JP7839434B2 - Coating solution, method for manufacturing the coating solution, and method for manufacturing grain-oriented electrical steel sheet - Google Patents

Description

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.

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

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 annealing is performed. After this, an annealing separating agent mainly composed of MgO and _Al₂O₃ is applied, and a final finish annealing is performed. This develops a crystalline structure with the {110}<001> orientation as the main orientation, _and a finish annealing film mainly composed of _Mg₂SiO₄ is formed on the surface of the steel sheet. Finally, an insulating coating liquid is applied, baked _, and then shipped.

Grain-oriented electrical steel sheets have the property that iron loss is 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 is 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.

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.

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.}

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.

Japanese Patent Publication No. 48-039338 Japan Special Publication No. 54-143737 Japanese Patent Publication No. 2000-169972 Japanese Patent Application Publication No. 2000-178760 International Publication No. 2015/115036 Japanese Patent Application Publication No. 06-065754 Japanese Patent Publication No. 06-065755 Japanese Patent Application Publication No. 06-306628 Japanese Patent Application Publication No. 08-325745 Japanese Patent Application Publication No. 09-256164 Japanese Patent Application Publication No. 2019-137874

Yoshitane Watanabe, "Synthesis and Properties of Oxide Sols," Gypsum and Lime, No. 211, p. 46 (1987) Taichi Sato, "On Aluminum Hydroxide and Alumina," Journal of the Mineralogical Society of Japan, Vol. 19, p. 21 (1989)

Conventional technologies each had the following problems:
The coating solution for forming an insulating film described in Patent Document 1 contains hexavalent chromium. Therefore, equipment considerations are necessary to improve the working environment in the process of forming an insulating film on grain-oriented electrical steel sheets. In recent years, with increasing environmental awareness, there has been a strong demand for the development of a coating solution for forming an insulating film on grain-oriented electrical steel sheets that does not contain hexavalent chromium and can produce an insulating film with excellent film properties such as tensile strength. The film tension of an insulating film obtained using an insulating film forming coating solution that does not contain chromic acid and uses additives other than chromic acid, as in the technologies of Patent Documents 2 to 5, is lower than that of an insulating film obtained using an insulating film forming coating solution that contains chromic acid. Furthermore, the additives used in the technologies of Patent Documents 2 to 5 are all more expensive than chromic acid.

The insulating films disclosed in Patent Documents 6 to 8 do not contain harmful substances such as chromic acid, as is clear from the components of the coating solution. Furthermore, compared to insulating films currently used as standard on grain-oriented electrical steel sheets, 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, the film tension imparted to the steel sheet is 1.5 to 2 times greater, and the iron loss improvement effect is greater than that of current films. However, aluminum borate films formed by baking a coating solution composed of alumina sol and boric acid still have room for further improvement in terms of corrosion resistance, as described in Patent Documents 9 and 10. Patent Documents 9 and 10 disclose a method of adding an alkali metal compound or alkaline earth metal compound to a coating solution composed of alumina sol and boric acid as a measure to improve corrosion resistance. Although these additives are effective in improving the corrosion resistance of the film, they increase the pH of the coating solution. Therefore, there is a problem in that the alumina sol component gels, making it difficult to apply it stably to steel plates.
The aluminum oxide disclosed in Patent Document 11 is a stable compound with poor reactivity, and it is not easy to obtain an aluminum borate compound even when mixed with boric acid and baked. The reason why the formation of aluminum borate films has been successful in Patent Documents 6 to 10 is that highly reactive alumina sol is used as the Al source. Alumina sol is not fine particles of alumina (aluminum oxide), but a dispersion of fine particles of aluminum oxide hydrate, and its reactivity increases when dehydrated by heating. Therefore, the industrial production of grain-oriented electrical steel sheets with aluminum borate as the main component and forming an insulating coating with good corrosion resistance has not been successful.

The present invention has been made in view of the above, and aims to provide a coating liquid for forming an insulating film on grain-oriented electrical steel sheets, a method for manufacturing the same, and a method for manufacturing grain-oriented electrical steel sheets, which do not use harmful substances such as chromic acid and have film properties that are high in film tension, excellent in magnetic properties, and excellent in corrosion resistance.

(1) A coating liquid according to one aspect of the present invention is 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 /g or more.
It is characterized by having a pH of 5.5 or higher.
(2) In the coating solution described in (1) above, the pH of the coating solution may be 6.0 or higher.
(3) In the coating solution described in (1) or (2) above, the aluminum hydroxide particles may consist of gibbsite, bayerite, boehmite, diaspore, or a combination thereof.
(4) In the coating solution described in any one of the above items (1) to (3), 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.
(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.
The present invention is characterized by producing a coating solution containing aluminum hydroxide particles and boric acid, wherein the specific surface area of the aluminum hydroxide particles is 20 ^m² /g or more, and the pH is 5.5 or higher.
(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, for a grain-oriented electrical steel sheet that has undergone final finish annealing,
A step of applying a coating solution containing aluminum hydroxide particles and boric acid, wherein the specific surface area of the aluminum hydroxide particles is 20 ^m² /g or more and the pH is 5.5 or more.
The method is characterized by comprising the step of applying a baking treatment to a grain-oriented electrical steel sheet coated with the aforementioned coating liquid at a temperature of 600°C to 1000°C.
(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.

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.

The 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 embodiments described below 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.

<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.

For example, the alumina sols disclosed in Patent Documents 6 to 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). Because 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.

Methods for producing alumina sol and boehmite sol include, as described in Non-Patent Literature 1, an ion exchange method in which excess anions (such as Cl) are removed from a basic aluminum salt (e.g., AlOH) x Cl) using an ion exchange resin; a neutralization method in which an acidic salt (e.g., _AlCl3 ) is neutralized and washed, and the resulting gel is disintegrated with an acid (e.g., HCl); a reaction method in which metallic Al is reacted with an acid (e.g., HCl) to create a basic aluminum salt (e.g., AlOH) x Cl), the resulting gel is hydrolyzed, and the resulting gel is disintegrated with an acid; and an organometallic hydrolysis method in which an aluminum alkoxide is hydrolyzed, and the resulting gel is disintegrated 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), and the sol becomes acidic. For example, Non-Patent Literature 1 shows pH 3.8 and pH 4.0 as specific examples of the pH of 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.

As mentioned earlier, although industrial coating operations become difficult, the inventors believe that the pH of the insulating coating solution influences 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.

Furthermore, while 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.

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' studies, 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 set 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.

Non-patent document 2 indicates that aluminum hydroxide is dehydrated at around 300°C or 500°C. Since aluminum hydroxide, like alumina sol, is activated during the heating dehydration process, 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. Furthermore, if the film is formed densely, it becomes an insulating film with high film tension and good corrosion resistance.

The following describes the materials that constitute the coating solution according to this embodiment.

(Aluminum hydroxide particles)
The coating solution according to this embodiment contains aluminum hydroxide particles. The aluminum hydroxide particles may be of one type or two or more types.

The aluminum hydroxide in this embodiment is preferably an alumina hydrate _such as gibbsite or bayalite in trihydrate form ( _{Al₂O₃ ・ 3H₂O} ), or boehmite 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 hydrazilite. Gibbsite, boehmite, 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 trihydrates such as gibbsite and bayerite is around 300°C, making them more reactive than monohydrates, which require dehydration temperatures around 500°C, thus offering advantages in terms of manufacturing costs such as time and expense. Gibbsite and bayerite also have the advantage of being readily available.

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.

The finer the aluminum hydroxide particles and the larger their 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 ^m² /g or more, more preferably 40 ^m² /g or more, and even more preferably 50 ^m² /g or more.

On the other hand, the upper limit of the specific surface area is not particularly limited; it may be 200 ^m² /g or less, 180 ^m² /g or less, or 150 ^m² /g or less. Keeping the upper limit of the specific surface area below the above limits makes it easier to maintain the dispersion stability (viscosity stability) of the coating liquid for forming the insulating film. The specific surface area of aluminum hydroxide particles is the specific surface area based on the BET method and is measured by a method compliant with JIS Z 8830:2013.

(Manufacturing of aluminum hydroxide particles with a specific surface area of 20 ^m² /g or more)
It is difficult to obtain commercially available aluminum hydroxide with a specific surface area of 20 ^m² /g or more for industrial use. For example, C-301N, manufactured by Sumitomo Chemical Co., Ltd., is among the smallest particle sizes of commercially available aluminum hydroxide powders, but its specific surface area is approximately 4 ^m² /g. By grinding such commercially available products, it is possible to obtain aluminum hydroxide fine particles with a specific surface area of 20 ^m² /g or more.

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.

In wet grinding processes, the viscosity of the dispersion increases as the specific surface area of the aluminum hydroxide particles increases. Furthermore, when the specific surface area exceeds 200 ^m² /g due to grinding, the viscosity of the dispersion increases, potentially causing gelation and hindering the grinding process. Therefore, a dispersant may be added to the dispersion as needed.

The increase in viscosity during the grinding process can be suppressed by adding a dispersant. However, among dispersants, if an organic dispersant is added, it may decompose and carbonize during the baking of the insulating film, potentially causing carburization in the grain-oriented electrical steel sheet. Therefore, when using a dispersant, an inorganic dispersant is preferable. Furthermore, in order 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.

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.

Preferably, the alkali metal content relative to the total mass of solids in the coating solution is 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) apparatus in accordance with JIS K 0116:2014 General Rules for Emission Spectroscopic Analysis.

(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 preferable. 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 4 g per 100 g 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 3.4 g of Al(OH) _₃ should be added to 4 g of orthoboric acid and 100 g of water. The anhydrous solid content concentration of this solution (calculated using _{B₂O₃ for} B and _Al₂O₃ for _Al ) is only 4%. Since the solubility of orthoboric acid increases to slightly over 8g at 40°C, warming the preparation of the coating solution and maintaining the temperature of the solution while coating will facilitate the coating process.

(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 made too large, the solid content concentration in the coating solution must be reduced, making it difficult to obtain the desired film amount. 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.

(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.

The solid content concentration of the insulating coating solution is not particularly limited, as long as it is within a 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.

Furthermore, the insulating film-forming coating liquid according to this embodiment may contain small amounts of other additives as needed, within a range that does 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.

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.

The viscosity of the coating solution for forming an insulating film is preferably 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.

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.

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.

Here, 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-grained electrical steel sheets. However, even if the insulating film-forming coating liquid according to this embodiment is applied to non-grained 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-grained electrical steel sheets.

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 is impossible or difficult to measure the pH. Therefore, the coating solution should be sampled, water added to make the weight ratio of water in the dispersion medium (solvent) 50% or higher, and then the pH should be checked.
pH is measured using a pH measuring device.

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 more, 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.

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.

Furthermore, other additives may be added and mixed as needed. The insulating coating solution can 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).

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.

(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.

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, where 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 silicate derived from the aluminum hydroxide particles. Next, the B concentration is determined by ICP-AES analysis (inductively coupled plasma-atomic emission spectroscopy) of the filtrate, and the boric acid concentration in the coating solution can be calculated from the B concentration. Furthermore, the concentration of aluminum hydroxide in the coating solution can be determined by measuring the weight of the residue and comparing it to the weight of the dispersion before filtration. The presence of Al can be measured by X-ray fluorescence measurement, and the presence of aluminum hydroxide particles can be measured 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.

<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.

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

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.

(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 need to have a finish annealing film.

(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.

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%.

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.

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.

The insulating film after the baking treatment 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 treatment can be determined by cross-sectional SEM observation.

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, the 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.

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 achieves similar effects is included within the technical scope of the present invention.

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

(Experimental Example 1)
First, commercially available gibbsite (hydrargillite), bayerite, boehmite, and diaspore powder (with specific surface areas of 4, 2, 16, and ^{1 m²} /g, respectively) were prepared. As shown in Table 1, comparative examples 5 to 8 were used as is, while the other comparative examples and examples were subjected to grinding by the respective grinding methods. 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 the grinding process. The dispersants used were sodium diphosphate or sodium hexametaphosphate, and the amount added was 3% by mass relative to the 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).

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.

The composition of the reference coating solution in Table 1 is as follows:
100 parts by mass of 20% by mass aqueous dispersion of colloidal silica, 60 parts by mass of 50% by mass aqueous solution of aluminum phosphate, and 6 parts by mass of chromic anhydride.

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.

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

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. A coating solution with the composition shown in Table 1 was applied and dried so that the amount of insulating film after baking was 5 g/ ^m² , and a baking treatment was performed at 850°C for 30 seconds.

The obtained grain-oriented electrical steel sheets with insulating coatings were evaluated for their coating properties (corrosion resistance and coating tension) and magnetic properties (magnetic flux density and iron loss). Table 2 shows the evaluation results.

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

(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.

(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 300 mm x 30 mm 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

(Iron loss and magnetic flux density)
Iron loss and magnetic flux density were measured according to the method described in JIS C 2550-1:2011. Specifically, iron loss per unit mass (W _17/50 ) was measured under conditions of a measured magnetic flux density amplitude of 1.7 T and a frequency of 50 Hz. For magnetic flux density (B ₈ ), the value of the magnetic flux density at a magnetization force of 800 A/m was measured.

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 to 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, and the film density was also insufficient, resulting in low film tension and poor corrosion resistance.

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 tensile strength 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.

Examples 1 to 32 in Table 1 are insulating films formed using an insulating film-forming 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 produced using a coating solution containing a chromium compound as shown in the reference example.

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

(Experimental Example 2)
In Experimental Example 2, corrosion resistance was evaluated under more severe corrosive conditions. New 150 mm x 60 mm 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 mass% NaCl aqueous solution adjusted to pH = 3.0 was continuously sprayed onto the specimens, and the rust formation after 48 hours was observed. The area ratio of the rusted area to the surface area of the test specimen was then calculated.

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. Examples 31 and 32 show that while no rust occurred in the neutral salt spray test of Example 1, rust formation was not completely suppressed in the acetic acid salt spray test. In contrast, Examples 6 to 10 show that rust formation was prevented even in the acetic acid salt spray test, which has more severe corrosive conditions.

(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 prepared to the same composition as Example 9 in Experimental Example 1 was applied and dried using the same procedure as in Experimental Example 1, so that the amount of insulating film after baking was 5 g/ ^m² . The baking temperature was changed to the conditions shown in Table 4, and the baking time was 30 seconds. The results are shown in Table 4.

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

While preferred embodiments of the present invention have been described above, the present invention is not limited to these examples. It will be clear 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.

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 (12)

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 /g or more.
A coating solution characterized by having a pH of 5.5 or higher. The coating solution according to claim 1, characterized in that the pH of the coating solution is 6.0 or higher. The coating solution according to claim 1 or 2, characterized in that the aluminum hydroxide particles consist of gibbsite, bayerite, boehmite, diaspore, or a combination thereof. The coating solution according to claim 1 or 2, characterized in that the content ratio of aluminum hydroxide particles to 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 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.
A method for producing a coating solution, characterized by producing a coating solution containing aluminum hydroxide particles and boric acid, wherein the specific surface area of the aluminum hydroxide particles is 20 ^m² /g or more, and the pH is 5.5 or more. The method for producing a coating solution according to claim 5, characterized in that the pH of the coating solution is 6.0 or higher. The method for producing a coating solution according to claim 5 or 6, characterized in that the aluminum hydroxide particles consist of gibbsite, bayerite, boehmite, diaspore, or a combination thereof. The method for producing the coating solution according to claim 5 or 6, characterized in that 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. For grain-oriented electrical steel sheets that have undergone final finishing annealing,
A step of applying a coating solution containing aluminum hydroxide particles and boric acid, wherein the specific surface area of the aluminum hydroxide particles is 20 ^m² /g or more and the pH is 5.5 or more.
A method for manufacturing a grain-oriented electrical steel sheet, characterized by comprising the step of applying a baking treatment to the grain-oriented electrical steel sheet coated with the aforementioned coating liquid at a temperature of 600°C to 1000°C. The method for manufacturing grain-oriented electrical steel sheets according to claim 9, characterized in that the pH of the coating solution is 6.0 or higher. The method for manufacturing grain-oriented electrical steel sheets according to claim 9 or 10, characterized in that the aluminum hydroxide particles consist of gibbsite, byerlite, boehmite, diaspore, or a combination thereof. The method for producing grain-oriented electrical steel sheets according to claim 9 or 10, characterized in that the content ratio of aluminum hydroxide particles to boric acid in the coating solution is 0.2 to 1.5 in terms of the molar ratio of boron to aluminum.