JPH06184762A - Formation of insulated film on grain-oriented silicon steel sheet - Google Patents

Abstract

PURPOSE:To obtain a film excellent in electrical insulating properties and applicability of tension by previously forming a thin SiO2 film on the surface of a silicon steel sheet and thereafter applying insulation coating high in tensile force imparting property. CONSTITUTION:A grain-oriented silicon steel sheet after being subjected to finish annealing is treated in such a manner that the oxygen potential and temp. are controlled in a weak oxidizing atmosphere to form an SiO2 film constituted only of an external oxidized film having >=0.001mum thickness on the surface. Then, tensile force imparting insulation coating is applied. As for the insulation coating, a coating soln. essentially consisting of colloidal silica and phosphate is used. As for the control of the weak oxidizing atmosphere, for a steel sheet having 2 to 4.8% Si content, the range of PH2O/PH2 <=0.5 is regulated in the temp. range of 500 to 700 deg.C. The range of PH2O/PH2 <=0.15 is regulated in the temp. range of >700 to 1000 deg.C. PH2O and PH2 respectively denote the partial pressure of vapor and hydrogen in the atmosphere.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a unidirectional silicon steel sheet not having a film of an inorganic mineral substance such as forsterite on the surface, and further a surface of a grain-oriented electrical steel sheet after finishing annealing adjusted to a mirror surface or a state close to the mirror surface. The present invention provides a method for forming a film having excellent insulation properties and tension imparting properties.

[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 low iron loss is particularly required to reduce energy loss. Applying tension to the steel sheet is effective for 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 thermal expansion coefficient smaller than that of the steel sheet at a high temperature. The film mainly composed of forsterite, which is generated by the reaction between the oxide on the surface of the steel sheet and the annealing separator in the finish annealing step, exerts a large tension on the steel sheet and is effective in reducing iron loss.

Further, the method of forming an insulating film by baking a coating solution containing colloidal silica and phosphate as disclosed in JP-A-48-39338 has the effect of imparting tension to a steel sheet. Greatly effective in reducing iron loss. Therefore, a general method for producing a grain-oriented electrical steel sheet is to leave a film formed in the finish annealing step and then apply a tensile insulating coating.

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, for example, JP-A-49-96920 and JP-A-4-96920
As disclosed in Japanese Patent No. 131326, a technique has been developed which attempts to further reduce iron loss by removing the forsterite film generated in the finish annealing step, further performing mirror finishing, and then applying a tension film again. It was

[0006] However, although the above coating solution gives a considerable film adhesion when applied on a film mainly composed of forsterite, it removes the forsterite film or is intentionally used in the finish annealing step. When forsterite formation is not performed on the film, the film adhesion is not sufficient.

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.

By the way, in JP-A-60-131956, the surface of a steel plate processed into a mirror surface is annealed in a weakly oxidizing atmosphere to form a SiO ₂ oxide layer corresponding to 0.05 to 0.5 μm. After that, there is a disclosure of forming an insulating coating.

According to this disclosure, this SiO ₂ oxide layer is described as “having a structure in which SiO ₂ grains are dispersed in steel”. However, as described later, when an oxide layer in which SiO ₂ particles are dispersed in steel, that is, an internal oxide layer is formed, the adhesion of the insulating coating is extremely poor. Therefore, if only the amount of SiO ₂ is specified, good film adhesion cannot always be obtained.

[0010]

DISCLOSURE OF THE INVENTION The present invention solves the problems in the prior arts, and even a steel sheet having no inorganic mineral substance film such as forsterite, has an insulating film excellent in electric insulation and tension imparting property. It is intended to provide a method of forming a.

[0011]

According to the present invention, a thin SiO ₂ film in the form of a film is formed on the surface of a steel sheet in advance, and thereafter an insulating coating having a high tension-imparting property is applied to the steel sheet, so that the strength of the unidirectional silicon steel sheet can be improved. It is a method of forming an insulating coating having film adhesion and tension.

That is, the gist of the present invention is as follows. (1) Prior to applying an insulating coating to a unidirectional silicon steel sheet having no inorganic mineral film on the surface of the steel sheet after finish annealing, the steel sheet is exposed to a weakly oxidizing atmosphere in either or both of oxygen potential and temperature of the atmosphere. A unidirectional silicon steel sheet characterized by forming a SiO ₂ film having a thickness of 0.001 μm or more consisting only of an external oxide film on the surface of the steel sheet by changing the control and then applying a tension-adding type insulating coating. Method for forming insulating film.

(2) Control of changing the oxygen potential and / or temperature of the atmosphere of the steel sheet in a weakly oxidizing atmosphere is controlled to 500 to 500 for a steel sheet having a Si content of 2 to 4.8%. In the temperature range of 700 ° C, PH ₂ O /
Within the range of P H ₂ ≦ 0.5 (P H ₂ O and P H ₂ are the partial pressure of water vapor and the partial pressure of hydrogen in the atmosphere), and 700
In the temperature range of over 1000 ℃ and over 1000 ℃, PH ₂ O / PH ₂ ≦
The method for forming an insulating coating on a unidirectional silicon steel sheet according to (1), wherein the control is performed within a range of 0.15.

(3) Prior to applying an insulating coating to a unidirectional silicon steel sheet having no inorganic mineral coating on the surface of the steel sheet after finish annealing, the steel sheet surface is formed with SiO 2 having a thickness of 0.001 μm or more by CVD or PVD method. _A method for forming an insulating film on a unidirectional silicon steel sheet, which comprises applying a tension-adding type insulating coating after forming _two films.

(4) The insulating coating uses a coating liquid mainly containing colloidal silica and phosphate (1) to
The method for forming an insulating film on a unidirectional silicon steel sheet according to any one of (3).

[0016]

The tension is applied to the unidirectional silicon steel sheet by forming a heat resistant oxide film at a 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. The conventional colloidal silica-phosphate based insulation coating is
Adhesion is good when a finish annealing film mainly composed of forsterite is present on the surface of the steel sheet, but film adhesion is weak when there is no finish annealing film.

The inventors of the present invention formed a layer having good adhesion between the current insulating coating and the steel sheet, and then formed a film having a large tension, and formed no inorganic mineral film such as forsterite. That is, the inventors have made repeated studies to secure high film adhesion on a steel sheet having exposed metal.

As a result, the surface of the steel sheet was previously subjected to SiO ₂
It was confirmed that forming a thin film improves the film adhesion of the insulating coating. Furthermore, this Si
The present invention has been completed by newly discovering that the O ₂ layer has no effect when it is a so-called internally oxidized structure that is granularly deposited inside the steel sheet, and has to have a film-like or externally oxidized structure.

Here, the external oxide film is an oxide film formed under a low oxygen partial pressure, and is an oxide film which becomes a film after the alloying element (Si) diffuses to the surface layer of the steel sheet. On the other hand, the internal oxide film is an oxide film formed under a relatively high oxygen partial pressure, and is an oxide in which the alloy elements are hardly diffused and are oxidized in the form of precipitates, and the oxide particles are dispersed in the steel sheet in a granular form. Refers to a membrane.

The results of the examination will be described in detail below with specific examples. A commercially available unidirectional silicon steel sheet containing 3% of silicon is removed by a method described in JP-A-4-131326, that is, after removing the finish annealing film by pickling, and then a silicon steel sheet having the finish annealing film is used as a spacer. As a result, a high-temperature long-time annealing in a reducing atmosphere (hereinafter referred to as pickling-planarization annealing) was performed to obtain a steel sheet having no finish annealing film and having a mirror surface state.

This steel sheet was used as P H ₂ O / P H ₂ = 0.02
750 ° C, 200 seconds (condition 1) and PH ₂ O / PH ₂
= 0.15, 750 ° C, 150 seconds (condition 2). In order to check the amount of each SiO ₂ produced, the residue (only oxide can be recovered) after potentiostatic electrolysis in a non-aqueous solvent
P analysis was performed (Table 1).

[0023]

[Table 1]

For each steel sheet, Japanese Patent Laid-Open No. 48-39
The aqueous solution of colloidal silica, aluminum phosphate, and dichromic acid disclosed in Japanese Patent No. 338 is applied, 80
Baked at 0 ° C.

The film annealed under the condition 1 formed a good film, and no peeling occurred even in the bending test with a curvature of 10 mm, but under the condition 2, the film peeled during baking.

It can be understood that the reason why the film formation under the condition 2 was not good despite the large amount of SiO ₂ produced was the structure of the oxide layer (FIG. 1).

That is, under the condition 1, only the external oxide film of SiO ₂ is formed (the thickness thereof is estimated to be 0.01 μm or less, so it is not observed in FIG. 1), whereas in the condition 2, it is remarkable. Internal oxidation state.

By similar tests under various annealing conditions, it was confirmed that the film formation of the insulating coating was always hindered or the film adhesion was not sufficient when the internal oxidation state was caused by the annealing.

Further, as shown in Table 1, the generated Si
If O ₂ is an externally oxidized form, the amount produced is very small (<
It can be seen that even with a thickness of 0.01 μm, the adhesion of the insulating coating is sufficient.

Further, such extremely thin externally oxidized Si
Since it is difficult to evaluate the amount of O ₂ film formed by the above-mentioned ICP analysis of the electrolytic residue, the infrared sensitive reflection method (Suetaka K.
Spectroscopic Research, Vol. 26, p251 (1977)) may be used. This method is suitable for the evaluation means of the present invention because it does not detect the oxides deposited inside the steel sheet and can obtain information only on the external oxide film. FIG. 2 is an infrared sensitive reflection spectrum of the SiO ₂ external oxide film formed by the annealing under the above condition 1.

It is clear from Table 2 that the formation of the external oxide SiO ₂ film does not cause the magnetic deterioration of the grain-oriented silicon steel sheet. Table 2 shows a commercially available unidirectional silicon steel sheet (sheet thickness 0.23 mm) mirror-finished by the above-mentioned pickling-planarization annealing, which was annealed under the condition that only external oxidation of SiO ₂ was formed, and before and after annealing. This is an examination of the difference in iron loss value.

[0032]

[Table 2]

The annealing conditions for ensuring the adhesion of the insulating coating, that is, the annealing conditions for forming only the externally oxidized SiO ₂ layer can be set from FIG. In the figure, an oxide layer formed in various annealing atmospheres and annealing temperatures was observed for a steel sheet having a silicon content of 2 to 4.8%,
The result is illustrated.

In each of the figures, the steel sheet on which the SiO ₂ oxide film having only the externally oxidized state was formed had a good insulating film formed and had sufficient film adhesion. According to the figure, when the annealing temperature is 700 ° C or lower, PH ₂ O / PH ₂ <
Above 0.5 and 700 ° C, PH ₂ O / PH ₂ <0.1
When annealing is performed in 5, a good tension-imparting insulating film can be formed with good adhesion.

The annealing temperature is 500 ° C. or higher and 1000
The reason for limiting the temperature to below ℃ is as follows. 500 ° C
Less than Si even under a relatively high oxygen potential
The productivity is low because the O ₂ oxide film formation is extremely slow and requires a long annealing time. On the other hand, if the temperature exceeds 1000 ° C, the steel sheet is softened and continuous annealing becomes difficult, and the annealing cost also increases.

On the other hand, the upper limit of the thickness of the externally oxidized SiO ₂ film has not been found so far. That is, as long as the internal oxidation does not occur, even if the SiO ₂ film thickness is increased, the adhesion of the insulating film is not deteriorated, and the magnetic properties and other properties of the silicon steel plate are not confirmed. .

In the above description, only the annealing in a weakly oxidizing atmosphere has been described regarding the method of forming a thin film SiO _{2 on the} surface of a steel sheet.
It is clear in the examples that good adhesion to the insulating coating can be secured even with those formed by CVD or PVD.

[0038]

【Example】

Example 1 A silicon steel rolled to a final plate thickness of 0.23 mm containing 3% Si was formed with an oxide layer containing SiO ₂ on the surface of the silicon steel for decarburization annealing, and then MgO was mainly contained. Then, the annealing separator was applied and the final finish annealing was performed. Since the film mainly composed of forsterite is present on the surface of the unidirectional silicon steel plate annealed in this way, the forsterite film was removed by immersing the steel plate in a sulfuric hydrofluoric acid solution (plate thickness 0.22 mm ).

Then, a silicon steel sheet having a finish annealing film was used as a spacer and annealed at a high temperature for a long time in a reducing atmosphere to mirror-finish the surface. Furthermore, P H ₂ O / P H ₂ = 0.3,65
An SiO ₂ external oxide film was formed by annealing at 0 ° C. for 70 seconds. The SiO ₂ film measured value by infrared reflection spectrum is
It was 0.002 μm.

Subsequently, 100 m of 20% colloidal silica
l, 60 g of 35% magnesium phosphate solution and 5 g of chromic anhydride were applied on one side at 8 g / m ² and 8
It was baked at 00 ° C.

As a comparative example, the insulating coating baking was also attempted on a steel sheet from which the annealing before the insulating coating baking was omitted (Comparative Example 1). The various characteristics of the unidirectional silicon steel sheet with an insulating coating produced in this way are
Table 3 shows an example in which the annealing after the mirror finishing is omitted as a comparative material.

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 to perform the final finish annealing. Since the film mainly composed of forsterite is present on the surface of the unidirectional silicon steel plate annealed in this way, the forsterite film is removed by immersing the steel plate in a sulfuric hydrofluoric acid solution, and further chemical polishing is performed to obtain a mirror surface. (Plate thickness 0.20 mm).

For this steel sheet, P H ₂ O / P H ₂ = 0.
An SiO ₂ external oxide film was formed by annealing at 1,800 ° C. for 70 seconds. The measured value of the SiO ₂ film by the infrared reflection spectrum was 0.03 μm. 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 on one side by a grooved rubber roll at 8 g / m ² and baked at 800 ° C.

As a comparative example, P H ₂ O / P H ₂ = 0.3,
Internally oxidized SiO ₂ by annealing at 800 ° C for 70 seconds
A steel sheet on which an oxide film was formed was also prepared, and the insulating coating was baked under the same conditions (Comparative Example 2). Table 3 shows various properties of the unidirectional silicon steel sheet with an insulating coating produced in this manner.

Example 3 Silicon steel rolled to a final plate thickness of 0.23 mm containing 3% Si was decarburized and annealed, and then an annealing separating agent mainly containing Al ₂ O ₃ was applied to the final finish annealing. I went. On the surface of the unidirectional silicon steel sheet annealed in this way, a film by annealing is not formed and a mirror surface state is exhibited (sheet thickness 0.23 mm).

On the surface of this steel sheet, SiH ₄ diluted with Ar was used.
By plasma CVD using + N ₂ O mixed gas,
A 01 μm SiO ₂ film was formed. The SiO ₂ film thickness was measured by infrared reflection spectrum.

Then 100 ml of 20% colloidal silica,
100 ml of 50% aluminum phosphate solution, chromic anhydride 5
8 g / m ² per side of a grooved rubber roll was applied to the treatment liquid consisting of g and baked at 800 ° C. Table 3 shows various properties of the unidirectional silicon steel sheet with an insulating coating produced in this manner.

Example 4 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 for decarburization annealing, and then MgO was formed. Was applied to perform the final finish annealing.

Since the film mainly composed of forsterite is present on the surface of the unidirectional silicon steel plate annealed in this way, the forsterite film is removed by immersing the steel plate in a sulfuric hydrofluoric acid solution (plate thickness). 0.22 mm).

Then, a silicon steel sheet having a finish annealing film was used as a spacer and annealed at a high temperature for a long time in a reducing atmosphere to mirror-finish the surface. On the surface of this steel sheet, Si in an O ₂ atmosphere
Si of 0.01μm by PVD targeting the plate
An O ₂ film was formed.

The SiO ₂ film thickness was measured by infrared reflection spectrum. Then 100% of 20% colloidal silica
A treatment liquid consisting of 100 ml of 50% aluminum phosphate solution and 5 g of chromic anhydride was applied at 8 g / m ^{2 on} one side by a grooved rubber roll and baked at 800 ° C. Table 3 shows various properties of the unidirectional silicon steel sheet with an insulating coating produced in this manner.

[0052]

[Table 3]

[0053]

The present invention provides a method for forming an insulating coating having high adhesion on a unidirectional silicon steel sheet without reducing the tension on the steel sheet. Therefore, according to the present insulating coating forming method, it is possible to produce a unidirectional silicon steel sheet with 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 enormous. is there.

[Brief description of drawings]

1A and 1B are optical micrographs showing the difference in the oxide layer structure depending on the annealing conditions of silicon steel sheets.

FIG. 2 is an infrared sensitive reflection spectrum of a thin external oxide SiO ₂ film formed by annealing a silicon steel sheet in a weakly oxidizing atmosphere.

FIG. 3 is a diagram showing an annealing condition and an oxide layer structure of a silicon steel sheet.

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[Procedure amendment]

[Submission date] December 10, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Figure 1

[Correction method] Change

[Correction content]

1 (a) and 1 (b) are optical micrographs of a metal structure showing a difference in an oxide layer structure depending on annealing conditions of a silicon steel sheet.

[Procedure Amendment 2]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 1

[Correction method] Change

[Correction content]

[Figure 1]

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshiyuki Ushigami 20-1 Shintomi, Futtsu City Nippon Steel Corporation Technology Development Division

Claims (4)

[Claims] 1. Prior to applying an insulating coating to a unidirectional silicon steel sheet having no inorganic mineral film on the surface of the steel sheet after finish annealing, the steel sheet is placed in a weakly oxidizing atmosphere and either one of the oxygen potential and temperature of the atmosphere or the temperature of the atmosphere is used. The unidirectionality is characterized in that a tension-adding type insulation coating is applied after forming a SiO ₂ film consisting only of an external oxide film with a thickness of 0.001 μm or more on the surface of the steel sheet by controlling to change both. A method for forming an insulating film on a silicon steel sheet. 2. A steel sheet having a Si content of 2 to 4.8% is controlled in a range of 500 to 700 by controlling one or both of oxygen potential and temperature of the atmosphere in a weakly oxidizing atmosphere. PH ₂ O / PH in the temperature range of ℃
Within the range of ₂ ≤ 0.5 (P H ₂ O and P H ₂ are the partial pressure of water vapor and the partial pressure of hydrogen in the atmosphere, respectively) and 700 ° C
In the temperature range of over 1000 ℃, PH ₂ O / PH ₂ ≤
The method for forming an insulating coating on a unidirectional silicon steel sheet according to claim 1, wherein the control is performed within a range of 0.15. 3. finish the grain oriented silicon steel sheet free of inorganic mineral coating the surface of the steel sheet after annealing before applying an insulating coating by CVD or PVD method, SiO ₂ of more than 0.001μm on the surface of the steel sheet thickness After forming the film,
A method for forming an insulating film on a unidirectional silicon steel sheet, which comprises applying a tension-type insulating coating. 4. The method for forming an insulating coating on a unidirectional silicon steel sheet according to claim 1, wherein the insulating coating uses a coating liquid mainly containing colloidal silica and phosphate.