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

Abstract

This grain-oriented electromagnetic steel sheet comprises a base steel sheet and an insulating coating film that is formed on the surface of the base steel sheet. The insulating coating film comprises: an intermediate layer that is formed on the base steel sheet side and that contains a crystalline metal phosphate salt; and a tension coating layer that is formed on the surface side of the insulating film. The intermediate layer has an average thickness of 0.10 μm to 5.0 μm, and the area coverage ratio by the intermediate layer with respect to the base steel sheet is 40% to 90%. The crystalline metal phosphate salt in the intermediate layer is one or more selected from zinc phosphate, manganese phosphate, iron phosphate and zinc calcium phosphate. The tension coating layer contains a metal phosphate salt and silica; and the content of the silica in the tension coating layer is 20% to 60% by 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. 2022-063398, filed on April 6, 2022, 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] 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 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 insulation but also function as an intermediate layer that ensures adhesion when forming a tension coating (tension-applying insulating coating). In other words, inorganic coatings are formed in a state where they penetrate deeply into the steel sheet, and therefore 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 an inorganic coating, the coating exhibits excellent adhesion. However, because bonding between metal and oxide is generally difficult, it has been difficult to ensure sufficient adhesion between a tension coating and the surface of an electrical steel sheet (base steel sheet) 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 (inorganic coating) and that 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, has excellent coating adhesion, excellent coating tension, and excellent magnetic properties, and further has sufficient corrosion resistance and elution resistance. 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 conducted research into the above-mentioned problems, and as a result, have found that in a grain-oriented electrical steel sheet that does not have a forsterite-based coating on its surface, by providing an intermediate layer containing a crystalline metal phosphate between the base steel sheet and the tension coating, and by setting the coverage area ratio of the intermediate layer to the base steel sheet within a predetermined range, it is possible to improve the coating adhesion, coating tension, and magnetic properties while maintaining sufficient corrosion resistance and elution resistance.

[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, the insulating coating being formed on the base steel sheet side and including an intermediate layer containing a crystalline metal phosphate, and a tensile coating layer formed on the surface side of the insulating coating; Made up of The intermediate layer has an average thickness of 0.10 to 5.0 μm, the coverage area ratio of the intermediate layer to the base steel sheet is 40 to 90%, 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, and the content of silica in the tensile coating layer is 20 to 60 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 [1], comprising: a finish annealing step of applying an annealing separator containing 10 to 100 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; a light pickling step of pickling the steel sheet after the annealing separator removing step with a mixed acid having a solution temperature of 25 to 85°C and a concentration of 0.5 to 10 mass % which is a mixture of one or more of sulfuric acid, phosphoric acid, and nitric acid for 5 to 30 seconds; and a light pickling step of pickling the steel sheet after the light pickling step with a fine particle emulsion of titanium-based or zinc-based colloid, or manganese phosphate. a dipping step in which the steel sheet after the surface conditioning step is immersed for 5 to 50 seconds in a treatment solution having a temperature of 20 to 85°C and containing 5 to 50 mass% of metal phosphate; a drying step in which the steel sheet after the dipping step is pulled out of the treatment solution, excess treatment solution is removed by rinsing with water, and then dried; and a tensile coating layer forming step in which a coating solution containing metal phosphate and colloidal silica in an amount of 30 to 150 mass parts of colloidal silica per 100 mass parts of the metal phosphate is applied to the steel sheet after the drying step, dried, and then maintained at a sheet temperature of 700 to 950°C for 10 to 50 seconds. [3] In the method for forming an insulating coating according to [2], the annealing separator is The Al 2 O 3 Contains 10 to 99.5 mass% of Furthermore, it may contain one or both of 5 to 90 mass % of MgO and 0.5 to 10.0 mass % of chloride. [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. Furthermore, this grain-oriented electrical steel sheet also has sufficient corrosion resistance and elution resistance. Furthermore, according to the present invention, it is possible to provide a method for forming an insulating coating on the grain-oriented electrical steel sheet. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 2 is a diagram showing an example of a cross-sectional view of the grain-oriented electrical steel sheet according to the present embodiment. 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 insulating coating 2 also has a tensile coating layer 22 formed on the surface side of the insulating coating 2 (i.e., the surface side of the grain-oriented electrical steel sheet 100), and an intermediate layer 21 containing a crystalline metal phosphate formed on the side of the base steel sheet 1. In Fig. 1, the insulating coating 2 is formed on both surfaces of the base steel sheet 1, but it may be formed on only one surface.

[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 the range of known grain-oriented electrical steel sheets. For example, in order to obtain the properties generally required of grain-oriented electrical steel sheets, 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 Carbon (C) is an effective element 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 of the base steel sheet of the finally obtained grain-oriented electrical steel sheet exceeds 0.010%, the magnetic properties of the 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 more preferable it is, but even if the C content is reduced to less than 0.0001%, the effect of grain 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 combines 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. In this case, 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% 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 combines 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%, an excess amount of inhibitor remains in the base steel sheet. In this case, 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% simply increases the manufacturing cost. 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, with the remainder 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 are contained, for example, one or more of W, Nb, Ti, Ni, Co, V, Cr, and Mo in a total amount of 1.0% or less, 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 is, for example, one that contains the above-mentioned elements with the balance being 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. However, if an insulating coating is formed on the surface, it must be 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.

[0035] <Insulating coating> In the grain-oriented electrical steel sheet 100 according to this embodiment, an insulating coating 2 is 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, and therefore the insulating coating 2 is formed in 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.

[0036] (middle class) The intermediate layer 21 is a layer (coating) containing a crystalline metal phosphate and having an average thickness of 0.10 to 5.0 μm. As described 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 base steel sheet.

[0037] 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, and may be 100 mass %. In terms of adhesion, the metal phosphate is preferably one or more of zinc phosphate, manganese phosphate, iron phosphate, and zinc calcium phosphate. The intermediate layer may contain oxides and elements such as Fe and Si diffused from the base steel sheet as the remainder of the metal phosphate. If the average thickness of the intermediate layer 21 is less than 0.10 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 5.0 μm, the magnetic properties will deteriorate significantly.

[0038] However, if the coverage area ratio of the intermediate layer to the base steel sheet is less than 40%, sufficient effect cannot be obtained, so the coverage area ratio is set to 40% or more. On the other hand, if the intermediate layer is formed uniformly, a coverage area ratio of 100% is acceptable. However, in the case of a typical dipping formation method, if the coverage area ratio exceeds 90%, the metal phosphate constituting the intermediate layer will partially stack, making the intermediate layer thicker in some places, which may result in a deterioration in the magnetic properties of the resulting grain-oriented electrical steel sheet. Therefore, the coverage area ratio is set to 90% or less, and preferably 85% or less.

[0039] (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 of the tensile coating layer 22 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. 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 preferably 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, resulting in deterioration of magnetic properties, or cracks can occur, resulting in reduced adhesion and reduced corrosion resistance.

[0040] The thickness of the intermediate layer 21, the thickness of the tensile coating layer 22, and the thickness of the insulating coating 2 are determined by the following method. The cross section of the sample is observed with a scanning electron microscope, and the average thickness is determined by measuring the thickness at five or more points. During measurement, the base steel sheet 1 and the insulating coating 2 can be distinguished by the reflection of the electron beam from the scanning electron microscope. Furthermore, within the insulating coating 2, the intermediate layer 21 and the tensile coating layer 22 can be distinguished by the presence or absence of silicon derived from silica (as mentioned above, the tensile coating layer contains silica, but the intermediate layer does not substantially contain silica). 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.

[0041] 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. Whether the metal phosphate in intermediate layer 21 is a crystalline metal phosphate can be determined by X-ray crystal structure analysis. For example, crystalline metal phosphate can be identified by measuring using an X-ray diffraction measurement device "SmartLab" manufactured by RIGAKU Corporation under conditions of a Cu tube, a voltage of 40 kV, a current of 40 mA, and a 2θ angle of 5 to 90°. The silica content of the tensile coating layer 22 can also be measured using a scanning electron microscope and an energy dispersive elemental analyzer.

[0042] The coverage area ratio of the intermediate layer is measured by the following method. If the intermediate layer can be exposed by mechanical polishing or the like before or after the formation of the surface tensile coating layer, the surface of the steel sheet (surface of the intermediate layer) is observed using an electron microscope over an area of ​​100 x 100 μm or more at a magnification of, for example, 1000x, and the Si concentration is measured by EDS to determine the area proportion of the metal phosphate layer that does not contain silica (Si concentration of 3 mass% or less), thereby calculating the coverage area ratio. On the other hand, when a tension coating layer is formed and it is difficult to expose the intermediate layer, the cross section of the surface layer portion of the grain-oriented electrical steel sheet in the sheet thickness direction is observed with an electron microscope at a magnification of, for example, 5000x, over a length of 20 μm or more on the surface of the base steel sheet, and the silica-free metal phosphate layer in contact with the metal surface (surface of the base steel sheet) is taken as the intermediate layer, and the length of the intermediate layer per measurement length is measured to calculate the coverage area ratio. In either case, calculations are made for five fields of view, and the average value is taken as the coverage area ratio.

[0043] <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 limited to this 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. The grain-oriented electrical steel sheet according to this embodiment is (I) a hot rolling step in which a steel billet having a predetermined chemical composition is hot rolled to obtain a hot-rolled sheet; (II) a hot-rolled sheet annealing step of annealing the hot-rolled sheet; (III) a cold rolling step of cold rolling the hot-rolled sheet after the hot-rolled sheet annealing 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) a light pickling step in which the steel sheet after the annealing separator removing step is pickled for 5 to 30 seconds with a mixed acid having a liquid temperature of 25 to 85°C and a concentration of 0.5 to 10 mass % which is a mixture of one or more of sulfuric acid, phosphoric acid and nitric acid; (VIII) a surface conditioning step of immersing the steel sheet after the light pickling step in a surface conditioner containing a titanium-based or zinc-based colloid or a manganese phosphate fine particle emulsion and having a liquid temperature of 25 to 50°C for 10 to 60 seconds; (IX) an immersion step of immersing the steel sheet after the surface conditioning step in a treatment solution having a liquid temperature of 20 to 85°C and containing 5 to 50 mass% of a metal phosphate for 5 to 50 seconds; (X) a drying step of removing the steel sheet after the immersion step from the treatment solution, removing excess treatment solution by rinsing with water, and then drying the steel sheet; (XI) a tensile coating layer forming step of applying a coating liquid containing 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 to the steel sheet after the drying step, drying the steel sheet, and then maintaining the steel sheet at a temperature of 700 to 950°C 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: (XII) a nitriding treatment step of nitriding the cold-rolled steel sheet between the decarburization annealing step and the finish annealing step; (XIII) 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: 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 (XI) tension coating layer forming process, which are mainly related to the formation of the insulating coating, and known conditions can be used for other processes or conditions not described. These steps will be described below.

[0044] <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: 0.010 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.

[0045] <Hot-rolled sheet annealing process> In the hot-rolled sheet annealing process, the hot-rolled sheet manufactured through the hot rolling process is annealed. By carrying out such annealing treatment, recrystallization occurs in the steel sheet structure, and good magnetic properties can be realized, 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.

[0046] <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 intermediate annealing), or may be multiple cold rollings with intermediate annealing between them, with the cold rolling interrupted and at least one or two or more intermediate annealings 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.

[0047] In the cold rolling step according to the present embodiment, the hot rolled sheet after the hot rolled sheet annealing step is cold rolled according to a known method to obtain a cold rolled sheet. For example, the final rolling reduction can 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.

[0048] <Decarburization annealing process> In the decarburization annealing step, the obtained cold-rolled sheet is subjected to decarburization annealing. In the decarburization annealing, the cold-rolled sheet is subjected to primary recrystallization, and the decarburization annealing conditions are not limited as long as 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, and the steel sheet can be held for 10 to 600 seconds.

[0049] <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 to set the nitrogen concentration of the steel sheet to 40 ppm or more by the nitriding process. On the other hand, if the nitrogen concentration of the steel sheet exceeds 1000 ppm, 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.

[0050] <Finishing annealing process> In the final annealing process, an annealing separator containing 10 to 100 mass% of Al2O3 is applied to a steel sheet (cold-rolled sheet) that has been subjected to a decarburization annealing process or further to a nitriding treatment, dried, and then final annealed. 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. The proportion of MgO is preferably 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.

[0051] <Annealing separator removal process> In the annealing separator removing step, excess annealing separator is removed from the steel sheet after the finish annealing step by rinsing with water.

[0052] <Light pickling process> In the light pickling process, the steel sheet after the annealing separator removal process is pickled with a mixed acid made by mixing one or more of sulfuric acid, phosphoric acid, and nitric acid. During pickling, the mixed acid concentration is 0.5 to 10 mass%, the solution temperature is 25 to 85°C, and the pickling time is 5 to 30 seconds. This removes unnecessary substances from the steel sheet surface and activates the surface.

[0053] <Surface conditioning process> In the surface conditioning process, the steel sheet after the light pickling process is immersed for 10 to 60 seconds in a surface conditioning agent containing a titanium-based or zinc-based colloid or a manganese phosphate microparticle emulsion at 25 to 50°C. This reduces the particle size of the crystalline metal phosphate, forming a dense intermediate layer and increasing the coverage area of ​​the intermediate layer. If the surface conditioner does not contain a titanium- or zinc-based colloid or a manganese phosphate microparticle emulsion, the particle size of the crystalline metal phosphate will become coarse, which may reduce the coverage area of ​​the intermediate layer and reduce the adhesion of the coating. Furthermore, if the temperature of the surface conditioner is below 25°C, the adhesion may deteriorate. Furthermore, if the temperature of the surface conditioner is above 50°C, the adhesion of the intermediate layer may become uneven. If the immersion time is less than 10 seconds, the adhesion may deteriorate, whereas if the immersion time is more than 60 seconds, the crystalline metal phosphate may grow in the thickness direction of the intermediate layer, making the intermediate layer too thick, increasing the coverage area ratio too much, or causing uneven adhesion of the intermediate layer.

[0054] <Soaking process> <Drying process> In the immersion step, the steel sheet after the surface conditioning step is immersed for 5 to 25 seconds in a treatment solution containing 5 to 50 mass% of a specified metal phosphate at a liquid temperature of 40 to 85°C. Then, in the drying step, the steel sheet is pulled out of the treatment solution, excess treatment solution is removed by rinsing with water, and the steel sheet is then dried. This forms an intermediate layer containing a crystalline metal phosphate on the surface of the steel sheet (base steel sheet). If the liquid temperature is below 20°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 above 85°C or the immersion time is more than 50 seconds, the coverage area ratio becomes excessive. 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. On the other hand, if the metal phosphate content exceeds 50% by mass, the crystal grains will become coarse, which may cause a decrease in adhesion. The metal phosphate contained in the treatment solution may be one or more of zinc phosphate, manganese phosphate, and zinc calcium phosphate.

[0055] <Tension coating 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 at a sheet temperature of 700 to 950°C for 10 to 50 seconds to form a tensile coating. The layer made of this tensile coating (tensile coating layer) and the intermediate layer form the insulating coating. If the sheet temperature is below 700°C, the tension will be low and the magnetic properties will be poor. Therefore, it is preferable that the sheet temperature be 700°C or higher. On the other hand, if the sheet temperature is above 950°C, the rigidity of the steel sheet will decrease and it will be prone to deformation. In this case, strain may be introduced into the steel sheet due to transportation, etc., resulting in poor magnetic properties. Therefore, it is preferable that the sheet temperature be 950°C or lower. Furthermore, if the holding time is less than 10 seconds, the resistance to elution 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 50 seconds, the adhesion of the tensile coating layer will be poor. Therefore, the holding time is preferably 50 seconds or less. 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, manganese phosphate, magnesium phosphate, nickel phosphate, copper phosphate, lithium phosphate, cobalt phosphate, molybdenum phosphate, etc. The coating solution may contain additional elements such as vanadium, tungsten, molybdenum, and zirconium. 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 caution is required 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 is relatively expensive due to the large number of processing steps required. It is preferable to use the appropriate type depending on the stability of the coating solution being prepared.

[0056] <Magnetic domain refining process> The method for manufacturing a grain-oriented electrical steel sheet according to this embodiment may further include a magnetic domain refinement step of subjecting the steel sheet to magnetic domain refinement. 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 refinement include narrowing the width of 180° magnetic domains (refining 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, or, if performed after the insulating coating layer formation step, narrowing the width of 180° magnetic domains (refining 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]

[0057] A slab containing, in mass %, C: 0.08%, Si: 3.31%, 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 cold-rolled sheet. This cold-rolled sheet was subjected to decarburization annealing at 830°C for 90 seconds in an atmosphere with a (PH2O / PH2) ratio of 0.4. Thereafter, an annealing separator containing 50 mass% of Al2O3, 45 mass% of MgO, and 5 mass% of bismuth chloride was applied to the cold-rolled sheet, dried, and then subjected to finish annealing at 1200°C for 20 hours.

[0058] 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. After removing the annealing separator, the steel was subjected to light pickling under the conditions shown in Table 1. After light pickling, the surfaces were immersed in one of the following conditioners 1 to 3 as shown in Table 1, with some exceptions, to condition the surfaces. Adjustment agent 1: Titanium colloid-containing liquid Adjuster 2: Manganese phosphate microparticle emulsion Adjuster 3: Zinc-based colloid-containing liquid After surface conditioning (or after light pickling if no surface conditioning was performed), the steel sheet was immersed in a phosphate treatment solution containing the metal phosphate salt shown in Table 1, and then heated and dried to form an intermediate layer. The intermediate layer had the thickness shown in Table 1. The thickness of the intermediate layer was taken as the average of the results of measurements at 10 points. X-ray crystal structure analysis showed that the metal phosphates in the intermediate layers of intermediate Nos. 1 to 11 were all crystalline metal phosphates. The metal phosphate (magnesium phosphate) in intermediate No. 12 was not crystalline metal phosphate.

[0059] [Table 1]

[0060] The steel sheets with the intermediate layer (Intermediate Nos. 1 to 12) were cut into multiple pieces as needed. Each steel sheet was coated with a coating solution containing the metal phosphate salt and colloidal silica shown in Table 2 and baked in a drying oven for the time shown in Table 2 to achieve the sheet temperature shown in Table 2, forming a tensile coating layer on the 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 this way, steel sheets (grain-oriented electrical steel sheets) Nos. 101 to 122 were produced.

[0061] The silica content and metal phosphate content of the tensile coating layer and the average thickness of the insulating coating were determined for these steel sheets using the methods described above. The thickness of the insulating coating was calculated as the average of the results of measurements at 10 points. The results are shown in Table 2. In addition, the chemical composition of the base steel plate was investigated and found to contain Si: 3.30%, 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.

[0062] [Table 2]

[0063] Furthermore, the adhesion of the insulating coating, coating tension, corrosion resistance, elution resistance, and magnetic properties of these steel sheets were measured using the methods described below. The results are shown in Table 3.

[0064] [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 ⊚ or ◯ was judged to indicate excellent coating adhesion. ◎: Peeling area rate 0-0.5% ○: Peeling area ratio: over 0.5% and 5.0% or less △: Peeling area rate over 5.0% and 20% or less ×: Peeling area ratio: over 20% and 50% or less ××: Peeling area rate over 50%

[0065] [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.

[0066] [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 0.10% or less) 8: The area ratio of rust is more than 0.10% and 0.25% or less 7: Area ratio of rust is over 0.25% and 0.50% or less 6: Area ratio of rust is over 0.50% and 1.0% or less 5: The area ratio of rust is over 1.0% and 2.5% or less 4: The area ratio of rust is over 2.5% and 5.0% or less 3: The area ratio of rust is over 5.0% and 10% or less 2: The area ratio of rust is over 10% and 25% or less 1: The area ratio of rust is over 25% and 50% or less

[0067] [Elution resistance] 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 amount of dissolved phosphoric acid per unit area (mg / m 2 ) was calculated and the resistance to elution was evaluated based on this. 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 40 mg / m 2 If the resistance was less than this, it was determined that the resistance to elution was excellent.

[0068] [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 2The magnetic properties were measured: B8 (magnetic flux density at a magnetizing force of 800 A / m) (not shown in the table) and W17 / 50 (iron loss per mass at a magnetic flux density amplitude of 1.7 T and 50 Hz). These property 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 0.70 W / kg or less, it was determined that the magnetic properties were excellent.

[0069] [Table 3]

[0070] As shown in Tables 1 to 3, inventive examples Nos. 101 to 113 had a base steel sheet and an insulating coating formed on the surface of the base steel sheet, with the insulating coating having a predetermined intermediate layer and a tensile coating layer, resulting in excellent coating adhesion, coating tension, magnetic properties, corrosion resistance, and elution resistance. On the other hand, in the comparative examples Nos. 114 to 122, the thickness or coverage of the intermediate layer was outside the range of the present invention, the intermediate layer did not contain crystalline metal phosphate, or the silica content of the tensile coating layer was outside the range of the present invention, and therefore one or more of the coating adhesion, coating tension, magnetic properties, corrosion resistance, and elution resistance were inferior. [Industrial Applicability]

[0071] The present invention provides 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, as well as a method for forming an insulating coating for the grain-oriented electrical steel sheet. 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, as well as sufficient corrosion resistance and elution resistance, and therefore has high industrial applicability. [Explanation of symbols]

[0072] 1 Base steel plate 2. Insulation coating 21 Intermediate Layer 22 Tension film layer 100 directional electromagnetic steel sheet

Claims

1. A base steel plate; an insulating coating formed on the surface of the base steel sheet; and 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 intermediate layer is 0.10 to 5.0 μm, a coverage area ratio of the intermediate layer to the base steel plate is 40 to 90%, 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 %; 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; a light pickling process in which the steel sheet after the annealing separator removal process is pickled for 5 to 30 seconds with a mixed acid having a liquid temperature of 25 to 85°C and a concentration of 0.5 to 10 mass% which is a mixture of one or more of sulfuric acid, phosphoric acid, and nitric acid; a surface conditioning step of immersing the steel sheet after the light pickling step in a surface conditioner containing a titanium-based or zinc-based colloid or a manganese phosphate fine particle emulsion and having a liquid temperature of 25 to 50°C for 10 to 60 seconds; an immersion step of immersing the steel sheet after the surface conditioning step in a treatment solution having a liquid temperature of 20 to 85°C and containing 5 to 50 mass% of a metal phosphate for 5 to 50 seconds; a drying step of removing the steel sheet after the immersion step from the treatment solution, removing excess treatment solution by washing with water, and then drying the steel sheet; a tensile coating layer forming step of applying a coating liquid containing a metal phosphate and colloidal silica to the steel sheet after the drying step, such that the colloidal silica is in an amount of 30 to 150 parts by mass per 100 parts by mass of the metal phosphate, drying the steel sheet, and then maintaining the steel sheet at a sheet temperature of 700 to 950°C for 10 to 50 seconds; Equipped with A method for forming an insulating coating.

3. the annealing separator contains 10 to 99.5 mass% of Al 2 O 3 and further contains one or two of 5 to 90 mass% of MgO and 0.5 to 10.0 mass% of chloride; 3. The method for forming an insulating coating according to claim 2.

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