JP7448819B2 - Grain-oriented electrical steel sheet and its manufacturing method - Google Patents

Description

The present invention relates to a grain-oriented electrical steel sheet and a method for manufacturing a grain-oriented electrical steel sheet.

Grain-oriented electrical steel sheets are often used as core materials for transformers, and materials with low iron loss are particularly desired in order to reduce energy loss. Since iron alloys containing iron and silicon have large magnetocrystalline anisotropy, application of external tension causes fragmentation of magnetic domains, which can reduce eddy current loss, which is the main element of iron loss. In particular, it is known that applying tension to the steel sheet is effective in reducing iron loss in grain-oriented electrical steel sheets containing 5% or less silicon. This tension is provided by a coating formed on the surface.

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

Aluminum borate coating has a high tension imparting effect when formed on the surface of steel sheets, so it is promising as a next-generation insulating coating for grain-oriented electrical steel sheets. However, it has a weaker anti-corrosion effect than the insulating coatings currently in use. It is necessary to improve this point.

Patent Document 2 discloses that after forming a conventional insulating film mainly composed of phosphate and colloidal silica, a low-crystalline alumina sol is used to form an aluminum borate film with good stability of the coating solution over time and good rust resistance. A method is disclosed.

Patent Document 3 discloses that after forming a conventional insulating film mainly composed of phosphate and colloidal silica, nitric acid is added when forming an aluminum borate film using a low-crystalline alumina sol. A method for forming an aluminum borate coating with good stability and good rust resistance is disclosed.

Japanese Patent Application Publication No. 6-65754 JP2002-309381A Japanese Patent Application Publication No. 2004-99929

According to the techniques disclosed in Patent Documents 2 and 3, the rust resistance is improved by increasing the thickness of the coating containing phosphate. However, this requires a complicated production process, such as multiple heat treatments at high temperatures. Further, although the rust resistance is good, there is a problem that the space factor is not sufficiently high due to the presence of an insulating coating mainly composed of phosphate and colloidal silica.

Therefore, an object of the present invention is to provide a simple method for improving the rust resistance of a grain-oriented electrical steel sheet having an aluminum borate coating.

The grain-oriented electrical steel sheet is generally a steel sheet with a forsterite film formed on its surface after finish annealing. This forsterite coating is the same as the coating generally called the primary coating or glass coating on grain-oriented electrical steel sheets. It is considered that sufficient rust resistance can be obtained if the steel plate is completely covered with a forsterite film. Therefore, the problem with the low rust prevention effect of the aluminum borate coating is that due to the presence of defects in the forsterite coating, parts of the steel plate are not covered with the forsterite coating and are in direct contact with the aluminum borate. This is thought to be because the rust resistance of that part is reduced.

Therefore, the present inventors thought that in order to improve the rust resistance of the aluminum borate film forming material, it would be sufficient to form a protective film on the surface of the steel sheet that is not covered with the forsterite film, and developed various methods. investigated. As a result, it has been found that desired rust resistance can be obtained by treating the surface of a steel plate with phosphoric acid before forming an aluminum borate film.

The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) A grain-oriented electrical steel sheet comprising a steel sheet, an intermediate layer mainly composed of forsterite provided on the surface of the steel sheet, and an aluminum borate coating provided on the surface of the intermediate layer, wherein the intermediate layer contains A grain-oriented electrical steel sheet, characterized in that a phosphate film containing iron phosphate is present in the grain-oriented electrical steel sheet, and the amount of the phosphate film deposited is 0.003 to 0.300 g/m ² .

(2) A method for manufacturing a grain-oriented electrical steel sheet according to (1) above, which includes a step of treating a steel sheet after finish annealing with a coating made of forsterite with an aqueous phosphoric acid solution, and washing and drying the steel sheet after the phosphoric acid treatment. and applying a coating liquid containing a boron source and an aluminum source to the surface of the steel sheet after cleaning and drying.

(3) The method for producing a grain-oriented electrical steel sheet according to (2) above, wherein the steel sheet is immersed in the phosphoric acid aqueous solution for a period of 30 to 300 seconds.

According to the proposal of the present invention, the rust resistance can be easily improved without affecting the ability of the aluminum borate coating to impart tension to the steel plate and the space factor.

The grain-oriented electrical steel sheet of the present invention comprises a steel sheet, an intermediate layer mainly composed of forsterite provided on the steel sheet, and an oxide containing aluminum and boron (hereinafter referred to as "boric acid") formed on the intermediate layer. It has an insulating coating consisting of an aluminum coating (referred to as "aluminum coating"). In the grain-oriented electrical steel sheet of the present invention, if there is a defect in the forsterite coating of the intermediate layer, a phosphate coating is formed in that portion. As a result, there is no direct contact between the steel plate and the aluminum borate coating. In the present invention, the forsterite coating and the phosphate coating are collectively referred to as an intermediate layer.

First, the inventors of the present invention will explain the investigation and study on the conditions for realizing the improvement in rust resistance of the insulating coating before realizing the above-mentioned coating structure.

The composition of the aluminum borate coating of the grain-oriented electrical steel sheet of the present invention is such that boric acid is in excess of the stoichiometric composition of aluminum borate crystals (Al ₄ B ₂ O ₉ ). When such a film is formed, if there are parts of the steel plate that are exposed due to defects in the glass film, and if the atmosphere in which the steel plate is exposed is humid, the parts of the steel plate that are exposed due to the effects of excessive boric acid may It is assumed that rust is formed because iron is eluted in an acidic environment. In order to prevent rust due to such a generation mechanism, the present inventors investigated a method of improving rust resistance by eliminating exposed portions of the steel plate. The idea behind this method is as follows.

If the forsterite coating completely covers the steel plate, even if the aluminum borate coating is exposed to a humid environment, an acidic environment due to excess boric acid will not be created on the steel plate surface. Therefore, it is believed that rust formation does not occur.

However, the forsterite film is a film made of an oxide mainly consisting of forsterite crystals (Mg ₂ SiO ₄ ) of about several micrometers, and this film usually has minute defects. It is thought that the steel plate is not covered with a forsterite film in this defective area, which is thought to be the cause of a decrease in rust resistance. Therefore, it is thought that the rust resistance can be improved by preventing the steel plate from being exposed at this defective portion and preventing direct contact with the aluminum borate coating.

As a result of the studies conducted by the present inventors, by forming a phosphate film on the defective parts of the forsterite film, a short-time treatment with phosphoric acid, which is available at low cost, can reduce the amount of phosphate deposited required for formation. It was found that it is possible to improve rust resistance easily and without affecting the space factor and tension.

<1. Grain-oriented electrical steel sheet＞
Hereinafter, the grain-oriented electrical steel sheet of the present invention will be explained.

The grain-oriented electrical steel sheet of the present invention includes a steel sheet, an intermediate layer mainly composed of forsterite provided on the surface of the steel sheet, and an aluminum borate coating provided on the surface of the intermediate layer. A phosphate film containing iron phosphate is present in the intermediate layer, and the amount of the phosphate film deposited is 0.003 to 0.300 g/m ² .

If the amount of deposited phosphate film is less than 0.003 g/m ² , the effect of the present invention cannot be obtained. If the amount of deposited phosphate film exceeds 0.300 g/m ² , the effect of the present invention is saturated, This is undesirable because the space factor tends to decrease.

The intermediate layer mainly composed of forsterite and the aluminum borate coating are the same as those included in known grain-oriented electrical steel sheets. The grain-oriented electrical steel of the present invention is characterized in that it includes a phosphate coating within the intermediate layer.

The phosphate coating in the intermediate layer is formed by a phosphoric acid treatment process described below, and mainly contains iron phosphate formed by the reaction of phosphoric acid and iron of the steel plate. That is, this coating is formed at the defective portion of the glass coating where the steel plate is exposed. In this process, leached iron forms phosphates containing iron phosphate on the steel plate surface within the forsterite coating. At this time, the phosphate film containing iron phosphate may contain iron oxide, phosphoric acid, or phosphates other than iron phosphate derived from impurities, but if iron is contained in the phosphate film, it may contain iron phosphate. It can be considered a phosphate and there is no problem. Defects in the forsterite film are usually about 1 to 10% in terms of area ratio, and the thickness of the phosphate film formed on these defects within the range of the amount of the phosphate film of the present invention deposited is 0.05 to 10%. It is about 0.5 μm.

As a phosphate film is formed at the exposed parts of the steel plate due to defects in the forsterite film, the parts of the steel plate that are not covered by the forsterite film are filled with phosphates and come into direct contact with the aluminum borate film. Rust resistance is improved.

If the amount of phosphate deposited is less than the above-mentioned amount, the thickness of the phosphate will be thin, and a sufficient rust prevention effect will not be obtained. If the amount of phosphate deposited is too large, there will be no change in the rust prevention effect, and the space factor will be adversely affected.

In the present invention, it is preferable that the phosphate film exists only in the defective parts of the forsterite film, and it is preferable to minimize the amount of the phosphate film that exists at the interface between the forsterite film and the aluminum borate film. In order not to reduce the space factor of the grain-oriented electrical steel sheet, the phosphate film at the interface between the forsterite film and the aluminum borate film (excluding defective parts of the forsterite film) should have an area ratio of 10% or less. is preferable, 5% or less is more preferable, and even more preferably 3% or less. Most preferably, the amount of phosphate deposited at the interface between the forsterite film and the aluminum borate film, excluding the defective portions of the forsterite film, is zero. If the amount of phosphate deposited is within the above range, the area ratio of the phosphate film present at the interface between the forsterite film and the aluminum borate film is considered to be 10% or less.

As described above, the phosphate film of the present invention is made of iron phosphate, phosphoric acid, phosphate containing a metal element other than iron, and iron oxide, but in the present invention, it is assumed that the phosphate film is entirely made of iron phosphate (FePO ₄ ). Define the amount of adhesion. Therefore, the amount of phosphate deposited can be obtained by analyzing the amount of iron contained in the coating.

Specifically, the coating of a steel plate that has undergone the process of the present invention is removed with an alkaline solution, and the amount of phosphate deposited is determined according to the following formula by measuring the amount of iron contained in the alkaline solution after the coating is removed. .

Here, the molar mass of FePO ₄ is 150.85 (g/mol), and the molar mass of iron is 55.85 (g/mol). Removal of the film can be achieved, for example, by immersing the film in a 20% by mass sodium hydroxide solution at 80° C. for 30 minutes. The amount of iron in the coating can be obtained by analyzing the amount of iron in this sodium hydroxide solution.

In the present invention, the amount of phosphate deposited by the above formula is set to 0.003 to 0.300 g/m ² .

Phosphates are formed in the defective areas of the forsterite coating, and the area ratio of phosphates formed at the interface between the forsterite coating and the aluminum borate coating excluding the defective areas of the forsterite coating is 10% or less. This can be confirmed by cross-sectional observation. In the grain-oriented electrical steel of the present invention, it is possible to prepare a cross-sectional sample, observe the interface between the forsterite film and the aluminum borate film at a magnification of 10,000 times using a scanning electron microscope, and perform energy dispersive X-ray analysis to confirm the interface. can,

<2. Manufacturing method of grain-oriented electrical steel sheet>
Next, a method for manufacturing a grain-oriented electrical steel sheet according to the present invention will be explained.

First, a steel plate that has been finish annealed is prepared. The steel sheet used in the method for producing a grain-oriented electrical steel sheet of the present invention is not particularly limited as long as it has undergone secondary recrystallization and has a primary coating mainly composed of forsterite.

Subsequently, the steel sheet having a primary coating mainly composed of forsterite is subjected to (i) formation of a phosphate coating on defective parts of the forsterite coating, (ii) cleaning, and (iii) formation of an aluminum borate coating.

(i) Formation of phosphate film on defective part of forsterite film

First, a steel plate having a forsterite coating that has been subjected to finish annealing is lightly pickled to remove unreacted MgO and the like remaining on the surface. For pickling, it is preferable to maintain the temperature of an aqueous solution containing 1 to 5% by mass of sulfuric acid at 50 to 90° C., immerse it in the solution for 10 to 60 seconds, wash it with running water for 10 to 30 seconds, and then dry it. If the concentration of sulfuric acid, the temperature of the aqueous solution, and the immersion time are outside the above ranges, unreacted MgO may not be removed, or the steel plate may be eroded and the primary coating may easily peel off.

Next, the steel plate is immersed in a phosphoric acid bath. The phosphoric acid treatment is carried out with a phosphoric acid solution. The concentration of phosphoric acid is preferably 0.1 to 15% by mass. If the concentration of phosphoric acid is low, a sufficient amount of phosphate film will not be formed on the steel sheet that is not covered with the forsterite film, and sufficient rust resistance may not be obtained. On the other hand, if the concentration of phosphoric acid is high, it becomes difficult to control the phosphate on the steel sheet, the amount of adhesion becomes too large, and the space factor may deteriorate.

In order to adjust the pH of the phosphoric acid solution, an alkaline solution such as sodium hydroxide may be added to the solution, and if the amount is in the range of 0.01 to 10% by mass in a solution without phosphoric acid, an iron phosphate film is likely to be formed.

In the treatment with a phosphoric acid solution, the temperature of the solution is preferably maintained at 40 to 60° C., and the treatment time is preferably 30 to 300 seconds. If the temperature of the solution is too low, a sufficient amount of phosphate coating may not be obtained. If the temperature is too high, the controllability of the amount of phosphate on the steel plate may become poor, and the space factor may deteriorate.

If the phosphoric acid treatment time is too short, a healthy phosphate layer may not be formed on the exposed steel sheet at the primary coating defect, and the rust resistance may not be improved. On the other hand, if the processing time is too long, no problem will occur as long as the space factor does not deteriorate, but since the processing time becomes longer, the advantage of the present invention of easily improving rust resistance is lost.

This phosphoric acid treatment can be carried out as an iron phosphate film treatment step, for example by dipping it in a tank filled with an aqueous solution of phosphoric acid, or by continuing to spray it for a certain period of time.

By the phosphoric acid treatment as described above, a steel plate is obtained in which a phosphate film is formed on the defective portions of the forsterite film with a phosphate coating amount of 0.003 to 0.300 g/m ² or less.

(ii) Cleaning It is important to wash a steel plate treated with a phosphoric acid aqueous solution with water or the like in order to remove excess phosphoric acid aqueous solution. This condition can be, for example, rinsing with running water at room temperature for 10 to 60 seconds, and immersing in pure water at room temperature for 10 to 60 seconds.

Without a cleaning process, after treatment with phosphoric acid, an excessive amount of phosphate layer may be formed in the defective areas of the forsterite coating, or a phosphate layer may be formed at the interface between the forsterite coating and the aluminum borate coating. However, the occupancy rate becomes a cause of deterioration. When washed with water, excess phosphoric acid is removed from defective parts of the forsterite coating, except for the extremely thin layer where the phosphoric acid and the steel plate have reacted. In addition, since phosphates are substantially removed from the interface between the forsterite film and the aluminum borate film, phosphates are formed only in the defective areas of the forsterite film, which is the objective of the present invention, and the space factor is reduced. The purpose of improving rust resistance can be achieved without any adverse effects. That is, according to the present invention, the space factor is almost the same as that of a grain-oriented electrical steel sheet in which an aluminum borate film is formed on a steel sheet having a forsterite film without any rust prevention treatment.

After the above water washing, the steel plate is heat treated at 50 to 80°C for 1 to 30 minutes, and then cooled once to 40°C or lower. This treatment simultaneously strengthens the phosphate layer and dries the steel sheet, making it possible to stably form the aluminum borate coating in the next step. In the present invention, there is no need for heat treatment at a high temperature after the phosphoric acid treatment, and the effect of improving rust resistance can be obtained at a low heat treatment temperature of 100° C. or less. Furthermore, the high tension effect due to the aluminum borate coating can be stably obtained.

(iii) Formation of aluminum borate film There is no particular limitation on the formation of the aluminum borate film, and known techniques can be used. For example, the following method can be adopted.

A coating solution for forming an aluminum borate film is applied to the steel plate after cleaning and drying. This coating solution may be a known coating solution for forming an aluminum borate film, and its composition includes a boron source and an aluminum source with a mass ratio in terms of Al ₂ O ₃ /B ₂ O ₃ of 1.5 to 2.6. including.

If the mass ratio in terms of Al ₂ O ₃ /B ₂ O ₃ is less than 1.5, the amount of boron in the insulating film becomes too large, and as a result, too much boron accumulates at the interface, and the amount of boron in the insulating film increases. The presence of aluminum borate becomes non-uniform, and aluminum borate crystals may not be formed sufficiently in some parts of the insulating film, resulting in a decrease in film tension. Furthermore, when the mass ratio in terms of Al ₂ O ₃ /B ₂ O ₃ exceeds 2.6, the aluminum source becomes too large, and as a result, the amount of boron near the interface between the insulation coating and the steel plate is not sufficient. , fewer aluminum borate crystals are formed, and the film tension may not become high.

As a boron source, orthoboric acid represented by H ₃ BO ₃ is the most preferred from the viewpoint of workability, cost, etc., but metaboric acid represented by HBO ₂ , boron oxide represented by B ₂ O ₃ , or any of these Mixtures of can also be used.

Examples of the aluminum source include aluminum oxide and aluminum oxide precursor compounds. As the aluminum oxide precursor compound, for example, hydrates of aluminum oxide such as boehmite, aluminum hydroxide, and various aluminum salts such as aluminum nitrate and aluminum chloride are preferably used.

These raw materials are dispersed in a dispersion medium to prepare a slurry as a coating liquid. The best dispersion medium is water, but organic solvents or mixtures thereof can be used if there are no particular problems in other steps. The solid content concentration of the slurry is appropriately selected depending on the workability and the like and is not particularly limited.

The obtained slurry (coating liquid) is applied to the surface of a grain-oriented electrical steel sheet that has been subjected to phosphate treatment by a conventionally known method such as a coater such as a roll coater, a dipping method, spraying, or electrophoresis.

As the baking conditions after drying the applied slurry, a known method for forming an aluminum borate coating can be used. For example, a steel plate is baked at The temperature is raised to ~1000°C, and heat treatment is performed in this temperature range for 20 to 120 seconds. The reason why it is necessary to perform heat treatment at 750 to 1000°C for 20 to 120 seconds is because crystal growth of aluminum borate occurs at temperatures above 750°C, and crystallization progresses. Note that although there is no problem if the temperature (soaking temperature) or heat treatment time exceeds this range, the effect of increasing the temperature or increasing the time will not be obtained.

In the manner described above, a grain-oriented electrical steel sheet having a high tensile strength and having an insulating coating as described above is obtained.

The present invention will be described in more detail below based on Examples, but the Examples shown below are merely examples of the present invention, and the present invention is not limited only to these Examples.

A solution of commercially available phosphoric acid (H ₃ PO ₄ ) having the concentration shown in Table 1 was prepared, sodium hydroxide of the concentration shown in Table 1 was added to each solution, and the solution was kept at 50°C. A unidirectional silicon steel plate containing 2% and having a thickness of 0.23 mm that had been finish annealed (with a forsterite primary coating) was immersed for the time shown in Table 1. After washing this steel plate with running water under the conditions shown in Table 1, it was immersed in pure water, then kept at a plate temperature of 70° C. for 20 minutes in an electric furnace, and then air-cooled.

Thereafter, a commercially available boric acid reagent and aluminum oxide (Al ₂ O ₃ ) powder (average particle size: 0.4 μm) were mixed at a ratio of 80.4 g of boric acid to 200 g of aluminum oxide, and distilled water was added to this. A slurry was prepared. Note that boric acid was weighed in terms of boron oxide (B ₂ O ₃ ) equivalent.

The obtained slurry was applied to a cooled steel plate so that the coating mass after baking was 4 g/m ² . After drying, the temperature was raised to 850° C. and baked at this temperature for 100 seconds to form an insulating film. The temperature reached by the steel plate during drying was 500°C. The atmosphere during drying, cooling, heating, and baking was a nitrogen atmosphere containing 10% hydrogen, and the dew point was 30°C.

A cross-sectional sample of the sample obtained in this way was examined using a scanning electron microscope to confirm the presence of phosphate at the interface between the forsterite film and the aluminum borate film, and to determine the rust resistance, space factor, and film tension. was evaluated.

To determine whether phosphate is present at the interface between the forsterite film and the aluminum borate film, a cross-sectional sample of the sample was prepared as described above, and the interface between the forsterite film and the aluminum borate film was examined using a scanning electron microscope. The interface between the forsterite film and aluminum borate was confirmed by observation at magnification and energy dispersive X-ray analysis. As a result, cases where the phosphate film present at the interface was 10% or less in terms of area ratio were evaluated as good (◯).

For rust resistance, the steel plate was held in a constant temperature and humidity chamber at 50° C. and 98% RH, and if no rust was visually observed after 48 hours, the rust resistance was evaluated as good (◯).

The space factor was measured by the method of JIS C2550-5, and 97.5% or more was considered good (◯).

The coating tension was calculated from the bending of the steel plate after removing the coating on one side of the steel plate on which the insulating coating was formed. This tension is the tension of only the aluminum borate coating, not including the intermediate layer. A sodium hydroxide aqueous solution was used to remove the insulating film. A tension of 12.0 MPa or more was defined as high tension.

From the results in Table 1, it can be seen that in the examples, coatings with rust resistance, space factors within the target range, and high tension were obtained.

Claims (3)

steel plate and
an intermediate layer mainly composed of forsterite provided on the surface of the steel plate;
A grain-oriented electrical steel sheet comprising an aluminum borate coating provided on the surface of the intermediate layer,
a phosphate coating containing iron phosphate is present within the intermediate layer;
A grain-oriented electrical steel sheet characterized in that the amount of the phosphate coating coated is 0.003 to 0.300 g/m ² . A method for manufacturing the grain-oriented electrical steel sheet according to claim 1, comprising:
a step of treating a finish annealed steel plate having a coating made of forsterite with an aqueous phosphoric acid solution;
A process of cleaning and drying the steel plate after phosphoric acid treatment,
A method for producing a grain-oriented electrical steel sheet, comprising the step of applying a coating solution containing a boron source and an aluminum source to the surface of a steel sheet after cleaning and drying. 3. The method for producing a grain-oriented electrical steel sheet according to claim 2, wherein the immersion time of the steel sheet in the phosphoric acid aqueous solution is 30 to 300 seconds.