Method for forming grain-oriented electrical steel sheets and insulating coatings

Abstract

This grain-oriented electrical steel sheet comprises a base material steel sheet and an insulating coating film that is formed on the surface of the base material steel sheet and contains a metal phosphate. The insulating coating film contains aggregates of silica microparticles. When the electrical steel sheet is boiled in boiling pure water for 10 minutes, the amount of phosphoric acid that has eluted into the pure water is measured, and the measured amount of phosphoric acid is divided by the area of the boiled insulating coating film, the resulting elution amount of phosphoric acid from the electrical steel sheet is less than 40 mg / m2.

Description

Technical Field

[0001] The present disclosure relates to a method for forming a grain-oriented electrical steel sheet and an insulating coating. This application claims priority based on Japanese Patent Application No. 2024-018749 filed in Japan on February 9, 2024, and incorporates its content herein by reference.

Background Art

[0002] Grain-oriented electrical steel sheets are mainly used in transformers. Transformers are continuously excited over a long period from installation to disposal and continue to generate energy losses. Therefore, the energy loss during magnetization with alternating current, that is, iron loss, becomes a major index determining the performance of transformers.

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

[0004] Also, applying tension to the steel sheet is effective in reducing iron loss. Forming a coating of a material with a smaller coefficient of thermal expansion than the steel sheet on the steel sheet surface at a high temperature is an effective means for reducing iron loss. In the finish annealing process of electrical steel sheets, a forsterite-based coating (inorganic coating) excellent in coating adhesion, which is generated by the reaction of the oxide on the steel sheet surface and the annealing separation agent, is a coating that can apply tension to the steel sheet.

[0005] For example, the method of forming an insulating coating by baking a coating liquid mainly composed of colloidal silica and phosphate on the steel sheet surface disclosed in Patent Document 1 is an effective method for reducing iron loss because the effect of applying tension to the steel sheet is large. Therefore, leaving the forsterite-based coating generated in the finish annealing process and applying an insulating coating mainly composed of phosphate thereon has become a general manufacturing method for grain-oriented electrical steel sheets.

[0006] However, in recent years, there has been a growing demand for smaller and more powerful transformers. To achieve this miniaturization, grain-oriented electrical steel sheets are required to exhibit excellent high-field iron loss performance, meaning that iron loss is low even at high magnetic flux densities. At the same time, it has recently become clear that forsterite coatings hinder the movement of magnetic domain walls and adversely affect iron loss. In grain-oriented electrical steel sheets, magnetic domains change under alternating magnetic fields as magnetic domain walls move. Smooth and rapid movement of these magnetic domain walls is effective in reducing iron loss, but forsterite coatings are nonmagnetic themselves and have an uneven structure at the steel sheet / coating interface. This uneven structure is thought to hinder the movement of magnetic domain walls and thus adversely affect iron loss.

[0007] Therefore, as a means of improving high-field iron loss, research is being conducted on techniques to manufacture grain-oriented electrical steel sheets without forsterite coatings by removing forsterite coatings using mechanical means such as polishing or chemical means such as pickling, or by preventing the formation of forsterite coatings during high-temperature finish annealing, as well as techniques to make the steel sheet surface mirror-like (in other words, techniques to magnetically smooth the steel sheet surface).

[0008] As a technique for preventing the formation of forsterite-based coatings, for example, Patent Document 2 discloses a technique in which, after normal finish annealing, surface deposits are removed by pickling, and then the steel sheet surface is polished to a mirror finish by chemical polishing or electrolytic polishing. It has been found that by forming a tension-imparting insulating coating on the surface of grain-oriented electrical steel sheets that do not have forsterite-based coatings, obtained by such known methods, an even better iron loss improvement effect can be obtained. In addition, the tension-imparting insulating coating can impart various properties other than iron loss improvement, such as corrosion resistance, heat resistance, and slipperiness.

[0009] However, forsterite-based coatings not only exhibit insulating properties but also act as an intermediate layer to ensure film adhesion when forming tension coatings (tension-imparting insulating coatings). In other words, because forsterite-based coatings are formed deeply embedded in the steel sheet, they exhibit excellent film adhesion to the steel sheet, which is a metal. Therefore, when a tension-imparting coating (tension coating) mainly composed of colloidal silica or phosphate is formed on the surface of a forsterite-based coating, excellent film adhesion is achieved. On the other hand, since bonding between metals and oxides is generally difficult, it was difficult to ensure sufficient film adhesion between the tension coating and the steel sheet surface when a forsterite-based coating was not present. Therefore, when forming a tension coating on grain-oriented electrical steel sheets that do not have a forsterite coating, it is being considered to provide a layer that replaces the role of the forsterite coating as an intermediate layer.

[0010] Patent Document 3 discloses a technique for ensuring the adhesion of a tension-impregnating insulating film by applying an intermediate coating beforehand when forming the tension-impregnating coating. However, the technology disclosed in Patent Document 3 has the problem that it is not possible to maintain good adhesion of a tension-applying insulating coating that has high tension.

[0011] Furthermore, for example, Patent Document 4 discloses a grain-oriented electrical steel sheet having a base steel sheet and an insulating coating formed on the surface of the base steel sheet, wherein the insulating coating is formed on the base steel sheet side and has an intermediate layer containing a crystalline metal phosphate salt and a tension coating layer formed on the surface side of the insulating coating. Patent Document 4 shows that the grain-oriented electrical steel sheet does not have a forsterite-based coating, has excellent coating adhesion, excellent coating tension, and excellent magnetic properties.

[0012] However, depending on the application, grain-oriented electrical steel sheets may be subjected to stress-relieving annealing after being processed into a predetermined shape. Our investigations revealed that the grain-oriented electrical steel sheets to which the technology described in Patent Document 4 is applied were not intended to undergo stress-relieving annealing, and that if stress-relieving annealing is performed, the coating adhesion may decrease. Furthermore, it was found that the crystalline metal phosphate in the intermediate layer may coarseen due to the chemical conversion treatment, potentially reducing the packing density when used as a core. [Prior art documents] [Patent Documents]

[0013] [Patent Document 1] Japanese Patent Publication No. 48-039338 [Patent Document 2] Japanese Patent Publication No. 49-96920 [Patent Document 3] Japanese Patent Application Publication No. 5-279747 [Patent Document 4] International Publication No. 2022 / 215709 [Overview of the project] [Problems that the invention aims to solve]

[0014] As described above, Patent Documents 1 to 4 did not disclose a grain-oriented electrical steel sheet that does not have a forsterite-based coating and that can obtain excellent coating adhesion even when strain-relieving annealing is performed.

[0015] Therefore, this disclosure aims to provide a grain-oriented electrical steel sheet that does not have a forsterite-based coating, while ensuring that the coating tension, corrosion resistance, phosphorus leaching from the coating, the packing ratio when used as a core, and iron loss are equivalent to or better than those of conventional materials, and that excellent coating adhesion can be obtained even after stress-relieving annealing. Furthermore, this disclosure aims to provide a method for forming an insulating coating that can be used to manufacture the above grain-oriented electrical steel sheet. [Means for solving the problem]

[0016] The inventors of the present invention studied the influence of strain relief annealing on the film adhesion of the insulating film. As a result, it has been found that by forming a phosphate film on the surface of a non-oriented electromagnetic steel sheet on which a forsterite-based film is not formed by chemical conversion treatment, fusing the phosphate film and the insulating film during strain relief annealing, and controlling the aggregation of silica fine particles, it is possible to suppress the deterioration of the magnetic properties and other film properties and suppress the decrease in the film adhesion of the insulating film layer without deteriorating them.

[0017] The present disclosure has been made in view of the above findings, and the gist thereof is as follows. [1] A non-oriented electromagnetic steel sheet having a base steel sheet and an insulating film containing a metal phosphate formed on the surface of the base steel sheet, wherein the insulating film contains aggregates of silica fine particles, and the amount of phosphoric acid eluted, which is obtained by boiling in boiling pure water for 10 minutes, measuring the amount of phosphoric acid eluted in the pure water, and dividing the measured amount of phosphoric acid by the area of the boiled insulating film, is less than 40 mg / m 2. 2 A non-oriented electromagnetic steel sheet characterized by this. [2] The non-oriented electromagnetic steel sheet according to [1] above, wherein the average particle size of the aggregates of the silica fine particles is 0.5 to 3.0 μm. [3] The non-oriented electromagnetic steel sheet according to [1] or [2] above, wherein the aggregates of the silica fine particles are amorphous. [4] A finishing annealing step of applying an annealing release agent containing 10 to 100% by mass of Al2O3 to a steel sheet, drying it, and then performing finishing annealing, An annealing release agent removing step of removing the excess annealing release agent from the steel sheet after the finishing annealing step, A light pickling step of pickling the steel sheet after the annealing release agent removing step with 0.1 to 5.0% by mass of an inorganic acid for 10 to 60 seconds, A water washing step of washing the steel sheet after the light pickling step with water and drying it, After the water washing step, the steel sheet is immersed in a treatment solution with a liquid temperature of 30 to 85°C and a concentration of metal phosphate of 1.0 to 20.0% by mass for 5 to 150 seconds. After washing away the treatment solution with water, the steel sheet is dried, in a first insulating film forming step; A coating solution containing a metal phosphate and colloidal silica is applied to the steel sheet after the first insulating film forming step. The content of colloidal silica having a particle size of 5 to 30 nm is 30 to 150 parts by mass with respect to 100 parts by mass of the metal phosphate, and the concentration is 10.0 to 40.0% by mass. After drying, it is held for 10 to 120 seconds in a state where the plate temperature is 750 to 950°C, in a second insulating film forming step; After the second insulating film forming step, the steel sheet is heated to a temperature range of 700 to 900°C in an atmosphere having a nitrogen content of 50 to 100% by volume and a hydrogen content of 0 to 50% by volume, and held in the temperature range for 30 to 240 minutes, in a heat treatment step; A method for forming an insulating film, characterized by including the above. [5] The method for forming an insulating film according to the above [4], characterized in that the annealing release agent further contains one or two of MgO: 5 to 90% by mass and chloride: 0.5 to 10.0% by mass.

Advantages of the Invention

[0018] According to the above aspect of the present disclosure, on the premise of a non-oriented electromagnetic steel sheet having no forsterite-based film, while ensuring that the film tension, corrosion resistance, elution property of phosphorus from the film, occupancy ratio when used as a core, and iron loss are equal to or better than those of the conventional ones, a non-oriented electromagnetic steel sheet with excellent film adhesion even after stress relief annealing can be provided. Further, according to another aspect of the present disclosure, a method for forming an insulating film capable of manufacturing the above non-oriented electromagnetic steel sheet can be provided.

Brief Description of the Drawings

[0019] [Figure 1] It is a diagram for explaining an example of an aggregate of silica fine particles. [Figure 2] It is a diagram for explaining another example of an aggregate of silica fine particles.

Embodiments for Carrying Out the Invention

[0020] This disclosure describes a grain-oriented electrical steel sheet (grain-oriented electrical steel sheet according to this embodiment) and a method for forming an insulating coating that can be used to manufacture the grain-oriented electrical steel sheet. However, this disclosure is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of this disclosure.

[0021] The individual elements of this disclosure are described in detail below. The numerical ranges indicated below, separated by a "~", include both a lower and upper limit. Numbers marked "less than" or "greater than" are not included in the numerical range.

[0022] <Grain-oriented electrical steel sheet> The grain-oriented electrical steel sheet according to this embodiment comprises a base steel sheet and an insulating coating containing a metal phosphate salt formed on the surface of the base steel sheet. The grain-oriented electrical steel sheet according to this embodiment may consist only of a base steel sheet and an insulating coating. That is, the grain-oriented electrical steel sheet according to this embodiment may have a two-layer structure consisting only of a base steel sheet and an insulating coating.

[0023] [Base material steel plate] The base steel sheet consists of a steel sheet having the following chemical composition.

[0024] (chemical composition) The chemical composition of the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment may be within the range known for grain-oriented electrical steel sheets, in order to obtain the properties generally required for grain-oriented electrical steel sheets. For example, it is preferable to include the following elements. In this embodiment, percentages relating to the chemical composition are mass percentages unless otherwise specified.

[0025] C: 0.010% or less Carbon (C) is an effective element for controlling the microstructure of steel sheets during the manufacturing process up to the completion of the decarburization annealing process. However, if the C content exceeds 0.010%, the magnetic properties of the grain-oriented electrical steel sheet, which is the finished product, 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. More preferably, the C content is 0.005% or less. While a lower C content is preferable, reducing the C content to less than 0.0001% will saturate the effect of microstructure control, only increasing manufacturing costs. Therefore, the C content may be 0.0001% or more.

[0026] Si: 2.50~4.00% Silicon (Si) is an element that increases the electrical resistance of grain-oriented electrical steel sheets and improves iron loss characteristics. If the Si content is less than 2.50%, a sufficient eddy current loss reduction effect cannot be obtained. Therefore, it is preferable that the Si content be 2.50% or more. More preferably, the Si content is 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 brittle, and its treadability deteriorates significantly. In addition, the workability of the grain-oriented electrical steel sheet decreases, and the sheet may break during rolling. For this reason, it is preferable to keep the Si content at 4.00% or less. More preferably, the Si content is 3.80% or less, and even more preferably 3.70% or less.

[0027] Mn: 0.01~0.50% Manganese (Mn) is an element that combines with sulfur (S) during the manufacturing process to form MnS. This precipitate functions as an inhibitor (an agent that suppresses normal grain growth) and causes secondary recrystallization in steel. Furthermore, Mn is an element that also improves the hot workability of steel. If the Mn content is less than 0.01%, the above effects cannot be fully obtained. Therefore, it is preferable that the Mn content be 0.01% or more. More preferably, the Mn content is 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. More preferably, the Mn content is 0.20% or less, and even more preferably 0.10% or less.

[0028] N: 0.010% or less Nitrogen (N) is an element that combines with Al during the manufacturing process to form AlN, which functions as an inhibitor. However, if an excess of inhibitor remains in the grain-oriented electrical steel sheet and the N content exceeds 0.010%, the magnetic properties will 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. More preferably, the N content is 0.008% or less. On the other hand, there is no specific lower limit for the N content, but reducing it to less than 0.001% would only increase manufacturing costs. Therefore, the N content may be 0.001% or higher.

[0029] sol.Al: 0.020% or less sol.Al (acid-soluble aluminum) is an element that, during the manufacturing process of grain-oriented electrical steel sheets, combines with N to form AlN, which functions as an inhibitor. However, if an excess of the inhibitor remains in the base steel sheet and the sol.Al content exceeds 0.020%, the magnetic properties deteriorate. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, the sol.Al content is preferably 0.020% or less. More preferably, the sol.Al content is 0.010% or less, and even more preferably less than 0.001%. There is no particular lower limit for the sol.Al content, but reducing it to less than 0.0001% would only increase manufacturing costs. Therefore, the sol.Al content may be 0.0001% or more.

[0030] 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 deteriorate due to the remaining inhibitor. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, it is preferable that the S content be 0.010% or less. It is even preferable that the S content in the grain-oriented electrical steel sheet be 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% would only increase manufacturing costs. Therefore, the S content in the grain-oriented electrical steel sheet may be 0.0001% or more.

[0031] Remainder: Fe and impurities The chemical composition of the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment contains the elements described above, with the remainder being Fe and impurities. However, for the purpose of improving magnetic properties, etc., Sn, Cu, Se, and Sb may be included in place of a portion of Fe within the ranges shown below. Furthermore, even if one or more of the following elements, such as W, Nb, Ti, Ni, Co, V, Cr, and Mo, are included in a total amount of 1.0% or less, it will not impair the effects of the grain-oriented electrical steel sheet according to this embodiment.

[0032] In this embodiment, impurities refer to elements that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during the industrial production of the base steel sheet, and are permitted to be present in amounts that do not adversely affect the function of the grain-oriented electrical steel sheet according to this embodiment.

[0033] Sn: 0~0.50% Tin (Sn) is an element that contributes to improving magnetic properties through the control of the primary recrystallization structure. To obtain the effect of improving magnetic properties, it is preferable to have a Sn content of 0.01% or more. More preferably, the Sn content is 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 the magnetic properties deteriorate. Therefore, it is preferable to keep the Sn content at 0.50% or less. More preferably, the Sn content is 0.30% or less, and even more preferably 0.10% or less.

[0034] Cu: 0~0.50% Copper (Cu) is an element that contributes to increasing the Goss orientation occupancy in the secondary recrystallized structure. To obtain the above effect, it is preferable to have a Cu content of 0.01% or more. More preferably, the Cu content is 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 brittle during hot rolling. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, it is preferable to have a Cu content of 0.50% or less. The Cu content is more preferably 0.30% or less, and even more preferably 0.10% or less.

[0035] Se: 0~0.020% Se (selenium) is an element that has the effect of improving magnetic properties. When Se is included, it is preferable to have a Se content of 0.001% or more in order to exhibit a good 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, it is preferable to keep the Se content at 0.020% or less. More preferably, the Se content is 0.015% or less, and even more preferably 0.010% or less.

[0036] Sb: 0~0.50% Antimony (Sb) is an element that improves magnetic properties. When Sb is included, it is preferable to have an Sb content of 0.005% or more in order to exhibit a good magnetic property improvement effect. More preferably, the Sb content is 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 deteriorates significantly. Therefore, it is preferable to keep the Sb content at 0.50% or less. More preferably, the Sb content is 0.30% or less, and even more preferably 0.10% or less.

[0037] As described above, in this embodiment, the chemical composition of the base steel sheet of the grain-oriented electrical steel sheet is exemplified as containing the above-mentioned elements, with the remainder being Fe and impurities.

[0038] 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 emission spectrometry method. However, if an insulating film is formed on the surface, it must be removed before measurement. The film can be removed by immersing the sheet in a high-concentration alkaline solution (for example, a 30% sodium hydroxide solution heated to 85°C) for 20 minutes or more. Whether the film has been removed can be determined visually. For small samples, the film may also be removed by surface grinding.

[0039] (plate thickness) The thickness of the base steel plate is not limited, but it is preferably 0.15 to 0.35 mm from the viewpoint of reducing iron loss.

[0040] [Insulating coating] In the grain-oriented electrical steel sheet according to this embodiment, an insulating coating is formed on the base steel sheet. That is, since no forsterite-based coating is formed, the insulating coating is formed in direct contact with the base steel sheet. As will be described later, the insulating coating is a single layer formed by the fusion of a first insulating coating and a second insulating coating through heat treatment.

[0041] The insulating coating contains aggregates of silica microparticles. To determine whether the insulating coating contains aggregates of silica microparticles, a transmission electron microscope (TEM) is used to examine a cross-section of the insulating coating, magnifying it 30,000 times and imaging the vicinity along the interface with a length of 10 μm or more. In this embodiment, silica nanoparticle aggregates refer to aggregates formed by primary silica particles coming together to form clumps, which are aggregated due to intermolecular forces, electrostatic attraction, etc. It is well known that silica forms aggregates, and in some cases, aggregates of primary particles may come together to form secondary aggregates. In this embodiment, however, the aggregates refer to aggregates formed by the coming together of silica nanoparticles with a primary particle diameter of 5 to 30 nm. Aggregates of silica microparticles contained within the insulating coating can be observed from the captured images. This is because colloidal silica containing Si elements within a phosphate matrix can be observed in the insulating coating. Furthermore, the difference from a matrix mainly composed of phosphates can be confirmed by elemental analysis. If the image resolution is low and a clear image cannot be obtained, it is possible to clarify it using an image processing device such as a Luzex. Regions with an SiO2 concentration of 50% or more are considered to be aggregates of silica microparticles. The aggregated particle size of colloidal silica is determined by measuring the particle size at three locations (i.e., a total of nine locations) within the TEM images obtained in this manner, and averaging the major and minor axes. If the aggregated particle is spread across the thickness of the insulating coating, the thickness of the insulating coating is used as the particle size of the aggregated particle.

[0042] In other words, aggregation, as used here, refers to a situation where silica nanoparticles derived from colloidal silica with a particle size of 5 to 30 nm aggregate densely within an insulating coating to form primary aggregated particles, as shown in Figures 1 and 2. These aggregated particles are defined as having a size of approximately 0.05 to 3.0 μm. Figures 1 and 2 are TEM images showing elemental analysis of Si. The white dots in Figures 1 and 2 represent silica nanoparticles. The shape of the aggregated particles can be anything, but due to their formation principle, the observed cross-section is often circular to elliptical, and in some cases, they may have irregularities or be composed of connected circles or ellipses. If the shape is irregular, the circumscribed circle is used as the aggregated particle size. Furthermore, if the average particle size of the silica microparticle aggregates is 0.5 to 3.0 μm, the elution properties can be improved, and at the same time, the film tension can also be improved. If the aggregates of silica microparticles are amorphous, the decrease in coating tension can be suppressed.

[0043] In the insulating coating, it is preferable that the metal phosphate salt content is 90-50% and the Si content is 10-40%.

[0044] The metal phosphate in the insulating film is preferably an Fe-PO-based or Fe-PO-Si-based metal phosphate. If the metal phosphate is an Fe-PO-based or Fe-PO-Si-based metal phosphate, the decrease in film adhesion after stress-relieving annealing can be suppressed. Examples of Fe-PO-based metal phosphates include Fe2P2O7, Fe3(PO4)2, and FePO4. An example of an Fe-PO-Si-based metal phosphate is FeOP2O5SiO2.

[0045] (Thickness) In the grain-oriented electrical steel sheet according to this embodiment, the thickness of the insulating coating is preferably 1 to 10 μm from the viewpoint of improving the space factor.

[0046] The thickness of the insulating coating is determined by the following method. In flat areas, 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. In this case, the point where the distribution of element P decreases sharply from the insulating coating side is defined as the interface between the insulating coating and the base steel plate.

[0047] Furthermore, by using a transmission electron microscope and an energy-dispersive elemental analyzer, it is possible to identify the type and mass ratio of metal phosphate salts. Measurements should be performed on at least three fields of view, and their average values ​​should be calculated. Furthermore, the Si content in the insulating film can also be measured using a transmission electron microscope and an energy-dispersive elemental analyzer.

[0048] Amount of phosphorus leached from the coating In this embodiment, the grain-oriented electrical steel sheet has suppressed phosphorus leaching from the coating, resulting in a phosphoric acid leaching amount of 40 mg / m². 2 It is less than. The amount of phosphoric acid eluted is obtained by boiling the film in boiling pure water for 10 minutes, measuring the amount of phosphoric acid eluted into the pure water, and dividing the measured amount of phosphoric acid by the area of ​​the boiled insulating film.

[0049] <Manufacturing method> The grain-oriented electrical steel sheet according to this embodiment can achieve the above-described effects regardless of the manufacturing method, but it can be preferably manufactured by a manufacturing method that includes, for example, the following steps. (i) Hot rolling process, in which the steel billet is heated and hot-rolled into a hot-rolled sheet, (ii) A hot-rolled sheet annealing step, in which the hot-rolled sheet is subjected to hot-rolled sheet annealing, (iii) Pickling step of pickling the hot-rolled sheet after the hot-rolled sheet annealing step, (iv) A cold rolling process in which the hot-rolled sheet after the pickling process is cold-rolled to form a steel sheet (cold-rolled sheet), (v) Decarburization annealing process, which involves subjecting the steel plate to decarburization annealing. (vi) A finish annealing step in which an annealing release agent containing 10 to 100% by mass of Al2O3 is applied to the steel sheet after decarburization annealing, and after drying, finish annealing is performed. (vii) An annealing separating agent removal step in which excess annealing separating agent is removed from the steel plate after the finish annealing step, (viii) A light pickling step in which the steel plate after the annealing separating agent removal step is pickled with 0.1 to 5.0% by mass of inorganic acid for 10 to 60 seconds. (ix) A water washing step in which the steel plate after the light pickling step is washed with water and dried, (x) The steel sheet after the washing step is immersed in a treatment solution having a liquid temperature of 30 to 85°C and a concentration of metal phosphate salt of 1.0 to 20.0% by mass for 5 to 150 seconds, the treatment solution is washed off with water, and the steel sheet is dried to form a first insulating film, a first insulating film forming step. (xi) A second insulating film formation step, wherein a coating solution containing a metal phosphate salt and colloidal silica is applied to the steel sheet after the first insulating film formation step, the content of colloidal silica with a particle size of 5 to 30 nm is 30 to 150 parts by mass per 100 parts by mass of the metal phosphate salt, and the concentration is 10.0 to 40.0% by mass, and after drying, the sheet is held at a temperature of 750 to 950°C for 10 to 120 seconds to form a second insulating film, and (xii) A heat treatment step in which the steel sheet after the second insulation forming step is heated to a temperature range of 700 to 900°C in an atmosphere having a nitrogen content of 50 to 100 vol% and a hydrogen content of 0 to 50 vol% and held in the said temperature range for 30 to 240 minutes, thereby fusing the first insulating film and the second insulating film to form an insulating film. The manufacturing method for grain-oriented electrical steel sheets according to this embodiment is characterized by (vi) the finish annealing process to (xii) the heat treatment process, and (i) the hot rolling process to (v) the decarburization annealing process is not particularly limited and known conditions can be applied.

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

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

[0052] [Cold rolling process] In the cold rolling process, the steel sheet after the pickling process is cold-rolled to produce a cold-rolled sheet. Cold rolling may be performed in a single cold rolling (series of cold rolling without intermediate annealing) or multiple cold rollings with intermediate annealing may be performed by interrupting the cold rolling before the final pass of the cold rolling process and performing at least one or more intermediate annealings.

[0053] The cold rolling conditions should follow known methods. The cold rolling ratio in grain-oriented electrical steel sheets greatly affects their magnetic properties. The final reduction ratio has a particularly significant effect, and it can be set to 80-95%. The final reduction ratio is the cumulative reduction ratio of cold rolling, and if intermediate annealing is performed, it is the cumulative reduction ratio of cold rolling after the final intermediate annealing.

[0054] For intermediate annealing, for example, the material is held at a temperature of 800-1200°C for 5-180 seconds. The annealing atmosphere is not particularly limited, but it is preferable to use a non-oxidizing atmosphere such as nitrogen, argon, or hydrogen to prevent oxidation of the steel sheet. The annealing method can be either continuous annealing or batch annealing in coil form, or other methods may be used. Considering manufacturing costs, it is preferable to perform intermediate annealing three times or less.

[0055] [Decarburization annealing process] In the decarburization annealing process, the cold-rolled sheet is subjected to decarburization annealing after the grinding process. In this decarburization annealing, carbon, which adversely affects the magnetic properties of the steel sheet, is removed (decarburized), and the cold-rolled sheet undergoes primary recrystallization.

[0056] While there are no restrictions on the decarburization annealing conditions, annealing is performed in a nitrogen-hydrogen mixed atmosphere with increased oxygen potential due to humidification. Furthermore, since it is necessary to form a primary recrystallized structure, the humidification temperature (dew point) is determined in terms of the annealing temperature required for recrystallization and the oxygen potential at which decarburization is possible at that annealing temperature. The annealing temperature is typically around 700-900°C, and since annealing is generally carried out in a continuous annealing process, a soaking period of about 60 seconds is performed.

[0057] [Nitriding process] Nitriding may be performed between the decarburization annealing process and the finish annealing process described later. In the nitriding process, for example, a decarburized annealed steel sheet is subjected to nitriding treatment by maintaining it at approximately 700-850°C in a nitriding atmosphere (an atmosphere containing gases with nitriding ability such as hydrogen, nitrogen, and ammonia). When using AlN as an inhibitor, it is preferable to raise the nitrogen concentration of the steel sheet to 40 ppm or higher through nitriding. On the other hand, if the nitrogen concentration of the steel sheet exceeds 1000 ppm, excess AlN will remain in the steel sheet even after secondary recrystallization is completed in finish annealing. Such AlN can cause iron loss degradation. Therefore, it is preferable to keep the nitrogen concentration of the steel sheet after the nitriding process below 1000 ppm.

[0058] [Finishing annealing process] In the finish annealing process, an annealing release agent containing 10-100% by mass of Al2O3 is applied to the steel sheet after the decarburization annealing process (or nitriding process), and after drying, the finish annealing is performed. In conventional methods for manufacturing grain-oriented electrical steel sheets, a forsterite-based coating was formed on the surface of the steel sheet (cold-rolled sheet) by applying an annealing separating agent mainly composed of MgO and performing finish annealing. In contrast, the manufacturing method of grain-oriented electrical steel sheets according to this embodiment uses an annealing separating agent containing Al2O3 so as not to form a forsterite-based coating.

[0059] On the other hand, while the proportion of Al2O3 may be 100% by mass, in order to prevent Al2O3 from burning onto the surface of the steel sheet, it is preferable that the annealing separating agent in the manufacturing method of grain-oriented electrical steel sheet according to this embodiment contains MgO. The amount of MgO may be 0%, but in order to obtain the above effect, it is preferable that the proportion of MgO be 5% by mass or more. When MgO is included, the proportion of MgO should be 90% by mass or less in order to ensure that 10% by mass or more of Al2O3 is secured. Preferably, the proportion of MgO is 50% by mass or less. The total of Al2O3 and MgO relative to the annealing separating agent should be more than 50% by mass in terms of solid content.

[0060] Furthermore, in the method for manufacturing grain-oriented electrical steel sheets according to this embodiment, the annealing separating agent may further contain chlorides. Including chlorides in the annealing separating agent has the effect of making it more difficult for forsterite-based coatings to form. The chloride content is not particularly limited and may be 0%, but to obtain the above effect, 0.5 to 10.0% by mass is preferred. Examples of effective chlorides include bismuth chloride, calcium chloride, cobalt chloride, iron chloride, nickel chloride, etc. The finish annealing conditions are not limited, but for example, conditions such as holding at a temperature of 1150-1250°C for 10-60 hours can be used.

[0061] [Annealing Separating Agent Removal Process] In the annealing separation agent removal process, excess annealing separation agent is removed from the steel sheet after the finish annealing process. For example, excess annealing separation agent can be removed by washing with water.

[0062] [Light pickling process] In the light pickling process, the steel sheet after the annealing separating agent removal process is pickled with 0.1 to 5.0% by mass of inorganic acid for 10 to 60 seconds. If the conditions for light pickling are unfavorable, excess annealing separating agent may remain on the steel sheet surface, increasing surface roughness and reducing the packing density. If the acid is too strong, the steel sheet surface may be etched, reducing its magnetic properties.

[0063] [Water washing process] In the water washing process, the steel plates that have undergone the light pickling process are washed with water and dried. This removes the pickling solution, making it possible to suppress rust formation. There are no restrictions on the conditions for washing and drying.

[0064] [First insulation film formation process] In the first insulating film formation step, the steel sheet after the water washing step is immersed for 5 to 150 seconds in a treatment solution with a liquid temperature of 30 to 85°C and a metal phosphate salt concentration of 1.0 to 20.0% by mass. After the treatment solution is washed off with water, the steel sheet is dried to form a first insulating film on the surface of the steel sheet.

[0065] If the processing solution temperature is below 30°C, the amount of the first insulating film formed is insufficient, resulting in a partial decrease in the adhesion of the insulating film. On the other hand, if the temperature exceeds 85°C, the first insulating film becomes partially too thick, ultimately leading to increased surface roughness and a decrease in the packing density. Furthermore, if the concentration of the metal phosphate salt is less than 1.0% by mass, the formation of the first insulating film takes too long, resulting in a cost disadvantage. On the other hand, if the concentration of the metal phosphate salt is greater than 10.0% by mass, the formation of the first insulating film becomes partially thick, ultimately resulting in an uneven insulating film. Furthermore, if the immersion time is less than 5 seconds, the amount of the first insulating film formed is insufficient, resulting in a partial decrease in the adhesion of the insulating film. On the other hand, if it exceeds 150 seconds, it takes too long and becomes costly.

[0066] [Second insulation film formation process] In the second insulating film formation step, a coating liquid (insulating film forming liquid) containing a metal phosphate salt and colloidal silica, wherein the colloidal silica content is 30 to 150 parts by mass with a particle size of 5 to 30 nm per 100 parts by mass of the metal phosphate salt and the concentration is 10.0 to 40.0% by mass, is applied to the steel sheet after the first insulating film formation step, and after drying, the sheet is held at a temperature of 750 to 950°C for 10 to 120 seconds to form a second insulating film on top of the first insulating coating.

[0067] If the plate temperature is below 750°C, the tension will be low and the magnetic properties will be inferior. Therefore, it is preferable to keep the plate temperature at 750°C or higher. On the other hand, if the plate temperature exceeds 950°C, the rigidity of the steel plate will decrease and it will become more susceptible to deformation. In this case, the steel plate may become distorted due to transportation, etc., and the magnetic properties may be inferior. Therefore, it is preferable to keep the plate temperature at 950°C or lower. Furthermore, if the retention time is less than 10 seconds, the elution performance will be inferior. Therefore, the retention time should be 10 seconds or more. On the other hand, if the retention time exceeds 120 seconds, the film adhesion will decrease, or productivity will be inferior if efforts are made to avoid the decrease in film adhesion. Therefore, a retention time of 120 seconds or less is preferable.

[0068] The coating solution contains a metal phosphate salt and colloidal silica, with a concentration of 30 to 150 parts by mass of colloidal silica having a particle size of 5 to 30 nm per 100 parts by mass of metal phosphate salt. The total amount of metal phosphate salt and colloidal silica in the coating solution should be more than 50% by mass in terms of solid content. If the amount of colloidal silica is less than 30 parts by mass, the packing density and iron loss may deteriorate. Therefore, it is preferable to have 30 parts by mass or more of colloidal silica. If the amount of colloidal silica exceeds 150 parts by mass, adhesion, film tension, elution, packing density, and iron loss may deteriorate. Therefore, it is preferable to have 150 parts by mass or less of colloidal silica. As the metal phosphate salt, one or more selected from, for example, aluminum phosphate, zinc phosphate, magnesium phosphate, nickel phosphate, copper phosphate, lithium phosphate, and cobalt phosphate can be used.

[0069] Furthermore, the coating solution preferably has a concentration of 10.0 to 40.0% by mass. If the concentration is less than 10.0% by mass, the elution properties will decrease. Therefore, it is preferable that the concentration be 10.0% by mass or higher. On the other hand, if the concentration exceeds 40.0% by mass, the elution properties, fill factor, and iron loss may deteriorate. Therefore, it is preferable that the concentration be 40.0% by mass or lower.

[0070] The coating solution may also contain additional elements such as vanadium, tungsten, molybdenum, and zirconium. Colloidal silica can be of type S or type C. Type S colloidal silica refers to a silica solution that is alkaline, while type C refers to a silica particle surface that has been treated with aluminum, resulting in a silica solution that is alkaline to neutral. Type S colloidal silica is widely used and relatively inexpensive, but caution is required as it may aggregate and precipitate when mixed with acidic metal phosphate solutions. Type C colloidal silica is stable even when mixed with metal phosphate solutions and does not precipitate, but it is relatively expensive due to the increased processing steps. It is preferable to choose the type depending on the stability of the coating solution being prepared.

[0071] [Magnetic domain refining process] In the magnetic domain subdivision process, energy rays may be irradiated onto the surface of the insulating coating (the surface of the insulating coating on the grain-oriented electrical steel sheet) to subdivide the 180° magnetic domains. By subdividing the magnetic domains, the iron loss of the grain-oriented electrical steel sheet can be further reduced.

[0072] A known method can be used for the magnetic domain subdivision process. For example, one method involves narrowing the width of the 180° magnetic domain (subdividing the 180° magnetic domain) by forming linear or dot-shaped grooves extending in a direction intersecting the rolling direction at predetermined intervals along the rolling direction.

[0073] When forming grooves, various methods can be applied, including mechanical groove formation using gears, chemical groove formation using electrolytic etching, and thermal groove formation using laser irradiation. If the insulating coating is damaged due to the formation of stress-strained areas or grooves, and its properties such as insulation deteriorate, the insulating coating may be reapplied to repair the damage.

[0074] [Heat treatment process] The steel sheet after the second insulation forming step is heated to a temperature range of 700 to 900°C in an atmosphere having a nitrogen content of 50 to 100% by volume and a hydrogen content of 0 to 50% by volume, and held at this temperature range for 30 to 240 minutes to fuse the first insulating film and the second insulating film to form an insulating film.

[0075] If the heat treatment temperature is below 700°C, the effect of stress-relieving annealing is low, resulting in inferior magnetic properties. On the other hand, if the heat treatment temperature exceeds 900°C, the rigidity of the steel sheet decreases, making it more susceptible to deformation, which can lead to a decrease in the packing ratio, reduced film adhesion, and deterioration of elution properties. Furthermore, if the holding time is less than 30 minutes, the effect of stress-relieving annealing is low, resulting in inferior magnetic properties. On the other hand, if the holding time exceeds 240 minutes, the annealing time becomes long, which is not only economically disadvantageous, but also deteriorates the adhesion or elution properties. A hydrogen-nitrogen mixture is preferred as the atmosphere during heat treatment, with a hydrogen content of 50% by volume or less. If the hydrogen content exceeds 50% by volume, not only will the cost increase, but the reducing properties of the atmosphere will become too strong, potentially causing a silica layer to form on the surface and leading to the delamination of the insulating coating. [Examples]

[0076] Next, the effects of one aspect of this disclosure will be described in more detail by reference to examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effectiveness of this disclosure, and this disclosure is not limited to these examples of conditions. This disclosure may adopt various conditions as long as they do not depart from the gist of this disclosure and achieve the objectives of this disclosure.

[0077] A slab containing C:0.08%, Si:3.31%, sol.Al:0.028%, and N:0.008% by mass was cast. After heating the slab, it was hot-rolled to obtain a 2.2 mm thick hot-rolled sheet. This hot-rolled sheet was then annealed by holding it at 1100°C for 10 seconds.

[0078] Subsequently, after pickling under known conditions, the steel was cold-rolled to 0.22 mm by continuous cold rolling without intermediate annealing to obtain a steel sheet (cold-rolled sheet). This steel sheet was then subjected to decarburization annealing by holding at 830°C for 3 minutes.

[0079] After decarburization annealing, an annealing separation agent containing 48% by mass of MgO, 48% by mass of Al2O3, and 4% by mass of BiCl3 was applied and dried. Then, a finish annealing was performed by heating to 1200°C and holding for 20 hours. After the finish annealing, the excess annealing separation agent was removed by washing with water, and no forsterite-based coating was found on the steel sheet surface.

[0080] A first insulating film was formed on this steel plate using a treatment solution prepared by mixing the metal phosphate salt and additives shown in Table 1. Subsequently, a coating solution containing the metal phosphate salt and colloidal silica shown in Table 2 in the proportions shown in Table 2 was applied and dried to form a second insulating film.

[0081] Subsequently, grooves with a depth of 20 μm and a width of 50 μm were formed on the steel plate surface at 6 mm intervals, in a direction inclined 80° from the rolling direction, using a tooth profile. Subsequently, heat treatment was performed under the conditions shown in Table 2 to obtain grain-oriented electrical steel sheets.

[0082] For the obtained grain-oriented electrical steel sheets, the aggregation of silica fine particles derived from colloidal silica in the insulating coating was observed, and the presence or absence of metal phosphate salts in the insulating coating was confirmed and identified using the method described above.

[0083] Although not shown in the table, the chemical composition of the base steel sheet (excluding the oxide layer) was Si: 3.31 mass%, C: 0.0020 mass%, Mn: 0.06 mass%, sol.Al: 0.002 mass%, with the remainder being Fe and impurities.

[0084] Furthermore, the following properties were measured for the obtained grain-oriented electrical steel sheets: coating adhesion, coating tension, corrosion resistance, elution resistance, space factor, and iron loss.

[0085] Coating adhesion A sample measuring 30 mm in width and 300 mm in length was taken from a grain-oriented electrical steel sheet. Cellophane tape (registered trademark) was attached to the inside of the sheet to be bent, and the sheet was bent using a 10 mm diameter cylinder. After bending, the cellophane tape was peeled off from the latter half of the sheet and the sheet was attached to white drawing paper for evaluation in a bending adhesion test. A result of A or higher was judged to indicate excellent coating adhesion and was deemed acceptable. On the other hand, a result of B or lower was judged to indicate poor coating adhesion and was deemed unacceptable. AA: No peeling A: Almost no peeling B: Peeling of several millimeters is visible. C: Peeling is observed on 1 / 3 to 1 / 2 of the surface. D: Almost completely peeled off

[0086] Coating tension The coating tension was calculated by working backward from the curvature when one side of the insulating coating was peeled off. If the obtained coating tension was 4.0 MPa or higher, it was judged to have high coating tension and was deemed acceptable. On the other hand, if the obtained coating tension was less than 4.0 MPa, it was judged to not have high coating tension and was deemed unacceptable.

[0087] Corrosion resistance The samples were evaluated using a salt spray test in accordance with JIS Z 2371:2015. A 5 vol% NaCl aqueous solution was dropped onto samples taken from grain-oriented electrical steel sheets in a 35°C atmosphere for 7 hours. The rusted area of ​​the samples was evaluated according to the following criteria. A score of 5 or higher was judged to be excellent corrosion resistance and was deemed acceptable. On the other hand, a score of 4 or lower was judged to be poor corrosion resistance and was deemed unacceptable. 10: No rust occurred. 9: Very little rust occurs (area ratio of 0.1% or less) 8. Area percentage where rust occurred is greater than 0.1% and less than or equal to 0.25%. 7: The percentage of rusted area is greater than 0.25% and less than or equal to 0.50%. 6. Area percentage where rust occurred: greater than 0.50% and less than or equal to 1.0% 5: Area percentage with rust is greater than 1.0% and less than or equal to 2.5%. 4: The area percentage where rust occurred is greater than 2.5% but less than or equal to 5.0%. 3: The area percentage where rust has occurred is greater than 5.0% but less than or equal to 10.0%. 2: The area percentage where rust occurred is greater than 10.0% and less than or equal to 25.0%. 1: The area percentage where rust occurred is greater than 25.0% and less than or equal to 50.0%.

[0088] Dissolution The elution rate was evaluated by the amount of phosphoric acid eluted from the sample. Samples taken from grain-oriented electrical steel sheets were boiled in boiling pure water for 10 minutes, and the amount of phosphoric acid eluted into the pure water was measured. The amount of phosphoric acid eluted (mg / m²) was calculated by dividing the measured amount of phosphoric acid by the area of ​​the insulating coating on the boiled grain-oriented electrical steel sheet. 2 The amount of phosphoric acid dissolved in the pure water was calculated by cooling the pure water (solution) in which the phosphoric acid had dissolved, diluting the cooled solution with pure water, and measuring the phosphoric acid concentration using ICP-AES. The amount of elution is 40 mg / m². 2 If the amount was less than 40 mg / m², it was judged to have excellent dissolution properties and was deemed acceptable. On the other hand, if the dissolution amount was 40 mg / m², it was judged to be acceptable. 2 If the above conditions were met, the product was judged to have poor elution properties and was deemed unacceptable.

[0089] Occupancy factor The packing factor was measured according to the method compliant with JIS C 2550-5:2020. Thirty samples were used, each measuring 30 mm in width and 320 mm in length. After measuring the total mass of the samples, the packing factor was calculated by measuring the distance between the upper and lower backing plates sandwiching the laminate while under a pressure of 1 MPa. If the occupancy rate was 97.0% or higher, it was judged to have a high occupancy rate when used as a core, and was deemed to pass. On the other hand, if the occupancy rate was less than 97.0%, it was judged to not have a high occupancy rate when used as a core, and was deemed to fail.

[0090] Iron loss The iron loss W17 / 50 (iron loss per unit mass at a magnetic flux density amplitude of 1.7T and 50Hz) was measured in accordance with the Single Sheet Tester (SST) method of JIS C2556:2015. If the iron loss W17 / 50 was 0.75 or less, it was judged to have excellent iron loss and was deemed acceptable. On the other hand, if the iron loss W17 / 50 was greater than 0.75, it was judged to not have excellent iron loss and was deemed unacceptable.

[0091] [Table 1]

[0092] [Table 2]

[0093] [Table 3]

[0094] Table 3 shows that the grain-oriented electrical steel sheet according to the present invention maintains coating tension, corrosion resistance, phosphorus leaching from the coating, packing ratio when used as a core, and iron loss at a level equivalent to or better than conventional materials, while also achieving excellent coating adhesion even after stress-relieving annealing. [Industrial applicability]

[0095] According to the above-described aspects of this disclosure, it is possible to provide a grain-oriented electrical steel sheet that can obtain excellent coating adhesion even after stress-relieving annealing. Furthermore, according to another aspect of this disclosure, it is possible to provide a method for forming an insulating coating that can manufacture the above-described grain-oriented electrical steel sheet.

Claims

1. Base material steel plate, A grain-oriented electrical steel sheet consisting only of an insulating coating containing a metal phosphate salt formed on the surface of the base steel sheet, The insulating coating contains aggregates of silica fine particles, The amount of phosphoric acid dissolved in the insulating coating is obtained by boiling it in boiling pure water for 10 minutes, measuring the amount of phosphoric acid dissolved in the pure water, and dividing the measured amount of phosphoric acid by the area of ​​the boiled insulating coating, resulting in a phosphoric acid dissolution amount of 40 mg / m². ２ A grain-oriented electrical steel sheet characterized by being less than [amount missing].

2. The grain-oriented electrical steel sheet according to claim 1, characterized in that the average particle size of the agglomeration of silica fine particles is 0.5 to 3.0 μm.

3. The grain-oriented electrical steel sheet according to claim 1 or 2, characterized in that the aggregate of silica fine particles is amorphous.

4. On the steel plate, Al ２ O ３ A finish annealing step involves applying an annealing release agent containing 10 to 100% by mass of [the substance], drying it, and then performing finish annealing. An annealing separating agent removal step is performed on the steel plate after the finish annealing step to remove any excess annealing separating agent, A light pickling step is performed in which the steel plate after the annealing separating agent removal step is pickled with 0.1 to 5.0% by mass of inorganic acid for 10 to 60 seconds. The steel plate after the light pickling process is washed with water and dried in a washing process, The first insulating coating formation step involves immersing the steel plate, after the water washing step, in a treatment solution with a liquid temperature of 30 to 85°C and a concentration of metal phosphate salt of 1.0 to 20.0% by mass for 5 to 150 seconds, then washing off the treatment solution with water, and finally drying the steel plate. A second insulating film formation step is performed, in which a coating solution containing a metal phosphate salt and colloidal silica is applied to the steel sheet after the first insulating film formation step, the content of colloidal silica with a particle size of 5 to 30 nm is 30 to 150 parts by mass per 100 parts by mass of the metal phosphate salt, and the concentration is 10.0 to 40.0% by mass, and after drying, the sheet temperature is maintained at 750 to 950°C for 10 to 120 seconds. A heat treatment step is performed in which the steel sheet after the second insulating film formation step is heated to a temperature range of 700 to 900°C in an atmosphere having a nitrogen content of 50 to 100% by volume and a hydrogen content of 0 to 50% by volume, and held at that temperature range for 30 to 240 minutes. A method for forming an insulating coating, characterized by including the following:

5. The method for forming an insulating film according to claim 4, characterized in that the annealing separating agent further comprises one or two of the following: MgO: 5 to 90% by mass, and chloride: 0.5 to 10.0% by mass.

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