Note: Descriptions are shown in the official language in which they were submitted.
1071039
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,
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'
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
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~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
- 7 -
" ~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).
- 8 -
.... ...... . ..
.
1071039
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
.
1071039
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 --
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
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.
~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
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.' . '- ' :'.-:. -' ' . ' . ' '" : : ' '
~ 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
- 14 -
. , . , - :
. . . , ~ - . . ~
~ 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
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... . ~ ~-- . -- . ~ - . .
. . , - ., : ,~ - - . : . -
.
~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
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. .. ... , . . _ .. ~.
~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
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~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
- 20 -
. . ~ ~ . .
'' - , ~
~ ~ 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%.
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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
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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).
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~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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Image
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