JPH07278826A - Low-iron-loss grain-oriented silicon steel sheet having oxide-base composite coating film and its production - Google Patents

Abstract

PURPOSE:To produce a grain-oriented silicon steel sheet reduced in iron loss by forming a coating film imparting high tension to the steel sheet on its surface. CONSTITUTION:A magnesium oxide-aluminum oxide-silicon oxide composite coating film or a magnesium oxide-aluminum oxide-silicon oxideamorphous oxide composite coating film is formed on the surface of a steel sheet. A remakable tension imparting effect is obtained when the film contains crystalline cordierite or crystalline sapphirine. A suspension contg. a magnesium oxide precursor compd., an aluminum oxide precursor compd. and a silicon oxide precursor compd. is applied, dried and then baked at a specified temp. to form the film.

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 in which iron loss is reduced by having a coating film which gives a large tension to the steel sheet, and a method for producing the same.

[0002]

2. Description of the Related Art One-directional silicon steel sheets are (110), [0
[01] is mainly used as a magnetic iron core material, and a material with a small iron loss is required to reduce energy loss. As a means for reducing the iron loss of the unidirectional silicon steel sheet, the steel sheet surface after finish annealing is irradiated with a laser beam to give a local strain,
A method for subdividing a magnetic domain by this is disclosed in Japanese Patent Laid-Open No. 58-26
It is disclosed in Japanese Patent No. 405. Further, there is a magnetic domain refining means which does not lose its effect even after performing stress relief annealing (stress relief annealing) after iron core processing, for example, JP-A-62-86175.
It is disclosed in the publication.

On the other hand, since iron alloys containing iron and silicon have large crystal magnetic anisotropy, application of external tension causes subdivision of magnetic domains, which reduces eddy current loss which is a main factor of iron loss. You can Therefore, it is effective to apply tension to the steel sheet in order to reduce the iron loss of the unidirectional silicon steel sheet containing 5% or less of silicon, and it is effective to apply the tension up to about 1.5 kgf / mm ^2. It is known that the loss can be reduced. This tension is usually applied by the film formed on the surface.

Conventionally, for unidirectional silicon steel sheets, a primary coating mainly composed of forsterite formed by a reaction between an oxide on the surface of the steel sheet and an annealing separator in the finish annealing step, and Japanese Patent Laid-Open No. 48-39338. A tension of about 1.0 kgf / mm ² is given by the two-layer coating film, which is a secondary coating film produced by baking the coating liquid mainly containing colloidal silica and phosphate disclosed in Japanese Patent Publication No. . Therefore, in the case of these existing coatings, there is still room for iron loss improvement by applying a larger tension, but increasing the applied tension by thickening the coating leads to a decrease in space factor, which is not preferable.

As another method for improving the iron loss of a unidirectional silicon steel sheet, the unevenness of the surface of the steel sheet after finish annealing and the internal oxide layer near the surface are removed to give a mirror finish, and the surface is made of metal. The method of plating is Japanese Patent Publication Sho 52-2
4499 discloses a method of further forming a tension film on the surface thereof, for example, JP-B-56-4150 and JP-A-6-61.
No. 1-201732, JP-B-63-54767, JP-A-2-213488, and the like. Even in these cases, the larger the tension applied to the steel sheet by the coating, the greater the iron loss improving effect. From these things, a coating film which is excellent in adhesion, thin, and capable of imparting a large tension to a steel sheet has been desired.

[0006]

SUMMARY OF THE INVENTION The present invention solves these problems in the prior art, and has a coating film that gives a large tension to the steel sheet on the surface thereof. It is an object to provide a manufacturing method thereof.

[0007]

According to the present invention, a unidirectional silicon steel sheet having a magnesium oxide-aluminum oxide-silicon oxide composite coating on its surface and a magnesium oxide-aluminum oxide-silicon oxide composite coating are crystalline cordier. Unidirectional silicon steel sheet containing light and / or crystalline sapphirine, unidirectional silicon steel sheet having magnesium oxide-aluminum oxide-silicon oxide-amorphous oxide composite coating on the surface, magnesium oxide-aluminum oxide-oxidation Unidirectional silicon in which the silicon-amorphous oxide composite coating contains crystalline cordierite and / or crystalline sapphirine and / or an amorphous phase containing at least one of silicon, boron and phosphorus as a component. The main point is steel plates.

Further, a suspension containing a magnesium oxide precursor compound, an aluminum oxide precursor compound and a silicon oxide precursor compound is applied to the surface of the steel sheet which has been subjected to finish annealing, dried and then baked at a temperature of 500 to 1350 ° C. The gist is a manufacturing method for forming an oxide film. Furthermore, the gist is a manufacturing method in which the aluminum oxide precursor compound is an aluminum oxide precursor sol and the silicon oxide precursor compound is a silicon oxide precursor sol. In addition, a suspension containing magnesium oxide precursor compound, aluminum oxide precursor compound, silicon oxide precursor compound, boric acid and / or borate, phosphoric acid and / or phosphate is applied to the surface of the steel sheet on which finish annealing has been completed. The gist is a manufacturing method in which after drying, baking is performed at a temperature of 500 to 1350 ° C. to form an oxide film. Furthermore, the gist is a manufacturing method in which the aluminum oxide precursor compound is an aluminum oxide precursor sol and the silicon oxide precursor compound is a silicon oxide precursor sol.

[0009]

The present invention will be described in detail below. The unidirectional silicon steel sheet of the present invention has a magnesium oxide-aluminum oxide-silicon oxide composite coating film on its surface. Conventionally, a film material having a small coefficient of thermal expansion has been selected for applying tension to a silicon steel sheet, and stress generated during cooling due to a difference in coefficient of thermal expansion from the steel sheet has been used. However, it has been pointed out that not only the thermal expansion coefficient, but also the Young's modulus of the coating material is a factor that affects the application of tension to the steel sheet. Although it is impossible to clearly define the role of the film-constituting component of the present invention, the aluminum oxide component has a relatively large Young's modulus, and the silicon oxide component can have a relatively small thermal expansion coefficient. It is speculated that a large amount of tension is applied to the steel sheet by compounding magnesium oxide with.

Although only what is called periclase is known as a crystalline phase of magnesium oxide, aluminum oxide has several crystal systems such as α-, γ-, δ-, and θ-. The effect of imparting the tension is not necessarily the same in each crystal system. However, the aluminum oxide of the present invention may be any of these. Further, the crystal does not necessarily have to have good crystallinity, and may be an amorphous crystal having poor crystallinity or a compound that serves as a crystal precursor. Silicon oxide is also known to have several types of crystal systems and an amorphous phase. Among them, an amorphous phase called silica glass has a relatively small thermal expansion coefficient and is particularly preferably used. Although crystalline SiO ₂ has a larger coefficient of thermal expansion than an amorphous phase, it may be used from the viewpoint of improving the sticking resistance of the coating.

Further, by containing crystalline cordierite and / or crystalline sapphirine in these coatings, it may be possible to give a larger tension. Cordierite is a crystal represented by the chemical formula of 2MgO · 2Al ₂ O ₃ · 5SiO ₂ and is presumed to bring high tensile strength to a steel sheet due to its properties of low thermal expansion coefficient and high Young's modulus. Sapphirine is 4MgO / 5Al
It is a crystal represented by the chemical formula of ₂ O ₃ .2SiO ₂ ,
For the same reason as in the case of cordierite, it gives high tension to the steel sheet.

Magnesium oxide in the composite coating of the present invention,
The abundance ratio of aluminum oxide and silicon oxide can be set in a relatively wide range, and can be set to any ratio. However, in order to maximize the features of the composite coating, it is preferable that the respective components are contained in a weight ratio of at least 5%, preferably 10% or more with respect to the entire coating. Further, the amounts of crystalline cordierite and sapphirine can be set to any ratio, and higher tension can be imparted by containing as many of these compounds as possible, but the surface properties of the coating tend to deteriorate. Therefore, it is preferable to determine the optimum amount as necessary. The content of the crystalline cordierite and the sapphirine is preferably 90% or less, more preferably 80% or less, and usually 5 to 7 by weight ratio with respect to the entire coating film.
It is selected from the range of about 5%.

The surface of another unidirectional silicon steel sheet of the present invention has a magnesium oxide-aluminum oxide-silicon oxide-amorphous oxide composite coating film. The roles of the magnesium oxide component, the aluminum oxide component, and the silicon oxide component are as described above. However, as the role of the amorphous oxide component, it does not have a great effect on the tension application to the steel sheet, and the surface smoothness and the substrate It is presumed that the adhesion to the steel sheet is greatly improved. Among them, it has been found that this effect is particularly remarkable in an amorphous phase containing at least one of silicon, boron and phosphorus as a component. In particular, a remarkably large effect is obtained when forming a glassy substance. The content of silicon, boron, and phosphorus in the amorphous phase is 50% or more, preferably 70% or more in terms of the total weight of the respective oxides in terms of the weight ratio to the entire amorphous phase.

Further, the amorphous phase may contain a trace amount of components other than silicon, boron and phosphorus.
Possible elements include Al and M, which are the main components of the film.
g, Fe as a base material constituent, and T as a primary film constituent
In addition to i, Mn, S, Li, Na, K, Ca, Sr, B
a, V, Cr, Ni, Co, Cu and other alkali metals, alkaline earth metals, transition metal elements, or S
Examples include n, Pb, Bi, Sb, and the like.

The content of the amorphous phase as a whole is not particularly limited, but if it is too much, the application of tension to the steel sheet will be insufficient, so the weight ratio to the entire coating is 90%.
Or less, more preferably 70% or less. On the other hand, if the amount is too small, a sufficiently smooth coating surface and good adhesion may not be obtained. Therefore, it is desirable that the content is 5% or more, more preferably 10% by weight or more with respect to the entire coating. The coating film on the surface of the unidirectional silicon steel sheet of the present invention is preferably as thin as possible according to the purpose because the space factor decreases if it is too thick, and one guideline is 5% or less of the steel sheet thickness. More preferably, it is 2% or less of the steel plate thickness. Further, from the viewpoint of applying tension, if it is extremely thin, a sufficient effect cannot be obtained, and 0.1 μm or more is desirable.

The method for suitably manufacturing the unidirectional silicon steel sheet of the present invention will be described below. One is to apply a suspension containing a magnesium oxide precursor compound, an aluminum oxide precursor compound, and a silicon oxide precursor compound to the surface of the steel sheet that has been subjected to finish annealing, and after drying, 500 to 1350 ° C.
It is a manufacturing method by baking at the temperature of 1 to form an oxide film.

The steel sheet that has been subjected to finish annealing as used herein means: a steel sheet that has been subjected to finish annealing by a conventionally known method and has a forsterite primary coating formed on its surface,
A steel sheet obtained by immersing an acid layer in a primary coating and an oxide layer that is additionally formed, and a steel sheet obtained by: being flattened and annealed in hydrogen, or by chemical polishing, electrolytic polishing, or the like. Polished steel plate: Alumina powder or the like which is inactive against film formation, or a conventionally known annealing separator to which a trace amount of additives such as chloride is added, and under the condition that a primary film is not formed. It includes steel sheets that have undergone finish annealing.

The magnesium oxide precursor compound is a general term for compounds that become magnesium oxide after baking (heat treatment). In addition to magnesium oxide, magnesium hydroxide, or various magnesium salts such as magnesium nitrate and magnesium chloride. Refers to. Similarly, the aluminum oxide precursor compound is also after baking (heat treatment).
Is a general term for compounds that become aluminum oxide, and not only aluminum oxide but also Al ₂ O ₃ such as boehmite.
-Aluminum oxide hydrate represented by nH ₂ O, aluminum hydroxide, or various aluminum salts such as aluminum nitrate and aluminum chloride. The silicon oxide precursor compound is also a general term for compounds that become silicon oxide after baking, and hydrates of silicon oxide, silicon oxyhydroxide, various silicon compounds and the like are preferably used.

These raw materials are dispersed in a dispersion medium to prepare a suspension (slurry). Water is most suitable as the dispersion medium from the viewpoints of workability and cost, but an organic solvent or a mixture thereof can be used if there is no particular problem in other steps. Some of the raw materials may dissolve at the time of making the slurry, but this is all right.

The slurry thus obtained is applied to the surface of the unidirectional silicon steel sheet which has been subjected to finish annealing by a conventionally known method such as a coater such as a roll coater, a dipping method, spraying, or electrophoresis. After drying, 500
An oxide film is formed on the surface by baking at ˜1350 ° C. The atmosphere during baking is preferably an inert gas atmosphere such as nitrogen, a reducing atmosphere such as a nitrogen-hydrogen mixed atmosphere, and an atmosphere containing air or oxygen may oxidize the steel sheet, which is not preferable. There is no particular limitation on the dew point of the atmospheric gas. When the baking temperature is lower than 500 ° C., the applied precursor may not become an oxide, and since the baking temperature is low, sufficient tension is not expressed, which is not preferable. On the other hand, when the temperature exceeds 1350 ° C, it is not economical although it is not particularly inconvenient, and more preferably 1250 ° C or less.

Among the above-mentioned slurries, a thin and uniform dispersion is obtained by using a so-called sol fine particle dispersion system as the aluminum oxide precursor and silicon oxide precursor compounds.
In addition, a film having good adhesion may be obtained. This is particularly remarkable when a non-metal substance does not exist on the surface and a film is directly formed on the metal surface. In this case, the fine particle dispersed sol described above or a sol containing a soluble component is preferably used. When a sol solution is used as the coating liquid, the so-called boehmite sol and / or alumina sol described above is particularly preferably used as the aluminum oxide precursor compound in terms of workability or cost. On the other hand, various kinds of silicon oxide precursor sols can be used, but SiO ₂ · nH ₂ O or S
The silica sol represented by the chemical formula iO _x · (OH) _y and / or the colloidal silica is also workable.
It is preferably used in terms of price.

Above all, Si is used as the silicon oxide precursor sol.
An alkyl silicate of the formula (OC _z H _{2z + 1} ) ₄ ,
And / or a hydrolyzate thereof is preferably used. Alkyl silicate is one kind of metal alkoxide,
Hydrolysis is relatively slow, and stable properties can be obtained as a precursor. Alkyl silicate usually forms a hydroxide of silicon or a hydrate of silicon oxide by hydrolysis, but in the case of forming a kind of film, the hydrolyzate is used rather than using the alkyl silicate as it is. There are cases where it is preferable. In such a case, the precursor sol after hydrolysis is preferably used. For this, there are several methods such as a method of preliminarily hydrolyzing and then mixing with other components, a method of parallelizing hydrolysis while mixing with other components, and adding aging if necessary, but In the invention, any of these is not a problem.

Among the preferred alkyl silicate compounds, those having a high hydrolysis rate and a small number of carbon atoms z are preferable, and z ≦ 3 is preferable, but a methyl silicate in which z = 1 is methyl produced by hydrolysis. Ethyl silicate with z = 2 is particularly preferably used because of the harmful nature of alcohol. When the aluminum oxide precursor sol is used as the aluminum oxide precursor compound and the silicon oxide precursor sol is used as the silicon oxide precursor compound as described above, the magnesium oxide precursor compound is an ultrafine magnesium oxide powder prepared by a vapor phase method or the like. It is preferable to use a fine powder prepared by a liquid phase method or the like, a dispersion system thereof, or soluble magnesium salts, whereby uniform mixing at an extremely micro level is realized.

When using the precursor sol of aluminum oxide or silicon oxide, it is possible to disperse it in a dispersion medium, especially water, as in the case of the above-mentioned slurry. In order to obtain particularly good dispersibility, pH control of the coating solution by addition of acid, alkali or the like is a technique often used, and the present invention can be carried out without any particular problems.
Further, there is no problem in adding a very small amount of a surfactant or the like to improve the coating property on the steel sheet. As another manufacturing method, a magnesium oxide precursor compound, an aluminum oxide precursor compound, a silicon oxide precursor compound, boric acid and / or borate, phosphoric acid and / or phosphoric acid salt are applied to the surface of the steel sheet after the finish annealing. It is a manufacturing method in which a suspension containing the composition is applied, dried, and then baked at a temperature of 500 to 1350 ° C. to form an oxide film.

H as boric acid and / or borate
Orthoboric acid represented by ₃ BO ₃ is the most preferable in terms of price, but metaboric acid represented by HBO ₂ and B ₂
Boron oxide represented by O ₃ or a mixture thereof can also be used. Further, as borate, alkali borate such as lithium borate and sodium borate, aluminum borate, zinc borate, barium borate, lead borate and the like are preferably used. As the phosphoric acid and / or the phosphoric acid salt, phosphoric acid and orthophosphoric acid are particularly preferably used. In addition to these, as phosphates, alkali phosphates such as lithium phosphate and sodium phosphate, alkaline earth phosphates such as magnesium phosphate, calcium phosphate, ammonium phosphate, aluminum phosphate, zinc phosphate, and phosphate. Barium acid, lead phosphate, iron phosphate, chromium phosphate and the like are preferably used.

In the second manufacturing method as well, as in the first manufacturing method described above, by using a fine particle dispersion system called a sol as the aluminum oxide precursor and silicon oxide precursor compounds, a thin and uniform composition can be obtained. A film with good adhesion may be obtained. In this case, the fine particle dispersed sol described above or a sol containing a soluble component is preferably used. The content of boric acid and / or borate, phosphoric acid and / or phosphate can be selected from a wide range, but in order not to impair the excellent feature of the present coating that high tension is imparted, oxidation is required. It is 50 wt% or less, preferably 30 wt% or less of the total solid content (oxide equivalent) in terms of the total amount in terms of the product. The present invention will be specifically described below with reference to examples, but the present invention is not limited to the examples.

[0027]

【Example】

Example 1 Commercially available magnesium oxide fine powder, aluminum oxide powder (α-Al ₂ O ₃ ), and silicon oxide powder were mixed in the proportions shown in Table 1, and distilled water was added to this to prepare a slurry.
This was applied to a unidirectional silicon steel sheet containing 3.2% Si and having a thickness of 0.2 mm and finished with finish annealing (having a primary coating of forsterite) so as to have 4 g / m ² on one side,
After drying, 800 in N ₂ atmosphere containing 5 vol% of H ₂
An oxide film was formed on the surface by baking at 5 ° C for 5 minutes.

From the results of chemical analysis, X-ray diffraction, electron microscopy, etc., it was found that the obtained coating mainly consisted of MgO, α-Al ₂ O ₃ and SiO ₂ -based amorphous phase. A winding test was conducted around a 20 mmφ cylinder so that the angle was 180 degrees, and the adhesion of the coating film evaluated from the peeling condition was very good. Table 1 shows the tension applied to the steel sheet measured from the bending of the sheet after removing the coating on one side and the magnetic properties. From the results in Table 1, it can be seen that the grain-oriented silicon steel sheets having significantly low iron loss are obtained. The chemical stability of the coating film formed on the surface was also very good.

[0029]

[Table 1]

Example 2 Commercially available magnesium oxide fine powder, boehmite powder (average particle size: 100 nm), colloidal silica (average particle size: 15)
nm) was mixed at a ratio shown in Table 2 in terms of solid content, and distilled water was added to this to prepare a mixed sol. This was applied to a 0.2 mm thick unidirectional silicon steel sheet containing 3.2% of Si, aluminum oxide applied as an annealing separator, and subjected to mirror finishing at the same time as secondary recrystallization. ²
After coating and drying so as to achieve the following, an oxide film was formed on the surface by baking at 1100 ° C. for 10 minutes in an N ₂ atmosphere containing 10 vol% of H ₂ .

Table 2 shows the constituent phases of the coating film measured from the results of chemical analysis, X-ray diffraction, electron microscopy and the like. The formation of cordierite was confirmed for some compositions in the table. The adhesion of the coating film evaluated in the same manner as in Example 1 was extremely good. Table 2 shows the tension applied to the steel sheet measured from the bending of the sheet after removing the coating on one side and the magnetic properties. From the results in Table 2, it can be seen that the grain-oriented silicon steel sheets having extremely low iron loss are obtained. The chemical stability of the coating film formed on the surface was also very good.

[0032]

[Table 2]

Example 3 Commercially available magnesium oxide fine powder, boehmite powder (average particle size: 100 nm), ethyl silicate and boric acid reagent were mixed in the proportions shown in Table 3, and distilled water was added thereto to prepare a mixed sol. did. This was applied to a 0.2 mm-thick finish-annealed unidirectional silicon steel sheet (having a primary coating of forsterite) containing 3.2% of Si so that one side was 4 g / m ² and dried. after, N ₂ containing H ₂ 3 vol%
An oxide film was formed on the surface by baking at 850 ° C. for 3 minutes in the atmosphere.

Table 3 shows the constituent phases of the coating film measured from the results of chemical analysis, X-ray diffraction, electron microscopy and the like. Since no crystalline phase containing boron is observed, it is understood that the boron component exists as an amorphous phase. Example 1
The adhesion of the coating film evaluated in the same manner as above was very good. Table 3 shows the tension applied to the steel sheet measured from the bending of the sheet after removing the coating on one surface and the magnetic properties. From the results in Table 3, it can be seen that the grain-oriented silicon steel sheets having extremely low iron loss are obtained. The chemical stability of the coating film formed on the surface was also very good.

[0035]

[Table 3]

Example 4 Commercially available magnesium oxide fine powder, boehmite powder (average particle size: 0.4 nm), ethyl silicate, boric acid reagent, and phosphoric acid were mixed in the proportions shown in Table 4, and distilled water was added thereto. To prepare a slurry. This was applied to a 0.2 mm thick unidirectional silicon steel sheet containing 3.2% of Si, aluminum oxide applied as an annealing separator, and subjected to mirror finishing at the same time as secondary recrystallization. ² so as coated, after drying, 950 ° C. in N ₂ atmosphere containing H ₂ 5 vol%,
An oxide film was formed on the surface by baking for 5 minutes.

Table 4 shows the constituent phases of the coating film measured from the results of chemical analysis, X-ray diffraction, electron microscopy and the like. The crystalline phase containing boron and phosphorus is not observed, and the crystalline phase containing boron and phosphorus is not observed, so that the boron and phosphorus components are present as an amorphous phase. I understand. The adhesion of the coating film evaluated in the same manner as in Example 1 was extremely good. Table 4 shows the tension applied to the steel sheet measured from the bending of the sheet after removing the coating on one surface and the magnetic properties. From the results in Table 4, it can be seen that the grain-oriented silicon steel sheets having extremely low iron loss are obtained. The chemical stability of the coating film formed on the surface was also very good.

[0038]

[Table 4]

[0039]

According to the present invention, it has a film of a specific component,
It is possible to provide a unidirectional silicon steel sheet which is chemically stable and whose iron loss is remarkably improved by its tension-imparting effect, and a method for producing the same. The present invention particularly exhibits good characteristics with respect to both a steel sheet having a primary coating that has been conventionally used and a mirror-finished steel sheet for the purpose of significantly reducing iron loss. The effect is enormous.

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[Procedure amendment]

[Submission date] June 14, 1995

[Procedure Amendment 2]

[Document name to be amended] Statement

[Name of item to be amended] Title of invention

[Correction method] Change

[Correction content]

Title: Low iron loss unidirectional silicon steel sheet having oxide-based composite coating and method for producing the same

Claims (8)

[Claims] 1. A low iron loss unidirectional silicon steel sheet having a magnesium oxide-aluminum oxide-silicon oxide based composite coating on its surface. 2. Magnesium oxide-aluminum oxide-
Silicon oxide-based composite coating is crystalline cordierite and / or
Alternatively, the low iron loss unidirectional silicon steel sheet according to claim 1, which contains crystalline sapphirine. 3. A low iron loss unidirectional silicon steel sheet having a magnesium oxide-aluminum oxide-silicon oxide-amorphous oxide composite coating film on its surface. 4. Magnesium oxide-aluminum oxide-
The silicon oxide-amorphous oxide-based composite coating contains crystalline cordierite and / or crystalline sapphirine, and / or
The low iron loss unidirectional silicon steel sheet according to claim 3, further comprising an amorphous phase containing at least one of silicon, boron and phosphorus as a component. 5. A suspension containing a magnesium oxide precursor compound, an aluminum oxide precursor compound, and a silicon oxide precursor compound is applied to the surface of the unidirectional silicon steel sheet on which finish annealing has been completed, dried and then 500 to 1350 ° C. A method for producing a low iron loss unidirectional silicon steel sheet, which is baked at the above temperature to form an oxide film. 6. The method for producing a low iron loss unidirectional silicon steel sheet according to claim 5, wherein the aluminum oxide precursor compound is an aluminum oxide precursor sol and the silicon oxide precursor compound is a silicon oxide precursor sol. 7. A magnesium oxide precursor compound, an aluminum oxide precursor compound, a silicon oxide precursor compound, boric acid and / or borate, phosphoric acid and / or phosphoric acid is formed on the surface of the unidirectional silicon steel sheet on which finish annealing has been completed. A method for producing a low iron loss unidirectional silicon steel sheet, comprising applying a suspension containing a salt, drying and baking at a temperature of 500 to 1350 ° C. to form an oxide film. 8. The method for producing a low iron loss unidirectional silicon steel sheet according to claim 7, wherein the aluminum oxide precursor compound is an aluminum oxide precursor sol and the silicon oxide precursor compound is a silicon oxide precursor sol.