Grain-oriented electrical steel sheet and method for forming insulating coating

Abstract

This grain-oriented electrical steel sheet has a base steel sheet and an insulating film formed on a surface of the base steel sheet. The base steel sheet has, on the insulating film side, an iron-based oxide layer containing an iron-based oxide. The insulating film has: an intermediate layer that is formed on the base steel sheet side and that contains a crystalline metal phosphate; and a tensile film layer formed on the surface side of the insulating film. The average thickness of the iron-based oxide layer is 0.10-1.50 μm, the average thickness of the intermediate layer is 0.3-10.0 μm, and the average thickness of the insulating film is 2.0-10.0 μm. The crystalline metal phosphate in the intermediate layer is one or two among zinc phosphate, manganese phosphate, iron phosphate, and zinc phosphate calcium. The tensile film layer contains a metal phosphate and silica. The contained amount of the silica in the tensile film layer is 20-60 mass%.

Description

[Technical Field]

[0001] The present invention relates to a grain-oriented electrical steel sheet and a method for forming an insulating coating. This application claims priority based on Japanese Patent Application No. 2021-064965, filed on April 6, 2021, the contents of which are incorporated herein by reference. [Background technology]

[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, (a) {110} <001> Many technologies have been developed to date, from the perspective of (a) increasing the concentration in the 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] In addition, 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 of reducing iron loss. Forsterite-based coatings (inorganic coatings) with excellent coating adhesion are produced by the reaction of oxides on the steel sheet surface with annealing separators during the finish annealing process of electrical steel sheets. This coating can apply tension to steel sheets.

[0005] Furthermore, for example, the method disclosed in Patent Document 1, in which an insulating coating is formed by baking a coating liquid mainly composed of colloidal silica and phosphate onto the surface of a steel sheet, 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 are nonmagnetic and have an uneven structure at the steel sheet / coating interface. This uneven structure is thought to hinder domain wall movement and adversely affect iron loss. Therefore, as a means of improving high magnetic field iron loss, research is being conducted on a variety of techniques, including methods for removing inorganic coatings by mechanical means such as polishing or chemical means such as pickling, and techniques for producing grain-oriented electrical steel sheets that do not have inorganic coatings by preventing the formation of inorganic coatings during high-temperature finish annealing, as well as 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 inorganic coatings, 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 by forming a tensioned insulating coating on the surface of an inorganic-coating-free grain-oriented electrical steel sheet obtained by such a known method, an even more excellent iron loss improvement effect can be obtained. Furthermore, in addition to improving iron loss, tensioned insulating coatings can also impart various properties such as corrosion resistance, heat resistance, and slip resistance.

[0008] However, inorganic coatings not only provide insulating properties but also function as intermediate layers that ensure adhesion when forming a tension coating (tension-applying insulating coating). In other words, inorganic coatings are formed so as to penetrate deeply into the steel sheet, resulting in 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 an inorganic coating, excellent coating adhesion is achieved. However, 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 an inorganic coating. Therefore, when forming a tension coating on a grain-oriented electrical steel sheet that does not have an inorganic coating, it is being considered to provide a layer that takes the role of the intermediate layer of the inorganic coating.

[0009] For example, Patent Document 3 discloses a technique in which grain-oriented electrical steel sheet having no inorganic coating is annealed in a weakly reducing atmosphere to selectively thermally oxidize the silicon inevitably contained in the silicon steel sheet, thereby forming an SiO2 layer on the steel sheet surface, and then a tension-applying insulating coating is formed. Also, Patent Document 4 discloses a technique in which grain-oriented electrical steel sheet having no inorganic coating is anodically treated in a silicate aqueous solution to form an SiO2 layer on the steel sheet surface, and then a tension-applying insulating coating is formed.

[0010] Furthermore, Patent Document 5 discloses a technique for ensuring the adhesion of a tension-applying insulating coating by applying a coating that serves as an intermediate layer beforehand when forming the tension-applying coating.

[0011] Patent Document 6 also discloses a grain-oriented electrical steel sheet comprising a base steel sheet and a tension-applying insulating coating, in which the tension-applying insulating coating is present on the surface of the grain-oriented electrical steel sheet, and an iron-based oxide layer with a thickness of 100 to 500 nm is present between the base steel sheet and the tension-applying insulating coating. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Publication No. 48-039338 [Patent Document 2] Japanese Patent Publication No. 49-96920 [Patent Document 3] Japanese Patent Publication No. 6-184762 [Patent Document 4] Japanese Patent Application Publication No. 11-209891 [Patent Document 5] Japanese Patent Application Publication No. 5-279747 [Patent Document 6] Japanese Patent Publication No. 2020-111814 Summary of the Invention [Problem to be solved by the invention]

[0013] However, the technology disclosed in Patent Document 3 requires the preparation of annealing equipment capable of controlling the atmosphere in order to perform annealing in a weakly reducing atmosphere, which results in a problem of processing costs. Also, in the technology disclosed in Patent Document 4, in order to perform anodic electrolysis in a silicate aqueous solution to obtain an SiO layer on the steel sheet surface that maintains sufficient adhesion with the tension-applying insulating coating, it is necessary to prepare new electrolysis equipment, which results in a problem of processing costs. Furthermore, the technique disclosed in Patent Document 5 has the problem that it is not possible to maintain a tension-applying insulating coating having a large tension with good adhesion. Furthermore, the technology disclosed in Patent Document 6 states that, in order to form an iron-based oxide layer, a surface-treated grain-oriented electrical steel sheet is heat-treated at a steel sheet temperature of 700 to 900°C for 5 to 60 seconds in an atmosphere with an oxygen concentration of 1 to 21% by volume and a dew point of -20 to 30°C. Therefore, when manufacturing steel sheets with inorganic coatings on the same line, it is necessary to change the atmosphere in the annealing furnace, which reduces workability.

[0014] As described above, assuming a method that does not impose equipment restrictions or degrade workability, it has been difficult to provide a grain-oriented electrical steel sheet that does not have a forsterite-based coating, has excellent coating adhesion, high coating tension, and excellent magnetic properties. Therefore, an object of the present invention is to provide a grain-oriented electrical steel sheet that does not have a forsterite-based coating and that has excellent coating adhesion, excellent coating tension, and excellent magnetic properties. Another object of the present invention is to provide a method for forming an insulating coating that is included in such grain-oriented electrical steel sheet. [Means for solving the problem]

[0015] The present inventors have investigated the above-mentioned problems and found that in a grain-oriented electrical steel sheet that does not have a forsterite-based coating, by forming an iron-based oxide layer on the surface layer of the base steel sheet and providing an intermediate layer containing a crystalline metal phosphate between the base steel sheet and the tension coating, it is possible to improve the coating adhesion, coating tension, and magnetic properties.

[0016] The present invention was made based on 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 comprises a base steel sheet and an insulating coating formed on a surface of the base steel sheet, wherein the base steel sheet has an iron-based oxide layer containing iron-based oxides on the insulating coating side, the insulating coating is formed on the base steel sheet side, an intermediate layer containing a crystalline metal phosphate, and a tensile coating layer formed on the surface side of the insulating coating, wherein the iron-based oxide layer has an average thickness of 0.10 to 1.50 μm, and the intermediate layer has an average thickness of 0.3 to 1.50 μm. 4.5 μm, the average thickness of the insulating coating is 2.0 to 10.0 μm, the crystalline metal phosphate of the intermediate layer is one or more of zinc phosphate, manganese phosphate, iron phosphate, and zinc calcium phosphate, the tensile coating layer contains metal phosphate and silica, the content of silica in the tensile coating layer is 20 to 60 mass%, and the proportion of hydrates in the entire insulating coating is less than 5.0 mass%. [2] A method for forming an insulating coating according to another aspect of the present invention is a method for forming the insulating coating provided on the grain-oriented electrical steel sheet according to the above item [1], the method comprising the steps of: a finish annealing step of applying an annealing separator containing 10 to 100% by mass of Al2O3 to a 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; and immersing the steel sheet after the annealing separator removing step in a treatment solution containing 5 to 50% by mass of a metal phosphate at a solution temperature of 40 to 85°C for 5 to 15 minutes. a drying step of removing the steel sheet after the immersion step from the treatment solution, removing excess treatment solution, and then drying the steel sheet; and a tensile coating layer forming step of applying a coating solution containing metal phosphate and colloidal silica such that the colloidal silica is 30 to 150 parts by mass per 100 parts by mass of metal phosphate to the steel sheet after the drying step, drying the steel sheet, and then holding the steel sheet at a sheet temperature of 800 to 950°C in an atmosphere with a dew point of 30°C or less for 10 to 100 seconds. [3] In the method for forming an insulating coating described in [2] above, the annealing separator may further contain one or both of MgO: 5 to 90 mass % and chloride: 0.5 to 10.0 mass %. [Effects of the Invention]

[0017] According to the above aspects of the present invention, it is possible to provide a grain-oriented electrical steel sheet that does not have a forsterite-based coating and that has excellent coating adhesion, excellent coating tension, and excellent magnetic properties. Also, according to the above aspects of the present invention, it is possible to provide a method for forming an insulating coating that is included in a grain-oriented electrical steel sheet that has excellent coating adhesion and excellent magnetic properties. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is an example of a cross-sectional view of a grain-oriented electrical steel sheet according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] A grain-oriented electrical steel sheet according to one embodiment of the present invention (grain-oriented electrical steel sheet according to the present embodiment) and a method for manufacturing the grain-oriented electrical steel sheet according to the present embodiment, including a method for forming an insulating coating provided on the grain-oriented electrical steel sheet according to the present embodiment, will be described. First, the grain-oriented electrical steel sheet according to this embodiment will be described.

[0020] As shown in FIG. 1, the grain-oriented electrical steel sheet 100 according to this embodiment has a base steel sheet 1 and an insulating coating 2 formed on the surface of the base steel sheet 1, and does not have a forsterite-based coating on the surface of the base steel sheet 1. The base steel sheet 1 has an iron-based oxide layer 11 on the insulating coating 2 side, and the insulating coating 2 has an intermediate layer 21 and a tensile coating layer 22 in this order from the base steel sheet side.

[0021] <Base material steel plate> (chemical composition) The grain-oriented electrical steel sheet 100 according to this embodiment is significantly characterized by the structure of the insulating coating 2 formed on the surface of the base steel sheet 1, and the base steel sheet 1 included in the grain-oriented electrical steel sheet 100 is not limited in terms of chemical composition, and may be within a known range. To obtain the properties generally required of a grain-oriented electrical steel sheet, it is preferable that the chemical components include the following: In this embodiment, % relating to the chemical components is % by mass unless otherwise specified.

[0022] C: 0.010% or less C (carbon) is an element effective for controlling the structure of steel sheets in the manufacturing process up to the completion of the decarburization annealing step. However, if the C content exceeds 0.010%, the magnetic properties of the finished grain-oriented electrical steel sheet will 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. The lower the C content, the better; however, even if the C content is reduced to less than 0.0001%, the effect of structural control will saturate and the manufacturing cost will simply increase. Therefore, the C content may be 0.0001% or more.

[0023] Si: 2.50 to 4.00% Silicon (Si) is an element that increases the electrical resistance of grain-oriented electrical steel sheets and improves their core 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 sheet becomes embrittled and the threading property deteriorates significantly. Furthermore, the workability of the grain-oriented electrical steel sheet deteriorates, and the steel sheet 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.

[0024] 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 steel. Mn also improves the hot workability of steel. If the Mn content is less than 0.01%, the above-mentioned effects cannot be fully obtained. 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.

[0025] 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 the N content exceeds 0.010%, an excessive amount of inhibitor remains in the grain-oriented electrical steel sheet, resulting in a deterioration in magnetic properties. 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% would only increase the manufacturing cost, so the N content may be 0.001% or more.

[0026] 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 sheet. However, if the sol-Al content of the base steel sheet exceeds 0.020%, excessive inhibitors remain in the base steel sheet, resulting in reduced magnetic properties. 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% simply increases manufacturing costs. Therefore, the sol-Al content may be 0.0001% or more.

[0027] 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. It is more preferable that the S content in the grain-oriented electrical steel sheet is as low as possible, for example, less than 0.001%. However, reducing the S content in the grain-oriented electrical steel sheet to less than 0.0001% will only increase the manufacturing cost. Therefore, the S content in the grain-oriented electrical steel sheet may be 0.0001% or more.

[0028] Remainder: 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 (basic elements), with the remainder being Fe and impurities. However, for the purpose of improving magnetic properties, etc., it may also contain one or more of Sn, Cu, Se, and Sb 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 amount of 1.0% or less (regardless of whether they are intentionally added or contained as impurities), this does not impair the effects of the grain-oriented electrical steel sheet according to this embodiment. Here, impurities refer to elements that are mixed in from raw materials such as ore or scrap, or the manufacturing environment, when the base steel sheet is industrially manufactured, and are permissible to be contained in amounts that do not adversely affect the function of the grain-oriented electrical steel sheet according to this embodiment.

[0029] Sn: 0 to 0.50% Sn (tin) is an element that contributes to improving magnetic properties by 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.

[0030] Cu: 0 to 0.50% Copper (Cu) 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.

[0031] 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 in order 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.

[0032] 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 in order 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.

[0033] As described above, the chemical composition of the base steel sheet of the grain-oriented electrical steel sheet in this embodiment may, for example, contain the above-mentioned basic elements with the balance consisting of Fe and impurities, or contain the basic elements and one or more other optional elements with the balance consisting of Fe and impurities.

[0034] 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. The silicon content is determined by the method (silicon determination method) specified in JIS G 1212 (1997). Specifically, when the above-mentioned chips are dissolved in acid, silicon oxide precipitates. This precipitate (silicon oxide) is filtered out with filter paper, and its mass is measured to determine the silicon content. The carbon and sulfur contents are determined by the well-known high-frequency combustion method (combustion-infrared absorption method). Specifically, the above solution is combusted in an oxygen stream by high-frequency heating, and the generated carbon dioxide and sulfur dioxide are detected to determine the carbon and sulfur contents. The N content is determined using the well-known inert gas fusion-thermal conductivity method. However, if an insulating coating is formed on the surface, it must be removed before measurement. The insulating coating can be removed by immersing the steel sheet in a highly concentrated alkaline solution (for example, a 30% sodium hydroxide solution heated to 85°C) for 20 minutes or more. Furthermore, if an iron-based oxide layer is formed on the surface of the base steel sheet, it must be removed before measurement. The iron-based oxide layer can be removed by immersing the steel sheet in acid (for example, a 20% hydrochloric acid solution heated to 75°C) for about 2 minutes. It is possible to visually determine whether the material has been peeled off or removed. In the case of small samples, it may be possible to remove the material by surface grinding.

[0035] (iron-based oxide layer) In the grain-oriented electrical steel sheet 100 according to this embodiment, an iron-based oxide layer 11 is present in the surface layer portion of the base steel sheet 1 (on the interface side with the insulating coating). The iron-based oxide layer 11 contains an iron-based oxide. Preferably, the iron-based oxide layer 11 is a layer containing 50 mass % or more of iron-based oxide. The proportion of iron-based oxide is preferably 60 mass % or more, and more preferably, the layer is made of iron-based oxide. The presence of the iron-based oxide layer 11 improves the adhesion of the insulating coating 2. Although the reason for this is not clear, it is thought that the iron-based oxide layer 11 alleviates stress concentration between the steel sheet and the intermediate layer formed thereon, thereby improving adhesion. However, if the average thickness is less than 0.10 μm, stress is not sufficiently alleviated, resulting in a decrease in coating tension. Therefore, the average thickness of the iron-based oxide layer 20 is set to 0.10 μm (100 nm) or more. On the other hand, if the average thickness of the iron-based oxide layer 20 exceeds 1.50 μm, the magnetic flux density of the steel sheet decreases, resulting in a decrease in magnetic properties. Therefore, the average thickness of the iron-based oxide layer 11 is set to 1.50 μm (1500 nm) or less. In this embodiment, the iron-based oxide is, for example, magnetite, wustite, hematite, fayalite, or clinoferrosilite.

[0036] The thickness of the iron-based oxide layer 11 can be measured by removing the insulating coating using the method described above and then measuring the depth profile of the oxidized iron element peak using ion sputtering (XPS). If the oxide layer thickness exceeds 0.5 μm, GDS (glow discharge optical emission spectroscopy) can also be used. In the case of GDS, sputtering is performed from the surface, and the region where both iron and oxygen elements appear is considered to be the oxide layer. The thickness of the oxide layer can be measured by actually measuring the depth of the sputtering marks that appear after measurement by cross-sectional observation. The above measurement is performed at three or more locations, and the average thickness is determined by averaging the obtained thicknesses. The proportion of iron-based oxides in the iron-based oxide layer is determined by measuring the iron-based oxide layer of a cross-section-polished base steel sheet using EDS (energy dispersive X-ray spectroscopy) and calculating the proportion of each element.

[0037] <Insulating coating> The grain-oriented electrical steel sheet 100 according to this embodiment has an insulating coating 2 formed on the surface of a base steel sheet 1. More specifically, the grain-oriented electrical steel sheet 100 according to this embodiment does not have a forsterite-based coating. Therefore, the insulating coating 2 is formed in direct contact with the base steel sheet 1. The insulating coating 2 is made up of an intermediate layer 21 and a tensile coating layer 22 in this order from the base steel sheet 1 side.

[0038] (middle class) The intermediate layer 21 is a layer (coating) containing a crystalline metal phosphate and having a thickness of 0.3 to 10.0 μm. As mentioned above, grain-oriented electrical steel sheets generally have a forsterite-based coating formed in the final annealing process and an insulating coating (tensile insulating coating) formed thereon. However, in recent years, it has become clear that this forsterite-based coating hinders the movement of domain walls and adversely affects iron loss. Therefore, grain-oriented electrical steel sheets without a forsterite-based coating are being studied to further improve magnetic properties. However, without a forsterite-based coating, it is difficult to ensure sufficient adhesion between the tensile coating and the surface of the base steel sheet.

[0039] In the grain-oriented electrical steel sheet 100 of this embodiment, an intermediate layer 21 containing a crystalline metal phosphate is formed between the base steel sheet 1 and the tensile coating, thereby improving the adhesion between the base steel sheet 1 and the tensile coating layer 22 via the intermediate layer 21. When the intermediate layer 21 contains crystalline metal phosphate, the tensile coating formed thereon (which becomes the tensile coating layer 22 after formation) also contains metal phosphate, resulting in high affinity and excellent adhesion between the intermediate layer and the tensile coating layer. Furthermore, when the intermediate layer is formed by immersion in a treatment solution containing metal phosphate, as described below, it can be formed on the surface of the base steel sheet 1 by utilizing a chemical reaction, and adhesion between the intermediate layer 21 and the base steel sheet 1 can also be ensured. If the intermediate layer 21 does not contain a crystalline metal phosphate, the above-mentioned effect cannot be obtained. The proportion of the crystalline metal phosphate in the intermediate layer is preferably 80 mass % or more, more preferably 90 mass % or more, and may be 100 mass %. In terms of adhesion, the metal phosphate is selected from one or more of zinc phosphate, manganese phosphate, iron phosphate, and zinc calcium phosphate. In terms of adhesion to the base steel sheet, the total amount (mol) of the metal phosphate (M) and Fe is preferably 2.0 times or more, more preferably 3.0 times or more, the amount (mol) of P. It is preferable that the metal phosphate is not a hydrate, since hydrates reduce corrosion resistance. In hydrates, the total amount (mol) of the above-mentioned metal (M) and Fe is generally 1.5 times or less the amount (mol) of P. In the grain-oriented electrical steel sheet according to this embodiment, hydrates inevitably produced during the formation of the intermediate layer may ultimately remain, but this amount is small (usually less than 5.0 mass% of the entire insulating coating 2). From the viewpoint of adhesion, the treatment solution used to form the intermediate layer does not contain colloidal silica. The remainder of the metal phosphate in the intermediate layer may contain oxides and elements such as Fe and Si diffused from the base steel sheet. However, as mentioned above, silica is not intentionally added, and therefore the Si content is, for example, 1.0 mass% or less. The intermediate layer 21 is formed at a different time from the tensile coating formed thereon, but the intermediate layer 21 and the tensile coating layer 22 together function as the insulating coating 2 .

[0040] The metal (M) amount (mol), Fe amount (mol), and P amount (mol) in the metal phosphate are determined by analyzing a cross section of the insulating coating in the thickness direction using EDS (energy dispersive X-ray spectroscopy). Measurements are taken at approximately three locations, and the average value is used as the respective amount (mol). The amount of hydrate can be roughly determined by measuring the amount of water by thermobalance.

[0041] The average thickness of the intermediate layer 21 is 0.3 to 10.0 μm. If the average thickness of the intermediate layer 21 is less than 0.3 μm, the effect of improving the adhesion between the base steel sheet and the insulating coating via the intermediate layer is insufficient. On the other hand, if the average thickness of the intermediate layer exceeds 10.0 μm, the magnetic properties will deteriorate significantly.

[0042] (Tension coating layer) In the grain-oriented electrical steel sheet 100 according to this embodiment, a tensile coating is formed on the surface of the intermediate layer 21 , so that a tensile coating layer 22 is provided on the surface side of the insulating coating 2 . The tensile coating layer 22 is not particularly limited as long as it is used as an insulating coating for grain-oriented electrical steel sheets, but from the viewpoint of adhesion to the intermediate layer 21 (adhesion to the base steel sheet 1 via the intermediate layer 21), it contains a metal phosphate and silica (derived from colloidal silica in the coating liquid) so that the silica content is 20 mass% or more. On the other hand, if the silica content in the tensile coating layer exceeds 60 mass%, it may cause powdering, so it is set to 60 mass% or less. The tensile coating layer 22 preferably contains a total of 70 mass % or more of metal phosphate and silica, and may contain ceramic particles such as alumina and silicon nitride as the remainder other than the metal phosphate and silica. Although the thickness of the tensile coating layer 22 is not limited, the average thickness of the insulating coating 2 (intermediate layer 21 + tensile coating layer 22) is set to 2.0 to 10.0 μm, assuming that the average thickness of the intermediate layer 21 is within the above range. If the average thickness of the insulating coating 2 is less than 2.0 μm, sufficient coating tension cannot be obtained. Furthermore, there is a large amount of elution of phosphoric acid. This can cause stickiness and reduced corrosion resistance, and may even lead to coating peeling. Furthermore, if the thickness of the insulating coating 2 exceeds 10.0 μm, the space factor can decrease, deteriorating magnetic properties, or cracks can occur, resulting in reduced adhesion and reduced corrosion resistance.

[0043] The thickness of the insulating coating 2 is determined by the following method. The average thickness can be measured by observing the cross section of the sample with a scanning electron microscope and measuring the thickness at five or more points. Of the insulating coating 2, the intermediate layer 21 and the tensile coating layer 22 can be distinguished by the content of silicon (Si) derived from silica (as mentioned above, the tensile coating layer contains silica). Furthermore, the average thickness of the insulating coating 2 can be obtained by adding the average thickness of the intermediate layer 21 and the average thickness of the tensile coating layer 22 together.

[0044] The mass proportion of the metal phosphate and the type of the metal phosphate in the intermediate layer 21 and the tensile coating layer 22 can be determined by the following method. Similar to the method for measuring the thickness of the intermediate layer 21 and the tensile coating layer 22, the mass proportion and type of metal phosphate can be identified by using a scanning electron microscope and an energy dispersive elemental analyzer. Furthermore, whether the metal phosphate in the intermediate layer 21 is a crystalline metal phosphate can be determined by X-ray crystal structure analysis. The silica content of the tensile coating layer 22 can also be measured using a scanning electron microscope and an energy dispersive elemental analyzer.

[0045] <Manufacturing method> The grain-oriented electrical steel sheet according to this embodiment can be suitably manufactured by a manufacturing method that satisfies the manufacturing conditions described below. However, it goes without saying that the grain-oriented electrical steel sheet according to this embodiment is not particularly limited to a manufacturing method. In other words, a grain-oriented electrical steel sheet having the above-described configuration is considered to be the grain-oriented electrical steel sheet according to this embodiment, regardless of its manufacturing conditions.

[0046] The grain-oriented electrical steel sheet according to this embodiment is (I) a hot rolling step in which a steel slab having a predetermined chemical composition is hot rolled to obtain a hot rolled sheet (hot rolled steel sheet); (II) a hot-rolled sheet annealing step of annealing the hot-rolled sheet; (III) a cold rolling step in which the hot-rolled sheet after the hot-rolled sheet annealing step is cold-rolled to obtain a steel sheet (cold-rolled sheet); (IV) a decarburization annealing step of performing decarburization annealing on the steel sheet; (V) a finish annealing step of applying an annealing separator containing 10 to 100 mass% of Al2O3 to the steel sheet after the decarburization annealing step, drying the steel sheet, and then finish annealing the steel sheet; (VI) an annealing separator removing step of removing excess annealing separator from the steel sheet after the finish annealing step; (VII) an immersion step of immersing the steel sheet after the annealing separator removal step in a treatment solution having a solution temperature of 40 to 85°C and containing 5 to 50 mass% of a metal phosphate for 5 to 150 seconds; (VIII) a drying step of removing the steel sheet after the immersion step from the treatment solution, removing excess treatment solution, and then drying the steel sheet; (IX) a tensile coating layer forming step of applying a coating liquid containing metal phosphate and colloidal silica in an amount of 30 to 150 parts by mass of colloidal silica per 100 parts by mass of metal phosphate to the steel sheet after the drying step, drying the steel sheet, and then maintaining the steel sheet at a sheet temperature of 800 to 950°C in an atmosphere with a dew point of 30°C or less for 10 to 50 seconds; The composition can be produced by a production method including the steps of: Furthermore, the method for producing a grain-oriented electrical steel sheet according to this embodiment further includes the steps of: (X) a nitriding treatment step of nitriding the steel sheet between the decarburization annealing step and the finish annealing step; (XI) a magnetic domain refining step for controlling the magnetic domains of the steel sheet after the tension coating layer forming step; may include either or both of the following: Furthermore, in the method for producing a grain-oriented electrical steel sheet according to this embodiment, the following step may be performed between the annealing separator removing step and the immersion step: (XII) a surface conditioning step for controlling the reactivity of the surface of the steel sheet; may also include. Of these, the manufacturing of the grain-oriented electrical steel sheet according to this embodiment is characterized by the (V) finish annealing process to the (IX) tension coating layer forming process, which are mainly related to the formation of the insulating coating, and other processes or conditions not described can be performed under known conditions. These steps will be described below.

[0047] <Hot rolling process> In the hot rolling process, a steel 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 steel billet is preferably within the range of 1100 to 1450°C, and more preferably 1300 to 1400°C. The chemical composition of the slab may be changed depending on the chemical composition of the grain-oriented electrical steel sheet that is ultimately desired to be obtained, but an example of such a chemical composition may include, in mass %, C: 0.01 to 0.20%, Si: 2.50 to 4.00%, sol. 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 appropriately set based on the desired properties. The thickness of the hot rolled sheet is preferably within the range of 2.0 mm to 3.0 mm, for example.

[0048] <Hot-rolled sheet annealing process> The hot-rolled sheet annealing process is a process of annealing the hot-rolled sheet manufactured through the hot rolling process. By performing such an annealing treatment, recrystallization occurs in the steel sheet structure, and good magnetic properties can be achieved, which is preferable. When hot-rolled sheet annealing is performed, the hot-rolled sheet produced through a hot rolling process may be 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 used. The annealing conditions are also not particularly limited. For example, the hot-rolled sheet may be annealed in a temperature range of 900 to 1200°C for 10 seconds to 5 minutes.

[0049] <Cold rolling process> In the cold rolling process, the hot-rolled sheet after the hot-rolled sheet annealing process is subjected to cold rolling to obtain a steel sheet (cold-rolled sheet). The cold rolling may be a single cold rolling (a series of cold rolling without annealing in between), or may be multiple cold rolling passes with intermediate annealing between them, with the cold rolling being interrupted and at least one or two or more intermediate annealing passes being performed before the final pass of the cold rolling process. When intermediate annealing is performed, it is preferable to hold the steel sheet at a temperature of 1000 to 1200° C. for 5 to 180 seconds. The annealing atmosphere is not particularly limited. In consideration of production costs, it is preferable to perform intermediate annealing three times or less. Furthermore, before the cold rolling step, the surface of the hot-rolled sheet may be subjected to pickling.

[0050] In the cold rolling step according to the present embodiment, the hot-rolled sheet after the hot-rolled sheet annealing step may be cold-rolled to obtain a steel sheet according to a known method. For example, the final rolling reduction may be in the range of 80 to 95%. If the final rolling reduction is 80% or more, the {110} <001> This is preferable because it is possible to obtain Goss nuclei with a high degree of orientation accumulation in the rolling direction. On the other hand, if the final rolling reduction exceeds 95%, it is not preferable because the secondary recrystallization is likely to become unstable in the subsequent finish annealing step. The final rolling reduction is the cumulative rolling reduction of cold rolling, and in the case where intermediate annealing is performed, it is the cumulative rolling reduction of cold rolling after final intermediate annealing.

[0051] <Decarburization annealing process> In the decarburization annealing step, the steel sheet obtained after the cold rolling step is subjected to decarburization annealing. In the decarburization annealing, the conditions for the decarburization annealing are not limited as long as the steel sheet undergoes primary recrystallization and C, which adversely affects magnetic properties, can be removed from the steel sheet. For example, the oxidation degree (PH2O / PH2) in the annealing atmosphere (furnace atmosphere) can be set to 0.3 to 0.6, and the annealing temperature can be set to 800 to 900°C for 10 to 600 seconds.

[0052] <Nitriding process> Nitriding treatment may be carried out between the decarburization annealing step and the finish annealing step described below. In the nitriding process, for example, the steel sheet after the decarburization annealing process is nitrided by maintaining it at approximately 700 to 850°C in a nitriding atmosphere (an atmosphere containing hydrogen, nitrogen, and ammonia or other nitriding gases). When AlN is used as an inhibitor, it is preferable that the N content of the steel sheet after the nitriding process be 40 ppm or more by the nitriding process. On the other hand, if the N content of the steel sheet after the nitriding process exceeds 1000 ppm, excess AlN will remain 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 that the N content of the steel sheet after the nitriding process be 1000 ppm or less.

[0053] <Finishing annealing process> In the final annealing step, an annealing separator containing 10 to 100 mass% of Al2O3 is applied to the steel sheet that has been subjected to the decarburization annealing step or further nitriding treatment (nitriding treatment step), dried, and then final annealing is performed. In conventional methods for producing grain-oriented electrical steel sheets, an annealing separator mainly containing MgO is applied and then finish annealing is performed to form a forsterite-based coating on the surface of the steel sheet (cold-rolled sheet). In contrast, in the method for producing grain-oriented electrical steel sheets according to the present embodiment, an annealing separator containing Al2O3 is used to prevent the formation of a forsterite-based coating. On the other hand, the proportion of Al2O3 may be 100% by mass, but from the viewpoint of preventing Al2O3 from seizing onto the steel sheet surface, in the method for producing grain-oriented electrical steel sheet according to this embodiment, it is preferable that the annealing separator contains MgO. MgO may be 0%, but to obtain the above effects, the proportion of MgO is preferably 5% by mass or more. If MgO is contained, the proportion of MgO is 90% by mass or less to ensure 10% by mass or more of Al2O3. Preferably, it is 50% by mass or less. Furthermore, in the method for producing a grain-oriented electrical steel sheet according to this embodiment, the annealing separator may further contain chloride. When the annealing separator contains chloride, the effect of making it more difficult for a forsterite-based coating to form can be obtained. The chloride content is not particularly limited and may be 0%, but in order to obtain the above effect, a content of 0.5 to 10 mass% is preferable. Effective chlorides include, for example, bismuth chloride, calcium chloride, cobalt chloride, iron chloride, and nickel chloride. The finish annealing conditions are not limited, but for example, conditions of holding at a temperature of 1150 to 1250° C. for 10 to 60 hours can be adopted.

[0054] <Annealing separator removal process> After the finish annealing step, excess annealing separator is removed from the steel sheet, for example, by washing with water.

[0055] <Surface conditioning process> A surface conditioning step for controlling the reactivity of the steel sheet surface may be carried out between the annealing separator removal step and the immersion step. The conditions for the surface conditioning step are not limited, but an example of the conditions is to immerse the steel sheet after the annealing separator removal step in a commercially available surface conditioning agent for 30 seconds to 1 minute.

[0056] <Soaking process> <Drying process> The steel sheet after the annealing separator removal step (or after a surface conditioning step, if necessary) is immersed for 5 to 150 seconds in a treatment solution containing 5 to 50 mass% of a specified metal phosphate at a liquid temperature of 40 to 85°C (immersion step). Thereafter, the steel sheet is removed from the treatment solution, and the excess treatment solution is removed, followed by drying (drying step). As a result, an intermediate layer containing a crystalline metal phosphate is formed on the surface of the steel sheet (base steel sheet). If the liquid temperature is less than 40°C or the immersion time is less than 5 seconds, an intermediate layer with sufficient thickness cannot be obtained. On the other hand, if the liquid temperature is more than 85°C or the immersion time is more than 150 seconds, the intermediate layer will be excessively thick. Furthermore, if the metal phosphate content of the treatment solution is less than 5% by mass, the formation of the intermediate layer will be slow, resulting in high industrial costs. Furthermore, to achieve a uniform thickness for the intermediate layer, the metal phosphate content is preferably 10% by mass or more. On the other hand, if the metal phosphate content exceeds 50% by mass, the crystal grains will become coarse, which may result in reduced adhesion. The metal phosphate contained in the treatment solution may be one or more of zinc phosphate, manganese phosphate, and zinc calcium phosphate. Furthermore, if the drying temperature is too high, voids may occur, resulting in poor adhesion, so the drying temperature is preferably 300°C or lower, more preferably 200°C or lower. The drying temperature is preferably 100°C or higher.

[0057] <Tension film layer formation process> In the tensile coating layer formation process, a coating liquid containing metal phosphate and colloidal silica is applied to the steel sheet (steel sheet with an intermediate layer formed on the base steel sheet) after the drying process, dried, and then held for 10 to 100 seconds in an atmosphere with a sheet temperature of 800 to 950°C and a dew point of 30°C or less, thereby forming a tensile coating. The layer made of this tensile coating (tensile coating layer 22) and the intermediate layer 21 form the insulating coating 2. In this tensile coating layer formation process, the surface layer of the base steel sheet is first dissolved very slightly by the coating liquid, allowing the coating liquid to be applied sufficiently, and then the coating liquid is dried. Subsequently, an iron-based oxide layer is formed on the base steel sheet by maintaining the temperature at a high temperature. The reason for forming the iron-based oxide layer after the application and drying is that if a tensile coating layer is formed on a steel sheet on which an iron-based oxide layer has already been formed, the iron-based oxide layer will be dissolved by the coating liquid containing phosphoric acid, and the desired iron-based oxide layer will not remain, or even if a partial iron-based oxide layer remains, the adhesion of the coating will be reduced. If the sheet temperature during holding is less than 800°C, the tension will be low and the magnetic properties will be inferior. Therefore, the sheet temperature is preferably 800°C or higher. On the other hand, if the sheet temperature is higher than 950°C, the magnetic properties may deteriorate and the corrosion resistance may decrease. Therefore, the sheet temperature is preferably 950°C or lower. Furthermore, if the holding time is less than 10 seconds, the elution property will be poor. Therefore, the holding time is set to 10 seconds or more. On the other hand, if the holding time is more than 100 seconds, the thickness of the iron-based oxide layer will be excessive. Therefore, the holding time is set to 100 seconds or less. Furthermore, if the dew point exceeds 30°C, the thickness of the iron-based oxide layer will be excessive. Therefore, the dew point of the atmosphere is set to 30°C or less. On the other hand, if the dew point is less than 0°C, it will take a long time to form the oxide layer, resulting in increased costs. Therefore, it is preferable to set the dew point to 0°C or higher. The coating liquid contains a metal phosphate and colloidal silica in an amount of 30 to 150 parts by mass of colloidal silica per 100 parts by mass of the metal phosphate. The metal phosphate may be one or a mixture of two or more selected from aluminum phosphate, zinc phosphate, magnesium phosphate, nickel phosphate, copper phosphate, lithium phosphate, barium phosphate, cobalt phosphate, strontium phosphate, etc. The coating liquid may contain additional elements such as vanadium, tungsten, molybdenum, zirconium, etc. When these elements are contained, they can be added to the coating liquid as, for example, an oxygen acid. Colloidal silica can be of either type S or type C. Type S colloidal silica refers to an alkaline silica solution, while type C refers to a silica particle surface that has been aluminum-treated, resulting in an alkaline to neutral silica solution. Type S colloidal silica is widely used and relatively inexpensive, but care must be taken as it may aggregate and precipitate when mixed with an acidic metal phosphate solution. Type C colloidal silica is stable even when mixed with a metal phosphate solution and does not precipitate, but it is relatively expensive due to the large processing steps required. It is best to use the appropriate type depending on the stability of the coating solution being prepared.

[0058] <Magnetic domain refining process> The method for manufacturing a grain-oriented electrical steel sheet according to this embodiment may further include a magnetic domain refining step of refining magnetic domains on the steel sheet after the tensile coating layer forming step. By performing magnetic domain refining treatment, it is possible to further reduce the iron loss of grain-oriented electrical steel sheets. Methods of magnetic domain subdivision include a method of narrowing the width of 180° magnetic domains (subdividing 180° magnetic domains) by forming linear or point-like grooves extending in a direction intersecting the rolling direction at predetermined intervals along the rolling direction, and a method of narrowing the width of 180° magnetic domains (subdividing 180° magnetic domains) by forming linear or point-like stress distortion portions or grooves extending in a direction intersecting the rolling direction at predetermined intervals along the rolling direction. When forming stress-strained portions, laser beam irradiation, electron beam irradiation, etc. can be applied. When forming grooves, mechanical groove formation methods using gears, etc., chemical groove formation methods in which grooves are formed by electrolytic etching, and thermal groove formation methods using laser irradiation can be applied. If the insulating coating is damaged by the formation of stress-strained portions or grooves, and the insulating properties and other characteristics are deteriorated, the insulating coating may be formed again to repair the damage. [Example]

[0059] A slab containing, in mass %, C: 0.08%, Si: 3.23%, sol. Al: 0.028%, N: 0.008%, Mn: 0.15%, S: 0.007%, and the remainder being Fe and impurities was cast. This slab was heated to 1350°C and then hot rolled to form a hot-rolled sheet having a thickness of 2.2 mm. This hot-rolled sheet was annealed at 1100°C for 10 seconds (hot-rolled sheet annealing), and then cold-rolled to a sheet thickness of 0.22 mm to obtain a steel sheet (cold-rolled sheet). This steel sheet was subjected to decarburization annealing at 830°C for 90 seconds in an atmosphere with a (PH2O / PH2) of 0.4. Thereafter, except for No. 115, an annealing separator containing 48 mass% Al2O3, 48 mass% MgO, and 4 mass% bismuth chloride was applied to the steel sheets, dried, and then subjected to finish annealing at 1200°C for 20 hours. For No. 115, an annealing separator consisting of only Al2O3 (100 mass%) was applied to the steel sheet, dried, and then subjected to finish annealing at 1200°C for 20 hours.

[0060] After the finish annealing step, the steel sheet was washed with water to remove excess annealing separator, and it was found that no forsterite-based coating had been formed on the surface of the steel sheet. This steel sheet was immersed in a treatment solution shown in Table 1, and then heated to 100 to 150°C and dried to form an intermediate layer (any of Nos. 1 to 10). The average thickness of the intermediate layer was as shown in Table 1. X-ray crystal structure analysis revealed that the metal phosphates in intermediate layers No. 1 to No. 9 were all crystalline metal phosphates. In these crystalline metal phosphates, the ratio of the total amount (mol) of metal (M) and Fe to the amount (mol) of P was approximately 2:1 or 3:1. The metal phosphate (magnesium phosphate) in intermediate layer No. 10 was not a crystalline metal phosphate.

[0061] [Table 1]

[0062] The steel sheet with the intermediate layer (any of Nos. 1 to 10) formed thereon was cut into multiple pieces as necessary. Each steel sheet was coated with an aqueous solution (coating solution) containing a metal phosphate salt and colloidal silica shown in Table 2 and baked in a drying furnace for the time shown in Table 2, in an atmosphere shown in Table 2, to achieve the sheet temperature. This formed an iron-based oxide layer on the steel sheet surface and a tensile coating on the steel sheet surface. When vanadium, tungsten, molybdenum, or zirconium was added to the coating solution, they were added as oxyacids (VO, WO, MoO, ZrO) in the molar ratios shown in Table 2. The thickness of the tensile coating layer was varied by changing the amount of coating solution applied. Some coating solutions contained alumina or silicon nitride as the remainder. In the atmospheres shown in Table 2, 4% H2 indicates a mixed atmosphere of 96% by volume of nitrogen and 4% by volume of hydrogen, 75% by volume of H2 indicates a mixed atmosphere of 25% by volume of nitrogen and 75% by volume of hydrogen, and "Dry" means an annealing atmosphere with a dew point of less than -20°C. In this way, a steel sheet (grain-oriented electrical steel sheet) was produced.

[0063] For the obtained steel sheets (Nos. 101 to 127), the silica and metal phosphate contents in the tensile coating layer, the average thickness of the iron-based oxide layer, and the average thickness of the insulating coating were determined using the methods described above. The results are shown in Table 2. In addition, the chemical composition of the base steel plate was investigated and found to contain Si: 3.21%, C: 0.001%, sol. Al: less than 0.001%, N: 0.001%, Mn: 0.07%, S: less than 0.0005%, with the remainder being Fe and impurities.

[0064] [Table 2]

[0065] The adhesion of the insulating coating, coating tension, corrosion resistance, elution, and magnetic properties of these steel sheets were also measured using the methods described below. The results are shown in Table 3.

[0066] [Adhesion] The adhesion of the coating was evaluated by taking a sample 30 mm wide and 300 mm long from the steel plate, annealing this sample for stress relief at 800°C for 2 hours in a nitrogen stream, then winding it around a 10 mm diameter cylinder and unwinding it, and then performing a bending adhesion test.The degree of peeling of the coating (area ratio) was measured. The evaluation criteria were as follows, and a rating of A or B was determined to indicate excellent coating adhesion. A: Peeling area ratio 0-0.5% B: Peeling area rate more than 0.5%, less than 5.0% C: Peeling area ratio more than 5.0%, less than 20% D: Peeling area ratio more than 20%, less than 50% E: Peeling area rate over 50%

[0067] [Coating tension] The coating tension was calculated by taking a sample from the steel plate and working backward from the state of curvature when the insulating coating on one side of the sample was peeled off. When the obtained film tension was 4.0 MPa or more, it was judged that the film tension was excellent.

[0068] [Corrosion resistance] In accordance with the salt spray test of JIS Z2371:2015, a 5% NaCl aqueous solution was allowed to fall naturally onto the sample for 7 hours in a 35°C atmosphere. Thereafter, the rust area was evaluated on a 10-point scale. The evaluation criteria were as follows, and a rating of 5 or more (5 to 10) was judged to be excellent in corrosion resistance. 10: No rust occurred 9: Very little rust occurs (area rate 0.1% or less) 8: Area ratio of rusted surface = over 0.1% and 0.25% or less 7: Area ratio of rusted surface = over 0.25% and up to 0.50% 6: Area ratio of rusted surface = over 0.50% and 1% or less 5: Area ratio of rusted surface = over 1% and up to 2.5% 4: Area ratio of rusted surface = over 2.5% and up to 5% 3: Area ratio of rusted surface = over 5% and up to 10% 2: Area ratio of rusted surface = over 10% and up to 25% 1: Area ratio of rusted surface = over 25% and up to 50%

[0069] [Dissolution] Samples were taken from the steel sheets and boiled in boiling pure water for 10 minutes, and the amount of phosphoric acid dissolved in the pure water was measured. The amount of phosphoric acid dissolved was divided by the area of ​​the insulating coating of the boiled grain-oriented electrical steel sheet to determine the elution rate (mg / m 2 ) was evaluated. The amount of phosphoric acid dissolved in the pure water was measured by cooling the pure water (solution) into which the phosphoric acid had dissolved, and then diluting the cooled solution with pure water to obtain a sample, and measuring the phosphoric acid concentration of the resulting sample using ICP-AES. The amount of elution per unit area is 140 mg / m 2 If the dissolution rate was less than this, it was determined that the dissolution rate was excellent.

[0070] [Magnetic properties] The iron loss was evaluated as a magnetic property. Specifically, the obtained steel sheet was irradiated with UA (irradiation energy density) of 2.0 mJ / mm 2 The magnetic domain refining process was carried out by irradiating the sample with a laser beam under the conditions of (a) and (b), and the iron loss after the magnetic domain refining process (iron loss W17 / 50 at 50 Hz at 1.7 T) was measured. If the iron loss was 0.70 W / kg or less, it was determined that the magnetic properties were excellent.

[0071] [Table 3]

[0072] As shown in Tables 1 to 3, Nos. 101 to 115, which are examples of the present invention, had excellent coating adhesion, excellent coating tension, and excellent magnetic properties. They also had sufficient corrosion resistance and elution properties. In contrast, Nos. 116 to 127 were poor in at least one of coating adhesion, coating tension, and magnetic properties. They also sometimes had poor corrosion resistance and elution properties. [Explanation of symbols]

[0073] 1 Base steel plate 2. Insulation coating 11 Iron-based oxide layer 21 Middle Class 22 Tension coating layer 100 grain-oriented electrical steel sheet

Claims

1. A base steel plate; an insulating coating formed on the surface of the base steel sheet; and The base steel plate is an iron-based oxide layer containing iron-based oxides on the insulating coating side; The insulating coating is an intermediate layer formed on the base steel sheet side and containing a crystalline metal phosphate; a tensile coating layer formed on the surface side of the insulating coating, the average thickness of the iron-based oxide layer is 0.10 to 1.50 μm; The average thickness of the intermediate layer is 0.3 to 4.5 μm, The average thickness of the insulating coating is 2.0 to 10.0 μm, the crystalline metal phosphate of the intermediate layer is one or more of zinc phosphate, manganese phosphate, iron phosphate, and zinc calcium phosphate; the tensile coating layer contains a metal phosphate and silica, and the content of the silica in the tensile coating layer is 20 to 60 mass %; the proportion of hydrates in the entire insulating coating is less than 5.0 mass%; A directional electrical steel sheet characterized by:

2. A method for forming the insulating coating provided on the grain-oriented electrical steel sheet according to claim 1, comprising: Steel plate, Al ２ O ３ a finish annealing process in which an annealing separator containing 10 to 100 mass% of the above is applied, dried, and then finish annealed; an annealing separator removing step of removing excess annealing separator from the steel sheet after the finish annealing step; an immersion step of immersing the steel sheet after the annealing separator removal step in a treatment solution having a solution temperature of 40 to 85°C and containing 5 to 50 mass% of a metal phosphate for 5 to 150 seconds; a drying step of removing the steel sheet after the immersion step from the treatment solution, removing excess treatment solution, and then drying the steel sheet; a tensile coating layer forming step of applying a coating liquid containing metal phosphate and colloidal silica in an amount of 30 to 150 parts by mass of colloidal silica per 100 parts by mass of metal phosphate to the steel sheet after the drying step, drying the steel sheet, and then maintaining the steel sheet at a sheet temperature of 800 to 950°C in an atmosphere with a dew point of 30°C or less for 10 to 100 seconds; A method for forming an insulating coating, comprising:

3. The annealing separator further contains one or two of MgO: 5 to 90 mass% and chloride: 0.5 to 10.0 mass%.

3. The method for forming an insulating coating according to claim 2.

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