Grain-oriented electrical steel sheet and method for manufacturing same

Abstract

This grain-oriented electrical steel sheet has: a base material steel sheet; and an insulating coating that is formed on the surface of the base material steel sheet and contains a metal phosphate, wherein the base material steel sheet has an oxide layer that includes an oxide of Si in a region in contact with the interface between the base material steel sheet and the insulating coating, and the average thickness of the oxide layer from the interface is 0.5-2.5 μm.

Description

Grain-oriented electrical steel sheet and its manufacturing method

[0001] This application claims priority to Japanese Patent Application No. 2024-018850, filed on February 9, 2024, the contents of which are incorporated herein by reference.

[0002] Grain-oriented electrical steel sheets are primarily used in transformers. Transformers are continuously excited over a long period of time, from installation to disposal, and continue to generate energy loss. Therefore, the energy loss during magnetization with AC, i.e., core loss, is a major indicator that determines the performance of a transformer.

[0003] To reduce the iron loss of grain-oriented electrical steel sheets, many technologies have been developed to date, including (a) increasing the concentration in the {110}<001> orientation (Goss orientation), (b) increasing the content of solid solution elements such as Si to increase the electrical resistance of the steel sheet, or (c) reducing the thickness of the electrical steel sheet.

[0004] Furthermore, applying tension to steel sheets is effective in reducing iron loss. Forming a coating made of a material with a smaller thermal expansion coefficient than the steel sheet at high temperatures on the steel sheet surface is an effective means for reducing iron loss. A forsterite-based coating (inorganic coating) with excellent coating adhesion is produced by the reaction of oxides on the steel sheet surface with an annealing separator during the finish annealing process of electrical steel sheets. This coating can apply tension to steel sheets.

[0005] For example, the method disclosed in Patent Document 1, in which a coating solution mainly composed of colloidal silica and phosphate is baked onto the surface of a steel sheet to form an insulating coating, is an effective method for reducing iron loss because it is highly effective in applying tension to the steel sheet. Therefore, a common method for producing grain-oriented electrical steel sheets is to leave the forsterite-based coating formed in the final annealing process and then apply an insulating coating mainly composed of phosphate on top of it.

[0006] However, in recent years, there has been an increasing demand for smaller and higher-performance transformers. To achieve this, grain-oriented electrical steel sheets are required to have excellent high-field iron loss characteristics, i.e., good iron loss even at high magnetic flux densities. At the same time, it has become clear that forsterite-based coatings hinder domain wall movement, adversely affecting iron loss. In grain-oriented electrical steel sheets, magnetic domains change due to domain wall movement under an AC magnetic field. Smooth and rapid domain wall movement is effective in reducing iron loss. However, forsterite-based coatings (also called glass coatings or primary coatings) are themselves nonmagnetic and have an uneven structure at the interface between the steel sheet and the coating. This uneven structure is thought to hinder domain wall movement and adversely affect iron loss. Therefore, as means for improving high magnetic field iron loss, research has been conducted on a variety of techniques, including methods for removing the forsterite-based coating by mechanical means such as polishing or chemical means such as pickling, techniques for producing grain-oriented electrical steel sheets that do not have a forsterite-based coating by preventing the formation of a forsterite-based coating during high-temperature finish annealing, and techniques for making the steel sheet surface mirror-finished (in other words, techniques for magnetically smoothing the steel sheet surface).

[0007] As a technique for preventing the formation of a forsterite-based coating, for example, Patent Document 2 discloses a technique in which, after normal finish annealing, the steel sheet is pickled to remove surface deposits, and then chemically or electrolytically polished to a mirror finish. It has been found that forming a tensioned insulating coating on the surface of a grain-oriented electrical steel sheet that does not have a forsterite-based coating and that has been obtained by such a known method can provide an even more excellent iron loss improvement effect. Furthermore, the tensioned insulating coating can impart various properties, such as corrosion resistance, heat resistance, and slip resistance, in addition to improving iron loss.

[0008] However, in addition to exhibiting insulating properties, forsterite-based coatings also function as intermediate layers that ensure adhesion when forming tension coatings (tension-applying insulating coatings). That is, because forsterite-based coatings are formed in a state where they penetrate deeply into the steel sheet, they have excellent adhesion to the metal steel sheet. Therefore, when a tension-applying coating (tension coating) containing colloidal silica, phosphate, or the like as a main component is formed on the surface of a forsterite-based coating, the coating exhibits excellent adhesion. On the other hand, because bonding between metal and oxide is generally difficult, it has been difficult to ensure sufficient adhesion between the tension coating and the steel sheet surface in the absence of a forsterite-based coating. Therefore, when forming a tension coating on a grain-oriented electrical steel sheet that does not have a forsterite-based coating, the provision of a layer that serves as an intermediate layer for the forsterite-based coating has been considered.

[0009] Patent Document 3 discloses a technique for ensuring the adhesion of a tensioned insulating coating by applying an intermediate coating beforehand when forming the tensioned coating. However, the technique disclosed in Patent Document 3 has a problem in that it is not possible to maintain good adhesion of a tensioned insulating coating that has a large tension.

[0010] Furthermore, for example, Patent Document 4 discloses a grain-oriented electrical steel sheet having a base steel sheet and an insulating coating formed on the surface of the base steel sheet, the insulating coating being formed on the side of the base steel sheet, an intermediate layer containing a crystalline metal phosphate, and a tensile coating layer formed on the surface side of the insulating coating. Patent Document 4 discloses that the grain-oriented electrical steel sheet does not have a forsterite-based coating, and has excellent coating adhesion, excellent coating tension, and excellent magnetic properties.

[0011] However, depending on the application, grain-oriented electrical steel sheets may be subjected to stress relief annealing after being processed into a predetermined shape. As a result of investigations by the present inventors, it was found that grain-oriented electrical steel sheets to which the technology of Patent Document 4 is applied are not intended to be subjected to stress relief annealing, and that if stress relief annealing is performed, the adhesion of the coating may be reduced.

[0012] Japanese Unexamined Patent Publication No. 48-39338 Japanese Unexamined Patent Publication No. 49-96920 Unexamined Japanese Patent Publication No. 5-279747 International Publication No. 2022 / 215709

[0013] As described above, Patent Documents 1 to 4 do not disclose grain-oriented electrical steel sheets that do not have a forsterite-based coating and that can obtain excellent coating adhesion even when subjected to strain relief annealing (after strain relief annealing). Therefore, an object of the present invention is to provide a grain-oriented electrical steel sheet that can obtain excellent coating adhesion even after strain relief annealing, while maintaining coating tension, corrosion resistance, resistance to phosphorus elution from the coating, and space factor and core loss when used as a core at least as good as conventional ones, based on the premise of a grain-oriented electrical steel sheet that does not have a forsterite-based coating, and a method for manufacturing the same.

[0014] The present inventors investigated the effect of stress relief annealing on the adhesion of an insulating coating, and found that by forming a phosphate coating, which serves as an intermediate layer and an insulating coating, by chemical conversion treatment on the surface of a grain-oriented electrical steel sheet that does not have a forsterite-based coating, and then fusing the intermediate layer and the insulating coating together during stress relief annealing, and forming a layer containing an oxide of Si (oxide layer) directly below the insulating coating obtained by the fusion, it is possible to suppress a decrease in the adhesion of the insulating coating without deteriorating the magnetic properties or other coating properties.

[0015] The present invention has been made in light of the above findings. The gist of the present invention is as follows. [1] A grain-oriented electrical steel sheet according to one aspect of the present invention is a grain-oriented electrical steel sheet having a base steel sheet and an insulating coating containing a metal phosphate formed on the surface of the base steel sheet, wherein the base steel sheet has an oxide layer containing an oxide of Si in a region in contact with the interface between the base steel sheet and the insulating coating, and the oxide layer has an average thickness from the interface of 0.5 to 2.5 μm. [2] In the grain-oriented electrical steel sheet according to [1], the ratio of the length of the interface between the oxide layer and the insulating coating to the length of the interface between the base steel sheet and the insulating coating may be 30% or more. [3] In the grain-oriented electrical steel sheet according to [1] or [2], the area ratio of voids in the insulating coating may be 30% or less. [4] A method for producing a grain-oriented electrical steel sheet according to another aspect of the present invention includes the steps of: adding Al to a steel sheet;２ O ３ a finish annealing process in which an annealing separator containing 10 to 100 mass % of the above is applied to the steel sheet, dried, and then finish annealed; an annealing separator removing process in which excess annealing separator is removed from the steel sheet after the finish annealing process; a light pickling process in which the steel sheet after the annealing separator removing process is pickled with 0.1 to 5.0 mass % of an inorganic acid for 10 to 60 seconds; a water rinsing process in which the steel sheet after the light pickling process is rinsed with water and dried; a first insulating coating forming process in which the steel sheet after the water rinsing process is immersed for 5 to 150 seconds in a treatment solution having a liquid temperature of 30 to 85°C and a metal phosphate concentration of 1.0 to 10.0 mass %, the treatment solution is removed by rinsing with water, and the steel sheet is then dried; [5] The method for producing a grain-oriented electrical steel sheet according to [4] includes a second insulating film forming step in which a coating liquid containing a metal phosphate and colloidal silica is applied to the steel sheet after the insulating film forming step, the coating liquid containing 30 to 150 parts by mass of colloidal silica per 100 parts by mass of the metal phosphate and having a solids concentration of 10 to 40% by mass, drying, and then holding the steel sheet at a sheet temperature of 700 to 950°C for 10 to 90 seconds, and a heat treatment step in which the steel sheet after the second insulating film forming step is heated to a temperature range of 700 to 900°C in an atmosphere having a dew point of 0 to 30°C, a nitrogen content of 50 to 100% by volume, and a hydrogen content of 0 to 50% by volume, and held at this temperature range for 10 to 180 minutes.

[0016] According to the above-described aspects of the present invention, it is possible to provide a grain-oriented electrical steel sheet that can obtain excellent coating adhesion even after stress relief annealing, and a method for producing the same.

[0017] 1 is an example of a photograph of a cross section of a grain-oriented electrical steel sheet according to an embodiment of the present invention, including an insulating coating and a base steel sheet.

[0018] A grain-oriented electrical steel sheet according to one embodiment of the present invention (grain-oriented electrical steel sheet according to this embodiment) and a method for manufacturing the same will be described.

[0019] 1 , the grain-oriented electrical steel sheet according to this embodiment has a base steel sheet 11 and an insulating coating 21 containing a metal phosphate formed on the surface of the base steel sheet 11. Each of these will be described below.

[0020] 1 , the base steel sheet 11 has an oxide layer 12 containing an oxide of Si in a region in contact with the interface IF between the base steel sheet 11 and the insulating coating 21. The portion other than the oxide layer 12 is made of steel sheet.

[0021] (Oxide Layer) In the grain-oriented electrical steel sheet according to this embodiment, an oxide layer containing an oxide of Si is formed in the region of the base steel sheet that contacts the interface between the base steel sheet and the insulating coating. This oxide layer contributes to improving adhesion between the base steel sheet and the insulating coating. Without the oxide layer, sufficient adhesion cannot be ensured. To obtain a high adhesion improvement effect and magnetic properties, the average thickness of the oxide layer from the interface between the base steel sheet and the insulating coating is 0.5 to 2.5 μm. If the average thickness is less than 0.5 μm, the effect of improving adhesion is small. On the other hand, if the average thickness exceeds 2.5 μm, the magnetic properties will be inferior.

[0022] Furthermore, the oxide layer is present in the region that contacts the interface between the base steel sheet and the insulating coating, i.e., constitutes the interface with the insulating coating, thereby improving adhesion (adhesion of the coating). To obtain a greater improvement in adhesion, the ratio of the length of the interface between the oxide layer and the insulating coating to the length of the interface between the base steel sheet and the insulating coating (coverage) is preferably 30% or more. The length ratio may be 100%.

[0023] The oxide layer mainly contains an oxide of Si, and the Si content in the Si oxide layer is preferably 60 to 70 mass %. ２ and FeSiO ４ The oxide layer contains Si-containing composite oxides such as those mentioned above. In addition to Si oxide, iron oxide, aluminum oxide, chromium oxide, etc. may also be contained. The P content of the oxide layer is 1.0 mass% or less, and the oxide layer can be clearly distinguished from an insulating coating. Furthermore, the oxide layer of the grain-oriented electrical steel sheet according to this embodiment is formed as part of the base steel sheet, and therefore has extremely high adhesion to the base steel sheet.

[0024] The presence or absence of an oxide layer, its average thickness, and its proportion of the interface (coverage) can be determined using the following methods. A transmission electron microscope (TEM) is used to photograph a cross section of a steel sheet, including the vicinity of the surface, at 30,000x magnification along the interface for a length of 10 μm or more. The resulting image allows for the distinction between the base steel sheet and the insulating coating. This is because the base steel is composed almost entirely of iron atoms, whereas the insulating coating contains phosphate, aggregated colloidal silica, and voids. Furthermore, the base steel sheet contains an internal oxide layer, but this layer is composed almost entirely of silica and contains little P. Because it is present within the base material, its difference from the phosphate-based insulating coating can be confirmed by elemental analysis. If the image resolution is low and a clear image cannot be obtained, it can be clarified using an image processing device such as Luzex. Alternatively, the interface between the insulating coating and the base material can be determined by the point where the distribution of P element rapidly decreases from the insulating coating side. This is because the P element exists in the form of phosphorus phosphate or phosphorus oxide in the insulating coating and hardly diffuses into the steel. Similarly, the interface between the insulating coating and the base material may be determined as the location where Al, Mg, Mn, Zn, Ca, Cu, Co, and Li, which are presumed to exist as cations in the insulating coating, rapidly decrease. Based on the three or more TEM images obtained as described above, the average thickness of the oxide layer is determined by averaging the thicknesses of the oxide layer measured at three locations in each image (i.e., a total of nine or more locations). The proportion of the oxide layer at the interface is determined by measuring the length of the interface between the oxide layer and the insulating coating at three locations in each of the three or more TEM images, within a photographable range of 10 μm, calculating the proportion, and averaging the obtained results.

[0025] The Si content of the oxide layer can be obtained by performing elemental analysis at three or more locations in each of three fields of view using an energy dispersive X-ray analyzer attached to a transmission electron microscope, and averaging the measurement results.

[0026] (Chemical Composition) The chemical composition of the portion (steel sheet portion) other than the oxide layer of the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment may be within a range known for grain-oriented electrical steel sheets in order to obtain the properties generally required of grain-oriented electrical steel sheets. For example, the following chemical components are preferably included as components constituting the chemical composition. In this embodiment, % relating to the chemical components is % by mass unless otherwise specified.

[0027] C: 0.010% or less C (carbon) is an element effective for controlling the structure of the steel sheet in the manufacturing process up to the completion of the decarburization annealing process. However, if the C content exceeds 0.010%, the magnetic properties of the finished grain-oriented electrical steel sheet deteriorate. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, the C content is preferably 0.010% or less. The C content is more preferably 0.005% or less. Although the lower the C content, reducing the C content to less than 0.0001% saturates the effect of structural control and merely increases manufacturing costs. Therefore, the C content may be 0.0001% or more.

[0028] Si: 2.50 to 4.00% Si (silicon) is an element that increases the electrical resistance of grain-oriented electrical steel sheets and improves their iron loss characteristics. If the Si content is less than 2.50%, a sufficient eddy current loss reduction effect cannot be obtained. Therefore, the Si content is preferably 2.50% or more. The Si content is more preferably 2.70% or more, and even more preferably 3.00% or more. On the other hand, if the Si content exceeds 4.00%, the grain-oriented electrical steel sheets become embrittled and their threading properties deteriorate significantly. Furthermore, the workability of the grain-oriented electrical steel sheets decreases, and the steel sheets may break during rolling. Therefore, the Si content is preferably 4.00% or less. The Si content is more preferably 3.80% or less, and even more preferably 3.70% or less.

[0029] Mn: 0.01 to 0.50% Mn (manganese) is an element that combines with S to form MnS during the manufacturing process. This precipitate functions as an inhibitor (a suppressor of normal grain growth) and induces secondary recrystallization in the steel. Mn also enhances the hot workability of the steel. If the Mn content is less than 0.01%, the above-mentioned effects cannot be fully achieved. Therefore, the Mn content is preferably 0.01% or more. The Mn content is more preferably 0.02% or more. On the other hand, if the Mn content exceeds 0.50%, secondary recrystallization does not occur and the magnetic properties of the steel deteriorate. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, the Mn content is preferably 0.50% or less. The Mn content is more preferably 0.20% or less, and even more preferably 0.10% or less.

[0030] N: 0.010% or less N (nitrogen) is an element that bonds with Al during the manufacturing process to form AlN, which functions as an inhibitor. However, if an excessive amount of inhibitor remains in the grain-oriented electrical steel sheet and the N content exceeds 0.010%, the magnetic properties deteriorate. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, the N content is preferably 0.010% or less. The N content is more preferably 0.008% or less. On the other hand, the lower limit of the N content is not particularly specified, but reducing it to less than 0.001% only increases the manufacturing cost. Therefore, the N content may be 0.001% or more.

[0031] Sol. Al: 0.020% or less Sol. Al (acid-soluble aluminum) is an element that bonds with N to form AlN, which functions as an inhibitor, during the manufacturing process of grain-oriented electrical steel sheets. However, if an excessive amount of inhibitor remains in the base steel sheet and the sol. Al content exceeds 0.020%, the magnetic properties deteriorate. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, the sol. Al content is preferably 0.020% or less. The sol. Al content is more preferably 0.010% or less, and even more preferably less than 0.001%. There is no particular restriction on the lower limit of the sol. Al content, but reducing it to less than 0.0001% only increases the manufacturing cost. Therefore, the sol. Al content may be 0.0001% or more.

[0032] S: 0.010% or less S (sulfur) is an element that combines with Mn during the manufacturing process to form MnS, which functions as an inhibitor. However, if the S content exceeds 0.010%, the magnetic properties will be reduced due to the remaining inhibitor. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, the S content is preferably 0.010% or less. The S content in the grain-oriented electrical steel sheet is preferably as low as possible. For example, less than 0.001%. However, reducing the S content in the base steel sheet of the grain-oriented electrical steel sheet to less than 0.0001% will only increase the manufacturing cost. Therefore, the S content in the base steel sheet of the grain-oriented electrical steel sheet may be 0.0001% or more.

[0033] Balance: Fe and impurities The chemical composition of the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment may contain the above-mentioned elements, with the balance being Fe and impurities. However, for the purpose of improving magnetic properties, etc., Sn, Cu, Se, and Sb may also be contained in the ranges shown below. Furthermore, even if other elements, such as W, Nb, Ti, Ni, Co, V, Cr, and Mo, are contained in a total of 1.0% or less, the effects of the grain-oriented electrical steel sheet according to this embodiment are not impaired. Here, impurities refer to elements that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during industrial production of the base steel sheet, and are allowed to be contained in amounts that do not adversely affect the function of the grain-oriented electrical steel sheet according to this embodiment.

[0034] Sn: 0 to 0.50% Sn (tin) is an element that contributes to improving magnetic properties through controlling the primary recrystallization structure. To obtain the effect of improving magnetic properties, the Sn content is preferably 0.01% or more. The Sn content is more preferably 0.02% or more, and even more preferably 0.03% or more. On the other hand, if the Sn content exceeds 0.50%, secondary recrystallization becomes unstable and magnetic properties deteriorate. Therefore, the Sn content is preferably 0.50% or less. The Sn content is more preferably 0.30% or less, and even more preferably 0.10% or less.

[0035] Cu: 0 to 0.50% Cu (copper) is an element that contributes to increasing the Goss orientation occupancy rate in the secondary recrystallized structure. To achieve the above effect, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.02% or more, and even more preferably 0.03% or more. On the other hand, if the Cu content exceeds 0.50%, the steel sheet becomes embrittled during hot rolling. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, the Cu content is preferably 0.50% or less. The Cu content is more preferably 0.30% or less, and even more preferably 0.10% or less.

[0036] Se: 0 to 0.020% Se (selenium) is an element that has the effect of improving magnetic properties. When Se is contained, the Se content is preferably 0.001% or more to effectively exhibit the effect of improving magnetic properties. The Se content is more preferably 0.003% or more, and even more preferably 0.006% or more. On the other hand, if the Se content exceeds 0.020%, the adhesion of the coating deteriorates. Therefore, the Se content is preferably 0.020% or less. The Se content is more preferably 0.015% or less, and even more preferably 0.010% or less.

[0037] Sb: 0 to 0.50% Sb (antimony) is an element that has the effect of improving magnetic properties. When Sb is contained, the Sb content is preferably 0.005% or more to effectively exhibit the effect of improving magnetic properties. The Sb content is more preferably 0.01% or more, and even more preferably 0.02% or more. On the other hand, if the Sb content exceeds 0.50%, the adhesion of the coating significantly deteriorates. Therefore, the Sb content is preferably 0.50% or less. The Sb content is more preferably 0.30% or less, and even more preferably 0.10% or less.

[0038] As described above, the chemical composition of the base steel sheet of the grain-oriented electrical steel sheet in this embodiment is, for example, one that contains the above-mentioned elements with the balance being Fe and impurities.

[0039] The chemical composition of the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment can be measured using a known ICP atomic emission spectroscopy. However, when an insulating coating is formed on the surface, it is removed before measurement. The removal can be achieved by immersing the sample in a highly concentrated alkaline solution (e.g., a 30% sodium hydroxide solution heated to 85°C) for 20 minutes or more. Peeling can be determined visually. For small samples, removal can also be achieved by surface grinding.

[0040] (Thickness) The thickness of the base steel plate is not limited, but is preferably 0.15 to 0.35 mm in terms of iron loss.

[0041] [Insulating Coating] In the grain-oriented electrical steel sheet according to this embodiment, an insulating coating is formed on the base steel sheet. That is, since a forsterite-based coating is not formed, the insulating coating is formed in direct contact with the base steel sheet. As described below, the insulating coating included in the grain-oriented electrical steel sheet according to this embodiment is a single layer formed by fusing the first insulating coating and the second insulating coating through heat treatment. This insulating coating contains a metal phosphate. Examples of the metal phosphate include aluminum phosphate, magnesium phosphate, manganese phosphate, zinc phosphate, calcium phosphate, iron phosphate, copper phosphate, and lithium phosphate. The insulating coating also contains silicon derived from colloidal silica. In the insulating coating, the metal phosphate content is preferably 90 to 50 mass % and the silicon content is preferably 10 to 40 mass %.

[0042] The proportions of the metal phosphate and Si content in the insulating coating can be measured by SEM-EDS, as described below.

[0043] (Voids) The area ratio of voids in the insulating coating is preferably 30% or less. By reducing the area ratio of voids, the effect of further improving the coating tension can be obtained.

[0044] The area ratio of voids in the insulating coating is determined by the following method. Based on an image of a cross section of a steel sheet, including the vicinity of the surface, taken with a transmission electron microscope (TEM) at 30,000 times magnification and measuring 10 μm or more in length, the phosphate-based insulating coating and voids are confirmed by elemental analysis, and the void area is determined. The percentage of the void area in the entire area of ​​the observation field is taken as the void area ratio (%). If the image resolution is low and a clear image cannot be obtained, it can be clarified using an image processing device such as Luzex.

[0045] (Thickness) In the grain-oriented electrical steel sheet according to this embodiment, the thickness of the insulating coating 21 is preferably 1.0 to 10.0 μm in terms of space factor.

[0046] The thickness of the insulating coating is determined as follows. A 10 mm square sample (10 mm in the rolling direction and 10 mm perpendicular to the rolling direction) is collected from the flat portion. A cross section of the sample in the thickness direction perpendicular to the rolling direction is observed using a scanning electron microscope. The thickness is measured at five or more points, and the average is taken as the thickness of the insulating coating. The P content is measured using an energy dispersive elemental analyzer. The interface between the insulating coating and the base steel sheet is determined as the position where the distribution of P element rapidly decreases from the insulating coating side (surface side). Alternatively, the interface between the insulating coating and the base steel sheet may be determined as the position where the distribution of Al, Mg, Mn, Zn, Ca, Cu, Co, and Li, which are presumed to be present as cations in the insulating coating, rapidly decreases.

[0047] In addition, the mass percentage of the metal phosphate and the type of metal phosphate are identified using a scanning electron microscope and an energy dispersive elemental analyzer. The Si content can also be measured using a scanning electron microscope and an energy dispersive elemental analyzer. The mass percentage of the metal phosphate and the Si content are measured at three locations and the average values ​​are used.

[0048] <Manufacturing Method> The grain-oriented electrical steel sheet according to the present embodiment can achieve the effects as long as it has the above-described configuration, regardless of the manufacturing method, but can be preferably manufactured by a manufacturing method including, for example, the following steps: (i) a hot rolling step of heating a steel slab and hot rolling it to form a hot-rolled sheet, (ii) a hot-rolled sheet annealing step of annealing the hot-rolled sheet, (iii) a pickling step of pickling the hot-rolled sheet after the hot-rolled sheet annealing step, (iv) a cold rolling step of cold-rolling the hot-rolled sheet after the pickling step to form a steel sheet (cold-rolled sheet), (v) a decarburization annealing step of decarburizing the steel sheet, and (vi) a step of adding Al to the steel sheet after the decarburization annealing. ２ O ３(ix) a water-rinsing step of rinsing the steel sheet after the water-rinsing step with a treatment solution having a temperature of 30 to 85°C and a metal phosphate concentration of 1.0 to 10.0 mass%, drying the steel sheet, and then performing a final annealing; (vii) an annealing separator removing step of removing excess annealing separator from the steel sheet after the final annealing step; (viii) a light pickling step of pickling the steel sheet after the annealing separator removing step with 0.1 to 5.0 mass% of inorganic acid for 10 to 60 seconds; (ix) a water-rinsing step of rinsing the steel sheet after the light pickling step with water and drying it; and (x) a first insulating coating forming step of immersing the steel sheet after the water-rinsing step in a treatment solution having a liquid temperature of 30 to 85°C and a metal phosphate concentration of 1.0 to 10.0 mass%, rinsing the treatment solution with water, drying the steel sheet, and forming a first insulating coating. (xi) a second insulating coating formation step of applying a coating liquid to the steel sheet after the first insulating coating formation step, the coating liquid containing a metal phosphate and colloidal silica, the content of the colloidal silica being 30 to 150 parts by mass per 100 parts by mass of the metal phosphate, and having a solid content concentration of 10 to 40% by mass, drying the coating liquid, and then holding the steel sheet at a sheet temperature of 700 to 950°C for 10 to 90 seconds to form a second insulating coating; and (xii) a heat treatment step of heating the steel sheet after the second insulating coating formation step to a temperature range of 700 to 900°C in an atmosphere having a dew point of 0 to 30°C, a nitrogen content of 50 to 100% by volume, and a hydrogen content of 0 to 50% by volume, and holding the steel sheet at this temperature range for 10 to 180 minutes, thereby fusing the first insulating coating and the second insulating coating to form an insulating coating and forming an oxide layer in the steel sheet. The method for producing a grain-oriented electrical steel sheet according to this embodiment is characterized by the (vi) finish annealing step to the (xii) heat treatment step, and the (i) hot rolling step to the (v) decarburization annealing step are not particularly limited, and known conditions can be applied.

[0049] [Hot Rolling Process] In the hot rolling process, a billet such as a slab having a predetermined chemical composition is heated and then hot rolled to obtain a hot-rolled sheet. The heating temperature of the billet is preferably within the range of 1100 to 1450°C. The heating temperature is more preferably 1300 to 1400°C. The chemical composition of the billet may be changed depending on the chemical composition of the base steel sheet of the grain-oriented electrical steel sheet that is ultimately to be obtained, and may be, for example, in mass %, C: 0.01 to 0.20%, Si: 2.50 to 4.00%, sol. An example of a chemical composition is Al: 0.01 to 0.040%, Mn: 0.01 to 0.50%, N: 0.020% or less, S: 0.005 to 0.040%, Cu: 0 to 0.50%, Sn: 0 to 0.50%, Se: 0 to 0.020%, Sb: 0 to 0.50%, and the balance being Fe and impurities. The hot rolling conditions are not particularly limited and may be set appropriately based on the desired properties. The thickness of the hot-rolled sheet is preferably, for example, in the range of 2.0 mm to 3.0 mm.

[0050] [Hot-rolled sheet annealing process] In the hot-rolled sheet annealing process, the steel sheet (hot-rolled sheet) after the hot-rolling process is annealed. This annealing process recrystallizes the steel sheet structure, enabling the steel sheet to achieve good magnetic properties. In the hot-rolled sheet annealing process of this embodiment, the hot-rolled sheet manufactured through the hot-rolling process is annealed according to a known method. The means for heating the hot-rolled sheet during annealing is not particularly limited, and known heating methods can be adopted. For example, so-called continuous annealing may be used, or the hot-rolled sheet may be coiled and subjected to batch annealing. The annealing conditions are also not particularly limited, but for example, the hot-rolled sheet can be annealed in a temperature range of 900 to 1200°C for 10 seconds to 5 minutes. The atmosphere is not particularly limited, but it is preferable to suppress oxidation of the steel sheet, and it is preferable to perform the annealing in a non-oxidizing atmosphere such as nitrogen, argon, or hydrogen.

[0051] [Pickling Process] In the pickling process, scale (oxides) formed on the surface of the steel sheet during hot rolling and hot-rolled sheet annealing is removed. In the pickling process of this embodiment, a known method is used. Known acids such as hydrochloric acid, sulfuric acid, and nitric acid are used as the pickling solution. If necessary, known pickling inhibitors, pickling accelerators, etc. may be added to the pickling solution. Furthermore, before contacting the steel sheet with the pickling solution, it is also possible to perform physical treatment such as shot blasting on the steel sheet before pickling in order to penetrate the pickling solution into the interface between the scale and the steel sheet and improve the pickling efficiency.

[0052] [Cold Rolling Process] In the cold rolling process, the steel sheet after the pickling process is cold-rolled to obtain a cold-rolled sheet (a steel sheet having the same sheet thickness as the base steel sheet of the grain-oriented electrical steel sheet). Cold rolling may be a single cold rolling process (a series of cold rolling processes without intermediate annealing), or multiple cold rolling processes with intermediate annealing may be performed by interrupting the cold rolling and performing at least one or more intermediate annealing processes before the final pass of the cold rolling process. The cold rolling conditions may be determined according to known methods. The cold rolling reduction of grain-oriented electrical steel sheets has a significant effect on their magnetic properties. The final reduction has a particularly large effect, and the final reduction can be set to 80 to 95%. The final reduction is the cumulative reduction of cold rolling, and in the case of intermediate annealing, it is the cumulative reduction of cold rolling after the final intermediate annealing. When intermediate annealing is performed, the steel is held at a temperature of 800 to 1200°C for 5 to 180 seconds, for example. The annealing atmosphere is not particularly limited, but it is preferable to perform the annealing in a non-oxidizing atmosphere such as nitrogen, argon, or hydrogen to prevent oxidation of the steel sheet. The annealing method may be either so-called continuous annealing or batch annealing in a coil shape, or other methods. The number of intermediate annealings is preferably three or less, taking into consideration the manufacturing cost.

[0053] [Decarburization Annealing Process] In the decarburization annealing process, the cold-rolled sheet after the grinding process is subjected to decarburization annealing. In this decarburization annealing, C, which adversely affects magnetic properties, is removed (decarburized) from the steel sheet, and the cold-rolled sheet undergoes primary recrystallization. The decarburization annealing conditions are not limited, but annealing is performed in a nitrogen-hydrogen mixed atmosphere for decarburization, with the oxygen potential increased by humidification. Furthermore, since it is necessary to form a primary recrystallized structure, the humidification temperature (dew point) is determined in terms of the annealing temperature required for recrystallization and the oxygen potential that allows decarburization at that annealing temperature. The annealing temperature is, for example, approximately 700 to 900°C. Since annealing is generally performed in a continuous annealing process, soaking is performed for 30 to 90 seconds.

[0054] [Nitriding Process] Nitriding may be performed between the decarburization annealing process and the finish annealing process described below. In the nitriding process, for example, the decarburization annealed steel sheet is maintained at approximately 700 to 850°C in a nitriding atmosphere (an atmosphere containing hydrogen, nitrogen, and a nitriding gas such as ammonia) to perform nitriding. When AlN is used as an inhibitor, it is preferable to set the nitrogen concentration of the steel sheet to 40 ppm (0.0040 mass%) or more by the nitriding process. On the other hand, if the nitrogen concentration of the steel sheet exceeds 1000 ppm (0.1000 mass%), excess AlN remains in the steel sheet even after the completion of secondary recrystallization in the finish annealing. Such AlN can cause iron loss degradation. For this reason, it is preferable to set the nitrogen concentration of the steel sheet after the nitriding process to 1000 ppm or less.

[0055] [Finish Annealing Process] In the finish annealing process, Al is added to the steel sheet after the decarburization annealing process (or the nitriding process). ２ O ３ In a conventional method for producing grain-oriented electrical steel sheets, an annealing separator containing 10 to 100 mass % of Al is applied, dried, and then finish annealed. In contrast, in the method for producing grain-oriented electrical steel sheets according to the present embodiment, an annealing separator containing mainly MgO is applied and finish annealed to form a forsterite-based coating on the surface of the steel sheet (cold-rolled sheet). ２ O ３ An annealing separator containing 10 mass % or more of Al is used. ２ O３ The content of Al is preferably 40 mass % or more. ２ O ３ The ratio of Al may be 100 mass %, but ２ O ３ From the viewpoint of preventing seizure of Al, in the method for producing a grain-oriented electrical steel sheet according to this embodiment, the annealing separator preferably contains MgO. Although the content of MgO may be 0%, in order to obtain the above effect, the content of MgO is preferably 5 mass% or more. When MgO is contained, the content of MgO is preferably 10 mass% or more. ２ O ３ The content of Al relative to the annealing separator is 90 mass % or less to ensure this. The content of MgO is preferably 50 mass % or less. ２ O ３ The total of MgO and MgO may be more than 50% by mass in terms of solid content. Furthermore, in the method for producing a grain-oriented electrical steel sheet according to this embodiment, the annealing separator may further contain chloride. The inclusion of chloride in the annealing separator provides the effect of making it more difficult for a forsterite-based coating to form. The chloride content is not particularly limited and may be 0%, but to obtain the above effect, 0.5 to 10.0% by mass is preferred. Examples of chlorides that are effective include bismuth chloride, calcium chloride, cobalt chloride, iron chloride, and nickel chloride. The finish annealing conditions are not limited, but for example, conditions of holding the steel sheet at a temperature of 1150 to 1250°C for 10 to 60 hours can be employed.

[0056] [Annealing Separator Removal Step] In the annealing separator removal step, excess annealing separator is removed from the steel sheet after the finish annealing step. For example, the excess annealing separator can be removed by washing with water.

[0057] [Light Pickling Step] In the light pickling step, the steel sheet after the annealing separator removal step is pickled with 0.1 to 5.0 mass % of inorganic acid for 10 to 60 seconds. If the conditions for the light pickling are not favorable, problems such as excess annealing separator remaining on the steel sheet surface, increasing the surface roughness and reducing the space factor, or if the acid is too strong, etching the steel sheet surface and reducing the magnetic properties may occur.

[0058] [Water-Rinsing Step] In the water-rinsing step, the steel sheet after the light pickling step is rinsed with water and dried. This removes the pickling solution, making it possible to suppress rusting. The conditions for water-rinsing and drying are not limited.

[0059] [First Insulating Coating Forming Step] In the first insulating coating forming step, the steel sheet after the water-rinsing step is immersed for 5 to 150 seconds in a treatment solution having a temperature of 30 to 85°C and a metal phosphate concentration of 1.0 to 10.0% by mass. The treatment solution is then rinsed off, and the steel sheet is dried to form a first insulating coating on the surface of the steel sheet. If the treatment solution temperature is below 30°C, the amount of the first insulating coating formed is too small, resulting in localized reduced adhesion of the insulating coating. On the other hand, if the solution temperature exceeds 85°C, the first insulating coating becomes too thick in some areas, ultimately resulting in increased surface roughness and a reduced space factor. Furthermore, if the metal phosphate concentration (concentration of metal phosphate) is less than 1.0% by mass, forming the first insulating coating takes too long, which is cost-effective. On the other hand, if the metal phosphate concentration exceeds 10.0% by mass, the first insulating coating is formed with a localized thickness, ultimately resulting in an uneven insulating coating. If the treatment time is less than 5 seconds, the amount of the first insulating coating formed will be too small, resulting in partial deterioration of the adhesiveness of the insulating coating, whereas if the treatment time is more than 150 seconds, the time will be too long and will be disadvantageous in terms of cost.

[0060] [Second Insulating Coating Forming Step] In the second insulating coating forming step, a coating liquid (insulating coating forming liquid) containing metal phosphate and colloidal silica is applied to the steel sheet after the first insulating coating forming step. The coating liquid contains 30 to 150 parts by mass of colloidal silica per 100 parts by mass of the metal phosphate, and has a solids concentration of 10 to 40% by mass. The steel sheet is then dried and held at a sheet temperature of 700 to 950°C for 10 to 90 seconds to form a second insulating coating on the first insulating coating. If the sheet temperature is below 700°C, low tension results, resulting in poor magnetic properties. Therefore, it is preferable to set the sheet temperature to 700°C or higher. On the other hand, if the sheet temperature is above 950°C, the rigidity of the steel sheet decreases, making it more susceptible to deformation. In this case, strain may occur in the steel sheet during transportation, resulting in poor magnetic properties. Therefore, it is preferable to set the sheet temperature to 950°C or lower. Furthermore, if the holding time is less than 10 seconds, elution resistance is poor. Therefore, the holding time is set to 10 seconds or more. On the other hand, if the holding time exceeds 90 seconds, adhesion will decrease, and attempts to avoid this decrease in adhesion will result in poor productivity. Therefore, a holding time of 90 seconds or less is preferred. The coating liquid contains a metal phosphate and colloidal silica in a ratio of 30 to 150 parts by mass of colloidal silica per 100 parts by mass of metal phosphate. The solids concentration of the coating liquid is set to 10 to 40% by mass, and the total of the metal phosphate and colloidal silica, calculated as solids, is preferably greater than 50% by mass of the coating liquid. As the metal phosphate, for example, one or a mixture of two or more selected from aluminum phosphate, zinc phosphate, magnesium phosphate, nickel phosphate, copper phosphate, lithium phosphate, cobalt phosphate, etc. can be used. The coating liquid may also contain additional elements such as vanadium, tungsten, molybdenum, and zirconium. Either S-type or C-type colloidal silica can be used. S-type colloidal silica refers to one in which the silica solution is alkaline, while C-type refers to one in which the surface of the silica particles is aluminum-treated and the silica solution is alkaline to neutral. S-type colloidal silica is widely used and relatively inexpensive, but care must be taken when mixing it with an acidic metal phosphate solution as it may aggregate and precipitate.Type C colloidal silica is stable even when mixed with a metal phosphate solution and there is no risk of precipitation, but it is relatively expensive due to the large number of processing steps required. It is preferable to use the type C colloidal silica depending on the stability of the coating solution to be prepared.

[0061] [Magnetic Domain Refinement Step] In the magnetic domain refinement step, the surface of the insulating coating (the surface of the insulating coating of a grain-oriented electrical steel sheet comprising a base steel sheet and an insulating coating) may be irradiated with energy rays to refine the 180° magnetic domains. Magnetic domain refinement can further reduce the iron loss of the grain-oriented electrical steel sheet. Known methods for the magnetic domain refinement process may be used. For example, linear or dot-like grooves extending in a direction intersecting the rolling direction may be formed at predetermined intervals along the rolling direction to narrow the width of the 180° magnetic domains (refining the 180° magnetic domains). Grooves can be formed by mechanical groove formation using gears or the like, chemical groove formation using electrolytic etching, or thermal groove formation using laser irradiation. If the formation of stress-strained portions or grooves damages the insulating coating, resulting in deterioration of its insulating properties, the damage may be repaired by forming a new insulating coating.

[0062] [Heat Treatment Step] In the heat treatment step, the steel sheet after the second insulating coating formation step is heated to a temperature range of 700 to 900°C in an atmosphere having a dew point of 0 to 30°C, a nitrogen content of 50 to 100% by volume, and a hydrogen content of 0 to 50% by volume, and is held at this temperature range for 10 to 180 minutes, thereby fusing the first insulating coating and the second insulating coating to form an insulating coating and forming an oxide layer in the steel sheet. This heat treatment step can also serve as stress relief annealing. When the heat treatment step also serves as stress relief annealing, the steel sheet may be processed to a predetermined shape (e.g., an iron core shape) before the heat treatment step. If the dew point is below 0°C, the oxide layer will not be sufficiently formed. On the other hand, if the dew point is above 30°C, the average thickness of the oxide layer will be excessive. If the hydrogen content in the atmosphere is above 50% by volume, there is a risk that the oxide layer will not be sufficiently formed. If the heat treatment temperature is less than 700°C, the rate of oxide layer formation is slow, and the oxide is not sufficiently formed. On the other hand, if the temperature exceeds 900°C, the rate of oxide formation becomes too fast, resulting in large variations and an excessively thick oxide layer. Furthermore, if the holding time is less than 10 minutes, the oxide formation is insufficient, resulting in large variations and an excessively thick oxide layer. On the other hand, if the holding time exceeds 180 minutes, the amount of oxidation becomes excessive, resulting in an excessively thick oxide layer. In the present application, additional annealing may be performed in conjunction with the formation of the oxide layer. For example, heating may be performed for 10 minutes in an atmosphere with a dew point of 20°C, followed by annealing for 120 minutes in an atmosphere of 100% nitrogen with a dew point of -20°C.

[0063] A slab containing, by mass%, 0.08% C, 3.31% Si, 0.07% Mn, 0.028% sol. Al, and 0.008% N, with the balance being Fe and impurities, was cast, heated, and hot-rolled to obtain a 2.2 mm hot-rolled sheet. This hot-rolled sheet was annealed at 1100°C for 10 seconds. After pickling under known conditions, it was cold-rolled to 0.22 mm by continuous cold rolling without intermediate annealing to obtain a steel sheet (cold-rolled sheet). This steel sheet was subjected to decarburization annealing at 830°C for 3 minutes. After decarburization annealing, the steel sheet contained 48% MgO and Al. ２ O ３ :48% by mass, BiCl ３An annealing separator containing 4% by mass of phosphate was applied to the steel sheet, dried, and then the steel sheet was heated to 1200°C and held there for 20 hours for finish annealing. After finish annealing, the steel sheet was rinsed with water to remove excess annealing separator, and no forsterite-based coating was formed on the steel sheet surface. This steel sheet was lightly pickled under the conditions shown in Table 1. The steel sheet was then rinsed with water and dried. This steel sheet was immersed in a treatment solution containing a metal phosphate salt shown in Table 1 mixed with additives such as preservatives and viscosity modifiers. The treatment solution was then rinsed off with water, and the steel sheet was dried to form a first insulating coating. Subsequently, a coating solution containing a metal phosphate salt and colloidal silica in the proportions shown in Table 2 was applied to the steel sheet and dried to form a second insulating coating. Subsequently, grooves 20 μm deep and 50 μm wide were formed on the steel sheet surface at intervals of 6 mm in a direction tilted 80° from the rolling direction according to the tooth profile. Thereafter, heat treatment was carried out under the conditions shown in Table 3 to obtain grain-oriented electrical steel sheets.

[0064] The grain-oriented electrical steel sheets were measured for the presence or absence of an oxide layer, the average thickness of the oxide layer, and the coverage of the oxide layer. The composition and thickness of the insulating coating were also measured. The results are shown in Table 4.

[0065] Although not shown in the table, the chemical composition of the base steel sheet (excluding the oxide layer) was 3.30 mass% Si, 0.0018 mass% C, 0.06 mass% Mn, 0.002 mass% sol. Al, and the balance being Fe and impurities.

[0066] The grain-oriented electrical steel sheets were also measured for coating adhesion, coating tension, corrosion resistance, elution resistance, space factor, and iron loss in the following manner. The results are shown in Table 5.

[0067] [Coating Adhesion] Coating adhesion was evaluated by the degree of peeling (area ratio) of the coating after a bending adhesion test in which a sample 30 mm wide and 300 mm long was taken from the steel plate and wrapped around a 10 mm diameter cylinder and unwound. The evaluation criteria were as follows, and coating adhesion was judged to be excellent in the cases of Ex, G, and F. Ex: Peeling area ratio 0-0.5% G: Peeling area ratio more than 0.5% and 5.0% or less F: Peeling area ratio more than 5.0% and 20% or less P: Peeling area ratio more than 20% and 50% or less B: Peeling area ratio more than 50%

[0068] [Coating Tension] The coating tension was calculated by back-calculating from the state of curvature when one side of the insulating coating was peeled off. When the obtained coating tension was 4.0 MPa or more, it was determined that the coating tension was sufficient.

[0069] [Corrosion Resistance] Corrosion resistance was evaluated by subjecting a sample to a salt spray test (JIS Z2371:2015) in which a 5% NaCl aqueous solution was allowed to fall naturally onto the sample for 7 hours in an atmosphere at 35°C. The rust area was then evaluated on a 10-point scale. The evaluation criteria were as follows: A score of 5 or higher was considered to indicate excellent corrosion resistance. 10: No rust occurred 9: Very little rust occurred (area ratio: 0.10% or less) 8: Area ratio where rust occurred = more than 0.10% but not more than 0.25% 7: Area ratio where rust occurred = more than 0.25% but not more than 0.50% 6: Area ratio where rust occurred = more than 0.50% but not more than 1.0% 5: Area ratio where rust occurred = more than 1.0% but not more than 2.5% 4: Area ratio where rust occurred = more than 2.5% but not more than 5.0% 3: Area ratio where rust occurred = more than 5.0% but not more than 10% 2: Area ratio where rust occurred = more than 10% but not more than 25% 1: Area ratio where rust occurred = more than 25% but not more than 50%

[0070] [Elution Resistance] Elution resistance was evaluated based on whether or not the elution of phosphoric acid from the sample could be suppressed. The amount of elution was measured by boiling the sample in boiling pure water for 10 minutes, measuring the amount of phosphoric acid eluted in the pure water, and dividing the amount of phosphoric acid by the area of ​​the insulating coating of the boiled grain-oriented electrical steel sheet. The amount of phosphoric acid eluted in the pure water was measured by cooling the pure water (solution) into which the phosphoric acid had eluted, and measuring the phosphoric acid concentration of the sample obtained by diluting the cooled solution with pure water using ICP-AES. When the elution amount was 40 mg / m ２ If the resistance is less than this, the resin is deemed to have excellent resistance to elution.

[0071] [Space Factor] The space factor was measured according to a method in accordance with JIS C 2550-5 (2020). Thirty test pieces, each 30 mm wide and 320 mm long, were used. After measuring the total mass of the sample, the distance between the upper and lower backing plates sandwiching the laminate was measured under a pressure of 1 MPa, and then calculated. If the space factor was 96.0% or higher, it was determined that a high space factor was ensured.

[0072] [Iron Loss] The obtained steel sheets (grain-oriented electrical steel sheets) were measured for B8 (magnetic flux density at a magnetizing force of 800 A / m) and W17 / 50 (iron loss per mass at a magnetic flux density amplitude of 1.7 T and 50 Hz). These characteristic values ​​were measured using a single sheet magnetic property measurement method (Single Sheet Tester: SST) in accordance with JIS C2556 (2015). If the iron loss was 0.74 W / kg or less, it was determined that the magnetic properties were excellent.

[0073]

[0074]

[0075]

[0076]

[0077]

[0078] As can be seen from Tables 1 to 5, the grain-oriented electrical steel sheets corresponding to the examples of the invention, obtained by the preferred manufacturing method, had a base steel sheet and an insulating coating containing a metal phosphate formed on the surface of the base steel sheet, and the base steel sheet had an oxide layer containing an oxide of Si in the region where the base steel sheet contacts the interface between the base steel sheet and the insulating coating, and the oxide layer had an average thickness of 0.5 to 2.5 μm from the interface, which resulted in excellent coating adhesion, coating tension, corrosion resistance, elution resistance, space factor, and iron loss. In contrast, the grain-oriented electrical steel sheets of the comparative examples did not form the required oxide layer because one or more of the conditions in the first coating formation process, second coating formation process, and heat treatment process were outside the ranges of the present invention, and therefore one or more of the coating adhesion, coating tension, corrosion resistance, elution resistance, space factor, and iron loss were below the target values.

[0079] 11 Base steel plate 12 Oxide layer 21 Insulation coating

Claims

1. A grain-oriented electrical steel sheet comprising: a base steel sheet; and an insulating coating containing a metal phosphate formed on the surface of the base steel sheet; wherein the base steel sheet has an oxide layer containing an oxide of Si in a region in contact with the interface between the base steel sheet and the insulating coating, and the average thickness of the oxide layer from the interface is 0.5 to 2.5 μm.

2. The grain-oriented electrical steel sheet according to claim 1, wherein the ratio of the length of the interface between the oxide layer and the insulating coating to the length of the interface between the base steel sheet and the insulating coating is 30% or more.

3. The grain-oriented electrical steel sheet according to claim 1 or 2, wherein the area ratio of voids in the insulating coating is 30% or less.

4. Steel plate, Al ２ O ３ a finish annealing step of applying an annealing separator containing 10 to 100 mass % of the above-mentioned compound to the steel sheet, drying the steel sheet, and then finish annealing the steel sheet; an annealing separator removing step of removing excess annealing separator from the steel sheet after the finish annealing step; a light pickling step of pickling the steel sheet after the annealing separator removing step with 0.1 to 5.0 mass % of inorganic acid for 10 to 60 seconds; a water rinsing step of rinsing the steel sheet after the light pickling step with water and drying the steel sheet; and a first insulating coating forming step of immersing the steel sheet after the water rinsing step in a treatment solution having a liquid temperature of 30 to 85°C and a metal phosphate concentration of 1.0 to 10.0 mass %, for 5 to 150 seconds, removing the treatment solution with water, and then drying the steel sheet. a second insulating coating forming step of applying to the steel sheet after the first insulating coating forming step a coating liquid containing a metal phosphate and colloidal silica, the content of the colloidal silica being 30 to 150 parts by mass per 100 parts by mass of the metal phosphate and having a solids concentration of 10 to 40% by mass, drying, and then holding the steel sheet at a sheet temperature of 700 to 950°C for 10 to 90 seconds; and a heat treatment step of heating the steel sheet after the second insulating coating forming step to a temperature range of 700 to 900°C in an atmosphere having a dew point of 0 to 30°C, a nitrogen content of 50 to 100% by volume, and a hydrogen content of 0 to 50% by volume, and holding the steel sheet at this temperature range for 10 to 180 minutes.

5. The method for producing grain-oriented electrical steel sheet according to claim 4, wherein the annealing separator further contains one or both of MgO: 5 to 90 mass % and chloride: 0.5 to 10.0 mass %.

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