a test specimen is cut using FIB (Focused Ion Beam) processing so that the cutting direction is parallel to the thickness direction and perpendicular to the width direction of the plate. The cross-sectional structure of this cut surface is then observed with FE-TEM at a magnification that allows each layer to be included in the observation field. If each layer does not fit in the observation field, the cross-sectional structure is observed using multiple consecutive fields. For example, a field of view of 1 μm × 1 μm or larger, preferably 2 μm × 2 μm, and a resolution of 1 nm / pixel or less with a pixel size should be used. The acceleration voltage should also be 200 kV. When observing with only one field of view, it is difficult to obtain average information about the steel plate, so 10 randomly selected locations spaced apart from each other are observed, totaling 10 μm. 2 You can make your decision based on the above perspective. 【0036】 To identify each layer in the cross-sectional structure, line analysis is performed along the thickness direction using the EDS (Energy Dispersive X-ray Spectroscopy) attached to the FE-TEM, and the chemical composition of each layer is quantitatively analyzed. The elements to be quantitatively analyzed are five elements: Fe, P, Si, O, and Mg. 【0037】 Based on the TEM-EDS quantitative analysis results described above, the layered region located at the deepest position in the thickness direction, and where the Fe content is 80 atomic percent or more after excluding measurement noise, is determined to be the base steel sheet, and the region excluding this base steel sheet is determined to be other coatings. 【0038】 When determining the region that is the base steel sheet as described above, precipitates, inclusions, and voids contained within each layer are not included in the determination. Instead, the region that satisfies the above quantitative analysis results as the matrix phase is determined to be the base steel sheet. For example, if precipitates, inclusions, and voids are confirmed to be present on the scanning lines of the line analysis from bright-field images, dark-field images, or line analysis results, this region is not included in the determination, and the determination is made based on the quantitative analysis results as the matrix phase. Note that precipitates, inclusions, and voids can be distinguished from the matrix phase by contrast, and in quantitative analysis results, they can be distinguished from the matrix phase by the abundance of constituent elements. When identifying the base steel sheet, it is preferable to identify it at a location where precipitates, inclusions, and voids are not included on the scanning lines of the line analysis. 【0039】 In the grain-oriented electrical steel sheet according to this embodiment, an insulating coating is placed on the base steel sheet. The surface side of the base steel sheet in the region determined from the quantitative analysis results of TEM-EDS is defined as the interface between the insulating coating and the base steel sheet. Furthermore, in the grain-oriented electrical steel sheet according to this embodiment, the interface between the insulating coating and the base steel sheet has sufficiently few irregularities in the analysis field described later. Therefore, during analysis, the average line of the interface between the insulating coating and the base steel sheet in the field of view may be treated as the interface. 【0040】 Specifically, regarding the region excluding the base steel sheet identified above, the region where the Fe content is less than 80 atomic%, the P content is 5 atomic% or more, and the O content is 30 atomic% or more, after excluding measurement noise, is determined to be an insulating coating (phosphate-based coating) based on the quantitative analysis results of TEM-EDS. In addition to the three elements mentioned above that are used to identify phosphate-based coatings, phosphate-based coatings may also contain aluminum, magnesium, nickel, etc., derived from phosphates. Furthermore, silicon derived from colloidal silica may also be included. 【0041】 When determining the region that is a phosphate-based coating as described above, precipitates, inclusions, and vacancies contained within each coating are not included in the determination. Instead, the region that satisfies the above quantitative analysis results as the matrix phase is determined to be a phosphate-based coating. For example, if precipitates, inclusions, and vacancies are confirmed to be present on the scanning line of the line analysis from bright-field images, dark-field images, or line analysis results, this region is not included in the determination, and the determination is made based on the quantitative analysis results of the matrix phase. Note that precipitates, inclusions, and vacancies can be distinguished from the matrix phase by contrast, and in quantitative analysis results, they can be distinguished from the matrix phase by the abundance of constituent elements. When identifying a phosphate-based coating, it is preferable to identify it at a position where precipitates, inclusions, and vacancies are not included on the scanning line of the line analysis. 【0042】 The grain-oriented electrical steel sheet according to this embodiment does not have an intermediate ceramic layer such as a forsterite coating or an external oxide film. Therefore, when the layer structure is identified by the method described above, the base steel sheet and the insulating coating placed in contact with the base steel sheet are identified. For example, the base steel sheet has a thickness of 0.17 to 0.29 mm, and the insulating coating has a thickness of 0.1 to 10 μm. 【0043】 However, if the electrical steel sheet has a forsterite coating as an intermediate ceramic layer, the forsterite coating will be confirmed between the base steel sheet identified by the above method and the insulating coating (phosphate-based coating). This forsterite coating satisfies, for example, the following on average for the entire coating: Fe content of less than 80 atomic%, P content of less than 5 atomic%, Si content of 5 atomic% or more, O content of 30 atomic% or more, and Mg content of 10 atomic% or more. Note that the quantitative analysis results of the forsterite coating are quantitative analysis results for the matrix phase and do not include analysis results for precipitates, inclusions, and vacancies contained in the forsterite coating. When identifying the forsterite coating, it is preferable to identify it at a position where precipitates, inclusions, and vacancies are not included on the scanning line of the line analysis. Generally, the forsterite coating has a thickness of 0.1 to 10 μm. 【0044】 Similarly, when the electrical steel sheet has an external oxide film as an intermediate ceramic layer, the external oxide film is confirmed between the base steel sheet identified by the above method and the insulating coating (phosphate-based coating). This external oxide film satisfies, for example, the following on average for the entire oxide film: Fe content of less than 80 atomic%, P content of less than 5 atomic%, Si content of 20 atomic% or more, O content of 30 atomic% or more, and Mg content of less than 10 atomic%. Note that the quantitative analysis results of the external oxide film are quantitative analysis results for the matrix phase, and do not include analysis results for precipitates, inclusions, and vacancies contained in the external oxide film. When identifying the external oxide film, it is preferable to identify it at a position where precipitates, inclusions, and vacancies are not included on the scanning line of the line analysis. Generally, the thickness of the external oxide film is 2 to 500 nm. 【0045】 In the grain-oriented electrical steel sheet according to this embodiment, the base steel sheet and the insulating film placed in contact with the base steel sheet can be confirmed by the method described above. Therefore, whether or not internal oxidized SiO2 is contained in the base steel sheet and the region in the base steel sheet that contains internal oxidized SiO2 can be confirmed using the EDS installed in the FE-TEM. 【0046】 For example, using TEM-EDS, a line analysis is performed along the thickness direction of the base steel sheet identified by the above method on precipitates observed in the bright-field image of the FE-TEM, and a quantitative analysis of the chemical composition is performed. The elements to be quantitatively analyzed are five elements: Fe, P, Si, Al, and O. From the quantitative analysis results of the TEM-EDS described above, after removing measurement noise, the region in which the Fe content is less than 80 atomic%, the Si content is 30 atomic% or more, and the O content is 55 atomic% or more is determined to be internal oxidized SiO2. In this embodiment of grain-oriented electrical steel sheet, since the internal oxidized SiO2 contained in the base steel sheet is amorphous, no clear diffraction spots are observed when electron diffraction is performed, and mainly broad, annular electron diffraction patterns are observed. 【0047】 Furthermore, regarding the voids and iron-phosphorus oxides contained in the insulating film described above, if the area ratio of the voids to the area of ​​the insulating film interface region is 0.010 to 3.0%, and the area ratio of the iron-phosphorus oxides to the area of ​​the insulating film is 0.10 to 5.0%, the adhesion between the base steel sheet and the insulating film will improve. The technical reason for this is presumed to be as follows. 【0048】 Generally, P and Al contained in the insulating film react with SiO2 on the surface of the base steel sheet formed by decarburization annealing and Fe in the base steel sheet during the formation of the insulating film to form a complex oxide. It is thought that this reaction reduces P and Al in the insulating film, leading to the formation of voids within the insulating film. Therefore, voids in the insulating film tend to form in the insulating film interface region, which is the main site where the above reaction takes place. In conventional technology, a large number of voids were formed in the insulating film interface region. Such voids in the insulating film interface region cause film delamination, significantly reducing film adhesion. 【0049】 In addition, in the grain-oriented electrical steel sheet according to this embodiment, in order to form internal oxidized SiO2 in the interface region of the base steel sheet, the atmosphere is controlled to an oxidizing atmosphere during the heating process of thermal oxidation annealing performed after finish annealing. However, if the heating process of thermal oxidation annealing is conducted in an oxidizing atmosphere, lumpy Fe oxides tend to form on the surface of the base steel sheet. It is thought that these Fe oxides on the steel sheet surface react with P contained in the insulating film during the formation of the insulating film to form an oxide containing iron and phosphorus, and at the same time, form voids within the insulating film. That is, oxides containing iron and phosphorus, and voids tend to form in the insulating film interface region, which is the main site where the above reaction takes place. In particular, when the heating process of thermal oxidation annealing is controlled to an oxidizing atmosphere, oxides containing iron and phosphorus and voids tend to form in the insulating film interface region. 【0050】 In the grain-oriented electrical steel sheet according to this embodiment, the heating process and soaking process of thermal oxidation annealing are optimally controlled to minimize voids and iron- and phosphorus-containing oxides formed in the insulating coating interface region. As a result, the adhesion between the base steel sheet and the insulating coating is improved. 【0051】 As described above, when voids are included in the insulating film interface region, the adhesion of the film is significantly reduced. On the other hand, voids included outside the insulating film interface region do not significantly reduce the adhesion of the film, and are thought to contribute to improved adhesion by relieving stress when the insulating film deforms. In the grain-oriented electrical steel sheet according to this embodiment, the inclusion of voids outside the insulating film interface region is permitted. 【0052】 If the void area ratio relative to the insulating film interface area exceeds 3.0%, the adhesion of the film will decrease. Preferably, the void area ratio is 2.5% or less, and more preferably 2.0% or less. A smaller void area ratio relative to the insulating film interface area is preferable. However, achieving a void area ratio of 0% is industrially difficult. Therefore, it is acceptable for the void area ratio to be greater than 0%, and may be 0.010% or more, or 1.0% or more. 【0053】 Similarly, if the area ratio of iron-and-phosphorus oxides relative to the area of ​​the insulating film exceeds 5.0%, the adhesion of the film will decrease. Preferably, the area ratio of iron-and-phosphorus oxides is 3.0% or less, and more preferably 2.0% or less. A smaller area ratio of iron-and-phosphorus oxides relative to the area of ​​the insulating film is preferable. However, it is industrially difficult to achieve a 0% area ratio of iron-and-phosphorus oxides. Therefore, it is sufficient to have an area ratio of iron-and-phosphorus oxides greater than 0%, and may be 0.10% or more, or 1.0% or more. 【0054】 The voids, iron and phosphorus-containing oxides, and their area ratios contained in the insulating film described above can be confirmed as follows. In the grain-oriented electrical steel sheet according to this embodiment, it is determined that the insulating film contains voids and iron and phosphorus-containing oxides when confirmed by the following method. 【0055】 Observe the cross-section where the cutting direction is parallel to the plate thickness direction and perpendicular to the plate width direction, and identify the base steel plate and the insulating coating (phosphate-based coating) in the same manner as described above. Whether or not voids are present in this insulating coating, and the regions within the insulating coating where voids are present, should be confirmed by FE-SEM. Furthermore, whether or not iron and phosphorus-containing oxides are present, and the regions within the insulating coating where iron and phosphorus-containing oxides are present, should be confirmed by FE-TEM. For example, voids should be observed with a field of view of 10 μm × 10 μm or larger and a resolution of 10 nm / pixel or less. Iron and phosphorus-containing oxides should be observed with a field of view of 2 μm × 2 μm or larger and a resolution of 5 nm / pixel or less. Since it is difficult to obtain average information about the steel plate when observing only one field of view, observe 10 randomly selected locations spaced apart from each other, totaling 40 μm. 2 You can make your decision based on the above perspective. 【0056】 For example, with regard to oxides containing iron and phosphorus, electron diffraction is performed by focusing the electron beam on precipitates observed within the insulating film in bright-field or dark-field images to obtain information from the target precipitates. The crystal structure and interplanar spacing of the target precipitates are then identified from the electron diffraction pattern. 【0057】 The crystal data, such as the crystal structure and interplanar spacing identified above, is compared with a PDF (Powder Diffraction File). This comparison allows confirmation of whether or not an iron-and-phosphorus oxide is present in the insulating film. For example, in this embodiment, examples of iron-and-phosphorus oxides include FeP oxide, Fe3(PO4)2 oxide, and Fe2P2O7 oxide. JCPDS number 03-065-2595 can be used to identify FeP oxide. JCPDS number 049-1087 can be used to identify Fe3(PO4)2 oxide. JCPDS number 01-076-1762 can be used to identify Fe2P2O7 oxide. In addition, a region having a similar brightness to the iron-and-phosphorus oxide identified by comparison with the above PDF may be considered an iron-and-phosphorus oxide. Furthermore, amorphous iron-and-phosphorus oxides may also be included in the above-mentioned iron-and-phosphorus oxides. 【0058】 Furthermore, regarding voids, a field emission scanning electron microscope (FE-SEM) can be used to observe defects and holes within the insulating coating using the secondary electron and backscattered electron images obtained from the SEM. Those skilled in the art can confirm whether or not voids are present within the insulating coating interface region based on the contrast of the secondary and backscattered electron images. Similar to TEM, the base steel plate and insulating coating can be identified from the contrast of the backscattered electron image obtained from the SEM. For example, observation should be performed with a field of view of 10 μm × 10 μm or larger and a resolution of 10 nm / pixel or less. Since it is difficult to obtain average information about the steel plate when observing only one field of view, 10 randomly selected locations spaced apart from each other should be observed, totaling 40 μm. 2 You can make your decision based on the above perspective. 【0059】 For example, by tilting the sample stage on which the sample to be observed is placed inside the SEM, a contrast corresponding to the depression on the surface of the void can be obtained in the secondary electron image, and the void can be identified from this contrast. Alternatively, to efficiently derive the area ratio, the void identified by the above method may be observed in a backscattered electron image, the region within the same field of view with similar brightness to the void is binarized, and the area can be derived using image processing techniques. Note that the image binarization may include manual work. 【0060】 Furthermore, the area ratio of voids to the area of ​​the insulating film interface region, and the area ratio of iron-and-phosphorus oxides to the area of ​​the insulating film, can be determined based on the observations and identifications described above. For example, the area of ​​the insulating film interface region, which is within a 0.5 μm range from the interface with the base steel plate in the thickness direction within the insulating film, and the total area of ​​voids contained in the insulating film interface region can be determined, and their area ratio can be calculated. Similarly, the area of ​​the insulating film and the total area of ​​iron-and-phosphorus oxides contained in the insulating film can be determined, and their area ratio can be calculated. The above areas and area ratios may also be determined by image analysis. For image analysis, the binarization of images may be performed by manually coloring the microstructure photographs with voids and iron-and-phosphorus oxides based on the identification results of voids and iron-and-phosphorus oxides described above, and then binarizing the images. 【0061】 In the grain-oriented electrical steel sheet according to this embodiment, if the base steel sheet interface region contains internally oxidized SiO2, and the area ratio of voids within the insulating film interface region is 0.010 to 3.0% or less, and the area ratio of iron and phosphorus-containing oxides within the insulating film is 0.10 to 5.0% or less, the adhesion between the base steel sheet and the insulating film is improved. 【0062】 Furthermore, in the grain-oriented electrical steel sheet according to this embodiment, when viewed as a cross-section where the cutting direction is parallel to the thickness direction and perpendicular to the width direction, it is preferable that the area ratio of internally oxidized SiO2 relative to the area of ​​the interface region of the base steel sheet is 2.0% or more. When the internally oxidized SiO2 satisfies the above conditions, it preferentially exists near the surface within the base steel sheet, and as a result, the adhesion of the coating is favorably improved. 【0063】 The above area ratio is preferably 5% or more, and more preferably 10% or more. There is no particular upper limit to the above area ratio, and a larger value is preferable. stomach. The above area ratio is preferably 80% or less, preferably 50% or less, preferably 20% or less, and more preferably 15% or less. 【0064】 The above area ratios can be determined based on the observation and identification performed using FE-TEM as described above. For example, the area of ​​the base steel sheet interface region, which is within a 2.0 μm range from the interface with the insulating coating in the thickness direction within the base steel sheet, and the total area of ​​internally oxidized SiO2 included in the base steel sheet interface region can be determined, and the area ratio can be calculated. The above area and area ratios can also be determined by image analysis. For image binarization for image analysis, the image may be binarized by manually coloring the microstructure photograph with internally oxidized SiO2 based on the identification results of internally oxidized SiO2 as described above. 【0065】 Furthermore, in the grain-oriented electrical steel sheet according to this embodiment, it is preferable that the internal oxidized SiO2 specified above has a dendrite shape when viewed as a cross-section where the cutting direction is parallel to the thickness direction and perpendicular to the width direction. Specifically, it is preferable that the isoperimetric constant derived from the area and perimeter of the internal oxidized SiO2 when viewed as such in the cross-section is less than 0.350. When the internal oxidized SiO2 satisfies the above conditions, the film adhesion is preferably improved. 【0066】 The dendrite shape (isoperipheral constant) of the internal SiO2 oxide can be confirmed by the following method. In the grain-oriented electrical steel sheet according to this embodiment, the internal SiO2 oxide is determined to have a dendrite shape when confirmed by the following method. 【0067】 The above isoperimeter constant is calculated as 4π × (area) ÷ (perimeter). 2The value is derived according to the formula and represents the degree of surface irregularities such as dendrite shapes. A value of 1 represents an ideal circle, with an upper limit of 1. Values ​​smaller than 1 indicate more complex surface irregularities. The area can be derived from FE-TEM micrographs or manually, similar to how the area ratio of internal oxidized SiO2 is calculated. The circumference can be obtained from the micrographs using image analysis software such as imageJ and obtained through image analysis. 【0068】 The above isoperimetric constant is preferably less than 0.20, and more preferably less than 0.10. The lower limit of the above isoperimetric constant is not particularly limited, and a smaller value is preferable. However, since it is not industrially feasible to set the isoperimetric constant to 0, the above isoperimetric constant may be set to 0.020 or higher. 【0069】 Furthermore, in the grain-oriented electrical steel sheet according to this embodiment, when the above-mentioned cross-section is observed at 10 observation points spaced apart from each other on the sheet surface, it is preferable that the above-mentioned internal oxidized SiO2 is contained in 5 or more observation points. When the internal oxidized SiO2 satisfies the above conditions, the internal oxidized SiO2 is widely distributed on the sheet surface of the grain-oriented electrical steel sheet, and as a result, the adhesion of the coating is favorably improved. 【0070】 It is preferable that there are eight or more observation sites containing the internally oxidized SiO2 described above. There is no particular upper limit to the number of observation sites containing the internally oxidized SiO2 described above, and the more the better, so it may be 10 sites. It is also possible that there are nine or fewer observation sites containing the internally oxidized SiO2 described above. 【0071】 Furthermore, in the grain-oriented electrical steel sheet according to this embodiment, when the above-mentioned cross-section is observed at 10 observation points spaced apart from each other on the sheet surface, it is preferable that there are 5 or more observation points where the area ratio of voids is 0.010% to 3.0% of the area of ​​the insulating film interface region, and the area ratio of iron and phosphorus-containing oxide is 0.10% to 5.0% of the area of ​​the insulating film. Note that the above-mentioned voids can be observed in a field of view of 10 μm × 10 μm, and the above-mentioned iron and phosphorus-containing oxide can be observed in a field of view of 2 μm × 2 μm. When the above conditions are satisfied for both voids and iron and phosphorus-containing oxide, the voids and iron and phosphorus-containing oxide are controlled over a wide area on the sheet surface of the grain-oriented electrical steel sheet, and as a result, the adhesion of the film is favorably improved. 【0072】 It is preferable that there are 7 or more observation sites where voids and oxides containing iron and phosphorus satisfy the above conditions, and more preferably 8 or more. There is no particular upper limit to the number of observation sites where voids and oxides containing iron and phosphorus satisfy the above conditions, and a larger number is preferable, so there may be 10 sites. However, since it is not easy to industrially produce 10 observation sites where voids and oxides containing iron and phosphorus satisfy the above conditions, it is sufficient if there are 9 or fewer observation sites where voids and oxides containing iron and phosphorus satisfy the above conditions. 【0073】 Furthermore, in the grain-oriented electrical steel sheet according to this embodiment, it is preferable that the coating area ratio when the grain-oriented electrical steel sheet is wound around a cylinder with a diameter of 20 mm and bent 180° is 90% or more, and more preferably 95% or more. The upper limit of the coating area ratio is not particularly limited, but for example, it may be 100%. 【0074】 The above-mentioned coating retention rate can be evaluated by wrapping the test specimen around a 20 mm diameter cylinder and bending it 180°. The area ratio of the remaining coating surface to the area of ​​the steel plate in contact with the cylinder can be calculated, and the area of ​​the steel plate in contact with the roll can be determined by calculation. The area of ​​the remaining surface can be determined by taking a photograph of the steel plate after the test and performing image analysis on the photographic image. 【0075】 Furthermore, in the grain-oriented electrical steel sheet according to this embodiment, the base steel sheet has a chemical composition that includes basic elements, optional elements as needed, and the remainder consists of Fe and impurities. 【0076】 For example, the base steel sheet has a chemical composition in mass percent, Si: 3.0~4.0%, Mn: 0.010~0.50%, C: 0~0.010%, N: 0~0.010%, Acid soluble Al: 0~0.020%, P: 0~0.040%, Total of S and Se: 0-0.010% Sn: 0~0.50%, Cu: 0~0.50%, Cr: 0~0.50%, Sb: 0~0.50%, Mo: 0~0.10%, Bi: 0~0.10%, It should contain [the specified substance], with the remainder consisting of Fe and impurities. 【0077】 Furthermore, the base steel sheet has a chemical composition in mass%, Sn: 0.0050~0.50%, Cu: 0.010~0.50%, Cr: 0.010~0.50%, Sb: 0.010~0.50%, Mo: 0.0050~0.10%, Bi: 0.00050~0.10%, It may contain at least one selected from the group consisting of the following. 【0078】 Si:3.0~4.0% by mass Silicon (Si) is a fundamental element for the base steel sheet. If the Si content is less than 3.0%, eddy current losses cannot be sufficiently reduced, and good magnetic properties cannot be obtained. Therefore, the Si content should be 3.0% or more. Preferably, the Si content is 3.10% or more, and more preferably 3.20% or more. On the other hand, if the Si content exceeds 4.0%, the steel sheet becomes brittle, and the passability during manufacturing deteriorates significantly, so the Si content should be 4.0% or less. Preferably, the Si content is 3.70% or less, more preferably 3.60% or less, and more preferably 3.50% or less. 【0079】 Mn:0.010~0.50% by mass Manganese (Mn) is a fundamental element for the base steel sheet. If the Mn content is less than 0.010%, it is difficult to form MnS and MnSe, which function as inhibitors, and secondary recrystallization does not proceed sufficiently, resulting in poor magnetic properties. Therefore, the Mn content should be 0.010% or more. Preferably, the Mn content is 0.030% or more, and more preferably 0.050% or more. On the other hand, if the Mn content exceeds 0.50%, the steel undergoes a phase transformation during secondary recrystallization annealing, and secondary recrystallization does not proceed sufficiently, resulting in poor magnetic properties. Therefore, the Mn content should be 0.50% or less. Preferably, the Mn content is 0.20% or less, more preferably 0.15% or less, and more preferably 0.10% or less. 【0080】 C:0~0.010% by mass Carbon (C) is a selective element for the base steel sheet. Although C is contained in the steel billet (slab), if an excessive amount of C remains in the base steel sheet after finish annealing, good iron loss characteristics may not be obtained. Therefore, the C content of the base steel sheet should be 0.010% or less. Preferably, the C content is 0.0050% or less, and more preferably 0.0030% or less. On the other hand, there is no particular lower limit to the C content of the base steel sheet, and it may be 0%. However, since it is not industrially easy to make the C content 0%, the C content may be greater than 0%, and may be 0.00010% or more. 【0081】 N:0~0.010% by mass Nitrogen (N) is a selective element for the base steel sheet. Although N is contained in the steel billet (slab), if an excess of N remains in the base steel sheet after finish annealing, it can adversely affect the magnetic properties. Therefore, the N content of the base steel sheet should be 0.010% or less. Preferably, the N content is 0.0090% or less, and more preferably 0.0080% or less. On the other hand, there is no particular lower limit to the N content of the base steel sheet, and it may be 0%. However, since N forms AlN and acts as an inhibitor during secondary recrystallization, the N content may be greater than 0%, and may be 0.00010% or more. 【0082】 Acid-soluble Al: 0~0.020% by mass Acid-soluble aluminum (sol.Al) is a selective element for the base steel sheet. Although acid-soluble Al is contained in the steel billet (slab), if an excess of acid-soluble Al remains in the base steel sheet after finish annealing, it may adversely affect the magnetic properties. Therefore, the acid-soluble Al content of the base steel sheet should be 0.020% or less. Preferably, the acid-soluble Al content is 0.0150% or less, and more preferably 0.010% or less. On the other hand, there is no particular lower limit to the acid-soluble Al content of the base steel sheet, and it may be 0%. However, since acid-soluble Al forms AlN and acts as an inhibitor during secondary recrystallization, the acid-soluble Al content may be greater than 0%, and may be 0.00010% or more. 【0083】 P:0~0.040% by mass Phosphorus (P) is a selective element for the base steel sheet. If the P content exceeds 0.040%, the workability of the steel sheet may decrease significantly. Therefore, the P content should be 0.040% or less. Preferably, the P content is 0.030% or less, and more preferably 0.020% or less. On the other hand, there is no particular lower limit to the P content, and it may be 0%. However, since P has the effect of improving the texture and the magnetic properties of the steel sheet, the P content may be greater than 0%, and may be 0.0020% or more. 【0084】 Total of S and Se: 0-0.010% by mass S (sulfur) and Se (selenium) are selective elements for the base steel sheet. S and Se are contained in the steel billet (slab), but if excess S and Se remain in the base steel sheet after finish annealing, it can adversely affect the magnetic properties. Therefore, the total content of S and Se in the base steel sheet should be 0.010% or less. On the other hand, there is no particular lower limit to the total content of S and Se in the base steel sheet, and it should be 0%. However, since S and Se form MnS and MnSe and act as inhibitors during secondary recrystallization, the total content of S and Se may be greater than 0%, and may be 0.0050% or more. 【0085】 Sn: 0~0.50% by mass Tin (Sn) is a selective element for the base steel sheet. If the Sn content exceeds 0.50%, secondary recrystallization becomes unstable, which can adversely affect the magnetic properties. Therefore, the Sn content should be 0.50% or less. Preferably, the Sn content is 0.30% or less, and more preferably 0.150% or less. On the other hand, there is no particular lower limit to the Sn content, and it may be 0%. However, since Sn has the effect of improving magnetic properties by increasing the concentration of Goss orientations, the Sn content may be greater than 0%, and may be 0.0050% or more. 【0086】 Cu:0~0.50% by mass Copper (Cu) is a selective element for the base steel sheet. If the Cu content exceeds 0.50%, the steel sheet may become brittle during hot rolling. Therefore, the Cu content should be 0.50% or less. Preferably, the Cu content is 0.30% or less, and more preferably 0.10% or less. On the other hand, there is no particular lower limit to the Cu content, and it may be 0%. However, since Cu has the effect of increasing the concentration of Goss orientation and improving magnetic properties, the Cu content may be greater than 0%, and may be 0.010% or more. 【0087】 Cr:0~0.50% by mass Cr (chromium) is a selective element for the base steel sheet. If the Cr content exceeds 0.50%, Cr oxide may form, which can adversely affect the magnetic properties. Therefore, the Cr content should be 0.50% or less. Preferably, the Cr content is 0.30% or less, and more preferably 0.10% or less. On the other hand, there is no particular lower limit to the Cr content, and it may be 0%. However, since Cr has the effect of improving magnetic properties by increasing the concentration of Goss orientation, the Cr content may be greater than 0%, and may be 0.010% or more. 【0088】 Sb:0~0.50% by mass Antimony (Sb) is a selective element for the base steel sheet. If the Sb content exceeds 0.50%, it may adversely affect the magnetic properties. Therefore, the Sb content should be 0.50% or less. Preferably, the Sb content is 0.30% or less, and more preferably 0.10% or less. On the other hand, there is no particular lower limit to the Sb content, and it may be 0%. However, since Sb functions as an inhibitor and has the effect of stabilizing secondary recrystallization, the Sb content may be greater than 0%, and may be 0.010% or more. 【0089】 Mo:0~0.10% by mass Mo (molybdenum) is a selective element for the base steel sheet. If the Mo content exceeds 0.10%, problems may arise with the rolling properties of the steel sheet. Therefore, the Mo content should be 0.10% or less. Preferably, the Mo content is 0.08% or less, and more preferably 0.05% or less. On the other hand, there is no particular lower limit to the Mo content, and it may be 0%. However, since Mo has the effect of improving magnetic properties by increasing the concentration of Goss orientation, the Mo content may be greater than 0%, and may be 0.0050% or more. 【0090】 Bi:0~0.10% by mass Bi (bismuth) is a selective element for the base steel sheet. If the Bi content exceeds 0.10%, the passability during cold rolling may deteriorate. Also, if the purification during finish annealing is insufficient and an excess of Bi remains, it may adversely affect the magnetic properties. Therefore, the Bi content should be 0.10% or less. Preferably, the Bi content is 0.050% or less, more preferably 0.020% or less, and more preferably 0.0010% or less. On the other hand, there is no particular lower limit for the Bi content, and it may be 0%. However, since Bi has the effect of improving magnetic properties, the Bi content may be greater than 0%, and may be 0.00050% or more. 【0091】 The base steel sheet of the grain-oriented electrical steel sheet according to this embodiment may contain impurities. "Impurities" refer to substances that are introduced during the industrial production of steel from raw materials such as ore and scrap, or from the manufacturing environment. 【0092】 The chemical composition of the base steel sheet described above can be measured using general analytical methods. For example, it can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Acid-soluble Al can be measured using ICP-AES with the filtrate obtained after heating and decomposing the sample with acid. Furthermore, C and S can be measured using combustion-infrared absorption spectroscopy, N using inert gas fusion-thermal conductivity spectroscopy, and O using inert gas fusion-nondispersive infrared absorption spectroscopy. 【0093】 The above chemical composition is that of the base steel sheet. If the grain-oriented electrical steel sheet used as the measurement sample has an insulating coating or the like on its surface, remove the coating or the like using the method described below before measuring the chemical composition. 【0094】 For example, one method for removing the insulating coating is to immerse the grain-oriented electrical steel sheet with the coating in a high-temperature alkaline solution. Specifically, the insulating coating can be removed from the grain-oriented electrical steel sheet by immersing it in a sodium hydroxide aqueous solution of NaOH: 30-50% by mass + H2O: 50-70% by mass at 80-90°C for 5-10 minutes, followed by rinsing with water and drying. The immersion time in the sodium hydroxide aqueous solution should be adjusted according to the thickness of the insulating coating. 【0095】 Furthermore, although the grain-oriented electrical steel sheet according to this embodiment does not have a forsterite coating, if it is desired to remove the forsterite coating, the electrical steel sheet from which the insulating coating has been removed by the above method can be immersed in high-temperature hydrochloric acid. Specifically, the preferred concentration of hydrochloric acid for removing the forsterite coating to be dissolved should be determined in advance, and the forsterite coating can be removed by immersing the sheet in hydrochloric acid of this concentration (for example, 30-40% by mass hydrochloric acid) at 80-90°C for 1-5 minutes, followed by rinsing with water and drying. Normally, an alkaline solution is used to remove the insulating coating, and hydrochloric acid is used to remove the forsterite coating, using different treatment solutions for each type of coating. 【0096】 Next, a method for manufacturing grain-oriented electrical steel sheets according to this embodiment will be described. Note that the method for manufacturing grain-oriented electrical steel sheets according to this embodiment is not limited to the method described below. The manufacturing method described below is one example for manufacturing grain-oriented electrical steel sheets according to this embodiment. 【0097】 Figure 2 is a flowchart illustrating a method for manufacturing grain-oriented electrical steel sheets according to this embodiment. The method for manufacturing grain-oriented electrical steel sheets according to this embodiment mainly includes a hot rolling step of hot rolling a slab (steel billet) having a predetermined chemical composition to obtain a hot-rolled steel sheet; a hot-rolled steel sheet annealing step of annealing the hot-rolled steel sheet to obtain a hot-rolled annealed sheet; a cold rolling step of subjecting the hot-rolled annealed sheet to one cold rolling or multiple cold rollings with annealing to obtain a cold-rolled steel sheet; a decarburization annealing step of subjecting the cold-rolled steel sheet to decarburization annealing to obtain a decarburized annealed sheet; a finish annealing step of applying an annealing separating agent to the decarburized annealed sheet and then performing finish annealing to obtain a finish annealed sheet; a thermal oxidation annealing step of subjecting the finish annealed sheet to thermal oxidation annealing to obtain a thermal oxidation annealed sheet; and an insulating film forming step of applying an insulating film forming liquid to the thermal oxidation annealed sheet and then performing heat treatment to form an insulating film on the surface of the thermal oxidation annealed sheet. 【0098】 Specifically, the method for manufacturing grain-oriented electrical steel sheets according to this embodiment is as follows: This process includes a hot rolling process, a hot-rolled steel sheet annealing process, a cold rolling process, a decarburization annealing process, a finish annealing process, a thermal oxidation annealing process, and an insulating coating formation process. In the above finish annealing process, After the decarburization annealing process described above, an annealing separation agent is applied to the steel sheet, which contains 20-99.5% by mass of alumina, 0.5-20% by mass of bismuth chloride, and the remainder being magnesia and impurities, on a solid content basis. After drying, finish annealing is performed. In the above thermal oxidation annealing process, As part of the heating process, the steel sheet after the above finish annealing process is heated from room temperature to a temperature range of 800 to 1100°C in an atmosphere where the oxygen concentration is less than 1.0 volume% and the oxygen potential PH2O / PH2 is 0.50 to 100. As a soaking process, the steel plate after the heating process described above is soaked for 5 to 200 seconds at a temperature range of 800 to 1100°C in an atmosphere where the oxygen concentration is less than 1.0 volume% and the oxygen potential PH2O / PH2 is less than 0.010 to 0.50. 【0099】 Each of the above steps will be explained in detail. Note that if the conditions for each step are not specified in the following explanation, publicly known conditions should be applied as appropriate. 【0100】 Hot rolling process In the hot rolling process, a steel billet (for example, a steel ingot such as a slab) having a predetermined chemical composition is hot-rolled. For example, the slab (steel billet) used in the hot rolling process has a chemical composition of, in mass%, Si: 3.0~4.0%, Mn: 0.010~0.50%, C: 0.020~0.20%, N: 0.0020~0.020%, Acid soluble Al: 0.010~0.050%, P: 0~0.040%, Total of S and Se: 0.0010~0.040% Sn: 0~0.50%, Cu: 0~0.50%, Cr: 0~0.50%, Sb: 0~0.50%, Mo: 0~0.10%, Bi: 0~0.10%, It should contain [the specified substance], with the remainder consisting of Fe and impurities. 【0101】 Furthermore, the slab (steel billet) described above has a chemical composition in mass%, Sn: 0.0050~0.50%, Cu: 0.010~0.50%, Cr: 0.010~0.50%, Sb: 0.010~0.50%, Mo: 0.0050~0.10%, Bi: 0.00050~0.10%, It may contain at least one selected from the group consisting of the following. 【0102】 Si:3.0~4.0% by mass Silicon (Si) is a fundamental element for steel slabs. If the Si content is less than 3.0%, eddy current losses cannot be sufficiently reduced, and good magnetic properties cannot be obtained. Therefore, the Si content should be 3.0% or more. Preferably, the Si content is 3.10% or more, and more preferably 3.20% or more. On the other hand, if the Si content exceeds 4.0%, the steel sheet becomes brittle, and the passability during manufacturing deteriorates significantly, so the Si content should be 4.0% or less. Preferably, the Si content is 3.70% or less, more preferably 3.60% or less, and more preferably 3.50% or less. 【0103】 Mn:0.010~0.50% by mass Manganese (Mn) is a fundamental element for steel slabs. If the Mn content is less than 0.010%, inhibitors such as MnS and MnSe are less likely to form, secondary recrystallization does not proceed sufficiently, and good magnetic properties cannot be obtained. Therefore, the Mn content should be 0.010% or more. Preferably, the Mn content is 0.030% or more, and more preferably 0.050% or more. On the other hand, if the Mn content exceeds 0.50%, the steel undergoes a phase transformation during secondary recrystallization annealing, secondary recrystallization does not proceed sufficiently, and good magnetic properties cannot be obtained. Therefore, the Mn content should be 0.50% or less. Preferably, the Mn content is 0.20% or less, more preferably 0.15% or less, and more preferably 0.10% or less. 【0104】 C:0.020~0.20%mass% Carbon (C) is a fundamental element for steel billets (slabs). C is included to increase the concentration of Goss orientation in secondary recrystallization. The C content required for improving magnetic properties is 0.020% or more, preferably 0.040% or more, in the slab. However, if excessive C remains in the final product, it can become a factor in iron loss degradation. Therefore, decarburization treatment is necessary in the decarburization annealing process, but if the C content in the slab exceeds 0.20%, decarburization treatment becomes difficult. The C content in the slab is 0.20% or less, preferably 0.15% or less, and more preferably 0.10% or less. 【0105】 N:0.0020~0.020% by mass Nitrogen (N) is a fundamental element for steel billets (slabs). N forms the inhibitor AlN and is necessary to increase the concentration of Goss orientation during secondary recrystallization. The N content required for inhibitor formation in a slab is 0.0020% or more, preferably 0.0040% or more, and more preferably 0.0060% or more. On the other hand, if the N content in a slab exceeds 0.020%, blisters (voids) may form in the steel sheet during cold rolling, the strength of the steel sheet may increase, and the passability during manufacturing may deteriorate. The N content in a slab is 0.020% or less, preferably 0.015% or less, and more preferably 0.010% or less. Like C, excess N remaining in the final product can cause magnetic degradation. Therefore, N needs to be purified during finish annealing. 【0106】 Acid-soluble Al: 0.010~0.050% by mass Acid-soluble aluminum (Al) (sol.Al) is a fundamental element for steel billets (slabs). Acid-soluble Al forms the inhibitor AlN, which is necessary to enhance magnetic properties. The content of acid-soluble Al in a slab is 0.010% or more, preferably 0.015% or more, and more preferably 0.020% or more. On the other hand, if the slab contains an excess of acid-soluble Al, embrittlement may become significant. The content of acid-soluble Al in a slab is 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less. Similar to N, acid-soluble Al needs to be purified from the base steel sheet during finish annealing. 【0107】 P:0~0.040% by mass Phosphorus (P) is a selective element for steel billets (slabs). If the P content exceeds 0.040%, the workability of the steel sheet may decrease significantly. Therefore, the P content should be 0.040% or less. Preferably, the P content is 0.030% or less, and more preferably 0.020% or less. On the other hand, there is no particular lower limit to the P content, and it may be 0%. However, since P has the effect of improving the texture and the magnetic properties of the steel sheet, the P content may be greater than 0%, and may be 0.0020% or more. 【0108】 Total of S and Se: 0.0010~0.040% by mass S (sulfur) and Se (selenium) are fundamental elements for steel billets (slabs). S and Se are the elements that form the inhibitor MnS. The total content of S and Se in a slab is 0.0010% or more, preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, if the total content of S and Se in a slab exceeds 0.040%, it can cause hot brittleness, making hot rolling difficult. The total content of S and Se in a slab is 0.040% or less, preferably 0.0350% or less, and more preferably 0.030% or less. If S and Se remain in excess in the final product, they can cause magnetic degradation. Therefore, S and Se also need to be purified from the base steel sheet during finish annealing. 【0109】 Sn: 0~0.50% by mass Tin (Sn) is a preferred element for steel billets (slabs). If the Sn content exceeds 0.50%, secondary recrystallization becomes unstable, which can adversely affect the magnetic properties. Therefore, the Sn content should be 0.50% or less. Preferably, the Sn content is 0.30% or less, and more preferably 0.150% or less. On the other hand, there is no particular lower limit to the Sn content, and it may be 0%. However, since Sn has the effect of improving magnetic properties by increasing the concentration of Goss orientations, the Sn content may be greater than 0%, and may be 0.0050% or more. 【0110】 Cu:0~0.50% by mass Copper (Cu) is a preferred element for steel billets (slabs). If the Cu content exceeds 0.50%, the steel sheet may become brittle during hot rolling. Therefore, the Cu content should be 0.50% or less. Preferably, the Cu content is 0.30% or less, and more preferably 0.10% or less. On the other hand, there is no particular lower limit to the Cu content, and it may be 0%. However, since Cu has the effect of increasing the concentration of Goss orientation and improving magnetic properties, the Cu content may be greater than 0%, and may be 0.010% or more. 【0111】 Cr:0~0.50% by mass Cr (chromium) is a preferred element for steel billets (slabs). If the Cr content exceeds 0.50%, Cr oxides may form, which can adversely affect the magnetic properties. Therefore, the Cr content should be 0.50% or less. Preferably, the Cr content is 0.30% or less, and more preferably 0.10% or less. On the other hand, there is no particular lower limit to the Cr content, and it may be 0%. However, since Cr has the effect of increasing the concentration of Goss orientation and improving magnetic properties, the Cr content may be greater than 0%, and may be 0.010% or more. 【0112】 Sb:0~0.50% by mass Antimony (Sb) is a preferred element for steel slabs. If the Sb content exceeds 0.50%, it may adversely affect the magnetic properties. Therefore, the Sb content should be 0.50% or less. Preferably, the Sb content is 0.30% or less, and more preferably 0.10% or less. On the other hand, there is no particular lower limit to the Sb content, and it may be 0%. However, since Sb functions as an inhibitor and has the effect of stabilizing secondary recrystallization, the Sb content may be greater than 0%, and may be 0.010% or more. 【0113】 Mo:0~0.10% by mass Mo (molybdenum) is a preferred element for steel billets (slabs). If the Mo content exceeds 0.10%, problems may arise with the rolling properties of the steel sheet. Therefore, the Mo content should be 0.10% or less. Preferably, the Mo content is 0.08% or less, and more preferably 0.05% or less. On the other hand, there is no particular lower limit to the Mo content, and it can be 0%. However, since Mo has the effect of improving magnetic properties by increasing the concentration of Goss orientation, the Mo content may be greater than 0%, and may be 0.0050% or more. 【0114】 Bi:0~0.10% by mass Bi (bismuth) is a selective element for steel billets (slabs). If the Bi content exceeds 0.10%, the passability during cold rolling may deteriorate. Also, if the purification during finish annealing is insufficient and an excess of Bi remains, it may adversely affect the magnetic properties. Therefore, the Bi content should be 0.10% or less. Preferably, the Bi content is 0.050% or less, more preferably 0.020% or less, and more preferably 0.0010% or less. On the other hand, there is no particular lower limit for the Bi content, and it may be 0%. However, since Bi has the effect of improving magnetic properties, the Bi content may be greater than 0%, and may be 0.00050% or more. 【0115】 The steel billets (slabs) used in the hot-rolling process may contain impurities. "Impurities" refer to substances that are introduced during the industrial production of steel, either from the raw materials (ore or scrap) or from the manufacturing environment. Furthermore, the chemical composition of the steel billets (slabs) used in the hot-rolling process can be measured using the same method as described above for the chemical composition of the base steel sheet. 【0116】 In the hot rolling process, the steel billet is first heat-treated. The heating temperature can be, for example, between 1200°C and 1600°C. Preferably, the lower limit of the heating temperature is 1280°C, and the upper limit is 1500°C. Next, the heated steel billet is hot-rolled. The thickness of the hot-rolled steel sheet after hot rolling is preferably in the range of, for example, 2.0 mm to 3.0 mm. 【0117】 Hot-rolled steel sheet annealing process In the hot-rolled steel sheet annealing process, the hot-rolled steel sheet obtained in the hot-rolling process is annealed. This hot-rolled steel sheet annealing induces recrystallization within the steel sheet, ultimately enabling the realization of good magnetic properties. The conditions for hot-rolled steel sheet annealing are not particularly limited, but for example, the hot-rolled steel sheet may be annealed at a temperature range of 900 to 1200°C for 10 seconds to 5 minutes. In addition, after hot-rolled steel sheet annealing but before cold rolling, the surface of the hot-rolled annealed sheet may be pickled. 【0118】 cold rolling process In the cold rolling process, the hot-rolled and annealed steel sheet undergoes either a single cold rolling or multiple cold rollings with an intermediate annealing in between. Since the hot-rolled and annealed steel sheet has a good shape due to the hot-rolled annealing process, the possibility of the steel sheet breaking during the first cold rolling is reduced. Furthermore, if intermediate annealing is performed between cold rollings, the heating method for the intermediate annealing is not particularly limited. Cold rolling may also be performed in three or more stages with an intermediate annealing in between, but since this increases manufacturing costs, it is preferable to perform cold rolling in one or two stages. 【0119】 The final cold rolling reduction ratio in cold rolling (cumulative cold rolling reduction ratio without intermediate annealing, or cumulative cold rolling reduction ratio after intermediate annealing) should be, for example, in the range of 80% to 95%. By setting the final cold rolling reduction ratio within the above range, the final {110} <001> This process increases the degree of concentration in a particular orientation and suppresses the destabilization of secondary recrystallization. The thickness of the cold-rolled steel sheet is usually the same as the thickness of the base steel sheet (final thickness) of the grain-oriented electrical steel sheet that is ultimately produced. The thickness of the cold-rolled steel sheet after cold rolling is preferably in the range of 0.17 mm to 0.29 mm. 【0120】 Decarburization annealing process In the decarburization annealing process, the cold-rolled steel sheet obtained in the cold-rolling process is decarburized and annealed. This decarburization annealing removes the carbon contained in the cold-rolled steel sheet, resulting in primary recrystallization. Decarburization annealing is preferably performed in a humid atmosphere to remove the carbon contained in the cold-rolled steel sheet. For example, annealing can be performed in a humid atmosphere at a temperature range of 700 to 1000°C for 10 seconds to 10 minutes. 【0121】 Alternatively, nitriding may be performed after decarburization annealing and before applying the annealing release agent. In nitriding, a nitrided steel sheet is produced by performing nitriding on the decarburized annealed steel sheet after decarburization annealing. For example, annealing may be performed for 10 to 60 seconds at a temperature range of 700 to 850°C in an atmosphere containing a gas with nitriding ability such as hydrogen, nitrogen, and ammonia. 【0122】 Finish annealing process In the finish annealing process, an annealing release agent is applied to the decarburized annealed sheet obtained in the decarburized annealing process, and then finish annealing is performed. Finish annealing can be performed by annealing the steel sheet for a long period of time while it is wound into a coil. To prevent the coiled steel sheet from seizing during finish annealing, an annealing release agent is applied to the decarburized annealed sheet and dried before the finish annealing process. 【0123】 The annealing separation agent contains magnesia (MgO), alumina (Al2O3), and bismuth chloride. In this annealing separation agent, the solid content is 20-99.5% by mass of alumina, 0.5-20% by mass of bismuth chloride, and the remainder is magnesia and impurities. The bismuth chloride can be bismuth oxychloride (BiOCl) or bismuth trichloride (BiCl3), etc. 【0124】 The annealing conditions for finish annealing are not particularly limited, and known conditions may be used as appropriate. For example, in finish annealing, a decarburized annealed plate coated with an annealing separating agent and dried may be held at a temperature range of 1000°C to 1300°C for 10 to 60 hours. The atmosphere during finish annealing may be, for example, a nitrogen atmosphere or a mixed atmosphere of nitrogen and hydrogen. After finish annealing, the surface of the finish annealed plate may be washed with water to remove dust. 【0125】 This finishing annealing process causes secondary recrystallization in the steel sheet, resulting in a crystal orientation of {110}. <001> The crystals are oriented in a specific direction. In this secondary recrystallized structure, the easy magnetization axes are aligned in the rolling direction, and the crystal grains are coarse. Excellent magnetic properties are obtained due to this secondary recrystallized structure. In this embodiment, since the annealing separating agent contains bismuth chloride, the formation of forsterite film is suppressed, resulting in a smooth surface on the finished annealed sheet. 【0126】 Alternatively, the atmosphere during finish annealing may be changed to a hydrogen atmosphere to perform a purification treatment. This purification treatment removes elements such as Al, N, S, and Se contained in the steel sheet as part of the steel composition, thereby purifying the steel sheet. 【0127】 Thermal oxidation annealing process In the thermal oxidation annealing process, the finished annealed sheet obtained in the finish annealing process is subjected to thermal oxidation annealing (heat treatment). Alternatively, a first surface treatment may be performed before the heat treatment, or a second surface treatment may be performed after the heat treatment. 【0128】 First surface treatment The finish annealed sheet obtained in the finish annealing process may be subjected to a first surface treatment as needed. While no specific pickling conditions are required for the first surface treatment, for example, the finish annealed sheet may be immersed in an acid of a specific concentration (first treatment solution). The first treatment solution may contain at least one of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid, with a total acid concentration of 1-20% by mass and a liquid temperature of 50-90°C. The finish annealed sheet may then be surface treated with this first treatment solution for 3-60 seconds. 【0129】 In the first surface treatment, it is preferable to perform the surface treatment under conditions that activate the surface of the finished annealed plate, while at the same time not creating etch pits on the surface of the finished annealed plate. To achieve this, the above conditions should be controlled in a complex and inseparable manner. For example, if the pickling strength is increased for one of the above conditions, then the pickling strength of the other conditions should be decreased to achieve both an activated surface and a smooth surface. A person skilled in the art can perform surface control including pickling behavior, and by considering the effect of each of the above conditions on the pickling strength, it is possible to control the surface state by combining the above conditions. 【0130】 Furthermore, if the total acid concentration of the first treatment solution is less than 1% by mass, it is difficult to achieve an active surface state on the surface of the finished annealed plate, and internal oxidation SiO2 is unlikely to form in the subsequent heat treatment. On the other hand, if the total acid concentration of the first treatment solution exceeds 20% by mass, etch pits are likely to form on the surface of the finished annealed plate. Similarly, if the temperature of the first treatment solution is less than 50°C, an active surface state cannot be obtained, and if the temperature of the first treatment solution exceeds 90°C, etch pits are likely to form. Likewise, if the treatment time for the first surface treatment is less than 3 seconds, an active surface state cannot be obtained, and if the treatment time for the first surface treatment exceeds 60 seconds, etch pits are likely to form. 【0131】 Heat treatment In the heat treatment, the finished annealed plate after the finish annealing process, or the finished annealed plate after the first surface treatment, is subjected to thermal oxidation annealing. In this heat treatment, the finished annealed plate is heated from room temperature and the heat treatment is carried out within the temperature range of 800 to 1100°C. The heating process from room temperature to the heating temperature within the 800 to 1100°C range and the soaking process at the soaking temperature within the 800 to 1100°C range are controlled individually. 【0132】 During the heating process, the steel sheet is heated from room temperature to a temperature range of 800 to 1100°C in an atmosphere where the oxygen concentration is less than 1.0 volume% and the oxygen potential PH2O / PH2 is 0.50 to 100. The oxygen potential PH2O / PH2 can be defined by the ratio of the partial pressure of water vapor PH2O to the partial pressure of hydrogen PH2 in the atmosphere. By controlling the heating conditions to the above conditions, sufficient internal oxidation SiO2 is preferably formed near the surface of the finished annealed sheet. 【0133】 In this embodiment, by including Bi chloride in the annealing separation agent and creating the atmosphere described above during the heating process, internal oxidized SiO2 is preferably formed near the surface of the finished annealed plate. On the other hand, it is preferable that no external oxide film is formed. To achieve this, the above conditions are controlled in combination and inseparably. For example, if the oxidizing properties are increased by controlling each condition within the above range, internal oxidation is likely to occur, and if the oxidizing properties are weakened by controlling each condition within the above range, external oxidation is likely to occur. Those skilled in the art can control the oxidation reaction by combining the above conditions to form the desired oxide. 【0134】 For example, if the oxygen concentration is 1.0 volume% or higher, internal oxidized SiO2 is not formed, and the entire surface of the steel sheet is prone to excessive oxidation. Therefore, the oxygen concentration should be less than 1.0 volume%. Preferably, the oxygen concentration is 0.50 volume% or less, and more preferably 0.10 volume% or less. On the other hand, there is no particular lower limit to the oxygen concentration; the smaller the better. However, since it is industrially difficult to achieve an oxygen concentration of 0 volume%, the oxygen concentration is set to 1.0 × 10⁻⁶. -20 It may be greater than or equal to a volume percent. The oxygen concentration is 1.0 × 10⁻⁶. -19 Preferably, it is 1.0 × 10% or more by volume. -18 It is more preferable that it be 1% by volume or more. 【0135】 Similarly, if the oxygen potential PH2O / PH2 is less than 0.50, sufficient internal oxidation of SiO2 will not be formed, and the effect of improving film adhesion will not be obtained. For this reason, the oxygen potential PH2O / PH2 should be 0.50 or higher. Preferably, the oxygen potential PH2O / PH2 is 0.60 or higher, and more preferably 0.70 or higher. On the other hand, if the oxygen potential PH2O / PH2 is greater than 100, internal oxidation of SiO2 will not be formed, and the entire surface of the steel sheet will be excessively oxidized, which will actually impair film adhesion. For this reason, the oxygen potential PH2O / PH2 should be 100 or lower. Preferably, the oxygen potential PH2O / PH2 is 90 or lower, and more preferably 80 or lower. The oxygen potential PH2O / PH2 can be derived from the hydrogen concentration and dew point in the annealing atmosphere. 【0136】 Similarly, if the temperature reached during heating is below 800°C, internal oxidized SiO2 is less likely to form. Therefore, the temperature reached during heating should be 800°C or higher. Preferably, the temperature reached is 830°C or higher, and more preferably 860°C or higher. On the other hand, if the temperature reached exceeds 1100°C, internal oxidized SiO2 is not formed, and the entire surface of the steel sheet tends to be excessively oxidized. Therefore, the temperature reached during heating should be 1100°C or lower. Preferably, the temperature reached is 1050°C or lower, and more preferably 1000°C or lower. 【0137】 The heating rate during the heating process is not particularly limited, but it is preferable that the heating rate is 10°C / second or more, and also preferable that it is 100°C / second or less. 【0138】 Furthermore, while the above heating conditions result in the formation of sufficient internal SiO2 oxide near the surface of the finished annealed sheet, these conditions also cause the formation of lumpy oxides such as Fe2SiO4 and FeO on the steel sheet surface. These oxides are thought to negatively affect the adhesion of the coating. Therefore, these oxides are reduced during the soaking process after the heating process. 【0139】 In the soaking process, the steel sheet is soaked in an atmosphere where the oxygen concentration is less than 1.0% by volume and the oxygen potential PH2O / PH2 is from 0.010 to less than 0.50 for 5 to 200 seconds within the temperature range of 800 to 1100°C. By controlling the soaking conditions to the above conditions, oxides such as massive Fe2SiO4 and FeO formed during heating can be reduced and preferably rendered harmless. 【0140】 In the soaking process, oxides such as Fe2SiO4 and FeO are reduced, but on the other hand, it is preferable that the internal oxidation SiO2 is not reduced. For this purpose, the above conditions are controlled in a complex and inseparable manner. For example, if the above conditions are controlled to enhance the reducibility, oxides such as Fe2SiO4 and FeO are easily reduced, but if the reducibility is increased too much, even the internal oxidation SiO2 may be reduced. A person skilled in the art can control the reduction reaction by combining the above conditions to preferentially reduce only oxides such as Fe2SiO4 and FeO. 【0141】 For example, when the oxygen concentration is 1.0% by volume or more, it is difficult to reduce massive Fe-based oxides. Therefore, the oxygen concentration is less than 1.0% by volume. The oxygen concentration is preferably 0.50% by volume or less, and more preferably 0.10% by volume or less. On the other hand, the lower limit of the oxygen concentration is not particularly limited, and the smaller the better. However, since it is industrially difficult to set the oxygen concentration to 0% by volume, the oxygen concentration may be 1.0×10 -20 % by volume or more. The oxygen concentration is preferably 1.0×10 -19 % by volume or more, and more preferably 1.0×10 -18 % by volume or more. 【0142】 Similarly, if the oxygen potential PH2O / PH2 is 0.50 or higher, oxides such as Fe2SiO4 and FeO are not sufficiently reduced. Therefore, the oxygen potential PH2O / PH2 should be less than 0.50. Preferably, the oxygen potential PH2O / PH2 is 0.40 or lower, and more preferably 0.30 or lower. On the other hand, if the oxygen potential PH2O / PH2 is less than 0.010, internal oxidation SiO2 may be excessively reduced. Therefore, the oxygen potential PH2O / PH2 should be 0.010 or higher. Preferably, the oxygen potential PH2O / PH2 is 0.020 or higher, and more preferably 0.030 or higher. 【0143】 Similarly, if the soaking temperature is below 800°C, the above oxides are not easily reduced. Therefore, the soaking temperature should be 800°C or higher. Preferably, the soaking temperature should be 830°C or higher, and more preferably 860°C or higher. On the other hand, if the soaking temperature exceeds 1100°C, the internal oxidized SiO2 may be reduced. Therefore, the soaking temperature should be 1100°C or lower. Preferably, the soaking temperature should be 1050°C or lower, and more preferably 1000°C or lower. 【0144】 Similarly, if the soaking time is less than 5 seconds, the above oxides are not easily reduced. Therefore, the soaking time should be 5 seconds or more. Preferably, the soaking time should be 10 seconds or more, and more preferably 15 seconds or more. On the other hand, if the soaking time exceeds 200 seconds, the internal oxidation SiO2 may be reduced. Therefore, the soaking time should be 200 seconds or less. Preferably, the soaking time should be 150 seconds or less, and more preferably 100 seconds or less. 【0145】 Second surface treatment A second surface treatment may be performed on the heat-treated, oxidized, and annealed plate, if necessary. While no specific pickling conditions are required for the second surface treatment, for example, the heat-treated, annealed plate may be immersed in an acid of a specific concentration (second treatment solution). The second treatment solution may contain at least one of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid, with a total acid concentration of 1-10% by mass and a liquid temperature of 50-90°C. The heat-treated, annealed plate may then be surface-treated using this second treatment solution for 3-60 seconds. 【0146】 In the second surface treatment, residual oxides on the surface of the hot-oxidized annealed plate are pickled, but it is preferable to perform the surface treatment under conditions that do not create etch pits on the surface of the hot-oxidized annealed plate. To achieve this, the above conditions should be controlled in a combined and inseparable manner. For example, if the pickling intensity of one of the above conditions is increased, the pickling intensity of the other conditions should be decreased to achieve both oxide pickling and a smooth surface. A person skilled in the art can perform surface control including pickling behavior, and by considering the effect of each of the above conditions on the pickling intensity, it is possible to control the surface state by combining the above conditions. 【0147】 Furthermore, if the total acid concentration of the second treatment solution is less than 1% by mass, it is difficult to pickle residual oxides from the surface of the heat-oxidized annealed plate. On the other hand, if the total acid concentration of the second treatment solution is greater than 10% by mass, etch pits are likely to form on the surface of the heat-oxidized annealed plate. Similarly, if the temperature of the second treatment solution is less than 50°C, it is difficult to pickle residual oxides, and if the temperature of the second treatment solution is greater than 90°C, etch pits are likely to form. Likewise, if the treatment time for the second surface treatment is less than 3 seconds, it is difficult to pickle residual oxides, and if the treatment time for the second surface treatment is greater than 60 seconds, etch pits are likely to form. 【0148】 Insulating film formation process In the insulating film formation process, an insulating film forming solution is applied to the heat-oxidized annealed plate after the heat oxidation annealing process and then heat-treated. This heat treatment forms an insulating film on the surface of the heat-oxidized annealed plate. For example, the insulating film forming solution may contain colloidal silica and phosphate. It is preferable that the insulating film forming solution does not contain chromium. 【0149】 This insulating coating reduces iron loss in the steel sheet by applying tension to the grain-oriented electrical steel sheet, and also reduces iron loss in the core by ensuring electrical insulation between the steel sheets when grain-oriented electrical steel sheets are used in laminated form. 【0150】 The insulating film is formed by applying an insulating film-forming solution, mainly composed of phosphate or colloidal silica, to the surface of a thermally oxidized annealed plate, and then performing a heat treatment at, for example, 350°C to 1150°C for 5 to 300 seconds. During film formation, the oxygen potential of the atmosphere (PH2O / PH2) can be controlled as needed. 【0151】 As the phosphate, phosphates of Ca, Al, Sr, etc. are preferred, and among these, aluminum phosphate is more preferred. The colloidal silica is not particularly limited to colloidal silica with specific properties. The particle size is also not particularly limited to a specific size, but 200 nm (number average particle size) or less is preferred. For example, 5 to 30 nm is acceptable. If the particle size exceeds 200 nm, sedimentation may occur in the coating solution. 【0152】 In this embodiment, the internal oxidized SiO2 contained in the base steel sheet and the voids and iron and phosphorus-containing oxides contained in the insulating coating are preferably controlled, so that the insulating coating formed in the insulating coating formation process adheres preferably to the heat-oxidized annealed sheet (base steel sheet). 【0153】 After forming the insulating film, flattening annealing may be performed as needed to correct the shape. Performing flattening annealing on the steel plate makes it possible to further reduce iron loss. 【0154】 Magnetic domain control process In this embodiment, magnetic domain control processing may be performed before or after the insulating film formation process, if necessary. By performing magnetic domain control processing, iron loss in the grain-oriented electrical steel sheet can be further reduced. 【0155】 When magnetic domain control processing is performed before the insulating film formation process, linear or dot-shaped grooves extending in a direction intersecting the rolling direction should be formed at predetermined intervals along the rolling direction. Alternatively, when magnetic domain control processing is performed after the insulating film formation process, linear or dot-shaped stress-strain areas extending in a direction intersecting the rolling direction should be formed at predetermined intervals along the rolling direction. Magnetic domain control processing narrows the width of the 180° magnetic domains (the 180° magnetic domains are subdivided). 【0156】 When forming grooves, mechanical groove formation methods using gears, chemical groove formation methods using electrolytic etching, and thermal groove formation methods using laser irradiation can be applied. Furthermore, when forming stress-strained areas, laser beam irradiation and electron beam irradiation can be applied. [Examples] 【0157】 Next, the effects of one aspect of the present invention will be described in more detail with reference to examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention. 【0158】 Slabs (steel billets) with the chemical composition shown in Table 1 were heated to 1350°C and subjected to hot rolling to obtain hot-rolled steel sheets with a thickness of 2.3 mm. These hot-rolled steel sheets were then annealed at 1100°C for 120 seconds and pickled. Subsequently, they were subjected to a single cold-rolling or multiple cold-rolling processes with intermediate annealing in between to obtain cold-rolled steel sheets of the final thickness. These cold-rolled steel sheets were then subjected to decarburization annealing at 830°C for 100 seconds in a humid hydrogen atmosphere. After decarburization annealing, nitriding treatment was performed as needed. 【0159】 The decarburized annealed plates obtained were coated with the annealing separating agents shown in Tables 3 to 7 and dried. In the tables, the alumina and bismuth chloride content is expressed on a solids basis, and the remainder represents magnesia and impurities. Subsequently, a finish annealing was performed by holding at 1200°C for 20 hours. The finish annealing atmosphere was a mixed atmosphere of nitrogen and hydrogen, followed by a hydrogen atmosphere. After the finish annealing, the steel plates were washed with water to remove excess annealing separating agent. 【0160】 The resulting finished annealed plates were subjected to thermal oxidation annealing, a heat treatment (heating process and soaking process) under the conditions shown in Tables 3 to 12. If necessary, first and second surface treatments were also performed as shown in Tables 3 to 12. 【0161】 The surface of the obtained heat-oxidized annealed plate was coated with an insulating film-forming solution containing colloidal silica and phosphate, and heat-treated at 850°C for 1 minute, resulting in a basis weight of 2.5 to 4.5 g / m² per side. 2 An insulating coating was formed to manufacture a grain-oriented electrical steel sheet. The resulting grain-oriented electrical steel sheet was subjected to a magnetic domain control treatment by irradiating it with a laser beam. 【0162】 For the obtained grain-oriented electrical steel sheets No. 1 to No. 69, the chemical composition of the base steel sheet, the internal oxidized SiO2 contained in the base steel sheet interface region, and the presence of voids and iron-phosphorus oxides in the insulating coating were confirmed based on the method described above. Note that the insulating coating was not evaluated for steel sheets with an intermediate ceramic layer or steel sheets without internal oxidized SiO2. Furthermore, the coating adhesion and magnetic properties of the obtained grain-oriented electrical steel sheets No. 1 to No. 69 were evaluated. 【0163】 For example, for SiO2, 10 μm × 10 μm evaluation samples were prepared by cutting out cross-sections parallel to the thickness direction and perpendicular to the width direction from 10 locations spaced apart on the plate surface. Line analysis was performed along the thickness direction using TEM-EDS at an acceleration voltage of 200 kV on the aforementioned base material steel plate interface region to quantitatively analyze the chemical composition, and it was identified from the chemical composition ratio. The elements quantitatively analyzed were five elements: Fe, P, Si, Al, and O. From the TEM-EDS results, after removing measurement noise, regions with an Fe content of less than 80 atomic%, a Si content of 30 atomic%, and an O content of 55 atomic%, were identified as internally oxidized SiO2. Next, using FE-TEM, dark-field images were acquired of the SiO2 identified by the method described above under the conditions of an acceleration voltage of 200kV, a magnification of 40,000x, and a pixel size of 1nm / pixel. The brightness values ​​of the dark-field images of SiO2 were obtained, and regions with similar brightness values ​​were identified as SiO2. The number of pixels included in the base steel sheet interface region and the number of pixels of SiO2 were converted from pixel size to area, and the area ratio of SiO2 to the base steel sheet interface region was determined. Dark-field images were acquired from a total of 10 locations, each with a field of view of 2μm × 2μm, from each evaluation sample. When identifying, regions smaller than the pixel size were removed from the analysis as noise. A FIB processing machine was used to prepare the evaluation samples. General image processing software such as ImageJ may be used for the identification of each phase and measurement of area, or manual correction may be included. 【0164】 Similarly, voids were identified by preparing 10mm × 10mm evaluation samples from 10 locations spaced apart on the plate surface. From each sample, a cross-section was cut out with the cutting direction parallel to the plate thickness direction and perpendicular to the plate width direction. Using an FE-SEM, voids were identified in the aforementioned insulating film interface region by the clear contrast of the voids obtained from the secondary electron image under the conditions of an acceleration voltage of 5kV, an irradiation current of 100pA, and a magnification of 10,000x. Next, backscattered electron images were acquired from the same region under the conditions of a pixel size of 5nm / pixel. A total of 10 locations with a field of view of 10μm × 10μm were acquired from each evaluation sample. The brightness values ​​of the backscattered electron images of the voids identified by the above method were obtained, and regions with similar brightness values ​​were identified as voids. The number of pixels in the insulating film interface region and the number of pixels in the voids were converted into area by the pixel size, and the area ratio of the voids to the insulating film interface region was determined. When identifying, regions smaller than the pixel size were removed from the analysis as noise. Furthermore, the observation surface of the evaluation sample was mirror-polished. For identifying each phase and measuring its area, general image processing software such as ImageJ may be used, or manual correction may be included. 【0165】 Similarly, for oxides containing iron and phosphorus, 10 μm × 10 μm evaluation samples were prepared by cutting out 10 spaced-apart locations on the plate surface, with the cutting direction parallel to the plate thickness direction and perpendicular to the plate width direction. Electron diffraction was obtained using FE-TEM with an acceleration voltage of 200 kV, focusing the electron beam on precipitates in the insulating film. Crystal data obtained from the diffraction pattern was compared with PDFs to identify whether the oxide contained iron and phosphorus. Specifically, comparisons were made with PDFs for Fe3(PO4)2 oxide (JCPDS number: 049-1087), Fe2P2O7 oxide (JCPDS number: 01-076-1762), and FeP oxide (JCPDS number: 03-065-2595). Next, using FE-TEM, dark-field images were acquired under the conditions of an acceleration voltage of 200kV, a magnification of 20,000x, and a pixel size of 2.5nm / pixel. Regions with brightness similar to the iron-phosphorus oxides identified by the method described above were identified as iron-phosphorus oxides. Image processing was used to identify the iron-phosphorus oxides within the insulating film. The number of pixels in the insulating film and the number of pixels in the iron-phosphorus oxides were converted into area by the pixel size, and the area ratio of iron-phosphorus oxides relative to the insulating film was derived. FE-TEM images were acquired from a total of 10 locations, each with a field of view of 2μm × 2μm, from each evaluation sample. During identification, regions smaller than the pixel size were removed from the analysis as noise. A FIB processing machine was used to prepare the evaluation samples. For the identification of each phase and measurement of area, general image processing software such as ImageJ may be used, or manual correction may be included. 【0166】 Coating adhesion was evaluated by measuring the percentage of the remaining coating area when a test specimen was wrapped around a 20 mm diameter cylinder and bent 180°. The area ratio of the remaining coating surface to the area of ​​the steel plate in contact with the cylinder was calculated. The area of ​​the steel plate in contact with the roll was determined by calculation. The area of ​​the remaining coating surface was determined by taking a photograph of the steel plate after the test and performing image analysis on the photographic image. A coating remaining area ratio of 95% or more was evaluated as Excellent (EX), 90% or more but less than 95% as Very Good (VG), and less than 90% as Poor. A coating remaining area ratio of 90% or more was judged as passing. 【0167】 The iron loss characteristics were evaluated using the Single Sheet Tester (SST) method for test specimens. Under conditions of AC frequency: 50 Hz and excitation magnetic flux density: 1.7 T, the iron loss W17 / 50 (W / kg), defined as the power loss per unit weight (1 kg) of the steel sheet, was measured. A value of less than 0.75 W / kg for iron loss W17 / 50 was considered acceptable. The magnetic flux density was measured by applying a magnetic field of 800 A / m to the test specimen and measuring the magnetic flux density B8 (T) in the rolling direction. 【0168】 Tables 1 to 22 show the manufacturing conditions, manufacturing results, and evaluation results. In the tables, a "-" next to the chemical composition indicates that no alloying elements were intentionally added, while a "-" next to anything other than chemical composition indicates that the process was not performed or is not applicable. 【0169】 Furthermore, in the table, "absent" for the intermediate ceramic layer means that there is no intermediate ceramic layer such as a forsterite coating, the insulating coating is placed in contact with the base steel sheet, and the base steel sheet has a smooth surface. "Present" for the intermediate ceramic layer means that an intermediate ceramic layer such as a forsterite coating is present, which adversely affects the magnetic properties. "Absent" for internal oxidized SiO2 in the table means that there was insufficient internal oxidized SiO2 in the base steel sheet interface region. The "area ratio" for internal oxidized SiO2 in the table represents the average value of the area ratio of internal oxidized SiO2 to the area of ​​the base steel sheet interface region. The "isoperipheral constant" for internal oxidized SiO2 in the table represents the average value of the isoperipheral constant of internal oxidized SiO2 present in the base steel sheet interface region. Furthermore, the "frequency of presence in 10 fields of view" for internally oxidized SiO2 in the table represents the number of locations where internally oxidized SiO2 was preferably present in the base steel sheet interface region when the cross-section was observed at 10 observation points spaced apart on the plate surface. The "area ratio" for voids in the table represents the average value of the area ratio of voids relative to the area of ​​the insulating coating interface region. The "area ratio" for Fe&P-containing oxides in the table represents the average value of the area ratio of iron and phosphorus-containing oxides relative to the area of ​​the insulating coating. The "frequency of presence in 10 fields of view" for the insulating coating in the table represents the number of locations where, when the cross-section was observed at 10 observation points spaced apart on the plate surface, the area ratio of voids observed in a 10 μm × 10 μm field of view was 0.010 to 3.0% of the area of ​​the insulating coating interface region, and the area ratio of iron and phosphorus-containing oxides observed in a 2 μm × 2 μm field of view was 0.10 to 5.0% of the area of ​​the insulating coating. 【0170】 Among Tests No. 1 to No. 69, the present invention example exhibited excellent coating adhesion and iron loss characteristics without relying on a forsterite coating. On the other hand, the comparative examples among Tests No. 1 to No. 69 did not exhibit excellent surface smoothness, coating adhesion, or iron loss characteristics. 【0171】 [Table 1] 【0172】 Table 2 【0173】 Table 3 【0174】 Table 4 【0175】 Table 5 【0176】 Table 6 【0177】 Table 7 【0178】 Table 8 【0179】 Table 9 【0180】 Table 10 【0181】 Table 11 【0182】 Table 12 【0183】 [Table 13] 【0184】 [Table 14] 【0185】 [Table 15] 【0186】 [Table 16] 【0187】 [Table 17] 【0188】 [Table 18] 【0189】 [Table 19] 【0190】 [Table 20] 【0191】 [Table 21] 【0192】 [Table 22] [Industrial applicability] 【0193】 According to the above embodiment of the present invention, it is possible to provide a grain-oriented electrical steel sheet and a method for manufacturing the same that have excellent coating adhesion without relying on a forsterite coating. In this grain-oriented electrical steel sheet, the surface of the base steel sheet is smooth because there is no forsterite coating, and the area near the surface of the base steel sheet is preferably internally oxidized and the insulating coating has a preferred form, resulting in excellent coating adhesion. Therefore, it is possible to favorably improve the iron loss characteristics. Consequently, it has high potential for industrial application. [Explanation of symbols] 【0194】 1 Grain-oriented electrical steel sheet 11 Base steel plate 11a Internal oxidation SiO2 12. Insulating coating 12a Void 12b Iron oxides containing phosphorus

Claims

[Claim 1] In a grain-oriented electrical steel sheet having a base steel sheet and a phosphate-based insulating coating disposed in contact with the base steel sheet, The aforementioned base steel sheet has a chemical composition of, in mass%, Si: 3.0 to 4.0%, Mn: 0.010 to 0.50%, It contains, with the remainder being Fe and impurities. When viewed as a cross-section parallel to the thickness direction and perpendicular to the width direction, the base steel sheet has internal SiO2 oxide in the base steel sheet interface region, which is within a range of 2.0 μm from the interface with the phosphate-based insulating coating toward the thickness direction. ２ It has, When viewed at the aforementioned cross-section, the phosphate-based insulating coating contains voids and an oxide containing iron and phosphorus. With respect to the area of ​​the insulating coating interface region which is within a range of 0.5 μm in the thickness direction from the interface with the base steel plate, the area ratio of the voids is 0.010 to 3.0%, and The area ratio of the iron and phosphorus-containing oxide relative to the area of ​​the phosphate-based insulating coating is 0.10 to 5.0%, and When viewed at the aforementioned cross-section, the area ratio of the internal oxidized SiO₂ relative to the area of ​​the interface region of the base steel sheet is 2.0% or more and 20% or less. A grain-oriented electrical steel sheet characterized by the following features. [Claim 2] The aforementioned base steel sheet has the following chemical composition in mass%, C: 0.010% or less, N: 0.010% or less, Acid-soluble Al: 0.020% or less, P: 0.040% or less, Total of S and Se: 0.010% or less. Sn: 0.50% or less, Cu: 0.50% or less, Cr: 0.50% or less, Sb: 0.50% or less, Mo: 0.10% or less Bi: 0.10% or less, including The grain-oriented electrical steel sheet according to claim 1. [Claim 3] When viewed from the aforementioned cross-section, the internal oxidized SiO ２ However, it has a dendritic shape A grain-oriented electrical steel sheet according to claim 1 or 2. [Claim 4] When the cross-section was observed at 10 observation points spaced apart on the plate surface, the internal SiO2 oxide observed in a field of view of 2 μm × 2 μm was observed. ２ However, it is included in five or more observation locations. A grain-oriented electrical steel sheet according to claim 1 or 2. [Claim 5] When the cross-section is observed at 10 observation points spaced apart on the plate surface, there are at least 5 observation points where the area ratio of the void observed in a 10 μm × 10 μm field of view is 0.010 to 3.0% of the area of ​​the insulating coating interface region, and the area ratio of the iron and phosphorus-containing oxide observed in a 2 μm × 2 μm field of view is 0.10 to 5.0% of the area of ​​the phosphate-based insulating coating. A grain-oriented electrical steel sheet according to claim 1 or 2. [Claim 6] A method for manufacturing grain-oriented electrical steel sheets according to claim 1 or claim 2, This process includes a hot rolling process, a hot-rolled steel sheet annealing process, a cold rolling process, a decarburization annealing process, a finish annealing process, a thermal oxidation annealing process, and a phosphate-based insulating coating formation process. In the aforementioned finish annealing process, After the decarburization annealing process, an annealing separation agent is applied to the steel sheet, which contains 20 to 99.5% by mass of alumina, 0.5 to 20% by mass of bismuth chloride, with the remainder being magnesia and impurities, on a solid content basis. After drying, finish annealing is performed. In the aforementioned thermal oxidation annealing process, As a heating process, the steel sheet after the finish annealing process is heated to an oxygen concentration of less than 1.0 volume% and an oxygen potential pH ２ O / PH ２ In an atmosphere where the ratio is 0.50 to 100, the food is heated from room temperature to a temperature range of 800 to 1100°C. As a soaking process, the steel plate after the heating process is heated to an oxygen concentration of less than 1.0 volume% and an oxygen potential pH ２ O / PH ２ In an atmosphere where the pH is between 0.010 and less than 0.50, the mixture is heated to a temperature range of 800 to 1100°C for 5 to 200 seconds. A method for manufacturing grain-oriented electrical steel sheets, characterized by the following features. [Claim 7] In the aforementioned thermal oxidation annealing process, As a first surface treatment before heat treatment, the steel sheet after the finish annealing process is immersed for 3 to 60 seconds in a first treatment solution containing at least one of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid, with a total acid concentration of 1 to 20% by mass and a liquid temperature of 50 to 90°C. As the heat treatment, the heating process and the soaking process are performed on the steel plate after the first surface treatment. The method for manufacturing grain-oriented electrical steel sheets according to quirt 6. [Claim 8] In the aforementioned thermal oxidation annealing process, As a second surface treatment after heat treatment, the steel sheet after the heating process and the soaking process is immersed for 3 to 60 seconds in a second treatment solution containing at least one of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid, with a total acid concentration of 1 to 10% by mass and a liquid temperature of 50 to 90°C. The method for manufacturing grain-oriented electrical steel sheets according to quirt 6.

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