CA1071039A - Method for producing coated electrical steel sheets having excellent punchability, weldability, electrical insulation and heat resistance - Google Patents

Abstract

Abstract of the Disclosure Coated electrical steel sheets having excellent punchability, weldability and insulation resistance are produced by applying an electrical steel sheet with a treating dispersion obtained by compounding the particle separating resin emullsion and the compatible acid in the particularly defined amount and baking the thus applied steel sheets. To the treating dispersion may be added saccharides or polyhydric alcohols as a reducing agent or boric acid or phosphoric acid.

Description

The present invention relates to a method fo~ producing coated electrical steel sheets having excellent punchability, weldability, electrical insulation and heat resistance.
The electrical steel sheets are applied with insulating coating and the coated sheets must be excellent in the heat resistance, corrosion resistance and resistance for refrigerant, such as Freon gas (Trademark) other than the electrical insulation. Furthermore, the coated sheets are expected to have improved punchability. In addition, the coated sheets must hot form blowholes in weld bead when a side of electrical steel sheet laminate is TIG
~Tungsten Inert Gas) welded. That is, a high weldability of the coated electrical steel sheets is required.
These properties are mainly provided by the insulating coating and the insulating coating for the electrical steel sheet is roughly classified into four classes of phosphate type, chromate type, organic resin type and chromate-organic resin mixture type. The characteristics of the steel sheets applied with these coatings are briefly explained as follows. ~c (1) Phosphate type:
This coating has been disclosed in U.S. Patent No. 2,501,846 and U.S. Patent No. 2,753,Z82 and classified as C-4 in AISI Standard. The weldability is excellent but the punchability cannot be improved. The insulation resistance depends upon the thickness of the coating but when the coating becomes thick, the coating is readily exfoliated by a heat treatment, such as a stress relief annealing.

~71~39 ~2) f;hromate t~8:
This coating has been di~closed in U.S. Patent No~ 3 7 5919425. The punchabili~y is good but the weldabili~y is not very satisfactory. The insulation resistance is fairly high but is considerably decreased by a stress relief annealing.
~3) Organic resin type:
This coating is classified as C-l or C-3 in AISI Standard. The punchability and electrical insulation are falrly satisfactory but this coating is composed of an organic substance, so that the heat resistance is poor an~
when welding, a large amount of gas is evolved and blow-holes are formed in the bead and when a stress relief annealing is effected, the coating is burnt or caTbonized.
(4) Chromate-organic resln mixture type:
This type coating has been mainly disclosed in U.S. Patent No. 3,666,568, which is composed of double layer coatings of a chromate and an organic resin~ In addi~ion, there has been a process wherein a mixture solution of a chromate and ~n organic resin is coated and then baked. The punchability is particularly excellent and 1,000,000 times punching can be conducted per one time grinding of a tool steel die. Some coatings are high in the insulation resistance but the weldability is generally not satisfactory. FurthermoTe, when the steel sheets applied with such a coating are annealed under an oxidizing atmosphere, such as DX lean gas, the coated sheets are readily mutually stuck.
Thus J the coatings heretofore used to not provide all of the properties of insulation resistance, ' 1C~7~039 punchability, weldability and heat resistance. The prbposal for improving these properties has been made but it has never been possible to satisfy all these ~equirements.
~or example, it has been proposed as method for improving the weldability of the organic resin type coating having an excellent punchability that before - preYiously applying-the insulating coating, a surface roughness of more than 20 ~ inch Hrms is provided on the steel sheet surface and then the insulating coating is applied to provide a moderate roughness on the surface of the formed electrical steel sheet, whereby the gas evolved from the weld bead is escaped. However, in this method, the thickness of the coating at the convex portion of the steel sheet is extremely thin and the electrical insulation at such portion lowers. Furthermore~
the punching of the steel sheet depends upon the thickness of the coating, so that the punchability of the steel sheet provided with the coating having such extremely thin portions is poor.
As a method for producing an electrical steel sheet in which these defects are obviated and which has an excellent weldability, it has been proposed in Japanese Paten~ Application Publication No. 19,078/74 that on smooth surface of a steel sheet is applied a coating having a moderate roughness by using a treating dispersion containing bakelite resin or melamine resin having a particle size of more than 2 ~ and the coated steel sheet is baked. But the particle size of the resin powder is generally coarse and it is difficult _ 4 _ - i - . --, . : .... . . . .
.-0-3~

to obtain fine powders of less than 10 ~. Thi~resin powder is hard and even if the resin powder is hea~ed, the deformation is difficult, the su~face roughness of the coating is extremely large and the space factor is considerably decreased. Furthermore, the resin powder is liable to be separated from the treating liquid due to the difference of the specific gra~ity between said resin powder and the treating liquid and it is difficult to effect the coating uniformly. In addition, according to this method, the resin particles, when annealing (stress relief annealing), are burnt or carbonlzed and the insulation resistance is extremely decreased.
The present invention provides a method for producing an electrical steel sheet having excellent electrlcal insulation, heat resistance~ punchability and weldability, by which the defects of such conventional process can be obviated and the object can be attained by applying a treating liquid in which fine particles are suspended and which is obtained by compounding to an aqueous solution of chromate with a resin emulsion which on compounding as above separates as fine agglomerated `~~
particles to a steel sheet surface, and baking the thus treated steel sheet to form a coating having a high surface roughness in which the resinous particles are uniformly dispersed.
The technical characteristic of the present in~ention becomes apparent from the claims and the following detailed explanation but is briefly explained as follows.
The present in~ention is particularly ~071039 defined as a method for producing coated electrical steel sheets having excellent punchability, weldability and insulation resistance, which comprises compounding an aqueous solution of chromate with a resin emulsion having a concentration of nonvolatile matter of 5-60%, which on compounding separates as fine agglomerated particles having an average particle diameter of 3-40 ~, and with a resin having a compatibility to the aqueous solu~ion of chromate, governed by the fact that both of the above resins are present in an amount such that the total amount of nonvolatile matter of both resins is 5-150 parts by weight based on 100 parts by weight of chromate expressed as chromic acid (CrO3), and further governed by the fact that the amount of the non-volatile matter of the former resin emulsion is at least 1 part by weight based on 100 parts of chromate expressed as chromic acid ~CrO3) and at least

2% by weight of the total amount of nonvolatile matter of both of the above resins, in order to effect the separation of the fine agglomerated particles from the emulsion, applying the resultant liquid suspension to an electrical steel sheet, and baking thus applied steel sheet.
The first object of the present invention is to provide the electrical steel sheet coated with a coating having an excellent punchability, so that the fundamental composition of the treating liquid is composed of chromate and an organic resin as publicly known. In order that these two components are applied by one time coating and subsequent baking to form the coating, chromate and the resin must coexist in an aqueous solution form or a suspension form. The inventors have made the following test to fine the resins capable of coexisting with chromate.
To 100 mQ of an aqueous solution of calcium .B

. .... . . .
.. .. ...

107103~

dichromate of a concentration of 18% tl4~ as CrO3) was added 10 mQ of a resin aqueous solution or a resin emulsion having a concentration of nonvolatile matter of 15-25~ in 5-10 seconds while stirring the aqueous solution of calcium dichromate.
The formed liquids are roughly divided into the following two classes.
(1) The resin is not substantially dissolved in the aqueous solution of chromate and the liquid is separated into two layers or macro-agglomerates having a diameter of more than several mm are formed.
~ (2) The resin is completely dissolred in the aqueous solution of calcium dichromate to form an opaque orange solution and even if said liquid is left to stand for several hours, any change is not observed.
However, when a further test has been made with respect to a large number of resins, the following type resin has been found.
t3) Fine agglomerates are formed and a suspension - is obtained.
If this suspension is left to stand for about 30 minutes, the agglomerates precipitate and a trans-pa~ent portion not containing resin is formed at the upper portion of the ressel.
Any one of the resins classified to this class

(3) are an emulsion type resin and are referred to as "particle separating resins" in the present invention.
The resins belonging to the above class (2) are also an emulsion type resin and are referred to as "compatible " ~07~039 resinsil .
The present invention can provide the elec~rical steel sheets having excellent insulation resistance and weldability by applying a treating dispersion suspending fine particles obtained by compounding particularly defined amounts of particle separating resin and the compatible resin to chromate, on electrical steel sheets and baking the thus applied steel sheet to form coatings.
Furthermore, the formed insulating coatings adhere tightly to the electrical steel sheets and even if a stre~s relief annealing is carried out, the adhesiop is not varied and the insulation resistance is still high and further the formed insulating coatings act to preven~ sticking of mutual steel sheets upon annealing.
As the particle separating resins to be used herein, the following resins are preferable.
Usually, the particle size of an emulsion is ultra-fine of less than 1 ~ but in the present invention, such an emulsion is utilized that when the resin emulsion is added to the aqueous solution of chromate, the agglomerates having a diameter of several microns '-.
to several ten microns are separated.
In the present invention, it is preferred that the particle size distribution of the agglomerates measur0d by the specific gravimeter method and the photosedimentation method, is 3-40 ~ in mode si~e diameter tparticle size showing the highest point of peak of the particle size distribution curve) or median size diameter (particle size corresponding to the center cumulative value (50%) of the cumulative curve).

.... ...... . ..
.

When the av~rage particle size is less than 2 ~, the surface roughness of the formed coating is small and the weldability cannot be remar~ably improved and when the average particle size exceeds 40 ~,- the dispersion is not sati~sfactory and the particles are precipitated on the bottom of the treating vesslsl and the amount of the particles not effectively used becomes large.
The invention will now be further described with reference to the accompanying drawings, in which:
Figure 1 illustrates a preferred particle size distribution of agglomerates, Figure 2 illustrates the compounding ratio of the particle separating resin and the compatible resin to the stripe pattern of the formed coating and the punchability of the coated steel sheets.
Figure 3 illustrates the compounding ratio of the particle separating resin and the compatible resin to the surface roughness of the formed coat-ing and the weldability of the coated steel sheets, Pigure 4, l(a) and l(b) are photographs of the cross-section and surface respectively of a coating obtained in the following Example 1, Pigure 4, 2(a) and 2tb) are photographs of the cross-section and surface respectively of a coating obtained in the following Example 2, Figure 4, 3(a) and 3(b) are photographs of the cross-section surface respectively of a coating obtained in the following Comparative Example 1, and Figure 5 illustrates the relation of burr height to number of punching timer for the coated steel strips of all the following examples.
A preferred particle size distribution of the agglomerates is shown in Figure 1. The agglomerates are soft and are very easily deformed, so that when the suspension dispersing the agglomerates having such a particle size is coated on an electrical steel sheet and baked, flat projections having a height of 1-10 ~ and a diameter of 3-50 ~ are formed and a moderate roughness is provided on the coating and the weldability and the insulation resistance can be improved. When the coated and baked agglomerates are analyzed by X-ray microanalyzer, chromium, oxygen and magnesium, calcium, zinc or aluminum which are elements for constituting the ~ _ 9 _ tB

1~71~39 chromate used in the treating liquid are detected in a fairly large amount together with carbon of a constituting element of the resin, so that it is cons:idered that the deposited particle reacts with the chromate during baking. Such excellent properties of the formed coa~ings according to the present invention that said coatings act to prevent sticking of mutual steel sheets upon a heat treatment at a high temperature, such as stress relief annealing, and have a high interlayer resistance even a~ter annealing~ are based on the fact that in the projections on the coating brought about by - 9a -B

.

the deposited particles, a large amount of chromate of an inorganic substance is contained.
As the resin emulsion which forms such agglomerates, for example, the following acrylic resin emulsion is preferable. 5-13 parts by weight of d ,~-e-thylenically unsaturated carboxylic acid, such as methacrylic acid and 95-87 parts by weight of ¢,~-ethylenical monomer, such as methyl methacrylate are copolymerized together with at least one of emulsifiers to form a resin emulsion. To the formed emulsion is added 0.8-5.5 parts by weight based on 100 parts by weight of the resin emulsion, of a water soluble amine.
The resin composition described above has been determined by a large number of tests and when the ranges defined herein are not satisfied, the agglomerated particles are not formed or when the amine is added, the viscosity is extremely increased and such a compo-sition is not preferable.
However, the resins to be used in the present invention are not limited to the above mentioned acrylic resin and any resin which forms the given agglomerated particles by the same test as described above, can be used. However, the concentration of the nonvolatile matter of the particle separating resin in the emulsion is important and is preferred to be 5-60%. When the concentration of the nonvolatile matter is less than 5%
even if such a resin emulsion is compounded to the aqueous solution containing chromate, the agglomerates having an average particle size of not less that 3U are not formed and when the concentration is more than 60%, the 10'7103~
macro-agglomerates are formed and such a concentration is not preferable.
On the other hand, the compatible resins are ones belonging to the above described class (2) but in the case of acrylic resin, the resin in which C(,~-ethylenically unsaturated carboxylic acid is less than 5 parts by weight and the water soluble amine is less than 0.5 part by weight, is preferable. However, the compatible resin is not limited to the acrylic resin and any resins which are completely dissolved to form an opaque orange solution, which does not show variation even after left to stand for several hours, when said resins are added to the aqueous solution containing chromate, may be used and at least one of vinyl resin, amino resin, alkyd resin, melamine resin, silicone resin and styrene resin may be used.
The mixture ratio of chromate and the resins, when the coating of the present invention is formed by using the particle separating resin and the compatible resin, is as follows. Namely, it is necessary that the total amount of the nonvolatile matters of the particle separating resin and the compatible resin is 5-150 parts by weight based on 100 parts by weight of chromate expressed as chromic acid (CrO3) and that the amount of the nonvolatile matter of the particle separating resin is at least 1 part by weight among the above described total amount and further is at least 5% by weight based on the total amount of the nonvolatile matters of both the particle separating resin and the compatible resin. These requirements are based on the ., . ~
: . - . : . . . - . .
: .: .. . . :
-: . . . : :

107~039 fol~wing ~xpcrimental data.
When the total amount of the nonvolatile ma~ters Gf both the resins based on 100 parts by weight of chromic acid (CrO~) is less than S parts, the S punchability is not improved. While, when said amount exceeds 150 parts, the ratio of the resins in the coating becomes too large, so that when stress relief annealing is effected, the film is carbonized and consequently the insulation resistance of the steel sheet becomes insufficient. Moreover, the punchability is deteriorated and even if any resin is selected, the weldability is deteriorated.
The punchability and the insulation resistance -can be ensured by selecting the total amount of the non-15 volatile matters of the particle separating resin and the compatible resin as defined above but the insurance of the weldability and the further impro~ement of the insulation resistance and the heat resistance can be accomplished by compounding the particle separating resin in an appropriate amount. That is, among the above mentioned amount of 5-150 parts of both the resins, at least ~ part by weight is occupied by the particle separating resin and further the amount of the non-volatile matter of the particle separating resin is at least 5% based on the total amount of the non-volatile matters of both the resins.
Even when the compatible resin is not used at all, the steel sheets with a coating having a high surface roughness and sufficient punchability and weldability can be obtained by compounding 5-lSO parts by weight of the .

~71039 particle separating resin.
However, if the treating liquid containing a large amount of particle separating resin is coated on an electrical steel strip industrially in a high speed, stripe 5 patterns having a width of several millimeters may be formed.
These stripe patterns are observed when the treating liquid dispersing particles is poured on the steel sheet surface or the steel strip is dipped in said treating liquid and then the steel strip applied with the trea-ting liquid is squeezed by grooved rolls to adjust the coating amount, as a process for coating said treating liquid, and when the rate of the steel strip to be coated becomes fast, the stripe patterns are liable to be formed on the steel sheet surface in contact with the lower squeezing roll.
The distance between the stripes with each other is several centimeters to several ten centimeters and the appearance of the product is deteriorated.
In order to prevent the formation of the stripe patterns, the inventors have made checked the composition of the treating liquid and as the result, it has been found that if the amount of the nonvolatile matter of the particle separating resin is limited to less than 30 parts by weight based on 100 parts by weight of chromic acid (CrO3), no stripe patterns are formed even if the coating is conducted at a high rate.
Furthermore, it has been found that when the compatible resin is added to the aqueous solution containing chromate in an amount of the nonvolatile matter of said resin being not less than 5 parts by weight based on 100 parts by weight of chr-omic acid (CrO3), the .' . '- ' :'.-:. -' ' . ' . ' '" : : ' ' ~ O 3 9 puncllabili~y ic improved.
Fi~s. 2 and 3 show the relations o~ the compounding ratio of the particle separating resin and the compatible resin to *he stripe pattern and surface roughness of the formed coating, and the punchability and weldability of the coated steel sheets. The steel strip, the treating liquids, the coating and baking conditions, and the test method are as follows~
S~eel strip:
0.4$ silicon steel strip.
Surface roughness 1 ~Hmax.
Treating liquid:
calcium dichromate ~ ethylene glycol.
Compounding ratio of the resins are the weight ratio of the nonvolatile matters of the resins based on 100 parts by weight of chromic acid.
Particle separatîng resin:
acrylic resin emulsion.
Compatible resin: ~ -acrylic resin emulsion. ~`~
Coated amount:
2-5 g/m2 (per one side after baking).
Baking:
400C, l minute.
Stripe pattern:
Formation degree when the coating is carried out at a rate of 60 m/min.
Weldability:
Compressing pressure100 kg/cm2 . , . , - :
. . . , ~ - . . ~

~ O 3~

Current 100A No groove.
Maximum welding rate when blowholes are not formed.
Punchability:
S 15 mm~ disc punching, using steel die.
Punched times until the burr height reaches 50 ~.
The formation of the stripe patterns becomes gradually remarkable, in the case of no addition of the compatible resin, when the amount of the particle separating resin exceeds lS parts by weight based on 100 parts by weight of CTO3. If the compatible resin is compounded, the tendency for forming the stripe patterns is restrained and when more than lS parts by weight of the compatible resin is compounded, even if a treating liquid compounded with 30 parts by weight of the particle separating resin is coated at a rate of 60 m/min, the stripe patterns are not formed but if said resin is compounded in an amount of more than 50 parts by weight, the formation of the stripe patterns cannot be avoided. Accordingly, when it is desired not to form thé stripepatterns, the amount of the particle separating resin is limited to less than ~c 30 parts by weight based on 100 parts by weight of CrO3~
On the other hand, when 1 part by weight based on 100 parts by weight of CrO~, of the particle separating resin is compounded, the surface roughness of the formed ilm becomes large and the weldability of more than 60 cm/min can be obtained and when 5 parts by weight is oompounded, the weldability of a high rate of more than 100 cm/min can be obtained. However, when the amount exceeds 15 parts by weight, the surface roughness ... . ~ ~-- . -- . ~ - . .
. . , - ., : ,~ - - . : . -.

~07~039 becomes too large and the space factor lowers, so that when such a drawback is not desirable, the amount is preferred to be less than 15 parts by weight. Considering these results, the compounding ratio of the particle separating resin is preferred to be 1-30 parts by weight based on 100 parts by weight of CrO3 and in order to obtain the best result, the amount is preferred to be 5-15 parts by weight.
As seen from Fig. 2, when only the particle separating resin is added to the aqueous solution of chromate as the resin within the range in which the stripe patterns are not formed, that is in the case of less than 15 parts by weight, even if the improvement of the punchability can be observed, the punchability of a high degree exceeding 1,000,000 times cannot be obtained.
This is because when only the particle separating resin is added to the aqueous solution containing chromate said resin forms fine agglomerates and is not dissolved in said aqueous solution, so that the solution portion in the formed treating liquid does not substantially contain said resin. When such a treating liquid is applied on the steel sheet, the fine agglomerates form convex portions of the formed coating and the coating of the other portion does not substantially contain the resin. Accordingly, when the compatible resin is compounded~in an amount of not less than 5 parts by weight, the punchability is suddenly improved. In this case, when the compounding amount of the particle separating resin is less than 2% based on the total amount of both the particle separating resin and the . .. ... , . . _ .. ~.

~07~ 9 compatible resin, the coating having the desired sllrface roughness cannot be formed and the high rate of weldability cannot be obtained.
When the compounding amount of the compatible 5 resin becomes too excess, the punchability is rather deteriorated and further the weldability and the heat resistance lower, so that the compounding ratio of the compatible resin is preferred to be less than 120 parts by weight based on 100 parts by weight of CrO3. The particularly preferable range in view of the punchability, weldability and heat resistance is 10-50 parts by weight of the compatible resin.
The aqueous solutions of chromates to be used in the present invention include the aqueous solution of at least one of chromates or dichromates of bivalent or trivalent metals of magnesium, clacium, zinc, aluminum and the like or chromic acid (CrO3) added thereto. For example, when a dichromate solution is prepared from a bivalent metal oxide and CrO3, if CrO3 is added in an amount of more or less larger than the stoichiometric quantity, the stability of the treating liquid, when the resins are added, is higher. Furthermore, to the aqueous solution of the chromates may be added the compounds of boric acid or phosphoric acid.
Particularly, boric acid improves an ability for preventing the sticking without deteriorating the other properties of the coating, so that the addition of boric acid is preferable.
The concentration of these aqueous solutions of chromates must be 3-40~ as CrO3. When the ' ' 107~ ~39 concentratlon o CrO~ is lower than 3%, even if the particle separating resin is compounded, the agglomerated particles having an average particle size of not less than 3 ~ ~re not substantially formed, while when said concentration exceeds 40~, the agglomerates having a macrodiameter are formed.
In the treating liquids containing chromates, a reducing agent of CrO3 is compounded but in the treating liquids in which an organic resin is compounded, the resin has an abiiity for reducing CrO3, so that it is not always necessary to add the reducing agent. However, if the coating formed after baking absorbs moisture, it is preferable to add polyhydric alcohols and saccharides, such as ethylene glycol, glycerine~ sugar and glucose as a reducing agent.
Although the compounding amount of these substances depends upon the compounding amount of the resins, 10-60 parts by weight based on 100 parts by weight of CrO~ in the aqueous solution containing chromate is preferred.
The steel sheets aimed at in the present inrention may be anyone which can be used as the electrical steel sheets and it is not necessaTy to adiust the surface roughness. As the pre-treatment, any specific treatment is not necessary, unless a large amount of oil or rust is deposited.
In the preparation of the treating liquid, while stirring an aqueous solution of a chromate containing about 3-40% of CrO~, a given amount of the particle separating resin emulsion having a , - l8 -1 ~ 7 1 ~ 3~

concentra~ion of the nonvolatile matter of the resin being 5-G0% is added thereto and then the compatible resin, if necessary, a reducing agent of the above described polyhydric alcohol or saccharide are added.
The compounding order may be varied.
The method for coating and baking the treating liquid of the present invention is completely the same method as the general method for coating and baking the treating liquid. Namely, the electrical steel sheet is dipped in the treating liquid or poured and then the coated steel sheet is squeezed by grooved rolls to adjus~ the thickness of the coa~ing and then baked in a furnace at a temperature of 300-700C for an appropriate time. In the treating liquid, the fine agglomerates are 15 dispersed in the treating liquid but unless the concentra~ion o the treating liquid is a low concentration, such as lower than 3%, the precipitation is not substantially noticed and it is not necessary to particularly effect stirring.
The following examples are given for the purpose of illustration of this invention and are '~
not intended as limitations thereof. In the examples, - "part" and "%" mean by weight.
~xample 1 An acrylic resin having a monomer composition composed of 82 parts of methyl methacrylate, lO parts of butyl acrylate and 8 parts of methacrylic acid was used as a particle separating resin. To 100 Q of an emulsion of this resin, whose concentration of non volatile matter was 42%, was added 100 Q of water to - . .

~1071039 prepare a diluted resin emulsion having a concen-tration of nonvGlatile matter of 21%.
To 100 ~ of a 32% aqueous solution of magnesium dichromate (concentration as CrO3 is 26.6%) was added gradually 20 ~ of the above diluted resin emulsion ~the amount of` nonvolatile matter of the particle separating resin emulsion is 12 parts based on 100 parts of CrO3) under stirring to separate out particles, and further to the resulting mass were added 10~ of a compatible acrylic resin emulsion having a concentration of nonvolatile matter of 50% (trademark Voncoat 4001) (the amount of nonvolatile matter of the compatible resin emulsion is 14 parts based on 100 parts of CrO3), 5 ~ of ethylene glycol (16 parts based on 100 parts of CrO3), 5 kg of boric acid (15 parts based on 100 parts of CrO3) and 300 ~ of water to prepare a treating dispersion.
An electrical steel strip of 0.5 mm thickness and 940 mm width containing 0.32% of Si and having a surface roughness of 0~9 ~ max was immersed in the treating dispersion at a rate of 60 m/min, squeezed by means of grooved rubber rolls and baked in a hot-air furnace kept at 400 C for 60 seconds to obtain a coating having no gloss and stripe pattern.
Properties of the resulting coating are shown in the following Table 1 together with properties of the coatings formed in the following Examples and Comparative Examples.
Example ?
A vinyl acetate-ethylene copolymer having . . ~ ~ . .
'' - , ~

~ ~ 7 ~ ~ 3~

a mononer co~posi~ion of vinyl acetate : ethylene -80 : 20 was used as a particle separating resin, in the form of an emulsion, whose concentration of a nonvolatile matter was 55~. To 100 Q of a 30~ aqueous S solution of calcium dichromate ~concentration as CrO~
is Z3.4~) was added gradually 4 Q of the above resin emulsion (the amount of nonvolatile matter of the particle separating resin emulsion is 6 parts based on 100 parts of CrO3) under stirring to separate out particles. Then, to the mass were added 40 Q of a compatible acrylic-styrene resin emulsion having a concentration of nonvolatile matter of 40~ (trademark Voncoat 4280) tthe amount of nonvolatile matter of the compatible resin emulsion is 48 parts based on 100 parts of CrO~) and 300 Q of water to prepare a treating dispersion.
An electrical steel strip of 0.5 mm thickness and 940 mm width containing 0.32% of Si and having a surface roughness o 1.4 ~HmaX was immersed in the treating dispersion at a rate of 80 m/min, squeezed by means of grooved rubber rolls and baked in a hot-air furnace kept at 500C for 45 seconds to obtain a ``
coating having no gloss and stripe pattern.
Ex_mple 3 An acrylic resin having a monomer composition of methyl methacrylate:ethyl acrylate:methacrylic acid- 70:20:10 was used as a particle separating resin.
To 100 Q of an emulsion of ~his resin, whose concentration of non~olatile matter was 43~, was added 200 Q of water to prepare a diluted resin emulsion having a concentration of nonvolatile matter of 14.3%.

~ 21 -iO71~39 To 100 R of a 29% aqueous solution of zinc dichromate (concentration as CrO3 is 20.6%) was added gradually 160 ~ of the above diluted emulsion (the amount of nonvolatile matter is 83 parts based on 100 parts of CrO3) under stirring to separate out particles, and further to the mass were added 2 ~
of glycerin (11 parts based on 100 parts of CrO3), 5 kg of boric acid (18 parts based on 100 parts of CrO3) and 250 Q of w~ter to prepare a treatlng dispersion.
An electrical steel strip of 0.5 mm thickness and 940 mm width containing 0.92% of Si and having a surface roughness of 1.5 ~ ax was coated at a rate of 30 m/min, and the above prepared treating dispersion was sprayed on the steel strip, squeezed by means of grooved rubber rolls and baked in a hot-air furnace kept at 350 C for 120 seconds to obtain a coating having no gloss and stripe pattern.
Comparative Example 1 To 100 ~ of a 30% aqueous solution of calcium dichromate were added, while stirring, 20 ~ of the same compatible acrylic resin emulsion as used in Example 1 (the amount of the nonvola-tile matter of the resin emulsion is 28 parts based on 100 parts of CrO3), 7 ~
of ethylene glycol (17 parts based on 100 parts of CrO3) and 300 ~ of water to prepare a treating liquid.
The resulting treating liquid was applied on an electrical steel strip (thickness 0.5 mm, width 940 mm, Si content 0.30%, surface roughness 1.4 ~HmaX) at a rate of 60 m/min by means of grooved rubber rolls, baked in ~071~39 a hot-air furnace kept at 400C for 60 seconds to obtain a glossy coating having a uniform appearance.
Comparative Example 2 To 100 Q of a 18~ aqueous solution of calcium S dichromate were added 5 Q of ethylene glycol and a very small amount of a surfactant to prepare a treating liquid.
The treating liquid was applied to the same electrical steel strip as used in Comparative Example 1 and treated in the same manner as described in Comparative Example 1 to obtain a uniform glossy coating.
Comparative Example 3 In 100 Q of a 35% aqueous solution of magnesium primary phosphate were dissolved 6 kg of CrO3 and 10 kg of AQtNO,)J-9H20, and then 100 Q of water was added to the solution to prepare a treating liquid.
The tTeating liquid was applied to the same s~eel strip as used in Comparative Example 1 and treated at a rate of 60 m/min by means of grooved rubber rolls and baked in an electric furnace kept at 450C for 60 seconds to obtain a glossy colorless transparent coating.
Photographs of the cross-section and surface of the coatings obtained in Examples 1 and 2 are shown ~, in Pigs. 4, lta), l(b), 2(a) and 2(b), respectively.
For comparison, pho~ographs of the cross-section and surface of the conventional coating obtained in Comparative Example 1, that is, obtained by using a treating liquid, which has been prepared by compounding only a compatible resin to an aqueous solution of chromate, are shown in Figs. 4, 3(a) and 3tb).

- ~3 -~0'7~039 The conventional coating obtained by using an aqueous solution of chromate compounded with only a compatible resin is very smooth, while the coating according to the present invention is rough. That is, the coating of Comparative Example 1 had a surface roughness of 1.3 ~HmaX, while the coatings of Examples 1, 2 and 3 had surface roughness of 4.3, 3.4 and 6.8 ~HmaX, respectively-It can be seen from the photographs of the surfaces of coatings shown in Figs. 4, l(b) and 2(b)that the coating according to the present invention contains a large number of deposited particles.
It can be seen from Table 1 that the coating according to the present invention has a very high insulation resistance, is excellent in the adhesion and in the heat resistance and does not exfoliate after stress-relief annealing.
The coated steel strips according to the present invention can be welded at a high speed and further are excellent in the punchability as shown in Fig. 5.
In the present invention, the weight of the coating should be 1.2-4.8 g/m per one side. When the coating weight is less than 1.2 g/m , the punchability and insulation resistance of the coated steel strip are poor. While, when the coating weight exceeds 4.8 g/m , the space factor of the coated steel strip is deteriorated.

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Claims (12)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for producing coated electrical steel sheets having excellent punchability, weldability and insulation resistance, which comprises compounding an aqueous solution of chromate with a resin emulsion having a concentration of nonvolatile matter of 5-60%, which on compounding separates as fine agglomerated particles having an average particle diameter of 3-40 µ, and with a resin having a compatibility to the aqueous solution of chromate, governed by the fact that both of the above resins are present in an amount such that the total amount of nonvolatile matter of both resins is 5-150 parts by weight based on 100 parts by weight of chromate expressed as chromic acid (CrO3), and further governed by the fact that the amount of the nonvolatile matter of the former resin emulsion is at least 1 part by weight based on 100 parts of chromate expressed as chromic acid (CrO3) and at least 2% by weight of the total amount of nonvolatile matter of both of the above resins, in order to effect the separation of the fine agglomerated particles from the emulsion, applying the resultant liquid suspension to an electrical steel sheet, and baking thus applied steel sheet.

2. The method as claimed in claim 1, wherein to said treating dispersion is added 10-60 parts by weight based on 100 parts by weight of chromic acid (CrO3), of at least one of polyhydric alcohols and saccharides as a reducing agent.

3. The method as claimed in claim l or 2, wherein said particle separating resin is added in such an amount that the nonvolatile matter of said resin is 1-30 parts by weight based on 100 parts by weight of chromic acid (CrO3).

4. The method as claimed in claim l or 2, wherein said compatible resin is added in such an amount that the nonvolatile matter of said resin is 5-120 parts by weight based on 100 parts by weight of chromic acid (CrO3).

5. The method as claimed in claim 1 or 2, wherein the particle separating resin and the compatible resin are added in such an amount that the nonvolatile matters of said particle separating resin and said compatible resin are 5-15 parts by weight and 10-50 parts by weight based on 100 parts by weight of chromic acid (CrO3), respectively.

6. The method as claimed in claim 1 or 2, wherein boric acid or phosphoric acid is further added.

7. The method as claimed in claim 1 or 2, wherein the baking temperature is 300-700°C.

8. The method as claimed in claim 1, wherein the particle separating resin is an acrylic resin emulsion obtained by copolymerizing 5-13 parts by weight of .alpha.,B-ethylenically unsaturated carboxylic acid and 95-87 parts by weight of .alpha.,B-ethylenical monomer together with at least one of emulsifiers added with 0.8-5.5 parts by weight based on 100 parts by weight of the emulsion, of a water soluble amine.

9. The method as claimed in claim 1 wherein the compatible resin is an acrylic resin containing less than 5 parts by weight based on 100 parts by weight of the total monomers of .alpha.,.beta.-ethylenically unsaturated carboxylic acid and less than 0.5 part by weight based on 100 parts by weight of the emulsion, of a water soluble amine.

10. The method as claimed in claim 1, wherein the chromate is at least one of salts of calcium, magnesium, zinc and aluminum.

11. The method as claimed in claim 1 or 2, wherein the aqueous solution containing chromate contains free CrO3.

12. The method as claimed in claim 1 or 2, wherein a weight of the coating is 1.2-4.8 g/m2 per one side.