US3649372A

US3649372A - Reagent for forming an insulating coating on the surface of electrical steel sheets

Info

Publication number: US3649372A
Application number: US816677A
Authority: US
Inventors: Mitsuo Imai; Hiroshi Shimanaka; Toshio Irie
Original assignee: Kawasaki Steel Corp
Current assignee: JFE Steel Corp
Priority date: 1968-10-28
Filing date: 1969-04-16
Publication date: 1972-03-14
Anticipated expiration: 1989-03-14
Also published as: DE1954216B2; BE740885A; JPS4812300B1; FR2021764A1; DE1954216A1; GB1218678A; SE348009B

Abstract

REAGENT FOR FORMING AN INSULATING COATING ON THE SURFACE OF ELECTRICAL STEEL SHEETS IS DISCLOSED WHEREIN ALUMINUM NITRATE AND/OR AMORPHOUS ALUMINUM HYDROXIDE ARE/ IS INCORPORATED INTO AN AQUEOUS SOLUTION OF PHOSPHATE COMPOUNDS IN AN AMOUNT OF 0.4-2.1 PARTS BY WEIGHT BASED

ON ALUMINUM. THE AQUEOUS SOLUTION OF PHOSPHATE COMPOUNDS INCLUDES 100 PARTS BY WEIGHT OF MONOBASIC MAGNESIUM PHOSPHATE AND 5 TO 30 PARTS BY WEIGHT OF CHROMIC ANHYDRIDE.

Description

March 14, 1972 rrsuo M EIAL 3,649,372

[(EAGIuNT FOR FORMING AN INSULATING COATING ON THE SURFACE OF ELECTRICAL STEEL SHEETS Filed Aprll 16, 1969 4 Sheets-Sheet 1 I C -x :AJZ(NO3)3 9H2O P (On gloss film) Imermminafion resis1oncen cm lominoflon .p

I 20 I I |o l I l I l A2(NO3)39HzO 0 IO 20 so 40 l l l I l March 14, 1972 MITSUO IMAI EFAL 3,649,372

REAGLLINT FOR FORMING AN INSULATING COATING ON THE SURFACE OF ELECTRICAL STEEL SHEETS filLecl April. lb, 1969 4 Sheets-Sheet 2 (Good) Z 6 B- Q. 8 a. e CrOa 6gr g f I CrOa 3gr g g CrOs L5gr 2 h 1 CrOa Ogr Amount of A(NO3)3 9H2O 2 4 6 9 added To reagent gr A 2 g/Mg (H2PO4)2 lOOgr March 1972 MITSUO IMAI E L 3,649,372

REAGENT FOR FORMING AN INSULATING COATING ON THE SURFACE OF ELECTRICAL STEEL SHEETS Fned April 16, 1969 4 Sheets-Sheet 5 A! (0H)3 A JI(NO3)39H2O A 0 No flowing m M M m u u m M m u u M U m M M M M M w w w w m.

llll ANNOa); 9H2O 0 March 14, 1972 MITSUO IMAI ETAL 3,649,372

RLAUQNF FOR FORMING AN INSULATING COATING ON THE SURFACE OF ELECTRICAL STEEL SHEETS u'lleu Aprn lb, 1969 4 Sheets-Sheet 4 FIIIIIIIIIIIIIIIIIIIIIIIA United States Patent U.S. Cl. 1486.16 9 Claims ABSTRACT OF THE DISCLOSURE Reagent for forming an insulating coating on the surface of electrical steel sheets is disclosed wherein aluminum nitrate and/ or amorphous aluminum hydroxide are/ is incorporated into an aqueous solution of phosphate compounds in an amount of 0.4-2.1 parts by weight based on aluminum. The aqueous solution of phosphate compounds includes 100 parts by weight of monobasic magnesium phosphate and to 30 parts by weight of chromic anhydride.

This invention relates to reagent for forming an insulating coating on the surface of electrical steel sheets and particularly to improvements on a reagent for use in a method of coating and baking phosphate solution on the surface of electrical steel sheets and providing an insulating coating having various excellent properties.

The insulating coating formed on the surface of electrical steel sheets is required to have a number of different properties in dependence with the magnetic property of the electrical steel sheets and the uses thereof.

For example, a grain oriented silicon steel sheet for use in wound type transformers is required to have a large space factor, high interlamination resistance and high heat resistance. Particularly, in the wound core type transformer in which the iron core is impregnated with resin, the steel sheet is required to have an insulating coating having an excellent adhesive strength for an adhesive agent, such as epoxy resin. Moreover, the grain oriented silicon steel sheet for use in transformers is required to have corrosion resistance against the transformer oil and non-inflammable synthetic insulating oil.

A non-oriented silicon steel sheet for use in domestic electrical machines and apparatuses is required to have excellent punchability, weldability and corrosion resistance etc., more than the above mentioned properties. The non-oriented silicon steel sheet for use in a motor core for refrigerators must resist against a refrigerant such as Freon gas.

Heretofore, a number of methods of forming an insulating coating on the surface of electrical steel sheets with the aid of phosphate or chromate have been proposed.

In the method of forming an insulating coating on the surface of electrical steel sheets with the aid of chromate it is necessary to use an organic compound as a reducing agent, with the result that the organic compound becomes occluded in the insulating coating, that when the insulating coating is formed on the surface of a low carbon electrical steel sheet, carburization occurs when effecting a stress relief annealing step thus deteriorating its magnetic property, that there is risk of the insulating coating being deteriorated by the refrigerant such as Feron gas, and that blow holes are liable to occur in weld beads formed by a TIG-arc welding.

3,649,372 Patented Mar. 14, 1972 The method of forming an insulating coating on the surface of electrical steel sheets with the aid of phosphate is not capable of easily providing an insulating coating having all of desired properties and beautiful surface appearance which is worth marketing.

This method makes use of a baking step under extremely strict conditions in order to produce a thin and smooth insulating coating having high corrosion resistance and insulating property, so that it is not suitable for producing industrially stable insulating coating on a mass production basis.

Moreover, the conventional phosphate coating has the disadvantage that deterioration to the adhesion of the coating occurs when it is subjected to a stress relief annealing step and hence the coating becomes flaked off from the steel sheet, and the interlamination resistance becomes remarkably decreased, and cannot be applied to grain oriented silicon steel whose insulating property must be high.

An object of the invention is to obviate the above mentioned disadvantages and provide a novel reagent for forming a coating having excellent properties such as insulating property, adhesive property, corrosion resistance, space factor, heat resistance, machinability, weldability, etc., in an industrially easy manner, which can be applied effectively to the formation of an insulating film having a high break down voltage on a glass film formed on a grain oriented silicon steel sheet.

A feature of the invention is the provision of such an improved reagent for forming an insulating coating on the surface of electrical steel sheets which comprises an aqueous solution consisting of parts by weight of monobasic magnesium phosphate and 5 to 30 parts by weight of chromic anhydride, and 0.4 to 2.1 parts by Weight based on aluminum of aluminum nitrate and/or amorphous aluminum hydroxide.

The surface condition of the electrical steel sheets on which the reagent according to the invention is coated must be cleaned preliminary to the coating treatment, but it is not necessary to apply special steps such as to remove a thin oxide film formed by an annealing step on the surface of the electrical steel sheets.

Several reagents consisting of magnesium phosphate and chromic anhydride or consisting of phosphoric acid, magnesium oxide and chromic anhydride have heretofore been proposed. But, such reagents could not provide an insulating coating having sufficient interlamination resistance and adhesive property.

The addition of specially defined aluminum compounds to a mixture of magnesium phosphate and chromic anhydride with a given mixing ratio in accordance with the invention results in a reagent that can obtain an insulating coating having remarkably improved interlamination resistance and adhesive property.

In accordance with the invention the aluminum compounds are limited to aluminum nitrate and/or amorphous aluminum hydroxide. Experimentation has shown that aluminum compounds other than aluminum nitrate and/or amorphous aluminum hydroxide or crystalline aluminum hydroxide can not attain the object of the invention.

Three kinds of aluminum hydroxide that is, monoclinic aluminum hydroxide ccAl(0H) hexagonal aluminum hydroxide fiAl(OH) and amorphous aluminum hydroxide gel are well known. The amorphous aluminum hydroxide is aluminum hydroxide gel available in the market as medical antiacid.

Aluminum nitrate can be aluminum nitrate nonahydrate Al(NO -9H O. It is a matter of course that an aqueous solution of Al(NO may also be used as aluminum nitrate.

It is preferable that to 100 parts by weight of monobasic magnesium phosphate is added to 30' parts by weight of aluminum nitrate nonahydrate Al(NO -9H O or 1 to 6 parts by weight of aluminum hydroxide.

Other objects, features and advantages of the inven tion will become apparent from a consideration from the following specification, when the specification is considered in conjunction with the accompanying drawing, in which:

FIG. 1 shows curves which explain the relationships between the amount of aluminum compounds added and interlamination resistances;

FIG. 2 shows curves which explain the relationships between the amount of aluminum compounds added and adhesive properties;

FIG. 3 shOWS a graph which explains the relationships between the amount of aluminum compounds added and baking times at 450 C.; and

FIG. 4 is a diagrammatic illustration of a circuit arrangement for measuring break down voltage of an insulating coating formed on the surface of an electrical steel sheet by a reagent according to the invention.

Referring now to FIG. 1, chain line curve a, full line curve 11 and dotted line curve 0 show the relation between the amount of aluminum salts added and interlamination resistances of insulating coatings formed on the surface of electrical steel sheets. The interlamination resistances of the insulating coatings each having a thickness of 0.8 to 1.2 1. were plotted in function of the amount of aluminum in the aluminum salts. In order to obtain the insulating coatings on the surface of an electrical steel sheet containing 1.2% by weight of silicon and having a thickness of 0.5 mm., this steel sheet is uniformly coated with an aqueous solution containing 100 cc. of water, g. of Mg(H PO and 3 g. of CrO and added with various amounts of aluminum salts. The insulating coatings thus obtained are baked in a furnace at a temperature of 450 C. for one minute.

As seen from FIG. 1, more than 0.4 part by weight based on aluminum of aluminum compound added is remarkably effective, but more than 1.5 parts by Weight is substantially non-effective.

In FIG. 1, the curves a and b show changes of the interlamination resistances in dependence with additions of aluminum hydroxide A1(OH) and aluminum nitrate nonahydrate Al(NO -9H O, respectively, the more the amount of aluminum is present, the higher the interlamination resistance becomes. In FIG. 1, the curve 0 shows the change of the interlamination resistance of a grain oriented silicon steel sheet covered with a heat resistance glass film on which is formed an insulating coating with the aid of a reagent containing aluminum nitrate nonahydrate Al(NO -9-H O. As seen from the curve 0, in this case the interlamination resistance be comes considerably increased. But, the correlation between the interlamination resistance and the amount of aluminum salt added is substantially the same as that shown by the curves a and b where the reagent is directly coated on the abovementioned steel sheet.

The solubility of the amorphous aluminum hydroxide in an aqueous solution of the monobasic magnesium phosphate is comparatively small. For example, more than 6 g. of amorphous aluminum hydroxide cannot be dissolved into 100 cc. of aqueous solution of the monobasic magnesium phosphate containing 12 g. of chromic anhydride. It is not preferable to add the amorphous aluminum hydroxide in an amount not soluble in the aqueous solution of the monobasic magnesium phosphate since such addition results in formation of a rough insulating coating without luster.

Moreover, addition of more than 30 parts by weight of aluminum nitrate is not preferable, because such addition causes the surface of the steel sheet and the treating liquid coated thereon to react each other to produce spot patterns which are easily rustable, thereby degrading the appearance and the value of the steel sheets available in market.

Under the above reasons, the amount of baked on aluminum of aluminum compounds added in accordance with the invention is limited to 0.4 to 2.1 parts by weight.

FIG. 2 shows the relation between the amount of aluminum compounds added and adhesive properties of the insulating coating. FIG. 2 was plotted from the re sults obtained by measuring the adhesive properties of the insulating coatings formed by reagents comprising cc. of water, 20 g. of Mg(H PO.;) 0 to 6 g. of CrO and O to 8 g. of aluminum nitrate.

As seen from FIG. 2, the addition of Al(NO -9H O causes an improvement of the adhesive property of the insulating coating and the addition of CrO is effective to further improve the adhesive property of the insulating coating.

Relationships substantially the same as those shown in FIG. 2 were obtained by using Al(OH) or a mixture of and instead Of Sing A].(NO3)3.

The addition of C1-C serves not only to improve the adhesive property of the insulating coating, but also to improve the corrosion resistance of the insulating coating. For the latter purpose, it is necessary to add more than 5 parts by weight of Cr0 with respect to 100 parts by Weight Of Mg(H PO Moreover, CrO is effective to make the insulating coating smooth and vitreous. When using Al(OH) it is possible to make its solubility higher by decreasing the pH of the reagent, while when using Al(NO CrO is capable of prohibiting the above mentioned reaction between the steel sheet surface and the reagent coated thereon and hence preventing occurrence of the spot patterns.

The addition of more than 30 parts by weight of CrO makes the reduction of CrO by means of the baking step difficult, and hence soluble Cr+ remains in the insulating coating to make it hygroscopic in air, thus liquefying and falling off the insulating coating from the steel sheet.

In the above mentioned composition ratio of the reagent, it is most preferable to use a range of 0.7 to 2.1 parts by weight based on aluminum of aluminum salts and 10 to 20 parts by weight of CrO The use of 6 to 25% concentration of the solution based on Mg(H PO permits of obtaining good workability and forming an excellent insulating coating in an industrially easy manner.

If necessary, proper amount of pulverized heat resistant substance such as colloidal silica, mica or surface active agent may be added to the reagent according to the invention.

The insulating coating formed by the reagent according to the invention and having a thickness of 0.3 to 2.5 may have excellent properties such as adhesive property, interlamination resistance etc. The insulating coating having a thickness less than 0.3 is low in interlamination resistance and inferior in corrosion resistance, while the insulating coating having a thickness more than 3.0;/. becomes opaque viridescent in color and flaky in shape, so that the insulating coating is liable to be flaked away from the surface of electrical steel sheets. But, in accordance with the invention it is possible to apply a reagent according to the invention again to the insulating coating which has been formed on the steel sheets and baked to obtain an insulating coating having a thickness which could not be obtained by one treatment, thus providing an insulating coating having an extremely high interlamination resistance and adhesive property. The composition, concentration and baking temperature of the reagent reapplied to the insulating coating may be the same as those of the reagent applied at the first time.

One of the advantages of the invention is that the reagent according to the invention after having been coated may be baked at a comparatively low temperature for a short time. If the baking temperature is lower than 300 C., the insulating coating thus baked becomes hygroscopic. It has been found out that if the insulating coating is baked in a furnace at a temperature of 300 to 600 C. for 30 to 120 seconds the insulating coating thus baked becomes non-hygroscopic and is particularly adapted for use in various electrical fields. FIG. 3 shows relationships between baking times at 450 C., for example, and amounts of aluminum compounds added. FIG. 3 was plotted by observing the surface condition of a steel sheet having a thickness of 0.5 mm. and coated with a reagent comprising 100 cc. of water, 20 g. of Mg(H PO 3 g. of CrO and various amounts of aluminum compounds and baked in a furnace at a temperature of 450 C. The surface condition of the steel sheet was observed after hours by the salt spray test. If the baking -step is not sufficient, the insulating coating becomes hygroscopic and flows off the steel sheet, while if the baking step is excessive, the adhesive property and corrosion resistance of the insulating coating become deteriorated.

In view of the above mentioned results, the use of Al(NO as the aluminum compounds can decrease the baking time by the order of to seconds.

The grain oriented silicon steel sheet for use in a large type transformer has recently been coated with an insulating coating adopted by steel sheet makers without being subjected to the varnish baking step. Such silicon steel sheet is required not only to have a high interlamination resistance, but also to have a high break down voltage. The insulating coating formed by the reagent according to the invention has a remarkably improved break down voltage as compared to other phosphate coatings and hence can be applied to the grain oriented silicon steel sheet for use in the large type transformers.

FIG. 4 shows a circuit arrangement for measuring the break down voltage. A grain oriented silicon steel sheet coated on the surface thereof with glass film is further subjected to be coated on the glass film with the reagent according to the invention and baked thus providing an insulating coating having a thickness of 2a. The thus treated sheet is annealed at a temperature of 800 C. in an N; atmosphere. The break down voltages of the insulating coating thus obtained and measured by the circuit arrangement shown in FIG. 4 are shown in the following table.

Amount of aluminum compound Break down added (based on Al):

In FIG. 4, 1 designates a 220 v. alternating current supply source, 2 a voltage regulating transformer, 3 an ammeter, 4 a resistor, 5 a circuit breaker, 6 a voltmeter, 7 an electrical steel sheet to be tested, 8 an insulating coating formed by a reagent according to the invention, 9 a measuring electrode having a diameter of 20 mm. and made of brass and 10 a measuring drill rod.

As seen from the above table, the addition of the aluminum compound serves to remarkably improve the break down voltage of the insulating coating.

The insulating coating formed on the surface of electrical steel sheets by the reagent according to the invention has various properties other than those described above, which read as follows.

The insulating coating provides a colorless glass-like film which is lustrous and beautiful in appearance and so hard that it is difiicult for it to be scratched.

The insulating coating is thin and smooth so that the space factor of the steel sheets having the insulating coating formed thereon is substantially the same as that of the steel sheets formed without the insulating coating.

The insulating coating has an excellent corrosion resistance and prevents formation of rust and has an excellent antichemical property and is stable against Freon gas and not damaged by transformer oil and punching oil.

The insulating coating is not flaked oil from the steel sheet when subjected to the stress relief annealing step and thus maintains a relatively high interlamination resistance even after the annealing step. The insulating coating does not contain any organic compound and hence has a good weldability and does not cause blow holes after being subjected to a TIG-arc welding.

An electrical steel sheet containing 1.2% by weight of silicon and having an insulating coating formed thereon by the reagent according to the invention could be punched through by a punching die made of steel and having a clearance of 40,14 to obtain discs over one hundred and ninety thousand punchings until burr formed along the periphery of the disc obtained reaches to a height of 100 Such number of punching is about two times larger than that'of the electrical steel sheet coated with conventional phosphate insulating coating and then baked.

The reason why the addition of aluminum nitrate and/ or amorphous aluminum hydroxide to an aqueous solution consisting of monobasic magnesium phosphate and chromic anhydride is capable of considerably improving the film properties such as interlamination resistance, adhesive property, etc., of the insulating coating is not clear at present. But, the presence of N0 radical is no longer detected in an insulating coating formed by a reagent containing aluminum nitrate, and aluminum nitrate and amorphous aluminum hydroxide are decomposed at 250 C. and 300 C., respectively, and hence it is believed that aluminum nitrate and amorphous aluminum hydroxide would be converted into a sort of oxides or hydroxy oxide. 4

But, the addition of alumina (A1 0 instead of these aluminum compounds results in a rough coating whose interlamination resistance and adhesive property etc., are not improved and whose punchability becomes considerably deteriorated.

If aluminum sulfate is used as a soluble aluminum compound, aluminum sulfate is not decomposed at a temperature less than 1,000" C. so that 80.; radical remains in the coating thus deteriorating its corrosion resistance and not improving its insulating property and adhesive property.

AlCl is also soluble in the magnesium phosphate, but the reagent containing AlCl when in contact with the steel sheets, immediately begins to react with them and spot-like patterns of tribasic phosphate are produced throughout the surface of the coating. Thus, after the steel sheets have been subjected to the baking step the coating is bad in appearance, corrosion resistance and adhesive property and cannot be used as an insulating coating.

-If the monobasic magnesium phosphate contains a free phosphoric acid, a portion of added amorphous aluminum hydroxide reacts with the free phosphoric acid to produce aluminum phosphate with the result that the desired film properties cannot be obtained. Moreover, if the monobasic magnesium phosphate contains free phosphoric acid, the above mentioned spot-like patterns of tribasic phosphate are liable to be produced on the coating so that it is preferable to make the amount of the free phosphoric acid in the monobasic magnesium phosphate as small as possible.

The invention will now be described with reference to examples.

EXAMPLE 1 To 100 l. of phosphoric acid was added 200 l. of water and then added gradually 24.2 kg. of magnesium oxide to produce about 30% monobasic magnesium phosphate solution. The amount of free phosphoric acid in the solution was measured. If the free phosphoric acid content was high, a small amount of magnesium carbonate was added to said solution so as to adjust the free phosphoric acid content.

6 kg. of chromic anhydride 7.5 kg. of aluminum nitrate (Al(NO -9H O) in 100 1. of water were added to 100 l. of said solution and dissolved therein. The reagent thus obtained was uniformly coated on the surf-ace of an electrical steel sheet containing 1.2% by weight of silicon with the aid of grooved rubber rolls. The steel sheet thus coated with the reagent was placed in a furnace and baked at a temperature of 450 C. for one minute.

Various properties of the coating thus obtained were measured, whose results are as follows.

INTERLAMINATION RESISTANCE [ASTM A344, 35 kgJcmfl] Test. After annealing (N 1, Thickpiece Before annealing 750 0., 3 hours) ness No. 1.-- 56.7 trawl/lamination". 19.4 Q-cm. /laminatin: 1. 0;; N o. 2. 31.7 Mini/lamination." 7.9 flcmflllamination 0. 6

CORROSION RESISTANCE salt spray test at 35 C.

No. 1-No rust gathers during 20 hours. No. 2--No rust gathers during 12 hours.

Weld ability Electrical steel sheets are laminated one upon the other and the side end surface of the laminated sheets is coated with a punching oil available in market as a trade name of Nihon Kosakuyu No. 6300. Then, the laminated sheets were subjected to TIG-arc welding using a mug sten electrode having a diameter of 1.6 mm. under Ar 5 l./min., a current of 120 a. and travel speed of the electrode of 500 mm./min. The beads obtained of the test pieces Nos. 1 and 2 were beautiful ones without blow holes and serpentine tumings.

Resistance against Freon Electrical steel sheets were immersed into Freon 22 enclosed in a hermetically sealed vessel made of stainless steel at a temperature of 40 C. for 1700 hours. Thus, the resistance against Freon of the steel sheets was measured. The appearance of the steel sheets was not changed owing to occurrence of rust etc., and the properties of the steel sheets were not also changed.

EXAMPLE 2 INTERLAMINATION RESISTANCE [ASTM A344, 35 kg.lcm.

Test. After annealing (N Thickpiece Before annealing 750 0., 3 hours) 11653 N o. 3..." 42.6 wand/lamination. 9.2 SZ-cm.-/lamination 0. 8s

CORROSION RESISTANCE 5% salt spray test at 35 C.

No. 3No rust occurs during 25 hours.

Weldability and anti-Freon resistance were the same as those obtained in the Example 1.

EXAMPLE 3 IN TERLAMINATION RESISTANCE [ASTM A344, 35 kgJcmfl] Test After annealing piece Before annealing (N2, 800 0., 5 hours) BREAK DOWN VOLTAGE After annealing163 v.

The description and examples given above are intended to illustrate the best mode of performing the invention. It is apparent that many modification thereof may occur to those skilled in the art, which will fall within the scope of the following claims.

What is claimed is:

1. A reagent for forming an insulating coating on the surface of electrical steel sheets comprising an aqueous solution consisting of parts by weight of monobasic magnesium phosphate and 5 to '30 parts by weight of chromic anhydride, and 0.4 to 2.1 parts by weight based on aluminum of aluminum nitrate and/or amorphous aluminum hydroxide.

2. A reagent as claimed in claim 1, wherein said aqueous solution consists of 100 parts by weight of monobasic magnesium phosphate having a concentration of 6 to 25% by weight and 10 to 20 parts by weight of CrO and added with 0.7 to 2.0 parts by weight based on aluminum of aluminum nitrate and/0r amorphous aluminum hydroxide.

3. A reagent as claimed in claim 1 and further comprising a pulverized heat resistance substance such as colloidal silica, mica or surface active agent.

4. A method of forming an insulating coating on the surface of electrical steel sheets comprising the steps of coating the surface of an electrical steel sheet with an aqueous solution consisting of 100 parts by weight of monobasic magnesium phosphate and 5 to 30 parts by weight of chromic anhydride, and 0.4 to 2.1 parts by weight based on aluminum of aluminum nitrate and/or amorphous aluminum hydroxide to form an insulating coating on the surface of the electrical steel sheets, and baking the insulating coating thus obtained in a furnace at a temperature of 300 to 600 C. for 30 to seconds.

5. A method as claimed in claim 4, wherein said aqueous solution is applied to the said insulating coating formed on the steel sheets and then baked.

6. A method as claimed in claim 4, wherein said insulating coating is formed on a heat resistant glass film covered on the surface of grain oriented silicon steel sheets.

7. An insulating coating formed on the surface of electrical steel sheets by the reagent as claimed in claim 1 and having a thickness of 0.3; to 2.5g.

8. An insulating coating formed by the method as claimed in claim 5 on the insulating coating which has been formed on the surface of electrical steel sheets.

9 10 9. An insulating coating formed by the method as 3,151,997 10/1964 Harvey 148113 X claimed in claim 6 on the heat resistant glass film covered 3,207,636 9/1965 Wada et a1. 148-616 on the surface of grain oriented silicon steel sheets. 3,395,027 7/ 1968 Klotz 1486.16 X

References Cited 5 RALPH S. KENDALL, Primary Examiner UNITED STATES PATENTS C. WESTON, Assistant Examiner 2,753,282 7/1956 Perry 1486.16 US. Cl. X.R.

3,151,000 9/1964 Schmidt et a1 148113 X 1486.2, 113