JPH05279747A - Formation of insulating film on grain oriented electrical steel sheet - Google Patents

Abstract

PURPOSE:To form the insulating films having high adhesion and the high tension to be imparted on a grain oriented electrical steel sheet by forming the films having the good adhesion to the steel sheet on the steel sheet, then applying insulation coatings of a colloidal silica-phosphate system thereon. CONSTITUTION:The insulating films having the high adhesion to the grain oriented electrical steel sheet are formed at <=4g/m<2> per one surface on the steel sheet. The insulation coatings consisting essentially of colloidal silica and phosphate are then applied thereon. Substrate films are formed by coating the steel sheet with an aq. primary phosphate soln. of >=1 kinds of Al, Mg, Sr, Ba and Fe or an aq. phosphoric acid soln. and baking the coatings at about >=350 deg.C. An aq. alkaline metal silicate soln. is also usable for forming the substrate films. The insulation coating liquid consists of (1) the colloidal silica, (2) >=1 kinds of the primary phosphate selected from Al, Mg, Sr, Ba and Fe and (3) >=1 kinds of chromic anhydride and chromates as basic components.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention has high adhesion to the surface of grain-oriented electrical steel sheet having no forsterite coating on the surface, and further to the surface of grain-oriented electrical steel sheet after finish annealing adjusted to a mirror surface or a state close to it. The present invention provides a method for forming a high-strength insulating film.

[0002]

2. Description of the Related Art Grain-oriented electrical steel sheets are widely used as a magnetic iron core material, and a material having a small iron loss is particularly required to reduce energy loss. It is known that applying tension to a steel sheet is effective in reducing iron loss.

In order to apply tension to the steel sheet, it is effective to form a film made of a material having a smaller coefficient of thermal expansion than the steel sheet at a high temperature. The film mainly composed of forsterite formed by the reaction between the oxide on the surface of the steel sheet and the annealing separator in the finish annealing step has a large tension applied to the steel sheet and has very good film adhesion.

Further, the method of forming an insulating film by baking a coating solution containing colloidal silica and a phosphate as disclosed in JP-A-48-39338 has the effect of imparting tension to a steel sheet. Large and effective in reducing iron loss. In this insulating coating solution, it is considered that the silica component contributes to the low thermal expansion of the insulating film, that is, imparts a high tensile strength to the steel sheet, and the phosphate component contributes to ensuring the adhesion of the insulating film to the base. Therefore, a general method for producing a grain-oriented electrical steel sheet is to apply a tensile insulating coating after leaving the film produced in the finish annealing step.

On the other hand, recently, it has been revealed that the disordered interface structure between the forsterite coating and the base iron cancels the coating tension effect on iron loss to some extent. Therefore, as disclosed in, for example, Japanese Patent Laid-Open No. 49-96920, by removing the fillsterite film produced in the finish annealing step, further performing mirror finishing, and then applying a tension film again, Techniques have been developed to try to reduce iron loss.

However, when the above coating liquid is applied on a film mainly composed of forsterite,
Although a considerable degree of adhesion can be obtained, sufficient adhesion cannot be obtained when the forsterite-based film is removed or when the forsterite is not intentionally formed in the final annealing step.

When the forsterite-based film is removed, it is necessary to secure the required film tension only by the insulating coating, and it is necessary to make the film thicker, and further the film adhesion is required. Therefore, it is difficult for the conventional insulating coating to achieve the film tension enough to bring out the effect of mirroring, and the iron loss has not been sufficiently reduced.

[0008]

DISCLOSURE OF THE INVENTION The present invention solves these problems in the prior art, and in particular, even for a steel sheet having no forsterite coating, an insulating coating having a high adhesion and a large tension applied to the steel sheet is provided. It is intended to provide a method of forming.

[0009]

According to the present invention, after forming a film having good adhesion to a steel sheet, a colloidal silica-phosphate-based insulating coating is applied again,
This is a method of forming an insulating coating having high film adhesion and tension on a grain-oriented electrical steel sheet.

Here, the colloidal silica-phosphate-based insulating coating liquid is, for example, JP-A-48-39338.
JP-A-53-28043, JP-A-50-
It is as disclosed in JP-A-79442 and JP-A-52-25296. That is, one or more kinds selected from colloidal silica, one or more kinds selected from Al, Mg, Sr, Ba and Fe, primary phosphate, anhydrous chromate and chromate. In addition to the above, the oxides such as SiO ₂ , Al ₂ O ₃ and TiO ₂ may be added to improve heat resistance.

[0011]

The present invention will be described in detail below. The application of tension to the grain-oriented electrical steel sheet is performed by forming a heat-resistant oxide film at high temperature and utilizing the difference in thermal expansion between the steel plate and the film during the cooling process. However, if tension is applied to the steel sheet, stress is generated at the interface, and if the adhesion of the coating is not sufficient, the coating will peel off. Therefore, the higher the tension on the steel sheet, the greater the adhesion of the coating to the substrate.

In the conventional colloidal silica-phosphate-based insulating coating, the composition is determined in order to satisfy both the tension by the film and the film adhesion.
That is, it is considered that the silica component contributes to the low thermal expansion and the high tension of the film and the phosphate contributes to the securing of the adhesive force with the base.

On the other hand, the affinity between oxides and metals is generally weaker than that between oxides and between metals. Therefore, in order to form a film having the same film tension on a steel plate without forsterite, further film adhesion is required.

The inventors of the present invention form a film having better adhesiveness to a steel sheet than the current insulating coating (hereinafter referred to as "undercoating"), and then form a film having high tension (hereinafter referred to as "tensioned film formation"). Then, we thought that it would be possible to secure a high film adhesion even on a steel plate without a forsterite film, that is, on a steel sheet with exposed metal, and conducted repeated studies.
As a result, if the base film is formed at a temperature of 350 ° C. or higher, not only the adhesion of the film is improved by forming the base film, but also the tension applied to the steel sheet by the formation of the tension film is increased by such treatment. The inventors completed the invention by newly discovering that the tension is not inferior to that expected when no base treatment is applied.

As a coating liquid which can be expected to form a film having a high adhesion to a steel sheet, phosphoric acid (a steel sheet reacts with phosphoric acid to produce iron phosphate on the surface of the steel sheet), an aqueous phosphate solution, an aqueous alkali metal silicate solution (water glass ) Is considered.

Table 1 shows a summary of experiments on the effect of forming a base film by these coating solutions prior to baking of colloidal silica-phosphate-based insulating coating.
Is.

[0017]

[Table 1]

All experiments were performed with 0.2 containing 3% Si.
For a grain-oriented electrical steel sheet having a final finish annealing of 3 mm thickness, the steel sheet from which the forsterite film was removed by pickling was used. The undercoat film is formed by applying a predetermined amount of the treatment liquid, drying it, and baking it at 800 ° C.

The adhesion of the coating was evaluated by conducting a 180 ° bending test of the sample using a steel round bar having a diameter of 20 mm and checking whether peeling occurred on the coating inside. O: No peeling of film Δ: 20% or less of film peeling area X: 20% or more of film peeling area The tension applied by the film formation was calculated from the radius of curvature of a sample having a film formed on only one surface.

As shown in Table 1, colloidal silica-
The adhesion of the film is remarkably improved by forming the base film prior to the formation of the phosphate coating film. Further, within the scope of this study, the tension applied to the steel sheet by the entire coating corresponds to the amount of the colloidal silica-phosphate coating coating formed, regardless of the underlying coating forming method and the coating amount used.

Therefore, it is understood that the film tension is not adversely affected by the base film forming treatment in the present invention. Presumably, it is because the thermal expansion coefficient of the undercoat according to the present invention is almost equal to that of the steel sheet.

Further, in Table 1, as can be seen by comparing Invention 1 with Comparative Example 3 and Invention 2 with Comparative Example 4,
When a phosphate was used, when the amount of underlying film formed exceeded 4 g / m ² , it became white powder after baking, making film formation difficult. Since the introduction of the undercoat film also causes the space factor to be deteriorated, it is preferable to suppress the amount of formation thereof to 4 g / m ² or less.

The baking temperature of the undercoat is 35
In the case of baking at 0 ° C. or lower, there was a phenomenon that the undercoat was peeled off when the subsequent coating solution for the tension film was baked. Therefore, the baking temperature of the base film must be 350 ° C. or higher.

[0024]

Example 1 After forming an oxide layer containing SiO ₂ on the surface of silicon steel also for decarburization annealing, the silicon steel rolled to a final plate thickness of 0.23 mm containing 3% Si was used. An annealing separator mainly composed of MgO was applied, and final finish annealing was performed. Since the film mainly composed of forsterite is present on the surface of the grain-oriented electrical steel sheet annealed in this way, the forsterite coating was removed by immersing the steel sheet in a sulfuric hydrofluoric acid solution (sheet thickness 0.22 mm). ..

A 35% magnesium phosphate solution was applied to the surface of the steel sheet at 1 g / m ² per side and baked at 800 ° C. Subsequently, a treatment liquid consisting of 100 ml of 20% colloidal silica, 60 ml of 35% magnesium phosphate solution, and 5 g of chromic anhydride was applied on one side at 8 g / m ² and baked at 800 ° C. Table 2 shows various characteristics of the grain-oriented electrical steel sheet with an insulating coating produced in this manner.

[0026]

[Table 2]

Example 2 For a silicon steel rolled to a final plate thickness of 0.23 mm containing 3% Si, an oxide layer containing SiO ₂ was formed on the surface of the silicon steel also for decarburization annealing, and then MgO was formed. Was applied, and a final finish annealing was performed. Since there is a film mainly composed of forsterite on the surface of the grain-oriented electrical steel annealed in this way, the forsterite film is removed by immersing the steel plate in a sulfuric hydrofluoric acid solution, and then chemically polished to give a mirror surface. (Plate thickness 0.20 mm).

A 50% aluminum phosphate solution was applied to the surface of this steel sheet at 1 g / m ² per side and baked at 800 ° C. Then, a treatment liquid consisting of 100 ml of 20% colloidal silica, 100 ml of 50% aluminum phosphate solution, and 5 g of chromic anhydride was 8 g / m on each side with a grooved rubber roll.
^Two coats were applied and baked at 800 ° C. Table 2 shows various characteristics of the grain-oriented electrical steel sheet with an insulating coating produced in this manner.

Example 3 On a silicon steel rolled to a final plate thickness of 0.23 mm containing 3% Si, an oxide layer containing SiO ₂ was formed on the surface of the silicon steel also for decarburization annealing, and then MgO was formed. Was applied, and a final finish annealing was performed. Since a film mainly composed of forsterite is present on the surface of the grain-oriented electrical steel sheet annealed in this way, the forsterite film is removed by immersing the steel sheet in a hydrofluoric acid solution, and the temperature is set to 1200 ° C. in a dry hydrogen atmosphere. The surface was flattened by annealing for 10 hours (sheet thickness 0.22 mm).

30% potassium silicate (K ₂
O / 3SiO ₂ ) solution is applied at 1 g / m ² per side,
It was baked at 800 ° C. Subsequently, a treatment liquid consisting of 100 ml of 20% colloidal silica, 100 ml of 50% aluminum phosphate solution, and 5 g of chromic anhydride was applied by a grooved rubber roll at 8 g / m ² per side and baked at 800 ° C.
Table 2 shows various characteristics of the grain-oriented electrical steel sheet with an insulating coating produced in this manner.

Example 4 Silicon steel rolled to a final plate thickness of 0.23 mm containing 3% Si was decarburized and annealed, and then an annealing separator mainly composed of Al ₂ O ₃ was applied to the final finish annealing. I went. On the surface of the grain-oriented electrical steel sheet annealed in this way, no film is formed by annealing, and a mirror-like state is exhibited (sheet thickness 0.23 mm).

30% sodium silicate (N
a ₂ O / 3SiO ₂ ) solution was applied at 1 g / m ² per side and baked at 800 ° C. Then 100 ml of 20% colloidal silica, 100 ml of 50% aluminum phosphate solution,
A treatment liquid consisting of 5 g of chromic anhydride was applied on one side by a grooved rubber roll at 8 g / m ² and baked at 800 ° C. Table 2 shows various characteristics of the grain-oriented electrical steel sheet with an insulating coating produced in this manner.

[0033]

The present invention provides a method for forming an insulating coating having high adhesion on a grain-oriented electrical steel sheet without reducing the tension on the steel sheet. Therefore,
By this insulating coating forming method, it is possible to manufacture a grain-oriented electrical steel sheet having excellent flatness at the interface of the coating base iron and low iron loss in which strong tension is applied to the steel sheet, and its industrial effect is great.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Osamu Tanaka 1-1, Toibatacho, Tobata-ku, Kitakyushu Nippon Steel Yawata Works Co., Ltd.

Claims (3)

[Claims] 1. When applying an insulating coating mainly composed of colloidal silica and phosphate to a grain-oriented electrical steel sheet, an undercoating having a larger adhesion to the steel sheet than that of the insulating coating is formed at 4 g / m ² or less per side. And then applying the insulating coating, the method for forming an insulating coating on a grain-oriented electrical steel sheet having excellent adhesion. 2. An undercoat film is formed using Al, Mg, Sr, B.
2. The direction according to claim 1, wherein any one or two or more primary phosphate aqueous solution or phosphoric acid aqueous solution selected from a and Fe is applied to the surface of the steel sheet and baked at a temperature of 350 ° C. or higher. Method for forming insulating film on magnetic electrical steel sheet. 3. A lithium silicate (Li ₂
O ・ nSiO ₂ ), sodium silicate (Na ₂ O ・ nSi
O ₂ ), potassium silicate (K ₂ O · nSiO ₂ ), or any one or two or more kinds of aqueous solutions selected from the above are applied to the surface of the steel sheet and baked at a temperature of 350 ° C. or higher. 1. The method for forming an insulating coating on a grain-oriented electrical steel sheet according to 1.