US3868280A

US3868280A - Method of forming electric insulating films oriented silicon steel

Info

Publication number: US3868280A
Application number: US388790A
Authority: US
Inventors: Takaaki Yamamoto; Kaneo Akanuma; Osamu Tanaka
Original assignee: Individual
Current assignee: Individual
Priority date: 1967-12-12
Filing date: 1973-08-16
Publication date: 1975-02-25
Anticipated expiration: 1992-02-25

Abstract

A method for producing a glassy film having an excellent insulating property on an oriented cold-rolled steel sheet by coating the surface of said silicon steel sheet with a mixture of a Ti compound and an Mg compound and possibly with the addition of a Mn compound thereto and then subjecting the coated silicon steel sheet to a heat-treatment.

Description

United States Patent [1 1 Yamamoto et al.

[4 Feb. 25, 1975 Filed: Aug. 16, 1973 Appl. No.: 388,790

Related U.S. Application Data Continuation of Ser. No. 207,524, Dec. 13, 1971, abandoned, which is a division of Ser. No. 781,963, Dec. 6, 1968, Pat. No. 3,627,524.

Foreign Application Priority Data Dec. 12, 1967 Japan 42-79243 U.S. Cl 148/113,1l7/135.1, 117/221,

148/31.57 Int. Cl. Holf 1/04 Field of Search 148/113, 112,111, 31.55;

[56] References Cited UNITED STATES PATENTS 2,533,351 12/1950 Carpenter 148/113 3,151,000 9/1964 Schmidt et a1 148/113 3,203,839 8/1965 Takahashi et a1... 148/113 3,389,006 6/1968 Kohler 148/113 3,418,710 12/1968 Seidel et a1. 148/113 3,627,594 12/1971 Yamamoto et a1. 148/113 7/1972 Matsumoto 61: a1. 148/111 Primary ExaminerWalter P. Satterfield Attorney, Agent, or Firm-Wenderoth, Lind & Ponack [5 7 ABSTRACT A method for producing a glassy film having an excellent insulating property on an oriented cold-rolled steel sheet by coating the surface of said silicon steel sheet with a mixture of a Ti compound and an Mg compound and possibly with the addition of a Mn compound thereto and then subjecting the coated silicon steel sheet to a heat-treatment.

6 Claims, No Drawings METHOD OF FORMING ELECTRIC INSULATING FILMS ORIENTED SILICON STEEL This application is a continuation of Ser. No. 207,524, filed Dec. 13, 1971 and now abandoned, which in turn is a division of application Ser. No. 781,963, filed Dec. 6, 1968 now U.S. Pat. No. 3,627,524.

This invention relates to a method for improving the brittleness of an oriented cold-rolled silicon steel sheet and at the same time forming an electrical insulating film having an excellent electric insulation, adhesion, bendability, space factor and heat resistance on the surface of the oriented cold-rolled silicon steel sheet. The

term oriented cold-rolled silicon steel sheet used in the present invention means a cold rolled silicon steel sheet consisting of 2 to 3.5 percent by weight Si, the rest being Fe and unavoidable impurities, and having a highly oriented crystal grain structure after being finally annealed. Further, it contains .in addition one or more of the materials Al (0.01 to 0.09 percent by weight so]. Al), sulfide, nitride of Mn and V and selenium compound, in small amounts as an inhibitor in order to effect a highly oriented secondary recrystallization during the final annealing. The expression oriented" means that .a .great part of crystal grains have (110) [001] crystal orientation, which is called cube on edge or (100) [001] crystal orientation, which is called cube on face.

Oriented cold-rolled silicon steel sheet is used for laminated iron cores or wound iron cores. Such individual silicon steel sheets arecoated with .electrical insulating films so as to be electrically insulated from each other.

Such electrical insulating film is required to be so tightly adhered to the sheet that-it will not be peeled off during working and to have a high space factor and excellent resistance to heat. On the otherhand, such insulating film should have no bad influence on the magnetic property ofthe steel sheet. The silicon steel sheet is generally subjected to a final annealing in which it is treated at a high temperature to about 1,200C for a long time.

Such final annealing is carried out on steel sheets which are laminated or wound in coils. In such case, in order to prevent the steel plates from sticking to each other due to heat, an annealing separator is used.

For such annealing separator there is utilized a substance which acts to prevent sticking of the steel sheet and at the same time forms a glassy electrical insulating film by reacting with an oxide on the surface of the steel sheet at the annealing temperature.

Substances used for such separators and methods for producing glass-like electrical insulating film are known from U.S. 'Pat. Nos. 2,354,123, 2,385,332, 2,492,682 and 2,533,351.

These known separators are provided not only for separating action, but also to form electrical insulating films. Therefore, various film characteristic improvin steps can be taken.

For example, there is known a process wherein, during a decarburizing annealing, when Si is selectively oxidized to form SiO on the steel sheet surface, the steel sheet surface is coated with a suspension consisting of MgO, Mg(OH) and H and is dried and the steel sheet is then final-annealed at a high temperature in a reducing atmosphere, and the MgO will act as annealing separators to prevent the steel sheets from sticking to each other and at the same time a part of the materials will react with the above-mentioned SiO to form a glassy electrical insulating film.

However, 'MgO is used as the substance to act as a separator and as a glassy film forming agent. When it is to be applied to a steel sheet, it is generally mixed with water so that itis in the form of a liquid suspension or slurry.

Therefore, there is a disadvantage that MgO acts with water to become Mg(Ol-l) which discharges water of hydration while being heated and oxidizes and embrittles the steel sheet.

Such embrittlement causes considerable trouble during the operation of cutting, perforating or winding steel sheets.

As a result of various research made with a view to eliminating the above-mentioned disadvantage and further to reduce the brittleness of steel sheets, the inv.en tors have found that, when a Ti compound such as an oxide or hydroxide of Ti and a Mn compound such as an oxide or hydroxide of Mn is mixed with a Mg compound such as an oxide or hydroxide of Mg, an excellent stable and strongly adherent glassyelectrical insulating film can be formed on an oriented cold-rolled silicon steel sheet and at the same time the brittleness of said steel sheet can be reduced.

An object of the present invention :is to provide an insulating film forming process for stably forming a film having an excellent electric insulation property and heat resistance and which is strongly adherent and at the same time reducingthe brittleness of the oriented cold-rolled silicon steel sheet itself.

It is known that, in order to form an adhesive insulating film,'SiO 'can be formed on the surface of a silicon steel sheet'by selective oxidization so as to be a silicious base for the later formation of a glassy film. Such a layer of the silicious base is formed generally at the time of decarburizing annealing. However, in general, the layer composition is so sensitive to the temperature, time, atmosphere and dew point of the atmosphere that it is difficult to obtain a high quality glassy film in a stable manner with known substances capable of serving both as a separator to prevent sticking and as a glassy film forming agent with the result that a film which is poorly adherent is often produced.

Particularly, when carrying out a treatment for forming .the above-mentioned silicious base layer, when Al in an amount of 0.01 to 0.09 percent by weight so]. Al

' is contained in the silicon steel sheet as an inhibitor, the

Al will also be partly oxidized 'to produce A1 0 which conjointly with the treating conditions will have a great influence on the formation of a uniform glassy "film such as impairing the stability of the glassy film and making it more difficult to form a film with excellent electric insulation characteristics, appearance and .adhesion.

Further, the water contained in the abovementioned substance will be discharged in the final annealing, which will improperly oxidize the steel sheet and prevent the formation of a high quality glassy film at a temperature particularly above 1,000C.

The present invention comprises heat-treating for a short time an oriented silicon steel sheet coldrolled to any desired dimensions, for example, in an atmosphere containing wet hydrogen gas so that a surface layer containing SiO from the selective oxidation of Si in the drogen so that a glassy insulating film will be formed According to the method of the present invention, a uniform glassy film which is strongly adherent is stably obtained without being influenced by the heat-treating conditions of the atmosphere and the like. Further, the

brittleness of the steel sheet itself is greatly reduced.

The method of the present invention shall be explained more particularly in the following.

As mentioned above, the present invention is' to be applied to oriented cold-rolled silicon steel sheets. Such an oriented cold-rolled silicon steel sheet is annealed continuously for a short time under a condition, for example, in a wet reducing atmosphere which prevents the oxidization of iron as much as possible, but allows the oxidization of silicon in the steel so that SiO will be formed on the surface of the steel sheet.

The heat-treatment for the formation of the abovementioned SiO- is carried out under a condition which will prevent the oxidization of iron. However, a slight oxidization ofiron is not always detrimental. But excess oxidization will reduce the quality of the glassy film and therefore must be avoided.

For the heat-treatment for forming 'SiO by the selective oxidization on or near the surface of an oriented cold-rolled silicon steel sheet, a reducing atmosphere of hydrogen only or dissociated ammonia is generally used, the dew point of which is 55 to 70C and the temperature of which is 700 to 900C. The longer the treating time, the thicker the oxidized layer and the greater the advantage in the formation of a glassy insulating film. But, the holding time at the abovementioned temperature is at least one minute or more than one minute. Needless to say, even when, instead of the short time treatment, a box-annealing is .carried out with open coils to form a layer containing Si on the surface of the oriented cold-rolled silicon steel sheet, the same favorable result will be obtained. But, in such case, the dew point is preferably below 55C.

During the above-mentioned heat-treatment for the formation of SiO the decarburization of the oriented cold-rolled silicon steel sheet can be simultaneously carried out.

The oriented cold-rolled silicon steel sheet surface on which a layer containing SiO has been formed is further coated with a substance having as main components one or more Mg compounds such as MgO and Mg(OH) and one or more such Ti compounds, such as TiO TiO .H O, TiO.(OH) and Ti(Ol-l) It is most economical to use this substance as a glassy filmforming agent and also as a material which prevents sticking of the steel sheets during the final annealing.

The above-mentioned substance is formed by mixing 0.5 to 40 parts by weight of a Ti compound with 100 parts by weight of an Mg compound.

When less than 0.5 part of TiO is used, there will be no effect of the Ti compound and the object of the present invention cannot be attained. On the other hand, when more than 40 parts of the Ti compound are used, no high quality glassy film will be obtained.

Therefore, 0.5 to 40 parts of the Ti compound are used to make the film agent.

Further, when the amount of the Ti compound is larger than 40 parts, the substance after being dried will be likely to peel off the sheet during handling, the formation of the insulating film will be difficult, a surface having a metallic luster will be exposed and no uniform glassy film will be produced, and even if a phosphatetreatment is thereafter used, no highly adherent film will be obtained.

Further, the Ti compound in the substance has an excellent effect in that it reduces the brittleness of the steel sheet. When 0.5 part of the Ti compound is mixed with 100 parts of the Mg compound, the effect of reducing the brittleness of the steel sheet will appear. When the amount is above the upper limit, there will be no substantial brittleness reducing effect. It is only when 5 to 20 parts of the Ti compound are used that the brittleness will be most reduced.

MgO, which is-one of the components of the substance to be used in the present invention, may be either a high-temperature baked product or a lowtemperature baked product. Howver, the lowtemperature baked MgO is more effective for attaining the object of the present invnetion. Its granularity can be as fine as about 325 meshes to obtain a favorable result.

TiO can be either the rutile type or the anatase type or can be obtained by either low-temperaturedehydrating or high-temperature-baking of metatitanic acid. lts granularity can be fine as about 325 meshes, the same as the above-mentioned MgO to obtain a favorable result.

The higher the purity of such film substance, the better the result. However, even with a reagent of about the first grade or a commercial industrial product, the object of the present invention can be attained.

The substance of the above-mentioned composition is preferably mixed with water so as to form a slurry and the slurry is applied to the surface of an oriented cold-rolled silicon steel on which a layer of SiO has been formed by selective oxidization. The amount applied is preferably such that the amount of the substance which remains after being dried will be '2 to 10 g./m When the amount is 4 to 8 g./m the best result will be obtained.

After the above-mentioned coating, the sheet is dried and is then heated at a high temperature in a reducing atmosphere so that a glassy insulating film will be formed. It is most economical to carry out this glassy insulating film forming treatment at the time of the final annealing of the oriented cold-rolled silicon steel sheet.

That is to say, the oriented cold-rolled silicon steel sheet coated with the above-mentioned substance and then dried is wound up in the form of a coil or sheets cut to proper dimensions are stacked and finalannealed during which it is held at a high temperature above l,100C, such as, for example, 1,200C for more than 5 hours in an annealing furnace in a reducing atmosphere such as, for example, of pure hydrogen.

In such case, SiO present on the surface of the oriented cold-rolled silicon steel will react with the substance to form a blackish lustrous, compact and rustproof, acidproof high quality glassy insulating film having a composition in the SiO -MgO-TiO system.

In the above-mentioned final annealing, the lower the dew point of the atmosphere as the temperature rises, the easier the formation of a high quality film. The substance applied to the surface of the oriented cold-rolled silicon steel sheet is also an annealing separator in the final annealing. That is to say, only a part of the applied substance reacts with Si0 formed on the surface of the steel sheet so as to form a glassy film and substantially the greater part of it remains as an annealing separator on the glassy film formed on the steel sheet surface. Therefore, in order to make the steel sheet a product, it is necessary to remove the powder by such a means as brushing or pickling.

By the above-mentioned treatment of the present invention, a high quality glassy insulating film is formed on the surface of an oriented cold-rolled silicon steel sheet and, at the same time, as another feature of the present invention, the brittleness of the oriented coldrolled silicon steel sheet itself is reduced. This is presumed to be because the mixture of MgO-TiO produced during the treatment reacts with SiO on the surface of the oriented cold-rolled silicon steel sheet to form a compact high quality glassy film, and therefore the oriented cold-rolled silicon steel sheet is not influenced by the annealing atmosphere and the Ti compound acts to directly purify the impurities in the oriented cold-rolled silicon steel sheet.

Further, in the case of selectively oxidizing an Al containing oriented cold-rolled silicon steel sheet so that SiO will be formed on the surface, even when Al is also oxidized to produce A1 0 the above-mentioned substance will be effective. However, the presence of this Al O will make it difficult to obtain a uniform glassy film and, with the slightest fluctuation of the treating conditions,'theformation of the glassy film will be influenced so as to be unstable and a film which is poorly adherent will be likely to be produced:

Therefore, by carrying various experiments to solve the above-mentioned problems, the inventors have discovered that, by using the same treatment as is described above by adding to the substance which is the mixture of a Mg compound and a Ti compount a Mn compound in the form of an oxide or a hydroxide, such as MnO Mn O- Mn (OH) or Mn0.0H or MnCO which becomes MnO when heated, a uniform lustrous glassy film which is strongly adherent can be stably formed on the surface of an oriented cold-rolled silicon steel sheet.

In the substance consisting of the Mg compound, Ti compound and Mn compound, the Mn compound is considered to act to suppress the influence of the A1 0 formed on the steel sheet surface. That is to say, when heated, the Mn compound will become MnO, will be further reduced by the atmosphere and will enter the steel. The reaction in such case is considered to effectively act on the formation of a film. Further, the oxygen produced by the decomposition of the Mn compound in the reducing atmosphere below 1,000C is also considered to effectively act. MnO is excellent in that it is a compound which discharges oxygen and is readily available. Its granularity should be about 325 meshes to obtain favorable results.

In an experiment, when an oriented cold-rolled silicon steel sheet produced from a hot-rolled silicon steel sheet containing 0.030 percent sol. Al was coated with a substance within the scope of the present invention, was then heat-treated and was taken out and the substance was analyzed, the content of Al O was 1.1 to 21.1 percent.

The substance effective for application to the abovementioned Al-containing oriented cold-rolled silicon steel sheet is formed by mixing 0.5 to parts, preferably 2 to 40 parts, of a Ti compound and 0.5 to 50 parts, preferably 2 to 25 parts, of a Mn compound with I00 parts of a Mg compound.

In the above-mentioned composition, when the Ti compound is below the above-mentioned range. no effect of the compound will be obtained and, on the other hand, when it exceeds 80 parts, the substance after being dried will be likely to be peeled off by the slightest contact or shock and the formation of the insulating film will become difficult.

Further, when the Mn compound is below the abovementioned range, it will not be effective. On the other hand, when it is used in excess of 50 parts, the amount of oxygen discharged during the heat-treatment will be so large that the atmosphere will become oxidative and will have a bad influence on the formation of the glassy insulating film;

The substance of the above .composition is not only effective for application particularly to an Alcontaining oriented cold-rolled silicon steel sheet, but also can be applied to such oriented silicon steel sheet containing no Al as is described above.

The substance of the present invention has been described in detail above. Furthere, a comparison of the differences in characteristics obtained, when known substances using MgO and the substances of the present invention are used, but the other conditions are exactly the same, will be described with reference to examples in the following.

EXAMPLE 1 An oriented cold-rolled steel sheet having 3.25% Si and reduced to a final thickness of 0.35 mm was annealed for a short time of 5 minutes at a temperature of 800C in a wet dissociated ammonia atmosphere having a dew point of 6lC so that SiO was selectively formed on the steel sheet surface, was coated with a substance at each of the mixing rates in Table 1 so that the amount coated on the sheet was 6 gJm". and was final-annealed for 2.0 hours at a temperature of l,200C in a dry hydrogen atmosphere. The characteristics of the thus formed glassy insulating films are shown in Table 2. The films made by the method of the present invention are more compact and much more strongly adherent than by the known processes.

Further, in Table 3 there are shown the results of bending tests by the ASTM method to show the effects of reducing the brittleness of the steel sheets after the formation of glassy films. It is clear that the brittleness is greatly reduced in the films made by the method of the present invention.

TABLE I COMPOSITIONS OF USED FlLM AGENTS present invention TABLE I-Continued COMPOSITIONS OF USED FILM AGENTS TABLE 4 COMPOSITIONS OF SUBSTANCES Composi- Composition tion Sample MgO Ti0 Water Sample MgO TiO Mn0 Water No. No.

3 100 g. 25 I lit. do. 1 100 I 4 100 40 1 m dfi. 8 2? g do. $31133:

Control Substance (A) 5 100 g. 100 I In. ($102 was not 3 do. 20 0 do. of the present formed) invention 4 do. 20 6 do. Substance (B) of the present 5 d 20 3 d inventioni o. I 0. 0. TABLE 2 6 do. 27 7 do. do. 7 do. 30 14 do. do. 8 do. 36 7 do. do. GLASSY FILM CHARACTERISTICS 9 do 40 15 do. do Proper- Layer Resistance Adhesion After Phosphate- 10 6 $9 3 no! ties Under 35 1 g./cm Coating as Bent by 180 m Sample by ASTM method 2 to a Diameter of 20 mm.

l 4.30!) cm.lsheet partly peeled off TABLE 5 2 1 1.5 do. not peeled off 3 19.4 l' l 4 928 3g g g g g gf g g GLASSY FILM CHARACTERISTICS 5 0.20 do. 1 almost peeled off charac- 4 teristics Layer resistance Adhesion after phosphate- $13111!212?KIRZZ2Q5 1$283831.itfifllfis'lfi fiZ1 13? I'IJFYKJZSPZ Sample (under 35 -l coming as hem y coating on the film was 3 g./m"'. N0. by ASTM method 2) IO 8 dlameter 0f 20 mm.

1 0.49 .Qcm ./sheet peeled off 2 0.91 do. partl peeled off TABLE 3 3 6.15 do. a litt e peeled off 4 10.5 do. do. BRI'ITLENESS OF STEEL SHEETS ON 5 0 Pee'ed WHICH GLASSY FILMS WERE FORMED 6 4 peeled 7 24.3 do. lIttle peeled off Charac- Bending frequency (by ASTM method) 5; g8 g8 teris- Less than l l to 2O 2l to 30 more than 10 0:13 m peeled fftIcs 10 tImes times times 31 tImes Sample l 20 2 6 I2 EXAMPLE 3 2 0 7 16 17 3 2 24 1| 2 40 An orIented cold-rolled sIlIcon steel havIng 0.3 perg 1, g 16 cent total Al and 2.85 percent Si was rolled to a sheet 0.15 mm thick, was continuously annealed for a short time of 4 minutes at a temperature of 850C in a wet EXAMPLE 2 decomposed atmosphere having a dew point of 65C so that SiO was selectively formed on the surface, was An oriented cold-rolled steel sheet having 0.32 per-- coated on the surface with a substance at each of the cent total A1 and 2.9 percent Si and reduced to a final below-mentioned mixing rates so that the amount of thickness of 0.30 mm. was continuously annealed for a the substance after drying would be 6 g./m was dried short time of 4 minutes at a' temperature of 850C in a and was then final-annealed for 20 hours at a temperawet dissociated ammonia atmosphere having a dew ture of 1,200C in a dry hydrogen atmosphere so that point of C so that SiO was selectively formed on the a glassy film was'formed. As a result, excellent electrisurface, was then coated on the surface with a subcal insulating films as described below were obtained.

Layer Resistance Surface Appearance Composition After Removing After Removing Powder Powder MgO TiO MnO In em .lsheet (under 35 kg./cm 10071 0.41 No glassy film was produced. 20g. 1g. 0.5g 3.95 A uniform lustrous glassy film was produced.

stance at each of the mixing rates and with the compositions shown in Table 4 so that the amount of the substance remaining after being dried was 5.5 g./m was dried and was then final-annealed for 15 hours at a temperature of 1,200C in a dry hydrogen atmosphere so that a glassy film was formed.

What is claimed is: l. A method for producing an electrical insulation film on an oriented cold-rolled silicon steel sheet containing 0.01 to 0.09 wt percent sol. A1, consisting essentially of the steps of first subjecting the silicon steel sheet to a heat-treatment at a temperature of 700 to 900C in a reducing atmosphere of either hydrogen or an atmosphere containing dissociated ammonia having a dew point of 55 to 70C for at least one minute; applying to the surface of the steel sheet an aqueous slurry of a mixture of l) 2 to 40 parts by weight ofat least one compound selected from the group consisting of an oxide of Ti and a hydroxide of Ti, and (2) 100 parts of by weight of a compound selected from the groups consisting of an oxide of Mg and a hydroxide of Mg; drying the steel sheet coated with the slurry, and annealing the said steel sheet in a hydrogen-containing atmosphere at a temperature above 1,100C.

2. The method claimed in claim 1 wherein the oriented cold-rolled silicon steel sheet contains 2 to 3.5 wt. Si.

3. The method claimed in claim 1 wherein the heattreatment for forming SiO on the surface of the steel sheet comprises holding said steel sheet at a temperature of 700 to 900C for at least one minute in a reducing atmosphere of hydrogen only.

4. The method claimed in claim 1 wherein the heattreatment for forming SiO on the surface of the steel sheet comprises holding said steel sheet at a temperature of 700 to 900C for at least one minute in a reducing atmosphere containing dissociated ammonia and having a dew point of 55 to C.

5. The method claimed in claim 1 wherein the aqueous slurry is prepared by mixing the mixture with water and is applied on the surface of the oriented silicon steel sheet on which SiO has been formed in an amount such that the amount of the deposited material left after it has been dried is 2 to 10 g/m*.

6. The method claimed in claim 1 wherein only one compound from each group is mixed in said slurry.

Claims

1. A METHOD FOR PRODUCING AN ELECTRICAL INSULATION FILM ON AN ORIENTED COLD-ROLLED SILICON STEEL SHEET CONTAINING 0.01 TO 0.09 WT PERCENT SOL. AL, CONSISTING ESSENTIALLY OF THE STEPS OF FIRST SUBJECTING THE SILICON STEEL SHEET TO A HEAT-TREATMENT AT A TEMPERATURE OF 700* TO 900*C IN A REDUCING ATMOSPHERE OF EITHER HYDROGEN OR AN ATMOSPHERE CONTAINING DISSOCIATED AMMONIA HAVING A DEW POINT OF 55* TO 70*C FOR AT LEAST ONE MINUTE; APPLYING TO THE SURFACE OF THE STEEL SHET AN AQUEOUS SLURRY OF A MIXTURE OF (1) 2 TO 40 PARTS BY WEIGHT OF AT LEAST ONE COMPOUND SELECTED FROM THE GROUP CONSISTING OF AN OXIDE OF TI AND A HYDROXIDE OF TI, AND (2) 100 PARTS OF BY WEIGHT OF A COMPOUND SELECTED FROM THE GROUPS CONSISTING OF AN OXIDE OF MG AND A HYDROXIDE OF MG; DRYING THE STEEL SHEET COATED WITH THE SLURRY, AND ANNEALING THE SAID STEEL SHEET IN A HYDROGENCONTAINING ATMOSPHERE AT A TEMPERATURE ABOVE, 1,100*C.

2. The method claimed in claim 1 wherein the oriented cold-rolled silicon steel sheet contains 2 to 3.5 wt. % Si.

3. The method claimed in claim 1 wherein the heat-treatment for forming SiO2 on the surface of the steel sheet comprises holding said steel sheet at a temperature of 700* to 900*C for at least one minute in a reducing atmosphere of hydrogen only.

4. The method claimed in claim 1 wherein the heat-treatment for forming SiO2 on the surface of the steel sheet comprises holding said steel sheet at a temperature of 700* to 900*C for at least one minute in a reducing atmosphere containing dissociated ammonia and having a dew point of 55* to 70*C.

5. The method claimed in claim 1 wherein the aqueous slurry is prepared by mixing the mixture with water and is applied on the surface of the oriented silicon steel sheet on which SiO2 has been formed in an amount such that the amount of the deposited material left after it has been dried is 2 to 10 g/m2.