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

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

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

[0001] This disclosure relates to a grain-oriented electrical steel sheet and a method for forming an insulating coating. This application claims priority to Japanese Patent Application No. 2024-018749, filed February 9, 2024, the contents of which are incorporated herein by reference.

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

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

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

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

[0006] However, in recent years, there has been an increasing demand for smaller and higher-performance transformers. To achieve this, grain-oriented electrical steel sheets are required to have excellent high-field iron loss characteristics, i.e., good iron loss even at high magnetic flux densities. At the same time, it has become clear that forsterite-based coatings hinder domain wall movement, adversely affecting iron loss. In grain-oriented electrical steel sheets, magnetic domains change due to domain wall movement under an AC magnetic field. Smooth and rapid domain wall movement is effective in reducing iron loss. However, forsterite-based coatings 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.

[0007] Therefore, as means for improving high magnetic field iron loss, research has been conducted on a variety of techniques, including methods for removing the forsterite-based coating by mechanical means such as polishing or chemical means such as pickling, techniques for producing grain-oriented electrical steel sheets that do not have a forsterite-based coating by preventing the formation of a forsterite-based coating during high-temperature finish annealing, and techniques for making the steel sheet surface mirror-finished (in other words, techniques for magnetically smoothing the steel sheet surface).

[0008] As a technique for preventing the formation of a forsterite-based coating, for example, Patent Document 2 discloses a technique in which, after normal finish annealing, the steel sheet is pickled to remove surface deposits, and then chemically or electrolytically polished to a mirror finish. It has been found that forming a tensioned insulating coating on the surface of a grain-oriented electrical steel sheet that does not have a forsterite-based coating and that has been obtained by such a known method can provide an even more excellent iron loss improvement effect. Furthermore, the tensioned insulating coating can impart various properties, such as corrosion resistance, heat resistance, and slip resistance, in addition to improving iron loss.

[0009] However, in addition to exhibiting insulating properties, forsterite-based coatings also function as intermediate layers that ensure coating adhesion when forming a tension coating (tension-applying insulating coating). That is, because forsterite-based coatings are formed in a state where they penetrate deeply into the steel sheet, they have excellent coating adhesion to the metal steel sheet. Therefore, when a tension-applying coating (tension coating) containing colloidal silica, phosphate, or the like as a main component is formed on the surface of a forsterite-based coating, excellent coating adhesion is achieved. On the other hand, because bonding between metal and oxide is generally difficult, it has been difficult to ensure sufficient coating adhesion between the tension coating and the steel sheet surface in the absence of a forsterite-based coating. Therefore, when forming a tension coating on a grain-oriented electrical steel sheet that does not have a forsterite-based coating, the provision of a layer that serves as an intermediate layer for the forsterite-based coating has been considered.

[0010] Patent Document 3 discloses a technique for ensuring the coating adhesion of a tensioned insulating coating by applying an intermediate coating beforehand when forming the tensioned coating. However, the technique disclosed in Patent Document 3 has a problem in that it is not possible to maintain good coating adhesion for a tensioned insulating coating that has a large 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, the insulating coating being formed on the side of the base steel sheet, an intermediate layer containing a crystalline metal phosphate, and a tensile coating layer formed on the surface side of the insulating coating. Patent Document 4 shows that the grain-oriented electrical steel sheet does not have a forsterite-based coating, and has excellent coating adhesion, excellent coating tension, and excellent magnetic properties.

[0012] However, depending on the application, grain-oriented electrical steel sheets may be subjected to stress relief annealing after being processed into a predetermined shape. As a result of investigations by the present inventors, it was found that the grain-oriented electrical steel sheets to which the technology of Patent Document 4 is applied are not intended to be subjected to stress relief annealing, and that if stress relief annealing is performed, the coating adhesion may be reduced. In addition, it was found that the crystalline metal phosphate in the intermediate layer becomes coarse due to chemical conversion treatment, and this may reduce the space factor when used as a core.

[0013] Japanese Unexamined Patent Publication No. 48-039338 Japanese Unexamined Patent Application No. 49-96920 Japanese Unexamined Patent Application No. 5-279747 International Publication No. 2022 / 215709

[0014] As described above, Patent Documents 1 to 4 do not disclose grain-oriented electrical steel sheets that do not have a forsterite-based coating and that can obtain excellent coating adhesion even when subjected to stress relief annealing.

[0015] Therefore, an object of the present disclosure is to provide a grain-oriented electrical steel sheet that does not have a forsterite-based coating, and that has excellent coating adhesion even after stress relief annealing while maintaining coating tension, corrosion resistance, phosphorus elution from the coating, and space factor and core loss when used as a core at least equal to or greater than conventional values. Another object of the present disclosure is to provide a method for forming an insulating coating that can produce the above-mentioned grain-oriented electrical steel sheet.

[0016] The present inventors have investigated the effect of stress relief annealing on the coating adhesion of an insulating coating, and have found that by forming a phosphate coating by chemical conversion treatment on the surface of a grain-oriented electrical steel sheet on which no forsterite-based coating has been formed, fusing the phosphate coating with the insulating coating during stress relief annealing, and controlling the aggregation of silica fine particles, it is possible to suppress a decrease in coating adhesion of the insulating coating layer without deteriorating the magnetic properties or other coating properties.

[0017] The present disclosure has been made in light of the above findings, and its gist is as follows: [1] A grain-oriented electrical steel sheet having a base steel sheet and an insulating coating containing a metal phosphate formed on the surface of the base steel sheet, wherein the insulating coating contains an aggregate of silica fine particles, and the amount of phosphoric acid eluted into the pure water is measured by boiling the steel sheet for 10 minutes, and the amount of phosphoric acid eluted is calculated by dividing the measured amount of phosphoric acid by the area of ​​the boiled insulating coating, and the amount of phosphoric acid eluted is 40 mg / m ２ [2] The grain-oriented electrical steel sheet according to the above [1], characterized in that the average particle size of the agglomerates of silica fine particles is 0.5 to 3.0 μm. [3] The grain-oriented electrical steel sheet according to the above [1] or [2], characterized in that the agglomerates of silica fine particles are amorphous. [4] The steel sheet contains Al ２ O ３a finish annealing step of applying an annealing separator containing 10 to 100 mass % of the above-mentioned compound to the steel sheet, drying the steel sheet, and then finish annealing the steel sheet; an annealing separator removing step of removing excess annealing separator from the steel sheet after the finish annealing step; a light pickling step of pickling the steel sheet after the annealing separator removing step with 0.1 to 5.0 mass % of inorganic acid for 10 to 60 seconds; a water rinsing step of rinsing the steel sheet after the light pickling step with water and drying the steel sheet; and a first insulating coating forming step of immersing the steel sheet after the water rinsing step in a treatment solution having a liquid temperature of 30 to 85°C and a metal phosphate concentration of 1.0 to 20.0 mass %, for 5 to 150 seconds, removing the treatment solution with water, and then drying the steel sheet. a second insulating coating formation step of applying to the steel sheet after the first insulating coating formation step a coating liquid containing a metal phosphate and colloidal silica, the content of colloidal silica having a particle size of 5 to 30 nm being 30 to 150 parts by mass per 100 parts by mass of the metal phosphate, and the concentration being 10.0 to 40.0 mass%, drying the steel sheet, and then maintaining the steel sheet at a sheet temperature of 750 to 950°C for 10 to 120 seconds; and a heat treatment step of heating the steel sheet after the second insulating coating formation step to a temperature range of 700 to 900°C in an atmosphere having a nitrogen content of 50 to 100% by volume and a hydrogen content of 0 to 50% by volume, and maintaining the steel sheet at the temperature range for 30 to 240 minutes. [5] The method for forming an insulating coating according to [4] above, wherein the annealing separator further contains one or both of MgO: 5 to 90 mass % and chloride: 0.5 to 10.0 mass %.

[0018] According to the above-described aspect of the present disclosure, 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 even after stress relief annealing while maintaining coating tension, corrosion resistance, phosphorus elution from the coating, and space factor and core loss when used as a core at least equal to or greater than conventional levels. Furthermore, according to another aspect of the present disclosure, it is possible to provide a method for forming an insulating coating that can produce the above-described grain-oriented electrical steel sheet.

[0019] 1A and 1B are diagrams illustrating an example of an aggregate of silica fine particles, and FIG. 1C are diagrams illustrating another example of an aggregate of silica fine particles.

[0020] A grain-oriented electrical steel sheet according to an embodiment of the present disclosure (grain-oriented electrical steel sheet according to the present embodiment) and a method for forming an insulating coating that can be used to manufacture the grain-oriented electrical steel sheet will be described below. However, the present disclosure is not limited to the configuration disclosed in the present embodiment, and various modifications are possible within the scope of the present disclosure.

[0021] Each of the constituent elements of the present disclosure will be described in detail below. Below, numerical ranges indicated with "to" include the lower and upper limits. Numerical values ​​indicated as "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 has a base steel sheet and an insulating coating containing a metal phosphate formed on the surface of the base steel sheet. The grain-oriented electrical steel sheet according to this embodiment may consist of only the base steel sheet and the insulating coating. In other words, the grain-oriented electrical steel sheet according to this embodiment may have a two-layer structure consisting of only the base steel sheet and the insulating coating.

[0023] [Base Steel Plate] The base steel plate is made of a steel plate 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 a range known for grain-oriented electrical steel sheets in order to obtain the properties generally required of grain-oriented electrical steel sheets. For example, it is preferable that the following elements are contained. In this embodiment, % relating to the chemical composition is % by mass unless otherwise specified.

[0025] C: 0.010% or less C (carbon) is an element effective for controlling the structure of steel sheets in the manufacturing process up to the completion of the decarburization annealing process. However, if the C content exceeds 0.010%, the magnetic properties of the finished grain-oriented electrical steel sheet deteriorate. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, the C content is preferably 0.010% or less. The C content is more preferably 0.005% or less. Although a lower C content is preferable, reducing the C content to less than 0.0001% saturates the effect of structural control and simply increases manufacturing costs. Therefore, the C content may be 0.0001% or more.

[0026] Si: 2.50 to 4.00% Si (silicon) is an element that increases the electrical resistance of grain-oriented electrical steel sheets and improves their iron loss characteristics. If the Si content is less than 2.50%, a sufficient eddy current loss reduction effect cannot be obtained. Therefore, the Si content is preferably 2.50% or more. The Si content is more preferably 2.70% or more, and even more preferably 3.00% or more. On the other hand, if the Si content exceeds 4.00%, the grain-oriented electrical steel sheets become embrittled and their threading properties deteriorate significantly. Furthermore, the workability of the grain-oriented electrical steel sheets decreases, and the steel sheets may break during rolling. Therefore, the Si content is preferably 4.00% or less. The Si content is more preferably 3.80% or less, and even more preferably 3.70% or less.

[0027] Mn: 0.01 to 0.50% Mn (manganese) is an element that combines with S to form MnS during the manufacturing process. This precipitate functions as an inhibitor (a suppressor of normal grain growth) and induces secondary recrystallization in the steel. Mn also enhances the hot workability of the steel. If the Mn content is less than 0.01%, the above-mentioned effects cannot be fully achieved. Therefore, the Mn content is preferably 0.01% or more. The Mn content is more preferably 0.02% or more. On the other hand, if the Mn content exceeds 0.50%, secondary recrystallization does not occur and the magnetic properties of the steel deteriorate. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, the Mn content is preferably 0.50% or less. The Mn content is more preferably 0.20% or less, and even more preferably 0.10% or less.

[0028] N: 0.010% or less N (nitrogen) is an element that bonds with Al during the manufacturing process to form AlN, which functions as an inhibitor. However, if an excessive amount of inhibitor remains in the grain-oriented electrical steel sheet and the N content exceeds 0.010%, the magnetic properties deteriorate. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, the N content is preferably 0.010% or less. The N content is more preferably 0.008% or less. On the other hand, the lower limit of the N content is not particularly specified, but reducing it to less than 0.001% only increases the manufacturing cost. Therefore, the N content may be 0.001% or more.

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

[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 be reduced due to the remaining inhibitor. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, the S content is preferably 0.010% or less. The S content in the grain-oriented electrical steel sheet is preferably as low as possible. For example, less than 0.001%. However, reducing the S content in the 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.

[0031] The balance: Fe and impurities The chemical composition of the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment contains the above-mentioned elements, with the balance consisting of Fe and impurities. However, in order to improve magnetic properties, etc., Sn, Cu, Se, and Sb may be contained in place of a portion of Fe in the ranges shown below. Furthermore, even if other elements such as W, Nb, Ti, Ni, Co, V, Cr, and Mo are contained in a total of 1.0% or less, this does 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 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.

[0033] Sn: 0 to 0.50% Sn (tin) is an element that contributes to improving magnetic properties through controlling the primary recrystallization structure. To obtain the effect of improving magnetic properties, the Sn content is preferably 0.01% or more. The Sn content is more preferably 0.02% or more, and even more preferably 0.03% or more. On the other hand, if the Sn content exceeds 0.50%, secondary recrystallization becomes unstable and magnetic properties deteriorate. Therefore, the Sn content is preferably 0.50% or less. The Sn content is more preferably 0.30% or less, and even more preferably 0.10% or less.

[0034] Cu: 0 to 0.50% Cu (copper) is an element that contributes to increasing the Goss orientation occupancy rate in the secondary recrystallized structure. To achieve the above effect, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.02% or more, and even more preferably 0.03% or more. On the other hand, if the Cu content exceeds 0.50%, the steel sheet becomes embrittled during hot rolling. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, the Cu content is preferably 0.50% or less. The Cu content is more preferably 0.30% or less, and even more preferably 0.10% or less.

[0035] Se: 0 to 0.020% Se (selenium) is an element that has the effect of improving magnetic properties. When Se is contained, the Se content is preferably 0.001% or more to effectively exhibit the effect of improving magnetic properties. The Se content is more preferably 0.003% or more, and even more preferably 0.006% or more. On the other hand, if the Se content exceeds 0.020%, the coating adhesion 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.

[0036] Sb: 0 to 0.50% Sb (antimony) is an element that has the effect of improving magnetic properties. When Sb is contained, the Sb content is preferably 0.005% or more to effectively exhibit the effect of improving magnetic properties. The Sb content is more preferably 0.01% or more, and even more preferably 0.02% or more. On the other hand, if the Sb content exceeds 0.50%, the coating adhesion deteriorates significantly. 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.

[0037] 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.

[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 atomic emission spectroscopy. However, when an insulating coating is formed on the surface, it is removed before measurement. The removal can be achieved by immersing the sample in a highly concentrated alkaline solution (e.g., a 30% sodium hydroxide solution heated to 85°C) for 20 minutes or more. Peeling can be determined visually. For small samples, removal can also be achieved by surface grinding.

[0039] (Thickness) The thickness of the base steel sheet is not limited, but 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 a forsterite-based coating is not 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 fusing a first insulating coating and a second insulating coating by heat treatment.

[0041] The insulating coating contains silica particle aggregates. Whether or not the insulating coating contains silica particle aggregates can be determined by using a transmission electron microscope (TEM) to photograph a cross section of the insulating coating at 30,000x magnification, along and around the interface with a length of 10 μm or more. In this embodiment, silica particle aggregates refer to aggregates of primary silica particles that have aggregated due to intermolecular forces, electrostatic attraction, or the like. It is well known that silica forms aggregates, and in some cases, aggregates of primary particles may further aggregate to form secondary aggregates. In this embodiment, aggregates refer to aggregates of silica particles with a primary particle diameter of 5 to 30 nm. The silica particle aggregates contained in the insulating coating can be observed from the photographed image. This is because colloidal silica containing Si element in a phosphate matrix is ​​observed in the insulating coating. Furthermore, the difference from a phosphate-based matrix can be confirmed by elemental analysis. If the image resolution is low and clear images cannot be obtained, it can be clarified using an image processing device such as Luzex. SiO ２ Regions with a concentration of 50% or more are considered to be agglomerates of silica fine particles. Colloidal silica agglomerates are measured at three locations (i.e., a total of nine locations) within the TEM image obtained in this manner, and the average diameter of the long and short axes is determined as the average particle diameter of the agglomerates. Furthermore, if the agglomerates extend throughout the thickness of the insulating coating, the thickness of the insulating coating is used as the particle diameter of the agglomerates.

[0042] Specifically, the term "aggregation" as used herein refers to a state in which colloidal silica-derived silica particles with particle sizes of 5 to 30 nm gather together densely in the insulating coating to form primary aggregates, as shown in Figures 1 and 2. The size of the aggregates is defined as approximately 0.05 to 3.0 μm. Note that Figures 1 and 2 show elemental analysis results for Si in TEM images. The white dots in Figures 1 and 2 represent silica particles. The shape of the aggregates can be any shape; however, due to the principles of their formation, most are circular or elliptical in cross section, and in some cases, they may have irregularities or even a series of circles or ellipses. In cases where the shape is distorted, the circumscribed circle is used as the aggregate particle size. Furthermore, when the average particle size of the silica particle aggregates is 0.5 to 3.0 μm, elution can be improved while simultaneously improving coating tension. When the silica particle aggregates are amorphous, a decrease in coating tension can be suppressed.

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

[0044] The metal phosphate in the insulating coating is preferably an Fe-P-O or Fe-P-O-Si based metal phosphate. If the metal phosphate is an Fe-P-O or Fe-P-O-Si based metal phosphate, it is possible to suppress a decrease in coating adhesion after stress relief annealing. Examples of Fe-P-O based metal phosphates include Fe ２ P ２ O ７ , Fe ３ (P.O. ４ ) ２ , FePO ４ Examples of Fe-P-O-Si based metal phosphates include FeOP ２ O ５ SiO ２ Examples include:

[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 as follows. For flat areas, the cross section of the sample is observed with a scanning electron microscope, and the average thickness can be measured by measuring the thickness at five or more points. The position where the distribution of P element rapidly decreases from the insulating coating side is taken as the interface between the insulating coating and the base steel sheet.

[0047] Furthermore, by using a transmission electron microscope and an energy dispersive elemental analyzer, it is possible to identify the type of metal phosphate and its mass ratio. Measurements are performed for at least three fields of view, and the average value is calculated. The Si content in the insulating coating can also be measured by using a transmission electron microscope and an energy dispersive elemental analyzer.

[0048] Amount of phosphorus eluted from the coating In the grain-oriented electrical steel sheet according to this embodiment, the amount of phosphorus eluted from the coating is suppressed, so the amount of phosphorus eluted is 40 mg / m ２ The amount of phosphoric acid eluted can be obtained by boiling the insulating coating 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 coating.

[0049] <Manufacturing Method> The grain-oriented electrical steel sheet according to the present embodiment can achieve the effects as long as it has the above-described configuration, regardless of the manufacturing method, but can be preferably manufactured by a manufacturing method including, for example, the following steps: (i) a hot rolling step of heating a steel slab and hot rolling it to form a hot-rolled sheet, (ii) a hot-rolled sheet annealing step of annealing the hot-rolled sheet, (iii) a pickling step of pickling the hot-rolled sheet after the hot-rolled sheet annealing step, (iv) a cold rolling step of cold-rolling the hot-rolled sheet after the pickling step to form a steel sheet (cold-rolled sheet), (v) a decarburization annealing step of decarburization annealing the steel sheet, and (vi) a step of adding Al to the steel sheet after the decarburization annealing. ２ O ３(viii) a light pickling step of pickling the steel sheet after the annealing separator removal step with 0.1 to 5.0 mass% of an inorganic acid for 10 to 60 seconds; (ix) a water rinsing step of rinsing the steel sheet after the light pickling step with water and drying; and (x) a first insulating coating formation step of immersing the steel sheet after the water rinsing step in a treatment solution having a liquid temperature of 30 to 85°C and a metal phosphate concentration of 1.0 to 20.0 mass% for 5 to 150 seconds, rinsing the treatment solution with water and drying the steel sheet, (xi) a second insulating coating formation step of applying a coating liquid to the steel sheet after the first insulating coating formation step, the coating liquid containing a metal phosphate and colloidal silica, the content of colloidal silica having a particle size of 5 to 30 nm being 30 to 150 parts by mass per 100 parts by mass of the metal phosphate, and the concentration being 10.0 to 40.0% by mass, and drying the coating liquid, and then maintaining the steel sheet at a sheet temperature of 750 to 950°C for 10 to 120 seconds to form a second insulating coating; and (xii) a heat treatment step of heating the steel sheet after the second insulating coating formation step to a temperature range of 700 to 900°C in an atmosphere having a nitrogen content of 50 to 100% by volume and a hydrogen content of 0 to 50% by volume, and maintaining the temperature range for 30 to 240 minutes to fuse the first insulating coating and the second insulating coating together to form an insulating coating. The method for producing a grain-oriented electrical steel sheet according to this embodiment is characterized by the (vi) finish annealing step to the (xii) heat treatment step, and the (i) hot rolling step to the (v) decarburization annealing step are not particularly limited, and known conditions can be applied.

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

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

[0052] [Cold Rolling Step] In the cold rolling step, the steel sheet after the pickling step is cold rolled to obtain a cold-rolled sheet. The cold rolling may be a single cold rolling (a series of cold rolling steps without intermediate annealing) or may be multiple cold rolling steps with intermediate annealing between them, in which the cold rolling is interrupted and at least one or two or more intermediate annealing steps are performed before the final pass of the cold rolling step.

[0053] The cold rolling conditions may be in accordance with known methods. The cold rolling reduction of grain-oriented electrical steel sheet has a significant effect on its magnetic properties. The final reduction has a particularly large effect, and the final reduction can be set to 80 to 95%. The final reduction is the cumulative reduction of cold rolling, and in the case where intermediate annealing is performed, it is the cumulative reduction of cold rolling after final intermediate annealing.

[0054] When intermediate annealing is performed, for example, the steel sheet is held at a temperature of 800 to 1200°C for 5 to 180 seconds. The annealing atmosphere is not particularly limited, but it is preferable to perform the annealing in a non-oxidizing atmosphere such as nitrogen, argon, or hydrogen to prevent oxidation of the steel sheet. The annealing method may be so-called continuous annealing, batch annealing in a coil shape, or other methods. The number of times intermediate annealing is performed is preferably three or less, taking into account production costs.

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

[0056] The decarburization annealing conditions are not limited, but annealing is performed in a nitrogen-hydrogen mixed atmosphere for decarburization, with the oxygen potential increased by humidification. Furthermore, since it is necessary to form a primary recrystallized structure, the humidification temperature (dew point) is determined from the viewpoint of the annealing temperature required for recrystallization and the oxygen potential at which decarburization is possible at that annealing temperature. The annealing temperature is, for example, about 700 to 900°C, and since annealing is generally performed in a continuous annealing process, soaking is performed for about 60 seconds.

[0057] [Nitriding Process] Nitriding may be performed between the decarburization annealing process and the finish annealing process described below. In the nitriding process, for example, the decarburization annealed steel sheet is maintained at approximately 700 to 850°C in a nitriding atmosphere (an atmosphere containing hydrogen, nitrogen, and ammonia or other nitriding gases) to perform nitriding. When AlN is used as an inhibitor, it is preferable to set the nitrogen concentration of the steel sheet to 40 ppm 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.

[0058] [Finish Annealing Process] In the finish annealing process, Al is added to the steel sheet after the decarburization annealing process (or the nitriding process). ２ O ３In a conventional method for producing grain-oriented electrical steel sheets, an annealing separator containing 10 to 100 mass % of Al is applied to the steel sheet, which is then dried and then finish-annealed, forming 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 10 to 100 mass % of Al is applied to the steel sheet, which is then finished and annealed, so as not to form a forsterite-based coating. ２ O ３ An annealing separator containing

[0059] On the other hand, Al ２ O ３ The ratio of Al may be 100 mass %, but ２ O ３ From the viewpoint of preventing seizure of Al, in the method for producing a grain-oriented electrical steel sheet according to this embodiment, the annealing separator preferably contains MgO. Although the content of MgO may be 0%, in order to obtain the above effect, the content of MgO is preferably 5 mass% or more. When MgO is contained, the content of MgO is preferably 10 mass% or more. ２ O ３ The content of Al relative to the annealing separator is 90 mass % or less to ensure this. The content of MgO is preferably 50 mass % or less. ２ O ３ The total amount of MgO and MgO may be more than 50% by mass in terms of solid content.

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

[0061] [Annealing Separator Removal Step] In the annealing separator removal step, excess annealing separator is removed from the steel sheet after the finish annealing step. For example, excess annealing separator can be removed by washing with water.

[0062] [Light Pickling Step] In the light pickling step, the steel sheet after the annealing separator removal step is pickled with 0.1 to 5.0 mass % of an inorganic acid for 10 to 60 seconds. If the conditions for the light pickling are not favorable, excess annealing separator may remain on the steel sheet surface, increasing the surface roughness and reducing the space factor, while if the acid is too strong, the steel sheet surface may be etched, resulting in reduced magnetic properties.

[0063] [Water-Rinsing Step] In the water-rinsing step, the steel sheet after the light pickling step is rinsed with water and dried. This removes the pickling solution, making it possible to suppress rusting. The conditions for water-rinsing and drying are not limited.

[0064] [First insulating coating formation step] In the first insulating coating formation step, the steel sheet after the water-rinsing step is immersed for 5 to 150 seconds in a treatment solution having a liquid temperature of 30 to 85°C and a metal phosphate concentration of 1.0 to 20.0 mass %, and after the treatment solution is removed by rinsing with water, the steel sheet is dried, thereby forming a first insulating coating on the surface of the steel sheet.

[0065] If the temperature of the treatment solution is below 30°C, the amount of the first insulating coating formed is too small, resulting in localized poor adhesion of the insulating coating. On the other hand, if the temperature exceeds 85°C, the first insulating coating becomes too thick in some areas, ultimately increasing the surface roughness and reducing the space factor. Furthermore, if the concentration of the metal phosphate is less than 1.0% by mass, it takes too long to form the first insulating coating, which is disadvantageous in terms of cost. On the other hand, if the concentration of the metal phosphate exceeds 10.0% by mass, the first insulating coating is formed in some areas and becomes uneven in the end. Furthermore, if the immersion time is less than 5 seconds, the amount of the first insulating coating formed is too small, resulting in localized poor adhesion of the insulating coating. On the other hand, if the immersion time exceeds 150 seconds, it takes too long, resulting in cost disadvantage.

[0066] [Second insulating coating formation step] In the second insulating coating formation step, a coating liquid (insulating coating formation liquid) containing metal phosphate and colloidal silica, the content of colloidal silica having a particle size of 5 to 30 nm being 30 to 150 parts by mass per 100 parts by mass of the metal phosphate, and having a concentration of 10.0 to 40.0 mass % is applied to the steel sheet that has undergone the first insulating coating formation step, dried, and then held at a sheet temperature of 750 to 950°C for 10 to 120 seconds, thereby forming a second insulating coating on the first insulating coating.

[0067] If the sheet temperature is below 750°C, the tension will be low and the magnetic properties will be inferior. Therefore, it is preferable that the sheet temperature be 750°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 more likely to deform. In this case, the steel sheet may be distorted due to transportation, etc., resulting in inferior 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 elution will be inferior. Therefore, the holding time is set to 10 seconds or longer. On the other hand, if the holding time is more than 120 seconds, the coating adhesion will be reduced, and if an attempt is made to avoid the reduction in coating adhesion, productivity will be inferior. Therefore, it is preferable that the holding time be 120 seconds or shorter.

[0068] The coating liquid contains a metal phosphate and colloidal silica in such a manner that 30 to 150 parts by mass of colloidal silica having a particle size of 5 to 30 nm is contained per 100 parts by mass of the metal phosphate. The total of the metal phosphate and colloidal silica, calculated as solid content, is sufficient as long as it exceeds 50% by mass of the coating liquid. If the colloidal silica content is less than 30 parts by mass, the space factor and core loss may deteriorate. Therefore, the colloidal silica content is preferably 30 parts by mass or more. If the colloidal silica content exceeds 150 parts by mass, the adhesion, coating tension, elution, space factor, and core loss may deteriorate. Therefore, the colloidal silica content is preferably 150 parts by mass or less. The metal phosphate may be, for example, one or a mixture of two or more selected from aluminum phosphate, zinc phosphate, magnesium phosphate, nickel phosphate, copper phosphate, lithium phosphate, cobalt phosphate, etc.

[0069] The coating liquid preferably has a concentration of 10.0 to 40.0% by mass. If the concentration is less than 10.0% by mass, the elution property decreases. Therefore, the concentration is preferably 10.0% by mass or more. On the other hand, if the concentration exceeds 40.0% by mass, the elution property, space factor, and iron loss may deteriorate. Therefore, the concentration is preferably 40.0% by mass or less.

[0070] The coating solution may contain additional elements such as vanadium, tungsten, molybdenum, and zirconium. S-type or C-type colloidal silica can be used. S-type colloidal silica refers to an alkaline silica solution, while C-type colloidal silica refers to a silica solution in which the silica particle surface is aluminum-treated and the silica solution is alkaline to neutral. S-type 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. C-type 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 to be prepared.

[0071] [Magnetic Domain Refinement Step] In the magnetic domain refinement step, the surface of the insulating coating (the surface of the insulating coating provided on the grain-oriented electrical steel sheet) may be irradiated with energy rays to refine 180° magnetic domains. By performing magnetic domain refinement, it is possible to further reduce the iron loss of the grain-oriented electrical steel sheet.

[0072] The magnetic domain subdivision process may be carried out by any known method, such as forming linear or dot-like grooves extending in a direction intersecting the rolling direction at predetermined intervals along the rolling direction to narrow the width of the 180° magnetic domains (subdividing the 180° magnetic domains).

[0073] The grooves can be formed by a mechanical groove forming method using gears or the like, a chemical groove forming method in which grooves are formed by electrolytic etching, a thermal groove forming method using laser irradiation, etc. If the insulating coating is damaged by the formation of stress-strained portions or grooves, causing deterioration of properties such as insulation, the damage can be repaired by forming the insulating coating again.

[0074] [Heat Treatment Step] 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 is held at this temperature range for 30 to 240 minutes, thereby fusing the first insulating coating and the second insulating coating to form an insulating coating.

[0075] If the heat treatment temperature is less than 700°C, the effect of stress relief annealing is low, resulting in inferior magnetic properties. On the other hand, if the heat treatment temperature is higher than 900°C, the rigidity of the steel sheet decreases, making it more susceptible to deformation, resulting in a lower space factor, reduced coating adhesion, and poor leachability. Furthermore, if the holding time is less than 30 minutes, the effect of stress relief annealing is low, resulting in poor magnetic properties. On the other hand, if the holding time is higher than 240 minutes, the annealing time becomes too long, which is not only economically disadvantageous but also deteriorates adhesion or leachability. A hydrogen-nitrogen mixed gas is suitable as the atmosphere during heat treatment, and the hydrogen content is 50% by volume or less. If the hydrogen content exceeds 50% by volume, not only is the cost high, but the atmosphere may become too reducing, forming a silica layer on the surface and causing peeling of the insulating coating.

[0076] Next, the effects of one embodiment of the present disclosure will be explained in more detail using examples, but the conditions in the examples are examples adopted to confirm the feasibility and effects of the present disclosure, and the present disclosure is not limited to these examples. Various conditions may be adopted in the present disclosure as long as they do not deviate from the gist of the present disclosure and the object of the present disclosure is achieved.

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

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

[0079] After decarburization annealing, MgO: 48 mass% and Al ２ O ３:48% by mass, BiCl ３ After drying, the steel sheet was heated to 1200°C and held for 20 hours for finish annealing. After finish annealing, the steel sheet was washed with water to remove excess annealing separator, and it was found that no forsterite-based coating was formed on the steel sheet surface.

[0080] A first insulating coating was formed on this steel sheet using a treatment liquid containing a mixture of metal phosphate and additives shown in Table 1. Thereafter, a coating liquid containing metal phosphate and colloidal silica in the ratios shown in Table 2 was applied and dried to form a second insulating coating.

[0081] Thereafter, grooves 20 μm deep and 50 μm wide were formed at intervals of 6 mm on the surface of the steel sheet in a direction tilted by 80° from the rolling direction using the tooth profile. Thereafter, heat treatment was carried out under the conditions shown in Table 2 to obtain grain-oriented electrical steel sheets.

[0082] The grain-oriented electrical steel sheets thus obtained were subjected to the above-described methods to observe the agglomerates of silica fine particles derived from colloidal silica in the insulating coating, and to confirm and identify the presence or absence of metal phosphate in the insulating coating.

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

[0084] The resulting grain-oriented electrical steel sheets were also measured for coating adhesion, coating tension, corrosion resistance, elution, space factor, and iron loss in the following manner.

[0085] Coating Adhesion A sample of 30 mm width and 300 mm length was taken from the grain-oriented electrical steel sheet, and Cellotape (registered trademark) was attached to the inside of the bent portion. After bending the sample using a 10 mm diameter cylinder, the Cellotape was peeled off from the rear half and attached to a piece of white drawing paper in a bending adhesion test to evaluate the adhesion. A score of A or higher was judged to be excellent in coating adhesion and was judged to pass. On the other hand, a score of B or lower was judged to be poor in coating adhesion and was judged to fail. AA: No peeling A: Almost no peeling B: Peeling of several mm observed C: Peeling of 1 / 3 to 1 / 2 observed D: Peeling over almost the entire surface

[0086] Coating tension was calculated by back-calculating from the state of curvature when one side of the insulating coating was peeled off. If the obtained coating tension was 4.0 MPa or more, it was judged to have high coating tension and pass. 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 fail.

[0087] Corrosion resistance was evaluated by a salt spray test in accordance with JIS Z 2371:2015. A 5% by volume NaCl aqueous solution was allowed to drip onto a sample taken from the grain-oriented electrical steel sheet in a 35°C atmosphere for 7 hours. The rust area of ​​the sample was evaluated according to the following criteria. A score of 5 or higher was considered to have excellent corrosion resistance and was judged to be acceptable. On the other hand, a score of 4 or lower was considered to have poor corrosion resistance and was judged to be unacceptable. 10: No rust occurred 9: Very little rust occurred (area ratio 0.1% or less) 8: Area ratio of rust occurred was more than 0.1% but not more than 0.25% 7: Area ratio of rust occurred was more than 0.25% but not more than 0.50% 6: Area ratio of rust occurred was more than 0.50% but not more than 1.0% 5: Area ratio of rust occurred was more than 1.0% but not more than 2.5% 4: Area ratio of rust occurred was more than 2.5% but not more than 5.0% 3: Area ratio of rust occurred was more than 5.0% but not more than 10.0% 2: Area ratio of rust occurred was more than 10.0% but not more than 25.0% 1: Area ratio of rust occurred was more than 25.0% but not more than 50.0%

[0088] Leachability Leachability was evaluated based on the amount of phosphoric acid eluted from the sample. A sample taken from the grain-oriented electrical steel sheet was boiled in boiling pure water for 10 minutes, and the amount of phosphoric acid eluted in the pure water was measured. The amount of phosphoric acid eluted (mg / m) was calculated by dividing the measured amount of phosphoric acid by the area of ​​the insulating coating of the boiled grain-oriented electrical steel sheet. ２ 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, diluting the cooled solution with pure water, and measuring the phosphoric acid concentration by ICP-AES. ２ On the other hand, when the amount of elution was less than 40 mg / m, it was judged to be excellent in elution property and was judged to be acceptable. ２ If the value was more than this, the sample was judged to have poor dissolution properties and was judged to have failed.

[0089] Space Factor The space factor was measured using a method in accordance with JIS C 2550-5:2020. Thirty samples were used, each 30 mm wide and 320 mm long. After measuring the total mass of the sample, the space factor was calculated by measuring the distance between the upper and lower backing plates sandwiching the laminate under a pressure of 1 MPa. When the space factor was 97.0% or higher, it was determined that the sample had a high space factor when used as a core, and was judged to have passed. On the other hand, when the space factor was less than 97.0%, it was determined that the sample did not have a high space factor when used as a core, and was judged to have failed.

[0090] Iron loss W17 / 50 (iron loss per mass at a magnetic flux density amplitude of 1.7 T and 50 Hz) was measured in accordance with the Single Sheet Magnetic Property Measurement Method (Single Sheet Tester: SST) of JIS C2556:2015. When the iron loss W17 / 50 was 0.75 or less, the sample was judged to have excellent iron loss and was judged to have passed. On the other hand, when the iron loss W17 / 50 was more than 0.75, the sample was judged not to have excellent iron loss and was judged to have failed.

[0091]

[0092]

[0093]

[0094] Looking at Table 3, it can be seen that the grain-oriented electrical steel sheets according to the examples of the present invention have coating tension, corrosion resistance, phosphorus elution from the coating, space factor when used as a core, and iron loss that are equal to or better than those of conventional steel sheets, and also have excellent coating adhesion even after stress relief annealing.

[0095] According to the above aspect of the present disclosure, it is possible to provide a grain-oriented electrical steel sheet that exhibits excellent coating adhesion even after stress relief annealing. Also, according to another aspect of the present disclosure, it is possible to provide a method for forming an insulating coating that can produce the above grain-oriented electrical steel sheet.

Claims

1. A grain-oriented electrical steel sheet having a base steel sheet and an insulating coating containing a metal phosphate formed on the surface of the base steel sheet, wherein the insulating coating contains an aggregate of silica fine particles, and the amount of phosphoric acid dissolved in the pure water is measured by boiling the steel sheet for 10 minutes, and the amount of phosphoric acid dissolved is calculated by dividing the measured amount of phosphoric acid by the area of the boiled insulating coating, and the amount of phosphoric acid dissolved is 40 mg / m ２ Grain-oriented electrical steel sheet, characterized in that 2. The grain-oriented electrical steel sheet according to claim 1, characterized in that the average particle size of the aggregates of silica fine particles is 0.5 to 3.0 μm.

3. The grain-oriented electrical steel sheet according to claim 1 or 2, wherein the aggregates of silica fine particles are amorphous.

4. Steel plate, Al ２ O ３ a finish annealing step of applying an annealing separator containing 10 to 100 mass % of the above-mentioned compound to the steel sheet, drying the steel sheet, and then finish annealing the steel sheet; an annealing separator removing step of removing excess annealing separator from the steel sheet after the finish annealing step; a light pickling step of pickling the steel sheet after the annealing separator removing step with 0.1 to 5.0 mass % of inorganic acid for 10 to 60 seconds; a water rinsing step of rinsing the steel sheet after the light pickling step with water and drying the steel sheet; and a first insulating coating forming step of immersing the steel sheet after the water rinsing step in a treatment solution having a liquid temperature of 30 to 85°C and a metal phosphate concentration of 1.0 to 20.0 mass %, for 5 to 150 seconds, removing the treatment solution with water, and then drying the steel sheet. a second insulating coating formation step of applying to the steel sheet after the first insulating coating formation step a coating liquid containing a metal phosphate and colloidal silica, the content of colloidal silica having a particle size of 5 to 30 nm being 30 to 150 parts by mass per 100 parts by mass of the metal phosphate, and the concentration being 10.0 to 40.0 mass%, drying the steel sheet, and then maintaining the steel sheet at a sheet temperature of 750 to 950°C for 10 to 120 seconds; and a heat treatment step of heating the steel sheet after the second insulating coating formation step to a temperature range of 700 to 900°C in an atmosphere having a nitrogen content of 50 to 100% by volume and a hydrogen content of 0 to 50% by volume, and maintaining the steel sheet at the temperature range for 30 to 240 minutes.

5. The method for forming an insulating coating according to claim 4, wherein the annealing separator further contains one or both of MgO: 5 to 90 mass % and chloride: 0.5 to 10.0 mass %.

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