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

Description

/ 48 GRAIN-ORIENTED ELECTRICAL STEEL SHEET AND METHOD FOR FORMING INSULATING COATING TECHNICAL FIELD

[001] The present invention relates to a grain-oriented electrical steel sheet and a method for forming an insulating coating.

[002] Priority is claimed in Japanese Patent Application No. 2023-064837, filed on April 12, 2023, the contents of which are incorporated herein by reference. FUNDAMENTALS OF THE TECHNIQUE

[003] A grain-oriented electrical steel sheet is mainly used in transformers. The transformer is continuously magnetized for a long period of time, from installation to disposal, and continues to generate energy loss. Therefore, the energy loss when the transformer is magnetized by alternating current, i.e., the iron loss, is the main index for determining the transformer's performance.

[004] To reduce iron loss from grain-oriented electrical steel sheet, many techniques have been developed so far from the point of view of (a) increasing development in the orientation {110} <001> (Goss orientation), (b) increasing the amount of a solid solution element, such as Si, to increase the electrical resistance of the steel sheet, or (c) reducing the sheet thickness of the electrical steel sheet.

[005] In addition, applying tension to the steel plate is effective in reducing iron loss. It is an effective means of reducing iron loss to form a coating made of a material with a coefficient of thermal expansion lower than that of the steel plate on a plate surface at high temperature. A forsterite-based coating (inorganic coating) with excellent coating adhesion, generated by a reaction between an oxide on the surface of a plate and an annealing separator in a step of Petition 870250091652, dated 07 / 10 / 2025, page 11 / 63 / 48 final annealing of an electrical steel plate, is a coating capable of applying tension to the steel plate.

[006] For example, a method for forming an insulating coating by baking a coating liquid containing mainly colloidal silica and a phosphate onto the surface of a sheet, described in Patent Document 1, is an effective method for reducing iron loss because the method has a large stress application effect on the steel sheet. Therefore, a general method for manufacturing a grain-oriented electrical steel sheet is to leave the forsterite-based coating generated in the final annealing step and form an insulating coating containing mainly a phosphate onto the forsterite-based coating.

[007] However, in recent years, there has been a growing demand for miniaturization and high performance of a transformer, and to miniaturize the transformer, it is necessary that a grain-oriented electrical steel sheet have excellent iron loss in a high magnetic field, so that the iron loss is favorable even when the magnetic flux density is high. At the same time, in recent years, it has been clarified that forsterite-based coating hinders the movement of a domain wall and negatively affects iron loss. In a grain-oriented electrical steel sheet, a magnetic domain changes by the movement of a domain wall under an alternating magnetic field.The smooth and rapid movement of the domain wall is effective in reducing iron loss, but the forsterite-based coating itself is a non-magnetic body and has an irregular structure at the interface between the steel plate and the coating, and this irregular structure hinders the movement of the domain wall. Therefore, forsterite-based coating is considered to negatively affect iron loss.

[008] Therefore, as a means of improving iron loss in high magnetic fields, a coating removal method was studied. Petition 870250091652, dated 07 / 10 / 2025, page 12 / 63 / 48 inorganic using mechanical means, such as polishing, or chemical means, such as pickling. In addition, a technique was studied for manufacturing a grain-oriented electrical steel sheet without inorganic coating, preventing the generation of an inorganic coating in the final high-temperature annealing, and a technique for bringing the surface of a sheet to a mirror-like surface state (in other words, a technique for magnetically smoothing the surface of a sheet).

[009] As a technique for avoiding the generation of an inorganic coating, for example, Patent Document 2 describes a technique in which a product formed on the surface is removed by pickling after normal final annealing, and then a sheet surface is brought to a mirror-like surface state by chemical polishing or electrolytic polishing. It has been found that a better iron loss improvement effect can be obtained by forming a voltage-applied insulating coating on the surface of a grain-oriented electrical steel sheet without an inorganic coating, obtained by a known method. Furthermore, the voltage-applied insulating coating can impart various characteristics such as corrosion resistance, heat resistance, and slip resistance, in addition to improving iron loss.

[0010] However, the inorganic coating has the effect of exhibiting insulating properties and the effect of an intermediate layer to ensure adhesion when a tension coating (tension-applied insulating coating) is formed. That is, since the inorganic coating is formed in a state of deep penetration into the steel sheet, the inorganic coating has excellent adhesion to the steel sheet, which is metallic. Therefore, when a tension-applied coating (tension coating) containing colloidal silica, a phosphate, or similar as a main component is formed on the surface of the inorganic coating, the coating adhesion is excellent. Petition 870250091652, dated 07 / 10 / 2025, page 13 / 63 / 48

[0011] On the other hand, since it is generally difficult to bond metal and oxide, it is difficult to ensure sufficient adhesion between the tension coating and the surface of a steel sheet when the inorganic coating is not present.

[0012] Therefore, in a case where a tension coating is formed on a grain-oriented electrical steel sheet without inorganic coating, it was studied to form a layer that plays a role as an intermediate layer of the inorganic coating.

[0013] For example, Patent Document 3 describes a technique in which a grain-oriented electrical steel sheet, without inorganic coating, is annealed in a weakly reducing atmosphere, and the silicon inevitably contained in a silicon steel sheet is selectively thermally oxidized to form a SiO2 layer on the surface of the sheet, and then a voltage-appliing type insulating coating is formed. Patent Document 4 describes a technique in which a grain-oriented electrical steel sheet without inorganic coating is subjected to an anodic electrolytic treatment in an aqueous silicate solution to form a SiO2 layer on the surface of a sheet, and then a voltage-appliing type insulating coating is formed.

[0014] However, in the technique described in Patent Document 3, it is necessary to prepare an annealing plant capable of controlling an atmosphere to perform annealing in a weakly reducing atmosphere, and there is a problem with the cost of the treatment. In the technique described in Patent Document 4, it is necessary to prepare a new electrolysis treatment plant to obtain a SiO2 layer that maintains sufficient adhesion to a voltage-applied type insulating coating on a sheet surface, performing an anodic electrolytic treatment in an aqueous silicate solution, and there is a problem with the cost of the treatment.

[0015] On the other hand, Patent Document 5 describes a grain-oriented electrical steel sheet, including: a base steel sheet; and a Petition 870250091652, dated 07 / 10 / 2025, page 14 / 63 / 48 insulating coating formed on the surface of the base steel sheet, wherein the insulating coating includes: an intermediate layer that is formed on the side of the base steel sheet and contains a crystalline metallic phosphate; and a tension coating layer formed on the side surface of the insulating coating. In grain-oriented electrical steel sheet, the intermediate layer may be formed by chemical conversion treatment.

[0016] Furthermore, Patent Document 6 describes a method for manufacturing a one-way silicon steel sheet in which a secondary recrystallised one-way silicon steel sheet is formed with a coating containing mainly zinc phosphate in an amount of 0.1 g / m2 or more and 10 g / m2 or less per steel sheet surface before a voltage-transmitting insulating coating is formed. List of Citations Patent Documents

[0017] Patent Document 1: Japanese Patent Application No. Examined, First Publication No. S48-039338

[0018] Patent Document 2: Unexamined Japanese Patent Application, First Publication No. S49-96920

[0019] Patent Document 3: Unexamined Japanese Patent Application, First Publication No. H06-184762

[0020] Patent Document 4: Japanese Patent Application No. Examined, First Publication No. H11-209891

[0021] Patent Document 5: International Publication PCT No. WO 2022 / 215710

[0022] Patent Document 6: Japanese Patent Application No. Examined, First Publication No. 2005-139481 SUMMARY OF THE INVENTION Technical Problem

[0023] However, the present inventors studied techniques Petition 870250091652, dated 07 / 10 / 2025, page 15 / 63 / 48 conventional, as described in Patent Documents 5 and 6, and found that when a grain-oriented electrical steel sheet obtained by a conventional technique is provided with a coating containing mainly a metallic phosphate (intermediate layer) to improve adhesion, the magnetic characteristics may deteriorate. At this point, the present inventors studied further and found that the magnetic characteristics deteriorate because the chemical conversion treatment to form an intermediate layer makes the precipitated metallic phosphate crystal thicker, and many voids are formed in the intermediate layer after a tension coating is applied and baked.It has been found that when many voids are formed in the intermediate layer, the actual transformer manufactured in this way may have a reduced space factor, so that the magnetic flux density per unit volume decreases and the iron loss of the transformer increases (deteriorates).

[0024] Therefore, an objective of the present invention is to provide a grain-oriented electrical steel sheet that is excellent in adhesion with a coating of voltage and magnetic characteristics, and does not reduce the space factor of a transformer (core), and a method for forming an insulating coating for it. Solution to the Problem

[0025] The present inventors have found that: when a layer containing a metallic phosphate is formed as an intermediate layer that improves the adhesion between the base steel sheet and the tension coating layer, the chemical treatment solution contains a specific oxidizing agent in a predetermined quantity; and thus, the intermediate layer obtained can have a reduced porosity, so that a grain-oriented electrical steel sheet that is excellent in adhesion with a tension coating and magnetic characteristics and does not reduce the space factor of a transformer (core) can be obtained. Petition 870250091652, dated 07 / 10 / 2025, page 16 / 63 / 48

[0026] The present invention was made in view of the above verifications. The essence of an embodiment according to the present invention is as follows.

[0027] [1] In one embodiment of the present invention, a grain-oriented electrical steel sheet includes: a base steel sheet; and an insulating coating formed on a surface of the base steel sheet, wherein the insulating coating includes: an intermediate layer that is formed on one side of the base steel sheet and contains a crystalline metallic phosphate; and a tension coating layer formed on one side of the insulating coating surface, and the intermediate layer has a porosity of less than 40%.

[0028] [2] In the grain-oriented electrical steel sheet according to [1], in the intermediate layer, the crystalline metallic phosphate may include at least one of the group consisting of manganese phosphate, ferromanganese phosphate, zinc phosphate and calcium zinc phosphate.

[0029] [3] In the grain-oriented electrical steel sheet according to [1] or [2], in the intermediate layer, the crystalline metallic phosphate can have a plate shape, a particle shape or a columnar shape.

[0030] [4] In the grain-oriented electrical steel sheet according to [1] or [2], in the intermediate layer, the crystalline metallic phosphate can have an average crystal grain size of 0.1 to 10.0 μm.

[0031] [5] In one embodiment of the present invention, a method for forming an insulating coating, which is a method for forming the insulating coating included in the grain-oriented electrical steel sheet according to [1], includes: a final annealing step involving the application of an annealing separator containing Al2O3 in an amount of 10 to 100% by mass. Petition 870250091652, dated 07 / 10 / 2025, page 17 / 63 / 48 regarding a steel plate, drying of the steel plate and then final annealing of the steel plate; A step to remove the annealing separator from the steel sheet after the final annealing step; a pickling step of the steel sheet after the removal step of the annealing separator with 0.1 to 5.0% by mass of an inorganic acid selected from the group consisting of sulfuric acid, chloric acid, nitric acid and phosphoric acid for 1 to 20 seconds; a step of immersing the steel sheet after the pickling step in a treatment liquid containing a metallic phosphate and an oxidizing agent for 2 to 60 seconds; A drying step involves pulling the steel sheet after the treatment liquid immersion step, removing excess treatment liquid, and then drying the steel sheet; and a tension coating layer formation step involves applying a coating liquid containing a metallic phosphate and colloidal silica, the metallic phosphate and colloidal silica contained in a total concentration of 10 to 40% by mass in terms of solid content, to the steel sheet after the drying step, drying the steel sheet and then maintaining the steel sheet at a sheet temperature of 750 to 950°C for 10 to 120 seconds, the treatment liquid contains: a metallic phosphate in a mixing quantity of 0.1 to 30.0 g / L; an oxidizing agent in a mixing quantity of 0.1 to 20.0 g / L;and a metallic ion at a concentration of 0.5 to 10.0 g / L, and the oxidizing agent is at least one from the group consisting of a nitrate, a nitrite, a chlorate, a chlorite, a bromate, a perborate, and a hydrogen peroxide solution.

[0032] [6] In the method for forming an insulating coating according to Petition 870250091652, dated 07 / 10 / 2025, page 18 / 63 / 48 with [5], in the immersion stage, the steel sheet after the pickling stage can be immersed in the treatment liquid for 2 to 60 seconds. Advantageous Effects of the Invention

[0033] According to one embodiment of the present invention, it is possible to provide a grain-oriented electrical steel sheet that is excellent in adhesion with a voltage coating and magnetic characteristics, and does not reduce the space factor of a transformer (core), and a method for forming an insulating coating for this purpose. BRIEF DESCRIPTION OF THE DRAWING

[0034] [FIG. 1] An example of a cross-sectional view of a grain-oriented electrical steel sheet according to the embodiment.

[0035] [FIG 2] A schematic view to illustrate how to determine porosity according to the modality. DESCRIPTION OF THE MODALITIES

[0036] The grain-oriented electrical steel sheet according to an embodiment of the present invention (grain-oriented electrical steel sheet according to the embodiment) and a method for manufacturing a grain-oriented electrical steel sheet according to the embodiment will be described, the method including a method for forming an insulating coating embedded in the grain-oriented electrical steel sheet according to the embodiment.

[0037] First, the grain-oriented electrical steel sheet will be described according to the modality.

[0038] As illustrated in FIG. 1, a grain-oriented electrical steel sheet 100 according to the embodiment has a base steel sheet 1 and an insulating coating 2 formed on the surface of the base steel sheet 1.

[0039] The grain-oriented electrical steel sheet 100 according to the embodiment does not have substantially any forsterite-based coating on the surface of the base steel sheet 1. That is, in the embodiment, a forsterite-based coating is not intentionally formed on Petition 870250091652, dated 07 / 10 / 2025, page 19 / 63 / 48 surface of the base steel sheet 1. However, when the amount of forsterite-based coating is 1 g / m2 or less, its presence is permitted (in this case, in a part between the base steel sheet 1 and the insulating coating 2). That is, the oriented grain electrical steel sheet 100 according to the embodiment may have a forsterite-based coating in an amount of 0 to 1 g / m2 on the surface of the base steel sheet 1.

[0040] The insulating coating 2 includes: a tension coating layer 22 formed on the surface side of the insulating coating 2 (i.e., the surface side of the grain-oriented electrical steel sheet 100); and an intermediate layer 21 that is formed on the side of the base steel sheet 1 and contains a crystalline metallic phosphate.

[0041] The configurations of each 100 grain-oriented electrical steel sheet will be described below. <Chapa de Aço Base> (Chemical composition)

[0042] The grain-oriented electrical steel sheet 100 according to the embodiment has a significant characteristic in the structure of the insulating coating 2 formed on the surface of the base steel sheet 1. The chemical composition of the base steel sheet 1 included in the grain-oriented electrical steel sheet 100 is not limited. However, to obtain characteristics generally required for a grain-oriented electrical steel sheet, the base steel sheet 1 preferably contains the following components as its chemical composition. In the embodiment, % referring to the chemical composition is % by mass, unless otherwise specified. C: 0.010% or less

[0043] C (carbon) is an effective element in controlling the microstructure of steel sheets in stages prior to the completion of the decarburization annealing stage in the manufacturing process. However, Petition 870250091652, dated 07 / 10 / 2025, page 20 / 63 / 48: When the C content exceeds 0.010%, the magnetic characteristics of the grain-oriented electrical steel sheet, which is a product sheet, are deteriorated. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to the embodiment, the C content is preferably 0.010% or less. The C content is more preferably 0.005% or less. The C content is preferably as low as possible, but even when the C content is reduced to less than 0.0001%, the microstructure control effect is saturated and the manufacturing cost is only increased. Therefore, the C content can be 0.0001% or more. Yes: 2.50 to 4.00%

[0044] Si (silicon) is an element that increases the electrical resistance of grain-oriented electrical steel sheet and improves iron loss characteristics. When the Si content is less than 2.50%, it is not possible to obtain a sufficient eddy current loss reduction effect. 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.

[0045] On the other hand, when the Si content exceeds 4.00%, the grain-oriented electrical steel sheet becomes brittle and the passability is significantly deteriorated. In addition, the workability of the grain-oriented electrical steel sheet is deteriorated, and the steel sheet may fracture 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. Mn: 0.01 to 0.50%

[0046] Mn (manganese) is an element that binds to S in the manufacturing process to form MnS. This precipitate acts as an inhibitor (inhibits normal grain growth) and causes secondary recrystallization in steel. Manganese is also an element that improves workability. Petition 870250091652, dated 07 / 10 / 2025, page 21 / 63 / 48 hot melt steel. When the Mn content is less than 0.01%, the above effect cannot be sufficiently obtained. Therefore, the Mn content is preferably 0.01% or more. The Mn content is preferably 0.02% or more.

[0047] On the other hand, when the Mn content exceeds 0.50%, secondary recrystallization does not occur and the magnetic characteristics of the steel are deteriorated. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to the 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. N: 0.010% or less

[0048] N (nitrogen) is an element that binds to Al in the manufacturing process to form AlN, which acts as an inhibitor. However, when the N content is greater than 0.010%, the inhibitor remains excessively in the grain-oriented electrical steel sheet, and the magnetic characteristics are deteriorated. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to the embodiment, the N content is preferably 0.010% or less. The N content is more preferably 0.008% or less.

[0049] On the other hand, the lower limit of the N content is not particularly restricted, but even when the N content is reduced to less than 0.001%, the manufacturing cost only increases. Therefore, the N content can be 0.001% or more. Al sol.: 0.020% or less

[0050] Al sol. (acid-soluble aluminum) is an element that binds to N in the manufacturing process of grain-oriented electrical steel sheet to form AlN, which acts as an inhibitor. However, when the Al sol. content in the base steel sheet exceeds 0.020%, the inhibitor remains excessively in the base steel sheet, deteriorating the magnetic characteristics. Therefore, in the base steel sheet of grain-oriented electrical steel sheet Petition 870250091652, dated 07 / 10 / 2025, page 22 / 63 / 48, guided by the modality, the content of Al sol. is preferably 0.020% or less. The content of Al sol. is more preferably 0.010% or less and even more preferably less than 0.001%. The lower limit of the Al sol. content is not particularly restricted, but even when the content is reduced to less than 0.0001%, the manufacturing cost only increases. Therefore, the Al sol. content can be 0.0001% or more. S: 0.010% or less

[0051] Sulfur (S) is an element that binds to Mn in the manufacturing process to form MnS, which acts as an inhibitor. However, when the S content exceeds 0.010%, the remaining inhibitor deteriorates the magnetic characteristics. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to the 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, the S content is less than 0.001%. However, even when the S content is reduced to less than 0.0001% in the grain-oriented electrical steel sheet, the manufacturing cost only increases. Therefore, the S content can be 0.0001% or more in the grain-oriented electrical steel sheet. Remaining: Iron and impurities

[0052] The chemical composition of the base steel sheet of the grain-oriented electrical steel sheet according to the embodiment may contain the elements described above, with the remainder being Fe and impurities. However, the base steel sheet may additionally contain Sn, Cu, Se, and Sb in the following ranges for the purpose of improving magnetic and similar characteristics. Furthermore, for example, even when the base steel sheet contains one or more of W, Nb, Ti, Ni, Co, V, Cr, and Mo in a total amount of 1.0% or less as elements other than these elements, the effect of the grain-oriented electrical steel sheet according to the embodiment is not impaired. Petition 870250091652, dated 07 / 10 / 2025, page 23 / 63 / 48

[0053] Here, impurities are contaminated by ore or scrap as raw material, or by a manufacturing environment or similar when the base steel sheet is industrially manufactured. Impurities mean elements that may be included in such a content that the action of the grain-oriented electrical steel sheet according to the modality is not adversely affected. Sn: 0 to 0.50%

[0054] Tin (Sn) is an element that contributes to the improvement of magnetic characteristics through the control of the primary crystallization structure. To obtain an effect of improving magnetic characteristics, 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.

[0055] On the other hand, when the Sn content exceeds 0.50%, secondary recrystallization is unstable and the magnetic characteristics are deteriorated. 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. Cu: 0 to 0.50%

[0056] Copper (Cu) is an element that contributes to increasing the occupation of the Goss orientation in a secondary recrystallization structure. To obtain 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.

[0057] On the other hand, when 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 the 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. Petition 870250091652, dated 07 / 10 / 2025, p. 24 / 63 / 48 less. If: 0 to 0.020%

[0058] Selenium is an element that has the effect of improving magnetic characteristics. When Se is included, the Se content is preferably 0.001% or more, so that Se favorably exhibits the effect of improving magnetic characteristics. The Se content is more preferably 0.003% or more and even more preferably 0.006% or more.

[0059] On the other hand, when the Se content exceeds 0.020%, the coating adhesion is deteriorated. 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. Sb: 0 to 0.50%

[0060] Sb (antimony) is an element that has the effect of improving magnetic characteristics. When Sb is included, the Sb content is preferably 0.005% or more, so that the Sb favorably exhibits the effect of improving magnetic characteristics. The Sb content is more preferably 0.01% or more and even more preferably 0.02% or more.

[0061] On the other hand, when the Sb content exceeds 0.50%, the coating adhesion is significantly deteriorated. 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.

[0062] As described above, for example, the chemical composition of the base steel sheet of the grain-oriented electrical steel sheet according to the embodiment contains the elements described above, with the remainder being Fe and impurities.

[0063] The chemical composition of the base steel sheet of the grain-oriented electrical steel sheet according to the modality can be measured Petition 870250091652, dated 07 / 10 / 2025, page 25 / 63 / 48 using a known ICP emission spectrometry. Note that, at the time of measurement, in a case where an insulating coating is formed on the surface, the measurement is performed after the insulating coating has been peeled off. As a peeling method, it is possible to remove the insulating coating by immersing the steel sheet in a high-concentration alkaline solution (e.g., a 30% sodium hydroxide solution heated to 85 °C) for 20 minutes or more. It is possible to visually determine whether or not they have been peeled off. In the case of a small sample, the insulating coating can be peeled off by grinding the surface.<Revestimento Isolante>

[0064] In the grain-oriented electrical steel sheet 100 according to the embodiment, the insulating coating 2 is formed on the surface of the base steel sheet 1.

[0065] The insulating sheath 2 has a structure in which the intermediate layer 21 and the tension sheath layer 22 are rolled in this order from the side of the base steel sheet 1. (Intermediate layer)

[0066] As described above, in general, a grain-oriented electrical steel sheet has a forsterite-based coating generated in a final annealing step and an insulating coating (voltage-insulating coating) formed over it. However, in recent years, it has been clarified that the forsterite-based coating hinders the movement of a domain wall and negatively affects iron loss, and therefore, to further improve the magnetic characteristics, a grain-oriented electrical steel sheet without a forsterite-based coating was studied. However, when the forsterite-based coating is not present, it is difficult to ensure sufficient adhesion between the voltage-insulating coating and the surface of the base steel sheet.

[0067] In the grain-oriented electrical steel sheet 100 according to Petition 870250091652, dated 07 / 10 / 2025, page 26 / 63 / 48, in this modality, the intermediate layer 21 containing a crystalline metallic phosphate is formed between the base steel sheet 1 and the tension coating layer 22 to improve the adhesion between the base steel sheet 1 and the tension coating layer 22 by means of the intermediate layer 21. This occurs because when the intermediate layer 21 contains a crystalline metallic phosphate, the tension coating formed over it (after being formed, it becomes the tension coating layer 22) also contains a crystalline metallic phosphate and therefore has a high affinity for it, thus exhibiting excellent adhesion between the intermediate layer and the tension coating layer.Furthermore, as described later, when the intermediate layer 1 is formed by immersion in a treatment liquid containing a metallic phosphate, the intermediate layer 1 can be formed on the surface of the base steel sheet 1 using a chemical reaction, and adhesion between the intermediate layer 21 and the base steel sheet 1 can also be ensured.

[0068] When the intermediate layer 21 does not contain a crystalline metal phosphate, the above effect cannot be obtained. The percentage of crystalline metal phosphate in the intermediate layer 21 is preferably 80% by mass or more, and may be 100% by mass. The metal phosphate is preferably one of the following: manganese phosphate, ferromanganese phosphate, zinc phosphate, and calcium zinc phosphate, from the point of view of adhesion.

[0069] The intermediate layer 21 may contain, as remnants of the metallic phosphate, an oxide or an element diffused from the base steel sheet 1, such as Fe or Si.

[0070] Here, the present inventors studied the adhesion and magnetic characteristics of the intermediate layer 21 using a metallic phosphate and found that, when an intermediate layer containing mainly a metallic phosphate is provided to try to improve the adhesion between the base metal and the insulating coating, the grain electrical steel sheet Petition 870250091652, dated 07 / 10 / 2025, page 27 / 63 / 48 oriented may have deteriorated magnetic characteristics. At this point, the present inventors studied further and found that the magnetic characteristics deteriorate because the chemical conversion treatment to form an intermediate layer makes the precipitated metal phosphate crystal thicker, and many voids are formed in the intermediate layer after a tension coating is applied and baked.

[0071] It has been verified that when many voids are formed in the intermediate layer, the actual transformer manufactured in this way may have a reduced space factor, so that the magnetic flux density per unit volume decreases and the iron loss of the transformer increases (deteriorates).

[0072] Therefore, when the grain-oriented electrical steel sheet according to the embodiment is manufactured, the treatment liquid to form an intermediate layer contains a specific oxidizing agent in a predetermined quantity to suppress the thickening of the crystalline metal phosphate crystal in the intermediate layer 21. As a result, it is possible to obtain an intermediate layer 21 that is suppressed in the thickening of the crystalline metal phosphate crystal and has a reduced porosity.

[0073] Intermediate layer 21 has a porosity of less than 40%. When intermediate layer 21 has increased porosity, the adhesion between the tension coating layer formed in intermediate layer 21 and the intermediate layer 21 (adhesion with the base steel sheet 1 through intermediate layer 21) deteriorates. Furthermore, when intermediate layer 21 has increased porosity, the space factor can be reduced, so the magnetic flux density per unit volume decreases and the transformer iron loss increases. Additionally, when the porosity exceeds 40%, the space factor decreases and the effect of the tension coating is reduced, so the iron loss becomes... Petition 870250091652, dated 07 / 10 / 2025, page 28 / 63 / 48 lower. Therefore, the intermediate layer 21 preferably has as low a porosity as possible, and the porosity can be 0%. However, it is practically impossible for the intermediate layer 21 to have a porosity of 0%. Therefore, the porosity can be greater than 0%.

[0074] Here, the mechanism by which voids are generated in intermediate layer 21 will be described below.

[0075] When the chemical treatment solution for intermediate layer formation 21 comes into contact with the surface of the steel plate, the iron ionizes and melts from the surface of the steel plate, and hydrogen gas is generated. The hydrogen gas is in a very fine gaseous state. Therefore, the generated hydrogen gas adheres to the surface of the steel plate to suppress contact between the chemical treatment solution and the steel plate; it forms a metallic phosphate crystal that develops in a specific direction that is easy to grow; and it is incorporated into the formed metallic phosphate to inhibit the densification of the space layer, thus generating an intermediate layer with large surface irregularities.It is assumed that when a treatment liquid for a tension coating layer is applied and baked into the intermediate layer with large surface irregularities, the remaining hydrogen gas in the intermediate layer and the tension coating treatment liquid that penetrates the intermediate layer expand during baking to generate voids in the intermediate layer and the tension coating.

[0076] In the intermediate layer 21, which has reduced porosity, the crystal of crystalline metal phosphate is naturally prevented from thickening. For example, in the intermediate layer 21, the average crystalline grain size of crystalline metal phosphate is 10.0 μm or less. To further suppress a decrease in the space factor, crystalline metal phosphate preferably has an average crystal grain size of 8.0 μm or less. The lower limit of the average crystal grain size of metal phosphate Petition 870250091652, dated 07 / 10 / 2025, p. 29 / 63 / 48 crystalline is not particularly limited, but can be 0.1 μm or more.

[0077] In intermediate layer 21, porosity is determined by the following method.

[0078] FIG. 2 is a SEM image of the intermediate layer 21 according to the thickness direction modality.

[0079] First, as shown in FIG. 2, SEM observation is performed at a magnification of 5000 to obtain an observation image including the interface between the base steel plate 1 and the intermediate layer 21 and the interface between the intermediate layer 21 and the tension coating layer 22. Then, in the observation image, the path of the interface between the intermediate layer 21 and the tension coating layer 22 (i.e., the curved path tracing the actual interface) is determined as the interface length L. Next, the total width W of the voids observed in the observation image is determined, and the ratio (W / L) of the void width W to the interface length L is calculated. The ratio (L / L) between the void width W and the interface length L is defined as the porosity of the intermediate layer 21.

[0080] The length of the interface L can be easily determined using a “LUZEX AP” application system created by NIRECO CORPORATION or similar on a cross-sectional image obtained by an electron microscope.

[0081] As described above, the “porosity” in this embodiment is the ratio between the width W of the voids and the length of the interface L. According to the present inventors’ study, the adhesion between the tension coating layer 22 and the intermediate layer 21 (adhesion with the base steel plate 1 through the intermediate layer 21) or the tensile property of the tension coating layer 22 is considered to correlate with the “width” of the voids in the intermediate layer 21. Presumably, this is because the adhesion or the tensile strength cannot be Petition 870250091652, dated 07 / 10 / 2025, p. 30 / 63 / 48 transmitted to the steel plate in the void area. Therefore, in this method, the “width” of the voids is reduced and the proportion of voids in the intermediate layer 21 is reduced, thus ensuring good adhesion and high tension.

[0082] The thickness of the intermediate layer 21 is preferably from 1.0 to 9.0 μm. When the average thickness of the intermediate layer 21 is less than 1.0 μm, the effect of improving the adhesion between the base steel sheet 1 and the insulating coating 2 by means of the intermediate layer 21 is sometimes not sufficiently achieved. On the other hand, when the average thickness of the intermediate layer 21 exceeds 9.0 μm, the magnetic characteristics may deteriorate.

[0083] The thickness of the intermediate layer 21 can be determined by the following method.

[0084] The thickness of the intermediate layer 21 can be determined by measurement using a scanning electron microscope (SEM) and an energy-dispersive element analyzer. That is, a sample containing a base steel sheet 1 and an insulating coating layer 2 is cut, and the polished cross-section is observed with a scanning electron microscope at a magnification of 5000, thus measuring the thickness of the insulating coating layer 2. At this point, using an energy-dispersive element analyzer, the insulating coating layer 2 can be calculated, so that the portion containing Si is the stress coating layer 22 and the portion not containing Si is the intermediate layer 21, to determine the thickness of the intermediate layer 21. Five or more positions are measured, and the average of these is defined as the thickness of the intermediate layer 21.

[0085] The shape of the crystalline metal phosphate contained in the intermediate layer 21 is preferably a plate shape, a particle shape, or a columnar shape. When the shape of the crystalline metal phosphate is a plate shape, a particle shape, or a columnar shape Petition 870250091652, dated 07 / 10 / 2025, page 31 / 63 / 48 columnar, the crystals are favorably placed in contact with each other, and the intermediate layer 21 can be densely formed.

[0086] As a result, adhesion is improved and the decrease in the space factor can also be suppressed. In the intermediate layer 21, all the crystalline metal phosphates contained do not necessarily have an identical shape and can have any plate shape, particle shape, and columnar shape. That is, crystalline metal phosphates with plate shape, particle shape, or columnar shape can be mixed. The shape of the crystalline metal phosphate can be formed by controlling the crystal growth rate. Specifically, a method of adjusting the concentration of each ion in the treatment liquid, the type of oxidizing agent, the temperature, the stirring force, or similar is exemplified, and the crystalline shape can be controlled by appropriately adjusting and combining these conditions.

[0087] Here, in the crystalline metallic phosphate format, “plate shape” refers to a flat shape in which two of the three directions: height, width, and depth are five or more times larger than the other direction. “Particle shape” refers to a substantially isotropic shape in which the three directions: width and depth are substantially as long as each other. “Columnar shape” refers to a shape in which one of the three directions: height, width, and depth is five or more times larger than the other two directions, and the other two directions are substantially as long as each other.

[0088] Although the intermediate layer 21 and the tension coating layer 22 formed over it are formed at different times, both the intermediate layer 21 and the tension coating layer 22 exhibit the effect of the insulating coating 2.

[0089] Regarding the crystalline metallic phosphate in the intermediate layer 21, the mass ratio of metallic phosphate and the type of metallic phosphate are Petition 870250091652, dated 07 / 10 / 2025, page 32 / 63 / 48 determined by measuring the cross-section of intermediate layer 21 along the thickness direction with a scanning electron microscope (SEM) and an energy-dispersive element analyzer. Whether the metallic phosphate of intermediate layer 21 is a crystalline metallic phosphate or not can be determined by X-ray crystallography.

[0090] The base steel sheet 1 and the insulating coating 2 can be determined by the presence or absence of phosphorus. In the insulating coating 2, the intermediate layer 21 and the tension coating layer 22 can be determined by the presence or absence of silicon. (Tension coating layer)

[0091] The grain-oriented electrical steel sheet 100 according to the embodiment, in which a tension coating is formed on the surface of the intermediate layer 21, has the tension coating layer 22 on the side of the insulating coating surface 2.

[0092] The tension coating layer 22 is not particularly limited, provided it is used as an insulating coating of a grain-oriented electrical steel sheet, but preferably contains a metallic phosphate from the point of view of adhesion to the intermediate layer 21 (adhesion to the base steel sheet 1 through the intermediate layer 21). In particular, it is preferable that the tension coating layer 22 mainly contains, as its composition, aluminum phosphate and silica.

[0093] The 22-tension coating layer preferably contains a metallic phosphate and silica (derived from colloidal silica in the coating liquid), such that the silica content is 20.0% by mass or more. Conversely, when the silica content in the 22-tension coating layer exceeds 60.0% by mass, pulverization occurs. Therefore, the silica content is preferably 60.0% by mass or less. The metallic phosphate and silica are preferably contained in a total amount of 70.0% by mass or more. As Petition 870250091652, dated 07 / 10 / 2025, page 33 / 63 / 48 remaining, in addition to metallic phosphate and silica, fine ceramic particles such as alumina and silicon nitride may be included. Metallic phosphate is preferably aluminum phosphate from the point of view of heat resistance.

[0094] The thickness of the tension coating layer 22 is not limited, but the average thickness of the insulating coating 2 (intermediate layer 21, tension coating layer 22) is preferably from 2.0 to 20.0 μm when the average thickness of the intermediate layer 21 is in the above range. When the average thickness of the insulating coating 2 is less than 2.0 μm, it will not be possible to obtain sufficient coating tension. In addition, the elution of phosphoric acid may increase. In this case, this may cause deterioration of adhesion and corrosion resistance, as well as causing peeling of the coating. When the thickness of the insulating coating 2 is greater than 20.0 μm, the space factor decreases to deteriorate the magnetic characteristics, adhesion decreases due to cracking or similar, or corrosion resistance decreases.

[0095] In the tension coating layer 22, the mass percentage of metallic phosphate and the type of metallic phosphate can be determined in the cross section along the thickness direction in the same way as in the intermediate layer 21.

[0096] As described above, the stress coating layer 22 and the intermediate layer 21 can be determined by the silica content method.

[0097] The thickness of the tension coating layer 22 can be determined in the same way as for the intermediate layer 21. The sum of the thickness of the tension coating layer 22 and the thickness of the intermediate layer 21 is determined as the thickness of the insulating coating 2. <Método de fabricação de chapa de aço elétrico de grão orientado>

[0098] According to the manufacturing method that satisfies the Petition 870250091652, dated 07 / 10 / 2025, page 34 / 63 / 48. Under the manufacturing conditions described below, the grain-oriented electrical steel sheet according to the embodiment can be properly manufactured. Note that, naturally, the grain-oriented electrical steel sheet according to the embodiment is not particularly limited in its manufacturing method. That is, a grain-oriented electrical steel sheet having the configurations described above is considered the grain-oriented electrical steel sheet according to the embodiment, regardless of its manufacturing conditions.

[0099] Grain-oriented electrical steel sheet according to the modality can be manufactured by a manufacturing method that includes the following steps: (I) a hot rolling step of a steel part with a predetermined chemical composition to obtain a hot band; (II) a hot band annealing step for hot band annealing; (III) a cold rolling stage of the hot strip after the annealing stage of the hot strip to obtain a steel sheet (cold strip); (IV) a decarburization annealing step for the steel sheet after the cold rolling step; (V) a final annealing step of applying an annealing separator containing STEEL3 in an amount of 10 to 100% by mass onto a steel sheet, drying the steel sheet and then final annealing of the steel sheet; (VII) an annealing separator removal step to remove excess annealing separator from the steel sheet after the final annealing step; (VIII) a pickling step of the steel sheet after the removal step of the annealing separator with 0.1 to 5.0% by mass of a Petition 870250091652, dated 07 / 10 / 2025, page 35 / 63 / 48 selected inorganic acid from the group consisting of sulfuric acid, chloric acid, nitric acid and phosphoric acid for 1 to 20 seconds; (IX) a step of immersing the steel sheet after the pickling step in a treatment liquid containing a metallic phosphate and an oxidizing agent for 2 to 60 seconds; (X) a drying step of pulling the steel sheet after the treatment liquid immersion step, removing the excess treatment liquid, then drying the steel sheet; and (XI) a tension coating layer formation step of applying a coating liquid containing a metallic phosphate and colloidal silica, the metallic phosphate and colloidal silica contained in a total concentration of 10.0 to 40.0% by mass in terms of solid content, to the steel sheet after the drying step, drying the steel sheet and then holding the steel sheet at a sheet temperature of 750 to 950°C for 10 to 120 seconds.

[00100] In the immersion stage, the treatment liquid contains: a metallic phosphate in a mixture quantity of 0.1 to 30.0 g / L; an oxidizing agent in a mixture quantity of 0.1 to 20.0 g / L; and a metallic ion in a concentration of 0.5 to 10.0 g / L, and the oxidizing agent is at least two of a nitrate, a nitrite, a chlorate, a chlorite, a bromate, a perborate, and a hydrogen peroxide solution.

[00101] In addition, during the immersion stage, the treatment liquid preferably meets the following conditions: (A) Concentration of metal ions: 0.5 to 10.0 g / L; (B) Phosphate ion concentration: 0.1 to 30 g / L; and (C) Oxidizing agent concentration: 0.1 to 20 g / L.

[00102] In addition, the manufacturing method for grain-oriented electrical steel sheet according to the modality may also include one or both of the following: Petition 870250091652, of 07 / 10 / 2025, page 36 / 63 / 48 (XII) a nitriding treatment step consisting of subjecting the steel sheet to a nitriding treatment between the decarburizing annealing step and the final annealing step, and (XIII) a magnetic domain refinement step to perform magnetic domain control of the steel sheet after the stress coating layer formation step.

[00103] Among these steps, (V) final annealing step to (XI) tension coating layer formation step (also collectively referred to as insulating coating formation step), which are mainly related to the formation of the insulating coating, are characteristic in the manufacture of grain-oriented electrical steel sheet according to the modality, and known conditions may be adopted for other steps or conditions not described.

[00104] These steps will be described next. [Hot Rolling Stage]

[00105] In the hot rolling stage, a steel workpiece with a predetermined chemical composition, such as a plate, is heated and then hot rolled to obtain a hot strip. The heating temperature of the steel workpiece is preferably in the range of 1100 to 1450°C. The heating temperature is more preferably 1300 to 1400°C.

[00106] The chemical composition of the steel part is altered according to the chemical composition of the grain-oriented electrical steel sheet to be finally obtained, but, for example, the chemical composition includes, in terms of % by mass, 0.01 to 0.20% C, 2.50 to 4.00% Si, 0.01 to 0.040% Al sol., 0.01 to 0.50% Mn, 0.020% or less N, 0.005 to 0.040% S, 0 to 0.50% Cu, 0 to 0.50% Sn, 0 to 0.020% Se, 0 to 0.50% Sb and a remainder including Fe and impurities.

[00107] Hot rolling conditions are not particularly Petition 870250091652, dated 07 / 10 / 2025, page 37 / 63 / 48 limited and can be adequately defined based on the required characteristics. The sheet thickness of the hot band is preferably, for example, in a range of 2.0 mm or more and 3.0 mm or less. [Hot Band Annealing Stage]

[00108] The hot band annealing step is a hot band annealing step manufactured by means of the hot rolling step. By performing such annealing treatment, recrystallization occurs in the metallographic structure and favorable magnetic characteristics can preferably be achieved.

[00109] When hot strip annealing is performed, the hot strip manufactured through the hot rolling step is annealed according to a known method. The means for heating the hot strip at the time of annealing are not particularly limited, and a known heating method can be adopted. The annealing conditions are not particularly limited. For example, the hot strip can be annealed in a temperature range of 900 to 1200°C for 10 seconds to 5 minutes. [Cold Rolling Stage]

[00110] In the cold rolling stage, the hot strip after the hot strip annealing stage is subjected to cold rolling to obtain a steel sheet (cold strip). Cold rolling can be performed once (performed continuously without intervening annealing(s)). Alternatively, before the final pass in the cold rolling stage, intermediate annealing can be performed at least once or two or more times by interrupting the cold rolling, i.e., cold rolling can be performed several times with intervening intermediate annealing(s).

[00111] When intermediate annealing is performed, it is preferable to maintain the hot strip at a temperature of 1000 to 1200°C for 5 to 180 seconds. The annealing atmosphere is not particularly restricted. The Petition 870250091652, dated 07 / 10 / 2025, page 38 / 63 / 48, the number of intermediate annealing times is preferably 3 or less, considering the manufacturing cost.

[00112] In addition, prior to the cold rolling stage, the surface of the hot strip may be subjected to pickling.

[00113] In the cold rolling stage according to the embodiment, the hot strip after the hot strip annealing stage is cold rolled according to a known method to form a steel sheet. For example, the final rolling reduction may be in the range of 80 to 95%. When the final rolling reduction is 80% or more, a Goss core in which the orientation {110} <001> A high degree of development in one rolling direction can be achieved, which is preferable. On the other hand, when the final rolling reduction exceeds 95%, there is a high possibility that secondary recrystallization will be unstable in the subsequent final annealing step, which is not preferable.

[00114] Final rolling reduction is a cumulative cold rolling reduction and, when intermediate annealing is performed, final rolling reduction is a cumulative cold rolling reduction after final intermediate annealing. [Decarburization Annealing Stage]

[00115] In the decarburization annealing step, the resulting steel sheet is subjected to decarburization annealing. In decarburization annealing, the conditions for decarburization annealing are not limited, provided that the steel sheet can be primarily recrystallized and the C, which negatively affects the magnetic characteristics, can be removed from the steel sheet. For example, the steel sheet is held at an annealing temperature of 800 to 900°C for 10 to 600 seconds with a degree of oxidation (PH2O / PH2) of 0.3 to 0.6 in an annealing atmosphere (furnace atmosphere). [Nitriding treatment step] Petition 870250091652, dated 07 / 10 / 2025, pages 39 / 63 / 48

[00116] A nitriding treatment can be carried out between the decarburization annealing step and the final annealing step described later.

[00117] In the nitriding treatment stage, for example, the steel sheet after the decarburizing annealing stage is maintained at approximately 700 to 850°C in a nitriding treatment atmosphere (an atmosphere containing a nitriding gas such as hydrogen, nitrogen, or ammonia) to perform the nitriding treatment. When AlN is used as an inhibitor, the N content of the steel sheet after the nitriding treatment stage is preferably 40 ppm or more. On the other hand, when the N content of the steel sheet after the nitriding treatment stage exceeds 1000 ppm, AlN is excessively present in the steel sheet even after the completion of secondary recrystallization in the final annealing. This AlN causes iron loss deterioration. Therefore, the N content of the steel sheet after the nitriding treatment stage is preferably 1000 ppm or less. [Final annealing stage]

[00118] In the final annealing stage, an annealing separator containing 10 to 100% by mass of AhO3 is applied to the steel sheet that is after the decarburization annealing stage or has been subjected to nitriding treatment (after the nitriding treatment stage) and dried, and then the final annealing is carried out.

[00119] In a conventional manufacturing method for a grain-oriented electrical steel sheet, a forsterite-based coating is formed on the surface of a steel sheet (cold band) by applying an annealing separator containing mainly MgO and performing the final annealing.

[00120] On the other hand, in the manufacturing method of grain-oriented electrical steel sheet according to the modality, an annealing separator containing Al2O3 is used to form virtually no Petition 870250091652, dated 07 / 10 / 2025, page 40 / 63 / 48 forsterite-based coating.

[00121] On the other hand, the percentage of Al2O3 can be 100% by mass. However, in the manufacturing method of grain-oriented electrical steel sheet according to the embodiment, the annealing separator preferably contains MgO from the point of view of preventing the sheet surface from being cooked with AbOs. The MgO can be 0%, but the percentage of MgO is preferably 5% by mass or more to obtain the above effect. When MgO is contained, the percentage of MgO is 90% by mass or less to ensure 10% by mass or more of Al2O3. The percentage of MgO is preferably 50% by mass or less.

[00122] Furthermore, in the manufacturing method of grain-oriented electrical steel sheet according to the embodiment, the annealing separator may also contain a chloride. When the annealing separator contains a chloride, a more difficult formation effect of a forsterite-based coating can be obtained. The amount of chloride is not particularly limited and can be 0%, but is preferably from 0.5 to 10% by mass when the above effect is to be obtained. As chlorides, for example, bismuth chloride, calcium chloride, cobalt chloride, iron chloride and nickel chloride are effective.

[00123] The final annealing conditions are not limited, but, for example, a condition of maintaining the steel sheet at a temperature of 1150 to 1250 °C for 10 to 60 hours may be adopted. [Annealing separator removal step]

[00124] In the annealing stripping removal stage, excess annealing strip is removed from the steel sheet after the final annealing stage by washing with water. For example, excess annealing strip can be removed by washing with water. [Pickling stage]

[00125] In the pickling stage, the steel sheet, after the stage of Petition 870250091652, dated 07 / 10 / 2025, page 41 / 63 / 48 removal of the annealing separator, is subjected to pickling with 0.1 to 10.0% by mass of an inorganic acid selected from the group composed of sulfuric acid, chloric acid, nitric acid and phosphoric acid, for 1 to 20 seconds, at a liquid temperature of 30 to 85 °C. Pickling under these conditions may have an effect that allows sufficient removal of the forsterite-based coating, and removal of MgO, if remaining, is possible. [Immersion stage] [Drying stage]

[00126] In the immersion stage, the steel sheet after the pickling stage is immersed in a treatment liquid containing a metallic phosphate and an oxidizing agent for 2 to 60 seconds. In the drying stage, the steel sheet after the immersion stage is removed from the treatment liquid, the excess treatment liquid is removed, and then the steel sheet is dried. In this way, the intermediate layer is formed on the surface of the base steel sheet.

[00127] In the immersion step, a metallic phosphate is contained in a mixture quantity of 0.1 to 30.0 g / L; an oxidizing agent is contained in a mixture quantity of 0.1 to 20 g / L; and the oxidizing agent is at least two of a nitrate, a nitrite, a chlorate, a chlorite, a bromate, a perborate, and a hydrogen peroxide solution. <Quantidade de mistura de fosfato metálico: 0,1 a 30,0 g / L>

[00128] In the treatment liquid, the amount of metallic phosphate in the mixture is from 0.1 to 30.0 g / L. When the amount of metallic phosphate in the mixture is less than 0.1 g / L, the metallic phosphate precipitates relatively slowly and iron ions are increasingly eluted from the steel sheet, and there is a possibility that the resulting intermediate layer 21 will have increased porosity and deteriorate magnetic characteristics and space factor. Therefore, the amount of metallic phosphate in the mixture is Petition 870250091652, dated 07 / 10 / 2025, page 42 / 63 / 48 preferably of 1.0 g / L or more, and more preferably of 3.0 g / L or more.

[00129] On the other hand, when the amount of metallic phosphate mixture is greater than 30.0 g / L, there is a possibility that the pH of the treatment liquid will increase, the precipitation rate of metallic phosphate will decrease, and precipitation will occur in the treatment liquid, and the intermediate layer will not be formed. Furthermore, when the amount of metallic phosphate mixture is greater than 30.0 g / L, there is a possibility that the resulting intermediate layer 21 will have increased porosity and deteriorate magnetic characteristics and space factor. Therefore, the amount of metallic phosphate mixture is preferably 25.0 g / L or less, and the concentration of phosphate ions is more preferably 20.0 g / L or less. <Tipo de agente oxidante>

[00130] The oxidizing agent added to the treatment liquid is at least one of the group consisting of a nitrite, a chlorate, a chlorite, a bromate, a perborate, and a hydrogen peroxide solution. In this embodiment, in the drying step described later, the hydrogen gas generated from the applied treatment liquid is oxidized to suppress void generation and reduce void size. That is, as the oxidizing agent to be used, a type capable of rapidly oxidizing hydrogen gas from the point of view of porosity reduction is preferably used. Specifically, the oxidizing agent includes a nitrate, a nitrite, a chlorate, a chlorite, a bromate, a perborate, and a hydrogen peroxide solution. Among them, bromate is particularly preferred.

[00131] In this method, in the immersion stage, the treatment liquid is preferably adjusted to satisfy the following conditions (A) to (C).

[00132] (A) Concentration of metal ions: 0.5 to 10 g / L (B) Concentration of phosphate ions: 1 to 30 g / L

[00133] The treatment liquid contains a metallic phosphate and a Petition 870250091652, dated 07 / 10 / 2025, page 43 / 63 / 48 oxidizing agent. Examples of metallic phosphate include zinc phosphate, manganese phosphate, calcium and zinc phosphate, and manganese and iron phosphate, and among them, zinc phosphate is preferable.

[00134] In the treatment solution, the concentration of metal ions and the concentration of phosphate ions are 0.5 to 10 g / L and 0.1 to 30 g / L, respectively. When the concentration of metal ions is less than 0.5 g / L, the metallic phosphate precipitates relatively slowly and the iron ions are increasingly eluted from the steel sheet, and there is a possibility that the resulting intermediate layer 21 will have increased porosity and deteriorate magnetic characteristics and space factor. Even when the concentration of phosphate ions is less than 1 g / L, there is a possibility that the phosphate precipitation rate will be relatively slow and the surface of the steel sheet will be etched. Therefore, the concentration of metal ions is preferably 0.5 g / L or more, and the concentration of phosphate ions is preferably 0.1 g / L or more.

[00135] On the other hand, when the concentration of metal ions is greater than 10 g / L, there is a possibility that the pH of the treatment liquid will increase and the precipitation rate of metal phosphate will decrease. Furthermore, when the concentration of phosphate ions is greater than 30 g / L, there is a possibility that the resulting intermediate layer 21 will have increased porosity and that the magnetic characteristics and space factor will deteriorate. Therefore, the concentration of metal ions is preferably 10 g / L or less, and the concentration of phosphate ions is preferably 30 g / L or less.

[00136] (C) Concentration of oxidizing agent (additive): 0.1 to 20 g / L

[00137] When the concentration of the oxidizing agent is less than 0.1 g / L, the ability to oxidize hydrogen gas is low, so the porosity may increase. On the other hand, when the concentration of the oxidizing agent is excessively high, the surface of the steel sheet is oxidized and Petition 870250091652, dated 07 / 10 / 2025, page 44 / 63 / 48, there is a possibility that the precipitation of metallic phosphate will be inhibited, the density of the intermediate layer 21 will be reduced, and the porosity will increase. Therefore, the concentration of the oxidizing agent is 20 g / L or less.

[00138] The immersion time in the treatment liquid is preferably from 2 to 60 seconds. When the immersion time is less than 2 seconds, there is a possibility that the intermediate layer is insufficiently formed and the adhesion is inferior. On the other hand, when the immersion time is greater than 60 seconds, there is a possibility that the intermediate layer has a region where the crystalline metallic phosphate is partially precipitated excessively and becomes inferior in space factor.

[00139] The temperature of the treatment liquid can vary from 20 to 85 °C. When the liquid temperature is below 20 °C, there is a possibility that the intermediate layer will be insufficiently formed and exhibit low adhesion. On the other hand, when the liquid temperature is above 85 °C, there is a possibility that the intermediate layer will have a region where the crystalline metallic phosphate is partially precipitated in excess and becomes inferior in space factor.

[00140] In the drying stage, when the drying temperature is high, there is a possibility of generating voids that increase the porosity of the intermediate layer 21, leading to poor adhesion. Therefore, the drying temperature is preferably 300°C or lower, and more preferably 200°C or lower. The drying temperature is preferably 100°C or higher. [Tension coating layer formation stage]

[00141] In the tension coating layer formation stage, a coating liquid containing metallic phosphate and colloidal silica at a concentration of 10 to 40% by mass is applied to the steel sheet. After the drying stage, the steel sheet is dried and then the steel sheet is Petition 870250091652, dated 07 / 10 / 2025, page 45 / 63 / 48 heated and maintained at a plate temperature of 700 to 950 °C for 10 to 50 seconds, thus forming a tension coating layer on the surface of the intermediate layer.

[00142] When the sheet temperature is kept below 700°C, the voltage is low and the magnetic characteristics are poor. Therefore, the sheet temperature is preferably 700°C or higher. On the other hand, when the sheet temperature is above 950°C, the stiffness of the steel sheet decreases and the steel sheet is easily deformed. In this case, the steel sheet may become distorted due to transfer or something similar, resulting in poor magnetic characteristics. Therefore, the sheet temperature is preferably 950°C or lower.

[00143] When the retention time is less than 10 seconds, elution is poor. Therefore, the holding time is 10 seconds or more. On the other hand, when the retention time is greater than 50 seconds, the adhesion of the tension coating layer may be poor. Therefore, the retention time is preferably 50 seconds or less.

[00144] The coating liquid (insulating coating solution) is adjusted so that the metallic phosphate and colloidal silica are contained in a total amount of 10 to 40% by mass in terms of solid content.

[00145] When the total concentration of metallic phosphate and colloidal silica is less than 10% by mass, the applied treatment liquid flows easily, which can cause uneven application. Furthermore, when the total concentration of metallic phosphate and colloidal silica is greater than 40% by mass, the viscosity becomes too high, which can cause a pattern or irregularity in the coating.

[00146] As metallic phosphate, for example, one or a mixture of two or more selected from aluminum phosphate, zinc phosphate, magnesium phosphate, nickel phosphate, copper phosphate, may be used. Petition 870250091652, dated 07 / 10 / 2025, page 46 / 63 / 48 lithium and cobalt phosphate. Among them, aluminum phosphate is preferable.

[00147] The coating liquid may contain vanadium, tungsten, molybdenum, zirconium and similar elements as additional elements. When these elements are present, they may be added to the coating liquid, for example, as an oxygenated acid.

[00148] As for colloidal silica, either type S or C can be used. Type S colloidal silica is alkaline in silica solution. Type C has its silica particle surface treated with aluminum and is alkaline to neutral in silica solution. Type S colloidal silica is widely used and has a relatively low price, but care must be taken because type S colloidal silica can aggregate and precipitate when mixed with an acidic metal phosphate solution. Type C colloidal silica is stable even when mixed with a metal phosphate solution, and there is no possibility of precipitation, but type C colloidal silica is relatively expensive because the number of treatment steps is large. It is preferable to select and use them according to the stability of the coating liquid to be prepared. [Magnetic Domain Refinement Stage]

[00149] The manufacturing method for grain-oriented electrical steel sheet according to the embodiment may additionally include a magnetic domain refinement step to perform magnetic domain refinement in the steel sheet after the tensile coating layer formation step. By performing magnetic domain refinement treatment, the iron loss of the grain-oriented electrical steel sheet can be further reduced.

[00150] The magnetic domain refinement treatment method includes: a method for narrowing the width of a 180° magnetic domain (performing 180° magnetic domain refinement) by forming linear or dotted groove pieces that extend in a direction that intersects a lamination direction at predetermined intervals in the direction of Petition 870250091652, dated 07 / 10 / 2025, page 47 / 63 / 48 lamination; and a method for narrowing the width of a 180° magnetic domain (performing the refinement of a 180° magnetic domain) forming linear or dotted stress-strain parts or groove parts that extend in a direction that crosses a lamination direction at predetermined intervals in the lamination direction.

[00151] In the case where a stress-strain part is formed, laser beam irradiation, electron beam irradiation, and similar methods can be applied. In the case where a groove part is formed, a mechanical groove forming method using a gear or similar, a chemical groove forming method to form a groove by electrolytic corrosion, a thermal groove forming method by laser irradiation, and similar methods can be applied.

[00152] In a case where the insulating coating is damaged due to the formation of a stress-strain part or a groove part and characteristics such as insulation properties are deteriorated, the insulating coating can be reformed to repair the damage. Examples

[00153] A plate was cast containing, in terms of mass %, 0.08% C, 3.31% Si, 0.028% Al sol., 0.008% N, 0.07% Mn, less than 0.0005% S and a remainder containing Fe and impurities.

[00154] This plate was heated to 1350°C and then hot-rolled to form a hot strip with a plate thickness of 2.2 mm.

[00155] The hot band was annealed (hot band annealing) under holding conditions at 1100°C for 10 seconds.

[00156] Next, the hot strip was cold rolled to obtain a cold strip with a plate thickness of 0.22 mm.

[00157] The cold band was annealed by decarburization under holding conditions at 830°C for 90 seconds.

[00158] After decarburization annealing, a separator of Petition 870250091652, dated 07 / 10 / 2025, page 48 / 63 / 48 annealing containing 95% MgO and Al2O3 and 5% BiCl3 as bismuth chloride was applied and dried, and then the final annealing is carried out at 1200°C for 20 hours.

[00159] After final annealing, the steel sheet was washed with water to remove excess annealing separator. As a result, a forsterite-based coating was not formed on the surface of the steel sheet.

[00160] The steel sheet was subjected to light pickling with 3% by mass of sulfuric acid at a liquid temperature of 85°C for 10 seconds.

[00161] After light pickling, an intermediate layer was formed using a treatment liquid in which a metallic phosphate and an additive (oxidizing agent) shown in Table 1 were mixed. The drying temperature was as shown in Table 1. The intermediate layer obtained was shown in Table 2.

[00162] Next, an insulating coating treatment liquid containing mainly metallic phosphate and colloidal silica, shown in Table 2, was applied and dried at 850°C for 20 seconds after application, to form a tension coating layer on the surface of the steel sheet.

[00163] The thickness of the insulating coating (intermediate layer and voltage coating layer) was shown in Table 2. The voltage coating layer was substantially made of metallic phosphate and silica.

[00164] The steel sheet obtained (grain-oriented electrical steel sheet) was irradiated with a laser beam under conditions where UA (irradiation energy density) was 2.0 J and the irradiation interval was 5.0 mm pitch, thus performing magnetic domain refinement.

[00165] After refining the magnetic domain, the W17 / 50 iron loss of the steel plate (iron loss at 50 Hz under 1.7 T) was measured by Petition 870250091652, dated 07 / 10 / 2025, pages 49 / 63 / 48 Single Sheet Tester: SST according to JIS C2556 (2015). When the W17 / 50 iron loss was 0.65 W / kg or less, it was determined that good magnetic characteristics were guaranteed.

[00166] In addition, the space factor was measured as follows. [Space Factor]

[00167] The space factor was measured by a method in accordance with JIS C 2550-5 (2020). The test piece was 30 mm wide and 320 mm long, and 30 pieces were used. The samples were measured for total mass and then pressurized to 1 MPa, at which point the space between the top and bottom cover plates enclosing the laminate was measured to calculate the space factor.

[00168] When the space factor was 96.0% or more, it was determined that a high space factor was guaranteed.

[00169] In addition, the coating adhesion, coating tension, corrosion resistance, and elution resistance of the steel sheet after magnetic domain refinement were evaluated by the following methods. The porosity of the intermediate layer was determined by the following method. The results are shown in Tables 2 and 3. [Porosity]

[00170] First, using a scanning electron microscope, a region including the interface between the base steel plate and the intermediate layer and the interface between the intermediate layer and the tension coating layer was observed at a magnification of 5000 to obtain an observation image. Then, in the observation image, the path of the interface between the intermediate layer and the tension coating layer was determined as the interface length L. Next, the total width W of the voids observed in the observation image is determined, and the ratio (W / L) of the void width W to the interface length L is calculated as the porosity. The length Petition 870250091652, dated 07 / 10 / 2025, page 50 / 63 / 48 of the L interface was determined using a “LUZEX AP” application system created by NIRECO CORPORATION or similar in a cross-sectional image obtained by an electron microscope. [Coating Adhesion]

[00171] For coating adhesion, a sample with a width of 30 mm and a length of 300 mm was collected from the steel plate, and the sample was subjected to stress-relief annealing at 800°C for 2 hours in a nitrogen stream. Then, the sample was subjected to a bending and adhesion test, in which the sample was rolled and unrolled around a 10 mmφ cylinder, and then the coating was evaluated for the degree of peeling (area fraction).

[00172] The evaluation criteria were as follows, and when a sample was evaluated as ® or o, the sample was determined to have excellent coating adhesion.

[00173] ®: Fraction of peeling area: 0 to 0.5% o: Fraction of peeling area: more than 0.5% and 5.0% or less Δ: Fraction of the peeling area: more than 5.0% [Coating tension]

[00174] The coating stress was calculated by reverse calculation from the curved state when a surface of the insulating coating was stripped. When the coating stress obtained was 4.0 MPa or more, it was determined that the coating stress was sufficient. [Corrosion resistance]

[00175] For corrosion resistance, a 5% aqueous solution of NaCl was naturally added to the sample for 7 hours in an atmosphere of 35°C according to JIS: Salt spray test methods (JIS Z2371:2015).

[00176] Subsequently, a rust area was evaluated at 10 points. Here, the evaluation criteria are as follows. For resistance to Petition 870250091652, dated 07 / 10 / 2025, page 51 / 63 / 48 corrosion, when the score was 5 or more, it was determined that the corrosion resistance was excellent.

[00177] 10: No rust was generated.

[00178] 9: Rust was generated in an extremely small quantity (the area fraction was 0.10% or less).

[00179] 8: Rust was generated in an area fraction of more than 0.10% and 0.25% or less.

[00180] 7: Rust was generated in an area fraction of more than 0.25% and 0.50% or less.

[00181] 6: Rust was generated in an area fraction of more than 0.50% and 1.0% or less.

[00182] 5: Rust was generated in an area fraction of more than 1.0% and 2.5% or less.

[00183] 4: Rust was generated in an area fraction of more than 2.5% and 5.0% or less.

[00184] 3: Rust was generated in an area fraction of more than 5.0% and 10% or less.

[00185] 2: Rust was generated in an area fraction of more than 10% and 25% or less.

[00186] 1: Rust was generated in an area fraction of more than 25% and 50% or less. [Resistance to elution]

[00187] Resistance to elution was evaluated by the possibility of suppressing the elution of phosphoric acid from the sample.

[00188] The method for measuring the amount of elution was as follows: the sample was boiled in pure boiled water for 10 minutes, the amount of phosphoric acid eluted in the pure water was measured, and the amount of phosphoric acid was divided by the area of ​​the insulating coating of the boiled grain-oriented electrical steel sheet. The amount of phosphoric acid eluted in Petition 870250091652, dated 07 / 10 / 2025, page 52 / 63 / 48 pure water was measured and calculated as follows: the pure water in which the phosphoric acid was eluted (solution) was cooled, and the cooled solution was diluted with pure water to prepare a sample, and the sample was measured for the concentration of phosphoric acid with ICP-AES.

[00189] When the elution quantity was less than 40 mg / m2, it was determined that the resistance to elution was excellent. Petition 870250091652, dated 07 / 10 / 2025, page 53 / 63 / 48 [Table 11] Observation Example of invention Example of invention Example of invention Example of invention Example of invention Example of invention Example of invention Example of invention Intermediate layer Thickness (pm) 6.4 Ch 8.8 4.1 o 2.8 2.2 6.3 5.3 Crystalline metallic phosphate Average crystal grain size (Pm) 4.5 6.7 7.5 3.4 3.8 2.1 o 5.5 4.1 Shape Particle Plate Columnar Columnar Plate Plate Particle Particle Particle Drying conditions Temperature (°C) 200 200 150 250 250 200 200 200 200 Immersion conditions Temperature (C) x Time (s) 85C x 10 seconds 85C x 20 seconds 85C x 20 seconds 80C x 12 seconds 80C x 12 seconds 40C x 20 seconds 30C x 20 seconds 85C x 20 seconds 85C x 20 seconds Additive (oxidizing agent) Amount of mixture (g / L) OO 3.5 4.8 o 1.2 C4 o 3.5 3.5 Type Sodium bromate Sodium perborate Hydrogen peroxide Potassium hypochlorite Potassium chlorate Sodium nitrite Sodium bromate Sodium bromate Sodium perborate Metal phosphate Amount of metal phosphate mixture (g / L) 5.5 4.8 5.5 7.5 24.5 15.5 5.5 13.5 8.5 Nitrate ion (g / L) oo C4 o -f- -f- Phosphate ion (g / L) -f- -f- -f- -f- C4 -f- Metal ion (g / L) o 0.8 o 3.5 8.5 3.5 OO 2.5 Type Manganese phosphate Manganese phosphate Manganese phosphate Calcium and zinc phosphate Calcium and zinc phosphate Zinc phosphate Zinc phosphate Zinc phosphate Manganese phosphate Zinc phosphate O CO -f- o r- oo OY, Petition 870250091652, dated 07 / 10 / 2025, pages 54 / 63 / 48 Example of invention Comparative Example Comparative Example Comparative Example Comparative Example Comparative Example Comparative Example Comparative Example 4.6 7.5 13.5 10.5 13.5 11.9 oo 3.8 10.5 12.5 8.6 13.5 Columnar Particle Columnar Needle Columnar Needle Plate 200 250 250 150 150 200 200 85C x 20 seconds 40C x 4 seconds 60C x 120 seconds 50C x 60 seconds 85C x 10 seconds 40C x 90 seconds 40C x 120 seconds 3.5 23 0.03 2.5 0.5 Hydrogen peroxide Potassium chlorate Potassium chlorate Hydrogen peroxide Hydrogen peroxide Sodium sulfate 8.5 5.0 15.5 3.3 36.0 3.8 5.5 -f- -f- -f- CO CO uo cn CO -f- 2.5 2.0 3.5 0.3 o 0.8 Zinc phosphate Manganese phosphate Manganese phosphate Zinc phosphate Manganese phosphate Zinc phosphate Zinc phosphate o cr CO -f- Observation Example of invention Example of invention Example of invention Example of invention Porosity (%) 20 O CO 20 Insulating coating Thickness (μm) 9.5 11.9 7.2 8.2 Tension coating layer Silica content (% by mass) 40.3 42.9 29.8 42.9 37.5 Tension coating layer formation step Insulating coating treatment liquid Total concentration of metallic phosphate and colloidal silica (% by mass) 35.3 35.0 30.8 35.0 35.6 Parts by weight 06 100 oo 100 O oo Colloidal silica type S type S type C type S type S Molar ratio of metallic element O Phosphate (100 parts by weight) Aluminum phosphate Zinc phosphate Manganese phosphate Molybdenum phosphate Aluminum / magnesium phosphate Intermediate layer formation stage Intermediate layer treatment no. C4 CO -f- O C4 CO -f- Petition 870250091652, dated 07 / 10 / 2025, pages 55 / 63 / 48 Example of invention Example of invention Example of invention Example of invention Example of invention Example of invention Example of invention Example of invention Example of invention Comparative Example Comparative Example Comparative Example Comparative Example Comparative Example Comparative Example Comparative Example O co OO 25 O CO OO 40 O CO O o 09 O o 09 O 70 5.9 5.3 9.4 8.6 7.9 9.5 o 11.5 7.2 o 16.6 13.6 16.6 o 11.1 28.6 23.1 34.4 32.8 38.9 32.8 32.8 25.2 34.4 37.5 37.5 37.5 42.9 47.4 31.0 31.1 32.5 35.9 36.1 35.4 32.1 32.1 33.8 35.9 35.6 35.6 35.6 35.0 34.5 30.5 O oo 09 70 OO oo OO 45 70 O oo O oo O oo 100 120 06 type C type C type S type S type S type C type C type C type S type S type S type S type S type S type COO 0.18 O 0.18 OOO 0,18 OO Aluminum / zinc phosphate Aluminum / copper phosphate Aluminum / vanadium phosphate Aluminum / tungsten phosphate Aluminum / zirconium phosphate Aluminum / lithium phosphate Aluminum / barium phosphate Aluminum / molybdenum phosphate Aluminum / vanadium phosphate Aluminum phosphate Aluminum phosphate Aluminum phosphate Aluminum phosphate Aluminum / copper phosphate Aluminum / copper r- oo O. O C4 CO Tl- C4 CO Tl- O r- oo O. o C4 CO Tl- O oo o. 20, Petition 870250091652, dated 07 / 10 / 2025, pages 56 / 63 / 48 Example of invention Example of invention Example of invention Example of invention Example of invention Example of invention Example of invention Example of invention Example of invention Comparative Example Comparative Example Comparative Example Comparative Example Comparative Example Comparative Example Comparative Example 0.60 3 o CO oo 0.60 0.59 0.62 CO oo 0.74 04 O 04 O 0.74 0.69 0.81 97.6 97.8 97.3 96.8 96.5 96.4 C4 96.5 97.2 95.4 93.5 95.2 94.4 94.7 93.2 32 28 O CO CO C4 23 26 32 44 K) 34 26 28 OO Ό Ch r- oo r- oo r- r- oo CO CO r- oo CO 20 Petition 870250091652, dated 07 / 10 / 2025, page 57 / 63 / 48

[00190] As can be seen in Tables 1 to 3, the Invention Examples are extremely excellent in each property, such as coating adhesion, and are improved in iron loss and space factor.

[00191] On the other hand, the Comparative Examples are inferior in at least one of the groups consisting of coating adhesion, magnetic properties, corrosion resistance, elution resistance and space factor of a transformer (core). LIST OF REFERENCE SIGNS

[00192] 100 Grain Oriented Electrical Steel Sheet Base steel plate Insulating coating Intermediate layer Tension coating layer INDUSTRIAL APPLICABILITY

[00193] According to the embodiment of the present invention, it is possible to provide a grain-oriented electrical steel sheet that is excellent in adhesion with a tensile coating and magnetic characteristics, and does not reduce the space factor of a transformer (core). Therefore, the resulting grain-oriented electrical steel sheet can be suitably applied to the iron core material of a transformer and thus has high industrial applicability. Petition 870250091652, dated 07 / 10 / 2025, pages 58 / 63

Claims

1 / 3 CLAIMS 1. Grain-oriented electrical steel sheet, characterized in that it comprises: a base steel sheet; and an insulating coating formed on a surface of the base steel sheet, wherein the insulating coating includes: an intermediate layer that is formed on one side of the base steel sheet and contains a crystalline metallic phosphate; and a tension coating layer formed on one side of the insulating coating surface, and the intermediate layer has a porosity of less than 40%.

2. Grain-oriented electrical steel sheet according to claim 1, characterized in that, in the intermediate layer, the crystalline metallic phosphate includes at least one of the group consisting of manganese phosphate, ferromanganese phosphate, zinc phosphate, and calcium zinc phosphate.

3. Grain-oriented electrical steel sheet according to claim 1 or 2, characterized in that, in the intermediate layer, the crystalline metallic phosphate has a plate shape, a particle shape or a columnar shape.

4. Grain-oriented electrical steel sheet according to claim 1 or 2, characterized in that, in the intermediate layer, the crystalline metallic phosphate has an average crystal grain size of 0.1 to 10.0 pm.

5. Method for forming the insulating coating included on the grain-oriented electrical steel sheet as defined in claim 1, characterized in that it comprises: a final annealing step of applying an annealing separator containing Al2O3 in an amount of 10 to 100% by mass onto a steel sheet, drying the steel sheet and then final annealing of the steel sheet; an annealing separator removal step to remove excess annealing separator from the steel sheet after the final annealing step; a pickling step of the steel sheet after the removal step of the annealing separator with 0.1 to 5.0% by mass of an inorganic acid selected from the group consisting of sulfuric acid, chloric acid, nitric acid and phosphoric acid for 1 to 20 seconds;a step of immersing the steel sheet after the pickling step in a treatment liquid containing a metallic phosphate and an oxidizing agent for 2 to 60 seconds; a drying step of pulling the steel sheet after the immersion step from the treatment liquid, removing the excess treatment liquid, then drying the steel sheet; and a step of forming a tension coating layer of applying a coating liquid containing a metallic phosphate and colloidal silica, the metallic phosphate and colloidal silica contained in a total concentration of 10 to 40% by mass in terms of solid content, to the steel sheet after the drying step, drying the steel sheet and then maintaining the steel sheet at a sheet temperature of 750 to 950°C for 10 to 120 seconds, the treatment liquid contains: a metallic phosphate in a mixing quantity of 0.1 to 30.0 g / L; an oxidizing agent in a mixture quantity of 0.1 to 20.0 g / L;and a metallic ion at a concentration of 0.5 to 10.0 g / L, and the oxidizing agent is at least one from the group consisting of a nitrate, a nitrite, a chlorate, a chlorite, a bromate, a perborate, and a hydrogen peroxide solution.

6. Method for forming an insulating coating according to claim 5, characterized in that, in the immersion step, the steel sheet after the pickling step is immersed in the treatment liquid for 2 to 60 seconds.

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