JP7481628B2 - Coating agent for forming tension coating on grain-oriented electrical steel sheet, method for producing same, and method for producing grain-oriented electrical steel sheet using same - Google Patents

Description

The present invention relates to a coating agent for forming a tension coating on grain-oriented electrical steel sheets, a manufacturing method thereof, and a manufacturing method thereof for grain-oriented electrical steel sheets.

Grain-oriented electrical steel sheets are characterized by having a crystal structure with a primary orientation of {110}<001>. Grain-oriented electrical steel sheets are primarily used as iron core materials for transformers, and materials with low iron loss are particularly sought after to reduce energy loss. Iron and iron alloys containing silicon have large crystalline magnetotropy, so when external tension is applied, magnetic domains are subdivided, and eddy current loss, the main factor in iron loss, can be reduced. In particular, it is known that applying tension to the steel sheet is effective in reducing the iron loss of grain-oriented electrical steel sheets containing 5% or less silicon. This tension is applied by a coating formed on the surface.

Patent Document 1 discloses a grain-oriented electrical steel sheet having a coating that generates particularly high tension, the coating being mainly composed of aluminum borate crystals on the surface.

For a coating to become a high-tensile coating, it is required that the coating have a high Young's modulus and a small coefficient of thermal expansion. In general, crystals have a higher Young's modulus than amorphous materials. A coating made of aluminum borate is primarily composed of crystals, so it has a higher Young's modulus than conventional amorphous coatings made of silica and phosphate. A coating made of aluminum borate also has a sufficiently low coefficient of thermal expansion, which, combined with the effect of the Young's modulus, makes it possible to obtain high tensile strength.

However, the technology in Patent Document 1 has a weaker rust prevention effect than the insulating coatings currently in use, and this point needed to be improved.

To solve these problems, Patent Document 2 discloses a method of applying two layers of an aluminum borate coating and an existing coating. Patent Document 3 discloses a base treatment prior to the formation of an aluminum borate coating by electroless nickel plating. Patent Document 4 discloses a method of applying a rust-preventive component onto an aluminum borate coating. Patent Document 5 proposes an aluminum borate coating component that contains a rust-preventive component.

Japanese Patent Application Laid-Open No. 6-65754 Japanese Patent Application Laid-Open No. 9-272982 Japanese Patent Application Laid-Open No. 9-279358 Japanese Patent Application Laid-Open No. 8-277474 Japanese Patent Application Laid-Open No. 9-256164

The technology of Patent Document 2 had the problem that the cost increased due to the increase in the number of processes and the space factor deteriorated. The technologies of Patent Documents 3 and 4 also had the problem of the increase in costs due to the increase in the number of processes. The technology of Patent Document 5 had the problem that it was difficult to overcome the problem of the reduction in tension due to the rust-preventing components.

The present invention has been made in consideration of the above circumstances, and aims to provide a simple method for improving the rust prevention properties of grain-oriented electrical steel sheets having an aluminum borate coating that is imparted with high tension.

The inventors believed that the rust resistance problem in aluminum borate coatings could be solved by suppressing the amount of unreacted boron contained in the coating, and investigated methods for reducing the amount of unreacted boron. As a result, they found that adding fine particles of aluminum borate to the coating agent used to form the aluminum borate coating was effective in promoting the crystallization of aluminum borate.

The present invention was made based on the above findings, and its gist is as follows:

(1) A coating agent for forming a tension coating on grain-oriented electrical steel sheets, characterized in that the solid content is 5-40% by mass, the solid content is 1-50% by mass of fine particles having an average particle size of 0.1-0.7 μm and made of aluminum borate crystals with a (220) interplanar spacing of 0.525-0.535 nm, and the remainder is an aluminum compound and a boron compound in a molar ratio of Al to B, Al/B, in the range of 1.9-2.1.

(2) A method for producing a coating agent for forming a tensile coating on grain-oriented electrical steel sheets as described in (1) above, comprising the steps of preparing a mixture containing an aluminum compound and a boron compound in a molar ratio of Al to B, Al/B, in the range of 1.9 to 2.1, firing the mixture at 800 to 1000°C to synthesize aluminum borate powder, pulverizing the obtained aluminum borate powder to obtain fine particles having an average particle size of 0.1 to 0.7 μm and consisting of aluminum borate crystals with a (220) interplanar spacing of 0.525 to 0.535 nm, and adding the obtained fine particles to a solvent.

(3) A method for producing grain-oriented electrical steel sheet, comprising: applying the coating agent for forming a tension coating for grain-oriented electrical steel sheet described in (1) to a steel sheet that has been subjected to finish annealing, drying the coating agent, and then heat treating the steel sheet at 750 to 1000°C for 20 seconds or more in an atmosphere having a dew point of 0 to 40°C, containing 0 to 30% by volume of hydrogen, and the remainder being one or both of nitrogen and an inert gas.

The present invention provides a coating agent for forming a tensile coating on grain-oriented electrical steel sheet that has sufficient rust resistance and can easily form an aluminum borate coating with high tension, a method for producing the same, and a method for producing grain-oriented electrical steel sheet on which an aluminum borate coating is formed using the same.

The present invention will be described in detail below based on a preferred embodiment of the present invention.

First, we will explain the coating agent for forming a tension coating on the grain-oriented electrical steel sheet according to this embodiment (hereinafter, simply referred to as "coating agent").

(Investigation by the inventor)
A known grain-oriented electrical steel sheet is one in which an aluminum borate coating having a high tension-imparting effect is formed on the surface of a steel sheet that has undergone secondary recrystallization annealing and has a so-called glass coating on its surface. The glass coating is a coating made of _an oxide mainly composed of forsterite crystals ( _Mg2SiO4 ) of about several μm in size.

This coating usually has tiny defects, and it is believed that in these defective areas the steel sheet is not covered by forsterite. In other words, defects in the glass coating leave the steel sheet exposed in places where it is in direct contact with the aluminum borate coating, and it is believed that in these areas, when the aluminum borate coating absorbs moisture, iron dissolves, causing rust.

In order to prevent rust from occurring through this mechanism, it is conceivable to prevent the pH of the steel sheet surface from being at a level at which iron dissolves. The inventors have investigated a method for improving rust resistance by preventing the pH of the steel sheet surface from being at a level at which iron dissolves. The idea behind this is as follows.

The inventors hypothesized that the pH at the steel sheet surface at which iron dissolves is due to the presence of boric acid in the coating, and that boric acid is produced for the following reasons:

The ratio of aluminum to boron in the coating agent for forming an aluminum borate film according to the prior art is a composition in which the amount of boron is greater than the stoichiometric composition of aluminum borate, Al ₄ B ₂ O _9. The reason for this is that if the ratio of aluminum to boron were the stoichiometric composition of aluminum borate, the temperature at which aluminum borate crystals are formed would be significantly higher, and a very high temperature would be required in the film formation process. Therefore, in the prior art, the Al/B is made smaller than the stoichiometric composition of aluminum borate, resulting in a composition in which aluminum borate crystals can be formed at about 850°C.

It is believed that excess boron compared to the stoichiometric composition remains in the coating as boric acid even after the coating is baked. When a steel sheet having a coating with residual boric acid is exposed to a humid atmosphere, the boric acid present in the coating absorbs moisture to form an acidic environment, which satisfies the conditions for iron to dissolve at the interface between the aluminum borate coating and the steel sheet, causing rust. Therefore, it is believed that the way to improve rust resistance is to eliminate the excess boric acid.

Because boric acid has a high vapor pressure at high temperatures, excess boric acid may be vaporized by heat treatment at high temperatures and removed from the coating. In fact, baking a coating agent with a composition containing more boron than the stoichiometric composition of aluminum borate at around 1200°C significantly improves rust resistance. However, in order to actually carry out production using this method, heat treatment at a high temperature of around 1200°C is required, and technological development is required to avoid the plastic deformation of the plate, which is a problem when continuous heat treatment is performed at a high temperature of around 1200°C. Therefore, when using a coating agent with a composition containing more boron than the stoichiometric composition of aluminum borate, it is currently not possible to realize a production technology that can remove excess boric acid from the coating after baking.

The inventors therefore investigated forming aluminum borate crystals at low temperatures without increasing the boron ratio. As a result, they discovered that adding fine particles of aluminum borate crystals to a coating agent for forming an aluminum borate film is effective. The reason why the temperature at which aluminum borate crystals are formed is lowered by adding fine particles of aluminum borate crystals to a coating agent for forming an aluminum borate film is thought to be as follows.

When an aluminum borate coating, which has a composition with a higher amount of boron than the stoichiometric composition, is applied to a steel sheet and dried, a mixture of boric acid and alumina is formed. When the temperature is raised from this state, the boric acid and alumina react, and it is believed that an amorphous phase of aluminum borate is formed first. If fine particles of aluminum borate crystals are already present in the system in which this reaction progresses, these act as nuclei to make it easier for aluminum borate crystals to form, and as a result, it is believed that a tensile coating made of aluminum borate crystals can be formed at low temperatures.

From the above, the mechanism by which the effects of the present invention are achieved is presumed to be as follows. That is, when a coating agent containing fine aluminum borate particles is baked, aluminum borate crystals are formed in the coating at a lower temperature than before, so it is possible to reduce the amount of boron source that was previously mixed in excess into the coating agent to lower the temperature at which aluminum borate crystals form. As a result, the amount of boric acid remaining in the coating after baking can be significantly reduced, so that even in the presence of moisture, the environment at the interface between the steel sheet and the coating is no longer one in which iron dissolves.

<Coating agent for forming coating on grain-oriented electrical steel sheets>
The coating agent for forming a tension film on grain-oriented electrical steel sheet according to the present invention will be described in detail below.

The coating agent for forming a tensile coating on grain-oriented electrical steel sheet of the present invention has a solids concentration of 5 to 40 mass %, contains 1 to 50 mass % of fine particles of aluminum borate crystals with an average particle size of 0.1 to 0.7 μm and a (220) interplanar spacing of 0.525 to 0.535 nm, and contains aluminum compounds and boron compounds in the balance with a molar ratio of Al to B, Al/B, in the range of 1.9 to 2.1. Here, the molar ratio of Al to B, Al/B, refers to the molar ratio of Al atoms and B atoms contained in the aluminum compounds and boron compounds. The aluminum compounds and boron compounds contained in the balance are, respectively, an aluminum source such as aluminum oxide and a boron source such as boric acid, which will be described later.

There are several types of aluminum borate crystals. The additive particles that can obtain high tension when included in the coating agent for forming a tensile film on grain-oriented electrical steel sheet in the present invention are crystals with a composition of Al ₄ B ₂ O _9. This is an aluminum borate crystal with a plane spacing (220) of the strongest ray in X-ray diffraction using a Cu target of 0.525 to 0.535 nm.

The (220) interplanar spacing of the added particles in the coating agent is 0.525-0.535 nm, which can be confirmed by X-ray diffraction using a powdered Cu target obtained by drying the coating agent at around 100°C. The dried coating agent contains boric acid and aluminum hydroxide in addition to fine aluminum borate particles, but as the diffraction lines of these are different from those of aluminum borate, it is possible to separate the diffraction lines of the added fine aluminum borate particles.

The aluminum borate crystals are fine particles with an average particle size of 0.1 μm to 0.7 μm. If the size of the fine particles of the aluminum borate crystals is too large, problems will arise with the smoothness of the coating, and the space factor will decrease after the formation of the tensile coating, which is not preferable. On the other hand, if the particle size is too small, it will be difficult to disperse them in the coating agent, and the particles will tend to aggregate together in the coating agent, making it difficult to achieve the effect of adding the fine particles. Therefore, the average particle size of the fine particles to be added needs to be 0.1 to 0.7 μm.

The particle size of the fine particles can be easily calculated from the half-width of the X-ray diffraction peak of the dried powder. This method estimates the crystal grain size t of the aluminum borate crystals using the Scherrer formula shown in formula (1).

Here, K is a constant (0.9), λ is the wavelength of the X-ray (0.1542 nm), B is the half-width (rad), and θ is the peak position (°) in the X-ray diffraction with a Cu target.

If the amount of fine particles of aluminum borate crystals in the coating agent is too small, the effect of lowering the temperature at which aluminum borate crystals form will not be obtained. On the other hand, if the amount is too large, a smooth tensile coating will not be obtained and the space factor of the product will decrease. Therefore, the amount of fine particles of aluminum borate crystals in the solid content is set to 1 to 50 mass%. In addition to the fine particles of aluminum borate crystals, the solid content also contains an aluminum source such as aluminum oxide and a boron source such as boric acid.

The amount of fine particles in the solid content can be determined by first determining the solid content of the coating agent, and then determining the amount of boron in the precipitate when the coating agent is allowed to stand. The solid content of the coating agent is calculated according to formula (2).

Solid content (g) = A + B ... (2)
Where A: mass (g) of the aluminum source converted into aluminum oxide
B: mass of boron source converted to orthoboric acid (g)

The fine particles in the present invention are considered to have a stoichiometric composition of aluminum borate Al ₄ B ₂ O _9. Therefore, the amount of fine particles in the solid content of the coating agent can be calculated from the amount of boron in the precipitate, the molar mass of aluminum borate, and the solid content of the initial coating agent. The calculation method is as follows.

First, the coating material to be analyzed is divided equally into two containers. The coating material in one container is dried, and the amount of boron and aluminum contained in the residue is determined by chemical analysis, etc., and the number of moles of each is designated p (moles) and q (moles).

The precipitate from the coating agent in the other container is collected and the amount of fine aluminum borate particles contained therein is determined. Since the entire amount of precipitate needs to be collected, the supernatant liquid after leaving it to stand is carefully discarded and the remaining precipitate is collected. When leaving it to stand, about 10% by mass of inorganic acid is added to the coating agent before leaving it to prevent the coating agent from gelling. Nitric acid or hydrochloric acid is used as the inorganic acid. Adding inorganic acid causes the alumina sol and other alumina sources in the coating agent to coagulate and become less dispersed, but this does not affect the analysis of the amount of boron in the precipitate.

In this method, it is necessary to thoroughly remove all boric acid components in the coating agent other than the boron contained in the fine particles. Since boron other than the fine particles is water-soluble boric acid, if there is a large amount of boric acid dissolved in the supernatant or precipitated in the precipitate, a large amount of boric acid will remain in the precipitate dried for analysis, and the amount of fine particles in the solid content cannot be accurately measured. For this reason, after discarding the supernatant, pure water three times the volume of the precipitate containing the remaining water is added, thoroughly stirred, and then allowed to stand again, and the supernatant is discarded. This process is repeated five times. By performing such operations, the amount of boric acid remaining in the solid content can be sufficiently reduced.

The number of moles of boron contained in the precipitate obtained in this manner is called r (moles). If the supernatant is discarded five or more times, the boric acid contained in the coating agent is sufficiently removed, so r can be considered to be derived from the boron in the aluminum borate microparticles.

The ratio of aluminum borate fine particles in the solid content is calculated as follows:

If the amount of boron other than the aluminum borate fine particles contained in the residue after drying is converted to orthoboric acid as X (g), and the amount of aluminum is converted to aluminum oxide as Y (g), then X = (p-r) x 59.8 from p and q, respectively.
Y = {(q - r x 2) / 2} x 102
In the above formula, 59.8 is the molecular weight of orthoboric acid, and 102 is the molecular weight of aluminum oxide.

If the weight of the aluminum borate fine particles in the precipitate is Z (g), the proportion M (mass%) of the aluminum borate fine particles in the solid content is Z = (r/2) × 146.5
M = Z x 100 / (X + Y + Z)
Here, 146.5 is the molecular weight of aluminum borate (Al ₄ B ₂ O ₉ ).

In this way, the proportion of aluminum borate fine particles in the solids can be determined.

As with known technologies, there is an appropriate range for the solids concentration of the entire coating agent to which fine particles have been added. If it is too low, bumping and other problems will occur during drying, resulting in coating defects and reduced tension of the coating. On the other hand, if it is too high, the stability of the coating agent will decrease and gelation will occur more easily, resulting in a coating with many defects and reduced tension. Therefore, the solids concentration must be between 5 and 40% by mass. Here, the solids concentration is defined according to formula (3).

Solid concentration (mass%) = {(W _AB + W _Al + W _B ) × 100} / (W _AB + W _Al + W _B + W _W ) ... (3)
here
W _AB : Mass of aluminum borate microparticles
W _Al : Mass of aluminum source converted to aluminum oxide
W _B : Mass of boron source converted to orthoboric acid
W _W : The water content of the coating agent.

The aluminum oxide-equivalent mass of the aluminum source means that the raw material serving as the aluminum source is replaced with the mass of aluminum _oxide ( _Al2O3 ) such that the molar amount of aluminum contained in the aluminum oxide is equal to that of the raw material serving as the aluminum source. Similarly, the orthoboric acid-equivalent mass of the boron source means that the raw material serving as the boron source is replaced with the mass of orthoboric acid ( _H3BO3 ) such that the molar amount of boron contained in the _orthoboric acid is equal to that of the raw material serving as the boron source.

The amount of aluminum borate fine particles in the solid content is calculated using formula (4).

Amount of aluminum borate fine particles in the solid content (wt%) = W _AB × 100 / (W _AB + W _Al + W _B ) ... (4)

The ratio of boron to aluminum other than the fine particles in the coating agent of the present invention must be less boron than in the prior art, and good results are obtained when Al/B is between 1.9 and 2.1. If Al/B is too small, the amount of excess boric acid in the coating increases, resulting in poor rust resistance, while if it is too large, sufficient tension cannot be obtained.

<Method of manufacturing coating agent for forming tensile film on grain-oriented electrical steel sheet>
The coating agent for forming a tensile film on the above-mentioned grain-oriented electrical steel sheet may be prepared by obtaining fine particles of aluminum borate having a (220) interplanar spacing of 0.525 to 0.535 nm, and adding the particles to a solvent so as to achieve the above-mentioned solid content concentration, Al/B.

Fine particles of aluminum borate with a (220) interplanar spacing of 0.525 to 0.535 nm can be obtained by synthesizing aluminum borate powder and then pulverizing it using a ball mill or the like. Aluminum borate powder can be obtained by drying and firing a sol with the same composition as a coating agent in known technology that forms an aluminum borate coating.

The firing temperature must be high enough to form aluminum borate crystals, and must be at least 800°C. If the temperature is higher, there is no particular problem in obtaining aluminum borate crystals, but it becomes difficult to crush into fine particles, so the upper limit is set at 1000°C.

In order to suppress the production of excess boric acid in the sol used to obtain fine particles, the Al/B ratio of the sol must be in the range of 1.9 to 2.1, as with the coating agent for the aluminum borate coating of the present invention. If this value is too large, the formation of aluminum borate crystals will be insufficient, as with the coating, and if it is too small, the excess boric acid will increase, resulting in inferior rust resistance of the aluminum borate coating used.

Below, we will explain in detail each component contained in the coating agent.

(Aluminum Source)
The aluminum source of the coating agent includes aluminum oxide and/or an aluminum oxide precursor compound. The aluminum oxide precursor compound is not particularly limited as long as it can form aluminum oxide in the aluminum borate coating to be formed, and examples thereof include aluminum oxide hydrates represented by _Al2O3.mH2O such as _boehmite , aluminum _hydroxide , etc., and one of these can be used alone or two or more can be used in combination.

The aluminum source may be dispersed in the coating agent, or may be dissolved in the coating agent. Usually, the aluminum source is dispersed in the coating agent. The aluminum source is preferably particulate so that it is stably dispersed in the coating agent. In this case, the volume-based average particle size (D50) of the aluminum source measured by the laser diffraction scattering method is, for example, 0.005 μm or more and 1.0 μm or less, preferably 0.015 μm or more and 0.7 μm or less.

The aluminum source may also be added to the coating agent in the form of a sol. By using such a fine particle dispersion system called a sol, a thin, uniform aluminum borate coating with good adhesion can be obtained. Examples of such sols include alumina sol and boehmite sol. Boehmite sol and alumina sol are particularly suitable in terms of workability and price.

(boron source)
Examples of the boron source for the coating agent include boron oxoacids (boric acid) such as orthoboric acid, metaboric acid, and _tetraboric acid, and boron oxide represented by _B2O3 , and one of these may be used alone or two or more may be used in combination. Of these, _orthoboric acid represented by _H3BO3 is preferred from the viewpoints of workability and cost.

The coating agent according to this embodiment contains less boron source relative to the aluminum source compared to conventional coating agents. Specifically, the coating agent contains an aluminum source and a boron source such that the molar ratio Al/B is 1.9 to 2.1. This is made possible by the inclusion of aluminum borate microcrystals in the coating agent, which is a feature of the present invention, and by achieving the effect of sufficient aluminum borate crystals being formed without the need for an excess of boron. If the boron source is too little, sufficient tension cannot be obtained, while if the boron source is too much, rust will occur due to deterioration of the water resistance of the aluminum borate coating.

(Silicon oxide and silicon oxide precursors)
The coating agent may contain known silicon oxide and/or silicon oxide precursors for the purpose of improving the adhesion of the aluminum borate coating. The silicon oxide and/or silicon oxide precursors contribute to the formation of a glassy network in the aluminum borate coating, and contribute to improving the adhesion of the resulting aluminum borate coating.

The silicon oxide is not particularly limited, but various known silicon oxides can be used. Colloidal silica, in particular, has excellent dispersibility in the coating agent.

Examples of silicon oxide precursors include compounds capable of forming silicon oxide, such as silane compounds. Examples of silane compounds include, but are not limited to, alkoxysilanes such as tetraethoxysilane, and other silicon oxide precursors, and one of these can be used alone or two or more can be used in combination. Alternatively, a portion of these silane compounds may be hydrolyzed in advance.

(solvent)
The coating agent contains a solvent, which functions as a solvent for decomposing each component and also functions as a dispersion medium for dispersing each component.

The solvent is not particularly limited, but examples thereof include water, alcohol-based solvents, ketone-based solvents, ether-based solvents, and hydrocarbon-based solvents, and one of these can be used alone or two or more can be used in combination. Water is preferred from the viewpoints of workability, suppression of defects during drying, and excellent dispersibility and solubility of each component.

The coating agent according to the present embodiment described above does not cause problems such as cost issues due to additional processes, deterioration of the space factor, or reduction in tension, and provides a coating agent that is highly rust-resistant and effective in applying tension to steel sheets. By using this coating agent, an aluminum borate coating film with sufficient rust resistance and high tension can be formed.

<Method of manufacturing grain-oriented electrical steel sheet>
The method for producing the grain-oriented electrical steel sheet according to the present embodiment will be described below. The method for producing the grain-oriented electrical steel sheet according to the present embodiment includes a step of forming an aluminum borate coating using the coating agent for forming a tensile coating for the grain-oriented electrical steel sheet according to the present embodiment described above.

(Preparation of base steel plate)
First, a base steel sheet on which an aluminum borate coating is to be formed is prepared. As the base steel sheet, for example, (1) a steel sheet on which a forsterite-based primary coating is formed by finish annealing using a conventionally known method, (2) a steel sheet on which the primary coating and the internal oxide layer formed concomitantly are removed by immersion in acid, (3) a steel sheet obtained by the above (2) performing flattening annealing in a hydrogen-containing atmosphere, or a steel sheet polished by chemical polishing, electrolytic polishing, or the like, or (4) a steel sheet on which a conventionally known annealing separator containing alumina powder or the like that is inactive against coating formation or a trace amount of chloride or the like is added, and finish annealing is performed under conditions that do not cause the formation of a primary coating, or a steel sheet whose surface is flattened by the method of (3), etc., may be prepared. The preparation of the base steel sheet may be performed before or after the preparation of the coating agent described below.

(Preparation of coating agent for forming tension coating on grain-oriented electrical steel sheet and formation of aluminum borate coating)
Next, a coating agent for forming a tension coating on a grain-oriented electrical steel sheet is prepared by the above-mentioned method. The obtained coating agent is used to form an aluminum borate coating on the surface of the steel sheet. The aluminum borate coating can be formed by applying the coating agent to the surface of the steel sheet, followed by drying and baking.

The coating on the steel sheet surface can be carried out by conventional methods such as using a coater such as a roll coater, dipping, spraying, or electrophoresis.

After the coating agent is applied to the steel sheet, the sheet is dried and then baked, forming an aluminum borate coating on the surface of the steel sheet. Baking can be performed at a temperature of, for example, 750°C or higher. If the baking temperature is less than 750°C, the applied precursor may not become an oxide, and sufficient tension may not be developed due to the low baking temperature, which is not preferred. The baking temperature is preferably 750°C or higher and 1000°C or lower, more preferably 800°C or higher and 1000°C or lower.

The baking time should be 20 seconds or more. If the baking time is shorter than this, aluminum borate crystals will not be sufficiently generated, and the coating will not have sufficient tension after baking. On the other hand, a longer baking time will not have any particular effect, but it will simply increase the heat treatment time without providing any new effects, so it is preferable to keep the baking time to 120 seconds or less.

The atmosphere during baking is preferably one containing 0 to 30% by volume of hydrogen, with the remainder being either or both of nitrogen and an inert gas. Specifically, reducing atmospheres such as a nitrogen gas atmosphere, an inert gas atmosphere such as argon, a mixed nitrogen-argon atmosphere, or a mixed nitrogen-hydrogen atmosphere are preferred. Atmospheres containing an excessive amount of air or oxygen are not preferred as they may cause the steel sheet to be excessively oxidized. In addition, there is no particular harmful effect if the hydrogen ratio exceeds 30% by volume, but an increase in the amount of hydrogen has no particular effect and only increases the cost of hydrogen gas, so the upper limit is set at 30% by volume.

Good results can be obtained by keeping the dew point of the ambient gas between 0 and 40°C.

In this way, grain-oriented electrical steel sheets can be manufactured that have an aluminum borate coating that provides sufficient rust prevention and tensile strength.

The present invention will be described in more detail below with reference to examples. However, the examples shown below are merely examples of the present invention, and the present invention is not limited to these examples.

[Example 1]
A commercially available boric acid reagent and _aluminum oxide ( _Al2O3 ) powder (average particle size: 0.4 μm) were mixed in the amounts shown in Table 1, distilled water was added to this, and then fine particles of aluminum borate crystals having the particle size shown in Table 1 and a (220) interplanar spacing of 0.525 to 0.535 nm in the amount shown in Table 1 were added and thoroughly stirred to prepare a slurry to serve as a coating agent.

The fine particles of aluminum borate crystals used here were made by weighing out aluminum oxide and boric acid in a composition of Al/B = 2.0, thoroughly mixing them, heat treating them in air at 900°C for 1 hour, and then pulverizing them in an alumina ball mill using pure water as a medium. The pulverized slurry was spray-dried, pulverized into primary particles in a jet mill, and then classified into the desired particle size in a cyclone.

The obtained slurry was applied to a 0.23 mm thick unidirectional silicon steel sheet (with glass coating) containing 3.2% Si and having been subjected to finish annealing, at a rate of 4 g/ ^m2 . This was dried in air at an atmospheric temperature of 100°C for 60 seconds, then heated to 800°C and baked at this temperature for a soaking time of 100 seconds to form an insulating coating. The baking atmosphere was a nitrogen atmosphere containing 10% hydrogen, with a dew point of 30°C.

The rust resistance, coating tension, and space factor of the obtained samples were measured.

Rust resistance was evaluated by exposing a 50 mm square sample to an atmosphere with a temperature of 50°C and a relative humidity of 98% for 48 hours and visually checking whether rust had developed. If there was no rust, the rust resistance was good and the sample was rated "good," and if there was rust, the sample was rated "poor."

The coating tension was calculated by removing the aluminum borate coating from each side of a sample measuring 30 mm in width and 300 mm in length, and from the bending of the steel plate that occurs during this process. A sodium hydroxide solution was used to remove the insulating coating, and the tension obtained does not include the glass coating. Here, a coating tension of 12 MPa or more was deemed to be high and "good," while a coating tension of less than 12 MPa was deemed to be "poor."

The space factor was measured using the JIS C2550-5 method, with 97.5% or more being considered "good" and less than 97.5% being considered "poor."

From the results in Table 1, it was confirmed that the coating obtained in the examples had rust resistance, a good space factor, and high tensile strength.

[Example 2]
A commercially available boric acid reagent and _aluminum oxide ( _Al2O3 ) powder (average particle size: 0.4 μm) were mixed in the amounts shown in Table 2, distilled water was added to the mixture, and fine particles of aluminum borate crystals having a particle size and a (220) interplanar spacing of 0.525 to 0.535 nm shown in Table 2 were added in the amount shown in Table 2 and thoroughly stirred to prepare a slurry to serve as a coating agent. The fine particles of aluminum borate crystals used here were prepared in the same manner as in Example 1.

The obtained slurry was applied to a 0.23 mm thick unidirectional silicon steel sheet (with glass coating) containing 3.2% Si and having been subjected to finish annealing, so that the coating mass after baking was 4 g/ ^m2 . This was dried in air at an atmospheric temperature of 100°C for 60 seconds, then heated to 800°C and baked at this temperature for a soaking time of 100 seconds. The atmosphere during baking was a nitrogen atmosphere containing 10% hydrogen, with a dew point of 30°C.

The obtained samples were measured for rust resistance, coating tension, and space factor. The evaluation methods were the same as in Example 1.

From the results in Table 2, it was confirmed that the examples provided a coating that was rust-resistant, had a good space factor, and had high tensile strength.

[Example 3]
A commercially available boric acid reagent and _aluminum oxide ( _Al2O3 ) powder (average particle size: 0.4 μm) were mixed in the amounts shown in Table 3, distilled water was added to this, and fine particles of aluminum borate crystals having a particle size of 0.4 μm and a (220) interplanar spacing of 0.525 to 0.535 nm were added in the amount shown in Table 3 and thoroughly stirred to prepare a slurry to be used as a coating agent.

The fine particles of aluminum borate crystals used here were prepared in the same manner as in Example 1, except that aluminum oxide and boric acid were weighed so that the Al/B value was the value shown in Table 3, and the firing temperature before crushing the aluminum borate fine particles was as shown in Table 3. However, in Comparative Example 3-4, the firing temperature before crushing the aluminum borate fine particles was too high at 1200°C, making crushing difficult, and the particle size of the aluminum borate fine particles was 1.1 μm.

The obtained slurry was applied to a 0.23 mm thick unidirectional silicon steel sheet (with glass coating) containing 3.2% Si and having been subjected to finish annealing, so that the coating mass after baking was 4 g/ ^m2 . This was dried in air at an atmospheric temperature of 100°C for 60 seconds, then heated to 800°C and baked at this temperature for a soaking time of 100 seconds. The atmosphere during baking was a nitrogen atmosphere containing 10% hydrogen, with a dew point of 30°C.

The obtained samples were measured for rust resistance, coating tension, and space factor. The evaluation methods were the same as in Example 1.

From the results in Table 3, it was confirmed that the coating obtained in the examples had rust resistance, a good space factor, and high tensile strength.

[Example 4]
A commercially available boric acid reagent and _aluminum oxide ( _Al2O3 ) powder (average particle size: 0.4 μm) were mixed in the amounts shown in Table 4, distilled water was added to the mixture, and fine particles of aluminum borate crystals having a (220) interplanar spacing of 0.525 to 0.535 nm and a particle size shown in Table 4 were added in the amount shown in Table 4 and thoroughly stirred to prepare a slurry to serve as a coating agent. The fine particles of aluminum borate crystals used here were prepared in the same manner as in Example 1.

The obtained slurry was applied to a 0.23 mm thick grain-oriented silicon steel sheet (with glass coating) containing 3.2% Si and having been subjected to finish annealing, so that the coating mass after baking would be 4 g/ ^m2 . This was dried in air at an atmospheric temperature of 100°C for 60 seconds and then baked under the conditions shown in Table 4.

The rust resistance and coating tension of the obtained samples were measured. The evaluation methods were the same as in Example 1.

From the results in Table 4, it was confirmed that the coating obtained in the examples had rust resistance, a good space factor, and high tensile strength.

Claims (3)

The solid content is 5 to 40% by mass, and the solid content contains 1 to 50% by mass of fine particles having an average particle size of 0.1 to 0.7 μm and made of aluminum borate crystals having a (220) interplanar spacing of 0.525 to 0.535 nm,
The remainder of the coating agent for forming a tension film on a grain-oriented electrical steel sheet is an aluminum compound and a boron compound, with the molar ratio of Al to B being in the range of 1.9 to 2.1 (Al/B). A method for producing a coating agent for forming a tension coating on a grain-oriented electrical steel sheet according to claim 1, comprising the steps of:
A mixture containing an aluminum compound and a boron compound in a molar ratio of Al to B, Al/B, of 1.9 to 2.1 is prepared;
The mixture is fired at 800 to 1000° C. to synthesize aluminum borate powder.
The obtained aluminum borate powder is pulverized to obtain fine particles having an average particle size of 0.1 to 0.7 μm and made of aluminum borate crystals having a (220) interplanar spacing of 0.525 to 0.535 nm;
A method for producing a coating agent for forming a tensile film on a grain-oriented electrical steel sheet, comprising the step of adding the obtained fine particles to a solvent. A method for producing a grain-oriented electrical steel sheet, comprising the steps of:
The coating agent for forming a tension coating for a grain-oriented electrical steel sheet according to claim 1 is applied to the steel sheet which has been subjected to the final annealing, and then dried.
A method for producing a grain-oriented electrical steel sheet, comprising heat treating the steel sheet at 750 to 1000°C for 20 seconds or more in an atmosphere having a dew point of 0 to 40°C, containing 0 to 30 volume% hydrogen, and the remainder being one or both of nitrogen and an inert gas.