Coating solution, method for manufacturing the coating solution, and method for manufacturing grain-oriented electrical steel sheet

Abstract

The present invention provides a coating liquid for forming an insulating coating used in a directional electromagnetic steel sheet, the coating liquid being characterized in containing aluminum hydroxide particles and boric acid, the specific surface area of the aluminum hydroxide particles being 20 m2 / g or above, and the pH value being 5.5 or above. The present invention also provides a method for producing a coating liquid for forming an insulating coating used in a directional electromagnetic steel sheet. The present invention also provides a method for producing a directional electromagnetic steel sheet in which the coating liquid is used.

Description

[Technical Field]

[0001] The present invention relates to a coating liquid, a method for manufacturing the coating liquid, and a method for manufacturing grain-oriented electrical steel sheets. This application claims priority based on Japanese Patent Application No. 2022-176194, filed in Japan on November 2, 2022, and the contents of that application are incorporated herein by reference. [Background technology]

[0002] Grain-oriented electrical steel sheets are {110} <001> It is a steel sheet having a crystalline structure with a dominant orientation and typically containing 2% or more by mass of Si. Its main use is as a core material for transformers and the like, and there is a particular demand for materials that minimize energy loss during transformation, i.e., materials with low iron loss.

[0003] The typical manufacturing process for grain-oriented electrical steel sheets is as follows: First, a slab containing 2% to 4% by mass of Si is hot-rolled, and the hot-rolled sheet is annealed. Next, it is cold-rolled once or twice or more with an intermediate annealing in between to obtain the final sheet thickness, and then decarburized annealed. After this, an annealing separating agent mainly composed of MgO or Al2O3 is applied, and a final finish annealing is performed. As a result, {110} <001> The crystalline structure is developed with the orientation as the dominant orientation, and a finish annealing film mainly composed of Mg2SiO4 is formed on the surface of the steel sheet. Finally, an insulating coating solution is applied, baked, and then shipped.

[0004] Grain-oriented electrical steel sheets have the property that iron loss can be improved by applying tension to the steel sheet. Therefore, by forming an insulating film of a material with a lower coefficient of thermal expansion than the steel sheet at a high temperature, tension can be applied to the steel sheet, and iron loss can be improved. Conventionally, various coating liquids for forming insulating films on electrical steel sheets are known.

[0005] For example, the insulating film obtained by baking a coating solution composed of colloidal silica, monophosphate, and chromic acid, as disclosed in Patent Document 1, exhibits excellent film properties such as tensile strength. For example, Patent Documents 2 to 5 describe a coating solution for forming an insulating film on grain-oriented electrical steel sheets, mainly composed of colloidal silica and monophosphate, with other additives used instead of chromic acid.

[0006] On the other hand, Patent Documents 6 and 7 disclose an insulating coating solution containing alumina sol and boric acid, and an insulating coating solution containing alumina sol, boric acid, and colloidal silica. The main components of the films obtained by baking these coating solutions are a composite oxide of aluminum oxide and boron oxide, or a composite oxide of aluminum oxide and boron oxide and silica. Such a composite oxide is crystalline aluminum borate represented by the chemical formula xAl2O3·yB2O3, as described in Patent Document 8, etc.

[0007] Patent documents 9 and 10 disclose a method of adding an alkali metal compound or alkaline earth metal compound to a coating solution consisting of alumina sol and boric acid. Patent document 11 discloses a method of forming an aluminum borate film with excellent water resistance and rust resistance by using an aqueous slurry prepared by mixing aluminum oxide with an average particle size of about 0.4 μm and boric acid in a molar ratio of Al to B within the range of Al / B = 1.25 to 1.81 as a coating solution. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 48-039338 [Patent Document 2] Japan Special Publication No. 54-143737 [Patent Document 3] Japanese Patent Publication No. 2000-169972 [Patent Document 4] Japanese Patent Publication No. 2000-178760 [Patent Document 5] International Publication No. 2015 / 115036 [Patent Document 6] Japanese Patent Application Publication No. 06-065754 [Patent Document 7] Japanese Patent Publication No. 06-065755 [Patent Document 8] Japanese Patent Application Publication No. 06-306628 [Patent Document 9] Japanese Patent Publication No. 08-325745 [Patent Document 10] Japanese Patent Publication No. 09-256164 [Patent Document 11] Japanese Patent Application Publication No. 2019-137874 [Non-patent literature]

[0009] [Non-Patent Document 1] Yoshitane Watanabe, "Synthesis and Properties of Oxide Sols," Gypsum and Lime, No. 211, p. 46 (1987) [Non-Patent Document 2] Taichi Sato, "On Aluminum Hydroxide and Alumina," Journal of the Mineralogical Society of Japan, Vol. 19, p. 21 (1989) [Overview of the project] [Problems that the invention aims to solve]

[0010] Conventional technologies each had the following problems: The coating solution for forming the insulating film of Patent Document 1 contains hexavalent chromium. Therefore, in order to improve the working environment in the insulating film forming process of the grain-oriented electrical steel sheet, considerations in terms of equipment are taken. In recent years, due to the increasing awareness of the environment, there is an urgent need for the development of a coating solution for forming an insulating film of a grain-oriented electrical steel sheet that can obtain an insulating film excellent in various film properties such as tension without containing hexavalent chromium. Like the technologies of Patent Documents 2 to 5, the film tension of the insulating film obtained by the coating solution for forming an insulating film that does not contain chromic acid and uses additives other than chromic acid is smaller than the film tension of the insulating film obtained by the coating solution for forming an insulating film containing chromic acid. Also, there is a problem that all of the additives used in the technologies of Patent Documents 2 to 5 are more expensive than chromic acid.

[0011] As is clear from the constituent components of the coating solution, the insulating film disclosed in the technologies of Patent Documents 6 to 8 does not contain harmful substances such as chromic acid. Furthermore, compared with the insulating film obtained by baking a coating solution composed of colloidal silica, primary phosphate, and chromic acid, which is the insulating film standardly used for the current grain-oriented electrical steel sheet, for example, the one disclosed in Patent Document 1, the film tension applied to the steel sheet is 1.5 to 2 times, and the iron loss improvement effect is greater than that of the current film. However, as described in Patent Documents 9 and 10, the aluminum borate film formed by baking a coating solution composed of alumina sol and boric acid still has room for further improvement from the viewpoint of corrosion resistance. In Patent Documents 9 and 10, as a measure for improving corrosion resistance, a method of adding an alkali metal compound or an alkaline earth metal compound to a coating solution composed of alumina sol and boric acid is disclosed. Although these additives are effective in improving the corrosion resistance of the film, the pH of the coating solution increases due to these additives. Therefore, there is a problem that the alumina sol component may gel and it may be difficult to perform a stable coating operation on the steel sheet. The aluminum oxide disclosed in Patent Document 11 is a stable compound with poor reactivity, and it is not easy to obtain a compound called aluminum borate even when mixed with boric acid and baked. The reason for the successful formation of the aluminum borate film in Patent Documents 6 to 10 is that a highly reactive alumina sol is used as the Al source. Alumina sol is a dispersion of aluminum hydroxide hydrate fine particles, not fine particles of alumina (aluminum oxide), and its reactivity increases when dehydrated by heating. Therefore, the industrial production of a grain-oriented electrical steel sheet with an insulating film mainly composed of aluminum borate and excellent corrosion resistance has not been successful.

[0012] The present invention has been made in view of the above, and an object thereof is to provide a coating liquid for forming an insulating film for a grain-oriented electrical steel sheet, a method for producing the same, and a method for producing a grain-oriented electrical steel sheet, which do not use harmful substances such as chromic acid, have a large film tension, excellent magnetic properties, and excellent film properties in terms of corrosion resistance.

Means for Solving the Problems

[0013] (1) The coating liquid according to one aspect of the present invention is a coating liquid for forming an insulating film used for a grain-oriented electrical steel sheet, and contains aluminum hydroxide particles and boric acid, the specific surface area of the aluminum hydroxide particles is 20 m 2 / g or more, and the pH is 5.5 or more, which is characterized. (2) In the coating liquid described in (1) above, the pH of the coating liquid may be 6.0 or more. (3) In the coating liquid described in (1) or (2) above, the aluminum hydroxide particles may be composed of any one of gibbsite, bayerite, boehmite, diaspore, or a combination thereof. (4) In the coating liquid described in any one of (1) to (3) above, the content ratio of the aluminum hydroxide particles and the boric acid in the coating liquid may be 0.2 to 1.5 in terms of the molar ratio of boron to aluminum. (5) A method for manufacturing a coating liquid according to one aspect of the present invention is a method for manufacturing a coating liquid for forming an insulating film used on grain-oriented electrical steel sheets, Mix a dispersion of aluminum hydroxide with a pH of 6.0 or higher with boric acid, or A boric acid solution, obtained by dissolving boric acid in a solvent, is mixed with aluminum hydroxide particles that, when mixed with water, have a pH of 6.0 or higher. It contains aluminum hydroxide particles and boric acid, and the specific surface area of ​​the aluminum hydroxide particles is 20 m². 2 To manufacture a coating solution that is 1 / g or more and has a pH of 5.5 or higher. It is characterized by the following: (6) In the method for producing the coating solution described in (5) above, the pH of the coating solution may be 6.0 or higher. (7) In the method for producing the coating solution described in (5) or (6) above, the aluminum hydroxide particles may consist of gibbsite, bayerite, boehmite, diaspore, or a combination thereof. (8) In the method for producing the coating solution described in any one of items (5) to (7) above, the content ratio of the aluminum hydroxide particles to the boric acid in the coating solution may be 0.2 to 1.5 in terms of the molar ratio of boron to aluminum. (9) A method for manufacturing a grain-oriented electrical steel sheet according to one aspect of the present invention involves applying a final finish annealing to a grain-oriented electrical steel sheet, It contains aluminum hydroxide particles and boric acid, and the specific surface area of ​​the aluminum hydroxide particles is 20 m². 2 A step of applying a coating solution that is 1 / g or more and has a pH of 5.5 or higher, The process includes a step of applying the aforementioned coating liquid to a grain-oriented electrical steel sheet and then baking it at a temperature of 600°C to 1000°C. It is characterized by the following: (10) In the method for manufacturing grain-oriented electrical steel sheets described in (9) above, the pH of the coating solution may be 6.0 or higher. (11) In the method for manufacturing grain-oriented electrical steel sheets described in (9) or (10) above, the aluminum hydroxide particles may consist of gibbsite, bayerite, boehmite, diaspore, or a combination thereof. (12) In the method for manufacturing grain-oriented electrical steel sheets described in any one of items (9) to (11) above, the content ratio of aluminum hydroxide particles to boric acid in the coating solution may be 0.2 to 1.5 in terms of the molar ratio of boron to aluminum. [Effects of the Invention]

[0014] According to the present invention, the coating solution, the method for manufacturing the coating solution, and the method for manufacturing grain-oriented electrical steel sheets, a coating solution for forming an insulating film for grain-oriented electrical steel sheets can be obtained without using harmful substances such as chromic acid, and the film has high film tension, excellent magnetic properties, and excellent corrosion resistance. [Modes for carrying out the invention]

[0015] Embodiments of the present invention will be described below with reference to examples, but it is obvious that the present invention is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be given as examples, but other numerical values ​​and materials may be applied as long as the effects of the present invention are obtained. In addition, each component of the following embodiments can be combined with one another. In this specification, numerical ranges expressed using "~" mean a range that includes the numerical values ​​written before and after "~" as the lower limit and upper limit. In this specification, the term "process" is included not only in the case of an independent process, but also in the case of a process that cannot be clearly distinguished from other processes, as long as the intended purpose of that process is achieved.

[0016] <Coating solution for forming an insulating film> In the coating solution according to this embodiment (also referred to as a coating solution for forming an insulating film), fine powder or dispersion thereof of aluminum hydroxide is used as the Al source when forming aluminum borate.

[0017] For example, the alumina sols disclosed in Patent Documents 6-10 are not fine particle sols of aluminum oxide, as described in Non-Patent Document 1, but rather fine particle sols of alumina hydrate. When the alumina hydrate fine particles have a crystal structure similar to that of boehmite (AlOOH), they are called boehmite sols. As described in Non-Patent Document 1, alumina sols and boehmite sols are dehydrated at around 500°C to become anhydrous alumina (aluminum oxide). Since the reaction is high immediately after dehydration, when mixed with boric acid and heated, it readily reacts to form aluminum borate. In other words, alumina sols, which are fine particle sols of alumina hydrate, can be said to be a readily available raw material for the synthesis of aluminum borate.

[0018] As described in Non-Patent Literature 1, methods for producing alumina sol and boehmite sol include ion exchange methods, in which excess anions (such as Cl) are removed from a basic aluminum salt (e.g., AlOH)xCly) using an ion exchange resin; neutralization methods, in which an acidic salt (e.g., AlCl3) is neutralized and the resulting gel is dissolved with an acid (e.g., HCl); reaction methods, in which metallic Al is reacted with an acid (e.g., HCl) to create a basic aluminum salt (e.g., AlOH)xCly) and the resulting gel is dissolved with an acid; and organometallic hydrolysis methods, in which an aluminum alkoxide is hydrolyzed to obtain a gel which is then dissolved with an acid. In all of these methods, an acid such as hydrochloric acid, nitric acid, or formic acid is always added to stabilize the sol state (prevent gelation), making the sol acidic. For example, Non-Patent Literature 1 shows pH 3.8 and pH 4.0 as specific examples of pH for alumina sol. If alkali or the like is added to alumina sol or boehmite sol to raise its pH above its original level, it becomes unstable and gels. Therefore, insulating coating solutions using alumina sol or boehmite sol must be kept acidic.

[0019] As mentioned earlier, although industrial coating operations become difficult, the inventors believe that the pH of the insulating coating solution affects the corrosion resistance after the insulating coating is baked on, as the pH of the insulating coating solution is increased by adding alkali metal compounds or alkaline earth metal compounds.

[0020] Furthermore, although the Al source exemplified in Patent Document 11 is aluminum oxide, anhydrous aluminum oxide has the problem of being less reactive compared to aluminum hydroxide.

[0021] On the other hand, an aqueous slurry of aluminum hydroxide is neutral (pH=6~7). When boric acid is added to it, the pH drops slightly to about 5.5. According to the inventors' research, in order to form a dense insulating film, it is necessary to use fine particles of aluminum hydroxide with a large specific surface area, which can be obtained by mechanical grinding. However, the dispersion of finely ground aluminum hydroxide obtained through grinding has high viscosity and is difficult to handle as an insulating coating solution, so a dispersant may need to be added. In this case, a weakly alkaline dispersant such as alkali metal polyphosphate or alkali metal silicate (water glass) can be used, and the pH of the aluminum hydroxide dispersion can be made to 8~11. When boric acid is further added to this dispersion, the pH becomes 6~9. In the coating solution according to this embodiment, the pH of the coating solution after the addition of boric acid is neutral to weakly alkaline, at 5.5 or higher, and when this is baked, an insulating film with good corrosion resistance is obtained.

[0022] Non-patent document 2 indicates that aluminum hydroxide is dehydrated at around 300°C or 500°C. Since aluminum hydroxide is activated during the heating dehydration process, similar to alumina sol, when aluminum hydroxide is mixed with boric acid and heated, it readily reacts to form aluminum borate. As described above, the present inventors have found that a coating solution with a pH of 5.5 or higher, obtained by adding boric acid to an aluminum hydroxide dispersion with a pH of 6.0 or higher, can form an insulating film made of aluminum borate by baking it at a temperature exceeding the dehydration temperature of aluminum hydroxide, and that if the film is formed densely, it will be an insulating film that combines high film tension and good corrosion resistance.

[0023] The following describes the materials that make up the coating solution according to this embodiment.

[0024] (Aluminum hydroxide particles) The coating solution according to this embodiment contains aluminum hydroxide particles. The aluminum hydroxide particles may contain one type or two or more types.

[0025] The aluminum hydroxide in this embodiment is preferably an alumina hydrate such as gibbsite or bayalite in trihydrate form (Al2O3·3H2O), or boehmaite or diaspore in monohydrate form (Al2O3·H2O). These aluminum hydroxides can be artificially produced, for example, by the method described in Non-Patent Document 2. Gibbsite is also called hydrazillite. Gibbsite, boehmaite, and diaspore also occur naturally as the main components of bauxite. These aluminum hydroxides may be used in combination. Of these, gibbsite, bayalite, or mixtures thereof are more preferred from the viewpoint of having a low dehydration temperature. For example, the dehydration temperature of gibbsite or bayalite trihydrate is about 300°C, so they are more reactive than monohydrates, which have a dehydration temperature of around 500°C, and are superior in terms of manufacturing costs such as time and expense. Furthermore, Gibbsite and Bayerite have the advantage of being readily available.

[0026] These aluminum hydroxides are activated by heating and dehydration, and react with boric acid to produce aluminum borate. As mentioned earlier, aluminum oxide has the problem of low reactivity and is therefore undesirable as a raw material for forming insulating films from the viewpoint of film formation. Alumina sols and boehmite sols need to be acidic to stabilize them as sols, and aluminum salts also exhibit acidity, so they have the problem of poor corrosion resistance when used as coating solutions for forming insulating films. For this reason, aluminum hydroxide is preferred as the Al source when forming aluminum borate.

[0027] The finer the aluminum hydroxide particles and the larger the specific surface area, the more easily the reaction with boric acid is promoted. Therefore, the specific surface area of the aluminum hydroxide particles is preferably 20 m 2 / g or more, more preferably 40 m 2 / g or more, and even more preferably 50 m 2 / g or more.

[0028] On the other hand, the upper limit value of the specific surface area is not particularly limited, and it may be 200 m 2 / g or less, may be 180 m 2 / g or less, or may be 150 m 2 / g or less. When the upper limit value of the specific surface area is the above or less, the dispersion stability (viscosity stability) of the coating liquid for forming an insulating film is likely to be maintained. The specific surface area of the aluminum hydroxide particles is the specific surface area based on the BET method and is measured by a method conforming to JIS Z 8830:2013.

[0029] (Production of Aluminum Hydroxide Particles with a Specific Surface Area of 20 m 2 / g or More) For aluminum hydroxide commercially available for industrial use, it is difficult to obtain those with a specific surface area of​​​​​​​​​Effective grinding methods for aluminum hydroxide include ball mills, vibratory mills, bead mills, and jet mills. These grinding methods may employ dry grinding, where the aluminum hydroxide particles are ground in powder form, or wet grinding, where the particles are dispersed in a slurry state using a dispersion medium such as water or alcohol. Both dry and wet grinding methods are effective. The specific surface area of ​​aluminum hydroxide particles increases with grinding time, regardless of the grinding method. Therefore, by controlling the grinding time, aluminum hydroxide particles and their dispersions with the required specific surface area can be obtained.

[0031] In the case of wet grinding, the viscosity of the dispersion increases as the specific surface area of ​​the aluminum hydroxide particles increases. And, after grinding, the specific surface area reaches 200 m². 2 If the amount exceeds [amount] / g, the viscosity of the dispersion will increase, causing gelation and potentially hindering the grinding process. Therefore, a dispersant may be added to the dispersion as needed.

[0032] The increase in viscosity during the grinding process can be suppressed by adding a dispersant. However, among dispersants, the addition of organic dispersants can cause them to decompose and carbonize during the baking of the insulating film, potentially leading to carburization in the grain-oriented electrical steel sheet. Therefore, when using a dispersant, an inorganic dispersant is preferable. Furthermore, to maintain a pH of 6 or higher after adding the dispersant, a neutral or weakly alkaline dispersant is desirable. Examples of such inorganic dispersants include alkali metal polyphosphates and alkali metal silicates (water glass). Specific examples of the former include sodium diphosphate and sodium hexametaphosphate. Specific examples of the latter include sodium silicate and potassium silicate.

[0033] The amount of these inorganic dispersants added is preferably limited to 20% by mass or less relative to the total mass (100% by mass) of aluminum hydroxide. By limiting the amount of inorganic dispersants to 20% by mass or less, changes in the film composition after baking are suppressed, and it becomes easier to obtain higher film tension. Since dispersants are optional added components, the lower limit of the dispersant is not particularly limited and may be 0% by mass. In other words, the coating solution may not contain dispersants such as polyphosphate or water glass. In the case of dry grinding, it is not necessary to add the dispersant during grinding; it can be added when preparing the aluminum hydroxide dispersion.

[0034] The alkali metal content relative to the total mass of solids in the coating solution is preferably 5% by mass or less. By keeping the alkali metal content within this range, it is possible to suppress components that do not contribute to film tension while ensuring an appropriate viscosity for the coating solution. More preferably, the alkali metal content relative to the total mass of the coating solution is 4% by mass or less, or 3% by mass or less. Examples of alkali metals include Na (sodium), K (potassium), and Li (lithium). The alkali metal content in the coating solution is measured using an ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopy) instrument in accordance with JIS K 0116:2014 General Rules for Emission Spectroscopic Analysis.

[0035] (Boric acid) Boric acid can be obtained by known manufacturing methods and may be either orthoboric acid (H3BO3) or metaboric acid (HBO2). Orthoboric acid is preferred. Boric acid may be used in particulate form, or it may be dissolved or dispersed in water before use. At 20°C, the solubility of orthoboric acid is slightly over 4g per 100g of water, and it is important to note that the solubility of boric acid in water and alcohol is low at around room temperature when preparing the coating solution. For example, if B / Al = 1.5, then 3.4g of Al(OH)3 is added to 4g of orthoboric acid and 100g of water. The anhydrous solid content concentration of this solution (calculated using B = B2O3 and Al = Al2O3) is only 4%. Since the solubility of orthoboric acid increases to slightly over 8g at 40℃, warming the preparation of the coating solution and maintaining the temperature of the coating solution while coating will make the coating process easier.

[0036] (Ratio of aluminum hydroxide particles to boric acid content) The ratio of aluminum hydroxide particles to boric acid in the coating solution is not particularly limited, but from the viewpoint of obtaining excellent film tension and excellent corrosion resistance, it is preferable that the molar ratio of boron (B) to aluminum (Al) (hereinafter also referred to as the B / Al molar ratio) is 1.5 or less. Note that boric acid and borate salts have relatively low solubility in water and alcohol. Therefore, if the B / Al molar ratio is too high, the solid content concentration in the coating solution must be reduced, making it difficult to obtain the desired film thickness. Accordingly, it is preferable to set the upper limit of the B / Al molar ratio to 1.5 or less, preferably 1.3 or less, and even more preferably 1.0 or less. Furthermore, from the viewpoint of obtaining excellent film tension and excellent corrosion resistance, it is preferable that the lower limit of the B / Al molar ratio be 0.2 or more. Accordingly, the ratio of aluminum hydroxide particles to boric acid is preferably 0.2 to 1.5 in terms of the B / Al molar ratio.

[0037] (Dispersion medium or solvent) In addition to water, alcohols such as ethyl alcohol, methyl alcohol, and propyl alcohol can be used as the dispersion medium or solvent in the coating solution for forming an insulating film. Water is preferred as the dispersion medium or solvent because it is non-flammable.

[0038] The solid content concentration of the insulating coating solution is not particularly limited, as long as it is within the range that can be applied to grain-oriented electrical steel sheets. For example, the solid content concentration of the insulating coating solution can be in the range of 5% to 50% by mass (preferably 10% to 30% by mass) of the total amount of the insulating coating solution. The solid content concentration in the insulating film-forming coating solution is the sum of the aluminum hydroxide particle concentration and the boric acid concentration in the coating solution, determined by the method described later. The amount of dispersion medium or solvent is obtained by subtracting the solid content concentration in the insulating film-forming coating solution from the total amount of the insulating film-forming coating solution.

[0039] Furthermore, the insulating film-forming coating liquid according to this embodiment may contain small amounts of other additives as needed, as long as they do not impair the properties of film tension and corrosion resistance. When small amounts of other additives are included, for example, the amount is preferably 3% by mass or less, and preferably 1% by mass or less, relative to the total solid content (100% by mass) of the insulating film-forming coating liquid according to this embodiment. Examples of other additives include surfactants that prevent the coating liquid from repelling on steel plates. Examples of surfactants include alkali metal salts of carboxylic acids and sulfonic acids, quaternary ammonium salts, fatty acid esters, polyethers, and higher alcohols.

[0040] Furthermore, the coating solution for forming an insulating film according to this embodiment may consist of aluminum hydroxide particles, boric acid, and a dispersion medium or solvent.

[0041] The viscosity of the coating solution for forming insulating films should ideally be between 1 mPa·s and 100 mPa·s, from the viewpoint of ease of application. If the viscosity is too high, it becomes difficult to apply, and if the viscosity is too low, the coating solution may flow, making it difficult to obtain the desired film thickness. The measurement is performed using a B-type viscometer (Brookfield type viscometer). The measurement temperature is 25°C.

[0042] Furthermore, from the viewpoint of the working environment, it is preferable that the coating solution for forming the insulating film according to this embodiment does not contain hexavalent chromium.

[0043] The insulating film obtained by the insulating film forming coating liquid according to this embodiment is baked at a high temperature (for example, 600°C or higher) to achieve high tension. Therefore, if the insulating film forming coating liquid contains resin, the resin will decompose and carburize during baking. As a result, the magnetic properties of the grain-oriented electrical steel sheet will deteriorate. From this viewpoint, it is preferable that the insulating film forming coating liquid does not contain organic components such as resin.

[0044] Herein, the insulating film-forming coating liquid according to this embodiment can impart tension to the steel sheet by baking, and is suitable as a coating liquid for forming an insulating film on grain-oriented electrical steel sheets. It should be noted that the insulating film-forming coating liquid according to this embodiment can also be applied to non-grain-oriented electrical steel sheets. However, even if the insulating film-forming coating liquid according to this embodiment is applied to non-grain-oriented electrical steel sheets, the insulating film does not contain organic components, and therefore there is no effect on improving the punchability of the steel sheet. For this reason, there is little benefit in applying it to non-grain-oriented electrical steel sheets.

[0045] In preparing the insulating coating solution according to this embodiment, aluminum hydroxide particles and boric acid should be mixed and stirred together with the dispersion medium (solvent). In the insulating coating solution according to this embodiment, the pH of the coating solution after stirring must be 5.5 or higher. If the dispersion medium (solvent) of the coating solution does not contain enough water, it will be impossible or difficult to measure the pH. Therefore, the coating solution should be sampled, water added to bring the weight ratio of water in the dispersion medium (solvent) to 50% or higher, and then the pH should be checked. pH is measured using a pH meter.

[0046] The order in which aluminum hydroxide particles and boric acid are added is not particularly limited. For example, a dispersion (a dispersion of aluminum hydroxide with a pH of 6.0 or higher) may be prepared by dispersing a predetermined amount of aluminum hydroxide particles in a dispersion medium, and then a predetermined amount of boric acid may be added and mixed. If the dispersion medium of the aluminum hydroxide dispersion does not contain enough water, the aluminum hydroxide dispersion may be taken and water added until the weight ratio of water in the dispersion medium (solvent) is 50% or higher, and then the pH may be checked. By using such an aluminum hydroxide dispersion, it is possible to make the pH of the coating solution after mixing with boric acid 5.5 or higher. Alternatively, a boric acid solution may be prepared by dissolving a predetermined amount of boric acid in a solvent, and then a predetermined amount of aluminum hydroxide particles may be added to the boric acid solution and mixed and mixed. The requirement for the aluminum hydroxide particles to be used is that when mixed alone with water without mixing with boric acid to make a 10% suspension by weight, the pH of the suspension will be 6.0 or higher. By using such aluminum hydroxide, it is possible to make the pH of the coating solution after mixing with boric acid 5.5 or higher.

[0047] From the viewpoint of ensuring more reliable corrosion resistance, the pH of the coating solution after the addition of boric acid is more preferably 6.0 or higher, greater than 6.0, or 6.2 or higher. The pH of the coating solution after the addition of boric acid is preferably 10 or 9 or less, from the viewpoint of suppressing the alkali metal content in order to prevent a decrease in film tension.

[0048] Furthermore, other additives may be added and mixed as needed. The coating solution for forming the insulating film may then be adjusted to the desired solid content concentration. The temperature of the coating solution may be heated (e.g., 50°C) or at room temperature (e.g., 25°C).

[0049] The dispersion of aluminum hydroxide particles should preferably have a pH of 6.0 or higher, and preferably 6.5 or higher, from the viewpoint of maintaining a pH of 5.5 or higher in the coating solution after the addition of boric acid. The dispersion containing aluminum hydroxide particles preferably has an upper pH limit of 12 or 11, from the viewpoint of suppressing alkali metal content.

[0050] (Analysis of the components of the application solution) In the insulating film forming coating solution according to this embodiment, the content of aluminum hydroxide particles and boric acid in the coating solution can be measured as follows.

[0051] Specifically, first, the coating solution for forming an insulating film is filtered. Below 100°C, the aluminum hydroxide particles and boric acid in the coating solution hardly react with each other. Therefore, below 100°C, the coating solution is in a slurry state, for example, in which aluminum hydroxide particles are dispersed in an aqueous boric acid solution. By filtering the coating solution, it is separated into a filtrate containing an aqueous boric acid solution derived from the boric acid before mixing, and a residue containing hydrated silicates derived from the aluminum hydroxide particles. Next, the concentration of B is determined by ICP-AES analysis (inductively coupled plasma atomic emission spectroscopy) of the filtrate, and the concentration of boric acid in the coating solution can be determined by calculation from the B concentration. In addition, the concentration of aluminum hydroxide in the coating solution can be determined by measuring the weight of the residue and comparing it with the weight of the dispersion before filtration. The presence of Al can be determined by X-ray fluorescence measurement, and the presence of aluminum hydroxide particles can be determined by X-ray diffraction. From the boric acid concentration and aluminum hydroxide concentration in the coating solution, the molar ratio of boron to aluminum (B / Al) can be determined. Furthermore, the specific surface area of ​​the aluminum hydroxide particles is determined from the residue separated above using the BET method described above.

[0052] <Grain-oriented electrical steel sheets and methods for manufacturing grain-oriented electrical steel sheets> Next, an example of a preferred embodiment of the grain-oriented electrical steel sheet and the method for manufacturing the grain-oriented electrical steel sheet according to this embodiment will be described. The grain-oriented electrical steel sheet according to this embodiment has a base material of grain-oriented electrical steel sheet and an insulating film provided on the base material of grain-oriented electrical steel sheet, the insulating film containing aluminum borate crystals composed of constituent elements including Al, B, and O. A glass film or an oxide film may be formed between the base material and the insulating film, or the insulating film may be formed directly on the base material.

[0053] The grain-oriented electrical steel sheet according to this embodiment is preferably obtained by the manufacturing method described below.

[0054] The method for manufacturing grain-oriented electrical steel sheets according to this embodiment includes the step of applying a coating solution for forming an insulating film for grain-oriented electrical steel sheets according to this embodiment to the grain-oriented electrical steel sheet after final finish annealing (i.e., the base material of the grain-oriented electrical steel sheet), and then performing a baking treatment at a baking temperature of 600°C to 1000°C.

[0055] (Grain-oriented electrical steel sheet after final finish annealing) The grain-oriented electrical steel sheet after final finish annealing is the grain-oriented electrical steel sheet that serves as the base material before the application of the above-mentioned coating liquid (i.e., the coating liquid for forming an insulating film according to this embodiment). The grain-oriented electrical steel sheet after final finish annealing is not particularly limited. As a preferred example, the grain-oriented electrical steel sheet that serves as the base material can be obtained as follows. Specifically, for example, a steel billet containing 2% to 4% by mass of Si is subjected to hot rolling, hot-rolled sheet annealing, and cold rolling, followed by decarburization annealing. After this, an annealing separating agent having an MgO content of 50% by mass or more is applied, and the sheet is obtained by performing final finish annealing. The grain-oriented electrical steel sheet after final finish annealing does not necessarily have a finish annealing film.

[0056] (Application and baking of insulating coating solution) After the final finish annealing of the grain-oriented electrical steel sheet, the insulating film-forming coating liquid according to this embodiment is applied, followed by a baking treatment. The amount of coating applied is not particularly limited. From the viewpoint of obtaining excellent film tension and excellent corrosion resistance, it is preferable to apply the coating so that the amount of coating after insulating film formation is in the range of 1 g / m² to 10 g / m² per side. More preferably, it is 2 g / m² to 8 g / m². The amount of coating applied after the baking treatment can be determined from the weight difference before and after the removal of the insulating film.

[0057] Excellent film tension and corrosion resistance may mean that they are equivalent to or better than conventional insulating films, particularly insulating films using coating solutions containing chromium compounds. In the reference example (insulating film using a coating solution containing chromium compounds) described later, the film tension is 8 MPa and the corrosion resistance is 0%. In comparative example 1 (insulating film using a coating solution consisting of alumina sol and boric acid) described later, the film tension is 15 MPa and the corrosion resistance is 100%. In the insulating film according to this embodiment, considering the acceptable likelihood, the film tension may be 8 MPa or more, preferably 10 MPa or more, and more preferably 12 MPa or more. Furthermore, the corrosion resistance may be 10% or less, preferably 5% or less, more preferably 1% or less, and may even be 0%.

[0058] The method for applying the insulating coating solution to the grain-oriented electrical steel sheet after final finish annealing is not particularly limited. For example, coating methods such as the roll method, spray method, and dip method can be used.

[0059] After applying the insulating coating solution, baking is performed. If the baking temperature (target plate temperature) is below 600°C, the formation of aluminum borate by the reaction between aluminum hydroxide particles and boric acid is insufficient. Therefore, the baking temperature should be 600°C or higher. The preferred lower limit for the baking temperature is 700°C or higher. On the other hand, if a baking temperature exceeding 1000°C is used, the grain-oriented electrical steel sheet will soften and become prone to distortion, so the baking temperature should be 1000°C or lower. The preferred upper limit is 950°C or lower. The baking time should be 5 seconds to 300 seconds, preferably 10 seconds to 120 seconds. The heating method for the baking treatment is not particularly limited and examples include radiant furnaces, hot air furnaces, induction heating, etc.

[0060] The insulating film after the baking process becomes a dense film. The thickness of the insulating film is preferably 0.5 μm to 5 μm (preferably 1 μm to 4 μm). The thickness of the insulating film after the baking process can be determined by cross-sectional SEM observation.

[0061] Through the above process, a grain-oriented electrical steel sheet with excellent film tension and corrosion resistance can be obtained using the insulating film-forming coating solution according to this embodiment, even without containing a chromium compound. Furthermore, a grain-oriented electrical steel sheet with an insulating film formed using the insulating film-forming coating solution according to this embodiment also exhibits excellent magnetic properties.

[0062] While examples of preferred embodiments of the present invention have been described, the present invention is not limited to those described above. The above is illustrative, and any configuration that is substantially identical to the technical idea described in the claims of the present invention and produces similar effects is included within the technical scope of the present invention. [Examples]

[0063] The present invention will be specifically described below with reference to examples, but the present invention is not limited thereto.

[0064] (Experimental Example 1) First, commercially available gibbsite (hydrargite), bayerite, boehmite, and diaspore powder (with specific surface areas of 4, 2, 16, and 1 m² respectively) 2 Prepared ( / g), the material was used as is in Comparative Examples 5-8, while the other Comparative Examples and Examples were subjected to grinding by the respective grinding methods, as shown in Table 1. Dispersants were used as needed during the grinding process. In wet grinding, the dispersant was added when preparing the aqueous slurry before processing, and in dry grinding, it was added when preparing the coating solution after grinding. The dispersants used were sodium diphosphate or sodium hexametaphosphate, and the amount added was 3% by mass relative to aluminum hydroxide. The pH of the slurry was measured after the grinding process. After drying at 120°C, the specific surface area of ​​the aluminum particles was measured according to the method described in JIS Z 8830:2013 (carrier gas method).

[0065] Boric acid was added to the aluminum hydroxide slurry described above to prepare a coating solution with the composition shown in Table 1, and its pH was confirmed. The B / Al values ​​shown in Table 1 are calculated values ​​obtained by mixing and adjusting the aluminum hydroxide particles and boric acid so that the molar ratio B / Al is as shown in each value. That is, the amount of boric acid to be added can be calculated from the molar ratio of boron to aluminum, B / Al, shown in Table 1. In Comparative Example 3, no boric acid was added, and B / Al was 0. In Comparative Example 4, a predetermined amount of acetic acid was added to the aluminum hydroxide slurry to adjust the pH to 5.0. The coating solutions shown in Examples 21 and 30 are examples of using a mixture of two types of aluminum hydroxide particles. Underlines next to numerical values ​​in the following tables indicate that the conditions deviate from the desirable conditions of the present invention.

[0066] [Table 1]

[0067] The composition of the reference coating solution in Table 1 is as follows: • 20% by mass aqueous dispersion of colloidal silica: 100 parts by mass • 50% by mass aqueous solution of aluminum phosphate: 60 parts by mass • Chromic anhydride: 6 parts by mass

[0068] Comparative Example 1 in Table 1 is an example using commercially available amorphous alumina sol. Similarly, Comparative Example 2 is an example using commercially available boehmite sol. As with the aluminum hydroxide slurry described above, pH measurement and specific surface area measurement of aluminum particles were performed.

[0069] The grinding methods in Table 1 are as follows: JM: Jet Mill (Dry Type) BD: Ball mill (dry type) BW: Ball mill (wet type) BM: Bead mill (wet type) The dispersants in Table 1 are as follows: SDP: Sodium Diphosphate SHMP: Sodium Hexametaphosphate

[0070] A 0.23 mm thick grain-oriented electrical steel sheet (B8=1.93T) with a finish annealed coating that had undergone final finish annealing was prepared, and a coating solution with the composition shown in Table 1 was applied, resulting in an insulating coating amount of 5 g / m² after baking. 2 The material was coated and dried, and then baked at 850°C for 30 seconds.

[0071] The obtained grain-oriented electrical steel sheets with insulating coatings were evaluated for coating properties (corrosion resistance and coating tension) and magnetic properties (magnetic flux density and iron loss). The evaluation results are shown in Table 2.

[0072] [Table 2]

[0073] The evaluation methods for each of the evaluations shown in Table 2 are as follows:

[0074] (corrosion resistance) Test specimens measuring 150 mm x 60 mm were prepared from grain-oriented electrical steel sheets with an insulating coating, and their corrosion resistance was evaluated using the "neutral salt spray test" described in JIS Z 2371:2015 Salt spray test method. Specifically, while the test specimens were kept at 35°C, a 5% by mass NaCl aqueous solution adjusted to pH=7.0 was continuously sprayed onto the specimens, and the rust formation was observed after 48 hours. The area ratio of the rusted area to the surface area of ​​the test specimen was then calculated.

[0075] (Film tension) The film tension was calculated from the curvature of the steel sheet when one side of the insulating film was peeled off. The specific conditions were as follows: A 300mm x 30mm test piece was prepared from a grain-oriented electrical steel sheet with insulating films formed on both sides. After applying protective tape to one side, the insulating film on only one side was removed by immersion in an alkaline aqueous solution. Subsequently, the curvature of the grain-oriented electrical steel sheet was measured to calculate the radius of curvature, and the film tension was determined using Equation 1 below. Coating tension (MPa) = 55000 × plate thickness (m) / radius of curvature (m) ... Equation 1

[0076] (Iron loss and magnetic flux density) Iron loss and magnetic flux density were measured in accordance with the method described in JIS C 2550-1:2011. Specifically, under conditions of a measured magnetic flux density amplitude of 1.7T and frequency of 50Hz, the iron loss per unit mass (W) was measured. 17 / 50 The magnetic flux density (B8) was measured at a magnetic force of 800 A / m.

[0077] As shown in Tables 1 and 2, the insulating films obtained using a coating solution containing alumina sol and boric acid in Comparative Example 1, and using a coating solution containing boehmite sol and boric acid in Comparative Example 2, exhibited excellent film tension and magnetic properties, but significantly poor corrosion resistance. In Comparative Examples 5-8, where aluminum hydroxide, which has a smaller specific surface area, was used instead of alumina sol, the reactivity with boric acid was poor, resulting in insufficient formation of aluminum borate. Furthermore, the film density was insufficient, resulting in low film tension and poor corrosion resistance.

[0078] Furthermore, in Comparative Example 3, where boric acid was not added, no film was formed, and fine powder of anhydrous aluminum hydroxide, i.e., aluminum oxide, adhered to the surface of the steel plate. In Comparative Example 4, where the pH of the coating solution was less than 5.5, a film with tension-impeding properties was formed on the steel plate, but the corrosion resistance was insufficient, resulting in the same outcome as when using alumina sol and boric acid.

[0079] Examples 1 to 32 in Table 1 are insulating films formed using an insulating coating solution containing pulverized aluminum hydroxide particles and boric acid. As shown in Table 2, the insulating films of each example exhibited high film tension and excellent corrosion resistance. Furthermore, the insulating films of each example also exhibited excellent magnetic properties. It was also found that the insulating films of each example achieved performance equivalent to or better than that of films obtained using a coating solution containing a chromium compound as shown in the reference example.

[0080] Therefore, it can be seen that the grain-oriented electrical steel sheet obtained using the insulating coating liquid of this embodiment has a dense insulating film, and even without using chromium compounds, it is possible to obtain a film with high film tension and excellent corrosion resistance. Furthermore, it can be seen that in addition to these film characteristics, the magnetic properties and packing factor are also excellent.

[0081] (Experimental Example 2) In Experimental Example 2, corrosion resistance was evaluated under more severe corrosive conditions. New 150mm x 60mm test specimens were prepared from the insulating coated electrical steel sheets of Examples 6-10 and Examples 31 and 32 in Table 1, and their corrosion resistance was evaluated by the "acetic acid salt spray test" described in JIS Z 2371:2015 Salt spray test method. Specifically, with the test specimens maintained at 35°C, a 5% by mass NaCl aqueous solution adjusted to pH=3.0 was continuously sprayed onto the specimens, and the rust formation was observed after 48 hours. The area ratio of the rusted area to the surface area of ​​the test specimen was then calculated.

[0082] Table 3 shows the corrosion resistance evaluation results for Experimental Example 2. The acceptable standard for the corrosion resistance of the coating was set at 1.0%, and coatings with a corrosion resistance of 1.0% or less were considered good in this experiment. In Examples 31 and 32, it can be seen that although no rust occurred in the neutral salt spray test of Example 1, rust occurrence was not completely suppressed in the acetic acid salt spray test. In contrast, in Examples 6 to 10, it can be seen that rust occurrence was prevented even in the acetic acid salt spray test, which has more severe corrosive conditions.

[0083] [Table 3]

[0084] (Experimental Example 3) In Experimental Example 3, the film properties and magnetic properties were evaluated by changing the baking temperature during insulating film formation. A coating solution adjusted to the same composition as Example 9 in Experimental Example 1 was used, and the insulating film amount after baking was 5 g / m², following the same procedure as in Experimental Example 1. 2 The coating was applied and dried, and then the baking process was performed by changing the baking temperature to the conditions shown in Table 4. The baking time was 30 seconds. The results are shown in Table 4.

[0085] [Table 4]

[0086] As shown in Table 4, Comparative Examples 9 and 10, where the baking temperature was less than 600°C, exhibited low film tension. This is thought to be due to insufficient reaction between the aluminum hydroxide particles and boric acid. On the other hand, Examples 33 to 36, where the baking temperature was 600°C or higher, were found to yield excellent film tension and magnetic properties.

[0087] Although preferred embodiments of the present invention have been described above, the present invention is not limited to such examples. It will be obvious to those skilled in the art that various modifications or alterations can be conceived within the scope of the ideas described in the claims, and these will naturally also fall within the technical scope of the present invention. [Industrial applicability]

[0088] The coating solution, its manufacturing method, and the manufacturing method for grain-oriented electrical steel sheets of the present invention do not use harmful substances such as chromic acid, and provide a coating with high film tension, excellent magnetic properties, and excellent corrosion resistance, making them extremely useful in industry.

Claims

1. A coating liquid for forming an insulating film used on grain-oriented electrical steel sheets, It contains aluminum hydroxide particles and boric acid. The specific surface area of ​​the aluminum hydroxide particles is 20 m². ２ / g or more, The pH is 5.5 or higher. A coating liquid characterized by the following features.

2. The pH of the coating solution is 6.0 or higher. The coating solution according to feature 1.

3. The aluminum hydroxide particles consist of gibbsite, bayerite, boehmite, diaspore, or a combination thereof. The coating solution according to claim 1 or 2.

4. The content ratio of the aluminum hydroxide particles to the boric acid in the coating solution is 0.2 to 1.5 in terms of the molar ratio of boron to aluminum. The coating solution according to claim 1 or 2.

5. A method for manufacturing a coating liquid for forming an insulating film used on grain-oriented electrical steel sheets, Mix a dispersion of aluminum hydroxide with a pH of 6.0 or higher with boric acid, or A boric acid solution, obtained by dissolving boric acid in a solvent, is mixed with aluminum hydroxide particles that, when mixed with water, have a pH of 6.0 or higher. It contains aluminum hydroxide particles and boric acid, and the specific surface area of ​​the aluminum hydroxide particles is 20 m². ２ Prepare a coating solution that has a concentration of 1 / g or more and a pH of 5.5 or higher. A method for producing a coating solution, characterized by the above.

6. The pH of the coating solution is 6.0 or higher. A method for producing a coating solution according to feature 5.

7. The aluminum hydroxide particles consist of gibbsite, bayerite, boehmite, diaspore, or a combination thereof. A method for producing a coating solution according to claim 5 or 6.

8. The content ratio of the aluminum hydroxide particles to the boric acid in the coating solution is 0.2 to 1.5 in terms of the molar ratio of boron to aluminum. A method for producing a coating solution according to claim 5 or 6.

9. For grain-oriented electrical steel sheets that have undergone final finishing annealing, It contains aluminum hydroxide particles and boric acid, and the specific surface area of ​​the aluminum hydroxide particles is 20 m². ２ A step of applying a coating solution that has a concentration of 1 / g or more and a pH of 5.5 or higher, The process includes a step of applying the aforementioned coating liquid to a grain-oriented electrical steel sheet and then baking it at a temperature of 600°C to 1000°C. A method for manufacturing grain-oriented electrical steel sheets, characterized by the following features.

10. The pH of the coating solution is 6.0 or higher. The method for manufacturing grain-oriented electrical steel sheets according to feature 9.

11. The aluminum hydroxide particles consist of gibbsite, bayerite, boehmite, diaspore, or a combination thereof. A method for manufacturing grain-oriented electrical steel sheets according to claim 9 or 10.

12. The content ratio of the aluminum hydroxide particles to the boric acid in the coating solution is 0.2 to 1.5 in terms of the molar ratio of boron to aluminum. A method for manufacturing grain-oriented electrical steel sheets according to claim 9 or 10.

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