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

Abstract

The present invention pertains to a grain-oriented electrical steel sheet having a base steel sheet, a glass cover film formed on the surface of the base steel sheet, and an insulating cover film formed on the surface of the glass cover film, the base steel sheet having a sheet thickness of 0.15-0.35 mm, the insulating coating film contains a metal phosphate, amorphous silica, and one or more oxyacid compounds selected from among tungsten, vanadium, molybdenum, and zirconium, the amorphous silica is 30-150 parts by mass per 100 parts by mass of the metal phosphate, the total of the oxyacid compounds is 1.0-50 parts by mass, and the moisture content of the insulating coating film is 0-0.04% by mass.

Description

Technical Field

[0001] The present invention relates to a directional electromagnetic steel sheet and a method for forming an insulating covering film on the directional electromagnetic steel sheet.

[0002] This application claims priority based on Japanese Patent Application No. 2023-061319, filed on April 5, 2023, the contents of which are incorporated herein by reference. Background Technology

[0003] Directional electromagnetic steel sheets are steel sheets mainly used as cores for transformers, etc. Typically, on such directional electromagnetic steel sheets, a magnesium olivine layer (also known as magnesium olivine cover film, glass cover film, primary cover film) is formed during high-temperature final annealing, and an insulating cover film is formed by coating with a treatment solution mainly composed of phosphates and baking during hot pressing and flattening of the steel sheet.

[0004] Insulating films are required to impart electrical insulation to directional electromagnetic steel sheets, reduce eddy current losses, and improve iron losses. In addition to insulation, insulating films also require various properties such as corrosion resistance, heat resistance, slip properties, and sealing properties. This is to ensure smooth manufacturing processes when processing directional electromagnetic steel sheets to produce transformer cores. For example, if the insulating film has poor heat resistance, slip properties, or sealing properties, it may peel off during stress-relief annealing in core manufacturing, sometimes failing to provide its original insulation properties or hindering the smooth lamination of steel sheets, thus deteriorating workability.

[0005] Furthermore, an important characteristic of the insulating covering film of directional electromagnetic steel sheets is the application of tension to the steel sheet. Applying tension to the steel sheet facilitates the movement of magnetic domain walls, thereby improving the iron loss of the directional electromagnetic steel sheet. Applying tension also reduces magnetostriction, one of the main causes of noise generated in transformers manufactured using directional electromagnetic steel sheets in their cores.

[0006] In order to improve the properties of the directional electromagnetic steel sheet as described above, specifically, the technologies disclosed in the following patent documents 1 to 7 have been researched and developed.

[0007] For example, Patent Document 1 discloses a method of coating an insulating coating film solution, mainly composed of phosphates, chromates, and colloidal silica, onto a magnesium olivine coating film formed on the surface of a steel plate after final annealing, followed by baking. According to the technology disclosed in Patent Document 1, a high-tensile insulating coating film can be formed on the surface of the steel plate, reducing iron loss and magnetostriction in directional electromagnetic steel plates.

[0008] Furthermore, Patent Document 2 discloses a method in which a treatment solution containing colloidal silica with a particle size of less than 8 μm, dihydrogen phosphate, and chromate in a specific ratio is coated onto a steel plate, followed by baking. According to the technology disclosed in Patent Document 2, the high tension of the insulating cover film can be maintained, thereby improving the lubricity of the cover film.

[0009] Furthermore, Patent Document 3 discloses the following technology: by attaching a specific amount of an insulating covering film mainly composed of phosphate, chromate and colloidal silica with a glass transition temperature of 950°C to 1200°C, a high-tensile insulating covering film is formed on the surface of a directional electromagnetic steel plate.

[0010] According to the technologies disclosed in Patent Documents 1-3, insulating covering films with exceptionally superior properties and improved tension can be formed. However, the technologies disclosed in Patent Documents 1-3 all contain chromate compounds, such as chromium compounds, in the insulating covering film. In recent years, with increasing attention to environmental issues, society has demanded the prohibition or restriction of the use of compounds such as lead, chromium, and cadmium.

[0011] Therefore, techniques for forming a good insulating coating film even without the aforementioned chromium compounds have been studied. For example, Patent Document 4 discloses a method for treating an insulating coating film on a directional electromagnetic steel sheet, wherein a treatment solution is baked at a temperature above 300°C, the treatment solution containing 20 parts by mass of colloidal silica (based on SiO2 content), 10 to 120 parts by mass of aluminum phosphate, 2 to 10 parts by mass of boric acid, and a total of 4 to 40 parts by mass of sulfates of one or more metallic elements selected from Mg, Al, Fe, Co, Ni, and Zn.

[0012] In addition, Patent Document 5 discloses a technology involving a coating agent for forming a cover film, which comprises a mixture of boric acid and alumina sol, and an organic solvent that is compatible with water, and has the effect of imparting tension to a directional electromagnetic steel sheet.

[0013] Furthermore, Patent Document 6 discloses a technique for adding an organic acid salt selected from one or more metallic elements chosen from Ca, Mn, Fe, Zn, Co, Ni, Cu, B, and Al to a surface treatment agent for directional electromagnetic steel sheets containing phosphates and colloidal silica. In addition, Patent Document 6 exemplifies formate, acetate, oxalate, tartrate, lactate, citrate, succinate, and salicylate as organic acid salts.

[0014] Furthermore, Patent Document 7 discloses the following technology: in an insulating covering film treatment agent for directional electromagnetic steel plates containing phosphate and colloidal silica, the metal components in the phosphate are set to a specific combination of divalent metal elements, trivalent metal elements, and metal elements with valence of four or more.

[0015] However, in the insulating coating film disclosed in Patent Document 4, the corrosion resistance of the steel plate sometimes decreases due to the action of sulfate ions in the sulfate. Furthermore, in the technology disclosed in Patent Document 5, the corrosion resistance of the insulating coating film and the baking temperature are too high, making the steel plate prone to defects. Additionally, in the technology disclosed in Patent Document 6, the surface treatment agent solution discolors under the action of organic acids in the organic acid salt, resulting in low liquid stability. Moreover, in the technology disclosed in Patent Document 7, the preparation of the coating solution is complex, and the concentration of the coating solution cannot be increased, making uniform coating difficult.

[0016] In addition, the chromate-free insulating covering films disclosed in patent documents 4-7 cannot impart sufficient tension to the steel sheet, and therefore cannot be said to sufficiently improve the iron loss of directional electromagnetic steel sheets. Therefore, the technology regarding the insulating covering films for these directional electromagnetic steel sheets requires further improvement.

[0017] Existing technical documents Patent documents Patent Document 1: Japanese Patent Publication No. 53-28375 Patent Document 2: Japanese Patent Application Publication No. 61-41778 Patent Document 3: Japanese Patent Application Publication No. 11-071683 Patent Document 4: Japanese Patent Publication No. 57-9631 Patent Document 5: Japanese Patent Application Publication No. 7-278828 Patent Document 6: Japanese Patent Application Publication No. 2000-178760 Patent Document 7: Japanese Patent Application Publication No. 2010-13692 Summary of the Invention

[0018] The problem that the invention aims to solve As mentioned above, there is still room for improvement regarding chromate-free insulating cover films.

[0019] In addition, in the case of chromate-free insulating cover film, if the above properties are to be improved, the method of adding phosphate containing a specific metal element can be considered. However, in this case, there are problems such as reduced chemical stability of the cover film and increased leaching of phosphoric acid from the insulating cover film.

[0020] Therefore, the objective of this invention is to provide a directional electromagnetic steel sheet with low leaching of phosphoric acid from the insulating cover film (excellent leaching resistance), based on a directional electromagnetic steel sheet having corrosion resistance, good cover film adhesion, and cover film tension equal to or greater than that of the conventional type, and having an insulating cover film that does not contain chromate, and to provide a method for forming the insulating cover film (insulating cover film forming method).

[0021] Methods for solving problems The inventors have investigated the suppression of phosphoric acid dissolution in chromate-free insulating coating films. As a result, it was found that phosphoric acid dissolution was suppressed in insulating coating films obtained by coating an insulating coating film solution containing an oxyacid compound of a specified metal element.

[0022] This invention was made in view of the above-mentioned insights. The gist of the invention is as follows.

[0023] [1] One aspect of the present invention relates to a directional electromagnetic steel plate having a base steel plate, a glass cover film formed on the surface of the base steel plate, and an insulating cover film formed on the surface of the glass cover film. The base steel plate has a thickness of 0.15 to 0.35 mm. The insulating cover film contains one or more oxyacid compounds selected from metal phosphate salts, amorphous silicon dioxide, tungsten, vanadium, molybdenum, and zirconium. The amorphous silicon dioxide is 30 to 150 parts by mass relative to 100 parts by mass of the metal phosphate salt, and the total oxyacid compounds are 1.0 to 50 parts by mass. The moisture content of the insulating cover film is 0 to 0.04% by mass.

[0024] [2] According to the directional electromagnetic steel plate described in [1] above, the oxyacid compound may also be tungstate, phosphotungstate, silicotungstate, vanadate, phospomolybdate or zirconate.

[0025] [3] Another aspect of the present invention relates to a method for forming an insulating cover film, comprising: a solution preparation step for preparing an insulating cover film solution, wherein the insulating cover film solution contains, relative to 100 parts by mass of a metal phosphate salt, 30 to 150 parts by mass of colloidal silica, and 1.0 to 50 parts by mass of one or more oxyacid compounds selected from tungsten, vanadium, molybdenum and zirconium, and the solid component concentration is 8 to 50 by mass; and a coating and drying step, wherein the insulating cover film solution is coated onto a steel plate and heated to a temperature range of 800 to 900°C at a heating rate of 40 to 200°C / second between 100 and 600°C, and held in the temperature range for 5 to 90 seconds.

[0026] [4] According to the insulating cover film forming method described in [3] above, the solution preparation step may also add 1 to 5 parts by weight of phosphonic acid to the insulating cover film solution relative to 100 parts by weight of the metal phosphate salt.

[0027] Invention Effects According to the above-described manner of the present invention, a directional electromagnetic steel sheet with a low leaching amount of phosphoric acid from the insulating covering film and a method for forming the insulating covering film can be provided. Detailed Implementation

[0028] A directional electromagnetic steel sheet according to one embodiment of the present invention (the directional electromagnetic steel sheet of this embodiment) has a base steel sheet, a glass cover film formed on the surface of the base steel sheet, and an insulating cover film formed on the surface of the glass cover film.

[0029] In addition, in the directional electromagnetic steel plate of this embodiment, the thickness of the base steel plate is 0.15 to 0.35 mm, the insulating covering film contains metal phosphate salt, amorphous silicon dioxide, and one or more oxyacid compounds selected from tungsten, vanadium, molybdenum and zirconium, and the moisture content of the insulating covering film is 0 to 0.04% by mass.

[0030] The following sections will explain each point.

[0031] [Base Material Steel Plate] This embodiment relates to a directional electromagnetic steel sheet, the insulating covering film of which has significant characteristics. The chemical composition of the base steel sheet used in the directional electromagnetic steel sheet is not limited and can be within a known range. For example, to obtain the characteristics typically required for a directional electromagnetic steel sheet, the chemical composition may include the following components. In this embodiment, unless otherwise specified, the percentage (%) of each element's content is expressed as a percentage (by mass).

[0032] C: Below 0.010% Carbon (C) is an effective element for controlling the microstructure of steel sheets throughout the manufacturing process, up to the decarburization annealing step. However, if the C content exceeds 0.010%, the magnetic properties of the directional electromagnetic steel sheet used as the finished product decrease. Therefore, in the base steel sheet of the directional electromagnetic steel sheet of this embodiment, the C content is preferably set to 0.010% or less. The C content is more preferably 0.005% or less. The lower the C content, the better, but even if the C content is reduced to less than 0.0001%, the effect of microstructure control saturates, which increases manufacturing costs. Therefore, the C content can also be set to 0.0001% or more.

[0033] Si: 2.00~6.00% Silicon (Si) is an element that improves the iron loss characteristics of directional electromagnetic steel sheets by increasing their resistance. When the Si content is below 2.00%, a sufficient reduction in eddy current losses cannot be achieved. Therefore, the Si content is preferably set to 2.00% or more. More preferably, it is 2.50% or more, and even more preferably 3.00% or more.

[0034] On the other hand, if the Si content exceeds 6.00%, the directional electromagnetic steel sheet becomes embrittled, and its through-sheet properties deteriorate significantly. Furthermore, the machinability of the directional electromagnetic steel sheet decreases, and the sheet may break during rolling. Therefore, the Si content is preferably set to 6.00% or less. More preferably, the Si content is 5.00% or less, and even more preferably 4.00% or less.

[0035] Mn: 0.01~0.50% Manganese (Mn) is an element that bonds with sulfur (S) during the manufacturing process to form MnS. These precipitates function as inhibitors (inhibitors of normal grain growth) and exhibit secondary recrystallization in the steel. Mn also further improves the hot workability of the steel. When the Mn content is below 0.01%, the effects described above cannot be fully obtained. Therefore, the Mn content is preferably set to 0.01% or more. The Mn content is more preferably 0.02% or more.

[0036] On the other hand, if the Mn content exceeds 0.50%, secondary recrystallization will not occur, thereby reducing the magnetic properties of the steel. Therefore, in the base steel sheet of the directional electromagnetic steel sheet of this embodiment, the Mn content is preferably set to 0.50% or less. More preferably, the Mn content is 0.20% or less, and even more preferably 0.10% or less.

[0037] N: below 0.010% Nitrogen (N) is an element that bonds with Al during the manufacturing process to form AlN, which functions as an inhibitor. However, if the N content exceeds 0.010%, the magnetic properties decrease due to the excessive residual inhibitor in the base steel sheet. Therefore, in the base steel sheet of the directional electromagnetic steel sheet of this embodiment, the N content is preferably set to 0.010% or less. More preferably, the N content is 0.008% or less.

[0038] On the other hand, there is no specific lower limit for the nitrogen content; even reducing it to below 0.001% would only increase manufacturing costs. Therefore, the nitrogen content can also be set above 0.001%.

[0039] sol.Al: 0.020% or less Al (aluminum) is an element that bonds with nitrogen (N) during the manufacturing process of directional electromagnetic steel sheets to form AlN, which functions as an inhibitor. However, if the sol.Al (acid-soluble aluminum) content of the base steel sheet exceeds 0.020%, the magnetic properties decrease due to the excessive residual inhibitor in the base steel sheet. Therefore, in the base steel sheet of the directional electromagnetic steel sheet of this embodiment, the sol.Al content is preferably set to 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 requirement for a lower limit for the sol.Al content; reducing it to less than 0.0001% would increase manufacturing costs. Therefore, the sol.Al content can also be set to 0.0001% or more.

[0040] S: below 0.010% Sulfur (S) is an element that bonds with Mn during the manufacturing process to form MnS, which functions as an inhibitor. However, when the S content exceeds 0.010%, the magnetic properties decrease due to the excessive residual inhibitor. Therefore, in the base steel sheet of the directional electromagnetic steel sheet of this embodiment, the S content is preferably set to 0.010% or less. The S content in the directional electromagnetic steel sheet is preferably as low as possible, for example, less than 0.001%. However, even if the S content in the directional electromagnetic steel sheet is reduced to less than 0.0001%, the manufacturing cost increases unnecessarily. Therefore, the S content in the directional electromagnetic steel sheet can also be 0.0001% or more.

[0041] P: below 0.030% Phosphorus (P) is an element that reduces workability during rolling. By setting the P content to 0.030% or less, excessive reduction in rolling workability can be prevented, and breakage during manufacturing can be suppressed. From this point of view, the P content is preferably set to 0.030% or less. More preferably, the P content is 0.020% or less, and even more preferably 0.010% or less.

[0042] The lower limit of phosphorus (P) content can include 0%, but since the detection limit for chemical analysis is 0.0001%, the practical lower limit of P content in practical steel plates is 0.0001%. Furthermore, P is also an element that improves texture and thus magnetic properties. To achieve this effect, the P content can be set to 0.001% or higher, or 0.005% or higher.

[0043] Remaining components: Fe and impurities The chemical composition of the base steel plate of the directional electromagnetic steel plate in this embodiment may also contain the aforementioned elements, with the remainder including Fe and impurities. However, for the purpose of improving magnetic properties, Cu, Cr, Sn, Se, Sb, and Mo may be further contained within the ranges shown below. These elements are also allowed to be contained as impurities. Since they may also be absent, the lower limit is 0%.

[0044] Furthermore, other than these elements, such as the presence of any one or more of W, Nb, Ti, Ni, Bi, Co, and V totaling less than 1.0% (whether intentionally added or contained as impurities), will not hinder the effect of the directional electromagnetic steel plate of this embodiment.

[0045] Here, "impurities" refers to substances that are mixed in from the ore, waste, or manufacturing environment used as raw materials during the industrial manufacturing of the base steel sheet. These are elements that are allowed to be present in a content that does not adversely affect the function of the directional electromagnetic steel sheet of this embodiment.

[0046] Cr: 0~0.50% Cr (chromium) is an element that helps increase the proportion of Goss orientation in secondary recrystallization structures, thereby improving magnetic properties. To achieve the above effect, it is preferable to set the Cr content to 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0047] On the other hand, when the Cr content exceeds 0.50%, Cr oxide is formed, which reduces the magnetic properties. Therefore, the Cr content is preferably set to 0.50% or less. More preferably, the Cr content is 0.30% or less, and even more preferably 0.10% or less.

[0048] Sn: 0~0.50% Sn (tin) is an element whose magnetic properties can be improved by controlling the recrystallization structure. To achieve the effect of improved magnetic properties, it is preferable to set the Sn content to 0.01% or more. More preferably, the Sn content is 0.02% or more, and even more preferably 0.03% or more.

[0049] On the other hand, when the Sn content exceeds 0.50%, secondary recrystallization becomes unstable, thereby deteriorating the magnetic properties. Therefore, the Sn content is preferably set to 0.50% or less. The Sn content is more preferably 0.30% or less, and even more preferably 0.10% or less.

[0050] Cu: 0~0.50% Cu (copper) is an element that contributes to increasing the proportion of Goss orientation in secondary recrystallization structures. Cu is an optional element in the base steel sheet of this embodiment. Therefore, its content has a lower limit of 0%, but to obtain the aforementioned effect, it is preferable to set the Cu content to 0.01% or more. More preferably, the Cu content is 0.02% or more, and even more preferably 0.03% or more.

[0051] On the other hand, when the Cu content exceeds 0.50%, the hot-rolled steel sheet becomes brittle. Therefore, in the base steel sheet of the directional electromagnetic steel sheet of this embodiment, it is preferable to set the Cu content to 0.50% or less. More preferably, the Cu content is 0.30% or less, and even more preferably 0.10% or less.

[0052] Se: 0~0.020% Selenium (Se) is an element that improves magnetic properties. Therefore, it may be included in the composition of the element. When Se is included, it is preferable to set the content to 0.001% or more to maximize the effect of improving magnetic properties. The Se content is preferably 0.003% or more, and more preferably 0.006% or more.

[0053] On the other hand, if the Se content exceeds 0.020%, the adhesion of the glass cover film deteriorates. Therefore, it is preferable to set the Se content to 0.020% or less. More preferably, the Se content is 0.015% or less, and even more preferably, 0.010% or less.

[0054] Sb: 0~0.500% Antimony (Sb) is an element that improves magnetic properties. Therefore, it may be included in the composition of Sb. When Sb is included, it is preferable to set the content to 0.005% or more in order to effectively improve magnetic properties. More preferably, the Sb content is 0.010% or more, and even more preferably 0.020% or more.

[0055] On the other hand, if the Sb content exceeds 0.500%, the adhesion of the glass cover film deteriorates significantly. Therefore, it is preferable to set the Sb content to 0.500% or less. More preferably, the Sb content is 0.300% or less, and even more preferably 0.100% or less.

[0056] Mo: 0–0.10% Mo (molybdenum) is an element that improves magnetic properties. Therefore, it may be included in the composition of the molybdenum. When Mo is included, it is preferable to set the Mo content to 0.01% or more in order to effectively improve the magnetic properties. More preferably, the Mo content is 0.02% or more, and even more preferably 0.03% or more.

[0057] On the other hand, if the Mo content exceeds 0.10%, the cold rollability deteriorates, potentially leading to breakage. Therefore, it is preferable to set the Mo content to 0.10% or less. More preferably, the Mo content is 0.08% or less, and even more preferably 0.05% or less.

[0058] As described above, the chemical composition of the base steel plate of the directional electromagnetic steel plate in this embodiment can be illustrated as follows: it contains the aforementioned essential elements, and the remainder includes Fe and impurities; or it contains the aforementioned essential elements, further containing one or more optional elements, and the remainder includes Fe and impurities.

[0059] The chemical composition of the base steel plate of the directional electromagnetic steel plate in this embodiment can be measured after removing the glass cover film and the insulating cover film formed on the surface.

[0060] Specifically, the directional electromagnetic steel sheet is immersed in an aqueous solution of sodium hydroxide containing 10-20% by mass and 80-90°C for 7-10 minutes to remove the insulating covering film.

[0061] The directional electromagnetic steel sheet with the insulating cover removed is washed with water, and then dried with a warm air blower for less than 1 minute (e.g., 5-60 seconds). The dried directional electromagnetic steel sheet (without the insulating cover) is then immersed in an aqueous solution of hydrochloric acid containing 5-10% by mass HCl at 70-90°C for 1-10 minutes to remove the glass cover.

[0062] The impregnated steel sheet is washed with water, and then dried with a warm air blower for less than 1 minute (e.g., 5-60 seconds).

[0063] Through the above procedures, the base steel plate can be removed from the directional electromagnetic steel plate.

[0064] The chemical composition of the base steel plate is determined using well-known compositional analysis methods. Specifically, chips are generated from the base steel plate using a drill bit, collected, and dissolved in acid to obtain a solution. ICP-AES is then performed on the solution to perform elemental analysis of its chemical composition.

[0065] Here, the Si content in the chemical composition of the base steel plate is determined by the method specified in JIS G1212 (1997) (quantitative method for silicon). Specifically, if the above-mentioned chips are dissolved in acid, silicon oxide precipitates out as a precipitate. This precipitate (silicon oxide) is then filtered with filter paper, and its mass is measured to determine the Si content.

[0066] The C and S contents were determined using a well-known high-frequency combustion method (combustion-infrared absorption method). Specifically, the solution was burned in an oxygen stream by high-frequency heating, and the produced carbon dioxide and sulfur dioxide were detected to determine the C and S contents.

[0067] The nitrogen content is determined using the well-known inert gas melting-thermal conductivity method.

[0068] <Plate thickness> Considering its use as the core of a transformer, the thickness of the base steel plate is 0.15–0.35 mm. The thinner the plate, the better the reduction in eddy current losses and the better the iron loss; therefore, the preferred upper limit for the base steel plate thickness is 0.35 mm. However, manufacturing base steel plates thinner than 0.15 mm requires specialized equipment, increasing manufacturing costs, making it less desirable for production. Therefore, the industrially preferred lower limit for the plate thickness is 0.15 mm.

[0069] [Glass Cover Film] In this embodiment, a glass cover film (sometimes also called a magnesium olivine cover film) is formed on the surface of the base steel plate of the directional electromagnetic steel plate. The glass cover film can be any known cover film. It is typically an inorganic cover film with magnesium silicate (magnesium olivine) as its main component.

[0070] The glass cover film is formed by reacting an annealing separating agent containing magnesium oxide (MgO) applied to the surface of a base steel sheet with the components of the base steel sheet surface during final annealing. It has a composition derived from both the annealing separating agent and the base steel sheet, and is structured with a microstructure comprising Mg₂SiO₄ and MgAl₂O₄ phases as the main phases (more than 50% by area). In addition to these phases, it sometimes contains precipitates of less than 1%.

[0071] Glass cover film helps improve the adhesion of insulating cover film.

[0072] [Insulating Covering Film] In the directional electromagnetic steel plate of this embodiment, an insulating covering film is formed on the surface of the glass covering film.

[0073] The insulating covering film contains a metal phosphate salt, amorphous silicon dioxide, and one or more oxyacid compounds selected from tungsten, vanadium, molybdenum, and zirconium. Regarding the proportions, the proportions of the metal phosphate salt, amorphous silicon dioxide, and oxyacid compounds are 1.0 to 50 parts by mass relative to 100 parts by mass of the metal phosphate salt, 30 to 150 parts by mass of the amorphous silicon dioxide, and 1.0 to 50 parts by mass of the oxyacid compounds.

[0074] The metal phosphate salt is not limited, and can be, for example, aluminum phosphate or magnesium phosphate. However, from the viewpoint of moisture resistance, the proportion of Mg in the metal of the metal phosphate salt is preferably 50% by mass or less, more preferably 30% by mass or less (for example, aluminum phosphate is 70% by mass or more and magnesium phosphate is 30% by mass or less).

[0075] If the proportion of amorphous silica derived from colloidal silica in the insulating cover film solution is too high, cracking of the cover film and an increase in the moisture content of the insulating cover film will occur. Conversely, if the proportion of amorphous silica is too low, the proportion of phosphate will increase beyond the necessary level, further increasing the moisture content of the insulating cover film.

[0076] The inventors conducted research and concluded that the reason for the increased dissolution of phosphoric acid in the absence of chromic acid is that when an insulating covering film solution without chromic acid is applied to a steel plate with a glass covering film, the magnesium olivine layer on the surface of the steel plate reacts with the solution to generate magnesium phosphate, which has high hygroscopicity and solubility.

[0077] Therefore, in the directional electromagnetic steel sheet of this embodiment, by incorporating a substance with higher acidity into its insulating cover film, the reaction with forsterite is suppressed, thus achieving both high cover film tension and suppression of phosphoric acid dissolution. A specific transition metal oxyacid compound is used as the substance that does not degrade the properties of the cover film while having higher acidity. Specifically, it contains 1.0 to 50 parts by mass of one or more of tungsten, vanadium, molybdenum, and zirconium, relative to 100 parts by mass of the metal phosphate salt. In this embodiment, the transition metal oxyacid compound refers to a compound formed by direct bonding of oxygen atoms to transition metal elements.

[0078] If the proportion of one or more oxyacid compounds among tungsten, vanadium, molybdenum, and zirconium is too low, the moisture content of the insulating coating increases, and the leaching resistance of the insulating coating decreases. On the other hand, if the proportion is too high, the corrosion resistance decreases or the solution becomes unstable.

[0079] The oxyacid compound must be one or more of the following: tungsten, vanadium, molybdenum, and zirconium. Other oxyacids will not provide sufficient resistance to the leaching of phosphoric acid.

[0080] The oxyacid compound is preferably a tungsten or vanadium oxyacid compound. For example, the oxyacid compound is a tungstate, phosphotungstate, silicotungstate, vanadate, phosphomolybdate, or zirconate. From the perspective of solution stability, phosphotungstate or tungstate is preferred. A vanadium oxyacid compound is more preferred.

[0081] In addition, considering the miscibility with phosphates, sodium salts such as sodium vanadate and sodium tungstate are preferred.

[0082] Even if tungsten, vanadium, molybdenum and / or zirconium are present in states other than oxyacid compounds, the desired effect cannot be achieved.

[0083] Oxyacid compounds exist as a matrix (i.e., in molecular form rather than particulate form) together with metal phosphate salts. For example, even if they exist in particulate form, the desired effect cannot be achieved.

[0084] Furthermore, if the oxyacid compound is an ammonium compound, there is a possibility that the corrosion resistance will decrease and the moisture content of the insulating covering film will decrease. Therefore, the oxyacid compound is preferably not an ammonium compound.

[0085] If the coating film contains highly hygroscopic substances such as boric acid along with oxyacid compounds, the moisture content is likely to increase. Therefore, it is preferable that the coating film does not contain any other compounds, or if it does, the amount should be 5 parts by mass or less relative to 100 parts by mass of the metal phosphate salt.

[0086] The proportions of metal phosphate salts, amorphous silicon dioxide, and oxyacid compounds in the insulating covering film can be determined by the following method.

[0087] By using an energy-dispersive X-ray analyzer to detect and analyze the elements in the insulating coating, the proportions of phosphates, silicon dioxide, and oxyacid compounds can be calculated. Furthermore, by using X-ray crystal structure analysis, the proportion of amorphous silicon dioxide in the silicon dioxide can be calculated.

[0088] When analyzing elements using an energy-dispersive X-ray analyzer, it is preferable to analyze approximately three points at a magnification of 1000x and calculate the average value. Furthermore, for crystal structure analysis, it is preferable to use, for example, the SmartLab manufactured by RIGAKU, with a Cu sphere under the following conditions: voltage 40kV, current 30mA, measurement angle (2θ) of 5–90°, step size 0.02°, scanning speed 1° / min, entrance slit 1 / 2 degree, and receiving slit 20mm.

[0089] <Moisture content> In the directional electromagnetic steel sheet of this embodiment, the moisture content of the insulating covering film is 0 to 0.04% by mass.

[0090] The leaching of phosphoric acid can be suppressed by reducing the moisture content of the insulating covering film. A significant effect can be achieved by setting the moisture content to below 0.04% by mass.

[0091] In typical insulating films, the moisture content is approximately 0.05 to 0.15% by mass. However, by setting the content of the oxyacid compound and the baking conditions to a specified range corresponding to the content of the oxyacid compound, the moisture content can be set to 0.04% by mass or less. Preferably, the moisture content is 0.03% by mass or less, and more preferably 0.02% by mass or less.

[0092] Moisture content can be determined using the Karl-Fischer method.

[0093] Specifically, approximately 3g of a sample collected from a directional electromagnetic steel plate with an insulating covering film was placed in a sealed heating furnace and heated to 105°C in a nitrogen atmosphere to vaporize the moisture. The vaporized moisture was then passed into 150ml of indicator solution containing Karl-Fischer reagent, allowing the moisture to dissolve in the reagent solution for 30 minutes. Quantitative analysis was then performed using the Karl-Fischer titration method described in JIS K0113 (2005).

[0094] <Amount of insulating covering film attached> In the directional electromagnetic steel sheet of this embodiment, there is no particular limitation on the amount of insulating covering film applied. For example, the coating amount of the insulating covering film is 1 to 10 g / m². 2 .

[0095] [Manufacturing Method] The directional electromagnetic steel plate of this embodiment does not depend on the manufacturing method. As long as it has the above-described structure, its effect can be obtained. However, it can be preferably manufactured according to a manufacturing method that includes the following steps.

[0096] (i) Hot rolling process, in which steel billets are heated and hot rolled to produce hot-rolled plates; (ii) A hot-rolled sheet annealing process, wherein the hot-rolled sheet is subjected to hot-rolled sheet annealing; (iii) Pickling process, which pickles the hot-rolled plate after the annealing process; (iv) A cold rolling process, wherein the hot-rolled sheet after the pickling process is subjected to one or more cold rolling processes, including annealing, to produce a cold-rolled sheet; (v) Decarburization annealing process, which decarburizes the cold-rolled sheet; (vi) The final annealing process, in which an annealing separating agent is applied to the front and back sides of the cold-rolled sheet after the decarburization annealing process, which is the base steel sheet, and then the final annealing is performed; (vii) Solution preparation process, which is used to prepare the insulating cover film solution; (viii) A coating and drying process, wherein the insulating covering film solution is coated onto the steel plate after the final annealing process, and then heated to form an insulating covering film; and (ix) Magnetic domain refinement process, which irradiates the surface of the insulating cover film with energy rays.

[0097] However, for processes other than (vii) solution preparation and (viii) coating and drying (which may also be collectively referred to as insulation cover film formation processes) which are related to the formation of the insulating cover film, there are no restrictions on the conditions, and they can be carried out under known manufacturing conditions for directional electromagnetic steel sheets.

[0098] [Hot rolling process] In the hot rolling process, a steel billet, such as a slab, having a specified chemical composition (a chemical composition corresponding to the chemical composition of the base steel plate of the directional electromagnetic steel plate of this embodiment) is heated and hot rolled to produce a hot-rolled plate.

[0099] The heating temperature is, for example, 1000–1400℃.

[0100] The chemical composition of the steel billet supplied for hot rolling can be determined by taking into account the changes in chemical composition in each process, based on the desired chemical composition as a directional electromagnetic steel sheet.

[0101] There are no restrictions on the method for obtaining steel billets. For example, molten steel with a specified chemical composition can be used for manufacturing. Slabs can be manufactured either by continuous casting or by using molten steel to create ingots, which are then rolled into billets. Alternatively, slabs can be manufactured using other methods.

[0102] The thickness of the steel billet is not particularly limited, for example, it is 150 to 350 mm. The preferred thickness of the steel billet is 220 to 280 mm. As a steel billet, so-called thin slabs with a thickness of 10 to 70 mm can also be used.

[0103] Hot-rolled steel sheets (hot-rolled plates) are obtained through hot rolling. The thickness (finished plate thickness) of hot-rolled plates is not particularly limited. However, hot-rolled plates undergo hot-rolling annealing, pickling, and then cold rolling. It is known that the so-called cold rolling rate affects the magnetic properties of directional electromagnetic steel sheets, therefore the plate thickness must be selected based on the cold rolling rate required for the final plate thickness. For example, the finished plate thickness of hot-rolled plates is 2.0–4.0 mm.

[0104] [Hot-rolled sheet annealing process] In the hot-rolled sheet annealing process, the steel sheet (hot-rolled plate) after the hot rolling process is annealed. By implementing this annealing treatment, the steel sheet structure recrystallizes, achieving good magnetic properties.

[0105] In the hot-rolled sheet annealing process of this embodiment, the hot-rolled sheet manufactured by the hot-rolling process is annealed according to a known method. There are no particular limitations on the method of heating the hot-rolled sheet during annealing; known heating methods can be used. For example, continuous annealing or batch annealing by forming the hot-rolled sheet into coils is also possible. Furthermore, there are no particular limitations on the annealing conditions; for example, the hot-rolled sheet can be annealed in a temperature range of 900–1200°C for 10 seconds to 5 minutes. Additionally, there are no particular limitations on the atmosphere; it is preferable to suppress the oxidation of the steel sheet, and preferably to carry out the annealing in a non-oxidizing atmosphere such as nitrogen, argon, or hydrogen.

[0106] [Pickling process] In the pickling process, the oxide scale (oxides) generated on the surface of the steel sheet during hot rolling and annealing are removed. In the pickling process of this embodiment, known methods can be used. Known acids such as hydrochloric acid, sulfuric acid, and nitric acid can be used as the pickling solution. Furthermore, known pickling inhibitors and pickling accelerators can be added to the pickling solution as needed. Moreover, before contacting the steel sheet with the pickling solution, the pickling solution is allowed to penetrate to the interface between the oxide scale and the steel sheet. To improve pickling efficiency, the steel sheet can also be subjected to physical treatments such as shot peening before pickling.

[0107] [Cold rolling process] In the cold rolling process, steel sheets after pickling are cold rolled to produce cold-rolled sheets. Cold rolling can be a single cold rolling process (a series of cold rollings without intermediate annealing), or it can be a series of cold rolling processes with intermediate annealing intermittently performed before the final pass of the cold rolling process.

[0108] Cold rolling conditions can be followed using well-known methods. The cold rolling ratio in directional electromagnetic steel sheets has a significant impact on their magnetic properties. In particular, the final reduction ratio has a considerable influence, and it can be set to 80-95%. The final reduction ratio refers to the cumulative reduction ratio of cold rolling; in the case of intermediate annealing, it refers to the cumulative reduction ratio of cold rolling after the final intermediate annealing.

[0109] In the case of intermediate annealing, the temperature is held at 800–1200°C for 5–180 seconds. The annealing atmosphere is not particularly limited, but to prevent oxidation of the steel sheet, it is preferable to perform the annealing in a non-oxidizing atmosphere such as nitrogen, argon, or hydrogen. Furthermore, the annealing method can be either continuous annealing, batch annealing in coil form, or other methods. Considering manufacturing costs, the number of intermediate annealing cycles is preferably limited to three or fewer.

[0110] [Decarburization annealing process] In the decarburizing annealing process, the cold-rolled sheet after the grinding process is decarburized and annealed. In this decarburizing annealing, carbon that has an adverse effect on magnetic properties is removed (decarburized) from the steel sheet, and the cold-rolled sheet undergoes a recrystallization.

[0111] There are no restrictions on the decarburization annealing conditions, but annealing is carried out in an atmosphere where the oxygen potential is increased by humidification, as a nitrogen-hydrogen mixed atmosphere is used for decarburization. In addition, since it is necessary to form a primary recrystallization structure at the same time, the humidification temperature (dew point) is determined from the viewpoint of the annealing temperature required for recrystallization and the oxygen potential that can be decarburized at that annealing temperature.

[0112] The annealing temperature is, for example, around 700 to 900°C. Annealing is usually carried out in a continuous annealing process, and therefore requires about 60 seconds of homogenization.

[0113] [Final annealing process] In the final annealing process, an annealing separating agent is applied to the cold-rolled sheet after the decarburization annealing process for final annealing.

[0114] Final annealing, due to its lengthy process, typically involves rolling the steel sheet into coils and annealing in batches. Since the steel sheet temperature reaches approximately 1200℃, an annealing release agent is applied to prevent the coiled steel sheet from sintering. MgO is commonly used as the annealing release agent. By using an annealing release agent primarily composed of MgO, a glass-like coating forms on the surface of the steel sheet after final annealing.

[0115] In addition, in the final annealing process, the steel plate is heated to cause the primary recrystallized grains obtained in the decarburization annealing process to recrystallize a second time, thereby obtaining grains with Goss orientation. The steel is then held at an annealing temperature close to 1200°C for a specified time, thereby removing (purifying) precipitates in the steel such as nitrides (e.g., AlN) and sulfides (e.g., MnS) that have ceased to act as inhibitors, without adversely affecting the magnetic properties.

[0116] The final annealing conditions are not limited. The temperature is increased from room temperature at a rate of 10–100 °C / h, within the range of 900–1000 °C, generally considered to be the temperature range for secondary recrystallization due to Goss orientation, at a rate of 5–20 °C / h, thereby promoting preferential growth in the Goss orientation (secondary recrystallization). Then, purification is performed near 1200 °C (e.g., 1150–1250 °C) as described above to terminate the effect of the inhibitor. Finally, the material is slowly cooled in a non-oxidizing atmosphere such as hydrogen or nitrogen, and the roll is removed from the furnace.

[0117] <Solution Preparation Procedure> In the solution preparation process, an insulating covering film solution is prepared, which contains 30 to 150 parts by mass of colloidal silica relative to 100 parts by mass of metal phosphate salt, and a total of 1.0 to 50 parts by mass of one or more oxyacid compounds selected from tungsten, vanadium, molybdenum and zirconium, with a solid content concentration of 8 to 50% by mass.

[0118] The proportions of metal phosphate salt, colloidal silica, and oxyacid compound are used to obtain an insulating capping film containing metal phosphate salt, amorphous silica, and oxyacid compound in the above proportions. Here, the oxyacid compound serves as the matrix of the insulating capping film, and therefore, in solution, it is not formed into an emulsion, dispersion, or suspension, but rather into a hydrated state.

[0119] If the solids concentration is below 8% by mass, the aqueous solution coated on the steel plate becomes unstable, potentially leading to coating defects such as patterns or uneven coating. On the other hand, if the solids concentration exceeds 50% by mass, it may shorten the pot life of the aqueous solution. Furthermore, the solids concentration also affects the amount of oxyacid compounds remaining in the insulating coating; this is also taken into account when setting the concentration.

[0120] In the insulating coating film solution, 1 to 5 parts by weight of phosphonic acid are preferably added relative to 100 parts by weight of metal phosphate salt. In this case, the coating properties can be improved.

[0121] On the other hand, it is preferable that the insulating covering film solution does not contain boric acid or other substances with high hygroscopicity as described above.

[0122] <Coating and Drying Process> In the coating and drying process, the insulating covering film solution is coated onto the steel plate (the steel plate with a glass covering film formed on the surface of the base steel plate after final annealing), and heated to a temperature range of 800-900°C at a heating rate of 40-200°C / second between 100-600°C, and held in this temperature range for 5-90 seconds, thereby forming the insulating covering film (drying and baking).

[0123] When the heating rate is below 40°C / second within the range of 100–600°C, moisture is difficult to evaporate from the surface of the coating film, increasing solubility and potentially leading to a higher moisture content. On the other hand, when the heating rate exceeds 200°C / second, boiling and other phenomena can easily occur, also increasing the moisture content and potentially reducing corrosion resistance.

[0124] Furthermore, when the heating temperature is below 800°C, the polymerization of phosphates with oxyacids is difficult to occur, which may lead to a decrease in the tension of the cover film. Above 900°C, thermal strain penetrates the steel sheet, which may lead to magnetostriction.

[0125] Furthermore, when the holding time is less than 5 seconds, the polymerization of phosphates with oxyacids is difficult to occur, which may lead to a decrease in the tension of the cover film. When the holding time exceeds 90 seconds, the cover film is prone to cracking, which may reduce corrosion resistance.

[0126] <Magnetic domain subdivision process> In the magnetic domain refinement process, energy rays are irradiated onto the surface of the insulating cover film (the surface of the insulating cover film of the directional electromagnetic steel plate having a base steel plate, a glass cover film and an insulating cover film) to perform 180° magnetic domain refinement.

[0127] By subdividing the magnetic domains, the iron loss of directional electromagnetic steel sheets can be further reduced.

[0128] As a method for magnetic domain subdivision processing, there are known methods. For example, there are methods that narrow the width of 180° magnetic domains (performing 180° magnetic domain subdivision) by forming linear or dot-shaped grooves extending in a direction intersecting the rolling direction at predetermined intervals along the rolling direction, and methods that narrow the width of 180° magnetic domains (performing 180° magnetic domain subdivision) by forming linear or dot-shaped stress-strain portions or grooves extending in a direction intersecting the rolling direction at predetermined intervals along the rolling direction.

[0129] When forming stress-strain regions, laser beam irradiation, electron beam irradiation, etc., can be applied. In addition, when forming grooves, mechanical trench forming methods based on gears, chemical trench forming methods based on electrolytic etching, and thermal trench forming methods based on laser irradiation, etc., can be applied.

[0130] In cases where the insulating cover film is damaged due to the formation of stress-strained sections or grooves, resulting in a deterioration of its insulation properties, the insulating cover film can be reformed to repair the damage.

[0131] Example Molten steel containing 3.2% by mass Si, 0.027% by mass Sol.Al, 0.08% by mass Mn, 0.008% by mass N, 0.08% by mass C, with the remainder being Fe and impurities, is cast to obtain a slab.

[0132] After heating the slab, it is hot-rolled to produce a steel plate (hot-rolled plate) with a thickness of 2.0 mm.

[0133] The steel plate was annealed at 1100℃ for 5 minutes (hot-rolled plate annealing).

[0134] The hot-rolled steel plate is pickled after annealing and then cold-rolled to produce a steel plate with a thickness of 0.23mm (cold-rolled plate).

[0135] The steel plate was subjected to decarburization annealing at 850°C for 3 minutes.

[0136] After decarburization annealing, an annealing separating agent with MgO as the main component (containing more than 90% by mass) is applied, and then heated to 1200℃, and the final annealing is carried out at this temperature in a hydrogen stream for 20 hours.

[0137] A sample measuring 7 cm in width and 30 cm in rolling direction was cut from the final annealed steel sheet. Residual annealing separating agent on the surface was removed by washing with water and light pickling. However, the glass coating formed during final annealing remained.

[0138] Then, the sample is annealed (stress-relief annealing) at 850°C for 2 hours in a nitrogen atmosphere to prepare the test material.

[0139] The oxyacid compounds of the metal elements shown in Table 1 were added to the insulating coating treatment solution, which mainly consisted of metal phosphate salts and colloidal silica, in the proportions shown in Table 2. The insulating coating treatment solution was then applied to the stress-relieved annealed test material and allowed to dry, thereby forming an insulating coating film on the surface. The adhesion amount of the insulating coating film was set at 5 g / m². 2 (Regarding the types and ratios of phosphate metal salts in Table 2, if it is Al:100%, it means that the phosphate metal salt contains 100% Al phosphate; if it is Al:75%, Mg:25%, it means that the phosphate metal salt contains 75% Al phosphate and 25% Mg phosphate.) This yields a directional electromagnetic steel sheet comprising a steel plate (the so-called base steel plate), a glass covering film, and an insulating covering film.

[0140] The resulting directional electromagnetic steel sheet is then irradiated with a laser beam to perform magnetic domain subdivision processing. The laser irradiation uses a continuous laser in a direction orthogonal to the rolling direction, with an irradiation spacing of 6 mm and an energy density of 2.0 mJ / mm². 2 It is carried out under the following conditions.

[0141] The chemical composition of the base steel plate of the obtained directional electromagnetic steel plate contains Si: 3.2% by mass, sol.Al: 0.01% by mass, Mn: 0.07% by mass, N: less than 0.001% by mass, C: 0.001% by mass, with the remainder being Fe and impurities.

[0142] In addition, the content (parts by mass) of amorphous silica and one or more oxyacid compounds among tungsten, vanadium, molybdenum and zirconium in the insulating cover film was determined using an energy-dispersive X-ray analyzer according to the above-described procedure. The oxyacid compounds form the matrix of the insulating cover film in a state of miscibility with the metal phosphate salt.

[0143] The results are shown in Table 3.

[0144] After the insulating covering film is formed, it is kept at room temperature, and the moisture content of the insulating covering film of the directional electromagnetic steel plate is determined by the Karl-Fischer method (current titration) after 24 hours.

[0145] Specifically, approximately 3g of sample is placed in a sealed heating furnace and heated to 105°C in a nitrogen atmosphere to vaporize the water. The vaporized water is then passed into 150ml of indicator solution containing Karl-Fischer reagent, allowing the water to dissolve in the reagent solution for 30 minutes. Quantitative analysis is then performed using the Karl-Fischer titration method described in JIS K0113 (2005).

[0146] The results are shown in Table 3.

[0147] In addition, for the obtained directional electromagnetic steel sheet, the tension of the covering film, the tightness of the covering film, the magnetic properties, the corrosion resistance, and the leaching resistance are evaluated according to the following criteria.

[0148] For corrosion resistance, the test was conducted after the insulating cover film was formed and kept at room temperature for 24 hours. For cover film tension, cover film tightness, magnetic properties, and leaching resistance, the test was conducted after 168 hours at a constant temperature and humidity of 50℃ and 80% to measure the properties after moisture absorption.

[0149] <Covering membrane tension> The tension of the covering film is calculated by inverse calculation based on the bending condition of one side when the insulating covering film is peeled off.

[0150] If the tension of the cover membrane is above 4.0 MPa, it is considered to have sufficient cover membrane tension.

[0151] <Covering film seal> For adhesion, samples with a width of 30 mm and a length of 300 mm were stress-relieved annealed at 800°C in a nitrogen atmosphere for 2 hours, and then evaluated using a bending adhesion test on a 10 mm φ cylinder. The evaluation criteria were set according to the peel width as follows: if it was 3 or higher (3 to 5), the film adhesion was considered sufficient.

[0152] 5: No peeling required 4: Almost no peeling (peeled portion less than 1mm) 3: Peeling can be seen with a width exceeding 1mm but less than 1 / 3 of the surface area. 2: We can see that peeling occurs at 1 / 3 to 1 / 2 of the width. 1: Peeling occurs across more than half the width of the surface. <Magnetic properties> B8 (magnetic flux density at a magnetization of 800 A / m) and W17 / 50 (iron loss per unit mass at a magnetic flux density amplitude of 1.7 T and 50 Hz) were measured. These characteristic values ​​were determined according to the Single Sheet Tester (SST) method of JIS C2556 (2015).

[0153] <Corrosion Resistance> After the insulating covering film is formed, it is kept at room temperature. For directional electromagnetic steel plates after 24 hours, the salt spray test method described in JIS Z2371 (2015) is followed, and the 5% NaCl aqueous solution is allowed to naturally drop to the sample in an atmosphere of 35°C for 7 hours.

[0154] The rusted area is evaluated on a scale of 1 to 10. The evaluation criteria are as follows. If the score is 5 or higher (5-10), it is considered to have sufficient corrosion resistance.

[0155] 10: No rust occurred. 9: Rust occurs in very small amounts (area rate less than 0.10%). 8: Rust area ratio = greater than 0.10% and less than 0.25% 7: Rust area ratio = greater than 0.25% and less than 0.50% 6: Rust area ratio = greater than 0.50% but less than 1.0% 5: Rust area ratio = greater than 1.0% and less than 2.5% 4: Rust area ratio = exceeding 2.5% but below 5% 3: Rust area ratio = exceeding 5% but below 10% 2: Rust area ratio = more than 10% but less than 25% 1: The area of ​​rust exceeds 25%. <Dissolution resistance> The amount of phosphoric acid dissolved from the sample was determined.

[0156] The sample was boiled in boiling pure water for 10 minutes, and the amount of phosphoric acid dissolved in the pure water was measured. The amount of phosphoric acid was divided by the area of ​​the insulating covering film of the directional electromagnetic steel plate being boiled to obtain the amount of phosphoric acid dissolved. The amount of phosphoric acid dissolved in the pure water was determined by cooling the pure water (solution) containing dissolved phosphoric acid, and then using ICP-AES to determine the phosphoric acid concentration of the sample obtained by diluting the cooled solution with pure water.

[0157] If the dissolution rate is less than 40 mg / m³ 2 If it is, it is judged to have excellent resistance to leaching.

[0158] As shown in Tables 1 to 4, in Invention Examples 1 to 8, the insulating cover film contains one or more oxyacid compounds selected from metal phosphate salts, amorphous silicon dioxide, tungsten, vanadium, molybdenum, and zirconium in a specified proportion, and the moisture content is 0.04% by mass or less. As a result, directional electromagnetic steel sheets with insulating cover films exhibiting excellent corrosion resistance, good film adhesion, sufficient film tension (equivalent to or better than conventional ones), and excellent leaching resistance were obtained. Furthermore, the magnetic properties of these directional electromagnetic steel sheets are also equivalent to or better than conventional ones.

[0159] On the other hand, in Comparative Examples 1, 3, 5, 7, and 15, the proportion of oxyacid compounds in the insulating covering film was too low, and the water content exceeded 0.04% by mass, resulting in poor leaching resistance.

[0160] In Comparative Examples 2 and 4, the amount of oxyacid compounds was too high, resulting in poor corrosion resistance and poor tension of the coating film.

[0161] In Comparative Examples 6 and 8, the amount of oxyacid compounds was too high, resulting in poor tension of the covering film.

[0162] In Comparative Examples 9-12, the oxyacid compounds were not more than one of tungsten, vanadium, molybdenum, and zirconium, resulting in increased moisture content in the insulating covering film. Consequently, the leaching resistance was poor.

[0163] In Comparative Example 13, the insulating cover film had too little amorphous silica and a high moisture content. As a result, it had poor corrosion resistance and leaching resistance.

[0164] In Comparative Example 14, the proportion of amorphous silicon dioxide in the insulating cover film was too high, resulting in poor adhesion of the cover film.

[0165] In Comparative Example 16, the heating rate during coating drying was slow, resulting in a higher moisture content in the insulating cover film. Consequently, the leaching resistance was poor.

[0166] In Comparative Example 17, the heating rate during coating drying was too fast, resulting in boiling and other issues, and the moisture content of the insulating cover film also increased. Furthermore, this resulted in poor adhesion and corrosion resistance of the cover film.

[0167] Industrial availability According to the present invention, a directional electromagnetic steel sheet with low phosphoric acid leaching from an insulating covering film and a method for forming the insulating covering film can be provided. Therefore, it has high industrial applicability.

Claims

1. A directional electromagnetic steel plate, characterized in that, have: Base material steel plate, A glass covering film formed on the surface of the base steel plate, and An insulating covering film formed on the surface of the glass covering film, The thickness of the base steel plate is 0.15–0.35 mm. The insulating covering film comprises a metal phosphate salt, amorphous silicon dioxide, and one or more oxyacid compounds selected from tungsten, vanadium, molybdenum, and zirconium. The amorphous silicon dioxide comprises 30-150 parts by mass relative to 100 parts by mass of the metal phosphate salt, and the total oxyacid compounds comprise 1.0-50 parts by mass. The moisture content of the insulating covering film is 0 to 0.04% by mass.

2. The directional electromagnetic steel plate according to claim 1, characterized in that, The oxyacid compound is a tungstate, phosphotungstate, silicotungstate, vanadate, phospomolybdate, or zirconate.

3. A method for forming an insulating covering film, characterized in that, have: The solution preparation process for preparing the insulating covering film solution, wherein the insulating covering film solution, relative to 100 parts by mass of the metal phosphate salt, contains 30 to 150 parts by mass of colloidal silica (based on silica content), and 1.0 to 50 parts by mass of one or more oxyacid compounds selected from tungsten, vanadium, molybdenum, and zirconium, and the solid content concentration is 8 to 50% by mass; and The coating and drying process involves coating the insulating covering film solution onto a steel plate, heating it to a temperature range of 800-900°C at a heating rate of 40-200°C / second between 100-600°C, and maintaining it in the temperature range for 5-90 seconds.

4. The method for forming an insulating cover film according to claim 3, characterized in that, In the solution preparation step, 1 to 5 parts by weight of phosphonic acid are added to the insulating covering film solution relative to 100 parts by weight of the metal phosphate salt.

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